Membrane distillation is a process that utilizes differences in vapor pressure to permeate water through a macro-porous membrane and reject other non-volatile constituents present in the influent water. This review considers the fundamental heat and mass transfer processes in membrane distillation, recent advances in membrane technology, module configurations, and the applications and economics of membrane distillation, and identifies areas that may lead to technological improvements in membrane distillation as well as the application characteristics required for commercial deployment. Keywordsa function of temperature, vapor pressure, and of the gas molecular mass K 0 membrane characteristic defined by Equation (9) Kn Knudsen number K(T) a function of temperature and molecular weight of the gas l mean free path of the molecules l m distance between parallel spacer fibres (m) LEP Limit Entry Pressure (kPa) M molecular mass (g/mol) M w molecular weights of water (g/mol) M a molecular weights of air (g/mol) n number of CNTs per unit cross section in bucky-paper P pressure in the air gap (kPa) half time to reach the maximum intensity-laser flash technique (s) t proportion of conductive heat (balance due to evaporative heat) loss through the membrane T mean temperature in the pores (K)

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Identification and mitigation of challenges associated with membrane distillation (MD) modeling are very crucial to the applicability of MD technology in the industry. Several research studies have been carried out on direct contact membrane distillation (DCMD) modeling because of its simplicity, while other MD configurations have gained little attention. Most studies conducted on MD modeling were achieved based on uniform membrane pore size and pore size distribution assumption. This study exploits the homogeneity of these assumptions to conduct a modeling review for temperature polarization (TP) and concentration polarization (CP), as they apply to MD configurations. TP and CP phenomena have been identified as two of the main challenges to advance MD modeling for further development of MD technology. Their impact are detailed in the heat and mass transfer mechanisms discussed. Thermal conductivity of common hydrophobic commercial membrane materials at different temperatures are presented in this study. The use of optimal operating flow rates, suitable membranes, and proper module design are recommended as viable solutions to reduce the effect of TP and CP on permeate flux decay.

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