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J une

1988

THE REMOVAL OF OI L FROM OI L - WATER MI XTURES

USI NG SELECTI VE OI L F I LTRATI ON

Paul a Magd i ch

Dr . Mi chael Semmens

Uni ver s i t y

o f

Mi n ne s o t a

De pa r t me nt o f Ci v i l and Mi ne r a l Engi ne er i ng

Mi n ne ap ol i s , MN 55455

Pr o j ec t Of f i c er

J ames S . Br i dge s

Of f i c e o f Env i r o nme nt a l E ng i n ee r i n g and Te c hno l o gy De mo ns t r a t i o n

Hazar dous Was t e Engi neer i ng Resear ch Labor a t o r y

Ci nc i nnat i , OH 45268

Thi s s t udy was conduct ed t h r ough

Mi n ne s o t a

Was t e Manage ment Boar d

S t .

P a ul , MN

5 5 1 0 8

and t he

Mi n ne s o t a T ec hni c a l As s i s t a nc e P r o gr a m

Uni v er s i t y of Mi nne s ot a

Mi n ne ap ol i s , MN 55455

HAZARDOUS WAST E ENGI NEERI NG RESEARCH LABORATORY

OFF I CE OF RES EARCH AND DEVEL OPMENT

U. S. ENVI RONMENTAL P ROTE CTI ON AGENCY

CI NCI NNAT I , OH 45268

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This project was partially supported with a United States

Environmental Pr otection Agency cooperat ive agreement through the

Minnesota Wa ste Management Board and the Minnes ota Technical

Assistance Program.

Although th e research described i n this report has been funded in

part

b y

the United States Environmental Protection Agency through

a cooperative agreement,

i t

has not been subjected t o Agency

review, and therefore does not necessarily reflect the views

of

the Agency and

no

official endorsement should be inferred.

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PROJ ECT SUMMARY

MlNNESOtA

q kHNICAL A k%STANCEPROGR

UNfVERSlTY OF MINNFSOTA

- - -

. .

BOX 197

MAYO, 420 DELAWARE

ST.

S.E.

MINNEAPCLIS, MINNESOTA 55455

T he Remov al of Oi l f r om Oi l - Wat er Mi x t ur e s Us i ng Se l e ct i v e Oi l

F i l t r at i on

Paul a Magdi ch and Mi chael Semmens

Bas ed on t he f i ndi ngs o f t hi s s t udy i t a pp ea r s t hat

t he

s el ec t i ve

oi l f i l t r a t i on ( SOF) pr o c es s h as po t e nt i a l a s an a l t e r na t i v e me t ho d

f or r emov i ng oi l f r om oi l y was t ewat er s

or

f r om ot her o i l - c ont ai ni ng

was t e s . T he r es ul t s i ndi c at e t hat t he SOF pr o ce s s i s c apa bl e of

r emo vi ng f r e e, me cha ni c al l y di s per s ed and c he m c al l y emul s i f i ed oi l

f r om oi l - wat e r m xt u r es . SOF i s not s ui t abl e f o r t he r e moval o f oi l -

wet s o l i ds , however , and pr i or t o f i l t r at i on, t he s us pended s ol i ds

must be r emoved t o pr event membr ane f oul i ng.

I nt r oduc t i on

Oi l - c o nt a i ni ng wa s t e wa t e r s a r e ge ne r a t e d at a n e s t i ma t e d r a t e o f

mo r e t han o ne bi l l i o n ga l l o ns per y ea r i n t he Uni t e d St a t e s . Ge ne r a -

t o r s .of t h es e wa s t e s t r e ams a r e e x t r e me l y v ar i e d and ma y i n c l u de

pet r o l eum r e f i ner i es , met a l f abr i c at i on pl ant s , r o l l i ng m l l s , c hem -

c a l pr o ce s s i ng pl a nt s , ma c hi ne s h o ps , and v ehi c l e ma i nt e na nc e s hops .

Ma ny di f f e r e nt t ypes o f o i l s ma y be pr e s ent i n o i l y wa s t e wa t e r s u c h

as di es el f ue l , c ut t i ng and gr i ndi ng o i l s , l ubr i c at i ng o i l s , wat er

s ol ubl e c ool a nt s , nat ur al ani mal or veget abl e f at s o r any ot her

o r g ani c i mm s c i bl e i n wat er . T he r e mov al o f t he s e o i l y wa s t e s f r om

wast ewat er i s o f i mpo r t anc e i n pr ev ent i ng pol l ut i on a nd me et i ng env i -

r o nme nt a l c o mp l i a nc e s t a nda r ds . Oi l y wa s t e r e mo v al ma y al s o b e

benef i c i al f or wa t e r a nd oi l r e co ve r y and r e us e.

Sev er al di f f er ent met hods a r e c ur r ent l y av ai l abl e f or s epar at i ng

o i l

f r om oi l y was t ewat er s , but t hey ar e gener al l y l i m t ed i n t he

f o r ms of o i l t hat t he y c a n r e mo ve . Oi l y wa s t e wa t e r s co nt a i ni ng

c hem c al l y emul s i f i ed oi l ar e par t i c ul ar l y di f f i c ul t t o s epar at e , and

t he opt i ons a vai l abl e f o r t r eat i ng t hem a r e f e w i n number .

I n t hi s s t udy ,

a

nove l membr ane separ a t i on p r ocess , t e r med

s el ec t i ve

o i l

f i l t r at i on ( SOF) was s t udi ed as an al t er nat i v e s epar a-

t i on t e c hni que f or r emov i ng emul s i f i ed oi l f r om oi l - wat er m xt ur es .

SOF

i s s i m l ar t o ul t r af i l t r at i on i n t hat m c r o por ous membr anes a r e

us ed. I t i s unl i ke ul t r af i l t r at i on, however , i n t hat oi l r a t her t han

wat er i s s e l e c t i v el y r e mo ve d by a ppl y i ng a pr e s s ur e a cr o s s t he

membr ane ( F i gur e 1) .

T he pur po s e of t hi s r e s ea r c h wa s t o de r i v e s o me unde r s t a ndi ng o f

how and why t he SOF pr o c es s wo r k s a nd t o de t e r m n e s o me o f i t s

l i m t at i ons . Mor e s pec i f i c al l y t he obj ec t i ves o f t hi s r e s ea r c h we r e

as f o l l ows : ( 1) d emons t r a t e t hat oi l c oul d be r e mov ed f r om oi l - wat er

m xt ur es us i ng s el ec t i ve oi l f i l t r a t i on; 2) det er m ne t he ef f ec t s of

v ar i ous o pe r a t i ng pa r a me t er s s uc h a s pr es s u r e , f eed f l owr a t e , o i l

v i s c os i t y and oi l c onc ent r a t i o n on pr oc es s p er f o r manc e,

( 3 )

det er m ne

t he ef f ec t o f e mul s i o n s t a bi l i t y and t h e pr es enc e o f emul s i f y i ng

agent s on t he abi l i t y of t he me mbr a ne t o s e l e c t i v el y r e mo ve oi l , a nd

4) det er m ne t he mec hani s m o f oi l t r ans por t f r om t he bul k s o l ut i on

t o t he pe r me at e s t r e am

1

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P r o c edu r e s

A

c o mpr e h ens i v e l i t e r a t u r e s e ar c h wa s c o nduc t e d on oi l - wat er

s e par at i on t e c hni que s and on t he t heor y o f e mul s i o ns . I n t he l a bo r a -

t or y s t u dy , m c r o po r o us h ol l o w- f i ber me mbr a n es we r e us e d

t o

s e pa r a t e

oi l

f r o m oi l - wat er m xt ur e s us i ng

S O F .

T hi s r es e ar c h was c ar r i ed out

i n t wo s t ages . St age one c onc er ned i t s el f wi t h qual i t at i v el y as s es -

s i ng t h e SOF pr ocess . Sever a l exper i ment s wer e conduct ed i n whi ch a

napht heni c bas e engi ne oi l was r emov ed f r om di f f er ent oi l - wat er

m x t ur es and t h e SOF pr ocess was eva l uat ed bot h i n t he p r esence and

absence of emu l s i f y i ng agent s . Two t ypes o f membr anes wer e t es t ed

and compar ed i n t hese exper i ment s . F r om t h i s wo r k some i n f e r ences

about t he S O F pr ocess wer e made and nar r ower r esear ch goa l s wer e

def i ned. T he ai m o f s t a ge t wo o f t hi s pr oj ec t was t o ac hi ev e t hes e

r e s e a r c h goal s .

Du r i n g s t a ge t wo t h e pe r f o r ma nc e o f t h e

S O F

pr ocess was

a s s e s s e d under mo r e c ar e f u l l y de f i ne d c o nd i t i o ns i n wh i c h do de c an e

was empl oyed as a model

oi l .

Thi s was done by s t udy i n g t he

e f f e c t s o f v ar i o us o pe r a t i ng par a me t e r s . Do de c an e wa s r e mo ve d f r om

do dec a ne- wa t e r m x t u r e s t o whi c h s o di u m dodec y l s u l f a t e wa s adde d a s

an emul s i f y i ng a ge nt . An a s s es s me nt o f e mul s i o n s t abi l i t y as a

f unc t i on o f e mul s i f i er c onc ent r a t i o n was a l s o ma de and c o r r el at e d t o

t he obs er v ed r es ul t s .

Re s ul t s a nd Di s c u s s i o n

Thi s s t ud y de mons t r a t ed t hat o i l c an b e s e l ec t i v el y r e mo ved

f r o m oi l - wat er m xt ur es us i ng m c r o por ous pol ypr opyl ene f i ber s , i n

bot h t h e pr e s enc e and a bs e nc e o f e mul s i f y i ng age nt s . T he e f f ec t o f

v ar i ous ope r a t i ng pa r a met er s on oi l r emov al wa s i n ve s t i ga t e d, and t he

c ont r ol l i ng mec hani s ms of oi l t r ans por t wer e i dent i f i ed under di f f e-

r ent o per at i ng c ondi t i ons . The i mpac t o f s ur f ac t a nt s on pr oc es s

pe r f o r manc e was e xa m ne d, and t he r o l e o f s u r f a c t a nt s i n t he

SOF

pr o c es s wa s de t e r m ne d.

I nf l uenc e of Oi l Vi s cos i t y: Whi l e bot h oi l s wer e ef f ec t i vel y

r emoved by t he membr ane pr ocess , t he r esponse o f t he o i l f l ux r a t es

t o c hanges i n oper at i ng c ondi t i ons s h owed s i gni f i c ant di f f er enc es i n

behav i or due t o v i s c os i t y . The hi gh v i s c os i t y napht heni c oi l c aus ed

l ow per me at e f l u xe s t o be o bt a i ne d. T he l o w v i s c o s i t y do de c ane

passed t h r ough t he membr ane mor e eas i l y , and hi gher per meat e f l uxes

we r e obt ai ne d.

I nf l uenc e o f Oi l Conc ent r at i on: Changes i n oi l c onc ent r at i on

appea r e d t o hav e a s i m l a r e f f ec t on t he r e mov al o f b ot h t he naph-

t he ni c oi l and do de ca ne .

As

t he

oi l

c onc ent r at i on i nc r eas ed, t he

r at e o f oi l r emov al al s o i nc r eas ed. Thi s i nc r eas e was not f ound t o

be l i nea r , ho wev er , o ver t he oi l c onc ent r at i on r a nge s t es t ed. I n

ge ner a l i t appe ar e d t hat t he ef f ec t o f o i l c onc ent r at i o n wa s mo r e

ma r ke d at l o w o i l c o nc e nt r a t i o ns a nd

i t

bec ame l es s s i gni f i c ant at

hi gher oi l c onc ent r at i ons . Ther e appear ed t o be a c r i t i c al c onc en-

t r at i on bey ond whi c h f ur t her i nc r eas es i n oi l c onc ent r at i on had

l i t t l e ef f ec t on t he per meat e f l ux and t he val ue o f t hi s cr i t i c al oi l

c onc ent r at i on dec r eas e d wi t h i nc r eas i ng f eed f l owr at e.

I n f l u en c e o f Sur f a c t a nt s : T he r e mo va l o f bo t h dodec a ne and

na pht he ni c oi l wa s a dv er s e l y af f e ct ed by t he pr e s enc e of s u r f a ct ant s

2

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and t he per meat e f l uxes decr eased as a r es ul t . The napht heni c oi l

e xper i me nt s de mo ns t r at ed t hi s be ha vi or qua l i t a t i v el y f or a ni o ni c and

noni o ni c emul s i f y i ng a ge nt s , whi l e t he dodec ane ex per i ment s quant i f i ed

t he ef f ec t of a s i ngl e ani oni c s ur f ac t ant by measur i ng t he per meat e

f l ux as a f unc t i on of s ur f ac t ant c onc ent r at i on. The per meat e f l ux

dec r eas ed wi t h i nc r eas i ng s ur f ac t ant c onc ent r at i on, al t hough t he f l ux

dec l i ne was mor e s i gni f i c ant f or s mal l addi t i ons o f s ur f ac t ant and

i t

bec ame l es s dr amat i c wi t h f ur t her s ur f ac t ant addi t i ons .

Concl us i ons and Recommendat i ons

The f o l l owi ng c onc l us i ons we r e dr awn f r o m t hi s s t udy on t he

s el ec t i ve f i l t r a t i on of oi l ac r os s m c r opor o us f i ber s

o f

pol y pr opy l ene.

1.

Sel ec t i ve oi l f i l t r at i on c an be us ed

t o

r ec o ve r a wa t e r - f r ee

oi l pr oduc t f r om oi l - wat er m x t u r es i n bot h t he pr es enc e and

a bs e nc e of emul s i f y i ng agent s .

2 .

L ow vi s c os i t y o i l s , s uc h as dode cane, a r e mor e ef f i c i ent l y

r emov ed t han hi gh v i s c os i t y oi l s s u c h as napht heni c o i l .

Thi s di f f er enc e i n r emov al ef f i c i enc y

i s

due t o di f f er enc es

i n t he c ont r ol l i ng mec hani s m of oi l t r ans por t .

3 . I n t hi s s t udy , t he t r ans por t o f hi gh vi s c os i t y oi l s was

membr ane l i m t ed wher eas t he t r ans por t o f l ow v i s c os i t y oi l s

wa s l i m t ed by t r a ns por t a nd a t t a c hme nt

t o

t he membr ane and

dr opl et c ol l aps e.

4. The r emov al o f l ow v i s c os i t y oi l s

i s

s t r ongl y af f ec t ed by

s ys t em hydr ody nam c s .

Oi l r emov al r at es i nc r eas e wi t h i n-

c r eas i ng f eed f l owr at e

as

t he oppor t uni t y f o r c o nt a ct be t wee n

t he di s per s ed dr opl et s a nd t he f i ber s

i s

i mpr oved.

5. The r at e of oi l r emov al

i s

af f ec t ed by t he f eed oi l c onc en-

t r at i on.

As

t he oi l c onc ent r at i on i nc r e as es , t he r at e of

oi l r emov al al s o i nc r e as es . Thi s s t udy s howed t hat t hi s

i nc r eas e

i s

not l i near o ver a br oad oi l c onc ent r at i on r ange

and t hat oi l r emov al

i s

l i m t ed at l ow oi l c onc ent r at i ons

( <l ) .

6.

Bot h ani oni c a nd noni o ni c emul s i f y i ng agent s h av e a del e-

t er i ous ef f ec t o n oi l r emov al . T he r at e o f oi l r emov al

de cr ea s es mar k edl y wi t h t he addi t i o n o f t hes e t y pe s o f emul -

s i f y i ng agent s .

7.

The exac t r ol e of emul s i f y i ng agent s i n s e l ec t i v e oi l

f i l

t r at i on

i s

not c l ear . The s e s t udi e s i ndi c at ed t hat emul -

s i f y i ng age nt s may af f ec t oi l r emov al by al t er i ng t he el ec -

t r i c al and/ or mec hani c al pr oper t i es o f t he dr opl et s and t h e

f i ber s . F ur t her s t udi es ar e r equi r ed t o det er m ne t he

ef f ec t o f mec hani c al s t abi l i t y .

8.

El ec t r os t at i c i nt er ac t i ons bet ween t he dr opl et s and t he

f i ber s pl ay an i mpor t ant r ol e i n t he oi l r emov al pr oc es s .

Repul s i v e el ec t r os t at i c i nt er ac t i ons bet ween t he negat i v el y

char ged oi l d r opl e t s and t he membr ane , wh i ch i s a l s o nega -

t i v el y c h ar ge d, hi nde r dr opl e t appr oa ch and at t ac hment .

3

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9. T he oi l r e mo va l pr o c es s c a n be i mpr oved by encour ag i ng

at t r ac t i ve el ec t r os t at i c i nt er ac t i ons bet ween t he dr opl et s

a nd t he f i ber s . On e way o f a c c o mpl i s h i ng t hi s i s

t o

app l y a

po s i t i v e c oa t i ng t o t he me mbr a ne s ur f a ce .

10.

The s el ec t i ve o i l f i l t r at i on pr oc es s has pot ent i al as an

al t er nat i v e oi l - wat er s epar at i on t ec hni que. Thi s pr oc es s

i s

bet t er s u i t ed t han ul t r af i l t r at i on t o t he t r eat ment

o f

o i l y

was t ewat er s wi t h a hi gh oi l c onc ent r at i on or f o r t he t r eat -

ment o f wat er - i n- oi l emul s i ons . I t cannot r epl ac e ul t r a-

f i l t r a t i on, however , f or t he r e moval

o f

oi l f r om di l ut e

o i l y

wa s t e wa t e r s .

4

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F igu re 1: The

SOF

Process

m b r a n e

P

(Oi l + Water)

Bulk

Oi l

C o n c e n t r a t i o n

(C)

Oil

Permeate

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THE REMOVAL OF OIL FROM

OIL-WATER

MIXTURES

USING

SELECTIVE

OIL

FILTRATION

A

THESIS

SUB MllTED TO THE GRADUATE SCHOOL

OF THE

UNIVERSITY

OF

MINNESOTA

BY

PAULA MAGDICH

IN

PARTIAL

FULFILLMENT

OF THE REQUIREMENTS

FOR

THE DEGREE

OF

MASTER OF SCIENCE IN CIVIL ENGINEERING

JULY,

1988

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I

A B S T R A C T

A novel membrane separation process, called selective oil filtration

(SOF),

was studied as a separation technique for removing oil from

oil-

water mixtures.

SOF is

similar to ultrafiltration

(UF)

in that microporous

membranes are used; it is unlike

UF,

however, in that oil rather than

water

is

selectively re mo ved by applying a p ressure across the

mem brane. The main advantage foreseen

in

using SOF is that a water-

free oil product can be obtained.

Rese arch was undertaken with the following objectives in mind: (1 )

demonstrate that oil can be separated from oil-water mixtures using

SOF; ( 2 )

determine the effects of various operating parameters such as

pressure, feed flowrate, oil concentration and module geometry on the

process performance;

(3)

determine the effect of emulsion stability and

emulsifying agents and

(4)

determine the mechanism by which oil is

transported from the bulk solution across the membrane.

.

Hydrophob ic m ic roporous ho l l ow- f i b re membranes were

employed.

The oil-water m ixtures were

fed

on the outside of the fibres

and the oil permeate was collected from the inside of the fibres.

Emulsions containing a l ight engine oil and dodecane were used to

study the behavior

of

the process. Mech anical ly and chem ically

stabilized emulsions were tested. Surfactant concentrations up

to 10%

by weight of oil were examined for

ABS,

Triton

X-102

and sodium

dodecyl sulfate.

The oi l permeate flux was measured as a function of various

operating parameters (pressure, oil concentration, feed flowrate,

surfactant concentration) and the rate controlling mechanisms for oil

transport were identified.

Emulsion stability was asses sed by m easuring

the zeta potential of the dispersed oil droplets and by measuring the

droplet size distribution.

These data were correlated to the results to

determine the role of emulsion stability in the

SOF

process. Finally, the

fibres were coated w ith

N(

-aminoe t hyl)-y-aminopropyl-t rimet hoxysi ane

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i i i

A C K N O W L E D G E M E N T S

There are several people whom wish to acknowledge for helping

to make this research project possible. First and foremost,

I

would like to

thank my advisor, Dr. Semmens, for his technical guidance and

assistance, and

also

for his friendship and patience.

1

would l ike to

thank my other committee members, Dr. Brezonik and Dr. lwasaki

for

taking the time to read my thesis and

for

offering their advice.

Many of the other graduate students in the Environmental

Eng ineering program have contr ibuted to this thesis indirectly, by

offering the ir encouragem ent and friendship. Specifically,

I

wou ld like to

thank Amy Zander and Janice T acconi for helping me in this way.

This research project was funded by the Minnesota Technical

Assistance Program

(MNTAP)

and The Donaldson Company, Inc.

I

would l ike to express my thanks to Cindy McComas of MNTAP for

devoting her time to this project, and for her cooperation and flexibility.

Finally, I would like to thank Andrew Keith for his encouragement

and support, particularly during the final writing stages.

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V

Page

(b) Module Design .......................................................................... 5 4

(c) Mod ule Cleaning

.......................................................................

60

4.3 Experimental Apparatus and Method

..........................................

60

4.4

Zeta Po tential Measurements

.......................................................

62

4.5 Droplet Size Distribution Determ ination...................................... 63

5

.

Experiments and Resu lts ........................................................................ 6 4

5.1 Naphthenic Oil Experiments .......................................................... 6 4

(a) AMT Experiments ...................................................................... 6 4

(c) Celgard Experiments ................................................................

76

(d) Additional Observations ........................................................... 88

5.2 Dodecane Experiments

..................................................................

89

(a) Preliminary Experiments

..........................................................

89

(b)

Countercurrent M0dule-5O/~Dodecane Experiments ......... 91

(c) Cross Flow Module-10% Dodecane Experiments

..............

95

(d)

Cross F low Modu le-5% Dodecane Experiments ............... 100

(b) AMT

Vs

Celgard Experiments

................................................. 71

(e) AEAPTMS Coated Fibre Experiment

...................................

107

6

.

Discussion ................................................................................................ 115

6.1 Discussion

of

Results ..................................................................... 15

6.2 Potential Uses and Limitations

of SO..........................................

126

6.3 futu re Research

Work

....................................................................

30

7

. Conclusions ............................................................................................. 132

References

.................................................................................................................

134

Appendices

................................................................................................................

139

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vi

LIST OF TABLES

Page

Table 1: Szmmary of Oily Wastewater Treatment Methods .............................

19

Table

2:

HLB Method of Emulsifier Selection ..................................................... 28

Table 3: Properties of Celgard Hollow-Fibres X-10 and X-20 .......................... 55

Table 4:

Properties

of

the Celgard Countercurrent

Hollow-Fibre Modules

.............................................................................

57

Table 5:

AMT Module: 100%

Oil;

Baseline Data ................................................ 65

Table 6: Zeta Potential A s a Function of SDS Concentration ......................... 104

Table 7:

A

Comparison of Permeate Fluxes for a 5% Dodecane-Water

Mixture

on

the AEAPTMS Coated and Uncoated Cross Flow

Modules

......................................................................................................

109

Table 8 : Flux Ratios for a 5 Dodecane-Water Mixture on he

AEAPTMS Coated and Uncoated Cross Flow Modu les...................110

Table 9:

Rem oval Ratios for the 5 Dodecane-Water Mixture on the

Uncoated Cross F low Module...............................................................

120

Table 10:

Correlation

of

the Data for the 5 Dodecane-Water M ixture

on the Uncoated Cross Flow Module With Empirical

Expressions for Coalescence Efficiency ............................................ 147

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vii

LIST

OF FIGURES

Page

Figure 1 The SOF Process

.....................................................................................

3

Figure

2:

The Ultrafiltration Process

......................................................................

14

Figure 3: The O riented Wedge Theory

.................................................................

32

(a) An

O M

Emulsion Stabilized By a Monovalent Soap

(b)

A

W/O Emulsion Stabilized By a Bivalent Soap

Figure

4:

Stern Double Layer Theory ................................................................... 36

(a) Distribution

of

Counter Ions

(b)

Potential as a Function of Distance From the Droplet

Surface

Figure

5:

Potential Diagram For the Interaction

of

Two Ch arged Droplets

....

36

Figure 6: Proposed Mechanism

of

Oil Transport for the SOF Process...........46

Figure 7: General Structure of ABS

.......................................................................

51

Figure 8: General Structure

of

Triton Surfactants

...............................................

51

Figure 9: Celgard Countercurrent Hollow-Fibre Module .................................. 56

Figure 10: AMT Countercurrent Hollow-Fibre Module

.......................................

58

Figure 11

:

Celgard Cross Flow Ho llow-Fibre Mod ule .......................................

59

Figure

12:

Experimental Apparatus

.......................................................................

61

Figure

13:

AMT Module; Flux

vs

Pressure

...........................................................

66

Figure 14: AMT Module; Flux

vs

Feed Flowrate.................................................. 67

Figure 15: AMT Module; Flux

vs

Feed Flowrate for a 19'0 Oil-W ater

Mixture

....................................................................................................

68

Figure 16:

AMT

Module; Flux

vs

Oil Concentration

...........................................

68

Figure 17: AMT Module Regeneration: Pure

Oil

Flux vs Tim e

.........................

71

Figure 18: AMT vs Celgard 1 Flux vs Pressure for Pure Oil

............................ 72

Figure

19:

AMT vs Celgard

2;

Flux vs T ime

for

a 1

o

Oil-Water Mixture

........75

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viii

Page

Figure 20: AMT vs Celgard 2; Flux

vs

Time for a 10% Oil-Water Mixture

......

75

Figure 21 : Celgard 2 Module; Flux vs Time for a 10 Oil-Water

Mixture Containing 1% Pet Mix 9 ................................................ 76

Figure 22: Celgard 2 Module; Flux vs Time for a 10% Oil-Water

Mixture Containing 1o/o ABS

...............................................................

7 9

Figure 23: Celgard 2 Module: Flux vs

10

Oil R emaining for a

10% Oil-Water M ixture Containing 1Yo BS

.....................................

79

Figure 24: Celgard 2 and C-#3 Modules; Flux vs Time for a 10%

Figure

25

Celgard

2

and C-#3 Modules; Flux

v s

OO Oil Rem aining for

a

Oil-Water M ixture Containing 1oo ABS .............................................

80

10% Oil-Water Mixture Containing 1Ol0 ABS

.....................................

8 1

.

Figure 26: Celgard 2 Module; Flux vs Time for a 10% Oil-Water Mixture

Figure 27: Celgard 2 Module; Flux vs 24

Oil Remaining for a 10% Oil-

Containing

1

YoABS at High and Low

pH

....................................... 8 3

Water Mixture Containing 1% ABS at High and Low pH .............. 84

Figure

28:

P -# l Module; Flux

vs

Time for a

10%

Oil-Water Mixture

Containing 1 h Triton X-1

02 .................................................................

