Znd. E n g . Chem. Res. 1995,34, 1102-1106
1102
Catalytic Activity of ZnS Formed from Desulfurization Sorbent ZnO for Conversion of COS to H2S Eiji Sasaoka" Faculty of Health and Welfare Science, Okayama Prefectural University, Kuboki-111, Soja, Okayama 719-11, Japan
Kazuo Taniguchi, Shigeru Hirano, Md. Azhar Uddin,Shigeaki Kasaoka, and Yusaku Sakata Faculty of Engineering, Okayama University, Tsushima-naka, Okayama 700, Japan
In order to understand the behavior of COS in the ZnS zone of a packed bed high-temperature desulfurization reactor, the catalytic character of ZnS for the conversion of COS to H2S in the presence of gases which are composed of coal-derived gas was studied. ZnS was active for the catalytic hydrolysis of COS in simulated coal-derived gases; ZnS was also active for the hydrogenation of COS to H2S, but the activity was considerably lower than that for the hydrolysis of COS. In simulated coal-derived gases, ZnS was also active for the reverse reaction; H2S was converted to COS over ZnS. The water gas shift reaction did not proceed in parallel with hydrolysis of COS over ZnS. From the results of this work, it is suggested that if the contact time of COS with ZnS was sufficient, the equilibrium of the conversion of COS to H2S was 2HzS CO COZ. controlled by the following reaction: 2COS H2 H2O
+ +
Introduction Solid oxide fuel cells and molten carbonate fuel cells as new technologies using coal-derived gas are receiving attention from thermal efficiency and/or environmental points of view as their high efficiency contributes to an abatement of C02 emission per unit electric power without SO2 emission. To establish these highly efficient processes, the development of a high-temperature process for the desulfurization of coal-derived fuel gas is an inevitable problem: for these processes, highly efficient sulfur compound removal from several thousand ppm down to ca. 1ppm is claimed (Lee, 1987; Lew et al., 1989;Minth, 1991). From the standpoint of desulfurization efficiency, zinc oxide is the most attractive among the sorbents reported because of its favorable sulfidation thermodynamics (Schrodt et al., 1975; Westmoreland and Harrison, 1976). For this reason, current practical research seems t o be concentrated on zinc oxide (Focht et al., 1988; Gangwal et al., 1989; Lew et al., 1989; Sa et al., 1989; Jothimurugesan and Harrison, 1990;Woods et al., 1990; Ayala et al., 1991; Silaban et al., 1991; Lew et al., 1992). In particular, a number of studies on desulfurization using zinc oxide stabilized by Ti02 and zinc ferrite have been reported. A number of reports of the practical application of sorbents containing ZnO to the removal of H2S can be found as described above. After HzS, COS is the main sulfur compound fmnd in coal-derived gases, and the removal of COS using MnO and FeO has been reported recently (Wakker et al., 1993). However, the removal of COS by ZnO has not been sufficiently clarified. Furthermore, the conversion of COS t o H2S over ZnS formed from ZnO has not been sufficiently clarified. When a fmed bed reactor or moving bed reactor is used for the removal of sulfur compounds in a coalderived gas, the gas has to flow through a zone of ZnS formed from ZnO by the reaction between ZnO and the sulfur compound. Therefore, if ZnS has some activity as a catalyst for COS conversion to H2S, COS in the
-
+
+
coal-derived gas will be converted to H2S by the catalytic reaction over ZnS before contact with ZnO. If most of the COS is easily converted to H2S by ZnS, it can be concluded that the reaction between ZnO and H2S is considerably more important for the desulfurization than the direct reaction between ZnO and COS. This work focuses on evaluating the catalytic activity of ZnS formed from ZnO for the conversion of COS t o H2S. Information obtained from this work is also useful for understanding the role of ZnS in partially sulfurized ZnO for conversion of COS to H2S. A number of studies on the catalytic conversion of COS to H2S have been done in the course of developing a process for sulfur recovery from sour gas; the conversion of COS to H2S is an important side reaction in the Claus process. In these studies, cobalt-molybdate on alumina catalysts (George, 1974a,b) and alumina catalysts (Akimoto and Dalla Lana, 1980) were used as catalysts. As far as we know, no previous studies of the catalytic reaction over ZnS have been carried out.
