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DISTILLATIONTOWERS

Eng. Ahmed DeyabFares

Senior Process EngineerSpecial thanks to: Eng. M. Abd El-Raof



Ahmed Deyab

Senior Process Engineer

Deliver Several Training Courses & Consultancy for process enginee

rs from several Oil, Gas & Petrochemicals companies in Egypt & Gulf countries.

Contact details:

Mobile (Egypt): +20 –1227549943

Mobile (a!di "#abia): +9$$500297503

E%ail: adeyab&adeyab'co%

ebsite: 'adeyab'co%

mailto:[email protected]

http://www.adeyab.com/

http://www.adeyab.com/

mailto:[email protected]



Courses Conducted to engineers from:



*!nda%entals o epa#ation

,oe# -nte#nals

C#!de .istillation

,#o!ble /ooting

Contents



FUNDAMENTALS OFSEPARATONN TO!ERS



Disti""ation

Separation by distillation implies a diference in

boiling points o two or more materials

We separate many things by detecting a

diference in a physical properties

color, size, weight, shape



The components or com#o$nds ma%ing $# cr$de oi" are

n$mbered in tho$sands

Many o& these com#onents ha'e simi"ar #hysica"

#ro#erties inc"$ding boi"ing #oints that may di(er by

on"y a &e) degrees. Thereore, it is dicult to separate

some pure compounds rom the complex mixture o

components in crude oil by distillation alone

There are other methods o& se#aration $sed in are*nery &or e+am#"e, e+traction )ith a so"'ent,

crysta""i-ation, and absor#tion

Fortunately, rare"y need #$re com#o$nds and it is oten

enough to separate groups o compounds rom each other by



we separate many compounds in crudeoil into groups we !nd that these groupsha"e characteristics that ma#e themconsiderably more "aluable than the whole

crude oilSome o these groups are products Some may be eedstoc# to other processing units where they are chemically changedinto more "aluable products

These products, in turn, are usually

separated or puri!ed by distillation



The basic principle o distillation issimple

When a solution o two or morecomponents is boiled

The lighter component $the one most

"olatile or the one with the greatesttendency to "aporize %"aporizespreerentially

rinci#"es O& Disti""ation



10

Tow component mixture is contained in a "essel

When heat is add until the more "olatile material

$ red dotes % start to "aporize. &ow the "apor

contains a higher proportion o red dots than dose

the original li'uid



t is important to note that an e'uilibrium in composition will

be established

at a gi'en tem#erat$re and #ress$re

(y e'uilibrium we mean there is a gi"en concentration as )red

dots* in the "apor and in the li'uid depending upon the

original concentration o each component in the li'uid and

their respecti"e properties in relation to each other



No), "ets de'e"o# this sim#"e disti""ation

conce#t into a #ractica" o#eration as it is $sedin the re*nery

First , let+s separate and remo"e the product

This results in the "apor abo"e the li'uid being

relati"ely rich in the lighter $ more "olatile material %

nd the li'uid is let with proportionately more o theless "olatile $ hea"ier li'uid %

Thus a separation, to some degree, has ta#en place



(y cooling the o"er head"apor, we condense andremo"e it rom the originalmixture

Thus to ha"e made a partialseparation, partial becauseyou will note that there are aew )blue dotes* in the

distillate product



This has occurred because at the temperature and

pressure we are conducting the distillation, the

hea"ier component still "aporizes to some extent

This is because the components o interest in a gi"en

distillation usually ha"e airly close boiling points



Thereore, to #$ri&y the

disti""ate #rod$ct, )e

may ha'e to cond$ct

a second disti""ation

as shown

-b"iously, we can

continue to cascade

these simple distillations

until we achie"e the

desired purity o product



The distillations depicted so ar are those we call

patch, and normally practical in the re!nery,

although it is done re'uently in the laboratory

Let $s ma%e o$r disti""ation e/$i#ment "oo%

more "i%e re*nery #ieces o& e/$i#ment and

"et $s ma%e contin$o$s instead o& #atch

o#eration



741$

This is called F"ash

0a#ori-ation s shown.

