PURPOSE: To separate impure oxygen product from air while operating a high pressure rectification column under an especially low pressure at the bottom part thereof because of a specified pressure at the bottom part of a low pressure rectification column.

CONSTITUTION: First stream of compressed, cooled air is introduced into a high pressure rectification column 12 through an inlet 14 and is separated into nitrogen vapor and oxygen enriched liquid. At least some of the nitrogen vapor is condensed in a condenser 22 and one stream of resulting liquid nitrogen is used as reflux in the column 12 and another stream as reflux in a low pressure rectification column 30. Oxygen enriched liquid withdrawn from the column 12 through an outlet 32 is separated in the column 30. Impure liquid oxygen product is withdrawn from the column 30 through an outlet 52. A less pure stream of liquid oxygen is withdrawn from the column 30 through an outlet 48, is reboiled in condenser-reboiler 16 by indirect heat exchange with a second stream of air and is reintroduced into from the column 30 at its bottom through an inlet 51.