Air separation with intermediate pressure vaporization and expansion

In a process for the separation of air in a cryogenic air separation system comprising a higher pressure distillation column and a lower pressure distillation column which is thermally linked with the higher pressure distillation column, wherein air is compressed and purified to remove higher boiling contaminants, at least a portion of the compressed purified air is cooled and distilled in the higher pressure column, at least a portion of the bottoms liquid from the higher pressure column is distilled in the lower pressure column, and at least one nitrogen-enriched stream and at least one oxygen-enriched stream are withdrawn from the system, a portion of the refrigeration required for operation of the air separation system is provided by a method which comprises:

• (a) vaporizing a condensed liquid containing at least about 20 mole % oxygen at a pressure between any pressure in the lower pressure column and any pressure in the higher pressure column to yield an intermediate pressure vapor;
• (b) work expanding the intermediate pressure vapor and introducing the resulting work-expanded stream into the lower pressure column;
• and
• (c) providing the heat for vaporizing the liquid in step (a) by indirect heat exchange with at least a portion of a sidestream vapor withdrawn from the lower pressure column to yield a cooled intermediate stream.