(155c) Desalination Studies Using Membrane Distillation

Membrane distillation for desalination employs heated brine in contact with a porous hydrophobic membrane to generate water vapor which is recovered in a variety of ways on the other side of the membrane. Direct contact membrane distillation (DCMD) employs colder distilled water on the other side to condense the water vapor. Air gap membrane distillation (AGMD) devices utilize a cold surface to condense the water vapor and generate the distillate. For brines at 50-93⁰C, this presentation will focus primarily on hollow fiber membrane-based devices for DCMD as well as AGMD; studies in DCMD for these brine temperatures included synthetic brines, simulated sea water as well as actual sea water. For brines at 100-130⁰C, this presentation will illustrate the DCMD performances of flat as well as hollow fiber membranes of PDVF and PTFE for a variety of salt concentrations, distillate in temperature, brine flow rate as well as simulated produced water from steam assisted gravity drainage (SAGD) process.  

 For AGMD the colder condensation surface is provided by a solid nonporous hollow fiber of polypropylene (PP). The evaporation fiber set consists of porous hydrophobic hollow fibers of either PP with a porous fluorosilicone coating or PVDF. Through the bores of the second set of solid nonporous hollow fibers flows the cooling fluid or the colder feed brine itself as it gets heated up. The performances of such two-hollow-fiber-based AGMD devices will be illustrated. The water vapor flux levels achieved in AGMD were quite high compared to those reported in literature. For the lower temperature DCMD studies, hot brine undergoes rectangular cross flow over the outer surface of highly porous hydrophobic PP hollow fibers whose outside surface was coated with porous plasma polymerized silicone-fluoropolymer coating to mitigate pore wetting and distillate contamination as cold distillate flows through the bores of fibers having a large wall thickness in DCMD. Extended pilot scale operation demonstrated no salt leakage, stable and repeatable performance with synthetic solutions having starting feed salt concentrations of up to 10% as well as with sea water concentrated up to 20% salt via batch recirculation. An overview of the device performance, resistance to scaling, concentration cascades and energy requirements will be provided for a variety of brines.