Abstract
High water vapor flux at low brine temperatures without surface fouling is needed in membrane distillation-based desalination. Brine crossflow over surface-modified hydrophobic hollow fiber membranes (HFMs) yielded fouling-free operation with supersaturated solutions of scaling salts and their precipitates. Surface modification involved an ultrathin porous polyfluorosiloxane or polysiloxane coating deposited on the outside of porous polypropylene (PP) HFMs by plasma polymerization. The outside of hydrophilic MicroPES HFMs of polyethersulfone was also coated by an ultrathin coating of porous plasma-polymerized polyfluorosiloxane or polysiloxane rendering the surface hydrophobic. Direct contact membrane distillation-based desalination performances of these HFMs were determined and compared with porous PP-based HFMs. Salt concentrations of 1, 10, and 20 wt% were used. Leak rates were determined at low pressures. Surface and cross-sections of two kinds of coated HFMs were investigated by scanning electron microscopy. The HFMs based on water-wetted MicroPES substrate offered a very thin gas gap in the hydrophobic surface coating yielding a high flux of 26.4–27.6 kg/m2-h with 1 wt% feed brine at 70 °C. The fluxes of HFMs on porous PP substrates having a long vapor diffusion path were significantly lower. Coated HFM performances have been compared with flat hydrophilic membranes of polyvinylidene fluoride having a similar plasma-polymerized hydrophobic polyfluorosiloxane coating.
Highlights
Water evaporation through a porous hydrophobic membrane is being intensively investigated for water desalination and a few other water treatment processes
This process, termed membrane distillation (MD), has several variants depending on how water evaporated from hot brine on the feed side of the membrane is condensed on the other side of the membrane
A total of 61 PP mini-modules and 24 PES mini-modules were tested for direct contact membrane distillation (DCMD) performance
Summary
Water evaporation through a porous hydrophobic membrane is being intensively investigated for water desalination and a few other water treatment processes This process, termed membrane distillation (MD), has several variants depending on how water evaporated from hot brine on the feed side of the membrane is condensed on the other side of the membrane. We are here concerned primarily with the direct contact membrane distillation (DCMD) process, where the evaporated water is condensed by a flowing stream of cold distilled water on the other side of the membrane. In DCMD, a porous gas-filled hydrophobic membrane exists between hot brine and cold distillate streams. Water evaporated at the hot brine-membrane interface is transported through gas-filled pores to the other side where it condenses in the flowing cold distillate. A few major recent developments in these and other aspects of MD will be identified first before we converge on the subject of this work involving water vapor flux in surface-modified hollow fiber membranes
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