Fractionation of low molecular weight component mixtures

Claim 1

IN APROCESS FOR FRACTIONATING A LOW MOLECULAR WEIGHT, NORMALLY GASEOUS, PREDOMINANTLY HYDROCARBON MIXTURE CONTAINING PRINCIPALLY C1-C4 HYDROCARBONS UTILIZING AT LEAST TWO DIMINISHING PRESSURE DISTILLATION ZONES, THE IMPROVEMENT WHICH COMPRISES THE STEPS OF WITHDRAWING AT LEAST TWO LIQUID FRACTIONS OF SAID MIXTURE FROM THE HIGHER PRESSURE DISTILLATION ZONE, ONE OF SAID FRACTIONS BEING A LIGHTER DISTILLATE FRACTION AND THE OTHER BEING A HEAVIER BOTTOMS FRACTION; PASSING THE BOTTOMS FRACTION AS FEED TO THE LOWER PRESSURE DISTILLATION ZONE; COOLING THE LIGHTER FRACTION AND PASSING IT AS REFLUX TO THE LOWER PRESSURE DISTILLATION ZONE; TAKING AN OVERHEAD PRESSURE STREAM FROM THE LOWER PRESSURE DISTILLATION ZONE; COOLING AND PARTIALLY CONDENSING THE OVERHEAD VAPOR STREAM; SUPPLYING THE RESULTING CONDENSATE AS ADDITIONAL REFLUX TO THE LOWER PRESSURE DISTILLATION ZONE; EXPANDING THE RESIDUAL UNCONDENSED OVERHEAD VAPOR STREAM TO DO WORK, THEREBY COOLING THE EXPANDED VAPOR STREAM; UTILIZING THE THUS COOLED EXPANDED VAPOR TO COOL THE OVER HEAD VAPOR STREAM FROM THE LOWER PRESSURE DISTILLATION ZONE; TAKING A RECYCLE PORTION OF THE EXPANDED VAPOR STREAM THROUGH A COMPRESSION ZONE WHEREIN WORK MADE AVAILABLE BY THE EXPANSION STEP IS UTILIZED TO COMPRESS THE EXPANDED VAPOR; COOLING THE THUS COMPRESSED VAPOR STREAM; RETURNING THIS THUS COMPRESSED VAPOR STREAM AS RECYCLE TO THE EXPANSION STEP AND WITHDRAWING THE RESIDUAL PORTION OF THE EXPANDED VAPOR STREAM AS PRODUCT.