Flue Gas Desulfurization - American Chemical Society

8 Kinetics of the Oxidation of Bisulfite Ion by Oxygen Downloaded by UNIV OF NEW SOUTH WALES on April 15, 2016 | http://pubs.acs.org Publication Date: July 1, 1982 | doi: 10.1021/bk-1982-0188.ch008

THOMAS G. BRAGA and ROBERT E. CONNICK University of California, Department of Chemistry, and Lawrence Berkeley Laboratory, Materials and Molecular Research Division, Berkeley,CA94720 The chain reaction between sulfur (IV) and oxygen has been studied in the pH region of 3.0 and 4.7 where bisulfite ion, HSO , is the principal species. Preliminary measurements were made with a two-phase gas-aqueous system. To avoid mass-transfer problems, the remaining studies were done on a single aqueous phase with no gas phase present and using an oxygen meter to follow the concentration of dissolved oxygen as a function of time. Empirical rate laws were determined for a variety of conditions, including the presence of ethanol, manganous ion and ultra violet light. Without additives the chain appears to be terminated by a bimolecular reaction of chain carriers, since the rate law contains powers of multiples of 1/2. Ethanol inhibits the reaction by chain termination involving a single chain carrier. Manganous ion is a strong catalyst, apparently through the introduction of a new propagating path as well as participation in the i n i tiation. Ultra violet light presumably initiates the chain. The "simplest" resolution of the rate laws into the three components: initiation, propagation and termination is suggested. The data do not establish the identity of the intermediates; other information will be necessary to fix mechanisms of the reaction. 3

The oxidation of bisulfite ion by oxygen: " + 2HS0 + 0 = 2S0i+ + 2H 2

3

2

is of importance in the lime/limestone processes for removing sulfur dioxide from stack gases of coal-burning power plants as well as in the conversion of atmospheric SO2 to sulfuric acid, the principal component of acid rain. In the various Flue Gas Desul0097-6156/82/0188-0153$6.00/0 © 1982 American Chemical Society Hudson and Rochelle; Flue Gas Desulfurization ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

Downloaded by UNIV OF NEW SOUTH WALES on April 15, 2016 | http://pubs.acs.org Publication Date: July 1, 1982 | doi: 10.1021/bk-1982-0188.ch008

154

FLUE

GAS

DESULFURIZATION

f u r i z a t i o n processes, there are advantages to be gained i n a c c e l e r a t i n g t h i s o x i d a t i o n i n some cases and i n slowing i t down i n others. Thus a b a s i c understanding of the k i n e t i c s of the r e a c t i o n i s a d e s i r a b l e goal. Since Backstrom (1,2) reported s t u d i e s of the thermal and photo-oxidation of sodium s u l f i t e s o l u t i o n s , the o x i d a t i o n of S(IV) species by oxygen has been known to proceed by a chain mechanism. The d e t a i l s of the mechanism, however, are s t i l l a matter of much controversy. A review of the extensive l i t e r a t u r e (_3,4) shows considerable disagreement concerning the r a t e law and r a t e constants. Most of the s t u d i e s were done on s u l f i t e s o l u t i o n s around pH 9. The r a t e law appears to vary depending on the experimental c o n d i t i o n s , and no c o n s i s t e n t law has been obtained. The r a t e has been reported to be p r o p o r t i o n a l to the s u l f i t e concentration and independent of the oxygen concentration (5-7) as w e l l as p r o p o r t i o n a l to both the s u l f i t e and oxygen concentrations (8-10). Although some reports (11) d i s t i n g u i s h between the r a t e laws at s u l f i t e concentrations l e s s than or greater than ^0.01 M, both expressions have been r e ported f o r both ranges. Dramatic e f f e c t s of many metal ions (1214) and organic molecules (15-16) have been c i t e d , however i n many cases the r o l e of these a d d i t i v e s has not been determined. The e f f e c t s of i m p u r i t i e s have been w e l l documented, with i n v e s t i g a t o r s r e p o r t i n g that c o n s i s t e n t r e s u l t s could only be approached a f t e r extensive p u r i f i c a t i o n (7,9), although even then the r a t e may have been c o n t r o l l e d by i m p u r i t i e s . T h i s s e n s i t i v i t y i s not s u r p r i s i n g s i n c e , as a chain r e a c t i o n , the process i n volves h i g h l y r e a c t i v e chain c a r r i e r s . We have chosen to attack the c o n t r o l of t h i s r e a c t i o n not by exhaustive p u r i f i c a t i o n , which has proven to be a d i f f i c u l t task owing to the l a r g e e f f e c t s produced by some c a t a l y s t s even i n t r a c e amounts, but by attempting to c o n t r o l or d e f i n e the chain processes by the i n t r o d u c t i o n of known c a t a l y s t s and i n h i b i t o r s . _ The present s t u d i e s were c a r r i e d out at a c i d i t i e s where HS0 i s the p r i n c i p a l S(IV) s p e c i e s . The dependences of the o x i d a t i o n r a t e on the concentration of b i s u l f i t e , oxygen and H were studied i n the pH range of 3.0 to 4.7. The e f f e c t s of ethanol, manganous i o n and u l t r a v i o l e t r a d i a t i o n on the r a t e and above dependences were i n v e s t i g a t e d i n order to gain information concerning the f e a s i b i l i t y of c o n t r o l l i n g the o x i d a t i o n r e a c t i o n through t h e i r presence. 3

