Date of Award
Doctor of Philosophy (PhD)
David A. Ladner, Committee Chair
To address the increasingly severe global water shortage and pollution problem, reverse osmosis (RO) has been widely used because of its ability to produce high quality water. Meanwhile, related technologies have been developed, called osmotically driven membrane processes (ODMPs). These include forward osmosis (FO), which has potential for wastewater purification and desalination, and pressure retarded osmosis (PRO), which has the capacity to produce energy. The main problems associated with RO and ODMPs are concentration polarization and membrane fouling which deteriorate the quality and quantity of the permeate flux and increase the operating cost of the system. The task of this study is to reduce concentration polarization and membrane fouling by providing more favorable hydrodynamics. The majority of the efforts that try to maximize the flux and minimize the membrane fouling for these membrane filtration processes focus on membrane modification; however, the possibility of optimizing the hydrodynamics inside the membrane channel has received less attention. The hydrodynamics inside membrane channels can be greatly influenced by the presence of spacers. Spacers play an important role in defining the hydrodynamics inside the membrane channel by creating vortices in the fluid flow. A mesh spacer design is the most common type of spacer used in the spiral wound RO module. Many attempts at optimizing the current mesh spacer by changing the flow angle, filament thickness, shape, etc. have been made. Those studies suggest that higher permeate production will incur higher pressure drop. In addition, the shape of the mesh spacer design will create dead zones and free surface area, which exacerbate the membrane fouling problem. In this dissertation is the development of a series of sinusoidal spacers to improve upon the conventional mesh spacers used in RO. This study also investigated the possibility of improving the performance of FO and PRO by using spacers. The research consists of three sections. The first two sections focus on RO membrane filtration with sinusoidal spacers where both experiments and 3D multiphysics CFD models were used. The first section investigates the hydrodynamics and mass transfer inside sinusoidal membrane channels during seawater desalination. The CFD models were verified by comparing the permeate flux obtained from the experiments. Permeate flux and pressure drop from different sinusoidal membrane channels and the mesh spacer-filled membrane channel were compared to evaluate the performance of the spacers. Because fouling was not taken into consideration in the first section, the CFD model only studied the steady state. The results showed that the permeate flux from simulation matched well with the experiments and sinusoidal spacers were able to enhance permeate flux and reduce pressure drop. The second section focuses on the performance of sinusoidal spacers for reducing humic acid membrane fouling. The degree of membrane fouling was evaluated through permeate flux decline over time and through imaging the fouling pattern on the membrane surface. The fouling pattern obtained from the experiments and the fouling pattern produced through CFD modeling were compared to verify the accuracy of the CFD models. Compared to the first section, the CFD models in the second section simulated various time points over a range of time instead of only one steady state point. In order to reduce the computational burden, it was assumed that the foulant formed a single layer on the membrane surface and flux decline caused by the single layer foulant was neglected. The results showed that the CFD modeling could predict the fouling pattern on the membrane surface. Compared to mesh spacers, sinusoidal spacers could reduce the flux decline caused by humic acid membrane fouling. The last section focuses on the impact of spacers in ODMPs. Compared to the CFD models for RO, the CFD models for ODMPs were comprised of three domains instead of one and the structure of the porous support layer of the membrane was considered in the model. With the additional domains and complexity of material transfer through the membrane the ODMP models were less stable and required more computational resources than the RO models. Thus for the bulk of the work it was necessary to use 2D simulations rather than 3D simulations for ODMPs. A few 3D simulations were successfully run in order to provide additional insight. The accuracy of the CFD models was verified by comparing the permeate flux in empty FO and PRO membrane channels obtained from experimental results in literature with the 2D CFD models. After verification another series of 2D CFD models were run to study the effect of spacers on hydrodynamics and mass transfer inside the membrane channel. A 3D model which studied only one unit of a mesh spacer was built to compare and verify the 2D CFD results. The results showed that (i) the permeate flux from CFD modeling from both FO and PRO matched well with the experimental results. (ii) The presence of spacers did not enhance the permeate flux significantly; however, the arrangement of spacer filaments played an important role in the hydrodynamics. In addition, the results from the 3D models and 2D models were very similar, suggesting that 2D modeling was generally accurate. The 3D model did provide additional insight in showing more details of the hydrodynamics, which are not fully captured in 2D. This study provides information on how to visualize and predict the mass transfer and hydrodynamics of RO and ODMPs which will benefit the future membrane research on the performance of RO and ODMPs. The investigation of building unobstructed membrane channels such as sinusoidal membrane channels will be helpful for novel membrane spacer designs.
Xie, Peng, "Simulation of Reverse Osmosis and Osmotically Driven Membrane Processes" (2016). All Dissertations. 1815.