The clean air from the molecular sieve adsorber enters the braking compressor to consume the work generated by the expander and at the same is pressurized. After being cooled to about 40? in the after-cooler of the braking compressor, the boosted air enters the main heat exchanger to be cooled to a 165 K and then goes into the expander for expansion. The expanded air is further cooled in the heat exchanger and then enters the upper column for rectification. The remainder of the air is directly sent to the main heat exchanger to be cooled to its dew-point of about 100 k and then enters the lower column for rectification.