87

Figure

29:

P-#l Module; Flux

Vs

YO

Oil Remaining for a 10% Oil-

Water Mixture Containing 1YOTriton X-102 .....................................

8 7

Figure

30:

Countercurrent Module; 5 Dodecane-Water M ixture

Pressure Effect......................................................................................

92

Figure 31: Countercurrent Module;

5%

Dodecane-W ater Mixture

Feed Flowrate Effect............................................................................. 93

Figure 32: Cross Flow Module; 10% Dodecane-Water Mixture

Pressure Effect ...................................................................................... 96

Figure 33:

Cross

Flow Module;

10

Dodecane-Water M ixture

Feed Flowrate Effe

...............................................................................

96

Figure 34: Cross Flow Modu le: Oil Concentration Effect for the

Diluted

10%

Dodecane-W ater Mixture .............................................. 99

Figure 35: Cross Flow Module; 5 Dodecane-Water M ixture

Feed Flowrate Effect............................................................................ 101

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i x

Page

Figure 36: Cross Flow Module; Oil Concentration Effect at

Different Feed Flowrates

....................................................................

103

Figure

37:

Droplet Size Distribution Data ........................................................... 105

Figure 38: AEAPTMS Coated Cross Flow Module; 5 % Dodecane-Water

Mixture; Feed Flowrate

Effect

............................................................ 1

08

Figure

39:

AEAPTMS Coated vs Uncoated Fibres: Flux vs Feed

Flowrate for the

5%

Dodecane-W ater Mixture Containing

No SDS and 100 mg/l

SDS............................................................... 1

1 2

Figure

40:

AEAPTMS Coated vs Uncoated Fibres: Flux vs F eed

Flowrate for the 5% Dodecane-W ater Mixture Containing

50mg/l and 200 mg/l SDS

.................................................................

113

1

Figure 41 : Oil Rem oval Efficiency vs 2(2-ln(R e)) for the 5

Dodecane-W ater Mixture on the Uncoated Fibres

........................ 122

Figure 42:

Oil

Rem oval Efficiency vs

(A

- 0.87A3)or the 5

Dodecane-W ater Mixture on the Uncoated Fibres......................... 123

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1

1. INTRODUCTION

1 . 1 Scope o f the Oi ly

Wastewater Problem

Oil containing wastewaters are generated at an estimated rate of more

than one billion gallons per year in the United States alone (Tabakin et

al.,

1978).

Generators of these wastestreams are extremely varied and they may

be

of industrial, comm ercial or domes tic origin. The m ajor contributors

however, are petroleum refineries, metal fabrication plants, roll ing mills,

chemical p rocessing plants, food processing p lants and dischargers of bilge

and ballast waters from ships (Tabakin et al.,

1978;

Wang et al., 1975).

Many different types of oils may be p resent in an oily wastewater, such as

diesel fuel, cutting and grinding oils, lubricating oils, water soluble coolants,

natural animal or vegetable fats or any other organic immiscible in water.

The

removal of these oily wastes from wastewater is of paramount importance in

preventing pol lution and

it

is required to meet the increasingly stringent

environm ental regulations. Oily waste removal may also be beneficial from the

point of view of water and oil recovery and re-use.

Oily wastes

can

have many adverse effects

if

discharged to a receiving

stream.

Among these are: (1) formation of a noticeable fi lm on the water

surface which also has potential for burning and creating

a

safety hazard;

(2)

exertion of an oxygen demand;

(3)

prevention of natural water reaeration; (4 )

toxicity to aquatic life and ( 5 ) objectionable taste and odor in fish and the water.

Oily wastes may

also

interfere with municipal wastewater treatment or w ater

purification opera tions (Tabakin et al.,

1978;

Nalco Technifax,

1985).

Current legislation requires that e ffluents discharge d directly to receiving

streams b e free of any visible floating oil and that the oil and grease content

be limited to levels as low as

5-15

mg/l (Nalco Technifax,

1985).

This limit

varies de pend ing upon local environm ental regulations. For discharge to

municipal sewer systems the oil and grease content

may have to be on the

order of

50-100

mg/l.

The

exact value depends on the p retreatment standards

required by individua l wastewater treatment facilities. Oil and grease refers to

-.

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2

any substance

Methods,

1985).

ext

facta b e

by

t

ric h

o rot ri luo

ro

et

hane

( freon) (Standard

1.2

Oi ly Wastewater Character is t ics

Oily wastewaters vary widely in composition and character depending

upon their source. Oil contents for example, may be as low as

50

ppm

or

as

high as 50%; values between 1% and 15% are more typical, however (Nalco

Technifax, 1985). In addition there are five different ways in which oil may exist

in water: (1) free; (2) mechanically dispersed; (3) chemically emulsified: (4)

dissolved or (5) adhered to particle surfaces (Freestone and Tabakin, 1975).

Free oil is that which readily separates from water under quiescent

conditions. It is generally the easiest type of oil to remove from water. Both

me cha nical an d chemically emulsified dispersions c ontain stabilized oi l

droplets with diameters ranging from microns to fractions of a millimeter. The

difference between them

is

that mechanical dispersions are stabilized only by

electrical forces, whereas chemical emulsions are stabilized by emulsifying

agents as well. As a result,

oil

is more difficult

to

separate from chemical

emulsions. Dissolved oil includes that which is truly dissolved in a chem ical

sense plus that oil dispersed in such fine droplets that removal by physical

means is impossible (Freestone and Tabakin, 1975). Rem oval of dissolved

oi l

requires more sophisticated techniques, and it is often considered an

advan ced treatment step. When oil adheres to particle surfaces, the product is

commonly called oil-wet solids and the removal of this type of oil from water

often occu rs with the rem oval of suspended solids.

The degree of difficulty in separating oil from an oily wastewater is

strongly affected by the form(s)

of

oil that are present.

Other wastestream

characteristics that affect the separation process include the suspended

solids

concentration and particle size distribution, oil and bulk fluid densities, the

presence or absence of various chemicals, pH and temperature (Nalco

Technifax, 1985;

Tabakin et al., 1978).

-

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3

1.3

The

Concept of

Selec t ive Oi l F i l t ra t ion

Several different methods are currently available for separating oil from

oily

wastewaters but they are generally limited in the forms of oil that they can

remove. Oily wastewaters containing chemically emulsified oil are particularly

problematic and the options available for treating them are not only few in

number, but they all have certain disadvantages associated with them. A novel

membrane separation process, called selective oil filtration (SOF), as studied

as

an alternative separation technique

for

removing emulsified

oil from

oil-

water mixtures.

SOF is similar

to

ultrafiltration in that microporous membranes are used. It

is

unlike ultrafiltration however, in that oil rather than water

is

selectively

removed

by

applying a pressure across the membrane. Figure 1 shows a

schematic diagram of the SOF

process.

Figure

1:

The

SOF

Process

P

Qf

c

(Oi l + Water)

Bulk Oi l

C o n c e n t r a t i o n

( C )

Oil Permeate

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4

An oil-water mixture with an oil concentration,

C,

flows along one side of

an oil-saturated membrane at a feed flowrate, Qf.

A

pressure,

P,

is applied

across the membrane, which causes the oil to pass through the membrane

pores while water is retained. Wa ter-free oil is collected from the other side of

the membrane at a Permeate rate, Qp, while the water retentate slowly

becomes depleted of oil. For this process to work the membrane must

be

hydrophobic in nature.

I f

the membrane loses its hydrophobicity during

the

course of operation, water will wet the surface and the separation process will

fail.

The

SOF

process is intended

to

remove free , mechanically or chemically

em ulsified and perhaps some dissolved oil from wastewaters. Oil-wet solids

can not be removed and they would have to be separated by conventional

means prior to

SOF.

The critical criterion for operating an SOF system is the oil permeate flux

which is the volume of oil passed per unit membrane area per unit time

(ml/min-A2). The purpose of this study was to evaluate the dependence of the

permeate flux upon the operating conditions, the chem istry of the em ulsion and

the system design.

The main advantage foreseen in using

SOF

is that a water-free oil product

can be o btained rather than a n oil-water concentrate or an oily sludge which is

usual ly recove red with the currently used separation methods. This is

important from the point

of

view of oi l recycl ing and reuse, and waste

minimiza tion. Possible limitations of the

SOF

process are

loss

of membrane

hydrophobicity due to the presence of chemical emulsifying agents and low

permeate fluxes at low bulk oil concentrations.

1.4 Research

Object ives

No

evidence was found in the literature to indicate that p revious work had

been done on the

SOF

process.

Consequently, much of the research work

conducted

in

this study was fundamental in nature. The intent of this research

was to de rive some understanding of how and why the

SOF

process works and

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5

to

determ ine some of its limitations. More specifically the objectives of this

research were as follows: (1) demonstrate that

oil

could be removed from oil-

water mixtures using selec tive oil filtration: (2 ) determine the effects of various

operating parameters such as pressure, feed flowrate, oil viscosity and oil

concentration on process performance,

(3)

determine the effect of emulsion

stability and the presence of emulsifying agents on the ability of the membrane

to selectively remove oil, and (4) determine the mechanism of oil transport from

the bulk solution to the permeate stream.

A

hypothesis for the mechanism of oi l transport was derived and

experiments were carried out to determine what the limiting step in the overall

transport

proces s is under various operating conditions. In doing this

it

was

hoped that a better understanding of the process could

be

achieved.

1.5 Scope o f

This

Research

In this study microporous hollow-fibre membranes were used to separate

oil from oil-water mixtures using SOF. This research was carried out in two

stages

and

will be presented as such in this thesis. Stage one concerned itself

mainly with achieving objective

( 1 )

above and with qua litatively assessing the

SOF

process. Several experiments were conducted in which a naphthenic

base engine

oil

was removed from different oil-water m ixtures and the

SOF

process was evaluated both

in

the presence and absence of emulsifying

agents.

Two types

of

membranes were tested and compared in these

experiments. From this work some inferences about the SOF process were

made a nd narrower research goals were de fined.

The aim of stage

two

of this

project was to achieve these research goals.

During stage two the performance of the SOF process was assessed

under more carefully defined conditions in which dodecane was employed as

a

mo del oil.

This was done by studying the effects of various operating

parameters. Dodecane was removed from dodecane-water mixtures to which

sodium dode cyl sulfate was added as an emulsifying agent. An assessment of

emulsion stability as a function of em ulsifier concentration also was made and

correlated to the observed results.

-

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where, Vr

= rise velocity

g = acceleration due to gravity

d = oil globule

or

particle diameter

pw = density of water

pp

= density

of

oil or particle

p = viscosity of water

Free oil droplets rise to the water surface where they can be removed by

skim ming while the settleab le solids sink. Most gravity separators are effective

at removing oil globules larger than 0.015 cm in diameter (Ford and Elton,

1977). The effectiveness depends, however, on the hydraulic design and the

retention time employed.

The most basic form of gravity separator

is

the so-called

API

unit which

consists of a rectangular or circular basin in which the wastewater flows

horizontally.

A

variation on this design is the addition of extended plate

surfaces.

Two units

of

this type are the corrugated-plate interceptor

CPI)

and

the parallel-plate interceptor (PPI). The purpose of the plates is to coalesce the

smaller oil droplets into larger drops that move up the plate to form a floating

layer on the water surface. The p lates effectively reduce the distance that the

oil droplets must rise to be collected. Consequently, CPI and PPI units require

less space than API units (Tabakin et al., 1978).

Effluent oil concentrations that can be achieved with gravity separators

typically range from 20 -100 mg/l (Ford and Elton, 1977; Tabakin et al., 1978);

higher or lower values may be obtained depending upon the predominant

forms

of oil

present in the wastewater.

The main advantages of gravity

separators are that they are economica l and relatively simple to operate. They

are limited however, in that they cannot remove emulsified or soluble oil.

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induced by a rotor disperser mechanism. Chem ical coagu lants may be added

to

agglom erate smaller oil droplets into larger flocs which are easier to remove.

This may greatly improve the effluent quality but it has the disadvantage of

producing a chemical sludge that must be disposed. For optimum clarification

it

is

necessary to remove the buik free

oil

and settleable so lids prior

to

flotation.

High quality effluents can be produced using flotation; values as

low

as

1-

20

mg/l oil

have been reported (Tabakin et a1.,1978; Ryan, 1986). Flotation

systems are capable of handling high suspended solids contents and shock

loads but they cannot remove soluble oil or emulsified oil unless an emulsion

breaking technique is employed.

Coalescence

Coalescers cause fine oil droplets to grow into larger droplets that are

more easily removed by gravity methods. For coalescence to occur, the oil

droplets must be forced into physical contact with one another to encourage

agglomeration and reduce their surface energies. Coalescers rely on a variety

of different physical and surface chemical mechanisms to accomplish this.

Coalescence is used primarily to remove free and mechanically dispersed

oil from water; chemically emulsified oil droplets are normally too stable to be

forced together.

Three different types of coalescers are used for oil-water

separations: fibrous-media, loose-media and plate coalescers (Ford and Elton,

1977).

Plate coalescers which use gravity separation and parallel plates have

already been discussed.

Fibrous-media coalescers have a fixed fi lter-l ike media of finely

intew ove n fibres that provide a tortuous path for the dispersed oil droplets. As

the wastewater filters through the media, the oil droplets collide with the fibres

which are hydrophobic.

Agglomeration of the droplets subsequently occurs at

the fibre surface. Once the drops have grown to a certain size they are

sheared off by the passing water and new drops begin to grow. These systems

are sensitive to the presence

of

particulate matter and clogging of the fibres

may occur, The presence of surface-active chem icals can also have an

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10

adve rse ef fect since they may alter the hyd rophob ic nature of the fibres.

Loose-media coalescers operate in much the same way as fibrous-media

coalescers except

the media is loose rather than fixed. Multi-grade sand

or

other loose media may be used.

Wide variations in oil removal efficiencies have been found

for

fibrous-

me dia and loose-me dia coalesce rs. These variations are due largely

to

fluctuations in the characteristics of the wastewater. Effluent oil concentrations

as low as 1-50 mg/l have been reported, however, indicating a potentially high

efficiency (Tabakin et al..

1978).

(b) Secondary Treatment : Remova l

o f

Chemica l ly Emuls i f ied

Oi l

Oily wastewaters to which chemical emulsifying agents have been added

are

more difficult to treat because of the e lectrical and mechanical barriers that

prevent the o il droplets from agglomerating. Traditionally this form of oil has

been separated from oily wastewaters by first breaking the oil- in-water

emu lsion an d then rem oving the freed oil by flocculation and gravity

separation. To brea k an emulsion the repulsive electrical forces on the oil

droplets m ust be neutralized and/or the effectiveness of the emulsifying agent

must be destroyed (Gambhir, 1983).

Several different techniques can be used

to break oil-water emulsions and separate emulsified oil from water.

These

may be chemical, physical

or

electrical in nature (Tabak in et

al.,

1978).

Chemical

Treatment

Chemical demulsification methods are the most widely used and they

usually include acidification and/or coagulation followed

by flocculation

(Tabakin et al., 1978; Berne, 1982). The conventional means of separating oil-

water emulsions is the acid-alum process.

In this process the pH is lowered

into the

2-4

range by the addition of acid. (usually sulfuric acid). This causes

most

of

the oil droplets to destabilize and separate out, and the freed oil is

subsequently removed by skimming. Aluminum sulfate (alum) is added as a

-

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1 1

coagulant and lime or caustic is added

to

raise the pH into the neutral range

which causes the aluminum to hydrolyze and form an insoluble aluminum

hydroxide precipitate. Other inorganic salts such as ferric chloride may be

used in place of alum. The hydroxide precipitate entraps the residua l oil which

is then removed by dissolved air flotation or other gravity separation

techniques (Bauer, 1976; Harlow et al., 1982).

This process works well but it has several disadvantages: oily wastes are

normally alkaline and they require large amounts of acid to drop the

pH

below

4 ; acid corrosion and handling problems prevail; pH adjustments with lime and

caustic increase the total dissolved solids content in the effluent and high alum

feed rates create large volumes of sludge which require dewatering and

disposal (Harlow et al., 1982).

Another chemical approach to oil-water emulsion separations is the

polymer-alum method. In this case, a cationic polymer is used in place of acid

for neutralizing the su rface charge on the stable oil droplets. Cationic polymers

are effective over

a

wide pH range which minimizes the need for pH

adjustments.

Alum is usually added after polymer addition to remove residual

oil droplets as in the acid-alum process. The po lymer-alum process offers the

following advantages over the acid-alum process:

(1)

reduced use

of

acid and

alum;

(2)

lower total dissolved solids in the effluent;

(3)

educed corrosion

problems and

(4)

reduced sludge production.

A

major advantage

of

both these

techniques is their ability to hand le high solids contents (Gambhir,

1983).

The disadvantages associated with chemical emulsion breaking

techniques have led

to

the development

of

a variety

of

non-chemical oil-water

emu lsion separation methods. These include electrolytic treatment, various

physical treatment methods and mem brane separation techniques. These are

discussed in turn below.

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12

EIec

rol

y t

c

Treat men

Several di fferent electrolytic methods have been investigated for

separating oil-water em ulsions over the years (Snyder and W illihnganz,

1

976;

Kramer et al., 1979; Oblinger, 1984). Electroflotation was one of the first

processes developed

in

which oil was removed from emulsions that had been

previously broken by chemical add itives. More recent efforts have focused on

the application of electrochemical techniques to break emulsions and separate

destab ilized oil without the addition of chem icals. The key process involved in

most of these methods is electrocoagulation which can be considered a two

step process:

(1)

aluminum or iron ions are introduced electrolytically to reduce

the repulsive forces on the negatively charged oil droplets and break the

emulsion and (2) a DC voltage is applied across the emulsion to cause the

cha rged droplets to migrate an d coalesce. The feasibi l ity of these

electrochemical methods

fo r

separating oi l-water emulsions has been

demonstrated

in

laboratory and pilot plant tests. Their applicability to large

scale operations is questionable, however.

Electrolytic methods are most

useful for treating smaller volumes of wastewater with a relatively constant

composition and character.

Phys ica l T rea tmen t Me thods

are available for treating oil-water

emulsions, including heating, high-speed centrifugation and magnetization

(Wang et

at., 1975).

Oil-water emulsion breaking by heating is technically

feasible bu t not economically practical because of the large amount of energy

that is required to vaporize the water before the

oil

can be removed. In the

case of high-speed centrifugation the maximum benefit of the centrifugal force

is

realized

at

the outer extremities of the centrifuge where the denser phases

accumulate.

It

is easier therefore,

to

separate a small amount of dispersed

water from a continuous oil phase, such as an oily sludge,

rather than

separate a small amount of dispersed oil from a continuous water phase

(Wang et al., 1975; Tabakin et al., 1978 ).

A

variety of physical methods

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13

Researchers have found a magnetization technique that is efficient at

separating oil-water emulsions (Wang et al., 1975). By adding a ferrofluid to a

wastewater the oil droplets become magnetically responsive and when the

emulsion is passed through a packed bed of magnetic particles the droplets

can be collected. This process has been carried out only on a laboratory scale

and its feasibility on an industrial scale is not known.

A number of other non-chemical oil-water separation processes have

been reviewed by Wang

et

al. (1975). These include solvent extraction, layer

filtration, crysta llization and freezing and adsorption flotation . An emerging

technology which is discussed in greater detail below is ultrafiltration.

UI

taf

l t r a t i on

Ultrafiltration i s a membrane separation technique that is becoming

increas ingly more important in treating oily wastewaters. It is a physica l

separation and concentration method in which free, emulsified and finely

dispersed oil are removed from a wastestream by forcing the water through a

mem brane und er low pressure. An essentially oil-free effluent can be

produced along with an oil-rich concentrate (Pinto, 1978).

An ultrafiltration membrane is a molecular filter which makes separations

based on size; small solutes

and

water are allowed to pass through the

membrane while larger oi l droplets and suspended

sol ids

are retained.

Typical pore sizes range from 0.001

to 0.02 microns in diameter which

corresponds to molecu lar weight cut-off values of 1000 to 100,000 (Applegate,

1984).

A schematic diagram of an ultrafiltration process is shown in Figure 2

(Dhawan, 1978). The oily wastewater enters the membrane unit under an

applied hydrostatic pressure

(20-60

psi),

and flows parallel to the membrane

surface.

The free oil, emulsified oil and suspended solids are retained and

concentrated inside the unit while water, and dissolved solids having a

molecular weight sm aller than the molecular weight cut-off, pass through the

membrane. The pores of the membrane are much smaller than the particles

-_

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14

that are reta ined and

this

prevents the particles from entering and plugging the

mem brane structure. The membrane also has

no

depth

to

its pore structure

and this minimizes plugging.

F igure

2:

The Ul t raf i l t ra t ion Process

*

.

E.

o t FEED

I/

011

e

The c ritical criterion for designing and operating an ultrafiltration system is

the capacity of the membrane to pass water. This is called the m embrane or

permeate flux and

is

defined as the volume of water pa ssed per unit membrane

area per unit time. To minimize both capital and operating co sts it is desirable

to maximize the membrane flux. For most ultrafiltration applications the

following relation

holds:

(Go ldsm ith et al.,

1974:

Michaels,

1968)

J - K ' I n T

*

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15

where,

J

=

membrane flux (gal/ft2/min)

C * =

a constant

C = feed concentration of oil

K

= aqueous phase mass transfer coefficient

The membrane flux is inversely related to the feed concentration and

directly related to the mass transfer coefficient. Temperature and feed flowrate

also affect the flux

as the

value of

K

depends on them. Increasing pressure

increases the flux up to certain point beyond which membrane compaction

occurs and the permea te rate is reduced (Pinto, 1978). pH also exerts an effect

on membrane flux in that higher rates have been obtained for acidified waters

containing anionic surfactants (Goldsmith et al., 1974).

The equation given abo ve assume s that reductions in permea te flux are

due

to

the formation of a concentrated boundary layer at the mem brane sutface

that has a concentration C*. This phenomena is known as concentration

polarization, and

it

is inherent in all membrane separation processes.

Concentration polarization cannot be eliminated but it can be minimized by

increasing the flowrate, w hic h redu ces the bounda ry laye r thickness.

A

compromise must be made, however, between pumping costs and increased

perm eate flux.

Membrane fouling, which may be caused by the build up of oil or

suspen ded solids at the mem brane surface, also reduces the pe rmeate flux by

creating

a

hydrody nam ic resistance.

This effect tends to be m ore pronounced

than concentration polarization at low oil concentrations, less than 10-15%

(Pinto,

1978).

Membrane fouling can be minimized by increasing the flow

velocity. Wastewater pretreatment and frequent mem brane cleaning also help

to prevent fouling.

Ultrafiltration mem branes are m ade from a variety of different polymeric

materials such as cellulose acetate, polyvinyl chloride, polysulfone or

poypropylene. Importan t criteria in selecting

a

membrane is com patibility with

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1 6

the

feed

solution and the cleaning agents. Polysulfone membranes are widely

used because they can tolerate tempe ratures as high as

93OC,

H's between

0.3 and 13 and they are compatible with many different cleaning agents

(Applegate, 1984).

Three different ultrafiltration configurations are gene rally used .

T h e

se

include tubular, spiral-wound and hollow-fibre units. Tubular system s are easy

to clean by back wash ing and they are best used to treat low flowrate

wastewaters in which large amounts of fouling are expected.

Hollow-fibre and

spiral-wound membranes offer a greater mem brane surface area per unit

volume and therefore they are capable of treating larger volumetric flowrates.

They cannot be cleaned by backwashing, however, and they require the use of

.

cleaning agents (Applegate, 1984).

The separation of oil from water is usually carried out as a semi-batch

operation. The oil concentrate is recirculated until the desired degree

of

concentration

or

dewatering has been achieved. An oil concentration of about

50%

can be attained quite easily; concentrations above this are increasingly

more difficult

to

achieve as the membrane flux decreases due to membrane

fouling (Nordstrom, 1974; Hockenberry, 1977). Up to 90%

of

the water is

remov ed from an oily wastewater, however, by the time a 60% oil concentrate

is p roduced (assuming a typical oily wastewater contains 1-10% oil).

The permeate, which is drawn

off

continuously, normally has

a

suspended

solids content less than 10 mg/l and an oil and grease content less than 100

mg /l (Pinto, 1978). The oil content of the effluent is directly related to the

soluble, low molecular weight organic content of the feed, and if this is low an

essentially oil-free effluent can be produced.

For

optimum performance the

wastew ater should be pretreated prior to ultrafiltration to remove the bulk free

oil and suspended solids. Suspended solids may damage the m embrane

surface and free oil may enhance membrane fouling.

The major application of ultrafiltration in treating oily wastewater is for

separating oil-water emulsions.

It

offers many advantages over conventional

techniques such as:

(1)

consistently high permeate quality can be achieved

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17

bec ause the mem brane acts as a physical barrier and

it

is not as susceptible

to

operational upset; (2) no chemical additions are required;

(3)

the

oil

concentrate, which may be reduced

to

less than 10%

of

its original volume, has

a

higher heating value, and in addition, the sludge hauling and disposal costs

are reduced and (4) capital and operating costs are relatively low. The major

disadvantage is the risk of membrane fouling which adversely affects the

perm eate flux and m embrane life. Ultrafiltration systems are also incapable of

recovering a directly reusable oil produc t. The oil-water concentrate obtained

requires further processing

to

recover the oil

(Goldsmith e t al., 1974;

Nordstrom, 1974; Pinto, 1978).

( c )

Tertiary Treatment

Biological treatment and carbon adsorption are two of the most common

tertiary treatment metho ds for separating oil from water. Both are aimed at

removing dissolved oil and any emulsified oil which was not removed by

previous treatment steps.

These metho ds are limited i n their application but

they ca n produc e extremely clean effluents when properly used.

Biological organisms are efficient at oxidizing soluble oil but they are

prone to upsets. Suitable pretreatment and dilution are nearly always required

prior to biological treatment.

Free oil concentrations in exces s of 0.01 Ib/lb

MLVSS

(mixed liquor volatile suspended solids) cannot be tolerated as the

free oil tends to coat the biological flocs and hinder oxygen transfer.

It

also

reduces sludge settleability. Trickling filters can treat wastestreams with oil

concentrations

as

high as 100 mg/l while activated sludge systems can only

handle about 25 mg/l. Effluents from biological treatment typically contain less

than

10

mg/l oil (Ford and Elton,

1977;

Tabakin et ai., 1978).

Soluble oil is also efficiently removed by carbon adsorption once the

wastew ater has been adequately pretreated. Free oil and solids may clog and

coat the activated carbon, which reduces its effectiveness.

