A comparison is made of countercurrent recycle membrane cascades (5-stage constant recycle and 6-stage no-mix) with continuous membrane columns, two-unit series, and one-unit recycle membrane modules for the enrichment of O-2 from air, assuming ideal crossflow stages. The crossflow stage equations permit easy comparisons and give insight into the inefficiencies associated with the "one compressor" module designs. For the separation specified (production of O-2 enriched up to y(P) = 0.983, at a rate P = 17.6, from a feed x(F) = 0.21, rate F = 100), the one compressor designs require from about 90 to 16 times more total compressor duty, and from about 50 to 10 times more membrane area than the no-mix cascade design, depending on module design, to make the same separation. The one unit recycle module is least efficient, followed by the continuous membrane column, and the two unit series module. The basic reasons for the inefficiencies associated with the one compressor modules relative to the recycle cascades are explored.