Experimental Section Preparation of Zinc Oxide. Zinc oxide was prepared by precipitation using a 20% aqueous Zn(NO& solution and 14 wt % aqueous NaOH solution (containing 10% excess of the theoretical amount of NaOH required for precipitation). The precipitation was carried out by adding the raw salt solution to the NaOH aqueous solution under vigorous mixing at room temperature. The product of the precipitation was washed, separated by filtration, dried at 110 "C for 25 h, and then calcined in an air stream (300 cm3/min at normal temperature pressure) from room temperature to 800 "C (10 "C/min, total 3 h). The product thus obtained was crushed and sieved to -0.85/+0.59 mm and -1.W +0.85 mm. ZnS was prepared from the ZnO by treatment with a gas mixture of H2S (0.2%)and N2 (at 500 "C for 14 h). In the pattern of XRD (X-ray diffraction) of the ZnS (not illustrated), clear ZnS peaks were observed and peaks for ZnO were not observed. The surface area and bulk density of the ZnS were 5.0 m2/g and 0.97 g/cm3,respectively.
0888-588519512634-1102$09.0010 0 1995 American Chemical Society
Ind. Eng. Chem. Res., Vol. 34,No. 4, 1995 1103 l I* OI - l 0.8.
I
0.41
0.6.
0 : cos 0 : H2S
cos
0: 0 :H2S
1
2
3
Time on stream / h Figure 1. Catalytic activity of ZnS for the conversion of COS to HzS. Catalyst particle size: -0.85/+0.59 mm. Reaction conditions: 520 ppm COS, 20% H2,30% CO, 10% C02,9.7% H20, Nz, 500 "C.
Apparatus and Procedure. The catalytic reaction experiments were carried out using a flow-type packed bed tubular reactor system under atmospheric pressure at 500 "C. The microreactor consisted of a quartz tube, 1.3 cm i.d., in which 0.5 mL of sorbent was packed. In these experiments, a mixture of COS (520 ppm), H2S (500 and 1100 ppm), Ha (20and 22%), CO (30 and 33%), H2O (4.7,9.7, and 18.1%),COZ(4 and lo%), and balance N2 was fed into the reactor at 200 cm3/min at NTP (normal temperature and pressure). The gas hourly space velocity (the volumetric flow rate of the gas introduced into the catalyst a t NTP divided by the volume of the catalyst) was 2.4 x lo4 h-l. H2S, COS, COS, and CO concentrations of inlet and outlet gases were measured using GC (equipped with a thermal conductivity detector and a flame ionization detector). Results and Discussion Catalytic Activity of ZnS for COS Conversion. Initially, the catalytic activity and stability of the ZnS (-0.85/+0.59 mm diameter) was examined under a system made up of 520 ppm COS, 20% H2, 30% CO, -9.7% H20, 10% C02, and balance N2 at 500 "C. The composition of a coal-derived gas can be affected by the specific type of coal, the gasification condition, or the feed method of coal (dry feed, slurry feed). Therefore, it was difficult to determine a typical composition for coal-derived gas. The gas composition used in this experiment was set so as to hopefully present an average gas composition of coal-derived gases from previous papers (Wakker et al., 1993; Sakurai et al., 1994). As shown in Figure 1, the amount of COS converted was almost the same as the amount of H2S produced. Since there is sulfur mass balance and the rate of H2S formation was constant during the experiment, it can be concluded that COS is catalytically converted to H2S in this system; ZnS was active for the catalytic conversion of COS t o H2S. Catalytic Activity of ZnS for hydrogenation of COS to H2S. COS can be converted t o H2S by both hydrogenation and hydrolysis (Wakker et al., 1993).The hydrogenation: COS hydrolysis: COS
+ H,
+ H20
-
-
+ CO
(1)
+ C02
(2)
H2S
H2S
catalytic character of ZnS for the hydrogenation of COS
Time on stream / h Figure 2. Catalytic activity of ZnS for the hydrogenation of COS. Catalyst particle size: -0.85/+0.59 mm. Reaction conditions: 520 ppm COS, 20% Hz, 30% CO, Nz, 500 "C.
