Diffusion through uniform barriers, with

A. Diffusion through Uniform Barriers with No Source... 

Pure gases. A typical diffusion barrier consists of a thin sheet of material perforated by a very large number of small holes of nearly uniform diameter. If the diameter of the holes and the thickness of the sheet are smaller than the mean free path of UFg at the pressure upstream of the barrier, individual molecules of UFg will flow through the holes without colliding with other molecules in what is known as molecular flow. The rate of molecular flow through a circular capillary is given by Knudsen s law [K3] ... 

If the gaseous mixture is forced under pressure against a suitable porous membrane or barrier, more of the lighter molecules will flow through the pores due to their higher velocity. To maximize the amount of separation achieved, the barrier material must meet exacting standards so that diffusive flow occurs, that is, pore sizes must be such that individual molecules collide only with the pore walls rather than with each other. Diffusive flow requires not only small barrier pores (less than two-millionths of an inch in diameter) but also uniformity of pore size. 

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