Ind. Eng. Chem. Res. 1992, 31,415-419 Vaska, L. Dioxygen metal complexes: Towards a unified view. Acc. Chem. Res. 1976,9, 175-183. Vogt, L. H.; Faigenbaum, H. M.; Wdberley, S. E. Synthetic reversible O2 carrying chelates. Chem. Reu. 1963,63, 269-277. Wilmarth, W. K.; Aranoff,S.; Calvin, M. The O2 carrying synthetic chelate compounds. I11 Cycling properties and 0 2 production.
415
J . Am. Chem. SOC. 1946,68, 2263-2266. Wolff, F.; Eyre, D. V.; Grenier, M. O2 plants: 10 years of development and operation. Chem. Eng. Prog. 1979, 7,83-86. Received for review August 12, 1991 Accepted August 28,1991
Simultaneous Removal of COS and H2S from Coke Oven Gas at Low Temperature by Use of an Iron Oxide Kouichi Miura,*" Kazuhiro Mae,?Tomohiko Inoue,?Tomoyuki Yoshimi,? Hiroyuki Nakagawa,?and Kenji Hashimoto! Research Laboratory of Carbonaceous Resources Conversion Technology and Department of Chemical Engineering, Kyoto University, Kyoto 606, Japan
A new method to remove a trace of COS from coke oven gas was developed. This method utilizes a special form of iron oxides prepared by partially dehydrating a-FeOOH at around 200 "C. We found that the iron oxide hydrolyzes COS to form H2S, even at 40-60 O C . The produced H2S is stabilized as iron sulfides on the iron oxide. Thus we could completely remove COS from coke oven gas by use of only the iron oxide. Furthermore, we clarified that this can be used to remove COS and H2S simultaneously from the coke oven gas at W O "C, although conventional processes employ separate processes for COS and H3 removal. The mechanisms of the COS and H2Sremoval reactions were also examined in detail by investigating the change of the properties of a-FeOOH during ita dehydration and by analyzing the deactivated catalysts by the X-ray photospectroscopy (XPS). Although the proposed method was developed for desulfurizing coke oven gas, it is applicable in purifying crude gas derived from partial oxidation of sulfur-bearing fossil fuels.
Introduction In Japan a part of the coke oven gas is utilized as town gas. The coke oven gas contains sulfur compounds, such as HzS, COS, and CSz, on the order of around 100 ppm. From the viewpoint of environment protection, the removal of such sulfur compounds is essential. To do so, steel-making companies intend to develop dry methods which are applicable at rather low temperatures from the energy-saving viewpoint. HZS, which is the main inorganic sulfur compound, is easily removed by iron oxides, even at room temperature, but the removal of the organic sulfur compounds, COS and CSz, is rather difficult, especially at low temperatures. The presence of COS together with the acid gases HzS and COPin raw gas from partial oxidation of coal, heavy oil, or other sulfur-bearing carbonaceous materials has been a significant constraint in utilizing the gas effectively (Dunlap and Galstaun, 1982). Then the development of the method to remove COS and HzSfrom COz-containing gas is important from this aspect also. Several dry methods to remove COS have been presented. The so-called base catalysts, such as 7-A1203and ZnO, are known to hydrolyze COS to form H2S at above 120 "C (Namba and Shiba, 1968; Chan and Dalla Lana, 1978; Akimoto and Dalla Lana, 1979). Recently a Japanese patent '(Nikki Co., 1988) has claimed that 7-A1203supported by KOH and/or NaOH hydrolyzes COS at around 50 "C. This method presented the possibility of reducing the treatment temperature significantly. All these methods, however, just convert COS to H2S. So, H2S must be removed through another treatment; namely, two processes are required to remove COS. In this paper we present a new method to remove COS by a single process utilizing an iron oxide at around 50 "C. t Research Laboratory of Carbonaceous Resources Conversion Technology. Department of Chemical Engineering.
*
0888-5885/92/2631-0415$03.00/0
Table I. Properties of Several Heat-Treateda-FeOOH H20 surface treatment content x area x H~O conditions 103/(kg/ 10+/(m2/ coverage sample temp/'C time/h ka) kd ratio a-FeOOH 110 2 10 10.8 3.4 sample A 220 1 9 11.3 2.9 sample B 220 2 11 39 0.97 sample C 220 19 23 90 0.94 sample D 200 24 17 70 0.87 a-Fe203 2
Iron hydroxides such as a-Fe00H and y-FeOOH are known to transform into a-Fez03upon heat treatment above 200 "C during which a special form of iron oxides having uniform micropore structures are formed (Ishikawa and Inouye, 1972; Naono and Fujiwara, 1980). By utilizing such micropore structures and unique surface properties formed during the transformation, the possibility of using them as the adsorbents for SOz, H20, NH3, NO, and so on has been examined (Kaneko et al., 1977; Hattori et al., 1979; Ishikawa and Inouye, 1980). In this paper we try to use such iron oxides as the hydrolysis catalysts of COS as well as the adsorbent of HzS at low temperature.
