Separation of air

An apparatus for separating air by rectification, including:

• a double rectification column comprising a higher pressure column and a lower pressure column;
• at least one air compressor for compressing the air to a first pressure;
• a main heat exchanger for cooling the first flow of the compressed air to a temperature suitable for its separation by rectification;
• an inlet to the higher pressure column for the first flow,
• an expansion turbine for expanding with the performance of external work a second flow of the compressed air and having an inlet for the second flow of the compressed air and an outlet communicating with the lower pressure column, the expansion turbine being loaded by an electrical generator;
• and
• an outlet from the lower pressure column for an impure oxygen product formed of a bottom fraction having an oxygen content in the range of about 50 to about 96 mole percent;
• there being no additional compressor for raising the pressure of either the said first flow or the said second flow of the compressed air above the first pressure, and the double rectification column additionally including a condenser-reboiler placing the higher pressure column in direct heat exchange relationship with the lower pressure column.