Membrane separations, combined multistage

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. 

Multistep and multistage membrane systems of the type shown in Figure 21.5 have been used to treat a number of vapor-gas streams. However, hybrid processes in which membrane separation is combined with another separation process, are attractive because they may enable each unit operation to operate in its preferred range, improving overall process efficiency. The most important of these hybrid processes is the combination of condensation under pressure with membrane separation, illustrated in Figure 21.6. In this process, a vapor-permeable membrane unit is combined with a vapor condensation-flash unit (Baker and Wijmans, 1994 Baker et al., 1998 Wijmans, 1993/1992). 

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