Experimental Section 1. Sample Preparation. A special grade of a-FeOOH (Nacalai Tesque Co.) was used as a starting material. It was calcined under different conditions to prepare samples of different dehydration levels. The calcination conditions, pore surface area, and water content of each sample are shown in Table I, where the pore surface area was measured by the nitrogen adsorption method and the water content was measured by the so-called Karl-Fischer titration method. Before the measurement of these properties, the samples were dried in vacuo at 110 "C for 2 h. Weight change and heat effects during the calcination were investigated by a thermobalance ( S h i m a h , TGA-50) 0 1992 American Chemical Society
416 Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992 1 .or
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vimetrically and calorimetrically. Figure 1 shows the weight decrease curve and the DSC profile of a-FeOOH heated in an inert atmosphere at the rate of 5 K/min. The weight of a-FeOOH started to decrease at around 200 "C and reached a constant value at 300 "C or so. The total weight decrease is about 1070, which corresponds to the weight decrease expected by the reaction 2FeOOH Fe203+ H20 (1) Two endothermic peaks appeared in the DSC profile, as shown in Figure lb, and the endothermic heats were 3800 and 1550 J/(g of water released) for the first and the second peaks, respectively. It has been reported that the dehydration process of a-FeOOH is greatly affected by its preparation conditions (Jurinak, 1964; Ishikawa and Inouye, 1972). This seems to suggest that the used a-FeOOH was not completely crystallized, though it was a special grade. Figure 1clearly shows that we can prepare samples of different dehydration levels of iron oxides by changing the heat treatment conditions. The temperature effect, however, is so sensitive that we changed the evacuation time at a constant temperature of around 220 O C to prepare the samples of different dehydration level. This method was easier for changing slightly the dehydration level. Thus we prepared four samples from a-FeOOH, as shown in Table I. Change of the Pore Distribution through Dehydration. The pore distributions of samples A-D and a-FeOOH are shown in Figure 2, where Dollimore's method (Dollimore and Heal, 1964) was used to calculate the distributions from nitrogen adsorption isotherms. The difference among the samples was found only in the micropore region of around 0.7 nm in pore radius, indicating that the micropores are created by dehydration from the hydrogen bonds between the layers forming a-FeOOH. Judging from the values of the surface area listed in Table I, sample C has the most developed micropores. Adsorption of Water on Dehydrated Samples. The surface of the partially dehydrated a-FeOOH is very reactive, so moisture in the ambient air is easily adsorbed on the partially dehydrated samples, even at room temperature. Then the amount of adsorbed water was measured by the Karl-Fischer titration method and is listed in Table I. Since structural OH groups are not detected by the Karl-Fischer method and since the sample was dried at 110 "C before the measurement, the water content thus measured corresponds to the chemisorbed water. The water contents of samples C and D were about twice as
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Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992 417 I
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2 3 L 5 B Time Chl Figure 4. (Left ordinate) Removal efficiency of COS for a-FeOOH and several heat-treated samples. (Right ordinate) Ratio of the amounta of COz evolved and COS converted for sample C. Conditions: T = 50 “C; W = 1.4 X lo4 kg; u = 2.0 X m3/min; Ccw = 3.85 x 10-3 m0i/m3.
