Flow and diffusion of a two-species gas mixture in a porous slab.

Problem of mass transport of oxygen is considered in a soil whose upper surface is open to air at atmospheric pressure and in which the oxygen is preferentially removed at some reaction surface within the body of the soil. The soil is assumed to be a permeable slab filled with a two-species gas mixture, the first of which is removed at one face of the slab, while the other face is maintained at constant pressure (~ atmospheric) . It is shown that the maximum removal rate of the first gas is largely determined by the diffusion coefficient of the second gas, but enhanced to some extent by gas flow due to the pressure difference set up by gas removal. This pressure difference is very small (~ 10-4 atmospheres) even at the maximum removal rate. It is also shown that under some conditions, which are satisfied in the case of oxygen removal from air, the concentration of the first gas has the same functional dependence on distance, as expected in the case of diffusion with no gas flow, but with an enhanced diffusion coefficient. For the problem of interest, this enhancement is about 20 per cent