Computational Fluid Dynamics simulations are conducted to study the vacuum membrane distillation process using the open-source library OpenFOAM. Swak4Foam is used to discretize the governing mass, momentum, heat, and mass transfer equations for a transient 3D turbulent flow in VMD modules containing a flat and sinusoidal (“wiggly”) sheet hydrophobic membrane. In our previous work, we have conducted steady-state laminar flow simulations in a two-dimensional VMD module consisting of a wiggly channel at low Reynolds numbers. In order to accurately resolve the flow structure at a higher flow rate, a turbulence model is adopted, where the LES model is employed to solve the filtered Navier Stokes equations. The module with a wiggly membrane has sinusoidal profiles with a specified wavelength, amplitude, and channel dimensions. The effect of the geometric alterations of the wavy membrane on the module separation performance will be the focus of this work. The membrane is treated as a patch where the local face temperature, concentration, and suction velocity are coupled. Both Knudsen and Viscous diffusion govern the mass transport through the membrane described using the Dusty Gas Model.&nbsp;The feed solution (a binary mixture of water and NaCl) will have a Reynolds number of 1,000 and 750, 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 boundary condition applied and zero gradients for both heat and mass flux. The symmetry condition is applied to the in and out of the plane direction. The membrane properties are taken from existing literature where the thickness is 180 microns, the mean pore size is 0.2 microns, the porosity is 75%, and the tortuosity is treated as a function of porosity.As pure water vapor is transported through the membrane, temperature and concentration polarization will occur near the membrane surface, impacting the flux performance. It is already shown that the difference in geometry between the two configurations will lead to different flow structures. Due to the curvature of the wiggly channel and inlet Reynolds number, unsteady vortical structures are present. By introducing vorticial activity to the flow, it is anticipated that the concentration and temperature boundary layers attached to the membrane surface will be disrupted, and the corresponding temperature and concentration polarizations will be reduced. Thus, the membrane flux performance should be improved. The contours of velocity, temperature, and concentration of the bulk fluid and profiles of temperature, concentration, and suction velocity (corresponding to flux) along the membrane surface at various times will be presented to assess the influence of the wavy membrane structure on the membrane performance.Membrane distillation has critical advantages over the commonly used Reverse Osmosis process. MD can operate at high feed concentrations while being less impacted by fouling/scaling and requires low-quality heat to operate versus a high-pressure head requirement. This is due to the hydrophobicity of the membrane (not allowing the salt solution to enter the membrane) instead of the small pore size utilized in RO applications. However, MD is prone to temperature polarization as water vaporizes near the membrane. This polarization reduces the flux performance of the module. Many studies, including this one, investigate means to mitigate the temperature polarization by inducing mixing and creating a way to quantify the gains from the mixing to establish an information base to help optimize future membrane desalination modules.

The Effect of Membrane Topology on Separation Performance of Vacuum 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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