Modeling concentration polarization and permeate flux in osmotic membrane processes / Chong Yie Kai

Faculty of Chemical and Process Engineering Technology

Thesis (Doctor of Philosophy) -- Universiti Malaysia Pahang Al Sultan Abdullah – 2025

Reverse osmosis (RO) is widely used in the desalination but its efficiency is limited by the concentration polarization (CP), which reduces the effective driving force to produce flux. Computational Fluid Dynamics (CFD) is employed to optimize RO membrane performance by modeling the CP. This research focuses on enhancing RO efficiency through spacer geometry modifications. CFD simulations were conducted to evaluate the hydrodynamic and mass transfer characteristics of various spacer designs, including column and spherical nodes, twisted spacers, and conventional ladder-type spacers at laminar flow (Reynolds numbers less than or equal to 200). Results indicate that column nodes outperform spherical nodes in term of Sherwood number by 29% due to improved flow diversion and higher local velocities near the membrane. Furthermore, when the dimensionless node diameter ratio of the column nodes increases from 0.3 to 1.2, Sherwood number increase by 25% at the expense of higher global friction factor (44%). Twisted spacers demonstrated superior performance (57% higher Sherwood number) compared to ladder-type spacers by inducing vortices and minimizing stagnant zones. The twisted spacer with right-handed twist direction, attack angle of 45° and twist ratio of 1 to 2 exhibited the best results. To dewatering hypersaline brine, the transmembrane pressure required to overcome the huge osmotic pressure difference can easily exceed the membrane burst pressure for conventional RO process. To address this limitation, osmotically assisted reverse osmosis (OARO) was explored. This study reveals that external concentration polarization (ECP) (contribute to 33% of the effective osmotic pressure) is more important than internal concentration polarization (ICP) (contribute to 32% of the effective osmotic pressure), especially for a system with a high transmembrane pressure (≥ 40 bar). Accurate modeling of effective structural parameter and flux is crucial for evaluating membrane processes. A comprehensive model was developed to address limitations of previous approaches. The model revealed the importance of considering pressure effects and porosity variations for precise predictions of effective structural parameters and flux in various membrane processes, including forward osmosis (FO), pressure assisted osmosis (PAO), pressure retarded osmosis (PRO), and OARO. Neglecting these effects can cause the deviation in effective structural parameter up to 222% and flux up to 50%. This research demonstrates the potential of CFD modeling to provide valuable insights into complex membrane phenomena, enabling the design of more efficient and effective membrane systems.