Recovery of solvent from a hydrocarbon extract

Claim 1

In a process for solvent refining a lubricating oil feedstock wherein said lubricating oil feedstock is contacted under pressure with N-methyl-2-pyrrolidone as a selective solvent for aromatic constituents of said feedstock in an extraction zone under solvent refining conditions thereby forming a raffinate phase comprising raffinate and a part of said solvent and an extract phase comprising extract and a part of said solvent, said raffinate phase is separated from said extract phase, and said solvent is removed from said extract phase by vaporization serially in a first solvent vaporization zone at a pressure less than that of said extraction zone and in a plurality of zones at progressively higher pressure and wherein heat from an external source is supplied only to said last high pressure vaporization zone and heat for each preceding vaporizaton zone is supplied by heat exchange with vapors from each succeeding vaporization zone, and the extract and solvent mixture from the high pressure vaporization zone is subjected to flash vaporization in a subatmospheric pressure flash zone followed by stripping with a gaseous stripping medium, the improvement which comprises heating the extract solvent mixture from the subatmospheric flash zone in a heating zone in admixture with added normally gaseous inert gas to a temperature at least 5° C. higher than the temperature of said last high pressure vaporization zone, introducing the resulting heated mixture comprising inert gas, solvent and extract into a stripping zone, and removing additional solvent from said extract solvent mixture by stripping with an inert stripping gas at a low superatmospheric pressure.