If this occurs

backwashing is required. A suitable means of regenerating the activated

carbon is also necessary. Effluents from carbon adsorption treatment may

have oil contents as low

as

2-10 mg/l (Ford and Elton, 1977). A major

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i a

drawback

to

this method is the expense involved as both capital and operating

costs are high. Consequently, it is used only for very specialized app lications

(Ryan,

1986).

Several different oil-water separation techniques have been discussed

here.

A

summary of the advantages, d isadvantages and applicabilities of each

is given in Table

1.

The information given in this table is fairly general and

should be regarded as such. The selection

of

one o il-water separation process

over another requires the consideration of many factors. The technique that

is

ultimately chosen must not only be technically feasible but it must be econom ic

as well.

2.2 Theory of

Emulsions

An emulsion is a colloidal system consisting

of

two im miscible liquids, one

of which is dispersed i n the other as fine droplets. Th e droplets, which

normally have diameters between

0.1

and

20 p,

possess a minimum stability

due

to

electrical forces an d this stability may be enhanced by the addition

of

emulsifying agents (Becher,

1965).

With respect

to

the oil-water system two

types of emulsions may exist: a water-in-oil

(W/O)

emulsion in which water

is

the dispersed phase and oil is the continuous phase

or

an

oil-in-water

OM)

emulsion in which oil is the dispersed phase and water is the continuous

phase. Both types are widely used in a num ber of different app lications.

A stable emulsion is one that will not separate into its

two

phases in a

reasonable period

of

time. This may range from minutes to years depending

on the emulsion in question. Surface-active

or

other agents may

be

added

to

an emulsion to increase its stability and these are known as emulsifiers or

e

mu si

yi

g agents.

Given here is a brief review of the theory of emulsions. Som e relevant

aspects of surface chemistry are discussed, followed by a description of the

formation of emulsions and their properties. A few of the theories explaining

emulsion stability are then described.

Finally, the methods that are used

to

assess emu lsion stability are addressed.

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8/10/2019 02432.pdf


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21

(a ) Re levant Aspec ts o f Sur face Chemis t r y

S u r f a c e T e n s i o n

When a pure liquid is in contact with air, short-range attractive forces

(London van der Waals) exist between all the molecules in the liquid. In

the

bulk liquid these forces tend

to

balance out but at the surface

they do not

because the molecules at the surface are

not

completely surrounded by other

liquid molecules.

This results in

a

net downward force which causes

the

surface to contract and this behavior

is

called surface tension.

Because of surface tension, molecules at the surface are at a higher

poten tial energy level than those in the bulk liquid. Consequently, work must

be done to bring a molecule from the interior

of

the liquid to the surface to

create new surface area. This work can be expressed as follows: (Adamson ,

1982)

d G - Y * d A

where,

dG

=

work required to crea te a unit of surface area

dA

= a

unit of surface area

Y

=

surface tension

The surface tension is defined as the work requ ired to create one square

centimeter of surface.

It

can

be

seen from the above equation that to minimize

the surface energy

of

the system the surface area must

be

kept as small as

possible.

I n te r f ac i a l T ens i on

Interfacial tension i s similar to surface tension except it exists between two

liquids rather than a liquid and a gas. It is also of much more importance in

understand ing the theory of emulsions. When two pure liquids are in contact

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22

with one another the forces acting on the molecules in the bulk liquids are

balanced while those at the interface are not. The net imbalance

of

forces on

the interfacial molecules is different from that

of

a simple air-liquid surface

however, because of van der Waals' interactions with molecules of the second

liquid. The interfacial tension be tween two liquids normally lies be tween

the

surface tensions

of

the individual liquids (Becher,

1965).

As with simple surfaces the work required to create new interfacial area

can be expressed in terms

of

the interfacial tension,

Y

as follows:

dG

=

Y 'dA.

To form an emulsion, which consists

of

a vast number of dispersed droplets

with a correspondingly high surface area, a very large input

of

energy is

required. An effective means of reducing this energy requirement is to reduce

the interfacial tension by adding a surface-active agent,

or

surfactant (Becher,

1965).

-

Surface Adsorp t ion

Compounds that lower the interfacial tension between two liquids

do so

by adsorbing and accumulating at the interface. The driving force for

adsorption is the minimization of surface free energy. This occurs when the

concentration

of

a species with a low interaction energy

is

greater at the

interface than in the bulk solution. Gibbs derived an equation based on

thermodynamic considerations which relates the concentration of adsorbed

species at the interface

to

the interfacial tension. He defined a quantity

r,

called the surface excess, which is the excess concentration of solute per unit

area

of

interface over that present in the bu lk solution. The m ost general form

of this equation

is:

(Rosen,

1978)

where,

dY =

change in interfacial tension of the solvent

Ti = interfacial excess concentration of component

i

dpi

=

change in chemical potential

of

component

i

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23

For

a two phase system and dilute solution this equation reduces to:

where,

C =

solute concentration in the bulk liquid

R =

universal gas constant

T

=

absolute temperature

For negative excess concentrations

or

negatively adsorbed

solutes

the

interfacial tension increases with increasing solute concentration which is

undesirable from the point of view of emulsification. For positive excess

concentrations

or

positively adso rbed solutes the interfacial tension decreases

with increasing solute concentration. Surface-active agents or surfactants fall

under this category (Becher, 1965).

Gibbs' equation can be used to indirectly determine the amount of

surfactant adsorbed per unit area of liquid-liquid interface. This is

done by

mak ing interfacial tension measurements and plotting interfacial tension as a

function of surfactant concentration.

The

slope

of

this plot is the surface excess

concentration or surfactant adsorbed per unit area (Rosen, 1978).

Surf

ace- Ac t

ve

Ag e n ts

Surface-active agents as men tioned above, are substances which when

present in low concen tration have the property of adsorbing onto surfaces, or at

interfaces, and of reducing the surface or interfacial energy. Surfactants have

a characteristic molecular structure consisting of a lyophobic (solvent hating)

group and a lyophilic (solvent loving) group. If the solvent is aqueous these

-.

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2 4

terms may be rep laced by hydrophobic and hydrophilic, respectively. This type

of

structure is known as amphipathic.

The hydrophil ic group of a surfactant is ionic

or

highly polar, and

depending on the nature

of

this group surfactants may be class ified as an ionic,

cationic or nonionic. The surface-active portion of an anionic surfactant bears

a negative charge, and a cationic surfactant has a positive charge. The

hydrophobic groups

of

each

of these

classes are similar in that they are long-

chain hydrocarbons (Rosen, 1978).

When a surfactant dissolves in an aqueous solution, the hydrophobic

portions

of

the molecules tend to cause an increased degree

of

structure in the

water, which increases the overall free energy

of

the system.

To

minimize the

free energy, the surfactant molecules migrate to an interface where they orient

themselves

so

that the hydrophobic groups extend away from the water and

into the o il phas e while the hydrophilic groups remain dissolved in the water.

As surfactant molecules continue to concentrate at the interface to reduce the

overall free energy of the system they also reduce the interfacial tension, which

allows for more interfacial area

to

be created. This in turn allows more

surfactant to be adsorbed an d the process continues as such.

An alternative method of removing the hydrophobic groups from contact

with water is to form organ ized clusters of surfactant molecules called m icelles.

In

a

micelle the hydrophobic groups are o riented away from the water and the

hydrop hilic grou ps are directed towards the water. The exact shape of a

micelle is still a debated issue an d there is evidence supporting both spherical

and laminar structures (Becher,

1965;

Adamson,

1982).

The efficiency of

a

surfactant in reducing interfacial tension depends on

several factors.

Most

important is the nature and concentration of the

surfactant. Generally, as the concentration

of

a surfactant increases the

surface or interfacial tension decreas es steadily up to

a

certain point, termed

the Critical Micelle Concentration (CMC), beyond which

it

remains relatively

constant.

At

this concentration micelle formation occurs. Above the CMC any

new surfactant entering solution will aggregate

to

form micelles rather than

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25

migrate to an interface. Thus, from the point of view of emulsification it is of no

benefit to add surfactants in amounts greater than the CMC.

Other factors that affect the ability of

a

surfactant to reduce interfacial

tension include the solubility limit of the surfactan t, tempe rature, pH and

additions of o rganics which alter the amphipathic nature of the surfactant.

(b)

Emuls ion Forma t i on

To form a stable emulsion two conditions must be satisfied. Sufficient

mechanical mixing must be p rovided to disperse one phase in the other as fine

droplets and once these d roplets have been formed they must be stabilized by

the addition of an emulsifying agent.

Most methods for preparing emulsions use brute force to break the

interface into fine droplets. Studies have shown that droplets in the 50-100

p

range may be produced by hand shaking (Becher, 1965). To get smaller

droplets, however, more vigorous agitation must be applied and most

commercial methods are designed to provide a very large velocity gradient to

achieve an appropriate droplet size.

Emulsification techniques can be divided into three broad categories: (1

mixing; (2) colloid milling and (3) homogenization (Sherman, 1968).

A

mixer

consists simply of a stirrer that rotates in a cylindrical vessel. Turbulent flow

is

required for effective mixing and this is best achieved by using vertical baffles

near the walls and a propeller shaped stirrer. Colloid mills emulsify liquids

under strong shearing flow in a narrow gap between a high speed rotor and a

stator surface.

The rotor can rotate at speeds of

1000-20,000

rpm. This,

together with the narrow gap, sets u p very strong shear flows which tears the

liquid interface apart and droplet diameters of about 2

p

can be obtained.

Homogenizers cause liquids to disperse in one another by forcing them

through an orifice under high pressure. Finely dispersed systems can be

achieved by homogenization with droplet diameters of 1 p or less. Mixers,

homogenizers and colloid mills are the standard methods of

producing

emulsions; several other methods are available for special uses, however.

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26

Sherman (1968) gave a detailed description of both the standard and less

commonly used emulsifying techniques.

Other aspects of emulsion formation must also be considered aside from

the brute force techniques involved in breaking up the interface. Among

these are the character and concentration of the emulsifying agent

,

the mode

of

addition

o f

emulsifier, the mode

of

adding the two phases, the time

and

intensity of agitation and temperature. These parameters greatly affect many of

the properties o f emulsions and they can be manipulated

to

produce an

emulsion with particular characteristics (Becher, 1965).

Emulsifying Agen ts

Emulsifying agents are added to an emulsion to ensure a certain stability.

Three types of emulsifying agents are used: (1) surface-active agents;

(2)

naturally occurring compounds and (3) finely-divided solids (Becher, 1965).

Surface-active compounds which were discussed previously are by far the

most commonly used emulsifiers.

They may be ionic or nonionic and they

stabilize dispersed droplets by adsorbing strongly at the o il-water interface.

Naturally occurring emulsifying agents, which include proteins, gums,

starches and derivatives of these substances, also stabilize emulsions by

adsorbing onto the oil-water interface.

Because

of

their macromolecular nature

and their multiplicity of hydrophobic and hydrophilic groups they can be very

strongly held and produce very stable emulsions.

Finely divided solids stick to the oil-water interface by surface tension

forces and they help stabilize emulsions by forming a protective monolayer

around the dispersed droplets. The requirements for sufficient stabilization are

that the solids have a particle size much smaller than the oil droplets and that

they have a substantial contact angle at the oil-water-solid boundary. The

latter is

to

ensure that the solids accumulate at the oil-water interface and do

not enter the oil or water phases.

A

variety of different materials may be use

including clays, powdered silica and basic salts of metals.

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2 7

Sherman (1968) summarized the desirable characteristics of an

emulsifying agent as follows:

(1) it

must sufficiently reduce the interfacial

tension;

(2)

it must adsorb quickly onto the dispersed droplets

to

form a film

which will not thin out when two drops collide; (3)

it

must have a specific

molecular structure with the polar end attracted

to

water and the non-polar end

attracted to oil: (4 ) it must be more soluble in the continuous phase so as to be

readily available for adsorption;

( 5 ) it

must have adequate electrokinetic

potential and (6) it must affect the viscosity of the emulsion. Further,

it

should

be efficient

at

low concentration and it should be relatively inexpensive.

Numerous emulsifying agents exist and the selection of one over another

for

a particular application is not a simple task. Different agents may be more

effective depending upon the emulsion in question and the existing conditions.

The best means of selecting an emulsifier is

to

test a number

of

agents

to

determ ine which yields

a

stable emulsion with desirable physical properties at

a reasonable cost.

This method is not always feasible, however,

as

it is time

consuming and labor intensive, and an alternative

means of emulsifier

selection

is

the hydroph ile-lipophile balance (HLB) method (Griffin,

1949).

In the HLB method a number is assigned to an emulsifying agent, based

on its emulsifying behavior, which is related to the balance between the

hydrophilic and lipophilic portions of the molecule. The concept u pon which

this method is based is that any emulsifier contains both hydrophobic and

hydrophilic groups and the ratio

of

their respective weights should influence

emulsification behavior.

The assigned numbers are related

to

a

scale of

suitable application as shown in Table

2

(Becher, 1965).

It

can be seen that

surfactants with high HLB numbers tend to form O M emulsions while those

with low HLB numbers form

W/O

emulsions. The HLB system only indicates

the type of emulsion that will be produced and it doe s no t give an ind ication of

the efficiency of emulsification.

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28

Table

2:

The HLB Method

of

Select ing an Emuls i f y ing Agent

HLB Number Oispersibi l i ty in Water

A m l i c a t i o n

1-4

nil

3-6 poor W/O emulsifier

6-8 milky

dispersion Wetting agent

8-1

0

stable dispersion Wetting agent

O M emulsifier

10-13

translucent dispersion

OM

emulsifier

13 clear dispersion O N emulsifier

Solubilizing agent

(c)

Phys ica l

Proper t ies o f

Emulsions

Emulsions have a number.

of

physical properties which are used

to

charac terize them. The se include emulsion type, volume ratio

of

inner to outer

phase, droplet size distribution, viscosity, electrical conductivity and dielectric

constant. These properties are largely dependen t on the composition

of

the

emulsion and its mode of preparation. In addition, many of them are not

entirely independent an d changes in one may affect another.

As men tioned previously an oil-water emulsion may be one

of

two types:

ONV or W/O. The type of emulsion that forms is dependent

both

on the relative

amounts of the two phases and the nature and amount of emulsifying agent

present. There are several ways of determining what the type of a given

emu lsion is.

One can distinguish between the two by diluting an emulsion with

one of the two phases; the emulsion can be readily diluted by its continuous

phase. Alternately, a dye which is soluble in only one

of

the phases may be

added and color will be imparted

to

the emulsion if the dye is soluble in the

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29

continuous phase. Electrical conductivity meas uremen ts may also be used

since O N emulsions conduct electricity much better than W/O emulsions

An important parameter used

to

describe an emulsion is the volume ratio

of

the inner to outer phase,

@.

As the volume of the dispersed phase exceeds

that of the continuous phase the emulsion will become less stable and

it

will

have a tendency t o invert (change types).

As shown later, a value

of

@ = 0.74

represents the closest packing of spheres and inversion should theoretically

occur beyond this value.

By definition an emulsion has a droplet diameter greater than 0.1 p

(Becher,

1965).

In practice the droplet size may be as large as 10-20

p,

and

the drop lets do not occur as a uniform size. Instead they are distributed over

a

size range and the droplet sjze distribution will change with time as the

emulsion stability changes.

Droplet size is also related to the mode of

preparation and the nature and concentration of emulsifying agent. More

intense mixing and higher concentrations of emulsifying agents for example,

will favor the formation of smaller droplets. The size of the dispersed droplets

determines the appearance of the emulsion to the naked eye due to the

scattering of light.

Most emulsions are opaque and milky colored; some may

be transparent

if

the droplets are fine enough or if the two phases have similar

refractive indices.

Viscosity is an important property of an emulsion from both a practical and

theoretical point o f view.

To

render an emulsion useful for a given application

a specific viscosity is usually required. The viscosity however, affects the

stability of an emulsion and other physical properties

so

it may be difficult to

meet the conflicting criteria

of

emulsion stabil ity and desired physical

properties. Six factors have been found to affect the rheology of an emulsion:

(1) viscosity

of

the internal phase; (2) viscosity

of

the external phase;

(3)

volume ratio of inner to outer phases; (4) nature

of

the emulsifying agent and

the interfacial film;

(5)

electroviscous effect and (6) particle size distribution.

Beche r (1965) and Sherman

(1968)

describe these in detail.

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30

Electrical conductivity, as previously mentioned, is one means of

determining emulsion type. Emulsions in which water is the dispersed phase

show little or no conductance while those with water as the continuous phases

show high conductance. The dielectric constan t of an emulsion was thought to

be linearly related to the dielectric constants of the individual phases but

studies have demonstrated that this is not the case and in fact the dielectric

constant is quite different from that expected (Becher, 1965).

( d ) Emuls ion Stab i l i t y

When two immiscible pure liquids are mixed vigorously they will form a

dispersion. When the mixing stops however, the dispersed droplets have a

natural tendency to recombine due to the thermodynamic instability of the

system.

To

form an emulsion an emulsifying agent is required

to

provide

stabil ity. Despite this, an emulsion is never comp letely stable in a

thermodynamic sense and the dispersed droplets stili tend to recom bine. The

stability of an emulsion therefore, may be defined as its ability to overcome

the forces which cause the dispersed droplets

to

recombine (Becher,

1965).

-

Two processes are involved in the recombination of dispersed droplets.

These include:

(1)

flocculation

in

which the droplets come together and

form

aggregates but still maintain their identity and (2) coalescence

in

which the

droplet aggregates fuse together to form single drops. Flocculation is

a

transport step involving long-range forces and/or Brownian motion and

coalescence is

a

transport an d destabilization step involving short-range forces

and film-film interactions (Weber,

1972).

Both Becher

(1965) and

Rosen

(1978)

described several theories which

have be en proposed over the years to explain emulsion stability and the role of

emulsifying agents.

Originally it was thought that the most important factor

leading to emulsion stability was the lowering of interfacial tension caused by

the adsorption of an emulsifying agent at the oil-water interface.

It

has long

since been realized how ever, that although this reduces the amount

of

energy

required to form an emulsion, it does not play a significant role in the

stab ilization of an emu lsion. Of primary importance is the oriented interfac ial

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32

Figure

3: The

Or iented Wedge Theory

(a)

O/W Em uls ion Stab i l i zed

(b)

W/O

Emu ls ion Stab i l i zed

B y

a Monova lent Soap

B y

a B ivalent Soap

Becher

(1965)

explained the volume phas e theory which was propose d

by Ostwald in an attempt to explain emulsion inversion.

It

can easily be

calculated that the phase volume ratio for the most densely packed

arrangement

of

solid spheres is 0.74. Ostwald concluded that this value also

corresponds

to the most

densely packed emulsion and that

i f t >

0.74, an

emulsion

would be more densely packe d than possible and therefore invert or

break. Theoretically, this behavior wou ld be expected but in reality

it

is

not

observed. The reason s

for

this are

that

the dispersed droplets are not

necessarily spherical in shape or of a uniform size distribution and in addition

they are deformable

so

they can be more closely pack ed than t =0.74.

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33

Modern Theor ies o f Emuls ion S tab i l i ty

emulsion stability: (i) mechanical and

(ii)

electrical.

The modern theories of emulsions have dealt with two aspects of

(i) Mechanica l Aspects

of

the

Inter facial

Film

The more modern theories of emulsions focus directly on emulsion

stability and in particular they consider the effect of the interfac ial film. King

(1941), who was one of the first investigators to realize the importance of the

interfacial film, considered the mechanical aspects of the film. He felt that the

most important factor controlling emulsion stability was the strength and

compactness of this film. He also thought that the quantitative source of the film

strength was the nature and concentration

of

emulsifying agent. The reduction

in. interfacial tension due

to

the addition of an emulsifying agent was

considered to play a minor role in emulsion stability. Subsequent workers

have drawn similar conclusions with respect to the importance of the physical

properties of the interfacial film (Becher, 1965; Horder, 1977).

To impart stability to an emulsion the mechanical properties of the

interfacial film must be such that colliding droplets resist rupture. The film must

be capable, therefore, of withstanding compaction and shearing forces. For

maximum mechanical stability the interfacial film should be condensed with

strong intermolecular forces and it should exhibit high film elasticity. A highly

viscous film may also contribute to the formation of a mechanical barrier; the

displacement of emulsifier molecules is required for coalescence and high film

viscosities hinder this process.

Highly purified surface-active agents generally do not produce close-

packed interfacial films and good emulsification is generally achieved using a

mixture of two

or

more surfactants. It is thought that complex formation occurs

at the interface which results in an interfacial film with greater strength.

Densest packing appears to be realized for an emulsion system containing

both a water soluble and an o il soluble surfactant. (Rosen, 1978).

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34

(ii)

Elect r ica l Aspects o f the In ter facia l Film

Electrical theories describing emulsion stability are based on the fact that

dispersed droplets carry an electrical charge that leads to electrostatic

repulsion between droplets. How closely two droplets may approach one

another depends on the magnitude of the repulsive force, and to keep an

emulsion stabilized the repulsive forces be tween droplets must be greater than

the attractive forces.

Droplets can become charged in one of three ways: ionization, adsorption

or

frictional contact (Becher, 1965).

A

surface-active agent with an ionized

hy :-ophilic group for example, may adsorb onto a dispersed droplet. Thus,

droplets in emulsions stabilized by anionic surfactants will possess a negative

surface charge such as an O M emulsion stabilized by a soap. For nonionic

emulsifiers, a general rule for predicting the surface charge is that the phase

having the h igher dielectric constan t will be positively charged. Regardless of

how these charges arise they result in the formation of an electrical double

layer which gives rise to the repulsive forces.

In the case

of

an anionic surfactant the oil-water interface has a net

negative charge and to maintain electrical neutrality this charge must be

balanced by a net positive charge on the water side

of

the interface. This

results in the formation of a potential across the interface and the rate of

change of this potential with distance away from the surface is determined by

the d istribution of coun ter charges in the water phase.

The electrical double layer theory proposed by Stern suggests that the

distribution of counter ions is as show n in Figure 4(a). Figure 4(b) shows the

corresponding potential drop as a function of distance away from the interface

(Rosen, 1978).

Adjacent to the interface is a f ixed layer of counter ions of

approximately a single ion thickness; in this layer the potential drops rapidly.

Beyond this is a diffuse layer which extends into the bulk aqueous phase and

within this layer the potential drops more gradually. Two com peting processes

lead to this distribution of counter ions; an electrostatic attraction towards the

interfacial area and Fickian diffusion away from the interface. The former is

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35

due to the charge on the interface and the latter is due to a concentration

gradient of counter ions in the water phase.

The effective thickness of the electrical double layer is the distance from

the charged surface into the aqueous phase within which the surface charge is

neutralized. This has been found to be directly proportional to the surface

potential, wo and the square root of the temperature, and inversely related to

the valence and concentration of ions in solution (Adamson, 1982).

Electrical

effects therefore, have a shorter range in the presence

of

high concentrations

of electrolytes and at lower temperatures under which conditions the double

layer is compressed.

A

term associated with the electrical double layer and one which is related

to various electrokinetic effects is the zeta potential, t; . This is the potential of

the charged surface at the plane of shear between the droplet and the

surrounding solution. This plane, as indicated in Figure 4(b), is in the diffuse

layer beyond the Stern layer boundary.

Zeta potentials are conveniently

measured and they can be related to the surface potential (Adamson, 1982).

Becher

(1

965) described the interaction of charged particles using the

D.L.V.O. theory, which considers emu lsion stability in terms of the attractive

an d repulsive forces acting on dispersed droplets. The repu lsive force

between droplets is due

to

the overlapping of electrical double layers and the

magnitude of this force depends on the double layer thickness and the surface

potential, vo. The attractive force is due

to

van der Waals' forces as discussed

previously.

The net interaction be tween two droplets is the sum of the

repulsive forces and the attractive forces and i f the repulsive force is greater

than the attractive force an energy barrier will exist as shown by the total

energy curve (2) in Figure

5

(Rosen, 1978). To form a stable emulsion it is

required that this energy barrier be formed. In curve

(1)

in Figure 5 an energy

barrier does not exist and a stable emulsion would not be formed.

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36

F igu re 4: Stern Elect r ica l Doub le Layer

(a) Dist r ibu t ion o f Counter Ions

(b) Potent ia l versus Distance

From the Inter face

+

m:+

r o p l e t

1 1 +

w--

++

+

+

+

+

+

+ +

+ +

/Stern Layer

Shear Plane

Dis tance From

Surface

x - 0

F i g u r e

5:

Potent ia l Energy Diagram For Two Dispersed Drop le ts

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37

In summary, one can say that the flocculation of droplets in an em ulsion is

prevented by the electrical barriers formed by the interfacial fi lm

of

an

emulsifying agent while coalescence is prevented by the mechanical barriers

formed. The classic D.L.V.O. theory can be used to describe the process of

flocculation and any factors minimizing the electrostatic repulsion between

droplets will favor flocculation. Coalescence cannot be described by the

D.L.V.O. theory, however, as it involves the rupturing of the interfacial film.

Whether

or

not coalescence occurs depends primarily on the physical nature of

this film and the m ethods used

to

destroy it.

(e) Assess ing Emu ls ion Stab i l i t y

To assess emulsion stability one must find a means o f measuring droplet

resistance to flocculation and coalescence.

According to Horder

(1

977) this

may be done in one of three ways:

(1)

directly by measuring the change of

state of the emulsion with time; (2) indirectly by measuring an emulsion

property and relating it to emulsion stability or

(3)

by directly determining the

stability under an applied stress. The latter of these methods is sometimes

referred to as accelerated aging (Becher, 1965).

One can directly assess emulsion stability simply by observing the change

in appearance of the emulsion with time.

As the emulsion becomes more

unstable the two phases will begin to cream and/or separate out. It may take a

long time however, for noticeable changes to be observed and

for

this reason

it is not always prac tical to apply this technique. Alternatively, the droplet size

distribution can be used to define the stability of an emulsion. An emulsion

with a small mean diameter and a narrow size distribution represents a

situation of maximum stability.

As

an emulsion becomes more unstable with

time the mean diameter will increase and the size distribution will broaden.

The d roplet size distribution may be measured microscopically or by using

a particle counter. Microscopic measurement, though extremely laborious,

allows for differentiation between spherical and polyhedral droplets and

between aggregates of droplets and single coalesced drops. Particle counting

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3a

methods are generally much quicker and less tedious, but they assume all

drops to be spherical and they cannot differentiate between aggregated and

coalesced droplets. Turbidity measuremen ts have also bee n used

to

determine the average droplet size of an emulsion (Reddy and Fogler, 1981).

