ULTRATHIN COMPOSITE MEMBRANES

MEMBRANE DIFFUSION IS AN INHERENTLY SLOW PROCESS. CONSEQUENTLY, MEMBRANE-BASED SEPARATION PROCESSES CAN BE ECONOMICALLY VIABLE ONLY IF EXTREMELY THIN, DEFECT-FREE MEMBRANES ARE USED. THIN-FILM COMPOSITE MEMBRANES WITH A PERMSELECTIVE LAYER THICKNESS DOWN TO ABOUT 0.5 MUM CAN BE MADE BY AVAILABLE TECHNIQUES. MEMBRANES WITH PERMSELECTIVE LAYERS THINNER THAN THIS WOULD DRAMATICALLY IMPROVE THE ECONOMICS OF MEMBRANE SEPARATION. CURRENT THIN-FILM COMPOSITE MEMBRANES ARE PREPARED BY COATING A DILUTE POLYMERSOLUTION ON TOP OF A SMOOTH, MICROPOROUS SUPPORT MEMBRANE. THE SOLVENT EVAPORATES, LEAVING BEHIND A THIN, DENSE FILM THAT ACTS AS A PERMSELECTIVE BARRIER TO GAS AND LIQUID PERMEATION. DURING THE FORMATION OF THE PERMSELECTIVE LAYER, SURFACE TENSION FORCES INDUCED BY MICROSCOPIC DEBRIS AND DEFECTS IN THE SUPPORT CAN GIVE RISE TO DISCONTINUITIES IN THE LIQUID FILM THAT RESULT IN MEMBRANE DEFECTS. FORMATION OF THESE DEFECTS LIMITS THE THINNESS OF THE PERMSELECTIVE LAYER. THE GOAL OF THIS PROJECT IS TO DEVELOPTHE TECHNOLOGY FOR MAKING INDUSTRIAL-SCALE COMPOSITE MEMBRANES WITH PERMSELECTIVE LAYER THICKNESSES DOWN TO 0.1 MUM OR SMALLER. IN PHASE I, MEMBRANES ARE BEING MADE BY MODIFYING THE SOLUTION COATING PROCESS USED TO APPLY THE THIN PERMSELECTIVE LAYER. THE COATING SOLUTIONS USED ARE CHARACTERIZED AS HAVING HIGH VISCOSITY AT LOW POLYMER CONCENTRATION. PREVIOUS EXPERIMENTAL DATA INDICATE THAT THEUSE OF SUCH SOLUTIONS ENABLES A MUCH THINNER DEFECT-FREE COATING TO BE APPLIED THAN IS POSSIBLE WITH CONVENTIONAL TECHNIQUES. IF THE TECHNIQUE CAN BE APPLIED TO MANY DIFFERENT POLYMERS, IT WILL ENABLE ECONOMICALLY VIABLE MEMBRANE-BASED SEPARATION SYSTEMS TO BE MADE AND APPLIED TO A BROAD RANGE OF GAS SEPARATION PROBLEMS.