The energy efficiency of the separation of air into an oxygen-enriched permeate and an oxygen-depleted residue stream in a polymeric hollow fiber membrane module is studied. On the basis of the modeled performance of the module, three different counter-current lumen-feed flow system configurations are compared and analyzed. The vacuum permeate configuration, where a vacuum is maintained on the permeate side of the membrane, is the most efficient single-stage process. A two-stage configuration, where the permeate of the first module is further enriched in a second stage, is also considered. By optimizing both stages the specific energy consumption can, in some cases, be further reduced. The optimization of the module geometry is not as important as the optimization of the feed parameters - the pressure ratio and stage cut. The results of polymeric membrane air separation are compared to cryogenic distillation, pressure-swing adsorption, and ion transfer membranes. Present-day polymeric membrane modules can compete with traditional techniques in specific energy consumption when the required oxygen molar fraction is low. A single-stage polymeric membrane module with a selectivity of 100 would be more efficient than other techniques up to an oxygen purity of approximately 92%. (C) 2012 Elsevier Ltd. All rights reserved.