The "i/$id is #$m#edcontin$o$s"y thro$gh aheater and into a dr$m

)here the #ress$re is"o)er

The lighter material ashes

instantaneously $"apor andli'uid ow rom the drumcontinuously%

The same system is showndiagrammatically in the in



S$##ose )e ha'e 123 o& the charge ta%en o'erhead

That is, we set the temperature and the pressure o the

system in such a way that hal the charge is boiled of

nd urther, s$##ose the res$"ting o'erhead #rod$ct

does not contain the desired concentration o& the

"ighter #rod$ct

s we ha"e seen beore, )e can increase the #$rity by

adding a stage o& disti""ation



19

Suppose we add twomore stages odistillation

lthough this isaccomplishing ourgoal o increasingthe #$rity o& the"ight &riction, we arealso ma#ing large

amounts o theintermediate product,each o whichcontains the same

light riction



n ob"ious simpli!cation in e'uipment can be made

i we allow the hot "apor rom the stage abo"e the

next higher $the intermediate product%

This eliminates the need or the intermediate

condensers and heaters

No) )e ha'e the contin$o$s, m$"ti4stage

disti""ation



To)er SectionsWe ha"e described stagingor the purpose oconcentrating the lightercomponent in the o"erhead

The same principles apply to

concentrating the hea"iercomponent in the bottomproduct

The upper stages are calledrecti&ying stages

These below the eed arecalled stri##ing stages



The upper rectiying section increases the

purity o the o"erhead product.

The lower stripping section increases the

reco"ery o the o"erhead product.n many cases, the bottom product is the

one o primary interest

For the bottom, or hea"y, product the

rectiying section impro"es reco"ery



/$i"ibri$m Stage

stage, or more speci!cally, an e'uilibrium stage,is de!ned as /

Any #ortion o& the disti""ation co"$mn s$chthat the "i/$id and 'a#or "ea'ing it ha'e

com#osition in e/$i"ibri$m )ith each other.

(y de!nition, then, a stage should be designed insuch a way as to pro"ide intimate contact, or

mixing, o the rising "apor and the descendingli'uid. The concept o an e'uilibrium stage iscon"erted to an actual mechanical separation trayby using an eciency actor which is less than oneand depends on the tray design.



Column Internals:

The plates or trays are contacting devices and used to hold up the liuid to

provide !etter contact !et"een vapour and liuid, hence !etter separation.

1.Trays !lates "# !ac$ings

V A L V E T R A Y S

S I E V E T R A Y S

B U B B L E C A P T R A Y S

T R A Y S W I T H D O W N C O M E R

D U A L F L O W T R A Y S

B A F F L E T R A Y S

D I S C & D O N U T T R A Y SR I P P L E T R A Y S

T R A Y S W I T H O U T D O W N C O M E R

P R O P R I E T A R Y D E S I G N S ( M V G , S U P E R F R A C )

T R I T O N , P R O V A L V E

N Y E T R A Y S

M U L T I D O W N C O M E R T R A Y S

N O N - F R A C T I O N A T I O N T R A Y S

C O L L E C T O R / C H I M N E Y T R A Y S

T R A Y S

R A S C H I GL E S S I N G

C R S S ! A R " I " I N R I N G

# E R L S A $ $ L E S

I G E N E R A " I N

! A L L R I N GH % ! A &

I M " !

C M R

N ' " " E R R I N G

I I G E N E R A " I N

R A N $ M

G E M ! A &M E L L A ! A &

I N " A L (

! A R L ! A &

I I I G E N E R A " I N

S " R ' C " ' R E $

! A C & I N G



Si



Sie%es



Sieve Tray This tray is a sheet of light metal &ith a large number of holes dril

led through it#

'a(or rising through the holes $ee(s the li)uid on the tray and bubbles u( through it#

The o%erflo& &eir $ee(s a constant de(th of li)uid on the tray#



A sieve tray is:

Inexpensive,Easy to clean, and

Maintains good liquid and vapor contact

as long as it is operated at its design load.

Because the sieve tray has fixed openings

and does not have covers over the holes, it

does not perform well if tower loads are

constantly changing.



#u!!le Cap $ %alve trays



#u!!le Cap $ %alve trays

In %al%e trays *ubble Ca( trays+ (erforations are co%ered

by liftable ca(s# 'a(or flo&s lifts the ca(s+ thus self creati

ng a flo& area for the (assage of %a(our# The lifting ca(

directs the %a(or to flo& hori,ontally into the li)uid+ thus

(ro%iding better mi-ing

ale ,#ays

iee ,#ays

!bble Cap

http://d/Industry/Industrial%20Photos/valve%20trays

http://d/Industry/dist/seive

http://d/Industry/dist/bubble%20caps

http://d/Industry/dist/bubble%20caps

http://d/Industry/dist/seive

http://d/Industry/Industrial%20Photos/valve%20trays



Valve Tray



Valve Tray

A valve tray has a variale opening for vapors to flowthrough.

