Abstract: The object of the present invention is to separate air at low power cost to produce high-pressure nitrogen gas and oxygen gas by utilizing cold of liquefied natural gas. Feed air is pre-purified and then introduced into a cryogenic separation unit, so that oxygen gas and nitrogen gas are separated. The nitrogen gas separated in the cryogenic separation unit is introduced into a second heat exchanger, and cooled by indirect heat exchange with a refrigerant. The nitrogen gas thus cooled is compressed in a low-temperature nitrogen compressor and supplied through a heater to an external plant for consumption. LNG is introduced into a first heat exchanger as a cold source. In the first heat exchanger, LNG cools and liquefies the refrigerant by indirect heat exchange. The liquefied refrigerant is sent to the second heat exchanger. The refrigerant is vaporized by indirect heat exchange with the nitrogen gas in the second heat exchanger. The vaporized refrigerant is returned to the first heat exchanger.

Abstract: A unit capable of simultaneously producing liquid nitrogen of ultra high purity and liquid oxygen of ultra high purity, is provided. The inside of a first rectification column 6 is demarcated to an upper rectifying part 12, a middle rectifying part 13 and a lower rectifying part 14. To the upper part 11 above the upper rectifying part 12 is connected a nitrogen condenser 8. A second rectification column 7 has a reboiler 24 provided under its rectifying part 22. Ultra high purity liquid nitrogen is recovered from between the upper rectifying part 12 and middle rectifying part 22 of the second rectification column 7, where it is brought in countercurrent contact with gas evaporated by the reboiler 24 provided below the rectifying part 22 so that lower boiling point components are separated therefrom. Thus, ultra high purity liquid oxygen is recovered from below the rectifying part 22 of the second rectification column 7.