Interactions between a mobile adsorbed phase and diffusing gases in porous media—an experimental study

Abstract

Experimental data were taken for both nitrogen‐helium and propane‐helium systems counterdiffusing through a plug of pelleted Graphon. The experimental pressure range was 1–20 atm for the nitrogen‐helium and 1–8 atm for the propane‐helium system; the temperature range was 299–408°K. The temperature and total pressure gradients were zero in the individual experiments (Wicke‐Kallenbach experiments). Because of the uniform nature of the Graphon surface, significant surface transport of the heavier vapor occurred in all experiments. In neither system was the helium diffusion behavior predictable by the dusty‐gas diffusion equation, as it should have been if the gas‐phase diffusion were unaffected by the moving adsorbed phase. The observed helium flux in the helium‐propane experiments also averaged about 20% lower than predicted from the fluxes in the helium‐nitrogen system. The first of these observations indicates that the presence of a mobile adsorbed phase significantly affects the diffusion behavior of the gas phase. The second reinforces this conclusion and further indicates that the greater surface flow of propane caused the helium flux to be significantly inhibited. Applications of these observations to catalysis, diffusion within porous materials, and surface diffusion indicate that these phenomena merit further intensive study. In the case of surface diffusion, these results indicate possible major doubts in all previously reported surface fluxes and surface diffusivities in porous materials.