!he hole has a cover

that consists of a capheld in place y guides

which go down through

the plate, or tray and

hoo" underneath it.



#hen there is no vapor flow, the caps sits over the

hole and close it.

$nder low pressure

the cap start to rise.

As the flow of vapors

increases, the cap rise

until it is stopped ythe guide tas.



!he valve tray is similar to the ale cap tray.

Both are more adaptale to variations in tower

loads than a sieve tray.

!he valve trays and ule cap trays are designed

to perform well with variale tower loads.



T



Trays Bubble Cap Tray

!he vapor is ro"en into small ules whichincreases the surface area for vapor%liquid contact.

!he ule cap sits on top of a riser.

!he riser channels vapors into the ule cap.



$5



&election of !ray !ype

!he principal factors to consider when comparingthe performance of ule%cap, sieve and valve

trays are:

'ost,

'apacity,

(perating range,

Maintenance and

)ressure drop.

'ost:



'ost:

Bule%cap trays are apprecialy more expensive

than sieve or valve trays.

!he relative cost will depend on the material of

construction used*

+or mild steel the ratios,

ule%cap: valve: sieve, are approximately

.- : ./ : .-



'apacity:

!here is little difference in the capacity rating for the

three types 0the diameter of the column required for

a given flow%rate1.

!he ran"ing is:

sieve, valve, and ule%cap



(perating range:



(perating range:

!his is the most significant factor.

By operating range is meant the range of vapour and

liquid rates over which the plate will operate

satisfactorily 0the stale operating range1.

&ome flexiility will always e required in an

operating plant to:

Allow for changes in production rate, and

'over start%up and shut%down conditions.



!bblecap trays have a positive liquid seal and cantherefore operate efficiently at very low vapour

rates.



iee t#ays rely on the flow of vapour through the

holes to hold the liquid on the tray and cannotoperate at very low vapour rates, ut, with good

design, sieve trays can e designed to give a

satisfactory operating range*

!ypically, from /- to 2- 3 of design capacity.

4alve trays are intended to give greater flexiility

than sieve trays at a lower cost than ule%caps.

Maintenance5



Maintenance5

For dirty ser"ices, bubble0caps are not

suitable as they are most susceptible to

plugging. Sie"e trays are the easiest to

clean.

Press$re Dro#5



Press$re Dro#5

!he pressure drop over the trays can e an importantdesign consideration, particularly for vacuum

columns.

!he trays pressure drop will depend on the detaileddesign of the tray ut.

In general,

sieve plates give the lowest pressure drop, followed y

valves, with ule%caps giving the highest.

&ummary



&ummary

&ieve trays are the cheapest and are satisfactory formost applications.

4alve trays should e considered if the specified

turn%down cannot e met with sieve trays.

Bule%caps should only e used where:

4ery low vapor 0gas1 rates have to e handled

and

A positive liquid seal is essential at all flow%

rates.

Typical Tray SectionTypical Tray Section



Typical Tray SectionTypical Tray Section

!tlet ei#

-nlet ei#

eal 6ot

.onco%e#

,#ays

Inlet Weirs



3

Inlet Weirs

!hese contriute to the uniform distriution of

liquid as it enters the tray from the down comer.

It is not recommended for

fluids that are dirty or tendto foul surfaces.

If a more positive seal is

required at the downcomer atthe outlet, an inlet weir can

e fitted or a recessed seal pan

used.

Do"ncomer sealing can !e achieved primarily !y means'



()* inlet "eirs

(* recessed seal pan.

These devices provide a positive

seal on the tray. The disadvantage is that they create a poc+et of s

tagnant liuid "here dirt, sediments, etc can !uild up. large a

mount of such !uild up can restrict the do"ncomer outlet area a

nd lead to premature flooding. Thus+ the use of these de%ices is n

ot recommended in fouling or corrosi%e ser%ices#

Outlet Weirs



Outlet Weirs

The function of a "eir is to maintain a desired liuid leve

l on the tray + thus insuring bubbling of %a(ors through li

)uid# Typical "eir height is !et"een - inches. /o" "

eirs are freuently used in lo" pressure or vacuum colu

mns. 0otched (rectangular or %-shaped* "eirs are com

monly used for lo" liuid loads



Do)ncomers



Do)ncomers5eflux flows down from one

tray to the next throughdowncomers.

6owncomers must e large

enough to allow for drainagefrom one tray to the next or

flooding might occur on some

of the trays.

6owncomers can e designed in

several different ways to provide

smooth flow from tray

to tray.



