Sulfur Removal and Recovery from Industrial Processes

12 Kinetics of the Reaction of Half-Calcined Dolomite with Sulfur Dioxide R. T. Y A N G , P. T. C U N N I N G H A M , W. I. W I L S O N , and S. A. J O H N S O N 1

2

Chemical Engineering Division, Argonne National Laboratory,

Downloaded by UNIV LAVAL on July 12, 2016 | http://pubs.acs.org Publication Date: April 1, 1975 | doi: 10.1021/ba-1975-0139.ch012

9700 South Cass Ave., Argonne, Ill. 60439

Kinetics of the reaction of sulfur dioxide with half-calcined dolomite have been studied using gravimetric

techniques.

The reaction rate depends significantly on the presence of water in the reactant gas mixture.

With water, the reaction

is first order with respect to the sulfur dioxide

concentration.

Without water, the reaction rate is slower, and the reaction is 0.76 order with respect to sulfur dioxide This suggests that the rate-determining

concentration.

step differs depend-

ing on whether or not water is present. The reaction has an apparent

activation

energy of 7.3 kcal/mole

with water

present in the reactant gas.

' " p h i s paper reports the initial results obtained by a small basic-chemistry A

support program associated with developing a fluidized-bed combustor

for high-sulfur coal. The combustion of fossil fuels in a fluidized bed containing a material that reacts with and fixes sulfur dioxide in the bed is only one of many processes presently being developed to permit the use of high sulfur fuels in an environmentally acceptable way. fluidized-bed

The

concept has several variations, but in most cases the sulfur-

fixing material is limestone in a form that reacts with sulfur dioxide to produce calcium sulfate. It is hoped that these initial results will also be useful in developing other processes for sulfur dioxide control such as panel-bed filters. The active material used in this program is half-calcined dolomite, which reacts with sulfur dioxide as indicated in Equation 1: Present address: Physical Chemistry Division, Alcoa Research Laboratories, Alcoa Center, Pa. 15069. To whom correspondence should be addressed. 1

2

149 Pfeiffer; Sulfur Removal and Recovery Advances in Chemistry; American Chemical Society: Washington, DC, 1975.

150

SULFUR

[CaC0

3

+ MgO] +

S0

+ 0.5 0

2

2

-> [ C a S 0

REMOVAL

4

AND RECOVERY

+ MgO] + C 0

F o r e c o n o m i c a n d e n v i r o n m e n t a l reasons, i t is d e s i r a b l e to

2

(1)

regenerate

the r e a c t i v e m a t e r i a l f r o m the p r o d u c t b y some s c h e m e s u c h as s h o w n i n Equations 2 and 3 [CaS0 [CaS0

+ MgO] + 4 H

4

4

2

-> [CaS + M g O ] + 4 H 0

+ M g O ] + 4 C O -> [CaS + M g O ] + 4 C 0

[CaS + M g O ] + H 0 +

C0

2

(2a)

2

2

-> [ C a C 0

3

(2b)

2

+ MgO] + H S 2

(3)

i n w h i c h the h y d r o g e n sulfide r e s u l t i n g f r o m E q u a t i o n 3 is c o n c e n t r a t e d e n o u g h to p e r m i t s u l f u r r e c o v e r y i n a C l a u s p l a n t . Downloaded by UNIV LAVAL on July 12, 2016 | http://pubs.acs.org Publication Date: April 1, 1975 | doi: 10.1021/ba-1975-0139.ch012

A c o n s i d e r a b l e a m o u n t of w o r k , i n c l u d i n g d e t a i l e d k i n e t i c studies (1, 2 ) , has b e e n r e p o r t e d o n the r e a c t i o n of c a l c i n e d limestones w i t h sulfur dioxide ( E q u a t i o n 4) CaO + S 0 i n connection w i t h both the

+ 0.5 0

2

2

fluidized-bed

-* CaS0

(4)

4

combustion and dry-limestone

i n j e c t i o n processes. T h e respective reactions of f u l l y c a l c i n e d a n d h a l f c a l c i n e d limestones w i t h h y d r o g e n sulfide [ C a O + M g O ] + H S -> [CaS + M g O ] +

H 0

2

[CaC0

3

+ M g O ] + H S -> [CaS + M g O ] + 2

(5)

2

H 0 + C0 2

2

(6)

h a v e b e e n s t u d i e d i n some d e t a i l i n c o n n e c t i o n w i t h other d e s u l f u r i z a t i o n schemes (2,3,4,5,6).

