Knudsen coefficient

The free parameters of this model are the ratio of porosity to tortuosity, e/r, the binary molecular diffusivities, D-j, the Knudsen coefficient, Ko (which determines specific effective Knudsen diffusivities, D j) and the permeability constant, Bo. 

Knudsen coefficient — The term relates to a particular type of mass transfer of gases through the pores of a specific porous medium. The gas transport characteristics depends on the ratio of the mean free path for the gas molecule, A, to the pore diameter, dpore, which is called the Knudsen number, Kn (Kn = -7 —). 

Ordinary diffusion coefficient for bulk gas or liquid (cm. /sec.) as opposed to the Knudsen coefficient Dk-... 

The process for gas separation by diffusion used in the Manhattan Project during World War II relies on the inverse relation of the Knudsen coefficient Dj, with molar mass M. It requires the use of a porous barrier or membrane, operated at a sufficiently low pressure to allow Knudsen diffusion to be the dominant transport mechanism. In addition, the downstream product is usually rapidly pumped away in order to minimize back-diffusion (C2 = 0). We can then write, for the enrichment process. 

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