Single Gas Permeation in Macroporous and Mesoporous Systems

The properties of gas flow in porous media depend on the ratio of the number of molecule-molecule collisions to that of the molecule-wall collisions. The Knudsen number Kn is a characteristic parameter defining different regions of this ratio. Its value is defined by Kn = Xl dp with X being the average free path length of the gas molecules and dp the characteristic pore diameter (sometimes the hydraulic pore radius is taken). 

The magnitude of Kn separates three main flow regimes of gaseous diffusion (see also Table 9.1)  

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