S o m e results h a v e also b e e n r e p o r t e d o n the r e a c -

tions o f w o r k discussed here (7,8,9).

E x p l o r a t o r y experiments a p p l y i n g

the system c o n s i d e r e d here to p a n e l - b e d

filtration

( 9 ) h a v e i n d i c a t e d its

f e a s i b i l i t y . T h e f o l l o w i n g conclusions c a n b e d r a w n f r o m this p r i o r w o r k : 1. D o l o m i t e limestones ( f u l l y c a l c i n e d or h a l f - c a l c i n e d ) are m o r e effective reagents t h a n c a l c i t e a n d s h o w c o n s i d e r a b l e p r o m i s e i n s u l f u r emission control. 2. T h e r e d u c t i o n of the s u l f a t i o n p r o d u c t to the sulfide ( E q u a t i o n 2 ) appears to b e satisfactory. 3. T h e a c t i v e reagent c a n be m o r e r e a d i l y r e g e n e r a t e d ( b y E q u a t i o n 3 ) f r o m sulfide p r o d u c e d b y d i r e c t s u l f i d a t i o n w i t h h y d r o g e n sulfide t h a n f r o m sulfide f o r m e d b y r e d u c t i o n of the s u l f a t e d p r o d u c t r e s u l t i n g from reaction w i t h sulfur dioxide. I n the l i g h t of this p r i o r w o r k , the goals of this p r o g r a m are to d e t e r m i n e the d e t a i l e d k i n e t i c s of E q u a t i o n s 1, 2, a n d 3, to e l u c i d a t e p l a u s i b l e m e c h a n i s m s f o r these reactions, a n d to d e t e r m i n e the c o n d i t i o n s t h a t o p t i m i z e e a c h of these reactions.

K i n e t i c results for E q u a t i o n 1 are

p r e s e n t e d here.

Pfeiffer; Sulfur Removal and Recovery Advances in Chemistry; American Chemical Society: Washington, DC, 1975.

12.

YANG

Half-Calcined

ET AL.

Dolomite

with Sulfur

151

Dioxide

Experimental Apparatus.

T h e e x p e r i m e n t a l a p p a r a t u s is s i m i l a r to t h a t u s e d b y

other w o r k e r s ( 3 , 5, 8 ) a n d is s c h e m a t i c a l l y d e p i c t e d i n F i g u r e 1.

The

reactant gas m i x t u r e , w h i c h is p r e p a r e d b y c o n t r o l l i n g the flow of e a c h constituent b y means of a d i a p h r a g m - t y p e r e g u l a t o r a n d c a l i b r a t e d r o tameters, flows u p w a r d t h r o u g h the h e a t e d r e a c t i o n t u b e , past the s a m p l e , a n d exits t h r o u g h a condenser a n d a series of scrubbers.

T o t a l flow c a n

b e c o n t r o l l e d f r o m 200 to 400 c m / m i n w i t h a n a c c u r a c y for t h e t o t a l 3

flow a n d for e a c h constituent of a b o u t ± 2 % .

T h e w a t e r content of the

reactant gas is c o n t r o l l e d b y a t h e r m o s t a t e d h u m i d i f i e r . S u l f u r d i o x i d e is a d d e d to the stream after h u m i d i f i c a t i o n . T h e s a m p l e is s u s p e n d e d i n a Downloaded by UNIV LAVAL on July 12, 2016 | http://pubs.acs.org Publication Date: April 1, 1975 | doi: 10.1021/ba-1975-0139.ch012

p l a t i n u m basket f r o m one a r m of a r e c o r d i n g b a l a n c e . T h e b a l a n c e , w h i c h p r o v i d e s c o n t i n u o u s w e i g h t d a t a f r o m 0.2 to 1.0 g w i t h a n a c c u r a c y of ± 0 . 1 m g , is p r o t e c t e d f r o m corrosive gases b y a p u r g e flow of n i t r o g e n . T e m p e r a t u r e i n the r e a c t i o n z o n e is c o n t r o l l e d b y a M a r s h a l l

furnace

w i t h a n a c c u r a c y of ± 5 ° C u p to a b o u t 9 5 0 ° C a n d is r e c o r d e d a l o n g w i t h s a m p l e w e i g h t o n a recorder. Materials. stainless steel.

