The computational fluid dynamics (CFD) simulations will be carried out to study the performance of hollow fiber direct contact membrane distillation (DCMD) for a desalination process. The hollow fiber membrane distillation separation modules typically consist of a shell containing a bundle of fibers. The feed solution typically flows inside the shell (outside of hollow fibers), and pure water flows in the hollow fibers in a counterflow arrangement. A single hollow fiber placed inside the circular cross-sectioned pipe shell will be considered in this study. The seawater with the salt concentration (NaCl) 35 g/L is taken as the feed solution. The Navier-Stokes, mass and energy transport equations in each stream coupled by the membrane flux conditions are solved in the shell and inside the hollow fiber. The DCMD hollow fiber system is investigated in this study to examine its sensitivity toward the module thickness, pore size, and inner hollow fiber diameter. The employed membrane is PTFE hydrophobic microporous membrane, which permits only the water vapor transported through the membrane.Two different thicknesses of 300 μm, 500 μm, and two membrane pores sizes of 0.2 μm, 0.45 μm are considered. The different inner diameters in the model of 1.2 mm and 1.8 mm are also examined. Laminar models are used to conduct these simulations in three-dimensional modules. The Reynolds number for the feed and the permeate channel is fixed at 500 and 250, respectively. The inlet temperature of the feed is set to&nbsp; &nbsp;while the inlet temperature of the permeate is set to . The membrane properties except thickness and pore size are fixed in all cases considered in the current work. The local concentration, temperature polarization, and mass flux along the membrane surface will be presented and discussed in detail. The presence of temperature polarization in both sides of the membrane and concentration polarization in the feed side cause a significant drop in the water permeation in the DCMD system due to reduced driving force across the membrane. The characterization of polarization in the shell and inside the hollow fiber will be the focus of this work.&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Several studies on membrane distillation (MD) systems have recently been carried out due to increased freshwater demand. MD is a thermally driven separation process where only water vapor molecules can pass through a microporous hydrophobic membrane. The system's primary driving force is the vapor pressure difference across the membrane induced by the temperature difference between the hot feed and the cold permeate solution. Direct contact membrane distillation systems are a cost-effective, viable alternative to Reverse Osmosis because they operate at low temperatures ( &nbsp;) and pressure. The potential use of waste heat or solar energy as a heat source makes DCMD operation environment friendly and cost-effective.

Performance Characterization of Hollow Fiber Direct Contact Membrane Distillation Module

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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