a method and apparatus is provided for casting a polymeric membrane on the inside surface of porous tubes to provide a permeate filter system capable of withstanding hostile operating conditions and having excellent selectivity capabilities . any polymer in solution , by either solvent means or melt processing means , is capable of being used in the present invention to form a thin polymer membrane having uniform thickness on the inside surface of a porous tube . multiple tubes configured as a tubular module can also be coated with the polymer solution . by positioning the longitudinal axis of the tubes in a substantially horizontal position and rotating the tube about the longitudinal axis , the polymer solution coats the inside surface of the porous tubes without substantially infiltrating the pores of the porous tubes , thereby providing a permeate filter system having enhanced separation capabilities .