Solvent extraction of hydrocarbon oils

Claim 1

In a method of solvent refining a petroleum based lubricating oil stock containing aromatic and non-aromatic components with N-methyl-2-pyrrolidone wherein said lubricating oil stock is contacted with N-methyl-2-pyrrolidone in a solvent extraction zone at a temperature in the range of 50° to 120° C. (120° to 250° F.) and a solvent to oil dosage in the range of 100 to 300 volume percent forming an aromatics-rich primary extract and a solvent refined oil raffinate having a predetermined refractive index, the improvement which comprises separating said primary extract from said raffinate, cooling the aromatics-rich primary extract to a temperature in the range of 10° C. (18° F.) to 45° C. (113° F.) below said solvent extraction temperature whereby two separate liquid phases are formed consisting of a secondary extract phase relatively richer in aromatic hydrocarbons than said primary extract and a secondary raffinate phase relatively poorer in aromatic hydrocarbons than said primary extract, separating said secondary raffinate from said secondary extract, returning 0.1 to 0.5 volumes of said secondary raffinate to said solvent extraction zone for each volume of lubricating oil stock supplied to said solvent extraction zone into admixture with said lubricating oil stock in said zone, supplying a substantially reduced dosage of solvent to fresh feed lubricating oil stock to said solvent extraction zone producing an increased yield of a solvent refined oil raffinate of said predetermined refractive index, and withdrawing said refined oil raffinate from said extraction zone.