produce refrigeration by work expansion their respective boosters first booster 50 and second booster 53 utilize the produced work to further compress their respective nitrogen streams fig2 provides an alternate embodiment of the invention which includes two recycle compressors recycle compressor 40 and second recycle compressor 45 recycle aftercooler 41 single column 20 and subcooler 30 in one embodiment single column 20 operates at about 5 bar in one embodiment nitrogen recycle 38 is partially compressed in recycle compressor 40 and cooled in recycle aftercooler 41 to form partially compressed nitrogen recycle 42 in one embodiment partially compressed nitrogen recycle 42 has a pressure of about 8 bar partially compressed nitrogen recycle 42 is then further compressed in second recycle compressor 45 and cooled in second aftercooler 43 to form compressed nitrogen recycle 46 which preferably has a pressure of about 25 to about 27 bar fraction of partially compressed nitrogen recycle 44 is withdrawn from partially compressed nitrogen recycle 42 and fed to the warm end of heat exchanger 10 whereas fraction of compressed nitrogen recycle 48 is only partially cooled in heat exchanger 10 fraction of partially compressed nitrogen recycle 44 is fully cooled in heat exchanger 10 in one embodiment the split between compressed nitrogen recycle 48 and compressed.