Fractionation of air by using refrigeration from open cycle evaporation of external cryogenic liquid

Claim 1

In a process for the low-temperature separation of air in an air fractionating column, wherein refrigeration is supplied by the evaporation of methane used as an external refrigerant, and wherein the transfer of the cold from the refrigerant to the air is conducted by way of an intermediate auxiliary gas, the improvement comprising the steps of: withdrawing nitrogen gas from said air fractionating column, dividing the withdrawn nitrogen into two partial streams, warming one of said partial streams of nitrogen to ambient temperature in indirect heat exchange with unfractionated air, precooling resultant warmed partial stream in heat exchange with said refrigerant, the latter being at a pressure of about 1 atmosphere absolute, passing both said precooled partial streams of nitrogen and the other of said two partial streams of nitrogen as said auxiliary gas, in indirect heat exchange relationship with said methane refrigerant, the latter evaporating at a pressure of 0.12 - 0.3 atmospheres absolute, thereby condensing said nitrogen, returning resultant liquid nitrogen to the air fractionating column, and employing said liquid nitrogen as reflux liquid, with the provision that the steps of withdrawing said nitrogen from the air fractionating column, condensing said nitrogen, returning said liquid nitrogen tO the column and of employing said liquid nitrogen as reflux are conducted at approximately the same pressure within the range of 4 - 8 atmospheres absolute, with the further provision that said methane be maintained out of heat exchange contact with said air through a common heat exchange wall.