Crosslinked PVDF based hydrophilic-hydrophobic dual-layer hollow fiber membranes for direct contact membrane distillation desalination: from the seawater to oilfield produced water

Abstract

Crosslinked polyvinylidene fluoride (PVDF) based hydrophilic-hydrophobic dual-layer hollow fiber membranes were fabricated for direct contact membrane distillation (DCMD). PVDF/ammonia/water dope solution was formulated with a spinning process delay of 9 days for the formation of a mechanically robust and hydrophobic crosslinked PVDF outer layer, and polyethylene glycol 6000 (PEG-6000) was introduced to create a thick hydrophilic PVDF/PEG-6000 inner layer. The influences of ammonia/water contents and the spinning process delay on the membrane morphology were studied. The DCMD performances were investigated using both simulated seawater and actual oilfield produced water as the feed solutions. The incorporation of ammonia/water mixture allowed a slow PVDF crystallization in the dope solution during the 9 days of spinning process delay. The obtained membrane DN2H2-9 showed contact angles of 133.6° and 47.1°on the surfaces of the crosslinked PVDF outer layer and PVDF/PEG-6000 inner layer, respectively. The membrane DN2H2-9 showed a superior permeate water flux of 97.6 kg m−2 h−1 and an energy efficiency of 92.8% to desalinate a 3.5% NaCl solution. The result from a 200 h continuous DCMD operation revealed a stable permeate water flux and more than 99.9% of salt rejection, which was attributed to a relatively high liquid entry pressure (LEP) of 1.95 bar and the simultaneously enhanced mechanical strength of the membrane. The membrane DN2H2-9 also demonstrated promising DCMD performance in the desalination of the real oilfield produced water with a high total dissolved solids (TDS), including almost 100% of permeate water recovery and more than 99.9% of salt rejection in 72 h of operation.