Time on stream / h Figure 3. Catalytic activity of ZnS for the hydrolysis of COS to HzS in the presence of COz. Catalyst particle size: -0.85/+0.59 mm. Reaction conditions: 520 ppm COS, 10%COz, 9.7% H20, Nz, 500 "C.
in the COS/H2/CO/N2 system was initially examined. As shown in Figure 2,only a little COS was converted to H2S in the system. From this result, it was concluded that the conversion of COS to H2S in the system of Figure 1 cannot be explained by the catalytic hydrogenation of COS to H2S over ZnS. Catalytic Activity of ZnS for the Hydrolysis of COS. From the above results, it is suggested that the conversion of COS to H2S in the simulated coal-derived gas (Figure 1) may be explained by the hydrolysis of COS (eq 2). Therefore, the catalytic activity of ZnS for the hydrolysis of COS was examined in the COS/COZ/ H20/N2 system. As shown in Figure 3, ZnS was active for the hydrolysis of COS and its activity was stable. The formation of H2S almost balanced with the decrease of COS a t the end of the experiment; however, in the early stages of the experiment, H2S formation did not balance with the decrease of COS. The fractional formation of H2S is the same as that in Figure 1. This confirms that the conversion of COS to H2S in the H2/ CO/H2O/C02/N2 system is mostly by hydrolysis over ZnS. Hydrolysis of COS over ZnS was also examined in the H20/N2 system. As shown in Figure 4, it was confirmed that the formation of H2S balanced with the formation of C02, but the formation of H2S was slightly smaller than the decrease of COS. A plausible explanation is the formation of a small amount of CS2 via the following reaction:
1104 Ind. Eng. Chem. Res., Vol. 34,No. 4,1995
1.0
1
I
0.6 0.4
-
0.2
-
.
30
-
............................................................................. i
Eq. 1
,
J
h
0:cos
.
0 :HZS
-
A:CO,
.
-U
CI
0)
1
ES.2
..............................................................................
J
1 2 3 Time on stream / h Figure 4. Catalytic activity of ZnS for the hydrolysis of COS. Catalyst particle size: -0.85/+0.59 mm. Reaction conditions: 520 ppm COS, 9.7% H20, N2, 500 "C. Ob
1.0 I
1
0 : cos 0 : HZS
1
2
3
4 5 Inlet Gas
6
7
Figure 5. Effects of coexistence gases on the conversion of COS
to H2S over ZnO in the presence of H2O. Catalyst particle size: -0.85/+0.59 mm. Inlet gas: (1)520 ppm COS, 9.7% HzO, Nz; (2) 520 ppm COS, 20% H2,9.7% H20, NO;(3) 520 ppm COS, 30% CO, 9.7% H20, Nz; (4) 520 ppm COS, 10% COZ,9.7% HzO, N2; (5) 520 ppm COS, 20% H2, 30% CO, 10%.C02,9.7% H20, N2; (6)520 ppm COS, 1100 ppm HzS, 20% H2,30% CO, 10% C02,9.7% HzO, N2; (7) 520 ppm COS, 1100 ppm HzS, 9.7% HzO, N2. Reaction temperature: 500 "C.
