CFD simulation of osmotic membrane distillation using hollow fiber membrane contactor: Operating conditions and concentration polarization effects

Inorganic salts, when present in high concentrations, can disrupt the osmotic balance of aquatic organisms, leading to physiological and ecological harm. Osmotic membrane distillation (OMD) is an emerging technique for recovering inorganic salts from wastewater. The process involves the transfer of water vapor from a feed solution (FS) to a permeate stream due to the difference in vapor pressure across a hydrophobic membrane. It thus results in a concentrated solution at the feed outlet and purified water at the permeate side. OMD is preferred due to its straightforward scale-up and low energy requirements. In this research, a steady state 2-D axisymmetric computational fluid dynamics (CFD) model is developed to study the recovery of sodium carbonate (Na2CO3) from aqueous feed solution through OMD in a hollow fiber membrane contactor (HFMC). Aqueous sodium chloride (NaCl) solution was taken as an osmotic solution (OS). The numerical convection-diffusion mass, momentum transport, and Happel equations were scripted in COMSOL Multiphysics™ version 6.0 software. The model computed the water transport from FS to OS. Computed water flux well matched with the literature based experimental results. Simulations were carried out to investigate the parametric effects, including Reynold’s number, FS and OS concentration and the module geometrical parameters to establish an optimized operating conditions and module geometry and to achieve the desired FS concentration. Results showed that a 2-fold rise in OS concentration increased the transmembrane water flux 4-fold. However, the flow rates of both FS and OS did not significantly affect the flux. On the other hand, concentration polarization (CP) showed dependence on the FS concentration, tortuosity, and Reynolds number. A membrane tortuosity of 1.6, feed concentration of 75 g L−1, OS concentration of 300 g L−1, and a FS Reynolds number in the turbulent flow range can result in higher transmembrane flux and lower chances of CP development.