Ind. Eng. Chem. Res. 1996, 35, 2389-2394
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Characterization of Reaction between ZnO and COS Eiji Sasaoka* Faculty of Health and Welfare Science, Okayama Prefectural University, Kuboki-111, Soja, Okayama 719-11, Japan
Kazuo Taniguchi, Md. Azhar Uddin, Shigeru Hirano, Shigeaki Kasaoka, and Yusaku Sakata Faculty of Engineering, Okayama University, Tsushima-naka, Okayama 700, Japan
In order to understand the behavior of COS in a ZnO desulfurization reactor, the reaction between ZnO and COS was studied in the presence of gases which compose a coal-derived gas. The behavior of COS in the reaction zone of a ZnO packed bed can be predicted as follows: H2S in coal-derived gas reacts more easily with ZnO than COS; most of COS is converted to H2S by catalytic hydrolysis and then reacts with ZnO, although a part of COS may react directly with ZnO; H2 accelerates the conversion of COS to H2S; the water-gas shift reaction accelerates the reaction between ZnO and COS; and CO2 does not affect the reaction. Introduction
controlled by the following reaction:
Solid-oxide fuel cells and molten-carbonate fuel cells, new technologies using coal-derived gas, are receiving attention from thermal efficiency and/or environmental points of view because their high efficiency reduces CO2 emission per unit of electric power without SOx emission. To establish these highly efficient processes, it is necessary to develop a high-temperature process for the desulfurization of coal-derived fuel gas. The highly efficient removal of sulfur compounds from several thousand ppm down to ca. 1 ppm has been reported (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 to 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; Sakurai et al., 1994). In particular, a number of studies of desulfurization using zinc oxide stabilized as zinc titanate and zinc ferrite have been reported. In these reports, the practical application of sorbents containing ZnO to the removal of H2S has been studied, but the reaction between ZnO and COS has not been clarified. Although a study of the reaction between MnO and COS was found (Wakker et al., 1993), the authors were unable to locate previous studies of the reaction between ZnO and COS. From previous papers (Wakker et al., 1993; Akimoto and Dalla Lana, 1980), it is thought that the catalytic hydrogenation of COS to H2S, the catalytic hydrolysis of COS, and the water-gas shift reaction will occur in addition to the gas-solid reaction of ZnO and COS. Therefore, to understand the behavior of COS in a desulfurization reactor, the catalytic activity of ZnO and ZnS (formed from ZnO) has to be clarified. We studied the catalytic activity of ZnS formed from ZnO in order to understand the behavior of COS in the ZnS zone of a packed-bed desulfurization reactor (Sasaoka et al., 1995). It was found that the ZnS formed from ZnO was active for the conversion of COS to H2S; if the contact time of COS with ZnS was sufficient, the equilibrium of the conversion of COS to H2S was S0888-5885(95)00370-8 CCC: $12.00
2COS + H2 + H2O S 2H2S + CO + CO2 This experimental work focuses on the characterization of the reaction between ZnO and COS in the presence of coal-derived gases. Experimental Section Preparation of Zinc Oxide. Zinc oxide was prepared by a precipitation method using a 20% aqueous solution of Zn(NO3)2 and a 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 STP) from room temperature to 800 °C (10 °C/min, total 3 h). The product thus obtained was crushed and sieved to 0.7 and 1.0 mm. In this study, two batches of ZnO prepared by the above method were used because the amount of catalyst prepared from one batch was not enough to test the characteristics of the reaction. Experimental results obtained from the two batches were somewhat inconsistent. This will be discussed later. The BET surface area of the sample ZnO of the first batch (ZnO(a)) was 3.2 m2/g and its bulk density was 0.84 g/cm3; the BET surface area and bulk density of the second batch ZnO(b) were 3.5 m2/g and 0.85 g/cm3, respectively. Apparatus and Procedure. The sulfidation 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 of 1.5 cm i.d., in which 0.5 mL of sorbent was packed. The main compounds in coal-derived gases are H2, CO, CO2, H2O, N2, H2S, and COS. There are large differences in gas composition depending on the type of gasifier, the type of coal used, and the kind of gasification reagent. The concentration of H2 varies from 29 to 40%; CO, from 16 to 65%; CO2, from 1 to 31%; and H2O, from 1 to 20%. In a paper published recently (Sakurai et al., 1994), a mixture of H2S (1%), H2 (30%), © 1996 American Chemical Society
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Ind. Eng. Chem. Res., Vol. 35, No. 7, 1996
Figure 1. Reactivity of ZnO with COS in the presence of H2S, H2, CO, CO2, and H2O. Sample, 0.7 mm. Reaction conditions: 520 ppm COS, 1100 ppm H2S, 20% H2, 30% CO, 10% CO2, 9.7% H2O, N2, 500 °C.
