Nitrogen liquefaction with plural work expansion of feed as refrigerant

Claim 1

IN A CYCLE FOR THE LIQUEFACTION OF LOW BOILING GAS WHICH COMPRISES THE STEPS OF: COMPRESSING A PRIMARY STREAM INCLUDING THE FEED STREAM OF SAID GAS TO A HEAD PRESSURE NOT EXCEEDING ABOUT TWO AND ONE-HALF TIMES THE CRITICAL PRESSURE OF SAID GAS, COOLING SAID COMPRESSED PRIMARY STREAM THROUGH A FIRST COOLING STEP IN A FIRST HEAT EXCHANGER, FURTHER COOLING AT LEAST A PORTION OF SAID COMPRESSED PRIMARY STREAM THROUGH A SECOND COOLING STEP IN A SECOND HEAT EACHANGER, WITHOUT SUBSTANTIAL LIQUEFACTION THEREOF, ISENTHALPICALLY EXPANDING SAID COMPRESSED PORTION AFTER SAID SECOND COOLING STEP TO A LOW PRESSURE TO FORM A LIQUID FRACTION AND A REMAINING VAPOR FRACTION, AND RETURNING THE REMAINING LOW PRESSURE VAPOR FRACTION OF SAID PORTION AT SAID LOW PRESSURE FOR RECOMPRESSION AND RECYCLING THROUGH A PATH INCLUDING SAID HEAT EXCHANGERS IN COUNTERCURRENT WITH STREAMS BEING COOLED INCLUDING SAID COMPRESSED PORTION; THE IMPROVEMENT WHICH COMPRISES: SEPARATING SAID COMPRESSED PRIMARY STREAM BEFORE SAID SEOND COOLING STEP INTO A MAJOR PORTION AND A MINOR PORTION, WHEREBY THE COMPRESSED PORTION FURTHER COOLED IN SAID SECOND HEAT EXCHANGER IS SAID MINOR PORTION, WITHDRAWING AND EXPANDING SAID MAJOR PORTION THROUGH A PLURALITY OF WORK EXPANSION STEPS WHEREIN SAID MAJOR PORTION IS COOLED AND EXPANDED TO A MEDIUM-LOW PRESSURE WITHOUT LIQUEFACTION, EACH OF SAID WORK EXPANSION STEPS BEING SEPARATED BY AN INTERVENING REHEAT STEP WHEREIN THE STREAM COMPRISING SAID MAJOR PORTION IS WARMED UP IN SAID SECOND HEAT EXCHANGER IN COUNTERCURRENT WITH STREAMS BEING COOLED INCLUDING SAID COMPRESSED MINOR PORTION, AND AFTER SAID FINAL WORK EXPANSION STEP, RETURNING SAID MAJOR PORTION AT A MEDIUM-LOW PRESSURE WITHOUT LIQUEFACTION THROUGH SAID HEAT EXCHANGERS IN COUNTERCURRENT WITH STREAMS BEING COOLED INCLUDING SAID COMPRESSED MINOR PORTION, FOR RECOMPRESSION AND RECYCLING.