(521d) Composite PVDF Hollow Fiber Membranes for Water Reuses and Desalination by Membrane Distillation

Membrane distillation (MD) is a relatively old technique; however it receives worldwide attention in the last decade as an alternative desalination technology. Due to the dramatically enhance of crude oil price, MD is being investigated extensively as an energy-saving and cost effective desalination and water purification compared to conventional approaches like multi-stage flash vaporization and reverse osmosis. However, the industry has not fully embraced MD for several reasons: low water flux (i.e., productivity) and shortage of long term performance data due to the wetting of the hydrophobic microporous membrane. In this respect, opportunities therefore beckon membrane researchers to improve the permeate flux through (1) module designs, and (2) materials breakthroughs on microporous MD membranes with desired porosity, hydrophobicity, low thermal conductivity and anti-fouling characteristic to bring MD closer to industrialization.

We started the MD research via membrane module designs. A series of systematic module configurations via spacers/baffled design and modified hollow fiber geometries (twisted and braided) were conducted for the optimization study. Experimental and simulation results show ~50% increase in water flux at 75°. Nevertheless, the improvement is still very limited as the initial permeability for the polypropylene (PP) is only 5.5 kg/m2 hr at 75°. Subsequently, the macrovoid-free composite polyvinylidene fluoride-polytetrafluoroethylene (PVDF-PTFE) hollow fiber membranes were successfully fabricated via non-solvent induced phase inversion. Composite membrane (modification by addition of micro- and / or nano-particles) is potentially an easy and promising approach to overcome material deficiencies and enhance membrane surface hydrophobicity. The incorporation of PTFE particles (<1 mm) into the polymeric matrix enhances the hydrophobicity of the membranes, yielding a resultant water contact angle of 103°Æ. All the fabricated fibers demonstrate high thermal efficiency of above 80 % when subjected to a hot feed solution of 80 ¨¬C. The PVDF-PTFE fibers spun at an air gap of 4 cm and with 50 % particle loading demonstrate an optimal separation performance of 40.4 kg/m2 hr permeation flux and ~99.8 % salt rejection at 80 ¨¬C, indicating that the newly developed hollow fibers have great potential application for seawater desalination.