+
H2S COS
-
CS2
+ H20
(3)
Unfortunately, CS2 was not looked for. The effect of the presence of gases which compose coalderived gas on COS hydrolysis was examined. As shown in Figure 5 (the steady state data are shown), the presence of Ha, CO, or C02 scarcely affects the formation of H2S from COS, although the fractional decrease of COS in these system is not exactly equal. In the COS/CO/H2O/N2 system in Figure 5, the amount of C02 produced almost balanced with the amount of COS reacted (not illustrated). From this result, it was confirmed that the water gas shift reaction did not proceed in parallel with hydrolysis of COS over ZnS. From these results, it is concluded that hydrolysis of COS over ZnS is the most important reaction for the conversion of COS to H2S in coal-derived gas. The effect of the presence of H2S on the conversion of COS was examined. As shown in Figure 5, H2S considerably inhibited the hydrolysis of COS in the presence of Hz, CO, and COZ. Furthermore, HzS inhibited hydrolysis in the absence of those gases. In the presence of H2S, the amount of H2S in the feed gas is considerably larger than the amount of H2S produced
from COS; therefore, only the fractional decrease of COS is illustrated in Figure 5. The inhibition of the conversion of COS by H2S may be explained by the reverse reactions of eqs 1 and 2; in the presence of H2S, the reaction of eq 1can be limited by the equilibrium of the reverse reactions of eqs 1and 2, or its rate can be inhibited due t o strong adsorption of H2S on the surface of the ZnS catalyst. Since the presence of H2, CO, and C02 does not inhibit the conversion of COS in the presence of H20 and H2S (Figure 5), the COS to H2S reaction under these conditions does not appear to be limited by the equilibrium. If the reaction of eq 2 contributes t o the inhibition of the conversion of COS, the presence of CO2 should have inhibited the conversion of COS. If the reaction described in eq l is a contributory factor, the presence of CO and Ha (mole ratio of CO/H2: 1.5) should also inhibit the conversion of COS. The inhibition due to a high concentration of H2S appears to be due to an increase of the adsorption term in a typical catalytic rate equation of the type rate = (kinetic term)(driving force)/ (adsorption term). To further clarify the inhibition by HzS, additional kinetic experiments and/or a more detailed analysis of the surface of the catalyst sample is needed. Catalytic Activity of ZnS for the Conversion of H2S to COS. As ZnS was active for the hydrolysis of COS, the conversion of H2S t o COS over ZnS (-1.M +0.85 mm diameter) was examined. In this experiment, two inlet gases were used. One of them is a simulated coal-derived gas produced from gasification operated by dry feed of coal: 500 ppm H2S, 22% H2,50% CO, 4% CO2,4.7% HzO, and balance N2. The other is a simulated coal-derived gas from gasification operated by slurry feed: 500 ppm HzS, 22%H2,33% CO, 10% COz, 18.1% H20, and balance Na. These detailed values for the gas composition were determined up to the limitations of the experimental apparatus. As shown in Figures 6 and 7, ZnS was active for the conversion of H2S to COS, and its activity was stable, although the conversion was very low. From a comparison of Figures 6 and 7, it is confirmed that the conversion proceeds more easily in the dry feed gas than in the slurry feed gas. This result is consistent with the thermodynamic properties of these reactions. As the conversion of the reaction was very low, it is possible that the conversion might be limited by thermodynamic equilibrium. Therefore, the equilibrium
Ind. Eng. Chem. Res., Vol. 34, No. 4, 1995 1106
E
P
a
20 ......................................................................
Eq. 2
Eq. 1
C 0-
0 m
l
$8j ZL. c.l
r3'm
w"
1..................................................................... 1
=
Eq.2
0.5 1.o 1.5 Time on stream / h Figure 7. Catalytic activity of ZnS for the reverse reaction of the conversion of COS to H2S under a simulated coal-derived gas (slurry feed). Catalyst particle size: -1.18/+0.85 mm. Reaction conditions: 500 ppm H2S, 22% H2,33% CO, 10%COz, 18.1%HzO, Nz, 500 "C. -
"0
Eq. 2
0.98 0.96
-
I
0.941
t
0*92 0.90' 400
Eq.4
0.90
1
1
600
700
Temperature I 'C
Figure 8. Equilibrium conversion for reactions 1, 2, and 4.Feed gas composition: 500 ppm COS, 22% Hz, 50% CO, 4% Cop, 4.7% HzO, Nz (dry feed coal-derived gas).
