A partially overlapping grain model for gas—solid reactions

A mathematical model is presented for gas—solid reactions occurring in porous media with a structure represented by a population of spherical dense grains, each having a hard core and a soft permeable shell. The development of the model is based on the scaled-particle approximation for the structural properties, porosity and surface area, of structures of partially permeable spheres and the randomly overlapping grain model of Sotirchos and Yu. A detailed parametric sensitivity analysis of the partially overlapping grain model is carried out, and its flexibility is demonstrated by applying it to a set of experimental data for the ZnO-H2S reaction. The results that the extent of grain overlapping has a strong influence on the variation of the reactivity of a solid of given porosity and internal surface area, which depends in a complex fashion on the intraparticle or product layer diffusional limitations.