Transport Processes and Separation Process Principles (Includes Unit Operations) Fourth Edition

7.6. DIFFUSION OF GASES IN POROUS SOLIDS AND CAPILLARIES

7.6A. Introduction

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

Diffusion of gases in small pores frequently occurs 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 H₂O 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 ...