MASS-TRANSFER DYNAMICS OF THE EVAPORATION STEP IN MEMBRANE FORMATION BY PHASE INVERSION

A diffusion model has been developed for the evaporation step of the phase inversion process which incorporates the effects of the moving air-film interface, concentration-dependence of the mutual diffusivity, and time-dependent mass transfer coefficient. Calculations are carried out for the cellulose acetate (CA)-acetone binary casting system to illustrate the effects of casting solution composition, casting film thickness and surface area, and initial vapor phase solvent composition on the evaporation rate. Results are shown to be in excellent agreement with experimental desolvation curve data. Such comparisons are a characteristic of the homogeneous diffusion process and therefore not subject to arbitrary assumptions regarding precipitation time.