Air separation method

A method of separating air comprising:

• fractionating argon, oxygen and nitrogen contained in at least one compressed, purified and cooled stream in an air separation system having a multiple column arrangement including a higher pressure column and a lower pressure column to separate the air into oxygen-rich and nitrogen-rich fractions and an argon column connected to the lower pressure column to receive an argon and oxygen-containing vapor stream and thereby to produce an argon-rich fraction as an argon-rich column overhead within said argon column for recovery of the argon;
• forming a two-phase stream containing a nitrogen-rich vapor phase and a liquid phase by expanding a liquid air stream or a crude liquid oxygen column bottoms stream composed of a liquid oxygen column bottoms formed within the higher pressure column, the liquid air stream being produced within the air separation system as a result of vaporization of a pressurized liquid stream made up of at least one of a liquid oxygen fraction and a liquid nitrogen fraction produced by the multiple column arrangement;
• disengaging at least part of the nitrogen-rich vapor phase from the liquid phase;
• recompressing said at least a portion of said nitrogen-rich vapor stream composed of the nitrogen-rich vapor phase and recycling the at least a portion of the nitrogen-rich vapor stream for fractionation in the multiple column arrangement of the air separation system;
• and
• introducing at least part of a liquid stream composed of the liquid phase disengaged from the nitrogen-rich vapor phase into at least one of the lower pressure column and the higher pressure column.