A 3D CFD comparative study between torsioned and non-torsioned net-type feed spacer in reverse osmosis

Abstract

Spacer-filled channels are incorporated in membrane modules in reverse osmosis applications where the role of feed spacer is to promote turbulence, improve mass transport and mitigate concentration polarization. The objective of this study is to emulate a realistic spacer, due to manufacturing conditions, spacers tend to have a special complex geometry (deformations) which is usually not adopted in computational fluid dynamics (CFD) studies, this deformation tends to be of a torsioned shape. A total of 18 simulations were conducted for torsioned and non-torsioned spacers evaluating concentration polarization and mass transfer across membrane walls at different inlet salinities (c o = 400, 500 and 600 mol/m 3) and different Reynolds number (Re = 100, 192 and 384) based on the inlet velocity using fully-coupled three dimensional CFD, where the solution-diffusion model was incorporated for the calculations of water and solute fluxes across reverse osmosis membrane. The current comparison illustrates that the torsioned spacers have an overall enhanced performance compared to non-torsioned spacers. It is found that concentration polarization is mitigated in torsioned spacers which is the contrary with non-torsioned spacers having a tendency to concentration polarization, consequently, the water flux in torsioned spacers was found higher than those in non-torsioned spacers. Generally speaking, all membrane modules are found to have a higher performance at higher Reynolds number and at lower inlet salinity. This study proves that spacer geometric details are critical to predict accurately the RO membrane performance. Keywords Reverse osmosis (RO) • Computational fluid dynamics (CFD) • Feed channel • Concentration polarization • Mass transfer List of symbols A Water permeability [m/(Pa s)] B Solute permeability [mol/(m 2 s)] c Concentration (mol/m 3) d h Hydraulic diameter (m) D s Diffusion coefficient (m 2 /s) J w Water flux (m 3 /m 2 s) J s Solute flux (mol/m 2 s) k Osmotic factor (m 3 Pa/mol) n Normal vector (dimensionless) p Pressure (Pa) R Gas constant [J/(mol K)] T Temperature (K) u Velocity vector (m/s) x Cartesian coordinate y Cartesian coordinate z Cartesian coordinate Greek letters Density (kg/m 3) Dynamic viscosity (Pa s) Osmotic pressure (Pa) Subscripts ave Average b Bulk f Feed low Lower membrane surface o Inlet condition

Simulation of Reverse Osmosis Process: Novel Approaches and Development Trends

“…The ANSYS CFX was applied to a lesser extent, for example in works [85-88, 93, 113, 128-131, 134, 146-147, 121]. Moreover, the ANSYS was used without pointing out the algorithm in works [84,99,121,123]. Among the other software un should be noticed the wide range of applications of COMSOL Multiphysics, for example, in works [98-99, 101-106, 111-112, 119, 122-123, 148], and open-source product OpenFOAM [92,94,114,132,135,137].…”

“…The result of the consideration of the mentioned publication confirms this statement. In contrast with the previous period, in 2011-2020, it was proposed a large number of novel spacer designs, in particular in many studies the design with the different thicknesses of the filament between the spacer nodes, in particular in works [99,101,102,112,113,118]. Also, it was proposed the sinusoidal spacer design [123] and the design with the screw thread on the filament [107].…”

Analysis of Concentration Polarisation in Full-Size Spiral Wound Reverse Osmosis Membranes Using Computational Fluid Dynamics

“…Three-dimensional CFD models have been developed to investigate the effect of geometrical characteristics of spacers on the performance of RO membranes [ 18 ]. Abdelbaky et al [ 19 ] performed CFD analysis on RO membrane modules with feed spacers having fixed or variable diameter under different inlet salinity and Reynolds number conditions. The results indicated that the RO membrane modules with variable-diameter feed spacers can reduce CP.…”