T h e a p p a r a t u s is f a b r i c a t e d f r o m q u a r t z a n d t y p e C o m m e r c i a l research-grade

304

c y l i n d e r gases are u s e d

m a k e u p the reactant gas stream. T h e stone u s e d i n these

to

experiments

was B C R - 1 3 3 7 d o l o m i t e o b t a i n e d f r o m C h a r l e s Pfizer a n d C o . , G i b s o n GAS HANDLfNG RECORDING MICROBALANCE" PURGE GAS IN—ROTOMETERS

REACTANT GAS OUT—=

- BAFFLES

REACTANT GAS IN—=

VENT

REACTOR SCRUBBERS

Figure 1.

Schematic of the

apparatus


152

SULFUR

b u r g , O h i o a n d has a n e m p i r i c a l f o r m u l a C a i .

1 4

REMOVAL

AND RECOVERY

M g . 8 G ( C 0 ) . Chemical 0

3

2


analysis a n d pétrographie characteristics of this stone h a v e b e e n r e p o r t e d by Harvey (10). P r o c e d u r e . I n a t y p i c a l e x p e r i m e n t , — 2 0 0 m g of 1337 d o l o m i t e p a r ticles h a v i n g d i a m e t e r s i n a n a r r o w range a r o u n d 1.1 m m ( —16 to -j-18 U . S . s t a n d a r d s c r e e n ) are p l a c e d i n the a p p a r a t u s u n d e r n i t r o g e n a n d c a r b o n d i o x i d e flow a n d are h e a t e d to 8 0 0 ° C at a b o u t 2 5 ° C / m i n to h a l f c a l c i n e the stone. A f t e r c a l c i n a t i o n the w e i g h t c h a n g e of the s a m p l e is o b s e r v e d . W h e n h a l f - c a l c i n a t i o n of the stone is c o m p l e t e ( u s u a l l y after a b o u t 45 m i n ) , the s a m p l e t e m p e r a t u r e is a d j u s t e d to that selected for the e x p e r i m e n t , the s a m p l e is i s o l a t e d u n d e r n i t r o g e n a n d c a r b o n d i o x i d e atmosphere, a n d the reactant gas m i x t u r e is a d j u s t e d to the a p p r o p r i a t e c o m p o s i t i o n a n d flow r a t e for t h e e x p e r i m e n t w h i l e b y p a s s i n g the r e a c t i o n tube. A t t i m e zero, the reactant gas is d i v e r t e d t h r o u g h the r e a c t i o n t u b e a n d the w e i g h t c h a n g e of the s a m p l e is o b s e r v e d as a f u n c t i o n of t i m e . T h e r e a c t i o n is f o l l o w e d u n t i l the rate of w e i g h t c h a n g e is n e g l i g i b l e , t y p i c a l l y a b o u t 2 h r for the experiments r e p o r t e d here. T o s t u d y the r e a c t i o n o f m a g n e s i u m oxide, a s a m p l e of reagent-grade m a g n e s i u m c a r b o n a t e w a s g r o u n d a n d pressed i n t o a dense p e l l e t w h i c h was t h e n b r o k e n u p to o b t a i n a s a m p l e c o n t a i n i n g p a r t i c l e s of the size used i n the other experiments. C a l c i n a t i o n a n d subsequent procedures w e r e i d e n t i c a l to those for other samples. T h e r e a c t i o n rate, r, at a n y t i m e d u r i n g the r e a c t i o n w a s c a l c u l a t e d f r o m the e q u a t i o n 1 r = w

·

dn -τ: at

ί η λ

(7)

w h e r e to is the t o t a l w e i g h t of the s a m p l e b e f o r e c a l c i n i n g , η is t h e a m o u n t of sulfate as moles of s u l f u r t r i o x i d e p r o d u c e d , a n d t is t i m e i n sec. Results and

Discussion

T y p i c a l e x p e r i m e n t a l results are s h o w n i n F i g u r e 2 w h e r e the f r a c t i o n of the stone r e a c t e d a c c o r d i n g to E q u a t i o n 1 is p l o t t e d against t i m e f o r s e v e r a l different s u l f u r d i o x i d e concentrations.

