the inlet of the second compressor stage 23 the pressurized stream from the first compressor stage is joined with the stream for entering the inlet of the second compressor stage intercoolers 32 33 are arranged to cool the streams after compressing fig4 shows a first triple expansion process scheme 50 according to an embodiment of the invention the liquefaction process can be further improved by adding a third pressure level and a third expansion step in this configuration four pressure levels would exist for the circulation of nitrogen streams u2014 high pressure from the compressor discharge two intermediate pressures and low pressure hp high pressure nitrogen would be cooled in the cold box and the first extraction stream would feed the hp expander he generating a cold n2 stream which is fed back into the heat exchanger system and returns to the third stage suction of the main nitrogen compressor 22 23 24 more cooled hp nitrogen is taken in a second extraction stream to feed the ip intermediate pressure expander ie generating a second cold n2 stream which is fed back into the heat exchanger system and returns to the second stage suction of the main nitrogen compressor the remaining sub cooled hp nitrogen is taken in a third extraction stream to feed the lp low pressure expander le generating a third cold n2 stream which is fed back into the heat exchanger system and returns to the first stage suction of the main nitrogen compressor compressed nitrogen from the third stage compressor discharge is further boosted in pressure using the compressors hc ic lc coupled to the three expanders he ie le respectively each compressor is coupled to a respective expander over a common drive shaft in this way three.