Grain Model of Effective Diffusion in Porous Media

A grain model has been developed to describe the effective diffusion of gas in porous media. The porous media is visualized as a random collection of near-spherical grains fixed in space. The diffusion of gas occurs through the interstices between the grains. The average effective gas diffusivity may be expressed as {formula}. The average pore radius re can be determined by Hg penetration measurements. The spherical factor γ{s} is defined as the ratio of the mean pore radius with a perfect spherical grain structure to the actual average pore radius from the integration based on the pore-size distribution, {formula}. The model gives results that are in good agreement with reported data with both a simple normal distribution and bimodal pore structure. The ratio of the mean radius with perfect spherical grains to the actual average pore radius, r{s}/r{e} was found to be a constant at about 0.07 for catalysts.