The straight, segmental, vertical do"ncomer is &idely us

ed as it provides good utili1ation of column area for do

"nflo" and has cost and sim(licity ad%antage. Sloped d

o"ncomer can !e used if vapour-liuid disengagement i

s difficult (e.g. due to foaming*. Sloped do"ncomer also

provide a slightly larger active area for vapour-liuid con

tact, !ut it is also more e2pensive.



do"ncomer must !e sufficiently large to allo" liuid to

flo" smoothly "ithout cho+ing. Sufficient time must also

!e provided in the do"ncomer to allo" proper vapour dis

engagement from the do"n-flo"ing liuid, so that the li

uid is relatively free of vapour !y the time it enters the tra

y !elo". Inade)uate do&ncomer area &ill lead to do&nc

omer cho$ing+ &hereby li)uid bac$s u( the do&ncomer i

nto the tray abo%e and e%entually flood the column#

Weep Holes



Weep Holes7oles for drainage must e adequate to drain the tower

in a reasonale time, yet not too large to interfere withtray action.

6raining of the tower

through the trays isnecessary efore any

internal maintenance

can e started.

!he ma8ority of the holes are placed ad8acent to the

outlet or down comer weir.

Bottom &trainer



&

6uring the operation of a tower:

!he ule caps,

Bolts, and

(ther foreign o8ects

may e dislodged and carried along with ottom

stream.

!o prevent these o8ects leaving the tower and



!o prevent these o8ects leaving the tower and

damaging pumps, a strainer is installed in the

ottom outlet line.

&trainers must have openings

small enough to catch small

o8ects, ut large enoughnot to hinder the flow of

liquid, or product, or oil

out of the tower.

!he holes in the strainers must e "ept open so that

the flow of liquid out of the tower will not e

stopped, or hindered.



5eflux distriutor



5eflux entering the top of the tower should e

spread evenly across the top tray to avoid dead spots.

(ne way to disperse

reflux is to place a

reflux distriutor in

front of the inlet line.



A reflux distriutor is simply a plate or affle that

prevents liquid from spraying across the tray.

5eflux entering the

tower is forced to flowunder the affle so that

the liquid is distriuted

evenly across the tray.



!op !ower 6emister



p

&ometimes small drops of liquid suspended in vapor

are carried up from one tray to the next or into theoverhead vapor line.

!his is called entrainment.

#hen the overhead product must e a dry vapor or

gas, entrainment is a more serious prolem.

Entrainment etween trays can usually e

prevented y controlling vapor velocity.

E t i t t th t f t t d



103

Entrainment at the top of a tower can e cut down

y placing a demister on the vapor outlet line.

6emisters are constructed

of fine%gauge wire "nitted

into mesh.

6emisters must e "ept

clean of dirt and foreign

matter, or the flow of

vapor will e restricted, or stopped.



Classification of Trays



#ased on the 0um!er of liuid paths

2. Two Pass

3. Three Pass

4. Four Pass

1. Single Pass



!ac$ing

Pac+ing:



6"88 -;

"C<-; -;

-,"8= "..8E

1' ando% 6ac>ing

2' t#!ct!#ed 6ac>ing



Str$ct$red



Pac%ingStructured packing are considerablymore expensiveper

unit volume than random packing. They come with

different sizes and are neatly stacked in the column.

Structure packing usually offer:

- less pressure drop and

- have higher efficiency and capacity than random

packing.



Reflu- . Reboiling



Reflu- . Reboiling

Re6$+ The word reux is de!ned as *6o)ing

bac% )

pplying it to distillation tower, reux is

the li'uid owing bac# down the tower

rom each successi"e stage

inds o& Re6$+



Co"d Re6$+

1old reux is de!ned as li'uid that is supplied at temperature a

little below that at the top o the tower

2ach pound o this reux remo"es a 'uantity o heat e'ual to the

sum o its latent and sensible heat re'uired to raise its temperature

rom reux drum temperature to the temperature at the top o the

tower

constant 'uantity o reux is recirculated rom the reux drum

into the top o the tower

t is "aporized and condensed and then returns in li#e 'uantity to

the reux



8ot



8otRe6$+

t is the reux that isadmitted to the tower atthe same temperature asthat maintained at the

top o the tower

t is capable o remo"ingthe latent heat because

no diference intemperature is in"ol"ed.



