CFD Simulation Of Membrane Distillation And Desalination System Design

Membrane distillation is capable to use the low-grade heat to process the high-salinity brine under atmospheric pressure, so it has been widely used in the integration with reversed osmosis desalination process together with the fields of solar energy, geothermal and so on During the treatment of high-salinity brine with membrane distillation, the high salinity will produce the precipitation of salts on the surface of membrane to block the transmembrane permeation. Also, an unfavorable polarization phenomena led by the high concentration will hinder the membrane distillation. The concentration profile in the feeding channel is critical to reveal the desalination of high-salinity brine, but few studies are existed to depict it in the literatures. In this thesis, the characteristics of unit operation for membrane distillation were simulated by computational fluid dynamics(CFD). The mechanism of heat and mass transfer was studied by analyzing the temperature and concentration profiles inside the membrane module, and the operating conditions were furtherly optimized to avoid the blockage of membrane pores. Based on the illustrated characteristics of membrane distillation in the single module, a complex system for high-salinity brine precessing was developed to show the importance of system design and optimization.Two parts are included in this thesis : CFD simulation of the membrane distillation and flowsheet simulation of desalination complex system.(1) A two-dimension CFD model was established for simulating the heat and mass transfer of membrane distillation in ANSYS FLUENT. The simulation results were verified with the literature data, and the relative mean deviation is near 6.0%. The profiles of temperature, concentration and supersaturation in the membrane module were investigated under different operating conditions, so the optimum operating conditions are concluded as: the low flowrates of(0.02~0.06m/s) are preferable in case of low feeding concentration of NaCl mass fraction of 0.15, while the high flowrates of over 0.07m/s will benefit to avoid the precipitation of NaCl on the membrane.(2) Two typical matrix schemes: type A, where the MD module has the identical length and varied number of fibers, and type B, where the equal fiber number is included in the MD module of each MD subsystem with varied length were designed to compare the water recovery rate, specific power consumption and water production cost with those in the simple system with the same total membrane area by the flowsheet simulation, developed in Aspen Plus with using our previously established model for simulating the heat and mass transfer in the hollow-fiber MD module. The matrix design of type A is preferable for the higher water production capacity, but type A scheme also produces a higher specific power requirement. In contrast, the matrix design of type B brings benefit to the lower energy consumption and production cost. The simulation results also reveal the minimal water production cost obtained by cautiously selecting the feed- and permeate-side flowrates. In the optimal flowrates, the type B scheme still has an advantage of 25 % cost reduction.