Permeation and separation in binary ternary gas mixtures

For multicomponent gas mixtures the generalised Maxwell-Stefan (GMS) equations should be used. Krishna [87b] derived an expression for the flux of specimen /  

The first term at the right-hand side represents the friction due to the counterexchange of adsorbed molecules, while the second term represents the friction with the zeolite. Note that this equation shows a strong similarity with the GMS equation (9.33) for gaseous diffusion. 

For small por s with a pore diameter of about the same size as the molecular diameter (situation b2 in Fig. 9.20) counter-exchange will not take place and terms with D12 are absent. This is the so-called single file diffusion for which can be written [89]  

The second component affects the diffusion of the first component in two ways (i) by occupying a number of sites which now are not any longer available for component 1 (term (1 - 62) in denominator of D), and (ii) by an entrainment contribution due to the gradient of component 2, which can be either positive or negative depending on conditions. 

If single file diffusion prevails the single component permeation data (which determine D,) and single component adsorption data are sufficient to predict the binary fluxes via Eq. (9.52). 

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