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ergamon Chemic al Enqineering Science Vol. 51, No. 16, pp. 3889 389 6, 1996Copyright ,~; 1996 Elsevier Science Ltd

Printed in Great Britain. All rights reservedP I I : S0009-2509(96)00222-9 0009 2509/96 $15.00 + 0.00

UNCATALYZED HETEROGENEOUS OXIDATION OF

CALCIUM BISULFITE

AMEDE O LANCIA,* DINO MUSMARRA* and FRANCE SCO P EPE ~

* Dipartimento di Ingegneria Chimica, Universit~idi Napoli Federico II , P.le Tecchio 80,80125 Napoli, Italy

t Istituto di Ricerche sulla Combustione, C.N.R., P.le Tecchio 80, 80125 Napoli, Italy: Facoll~i di Scienze Ambientali, Seconda Universitfi di Napoli, Via Arena 22, 81100 Caserta, Italy

F i r s t r e c e i v e d 8 A u g u s t 1995; r e v i se d m a n u s c r i p t r e c e i v e d a n d a c c e p t e d 1 F e b r u a r y 1996)

Abstract--Wet limestone scrubbing is the most common flue gas desulfurization process for control ofsulfur dioxide emissionsfrom combustion of fossil fuels. Forced oxidation in the scrubber loop improves thedewatering properties of the sludge, leading to the formation of gypsum (CaSO4.2H20). A literature

analysis revealed that uncertainties on the mechanisms of the oxidation reaction and on the values of thekinetic parameters still remain. In the present work the oxidation rate was experimentally studied bycontacting pure oxygen or mixtures of oxygen and nitrogen with a calcium bisulflte solution. Theexperiments were carried out in a well-mixedbubbling reactor varying temperature, oxygen partial pressureand sulfite concentration, in the absence of solid calcium sulfite and of catalytic species. It was shown thatthe rate of the process is controlled by reaction kinetics, and that the reaction rate is zero order in dissolvedoxygen and 3/2-order in bisulfite ion. Copyright ~) 1996 Elsevier Science Ltd

I N T R O D U C T I O N

Desulfur ization of flue gas is required in order

to minimize the impact of the combustion of fossil

fuels on the env ironm ent. Wet limestone scrubbingis the flue gas desulfurization (FGD) process which

has reached the widest diffusion. This process re-

quires, downstream of the absorber, a hold tank

where crystallization of CaSO3 and CaSO4 and dis-

solution of make-up CaCO3 occur. Forced oxidation

of sulfite in the hold t ank allows the main prob lem of

the process to be solved i.e. the disposal of the solid

by-product, a sludge composed of calcium sulfite and

sulfate.

Forced oxida tion is carried out by injecting air into

the liquid phase, so that the following reactions take

place:

HSO3- + ½02 = SO~- + H + (1)

s o l - k o a = s o l - . (2)

The kinetics of such reactions, and particularly of

the absor ption of oxygen by basic solutions of sodium

sulfite in presence of catalysts, received much atten-

tion during the last 30 years; Linek and Vacek (1981)

presented a detailed review of the literature for the

period 1960-1980. The researchers who studied

the reaction of sulfite oxidation pointed out theextreme sensitivity of its kinetics to experimental

conditions, which often prevented the achievement

of reproducible results. It has been shown that

liquid-phase composit ion (sulfite concentrat ion, dis-

solved oxygen, pH), temperature, and the presence,

even in traces, of catalysts ( C o 2 + , C u 2 + , Mn 2+) and

inhibitors (alcohols, phenols, hydroquinone) strongly

affect the reaction rate.

The kinetics of sulfite oxidation were studied inhomogeneous conditions or in heterogeneous condi-

tions. Results for homogeneous conditions, obtained

by contacting a sulfite solution with an oxygen

saturat ed solut ion, are relatively consistent, indica ting

a 3/2-order dependence from sulfite and a zero-order

dependence from oxygen, both in the absence and in

the presence of catalysts (Barton and O'Hern, 1966;

Matsuura e t a l . , 1969; Chen and Barron, 1972; Mishra

and Srivastava, 1975, 1976; Bengtsson and Bjerle,

1975). The dependence of the reaction rate on the

catalyst concentration is more uncertain; while the

reaction rate is proportional to the square root of theconcentra tion of Co 2+ or Mn 2+, the description of

the catalytic activity of copper appears more difficult

(Greenhalg e t a l . , 1975).

On the basis of the results reported in the literature,

Biickstrom [1934; see also Hayon e t a l . (1972)] pro-

posed a chain react ion mechanism which leads to the

following rate equation:

_ _ b . A / 2 ~ 3 / 2I . . . M t~ S ( IV) (3 )

where r is the reaction rate expressed as moles

of SO ] - produced per unit time and volume, k is the

kinetic constant, c u the catalyst concentration, andCs~iv) the total sulfite concen tration. On the other

hand, dis agreement exists for the values of the kinetic

constant evaluated at 25°C and of activation energy,

that were found in the ranges of 2× 106-35 x

106 m3/mol s and 50-150 k J/tool, respectively.

3889



3890

T h e s t u d y o f s u l f i t e o x i d a t i o n i n h e t e r o g e n e o u s

c o n d i t i o n s r e ce i v e d m u c h a t t e n t i o n i n th e l a s t

d e c a d e , s i n c e th e s e c o n d i t i o n s a r e c l o s e r t o t h o s e

e n c o u n t e r e d i n F G D p r o c e s s e s , w h e r e t h e r e a c t i o n i s

c a r r i e d o u t b y b u b b l i n g a i r i n t o a s o l u t i o n s a t u r a t e d

w i t h r e s p e c t t o c a l c i u m s u l f i t e a t p H 3 . 5 - 5 ( L a n c i a e t

a l . , 1 99 3) . I n F G D p l a n t s , d u e t o t h e l o w e r p H , t h e

p r e v a i l i n g s u l f u r o u s s p e c i e s i s b i s u l f i t e i o n H S O 3

i n s t e a d o f s u l f i t e i o n S O Z 3 - ; b e s i d e s , s i n c e c a l c i u m

s u l f it e s o l u b i l i t y i s q u i t e l o w , t h e c o n c e n t r a t i o n s i n -

v o l v e d i n th e r e a c t i o n a r e l o w e r t h a n t h o s e t a k e n i n t o

a c c o u n t b y t h e r e se a r c h e r s w h o w o r k e d w i t h s o d i u m

sul f i t e .

