Session: 01-07-01 Industrial Fluid Mechanics

Paper Number: 65611

Start Time: August 11th, 12:50 PM

65611 - The Effect of Membrane Topology on Separation Performance of Vacuum Membrane Distillation Module 

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 2D laminar flow in VMD modules containing flat and sinusoidal (“wiggly”) sheet hydrophobic membranes on the top and bottom of the channel. The flat sheet module is of height 1mm and length 120mm. The module with wavy membranes has sinusoidal profiles with a specified wavelength and amplitude. The effect of the wavelength and amplitude of the wavy membrane on the module separation performance will focus on this work. The height and width of the wavy channel are the same as the flat sheet membrane module. The membrane is treated as a patch where the local face temperature, concentration, and suction velocity are coupled. The mass transport through the membrane governed by both Knudsen and Viscous diffusion is described using the Dusty Gas Model. 

The feed solution (a mixture of water and NaCl) will have a Reynolds number of 1,000, 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 membrane properties are taken from a previous VMD study where the thickness is 180 microns, the maximum pore size is 0.2 microns, the porosity is 75%, and the tortuosity is taken as a function of porosity.

As the water vapor is transported through the membrane, temperature and concentration polarization will occur near the membrane surface, impacting the flux through the membrane. It is expected that the difference in geometry between the two configurations will lead to different flow structures. For flow around a corner, the channel's curvature causes a pressure gradient in the crossflow direction of the channel, which can induce vorticial activity. By breaking the parallel flow pattern, 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 will be improved. The contours of velocity, vorticity, temperature, and concentration of the bulk fluid and profiles of temperature, concentration, and suction velocity (corresponding to flux) along the membrane surface will be presented to assess the influence of the wavy membrane structure on the membrane performance.

The membrane distillation has many advantages over a widely used Reverse Osmosis process. MD can operate at high feed concentrations while being less impacted by fouling/scaling, 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 at the entrance of the membrane pores. 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.

Presenting Author: Justin Caspar Lehigh University

Authors:

Justin Caspar Lehigh University
Guanyang Xue Lehigh University
Robert Krysko Lehigh University
Alparslan Oztekin Lehigh University