Simulations are conducted to characterize the flux performance of a vacuum membrane distillation (VMD) process using a hollow fiber membrane. OpenFOAM v2006 alongside Swak4Foam is used to solve the governing mass, momentum, heat, and mass transfer equations. Two different modes of operation, feed inside the lumen versus the shell side of a vacuum membrane distillation module, are considered for the single hollow fiber membrane in the module. The effect of the membrane thickness and the pore size will be studied for both configurations using the laminar flow model.The hollow fiber is treated as a functional surface where the local face temperature, concentration, and suction velocity are coupled. The relationship between fields is described using the Dusty Gas Model. The Knudsen and Viscous diffusion will be considered for vapor transport through the membrane. The hollow fiber has a circular cross-section with a fixed length of 120mm and an inner diameter of 1 mm. The wall thicknesses will be varied from 150 to 450μm, and the pore size will be varied from 0.1 to 0.3μm. The porosity will be held constant at 70%, and the tortuosity of a function of the porosity will be kept at 1.30. The range of parameters is taken from existing studies and commercial membranes on VMD/hollow fiber systems. The feed solution (an aqueous solution of water and NaCl) will have a Reynolds number of 500, an inlet temperature of 65°C, and an inlet concentration of 35g/L. The vacuum pressure is 5,000 Pascals. The outlet has a zero pressure condition applied and zero gradients for both heat and mass flux.The vapor transport through the membrane induces temperature and concentration polarizations, impacting the flux through the membrane. The present study will address how sensitive the polarizations are to the various membrane properties at fixed operational conditions. Contours of velocity, temperature, and concentration along the streamwise direction and profiles of temperature, concentration, and suction velocity (corresponding to flux) along the membrane surface will quantify the effects of membrane properties on the module performance.Membrane distillation has several favorable advantages over the industrially popular reverse osmosis process. It is less prone to fouling for high concentration separation applications; it utilizes low-grade heat for operation; and it has a lower operational and capital cost. Computational and experimental studies on the hollow fiber VMD process are lacking. Hollow fiber membrane separation modules are utilized in desalination studies due to the high packing (membrane area to volume ratio) that can be employed versus a flat sheet or spiral wound membrane. This study will help design and optimize future hollow fiber VMD systems for desalination and water treatment applications.

Performance Characterization of Hollow Fiber Vacuum Membrane Distillation Module for Desalination

Paper Proceedings: https://asmedigitalcollection.asme.org/FEDSM/FEDSM2021/volume/85284

ASME 2021 Fluids Engineering Division Summer Meeting (FEDSM2021)

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