Recovery of dewaxing aid using asymmetric polyimide ultrafiltration membrane and method for producing said membrane

Claim 1

In a method for recovering dewaxing aids used in the solvent dewaxing of waxy hydrocarbon oils wherein the dewaxing aid is present in a dewaxed oil stream, a precipitated wax stream or both and wherein the dewaxing aid present in said dewaxed oil stream, precipitated wax stream or both is recovered by heating said stream so that the wax is in the liquid state and contacting said heated stream with an ultrafiltration polymeric membrane to selectively permeate through said membrane the oil or wax components of said heated stream while producing a retentate containing the dewaxing aid which retentate can be recycled to the solvent dewaxing process, the improvement comprising ultrafiltering said stream with a membrane made from polyimide polymer wherein the polyimide ultrafiltration membrane is produced by dissolving the polyimide polymer in a dual component solvent system comprising a pro-solvent and an anti-solvent pair used in a pro-solvent/anti-solvent ratio of between about 10:1 to 1:1 and adding 1 to 6 wt. % inorganic salt to produce a clear, non-turbid casting solution, using said casting solution to produce a thin layer of said polyimide polymer/dual solvent solution mixture, partially evaporating the solvent for from 1 to 60 seconds and thereafter quenching the film in a quenching solvent, and performing the ultrafiltration dewaxing aid recovery process at a temperature of about 70° to 250° C. and an applied pressure of about 5 to 200 psi.