This work presents numerical modeling results and flux experiments for a novel membrane distillation configuration called conductive gap membrane distillation (CGMD), as well as permeate gap membrane distillation (PGMD). CGMD has a conductive spacer in the gap between the membrane and condensing surface rather than more commonly used insulating materials. Flux measurements with two experimental systems are used to validate the numerical models for PGMD and CGMD. PGMD has 20% higher GOR (energy efficiency) than an air gap membrane distillation (AGMD) system of the same size, whereas CGMD can have two times higher GOR than even PGMD. Increasing gap effective thermal conductivity in CGMD has negligible benefits beyond k gap ≈ 10 W/m·K under the conditions of this study. The direction of pure water flow in the gap has a significant influence on overall system energy efficiency, especially in the case of CGMD. Using a countercurrent configuration for the pure water flow in the gap relative to the cold stream leads to 40% higher GOR than flow cocurrent with the cold water stream.
Polyvinylidene fluoride (PVDF) is a popular polymer material for making membranes for several applications, including membrane distillation (MD), via the phase inversion process. Non-solvent-induced phase separation (NIPS) and vapor-induced phase separation (VIPS) are applied to achieve a porous PVDF membrane with low mass-transfer resistance and high contact angle (hydrophobicity). In this work, firstly, the impacts of several preparation parameters on membrane properties using VIPS and NIPS were studied. Then, the performance of the selected membrane was assessed in a lab-scale direct-contact MD (DCMD) unit. The parametric study shows that decreasing PVDF concentration while increasing both relative humidity (RH) and exposure time increased the contact angle and bubble-point pore size (BP). Those trends were investigated further by varying the casting thickness. At higher casting thicknesses and longer exposure time (up to 7.5 min), contact angle (CA) increased but BP significantly decreased. The latter showed a dominant trend leading to liquid entry pressure (LEP) increase with thickness.
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