Abstract: Nitrogen is rejected from a pressurized feed gas stream comprising methane and nitrogen by cooling the feed gas stream in a main heat exchanger and rectifying the cooled feed gas stream in a double rectification column comprising a higher pressure rectification column, a lower pressure rectification column, and a condenser-reboiler placing the higher pressure column in heat exchange relationship with the lower pressure column. A pump withdraws a methane product stream from the column. The methane product stream is pressurized by the pump and is vaporized in the main heat exchanger. The pressurized feed gas stream is expanded with the performance of external work in an expansion turbine 6 upstream of the main heat exchanger.

Abstract: Air is compressed in compressors, cooled in a main heat exchanger, and separated into oxygen and nitrogen products in a double rectification column comprising a higher pressure rectification column and a lower pressure rectification column. A liquid oxygen product is withdrawn from the lower pressure column via a conduit. A liquid nitrogen product is also formed. An argon-enriched oxygen vapour stream is withdrawn from the lower pressure column through an outlet and has argon separated from it in an argon column. In order to help meet the requirements of the higher pressure column for reflux, a nitrogen vapour stream is withdrawn from the top of the lower pressure column, is warmed by passage through the heat exchanger, is recompressed in a compressor, and is liquefied by passage back through the heat exchanger from its warm end to its cold end and passage through a valve. A high liquid make and a high argon recovery can both be achieved.