T h e reaction tempera

t u r e a n d reactant gas c o m p o s i t i o n are g i v e n i n the figure l e g e n d .

The

flow rate of the reactant gas w a s m a i n t a i n e d w e l l a b o v e the rate at w h i c h gas phase d i f f u s i o n affected

the r e a c t i o n rate.

It is e v i d e n t that t h e

r e a c t i o n rate, a n d h e n c e the extent of the r e a c t i o n at a g i v e n t i m e , is a f u n c t i o n of s u l f u r d i o x i d e c o n c e n t r a t i o n . C u r v e a i n F i g u r e 3 shows these same d a t a , i n a p l o t of the l o g a r i t h m of the i n i t i a l r e a c t i o n rate vs. the l o g a r i t h m of the s u l f u r d i o x i d e c o n c e n t r a t i o n . e v a l u a t e d at t =

I n i t i a l rates w e r e a c t u a l l y

1 m i n b e c a u s e of scatter i n the d a t a near t =

0.

The

straight l i n e thus o b t a i n e d has a slope of 1.08 a n d i n d i c a t e s t h a t the r e a c t i o n is first o r d e r w i t h respect

to s u l f u r d i o x i d e c o n c e n t r a t i o n

the reactant gas u n d e r these r e a c t i o n c o n d i t i o n s .

Similar

first-order


in de-


12.

YANG

Half-Calcined

ET AL.

Dolomite

with Sulfur

153

Dioxide

TIME, minutes

Figure 2. Percent conversion vs. time for various sulfur dioxide concentrations in the reactant gas at 750°C. Gas composition in %—S0 : a = 3.1; b = 1.1; c = 0.65; d = 0.39; e = 0.20; f = 0.10. C0 : 15.0. 0 : 5.0. H 0: 2.9. N, balance. 2

2

2

2

2

p e n d e n c e for the r e a c t i o n of f u l l y c a l c i n e d 1337 d o l o m i t e w a s r e p o r t e d b y B o r g w a r d t ( J ) , a n d , i n fact, the r e a c t i o n rates r e p o r t e d b y B o r g w a r d t are s i m i l a r to those o b s e r v e d here. W e n o t i c e d i n several e a r l y experiments that the c o n c e n t r a t i o n

of

w a t e r i n the reactant gas a p p e a r e d to i n f l u e n c e the r e a c t i o n rate.

A

series of experiments was p e r f o r m e d i n w h i c h the w a t e r

concentration

i n the reactant gas w a s v a r i e d f r o m about 1.0 to 40 m o l e % c o n c e n t r a t i o n of other reactants w a s constant. however,

w h i l e the

T h e results i n d i c a t e d ,

that the r e a c t i o n w a s essentially zero o r d e r w i t h respect

to

w a t e r c o n c e n t r a t i o n . A n o t h e r series of experiments, i n w h i c h the reactant gas w a s d r y a n d the c o n c e n t r a t i o n of s u l f u r d i o x i d e w a s v a r i e d , gave the results s h o w n i n F i g u r e 3, C u r v e b . T h e o b s e r v e d slope of 0.76 indicates that the rate varies w i t h the three-fourths p o w e r of s u l f u r d i o x i d e c o n centration.

T h u s , i t appears t h a t the r a t e - d e t e r m i n i n g step is different

d e p e n d i n g o n w h e t h e r or not w a t e r is present i n the reactant gas. w a t e r present, t h e r e a c t i o n is 0.22 order w i t h respect to o x y g e n

With

concen-

t r a t i o n i n the reactant gas stream. T h e t e m p e r a t u r e d e p e n d e n c e of the r e a c t i o n rate w i t h w a t e r present w a s e x a m i n e d f r o m 550 to 8 5 0 ° C .

