Gaseous Diffusion in Porous Solids at Elevated Temperatures

The Wlcke-Kallenbach (WK) technique was applied to the measurement of the binary gaseous diffusivity in a porous carbon at temperatures up to 700 °C. The severe leaking problem associated with the high-temperature application of the technique was solved by electroplating of soft metal layers on the sample. The diffusivity was also independently calculated from the reaction rate at a very low conversion through the use of a model. Comparisons of the two values showed that the WK diffusivities were substantially lower (by about 60%) than those calculated from reaction conditions. The commonly used expression in modeling and design studies, D_eff= ε²D_mol, was found to yield values by over an order of magnitude too high In the temperature range of practical interest, e.g., 500-1200 °C. A remedy to this problem is to replace the molecular diffusivity (D_mol) in the expression by a transition diffusivity (D_t). D_t, may be approximated by using a hypothetical single pore size.