S o m e o f t h e r e s u l t s r e l a t i v e t o s u l f it e o x i d a t i o n i n

c o n d i t i o n s ty p i c a l o f F G D p r o c e s se s a r e r e p o r t e d i n

T a b l e 1. T h e t a b l e s h o w s t h a t t h e i n t e r p r e t a t i o n o f t h e

e x p e r i m e n t a l r e s u l t s i s s o m e w h a t c o n t r a d i c t o r y .

P r o b a b l y , t h i s is d u e t o t h e i n t e r a c t i o n s b e t w e e n t h e

r e a c t i o n s t e p s o n t h e o n e s i d e , a n d t h e d i f f u s i v e t r a n s -

p o r t o f r e a c ta n t s , p r o d u c t s a n d c a t a l y s ts o n t h e o t h e r

( S h u l t z a n d G a d e n , 1 9 56 ).

W i t h t h e a i m o f g a t h e r i n g a b e t t e r u n d e r s t a n d i n g o f

t h e f u n d a m e n t a l p h e n o m e n a i n v o l v e d i n f o rc e d o x i -

d a t i o n , i n t h e p r e s e n t p a p e r t h e a t t e n t i o n i s fo c u s e d o n

t h e k i n e t ic s o f t h e o x i d a t i o n r e a c t i o n i n c o n d i t i o n s o f

p H a n d t e m p e r a t u r e c o m p a r a b l e t o t h o s e e n c o u n -

t e r e d i n t h e w e t l i m e s t o n e F G D p r o c e s s . A n e x p e r i -

m e n t a l w o r k o n c a l c i u m s u l fi te o x i d a t i o n is p r e s e n t e d ,

i n w h i c h p u r e o x y g e n o r m i x t u r e s o f o x y g e n

a n d n i t r o g e n a r e c o n t a c t e d w i t h a c le a r s o l u t i o n o f

c a l c i u m s u l f i t e , a n d t h e r e s u l t s o f s u c h w o r k a r e u s e d

t o o b t a i n a k i n e t i c e q u a t i o n f o r t h e r a t e o f u n -

c a t a l y z e d s u l f i t e o x i d a t i o n . T h e i n t e r a c t i o n s b e t w e e no x y g e n m a s s t r a n s f e r a n d s u l f i t e o x i d a t i o n a r e d e -

s c r i b e d u s i n g t h e w e l l- a s se s s e d t h e o r y o f m a s s t r a n s fe r

w i t h c h e m i c a l r e a c t i o n ( D a n c k w e r t s , 1 9 7 0 ; C h a r p e n -

t ier , 1981).

A . L A N C I A et a l .

b o t h g a s a n d l i q u i d p h a s e . T h e r e a c t o r , m a d e o f

P y r e x g l a s s , i s a j a c k e t e d , 0 . 1 3 m I D c y l i n d e r w i t h

a h e m i s p h e r i c a l b o t t o m , f i t t e d w i t h t w o v e r t i c a l

b a f fl e s a n d a l i q u i d o v e r f l o w . A n a x i a l s t i r r e r w a s u s e d

t o p r o v i d e t h o r o u g h m i x i n g in th e l i q u i d p h as e . T h e

s t i r r e r s p e e d n w a s k e p t c o n s t a n t i n t h e e x p e r i m e n t s

as 13.3 s 1.

T h e g a s p h a s e w a s p u r e o x y g e n o r m i x t u r e s o f

o x y g e n a n d n i t r o g e n w i th o x y g e n c o n c e n t r a t i o n s o f

4 0 o r 2 1 % ; i t w as t a k e n f r o m c y l i n d e r s a n d b u b b l e d a t

t h e b o t t o m o f t h e r e a c t o r. T h e v o l u m e t r i c f lo w r a t e o f

t h e g a s f e d t o t h e r e a c t o r , m e a s u r e d b y a r o t a m e t e r ,

w a s k e p t c o n s t a n t a t 1 .3 9 × 1 0 - 4 m 3 / s . S u c h g a s f lo w

r a t e , in c o n j u n c t i o n w i t h t h e s t i r r e r s p e e d o f 1 3 .3 s 1 ,

g a v e a l i q u i d h o l d u p V o f 3 . 9 × 1 0 - 4 m 3 . T h e l i q u i d

p h a s e w a s a c l e a r s o l u t i o n p r e p a r e d b y d i s s o l v i n g

a n a l y t i c a l - g r a d e c a l c iu m h y d r o x i d e i n t o a n a l y t i c a l -

g r a d e s u l f u r d io x i d e s o l u t i o n a n d b y d i l u t i n g w i t h

b i d i st i ll e d w a t e r. T h e C a 2 + c o n c e n t r a t i o n r a n g e d

f r o m 1 to 8 0 m o l / m 3, w h i l e th e t o t a l S ( I V ) c o n c e n t r a -

t i o n r a n g e d f r o m 1 t o 1 60 m o l / m 3 , w i t h t h e p H i n t h e

r a n g e o f 2 . 5 - 3 . 5 . T h e l i q u i d f l o w r a t e L w a s k e p t

c o n s t a n t i n e a c h e x p e r i m e n t a n d i t w a s v a r i e d u s i n g

a p e r i s t a l t ic p u m p f r o m 2 . 5 7 t o 1 3 .6 × 1 0 7 m 3 / s , c o r -

r e s p o n d i n g t o r e s id e n c e t i m e s in t o t h e r e a c t o r, w i t h

r v a r y i n g f r o m 2 9 0 t o 1 5 4 0 s . T h r e e d i f f e r e n t te m p e r -

a t u r e l e v e ls w e r e e x p l o re d , o f 2 5, 4 5 a n d 6 3 C , u s i n g

t h e t h e r m o s t a t i c b a t h .

