Three column cryogenic cycle for the production of impure oxygen and pure nitrogen

A method of operating a cryogenic distillation column having a higher pressure stage, a lower pressure stage, and a medium pressure stage, to produce at least one of nitrogen and impure oxygen, said method comprising the steps of:

• (a) compressing and cooling feed air to provide (i) a first feed air stream having a first pressure and (ii) a second feed air stream having a second pressure less than said first pressure;
• (b) introducing said second feed air stream into said medium pressure stage for rectification into a medium pressure, oxygen-enriched liquid and a medium pressure nitrogen overhead stream;
• (c) introducing a first fraction of said first feed air stream into said higher pressure stage for rectification into a higher pressure, oxygen-enriched liquid and a higher pressure nitrogen overhead stream;
• (d) condensing said higher pressure nitrogen overhead stream against a liquid from said lower pressure stage to form higher pressure nitrogen condensate and returning a first portion of said higher pressure nitrogen condensate to said higher pressure stage as reflux and introducing a second portion of said higher pressure nitrogen condensate to said lower pressure stage as reflux;
• (e) withdrawing an oxygen-enriched product from a position near the bottom of said lower pressure stage;
• and
• (f) withdrawing a nitrogen-enriched product from a position near the top of said lower pressure stage, characterized in that the method further comprises:
• (g) reducing the pressure of at least a portion of at least one of said medium pressure, oxygen-enriched liquid and said higher pressure, oxygen-enriched liquid to form a first reduced-pressure, oxygen-enriched liquid;
• (h) condensing said medium pressure nitrogen overhead stream against said first reduced-pressure, oxygen-enriched liquid, resulting in an oxygen-enriched vapor stream and a medium pressure nitrogen condensate, and returning a first portion of said medium pressure nitrogen condensate to said medium pressure stage as reflux and introducing a second portion of said medium pressure nitrogen condensate to said lower pressure stage as reflux;
• and
• (i) introducing said oxygen-enriched vapor stream to said lower pressure stage as feed.