arrangement a combined stream that is derived from an exhaust stream of a turboexpander could be supplied to the lower pressure column or an exhaust stream produced from high pressure nitrogen expansion in the illustrated embodiment these streams are rectified within higher pressure distillation column 52 by contacting an ascending vapor phase of such mixture within mass transfer contacting elements 66 and 68 with a descending liquid phase that is initiated by reflux stream 62 this produces a crude liquid oxygen column bottoms 70 and the nitrogen rich column overhead that has been previously discussed a stream 72 of the crude liquid oxygen column bottoms is expanded in an expansion valve 74 to the pressure of the lower pressure column 54 and introduced into such column for further refinement along with the second liquid stream 48 second liquid stream 48 is passed through an expansion valve 76 and expanded to the pressure of lower pressure column 54 lower pressure column 54 is provided with mass transfer contacting elements 78 80 82 and 84 that can be trays or structured packing or random packing or other known elements in the art as stated previously the separation produces an oxygen rich liquid column bottoms 58 and a nitrogen rich vapor column overhead that is extracted as a nitrogen product stream 86 additionally a waste stream 88 is also extracted to control the purity of nitrogen product stream 86 both nitrogen product stream 86 and waste stream 88 are passed through a subcooling unit 90 subcooling unit 90 subcools reflux stream 64 part.