T h e p r o d u c t b e t w e e n t h e l i q u i d s i d e m a s s t r a n s f e r

c o e f f i c i e n t a n d t h e s p e c i f i c g a s - l i q u i d i n t e r f a c i a l a r e a

k ° a w a s e v a l u a t e d b y m e a n s o f t h e f o l l o w i n g d i m e n -

s i o n l e s s e q u a t i o n , e x p e r i m e n t a l l y o b t a i n e d b y

A n s e l m i e t a l . ( 1 9 84 ) u s i n g a r e a c t o r s i m i l a r t o t h e o n eus e d he r e :

k ° a d e _ =o

D \ a / \ v /4 )

E X P E R IM E N T A L A P P A R A T U S A N D P R O C E D U R E

T h e r a t e o f s u l fi te o x i d a t i o n w a s m e a s u r e d u s i n g

t h e l a b o r a t o r y - s c a l e a p p a r a t u s s k e t c h e d i n F i g . 1 . S u c h

a p p a r a t u s c o n s i s t s o f a t h e r m o s t a t e d s t i r r e d r e a c t o r

w i t h l in e s f o r c o n t i n u o u s fe e d i n g a n d d i s c h a r g i n g o f

I n t h is e q u a t i o n d i s th e r e a c t o r i n t e r n a l d i a m e t e r ,

v~ i s t he s up e r f i c i a l ve l o c i t y o f t he g a s , D i s t he O 2

d i f f u s i v i ty i n w a t e r , a n d / ~ , v a n d o a r e t h e v i s c o s i ty ,

t h e k i n e m a t i c v i s c o s i t y a n d t h e s u r f a c e t e n s i o n o f

w a t e r , r es p e c ti v e ly . U s i n g d a t a t a k e n f r o m B i r d e t a l .

Table 1 . Li te ra ture resul ts for the sul fi te ox ida t ion k ine tics in FG D con di t ions

T S( IV) 02 H + M n 2 +A ut ho r s ( C ) pH o r de r o r de r o r de r o r de r

Weisnicht e t a l . (1980) 40 4.6--5.0 3/2 . . . . .Pas iuk-Bronikowska and Bronikowski

( 1981) 4 0 <3 0 0 -1 - 1 0 0 2H us s e t a l . (1982) 25 1-4 (~1 0 1-2Pas iuk -Broniko wska and Zia jka (1985) 26 1 .5 0 1 2U l r i c h e t a l . (1986) 25 75 ~ 5 0 0~1 1/2Pas iuk-Bronikowska and Bronikowski

(1989) 26 3.9-4.8 0 1 0

26 4.8 6.6 3/2 0 1/2Pas iuk-B ronikow ska and Zia jka (1989) 26 4 .5 6 .0 3 /2 0 0 1 /2

26 3.8-4.2 0 1 0 0

N o t e : The da sh indica tes tha t the depend ence i s no t s tudied.



Uncatalyzed heterogeneous oxidation of calcium bisulfite

eactor

F Feed tank

D Discharge tankC Temperature controllerAn SO2 analyzerT Water trap

Fig. 1. Sketch of the experimental apparatus.

- ' ' A~-

3891

(1960) and from Perry and Chilton (1973), k ° a was

found to be equal to 1 .74x1 0 -z, 5 .88x1 0 2 and

1.16 x 10 - t s- t at 25, 45 an d 63°C, respectively.

At the begin ning of each experiment, as soon as theliquid in the reactor reached the overflow, agitation

was started and the gas stream was introduced . It was

assumed that steady state was reached after a time

longer than 5r had elapsed. The oxidation rate at

steady state was evaluated by measuring, in the inlet

and in the outlet liquid streams, the total sulfate

concentration. Furthermore, in both streams the con-

centra tions of total sulfite and Ca 2 + ion were meas-

ured. Total sulfite concentration was measured by

iodometric titration using starch as an indicator,

Ca 2+ ion conc entration was measured by EDTA ti-

trat ion using muresside as an ind icator, while the totalsulfate concent ration was measured by means of a tur-

bidimeter (Hach DR/3) at 450 nm wavelength. Since

the gas fed to the reactor removes part of the sulfite as

SO2, the sulfur material balance was checked by

measuring the SO2 concentration in the outlet gas

stream by means of an UV analyzer (Hartmann and

Brown Radas 1G), and a difference not larger than

5% was found.

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

The experimental results are reported in Tables

2-6. Namely, in Tables 2 4 there are the results of

experiments carried out using pure oxygen in the gas

phase, at liquid-phase temperatures of 25, 45, and

63°C, respectively, while in Tables 5 and 6 there are

the results relative to expreriments carried out using

mixtures of oxygen and nitrogen as gas phase at

a liquid tempera ture of 45°C, with oxygen concentra -

tions of 40 and 21%, respectively. In each table, to-gether with the reaction rate, the total concentrations

of sulfite, sulfate and calcium, are also reported.

Since sulfite oxidation interacts with gas-liquid dif-

fusive transport of oxygen, the analysis of the experi-

mental results is made complex by the problem of

finding out whether oxygen absorption takes place in

the fas t r eac t ion reg im e or in the dif fus ional or k ine t ic

reac t ion subregim es . With this aim it is necessary to

compare the experimentally measured reac tion rate to

the rate characteristic of the diffusional subregime, ro

since, if the reaction rate r is definitely greater than ro

it can be assumed that the process takes place in thefast react ion regime, while if r is definitely smaller than

rD, it can be assumed that the process takes place

in the kinetic subregime. The diffusional rate, con-

sidering that the oxidat ion reaction is irreversible, can

be evaluated by means of the following equation

(Astarita, 1967):

rD = 2k~ac~2 (5)

i is the interfacial oxygen concen trati on. Us-here c%

ing the values o f k ° a calculated by means ofeq. (4) and

i given by Henry' s law, the valueshe values of Co2

reported in Table 7 are obtained for rD.

