Patent GB-700689-A (GB)
Title: Improvements in or relating to the method of fractionating mixtures
Inventor: KNIEL LUDWIG
IPC: F25J3/02
Filed: 1950-12-12
Abstract: <PICT:0700689/III/1> A fractional distillation system wherein a liquid refrigerant is used to cool the reflux stream whereby the refrigerant is vapourized and these vapours are compressed and used to heat the reboiler, characterized in that a part of the compressed vapours are further compressed and passed to a condenser, vapour from which is passed to the reboiler in accordance with the heat requirements in the bottom of the fractionation column.  The mixture, e.g. a hydrocarbon mixture, is fed to a column 10 via line 11, some of the overheads being recycled via line 12, reflux condenser 13, storage drum 14 and line 16 and some of the bottoms being recycled via line 19, reboiler 18 and line 20.  A liquid refrigerating medium is circulated from accumulator 32, via line 34, heat-exchanger 26, expansion valve 35, reflux condenser 13 wherein it is vapourized, line 36, and a drum 24 where entrained liquid is separated and returned as vapour to the accumulator 32. Refrigerant vapours from the drum 24 pass via the heat-exchanger 26, the low pressure stage of a compressor 22 where their temperature is raised, line 50, the reboiler 18 wherein the vapours are liquified, valve 29 actuated by the temperature or pressure in column 10, line 30 and back to the accumulator 32. Some vapours in line 50 are cycled via line 52, the high pressure stage 54 of compressor 22, line 56, a condenser 44 also supplied with cooling water, a trim tank 47, supplied with a liquid level control valve 45, and back to accumulator 32.  Liquid refrigerant is also cycled from accumulator 32 via a pump 42, condenser 44 and trim tank 47 and back to accumulator 32.  Vapours from tank pass via a pressure controlled valve 46 to line 50.  If the temperature in column 10 is at the desired level, pressure in line 50 (as determined by valve 29) is such that valve 46 is partially closed and the pressure in condenser 44 is such that vapours from line 56 are not condensed and valve 45 is thus controlled to be open.  If too much heat is supplied to the reboiler 18, the actuated valve 29 closes, the pressure in line 50 increases, valve 46 closes, pressure in condenser 44 increases and thus vapours passing therethrough are condensed; thereby decreasing the vapour passing through line 50 to the reboiler.  If too little heat is supplied to the reboiler 18, valve 29 opens, pressure in line 50 decreases, valve 46 opens, pressure in condenser 44 drops and thus liquid refrigerant passing therethrough is vaporized; thereby increasing the vapours supplied to the reboiler 18 via line 50.  Increase of pressure of liquid refrigerant in the accumulator 32 is compensated for by the opening of a pressure controlled valve 39 in line 38 which passes excess vapours to the low pressure stage of compressor 22.  A temperature controlled valve (not shown) in line 36 may control the amount of refrigerant entering the reflux condenser 13.  The refrigeration system may control the cooling fluid for the reflux condensers and the heating fluid for the reboilers of a plurality of fractionating columns.  The described system is suitable for the separation of ethylene from ethane using a pressure of 170lbs./sq. in., a top column temperature of -40 DEG F. and a reflux ratio of 4.5.