irc$"ating Re6$+



t is also able toremo"e only thesensible heat which isrepresented by its

change in temperatureas it circulates

The reux is withdrawn

and is returned to thetower ater ha"ingbeen cooled

Re6$+ Ratio



t is de!ned as the amount o actual

reux di"ided by the amount o top

product

t is denoted by 3 which e'uals 456



)o #oints to consider



7. minimum number o plates $stages% re'uired at total reux

8. There is a minimum reux ratio below which it is impossible toobtain the desired enrichment howe"er many plates are used

Tota" Re6$+



Total reux is the conclusion when all thecondensate is returned to the tower asreux, no product is ta#en of and there isno eed

t total reux, the number o stagesre'uired or a gi"en separation is theminimum at which it is theoreticallypossible to achie"e the separation

Total reux is carried out at /

7. Towers start0up

8 Testin o the tower



O#tim$m Re6$+ Ration



9ractical reux ratio will lie between

The minim$m or the speci!ed separation and Tota" reux

The optimum "alue will be the one at which thespeci!ed separation is achie"ed at the lowest annualcost

For many systems, the optimum "alue o reux ratio

will lie between

7.8 to 7.: times the minimum reux ratio

Reboi"ing



n all distillations processes

;eat been added by <eans the eed

<eans o a reboiler



Types of re!oilers



yp

The most critical element of re!oiler design is the selection of th

e proper type of re!oiler for a specific service. 4ost re!oilers ar

e of the shell and tu!e heat e2changer type and normally steam

is used as the heat source in such re!oilers. 7o"ever, other hea

t transfer fluids li+e hot oil or Do"therm (T4* may !e used. 8ue

l-fired furnaces may also !e used as re!oilers in some cases.



:ettle re!oilers



Thermosyphon re!oilers



8orced circulation re!oilers



8orced circulation re!oilers are used for re!oiler duties "here viscous

and$or heavily contaminated media are to !e e2pected in the !otto

m product.

7igh liuid velocities in the tu!es and the resulting shearing forces e

nsure that this type of heat e2changer is operated "ithin its optimu

m performance range, "hile +eeping fouling to a minimum. Pump s

election influences performance and efficiency. 8orced circulation re

!oilers can !e designed for either hori1ontal or vertical installation.

1 ? 'olumn

2 ? ,#ays



3 ? .onco%e#

4 ? eboiling

ci#c!lation line

5 ? Man/ole

$ ? +orced

circulation

reoiler

7 ? tea% inlet

? ales

9 ? <eating t!bes



h b il i h h h h



The 3eboiler is a heat exchanger through

which the bottom li'uids circulate

;eat is transerred to the bottom materialswhich cause "aporization o the lighter

components

This "apor tra"els up the column to pro"ide

The stripping action and The additional heat necessary to

"aporize the down coming reux



9. CRUDE DSTLLATON

The #$r#ose o& cr$de oi"disti""ation



disti""ation

is primarily to split the crude into se"eral

distillate ractions o a certain boiling range

Sharpness o ractionation is o secondaryimportance

crude distillation tower, producing =ractions has >? to :? trays

into



gasoline,

naphtha,

#erosene,

diesel oil,

gas oil, and other products, by

distillation at atmospheric



741$

147

1rude is generally pumped to the unit

rocess Descri#tion



1rude is generally pumped to the unitdirectly rom a storage tan#, and it isimportant that charge tan#s bedrained completely ree rom waterbeore charging to the unit

water is entrained in the charget will "aporize in the exchangers and

in the heater, and cause a highpressure drop through that e'uipment



The mixture o oil and water ist d i d lt hi h i



150

separated in a desalter, which is a

large "essel in which may beaccelerated by the addition ochemicals or by electrical de"ices

the oil entering the desalter is notenough heated, it may be too "iscousto permit proper mixing and completeseparation o the water and the oil,and some o the water may be carriedinto the ractionators

th th h d th il i t h t



, on the other hand, the oil is too hot,

some "aporization may occur, and theresulting turbulence can result inimproper separation o oil and water

The desalter temperature is thereore'uite critical, and normally a bypass ispro"ided around at least one o theexchangers so that the temperaturecan be controlled

The optimum temperature depends upon



The desalter pressure and'uantity o light material in the crude

but is normally about :;2<C :2<C

being lower or low pressure and lightcrudes

The a"erage water inAection rate is

B0:C o the charge

±

8eat E+change

d t d th t ti



n order to reduce the cost o operating a

crude unit

s much heat as possible is reco"ered rom

the hot streams by heat exchanging them

with the cold crude charge

The number o heat exchangers within the

crude unit and cross heat exchange with

other units will "ary with unit design

Cr$de F"ashing =F$rnace>



6esalted crude is heat exchanged against what e"er

other heat sources are a"ailable to reco"ermaximum heat beore crude is charged to theheater, which ultimately supplies all the heatre'uired or operation o the crude unit

