GB695501A - Improvements in or relating to the extractive distillation of hydrocarbons - Google Patents

Abstract

Close-boiling hydrocarbons, e.g. orthoand para-xylenes, are separated from each other by subjecting the mixture to extractive distillation using as solvent an aqueous <PICT:0695501/III/1> solution containing a salt of a polar organic compound (a "solubilizer"), such as a sodium salt of an organic sulphonic acid, the relative proportions of the hydrocarbon mixture and the solvent being adjusted so as to maintain a single liquid phase during fractionation. An aqueous solution containing at least 40 per cent by weight of the organic compound, is preferably employed. The aqueous solution may also contain an organic solvent such as aniline, and an auxiliary solubilizer such as potassium sulphate. The best separations are achieved when the hydrocarbon mixtures have narrow boiling ranges, e.g. 20 DEG C. A mixture comprising p-xylene (80 per cent) and o-xylene (20 per cent) is fed through line 1 into column 2, which is heated by reboiler 3; and a 40 per cent aqueous solution of sodium o-xylene sulphonate is supplied to the column through line 15. Aqueous vapours rich in p-xylene pass off overhead, and are led to condenser 7. The condensate is passed to decanter 17 in which there is formed an upper hydrocarbon phase and a lower aqueous phase. The hydrocarbon phase (p-xylene enriched fraction) is in part withdrawn from the system through line 19, and in part returned to column 2 as reflux through lines 20 and 15. All or part of the water layer from the decanter is returned to column 2 through lines 23 and 15. If desired a part of the water layer may be withdrawn through line 22 in order to control salt concentration. Aqueous solvent enriched in o-xylene is withdrawn from the base of reboiler 3, and is pumped through line 10 into fractionating column 11, which is heated by coil 12. The vapours taken overhead from column 11 are led to condenser 14; and the condensate is passed to decanter 31, where it separates into two liquid phases. The upper hydrocarbon phase (o-xylene enriched fraction) is in part withdrawn from the system through line 26, and in part refluxed to column 11 through line 27. The majority of the water phase is returned to column 11 through lines 28, 30 and 27. Aqueous salt solution is withdrawn from the base of column 11, and is recycled to column 2 through lines 9 and 15. Other examples relate to the following separations:-p-from m-xylene, p-xylene from n-nonane, cyclohexane from benzene, n-heptane from methylcyclohexane. The separation of naphthalene and naphthalene derivatives from benzene derivatives is referred to, also the separation of propylbenzene, methylethylbenzenes, and trimethylbenzenes. Ordinary fractional distillation, adsorption and crystallization processes may be used in conjunction with the extractive distillation.