Abstract: Helium contaminated with minor amounts of air, water vapor and carbon dioxide and from an envelope (10) is compressed in a compressor (12) and thereafter cooled in cooler (18) and fed via water separator (20) to a first of two substantially helium-permeable, oxygen/nitrogen-impermeable membranes (28,34), the permeate from which is substantially pure helium. The impermeate still containing helium is fed to the second membrane (34), the permeate from which is recycled to the first membrane while the impermeate is rejected with little loss of helium. The method is useful in purifying helium in an airship or balloon.

Abstract: Carbon dioxide is recovered from gas mixtures containing it in a concentration not greater than the equilibrium concentration at the freezing temperature of the mixture, by subjecting the gas mixture to membrane separation and recovering from the membrane separation a permeate having a carbon dioxide concentration between the equilibrium concentration and 98% (by volume), distilling said permeate at subambient temperature above the freezing temperature of the permeate and recovering substantially pure carbon dioxide as a liquid bottoms product of the distillation.

Abstract: In the single column process for the recovery of nitrogen from air wherein reflux for the distillation is provided by condensing a first portion of the overhead stream by indirect heat exchange in a reflux condenser with oxygen-rich liquid from the bottom of the column which has been subjected to a first expansion to an intermediate pressure, refrigeration for the process is provided by work expanding evaporated oxygen-rich liquid from the reflux condenser, and prior to said first expansion, the oxygen-rich liquid recovered from the bottom of the column is sub-cooled by indirect heat exchange in a sub-cooler with the work-expanded evaporated oxygen-rich liquid, nitrogen recovery is increased by work expanding the evaporated oxygen-rich liquid at a temperature not substantially greater than that at which it is recovered from the reflux condenser or by first passing it through said sub-cooler at a temperature not substantially greater than that at which it is recovered from the reflux condenser and thereafter w