Knudsen flow

Knudsen flow describes the movement of fluids with a high Knudsen number, that is, where the characteristic dimension of the flow space is of the same or smaller order of magnitude as the mean free path.

Separation processes using Knudsen Flow

For a gas passing through small holes in a thin wall in the Knudsen Flow regime, the number of molecules that pass through a hole is proportional to the pressure of the gas and inversely proportional to its molecular weight. It is therefore possible to effect a partial separation of a mixture of gases if the components have different molecular weights. The technique is used to separate isotopic mixtures such as uranium using porous membranes.^[1] It has also been successfully demonstrated for use in hydrogen production, as a technique for separating hydrogen from the gaseous product mixture created when water is heated at high temperatures using solar or other energy sources.^[2]

References

1. ^ Villani, S. (1976). Isotope Separation. Hinsdale, Ill.: American Nuclear Society. 
2. ^ Kogan, A. (1998). "Direct solar thermal splitting of water and on-site separation of the products - II. Experimental feasibility study". Int. J. Hydrogen Energy (Great Britain: Elsevier Science Ltd) 23 (2): 89–98. doi:10.1016/S0360-3199(97)00038-4.