Low temperature distillation of normally gaseous substances

Claim 1

IN A PROCESS FOR PRODUCING COMPRESSED GASEOUS OXYGEN BY THE LOW TEMPERATURE RECTIFICATION OF AIR IN A DOUBLE RECTIFICATION COLUMN HAVING A BOTTOM HIGH PRESSURE COLUMN AND A TOP LOW PRESSURE COMUMN WHEREIN CIRCULATING NITROGEN IS UTILIZED AS A HEAT TRANSFER MEDIUM, AND WHEREIN AT LEAST A PORTION OF LIQUEFIED OXYGEN FROM THE BOTTOM OF THE LOW PRESSURE COLUMN IS COMPRESSED AND THEN PASSED IN HEAT TRANSFER RELATIONSHIP WITH THE CIRCULATING NITROGEN, SAID OXYGEN BEING THEREBY VAPORIZED, THE IMPROVEMENT WHICH COMPRISES (1) WITHDRAWING A NITROGEN ENRICHED STREAM FROM THE TOP ZONE OF THE HIGH PRESSURE COLUMN, (2) PREHEATING SAID STREAM BY HEAT EXCHANGE PARTLY WITH AIR TO BE SEPARATED AND PARTLY WITH A PORTION OF SAID STREAM AFTER BEING COMPRESSED, (3) COMPRESSING SAID PREHATED NITROGEN ENRICHED STREAM, (4) EXPANDING AT LEAST A PORTION OF SAID NITROGEN ENRICHED STREAM THROUGH AN EXPANSION TURBINE TO PRODUCE EXTERNAL WORK AFTER BEING PRECOOLED BY SAID HEAT EXCHANGE, (5) RECYCLING THE RESULTANT EXPANDED NITROGEEN STREAM TO THE HIGH PRESSURE COLUMN, (6) SIMULTANEOUSLY PASSING A SECOND PORTION OF THE COMPRESSED NITROGEN ENRICHED STREAM IN HEAT EXCHANGE RELATIONSHIP WITH AT LEAST ONE LOW TEMPERATURE FRACTION FROM THE DOUBLE COLUMN, THEREBY COOLING THE SECOND NITROGENENRICHED STREAM, AND (7) EXPANDING THE RESULTANT COOLED SECOND STREAM INTO THE TOP ZONE OF THE HIGH PRESSURE COLUMN.