Gas permeation is a method of separating gases based on their permeation rates through a membrane. A high-pressure feed stream contacts the membrane. A high-pressure feed stream contacts the membrane. Permeable fraction of flow preferentially crosses membrane due to partial pressure difference between components. The permeate is enriched in the most permeable gases (those with the permeation the highest). The retentate (or residue) fraction is concentrated in the least permeable species (lowest permeation rates). In some configurations, a purge flow circulates on the same side as the permeate flow, increasing the partial pressure difference on either side of the membrane.