Study of the effective thickness of the water-intrudable hydrophilic layer in dual-layer hydrophilic-hydrophobic hollow fiber membranes for direct contact membrane distillation

Dual-layer hydrophilic-hydrophobic hollow fiber membrane is a promising membrane that simultaneously shows enhanced vapor transfer coefficient and inhibited conductive heat loss in direct contact membrane distillation (DCMD). The presence of an effective water-intrudable hydrophilic layer enables to reduce the vapor transportation distance and improve the water flux in DCMD. The aim of this study is to estimate the water-intrudable hydrophilic layer thickness in a dual-layer hydrophilic-hydrophobic hollow fiber membrane comprising a PVDF/PEG inner layer and a PVDF outer layer. The water-intrudable hydrophilic layer thickness was precisely measured by a dye solution permeation method under simulated DCMD conditions. The results showed that the water-intrudable hydrophilic layer thickness was highly determined by the pore size and effective porosity of the PVDF/PEG inner layer. The applied hydraulic pressure contributed significantly to the difference in the thickness of the water-intrudable hydrophilic layer. The membranes with higher thickness percentages of the water-intrudable hydrophilic layer exhibited simultaneously enhanced permeate water flux and energy efficiency in DCMD. This study offered a simple and reliable approach for the estimation of the effective thickness of the water-intrudable hydrophilic layer for the dual-layer hydrophilic-hydrophobic hollow fiber membrane that contributes substantially to the desalination performance enhancement in DCMD.