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C
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large as those of the other samples. The surface coverage ratio of water was calculated from the water content and the surface area by assuming that one water molecule occupies 0.108 nm2and is listed in the last column of Table I. For a-FeOOH and sample A, the surface coverage ratio was around 3, whereas it was almost unity for samples B, C, and D. This indicates that water was adsorbed as a single layer on the dehydrated surface for samples B, C, and D. The water content of a-FeOOH is much smaller than the weight decrease observed during the heat treatment, as shown in Figure la This means that structural OH groups were not detected by the Karl-Fischer method as expected. Then the water content for a-FeOOH and sample A listed in Table I is probably due to weakly chemisorbed water. The FTIR spectra shown in Figure 3 give the information on the state of water retained. The distinct adsorption peak at around 3130 cm-I for cy-FeOOH clearly indicates the existence of OH groups (Russel et al., 1974), though thia was not detected by the Karl-Fischer titration method. The peak strength is weakened and another peak appears at around 3400 cm-’ with the increase of severity of the heat treatment, and sample C has the peak at only 3420 cm-’ or so. This peak corresponds to an weak hydrogen bond (Russel et al., 1974) and is judged to derive from the strongly chemisorbed water. By further dehydrating cyFeOOH to convert it to a-Fe203perfectly, no peaks were found. Thus the changes in the amount and the state of water retained by the iron oxides were clarified. Then it was examined how the changes in the properties of the iron oxides affect the removal of COS. 2. Removal Efficiency of COS. Figure 4 shows the removal efficiency of COS measured at 50 O C under the same operating conditions for the samples listed in Table I. Sample C,which has the largest surface area, gave the best removal efficiency of COS. The efficiency was almost proportional to the pore surface area or the amount of chemisorbed water. The efficiency of a-FeOOH is large at the initial stage, but it decreases rapidly. The efficiency of cy-Fe203was very small. Next, we examined how COS is removed by the samples. No H2S was detected in the effluent stream for all the samples examined. Inorganic gases in the effluent stream were also analyzed for sample C, which showed the maximum removal efficiency of COS, and C02,whose amount
v)
0.5
r”
Time Chl Figure 5. Removal efficiency of H2S for a-FeOOH and several heat-treated samples. Conditions: W = 1.3 X lo-’ kg; u = 1.00 X 1 P mol/m3. m3/min; CH6 = 3.96 x
is almost the same as that of removed COS,was detected while COS was being removed, as shown in Figure 4. This confirms that COS is converted to HzS by the hydrolysis reaction COS + HzO H2S + COP (2)
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The water required to proceed with the above reaction is expected to be supplied from the chemisorbed water, since water was not supplied at the reactor inlet. The H2S produced by the hydrolysis reaction (2) was considered to be retained by the samples. To confirm this, the removal of H2S by the samples was also performed, as shown in Figure 5, under the flow rate of u = 1.00 X lo4 m3/min, which is 5 times larger than that employed in Figure 4. The order of the removal efficiency is the same as that of COS, but the capacity of HzS removal seems to be larger than that of COS removal. 3. Estimation of the Life of Sample C. To estimate the life of the iron oxide, the removal efficiency of COS for sample C, which has the best efficiency, was examined by changing the sample weight, as shown in Figure 6. The removed amount of COS calculated from the data of W = 0.008 X low3kg is estimated to be 0.54 mol/kg. Since the water content of sample C is 1.28 mol/kg (=0.023/ 0.018), less than half of the chemisorbed water is judged to be utilized to hydrolyze COS to H2S. By using 0.54 mol/kg as the capacity of COS removal of sample C, the life of sample C can be estimated as 430 h if we assume that the density of sample C is 3000 kg/m3 and that the space velocity is 1000 h-l, which is the typical value employed to remove H2S in practical operations. The iron oxide bed which is utilized to remove HzS from coke oven
418 Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992 m
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Figure 8. Five series of experiments to distinguish the active sitas between COS and HzS on the iron oxide: runs 1-111, for examining the competing site (a), run IV (b) and run V (c), for finding, respectively, the independent sites of HzS and COS. conditions: run I, C C W = 1.92 x 10-3 m0i/m3; run 11, cH, = 1.98 x 10-3 m0i/m3; run 111, cCOS = 1.92 x 10-3 m01/m3; cH, = 1.98 x 10-3 m0i/m3; run IV, Ccos = 3.85 X mol/m3; CH, = 3.96 X mol/m3; run V, CH,~ = 3.96 x 10-3 m0i/m3; ccos= 3.85 x 10-3 m0i/m3.
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Figure 7. Retardation effect of COz on the removal efficiency of COS for sample C.
