Computational Study of Desalination by Membranes

Abstract Water desalination by membranes constitutes the majority of the low-quality water purication systems that extends across many different techniques. This study considers transport phenomena in reverse osmosis (RO) and vacuum membrane distillation (VMD) modules using computational techniques. Reverse osmosis is a pressure-driven separation method using semi-permeable membranes featuring nanoporous structures. Vacuum membrane distillation is another emerging separation method relying on the temperature difference across the microporous hydrophobic membranes.The membrane separation process intrinsically gives rise to temperature polarization (TP) and concentration polarization (CP) which could be severe limitations in these systems. To eliminate these polarizations and increase the module performance various design adaptations are suggested, and the effectiveness of this modications are investigated using incompressible laminar/turbulent steady/unsteady computational fluid dynamics simulations. Fully coupled membrane transport models, which is Solution-Diffusion model for RO and Dusty Gas model for VMD, are employed where the local flow properties - concentration, temperature, pressure, and suction rate - are implicitly solved along the membrane surface to predict the permeate flux accurately.It is shown how a simple design adaptation is achieved by twisting the hollow fiber membrane module that creates desirable flow structures result in dramatic performance enhancement of hollow ber membrane (HFM) RO water desalination modules. Twisted HFM bundles induce swirling flow structures inside desalination modules that increase momentum mixing throughout. It is found that the twisted HFM module mitigates concentration polarization (CP) effects and increases transmembrane permeate flux by 5 - 9% for three flow rates considered. Frictional energy losses and increased pumping power associated with this subtle design alteration are small relative to projected gains in clean water production and there are in principle no additional required components associated with this geometry.Complex nature of the permeate transport through the hydrophobic membranes in VMD process is modeled, and the effect of membrane properties and spacers on the TP and CP mitigation are explored. Large-Eddy Simulations (LES) are employed to gain more insight into the unsteady nature of the flow phenomena. It is found that coupling not only the local temperature but also the local concentration along the membrane surface affects the permeate flux since elevated concentration reduces the water vapor pressure. Membrane pore size is the dominant membrane parameter which mandates the true transport mechanisms. TP and CP are more pronounced in higher feed temperatures, and spacers dramatically mitigate these polarizations by inducing turbulent and momentum mixing. Submerging the spacers into feed channel also increases the pressure drop which is a potential threat for membrane wetting so that it needs to be examined along with other performance criteria while designing an optimum VMD module.