Multiple expander process to produce oxygen

In a process for the cryogenic distillation of air in a distillation column system that contains at least one distillation column wherein the boil-up at the bottom of the distillation column producing the oxygen product is provided by condensing a stream whose nitrogen concentration is equal to or greater than that in the feed air stream, the improvement which comprises the steps of:

• (a) generating work energy which is at least ten percent (10%) of the overall refrigeration demand of the distillation column system by at least one of the following two methods:
• (1) work expanding a first process stream with nitrogen content equal to or greater than that in the feed air and then condensing at least a portion of the expanded stream by latent heat exchange with at least one of the two liquids:
• (i) a liquid at an intermediate height in the distillation column producing oxygen product and (ii) one of the liquid feeds to this distillation column having an oxygen concentration equal to or preferably greater than the concentration of oxygen in the feed air;
• and
• (2) condensing at least a second process stream with nitrogen content equal to or greater than that in the feed air by latent heat exchange with at least a portion of an oxygen-enriched liquid stream which has oxygen concentration equal to or preferably greater than the concentration of oxygen in the feed air and which is also at a pressure greater than the pressure of the distillation column producing oxygen product, and after vaporization of at least a portion of oxygen-enriched liquid into a vapor fraction due to latent heat exchange, work expanding at least a portion of the resulting vapor stream;
• (b) work expanding a third process stream to produce additional work energy such that the total work generated along with step (a) exceeds the total refrigeration demand of the cryogenic plant and if the third process system is the same as the first process system in step (a)(1) then at least a portion of the third process stream after work expansion is not condensed against either of the two liquid streams described in step (a)(1).