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Novel porous spacer fabrication for reverse osmosis membrane by non-solvent induced phase separation (NIPS)

Novel porous spacer fabrication for reverse osmosis membrane by non-solvent induced phase separation (NIPS)
Alternative Author(s)
Issue Date
2021. 2
In recent decades, many countries suffer from water shortages, one of the most significant issues, the main challenge is to produce fresh drinking water from seawater using various desalination technologies. Seawater desalination by membrane-based technologies is one of the promising methods to obtain pure quality of drinking water. Among these methods, reverse osmosis (RO) membrane which is a pressure-driven process has been increasingly applied in desalination fields due to its prominent properties of water flux and salt rejection. However, an organic fouling which is the major cause of membrane fouling interrupts stable operation during long-term of the membrane filtration process, leading serious economic problem. The fouling generally initiates from the feed spacer surface, which is one of the main components of the RO membrane module, thereby expanding the fouling range to the entire membrane surfaces due to contact between the specific membrane area and spacer strands. Since commercial polypropylene (PP) feed spacer has hydrophobic properties and dead zones due to non-porous structure of the fiber, it may induce accumulation of fouling and finally decrease water permeability. For achieving the stable RO membrane operation, fouling management technique of feed spacer is necessary. To control the fouling, surface coating with hydrophilic materials and modification of spacer geometry are commonly used. However, coating materials could easily desorb from the surface during the high-pressurized membrane filtration process and modifying the spacer geometry has no reproducibility and practicality. Based on those limitations, in this study, fabrication of feed spacer by phase inversion and its performance evaluation were conducted. Phase inversion technology, especially non-solvent induced phase separation (NIPS) method, has been commonly applied for the synthesis of commercial polymeric membranes due to its simplicity and reproducibility. For NIPS method, homogeneous polymer solution is casted on a substrate and then immersed in a coagulation bath containing non-solvent. Various factors could affect the structures such as the type of polymer/solvent/non-solvent, initial polymer solution concentration, and temperature. This technology has widely been studied for membrane preparation, yet there are no results for feed spacer fabrication. In this experiment, Polyethersulfone (PES), N-Methyl-2-pyrrolidone (NMP), and distilled water (DI) are chosen for polymer, solvent, and non-solvent, respectively. For improving mechanical strength and microporous porous structure, 18 wt% of PES/NMP solution was used. To produce optimal pore shape of feed spacer, casting plate was manufactured from 3D printed technique using computational fluid dynamics (CFD) results. A lab-scale RO module with high pressure of 5-6 MPa was operated to confirm efficiency of porous phase inverted feed spacer. For the fouling experiment, artificial seawater containing humic acid as an organic foulant was prepared. Various surface analyzations are also carried out to identify the phase inverted feed spacer surface properties compared pristine PP feed spacer. Phase inverted spacer showed higher water permeability than pristine feed spacer of 15.83% due to its smoother surface, porous fiber structure, and hydrophilicity from atomic force microscopy (AFM), scanning electron microscope (SEM), and contact angle results, respectively. Finally, the porous phase inverted feed spacer by NIPS method was considered as the suitable for compensating the commercial PP feed spacer.
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