Air boiling process to produce low purity oxygen

In a process for the production of oxygen by the fractionation of air, wherein feed air at greater than atmospheric pressure and substantially free of water and carbon dioxide is introduced (a) into a medium pressure column where it is separated by rectification into a nitrogen-rich fraction and an oxygen-enriched fraction, and (b) into a low pressure column where it is separated by rectification into a nitrogen-rich fraction and a product oxygen fraction;

• and wherein the low pressure column bottoms are reboiled in a main condenser to produce vapor reflux by indirect heat transfer with a higher pressure condensing feed air stream, the improvement comprising:
• (1) introducing from about 50 to 75 percent of the gaseous feed air into the main condenser as said condensing feed air stream;
• (2) introducing from about 25 to 50 percent of the gaseous feed air into the medium pressure column at a pressure lower than the pressure of said condensing feed air stream;
• (3) expanding the condensed feed air stream to a lower pressure;
• (4) introducing from about 50 to 75 percent of the condensed feed air stream into the medium pressure column as additional feed;
• and
• (5) introducing from about 25 to 50 percent of the condensed feed air stream into the low pressure column as feed;
• thereby enabling both the low pressure column and the medium pressure column to be operated at down to the lowest effective pressure and up to the most efficient reflux ratio.