Multistage membrane separations

E.J. Hoffman, Membrane Separations Technology Single-Stage, Multistage, and Differential Permeation, Gulf Professional Publishing, an Imprint of Elsevier Science, Amsterdam, 2003. 

With the proper choice of membrane material, a membrane separator can achieve significant separations in a single stage. Indeed, as we shall see, it usually turns out that multistage operations with membranes are not economical (compared to cryogenic distillation). Thus most industrial applications of membranes do not require very high purity products. 

In the majority of multistage separation processes, the phases in question are vapor and liquid. Vapor-liquid-liquid equilibrium (VLLE) can exist when two partially immiscible liquids are at equilibrium with each other, as well as with a vapor phase. This is a phenomenon that can occur under certain conditions in phase separation processes. Applications in this book cover vapor-liquid, liquid-liquid, and vapor-liquid-liquid separation processes. Chapter 18 discusses membrane separation processes, where the separation is not brought about as a result of different phase compositions, but by the action of membrane barriers. 

I. Nods and C. C. Giyte. Multistage Membrane Separation Processes for the Contiguous Frac-... 

S. Okonogi, Progress in practice of membrane separation techniques in dairy industry, Nihon Shokuhin Kougyou Gakkaishi (J. Food Ind. Jpn.), 1985, 32, 144—155 Y. Kobuchi and H. Motomura, Demineralization of whey by multistage continuous electrodialysis, International Membrane Technology Conference, Nov. 8-10 (1983), Sydney. 

Membrane separation can provide 99% pure nitrogen from air, in a multistage process. The membrane selectivity a is defined from its permeabilities o and N for oxygen and nitrogen... 

It is theoretically possible to accomplish a complete separation of a pair of gases by membrane separation, either by very highly selective membranes combined with very low partial pressure of the permeating component on the permeate side of the membrane, or by multistage cascades. However, it usually is not economical to accomplish high purity and high recovery of both components of a binaiy by membrane separation alone. Therefore, it usually b necessary to couple membrane gas separation widi other processing steps when neatly complete separation te required. 

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