The Effect of Porous Support Layer in Forward Osmosis Membranes A Computational Fluid Dynamics Simulation

Computational fluid dynamics (CFD) simulations are conducted to study the transport phenomena in spiral wound membranes (SWM) within a Forward Osmosis (FO) module. The effect of the porous layer on the membrane performance is examined. Simulations are prepared for three different porous layer thicknesses by having the porous layer facing the draw channel, a mode known as AL-FS (active layer facing feed solution). In the current study, a Reynolds number range from 2 to 500 is considered. The Navier-Stokes and the mass transport equations are used to obtain the velocity, pressure and concentration fields in the flow channels. The local osmotic pressure and the membrane properties are used to calculate the water permeation over the membrane surface. The membrane is considered as a semipermeable functional surface of zero thickness. The effect of the porous layer is included in the flux model, but the flow and concentration fields in the porous layer are not resolved. The results suggest that increasing the streamwise velocity decreases the level of the external concentration polarization on both sides of the membrane which in turn leads to higher water flux through the membrane. Also, the existence of the porous layer reduced the membrane performance. The water flux didn't improve much with increasing streamwise velocity at the same porous layer thickness. The suction velocity over the membrane starts at a high value at the inlet of the draw channel and decreases until reaching the outlet of the draw channel then it starts to increase slightly from the effect of the inlet of feed solution. Moreover, by increasing the net osmotic pressure difference, the water flux exhibited a non-linear increase.