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Title: Preparation, characterization and optimization of thin-film composite hollow fiber membranes for osmotically driven membrane processes
Authors: Chou, Shuren
Keywords: DRNTU::Engineering::Environmental engineering::Water treatment
Issue Date: 2013
Abstract: Osmosis or forward osmosis (FO) is a natural process for water transfer through a semi-permeable membrane under an osmotic pressure gradient across the membrane. Recently there has been a renewed interest in osmotically driven membrane processes (ODMPs) as a potential means of desalination, dewatering and power generation in pressure retarded osmosis (PRO). The renewed interest comes from their potential to either reduce energy consumption or produce energy (by PRO). First of all, a novel thin-film composite (TFC) hollow fiber membrane has been fabricated by interfacial polymerization on the inner surface of a polyethersulfone (PES) hollow fiber. We describe the characteristics and potential applications of the newly developed FO hollow fiber membrane. This FO membrane presents excellent intrinsic separation properties, with a water flux of 42.6 L/m2•h using 0.5 M NaCl as the draw solution and DI water as the feed with the active layer facing the draw solution orientation at 23 °C. The corresponding ratio of salt flux to water flux was only 0.094 g/L. To evaluate different application scenarios, various NaCl solutions (500 ppm (8.6 mM), 1wt% (0.17 M) and 3.5wt% (0.59 M)) were used as the feed water to test the performance of the FO membrane. The membrane can achieve a water flux of 12.4 L/m2•h with 3.5 wt% NaCl solution as the feed and 2 M NaCl as the draw solution, respectively, suggesting it has good potential for seawater desalination. Further, a specially designed PRO hollow fiber membrane has been successfully developed and applied in the PRO process to demonstrate its potential for power generation. The membrane fabrication method is similar to that used for making TFC FO hollow fiber membranes, but further optimization and improvement have led to a new type of TFC hollow fiber membranes with much stronger mechanical strength in addition to its excellent separation property and high water flux. A power density as high as 10.6 W/m2 can be achieved using seawater brine (1.0 M NaCl) and wastewater brine (40 mM NaCl), which suggests that the newly developed PRO hollow fiber membrane has great potential to be applied in PRO processes to harvest salinity gradient energy. A higher pressure is preferred as it allows generation of higher power density (pressures of 12 bar may be optimal for seawater as the high salinity stream), and this can be realized by reduced fiber dimension. Moreover, we have developed a novel hollow fiber membrane by balancing these competing factors based on our prior experience in TFC membranes. The newly developed TFC hollow fiber membrane can achieve a power density of 20.9 W/m2 at a pressure of 15 bar, using synthetic seawater brine (1.0 M NaCl) as the draw solution and synthetic river water (1 mM NaCl) as the feed water, respectively. The simultaneous specific reverse salt flux was found to be 0.03 mole/L, which is much lower than reported flat-sheet membranes, as the self-supported hollow fiber membrane can eliminate the deformation-enhanced reverse salt diffusion that is typical for flat-sheet membranes. However it was observed that the water permeability and structure parameter of the TFC hollow fiber membrane varied noticeably corresponding to the change in the hydraulic pressure imposed in the fiber lumen. Over a certain range (<15 bar), such variations proved to have positive impacts, facilitating the water permeation and reducing the ICP in the PRO process. Finally, a theoretical model is developed to understand the ICP for hollow fiber membranes more deeply. The ICP is unique in ODMPs, and is believed to associate with the thickness, porosity and tortuosity of the flat-sheet membrane, but the situation of hollow fiber membrane is somehow more complicated. Based on the newly developed model, the geometry parameter of hollow fiber membranes is found to impact the ICP, and the argument on the membrane configurations including inner and outer location of the active layer is conducted. Moreover, the external concentration polarization (ECP) in the lumen of hollow membranes is found to relate to inner diameter, membrane module length and cross flow hydrodynamics by theoretical calculation. However, the gain in membrane performance may be accompanied by the loss in energy efficiency, which is the limiting factor behind the optimization. Based on these findings, this work provides a comprehensive insight on tailoring hollow fiber membranes in ODMPs for better performance and higher energy efficiency.
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