A distillation method and apparatus having application to the distillation of air in which liquid production make is varied by varying the pressure ratio across a turbo expander (24) used in generating refrigeration. The pressure ratio is varied by varying the pressure of a compressed stream fed to the turbo expander. This is done by solely compressing such compressed stream by a first booster compressor (36) during a low rate of production of liquid products. During a high rate of production of liquid products, the compressed stream is also compressed within a second booster compressor. The second booster compressor (40) is driven by a variable speed drive to allow a variety of liquid production rates between the low level of liquid production and the high level of liquid production.