The heater transer temperature is merely acon"enient control, and the actual temperature,which has no great signi!cance, will "ary rom B8:D1to as high as >B?D1, depending on

The ty#e o& cr$de and

The #ress$re at the bottom o& the&ractionating to)er

Fractionation



1rude entering the ash zone o the ractionating

column ashes into/

The "apor which rises up the column and The li'uid residue which drops downwards

This ash is a "ery rough separation

The "apors contain appreciable 'uantities o hea"y

ends, which must be reAected downwards intoreduced crude, while the li'uid contains lighterproducts, which must be stripped out

Prod$ct Stri##ing =SideStri##ers>



Stri##ers>

The ashed residue in the bottom o theractionators and the side cut products ha"ebeen in contact with lighter boiling "apors

These "apors must be remo"ed to meet ashpoint speci!cations and to dri"e the lightends into lighter and more "aluable products

Steam, usually superheated steam, is used tostrip these light ends



?enera""y on"y eno$gh steam is$sed to meet a 6ash #oints#eci*cation

!hi"e &$rther increases in the/$antity o& steam may raise the@P o& the #rod$ct s"ight"y

ll the stripping steam is condensed inthe o"erhead recei"er and must be



the o"erhead recei"er and must be

drained of

3euxing water will upset the

ractionators

& the end#oint o& the o'erhead#rod$ct is 'ery "o), )ater may not#ass o'erhead, and )i""acc$m$"ate on the $##er traysand ca$se the to)er to 6ood

Prod$ct Dis#osa"



ll products are cooled beore beingsent to storage

4ight products should be below =?D1

to reduce "apor losses in storage, but

;ea"ier products need not be as cold

a product is being charged to anotherunit, there may be an ad"antage in



sending it out hot

product m$st ne'er "ea'e a $nit ato'er :22<C

there is any possibility o it entering atan# with water bottoms

The hot oi" co$"d readi"y boi" the)ater and b"o) the roo& o(

)roduct &pecifications



The composition o a distillation product is

determined by perorming laboratory

tests on samples o that product.

These test results are then compared with

product speci!cations or standards that

ha"e been set or the product

the product is meeting speci!cations, column



operations do not ha"e to be adAusted

(ut, i the products are of0speci!cation, a change

in column operations must be made

-ne can see that the control o the tower is a

rather complicated simultaneous solution o

material and heat balances

<aterial (alance



t each draw we must draw the 'uantity omaterial in the crude that boils within thespeci!ed boiling range

we draw too much, or too little, theproduct abo"e or below will ha"e to shit bythat amount, thereby possibly putting it ofspeci!cation

To stay on speci!cations the materialbalance must be maintained



Suppose speci!cations on the bottomproduct call or an (9 between 7??



product call or an (9 between 7??0

77?EF

4ab tests show an (9 o :EF

Gou #now that light material boils atlower temperatures than hea"ymaterial



o@ t/e botto% p#od!ct in t/is eAa%ple

contains %ate#ial t/at is too lig/t

n order to raise the (9 o a product



n order to raise the (9 o a product,

we must ma#e the product hea"ier

-ne way is to strip some lightcomponents of with steam

nother way is to increase thetemperature o the eed or the

reboiler temperature so more lightcomponents are "aporized

End @oi"ing Point =EP>

h hi h h l



s the temperature at which the last

drop o li'uid "aporizes during the test

n a mixture o hydrocarbons, the

last molecules to "aporize are thehea"y ones

So, the 2(9 $29% test is used to chec#or hea"y hydrocarbons that arepresent in a product



