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Gas separation membranes are expected to play a significantrolein green house reduction, renewable energy production, and energysavings for separations. However, newly developed membrane materialsare constrained by the trade-off between selectivity and permeabilityand existing body of literature lacks the clear guidance on directionsfor the investigation. In this paper, we first systematically reviewedthe effects of mixed gases and contaminants on membrane selectivitiesfrom gas transport mechanism. We then examined cascade membrane processdesign to achieve the desired product recovery and purity, utilizingtwo types of membranes and utilizing pressure ratio as the guidanceto design the stages. From these examinations, we concluded that highmembrane selectivity is critically needed, while high membrane permeabilityhas limited impact for practical applications. Process design andeconomics for biogas purification was utilized as an example to demonstratethe need for high selectivity membranes. We further identified somegas separation applications that are critically in need of high membraneselectivities, such as CO2 capture from flue gases, andhighlighted recent progress in membrane materials with high gas selectivitiesfor these applications.
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