Process for recovering nitrogen in low pressure type air separation apparatus

A process for recovering a product nitrogen gas in a low pressure type air separation apparatus provided with a reversing heat exchanger and a duplex type rectification tower having a high pressure column and a low pressure column, which comprises leading a nitrogen gas taken out of said high pressure column of said duplex type rectification tower to an expander for generating subcooling for the air separation apparatus, taking out the nitrogen gas from the outlet of the expander under a pressure higher than the pressure of a nitrogen gas to be taken out of said low pressure column of said duplex type rectification tower, adjusting the temperature of the nitrogen gas from the expander to a necessary inlet temperature for the reversing heat exchanger by passage through a heat exchanger, then passing the temperature adjusted nitrogen gas through the reversing heat exchanger, and taking the nitrogen gas out of the reversing heat exchanger as said product nitrogen gas.