This

method is relatively easy and inexpensive, but

it

yields an average drop

diameter rather than a size distribution, and

it

also requires a significant

difference in the refractive indices of the

two

phases.

Accelerated aging tests are used to speed up the rate of coalescence

so

that a direct assessment of emulsion stability may be made more rapidly. Two

kinds of stresses are generally applied to accelerate droplet coalescence:

abno rmal temperatures and centrifugation. Cen trifugation accelerates the

effect of gravity separation.

It

also subjects the droplets to very close packing

compared with normal conditions, and therefore centrifugation provides

information on the final stage of coalescence: drop let rupture. Elevated

temperatures accelerate the rate of droplet collision by increasing the kinetic

energy due to Brown ian motion. In using accelerated aging tests one must

take care to ensure that the applied stress does not alter the mechanism of

coalescence or introduce additional mechanisms; some researchers criticize

this m ethod because there is often little direct correlation between accelerated

tests and behavior under normal conditions (Horder, 1977).

The stability

of

an emulsion has been shown to be a function of the

electrical and mechanical properties of the interfacial film.

As a

result, the

measurement of these properties may be used

to

assess emulsion stability.

It

must be borne in mind, however, that the relationship between an emulsion

property and emulsion stabil ity depends on the mode of action of the

emulsifying agent.

If

the mechanical aspects of the interfacial fi lm are

principally responsible

for

emulsion stability, then interfacial rheometry should

be measured:

if

the emulsion ow es its stability mostly to electrostatic repulsion,

however, the electrical properties of the emulsion should be measured

(Horder,

1977).

Methods of measuring the interfacial rheometry of an emulsion have been

reviewed by Sherman (1953). The kinds of measurements that are made

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39

include interfac ial viscosity, interfac ial tension and spreading coefficients. In

addition, the response

of

an emulsion to an applied stress may also be

measured

to

determine film strength.

The electrical properties o f an emulsion can be determined using

electrophoresis techniques. In this method a known D.C. voltage is applied

across a sample of emulsion which causes the charged droplets to migrate

towa rds the electrode of opposite charge. The electrophoretic mob ility of a

droplet depends on the magnitude of the charge density at the droplet surface.

Zeta potentials are calculated from the measured droplet velocity and from

knowledge of other emulsion properties.

The zeta potential can be calculated from the measurement of the

electrophoretic velocity using the Helmholtz-Smoluchouski equation as follows

(Sherman, 1968):

where,

4mv

c

&E

=

zeta potential

q

= viscosity

v

=

electrophoretic velocity

e

= dielectric constan t of the continuous medium

E = applied voltage

A more detailed discussion of electrophoretic techniques was given by both

Sherman (1968) and Becher 1 965).

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40

3. THEORY RELEVANT TO SOF

3.1

Droplet Coalescence on

Fibres

A review of the literature addressing droplet coalescence on fibres was

made in an attempt to gain a better understanding of the possible role of oil

droplet coalescence on fibres in the

SOF

process. In the context of this

discussion, coalescence refers

to

both the aggregation and collapse of

dispersed droplets.

It

was hoped that some of the observations and theories

presented in the literature would be app licable to the

SOF

process in order that

predictions could be made about the effects of various system parameters on

the

SOF

performance.

Numerous studies of oil and water droplet coalescence on fibres have

been conducted.

Most

of the studies reported were aimed ultimately at

understanding the mechanism of coalescence in fibrous filter beds. Fibrous

filters differ fundamentally from the SOF process in that the dispersed droplets

coalesce a s they pass through the filter bed, and a final separation of the two

phases is made following the filtration process

so

that the dispersed phase

remains in contact with the continuous phase.

In the SOF process the

coalesced

or

collapsed oil droplets are separated from the water phase

immediately by forcing them through the mem brane pores. Desp ite this

difference, some aspects of the reported studies were found to be relevant.

The findings of several studies are reported below.

Hazlett (1965) examined the steps involved in the coalescence of water

droplets in a fibrous filter bed. Three steps were considered: (1 ) approach of a

droplet to a fibre;

(2)

attachment of a d roplet to a fibre and (3) release of an

enlarged droplet from a fibre surface.

He found that interception is the

predominant mechanism of approach and the l imit ing step in overal l

coalescence. He described the interception mechanism by an equation

derived by Langmuir (1942):

4 4

I

[2(1+R)ln(l+R)

- (1+R)

+i$$

Es = 2(2-ln(Re))

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41

where,

Es =

collection efficiency by a single isolated fibre

-

Oilfeed'OiJoroduct

-

Oilfeed

drvp

Re

=

Reynolds number

=-

P

R =

interception parameter

=

d

=

particle diameter

df

=

fibre diameter

v

=

supe rficial velocity

p =

oil-water mixture viscosity

p =

oil-water mixture density

9 2

df

This equation indicates that the efficiency of interception on a single fibre is

affected by both the fibre and droplet diameters and the flow velocity. The

collection efficiency increases as flow velocity increases, fibre diameter

decreases and drop diameter increases. Based on this equation Hazlett

concluded that hydrodynam ic factors control interception.

The attachment process was considered to involve droplet film drainage

and rup ture prior to attachment. Haz lett speculated that surfactan ts may affect

the a ttachment of droplets by reducing the rate of film drainage and by lowering

the interfacial tension which increases the deform ability of the droplets. He

also suggested that surfactants decreased the wettability of the fibres which

contributed to reduce d coalescer performance. In addition, he predicted

improved coalescence with increasing surface ene rgy of the fibre.

The extent of growth of coa lesced droplets and their release from the fibre

was found to depend on the interfacial forces between the continuous oil

phase and the water. Small amounts of sodium petroleum sulfonate were

found to have a deleterious effect on the coalescer efficiency and it was

thought to be due to changes in interfacial forces which tend to keep the

coalesced droplets small in size and causes the droplets to redisperse after

coalescence.

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4 2

Bitten

(1970)

conducted a visual study of the coalescence of water

droplets from fuel oil on five different kinds of fibres. Single fibres were used

and they were placed perpendicular

to

the flow of the oil-water mixture . He

found that a minimum velocity existed below which little or no coalescence

occurred and that the maximum size droplet that could be held by the fibre

varied considerable among the different fibres. He also found that small

amounts of sodium sulfonate greatly inhibited coalescence.

Moses and Ng

(1985)

also conducted a visual study o f droplet

coalescence, but they looked at the coalescence of silicon oil droplets from

water. They considered the effects of wettability, emulsion and collector zeta

poten tials and emulsion droplet size on the coalescence process. Larger

droplets were found to enhance the collection efficiency through inertial

impaction and interception. Globule formation was encouraged when the

outlet

of

the coalescer was non-wetting to the dispersed phase. Moses and N g

found that the relative signs and magnitudes

of

the zeta potentials of the

droplets and the fibres were important; improved coalescence

of

the negatively

charged oil droplets occurred when a

less

negative or positive collector was

employed.

Several theoretical analyses have been made to predict coalescence

efficiencies under different conditions. Spielman and Goren

(1970)

developed

transport equations for coalescence on a cylindrical collector in which they

accounted for co lloidal and hydrodynam ic particle-collector interactions at

low

Reynolds numbers. Later Spielman and Su

(1

977)

presented another

theoretical analysis focusing on the effects

of

droplet release and transport

through the filter bed.

Adamczyk and

van

de Ven

(1981)

predicted deposition rates of Brownian

particles flowing past an isolated cylindrical collector and a fibrous filter.

They

developed complete transport equat ions taking into account sur face

interactions and external forces. Their numerical calculations indicated that

particle deposition rates increase significantly when strong attractive double

layer forces exist be tween the particles and the co llector.

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43

Weber and Paddock (1

983)

predicted collision efficiencies for spherical

particles on spherical and cylindrical collectors. They considered only

gravitational and interception mechanisms of approach, and they did not

consider electrical or surface forces. Their equations were developed for

Reynolds numbers up to 100, however, and the solutions to these equations

predict an increase in collision efficiency with decreasing fibre diameter,

increasing drop diameter and increasing flow velocity. These results are in

agreem ent with those found by Hazlett.

Albery et al.

(1985)

studied the kinetics of colloid deposition on

microscop ic slides. They measured the rate of deposition of negatively

charged carbon particles onto clean glass and onto clean glass treated with

N

p

-a m no et hy

I

-y-a m

n

o p ro py I rimet h

ox

ys ane (A

E

A P

TMS ) w

h h gav e t he

surface a positive charge. The kinetics of deposition on the clean glass slides

was found to be about an order

of

magnitude slower than that for the surface-

coated slides. This implied the existence of a kinetic barrier due to electrostatic

repulsion between the similarly charged particles and collector surface. The

coated surface carried a charge of opposite sign to the particles and thus a

kinetic barrier was not observed.

Clayfield et

al.

(1985) conducted

a

comprehensive study to elucidate the

mechanisms o f coalescence and to determine the roles of surface chemistry

and eiectrostatic interactions between the droplets and the collector surface.

They measured the coalescence efficiency

of

kerosine droplets from water onto

glass fibres that had been surface treated with a variety of polymeric coatings.

Their results indicated that coalescence was not correlated with surface energy

or wettability as predicted by previous investigators.

They did find a strong

influence of surface charge effects, however, in that coalescence improved

when the dispersed droplets and the fibres were of opposite charge.

Clayfield et al. derived a model based on the three step mechanism

proposed by Hazlett.

They considered droplet approach to be primarily an

interception process and that this step plays a critical role in the overall

efficiency of coalescence.

The probability

of

interception depends on the

hydrodynamics

of

the system and the electrostatic interaction between the

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44

droplets and fibres. Clayfield et at. proposed that coalescence w ill take place

at a hydrodynamic limiting rate when there is no electrostatic repulsion and at a

rate less than this when there is an electrostatic effect, They also speculated

the e ffect of surfactants, suggesting that they modifiy the electrical properties of

the droplets and the fibres, and that they affect droplet film drainage and

interfacial tension

.

Hughes and Foulds

(1986)

studied the coalescence of kerosine, Nigerian

light crude oil and Brienenard crude oil on clean polypropylene fibres and

fibres coated with AEAPTMS.

Electrokinetic measurements were made to

assess the effect

of

electrostatic interactions. The effect of surfactants, pH and

ionic strength on droplet interception were a lso examined. A theoretical model

was deve loped to predict coalescence efficiency on a filter bed as a function of

system parameters.

-

As observed previously, the electrostatic interaction between oil droplets

an d the fibre surface was significant. Fibres to which AEAPTMS had been

applied had

a

highly positive zeta potential, in contrast to the highly negative

zeta potential on the oil droplets. Consequently, high coalescence efficiencies

were obtained.

Surfactants affected the electrostatic interaction by adsorbing

onto both the droplets and the fibre surfaces. Dissolved salts com pressed the

electrical double layer and at higher salt concentrations the electrokinetic

properties were less important.

Surface potential and the double layer

thickness were both affected by

pH.

A theoretical model derived by Hughes and Foulds is based on the

assum ption that interception is the determining step in coalescence.

They

began the development of his theory with Langmuir's equation (Eq. 3.1) and

they then added a factor wh ich includes the effects of electrostatic

i n eractions

so

that:

where,

Est= oEs

o = 1.O or

no

repulsive interaction

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45

CT e

1

.O

or same charge on particles and fibres

They extended equation

(3.2) from

application

to

an isolated single fibre to a

bed of fibres so that:

where,

Eb = coalescence efficiency for a bed of fibres

n = number of fibres encountered by a droplet

The value of n is calculated as follows:

where,

d

n = 2(1-E)T

d =

bed depth

df =

fibre diameter

e =

bed porosity

(3.4)

From the studies described above the following statements can be made

about the coalescence of d roplets on fibres: (1) the interception of droplets by

fibres

is of primaty

importance in determining the overall coalescence

efficiency;

(2)

the hydrody nam ics of the system de term ine the limiting

coalescence efficiency;

(3)

repulsive electrostatic interactions reduce the

coalescence efficiency; (4) surfactants may alter the electrostatic interaction

between the droplets and fibres, and they may reduce the growth of coalesced

droplets and affect their release; (5) pH and ionic strength also affect the

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4 6

electrostatic interaction by modifying surface charges and double layer

interactions; (6) surface energies and wettabilities may or may not play a role in

the coalescence process and ( 7 ) several theoretical analyses have been made

to predict coalescence efficiencies and care must be taken in selecting one

of

these models to ensure that the appropriate operating conditions and

coalescing mechanisms have been taken into account.

3.2

Proposed Mechan ism of Oil Transport i n the SOF Process

A hypothesis for the transport of oil from the bu lk solution to the permeate

stream using

SOF

was derived.

A

schematic diagram of this conceptual model

is shown in Figure

6.

Three steps are considered in the overall transport

process: (1) transport to the membrane surface, J; (2) attachment and collapse

at the membrane surface, A, and (3) transport through the membrane pores,

Jm.

Figure 6: Proposed Mechan ism o f Oi l Transpor t in the SOF Process

Bulk

Solution

( C )

c

J

.------..

cap

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4a

It was the intent of this study to determ ine the conditions under which each

of these three steps limits the ove rall transport process. This information is

valuable as it allows for appropriate measures

to

be taken to increase the

permeate rate and improve the process performance. This was determined by

studying the influence of various operating parameters on the permeate flux.

Oil transport to the membrane would be expected

to

l imit under the

following conditions: low bulk oil concentration and low feed flowrate. The

transport step may also limit

if

there is a strong electrical repulsion between the

oil droplets and the membrane, as may be the case in the presence of an ionic

emulsifying agent.

Transport through the membrane should l imit at low

operating pressures and for high viscosity oils.

Low

operating temperatures

would also promote this situation since viscosity is inversely related to

temperature.

The attachment and collapse step should limit

if

the oil droplets

are electrically and/or mechanically stable.

This would be expected in the

presence of high concentrations of emu lsifying agents.

.

If transport to the membrane surface is l imiting, one would expect the

permeate flux to be dependent on the hydrodynamics

of

the system and

relatively independent of pressure,

oil

viscosity

or

temperature.

If

transport

across the membrane were l imiting, the reverse would be true, and

i f

the

attachment step l imits, the flux should depend on surfactant type and

conce ntration and/or emulsions stability.

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49

4. MATERIALS AND METHODS

4.1

Oi l -Water Mix tures

A

variety of oil-water mixtures were tested with different oils and

emulsifying agents. The materials used to make these mixtures along with a

description of the method of preparation.are described below.

(a) Oi ls

(ii) dodecane.

Two different

oils

were used in these experiments: (i) naphthenic oil and

(i)Naphthenic O i l :

A

naphthenic lube base stock engine oil was

supplied by Farm Oyl (St. Paul,

MN.)

and used in the first stage of this research.

Lube oils are not pure compounds but rather made u p of a number of different

compounds such as paraffins, naphthenes an d aromatics. The exact nature

and cha racter of a lube oil varies with the selection of crude from which it is

derive d an d the type and seventy of processing. Typically, naphthenic lube

oils contain 50-60% naphthenes and

4040%

paraffins and aromatics. The

naphthenic

oil

received was light brown

in

color and

it

had a viscosity of

100

SUS at 100' F or 53 CP at 20' C. The density of this oil was calculated to be

0.88 g/cm3- The naphthenic oil was selected for use as it represents one type

of oil typically found in oily wastewaters and

it

emulsifies easily with many

different emulsifying agents.

(ii) Dodecane:

Dodecane used in stage two of this research

is

a

straight chain n-alkane.

Its chemical formula is CH 3(CH2 )10CH 3 and it has

chemical a nd physical properties as follows:

Molecular Weight: 170.34 g/mole

Density: 0.749 g/cm3

Melting Point: -9.6' C

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51

(Minneapolis, MN). It has a random distribution of benzene rings and an

average alkyl chain length of about

12.

The anionic functional group

is SO3-

which results from the disassociation of the sulfonic acid. The general structure

of

ABS may be represented as shown in Figure

7.

Because

of

the acidic

nature of this su rfactant emulsions prepared with it have a very low pH.

Figure 7:

General Structure of

Alkylbenzene Sul fon ic

Acid

(c)

Triton X-102:

Triton X-702 is a nonionic surfactant supplied

by

Rohm and Haas (Chicago,

IL).

It is prepared by the reaction of octylphenol with

ethylene oxide and the product is often referred

to

as an alkylaryl polyether

alcohol. The general structure of

this

class of surfactants is

as

shown

in

Figure

8 (Rohm

and Haas Information Bulletin,

1977).

Figure

8:

General

Struc ture o f Tr i ton Surfactants

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52

Triton X-102 has an

x

value

of

12-13, where

x

is the average number of

ethylene oxide units in the ether side chain. This surfactant was rece ived in the

form of a clear viscous liquid and it has the following properties: (Rohm and

Haas Information Bulletin, 1977)

Density: 8.9 Ib/gal (5.17 g/cm3)

Viscosity:

330 cps

Pour Point:

6OoC

Flash Point:

>300°

C

Calculated HLB: 14.6

(c) Sodium Dodecyl

Sulfate:

Sodium dodecyl sulfate

(SDS)

which i s

an

anionic surfactant was supplied by Aldrich Che mical Company (Milwaukee,

WI).

It is derived from the reduction of natural fatty acids and it has the

mo lecular formula CHa(CH2)11OSO3Na. The SDS was received in the form

of

a w hite powder and som e of its properties are given below:

Mo lecular Weight: 288.38 g/mole

Melting Point: 204-207OC

CMC: 9.7 x 1

Oe3

M at 40'

C

(Rosen, 1978)

HLB: 40 (Becher, 1965)

SDS dissolves readily in water but remains undissolved in dodecane.

It

was

selected as

it

is

a

pure compound and

it

is a widely used detergent and

therefore typical of surfactants one might encounter in practice.

(d) Prepara t ion o f Oil-Water Mixtures

Oil-water mixtures ranging from

1-100%

oil by weight were tested.

Emulsifying agents were employed for some experiments and omitted in

others. In

all

cases however, a h igh-speed rotary vane type pump was used to

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5 3

disperse the

oi l

in water. The pump

i s

made by Procon Products

(Murfreesboro, TN) and it has a capacity of 15-1

00

GPM. A reducer connector

was placed on the outlet tubing from the pump to constrict the flow and produce

a high velocity jet of solution which provided sufficient mixing conditions.

In most experiments, the surfactants were mixed with the oil phase and

then the oil phase and then the oil+ emulsifier mixture was mixed with the

water.

However, SDS was added directly to the oil-water mixture due to its

insolubility with oil , Mixing was carried out using the pump in recirculating

mode for about five minutes initially and during the course of an experiment the

oil was kept dispersed by either magn etic stirring

or

intermittent mixing with the

Pump.

4.2 Hol low-F ibre Modu les

Selective oil f i l tration was carried out using a hollow-fibre membrane

system.

In

all

the experiments the oil-water mixtures were made

to

flow on the

outside of the fibres

and

the oil permeate was collected inside the fibres. Many

different m odules were u sed in this study, and these are described below.

Also

given is a description of the membranes used and the method by which the

modu les were made.

(a)

Membrane Se lec t ion

The most important criteria

in

the selection of a membrane for the SOF

process is that the m embrane be hydrophobic in nature. In addition to this, the

mem brane should b e resistant to chem ical attack b y a variety of acids, bases

an d organic solvents. Finally, the mem brane configuration should provide a

high surface area to volume ratio to promote efficient mass transport and this

can be accom plished through the use of hollow m embrane fibres.

D’Elia (1985) and Dahuron (1987) found that only a few hydrophobic

microporous hollow-fibre mem branes were available. These included a

polypropylene membrane made by Questar Corporation (Charlotte, NC) , a

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5 5

Table

3:

Proper t ies

o f

Celgard Hol low-Fibres X-10 and

X-20

-

. -

~

- s i

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56

(i) Celgard Countercur ren t Modu les

Figure 9 shows a schematic diagram of a Celgard countercurrent hollow-

fibre modu le. The modu le

looks

l ike a small tube heat exchanger and

it

consists of a glass tube or shell with an inside diameter of 0.8 cm and an

outside diameter of 1

.O

cm. The length of the tube is about

20

cm.

F i gu r e 9: Celgard Countercur ren t Ho l low-F ibre Modu le

:ob

F ibres

I I

Threaded

E p o x y E n d

To prepare these modules a desirable number of hollow fibres are

bun dled together and threaded through the glass shell. The fibres are then

glued in place at either end with a potting compound. The po tting compound

used was a n epoxy (FE5045) y H.B. Fuller (St. Paul,

MN).

Dahuron (1987)

and D'El ia (1985) examined the properties of several di fferent pott ing

compounds and they found FE5045 to be the best. It adheres well to the fibres

and the glass shell, it has excellent resistance to chem ical attack and it has a

low viscosity and long curing time which provides sufficient time for module

asse mb y

.

The fibres were potted into a threaded Teflon mou ld so that upon removal

of the mould end the epoxy caps were threaded. After the epoxy had hardened

for several hours at room temperature the fibres protruding from the epoxy

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57

were cut with a razor. A more detailed description of the module assembly

technique is given by Dahuron (1987). For the SOF experiments one end of

the module was closed off with epoxy to allow oil to flow in only one direction

within the fibres.

Several different Celgard countercurrent modules were used in this study

and they varied in the type of mem brane and num ber of fibres used. The

properties of each of these modules are given in Table 4 and they

will

be

referred to subsequently by the nam es given in this table.

Table

4:

Proper t ies o f the Ce lgard Countercur ren t Modu les

Modu le Membr ane Type o f Fibres Mem bran e Area

Ce gard-1 x -2 0 6 0 0.14 ft2

4 0 0 p

1.0.

130 cm2

Ce g

a rd-2

x-2 0 60 0.14 ft2

400~.0. 130 cm2

c-#3 X -1 0

or

X-20 12 0 0.28

ft2

400p

I.D.

260

cm2

P -# l x-1 0 120 0.20 ft2

200 p 1.0. 186

cm2

P-#5

x -2 0

72 0.181 ft2

400

p 1.0.

168 cm2

The surface area available for transport was calculated based on the

outside diameter of the fibres and i t was assumed that all

the

fibres were

contacting the oil-water mixture and that they were

all

available for transport

(i.e. none were plugged).

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59

length,

L

= 10.2 cm

width, W = 1.O cm

height,

H

=

1.8

cm

The fibres were threaded parallel

to

the length of the module while the flow of

oil-water mixture was perpendicular

to

the length.

Figure 1 1 : Celgard Cross Flow Module

/

\

I- Closed End

Oil

Ho l lo w Fibres

Glass Beads

t

Feed In let

Only one

cross

flow module was used in this study and the module

contained 720,

X-20

fibres having an inside diameter

of

400

p.

As

with the

countercurrent modules the fibre bundle was threaded through the module

shell and potted at either end with

FE5045

epoxy.

Glass beads were placed in

the inlet of the module as shown in Figure 11

to

help disperse the oil-water

mixture flow evenly along the length

of

the fibres. One end

of

the fibre bundle

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60

was closed off with epoxy. The surface area available for transport was

calculated to be 1.15 ft2 (1068 cm2).

( c ) Module C leaning

Module cleaning was carried out occasionally between experiments. The

method

of

cleaning was as follows. The module was flushed with warm soapy

water for 10-15 minutes under an applied pressure of about

5-1

0 psi; tap water

and Dawn dishsoap were used.

This removed oil from the system. The

module was rinsed with warm tap water for 10-15 minutes to remove the

detergent and then rinsed with distil led water for another 5-10 minutes.

Isopropy l a lcoho l was f lushed th rough the m o d u I e

to remove any water in the membrane pores and finally the module was rinsed

with pentane and dried thoroughly. After cleaning the m embranes retained

their original size and shape and their hydrophobic nature.

4.3

Exper imenta l Appara tus and Method

Figure 12 shows a schematic diagram of the experimental apparatus used

in this study.

The oil-water reservoir was mixed continuously using a magnetic

stirrer during stage one of the research.

During stage two, he Procon pump

was used to m ix the resewoir intermittently. The pump was programm ed to run

on a

0.5

min on/

2.5

min off cycle.

The oil-water mixture was pumped through the hollow-fibre module using

one

of

two kinds of pum ps. In stage one a positive displacement pum p made

by Fluid Metering, Inc. (Oyster Bay, NY) was used and in stage two a Master-

Flex pump by Cole Parmer Instrument Company (Chicago, IL) was used. For

the dodecan e experiments special tubing that would not react with the

dodecane was required. The tubing used, called Tygon Special was supplied

by C ole Parmer Instrument Company (Chicago, IL). Regular Tygon tubing was

employed for the naphthenic oil experiments.

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6 1

In

P

L

Figure 12: Exper imenta l Apparatus

V a l v e

H o l l o w Fibre

P o u t

t

R es e rv o i r

+

Oi l Permeate

A

needle valve was used to control the flow and pressure differential

across the hollow-fibres. The pressu re inside the fibres was assu med to be

atmos pheric. The p ressure applied on the outside of the fibres was m onitored

at both the inlet and outlet of the module. Test gages with an accuracy of

0.25%

were used. By measuring the inlet and outlet pressures an average

applied pressure could be determined and the pressure drop across the

module could also be measured. After the oil-water mixture passed through

the module

it

was recycled back

to

the oil-water reservoir.

The naphthenic oil experiments were carried out

at

room temperature,

which vaned between 23-32'

C.

The temperature of the reservoir increased

due to heat inputs from the pumps.

For

the dodecane experiments, this

problem was el iminated

by

using an aquarium glass rod heater with

a

tempe rature controller to keep the temperature constant.

The hollow-fibre module shown in Figure 12

is

countercurrent in design.

With both the countercurrent

or

cross flow module however, the oil-water

mixture flowed outside the fibres. This flow co nfiguration was chose n to avoid

excessive pressure drops across the module and for ease of permeate

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62

collection. Prior to each experiment with a new module, the fibres were wetted

with oil to facilitate oil transport. The permeate rate of oil passing through the

fibres was measured using a 10 ml or 20 ml graduated cylinder and a digital

stopwatch.

Depending on the type of experiment being conducted the oil

permeate was either removed continuously from the system or recycled back

to

the reservoir to maintain a constan t oil concentration.

4.4 Zeta Potent ia l Measurements

Zeta potential measurements were m ade using a Laser Zee Meter, Model

501

made by Penkem Incorporated (Bedford Hills. NY). The method of

measurement used with this instrument

is

microelectrophoresis as described in

Section 2.1.

Zeta potential measurements were made as outlined in the instruction

manual; these are described briefly here. The cell chamber was rinsed with

distilled water and then filled with the oil-water sample using a syringe. All oil-

water samples measured were diluted approximately 1000 times to allow for

easier viewing of the droplets. The chamber was placed on the microscope

stage and the microscope was focused on the top outer surface of the cell.

