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Ind. Eng. Chem. Res. 2002, 41, 1916
Response to Comments on “Design of Entrained-Flow and Moving-, Packed-, and Fluidized-Bed Sorption Systems: Grain-Model Kinetics for Hot Coal-Gas Desulfurization with Limestone” by Hartman et al. Scott Lynn Department of Chemical Engineering, University of California, Berkeley, California 94720-1462
Sir: We thank Hartman et al., for pointing out the availability of the new thermodynamics data by Barin and Platzki,1 which appeared after our paper2 was submitted for publication and which may contain improved enthalpy data. The work of Hartman et al. confirms one of our major points: that there is a relatively narrow window of temperature, pressure, and gas composition in which the level of H2S attainable with sorption by lime or limestone is minimized by the thermodynamics of the system. Maximum potential H2S uptake occurs precisely at the calcination temperature of calcium carbonate, which is a function of the above parameters. It thus follows that the use of a separate sorption step operated under nearly uniform, appropriate conditions will maximize both sorbent utilization and H2S removal. Within a gasifier, where lime or limestone are frequently injected together with coal for the purpose of desulfurization, the temperature field is by no means uniform and neither goal can be met. A second major point of our paper is that in addition to the thermodynamic constraints the sorption is diffusion-limited. A few degrees above the calcination temperature lime is a far more effective sorbent than is limestone a few degrees below. This is true not because of equilibrium considerations, as implied by Hartman et al., but because CaO has a far greater porosity. Our paper applies both the shrinking-core and the grain model to small pellets of lime in fixed-bed, moving-bed, fluidized-bed, and entrained-flow reactors to model the H2S sorption that would be expected. Because no experimental data were available for any
of these systems, no conclusion could be drawn regarding the best fit. It is hoped that this study will be of use when such experiments are undertaken. As noted both in our paper and by Hartman et al., CaO can reduce the H2S content of a typical gas only to the order of 100-200 ppmv at temperatures, pressures, and compositions typical of coal gas. This is not adequate to meet process requirements in most cases. However, because of its low cost, it may be economically attractive to use lime for removing the bulk of the sulfur in a coal gas employing a continuous operation. One could then use a more expensive sorbent such as ZnFeO2 in a fixed-bed polishing step. This approach would be particularly appealing when gasifying a high-sulfur coal because elemental sulfur and recyclable calcium carbonate can be recovered readily from CaS (Brooks and Lynn3). In contrast, the regeneration of sulfided ZnFeO2 is an oxidative process that makes SO2, which in turn requires additional processing and may result in formation of a waste. Literature Cited (1) Barin, G.; Platzki, G. Thermochemical Data of Pure Substances, 3rd ed.; VCH: Weinheim, Germany, 1995; Vol. 1. (2) Fenouil, L. A.; Lynn, S. Design of Entrained-Flow and Moving-, Packed-, and Fluidized-Bed Sorption Systems: GrainModel Kinetics for Hot Coal-Gas Desulfurization with Limestone. Ind. Eng. Chem. Res. 1996, 35, 1024-1043. (3) Brooks, M. W.; Lynn, S. Recovery of Calcium Carbonate and Hydrogen Sulfide from Waste Calcium Sulfide. Ind. Eng. Chem. Res. 1997, 36, 4236-4242.
IE020021X
10.1021/ie020021x CCC: $22.00 © 2002 American Chemical Society Published on Web 02/21/2002
