(152be) An Investigation of a 3D CFD Study Exploring the Influence of Feed Spacer Designs on the Effectiveness of High-Permeance RO Membranes

The increasing concerns over securing fresh water resources in order to meet demands for drinking water have driven the search for sustainable methods. One such method is reverse osmosis (RO), a process which involves separation of fresh water from seawater by using a semi-permeable membrane. The choice and improvement of the membrane is crucial in the performance of allowing water permeate from the feed side, while rejecting salts to obtain high-purity water, arising to many researches in the materials science field. Researches regarding the RO technology in the process-level are also present to reduce the energy required to produce fresh water. Despite several researches conducted to enhance the permeance and selectivity of the membranes, the interdependence between the performance of the membrane and the process as whole is rather scarce. Furthermore, the performance of the RO process tends to plateau compared to the enhancement of the membrane, due to concentration polarization occurring near the membrane. While several methods are introduced to resolve this issue, applying adequate types of spacers is mentioned as a remedy for the high-permeance membrane to exert its performance.

Spacers are mesh-like structures that keeps the membrane sheets apart and enhancing fluid mixing near the membrane surfaces, especially used in spiral-wound modules. However, spacers also create stagnant zones within feed channels, making such areas more prone to fouling. The presence of spacer filaments results in increased flow resistance in the feed channel, leading to an increased pressure drop. Therefore, well-designed spacers with effective mixing capability can assist in overcoming aggravated concentration polarization (CP) in high permeance membranes, while balancing the mixing effects and pressure loss linked to permeation performance and energy consumption. A number of studies on the effects of spacers on module performance have been undertaken both experimentally and computationally. Different filament designs have been proposed to remedy stagnation zones and mitigate CP. Ultimately, quantitatively evaluating fouling propensity, CP, and pressure loss under a wide range of water permeance depending on different spacer designs is necessary.

In this talk, we present a computational fluid dynamics (CFD) simulation of feed channels filled with spacers for permeable membrane walls, considering a wide range of membrane permeances and various spacer designs. The study specifically examines the effect of water permeance, which is the critical parameter affecting concentration polarization (CP) and water flux, by varying it from the current polymeric RO membrane permeance to 20 times larger values representing ultra-high flux membranes. The simulation result includes a hypothetical spacer design that may reduce flow stagnation, along with conventional two-layered woven and unwoven configurations with different mesh angles. The methodology for coupling the CFD governing equations and permeable wall conditions is adapted from a previous study to assess water and salt fluxes, pressure drops, flow and concentration patterns, and shear rates at the membrane walls indicative of fouling tendency. Local water fluxes and wall shear rates throughout the total membrane areas are quantified to determine the effectiveness of each spacer design in combination with the membrane. The simulation framework is expected to facilitate the prediction of averaged water and salt fluxes for different spacers and membrane properties and provide insights into selecting appropriate spacer designs based on membrane permeance in terms of permeation efficiency, pressure loss, and fouling risk.