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I n d . Eng. Chem. Res. 1991,30, 55-60
give proper product distributions since the main effect of radial variations and mass transfer is to increase the effective length of the reactor. As shown in these calculations, the reactor length is not an important variable, so the results are not particularly sensitive to these problems. Summary This model gives results that appear to agree well with known properties of the Ostwald process. Thus, the assumptions on which the model is based either accurately describe the real system or at least do not introduce large errors into the calculations. Apparently, the high temperature and relatively small number of reactions in the gauze reactors justify these simpliciations, which make these reactors the first to yield to quantitative prediction. These calculations may therefore have utility in designing other types of reactors. Similar models of other catalytic reactors with low area catalysts such as catalytic combustors may also be successful in predicting behavior. Acknowledgment This material is based upon work supported under a National Science Foundation Graduate Fellowship. This research was also partially supported by DOE under Grant DE-FG02-88ER13878-AO2. Registry No. NH3, 7664-41-7; NO, 10102-43-9.
Literature Cited Blieszner, J. Ph.D. Thesis, University of Minnesota, Minneapolis, 1979.
Handforth, S. L.; Tilley, J. N. Catalysts for Oxidation of Ammonia to Oxides of Nitrogen. Ind. Eng. Chem. 1934,26 (12), 1287-1292. Hasenberg, D.; Schmidt, L. D. HCN Synthesis from CH, and NH3 on Clean Rh. J. Catal. 1985,91, 116-131. Hasenberg, D.; Schmidt, L. D. HCN Synthesis from CHI and NH3 on Platinum. J. Catal. 1986, 97, 156-168. Hasenberg, D.; Schmidt, L. D. HCN Synthesis from CHI, NH3, and O2 on Clean Pt. J . Catal. 1987,104, 441-453. Heck, R. M.; Bonacci, J. C.; Hatfield, W. R.; Hsiung, T. H. A New Research Pilot Plant Unit for Ammonia Oxidation Processes and Some Gauze Data Comparisons for Nitric Acid Process. Ind. Eng. Chem. Process Des. Dev. 1982,21, 73-79. Hwang, S. Y.; Schmidt, L. D. Decomposition of CH3NH2on Pt(ll1). Surf. Sci. 1987, 188, 219-234. Loffler, D. G.; Schmidt, L. D. Kinetics of NH3 Decomposition on Polycrystalline Pt. J. Catal. 1976,41, 440-454. Mummey, M. J.; Schmidt, L. D. Decomposition of NO on Clean Pt Near Atmospheric Pressures. Surf. Sci. 1981, 109, 29-59. Pignet, T.; Schmidt, L. D. Selectivity of NH3 Oxidation on Pt. Chem. Eng. Sci. 1974,29, 1123. Pignet, T.; Schmidt, L. D. Kinetics of NH3 Oxidation on Pt, Rh, and Pd. J. Catal. 1975,40, 212-225. Satterfield, C. N. Heterogeneous Catalysis in Practice; McGraw-Hill: New York, 1980; pp 214-221. Takoudis, C. G.; Schmidt, L. D. Reaction between Nitric Oxide and Ammonia on Polycrystalline Platinum. 1. Steady-State Kinetics. J. Phys. Chem. 1983,87, 958-963. Trimm, D. L. Catalytic Combustion (Review). Appl. Catal. 1983, 7, 249-282. Twigg, M. V. Catalyst Handbook; Wolfe Publishing Ltd.: London, 1989; pp 470-489. Waletzko, N.; Schmidt, L. D. Modeling Catalytic Gauze Reactors: HCN Synthesis. AIChE J. 1988,34 (7), 1146-1156.
Received for review March 1, 1990 Revised manuscript received June 25, 1990 Accepted July 5, 1990
Characterization and Long-Range Reactivity of Zinc Ferrite in High-Temperature Desulfurization Processed R a u l E.Ayala* and Donald W . M a r s h C E Corporate Research and Development, P.O.Box 8, Schenectady, New York 12301 The chemical reactivity of zinc ferrite was studied experimentally to demonstrate the potential use of zinc ferrite as a sorbent in high-temperature desulfurization of coal gases. Fifty cycles of H2S absorption from simulated coal gas and regeneration under 4.5% oxygen were conducted in a laboratory-scale, packed-bed reactor system simulating gas compositions of a fixed-bed, air-blown gasifier and a regeneration scheme typical of a moving-bed process. Approximately 70% of the theoretical fractional conversion, as determined by thermogravimetric analysis (TGA), was maintained by the sorbent. Less than 1% residual total sulfur and total carbon were measured in the sorbent. Undesired solid phases, (e.g., metal carbides, sulfates, and elemental iron) were absent in the samples as determined by powder X-ray diffraction. Introduction A major cost in the integrated gasification combined cycle (IGCC) for generation of electricity from is the gas 'leanup to remove contaminants such as solid particulates, species (e*g*tH2S and COS),and tars. Conventional cleanup systems use energy-inefficient, low-temperature scrubbing processes where costly heat exchanger systems are necessary to cool off the gas from the gasifier, the water 'Ondensate, and reheat the gases to match the required turbine inlet tern'Presented at the Symposium on Adsorption and Reaction on Oxide Surfaces of the 198th ACS National Meeting, Miami Beach, FL, Sept 10-15, 1989. 0888-5885/91/2630-0055$02.50/0
peratures (Corman, 1986). A novel approach is the use of high-temperature desulfurization utilizing mixed-metal oxides as sulfur sorbents (Grindley and Steinfeld, 1982, 1983, 1984; ~ ~ ~ z a n ~ ~ ~ ~ e p ~eta n o1987; p o u~~~~l ~os et 1989). Sulfur absorption is accomplished at ternperaturesand pressures matching those of gasifier and turbine components in the IGCC system, thus minimizing energy and cost expenditures in the process, Zinc ferrite, ZnFe20r,is a leading mixed-metal sorbent for high-temperature desulfurization of coal gas having a crystalline spinel structure formed by two metal oxides: ZnO and Fe;O3 (Kolta et al., 1980). Zinc oxide is able to reduce H2S and COS levels below 10 parts per million by volume (ppmv) in coal gas while iron oxide has twice the sulfur absorption capacity of zinc oxide on a molar basis; 0 1991 American Chemical Society
56 Ind. Eng. Chem. Res., Vol. 30, No. 1, 1991
hence, 1 mol of zinc ferrite removes a total of 3 mol of sulfur. Both oxides exhibit easy regenerability over extended cyclic use. The overall reaction describing the absorption of H2S from coal gas by ZnFe204is given by ZnFe204 3H2S H2 ZnS + 2FeS + 4H20 (1)
+
-
+
Reaction 1can be considered essentially irreversible since the equilibrium vapor pressure of H a over ZnFe204is very low (i.e.,
