Date

2016

Document Type

Dissertation

Degree

Doctor of Philosophy

Department

Mechanical Engineering

First Adviser

Oztekin, Alparslan

Other advisers/committee members

Kazakia, Jacob; Webb, Edmund; Cheng, Xuanhong

Abstract

The reverse osmosis (RO) desalination process has been widely used to overcome the fresh water shortages around the globe. Spiral wound membranes are utilized to desalinate sea water and brackish water. In a typical spiral wound desalination module, the feed channel contains spacers with different arrangements. The membranes are semi-permeable which allows the pure water to pass through and hold back the dissolved salt ions from passing. In order to model desalination process properly in these modules accurate membrane flux model should be employed. The membrane is treated as a functional surface where the rate of water permeation is determined by the local pressure, osmotic pressure and local concentration. The rejection of salt from membrane causes an accumulation of salt near the membrane surface. Such phenomena are referred as concentration polarization occurring at the surface or near the surface of the membrane. The concentration polarization adversely affects the membrane performance and reduces the lifetime of the desalination module. The concentration polarization causes an increase in the osmotic pressure across the membrane and in turn leads to reduction of water flux through the membrane. The fouling potential also increases in regions where high concentration polarization occurs. Fouling build ups and scaling are also known as influencing the membrane performance adversely. This study demonstrates that membrane mass flux performance, concentration polarization and the fouling buildup/scaling are profoundly influenced by the flow structures in the feed channel. It has been shown by the present study that the flow inside the feed channel containing spacers can be strongly three dimensional and transient. Numerical simulations are conducted to characterize three dimensional and transient nature of the flows. Navier-Stokes equations and the mass transport equation are solved to determine the flow and concentration fields in the feed channel. The laminar flow model is utilized at low flow rates while turbulent flow models are utilized at higher flow rates. The Shear Stress Transport (SST) �-� turbulence model and large eddy simulation (LES) methods are employed to characterize the turbulent flow structures. The presence of spacers in the feed channel enhances membrane performance significantly. The influence of the spacer is more pronounced at higher flow rates. The spacing and the arrangement of spacers have strong influence on the membrane performance. Spacers also aid in mitigating concentration polarization and fouling in these modules. Momentum mixing induced by the presence of spacers is the reason for improvement of membrane performance. Modules including the mesh type of spacers with 30° angle performs best. This study clearly illustrated that the type, shape and arrangements of spacers are integral part of design and optimization of reverse osmosis desalination modules.

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