With few exceptions the reaction rates reported in

Tables 2-6 are smaller tha n the diffusional rates,

and this indicates that the process takes place in the



3 8 9 2 A . LA N CIA e t a l

T a b l e 2 . E x p e r i m e n t a l r e s u l ts w i t h p u r e o x y g e n a n d l i q u id t e m p e r a t u r e o f

2 5 C

Cs(iv CS(VI CCa + r X 103N O . ( m o l / m 3 ) ( m o l / m 3 ) ( m o l / m 3 ) ( m o l / m 3 s )

1 1 2 6 1.87 7.2 1 4 2

2 13.2 2 .54 7.6 1 .24

3 1 2 .3 2 .5 0 7 .0 0 .9 5

4 45 .0 2 .1 9 2 2 .0 2 .8 35 24.4 2 .83 12 .6 2 .21

6 3 5 .0 4 .8 3 2 1 .4 3 .9 2

7 27.2 8 .83 21 .0 4.86

8 5 .3 1 .44 2 .6 0 .08

9 5 6 .0 6 .49 3 3 .0 9 .1 9

1 0 6 2 .2 6 .8 8 3 6 .0 9 .6 9

T a b l e 3 . E x p e r i m e n t a l r e s u l t s w i t h p u r e o x y g e n a n d l i q u i d t e m p e r a t u r e o f

4 5 ° C

N O . CS(IV CS(VI CCa2+ r )< 103

( m o l / m 3 ) ( m o l / m 3 ( m o l / m 3 ) ( m o l / m 3 s )

11 2 .2 1 .00 1 .9 0 .64

12 25.5 11 .3 20 .5 18 .1

13 11.8 3 .53 8 .0 3 .69

14 30.5 17.4 30 .0 28 .9

15 6.3 1.52 3.7 1.18

T a b l e 4 . E x p e r i m e n t a l r e s u l t s w i t h p u r e o x y g e n a n d l i q u i d te m p e r a t u r e

o f 6 3 ° C

CS(IV CS(VI CCa + r × 103N o . ( m o l / m 3 ) ( m o l / m a ) ( m o l / m 3 ) ( m o l / m 3 s )

1 6 3 .6 3 .8 3 4 .0 5 .1 7

17 1 .6 3 .14 2 .7 1 .83

18 6 .0 12 .2 13 .5 21 .0

19 6 .5 6 .67 7.8 10 .7

20 5 .2 12 .7 13 .5 22 .1

T a b l e 5 . E x p e r i m e n t a l r e s u lt s w i t h 4 0 % o x y g e n a n d l iq u i d t e m p e r a t u r e o f

4 5 C

N o . C s ( i v } C s ( v i CCa2 + r × 10 3( m o l / m 3 ( m o l / m 3 ) ( m o l / m 3 ( m o l / m 3 s )

21 3 .48 1 .47 2 .7 0 .63

2 2 5 .8 9 3 .0 7 5 .40 1 .44

2 3 2 .0 8 0 .5 4 1 .35 0 .43

2 4 7 .5 7 2 .1 5 5 .40 2 .51

2 5 1 0 .7 4 .5 9 8 .9 5 4 .2 92 6 3 .73 0 .6 0 2 .2 0 .70

27 6 .28 1 .28 5 .40 1 .60

28 12.11 3 .07 8 .65 5 .18

29 16.93 6 .14 12 .67 8 .42

3 0 72 .0 5 3 0 .6 9 5 4 .0 5 5 3 .70

3 1 8 9 .2 0 2 9 .3 7 70 .0 0 5 3 .0 0

k i n e t i c s u b r e g i m e . H o w e v e r , i t h a s t o b e o b s e r v e d t h a t

f o r th e h i g h e s t s u l f i t e c o n c e n t r a t i o n s t h e a b s o r p t i o n

r a t e s a r e q u i t e c l o s e t o t h e l i m i t i n g d i f f u s i o n a l r a t e s ,

c o n f i r m i n g t h e e x is t e n c e o f a b o u n d a r y a t t h e

t r a n s i t i o n b e t w e e n t h e k i n e ti c a n d t h e d i f fu s i o n a l s u b -

r e g i m e s . I n o r d e r t o f i n d a k i n e t i c e q u a t i o n f o r t h e

r e a c t i o n c o n s i d e r e d , i t i s n e c e s s a r y t o s p e c i a t e t h e

s o l u t i o n c o m p o s i t i o n . W i t h t h i s a i m t h e e q u i l i b r i u m



Unca t a l yz ed he t e r oge neous ox i da t i on o f c a l c ium b isu lf it e

Tab l e 6 . Ex pe r i me n t a l r es u lt s w i t h 21% ox ygen a nd l i q u id t empe r a t u r e o f4 5° C

C s ( l v ) C s ( v l ) C c a 2 + r X 1 0 3

No . (mo l /m 3) (mo l /m 3) (mo l /m 3) (mo l /m 3 s )

32 1.20 2.35 5.40 2.8233 2.36 0.53 1.27 0.2734 3.91 1.91 2.65 0.95

35 2.36 0.59 1.23 0.2036 7.3 3.21 5.45 3.6637 4.11 1.48 2.47 1.2738 7.60 2.74 5.40 3.3939 4.84 1.90 3.38 1.6040 7.77 3.85 6.50 2.9741 12.37 8.66 14.34 6.0542 22.15 13.08 2 2.22 11.6043 31.02 12.08 27.76 15.5044 42.41 17.13 35.15 21.7045 152.00 15.78 75.67 20.5046 95.70 13.53 54.05 19.10

3893

Tab l e 7 . D i f fu s i ona l r a t e s eva l ua t ed b y means o feqs (4) and (5)

02 ga s - pha seT concen t r a t i on , r ~

(°C) d imens ionless (mo l /m 3 s )

25 100% 4.42 x 10 245 100% 0.11763 100% 0.20245 40% 4 .68 x l0 -z45 21% 2.45 x 10 -z

e q u a t i o n s f o r t h e f o l l o w i n g r e a c t i o n s w e r e u t i l i z e d ( s ee

t h e a p p e n d i x ) :

SO2( aq ) + H 2 0 = H + + H S O ~ - ( 6)

