8/13/2019 chemical eng paper 4 http://slidepdf.com/reader/full/chemical-eng-paper-4 1/8 ergamon Chemical Enqineering Science Vol. 51, No. 16, pp. 3889 3896, 1996 Copyright ,~; 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved PII: S0009-2509(96)00222-9 0009 2509/96 $15.00 + 0.00 UNCATALYZED HETEROGENEOUS OXIDATION OF CALCIUM BISULFITE AMEDEO LANCIA,* DINO MUSMARRA* and FRANCESCO PEPE ~ * Dipartimento di Ingegneria Chimica, Universit~i di 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~idi Scienze Ambientali, Seconda Universitfidi Napoli, Via Arena 22, 81100 Caserta, Italy First received 8 August 1995; revised manuscript received and accepted 1 February 1996) Abstract--Wet limestone scrubbing is the most common flue gas desulfurization process for control of sulfur dioxide emissions from combustion of fossilfuels.Forced oxidation in the scrubber loop improves the dewatering 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 the kinetic parameters still remain. In the present work the oxidation rate was experimentally studied by contacting pure oxygen or mixtures of oxygen and nitrogen with a calcium bisulflte solution. The experiments were carried out in a well-mixed bubbling reactor varying temperature, oxygenpartial pressure and sulfite concentration, in the absence of solid calcium sulfite and of catalytic species. It was shown that the rate of the process is controlled by reaction kinetics, and that the reaction rate is zero order in dissolved oxygen and 3/2-order in bisulfite ion. Copyright ~) 1996 Elsevier Science Ltd INTRODUCTION Desulfurization of flue gas is required in order to minimize the impact of the combustion of fossil fuels on the environment. Wet limestone scrubbing is 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 tank allows the main problem of the process to be solved i.e. the disposal of the solid by-product, a sludge composed of calcium sulfite and sulfate. Forced oxidation is carried out by injecting air into the liquid phase, so that the following reactions take place: HSO3- + ½02 = SO~- + H + (1) sol- koa = sol-. (2) The kinetics of such reactions, and particularly of the absorption 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 the extreme sensitivity of its kinetics to experimental conditions, which often prevented the achievement of reproducible results. It has been shown that liquid-phase composition (sulfite concentration, dis- solved oxygen, pH), temperature, and the presence, even in traces, of catalysts (Co 2+, Cu 2+, Mn 2+) and inhibitors (alcohols, phenols, hydroquinone) strongly affect the reaction rate. The kinetics of sulfite oxidation were studied in homogeneous conditions or in heterogeneous condi- tions. Results for homogeneous conditions, obtained by contacting a sulfite solution with an oxygen saturated solution, are relatively consistent, indicating 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 et al., 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 the concentration of Co 2+ or Mn 2+, the description of the catalytic activity of copper appears more difficult (Greenhalg et al., 1975). On the basis of the results reported in the literature, Biickstrom [1934; see also Hayon et al. (1972)] pro- posed a chain reaction mechanism which leads to the following rate equation: __ b.A/2~3/2 I .. . 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, cu the catalyst concentration, and Cs~iv) the total sulfite concentration. On the other hand, disagreement 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
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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
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
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
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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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
p r e s e n c e o f c a t a l y s ts .
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a c t i v i t y o f t h e I s p e c i e s , m o l / m 3
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 ,
m o l / m ~
r e a c t o r d i a m e t e r , m
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i o n i c s t r e n g t h , m o l / m 3
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
G r e e k l e t t e r s
~ t s t o i c h i o m e t r i c c o e f f i c i e n t o f t h e I s p e c i e s,d i m e n s i o n l e s s
71 a c t i v i t y c o e f f i c i e n t o f t h e I s p e c i e s, d i m e n -
s i o n l e s s
p v i s c o s i ty , k g / m s
v k i n e m a t i c v i s c o si ty , m 2 / s
a s u r fa c e t e n s i o n , N / m
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
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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 :