+

Experimental. Two Phase Experiments. The r a t e of O2 uptake by b i s u l f i t e s o l u t i o n s was measured by f o l l o w i n g the change i n the volume of oxygen gas at one atmosphere pressure i n a c l o s e d , thermostated system a t 25°C. The s o l u t i o n was v i o l e n t l y a g i t a t e d using a Vibro Mischer (Ag. f u r Chemie-Apparatebau, Z u r i c h , Switzerland) which v i b r a t e d a p e r f o r a t e d p l a t e v e r t i c a l l y about 5 mm below the

Hudson and Rochelle; Flue Gas Desulfurization ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

8.

BRAGA

A N D CONNICK

155

Oxidation of Bisulfite Ion


surface of the l i q u i d a t 7200 rpm with ca. 2 mm amplitude. T h i s a c t i o n f o r c e s many small gas bubbles i n t o the s o l u t i o n below the p l a t e and produces almost a f r o t h above the p l a t e . The mixing was f u r t h e r enhanced by a magnetic s t i r r i n g bar operating on the bottom of the v e s s e l at i t s maximum speed. A l i q u o t s of HS07 s o l u t i o n were introduced i n t o the system by means of a pressure e q u a l i z i n g buret. S i n g l e Phase Experiments. In order to e l i m i n a t e the p o s s i b i l i t y of g a s - l i q u i d mass t r a n s f e r c o n t r o l , t h e r a t e of the d i s appearance of d i s s o l v e d oxygen i n b i s u l f i t e s o l u t i o n s was d e t e r mined i n the l i q u i d phase i n the absence of a gas phase using the v e s s e l shown i n Figure 1. Concentrations were v a r i e d by i n t r o ducing a l i q u o t s of the reagents through the port of the v e s s e l both a t the beginning and sometimes during a k i n e t i c run. The i n i t i a l oxygen c o n c e n t r a t i o n was u s u a l l y 2.6 χ 10" * M but i n a few cases was as high as 4.3 χ 1 0 M. The d i r e c t measurement of the change i n the c o n c e n t r a t i o n of oxygen with time was achieved by using a Yellow Springs Instrument model 57 oxygen meter and model 5739 probe. T h i s probe c o n s i s t s of a C l a r k - t y p e membrane covering a gold and AgCl e l e c t r o d e system. Output was recorded on a Leeds and Northrup Speedomax s t r i p chart recorder. The response time of the oxygen probe was determined by mea s u r i n g the response of the probe to a sudden change i n oxygen con c e n t r a t i o n . T h i s was accomplished by p h y s i c a l l y t r a n s f e r i n g the probe from a i r saturated water to deoxygenated water. A n a l y z i n g these data i n terms of a two l a y e r d i f f u s i o n model (17) i n d i c a t e d that a zero order r a t e of O2 disappearance of l e s s than ^3 χ 10" M s e c " could be determined without applying any c o r r e c t i o n s due to d i f f u s i o n a l processes. Larger zero order r a t e s were determined by g r a p h i c a l l y f i t t i n g the decay to a s e r i e s of p l o t s c a l c u l a t e d using a mathematical treatment analogous to that of Benedek and Heideger (17) f o r the oxygen probe. Rates l e s s than ^2 χ ΙΟ"" M sec" were measurable, i . e . , a p p r e c i a b l y slower than the d i f f u s i o n control limit. The pH of the s o l u t i o n was measured with an O r i o n Research model 601A d i g i t a l meter and a Markson model 788 combination e l e c trode. U l t r a v i o l e t r a d i a t i o n was produced by a General E l e c t r i c H100A4/T bulb, with the g l a s s outer casing removed, l o c a t e d ap proximately 6 cm above the upper s o l u t i o n . The UV l i g h t was a l lowed t o shine i n t o the s o l u t i o n through the quartz tube i n d i c a t e d (Figure 1). The i n t e n s i t y of the l i g h t on the s o l u t i o n was v a r ied by masking the cross s e c t i o n a l area of the tube c l o s e to the l i g h t source with a f o i l p e r f o r a t e d with h o l e s . A l l experiments were a t 25°C. 1