concentrations of COS for eqs 1and 2 and the following reaction (eq 4) under the experimental conditions were calculated considering the thermodynamic properties of the composition gases (Nagamawari et al., 1972). Equa2HzS
+ CO + CO,
-
2COS
+ H2 + H 2 0
600
I 700
As a fundamental study of zinc oxide high-temperature desulfurization sorbent, this work focuses on the catalytic character of ZnS for the conversion of COS to H2S. COS was catalytically hydrolyzed to H2S over ZnS, and COS was also catalytically hydrogenated t o H2S, but the rate of hydrogenation was considerably lower than that of the hydrolysis. The water gas shift reaction did not proceed in parallel with the hydrolysis of COS over ZnS. HzO played the most important role in the conversion of COS to HzS. If the residence time is long enough, it is concluded that the concentration of COS is controlled by the equilibrium of reaction 4. Generally speaking, a packed bed or moving bed reactor is used in high-temperature desulfurization processes. The outlet gas composition from the zone in which ZnO has been converted to ZnS is not the same as that of the inlet gas since, if ZnS is catalytically active, the inlet gas composition will be changed by ZnS formed from ZnO. From this study of the COS/H2S system, it is suggested that the COS/HzS ratio of the outlet gas from the ZnS zone is determined by the equilibrium reaction of eq 4 when the residence time of the gas in the ZnS zone is long enough. As the inlet gas to the reaction zone of the reactor (a mixture of ZnO and ZnS) is the outlet gas of the ZnS zone, the composition of the inlet gas is controlled by the equilibrium reaction of eq 4. In the case of the packed bed reactor, the reactor volume is designed to allow sufficient time before the breakthrough of sulfur compounds. Therefore, it is possible that the residence time of the gas in the ZnS zone is long enough for the reaction of eq 4 to reach equilibrium, except during the initial startup stage. In order to develop a highly active ZnO desulfurization sorbent, the reactivity of the sorbent with H2S is the most important factor. High reactivity will allow HzS to be removed in a shallow zone and thus enable COS t o be converted to H2S which will be adsorbed slightly downstream.
1 500
500
Temperature / "C Figure 9. Equilibrium conversion for reactions 1 , 2 , and 4.Feed gas composition: 500 ppm HzS,22% H2,33% CO, 10% COz, 18.1% HzO, Nz (slurry feed coal-derived gas).
0.92
400
Eq.4
Conclusion
-: 1-
1.00.
O
0.98-
(4)
tion 4 is the combined reaction of eq 1and 2. As shown in Figures 6 and 7, in both experimental cases, the data are located between the equilibrium concentration of COS for eqs 1 and 4; the experimental data exceeded the equilibrium concentration of COS for eq 1. It is suggested that initially, H2S is converted to COS by the reverse reaction of eq 2, and the COS thus formed is converted t o H2S by H2O according to the reaction of eq 1. It is concluded that if the residence time is long enough, the concentration of COS will be determined by the equilibrium of eq 4. For the prediction of the conversion of COS to HzS in the temperature straddling 500 "C, the equilibrium conversions for the reactions of eqs 1, 2, and 4 were calculated. As shown in Figures 8 and 9, for both the dry and slurry feed coal-derived gas, lower reaction temperatures are more favorable t o the forward reactions of eqs 2 and 4. Conversely, lower reaction temperatures are more favorable to the reverse reaction of eq 1. The slurry feed coal-derived gas was more favorable to the forward reactions of eqs 1,2, and 4 than the dry feed coal-derived gas.
Acknowledgment We gratefully acknowledge that this work was supported by the Ministry of Education, Science and Culture, Japan through the Grant in Aid for Scientific Research No. 04203111.