gas now is said to be replaced every 30 days. As compared with this value, the life of sample C for removing COS is 60% or so. 4. Retardation Effect of COP Coke oven gas and the crude gas from partial oxidation of sulfur-bearing fossil fuels contain many other gas components in addition to COS and HzS. Of the gas components, COPcontained by percent order has the largest retardation effects on the hydrolysis reaction, since COz is the product of the hydrolysis reaction. Then the retardation effect of COPwas examined by use of sample C in Figure 7. For run A, only COS was fed at the inlet. Then 100% removal efficiency was attained for more than 5 h. When 5% COz (=1.89 mol/m3) was fed in addition to COS by keeping the COS feeding rate and the sample weight constant, the removal efficiency decreased to 50% or 90 (run B). But the removal efficiency could be increased up to 90% or so by increasing the sample weight up to 0.49 X lr3 kg (runC). The space velocity for run C is still more than loo00 h-l, which is much larger than the commercially employed values, and COz was fed in excess to see if there is the retardation effect. Therefore, we can safely say that sample C can be used commercially, although COPretards the hydrolysis reaction slightly. 5. Mechanism of COS Removal by the Iron Oxide. The above discussions indicate that COS is first hydrolyzed to HzS on the special form of iron oxides, and second the produced HzS is stabilized as iron sulfides on the iron oxide. Here we examined the mechanism of COS removal by iron oxide from two aspects: the first one is to examine the active sites for COS and HzS, and the second one is to specify the products formed through the treatment of COS and/or HzS. Five series of experimental runs were performed to distinguish the active sites on sample C. Only COS of 50
ppm and only HzS of 50 ppm were fed in runs I and 11, respectively. In run I11 a mixture of COS (50 ppm) and HzS (50 ppm) was fed. These runs were performed to examine if there is a competing site between COS and HzS. The W / u ratio, which was kept constant in these runs, was selected to be small enough to detect the deactivation of active sites within a measurable time. Figure Sa shows the removal efficiency of COS and HzS in these runs. Comparing runs I and I1 shows that the number of active sites for HzS is larger than that for COS. In run I11 both COS and Ha removal efficiencies are smaller than in runs I and 11. This shows that there are competing active sites for COS and HzS. Runs IV and V were performed to examine if there are independent active sites for COS and/or HzS. In run IV sample A was deactivated completely by COS, after that the influent stream was changed from COS to HzS. On the other hand COS and HzS were fed in the reverse order in run V. Parta b and c of Figure 8 show the regulta of runs IV and V, respectively. In Figure 8b about 75% of H2S was still removed even after sample A was deactivated by COS. This shows that there are independent active sites for HzS. In Figure 8c around 10% of COS was removed after sample C was deactivated by HzS, but the produced HzS by the hydrolysis reaction was not stabilized at all on sample C and was detected in the effluent stream. This shows the independent active sites for COS are very few. The above considerations indicate that there are at least two active sites: one is active for only HzS, and the other is active for both COS and HzS. Then we can write the following stoichiometric equations as possible reactions COS + HzO HzS + C02 on Fez03.xHz0 (3) Fez03.xHz0+ 3HzS FezS3+ (3 + x)HzO (4) Fez03 + 3HzS -.+ 2FeS + 3H20 + s (5) Fez03 + 3HzS FezS3+ 3Hz0 (6) Fez03 + 4H2S 2FeSz + 3H20 + HZ (7) where the site active for only HzS and the site active for
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Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992 419 ables one to use only a single process to remove both COS and H2S a t low temperatures.
s-s
178
170 Binding energy
1
162 CeVl
Figure 9. XPS S (2p) spectrum for sample C deactivated by H2S.
both COS and H2S were represented by Fe2O3 and Fez03.xH20,respectively. Reaction 3 is the hydrolysis reaction of COS. Reaction 3 was judged to be essential on the basis of the facts that competing sites existed for COS and H2S and that the independent sites for COS were very few. Reactions 5-7 were involved as possible ieactions because the independent active site for H2S was found to exist. The amount of the site available for H2S is 4 to 5 times larger than that for COS, judging from Figures 4 and 5. To examine which reaction is prevailing of (5)-(7), sample C deactivated by H2S was examined by X-ray photospectroscopy (XPS)first. Figure 9 shows the spectra of S (2p). The existence of F e S and S-S bondings are clearly shown in the figure, indicating that reaction 5 is occurring at least. Reaction 6 is known as a main reaction to remove H2Sat room