Using the fine focus, the microscope objective lens was lowered until the

stationary layer was reached. The distance from the top surface to the

stationary layer was previously calculated using a technique outlined in the

manual to be 1230

p.

Once at the stationary level a voltage of

150 V

was

applied and the rotating prism was adjusted unti l the focused droplets

appeared stationary. At this time an average zeta potential was calculated by

the meter.

The temperature correction factor and the specific conductance

were measured. After calculating these values the cell chamber was removed

and cleaned with a solution provided by Penkem Incorporated. The cell was

then rinsed well with distilled water and stored with water in it.

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63

4.5 Drop le t Size Dis t r ibu t ion Determinat ion

A Hiac/Royc o Particle Counter, M odel 4100A equipped with a model HR-

60HC sensor was used to determine the droplet size distribution of the

oil-

water mixture. This microproce ssor based counter acquires count data on

colloids dispersed in a liquid medium in six different particle size ranges which

can be set independe ntly. The overall particle size range that could

b e

measured was 1 to 60 p.

The counter was operated in a constant volume mode. A volume sampler

was used which issue s sta ds top counting signals

to

the counter at a lower and

upper detec tor which a re set by the operator in

a

metering tube. The volume of

sample ana lyzed in these experiments was 30 mls.

The oil-water mixtures were diluted to an the appropriate concentration

range prior to counting. This required dilutions of at least lo6 t imes and a

dilution

of

2 x

lo6

was used.

Dilution water consisted of distilled water

containing 10 mg/l SDS i ltered through a 0.2 p filter.

Prior

to

each run

baseline da ta was collecte d on the filtered water. These counts were then

subtracted from the counts o btained for the sam ples prepared with the water.

In this way the true count data for a sample was obtained.

The cell chamber was cleaned after running

a

sample, by flushing with

filtere d distilled water

at a

higher feed flowrate (40-50 mWmin). Occasionally

the sen sor cell required more vigorous cleaning. This involved rinsing with

warm soapy wa ter, distilled water and then freon to rem ove any rem aining oil.

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6 4

5.

EXPERIMENTS

AND RESULTS

A description of the experiments conducted in this study and the results

obtained are given below. The experimental work, as me ntioned previously,

was carried out in two stages: (1) naphthenic oil experiments and (2) dodecane

experiments.

5.1 Naphthenic Oil Exper iments

The naphthenic oil experiments were generally quite qualitative in nature,

and they were aimed primarily at identifying the feasibility of the

SOF

process,

and testing process performance under a wide variety of conditions. Three

sets of experimen ts were conducted. Initially, work was begu n with only the

coated AMT fibres, and studies were made to determine whether or not oil

could be separated from oil-water mixtures.

These tests determined the effect

of various operating parameters on oil separation in the absence of surfactants.

In the second set of experiments, a comparative study was made between the

coated

AMT

fibres and the uncoated Celgard fibres in both the presence and

absence of surfactants.

Finally, research was directed towards

a

detailed

examination of the behavior of the uncoated Celgard fibres in the presence of

surfactants.

(a) AMT

Exper iments

The hollow-fibre module supplied by

AMT

was used for all of these

experiments. Preliminary experiments were carried out using pure naphthenic

oil to obtain baseline

data

and to determine the ability of the fibres to pass oil in

the absence of water or surfactants. The effect of

oil

viscosity was assessed

qualitatively by varying the operating temperature. It was found that as the

temperature increased the permeate rate also increased.

The effects of

pressure and feed flowrate were determined and these results are given in

Table 5. It can be seen that while pressure exerted a near linear effect, feed

flowrate exerted little i f any effect. These results were expected and they agree

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6 5

with Equation (3.5) which expresses t h e membrane flux as a function of

pressure, oil viscosity, membrane 2ermeability and membrane thickness.

Table

5:

AMT

Module:

100% Oil; Baseline

Data

01 (ml lmin)

p

(Psi) FIUX

(ml/min-ft2)

30

5

0.65

30

13

1.49

30

21

2.36

75 5

0.69

75

13

1.51

75

21

2.46

150

5

0.69

150

11

1.29

150

21

2.27

190

5

0.81

190

9 1.23

190

21

2.47

The removal of oil from oil-water mixtures of varying oil concentration was

assessed under the following experimental conditions:

Oil

Concentration: 1%, 1O , 50% (w/w)

Pressure: 5-25 psi

Feed Flowrate: 30-330 mI/min

Temperature: 32-38' C

Surfactants: NONE

Oil was removed from the 1% and 10% oil-water mixtures over a relatively long

time period

of

92 and

50

hours, respectively, and from the

50%

oil-water

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66

mixture for a shorter time period of only

4

hours. For each of these experiments

the permeate rate was measured as a function of pressure and feed flowrate.

In all cases, the oil permeate was continuously recycled to the oil-water

reservoir to maintain a constant

oil

concentration. The resu lts of these

experiments are summ arized in Appendix I.

The effect of pressure is shown graphically in Figure

13.

For

oil

concentrations between

1%

and

100%

the effect of pressure appears

to

be

linear over the experimental range. This suggests that transport across the

membrane,

Jm,

is limiting under these conditions (Eq.

3.5).

.

Figure

13:

AMT Module; Flux

vs

Pressure

(Y

3

p5-I00% Oil

1

2-

/

/

0

szzi

5 10 15

20 25

Pressure

(psi)

Qf

=3

ml lmln

The e d c t of feed f lowrate is shown graphically

in

Figure 14 for

1

%,10%,50% and

100%

oil concentrations. Fee d flowrate appears

to

exert

very little effect on the pe rmeate flux excep t at low oil concentrations. Figure

15

shows the significance of this effect for the 1

Yo

il-water mixture. These results

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6 7

suggest that the overall transport of oil is not limited by transport to the

membrane, J, for oil concentrations of

10% or

greater but that it is limited by

J

for oil concentrations

of

1%

or less. Further experiments in the range of

1%-

1 0 % oil would be required to determine at what oil concentration this shift

in

the transport limiting step occurs.

The effect of oil concentration on permeate flux is shown in Figure 16. It

can be seen that the the fluxes obtained for the oil-water mixtures are well

below that for pure oil.

A

plateau in the permeate flux is observed between

10%

and 50% oil which implies that

oil

is being transported to the membrane

faster than it is passing through the membrane in this oil concentration range.

The plateau flux value is approached m ore rapidly at h igher feed flowrates as

the o il droplets are brought into better contact with the hollow-fibres. These

results suggest that the overall transpo rt of oil is membrane limited at higher oil

concentrations and higher feed flowrates.

Figure 14:

AMT

Module; F lux vs Feed Flowrate

0

100 200 3 400

Q. (ml/min)

P=12

PSI

I '

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68

0 .00 -

Figure 15:

-

' *

'

-

'

.

I - I 1

.

CY-

c

U

X

a

i i

0.1

6

0.1

2

0.08

0.04

AMT Module; Flux

v s

Feed Flowrate

for

1

% Oil-Water Mixture

P=12 P S I

Figure

16:

AMT

Module;

Flux vs

Oi l

Concent rat ion

2

el

0

Flux

Corresponding

to 100% 011

c

-a

. - - I - - - - .

-

0 5

1 5 2 5

35

4 5 5 5

Oi l Concent ra ion (%)

P=12

PSI

A 75 ml/mln

300 mi/min

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69

Bas ed on the above results, the remo val of naphthenic oil from oil-water

mixtures using the AMT module can be explained by the proposed m echanism

of oil transport as follows. Oil transport is membrane limited

Jm)

excep t at low

oil concen trations and at low feed flowrates. This is not surprising as the

naphthenic oil has a relatively high viscosity which hinders oil transport

through the mem brane pores. At low oil concentrations the overall transport of

oil

is

l imited by both transport to the membrane (J) and transport across the

membrane (Jm); his is evidenced by the effect of feed flowrate and pressure

on

the permeate flux in the 1 oil-water mixture experiment. Little information

regarding the importance of the attachment m echanism can be drawn from the

above e xperiments. It is likely however, that droplet attachment and collapse

is

encouraged at high feed flowrates as the droplets are brought into better

contact with the fibres.

The fact that the permeate flux for the oil-water mixtures was m uch lower

than for pure oil indicates that o il dispersed

in

water is more difficult to remove

than pure oil. This result was expected as there is obviously less probability of

dispe rsed oil droplets contacting the fibres than pu re oil. The p robability of

contact should also diminish as the concentration

of

oil droplets is reduced and

this was observed as the permeate f lux decreased with decreasing

oil

concentrat ion below

10%

oil

as shown in Figures

14

and

16.

Another

contributing factor to the reduced rate of transport for the oil-water mixtures may

be that the oil droplets m ust first attach to the fibre surface and collapse before

they can pass through the membrane. In this case the attachment mechanism

clearly becom es important.

One other possible explanation for the difference between the permeate

flux for pure oil and the oil-water m ixtures is that water molecules m ay adhere

to the membrane surface and bl ind the membrane pores making it more

difficult

for oil

to

pass through the membrane. Murkes

(1986),

in his study of

water removal from water-in-oil emulsions using microporous fi l ters also

observed water blinding.

At no time

in

the experiments, however, did water

pass through the fibres.

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70

Evidence sup porting the water blinding explanation was found in the 1%

oil-water mixture experiment. After running the o il-water mixture for several

days the module was cleaned with isopropyl alcohol and a pentane-water

mixture and then replaced by pure oil. In spite

of

the extensive cleaning

procedure the pure oil became cloudy as it passed through the module,

suggesting that water may have been sheared off the membrane surface or

removed from inside the membrane pores.

An unexpected result in these experiments was a decline in permeate flux

with time. This was observed in all of the experiments, and in some cases this

decline was quite marked. In the 1% oil-water experiment, for example, the

perme ate flux drop ped by an order of m agnitude over a four day period. This

flux decline imp lies that steady state was not reached, and this may have been

due to the build up of water in the membrane pores or at the membrane

surface.

Murkes (1986) also observed a decline in oil permeate flux which he

attributed to w ater binding.

To reach steady state more quickly he suggested

using higher feed flowrates.

This variation in permeate flux with time made the analysis of the data

more difficult. It was found that over

a

short time period consistent results cou ld

be obtained and the trends that are shown in Figures 14 and 15 were

observed. Ove r longer time periods, howe ver, the flux values change d quite

markedly, and this must be taken into consideration when interpreting these

results. To compou nd this effect, the AMT module was not cleaned

or

regenerated between experiments for

fear

of altering the fluorocarbon

coating. The condition

of

the m odule therefore, may have changed with time,

which in turn could

affect

the membrane's ability to pass oil.

Following these experiments, the AMT module was regenerated with

isopropyl alcohol and an oil-pentane mixture, which

was

run through the fibres

for several hours.

The pentane was added to reduce the oil viscosity and

facilitate oil transport.

The oil-pentane mixture was replaced by pure

oil

and

the pure oil flux was m easured

as

a function of time.

As mentioned previously,

water appeared in the perme ate initially. Show n in Figure

17

is the oil flux for

the water-free oil permeate. It can be seen that several hours were requ ired

to

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71

regenerate the AM T module; a permeate flux similar

to

that previously

observed however, was eventually obtained.

It was decided that a comparative study should be made between the

coate d and unco ated f ibres to determine whether o r not the coating

significantly improved the ability of the membrane

to

pass oil.

Figure

17:

AMT Mo du le Regenerat ion; Pure Oil Flux

vs

Time

1.6

1.4

1.2

1

o

P=13

PSI

Q

=140 ml/mln

f

0.8

0 20 4 0 6 0 8 0

Time (hr)

(b) AMT vs Ce lgard Exper iments

The performance

of

the coated AMT fibres was com pared with that of the

uncoated Celgard fibres.

Preliminary experiments were carried out on the

Celgard 1 module

to

compare its behavior with that

of

the AMT module. Oil-

water mixtures containing

1%, 10%

and

100%

oil by weight were tested, and

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72

the results are summarized in Section 1 of Appendix

II. As

with the

AMT

module, feed flowrate exerted little or no effect on the permeate flux for the 10%

and

100%

oil-water m ixtures and pressure exerted

a

near linear effect on the

permeate flux for the 10% oil concentration. A decline in permeate flux with

time also was observed with the Celgard 1 module, and this decline was

significant for the 1

OO

oil-water mixture.

Before comparative studies were conducted, both the Ce lgard 1 and AMT

modules were regenerated with pure oil. The permeate

flux

for pure oil was

measured

as

a function of pressure an d these results are shown graphically in

Figure

18 .

Figure 18:

AMT vs

Celgard 1; Flux

v s

Pressure For Pure Oil

X

3

c

6 -

5 -

4 -

3 -

2 '

0

I

I

i

0 1 0 2 0

Pressure (psi)

Q =200

ml/min

The uncoated Celgard f ibres yielded higher permeate f luxes at al l

pressures. This cou ld be

due

to differences in membrane porosity or a

reduction in membrane permeabil i ty in the AMT module caused by the

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7 3

fluorocarbon coating.

(The coating may have reduced the po re size and thus

the overall porosity of the m embrane.) The plot for the Celgard module

suggests some curvature at higher pressures. This type

of

behavior would not

be unexpected as membrane compaction could occur at these pressures.

Theoretically these plots should pass through the origin as pressure is the

driving force for oil transport through the membrane.

Oil-water mixtures containing 1% and

10%

oil, and no surfactants were

run on the two modules in parallel and permeate rates were collected over a

25 hour and 49 hour period, respectively. The following operating conditions

were used:

Pressure: 12-13 psi

Feed Flowrate; 200 ml/min

Temperature: 32-38' C

Surfactants: NONE

The temperature varied due

to

heat inputs from the mixing devices: both

experiments were conducted over sim ilar temperature ranges, however.

The data obtained are given in Section 2 of Appendix II. Between runs

the m odules were regenerated with an oil-pentane mixture and then pure oil.

The pure oil fluxes were measured and the flux values recorded at time

zero

in

Appendix

II

correspond to those immediately after pure o il regeneration.

It

can

be seen that the pure oil

flux

dropped slightly between runs.

Figure 19 shows a plot of pe rmeate flux versus time for the 1% oil-water

mixture and Figure 20 for the 10% oil-water mixture. The uncoated fibres

yielded higher permeate fluxes than the coated fibres for both these oil

concentrations

over

the time period tested. Again, this could have been due to

differences in membrane porosity. It appears from Figure

19,

that the uncoated

fibres

took

somewhat longer to reach a steady state flux than the coated

AMT

fibres. The Celgard 1 module and the AMT module both showed an initial drop

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7 4

in flux over the first 10 hours of operation. How ever, the flux in the Celga rd 1

module de clined dramatically between 10 and 24 hours. This flux decline may

have bee n due to water blinding. Following this resu lt it was decided that the

10 oil-water m ixture should be tested over a long er period of time.

The results shown in Figure

20

indicate a very similar decline in flux for

the two modules. In this case, the dramatic decline in flux in the C elgard 1

module after

10

hours was not observed. Interestingly, however, the absolute

drop in the flux measured in the Celgard module was greater than that

observed in the AMT module; the pe rcent drop in flux was

49%

and

74% for

the

Celgard

1

and AM T modules, respectively.

These comparative studies demonstrated that C elgard fibres are capable

of selectively removing oil from oil-water mixtures without the application of a

fluorocarbo n coating. Further, the uncoa ted fibres were foun d

to

be more

efficient at remov ing oil from surfactant-free oil-water mixtures.

At this point a decision was made to continue experimental work with only

the uncoated Celgard fibres. The reasons for this decision were as follows: (1)

the exact nature of the AMT fluorocarbon coating was proprietary ;

(2)

the AMT

module had been us ed previously and was received

in

unknown condition;

(3)

the coated fibre made by AMT was over

a

year old and changes in the

surface character of the coating may have occurred; (4) AMT could not provide

sufficient technical support to continue working with the A M T module, and they

had no intention of making new coated fibre in the near future; and (5) the

uncoa ted Ce lgard fibres were ca pable of selectively removing oil from oil-water

mixtures.

Subsequent experiments with the unco ated Celgard fibres w ere aime d at

charac terizing their behavior in the presence of different surfactants.

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75

Figure

19:

AMT

vs

Celgard 1; Flux vs Time for a

1%

Oil-Water Mixture

I I

0

1 0

2 0 30

Figure 20:

Time (hr)

AMT

vs

Celgard

1;

Flux

vs

Time for a

10% Oi l -Water Mix ture

3

n . I

I

I

I

u -

0

1 0

20

30

4 0

Time

(hr)

5 0

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76

( c )

Celgard Exper iments

Preliminary experiments were carried out

to

determine the effect of

emu lsifying agen ts on the permeate flux for the Celgard fibres. An experiment

was co nducted in which a 10% oil-water mixture containing 1% Pet Mix#9 by

weight of oil was tested on the Celgard

2

module. The Celga rd 2 module was

used

for

these experiments as the Celgard 1 module was damaged . The

results

of

this experiment are shown g raphically in Figure 21.

F i gu r e 21: Celgard 2 Module; Flux

vs Time

for a 10% Oil-Water

M i x tu r e C on ta i n i ng 1% Pet Mix#9

4.0

3.0

2.0

Water

in

Permeate

3

'

, l

.0

-

0.0

0 1

2

Time ( h r )

The permeate flux declined rapidly and water appeared in the permeate

of

the Celgard

2

module a fter only

2

1/2

hours. The experiment on this m odule

was terminated

at

this time. The m odule was cleaned

with

detergent, isopropyl

alcohol and pen tane, and then dried thoroughly. After cleaning, pure oil fluxes

comparable

to

those previously observed were obtained.

This demonstrated

that the uncoa ted fibres can be regenerated after the use of a surfactant.

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7 7

The effect of surfactants was further tested by running a 10% oil-water

mixture containing 1% ABS by weight of oil on the Celgard 2 module. Water

appeared in the permeate stream after only about

5

minutes, and the

experiment was terminated shortly thereafter.

It was clear that water could not be prevented from passing through the

uncoated membrane in the presence of a surfactant. It was observed,

however, that the permeate separated quite readily into two distinct phases, oil

and water, respectively. Although this behav ior is not ideal, it is favorable since

the resulting permeate could easily be treated using gravity separation

techniques to recover the oil from the water. Additional Celgard experiments

were conducted therefore, to determine whether

or

not this behavior is

observed over longer periods of t ime and in the presence of di f ferent

surfactants.

Tests were initiated with the Celgard 2 module. The module was cleaned

and dried as described above and pure

oil

was run through the module to

check the pure oil f lux. A value similar to that achieved previously was

obtained (4.2 ml/min-ft2 at 12 psi). A

10

oil-water m ixture containing 1YOABS

by weight of o il was then prepared

and

run on this module.

This em ulsion was tested over a 70 hour period at a feed flowrate of

240

mumin and

a

pressure

of 12

psi. Permeate samples were collected and the

permeate rate was measured at different time intervals.

In determining the

permeate rates, permeate samples were collected for roughly 5 minutes. Oil

was removed continuously from the system to determine the permeate flux as a

function of oil concentration

and

to determ ine to what p ractical oil concentration

the oi l -water mixture could

be

reduced. The data col lected from this

experiment is summa rized

in

Section 1 of Appendix

111

Water passed through the m embrane almost immediately after beginning

the experiment. At this time the permeate was water rich and it separated

readily into two phases.

As time passed however, the permeate became

depleted in water and enriched in oil until a cloudy o il permeate was obtained

from which water would not separa te. Finally, after 3

or

4 hours, a pure oil

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permeate was obtained and it continued for the remainder of this experiment.

No water p assed through the membrane after about 3 hours.

Figure 22 show s a plot of flux versus time for this experiment. The flux at

time zero corresponds to that immediately after pure

oil

was run and so the

sharp initial drop

in

flux was anticipated. Thereafter, the flux increased slightly

due to a rise in temperature from 32Oto 38' C, but remained relatively constant

for about 18 hours (fr om 4 to 22 hours). During this time the oil concen tration

dropped from about 10.4% to 2.8% by volume.

This is shown in Figure 23

which presents a plot

of

permeate flux versus the residual oil concentration.

(The Yo oil remaining was determined using a mass balance rather than by

direct

oil

analysis. The volume of oil collected was sub tracted from that initially

* present an d the oil remaining was calculated.) At oil concentrations less

than about 2% the permeate flux decreased dramatically. After about 40 hours

an oil concentration of less than

1%

remained and at this point the permeate

flux was extremely

low

(<

0.01

ml/min-ft2). This suggests that there is a lower

practical limit be yond which

SOF

s not feasible for o il removal.

After runn ing this expe riment

for

about 20 hours it was observed that the

permeate oil had become darker in color, and it continued

to

do so for the

remainder of the experiment.

After 40 hours the permeate was very dark

brown, which implies that ABS, which is also dark colored, was passing

through the fibres. No specific tests were m ade at the time to verify this.

After 70 hours the reversibility of the separation process was tested by

adding fresh oil bac k to the emulsion to increase the o il concentration to that

initially used (1 0% by weight

or

11.2% by volume).

Following this, a permeate

flux similar to that previously measured was obtained.

This

is shown in Figure

22. The flux rates are therefore concentration dependen t and reversible.

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The above experiment was repeated on the C- 3 module to check the

reproducibility of the results.

This module had been made by a previous

researcher. The results of this reproducibility study are given in Section

2 of

Appendix

I l l .

The same general trends were observed

in

this experiment in that w ater

passed through the fibres initially, and after a few hours only oil passed

through.

However, lower permeate fluxes were obtained and this may have

been due

to

differences in membrane porosity. (It was not known fo r sure if the

fibres were of the X-10 or X-20 type). The resu lts of the two experiments are

shown graphically in Figures

24

and 25, which give permeate flux as a function

of

t ime and

O h

oil

remaining, respectively. Although the absolute fluxes are

somewhat different the system response was similar.

Figure 24:

Celgard 2 and

C-#3 Modules;

Flux

vs Time for

a

10%

Oil-Water

Mixture

Conta in ing

1% A B S

3.5

3.0

2.5

2.0

1.5

1 o

0.5

0.0

-

Celgard 2

Q =200 ml/min

P=12 PSI

f

0

1 0

20 3 0 4 0 5 0

Time (hr)

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81

F igure

25:

Celgard 2 and C-#3 Modules; F lux vs % Oi l Remain ing

for a

10% Oi l -Water Mix ture Conta in ing

1%

ABS

21

Q

=200 ml l m l n

f

P=12 PSI

- 0

2

4

6 8

1 0

1 2

70 Oil

Remain ing

The permeate flux for the

C- 3

module remained relatively constant for

about 22 hours (4 to 26 hours).

Bey ond this the permeate flux declined. This

behavior was different from that obsewed for the Celgard 2 module

in

that the

flux decline began at

a

higher oil concentration

(5.3%)

and

it

occurred more

gradually.

As a result a much longer time was required

to

obtain an oil

concentration of

2.0%

(46 hours

as

opposed to 25 hours).

Again, the

SOF

process appears to be limited at low oil concentrations.

The behavior of the Celgard membrane appeared to be reproducible for

the

ABS

naphthenic oil experiments. In both cases water permeation occurred

initially followed by a reversal to oil permeation only. The reas ons for this

behavior are unclear. The system is complicated with a comp lex engine oil

and a commercial surfactant blend of alky benzene sulfonates. The exact

-_

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83

Figure 26 presents a plot of permeate flux versus time for both the high

and

low

pH experiments. The permeate flux for the low pH experiment is

greater than that for the high pH experiment over the entire time period tested

(except where additional oil was added to check revers ibility). This implies that

oil rem oval is adverse ly affecte d as emulsion stability increases and that the

coalescence and attachment mechanism is important in the overall transport

process. Figure 27, which is a plot of permeate flux versus

Ol0

oil remaining,

also

shows higher fluxes for the low pH emulsion except at a few of the higher

oil concentrations. A plateau in the flux was not observed for the oil-water

mixture at high p H a nd oil removal efficiency dropped m ore rapidly with time.

The permeate was free of water, however.

Figure 26: Celgard

2

Module; Flux vs

Time f o r

a

10%

Oil-Water

Mixture Contain ing

1%

A B S at

High

and

Low pH

Q ~ 2 0 0

l/min

P=12

PSI

f

0

1 0

2 0

30

4 0 5 0

Time (hr)

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85

i i i ) Sal t Add i t i on

A

final experiment on em ulsion stability was carried out by adding 0.1 M

KCI

to the

10%

ABS emulsion. The addition of salt should result in a

compression

of

the electrical double layer surrounding the oil droplets and

therefore cause a reduction in emulsion stability. It was anticipated that this

would lead to an increase in permeate flux as observed for the low pH tests.

The results were unexpected.

After the addition of KCI the membrane

immediately began to pas s water and retain oil. In other words, the membrane

becam e an u ltrafilter.

This behavior continued for a few hours after which time

the test was terminated.

A hypothesis for this behavior is that the salt addition may have caused an

apparent depression in the pK o f the sulfonic groups attached to the membrane

surface (H elfferich,

1962).

At an ABS concentration

of 10%

one would expect

a

large amount of surfactant to be present at the membrane surface and

oriented in such a way tha t the hydroph ilic heads of the surfactant

(RSO3-H+)

extend out from the surface. The K+ ions could displace some

of

the

H+

ons

and cause a shift in the dissociation equilibrium of the sulfonic acid groups

which w ould effectively lower the pK and result in an increase in the am ount of

unprotonated RSO5 functional groups. Thus, the fibres would acquire a more

negative charge which would not only repel the negatively charged dispersed

oil droplets but the

RSOf

ions would attract water molecules and become

hydrated. In

so

doing, the surface would appear strongly hyd rophilic and the

passage

of

water wou ld be encouraged.

The separation

of

oil from oil-in water emulsions appears to be strongly

influenced by the presence

of

emulsifying agents and the stability of the oil

droplets.

The experiments with increased pH and with increased ABS

concentrations clearly dem onstrate this effect. The influence of KCI raises

additional questions about the m echanism, however.

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86

Non ion ic

Sur factant Ef fect

A final test was conducted on a nonionic surfactant, Triton X-102.

Communications w ith

R.

Callahan (Questar) suggested that this surfactant has

a high affinity for polypropylene membranes and as such it was anticipated that

the Celgard fibres would be ineffective in removing oil from emulsions

stabilized with this surfactant

An experiment similar to those described above was carried out on a 10%

oil-water mixture containing 1% Triton

X-102

by weight of oil to determine the

effect of a nonionic surfactant.

