Computational Study of Forward Osmosis Processes Using Hollow Fibers Membrane

Abstract Computational fluid dynamic simulations were conducted to investigate the transport phenomena in hollow fibers forward osmosis membrane module. Navier-Stokes and mass transport equations were solved to obtain the laminar velocity and concentration field within the FO modules for various values of flow rates and concentration of draw solution. The hollow fiber membrane was considered as a semipermeable functional surface of zero thickness, but the porous layer effect was included in the flux model. The solution–diffusion model was employed to determine the rate of water passage through the hollow fiber membrane. The unique contribution of this work is that simulations include flow and mass transport of each stream coupled with the membrane flux condition and the module consists of multiple membranes placed inside the shell. The parametric study was performed to study the effect of membrane properties and the operational parameter on the flux permeation, and concentration polarization for the feed and the draw streams. Flow disruptors such as disks and orifices and twisted membrane were considered to promote mixing and improve the module performance. The range of flow rate is 1.8 ≤ Re ≤ 10 for the feed stream, and 2.5 ≤ Re ≤ 100 for the draw stream. The novel idea of inserting baffles in the draw channel, and twisted hollow fibers were introduced for the first time in this study to assist to promote mixing in the draw and feed channel and increase the flux performance of FO hollow fiber module. The result showed that the membrane properties and the operational parameter have a strong effect on the flux permeation and concentration polarization. The water permeation were increased as the membrane thickness and the tortuosity were reduced. The concentration polarization became more intense in feed and draw stream as the draw concentration is increased. The parametric study concluded that the optimum properties as membrane thickness of 15 μm and a tortuosity of 1.6 should be used when manufacturing hollow fiber membranes. The maximum flux permeation were in the module with no mixing promoting as 20.25 (kg⁄m^2 h) at membrane thickness of 15 μm, the membrane tortuosity of 1.6, the porosity of 0.33, the inlet draw concentration of 0.01, and draw Reynolds number of 5.0. It is evident that concentration polarization hindered the separation process, mixing promoting is needed in the module. Generally, the water permeation is enhanced in the module containing disk baffles configuration, and become more effective with the combined configuration at all flow rates. It indicates that the existence of disk baffles in the shell mitigated the concentration polarization and resulted in high water permeation. The concentrative concentration polarization increased by 29.1% for the feed side (〖CP〗_b), decreased by nearly 3% for the draw side (〖CP〗_d), and increased by 41.2% with higher draw concentration solute. Additionally, the combined baffles in the shell mitigated the concentration polarization and resulted in higher water permeation; the concentrative concentration polarization increased by 28.5% for the feed side (〖CP〗_b), decreased by almost 3.4% for the draw side (〖CP〗_d), and increased by 83.2% with higher draw concentration solute. As the flow rate increased, the water permeation enhanced by more than 15% at the highest flow rate. The result showed that the water permeation increased by 60.1% with 30° twisting angle, the concentrative concentration polarization increased by 29.8% for the feed side (〖CP〗_b), and decreased by almost 2.9% for the draw side (〖CP〗_d). Once the flow rate increased, the water permeation enhanced by 14% at the highest flow rate. It is evident here twisting the hollow fiber membrane modules is strongly enhanced the separation performance and it is practical to be used in the field.