Date

2019

Document Type

Dissertation

Degree

Doctor of Philosophy

Department

Mechanical Engineering

First Adviser

Oztekin, Alparslan

Abstract

Forward Osmosis is a natural phenomenon that takes places across a semi-permeable membrane when there is a concentration difference across the membrane. Pure water permeates to the highly concentrated channel until the concentration across the membrane equilibrates. In water desalination applications, the same principle is applied. Spiral-wound membrane, flat sheet, or hollow fiber module are typical configurations in forward osmosis desalination modules. The application of water desalination using forward osmosis requires the existence of two channels separated by a suitable forward osmosis membrane. Sea or Brackish water is introduced in one side while the other side has a suitable draw solution. The concentration of the draw solution must possess several properties for optimum performance. The vital property is that the draw solution should have a concentration greater than the sea or the brackish water. The forward osmosis membrane consists of an active dense layer and a porous support layer. The membrane should be designed to have high pure water permeability with a low solute permeation coefficient and a low structural parameter. Low values of the structural parameter ensure that the effect of the internal dilutive concentration polarization is neglected.The process of water permeation in a forward osmosis membrane module has been modeled using computational fluid dynamic simulations. The local variation of the water flux along the membrane surface as a function of the local concentration of both the feed and the draw channels was calculated. The permeation of the water flux causes the concentration in the feed channel to increase as the salt start to accumulate over the membrane surface. While the concentration of the draw solution is diluted as the water mixes with the draw solution.In the simulations with a flat membrane and no mixing promoters, a laminar model was used. In the model, Navier-Stokes equations along with a mass transport equation were used to model the flow and the variation of the concentration inside the channels. The flow rate was varied to study the effect of the concentration boundary layer growth in forward osmosis membrane systems. Also, the thickness of the porous support layer was varied. The results indicate that increasing the flow rate indeed improved the water flux by reducing the growth of the concentration boundary layer. The presence of the porous support layer drastically reduces the performance of the system because of the high level of the internal dilutive concentration polarization (active layer facing feed solution orientation). It is important to reduce the thickness of the porous support layer, increase the porosity or improve the tortuosity of the porous support layer. The optimum solution is to remove the porous support layer completely.Corrugating the membrane should mix the feed and the draw solutions near the membrane surface. The membrane was corrugated in a chevron manner. Four different types of corrugations were considered: (1) a single corrugation where the peak of the corrugation is towards the feed side to avoid fouling; (2). a double membrane corrugation so that both the feed and the draw solutions are mixed; (3) a channel corrugation in which the membrane is left flat: and (4). a combined corrugation in which the double and channel corrugations are combined. In each set of simulations, three Reynolds number were considered giving twelve sets of simulations in total. The k-ω SST model is utilized to characterize the steady state turbulent structures inside the modules containing corrugations for Re of 300, 800, and 1500. The results indicate that the porous support layer is still reducing the performance of the membrane system even with the introduction of the corrugations. However, there has been improvement in the water flux up to 15% in the combined corrugation case.The final part of the dissertation research focused on removing the effect of the porous support layer and introducing embedded spacers within the structure of the membrane. Net-type spacers of 45° and three different spacer strand diameters are used. The diameters of the spacer are chosen as 0.1h, 0.2h, and 0.3h. The flow rate was changed so that Re is 300, 800, and 1500. The results indicate that the case with D = 0.3h and Re = 300 had the best performance.

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