The P-#l Module, which contained X-10 fibres

with a 200 p diameter, was used for this test. The experiment was run for 29

hours during which permeate rates were m easured. The results are shown in

Section 4 of Appendix 1 1 1

The results of the Triton X-102 experiment differed from those of the

ABS

experiments

in

that water did not pass through the membrane initially and a

pure oil permeate was collected throughout the experiment. The permeate flux

was also found to be lower than that

for

the ABS experiments over the entire

time tested. This effect which appears to be due to the presence of a nonionic

surfactant may also b e due to the differences in the membrane types used in

the modules. The results

of

this experiment are shown graphically in Figures

28

and

29

which give plots of the permeate flux versus time and

Yo

oil

remaining.

These p lots show a continuous decrease

in

permeate flux with time and o/o

oil remaining

for

the emulsion stabilized with Triton X-102 rather than a plateau

region as observed in Figures

24

and

25

for the ABS emulsions.

The oil flux

was not measured however, between 6 and 20 hours for the Triton X-102

experiment, and so the flux may have remained relatively constant for some

time

and

not been observed. Nevertheless, the flux fell dramatically below an

oil concentration of about 2%, which suggests that the process

is

limited fo r

nonionic surfactants as well. When the concentration of oil in the emulsion was

increased it was observed that the flux of o il rebounded

to

i ts initial values as

shown in Figure

28.

Thus the process appears to be reproducible and

reversible.

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a 7

Figure 28:

el

F

E

E

C

.-

W

x

a

i i

Figure 29:

P-# l Module ; F lux v s Time for a 10% Oil-Water

Mixture Conta in ing

1%

Triton X-102

More Oil

Added

P=12

psi

5

1 0 1 5 2 0 2 5 3 0

3 5

Time (hr)

P- l Module; Flux vs YOOi l Remaining for a

10%

Oil-Water Mixture Contain ing 1% Triton X-102

1.00

1

o.*o/

0.60

0.40 -

0.20 -

-

f

0

2

4

6

a

1 0 1 2

%

Oil Remaining

P=12

p s i

Qf=200 ml/mln

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(d )

Additional

Observa t i ons

An interesting observation made during all of the Celgard experiments

was that the permeate flux tended

to

increase whenever the Procon pump was

used in conjunction with the magnetic stirrer. The pum p, which was switched

on occasionally to provide additional mixing, resulted in the formation of a

significant amount of air bubbles in the oil-water mixture. The air bubbles may

have enhanced oil transport by increasing the opportunity for contact between

the dispersed oi l droplets and the membrane surface by providing more

turbulent flow conditions. Also, the air bubbles may have altered

the

contact

angle between the oil droplets and the fibres in such a way that droplet

attachment and coalescence was encouraged.

The Celgard experiments described above demonstrated that oil can be

selectively removed from chemically emulsified oil-water mixtures using SOF.

The e fficiency of the separation however, appears

to

be dependent on the type

and concentration of emulsifying agent and perhaps on emulsion stability. The

SOF

process also seem s to be limited at low oil concentrations.

These studies raised many basic questions regarding the mechanism of

oil sepa ration and the role of emulsion stability. Unfortunately, the practical

systems studied above were too com plicated to extract answers. The base

engine oil was

a

blend of hydrocarbons and the surfactants were also mixtures

of products. In an attempt

to

simplify the system additional tests were

conducted with pure m odel compounds.

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89

5.2 Dodecane Exper iments

The dodecane experiments were carried out

to

assess the SOF process

under more carefully controlled conditions. Purer compounds were employed

in order to m inimize experimental uncertainties. Dodecane was used as a

model oil and sodium dodecyl sulfate (SDS) was em ployed as a surfactant.

The experiments on dodecane-water mixtures stabilized with

SDS

were

intended initially to duplicate some of the preliminary studies with naphthenic

oil: the influence of operating pressure, feed flowrate, dodecane concentration

and system design were tested. The permeate flux was m easured as a

function of these operating parameters, and an attempt was made

to

characterize the mechanism of oil transport under different conditions and

to

determine som e of the controlling factors in the SOF process. Following these

studies

a

more careful examination of emulsion stability and its influence on

dodecane separation was conducted.

(a) Prel iminary Exper iments

Preliminary experiments were carried out to verify that dodecane c ould be

emulsified using

SDS

as an emulsifying agent and to obtain some baseline

data on the removal

of

pure dodecane using the SOF process.

SDS

was

found to emulsify dodecane very easily in distilled water.

A

5% dodecane-

water mixture was made and tested

on

the Celgard 2 module which had been

saturated with dodecane.

Only dodecane passed through the fibres under an

applied pressure of 10 psi and

a

feed flowrate of 200 ml/min.

It was concluded

that dodecane-water mixtures stabilized by

SDS

were suitable for use in these

experiments.

The

fibres

of

the

P-#5

module were wetted with pure dodecane, and the

dodecane flux was measured. Fluxes of approximately

58

ml/min-ft2 and

120

ml/min-ft2 were obtained at operating pressures of 5 and 10 psi, respectively.

Though the data are limited, it appears that pressure exerted a linear effect on

the flux for pure dodecane, as predicted by equation (3.5) which describes

membrane transport. The flux values for dodecane were considerably higher,

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9 0

however, than those obtained for pure naphthenic oil on the Celgard fibres,

which were about 2.10 ml/min-ft2 and 4.30 mI/min-ft2 at

6

and 13 psi,

respectively. This is due most probably to the large viscosity difference

betw een the two oils.

As

shown in equation

(3 . 5 ) ,

he rate of oil transport through a membrane

may be defined as, Jm-

.

For a given membrane one might assume that the

membrane permeability, K, and the membrane thickness, 6, remain constant

under different operating conditions, and thus one wou ld expect

J m

a-. If the

same operating pressure was used

for

two different oils with varying

viscosities, one should find that (Jm*p) l = (Jm*p)2,where

1

and 2 denote oils 1

and 2, respectively. To determine whether or not this relationship holds true in

the

SOF

process, a comparison of the fluxes and viscosities

for

the pure

naphthenic o il and the pure dodecane was m ade.

KAP

P6

A?

CI

The comparison was made on the basis of pure oil flux measurements

taken at room temperature, which was assumed to be about 25OC. The

viscosity of dodecane at this temperature is 1.35 CP (CRC Handbook of

Chem istry and Physics, 1982-1983).

The viscosity for the naphthenic oil was

measured to be 52.6 CP at 2OoC (by AMT), and thus an extrapolation had to be

made to 25OC. Based on viscosity d ata given in the CRC Handbook for

a

light

ma chine oil, the naphthenic oil viscosity was estimated to be 40 cP.

Pure oil

fluxes at an operating pressure of 5 psi were use d in this calculation.

The flux

values for the naphthenic oil were extrapolated to 5 psi assum ing a linear flux-

pressure relationship for pure oil. A value of

1.8

ml/min-ft2 was use d for the

naphthenic o il and a value of

58

ml/min-ft* was used for dodecane..

It was

found that ( J m * p ) ~ = 7 2nd ( Jm*p )o=78 under the conditions

described above, for the naphthenic oil and dodecane, respectively

.

These

values are quite similar which suggests that the properties of the membrane

did not change significantly for the two different oil. One might have anticipated

a cha nge in m embran e permeability in the presence of different hydrocarbons

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91

due to differenc es in solvent-mem brane interactions. This does not appear to

be the case here. however.

Following these preliminary tests, four suites

of

experiments, which are

described in turn below, were conducted under the following operating

conditions:

Oil Concentration = 5%, 10% dodecane (w/w)

Pressure = 5-20 psi

Feed F lowrate = 100-2400 ml/min

Temperature = 36-38' C

SDS Concentration = 0-500 mg/l

Module Geometry = Countercurrent and Cross Flow

A constant temperature was maintained to prevent changes in oi l

viscosity, w hich could cause inconsistencies in the experimental results. Two

different module geometries were tested to compare the influence of hydraulic

design on the process performance; it was anticipated that the cross flow

module would yield higher permeate fluxes as it provides better contact

between the o il droplets and the fibres.

For consistency in measuring the permeate rates the following procedure

was adopted. After chang ing either the operating pressure or feed flowrate, 15

minutes was allowed

for

equilibration. The permeate rate was then measured

several times over a half hour period. In any given measurement roughly 10 ml

of dod ecan e were collected. The average of the permeate rate values

obtained over this time was used. After changing the SDS concentration,

several hours were allowed for equil ibration; usually the system was left

running overnight.

(b) Countercurrent Module; 5 Dodecane Exper iments

Initially a 5 % dodecane-water mixture was tested on the countercurrent

module P-#5.

SDS concentrations of 50, 100 and

500

mg/l were tested. At

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92

each of these surfactant concentrations the effect of pressure and feed flowrate

on permeate flux was determined over the ranges

of

5-20 psi and

100-750

ml/min, respectively. The results of these experiments are given in Appendix

IV.

Figures 30 and 31 show the effect

o f

pressure and feed flowrate

respectively, on permeate flux. The flux did not increase linearly with pressure

as previously demonstrated in the naphthenic oi l -water experiments.

Unexpectedly, the permeate flux tended

to

decrease with increasing pressure.

One possible explanation for this is that m embrane compaction occurred at the

higher pressures; this would tend to reduce the m embrane porosity. The

permeate flux appears

to

have increased linearly with feed flowrate, however,

over the experimental range tested.

Figure

30:

Countercurrent Module; 5 Dodecane

Pressure Effect

4.0

3.5

3.0

2.5

2.0

1.5

1 o

0.5

0.0

-

Qf

=300

T=37

C

0

mllmin

0 1 0 2 0

Pressure (ps i )

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93

F i gu r e 31:

Countercur rent Module;

5

Dodecane

Feed F lowra te E f fec t

8 -

6 '

4 -

2 -

10 I

50

mg/ l SDS

o

100 m g / i

SDS

4500 m g / l

SDS

0 200 400 600 80

P=10 p s l

T=37 C

I O

Flowra te

(ml/min)

These results imply that the SOF process is not membrane limited for the

transport

of

dodecane from

a 5%

dodecane-water mixture and that dodecane

passes through the membrane pores faster than

it

can be supplied to the

mem brane surface.

This is no t surprising since dodecane has

a

low

viscosity,

which facilitates membrane transport. Increasing the feed flowrate not only

increases the rate of o il transport to the m embrane but it may a lso enhance the

attachment and coalescence step as the dodecane droplets are brought into

more intimate contact with the fibres.

It

would be of interest to study the effect of

feed

flowrate over a broader

range

to

determine at what flowrate the permeate flux stops increasing. One

would expect the permeate

flux

to level out at some point which corresponds to

a shift in the rate controlling process from transport to the membrane surface to

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94

transport throug h the mem brane. Beyond this flowrate the permeate flux

should become dependent on pressure, oi l viscosity and membrane

characteristics.

f igures 30 and 31 both show a decrease in permeate flux with increasing

SDS oncentration. The exact cause for this effect was not clear and it may be

due either to an increase in emulsion stability which is accompanied by

reduced coalescence and attachment,

or

to a build up of surfactant at the

membrane surface which presents a mechanical and/or electrical barrier

to

mem brane transport. If the m embrane were to become coated with SDS,

t

would acquire a negative charge and thus have a tendency to repel the

similarly charged

oil

droplets.

The linear regressions shown in Figure 31 for the flux versus feed flowrate

data show that the plots do no t pass through the origin. The positive intercept

on the absicissa indicates that there is some critical feed flowrate below which

oil cannot be transported across the membrane. This sugges ts that a minimum

amount of kinetic energy is required for the dodecane droplets to collide with,

coalesce and attach to the fibres.

One would expect the value of this critical

flowrate to vary with dodecane concentration and SDS concentration. At

higher oil concentrations the critical flowrate should decrease as there are

more dodecane droplets available for transport to the fibre surface and thus a

higher probability of contact.

At

higher SDS concentrations the c ritical flowrate

should increase as more kinetic energy is required to force the stable oil

drop lets into intimate contact with the fibres. The plots in Figure 31 show no

increase in the critical flowrate between

50

mg/l and 100 mg/l SDS and only a

slight increase at 500 mg/l SDS.

Permeate samples were taken at different SDS concentrations and

qualitatively tested for the presence of surfactant. This was done by adding

water to the samples, hand shaking them and observing whether or not the

dodecane readily re-emulsified.

SDS

was found to be present in all the

permeate samples.

In practice, this may

or

may not be desirable depending

upon the intended use

of

the recovered oil.

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95

(c)

Cross Flow

Module;

10%

Dodecane Exper imen ts

In the second suite of experiments a 10% dodecane-water mixture was

tested on a cross flow module. This module, as described previously, had a

surface area of 1.1

5

ft2.

SDS

concentrations of 0, 50, 100 and

200

mg/l were

tested and the effects of p ressure and feed flowrate on the perm eate flux were

determined over the ranges of 5-20 psi and 100-750 m lh in , respectively. The

results of these experiments are given in Appendix V.

Before running these experiments the module was wetted with dodecane

and the permeate flux for pure dodecane was measured. The permeate flux

was found to be similar

to

that obtained for the countercurrent module

(60

ml/min-ft2

at

5

psi). This was expected as system design should have no effect

on memb rane transport, Jm.

Figures 32 and 33 present the effects of pressure and feed flowrate,

respectively.

The same general trends were observed for the 10% dodecane-

water mixture

on

the cross flow m odule as for the 5% dodecane-water mixture

on the countercurrent module. Increasing pressure caused a decrease in

permeate flux and increasing feed flowrate caused an increase in permeate

flux. These resu lts indicate that the SOF process is not membrane limited even

at a dodecane concentration of

10%

and suggests that dodecane

is

not

reaching the membrane surface as fast a s it

can

be transported through the

membrane. The permeate flux decreased with increasing

SDS

concentration

and the decline was not

linear. The first add ition of SDS caused a dramatic

drop in permeate flux whereas subsequent additions of SDS had a less

marked effect.

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9 7

As

shown in Figure 33 the increase in permeate flux with feed flowrate was

linear except in the absence of

SDS.

In this case, the permea te flux declined

more grad ually at higher feed flowrates.

At

a dodecane concentration of

10%

the opportunity for contact between dodecane droplets and the fibres is

improved by the presence of more dodecane droplets. The transport of oil to

the membrane therefore, is not as strongly enhanced at high feed flowrates

compared to a 5 dodecane-water mixture which contains fewer dodecane

drop lets, At a perm eate flux of about 2-3 ml/min-ft* one could expect a shift in

the transport limiting step for the No

SDS

case, to membrane control; this

should occur at some feed flowrate beyond 750 ml/min.

As

with the 5% dodecane experiment on the countercurrent module, there

appears to be a critical feed flowrate below which oil transport cannot occur.

(The plots in Figure 33 do not pass through the origin.) Figure 33 shows a

slight increase in the critical flowrate with increasing SDS concentration and

this may

be

compared to Figure 31, which shows

a

similar trend. A comparison

of

Figures 31 and 33 'also indicates a slight decrease in the crit ical feed

flowrate with increas ing dodecane concentration. Different module geometries

were used however, which wo uld affect the hydraulics of the system and thus

affect the efficiency

of

oil transport to the m embrane surface.

A direct comparison

of

the

data

for the two module geometries indicated

that

the coun tercurrent design was more effective at remo ving dodecane from

dodecan e-water mixtures.

For the

same pressures and feed flowrates the

countercurrent module yielded higher permeate fluxes, even though the oil-

water mixtures contained only h alf as m uch oil as those run on the cross flow

module. This type of comparison is misleading, however, as the Reynolds

numbers corresponding to a given feed flowrate are very different for the two

modules. The Reynolds number may be defined as follows (Hughes and

Foulds,

1986):

VSPdf

Re =-

P

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98

where,

vs

= superficial velocity

p = oil-water mixture density

df

=

fibre diameter

p =

oil-water mixture viscosity

The cross sectional area

of

the countercurrent module was much smaller

than that of the cross flow module (0.785 cm2 versus 17.85 cm').

As

a result

the superficial velocities, and hence the Reynolds numbers, were much greater

for the countercurrent module for a given feed flowrate. This is the reason for

the higher permeate fluxes observed

for

the countercurrent module. It was not

possible to compare permeate fluxes of the two modules at similar Reynolds

numbers because this would require the use of feed flowrates greater than 3

Vmin for the cross flow module; these flowrates could not be achieved without

producing excessive head losses across the module (>

10

psi).

N o

further

attempts were made therefore, to characterize the effect of modu le geometry

and all subsequent experiments were carried out on the cross flow module.

To determine the influence of oil concentration on permeate

flux

the

10%

dodecane-water mixture containing 200 mg/l SDS was diluted with distilled

water to half its original dodecane concentration. The

SDS

concentration

of

the

5

dodecane-water m ixture was 100 mg/l as the SDS concentration was

also reduced upon dilution.

The permeate flux was m easured as a function

of

feed flowrate and the results were compared to those obtained for the 10%

dodecane-water mixture. This is shown graphically in Figure

34.

Figure 34 shows that the permeate flux was lower for the 5 dodecane-

water m ixture even though the

SDS

concentration was half that of the 10%

dodecane-water mixture. This result was expected as the opportunity for

contact between the oil droplets and the fibres should decrease as the

concentration of

oil

droplets decreases. This result also supports the

hypothesis that the dodecane flux is transport controlled within this oil

concentration range.

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/

99

F i gu r e 34: Cross Flow Module;

O i l

Concentrat ion Ef fec t

0.6

-

0.4

-

0.2 -

0.8

0.0

*< '

I I 8

0 200

400 600

8 0 0 1000

P=10 p s l

0

T=37

C

perme

te flux v

Flowrate

(ml /min)

lues were on average only about

3 times as

1

for

the

10%

dodecane-water mixture rather than twice as high. This suggests

that the permeate flux is not linearly related to dodecane concentration in this

concen tration range. The 5% dodecane values shown here however, may be

high since less SDS was present than with the

10%

dodecane mixture. Figure

34

also shows that the critical feed flowrate below which oil transport cannot

occur is higher for the

5

dodecane mixture, as expected

These tests for the countercurrent and cross flow m odules agreed well,

and the data indicate that the

SOF

process appears to

be

controlled by the

transport and attachment mechanisms. In the next experiments, additional

studies were conducted to characterize the emulsion

in

order that performance

could be related to emulsion stability

as

well

as

normal ope rating parameters.

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100

(d)

Cross

F l o w Module;

5%

Dodecane Exper iments

In these experiments, which were carried out on the cross flow module

using a

5%

dodecane-water mixture, the effects of feed flowrate and pressure

were determined as they were previously. SDS concentrations of 0,

50,

100,

and

200

mg /l were tested, and the permeate flux was measured over a slightly

broader feed flowrate range of

300-2400

ml/min.

An attempt was made to

assess emulsion stability as a function

of

SDS concentration by measuring the

droplet size distribution and zeta potential. These data then were correlated to

the observed effect of increasing SDS concentration on the permeate flux with

the intent of clarifying the role of SDS in the process.

The results of these

experiments are given in Appendix

VI.

Before running these experiments the cross flow module was flushed with

distilled water for about 30 minutes to remove any residual SDS from the

system.

Dode cane was then run through the module and the permeate flux for

pure dodecane was measured.

The flux was found to be considerably lower

than that obtained prior to the 10% dodecane-water experiments (36 ml/min-ft2

versus 60 ml/min-ft2 at 5 psi). This implied that the fibres had lost some of their

membrane permeability during operation.

Possible causes of this are water

blinding or surfactant coating of the membrane.

Figure 35 shows a plot of permeate flux versus feed flowrate for the 5

dodecane-water mixture at the various

SDS

concentrations. As with the

10%

dodecane-water experiments, the f lux decreased with increasing SDS

concentration.

The flux decrease was marked initially and then it became m ore

gradual with subsequent SDS additions.

Unlike the 10% dodecane-water

mixture, the permeate flux increased linearly with feed flowrate in both the

presence and absen ce of SDS even up

to 2400

ml/min.

(No eveling off was

observed for the No SDS lux at higher feed flowrates.)

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101

Figure 35: Cross Flow Module; 5 % Dodecane

Feed Flowrate Effect

J

a-

8 -

?

E

6 -

E

= 4 '

C

X

u,

2 -

n 50 m g l l

SDS

100

m g l l

SDS

o I

P=10

P S I

T=37

C

-

0

1000 2000

3000

Flowrate (ml /min)

This linear increase in permeate flux would be expected since the

probability of droplet-fibre contact is lower at low dodecane concentrations.

Feed flowrate should have an effect over a broader range therefore, as more

kinetic energy is requ ired to result in droplet attachment and collapse.

A comparison of Figures 33 and

35

shows however, that the permeate flux

for the

5%

doecane-water mixture at feed flowrates above

1000

mI/min was

greater than that ob tained for the

10%

dodecane-water mixture. This implies

that the limiting transport step does not shift to membrane control at a flux of

about 2-3 ml/min-ft2 as suggested by the 10% dodecane-water results. The

transport controlling mechanism should shift at the same permeate flux

regardless

of

the oil concentraiton, although higher feed flowrates may be

required at lower

oil

concen trations to obtain this critical flux. These data lead

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102

one to conclude that the value of point A in Figure 33 is low, and that

a

plot of

flux versus flowrate for the

10%

dodecane-water mixture should be linear up to

a

flux of at least

9

ml/min-ft*, as shown in Figure

35

for the 5% dodecane-water

mixture.

The linear regression data given in Figure 35 shows that a critical feed

flowrate exists, below which oil cannot be transported across the membrane,

for the 5% dodecane-water mixture at all

SDS

concentrations. Comparing this

data

to

that shown in Figure 33, it appears that the critical flowrate values are

higher for the

5%

dodecane mixture than for the

10%

dodecane mixture.

A comparison of the data for the 5% d odecane-water experiments and the

10% dodecane-water experiments on the cross flow module show that oil

concentration has an effect on the perm eate flux.

It

is not possible

to

quantify

the exact nature of this effect as only two dodecane concentrations were tested.

However,

it

appears that the permeate f lux increased with increasing

dodecane concentration and that this effect became more pronounced at

higher feed flowrates. This is illustrated in Figure 36, which shows a plot

of

flux

versus dodecane concentration for two different SDS concentrations at two

different f lowrates.

The slopes

of

the lines at a feed flowrate

of

500

ml/min are steeper than

those at 300 ml/min. The slopes of the lines for the higher SDS concentration

also appear shallower, which further suggests that oil transport is more difficult

as the SDS concentration increases.

The oil concentration effect may have

been accentuated somewhat by the loss in mem brane permeability for the 5%

dodecane-water experiments,

as

indicated by the decline in pure dodecane

flux.

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103

F i gu r e

36:

Cross F low Module; Oi l Concentrat ion Ef fec t at

D i f fe ren t Feed F lowra tes

1

oo

a

300

m l l m i n

0.60 -

0.00

I . , . , . , . , . , .

P=10

psi

0

T=37

C

4 5

6 7

8

9

1 0 1 1

YO

Oil

A

comparison of Figures

34

and

35

shows that the fluxes obtained for the

diluted

10%

dodecane-water m ixture were slightly higher than those obtained

in these experiments. This could be due to a

loss

of membrane permeability as

discussed above.

Table

6

shows the zeta potential data that were obtained in these

experiments. The values ranged from about -56 mV to -70 mV for no SDS and

200 mg/l SDS, respectively.

Hughes and Foulds

(1986)

conducted zeta

potential measurements on

a

Nigerian light crude oil and kerosine and they

found the zeta potential of the oil droplets to be between

-40

mV and -70 mV in

the absence of surfactant and at a pH of about 6.0-7.0.

The pH of the 5%

dodecane-water mixture was m easured

to

be

6.4.

Although the ze ta potential

for the dodecane-water mixture containing

no

SDS fell within this range, it is

possible that this zeta potential value may have been low as a result

of

con tam ination of the system with SDS from previous experiments. (Even

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1 0 4

though the system was flushed with

SDS

it is possible that a very sm all amount

of SDS still rem ained which could have been sufficient to impart some stability

to

the oil droplets.)

Table 6:

Zeta Potent ia l as a Func t ion

of SDS

Concent ra t i on

~~

SDS C o n c e n t r a t i o n (mgl l ) Zeta Potent ia l

(mV)

0

-56.7

50 -65.7

100

-66.1

The zeta potential

of

the dodecane droplets became only slightly more

negative with increasing SDS concentration; a larger decrease in zeta

potential was anticipated. The change in zeta potential with increasing

SDS

concentration however,

corresponded quite well to the observed changes in

the permea te flux values since the largest decrease in zeta potential occurred

between

0

and

50

mg/l SDS and then the decrease was more gradual. (See

Figure

35.)

This apparently small effect of SDS concentration on emulsion stability

may have been sufficient to significantly limit the transport of oil across the

membrane by increasing the electrical repulsion between the oil droplets

themse lves and/or between the drop lets and the fibres which would reduce the

rates of droplet attachment coalescence and collapse. Hughes and FouIds

(1

986) made zeta potential measurements on polypropylene fibres in an

aqueous medium as a function of

pH

and found the fibres to be strongly

negatively charged, with a zeta poten tial of about -100 mV at a pH of 6-7. This

sugges ts that th e e lectrostatic repulsion between the fibres and the dodecane

drop lets may be very strong.

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105

The droplet size distribution data is given in Figure 37. The percentages

of the total oil volume w ithin each of the different droplet size ranges are shown

for the different SDS concentrations. It can be seen that the droplet size

distribution did not change significantly with increasing SDS concentration. In

all cases, the largest percentage (on a volume basis) of the droplets was in the

10-20

p

range and the smallest percentage was in the 1-3 p range.

A

slight

increase in emulsion stability was indicated by a small

increase in the

percentage of oil in the 1-3 p range and a small decrease in the percentage in

the 10-20 p range. This suggests that the smaller, more stable droplets are

more difficult to remove by

SOF

because of a reduced tendency for these

droplets to coalesce and attach to the fibres, and collapse. This is in

agreement with the theory

of

droplet coalescence on fibres (Haz lett,

1969).

Figure

37:

O h Tota l

Volume

1-3

3-5 5 - 7

7-10 10-20

>20

Droplet Diameter

(p)

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106

The nearly constant shape of the droplet size distribution curves implies

that the stability of the dodecane-water emulsions was not significantly

increased by the addition of SDS. This seems unlikely, however, and it is

possible that the droplet size distribution rema ined constant due to limitations

impo sed by the mixing pump. The pump, for example, may not have been

capable of producing a finer emulsion even though the interfacial tension was

sufficiently reduced by the addition of SDS. Alternatively, it is possible that

SDS

was migrating to the fibre surface and therefore, unavailable for droplet

stabilization.

Together, Table

6

and Figure 37 show that there is little increase in

emulsion stability with increasing

SDS

concentration, which

in

turn implies that

em ulsion stability has little effect on the perm eate flux. The mech anica l

aspects of emulsion stability may be important however, and they may change

with increasing

SDS

concentration.

.

