GB855116A - Improvements in or relating to extractive distillation processes - Google Patents

Abstract

<PICT:0855116/III/1> Hydrocarbon mixtures are separated by an extractive distillation process in which they are contacted with a solvent in a distillation zone to produce a raffinate phase and an extract phase, the raffinate phase is rectified to reduce its solvent content and is then condensed, a part of the condensate is refluxed for admixture with solvent before the entry of the latter into the distillation zone, and another part is refluxed to the top of the rectification zone. In an example, a C4 feedstock containing butanes and a molar proportion of 50 to 55% butenes is fed at about 66 DEG C. as a vapour or liquid just below its boiling point into a column 12 at 10, and an aqueous acetonitrile solvent in a solvent: feed ratio of 9:1 and containing 15% by weight water is fed at about 88 DEG C. from line 16 into the column at a point between the distillation zone 14 and rectification zone 15. A reboiler 17 heats the column which is maintained at a pressure of about 7 atmospheres and a temperature of about 60 DEG C. The liquid solvent extracts the butene from its admixture with butane by downward flow countercurrent to the hydrocarbon stream, to form a solvent extract phase which is removed by line 19 to a stripping tower 20, from which butene is discharged by line 23 and stripped solvent is returned to the column 12. The raffinate phase is rectified in zone 15 by plates sufficient in number, e.g. 3 to 5, to separate much of the solvent from the butane, after which the raffinate is condensed at 24 and allowed to separate at 28 into an upper hydrocarbon-rich layer and lower water-rich layer. The latter is discharged at 40 for further processing while the former is discharged at 30 into lines 33 and 34. Reflux line 33 is fed into solvent inlet line 16 to give a reflux: solvent ratio of about 3.4:1, and line 34 is split into a butane product line 38 which is further processed, and a second reflux line 36 which feeds into the rectification zone 15. Line 33 carries between 50% and 85% of the total reflux. Other solvents, e.g. aniline, furfural, phenol, nitrobenzene or chlorex (dichloroethyl ether), may be employed.