The Catalytic Conversion of Sulfur Dioxide in Wet Stack Gases to

46 The Catalytic Conversion of Sulfur Dioxide in Wet Stack Gases to Elemental Sulfur M.

F.

MOHTADI

and

H.

B.

DINGLE

1

Downloaded by CORNELL UNIV on May 18, 2017 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/ba-1974-0133.ch046

Department of C h e m i c a l Engineering, University of Calgary, Alberta Canada The catalytic conversion of sulfur dioxide in wet stack gases to elemental sulfur, using carbon monoxide as reducing agent, was studied theoretically and experimentally. Computer models were developed to calcule equilibnum compositions for the reaction system. Experiments were done in a reactor designed to simulate equipment and operating conditions used in sulfur recovery plants. The method studied can remove up to 90% SO in the wet stack gas. Complete conversion to elemental sulfur in the presence of water vapor, is, however, hampered by side reactions which produce carbonyl sulfide and hydrogen sulfide. 2

onventional Claus-type sulfur recovery plants operate w i t h overall efficien cies of 9 3 - 9 7 % ( I ) although their potential efficiencies could be 9 9 % (2). Thus, their tail gases contain much H S and S 0 plus smaller amounts of C O S and C S . T h e tail gas is fired w i t h air and methane to oxidize a l l sulfur com pounds to S 0 and to elevate the gas temperature to the level required. T i g h t ening controls on S 0 emission have caused the sulfur gas plants to look for further means to desulfurize their effluent gas streams. This work studies the feasibility of C O as a reducing agent for S 0 i n wet stack gases. Specific objectives were: (1) T o develop a suitable catalyst for this conversion. (2) T o verify the conversion i n a fixed bed reactor, under the same pres sure, temperature, and space velocity used i n conventional sulfur recovery plants. (3) T o investigate the effect of large amounts of water vapor i n the feed stream on the conversion reaction. T h e literature reports at least five reducing agents suitable for converting S 0 to elemental sulfur: hydrogen sulfide, carbon, hydrogen, methane, and carbon monoxide. Direct conversion by catalytic reduction w i t h C O appeared to be the most applicable to sulfur recovery plants. T h e catalytic conversion w i t h C O was first studied i n 1918 b y Ferguson (3) who used ceramic chips as catalyst. Yushkevich and Karzhavin (4) noted later that the non-catalyzed reduction of S O w i t h C O was ca. 9 0 % complete at 400°-1200°C and that reaction velocity was enhanced b y f e r r i c / a l u m i n u m oxides as catalysts. Also, water vapor promoted H S formation. Ryason and Harkins (5) studied the simultaneous removal of S 0 and N O ^ from combus-

C

2

2

2

2

2

2

2

s

2

2

1

Present address: Imperial O i l L t d . , Sarnia, Ontario, Canada. 612

Hulburt; Chemical Reaction Engineering—II Advances in Chemistry; American Chemical Society: Washington, DC, 1974.

46.

MOHTADi

AND

Sulfur Dioxide

DINGLE

613

Conversion

tion gases. F o r a synthetic gas composed of S 0 , C O , C 0 , N , and nitrogen oxides, they observed a maximum of 9.7% at 1000°F. Simultaneous catalytic reduction of N O and S 0 by C O was also studied by Quinlan, Okay, and Kittrell ( 6 ) . They showed that the maximum attainable removal of sulfur compounds is 7 5 - 8 5 % i n a single-bed reactor. In a recent publication on S 0 removal from stack gases using C O , Querido and Short (7) describe a process particularly suitable for treating of gases at typical power plant concentrations and temperatures. Their process re moves 9 7 % sulfur, using a dual copper-on-alumina catalyst reactor system and a C O : S 0 ratio of 1.03. However, elemental sulfur and C O form C O S — a harmful side reaction. Short and O k a y (8) subsequently described the effect of water vapor on S 0 reduction by C O and show that S 0 reduction activity is adversely affected by water. O u r studies on sulfur removal were i n two phases: (a) the theoretical phase (thermodynamic equilibrium studies), and (b) the experimental phase (conversion process under non-equilibrium conditions). T h e theoretical equi librium studies delineated the absolute conversion limit. The experimental studies showed the overall conversion attainable under simulated plant conditions. 2

