Air separation with medium pressure enrichment

In a process for the subambient distillative separation of a mixture of noncondensable gases comprising distilling the feed mixture at a single supply pressure in at least one refluxed high pressure (HP) distillation column to a nearly pure overhead liquid and an enriched liquid containing the bottom product;

• further distilling at least one enriched fluid containing the bottom product in a reboiled low pressure (LP) column to nearly pure fluid bottom product and nearly pure gaseous overhead product including directly injecting at least part of said nearly pure overhead liquid into said LP column as reflux;
• and causing HP column reflux and LP column reboil by indirect heat exchange between the two columns, the improvement comprising;
• (A) distilling a single supply pressure feed mixture in the HP column;
• (b) providing a medium pressure (MP) distillation column operating at a pressure intermediate to said high and low pressures;
• (c) distilling in the MP column essentially all the enriched liquid containing the bottom product produced by the HP column to a relatively pure gaseous overhead product at medium pressure and a further enriched fluid containing bottom product;
• (d) refluxing the MP column by directly injecting part of said overhead liquid into the upper portion thereof;
• (e) reboiling the MP column by indirect heat exchange with the HP column;
• (f) routing the further enriched fluid obtained in step (b) into the LP column;
• (g) recovering a bottom product from the LP column of up to 99.5% purity.