Patent GB-748637-A (GB)
Title: Improvements in defrosting a gas mixture separation system including a gas rectification or fractionating column
Inventor: SCHILLING CLARENCE JOHN
IPC: F25J3/08
Filed: 1953-12-18
Abstract: 748,637. Cold separation of gas mixtures. AIR PRODUCTS, Inc. Dec. 18, 1953, No. 35355/53. Class 8(2). In a process for defrosting a low temperature gas separation plant including a conventional two-stage air separation column 69 comprising passing therethrough warming-up air and thereby melting condensed impurities e.g., carbon-dioxide, removal from said warming-up air of impurities having a freezing point above the normal working temperature of the column is effected by cooling said warming-up air to below the freezing point of said impurities but above the normal working temperature by heat exchange with defrosting air leaving the column 69. Normal separation of air from a compressor 15, 16 is effected by passing it through after coolers 17, 18 to a point 20 where it is divided in an 80: 20 ratio. The major portion m a manitold 21 traverses tubes 25 of one or other of interchangers 23A, 23B according to the setting of reversing valves 29A, 29B ; the shell of the interchanger in use, e.g., 23A being traversed by returning nitrogen from the head of low pressure column 71 as are also the tubes 25 of the other exchanger 23B being derimed. Auxiliary passages 26 of both interchangers are continuously traversed by nitrogen from the head 85 of the high pressure column 70 and this nitrogen then traverses ducts 94, 95 a valve 98, duct 96, an expander 97 and a valve 123 to join the low pressure nitrogen from the column 71. The minor portion of air leaving the point 20 traverses one or other of interchangers 50A, 50B which are similar to interchangers 23A, 23B but have no auxiliary passages and are traversed by oxygen from the base of the low pressure column 71. The so cooled minor portion of air then combines with the major portion in a duct 48 and after further cooling by returning gas in an exchanger 84 is fed to the base of the high pressure column 70. To effect defrosting of the column 69 air is supplied thereto as for normal separation except that cooling water is shut off from the aftercoolers 17, 18, the expander 97 is rendered idle and drain valves 120, 121, 122 at the bases of the two sections of the column are opened. When the temperature of the column 69 reaches -40‹C. and that of the interchangers is such that no condensation of impurities in the warming-up air can occur then all the incoming air leaving the interchangers is diverted from duct 48 into ducts 110, 90, 106, 95, through a valve 99 and a conventional air drier 107 into duct 96 and a portion is then fed through a valve 119 to column 71 and a further portion is fed through a valve 124, idle expander 97 and conduits 126, 48 and a valve 108 to the high pressure column 70 ; this diversion being effected by having valves 52, 93, 99, 100, 124 open and valves 125, 123, 98 closed. If during the initial warming up period the interchangers 23 A, B, 50, A, B, warm up more quickly than the column is being defrosted additional cold is added to the interchangers by passing air from the head of column 70 through ducts 89, 106,95, 96, valve 124, expander 97 which is restarted for this purpose valve 123 and duct 49 leading to the interchangers. An alternative method of adding cold is to by-pass a portion of air from line 48 through ducts 110, 90, 106, valve 93, duct 95, valve 98, duct 96 and valve 124, the restarted expander 97 and duct 101 leading to the interchangers. Specification 666,914 is referred to.