4 Limestone Dissolution Effects
of pH, CO , a n d Buffers M o d e l e d b y M a s s 2
Transfer
PUI K. C H A N and GARY T. R O C H E L L E
Downloaded by CORNELL UNIV on October 23, 2016 | http://pubs.acs.org Publication Date: July 1, 1982 | doi: 10.1021/bk-1982-0188.ch004
University of Texas at Austin, Department of Chemical Engineering, Austin, TX 78712
The rate of CaCO dissolution in slurry scrubbers for flue gas desulfurization affects SO absorption, CaSO /CaSO scaling, and ultimate CaCO utilization. The dissolution rate of reagent CaCO has been measured in 0.1 M CaCl at constant pH and CO partial pressure by batch titration with HCl. A mass transfer model has been developed assuming that the calcite particles behave as spheres in an infinite stagnant solution. The model incorporates the effects of several equilibrium acid/ base reactions and also includes the finite rate reaction involving CO and HCO . The cumulative rate of mass transfer is calculated by integrating over a particle size distribution obtained by a Coulter counter. The results of this investigation show that CaCO3 dissolution is controlled by mass transfer and not surface reaction kinetics. Buffer additives such as adipic acid enhance mass transfer by increasing acidity transport to the limestone surface. Dissolution is enhanced at low sulfite concentration but inhibited at high sulfite concentration, indicating some kind of surface adsorption or crystallization phenomenon. The rate of dissolution is a strong function of pH and temperature as predicted by mass transfer. At high CO2 partial pressure, the rate of dissolution is enhanced due to the CO hydrolysis reaction. 3
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Limestone (CaC03) dissolution is an important phenomenon in stack gas desulfurization processes using limestone slurry to absorb SO2 and produce CaS03/CaS04 waste solids (1). The rate of dissolution directly determines the need for excess limestone and interacts strongly with SO2 removal and scale-free operation in the absorber. There is a need to know the dependence of dissolution rates on both solution composition and the type and grind of limestone. This paper presents a mass transfer model and 0097-6156/82/0188-0075$6.75/0 © 1982 American Chemical Society Hudson and Rochelle; Flue Gas Desulfurization ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
Downloaded by CORNELL UNIV on October 23, 2016 | http://pubs.acs.org Publication Date: July 1, 1982 | doi: 10.1021/bk-1982-0188.ch004
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GAS
DESULFURIZATION
experimental r e s u l t s on the d i s s o l u t i o n of reagent CaC03 ( c a l c i t e ) as a f u n c t i o n of s o l u t i o n composition (2). A l a t e r paper w i l l d i s c u s s the a p p l i c a t i o n of t h i s mass t r a n s f e r model to n a t u r a l l y o c c u r r i n g limestones (_3,_4) · In the s l u r r y scrubbing process, limestone d i s s o l v e s at pH 4 to 6 and 55°C i n both absorber and the hold t a n k / c r y s t a l l i z e r . Because of HC1 accumulation from the f l u e gas, t y p i c a l scrubbing s o l u t i o n contains 0.01 to 0.2 M CaCl2. C02 p a r t i a l pressure can vary from near zero with f o r c e d o x i d a t i o n to one atmosphere with CO2 e v o l u t i o n from the hold tank and i s t y p i c a l l y 0.1 atm i n the absorber. S u l f i t e / b i s u l f i t e b u f f e r can be present i n concentrat i o n s up to 0.1 M. CaS03 and/or CaS04 c r y s t a l l i z a t i o n must occur simultaneously with limestone d i s s o l u t i o n . B u f f e r a d d i t i v e s such as a d i p i c a c i d should enhance both SO2 removal and CaC03 d i s s o l u t i o n at concentrations of 3 to 10 mM ( 5 ) . Previous Work The most s i g n i f i c a n t previous work on c a l c i t e d i s s o l u t i o n has been done by geochemists i n pure water or a seawater environment as reviewed by Plummer et a l (6). Berner and Morse (7) measured d i s s o l u t i o n r a t e s of reagent CaC03 using the pH-stat method. Plummer et a l (8) s t u d i e d d i s s o l u t i o n of coarse Iceland spar u s i n g the f r e e d r i f t and pH-stat methods. These i n v e s t i g a t o r s both found a l i n e a r dependence of d i s s o l u t i o n r a t e on concentration and PCO2 * a constant forward r a t e i n the near absence of ï& and CO2. Near e q u i l i b r i u m pH Plummer observed a reverse r e a c t i o n r a t e p r o p o r t i o n a l to the product of Ca"*"*" and HC0 ~. a n c
3
Some previous i n v e s t i g a t o r s have modeled c a l c i t e d i s s o l u t i o n as l i m i t e d by d i f f u s i o n of H+ at pH l e s s than 4 to 5 (]_,9). Other i n v e s t i g a t o r s have reported a dependence on a g i t a t i o n (10, 11). The reported a c t i v a t i o n energy of c a l c i t e d i s s o l u t i o n v a r i e s from 1.5 to 14 kcal/gmol (8,9,10,12,13). The trend of data gives low a c t i v a t i o n energy at pH 2 - 4 and high a c t i v a t i o n energy at pH 8 - 10. Several i n v e s t i g a t o r s have s t u d i e d i n h i b i t i o n of c a l c i t e d i s s o l u t i o n near e q u i l i b r i u m pH by s u r f a c e a d s o r p t i o n of insoluble salts. Berner and Morse (7) showed i n h i b i t i o n by phosphate at 10~ M. Terjesen et a l (14) modeled the i n h i b i t i n g e f f e c t s of metal ions as an apparent r e d u c t i o n i n the e q u i l i b r i u m pH. Koss and M o l l e r (15) s t u d i e d the apparent e q u i l i b r i u m pH i n the presence of N i , Fe, Mg, and other metal i o n s . Sjoberg (13) measured i n h i b i t i o n by phosphate and by Mg"*""*". 6
Theory The r a t e of CaC03 d i s s o l u t i o n can be c a l c u l a t e d by mass t r a n s f e r theory assuming that the s o l u t i o n i s i n e q u i l i b r i u m with
Hudson and Rochelle; Flue Gas Desulfurization ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
Downloaded by CORNELL UNIV on October 23, 2016 | http://pubs.acs.org Publication Date: July 1, 1982 | doi: 10.1021/bk-1982-0188.ch004
4.