temperature, although F e a 3could not be detected by the XPS measurement. It is known that Fe2S3is easily regenerated to Fez03 in humid air. Actually, the color of the deactivated sample changed from black to brown while the sample was stored, indicating the change from Fe2S3to Fe203. This may be the reason we could not detect Fe2S3by the XPS measurement. From this discussion, reaction 6 was judged to be prevailing. Next, to judge if reaction 7 was occurring, we tried to examine if H2 was formed during the treatment of H2S. Careful analysis of the product gas denied the formation of H2. Summarizing the above discussions, we could conclude that COS is removed via reactions 3-6, whereas H2Ssupplied a t the reactor inlet would be removed by reactions 4-6. These mechanisms suggest that the COS hydrolysis reaction 3 is terminated with the exhaustion of Fe203.xH20 and that the water produced by eqs 4-6 is not effective in hydrolyzing COS. Then these mechanisms explain well the fact that the amount of COS hydrolyzed by sample C is less than half the amount of chemisorbed water. 6. Recommendation. A commercially intended scheme to remove COS and H2S from the coke oven gas is to employ separate processes for COS and H2S,respectively. There are several optional processes to remove COS, such as hydrolysis by use of base catalysts at high temperatures, the absorption by base absorbents, and so on. On the other hand the removal of H2Sis generally performed by use of iron oxides as adsorbents at room temperature. The steel industry wants to develop a dry method to remove COS at low temperatures. The proposed method in this paper presents one possibility for the demand. Furthermore, the proposed method simultaneously removes H2S,which en-
Conclusions A new dry method to remove a trace of COS at around 50 "C from coke oven gas was developed. This utilizes a special form of iron oxides which is prepared from aFeOOH. When a-FeOOH is calcined for 20 h or so a t around 200 OC, its surface area reaches a maximum of around 100 m2/g and water is chemisorbed on it by the monolayer. This iron oxide hydrolyzed COS effectively to H2Sby utilizing the chemisorbed water, even at 50 "C, and immediately stabilized the produced H2S as Fe2S3. Thus COS is found to be completely removed by use of only this special form of iron oxides. Since the iron oxide has the sites which are active for only H2S in addition to the sites active for both COS and H2S, it can be used to remove COS and H2Ssimultaneously from the coke oven gas. Acknowledgment We express our sincere thanks to Mrs. N. Shiraishi and F. Yoshikawa of the Kawasaki Steel Co. for their cooperative discussions. This work was financially supported by the Ministry of Education, Science and Culture, Japan, through the Grant-in-Aid for Scientific Research (No. 02650706). Registry No. COS,463-58-1; SH2,7783-06-4; Fe02H, 2034449-4; iron
oxide, 1332-37-2.
Literature Cited Akiioto, T.; Dalla Lana, I. G. Participation of Acidic Sites and Reduction Sites in Vapor-Phase Hydrolysis of Carbon Desulfide over Alumina Catalyst. Nippon Kagakukai-shi 1979, 579-585. Chan, V. A. Y.;Dalla Lana, I. G. On the Catalytic Hydrolysis of Carbonyl Sulphide over Gamma-Alumina. Can. J. Chem. Eng. 1978,56, 751-753. Dollimore, D.; Heal, G. R. An Improved Method for the Calculation of Pore Size Distribution from Adsorption Data. J. Appl. Chem. 1964,14, 109-113. Dunlap, M. K.; Galstaun, L. S. Improved Process for COS Conversion. Energy Rog. 1982,2 (4), 191-196. Ewing, F. J. J. The Crystal Structure of Lepidocrocite. Chem. Phys. 1935,3,420-424. Hattori, T.; Kaneko, K.; Ishikawa, T.; Inouye, K. Adsorption of Nitrogen Monoxide on Iron Oxide Hydroxides and Their Calcinated Products. Nippon Kagakukai-shi 1979,423-426. Ishikawa, T.; Inouye, K. The Structural Transformation of Ferric Oxyhydroxides and Their Activity to Sulfur Dioxide. Bull. Chem. SOC. Jpn. 1972,45, 2350-2354. Ishikawa, T.; Inouye, K. Adsorption Sites for Sulfur Dioxide and Water on Iron Hydroxide Oxides. Nippon Kagakukai-shi 1980, 681-685. Jurinak, J. J. Interaction of Water with Iron and Titanium Oxide Surfaces: Goethite, Hematite, and Anatase. J. Colloid Sci. 1964, 19,477-487. Kaneko, K.; Ishikawa, T.; Inouye, K. The Electrical Conductivity of Iron(II1) Hydroxide Oxides with Chemisorbed Sulfur Dioxide. Nippon Kagakukai-shi 1977,162-166. Namba, S.; Shiba, T. Hydrolysis Reactions of COS by Use of Alumina Catalysts. Kogyo-Kagaku Zasshi (Japan) 1968,71,93-96. Naono, H.; Fujiwara, R. Micropore Formation due to Thermal Decomposition of Acicular Microcrystals of a-FeOOH. J. Colloid Interface Sci. 1980,26, 406-415. Nikki Co. Ltd. Japanese Pat. No. 224736,1988. Russell, J. D.; Parfitt, R. L.; Frasen, A. R.; Farmer, V. C. Surface Structures of Gibbsite Goethite and Phosphated Goethite. Nature 1974,248, 22Cb-221. Received for review April 22, 1991 Revised manuscript received August 26, 1991 Accepted September 14, 1991
