Znd. Eng. Chem. Res. 1994,33, 1654-1656
1654
Decrease in Carbonyl Sulfide in the Feed to Claus Converters by Shift Catalysts T. A. Gens BOC Group Technical Center, 100 Mountain Avenue, Murray Hill, New Jersey 07974
Alumina or commercial shift catalysts were used downstream of a Claus burner to shift carbon monoxide to carbon dioxide before it could react t o form carbonyl sulfide. The shift catalysts were effective in preventing formation of carbonyl sulfide, thereby decreasing sulfur losses.
Introduction
3H,S
The principal reaction in the burner in a Claus plant is the combustion of part of the hydrogen sulfide to form sulfur dioxide, which can react with more hydrogen sulfide to form sulfur. The Claus catalytic converters downstream from the burner bring this sulfur-forming reaction close to equilibrium, where nearly quantitative recovery as condensed sulfur is possible. In practice, both the burner and converter effluents contain much carbonyl sulfide (COS) and carbon disulfide, which cannot be efficiently converted by the Claus catalysts and which must be removed by additional costly treatment. These compounds originate from homogeneous reactions of sulfur vapor with carbon dioxide or hydrocarbons at burner temperatures. The conventional treatment for removal of these compounds occurs downstream of the Claus converters and involvescatalytic reduction with hydrogen or reducing gas mixtures to convert the sulfur content to hydrogen sulfide, as in the Beavon sulfur removal process (Beavon and Fleck, 1975)and the SCOT process (Swaim, 1975). A principal route for making COS is the reaction of carbon monoxide and sulfur vapor below 500 "C (Weil, 1983). Luinstra and d'Haene (1989) observed that alumina at 1000 O C effectively decomposes carbonyl sulfide and carbon disulfide in the effluent gases from a Claus burner and upstream of the Claus converters. The nature of the decomposition reaction was not discussed. Laboratory results presented below indicate that alumina can catalyze the water-gas shift reaction at rates that are fast enough to actually prevent the formation of COS. Further work using shift catalysts not only supports this possibility but also suggests use of the shift catalysts to improve upon alumina in preventing formation of COS. Thermodynamics show that competing shift and COS formation are both favorable reactions, suggesting that the shift reaction could be encouraged by use of selective catalysts.
Experimental Section The reactor was a vertical stainless steel tube 0.4 in. i.d. by 1 f t long with a volume of 25 cm3. A heated porous alumina disc (Coor's, 100-pm pores, 40% porosity) of 114 in. diameter by 112 in. length in the bottom gas entrance served to mix and ignite a mixture of oxygen and feed gas containing hydrogen sulfide. This disc had been dipped in dilute potassium permanganate solution to aid ignition and combustion. The feed was an equimolar mixture of carbon dioxide and hydrogen sulfide with oxygen adjusted (at about 20 % ) for maximum conversion to sulfur: 3H,S
--
+ 1.50, 2H,S + SO, + H,O 2H,S + SO, 3 s + 2H,O
(1) (2)
1.50,
-+
3s
3H2O
(3)
where much of reaction 2, called the Claus reaction, occurs downstream of the burner and reaction 3 is the sum of 1 and 2. This equimolar carbon dioxide-hydrogen sulfide mixture in the feed, therefore, produced a product gas from the Claus converter, which, after condensation of sulfur, was very close to equimolar in carbon dioxide and water. The gas temperature in the reactor was measured by a thermocouple, sheathed in a sintered alumina tube and placed just above the porous aluminadisc. The reactor was wound with heating tape and insulation to decrease the rate of cooling. This produced a temperature drop between the combustion zone above the porous disc, at temperatures measured as high as 970 OC, and the reactor exit, where the effluent gas was approximately 300 OC. Runs were made with the reactor empty, except for the alumina disc, or filled either with small activated alumina spheres (Alcoa, surface area about 250 m2/g) or with shift catalysts. Two kinds of shift catalysts, both obtained from United Catalyst Company, were used: one was principally iron and