Crude He obtained from natural gas by low temperature liquefaction, containing approximately 70 pct He and 30 pct N2, is either purified to commercial grade (>99.99 pct He) or stored in underground reservoirs. Few suitable reservoirs are available so it is desirable to further enrich the gas to increase storage capacity. An economical enrichment method is described, consisting of cooling the crude he (e.g., to -306 deg-F) by indirect heat exchange without reduction of pressure (e.g., 1,200 psia). This causes partial liquefaction, e.g., 28 pct liquid. The He-rich vapor (e.g., 96.1 pct He) is separated from the liquid and passed through an expansion engine in which the pressure is reduced to 1,000 psia and the temperature to -311 deg-F. The vapor from the engine is used to cool the incoming gas and is then sent to storage. The liquid from the separation step is treated to strip out the residual He, cooled by passing through an expansion valve, and the resulting gas is also used to cool the incoming gas.
U.S. Pat. 3,599,438; Aug. 17, 1971; Chem. Abstr. 75:99,553R