CHAN
AND ROCHELLE
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Limestone Dissolution
c a l c i t e a t the limestone surface. A steady-state s o l u t i o n of mass t r a n s f e r theory i s e a s i l y obtained by assuming s p h e r i c a l CaC03 p a r t i c l e s i n an i n f i n i t e stagnant s o l u t i o n , corresponding to a mass t r a n s f e r c o e f f i c i e n t equal to the r a t i o of d i f f u s i v i t y and p a r t i c l e r a d i u s (D/r). The c a l c u l a t e d r a t e s from the stagnant model have been a b i t r a r i l y increased by a f a c t o r of 1.88 to f i t the experimental r a t e s . A f a c t o r of 1.25 would be c o n s i stent with the expected e f f e c t of a g i t a t i o n on mass t r a n s f e r (_3 ). The balance of the c o r r e c t i o n may be due to non-spherical shape or other f a c t o r s . The general model assumes instantaneous e q u i l i b r i a i n the boundary l a y e r of a l l s o l u t i o n species except C02« I t uses a d i f f e r e n t d i f f u s i v i t y f o r each species. I t accounts f o r the f i n i t e - r a t e , r e v e r s i b l e r e a c t i o n of CO2 and H2O to give IT*" and HC03~ by i t e r a t i v e , numerical i n t e g r a t i o n of a second-order, nonlinear d i f f e r e n t i a l equation and a set of nonlinear a l g e b r a i c equations. For many cases the CO2 r e a c t i o n can be neglected, and s o l u t i o n of the mass t r a n s f e r theory r e q u i r e s i t e r a t i v e s o l u t i o n of a much simpler s e t of nonlinear a l g e b r a i c equations. Modeling of experimental data r e q u i r e s i n t e g r a t i o n of the d i s s o l u t i o n r a t e over a p a r t i c l e s i z e d i s t r i b u t i o n . This i s s i m p l i f i e d by assuming that the d i s s o l u t i o n r a t e per p a r t i c l e i s d i r e c t l y p r o p o r t i o n a l to the p a r t i c l e diameter. Because of the CO2 r e a c t i o n , the d i s s o l u t i o n r a t e of a p a r t i c l e i n gmol/sec i s not e x a c t l y p r o p o r t i o n a l to the p a r t i c l e diameter. Therefore, the e f f e c t of the CO2 r e a c t i o n i s assumed to be the same f o r a l l p a r t i c l e s as f o r a 10 ym ( e f f e c t i v e diameter) p a r t i c l e . The general mass t r a n s f e r model i s used to c a l c u l a t e r a t e s f o r 10 ym p a r t i c l e s as a f u n c t i o n of s o l u t i o n composition. General Model. The general mass t r a n s f e r model c a l c u l a t e s d i s s o l u t i o n r a t e as a f u n c t i o n of bulk s o l u t i o n composition, p a r t i c l e diameter, and temperature. I t assumes instantaneous e q u i l i b r i u m o f the s o l u t i o n species H , H2O, OH", CaC03°, C a , C03~, HCO3 , S03 , HSO3", and a general b u f f e r represented by the species H2A, HA"", and A~. CaHC03° was not included, but should not a f f e c t the d i s s o l u t i o n r a t e at most of the conditions modeled This assumption gives the f o l l o w i n g e q u i l i b r i a which apply througj*out the mass t r a n s f e r boundary l a y e r (constants at 25°C): 4-
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Hudson and Rochelle; Flue Gas Desulfurization ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
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o -14 At 55 C, K H 0 taken to be 7.26 χ 10 and the other constants were evaluated according to Lowell et a l (19). A c t i v i t i e s were c a l c u l a t e d as the product of c o n c e n t r a t i o n and a c t i v i t y c o e f f i c i e n t . A c t i v i t y c o e f f i c i e n t s were estimated u s i n g an extended Debye-Huckel l i m i t i n g law as implemented by Lowell et a l (19). To account f o r the CaS03 i o n p a i r , an e f f e c t i v e K - was defined as: w
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Downloaded by CORNELL UNIV on October 23, 2016 | http://pubs.acs.org Publication Date: July 1, 1982 | doi: 10.1021/bk-1982-0188.ch004
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