Diffusion through porus media in transition region
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Chemical engineering is the design of industrial processes in which raw materials are under physical and/or chemical treatment. Catalysis is very importment in these processes. Therefore, a knowledge of the movement of molecules of fluids into a catalyst particle, usually in pores, is important. The diffusion problem in the bulk phase has been well developed. However, when the gas molecules, henceforth called particles, diffuse through a porous medium, the mechanism of the diffusion is no longer described by particle - particle collisions. The collisions between particles and the wall of the porous medium become more and more important when the mean free path is equal to or greater than the mean pore diameter. The properties of porous solids, such as pore size distribution, tortuosity, porosity, etc. then influence in the diffusion mechanism. This study begins with a discussion of ordinary diffusion because ordinary diffusion often occurs in porous media. The theory of diffusion through capillaries is then surveyed. Many models of diffusion in porous media based on the capillary model. A modified model is suggested by the author . Experimental data are collected from published papers. The effective diffusion coefficients of gases in fifteen porous pellets were obtained by experiments. These data are then described by parallel pore models bidisperse model, tridisperse model, and revised bidisperse model. These result are discussed and compared. The methods to measure and characterize the properties of porous solids are reviewed. The apparatus of the diffusion measurements are also discussed. Several recommendations are made in the last chapter of this work.--Introduction.
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