Diffusion of Gases in Graphite Pores. Dragon Project Report

A system of equations have been adopted for the diffusion of gases in graphite pores in the absence of temperature and pressure gradients. Portions of the medium are visualized as a collection of fixed ''dust'' particles which are uniformly distributed. The results apply over the entire pressure range from the Knudsen region to the normal diffusion region. This approach permits a satisfactory explanation of the fact that over wide pressure ranges the flux ratio of two counter-diffusing gases is (m₂/m₁)^1/2 in porous media under steady-state and uniform—pressure conditions. The effect of non-uniform pressures on the diffusion equations is to introduce both a pressure diffusion term and an external force term. However, the final diffusion equation has the same form as in the uniform pressure case. No additional parameters are required to compute the diffusion rates when pressure gradients are present. A complete solution requires also a forced flow equation giving J (the net flux) as a function of the pressure gradient. A forced flow equation is derived on the basis of the dusty-gas concept, but one parameter must be made disposable — in order to compensate for the fact that the diffusion equations permit only a diffusive mechanism for flow, never a viscous mechanism. A series of experiments with one gas mixture is discussed. (author)