Date of Award

Spring 5-15-2015

Author's School

School of Engineering & Applied Science

Author's Department

Mechanical Engineering & Materials Science

Degree Name

Master of Science (MS)

Degree Type

Thesis

Abstract

Forward Osmosis (FO) driven asymmetric membrane filtration is a developing technology which shows promise for seawater desalination and wastewater treatment. Due to the fact that asymmetric membranes are widely used in conjunction with this technology, internal concentration polarization (ICP), a flow-entrainment effect occurring within such membranes, is a significant if not dominant source of overall osmotic pressure loss across the membrane. Accurate modeling of ICP effects is therefore very critical for accurate Computational Fluid Dynamic (CFD) modeling of asymmetric membranes. A related, dilutive effect known as external concentration polarization (ECP) also develops on both the rejection and draw sides of the membrane, further contributing to osmotic pressure loss. In order to increase the overall water flux, circular spacers known as baffles can be implemented in the draw channel of FO cross-flow membrane exchange units to decrease the effects of ICP and draw ECP. The drawback of baffle inclusions is an increased pressure loss across the length of the draw channel. The system efficiency gained by the decrease in ECP must therefore be weighed against the energy cost of hydraulically making up lost channel pressure. To model the geometry of a FO cross-flow channel, the open source CFD package OpenFOAM is used. A compressible flow model with explicit boundary conditions is developed to simulate the flux transfer, ICP and ECP effects present within an asymmetric membrane when exposed to a NaCl solution. Results are validated by comparison with the numerical data generated by earlier models of asymmetric membranes implemented by other investigators using similar simulation conditions. Mesh convergence and OpenFOAM flow-variable solver studies are then conducted to minimize solution inaccuracy and computation time. Finally, a genetic algorithm is implemented via the open source optimization toolkit DAKOTA which allows optimized draw-channel baffle placement such that the pressure loss along the draw channel is minimized and the water flux through the Asymmetric, FO membrane is maximized. The results of this optimization are then analyzed and discussed.

Language

English (en)

Chair

Ramesh K Agarwal

Committee Members

Spencer P Lake

Comments

Permanent URL: https://doi.org/10.7936/K76M350H

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Engineering Commons

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