Fluid–Solid Reaction Kinetics for Solids of Nonbasic Geometries: Comparison of the Sohn–Wall Method and the Shape-Factor Method

The shape of solids undergoing a reaction with a fluid influences how the reaction proceeds with conversion. Complex numerical solutions are needed in general to calculate the conversion–time relation for reactions involving nonbasic solids. This paper presents two simplified, although approximate, methods for obtaining conversion–time relations for such solids. In the shape-factor method, a shape factor is determined for a nonporous solid of the same shape reacting under interfacial reaction control and this shape factor is used to predict the conversion–time relation under pore-diffusion control. In the Sohn–Wall method, the progress of the reaction front under pore-diffusion control is simplified so that a closed-form equation approximately describing its position, and thus the conversion, may be found. Good agreement was observed between the exact numerical solution of nonbasic solids and these two approximate methods. Either of these methods can be combined with Sohn’s law of fluid–solid reactions to describe the overall conversion–time relationship of a fluid–solid reaction under a simultaneous rate control by chemical reaction and pore diffusion.