Dual pressure cycle for air separation

768,923. Cold separation of gas mixtures. UNION CARBIDE & CARBON CORPORATION. April 29. 1955 [May 4, 1954], No. 12558/55. Class 8(2) In a binary gas mixture. e.g., air rectification process for the production of gaseous and liquid oxygen wherein low pressure air from a compressor 10 e.g., at 70 p.s.i. after being cooled and partially liquefied in regenerators 12 to 15 traversed by a first portion of cold returning nitrogen and oxygen is joined before passing to a conventional two stage rectifying column 59 by a second stream of air previously compressed by a compressor 40 to about 2,000 p.s.i. and divided into a first portion which is cooled by expansion in an engine 47 to said low pressure and into a second portion which is cooled in an exchanger C traversed by a second portion of cold returning oxygen and nitrogen and by expansion at a valve 55 to said low pressure, the unliquefied portion of the combined low pressure air streams is superheated in a coil 130 and expanded in a turbine 57 to yield any saturated vapour which is fed either through a valve 124 to the column 59, or through a valve 92 to join nitrogen effluent in a duct 81 or is fed through both valves according to the desired proportions of liquid and gaseous oxygen products which issue through valved ducts 76, 91, 75. The combined low pressure air streams traverse a scrubber 17 and scrubbed liquid is expanded at a valve 24 and is freed from residual impurities by one or other of filters 25, 25a before passing to the low pressure column 61. A portion of vapour from the scrubber 17 traverses a duct 26 leading to the base of the high pressure column 60, a second portion traverses a duct 27 to a condenser 28 and is returned as liquid through a duct 29 whilst the remaining vapour undergoes the superheating in coil 130 and expansion in turbine 57. The proportions of liquid and gaseous oxygen yield is controlled by adjustment of the nozzle valves of the turbine 57, the by-pass valve 92 and valves 150, 89, 93. For the production of liquid oxygen only the compressor 40 is operated at full pressure and volume, oxygen regenerators 12, 13 are closed, valve 92 is opened wide, valves 124, 150, 89 are closed and liquid oxygen outlet valve 93 is opened. Heat balance is maintained by increasing the effluent nitrogen flow through a line 52 and decreasing that through regenerators 14, 15. For large gas and small liquid oxygen yield the compressor 40 is operated at minimum pressure and volume, the quantity of ai