(419e) Highly Hydrophobic Polyvinylidene Fluoride (PVDF) Membrane for Direct Contact Membrane Distillation through Diffusion Induced Phase Inversion

PVDF membrane for direct contact membrane distillation (DCMD) with good mechanical stability was fabricated through diffusion induced phase inversion (DIPS).  The scalability and simplicity of the phase inversion and among other hydrophobic polymer the tractability of PVDF in common organic solvents makes PVDF an interesting candidate for the MD membrane fabrication. Nonetheless there exist significant challenges in tuning the important structural parameters that influence the MD membrane performance. To understand the influence of the major parameters contributing to the performance of the flat sheet membranes in direct contact mode operation, the pores size and the surface roughness of the membranes were controlled by using different solvents and different coagulation bath compositions, and the membrane performance were evaluated using a laboratory-scale DCMD unit. Dope solutions with polymer concentration ranging from 8 to 15 weight % in N,N-dimethylacetamide (DMAc), N,N-dimethylformamide (DMF), N-Methyl-2-pyrrolidone (NMP), and triethyl phosphate (TEP) were used for casting of the membranes.  A mixture of isopropanol in water was used as the coagulation medium. By adjusting isopropanol to water ratio, PVDF phase inversion was directed toward incipient precipitation.. Minor improvement in the contact angle was observed when NMP, DMAc, and DMF were used as solvents. However, by using TEP as solvent, it was found that the surface roughness can be enhanced without the penalty of losing the porosity and the contact angle found to be tunable and sensitive to the isopropanol content in the coagulation bath. By exceeding a specific volume fraction of isopropanol in the coagulation medium, the incipient precipitation becomes the dominant process and films with poor mechanical integrity and low porosity were obtained. Nevertheless, at the optimal condition, free standing membranes with good mechanical stability and contact angles as high as 143˚ and 140˚ on the top and the bottom side, respectively, were fabricated. By reducing the thickness to 70 µm and increasing the porosity to 70%, we were able to achieve high water flux and complete salt rejection. The maximum water flux achieved was 45 LMH at temperature difference of 40 °C and a cross-flow velocity of 12.8 cm/s. Here we demonstrate, that controlling the wetting by increasing the contact angle and keeping the porosity high are the major parameters for designing an efficient membrane for DCMD. Increasing the liquid entry pressure by reducing the surface pore size accompanied by decline in the water flux, so regardless of the bulk porosity of the membrane, effective porosity  of the membrane at the surfaces is a critical design parameter for the DCMD application.