GB1066342A - Improvements in and relating to fractional distillation - Google Patents

Abstract

<PICT:1066342/C4-C5/1> In a method of separating a liquid mixture into fractions by distillation in a rectification system comprising bubble trays, provision being made for the independent control of the reflux ratio and the pressure at two or more sections of the system, the reflux ratio and pressure required for the separation are applied to the section where the composition of the mixture undergoing rectification is relatively constant from tray to tray in the section, and a lower reflux ratio and higher pressure are applied to the ends of the system where there is a relatively rapid change in composition from tray to tray. Thus the reflux ratio is highest and the pressure is lowest in the section where the separation is most difficult. The method is particularly applicable to superfractionation, such as is required in the separation of ethylbenzene from a mixture with xylene isomers. The rectification may be carried out in a system comprising two rectifying columns 1, 2, a pinch column 3 and a stripping column 4. The liquid to be separated is pumped through line 74 and preheater 78 into column 3, which is heated by reboiler 68 and by the vapours conducted through line 40 from the head of column 4. The overhead vapour from column 3 passes to condenser 50. Part of the condensate is pumped through valve 62 to the top tray of column 3, the remainder being pumped through valve 110 and line 104 to the reboiler 106 of column 2. Some of the liquid withdrawn from the base of column 2 passes to reboiler 106, and the remainder is pumped through line 52 to mix with the main reflux for column 3. The overhead vapour from column 1 passes to condenser 12. Part of the condensate is refluxed through line 122 to the top tray of column 1, and the remainder is withdrawn from the system as overhead product. Columns 1, 3 and 4 are provided with pressure-controllers 100, 64 and 36, respectively; and columns 1, 2 and 4 are operated at higher pressure than column 3. The bottoms product is withdrawn from the base of column 4. In a modification of the system, the liquid passing from the base of column 1 to the top of column 2 flows in counter-current heat exchange with the vapour which has come from the head of column 2. The vapour is then compressed before being introduced into the base of column 1. Part of the vapour from the head of column 3 is passed in heat exchange with the liquid being conducted from the base of column 2 to the top of column 3; the vapour is then compressed and introduced into the base of column 2.