H S O ; = H + + S O 2 - (7 )

H S O ~ - = H + + S O 2 - ( 8)

H 2 0 = H + + O H - (9 )

f o r w h i c h t h e v a l u e s o f t h e t h e r m o d y n a m i c e q u i l i b -

r i u m c o n s t a n t s w e r e c a lc u l a t e d u s i n g d a t a r e p o r t e d b y

G o l d b e r g a n d P a r k e r ( 1 9 8 5 ) [ r e a c t i o n s ( 6 ) a n d ( 7 ) ]

a n d b y B r e w e r ( 1 98 2 ) [ r e a c t i o n s ( 8 ) a n d ( 9 )] . T o g e t h e rw i t h t h e e q u i l i b r i u m e q u a t i o n s r e l a t i v e t o r e a c t i o n s

( 6 ) - ( 9 ) , t h e s t o i c h i o m e t r i c e q u a t i o n s f o r t o t a l s u l f i t e

a n d t o t a l s u l f a t e c o n c e n t r a t i o n s a n d t h e e l e c t r o -

n e u t r a l i t y e q u a t i o n w e r e c o n s i d e r e d :

Cso . . .. + Cnso~ + Cso~ = Csav) (10)

Cnso~ + Cso~- = Cslvlj (11)

~ I z l c l = 0 (12)

w h e r e z t i s t h e e l e c t r i c c h a r g e o f t h e 1 s p e c i e s , w i t h

I = C a 2 + , H +, H S O £ , S O 2 - , H S O g , S O 42 -, O H - .

T h e c o n c e n t r a t i o n s e v a l u a t e d b y m e a n s o f e q s( 6 ) - ( 1 2 ) w e r e u s e d i n a r e g r e s s i o n a n a l y s i s t o f i n d

a p o w e r - l a w k i n e t i c e q u a t i o n f o r b i s u l f i t e o x i d a t i o n .

B y m e a n s o f t h is a n a l y s i s it w a s c o n c l u d e d t h a t t h e

o n l y s p e c i e s s ig n i f i c a n t l y a f f e c t in g t h e o x i d a t i o n r a t e i s

t h e b i s u lf i te i o n H S O 3 , t h a t t h e r e a c t i o n i s o f o r d e r

3 / 2 i n s u c h i o n , a n d t h a t t h e a c t i v a t i o n e n e r g y E i s

8 6 k J / m o l , a c c o r d i n g t o t h e f o l l o w i n g e q u a t i o n :

- t . ~ ~mT~ 3/2 (13)r - - / x, o ~ LHSO ~

w h e r e k o i s t h e p r e e x p o n e n t i a l f a c t o r , t h e v a l u e o f

wh i ch i s 1 .95 × 10 l ° m 3 / Z / mo l 1 /2 s .

I n F i g s 2 a n d 3 t h e r e a c t i o n r a t e i s r e p o r t e d o n

a l o g a r i t h m i c p lo t a s a f u n c ti o n o f H S O 3 c o n c e n t r a -

t i o n ; in p a r t i c u l a r , F i g . 2 r e f e rs t o t h e r u n s c a r r i e d o u t

u s i n g p u r e o x y g e n a n d v a r y i n g t h e e x p e r i m e n t a l t e m -

p e r a t u r e , w h i l e F i g . 3 re f e rs t o t h e r u n s c a r r i e d o u t a t4 5 ° C w i t h d i f fe r e n t o x y g e n p a r t i a l p r e s s u r e s. I n F i g . 2 ,

t o g e t h e r w i t h t h e e x p e r i m e n t a l r e s u l t s , t h r e e s t r a i g h t

l i n es o f s l o p e 3 / 2 , o b t a i n e d f r o m t h e k i n e t i c l a w

[ E q . ( 1 3) ] c o n s i d e r i n g t h e t h r e e t e m p e r a t u r e s o f 2 5 ,

4 5 , 6 3~ 'C a r e r e p o r t e d . O n t h e o t h e r h a n d , i n F i g . 3

j u s t o n e s i n g l e s t ra i g h t l i n e i s c a p a b l e o f d e s c r i b i n g t h e

e x p e r i m e n t a l r e s u l ts r e l a t iv e t o p u r e o x y g e n a n d t o

t h e m i x t u r e s c o n t a i n i n g 4 0 a n d 2 1 % o x y g e n , c o n f i r m -

i n g t h a t s u l f i t e o x i d a t i o n i s z e r o o r d e r i n o x y g e n .

H o w e v e r in s u c h a f i g u re , t o g e t h e r w i t h t h e l i n e r e l a -

t i v e t o e q . ( 13 ), t h r e e s t r a i g h t l i n e s a r e a l s o r e p o r t e d

r e l a t i v e t o t h e d i f f u s i o n a l r a t e s c o r r e s p o n d i n g t o1 0 0 , 4 0 , a n d 2 1 % o x y g e n , o b t a i n e d f r o m t h e v a l u e s

r e p o r t e d i n T a b l e 7 . S u c h l i n e s c l e a r l y in d i c a t e t h a t

t h e d i f f u s i o n a l r a t e c o n s t i t u t e s a n u p p e r l i m i t f o r t h e

o x i d a t i o n r a t e a t t h e t r a n s i t i o n b e t w e e n t h e k i n e t i c

a n d t h e d i f f u s i o n a l s u b r e g i m e s .