2

2

2

2

2


2

Theoretical

2

Study

Suitable computer models were developed to: (1) calculate the equi librium compositions for reaction systems and chemical species; (2) establish w h i c h reactions contributed significantly to the overall process; (3) evaluate the equilibrium compositions arising from changes i n the feed gas compositions. T w o methods are available for calculating equilibrium gas compositions from thermodynamic principles. The first is based on successively correcting an initial, non-equilibrium composition by adjusting factors derived from the equilibrium expressions for each reaction—the simultaneous chemical equilibria ( S C E ) method. T h e second method relies on the minimization of the total free energy of the given system ( 9 ) — t h e free energy minimization ( F E M ) method. Details are given elsewhere (10). T h e two methods can be used i n a complementary way. Overall Reaction System. T h e principal reactions i n the reduction of S 0 w i t h C O are: 2

S0

2

+ 2CO = 2 C 0

CO + ί S

e

+ i S (basic reaction)

2

e

= COS (dry gas system)

(1) (2)

Neglecting the presence of water vapor, the additional reactions w h i c h are thermodynamically dependent but w h i c h might be important kinetically are: S0

2

+ 2COS = 2 C 0

2COS «

C0

2

+ CS

2

2

+ I S

e

) f )

(3) dry gas system *

(4)

W a t e r vapor i n the feed allows three further reactions w i t h the product sulfur, the by-product C O S , and the reducing agent C O , respectively:


614

CHEMICAL REACTION ENGINEERING

2H 0 + - S e 2

= S0

e

2

+ 2H S

(5)

2

H 0 + COS = C 0 2

H 0 + CO = C 0 2

2

2

II

+ H S ) > + H ) 2

(6)

wet gas system

(7)

2

Reaction 7, the water gas shift reaction, is important since the hydrogen pro duced w i l l react w i t h other system components: H


2H H

+ - S e

2

2

2

+ S0

e

2

= H S

(8)

2

= 2H 0 + - S 2

(9)

e

e

+ COS = CO + H S

(10)

2

In addition, sulfur vapor exists i n a complicated equilibrium between S , S3, S , S , S , S , and S ; the distribution of these species is a function of tem perature and the total sulfur partial pressure (11). Therefore, we must include i n the overall reaction system: 2

4

5

6

7

8

I S = S ; (x = 2,3,4,5,6,7,8) (molecular sulfur equilibrium) e

(11)

c

x

Earlier workers (12, 13) considered only the equilibrium between S , S , and S owing to the lack of accurate thermodynamic data for the remaining sulfur species. D a t a have become available recently for a l l the polymeric species of elemental sulfur (14). However, for convenience, the equilibrium between S , S , and S are considered here. Application of the Computer Programs. The S C E program was used to calculate the equilibrium gas compositions from the individual chemical reac tions assumed to occur i n the overall reaction system. These calculations were verified, where possible, w i t h the F E M program. T h e feed compositions for each reaction are listed i n Table I. A typical computation result is shown i n Figure 1 (percent conversion (at equilibrium) of S 0 to elemental sulfur as a function of temperature). S C E and F E M programs were applied to the overall reaction system at 400°-1000°K. Pressure was assumed as 1 atm absolute. Although the pro grams were used to calculate equilibrium compositions for temperatures as low 2

6

8

2

6

8

2

Table I. 1

00

(2)

0.00

0.00 7.69 15.38 76.93 — — — — — —

Specie S« so CO C0 COS CS H H 0 H S N

1

33.33 66.67 0.00 — — — — — —

2

2

2

2

2

2

2

1

(8) 0.00 1.00 2.00 10.00 — — — — — 87.00

2 0)

F e e d Compositions (vol %) 2 (2)

3

4

1.00 0.00 — — — — 30.00 0.00 69.00

50.00 — 50.00 — 0.00 —

1.00 — 2.00 — 0.00 —

0.00 1.00 — 10.00 2.00 —

— — — 10.00 2.00 0.00

— —

— 97.00

— 87.00

— 88.00

« S = 2S + 6S + 8S . 2

6

8


5

46.

MOHTADi

Sulfur Dioxide

A N D DINGLE

615

Conversion

100

99-

98-

l

97-

96-


o

93

ο

M.

92

91

90

. 500

400

, 700

600

800

900

1000

TEMPERATURE (°Κ)

Figure 1.

