Asymmetric polyimide reverse osmosis membranes of high flux and selectivity for organic liquid separations are described. These membranes are prepared from undegraded polyimide by dissolving from 14-30 wt. % of the undegraded polymer in a dual solvent system comprising a polymer pro-solvent/anti-solvent wherein the pro-solvent is DMF and the anti-solvent is dioxane, wherein the ratio of anti-solvent/pro-solvent ranges from about 10:1 or more-1:1, preferably about 8:1-1.5:1, most preferably about 7:1-3:1; the polymer-solvent mixture is spread into a thin film of the desired thickness and permitted to evaporate for a time just sufficient to permit formation of an asymmetric dense active layer, i.e., within the range 2-120 seconds, preferably 2-60 seconds, most preferably 2-20 seconds before being immersed in a gelation bath.
The membranes may be fabricated in the form of sheets, tubes, hollow fibers, etc.
Mixtures of organic liquids, and especially mixtures of dewaxing solvents such as ketones, aromatic hydrocarbons and mixtures thereof, halogenated hydrocarbons such as dichloromethane, dichloroethane and mixtures of such halogenated hydrocarbons with ketones or aromatic hydrocarbons or other polar dewaxing solvents and dewaxed oils, are separated after completion of dewaxing operations, using the asymmetric polyimide reverse osmosis membrane described above.