T h e d e p e n d e n c e o f t h e r e a c t i o n r a te o n t h e c o n c e n -

t r a t io n s o f O 2 a n d H S O 3 i s i n a g r e e m e n t w i t h th e

r e s u l t s r e p o r t e d b y P a s i u k - B r o n i k o w s k a a n d Z i a j k a

( 1989 ) , W e i s n i ch t e t a l ( 1 9 8 0 ) , a n d P a s i u k -

B r o n i k o w s k a a n d B r o n i k o w s k i (1 9 89 ), w h o i n d i c a te

t h a t t h e o x i d a t i o n r e a c t i o n i s z e r o o r d e r i n d i ss o l v e d

o x y g e n a n d 3 / 2 - o r d e r i n b i s u l fi t e i o n . T h i s k i n e t i c

e q u a t i o n a p p e a r s t o c o n f i r m t h e c h a i n m e c h a n i s m

p r o p o s e d b y B h c k s t r o m . H o w e v e r , w i t h r e fe r e nc e t o

t h e B ~ i c ks t ro m ' s m e c h a n i s m , t h e p r o b l e m r e m a i n s

o p e n o f c la r i f y i n g w h e t h e r t h e f re e r a d i c a l s w h i c h



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F i g. 2 . Rea c t i on r a t e v s HS O y conc e n t r a t i on f o r e x pe ri me n t s c a rr i ed ou t w i t h pu r e ox yge n a nd t e mpe r -

a tures o f 25 , 45 , and 63 C. • : 25~C; O: 45°C; A: 63°C.

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F i g. 3 . Reac t i on r a t e v s HSO~ - concen t r a t i on f o r ex pe r i men t s c a r r i ed ou t w i t h t empe r a t u r e o f 4 5 C a ndox yge n conc e n t r a t i ons o f 100, 4 0 , a nd 21% . • : 100% O2 ; 0 : 4 0% 02 ; A : 21% 02 .

i n i t i a t e t h e c h a i n a r e g e n e r a t e d b y a r e a c t i o n w i t h t h e

m e t a l l i c i m p u r i t i e s p r e s e n t i n t h e s o l u t i o n , o r b y t h e

a c t i o n o f U V r a d i a t i o n . M o r e i n g e n e r a l , it w i l l b e

u s e f u l t o a d d r e s s t h e f u t u r e w o r k t o w a r d a d e e p e r

u n d e r s t a n d i n g o f t h e r o l e w h i c h m e t a l l i c i o n s , s u c h a s

M n z + a n d F e z + , p l a y i n t h e o x i d a t i o n r e a c t i o n .

CONCLUSI ONS

D e s p i t e w e t l i m e s t o n e s c r u b b i n g w i t h f o r c e d o x i d a -

t i o n i n th e s c r u b b e r l o o p b e i n g o n e o f t h e m o s t c o m -

m o n F G D p r o c e s s e s , s t il l g r e a t u n c e r t a i n t y e x i s ts

a b o u t t h e k i n e t i c s o f t h e r e a c t i o n o f s u l fi t e o x i d a t i o n

i n c o n d i t i o n s t y p i c a l o f w e t l i m e s t o n e s c r u b b i n g .

I n t h e p r e s e n t w o r k o x i d a t i o n o f c a l c i u m b i s u l f it e

w a s s t u d i e d i n a s t i r r e d r e a c t o r a t l a b o r a t o r y s c a l e i n

t h e a b s en c e o f c a t al y s ts . T h e c o m p a r i s o n b e t w e e n t h e

o b s e r v e d r e a c t i o n r a t e a n d t h e d i f f u s i o n a l r a t e o f

o x y g e n a b s o r p t i o n i n d i c a te d t h a t i n th e c o n d i t i o n s

c o n s i d e r e d , t h e r a t e o f u n c a t a l y z e d s u l fi t e o x i d a t i o n i s

c o n t r o l l e d b y t h e k i n e t i c s o f t h e r e a c t i o n i ts e lf , r a t h e r

t h a n b y d i f f u s io n a l p r o c e s s e s . T h e a n a l y s i s o f th e

e x p e r i m e n t a l r e s u l t s a l l o w e d u s t o i n d i v i d u a t e t h e

d e p e n d e n c e o f t h e r e a c t io n r a t e o n t h e t e m p e r a t u r e

a n d o n t h e c o n c e n t r a t i o n o f b i s u lf i te io n , s h o w i n g a l s o

t h a t t h e r e a c t i o n r a t e i s o f z e r o o r d e r i n d i s s o l v e d

o x y g e n . T h e r e s u l t s r e p o r t e d i n d i c a t e t h a t , e v e n w h e n

n o c a t a l y t i c s p e c i e s a r e p r e s e n t , a f i n i t e r e a c t i o n r a t e

f o r s u l f i t e o x i d a t i o n c a n b e o b s e r v e d . H o w e v e r , t h e

f a c t t h a t i n in d u s t r i a l a p p l i c a t i o n s s o m e m e t a l l i c i o n s

a r e p r e s e n t ( i. e. M n z + , F e z + , e t c .) , w h i c h c o m e a s

i m p u r i t ie s o f l i m e s t o n e a n d m a y h a v e a c a t a l y t i c



U n c a t a l y z e d h e t e r o g e n e o u s o x i d a t i o n o f c a l c iu m b i su l fi te

a c t i v i ty o n s u l fi te o x i d a t i o n , s u g g e s t s t h e o p p o r t u n i t y

o f e x t e n d i n g t h e s t u d y t o t h e o x i d a t i o n r e a c t i o n i n t h e

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D e b y e - H i i c k e l c o n s t a n t , m 3 / 2 / m o l 1/2D e b y e - H i i c k e l p a r a m e t e r

c o n c e n t r a t i o n o f t h e I s p e c i e s, m o l / m 3

i n t e r fa c i a l c o n c e n t r a t i o n o f t h e I s p e c i es ,

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k i n e t i c c o n s t a n t , m 3 / m o l s

k i n e t i c c o n s t a n t , m 1 5 / 2 / m O 1 5 / 2

l i q u i d s i d e m a s s t r a n s f e r c o e f f i c i e n t , m / s

e q u i l i b r i u m c o n s t a n tl i q u i d f l o w r a t e , m 3 / s

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r e a c t i o n r a te , m o l / m 3 s

d i f f u s i o n a l r a t e , m o l / m 3 s

g a s c o n s t a n t , J / t o o l K

t e m p e r a t u r e , C

l i q u id h o l d u p , m 3

g a s s u p e r f i c i a l v e l o c i t y , m / s

e l e c t ri c c h a r g e o f t h e I s p e c i es , d i m e n s i o n l e s s

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z l i q u i d - p h a s e m e a n r e s i d e n c e t i m e , s

R E F E R E N C E S

Abdul sa t t ar , A. H. , Shr idar , S . and Bromley, L . A. , 1977 ,T h e r m o d y n a m i c s o f t h e s u l f u r d i o x i d e - s e a w a t e r s y s t e m .A . I . C h . E . J . 23, 62-68.

