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 turboexpander used in generating refrigeration. The pressure ratio is varied by varying the pressure of a compressed stream fed to the turboexpander. This is done by solely compressing such compressed stream by a first booster compressor 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 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.