Diffusion of Gases in Porous Solids and Capillaries

In Section 6.5C diffusion in porous solids that depends on structure was discussed for liquids and for gases. For gases it was assumed that the pores were very large and Fickian-type diffusion occured. However, often the pores are small in diameter and the mechanism of diffusion is basically changed. 

Diffusion of gases in small pores occurs often in heterogeneous catalysis where gases diffuse through very small pores to react on the surface of the catalyst. In freeze drying of foods such as turkey meat, gaseous HjO diffuses through very fine pores of the porous structure. 

Since the pores or capillaries of porous solids are often small, the diffusion of gases may depend upon the diameter of the pores. We first define a mean free path 2,-which is the average distance a gas molecule travels before it collides with another gas molecule. 

In the next sections we shall consider what happens to the basic mechanisms of diffusion in gases as the relative value of the mean free path compared to the pore diameter varies. The total pressure P in the system will be constant, but partial pressures of A and B may be different. 

7 Principles of Unsteady-State and Convective Mass Transfer 

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