chromium oxide and the other was principally cobalt and molybdenum oxide. In runs withshift catalysts, 5 g of alumina spheres, which would not sinter in the hot zone, was loaded into the hot lower part of the reactor and 22 g of shift catalyst was loaded above the alumina. A volume of about 4 cm3 remained empty above the shift catalyst. All runs were made at 6 atm absolute pressure. Pressure was needed to prepare the feed for a pressurized Claus reaction operated at 120 "C under water. Results with this novel Claus converter have been described elsewhere (Gens et al., 1989). Analyses were made on the product gas from the Claus converter, after sulfur was removed, using a Hewlett Packard 5880-A gas chromatograph equipped with a 13X molecular sieve column for the lighter nonpolar gases and an Altech Super Q column for other gases. Results ShowingThatAlumina Affects the WaterGas Reaction. The reactants and products of the watergas reaction CO + H,O
-
CO,
+ H,
(4)
are all present in a burner in a Claus plant, so this reaction might be expected to influence the final product composition. This reaction was investigated by measuring the hydrogen and carbon monoxide content of the product gases leaving the Claus converter. Alumina spheres in the reactor caused a large increase in the Hp/CO ratio, as compared to this ratio with the empty reactor (Figure 1, feed flow rate of 3.5 L/min). The increased ratio was observed at reaction temperatures exceeding 700 O C , measured just above the porous alumina disc, and the ratio increased with the disc temperature. The alumina apparently caused reaction 1 to be shifted
QSSS-5SS5/94/2633-1654$Q4.50/0 0 1994 American Chemical Society
Ind. Eng. Chem. Res., Vol. 33, No.7,1994
10
1655
5
12
.
..
."
m
0
am
7m
om
(UIO
400
1W
8W
6M)
TEMPERATURE(C)
rm
TEMPEMTURE IC1 ABOVE POROUS PLUG
-1
ILOMEO Wrm MUUNA (Wll
1 -
EuPp(0.n)
Figure I. Aluminacatalyzesthewatergass~reaction.Thereactor erittempraturewasabout300~C,wherethevolumeratioofHdC0 would be near 40 at equilibrium.
in the forwarddirection. Thisshiftislarge,corresponding to equilibrium in reaction 4 at a temperature not much more than 100 O C above the 300 "C gas exit temperature. (An equilibrium HdCO ratio of 10 would be expected in reaction 4 at 420 "C with equal water and earbon dioxide partial pressures). It was hypothesized that if reaction 4 could be shifted sufficiently, the supply of carbon monoxide, which normally forms in the burner from the reduction of carbon dioxide, might be depleted enough to depress the formation of undesirable contaminants, since carbon monoxidecan reactfurthertomakecabonylsulfide and possibly other carbon-containing sulfur compounds. Not only might this explain Luinstra and dHaene's observation but it would also suggest use of better shift catalysts for improved results, since alumina is not a particularly effective shift catalyst. Thermodynamics of Competing Reactions of Carbon Monoxide The free energy changesforcarbonmonoxidereactions, either with water in the water-gas reaction or with sulfur to form carbonyl sulfide, are both favorable below about 900"C, as shown in Figure 2 (free energy data taken from the JANAF Tables). This situation, where both expected reactions are favorable, is ideal for use of catalysts to try to accelerate the desired reaction, which is the water-gas reaction, in preference to the undesired reaction to form COS, as the gaseous effluent from the reaction furnace is cooled. Results Using Shift Catalysts Much lower COS concentrations were observed in the product gas from runs using shift catalysts than in runs using alumina catalyst or, especially, in runs with no catalyst in the reactor (Table 1). Either shift catalyst was effective in decreasing the COS concentration. Table 1 resultswithaluminawereaveragedfrom thedataofFigure 1, and similar data, not shown, were taken at the lower flow rate of 2.5 L/min. Other data in Table 1 are from individual runs. The addition of about I%,individually, of methane, ethane, ethylene, or methyl mercaptan to the feed had no detectable effect upon the product gas over shift catalysts
Figure 2. Reactions of CO with either sulfur or water M favored below 900 oc.
