(287e) Direct Contact Membrane Distillation with Hollow Fiber Membranes: Studies On Scaling, Antiscalants and Countercurrent Cascades

We have recently developed novel hollow fiber membranes and devices for recovering pure water from hot brine via direct contact membrane distillation (DCMD). Hot brine undergoes rectangular crossflow over the outer surface of highly porous hydrophobic polypropylene 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. The DCMD studies were successfully carried out earlier at three levels: small laboratory modules, large laboratory modules, larger pilot plant units. Extended pilot scale operation demonstrated no salt leakage, stable and repeatable performance with synthetic solutions having salt concentrations of 10% as well as with sea water concentrated up to 20% salt.

If such a technique is to be used to further concentrate the reverse osmosis desalination process concentrate, we should know how it behaves with antiscalants. Extensive scaling experiments with the addition of antiscalant were conducted to see the effects of concentration and different kinds of antiscalants on inhibiting scaling from deposits of CaSO₄ and CaCO₃. The parameters of the induction period, calcium concentration, distillate conductivity and the water vapor flux were investigated.

Cost-efficient desalination technology was also developed successfully by integrating a countercurrent cascade of the novel crossflow DCMD devices and solid polymeric hollow fiber-based heat exchange devices. Simulations have been carried out for the whole DCMD cascade to project values of gained output ratio (GOR) as a function of the number of DCMD stages as well as other important factors in the cascade vis-à-vis the temperatures and flow rates of the incoming hot brine and cold distillate streams. The simulation results were verified with the experimental results from cascades consisting of 2 to 8 stages. An artificial sea water was concentrated 8 times successfully when a countercurrent cascade composed of 4 stages of the DCMD modules and a heat exchanger was employed during the DCMD process.