Effects of Aqueous Chemical Equilibria on Wet Scrubbing of Sulfur

11 Effects of Aqueous Chemical Equilibria on Wet Scrubbing of Sulfur Dioxide With MagnesiaEnhanced Limestone C L Y D E H. ROWLAND, ABDUL H. ABDULSATTAR, and DEWEY A. BURBANK

Downloaded by UNIV QUEENSLAND on May 28, 2014 | http://pubs.acs.org Publication Date: October 29, 1980 | doi: 10.1021/bk-1980-0133.ch011

Bechtel National, Inc., P.O. Box 3965, San Francisco, CA 94119

In wet scrubbing of S0 from boiler flue gas by limestone slurry, the concentration of dissolved species in the scrubbing liquor that can react with incoming S0 gas is very low, about one to two m-mole/1. This is far below the S0 make-per-pass in the scrubber, typically about 10 m-mole of S0 absorbed per liter of liquor for one pass through the scrubber. Therefore, the S0 absorption rate is largely dependent upon the slow rate of limestone dissolution into the liquor passing through the scrubber. Addition of magnesia to the scrubbing liquor increases the concentration of two dissolved sulfite species,SO= and MgSOo, with CaSOo remaining constant. This increase in dissolved sul fite concentration makes'the SO absorption rate more dependent on the very fast liquid phase reactions of the basic sulfite species with the strong dibasic acid SO (aq): 2

2

2

2

2

3

3

3

2

2

base

+ dibasic acid

SO=

+ SO(aq) +

HO 2

-->

2 HSO-

MgSOo + SO(aq) +

HO 2

-->

2HSO- + Mg

CaSOo + SO (aq) +

HO

-->

2 HSO- + Ca

3

2

3

2

3

2

-->

2

monobasic acid 3

++

3

++

3

Conversion of SO(aq) to HSO- by the above reactions in the scrubber facilitates the absorption of more S0 (g) into the liquor as S0 (aq), thus increasing S0 removal. A chemical model for magnesia wet scrubbing of S0 was previously verified with experimental data for aqueous solu tions of magnesium, S0 , and sulfate over the temperature range 15-60°C for dissolved magnesium concentrations as high as 1200 m-mole/1 (JJ. The present chemical model for limestone wet scrubbing with magnesia enhancement is applicable for liquors containing dissolved magnesium, calcium, S0 , sulfate, and chloride, where the molality of chloride is less than twice that of magnesium. The limestone/magnesia scrubbing model 2

3

2

2

2

2

2

2

0-8412-056W80/47 1.23^$05.25/0 © 1980 èHénftbXl Society f

In Thermodynamics of Aqueous Systems with Industrial Applications; Newman, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

248

THERMODYNAMICS OF AQUEOUS SYSTEMS WITH INDUSTRIAL APPLICATIONS

consists chemical chloride equations

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 a among t h e i m p o r t a n t s p e c i e s , t h e mass b a l a n c e s f o r t o t a l magnesium and s p e c i e s , t h e e l e c t r o n e u t r a l i t y r e l a t i o n s h i p , and for calculating significant activity coefficients.


F o r m u l a t i o n and V e r i f i c a t i o n o f C h e m i c a l

Model

S p e c i f i e d and C a l c u l a t e d V a r i a b l e s . The v a r i a b l e s t h a t are s p e c i f i e d * a s input t o the limestone/magnesia chemical model a r e t h e s c r u b b e r t e m p e r a t u r e , t h e l i q u o r p H , t h e t o t a l d i s s o l v e d magnesium c o n c e n t r a t i o n , t h e t o t a l d i s s o l v e d c h l o r i d e c o n c e n t r a t i o n , and t h e f r a c t i o n a l d e g r e e o f s a t u r a t i o n o f t h e l i q u o r w i t h c a l c i u m s u l f i t e (CaSu3»l/2 H 0) and gypsum ( C a S O ^ 2H 0) s o l i d s . An e q u i l i b r i u m c o m p u t e r program c o n t a i n i n g t h e e q u a t i o n s i n t h e model c a l c u l a t e s t h e e q u i l i b r i u m S0 p a r t i a l pressure; the total dissolved concentrations of calcium, s u l f i t e , S0 s p e c i e s ( s u l f i t e p l u s b i s u l f i t e ) , and s u l f a t e ; and t h e c o n c e n t r a t i o n s o f t h e i n d i v i d u a l s p e c i e s . The 12 i n d i v i d u a l s p e c i e s t h a t a r e c a l c u l a t e d b y u s e o f t h e c h e m i c a l model a r e : S 0 ( g ) , S 0 ( a q ) , HSOÔ, SOÔ, SO4, M g , C a , CaSOS, MgSO^, MgS0$, CaSO^, and C T " . The l i q u o r pH i s used t o d e t e r m i n e t h e r a t i o s o f t h e a c t i v i t i e s o f S 0 ( a q ) , HSO2 and SO3, b u t t h e h y d r o n i u m i o n ( H ) and h y d r o x i d e i o n (OH ) c o n c e n t r a t i o n s a r e n e g l i g i b l y s m a l l , and c a n be e x c l u d e d from t h e i o n i c b a l a n c e . As i n p r e v i o u s work {2_ 3), c h l o r i d e i s assumed n o t t o f o r m c o m p l e x e s , so t h a t a l l c h l o r i d e i s p r e s e n t as c h l o r i d e i o n , C I " . O t h e r s p e c i e s , s u c h as b i c a r b o n a t e i o n , b i s u l f a t e i o n , and t h e c a l c i u m and magnesium c o m p l e x e s o f b i c a r b o n a t e and h y d r o x i d e , have n e g l i g i b l y s m a l l c o n c e n t r a t i o n s in limestone/magnesia scrubbing l i q u o r s . 2