1106 Ind. Eng. Chem. Res., Vol. 34,No.4,1995
Literature Cited Akimoto, M.; Dalla Lana, I. G. Role of Reduction Sites in VaporPhase Hydrolysis of Carbonyl Sulfide over Alumina Catalysts. J. Catul. 1980,62,84-93. Ayala, R. E.; Marsh, D. W. Characterization and Long-Range Reactivity of Zinc Ferrite in High-Temperature Desulfurization Process. Ind. Eng. Chem. Res. 1991,30,55-60. Focht, G. D.; Ranade, P. V.; Harrison, D. P. High-Temperature Desulfurization Using Zinc Ferrite: Reduction and Sulfidation Kinetics. Chem. Eng. Sci. 1988,43,3005-3013. Gangwal, S. K.; Harkins, S. M.; Woods, M. C.; Jain, S. C.; Bossart, S. J. Bench-Scale Testing of High Temperature Desulfurization Sorbents. Environ. Prog. 1989,8, 265-269. George, Z. M. Kinetics of Cobalt-Molybdate-catalyzed Reaction of SO2 with H2S and COS and the Hydrolysis of COS. J . Cutul. 1974a,32,261-271. George, Z. M. Effect of Catalyst Basicity for COS-SO2 and COS Hydrolysis Reaction. J . Cutal. 1974b,35,218-224. Jothimurugesan, K.;Harrison, D. P. Reaction between H2S and Zinc Oxide-Titanium Oxide Sorbent. 2. Single-pellet Sulfidation Modeling. Ind. Eng. Chem. Res. 1990,29,1167-1172. Lee, A. L. Internal Reforming Development for Solid Oxide Fuel Cells - Final Report; Department of Energy Report MCl2200452364; U S . Government Printing Office: Washington, DC, 1987. Lew, S.; Jothimurugesan, K.; Flytzani-Stephanopoulos, M. HighTemperature H2S Removal from Fuel Gases by Regenerable Zinc Oxide-Titanium Dioxide Sorbents. Ind. Eng. Chem. Res. 1989,28,535-541. Lew, S.; Sarofim, A. F.; Flytzani-Stephanopoulos, M. Sulfidation of Zinc Titanate and Zinc Oxide Solids. Ind. Eng. Chem. Res. 1992,31,1890-1899. Minth, N. Q. High-Temperature Fuel Cells. CHEMTECH 1991, 120-126.
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Sa, L. N.; Focht, G. D.; Ranade, P. V.; Harrison, D. P. Modeling High Temperature Desulfurization in a Fixed-Bed Reactor. Chem. Eng. Sci. 1989,44,215-224. Sakurai, T.; Okamoto, M.; Miyazaki, H.; Nakao, K. High-Temperature Desulfurization Performance and Durability of Fine ZincIron-Aluminum Oxide. Kaguku Kogaku Ronbunshu 1994,20, 275-282.
Schrodt, J. T.; Hilton, G. B.; Rogge, C. A. High-temperature Desulfurization of Low-CV Fuel Gas. Fuel. 1975,54,269-272. Silaban, A.; Harrison, D. P.; Berggren, M. H.; Jha, M. C. The Reactivity and Durability of Zinc Ferrite High Temperature Desulfurization Sorbents. Chem. Eng. Commun. 1991,107,555 71.
Wakker, J. P.; Gerritsen, A. W.; Moulijin, J. A. High Temperature H2S and COS Removal with MnO and FeO on y - A l 2 0 3 Acceptors. Znd. Eng. Chem. Res. 1993,32,139-149. Westmoreland, P.R.; Harrison, D. P. Evaluation of Candidate Solids for High-Temperature Desulfurization of Low-Btu Gases. Environ. Sci. Technol. 1976,10, 659-661. Woods, M. C.; Gangwal, S. K.; Jothimurugesan, K.; Harrison, D. P. Reaction between HzS and Zinc Oxide-Titanium Oxide Sorbent. 1.Single-pellet Kinetic Studies. Ind. Eng. Chem. Res. 1990,29,1160-1167.
Received for review August 15, 1994 Revised manuscript received January 9, 1995 Accepted January 20, 1995@ IE940491U ~
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~~
Abstract published in Advance ACS Abstracts, March 15, 1995. @