Table 1. Efleet of Catalysts on COS Content. vol % of each gas in the product stream
p flow
catalyst (L/min) none 1.9 none 3.5 none 5.0 none 6.3 All03
Altos CwMo CwMo
FeCr FeCr
2.5 3.5 2.5 3.5 2.5 3.5
HI CO 0.20 0.21
0.43 0.84 0.33 0.52 0.00 0.00
COS
HS SO*
0.21 0.22 0.81 1.03 0.03 0.06 0.01
3.12 2.36 6.07 6.35 0.31 0.29
0.01
0.13 0.13 0.17
0.42 0.73 0.02 0.07 0.15 0.05 0.28 0.89 0.53 0.01 0.48 0.00 0.03 0.01 1.00 0.00 0.01 0.00 0.01 0.00
0.07 0.00 0.08 0.01
0.08
max bed T(OC)
805
Figure 1 704
741 797 874
'Feed was 50%carbon dioxide and 50% hydrogen sulIide with oxygen added o needed to optimize sulfur removal. This produced aproductgaswhichwocloseto50% carbondioxideand 50%water after sulfur removal. The volume of the reactor was 25 ems.
(sulfur removals of > 99%), provided that the oxygen content of the feed was carefully readjusted to optimize sulfur removal. In commercial practice, the presence of hydroearbons can cause COS and CS2 concentrations to increase in theeffluent (Luinstra and dHaene, 1989).The absence of this effect here is credited to efficient mixing and combustion with the porous alumina disc. Very large heat losses on this small scale permitted operation below the alumina melting point, even at flow rates several times greater than used in this study. Conclusions a n d Comments
These resulta show that alumina, and especially shift catalysts, were affective in preventing build-up of COS in the effluent when hydrogen sulfide is combusted in the presence of carbon dioxide. The ability of these catalysts to preferentially catalyze the water-gas reaction during the rapid cooling that occurs downstream of the combustion reactions, thereby preventing the reaction of carbon monoxide and sulfur to form COS, is presented as the probable explanation. This explanation implies that prevention of COS formation may be an effective altemative to the conventional treatments for its removal subsequent to formation. High removals of sulfur compounds, well above 99%. were achieved using the shift catalysts. These high removals were, in part, due to the efficiency of the novel
1656 Ind. Eng. Chem. Res., Vol. 33, No. 7, 1994
Claus converter (Gens et al., 19891, but this converter is ineffective in converting COS. The avoidance of COS through the use of shift catalysts was required in order to realize the benefit of efficient conversion in the Claus reaction. Conversion by the Claus reaction appears to be less efficient, in the data of Table 1, for the streams containing larger COS concentrations. This is probably not actually the case; rather, the larger H2S and SO2 concentrations in these streams probably originate from hydrolysis of COS downstream of the Claus converter, possibly even in the analysis train.
Literature Cited Beavon,D. K.; Fleck, R. N. Beavon Sulfur Removal Process for Claus Plant Tail Gas; Pfeiffer, J. B., Ed.; Advances in Chemistry 139; American Chemical Society: Washington, DC, 1975; pp 93-99.
Gens, T.; Tucker, M.; Grob, J. Process for the Production of Sulfur from Hydrogen Sulfide Using High Concentration Oxygen and Recycle in Combination with a Scrubbing Tower. U.S.Patent 4,844,881, July 4, 1989. Luinstra, E.; d"aene, P. Catalyst Added to Claw Furnace Reduces Sulfur Losses. Hydrocarbon Process. July 1989, 53-57. Swaim, C. D. The Shell Clam Ojjgas Treating (SCOT) Process; Pfeiffer, J. B., Ed.; Advances in Chemistry 139;American Chemical Society: Washington, DC, 1975; pp 111-119. Weil, E. D. Sulfur Compounds. In Kirk-Othrner Encyclopedia of Chemical Technology, 3rd ed.; Grayson, M., Ed.; John Wiley & Sons: New York, 1983; Vol. 22, p 109.
Received for review December 20, 1993 Revised manuscript received March 31, 1994 Accepted April 29, 1994. Abstract published in Advance ACS Abstracts, June 1,1994.