Speci!cations call or an o"erheadproduct with an 29 between 7:?07=?EF

4ab results indicate an 29 o 7H?E F



Bo! >no t/at /eay %ate#ial boils at

/ig/e# te%pe#at!#es t/an lig/t %ate#ial

o@ t/e top p#od!ct does not %eet

speciications beca!se it contains %ate#ial

t/at is too /eay

n order to lower the 29 o a product,



we must ma#e the product lighter

-ne way is to decrease the eed orreboiler temperature so that ewer

hea"y components "aporize

nother way is to lower the top

temperature by increasing the reuxrate

F"ash Point



s the temperature at which a petroleumproduct generates ignitable "apors

4ight hydrocarbons tend to ash moreeasily than hea"y hydrocarbons

sample that contains traces o light

hydrocarbons ashes at a lowertemperature than a sample without thesetraces



side draw product carries ash pointspeci!cations o 78:07B?E F

The lab test shows a ash point o77?E F



,/e sa%ple contains %ate#ial t/at is too lig/t

e can b#ing t/e p#od!ct bac> to speciication by

dec#easing t/e #el!A #ate@ o#

!sing %o#e st#ipping stea%@ o#

inc#easing t/e #eboile# te%pe#at!#e

AP ?ra'ity

s used to designate the *hea"iness* or



s used to designate the hea"iness or

*lightness* o products

Ierosene is measured at about >8E 9Jasoline is measured at about =?E 9

The lighter the oil, the higher 9

gra"ity

o 1rude oil 9 gra"ities typically range from 7 to 52 corresponding



to about H? #g5mBto H:? #g5mB,

o (ut most fall in the 20 to 45 API gra"ity range.

o Although light crude (ie 40!45 degrees API" is considered

the #est , lighter crude $i.e., >= degree 9 and abo"e% is generally

no better or a typical re!nery.

o As the crude gets lighter than 40!45 degrees API, it contains

shorter molecules, which means a lower carbon number. This also

means it contains less of the molecules useful as high octane

gasoline and diesel fuel$ the %roduction of &hich most

re'ners try to maimi)e.

o If a crude is heavier than *5 degree API, it contains longer and

#igger molecules that are not useul as high octane gasoline and

diesel uel without urther processing



!ppose speciications call o# a p#od!ct it/ an "6- g#aity o 3035

,/e p#od!ct sa%ple tests 2

,/e p#od!ct is too /eay

Co"or

4ight hydrocarbons are light colored



g y g

while ;ea"y hydrocarbons are dar# incolor

light hydrocarbon product that is dar#

colored probably contains too many hea"ymolecules

2xcessi"e "apor rates can cause small

drops o li'uid to become entrained in the"apor and be carried up the tower

2ntrainment o hea"y materials maycontaminate the o"erhead product



p

and ma#e it too dar# in color

;ydrocarbons will decompose andchange color at "ery hightemperatures

So, an of0color product may

indicate that a tower is operating attoo high a temperature



RUDE DSTLLATON OPERAT

Re6$+ Rate Changing

S h i i d

9$E&!I( 01



193

Suppose the reux rate is increasedrom 7,??? to 7,8?? barrels per hour,and the other tower operatingconditions are held constant which of the

following will occur

"' 8ig/te# oe#/ead@ botto%@ and side d#a

p#od!cts'' <eaie# oe#/ead and side d#a p#od!cts'

C' <eaie# botto% p#od!ct'

. M / d d t



,/is eAt#a #el!A loing don t/e toe#

ca!ses t/e te%pe#at!#e on eac/ t#ay to

dec#ease

Some o the hea"ier hydrocarbons in the upward

owing "apors will now condense and all bac# down

the tower



741$

195

the tower

The extra reux owing down the tower reduces the

temperature o the li'uid at the bottom o the

column. When the bottom temperature decreases,

the amount o light material "aporized out o the

li'uid at the bottom o the tower is dec#eased

(ecause ewer "apors are now going o"erhead, the

amount o top product ormed is decreased, or less.

4ighter o"erhead, bottom, and side draw products

are produced by increasing the reux rate

9$E&!I( 021



we decrease the reux raterom 7,??? barrels to K??

barrels,

,/ t i t / d ,/



,/e c!t point c/anges a#e #ee#sed' ,/e

te%pe#at!#e on eac/ o t/e t#ays inc#eases@

and a /ig/e# toe# te%pe#at!#e %ean

/eaie# p#od!cts' o oe#/ead@ botto%@and side d#a p#od!cts beco%e /eaie# '

,/e a%o!nt o oe#/ead p#od!ct p#od!ced

inc#eases and t/e a%o!nt o botto% p#od!ct o#%ed dec#eases



pe#ation 6#oble%s

+looding

(ccurs when the pressure drop across a tray is so high



(ccurs when the pressure drop across a tray is so high

that the liquid cannot flow down the tower as fast asrequired.

!he pressure drop across the tray increases to very

high values, and the tray efficiency drops mar"edly.

#hen the froth and foam in the down comer ac" up

to the tray aove and egin accumulating on this tray.