The most likely explana tion for the lack of correlation between the change

in permeate flux and emulsion stability with increasing

SDS

concentration is

that some of the

SDS

migrates to the fibre surface and forms electrical and/or

mechanical barriers which oppose the transport

of

oil. As the polypropylene

fibre s are already negatively ch arged (Hu ghe s an d Foulds,

1986),

an

accumulation of anionic surfactant at the surface should result in an even

stronger negative charge and

one

would expect a reduction in oil transport due

to electrical repulsion.

To verify this hypothesis, the electrical charge on the

fibre surface would have to be m easured as a function of SDS oncentration in

solution to determine the electrostatic repulsion between the dispersed

droplets and the fibres.

The

exact

effect of

SDS on the

SOF

process was still not clear after the

completion of these experiments. Subsequent experiments were conducted to

further investigate the importance of electrostatic interactions between the

dispersed droplets and the fibres.

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107

(e)

AEAPTMS

Coated F ib re Exper iments

The cross flow module was coated with

N(P-aminoethy1)-y-aminopropyl-

trimethoxysilane (AEAPTMS), which is a cationic polymer,

to

impart a positive

charge to the fibre surface. It was anticipated that

if

the fibres were positively

charged, the negative dodecane droplets would have a stronger attraction

for

them , and hence a higher permeate flux could be achieved. The fibres were

coated by immersing the cross f low module into a

1%

solution (v/v) of

AEAPTMS for about 10 minutes and then drying overnight at 55-60'

C.

This

procedure was similar to that outlined

by

Hughes and Foulds (1986).

Before

the AEAPTMS coating was applied, the module was thoroughly cleaned and

dried.

The coated fibres were saturated with dodecane and the pure dodecane

flux was measured at a pressure of

5

psi.

The permeate flux was found

to

be

5 4

ml/min-ft2, which was com parable to that obtained initially on the uncoated

mod ule before it had been used. This sugges ted that the cleaning process

was effective in regenerating the fibres.

A 5%

dodecane-wa ter mixture with

SDS

concentrations of

0, 50,

100 and

200

mg/l was tested on the coated module.

The permeate flux was measured

at a p ressure of

10

psi and

at

feed flowrates

of

300,

500,

and

1000

ml/min. The

da ta from these experiments

is

given in Appendix

VII.

Figure

38

shows the effect of feed flowrate on the permeate flux for the

coated fibres. As with the 5%' dodecane-water mixture on the uncoated fibres

the pe rmeate flux increased linearly with increasing feed flowrate. The flux

also

decreased with increasing

SDS

concentration but the rate of decline was

unlike that observed for the uncoated fibres (See Figure

35).

With the coated

fibres, the flux decreased markedly from both 0 to

50

mg/l

SDS

and from

100

to

200 mg/l

SDS.

These data compared very well with the zeta potential data

shown in Table 6 in that a small change in zeta potential occurred between 50

mg/l and

100

mg/l SDS.

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108

Figure 38:

AEAPTMS Coated Modu le ;

O h

Dodecane-Water

Mixture; Feed Flowrate Effect

n

N

.

U

K

3

is

P=10 p s i

T=37

C

c

I

I

I I

I

I

0 200

400 600

8 0 0

1000 1200

9

(mllmin)

In

contrast to the uncoated fibres there does not appear to be a critical

feed flowrate below which oil transport is l imited, except for an SDS

concentration of 200 mg/l.

At

this concentration, the effect of

SDS

must be

significant enough to severely limit the transport an d attachment m echanisms.

The positive intercepts on the ordinate axis in Figure 38 suggest that under

stagnant flow cond itions oil transport is still possible. This is probably due to a

strong attractive interaction between the dodecane droplets and the coated

fibres.

A comparison of the permeate f luxes for the coated and uncoated

modules is given in Table 7. The AEAPTM S coating had a significant effect on

the permeate flux particularly at low feed flowrates.

Flux values u p to an order

of

magnitude greater were obtained for the coated fibres.

A

contributing factor

to this may have been the increase in membrane permeability due to module

.

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109

cleaning but the major effect is thought to be the attractive electrostatic

interactions between the droplets and the fibres.

Table 7: Permeate F luxes for the AEAPTMS Coated and Uncoated

Cross Flow Modu les fo r a

5%

Dodecane-Water Mixture

FIUX (m l /m in - f t21

O f

(m l /m in ) No SDS 50 mg/ l 100 m g / l 2 0 0 m g / l

S D S S D S

S D S

C

U

c u

C

U

c

U

300 2.08 0.87 1.44 0.16 1.20 0.11 0.26 0.09

500

2.90 1.31 2.03 0.23 1.76 0.19 0.44

0.15

1000 5.34 3.63 3.12 1.12 2.78 0.59

1.00

0.59

C = AEAPTMS coated module;

U =

uncoated module

P

=

10 psi;

T =

37

The increase in dodecane removal caused by the appl ication of

AEAPTMS was not consistent over the SDS concentration and feed flowrate

ranges tested.

In fact, the observed effect

of

the coating vaned dramatically, as

seen in Table 8, which gives the ratio of the permeate fluxes for the coated and

uncoated fibres, respe ctively. The improvem ent in dodecane remo val varied

from 1.7 imes for an SDS concentration

of

200 mg/l and a feed flowrate of

1000

mI/min to 10.9 times for an SDS concentration of

100

mg/l and a feed

flowrate of 300 ml/min.

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111

When

SDS

is added

to

the dodeca ne-wa ter mixtures the dode can e

droplets become more negatively charged and their mechanical stabil i ty

increases . At the same time some of the SDS may migrate

to

the fibre.surface

and cause the uncoa ted fibres to be com e more negative and the coated fibres

to

becom e less positive. The very low fluxes shown in Table

7

suggest that at

intermediate SDS concentrations of

50

and 100 mg/l

a

strong electrostatic

repulsion is ob served between the droplets and the uncoated fibres. The data

also indicate a strong attractive interaction for the coated fibres. This suggests

that the migration of

SDS to

the coated fibre surface may not have been

suff icient to signif icantly reduce the posit ive charge at these SDS

concentrations.

At an

SDS

concentration of 200 mg/l the ability of the AEAPTMS coating

to improve dodecane removal appears

to

have diminished. One explanation

for this is that sufficient

SDS

has migrated

to

the fibre surface to significantly

reduce the positive charge on the fibres.

This could be verified by measuring

the zeta potential of the fibre surface. With further additions of SDS, one would

expect the ratio of flux values to approach unity as the coated fibres become

more negatively charged; in other words the benefit of AEAPTMS may be

negated at high surfactant concentrations.

The effect of the AEAPTMS coating is shown graphically in Figures 39

and 40, which plot the dodecane flux versus feed flowrate for the coated and

unc oated fibres at the different SDS concentrations.

The AEAPTMS coating

clearly ha d a dramatic effect at

low

feed flowrates as sho wn by the differences

in the y-intercepts

for

the coated and unc oated fibres. The vertical distance

betw een the plots for the coa ted and u ncoated fibres reflects the increase in

flux attributable to the AEAPTMS coating. The improvem ent varied with SDS

concentration,

with

the greatest improvement in dodecane removal being

observed

for

SDS

oncentrations of

50

and 100 mg/l as discussed.

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112

Figures

39

and

40

show that the slopes of the plots for the co ated fibres

are only slighlty greater than those for the uncoated fibres.

This similarity in

slopes may reflect the separate

role

of droplet transport to the mem brane and

droplet attachment and collapse at the m embrane surface in the SO F process.

Feed f iowrate strongly affects both the transport and attachment

mechanisms, but the appl ication of the coating primarily affects droplet

transport and attachment, and perhaps collapse.

The observed differences in

the y-intercepts and the vertical distances between the p lots appear therefore,

to be caused by the change in the electrostatic interactions between the

drop lets and fibres for the coa ted and unco ated fibres. The similarity of the

slopes on the other hand, may be due to the insensitivity of the droplet collapse

mechanism to the application of the AEAPTMS coating.

F igu re 39

AEAPTMS

Coated F ibres

vs

Uncoated F ibres;

F lux

vs

Feed Flowrate for the

5%

Dodecane-Water

Mix ture Conta in ing

No SDS

and 100 mg/ l

SDS

4 '

100 mg/ l

P

=

10 PSI

0 2 0 0 400

600

800 1000 1 2 0 0

q (ml/min)

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113

Figure 40: AEAPTMS Coated F ibres

vs

Uncoated F ibres:

Flux

vs Feed Flowrate for a 5 % Dodecane-Water

Mix ture Conta in ing 50 mg/ l and 200 mg/ l SDS

x

3

ii

1

0

I I

I

0 200 4 0 0

600

800 1000 120

Qf (mllmin)

P =

10

ps i

200 m g l l

SDS

IO

Hughes and Foulds

(

986)

conducted streaming potentia, measurements

on AEAPTMS coated polypropylene fibres in aqueous solution and found the

zeta potential to be about +40 mV at a pH of 6-7. He did not conduct

measurem ents in the presence of surfactants, however.

Without zeta potential

data on the coated and uncoated fibres at different

SDS

concentrations it is

difficult to quantify the mag nitude of the e lectrostatic interactions be tween the

dodecane droplets and the coated fibres and their importance on the SOF

process. Stream ing potential measureme nts shou ld be done to verify the

hypothesis described above.

Zeta po tential measurem ents were m ade on the dod ecane-water mixtures

at SDS concentrations of

50

mg/l and 100 mg/l. Values similar to those

reported earlier for the unco ated fibre experimen ts were obtained. The

observed increase in dodecane removal therefore, can be attributed to the

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1 1 4

application of the AEAPTMS coating on the fibres rather than differences in

draplet stability.

These experiment demonstrated the importance of e lectrostat ic

interactions between the o il droplets and the fibres on the S OF process. In

effect, they confirmed what was suggested by much of the work done by

previous researchers on droplet coalescence on fibres. The results of these

experiments also imply that surfactants affect the

SOF

process primarily by

coating the fibre surfaces.

In

so doing,

they

present electrical, and possibly

me chanica l barriers, which oppose droplet approach and attachme nt. The role

of emulsion stabil ity on the

SOF

process appears to be secondary. Further

tests must be done to verify this hypothesis and to quantify the effect

of

SDS

on

the SOF process.

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115

6. DISCUSSION

6.1

D i scu ss i o n of Results

The fundamental research conducted in this study demonstrated that oil

can

be selectively removed from oil-water mixtures using microporous

polypropylene fibres, in both the presence and absence of em ulsifying agents.

The effect of various operating parameters on

oil

removal was investigated,

and inferences as to the controlling m echanisms

of

oil transport were identified

und er different operating conditions. The impact

of

surfactants on process

performance was examined and the role of surfactants in the SOF process was

determined. The principal findings of the study are discussed below.

I n f l u e n ce

o f

Oi l

Viscos i t y

Two oils were selected for study: a naphthenic oil with

a

viscosity of

52.6

CP at

2OoC

and dodecane with a viscosity of 1.35 at

25OC.

Both oils were

effectively removed by the mem brane process; however, the response of the oil

f lux rates to changes in operating conditions showed marked differences in

behavior. Rem oval of the naphthenic

oil

appeared to be limited primarily by

the membrane since the permeate flux was dependent on operating pressure

and insensitive

to

feed flowrate (exce pt at low oil conce ntrations). This

behavior may be attributed to the high viscosity of the naphthenic oil which

caused low permeate fluxes to be obtained. By comparison the low viscosity

dodeca ne pa ssed through the mem brane more easily, and thus h igher

perm eate fluxes were obtained. The strong dependence of dodecane flux rate

on feed flowrate and

its

insensitivity to operating pressure clearly indicate that

the do decane was not memb rane limited but rather limited by the transport and

attachment mechanisms responsible for bringing the oil to the membrane

surface.

Direct comparison of dodecane and naphthenic oil to evaluate the

influence of viscosity requires an assumption that any interaction between the

oil and membrane is essentially equal for both oils in spite of their structural

differences. This assumption is questionable. The

oil

transport through the

,

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1 1 6

mem brane is not directly analogous to ultrafiltration, where mem brane-solvent

interactions are genera lly unimportant. In the SOF process the membrane

swells and dissolves in the oil phase. The degree of swelling and the affinity of

the oil for the m embrane may have an important effect on separation rates. In

this sense the mem brane is more analogous to reverse osmosis.

An effective way of assessing the role of oil-memb rane interaction would

be to com pare oils having very different structures and/or molecular weight but

similar viscosities.

I n f l u e n c e o f

Oil

Co n c e n t ra t i o n

Changes

in

oil concentration appeared to have a similar effect on the

rem ova l of both the naphthenic oil and dodecane. As the oil concentration

increase d the rate of oil removal also increased. This increase was not found

to be linear, however, over the oil concentration ranges tested.

In general it

appeared that the effect

o f oil

concentration was more marked at

low

oil

concentrations and beca me less significant at higher oil concentrations. This is

shown

in

Figure 16 for the naph thenic oil. There appe ared to be a critical

concentration beyond which further increases

in

oil concentration had little

effect on the permeate flux and the value

of

this crit ical oil concentration

decreased with increasing feed flowrate. In the dodecane experiments a

similar effect of feed flowrate was observed.

For both oils the permeate flux

droppe d dramatically at low oil concentrations, which suggests that there is a

limiting oil concentration below w hich SOF is no t feasible.

I n f l uence o f Sur fac tan ts

Th e rem oval of both dodecane and nap hthenic oil was adversely affected

by the presence of surfactants, and the permeate fluxes decreased as a result.

The naphthenic oil experiments demonstrated this behavior qualitatively for

anionic and nonionic emulsifying agents, while the dodecane experiments

quantified the effect of

a

single anionic surfactant by measuring the permeate

flux as a function of SDS concentration. The permeate

flux

decreased with

increasing

SDS

concentration, although the flux decline was more significant

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117

for small additions of surfactant and became less dramatic with further SDS

additions.

These findings agree with those of other researchers who studied the

effect of surfactants

on

oil and water droplet coalescence on fibres (Hazlett,

1969; Bitten,

1970;

Clayfield et al., 1985; Hughes and

Foulds,

1986). They

found that the presence of surfactants greatly reduced the coalescence

effic ienc y. Clayfield et al. (1

985)

speculated that surfactants modify the

electrical properties of the droplets and the fibres and also that they affect

droplet film drainage and rupture. If this were the case in these experiments,

then both the transport and attachment mechanisms would be adversely

affected. The zeta potential measurem ents which were made in our study

indicated a slight increase in the charge on the dodecane droplets with

increasing

SDS

conce nt ation.

The dramatic influence of

SDS

is clearly not solely attributable

to

the

small change in the charge on the oil droplets. The ze ta potential droppe d only

from

-56.7

mV

to

-70.4 mV when the SDS concentration was chang ed from 0 to

200

mg/l

SDS.

In addition, measurements of the droplet size distribution

showed l itt le difference in the presence of

SDS.

These measurements

suggested that the emulsion stability had not increased dramatically in the

presence

of

SDS; howe ver it was apparent that the o il permea te rate declined

dramatically.

The me cha nical aspects of emulsion stability may have bee n

overlooked, and their role in preventing droplet attachment and collapse

should be assessed before conclusions as to the predominant role of

surfactants o n the

SOF

process are made.

It

is

possible that the dramatic impact that

SDS

had on oil flux across the

fibres resulted from some modification

of

the fibre surface.

SDS

may adsorb

onto the fibres and render

the

fibres more negatively charged.

Hughes and

Foulds

(1

986) showed that polypropylene fibres are negatively charged in

water with a zeta potential of about

-80

to -100 mV in the neutral pH range,

and the adsorption of

SDS

to the fibres could be expec ted

to

depress the zeta

potential

to

mo re negative values. The comb ined effect of

SDS

on the droplets

and mem brane surface may explain the obse rved behavior.

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119

me to believe that the nonionic surfactant molecules employed in this Triton

formulation wo uld adhere strongly to the polypropylene fibres. It was expected

that these molecules would cause water wetting

of

the membrane and convert

the membrane to an ultrafilter. This

was

not observed.

O i l Remova l Ef f ic iency

In

an effort to characterize the efficiency

of

the SOF process,

oil

removal

efficiencies were calculated for the

5%

dodecane-water experiments on the

uncoated cross flow module. The rem oval efficiency is defined as follows:

n

Where,

Q, = rate of oil permeation

Qd = rate of oil delivery to the modu le

= (Qf) (Volume Yoof oil in feed)

Table 9 gives the oil rem oval efficiency as a function of feed flowrate and

SDS

concentration. Obviously as the feed flowrate increases, the rate of oil

delivery to the mem brane also increases. (The rate of oil delivery was

calculated as Qf (ml/min)x6.6 vel.% dodecane.)

The results in Table 9 show

that a s the fee d flowrate increases the efficiency of oil removal also increases;

this increase do es n ot appear to be linear, however.

As

the

SDS

oncentration

increases

the

efficiency of

oil

rem oval decreases. These results were already

suggested by the flux versus flowrate data for this experiment. (See Section

5.2

c).)

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120

Table 9: Oi l Removal Ef f i c ienc ies for the

5

Dodecane-Water

Mix ture on the Uncoated

C ro ss

F l o w M o d u l e

Oi l Remova Ef f i c iencv

(16)

Qf Qd

N o

50 mg/ l 100 mg/ l

200

mg/ l

( m l /m i n ) (m l /m i n ) S D S S D S

S D S S D S

300

19.8

5.1

0.9

0.640

0.52

500 33.0

4.6

0.8

0.66

0.52

1000 66.0

6.3

2.0

1 o 1 o

2000

132.0

6.5 2.9 1.7

1.7

2400 158.4

7.0 3.4

1.9

1.5

Overall, the removal efficiency values were very

low

for the dodecane-

water experiments.

Coalescence efficiencies much greater than this are

normally achieved (Hughes and F oulds, 1986).

This result is not surprising,

however, as the fibres used in these experiments had a much larger diameter

than those typically used in oil coalescence.

The fibres used here had an

outside diameter of

425 p

whereas those us ed by H ughes and Foulds (1986)

had a diam eter of

7.5

p.

Rem oval efficiencies as calculated above are valid only for

oil

separation

processes limited by the transport and attachment mechanisms. Oil removal

that i s limited by the membrane, such as for the naphthenic oil studied in these

experiments, cannot be described in this way since the oil flux is insensitive

to

changes in feed flowrate. Another m eans of asse ssing the effective ness of the

SOF

process is needed

to

include this type of transport control

so

that

com parisons can be made with other oil-water separation methods.

The I n f l uence o f Sys tem Hydrodynamics

The importance

of

system hydrodynamics on droplet coalescence on

fibres has been demonstrated by several researchers, and i t is generally

accepted that the interception mechanism of droplet approach

is of

primary

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121

importance in promoting droplet coalescence.

Langmuir

(1 942)

derived an

equation to describe droplet coalescence on a single fibre strictly in terms

of

hydrodynam ic considerations. This equation was described in Section 3.1.

Subsequent workers have used

this

equation and made modifications to

account for electrostatic interactions and

to

consider a bed of fibres (Hughes

and Foulds,

1986) ,

and sti l l others have developed their own empirical

relat ions (Weber and Paddock,

1983).

Al l these empirical correlations,

however, relate the coalescence efficiency to the Reynolds number and the

ratio of droplet to fibre diameters . These correlations indicate an increase in

droplet coalescence with increasing Reynolds number and increasing droplet

to fibre diameter ratio.

Al l

but Weber and Paddock's correlation were given in Section 3.1; this

equation is a s follows:

where

E,

= (A

- 0.87A3)*

3

E,

=

coalescence efficiency for a single fibre

A = (2.022

-

In(Re))''

droplet diameter

= f ibre d iameter

An attempt was made to correlate some of the data with the equations

derived by Langmuir (1942), Hughes and Foulds (1986), and Weber and

Paddock (1983).

The data from the naphthenic oil experiments were not

amenable to this because the removal of naphthenic oi l is membrane

controlled.

Data from the

5

dodecane-water experiments on the uncoated

cross

flow

module were selected to make these correlations. Details of the

assumptions and calculations which were made in correlating this data are

given in Appendix

VIII.

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122

Figures 41 and 42 give plots of the oil removal efficiency,

ER,

versus

1

and

(A - 0.87A3),

espectively, for the differen t

SDS

concentrations.

2 (2-ln(Re))

A s

shown in Appendix

Vli l ,

the be d coalescence efficiency, Eb, is equivalent

to

the oil removal efficiency,

ER,

which is related to the single fibre efficiency,

E,

by the relationship E = R where n is the number of d roplet-fibre contacts.

Figures 41 and 42 show that the equations developed by Langmuir

(194 2) and We ber an d Paddock (1983) yield similar correlations.

Hughes and

Foulds (1986) pointed out this similarity between the two expressions, and they

found that despite their apparent difference these equations gave similar

E

values. Lang mu irs' equation, howeve r,

is

l imited to very low Reynolds

numbers,

while

Weber and

Paddock's

equation was developed for a broader

range of Reynolds num bers (up to 100).

for the 5%

igure 41:

Removal Ef f ic iency vs 2 ( 2 - , n ( R e ) )

Dodecane-Water Mix ture on the Uncoated Fibres

1

5% Dodecane

o 50

mg/l

SDS

A

200

mgll

SDS

A

100

mg/l SDS

0.07 0.08 0.09

0.10

0.11

0.12

1

2 (

2-1

n ( R e) )

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123

Figure 42:

Removal Efficiency v s (A

-

0.87A3) or

the 5

Oodecane-Water

Mixture o n

the

Uncoated

Fibres

8 1

1

6

R

E

4

1

5

Dodecane

100 mg/l SDS

200 m g / l

SDS

2

I

I

0.14 0.1 6 0.1 8 0 . 2 0 0.22

3

(A-0.87A

)

The data correlated well with the expressions developed by Langmuir

(1942),

Hughes and Foulds (1986) and Weber and Paddock (1983). This is

illustrated by the linearity of the plots shown in Figures 41

and

42. The da ta fits

the general form of these equations, suggesting that the behavior of selective

oil

filtration is compatible

to

the theories developed for droplet coalescence on

fibres. This provides further evidence that oil removal is transport and

attach ment limi ed.

In the dodecane experiments on the uncoated f ibres

a

crit ical feed

flowrate was identified below which no dodecane was transported across the

module. The value of this cr it ical f lowrate appeared to decrease with

increasing dodecane concentration and decreasing SDS concentration.

Bitten

(1970) also determined that there was a critical feed flowrate below which no

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124

coalescence occurred. N o critical flowrate was exhibited for the AEAPTMS

experiments except at high

SDS

concentrations because of the reduction in

electrostatic repulsion of the dodecane droplets and the fibres. These da ta

suggested that dodecane removal was possible even under stagnant flow

conditions.

The effect of fibre and droplet diameter on the removal

of

oil using

SOF

was not examined in this study.

The theory of droplet coalescence on fibres

suggests however, that as decreases droplet coalescence increases. With

respect to the SOF process one would expect an improvement in performance

for large

oil

droplets which are less stable. The effect of decreasing fibre

diame ter, however, is not obvious. While the efficiency of coalescence may

increase with a decrease in fibre diameter, it is possible that the rate of oil

transport within the fibre lumen

may be reduced. There may be a tradeoff

between imp roved coalescence and oil removal from the lumen in which case

an optimum fibre diam eter should exist for a given oil.

df

Wate r B l ind ing

The mem brane was always wetted with the pure oil to be separated at the

beginning of a test, and it was obse rved that the flux of oil declined over the first

hours of exposure to the oil-water mixture.

This decline

in

flux was

not

solely

attributable

to

the removal of excess oil from the membrane during the first

hours

of

operation.

It appears that the water adheres to the membrane and

occludes a fraction of the surface. Murkes

(1986)

discussed water blinding in

his studies. Microscopic examination

of

the fibre surface may be useful in

further characterizing this phenomena.

A i r

Ent ra inmen t

The oil f lux through the membrane was always observed to increase in

the pre sence of entrained air. This ma y result from a mod ification of the oil-

mem brane contact angle. This e ffect is worthy of additional study.

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125

I n f l uenc e

o f

S ur f ac e Coa t i ngs

The AMT experiments initially conducted did not show an improvement in

oil removal for f ibres to which a hydrophobic f luorocarbon coating had been

applied. Theoretically, an increase in the hydropho bicity of the membrane

shou ld result in a more effective oil-water separation. My results did not

demonstrate this, and this behavior was attributed

to

a

loss

of membrane

permea bility due

to

the app lication of the coating.

The importance of e lectrical interactions between the dodecane droplets

and the fibres was dem onstrated by the AEAPTMS experiments. The positive

coating on the fibre surface greatly increased the dodecane flux. Clayfield et

al.

(1985)

and Hughes and Foulds

(1986)

both found an improvement in the

coalescence efficiency of oil droplets on AEAPTMS coated surfaces, and they

stressed the importance of electrostatic interactions. These results sugges t that

the ap plication of a positively charged coating, such as AEAPTMS, is beneficial

to selective oil removal.

-

pH Effect

Hughes and Foulds

(1986)

i l lustrated the importance of pH on the

coalescence of

oil

droplets. The magnitude

of

the charge on both the

dispersed

oil

droplets and the fibres that they examined varied signif icantly

with pH.

In our study pH was not

a

variable except in one experimen t with the

AB Sn aph then ic oi l-water mixture. This experiment indicated a decline in

perm eate flux at high pH.

Hughes and Foulds

(1986)

found that both the oil droplets and the

polypropylene f ibres became more negatively charged with increasing pH.

The decrease in f lux which was observed therefore, may be the result of

stronger repulsive electrostatic interactions between the droplets and the

fibres. In addition, the flux decline may be due to an increase in emulsion

stability as discussed previously.

Further experiments should be conducted to

study the effect of pH in m ore detail. The effect of pH will vary depend ing upon

the nature of the surfactant

,

the oil and the m embrane surface character.

.

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126

Cont ro l l i ng

Mechanisms

o f Oi l T ranspor t

Based on the results of this study the following observations were made

as to the conditions under which the different mechanisms of oil transport

control the overall process. Transp ort to the m embrane, Jm, appears to limit for

high viscosity oils and at low operating temperatures. Oil transport through the

mem brane may also limit for mem branes with low porosity.

For

low viscosity oils, Jm is not likely to limit, but rather transport to

the

mem brane, J, or attachment and collapse at the m embrane,

A.

If either the o il

concentration

or

feed flowrate is low the probability of contact between the oil

droplets and the fibres is reduced and the rate of oil removal is decreased.

Electrical interactions between the droplets and the fibres may also affect how

closely the droplets may approach the fibres and repulsive interactions will

deleteriously affect the transport of oil to the mem brane.

The attachment and

collapse step requires that the droplets have a strong affinity for the fibres and

that they readily collapse.

