Gas transport rate-controlling

Unless the pressure is so low that the molecular mean free path (Appendix E) is comparable to the dimensions of the chamber enclosing the gas, the rates of the first and last steps, 1 and 5, are governed by molecular diffusion rates (or by rates of mass transfer through a boundary layer, if the gas is in motion). Although these macroscopic transfer processes, which lie within the realm of fluid mechanics (Appendixes C-E), often are the ratedetermining steps [62] (see also Chapter 12), we shall assume here that they are so fast that they can be neglected. 

We shall now consider systems in which steps 2 or 4 are rate-con-trolling and later investigate systems in which step 3 determines the overall rate. Much additional information on these topics is available [63]-[66]. 

The rate of adsorption of a molecular species depends on the nature of the adsorption process. Gas molecules may be held on a surface by the weak van der Waal attractive forces which exist between all molecules or by stronger chemical types of bonds (chemisorption) which depend on the 

For ideal gases, the number of moles of chemical species i in the gas striking the surface per unit area per second is 

Adsorbed molecules will also return to the gas because of thermal agitation. If denotes the number of moles of adsorbed molecules of species i per unit area per second which would return to the gas from a surface completely covered by molecules of species i, then the number of moles of species i per unit surface area per second returning to the gas is 

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