CO (40%), H2O (10%), CO2 (10%), and N2 was used as a simulated coal-derived gas for molten carbonate fuel cells. In these experiments, a mixture of COS (520 ppm), H2S (1100 ppm), H2 (20%), CO (30%), H2O (9.7%), CO2 (10%), and N2 was fed into the reactor at 200 cm3/min at STP. The space velocity, SV, was 2.4 × 104 h-1. H2S, COS, CO2, and CO concentrations of inlet and outlet gases were measured using GC equipped with TCD (column packing: Porapak QS + PorapakT for H2S, COS, CO2; Active carbon for CO). Results and Discussion Reaction between ZnO and COS in a Simulated Coal-Derived Gas. The reactivity of ZnO was examined in the presence of the above gas mixture using ZnO(a) and ZnO(b). As shown in Figure 1, COS and H2S simultaneously decreased on ZnO. The production of elemental sulfur and the vaporization of Zn were not observed. The fractional decrease of COS or H2S was calculated using the following equation:
fractional decrease of COS (or H2S) ) [outlet concentration of COS (or H2S)] (1) 1[inlet concentration of COS (or H2S)] Apparently, H2S reacted more easily with ZnO than COS. The fractional conversions of ZnO to ZnS (from start to 5 h) were calculated as ca. 0.57 [ZnO(a)] and ca. 0.50 [ZnO(b)] from the data in Figure 1. The activity dependence on time on stream was similar for the two samples. Samples a and b were identical within the author’s ability to reproduce preparation and analytical testing (Sakurai et al., 1993). From the results shown in Figure 1, it can be concluded that COS and H2S are simultaneously removed in the presence of the coal gases, but it is difficult to compare the reactivity of COS and H2S with ZnO because of the existence of H2S: COS may directly react
Figure 2. Reactivity of ZnO with COS. Sample, 0.7 mm. Reaction conditions: 520 ppm COS, N2, 500 °C.
with ZnO but may also be converted to H2S, and a part of the H2S formed from COS may escape from the sorbent without being trapped by the sorbent; if H2S forms from COS, it cannot be distinguished from the H2S feed. Therefore, to understand the behavior of COS in the packed bed, it is necessary to examine the catalytic hydrogenation of COS to H2S and the hydrolysis of COS to H2S (Yumura and Furimsky, 1985; Sasaoka et al., 1994b) in addition to the gas-solid reaction of ZnO and COS in the absence of H2S. Reaction between ZnO and COS. Initially, the reaction between ZnO and COS in the COS-N2 system (the gas-solid reaction) was examined at 500 °C using the two samples of ZnO. As shown in Figure 2, ZnO reacted with COS and produced CO2. The fractional formation of CO2 was calculated using the following equation:
fractional formation of CO2 ) [outlet concentration of CO2] [inlet concentration of COS]
(2)
In both cases, a yellowish material was deposited as a thin film on the inside wall downstream from the reactor tube, a part of the reactor which was cooled by the atmosphere. This material could not be physically removed from the wall because the amount of material was too small. Solid-state byproducts which can be predicted in this system are elemental sulfur and metal zinc. Therefore, the reactor was taken off from the reactor system and dipped in HCl (aq). The material was insoluble in the solution. When the reactor part was heated, the material melted and changed color from yellow to dark reddish black (brown) and smelled of elemental sulfur. From these results, it was confirmed that elemental sulfur was produced as a byproduct in both experiments. This is similar to the results reported in the case of the reaction between ZnO and H2S (Yumura and Furimsky, 1985; Sasaoka et al., 1994b). The production of elemental sulfur suggests that the catalytic decomposition of COS proceeds over ZnO.
Ind. Eng. Chem. Res., Vol. 35, No. 7, 1996 2391
Figure 3. Reactivity of ZnO with COS and H2S. Sample, for COS: ZnO (a, 0.7 mm); for H2S: ZnO (a, 1.0 mm). Reaction conditions: 520 ppm COS or 500 ppm H2S, N2, 500 °C.
Figure 4. Reactivity of ZnO with COS in the presence of H2. Sample, ZnO (a, 0.7 mm). Reaction conditions: 520 ppm COS, 20% H2, N2, 500 °C.
CO2 was observed in outlet gases from the reactor, as shown in Figure 2, but only traces (
