THE EFFECT OF PORE STRUCTURE ON THE KINETICS OF FLUID-SOLID REACTIONS.

摘要

For fluid-solid reactions a random pore model is developed which accounts for arbitrary pore-size distributions as well as pore intersections in a reacting solid. The model is quite versatile and subsumes the prior grain models as special cases. It is found that a rate maximum may be expected for certain pore size distributions but not for others. In the initial stages of reaction the rate increases with conversion due to increase in the surface area as pore growth occurs, but this effect is overshadowed in later stages by pore intersections which reduce surface area. The model indicates that given the porosity and surface area, a solid possessing uniform size pores is intrinsically the least reactive. The data of Hashimoto, et al. (1979) on the surface area development and conversion-time behavior during char gasification are found to produce correlations consistent with the expectations of the model.; The model is formulated to include as well transport effects arising from boundary layer, intraparticle and product layer diffusion. It is found that the rate and surface area maxima predicted in the kinetic regime are shifted to lower conversions as intraparticle or product layer diffusional resistances increase. For reactions accompanied by an increase in the volume of the solid phase it is shown that incomplete conversions may be expected due to pore closure, the ultimate conversion decreasing with an increase in the intrapellet diffusional resistance. The model is also applied to the data of Borgwardt (1970) and of Hartman and Coughlin (1976) on the reaction between CO(,2) and lime in the presence of oxygen, where the calcium sulfate produced forms a protective product layer. It is seen that the reaction is diffusion controlled under the conditions of Hartman and Coughlin (1976) consistent with their own findings, and a prior Pigford and Sliger (1973) interpretation. The temperature behavior of the diffusion coefficient in the product layer suggests the participation of an activated process, possibly a solid state step.; TGA experiments were conducted with the reaction between CO(,2) and lime, in the range of 400(DEGREES)C to 725(DEGREES)C, with a view to examine the effect of product layer deposition and of varying pore structures on the kinetics. Limes with different pore structures were obtained for reactions by calcining limestone under varying partial pressures of carbon dioxide in nitrogen. It is found that the recarbonation reaction is initially chemically controlled, before undergoing a rapid transition to a much slower regime controlled by product layer diffusion. The magnitude of the product layer diffusivity, in the range of 10('-14) to 10('-17) cm('2)/sec, and the activation energy of about 20 kcal/gmole below 515(DEGREES)C, and 40 kcal/gmole above 515(DEGREES)C, are suggestive of solid state diffusion. Most likely the mechanism involves countercurrent motion of carbonate and oxygen ions below 515(DEGREES)C, and sitewise motion of decomposition-produced CO(,2) above this temperature.