All membrane processes are nonequilibrium processes. The transport equation describing a particular membrane process must satisfy the principles of nonequilibrium thermodynamics. However, many expressions for the flux through a membrane as functions of the driving forces can be found in the literature without resorting to nonequilibrium thermodynamics. In fact, the choice of fluxes and driving forces for a particular membrane process frequently seems to be arbitrary and accidental; this is attributed to historical developments. Some exceptions are the cases of reverse osmosis and ultrafiltration processes. Katchalsky and coworkers successfully applied the principles of nonequilibrium thermodynamics by Onsager to analyze reverse osmosis and ultrafiltration processes. A generalized treatise of nonequilibrium thermodynamic analysis is given for all different membrane processes including gas permeation, pervaporation, dialysis, reverse osmosis, ultrafiltration, microfiltration, and electrodialysis. Starting from the entropy production term, fluxes and driving forces are ascertained for each membrane process and the linear expressions between fluxes and driving forces are presented with corresponding coefficients. These are identified with the conventional entities whenever possible. As a consequence of this treatment, flux equations are more generalized to contain additional terms with additional driving forces representing the coupling phenomena. 2004 American Institute of Chemical Engineers AIChE J, 50:862–870, 2004
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