Novel Membrane and Device for Direct Contact Membrane Distillation-Based Desalination Process: Phase II

摘要

Although research and development studies to improve commercialized reverse osmosis and thermally-driven desalination processes are continuing, there exists a need to develop and evaluate alternate desalination technologies, e.g., membrane distillation (MD) which utilizes waste heat. In one variety of MD, direct contact membrane distillation (DCMD), hot brine flows on one side of a gas-filled porous hydrophobic hollow fiber membrane and cold distillate flows on the other side of the membrane. This technique is of interest here. The primary deficiencies of this technique are flux reduction due to long-term pore wetting and reduced brine-side heat and mass transfer coefficients. To overcome these, this research has made a preliminary investigation of the DCMD process where a number of changes were introduced: (1) The porous hydrophobic hollow fiber membrane has a thin water-vapor permeable hydrophobic microporous coating of a siliconefluoropolymer plasmapolymerized on the fiber outside diameter on the hot brine side to prevent pore wetting; (2) to increase the brine-side heat transfer coefficient drastically, the hot brine feed is in a rectangular cross flow mode vis-a-vis the hollow fiber membranes; (3) the hydrophobic porous hollow fibers had thick walls and high porosity; (4) the module design ensured that the temperature rise of the cold distillate was minimal. It is known that the vacuum membrane distillation (VMD) technique, wherein there exists vacuum instead of cold distillate flow on one side of the membrane, the other side having hot brine flow, can illuminate many features of the brine side of a DCMD process. Therefore extensive data were obtained for the DCMD as well as the VMD process.