Abstract: Air is compressed in a first compressor and has carbon dioxide and water vapor removed therefrom in a purification apparatus. The air is then cooled by passage through main heat exchangers and to a temperature suitable for its separation by rectification. The cooled air is separated in a single rectification column. Liquid oxygen is withdrawn from the column by a pump and is passed through the heat exchangers and countercurrently to the air stream and is thereby vaporized, a high pressure gaseous oxygen product thus being formed. Nitrogen vapor is withdrawn from the top of the column through an outlet is warmed by passage through a further heat exchanger and the heat exchanger. The nitrogen is then divided. One part is further warmed in the heat exchanger, is compressed in a compressor, and is returned through the head exchangers as a heat exchange stream countercurrently to the oxygen product stream.