Abstract: A compressed, purified air stream is reduced in temperature by heat exchange with returning streams in heat exchanger. It is passed through a Joule-Thomson valve, and a resulting stream comprising liquid and vapor is separated in a phase separator. A resulting vapor stream is introduced through inlet into the higher pressure column of a double distillation column comprising the higher pressure column and a lower pressure column. Oxygen-rich liquid is withdrawn from the bottom of the higher pressure column through an outlet and introduced into the lower pressure column through an inlet. Pure liquid oxygen and gaseous oxygen products are withdrawn from the lower pressure column through outlets. A liquid steam is withdrawn from the phase separator and introduced into the lower pressure column through an inlet at a level above that of the oxygen-rich liquid inlet, thereby making possible more efficient operation of the lower pressure column.