Gaseous diffusion in porous media. Part 2. - Dry granular materials

The diffusion of hydrogen through cylindrical samples of porous granular materials, 3 in. long and 2 in. diameter, was measured by the non-steady state technique of Part I, and reduced coefficients of diffusion D, referred to the value D0 in free air, were calculated for over twenty materials with porosity epsilon between 0.18 and 0.98. Results are comparable with those obtained by other methods, and with those for analogous electrical properties of porous media. Replicate determinations on a given sample agree to within 1%, but larger differences, caused by variations in packing, are observed between duplicate samples of the same porosity. It is shown theoretically that D/D0 = (l/le)2f, where f and le/l are factors for non-uniformity of cross-section and the increased length of the tortuous path followed by the gas, respectively. The influence on D/D0 of not only porosity, but also particle shape, was clearly shown and there can therefore be no unique relationship for all materials, as often supposed, between diffusion rates and porosity. The equations of Burger and Bruggeman, both of which include a particle shape factor, were tested on the experimental data and neither was entirely satisfactory. An empirical equation of the form D/D0 = γμ fits all materials where γ(≤ 1) and μ(≥ 1) are constants for a specific type of granular material, and probably represent measures of pore shape. Two particle shape parameters, relative surface area and equivalent settling radius, were measured for a limited number of materials, but at best, only a general trend between these and the experimental values for the Bruggeman shape factor was observed.