Theoretical equilibrium: SO, + 2C0 = 2CO 1/eS.

t

+

as 400° K , the lower temperature limit for each case h a d to be corrected to correspond w i t h the d e w point of sulfur vapor i n that system. Neither program was designed to treat a two-phase system. T h e dew points of sulfur vapor were taken from literature partial pressure/vapor pressure data. F e e d compositions used for the overall reaction system are given i n Table II. T y p i c a l computed results are shown i n Figures 2 - 6 , where temperature is plotted vs. equilibrium conversion of feed sulfur dioxide to either elemental sulfur, H S , or C O S at various C O : S 0 . In a l l computer studies, equilibrium gas compositions were calculated at 25°Κ intervals. The thermodynamic data were obtained mainly from the paper by Gamson and Elkins (12). 2

2

for I n d i v i d u a l C h e m i c a l Reactions 6

7

8

9

10

11

— — —

— —

1.00

0.00 1.00

— —

1.00

10.00 2.00

— 30.00 0.00 58.00

2.00 10.00

— 0.00 30.00

—

58.00

— — — — 2.00

—

0.00 97.00

— —

0.00

—

2.00

2.00 30.00

2.00

—

67.00

—

—

0.00 96.00

— — — — —

—

99.00


616

CHEMICAL

REACTION

Table II.

ENGINEERING

F e e d Compositions

Case Number Species

1

2

3

4

5

6

7

8

1.00 2.00 10.00

1.00 2.50 10.00

1.00 3.00 10.00

1.00 3.50 10.00

1.00 4.00 10.00

1.00 2.00 10.00 2.00 85.00

1.00 2.00 10.00 3.00 84.00

—

—

—

—

—

87.00

86.50

86.00

85.50

85.00

1.00 2.00 10.00 1.00 86.00

2CO/SO2

1.00

1.25

1.50

1.75

2.00

1.00

1.00

1.00

%

0.00

0.00

0.00

0.00

0.00

1.00

2.00

3.00

S0 CO C0 H 0 N, 2

2

2


0

H

2

0

Ρ = 1.00 atm; Τ = 400°-1000°Κ.

400

450

500

550

600

650

700

750

800

850

II

900

950

1000

TEMPERATURE (°K)

Figure 2.

Percent feed sulfur dioxide reduced to elemental sulfur

Figure 3.

Percent feed sulfur dioxide converted to hydrogen sulfide


46.

MOHTADi

Sulfur Dioxide

A N D DINGLE

617

Conversion

(vol %) for General System"


Case Number 9

10

11

12

15

16

1.00 2.00 10.00 5.00 82.00

1.00 2.00 10.00 10.00 77.00

1.00 2.00 10.00 20.00 67.00

1.00 2.00 10.00 30.00 57.00

1.00 2.50 10.00 30.00 56.50

1.00 3.00 10.00 30.00 56.00

1.00 3.50 10.00 30.00 55.50

1.00 4.00 10.00 30.00 55.00

1.00

1.00

1.00

1.00

1.25

1.50

1.75

2.00

5.00

10.00

20.00

30.00

30.00

30.00

30.00

30.00

Figure 4.

13

14

Percent feed sulfur dioxide converted to carbonyl sulfide

Figure 5. Percent feed sulfur dioxide converted to carbonyl sulfide as a function of feed CO:SO ratio and temperature t


618

CHEMICAL REACTION ENGINEERING H

Analyses of the Computer Studies. E q u i l i b r i u m gas compositions com puted for the individual reactions assumed showed the equilibrium performance of a single important species i n the given reaction. N o results were obtained for the reaction leading to C S formation since the computer studies indicated virtually no conversion of C O S to C S under the prescribed conditions. T h e results for the overall system (Figures 2 - 6 ) are those computed from the F E M program. T h e S C E program gave virtually identical results and was used for random point checks. Figure 5 shows the effect of increasing C O concentration on C O S production i n a dry gas system while Figure 6 indicates this effect o n H S production for a wet gas. Water vapor i n the feed gas is obviously significant. F o r example, F i g u r e 2 shows that w i t h 3 0 . 0 % water 2

2


2

0

«

ι

1

I

I

400

450

500

550

600

I

I

650

I

700

750

I

800

I

I

I

850

900

950

1

1000

TEMPERATURE (°K)

Figure 6. Percent feed sulfur dioxide converted to hydrogen sulfide as a function of feed CO:SO ratio and temperature t

vapor, elemental sulfur production approaches zero at Τ > 550° Κ. T h e maxi m u m adverse effect of water vapor is at 500°-600°K. In the d r y reaction system excess C O reacts with product sulfur to form C O S ; i n the wet gas system, C O promotes H S production. T h e former obser vation agrees w i t h Querido and Short ( 7 ) . The latter can be attributed to the water-gas shift reaction where C O reacts w i t h water vapor to produce hydro gen; the hydrogen then reacts w i t h product sulfur to form H S . Thus, this process must operate at or below the d e w point of sulfur vapor. 2

2

Experimental

Study

T h e only published data on the kinetics of S 0 reduction b y C O is b y Quinlan et ai. a n d recent ( 6 ) . However, it has been k n o w n for some time that reactions between C O and S 0 proceed satisfactorily i n a few tenths of a second. O u r experimental apparatus (Figure 7) was basically a fixed-bed reactor system, w i t h flow control and gas analysis. It included a feed gas preheater, a fixed-bed catalytic reactor, and a sulfur/water recovery condenser. Details are given elsewhere (10). A l l gas mixtures were analyzed on a Varian Aerograph dual column gas chromatograph (series 1 7 0 0 / m o d e l 20) w i t h thermal conductivity detectors, 2

2



46.