F"ooding is brought about by excessi"e "apour

ow, causing li'uid to be entrained in the "apour up



the column. The increased pressure rom excessi"e

"apour also bac#s up the li'uid in the downcomer,

causing an increase in li'uid holdup on the plate

abo"e. 6epending on the degree o ooding, the

maximum capacity o the column may be se"erely

reduced. Flooding is detected by sharp increases in

column diferential pressure and signi!cant decreasein separation eciency.

Entrainment +looding *o# a constant liD!id #ate@ inc#easing

t/e gas #ate #es!lts eent!ally in eAcessie ent#ain%ent and looding'

"t t/e lood point it is diic!lt to obtain net dona#d lo o liD!id



"t t/e lood point it is diic!lt to obtain net dona#d lo o liD!id@

and any liD!id ed to t/e col!%n is ca##ied o!t it/ t/e oe#/ead gas'

*!#t/e#%o#e@ t/e col!%n inento#y o liD!id inc#eases@ p#ess!#e d#op

ac#oss t/e col!%n beco%es D!ite la#ge@ and cont#ol beco%es diic!lt'

6ownflow +looding *looding %ay also be b#o!g/t on by

inc#easing t/e liD!id #ate /ile /olding t/e gas #ate constant' EAcessie

liD!id lo can oe#taA t/e capacity o donco%e#s o# ot/e# passages@

it/ t/e !lti%ate #es!lt o inc#eased liD!id inento#y@ inc#eased

p#ess!#e d#op@ and t/e ot/e# c/a#acte#istics o a looded col!%n'

#eeping;6umping

,/is p/eno%enon is ca!sed by lo apo!# lo' ,/e



p#ess!#e eAe#ted by t/e apo!# is ins!icient to /old !p t/eliD!id on t/e t#ay' ,/e#eo#e@ liD!id sta#ts to lea> t/#o!g/

pe#o#ations' EAcessie eeping ill lead to d!%ping' ,/at

is t/e liD!id on all t#ays ill c#as/ (d!%p) t/#o!g/ to t/e

base o t/e col!%n (ia a do%ino eect) and t/e col!%n

ill /ae to be #esta#ted' eeping is indicated by a s/a#p p#ess!#e d#op in t/e col!%n and #ed!ced sepa#ation

eiciency'

#eeping;6umping

(ccurs at:



7igh liquid rates, and<ow vapor loads.

&ome of slots or holes will dump liquid instead of

passing vapor, resulting in poor tray efficiency.

+or towers with conventional down comers,

dumping usually occurs at the upstream raw of caps

or holes, where the liquid has the largest head and"inetic energy.

+oaming

*oa%ing #ee#s to t/e eApansion o liD!id d!e to passage o



204

apo!# o# gas' "lt/o!g/ it p#oides /ig/ inte#acial liD!id

apo!# contact@ eAcessie oa%ing oten leads to liD!id

b!ild!p on t#ays' -n so%e cases@ oa%ing %ay be so bad

t/at t/e oa% %iAes it/ liD!id on t/e t#ay aboe' /et/e#

oa%ing ill occ!# depends p#i%a#ily on p/ysical

p#ope#ties o t/e liD!id %iAt!#es@ b!t is so%eti%es d!e to

t#ay designs and condition' /atee# t/e ca!se@ sepa#ation

eiciency is alays #ed!ced'

'oning

(ccurs at low liquid rate or seals.



(ccurs at low liquid rate or seals.

!he vapor pushes the liquid ac" from the slots or

holes and passes upward with poor liquid contact.

!his causes poor tray efficiency.



5educing the liquid flow rate will usually eliminate

pu"ing.



)u"ing should not e confused with carryover.

)u"ing occurs almost instantaneously.

+urthermore, if the liquid rate is not reduced, the

tower will pu"e again when the liquid stac"s up.

'arryover is usually caused y a high vapor flow rate0It happens continuously, whereas pu"ing is an

intermittent thing1.

(perating 5ange

&atisfactory operation will only e achieved over a



20

&atisfactory operation will only e achieved over a

limited range of vapour and liquid flow rates.

!he upper limit to vapour flow is set y the condition

of flooding.



At flooding there is:

A sharp drop in plate

efficiency and

Increase in pressure drop.

+looding is caused y either:

Excessive carry over of liquid to the next plate

y entrainment, or

<iquid ac"ing%up in the down comers.

!he lower limit of the vapour flow is set y condition

of weeping.



p g

#eeping occurs when the vapour flow is insufficient

to maintain a level of liquid on the plate.

='oning= occurs at low liquidrates, and is the term given

to the condition where the

vapour pushes the liquid

ac" from the holes and 8ets

upward, with poor liquid contact.



Than$ 0ou

towers.ppsx

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