GB776154A - Improvements in and relating to the fractional distillation of mixtures of liquids - Google Patents

Abstract

<PICT:0776154/III/1> <PICT:0776154/III/2> In a process of fractionating a liquid mixture in a distillation system comprising one or more fractionating columns wherein the sump of the system is heated in known manner, a vapour fraction withdrawn from a column is subjected to indirect heat-exchange with an auxiliary liquid whereby the vapour fraction is condensed and the auxiliary liquid is vaporized, the vapours of the auxiliary liquid are raised in temperature by compression and used to effect vaporization, by heat-exchange, of part of the liquid to be rectified, the temperature of this liquid being higher than that of the vapours withdrawn from the column by an amount which is less than the maximum temperature-difference in the system. In a modification the auxiliary vapours are absorbed in a liquid whereby heat is evolved, and this heat is transferred to the liquid to be rectified. In an example, a liquid mixture comprising 20 per cent (by volume) of ethyl alcohol and 80 per cent of water is introduced at a temperature of 80 DEG C. through pipe 1 (Fig. 1) into the lower part of a fractionating column which is heated at the base. A vapour mixture containing 86 mol. per cent of alcohol is withdrawn from the head of the column. About 25 per cent of this vapour mixture is condensed in condenser 6, the condensate being withdrawn from the system. The remainder of the vapour mixture is passed to heat-exchanger 4 where it is condensed by transmission of heat to benzene supplied to the heat-exchanger via pipe 7. The benzene, which is under an absolute pressure of 650 mm., vaporizes at a temperature of 75 DEG C. Part of the concentrated alcoholic condensate is returned to the column as reflux, and the remainder is withdrawn through pipe 5. The benzene vapours are compressed to 1.35 atm. in compressor 9, whereby the temperature is raised to 88.5 DEG C.; and the compressed vapours are introduced into coil 10 which is immersed in liquid flowing from the upper section of the column into collector 11. The benzene is condensed giving up heat to the column-liquid which, having a boiling point of 80 DEG C., is partially vaporized. The vapours pass up the column, while the unevaporated liquid weirs over into the lower section of the column. Water is withdrawn from the base of the column. The condensed bnzene is returned to heat-exchanger 4 after being relieved from pressure in vapour-trap 12. The apparatus shown in Fig. 1 can also be used for separating anthracene and carbazole. The separation of naphthalene and methylnaphthalene from a tar residue is also exemplified. In another example, a mixture of benzene and toluene at 90 DEG C. is introduced through line 13 (Fig. 2) into the lower section of a fractionating column which is heated at the base. Benzene vapours are withdrawn from the top of the column and introduced into heat-exchanger 16 where they are condensed by heat-exchange with ammonia boiling under a pressure of 16 atm. Part of the condensed benzene is withdrawn as product, and the the remainder is returned to the column as reflux. The ammonia vapours withdrawn from exchanger 16 are absorbed by water in absorber 17, which is immersed in liquid flowing down from the upper part of the column into collector 18. The heat set free by this absorption is transmitted to the liquid in the collector, which boils at 90 DEG C. Ammonia solution is pumped from absorber 17 to stripper 20. The water, from which ammonia has been expelled by heating, flows back through reducing-valve 21 to the absorber. The ammonia gas from the top of the stripper is condensed in heat-exchanger 22 at a temperature of 95 DEG C. and a pressure of 26 atm., the heat being transmitted to the liquid in collector 18. After reduction of pressure in reducing valve 23, the liquid ammonia is returned to heat-exchanger 16. Toluene is withdrawn from the base of the column.