Repulsive electrical interactions will hinder droplet

attachment w hile increased m echanical strength of the dro plets will affect the

rate of droplet rup ture and collapse.

6.2 Potent ia l

Uses

and L im i ta t i ons

o f

t h e SOF P ro ce ss

Based on the findings of this study it appears that the SOF process has

potential as

an

alternative method for removing oil from oily wastewaters or

frcm other oil-containing wastes.

My

results indicate that the

SOF

process is

capable of removing free, mechanically dispersed and chemically emulsified

oil

from o il-water mixtures.

SOF

is not suitable for the removal of oil-wet solids,

however, and prior to fi l tration the suspended solids must be removed to

prevent m embrane fouling.

The results of this study indicate that the

SOF

process is m ost effective at

remov ing oil from wastew aters containing high oil conce ntrations. The

ultrafiltration

(UF)

process, which is conventionally used to treat wastewaters

containing chemically emulsified oils, is limited in that it cannot handle high oil

concentrations.

As

men tioned previously, U F performance deteriorates as the

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127

feed oi l concentrat ion increases, and

3eyond about 30% oil the water

permeation rate begins

to

drop dramatically. The SO F process, on the other

hand, improves in perfo rm anc e as the oil concentration increases. Thus, the

SOF

proc ess could replace UF in certain applications. Also, the UF process

requires the removal of free oil as a pretreatment step; with SO F this step is not

necessary.

SOF would not be a viable alternative to UF for the removal of oil

from

dilute oily wastewaters

(<5-10%

oil). A s shown in this study, the rate of oil

removal is l imited in the SOF process at low oil concen trations whereas UF is

most efficient under these conditions.

A

comparison of the relative efficiencies of SOF and UF can be made by

comparing the membrane areas required to treat the same oily wastewater.

Given below is

a

rough calculation e stimating the membrane areas required to

treat

a

typical oily wastewater:

Considered here is a

1

liter volume

of

a typical oily wastewater containing

5%

oil by volume.

If

Selective Oil Filtration were used to reduce the oil content to e l ,

approximately 45 m l of oil would have

to

be removed.

Assuming an

average oil rem oval rate of 2 ml/min-ft2, the membrane area required to

treat 1Vmin of oily wastewater is,

If Ultrafi ltration were use d to concentrate the oil

to

50%, approximately

900 m l of water would have to be removed. Assuming an average water

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128

permeation rate of 100 ml/m in-ft2 (Pinto, 197 8), the mem brane area

required to treat 1 I/min of oily wastewater is,

This calculation suggests that

U F

is more efficient at treating an oily

wastewater with an oil concentration

of 5%.

It must be borne in m ind however,

that higher operating pressures are required with

U F

and in addition a pure

oil

.

product cannot be obtained.

Another option for treating dilute oily wastewaters is to combine U F and

SOF. Ultrafi ltration co uld be used

to

concentrate the oily wastewater and

produce an oil-free effluent, while SOF could be used to recover a water-free

oil product.

An application for which the S OF process appears to be very p romising is

the removal of water from water-in-oil emulsions. Fuel oils, for example, are

often contam inated with water droplets that must be removed prior to use.

U F

cann ot be used for this separation as the water pe rmeate flux is virtually zero

under these conditions.

Presently, coalescing filters and centrifuges are the

methods

of

treatment.

It is thought that a pure oil product could readily be

obtained using SO F; the oil f luxes should be high with o il as the continuous

phase.

The

SOF

process may be l imited in i ts abi l i ty to remove oi l from

wastewaters containing high concentrations of surfactants. The results of this

study indicate that as the concentration

of

surfactant increases, the rate of oil

rem ova l decrea ses. The results also indicate that some of the surfactant

passe s through the membrane with the oil. This may l imit the process if

surfactants are undesirable in the oil. In some applications, however, the

presence of surfactants may be desirable; one such application is in the area of

metal working fluids which are purchased with su rfactants in them.

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129

Water blinding of the mem brane is another possible limitation of the

SOF

process. In some

of

the experiments a dram atic decline in oil removal with time

was obse rved, which was attributed to water blinding. This behavior would be

expected to be significant at lower oi l concentrations. Further studies are

necessary to characterize this phenomena and

t o

determine methods

of

minimizing

its

effect.

The

SOF

process may also be limited in the types

of

oils that it can

remove. This study showed that high viscosity oils were m ore difficult to

remove as transport through the mem brane controlled the overall process. Oil

viscosity is not a critical parameter in

UF,

although it m ay affect the deg ree of

concentration polarization and /or mem brane fouling.

The main advantages foreseen in using SOF are that a water-free oil

product can be recovered and that high concentrations of oil can be handled.

The main disadvantages are that the process appears to be limited for high

viscosity oils, low concentrations of oil and it is adversely affected by the

presence

of

surfactants and water b linding.

The SOF process can be opt imized in one of several ways depending

upon the oily wastewater in question and the controlling mechanism

of

oil

transport. The controlling mechanism, which is dependent

on

the operating

conditions, will dictate the degree to which the process can b e improved. The

most difficult case is for membrane limited transport, Jm.

The only way to

improve oil removal is to increase the operating pressure and/or temperature,

and the membrane i s restricted in the pressures and temperatures that it can

withstand (typically

30-50

psi

and 90°C, respectively).

In addition, one might

experiment w ith m embranes having different pore size distributions to find one

with a high permeability for a given oil.

The removal

of

oils that are limited by transport to the membrane, J, and

attachment and collapse at the membrane , A , can be optimized more easily by

improving the system hydraulics and encouraging droplet-membrane contact.

The feed f lowrate could be increased

or

the module geometry could be

adjusted.

The stability of the dispersed droplets could be reduced by varying

.

8/10/2019 02432.pdf


130

the pH or ionic strength

of

the oil-water mixture. In addition, a coating could be

applied to the membrane

to

enhance the attractive electrostatic interactions

between the d roplets and the fibres.

6.3 Future

Research

It is evident that a considerable amount of research has yet to be done,

both fundamentally and on a more practical level, to better characterize and

understan d the

SOF

process and to better assess its potential as an alternative

oil separation technique.

A s

a continuation of the dodecane experiments conducted in this study,

the role of electrostatic interactions between the dispersed dodecane d roplets

and the fibres shou ld be quantified. This would requ ire streaming potential

measurements to be made on the coated f ibres at d i f ferent SDS

concentrations.

Also,

the role of

SDS

in the

SOF

process must be clarified

further; in particular, the importance of droplet stability must be determined.

The mechanical stability of the dodecane droplets should be assessed along

with further studies of the electrical stability. It would b e worthwhile to

look

more closely at other means of varying emulsion stability besides surfactant

concentration, to separate the effects of emulsion stabil i ty from those of

surfactant adsorption at the fibre surface. Both pH adjustment an d salt

additions could be made.

Also,

the anomalous results observed in these

experiments should be veri f ied, and the removal of surfactant must be

quantified.

In

a

broader sense, much more fundamental research could be done on

the SOF process . Different oils and surfactants co uld be tested, different

module geom etries could be assesse d and the effect of alternative membrane

configurations could be examined (i.e. flat sheets as opposed to fibres). In

particular, the effect of hydrocarbon character and nonionic surfactants should

be studied.

The influence of fibre diameter and sy stem hydraulics should be

further investigated, and an empirical expression relating

SOF

efficiency to the

system param eters should be formulated.

-

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131

The influence

of

membrane characteristics is important and needs to be

examined carefully. For example, the influence of membrane porosity and

pore size distribution should be assessed. Membrane coatings or surface

functionality may b e imp ortant in modifying oil droplet attachment an d rupture.

Also, selected m odel organic compounds might be us eful to determine whether

the membrane exhibits any selectivity beyond the influence of viscosity.

From

a

practical point of view, tests should b e run on representative

oily

wastewater samples, and methods

of

optimizing the oil separation process

shou ld be considered. Also, it would be informative to interface a n SOF system

with

a UF

system to determ ine the feasibility of this process combination, and

carry out studies to compare the performance of the SOF process to that

of

other oil-water separation techniques.

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133

8,

9.

Electrostatic interactions between the droplets and the fibres play an

importan t role in the oil removal process. Repulsive electrostatic

interactions between the negatively cha rged oil droplets and the

membrane, which is also negatively charged, hinder droplet approach and

attachment.

The

oil

removal process can be improved

by

encou raging attractive

electrostatic interactions between the drop lets and the fibres. One way of

accom plishing this is to apply a positive coating to the m embrane surface,

such

as

the cationic polymer,

AEAPTMS.

10.

The selective oil filtration process has p otential as an alternative oil-water

separa tion technique. This process is better suited than ultrafiltration to the

treatmen t of oily wastewaters w ith a high oil concentration

or

for the

treatm ent of water-in-oil emulsions.

It canno t replace ultrafiltration

however, for

the

removal of oil from dilute oily was tewaters.

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8/10/2019 02432.pdf


1 3 5

Dahu ron, L., Designing Liquid-Liquid Extractions in

Hollow

Fiber Modules,

PHD Thesis, Department of Chemical Engineering, University of Minnesota,

(1

987)

Dhaw an, G.K., Em ulsified Oily Waste Wa ter Treatme nt by Ultrafiltration,

International Waste Treatme nt 8 Utilization Conference,

(July, 1978).

Emulsifiers and Detergents, North American Edition, McC utcheon s Division,

Ed.,

M.C. Publishing Company,

(1 981).

Ford, D.L. an d Elton, R.L., Re mo val

of Oil

and Grease From Industrial

Wastewaters, Chem. Eng., Deskbook Issue,

49,

(Oct.,

1977).

Freestone, F.J. and Tabakin, R.B., Review of U.S. Environmental Protection

Agency Resea rch in Oi l-Water Separation Technology, Conference on

Preven tion and Control of O il Pollution, San Fransisco, 437, (1975).

Gambhir, S.P., Resource Recovery-Oil From Sludge, 38th Purdue Industrial

Wa ste Conference, Lafayette, IN, 23,

1

983).

Goldsmith, R.L., Roberts,

D.A.

an d Burre, D.L., Ultrafiltration

of

Soluble Oil

Wastes,

J.

of WPCF,

4,

9), 21 38,

(1

974).

Griffin, W.C., Classification of Surface-Active Agents By HLB

, J.

SOC.Cosmet.

Chem.,

1,311 (1949).

Harlow, B.D., Hubbe ll, J.W. an d Doran,T.M., O il Waste Trea tme nt and

Rec lama tion in the Autom otive Industry, 37th Purdue Industrial Waste

Conference, Lafayette,

IN, 197,

(1

982).

Hazlett, R.N., Fibrous Be d Coalescence of Water, Ind. Eng. Chem. Fundam .,

8,

(4),62 5, (1969).

Hockenberry, H.R., Practical Applications

of

Mem brane Techniques of W aste

Oil Treatment, J. Am. Sic. Lub. Eng., 3

5), 47, (1977).

8/10/2019 02432.pdf


136

Horder,

R.L.,

An Evaluation of Som e M ethods For Assessing The Stability of

Oil-In-Water Emulsions, PHD Thesis, Department of Pharmac eutics, The

School

of

Pharmacy, University of London, London,

(1

977).

Hughes, V.B. and Foulds,

A.W.,

Coalescence of Oil-In-Water Emulsions:

Development of a Novel Fibrous Bed Coalescer Using Surface Treated

Poyp ropylene Fibres, Procee dings of the 4th World Filtration Congress,

Oostend, (1986).

King, A., Some Factors Governing the Stability

of

Oil-in-Water Emulsions,

Trans. Farada y SOC.,

168,

1

941

).

Kramer, G.R., Buyers,

A.

and Brownlee, B., Electrolytic Treatment for Oily

Wastewater, 34th Purdue Industrial Waste Confere nce, Lafayette,

IN,

673,

(1979).

Langmuir,

I,

OSRD

Rept. 865, (Sept., 1942).

Manual on Disposal

of

Refinery Wa stes-Volum e on Liquid Wastes: Ch apter

5-

Oil Wa ter Sepa rator Design, American Petroleum Institute, (1969).

Michaels, A.S., New Separation Technique for the CPI, Chem. Eng. Prog.,

fi

31, (1968).

Moses, S.F. an d Ng, K.M., " A Visual Study of the Breakdow n of E mulsions in

Porous Coalescers, Chem. Eng. Sci.,

a

12), 2239,

(1985).

Murkes,

J.,

Cross-Flow Filtration

of

Em ulsions Com bined With Coalescing.

A

Ne w Filter-Coalesce r Conc ept, Procee dings of the 4th Wo rld Filtration

Con gress, Oostend,

(1

986).

N

alco

Tech ni

ax,

Oi

I-

n-Wat er

E

mu sion

6

eaki ng

,

Nalco C he mica1 Com pany

,

(1

985).

Nordstrom, R.P.,Ultrafiltration R em ova l of Soluble Oil, Poll. Eng., 46, (Oct.,

1974).

8/10/2019 02432.pdf


8/10/2019 02432.pdf


138

T i o

n

Surf

ace-Active Ag e n

s No

n

o

n c

A Ik

y p

h

e ny

Po

yet he

r A Ico

h

oIs,

Information Bulletin, Rohm and Haas Company,

(1

977).

Wang,

L.K.,

Yang, J.Y. an d Dahm, D.B., Separation of Emu lsified O il From

Water, Chem.

&

lnd., 662,

(July, 1975).

Weber, M.E. and Paddock, D., lnterceptional and Gravitational Collision

Efficiencies for Single Collectors at Intermediate Reynolds Numbers, J. Colt.

Int.

Sci.,

9f1,

(2) 328,

(1983).

Weber,

W.J.,

Phys icochem ical Process for Water Quality Control, John Wiley

& Sons, N.Y., (1972).

Yang, M.C., Hollow Fiber Ga s Treating, PH D Thesis, Department of Chemical

Engineering, University

of

Minnesota,

(1

986).

8/10/2019 02432.pdf


139

A P P E N D I C E S

8/10/2019 02432.pdf


1 4 0

Append ix I

N a p h t h e n i c Oi l Exper imen ts

AMT Modu le

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8/10/2019 02432.pdf


142

10 Oi l -Water Mix ture

Time

(hr)

Qt

(ml /min)

P

(ps i )

FIUX

(mt/min-f t2)

0 75 12 0.35

19

1

II

8

10

2 4

2 7

2 8

30

I9

n

I9

3 2

75

75

75

75

75

75

75

330

330

31

0

31

0

320

30

6

12

2 0

12

1 2

12

12

12

12

20

6

12

2 0

0.20

0.40

0.62

0.38

0.38

0.38

0.36

0.39

0.36

0.58

0.20

0.37

0.28

30 12 0.33

0

T=

32-38' C

50% Oi l -Wa er Mix tu re

T i m e ( h r )

Q f

(ml /min) P

(psi )

FIUX (ml/min-ft2)

0 200 13 0.57

0.5 200

11 0.47

2.0 200

12 0.49

2.5 75

1 2

0.44

75

1 1

0.45

75

13 0.46

3.5 30

13 0.42

n

I1

4.0 200 13 0.47

T =

32-38'

C

8/10/2019 02432.pdf


143

Append ix

il

Na p h t h e n i c Oi l E x p e r i m e n t s

AMT Versus

Celgard

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144

Sect ion

1:

Basel ine Data For Celaard

1

Module

Pure

Oil

Data

T ime (h r ) Of (m l lm in ) P (ps i ) FIUX (mt/min- f tZ)

0 78 12.5 4.31

0.5 78 12.5 4.37

1.5 155 12.5 4.33

11

155 13.5 4.63

305 13.0 4.40

305 14.0 4.70

2.0 3 05 12.0 4.46

7.0 3 05 12.5 4.23

1V

1,

24.0 303 12.5 3.74

T

= 32-38' C

10%

Oi l -Water Mix ture

Time (hr ) Q f (m l lm in ) P (psi ) FIUX (ml /min- f tZ)

0

0.5

1

o

2.0

3.0

4.5

5.5

22.0

303

160

160

310

8 0

8 0

400

155

1 2

1 2

7

1 2

1 2

1 2

1 2

1 2

3.66

3.06

1.42

2.74

2.56

2.54

2.50

1

85

23.0 390 12 1.83

T

= 32-38' c

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145

Sect ion

1

c o n t i n u e d ....

1%


T ime (h r )

Q f

(ml lmin)

P

(ps i )

FIUX

(mt /min- f t * )

0

280 12 2.92

0.25

1.25

1.50

2.0

17.25

280

280

280

280

280

1 2

12

12

12

12

2.32

1.73

1.19

1.49

0.52

17.75 280

12

0.41

T

= 32-38'

C

Sect ion 2:

AMT

Vs Celaard

1

t e r M ix tu ra

FIUX (mi /min- f t2)

T ime (h r ) AMT Flux Celgard

1

Flux

0.0

1.22

3.03

0.5

1

o

2.0

3.0

4.0

5.0

6.0

7.0

8.0

47.5

1.02

0.60

0.64

0.63

0.62

0.58

0.57

0.59

0.33

0.78

2.82

2.86

2.78

2.52

2.36

2.42

2.28

2.50

2.02

1.74

48.75 0.32 1.54

T

= 32-38'

C

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146

Sec t ion 2 C on t i nued...

1%


FIUX (ml /min- f t2 )

Time (h r ) AMT Flux Celgard 1 Flux

0

1.19 2.38

0.25 0.80 2.32

1

0.43 2.24

2 0.26 1.72

3 0.24 2.06

4 0.23 1.92

5 0.20 2.10

6 0.1 9 2.06

7

0.20 2.02

8 0.1

9

2.02

9

0.18 2.02

24 0.1

3

1.34

25 0.1

3 1.36

T =

32-38'

C

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147

A p p e n d i x

Naph then ic O i l Exper imen ts

Sur fac tant Ef fec ts

Cel g ard Ex per

i

m e n s

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1 4 8

Sect i on 1: Celaard 2 Module

10% O/ W E m u l s i o n C o n t a i n i n a

1% ABS

Time

Flux

O h Oi l

( h r ) (m l l m i n oft2) R e m ain in g ( v h ) C o m m e n t s

0

0.25

1 o

2.0

3.0

4.0

5.0

6.0

7.0

17.0

18.0

19.0

22.0

24.0

25.0

26.0

41 .O

70.0

70.5

71

.O

72.0

4.21

2.57

0.93

0.86

0.93

1.21

1.29

0.93

1.29

1S O

1.50

1.43

1.64

0.86

0.64

0.50

0.01

0.0001

1.71

1.86

1.92

100.0

11 .1

----

10.8

10.6

10.4

10.2

9.9

9.6

4.8

4.4

3.9

2.4

2.1

1.9

1.7

0.44

------

11.2

9.5

7.9

pure oil flux

water in permeate;

two distinct phase s

50%

OMI

permeate

phases

not

separating

pure oil permeate

It

I

It

I t

It II

I t

I t I t

I t

II I t

It

I t It

flux still high overnight

permeate darker in color

flux starting to decrease

flux very low overnight

flux virtually zero

more oil added

permeate dark brown

T

= 32-38' C;

P

= 12 psi;

Qf =

200 mI/min

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149

Sec t ion

2:

C 3

Module

ReDroduc ib i l i t v S t u d y

10 OIW Emuls ion Conta in ina 1% A B S

~

Ti

m e Flux

YO

Oil

( h r ) (m l / m in - f t 2 ) R ema in ing ( v l v ) C o m m e n t s

0 5.32

100.0

pure

oil

flux

0.25 0.56 1 1 . 1 water in permeate;

two

distinct phases

1

.o

0.31

1 1

o

2.0

2.5

3.5

4.0

5.5

6.0

7.5

8.0

22.0

23.0

24.0

0.44

0.48

0.53

0.54

0.68

0.69

0.65

0.63

0.52

0.41

0.37

pure oil permeate

flux lower overnight

26.0 0.47 ----

27.5 0.32 3.8

29.0 0.31

----

flux decreasing rapidly

31.5 0.25 3.3

46.0 0.14 1 .a

47.5 0.06

----

T

= 32-38' C; = 12 psi;

Qf

= 200 ml/min

8/10/2019 02432.pdf


150

Sect ion 3: Ce aard 2 Modu le : pH E f fec t

10%


Conta in ina 134, A B S

T i m e (h r )L o w p H F lu x H i g h p H Fl u x 06 Oil L o w p H

O h

Oil Hi g h p H

0

4.21 3.01 11.2 11.2

0.25

1

o

2.0

3.0

4.0

5.0

6.0

7.0

17.0

22.0

24.0

25.0

26.0

29.0

30.0

41

70.0

70.5

2.57

0.93

0.86

0.93

1.21

1.29

0.93

1.29

1.50

1.64

0.86

0.64

0.5

.....

.....

0.01

1.71

1.86

2.39

1.80

0.91

.....

0.55

0.66

0.73

0.55

.....

0.07

.....

0.06

0.04

1.21

1.31

0.66

.....

.....

11.1

.....

10.8

10.6

10.4

10.2

9.9

9.6

4.8

2.4

.....

1.9

1.7

.....

.....

0.44

11.1

9.5

11.1

9.5

8.6

...,.

7.1

6.5

6.0

5.3

.....

4.0

.....

3.9

3.8

10.9

10.6

9.7

.....

.....

7.9 .....

.....

1 O 1.93

T

= 32-38'

C; p

=

12 psi;

Qf = 200 mI/min

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8/10/2019 02432.pdf


1 5 2

Appendix IV:


Coun te rcu r ren t Modu le : 5% Oodecane

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153

Pressu re Effect o

n

Permeate Flux

P

(ps i ) FIUX

(ml /min- f t2 )

50

mg/ l

100 mg/ l

500

m g / l

S D S

S D S SD S

5 .....

3.342 0.188

10 2.652

2.61 3 0.249

20 .....

2.1 10 0.1 71

T = 37' C; Qf = 300

ml/min

Feed F lowra te m fec t o n Per m eate F u x

Q f (ml lm in) F lux (ml /min - f t2 )

50 mg/ l

100 mg/ l 500 mg/ l

S D S

SDS

S D S

0.243 0.028

00 0.243

30 0 2.641 1.597 0.334

500

5.535 2.220 0.699

750 9.130 - 3.920 1.21

0

T =

370 C;

P =

10

psi

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1 5 4

Appendix V:

Do d e c a n e E x p e r i m e n t s

Cro s s F low Modu le : 10% Dodecane

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155

Pressu e Effect on Permeate Flux

P

(ps i )

FIUX (ml /min- f t2 )

No SDS

50

mg/ l 100

mg / l 200 m g/ l

S D S S D S

S D S

1 2.10 .....

5 2.30 0.77 ..... 0.36

20

3.68

0.44 0.13

0.1

7

5 2.39

0.75 0.63 0.25

.....

.....

10 2.78

0.54 0.36

0.22

10 1.46

0.63 0.35

0.1

9

T

=

37' C; Qr = 300

ml/min

Feed Flowrate E fect

o

n Permeate

Flu3

Q f (ml /min) FIUX (ml /min- f t2 )

No SDS 50 mg/ l

100

mg/ l 200 mg/ l

S D S

SDS

S D S

300

1.37

0.61 0.35

0.21

100

0.38 0.14

0.09 0.036

30 0 1.22 0.58 0.31 0.23

500

1.99 1.01

0.67

0.50

750 2.51 1.51 1 oo 0.72

50 0 1.42 0.91 0.71 0.50

T

= 370 C;

P

= 10 psi

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156

Oi l Concentrat ion E f fec t

D i lu t i on o f 10% Dod eca n e-Water Mixture

Q f (ml /min) F lux (ml /min0f t2 )

5 % Oil

10%

Oil

100

.....

0.036

300 0.18 0.22

500 0.32 0.50

750 0.56

0.71

1250

1.15

.....

T

=

370

C;

P = 10 psi ;

100

mg/l

SDS

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1 5 7

Append ix V I


Cro s s F l o w Module:

5%

Dodecane

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158

Feed Flowrate Ef fec t o n Permeate Rate

Q f (ml /min) FIUX (ml/min-ft*)

50

m g / l

100

mg/ l

200

m g / l

S D S SDS S D S

300 0.87 0.16 0.1 1 0.089

500 1.31 0.23 0.19

0.1

5

10 00 3.63 1.12 0.59 0.50

2000

7.49 3.28 1.99 1.90

240 0 9.59 4.73 2.62 2.04

No SDS

.

T = 370

C;

=

10

psi

Zeta Potent ia l Va lues

Zeta

Potent ia l (mV)

No SDS 50

mg/ l 100

mgl l 200 mg/ l

S D S

S D S

S D S

Run

1

-55.1 -67.8 -66.5

-71.6

-58.3 -64 .6 -66.1 -69.2

Ave.

-56.7 -66.2 -66.3 -70.4

Run 2 .....

-68.2 -68.4 -69.9

..... -62.2

-63.4 -67.2

A v e

.....

-65.2 -65.9 -68.5

Overa l l Ave

-56.7

-65.7

-66.1 -69.5

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159

DroDlet

S

ze

Data

~~

Size

(p)

OO Total Oil

Volume

No SDS 50 mg/ l 100 mg/l 200

mg/l

S D S S D S S D S

1-3

78.1 76.0 78.7 78.9

3-5

12.6

16.1

13.8 14.9

5-7

4.1

5.8

4.4 4.4

7-10

3.4

2.5

1.7 1.1

10-20

2.4

1.3

1.3 0.7

>20

0.33

0.07

0.09

0.03

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160

Append ix

VII:

AEAPTMS

Coated Fibres

Cross Flow Modu le : 5 Dodecane

8/10/2019 02432.pdf


1 6 1

Effect o Feed Flowr ate on Permeate Flux

No

SDS

50

m g / l

100

m g / l

200

m g l l

S D S

SDS S D S

300 2.08 1.44 1.20 0.26

500

2.90

2.03

1.76

0.44

1000 5.34 3.12 2.78 1

.oo

T

=

370 C;

P = 10

psi

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8/10/2019 02432.pdf


1 6 4

where,

Q = oil permeate rate

Qf

= oil feed rate

C,

= feed oil concentration

The equation presen ted by H ughes and Foulds (Eq. i i) above) can

be

simplified by using a binomial expansion since E,' values cc

1

for our

experiments. This expa nsion yields,

Eb =

1 ( 1

nE,'

)

Wh ere higher order terms are considered to be negligible; the above equation

becomes,

Thus,

Reynolds numbers were ca lculated for feed flowrates ranging from 300 ml/min

to

2400 mVmin assu ming a flow area of 17.85 cm2. The value of n was

calculated to be

30.83

assu ming a porosity

of

0.82, a b ed depth of 3.6 cm and

a fibre diameter of 0.0425 cm.

The empirical equations

(i), (ii)

and (iii) can be simp lified as follows:

Langmui :

K

ER

a 2(2-Ln(Re))

K = f (r, n)

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