AbstractThe permeation behavior of the high‐flux asymmetric membrane differs from that of the conventional symmetric membrane. A calculation method for predicting the gas separation performance of a permeator with asymmetric membrane is presented. The model takes into account the permeate pressure drop and is applicable to both hollow‐fiber and spiral‐wound modules. The effect of permeate‐feed flow pattern on module performance is analyzed. It is shown that for the high‐flux asymmetric membrane, the countercurrent flow pattern is not necessarily always the preferred operating mode. The mathematical model is verified by large‐scale field pilot‐plant experiments for helium recovery from natural gas using large hollow‐fiber modules (220 m2/unit).