A t h i g h e r temperatures, the c a r b o n



154

SULFUR

S0

2

CONCENTRATION Χ I0

7

REMOVAL

AND

RECOVERY

(mol/cc)

Figure 3. Curve a: initial reaction rate (see text) vs. sulfur dioxide concentration on logarithmic scales. Re action conditions as noted under Figure 2. Curve b: initial reaction rate vs. sulfur dioxide concentration on logarithmic scales. Reaction conditions as noted under Figure 2 except that water is absent. d i o x i d e c o n c e n t r a t i o n necessary to p r e v e n t c a l c i u m carbonate tion

(11)

decomposi

c o u l d n o t be m a i n t a i n e d w i t h the present a p p a r a t u s .

The

i n i t i a l r e a c t i o n rate i n c r e a s e d s i g n i f i c a n t l y w i t h t e m p e r a t u r e over this range. A n A r r h e n i u s p l o t of the d a t a ( F i g u r e 4 ) shows a l i n e a r d e p e n d ence of rate o n 1 / T

a n d y i e l d s a n a p p a r e n t a c t i v a t i o n energy of

7.3

k c a l / m o l e . A l t h o u g h s u c h a v a l u e does not p o i n t c o n c l u s i v e l y to a m e c h a n i s m i n w h i c h some c h e m i c a l r e a c t i o n is rate c o n t r o l l i n g , the v a l u e is s o m e w h a t greater t h a n one m i g h t expect i f the r e a c t i o n w e r e d i f f u s i o n controlled. I n some e x p e r i m e n t a l r u n s , the c o n v e r s i o n to sulfate b a s e d o n E q u a t i o n 1 w a s greater t h a n t h e o r e t i c a l l y p r e d i c t e d . I t has b e e n r e p o r t e d that m a g n e s i u m o x i d e undergoes

s u l f a t i o n (8).

A c c o r d i n g l y , to assesss

the

extent to w h i c h this r e a c t i o n m i g h t interfere w i t h d e t a i l e d analysis of o u r results, the s u l f a t i o n of c a l c i n e d m a g n e s i u m c a r b o n a t e was

examined.

T y p i c a l results are s h o w n i n F i g u r e 5 together w i t h results for the h a l f c a l c i n e d stone.

T h e extent of m a g n e s i u m o x i d e s u l f a t i o n is not


great,

12.

YANG

Half-Calcined

ET AL.

Dolomite

with Sulfur

Dioxide

u

V>


D»

D _

5 0.6 h

Ό.7

0.8

0.9

I.I

1.0 3

I0 /T

1.2

1.3

1

(K" )

Figure 4. Arrhenius plot for the reaction of sulfur dioxide with half-calcined 1337 dolomite

TIME (minutes)

Figure 5. = {CaO and • = %—S0 : 0.0. 2

Percent conversion vs. time for: Δ + M g O ] ; Ο = [CaC0 + MgO]; MgO at 750°C. Gas composition, in 1.1. COy. Δ = 0.0, Ο = 15.0, Π = 0 : 5.0. H 0: 2.9. N , balance. 3

2

2

2


155

156

SULFUR

REMOVAL

AND RECOVERY

a n d t h e r e a c t i o n rate is m u c h s l o w e r t h a n f o r t h e h a l f - c a l c i n e d stone. N e v e r t h e l e s s , t h e effect is too great to b e i g n o r e d i n a n y d e t a i l e d treat m e n t of t h e d a t a f o r h a l f - c a l c i n e d stone.

F i g u r e 5 also shows

results

o b t a i n e d f o r t h e s u l f a t i o n of f u l l y c a l c i n e d stone u n d e r the same c o n d i tions except that c a r b o n d i o x i d e w a s absent i n the reactant gas. T h e s e results a r e v e r y s i m i l a r to those f o r the h a l f - c a l c i n e d stone, as one m i g h t e x p e c t i n v i e w of t h e g e n e r a l l y close c o r r e l a t i o n b e t w e e n this w o r k a n d that of B o r g w a r d t ( 1 ). T o date, there h a v e b e e n several u n s u c c e s s f u l attempts to fit these results to a s i m p l e m o d e l — f o r

e x a m p l e , one b a s e d o n a s h r i n k i n g u n -

r e a c t e d core or o n r e a c t i o n of a porous s o l i d . T h e a p p a r e n t role of w a t e r i n t h e m e c h a n i s m suggests that s u l f u r d i o x i d e m a y b e o x i d i z e d to s u l f u r Downloaded by UNIV LAVAL on July 12, 2016 | http://pubs.acs.org Publication Date: April 1, 1975 | doi: 10.1021/ba-1975-0139.ch012

t r i o x i d e o n t h e surface a n d that s u l f u r t r i o x i d e diffuses t h r o u g h a p r o d u c t l a y e r to react w i t h c a l c i u m carbonate.