2

2

2

=

2

+ +

2

+ +

2

+

9

E q u i l i b r i a , Mass B a l a n c e s , and A c t i v i t y C o e f f i c i e n t s . Nine aqueous t h e r m o d y n a m i c e q u i l i b r i a a p p l y , one each f o r t h e f o r m a -

t i o n o f S 0 ( g ) , S 0 ( a q ) , HSOÔ, MgS0§, MgSOfl, CaS0§, CaS09, 2

2

CaS0 *l/2 H 0 ( s ) , and C a S 0 * 2 H 0 ( s ) . These e q u i l i b r i a a r e shown b e l o w . The c o n s t a n t s f o r t h e s e e q u i l i b r i a a t 25°C and 1 a t m o s p h e r e t o t a l p r e s s u r e a r e g i v e n i n T a b l e I. The t e m p e r a t u r e dependence o f t h e s e c o n s t a n t s was p r e s e n t e d p r e v i o u s l y (_3)· L i m e s t o n e wet s c r u b b e r s o p e r a t e a t e s s e n t i a l l y a m b i e n t p r e s s u r e , so t h a t no p r e s s u r e c o r r e c t i o n o f t h e c o n s t a n t s i s r e q u i r e d . The v a l u e o f 3 . 5 χ 10"' f o r t h e s o l u b i l i t y p r o d u c t o f CaS0j*l/2 H 0 ( s ) was d e r i v e d f r o m l i m e s t o n e / m a g n e s i a s c r u b b e r i n l e t l i q u o r a n a l y s e s o b t a i n e d a t t h e E n v i r o n m e n t a l P r o t e c t i o n Agency (EPA) Shawnee t e s t f a c i l i t y ( 9 ) , and may i n c l u d e k i n e t i c , as w e l l as t h e r m o d y n a m i c , i n t e r a c t i o n between s l u r r y s o l i d s and l i q u o r . 3

2

4

2

2


11.

ROWLAND ET AL.

249

Wet Scrubbing of Sulfur Dioxide Table

I

E q u i l i b r i u m C o n s t a n t s a t 2 5 °C and 1 A t m o s p h e r e

Species

E q u i l i b r i u m Constant

Formed

S0 (g)

1.23*

(£)

S0 (aq)

0.0130

(1)

HSO3

6.24

MgS0§

1.2 χ 1

MgSO .

5.6 χ

1 0 "

3

CaS0°j

4.0 χ

1 0 "

4

CaSO .

4.9

x 10"

3

CaS0 »l/2 H 0 ( s )

3.5 χ 1 0

CaS0 «2H 0(s)

2.4 χ 1 0 "


2

2

0

0

3

4

*

Reference

2

2

χ1 0 0 "

-

8

(1)

3

- 7

5

(5)

(6) (1) (Z)

*• (8)

Henry's constant f o r S 0 i s the inverse o f t h i s value, or 0.813 atm/ (g-mole/1). 2

** Derived from limestone/magnesia scrubber i n l e t l i q u o r analyses o b t a i n e d d u r i n g 1976 a t t h e EPA Shawnee A l k a l i S c r u b b i n g T e s t Facility.


250

THERMODYNAMICS OF AQUEOUS SYSTEMS WITH INDUSTRIAL APPLICATIONS

S0 (aq) 2

HSO^ + H


SO3

+ H

+

*·-

—*-

S0 (g)

—*»

S0 (aq) +

2

2

H0 2

—HSO3

+

Mg

+ +

+ SO3 •

Effects of Aqueous Chemical Equilibria on Wet Scrubbing of Sulfur

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