Counterflow/sweep membrane module

In the case of the counterflow/sweep membrane module illustrated in Figure 8.5 (c), a portion of the dried residue gas stream is expanded across a valve and used as the permeate-side sweep gas. The separation obtained depends on how much gas is used... 

Figure 8.5 Comparison of (a) crossflow, (b) counterflow and (c) counterflow sweep module performance for the separation of water vapor from air. Membrane water/air selectivity = 100, water permeance = 1000 gpu.

Figure 8.6(b) shows an equivalent figure for a counterflow module in which 10% of the residue gas containing 500 ppm water vapor at lObar is expanded to lbar and introduced as a sweep gas. The water-vapor concentration in the permeate gas at the end of the membrane then falls from 4500 to 500 ppm, producing a dramatic increase in water-vapor permeation through the membrane at the residue end of the module. The result is a two-thirds reduction in the size of the module required for the separation. 

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