T h i s c o n c e p t w o u l d b e consistent

w i t h t h e s i m i l a r k i n e t i c s o b s e r v e d f o r h a l f - a n d f u l l y c a l c i n e d stone since t h e r a t e - d e t e r m i n i n g step w o u l d p r e s u m a b l y b e t h e same i n either case. T h i s v i e w is s u p p o r t e d b y t h e o b s e r v a t i o n that r e a c t i v i t y i n a

fluidized

b e d decreases s o m e w h a t a b o v e about 850 ° C because t h e t h e r m o d y n a m i c s of s u l f u r d i o x i d e o x i d a t i o n b e c o m e less f a v o r a b l e .

O n t h e other h a n d ,

B o r g w a r d t ' s observations w i t h f u l l y c a l c i n e d stone ( 1 ) suggest that t h e decreased r e a c t i v i t y is c a u s e d b y h a r d - b u r n i n g of t h e stone. F u r t h e r w o r k is n e e d e d to u n d e r s t a n d t h e r o l e of w a t e r i n t h e s u l f a t i o n m e c h a n i s m m o r e f u l l y a n d to extend t h e k i n e t i c studies to t h e r e d u c t i o n a n d r e g e n e r a t i o n reactions o u t l i n e d above. T h e p o t e n t i a l advantages of a process u s i n g d o l o m i t e i n a closed c y c l e f o r s u l f u r d i o x i d e c o n t r o l are sufficiently great to w a r r a n t c o n t i n u e d effort. Literature

Cited

1. Borgwardt, R. H . , Environ. Sci. Technol. (1970) 4(1), 59. 2. O'Neill, E . P., Keairns, D . L . , Kettle, W . F . , Intern. Conf. Fluidized-Bed Combust., 3rd, Hueston Woods, Ohio, Oct. 29-Nov. 1, 1972. 3. Pell, M . , Ph.D. Thesis, City University of New York, 1970. 4. Pell, M . , Graff, R. Α., Squires, A. M . , in "Sulfur and S O Developments," AIChE (1971). 5. Squires, A. M . , Graff, R. Α., Pell, M . , Chem. Eng. Prog. Symp. Ser. (1971) 67, 23. 6. Ruth, L . Α., Squires, A. M . , Graff, R. Α., Environ. Sci. Technol. (1972) 6(12), 1009. 7. Bertrand, R. R., Frost, A. C., Skopp, Α., "Fluid Bed Studies of the Lime stone Based Flue Gas Desulfurization Process," Interim Report, Contract No. P H 86-67-103, National Air Pollution Control Administration, Esso Research and Engineering Co., Linden, N . J., Oct. 31, 1968. 8. Narayanan, S. G . , M.S. Thesis, City University of New York, 1971. 9. Squires, A. M . , Graff, R. Α., J. Air Pollut. Control Ass. (1971) 21, 272. 10. Harvey, R. D . , "Petrographic and Mineralogical Characteristics of Car bonate Rocks Related to Sulfur Dioxide Sorption in Flue Gases," Final 2


12.

YANG E T A L .

Half-Calcined Dolomite with Sulfur Dioxide

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Report, Contract No. C P A 22-69-65, Environmental Protection Agency, Illinois State Geological Survey, Urbana, Ill., July 15, 1971. 11. Hill, K. J., Winter, E . R. S., J. Phys. Chem. ( 1 9 5 6 ) 60, 1361.


RECEIVED April 4, 1974. Work performed under the auspices of the U . S. Atomic Energy Commission and the Environmental Protection Agency, Office of Research and Monitoring, Control Systems Laboratory, Contract No. EPA-IAG0199.


Sulfur Removal and Recovery from Industrial Processes

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