Anse lmi , G. , Ligno la , P . G., Rai t a no, C. an d Volpicel li , G. ,1984 , Ozone mass t r ans fer i n s t i r r ed ves se l . Ozone Sci .

Engng 6, 17 28.A s t a r i t a , G . , 1 9 6 7 , Mass Trans fer w i th Chemica l Reac t ion .

E l s ev ie r , A m s t e r d am .B ~ ick st ro m , H . L . J. , 1 9 3 4 , D e r k e t t en m ech an i s m u s b e i d e r

a u t o x y d a t i o n y o n n a t r i u m s u l f i t l 6 s u n g e n . Z. Phys . Chem.

B 2 5 1 2 2 1 3 8.

B ar r o n , C . H . an d O ' H er n , H . A . 1 9 6 6, R eac t i o n k i n e t i c s o ft h e s o d i u m s u lf it e o x i d a t i o n b y t h e r a p i d - m i x in g m e t h o d .

Chem. Enyng Sci . 2 1 , 3 9 7 4 0 4 .B en g t s s o n , S . an d B j er le , I ., 1 9 7 5 , C a t a l y t i c o x i d a t i o n o f

s u l p h i t e i n d i l u t ed aq u e o u s s o l u t i o n s . Chem. Eny ny Sc i . 30,1429-1435.

Bi rd , R. B. , S t ewar t , W. E. and Light foot , E . N. , 1960,Transp or t Phenomena . W i l ey , N ew Y o r k .

B r ew er , L ., 1 9 8 2 , T h e r m o d y n am i c v a l u e s fo r d e s u l p h u r i z -a t i o n p r o ces s e s . I n Flue Gas Desu l fur i za t ion ( E d i t ed b yJ . L . H u d s o n an d G . T . R o ch e l l e ) . ACS Syrup. Ser. ,

V o l . 1 8 8 , W as h i n g t o n .

3895

C h a r p en t i e r , J . C ., 1 9 8 1 , M as s - t r an s f e r r a t e s i n g a s - l i q u i da b s o r b e r s a n d r e a c to r s . I n Advances in Chemical Engineer-

ing, V o l . 1 1 ( E d i t ed b y T . B . D r ew , G . R . C o k e l e t an dT . V e r m eu l en ) . A cad em i c P r e s s , N ew Y o r k .

C h en , T . I . an d B a r r o n , C . H . , 1 9 7 2 , S o m e a s p ec t s o f t h eh o m o g en eo u s k i n e t i c s o f s u l f i t e o x i d a t i o n . Ind . EngngChem. Fundam. 1 1, 4 6 6 4 7 0 .

C o l i n , E . , C l a r k e , W . an d G l ew , D . N . , 1 9 8 0 , E v a l u a t i o n o fD eb y e - H t i ck e l l i m i t i n g s l o p es f o r w a t e r b e t w een 0 an d

1 5 0 ' C . J . Chem. Soc . Faraday Trans . 76, 1911 1916.D an ck w er t s , P . V . , 1 9 7 0 . Gas-L iqu id Reac t ions . M c G r a w -

H i l l, N ew Y o r k .G o l d b e r g , R . N . a n d P a r k e r V . B ., 1 9 85 , T h e r m o d y n a m i c s o f

s o l u t i o n o f S O 2 t~ l i n w a t e r an d o f aq u eo u s s u l p h u r d i o x -ide solu t ions . J. Res. Nat . Bur. Stan, 90, 341 358.

G r ee n h a l g h , S. H . , M cM an a m e y , W . J . an d P o r t e r , K . E . ,1 9 7 5 , A n a b s o r p t i o n p e a k i n t h e o x i d a t i o n o f s o d i u ms u l p h i t e s o l u t i o n s . Chem. Engng Sci . 30, 155 157.

H ay o n , E . , T r en i n , A . an d W i lf , J. , 1 9 7 2 , E l ec t r o n i c s p ec t r a ,p h o t o c h e m i s tr y , a n d a u t o x i d a t i o n m e c h a n i s m o f t h e s u l-f i t e -b i s u l f i te p y r o s u l f it e s y s tem s . T h e S O 2 , S O { ; S O 4 ,an d S O ~ r ad i ca l s . J. Am. Chem. Soc. 94 , 47-57 .

Huss , A. J r . , L im, P . K. and Ecker t , C . A. , 1982, Oxidat ion ofa q u e o u s s u l p h u r d i o x i d e. 1 . H o m o g e n e o u s M a n g a n e s e ( I I)

an d I r o n ( I I ) c a t a l y s i s a t l o w p H . J. Phys. Chem. 86,4 2 2 4 4 2 2 8 .

Lancia , A. , Musmar ra , D. , Pepe, F . and Volpice l l i , G. , 1993,Oss idaz io ne del b i sol f i t o d i ca l c io nel proces s () d i de-s o l f o r az i o n e a u m i d o d e i f u m i d e l l a co m b u s t i o n e . Chim.

Ind . (Mi lan) 75, 177 181.L i n ek , V . an d V acek , V . , 1 9 81 , C h em i ca l en g i n ee r i n g u s e o f

ca t a l y z ed s u l f i t e o x i d a t i o n k i n e t i c s f o r t h e d e t e r m i n a t i o no f m as s t r an s f e r ch a r ac t e r i s t i c s o f g a s l i q u i d co n t ac t o r s .Chem. Engn9 Sci. 36, 1747-1768.

M a t s u u r a , A ., H a r ad a , J ., H ak eh a t a , T . an d S h i r a i , T . , 1 9 6 9 ,R a t e o f a m m o n i u m s u lf it e o x id a t i o n i n a q u e o u s s o l u ti o n s .J . Chem. Engn 9 Japa n 2, 199-203.

