Gas liquefaction method

In the liquefaction of a permanent gas comprising nitrogen the temperature of a permanent gas stream at a pressure in the range of 75 to 90 atmospheres is reduced to below its critical temperature by passage through heat exchangers 10, 12 and 14. Providing at least part of the necessary refrigeration, a single nitrogen working fluid cycle 36 is performed in which the nitrogen working fluid is compressed to a pressure in the range of 75 to 90 atmospheres, cooled to a temperature in the range of 170 to 200 K, work expanded in expansion turbine 38 to a temperature in the range of 107 to 120 K, and warmed by heat exchange countercurrently to the permanent gas stream, thereby providing refrigeration for the permanent gas stream.