of third column 115 within third column 115 the feeds to the third column are separated by cryogenic rectification into nitrogen containing fluid having a nitrogen concentration generally within the range of from 99 to 99 999 mole percent and into oxygen containing fluid having an oxygen concentration generally within the range of from 35 to 50 mole percent oxygen containing fluid is withdrawn as liquid stream 13 from the lower portion of third column 115 passed through valve 121 and into the vaporizing section of top condenser 122 second oxygen enriched liquid is withdrawn from the lower portion of second column 107 as stream 113 subcooled by passage through heat exchanger 123 and passed as stream 114 into the vaporizing section of top condenser 122 nitrogen containing fluid is passed as vapor stream 124 from the upper portion of third column 115 into the condensing section of top condenser 122 within top condenser 122 the nitrogen containing fluid is condensed by indirect heat exchange with the liquids passed into the vaporizing side to produce liquid nitrogen and waste gas the waste gas is withdrawn from top condenser 122 as stream 45 progressively warmed by passage through heat exchangers 123 105 and 102 and removed from the system as stream 48 the condensed nitrogen containing fluid i e liquid nitrogen is passed as steam 125 into third column 115 as reflux preferably a portion 31 of stream 125 is increased in pressure by passage through liquid pump 126 to about the operating pressure of first column 104 resulting pressurized stream 32 is warmed by passage through heat exchanger 123 and resulting stream 33 is passed through valve 127 and into the upper portion of first column 104 wherein it serves as additional reflux for the cryogenic rectification.