_1+

6

1

5

1

Results Two Phase Experiments.

F i g u r e 2 shows t y p i c a l data f o r the


GAS

DESULF URIZATION


FLUE

Figure 1.

Reaction vessel used for single phase experiments.



BRAGA


A N D CONNICK

ΙΟΌ

Ê >

>» ι >

2. ο ο _o. ο

ο

ο

ο

ι 1.0 2.0 TIME , min.

ι.υ 0.0 ο

•

3Ό

ο ο ο

-

ο Ο

I

0.0

I

100.0

I

200.0

I

I

Ο

300.0

TIME , min. Figure 2. Typical gas volume data for the absorption of gaseous 0 into a bisulfite solution. Sodium acetate-acetic acid buffer at 0.5 M ; initial [HS0 ~] = 0.012 M ; [0 ] = 1.23 χ ΙΟ M . Insert: O, 0 absorption into H 0; Φ, initial 0 absorp tion into HS0 ~ solution. 2

3

3

2

2

2

3


2


158

FLUE

GAS D E S U L F U R I Z A T I O N

absorption of gaseous O2 i n t o a b i s u l f i t e s o l u t i o n . To maintain pH c o n t r o l , an a c e t i c acid-sodium acetate b u f f e r (pH = 3.7 t o 4.7) was used. Since the s o l u t i o n was prepared from oxygen de p l e t e d water, the i n i t i a l r a p i d drop i n volume of oxygen occurs as the s o l u t i o n i s q u i c k l y being saturated, followed by the slower decrease i n volume as the o x i d a t i o n proceeds. When the data f o r the f i r s t few minutes have subtracted from them values of the smoothly e x t r a p o l a t e d remainder of the curve, i t i s found that the f i r s t order r a t e agrees c l o s e l y with the r a t e a t which 02(g) goes i n t o water under the same mixing c o n d i t i o n s ( i n s e r t , F i g u r e 2 ) . The r a t e of r e a c t i o n between HSO3 and O2 i s shown to be much slower than the mass t r a n s f e r r a t e and t h e r e f o r e i s k i n e t i c a l l y c o n t r o l l e d . T h i s r a t e , on a n a l y s i s , i s found to be 3/2 order i n b i s u l f i t e c o n c e n t r a t i o n during a s i n g l e experiment (Figure 3 ) . Rates were not r e p r o d u c i b l e between runs, v a r y i n g u s u a l l y 10 to 20 percent but sometimes much more. Varying the source and p u r i t y of the water and the source of b i s u l f i t e f a i l e d to e l i m i nate the problem. However, comparison of r a t e s between e x p e r i ments seemed c o n s i s t e n t with 3/2 order f o r H S O 3 and i n d i c a t e d the r a t e was i n v e r s e f i r s t order i n H c o n c e n t r a t i o n . The oxygen de pendence was not t e s t e d . +

S i n g l e Phase

Experiments.

