The role of in-pore mass transport resistance in the reaction of porous solids with gases The air oxidation of large tubes of graphite

Abstract The rate of reaction between a gas and a porous solid at elevated temperatures is limited by in-pore mass transport resistance. A mathematical model is presented which allows the magnitude of this effect to be estimated. The mass transport resistance is considered as three constituent parts, diffusion, bulk flow due to superimposed pressure gradients and bulk flow due to volumetric changes in the chemical reaction. The effect of diffusional resistance is determined on the basis of a free gas model and Knudsen diffusion is not considered. The chemical rate is assumed to obey a simple order relationship with respect to the reactant gas. The theory has been verified by a study of the oxidation of large tubes of graphite exposed to air at the bore surface. Evidence of a diffusional effect was obtained from gas concentrations in the bore and at the external surface. The effect of superimposed bulk flow of gas through the graphite was examined, Good agreement between the observed results and those predicted from the theory was obtained at temperatures in the region 400–550°C. The effects on the calcualted results of carbon monoxide formation and the concentration dependence of the diffusion coefficient were examined. Under the experimental conditions employed these were found to be of the second order. The commonly assumed first order dependence of the graphite—oxygen reaction was found to be incorrect and support for a 0·6 order dependence obtained.