PURPOSE: To provide a process comprising contacting a fed solution which consists of hydrocarbon dissolved in an aromatic solvent and a polar aliphatic solvent, with one surface of a dense supporting membrane heated in a low temperature, low pressure plasma phase, in a high pressure, and separating hydrocarbon, and recovering the solvent from other sites of membranes.

CONSTITUTION: For example, a separation of Brightstock/solvent in a wax removal methods is performed by use of a plasma-treated, dense supporting membrane. The supporting membrane consists of a dense selective layer of fluorosilicone compounds and a porous supporting membrane (desirably, polypropylene), and the plasma-treating is performed at the pressure of less than 200 Pa for 0.5-30 minutes. The plasma-treating is performed in argon, argon/toluene (2:1-11.5:1), or argon/toluene/Freon. By the plasma-treating, a selectivity of the membrane is significantly improved.