Concentration polarization is an important factor that limits separation performance in nearly all cross flow membrane filtration processes. For example, polarization of rejected solutes in reverse osmosis (RO), nanofiltration (NF), and ultrafiltration (UF) separations causes elevated concentrations at the membrane-solution interface, which increases solute passage and trans-membrane osmotic pressure. In addition, concentration polarization (CP) exacerbates all forms of surface fouling phenomena including scale formation by sparingly soluble mineral salts, cake formation by colloids, gel formation by organics, and biofilm formation by bacteria. Therefore, accurate description of CP phenomena is critical for design and optimization of membrane separations. We have developed a finite element model to study momentum and mass transfer in cross flow membrane filtration systems with open and spacer-filled channels. Simulation conditions represent practical reverse osmosis operating conditions and membrane properties considering both open and spacer-filled channels. Although the model is two-dimensional and somewhat simplified, the results provide valuable insight to guide further exploration in feed spacer design and optimization.

In straight-through open channels, finite element numerical model results agree reasonably well with classical analytical models for predicting pressure drop and wall shear rate, but differ from the film theory-based concentration polarization model for most combinations of cross flow and permeate hydrodynamics. In spacer-filled channels, axial frictional pressure losses are always higher than in open channels, whereas concentration polarization is (on average) reduced for all spacer geometries considered. Preliminary results suggested that certain spacer geometries create zones where fluid mixing stagnates near the membrane surface. This leads to “spacer-enhanced concentration polarization,” which (theoretically) could promote localized scale, cake, gel, and biofilm formation. As a result, we have begun exploring novel feed spacer geometries that may lead to improved spacer designs for practical membrane modules. Results from these and other simulations of fluid flow, pressure drop, concentration polarization, and particle transport can help to describe the efficacy of a given feed spacer geometry.

It is hypothesized that one combination of channel height and spacer geometry may not be optimum for every application, but rather that feed water composition and operating conditions will dictate the optimum spacer-filled channel geometry. Simulations were performed representing seawater (SWRO), brackish water (BWRO), and reuse water (RWRO) reverse osmosis applications in order to elucidate the roles of feed water chemistry and operating conditions on feed spacer performance. The results of these simulations suggest that the influence of concentration polarization on trans-membrane osmotic pressure is the main source of energy loss in SWRO applications, while axial frictional losses cause the greatest proportion of energy loss in RWRO applications. In BWRO applications, the balance of frictional and osmotic pressure losses are dependent on feed water osmotic pressure, membrane properties, and operating conditions. To further explore this idea, simulations varying feed channel height and feed spacer thicknesses were performed. Results from all simulations will be presented at the conference along with practical guidance for the design and optimization of spacer filled channels in RO membrane desalination processes.