Abstract: In a low pressure type air separation apparatus provided with a reversing heat exchanger for removing water vapor and carbon dioxide gas in feed air, a nitrogen gas taken out of a high pressure column in a duplex type rectification tower is led to an expander for generating subcooling for the air separation apparatus, and the effluent nitrogen gas is obtained under a predetermined pressure by controlling a pressure of the effluent gas from the expander. The effluent nitrogen gas is adjusted to a temperature satisfactory for the inlet temperature conditions for the reversing heat exchanger by passing the effluent nitrogen gas through a heat exchanger, and then is passed through the reversing heat exchanger, whereby the effluent gas from the reversing exchanger is obtained as a product nitrogen gas under a predetermined pressure.

Abstract: In a process of and a system for liquefying air to separate its components employing a so-called compound fractionating column, the system includes a fractionating medium pressure column into which a part of feed air is introduced to flow therethrough as an upwardly flowing gas for contact with a returning liquid, so as to obtain nitrogen gas at the top thereof and change the returning liquid to liquefied air as it reaches the bottom thereof, a first condenser adapted for a heat exchange between a liquefied air of a reduced pressure, which has been obtained by allowing an expansion of the liquefied air available at the bottom of the medium pressure column, and the nitrogen gas obtained at the top of the medium pressure column, so as to liquefy the nitrogen gas to make use of it as the returning liquid for the medium pressure column, a second condenser adapted for a heat exchange between the remainder part of feed air and liquefied oxygen to evaporate the oxygen, and a fractionating low pressure column through