MOHTADI AND DINGLE

Figure 7.

Sulfur Dioxide

Conversion

619

Reaction system schematic

a column switching valve, and a linear programmed oven. T w o columns were necessary to analyze a l l the gas components since no single column packing was suitable for a l l . Nitrogen, hydrogen, and C O were analyzed on a 6-ft by Ve-inch Teflon column packed w i t h 8 0 / 1 0 0 mesh, 5 A molecular sieve at 125°C. Water vapor, C 0 , H S , C O S , and S 0 were analyzed on a 6-ft by Vs-inch Teflon column packed w i t h 8 0 / 1 0 0 mesh, Porapak QS at 125°C. T y p i c a l responses are i n Figure 8. A l l gas chromatographic analyses were recorded by a Varian Aerograph single-pen chart recorder (model 20) w i t h a 1.0-mv response. A disc integrator was used to calculate peak area. Catalysts. T w o catalysts types were used: (1) 3 / 1 6 - i n c h , cylindrical silicaalumina pellets ( S M R 7-2423/grade 970: Davison Co.) w i t h 0.3 wt % copper; (2) 8 to 14 mesh, hydrated alumina ( T 6 1 : Canada Colours and Chemicals L t d . ) w i t h 0.3 wt % copper. H y d r a t e d alumina catalyst was also used by R a y a son and Harkins and by Querido and Short. W e used it as a comparison for the silica-alumina pellets w h i c h represent an industrial-sized catalyst w i t h great resistance to attrition. Elemental copper was deposited on the respective supports by soaking the support i n cupric nitrate solution, drying i n nitrogen, heating at 300°F to reduce the cupric nitrate to cupric oxide, and reducing the cupric oxide i n hydrogen at 1000° F to give elemental copper. F i n a l reduction was done just before each run. Procedure. Seventeen separate experimental series were designed and conducted. The term series defines the collection of data points pertaining to various temperatures (500°-1100°K) for a single, fixed combination of the following variables: (1) Catalyst: alumina, silica-alumina, copper on alumina, or copper on silica-alumina 2

2

2


620

CHEMICAL REACTION ENGINEERING

II

(2) Contact time: 0.1, 0.2, 0.4, or 3.35 sec. (3) F e e d gas composition: v o l % of N , C 0 , C O , H , S 0 , a n d water vapor. T h e feed gas flow rate was fixed at 25 c m / s e c at standard temperature ( 0 ° C ) a n d pressure (1.0 atm) for a l l experiments. F o r each series, a fresh charge of catalyst, sufficient to give the required contact time, was placed i n the reactor unit. T h e reactor was heated, and the reactant gases were successively added i n proportions to make the total volumetric flow rate 25 c m / s e c . Results. T h e first three series were done to determine the effect of catalyst support. T h e gas composition d i d not change on going through the reactor at reactor temperatures of 560°, 755°, and 975°K. Thus, the silica-alumina support was inert for the reaction between S 0 and C O at contact times up to 3.5 sec. Series 4, 5, 5a, 5b, a n d 6 involved d r y gas mixtures a n d were designed to study the effectiveness of the catalyst supports for the active copper and the stability of the C u / S i - A l catalyst. These series were also done to see i f the deactivated catalyst could be regenerated i n a hydrogen atmosphere. T h e effects of water vapor on the efficiency of S 0 reduction w i t h C O were studied i n the remaining series. I n series 7, 8, a n d 9 the water vapor i n 2

2

2

2

3

3


2

2

9

*f

Ï S •

1 1Ι

0

1 ο

Ï

MOLECULAR SEVE: 80/100 m»sh, Grade 5 6'xL/8" solium at 12 35 ml/mi ι He carrier TC curre it at 200 mA

«

c O

y detector ρ >larity 2 3 4 MINUTES AFTER SAMPLE INJECTION PORAPAK OS ( 50:50 Q:S ): 80/100 πssh 6«xl/8« solium at 12 5°C 30 ml/ml ι He carrier TC curr« lit at 200 mi

8

3

5

s 9

i ί

UV J\

The Catalytic Conversion of Sulfur Dioxide in Wet Stack Gases to

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