Multiscale phase change, heat and mass transfer in direct contact membrane distillation
Abstract
The global water shortage has become a serious threat for the world and the most promising solution for the water issue is the desalination of seawater or brackish water. In this work, direct contact membrane distillation (DCMD) as one of thermal desalination technologies was numerically and exprimentally analyzed to study its performance. A large DCMD system with multiple membrane modules in a parallel arrangement running on the waste heat from a diesel power generator was numerically analyzed using a thermo-fluid network model to study the technical feasibility of the use of the low-grade engine waste heat and simulate the distillation performance of the DCMD system. Next, a small DCMD experimental apparatus was fabricated to test for the distillation performance for various operating conditions (inlet temperatures, flow rates of feed and permeate streams and NaCl concentration) and design variables (filament spacing of a screen spacer in the flow channels and flow configuration). In the DCMD, two different regimes were observed in the water flux behavior regarding the salinity of feed water. In the first regime, from low NaCl concentration to 90% saturated NaCl concentration, there was a gradual decrease in the water flux due to the suppression of vapor pressure at the feed water which is simulated by a CFD model. In the second regime, at higher 90% saturated NaCl concentration, there was a sharp drop in the water flux due to the deposition of NaCl crystals on the membrane surface which is simulated by an analytical model using the adjusting parameter from the experiment. Finally, a nanoscale DCMD using Carbon Nanotube (CNT) membrane was numerically analyzed using non-equilibrium molecular dynamics (NEMD) simulation for different diameters and lengths of the CNT and operating conditions such as system temperature, temperature difference between the feed (hot) and permeate (cold) reservoirs, and sodium chloride (NaCl) concentration in the feed reservoir. The distillation performance of the DCMD systems is enhanced by increasing system temperature, temperature difference between feed and permeate streams, and decreasing the NaCl concentration. The permeability of the CNT membrane (1.8 x 10-5 liter/m2-s-Pa) was found two orders-of-magnitudes higher than a Polytetrafluoroethylene (PTFE) membrane (1.7 x 10-7 liter/m2-s-Pa ) used in our experimental work.
Degree
Ph. D.
Thesis Department
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OpenAccess.
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