M i s h r a , G . C . an d S r i v a s t av a , R . D ., 1 9 7 5 , K i n e t i c s o f o x i d a -t i o n o f a m m o n i u m s u lf it e b y r a p i d - m i x i n g m e t h o d . Chem.

Engng Sci. 30, 1387 1390.M i s h r a , G . C . a n d S r i v a s ta v a , R . D ., 1 9 7 6 , H o m o g e n e o u s

k i n e t i c s o f p o t a s s i u m s u l fi te o x i d a t i o n . Chem. Engng Sci .3 1 , 9 6 9 - 9 7 1 .

P as i u k - B r o n i k o w s k a , W . an d B r o n i k o w s k i , T . , 1 9 8 1 , T h er a t e e q u a t i o n f o r S O 2 a u t o x i d a t i o n i n a q u e o u s M n S O 4s o l u t io n s c o n t a i n i n g H z S O 4 . Chem. Engng Sci . 36,215 219.

P as i u k - B r o n i k o w s k a , W . an d B r o n i k o w s k i , T . , 1 9 89 , K i n e t i cm o d e l f o r s u l p h i te a u t o x i d a t i o n u n d e r h e t e r o g e n e o u s c o n -di t i ons . Chem. Engng Sci . 44, 1361-1368.

P as i u k - B r o n i k o w s k a , W . an d Z i a j k a , J. , 1 9 8 5 , O x y g en ab -s o r p t i o n i n t o a q u e o u s s u l p h u r d i o x i d e s o l u t i o n s . Chem.

Engmj Sci . 40, 1567 1572.P as i u k - B r o n i k o w s k a , W . an d Z i a j k a , J ., 1 9 8 9 , K i n e t i c s o f

aq u eo u s S O 2 o x i d a t i o n a t d i f fe r en t r a t e co n t r o l l i n g s t ep s .Chem. Enyn9 Sci . 44, 915-920.

P e r r y , R . H . an d C h i l t o n , H . C . , 1 9 7 3 , Chemical Engineers

Handbook , 5 t h E d i ti o n , M c G r a w - H i l l K o g a k u s h a , T o k y o .S h u l t z , J. S . an d G a d en , E . L . , 1 9 5 6 , S u l f it e o x i d a t i o n a s

a measure of aera t ion ef fec t ivenes s . Ind . Engn9 Chem. 48,2209 2212.

U l r i ch , R . K ., R o ch e l l e , G . T . an d P r ad a , R . E ., 1 9 8 6 , E n -h a n c e d o x y g e n a b s o r p t i o n i n t o b i su l fi te s o l u t io n s c o n t a i n -i n g t r an s i t i o n m e t a l c a t a l y s t s . Chem. Engng Sci . 41,2183 2191.

W ei s n i ch t , W . L . , O v e r m a n , J . , W an g , C . C . , W an g , H . J . ,E r w i n , J. an d H u d s o n , J . L . , 1 9 8 0 , C a l c i u m s u l fi t e o x i d a -t i o n i n a s l u r r y r eac t o r . Chem. Engng Sci . 3 5 , 4 6 3 4 6 8 .

A P P E N D I X

T h e c h e m i c a l r e a c t io n s t a k e n i n t o a c c o u n t c a n b e w r i tt e ni n t h e f o l l o w i n g g en e r a l f o r m :

~ x ~ 1 l = 0 ( A I )



3 8 9 6 A . LA N C1 Ae t a l .

w h e r e s i i s t h e s t o i c h i o m e t r i c c o e f f i c ie n t o f t h e I s p e c i e s a n d

i s a s s u m e d p o s i ti v e f o r t h e r e a c t a n t s a n d n e g a t i v e f o r t h e

p r o d u c t s . T h e e q u i l i b r i u m c o n d i t i o n f o r r e a c t i o n ( A I ) is

K = I l l a l 5 , ( A 2 )

w h e r e a I i s t h e a c t i v i t y o f t h e 1 s p e c ie s .

T h e a c t i v it y a 1 i s r e la t e d t o t h e m o l a r c o n c e n t r a t i o n b y

a l = c i , , x (A 3 )

wh ere 3 , t i s t h e ac t i v i t y co e f f i c i en t .

V a l u e s o f t h e a c t i v it y c o e f f ic i e n ts fo r a n i o n s ( M ) a n d c a -

t i o n s (x ) c a n b e c a l c u l a t e d u s i n g t h e e x t e n d e d v e r s i o n o f t h e

D e b y e - H i i c k e l t h e o r y p r o p o s e d b y B r o m l e y a n d c o w o r k e r s

( A b d u l s a t t a r e t a l . , 1 97 7) . A c c o r d i n g t o t h o s e a u t h o r s i t i s

A,~z~a(F)1/2l o g ( T u ) = 1 + ( F )' ~ + B M ~ x c " + ~ x B xc x (A 4)

A~,ZM(F) t 2l o g (Tx) 1 + (F )' ~ + B x ~ M C M q- ~MBMCM (A 5 )

I n t h e s e e q u a t i o n s A~. i s t h e D e b y e H i J ck e l c o n s t a n t , t h e

v a l u e o f w h i c h i s g i v e n b y C o l i n e t a l . (1 9 80 ) , an d F i s t h e

T a b l e A 1 . D e b y e H i i c k e l p a r a -

m e t e r s f o r e q s ( A 4 ) a n d ( A 5 ) [ f r o m

A b d u l s a t t a r e t a l . (1 9 77)]

S p e c i e s B

H + 0 .0 8 7

O H - - 0 . 0 12

H S O 3 - 0 . 01 3

so~- 0.087H S O 4 - 0 . 01 3SO42 - - - 0 .09

C a 2 + - - 0 . 0 3 5

i o n i c s t r e n g t h , w h i c h c a n b e e v a l u a t e d b y m e a n s o f t h e

f o l lo w i n g e q u a t i o n :

= ~ y , , z~ c , . (A 6 )

T h e v a l u e s o f th e D e b y e - H i J c k e l p a r a m e t e r s B a r e r e -

p o r t e d i n T a b l e A 1 , ta k e n f r o m A b d u l s a t t a r e t a l . (1977).

chemical eng paper 4

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