Buffered S o l u t i o n s . S i n g l e phase experiments i n 0.5 M a c e t i c a c i d - 0 . 5 M sodium acetate b u f f e r s o l u t i o n s , with HS07 ( 0 . 0 1 to 0.04 M) i n l a r g e excess over oxygen, gave approximately a zero order dependence on oxygen. The data a c t u a l l y i n d i c a t e d a some what l e s s than zero order i n i t i a l l y which g r a d u a l l y became zero order as the r e a c t i o n approached completion. The complete r a t e law f o r these b u f f e r e d s o l u t i o n s at pH ^4.7 appears to be - j-°2] d

=

R

a

t

=

e

d t

k[HS07] / [0 ]° 3

2

2

(

2

)

+

[H ]

with k = 3 χ 1 0 ~ s ^ . The a d d i t i o n of 0 . 2 M CuSOi* increased the r a t e by a f a c t o r of 3 f o r 0.016 M HSO3 i n the equimolar buf fer. 8

1

Buffered S o l u t i o n s with MnSOu and Ethanol. A s e r i e s of ex periments at constant [HSO^], b u f f e r and i n i t i a l [O2] showed a f i r s t order dependence of the r a t e on c o n c e n t r a t i o n of manganous ion (Figure 4, curve A ) . A s e r i e s of experiments a t constant [HS07], b u f f e r and i n i t i a l [O2] gave an inverse dependence of the r a t e on ethanol con c e n t r a t i o n as shown i n F i g u r e 5 where the r e c i p r o c a l of the r a t e , normalized to the value a t zero ethanol i s p l o t t e d . For s u l f i t e s o l u t i o n s Backstrom (1) found an a l c o h o l dependence of the form



8.

BRAGA

«

I

0.0

159


A N D CONNICK

ι

5C.0

100.0

ι

ι

I50J0

TIME ,min

200.0

ι

J

250.0

I

300.0

Figure 3. Plot indicating three-half-order dependence of rate on [HSO ~] for the reaction: HSO ~ + 0 in 0.5 M acetic acid-sodium acetate buffer. Initial [HS0 ~] = 0.012 M ; [0 ] = 1.23 χ ΙΟ M. s

s

2

3

3

2

[Mn ] x Ι Ο , M 2+

5

Figure 4. Effect of Mn on the rate of oxidation of HS0 ~. Curve A: 0.25 M acetic acid-0.25 M sodium acetate buffer, 0.02 M HSO ' 1.8 X 10' M 0 , rate without Mn (R ) = 9.6 χ ΙΟ M/s. Line of slope 1.0 indicated. Curve B: 9.0 χ 10~ M HSOi, pH 4.2, R = 5.70 χ 10 M/s. Line of slope 1.5 indicated. Curve C: 1.04 χ 10' M HSOf, pH 3.00, R = 2.40 χ ΙΟ M/s. Line of slope 1.0 indi cated. 2+

3

4

s f

2+

1

0

3

8

0

2

1

0


2


160

FLUE

GAS

DESULFURIZATION

CEtOHH, M Figure 5. Effect of EtOH on the rate of oxidation of HSOf in a 0.25 M acetic acid-0.25 M sodium acetate buffer. [HSOf] = 0.02 M ; [0 ] = 1.8 χ 10 M . Rate without EtOH (R ) = 8.7 χ 10 M A . A

2

1

0


8.


BRAGA A N D CONNICK


Rate °c

1 1 + k [EtOH]

161

(2)

The present r e s u l t s a t low ethanol concentrations are probably c o n s i s t e n t with t h i s r e l a t i o n s h i p . At higher e t h a n o l , a new r e a c t i o n path becomes observable (Figure 5 ) . T h i s path i s roughly 100 f o l d slower than the o r i g i n a l path and i s independent of the ethanol c o n c e n t r a t i o n . Further experiments showed that the new path had a 3/2 order dependence on oxygen and increased i n r a t e as the b u f f e r c o n c e n t r a t i o n was i n c r e a s e d . Experiments were attempted with both ethanol and manganous i o n present. The r e s u l t s conformed to no simple r a t e law and seemed only understandable i f i t was assumed that the manganous i o n (at

Flue Gas Desulfurization - American Chemical Society

Recommend Documents