Kinetics of Vapor Permeation

In a partial pressure-driven vapor permeation process, the NC-dimensional vector of mass fluxes through the porous membrane can be expressed as [33]  

In general, all elements of the mass transfer matrix depend on the process variables, and in particular on the vapor phase composition. The mass transfer mechanisms in membranes can be rather complicated. However, for the conceptual analysis of the considered membrane process, it is not advantageous to go into the details of mass transport. Therefore, in the following the effective binary mass transfer coefficients k,j are assumed to be constants. 

Three cases can be distinguished concerning the mathematical structure of the matrix [k]  

Diagonal [k] if the permeating components are only driven by their own partial pressure differences this case is valid, for example, for a Knudsen-membrane. 

Nondiagonal [k] if the permeating components interact with each other within the membrane this will be the case for example if bulk diffusion and/or competitive adsorption effects are involved. 

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