Integrated CFD simulation of concentration polarization in narrow membrane channel

A novel corrugated wall channel module for external concentration polarization mitigation in forward osmosis process

Much work has been conducted on the topic of forward osmosis (FO), but only a few studies have focused on mitigating external concentration polarization (ECP). This study introduced a simple structure, the corrugated wall channel, to the design of FO module, to induce vortex, and then mitigate ECP. In this study, the corrugated wall channel module (CWCM) was tested under given conditions, with a traditional flat membrane module (FMM) as control. CWCM could mitigate ECP and then enhance water flux. When deionized water was taken as feed solution (FS) and 2-M NaCl solution as draw solution (DS), the water flux enhancement was 16.49 and 18.51% in FO mode (active layer facing FS) and PRO mode (active layer facing DS), respectively. When 0.5-M NaCl solution was taken as FS, the corresponding values were 15.92 and 17.13%, respectively. Computational fluid dynamics (CFD) analysis showed that the CWCM could induce vortex, promote the mixing of the solution in the module, and further contribute to the increase of water flux. The specific shape of CWCM affected its performance on mitigating ECP. Also, the more tortuous CWCM exhibited higher water flux. In addition, CWCM could lessen membrane fouling.

Temperature and Velocity Effects on Mass and Momentum Transport in Spacer-Filled Channels for Reverse Electrodialysis: A Numerical Study

Concentration polarization is one of the main challenges of membrane-based processes such as power generation by reverse electrodialysis. Spacers in the compartments can enhance mass transfer by reducing concentration polarization. Active spacers increase the available membrane surface area, thus avoiding the shadow effect introduced by inactive spacers. Optimizing the spacer-filled channels is crucial for improving mass transfer while maintaining reasonable pressure losses. The main objective of this work was to develop a numerical model based upon the Navier–Stokes and Nernst–Planck equations in OpenFOAM, for detailed investigation of mass transfer efficiency and pressure drop. The model is utilized in different spacer-filled geometries for varying Reynolds numbers, spacer conductivity and fluid temperature. Triangular corrugations are found to be the optimum geometry, particularly at low flow velocities. Cylindrical corrugations are better at high flow velocities due to lower pressure drop. Enhanced mass transfer and lower pressure drop by elevating temperature is demonstrated.

Modeling Dynamics of Colloidal Fouling of RO/NF Membranes with A Novel Collision-Attachment Approach

We report a novel collision-attachment approach for modeling the dynamics of colloidal fouling. The model treats fouling as a two-step process: colloidal particles colliding with a membrane surface followed by their attachment onto the surface. An attachment coefficient is adopted to describe the probability of successful foulant attachment for any given collision event, the value of which can be determined by the classical Boltzmann distribution. Our model shows excellent agreement with experimental data in terms of both the kinetics of flux decline and foulant mass deposition. Modeling results reveal the critical roles of water flux and energy barrier in governing colloidal fouling. Greater water flux or lower energy barrier can lead to a collision-controlled condition, where severe fouling occurs and nearly all collision events lead to successful foulant attachment. On the contrary, fouling is increasingly controlled by the probability of successful attachment at lower water flux and/or greater energy barrier. Our model provides deep insights into the various mechanisms governing the dynamics of colloidal fouling (i.e., concentration polarization, collision, and attachment) and the self-limiting fouling behavior under constant-pressure mode.

Flow behavior in weakly permeable micro-tube with varying viscosity near the wall

Weakly permeable micro-tubes are employed in many applications involving heat and/or mass transfer. During these processes, either solute concentration builds up (mass transfer) or steep change in temperature (heat transfer) takes place near the permeable wall causing a change in the viscosity of the fluid. Results of the present work suggest that such change in viscosity leads to a considerable alteration in the flow behavior, and the commonly assumed parabolic velocity profile no longer exists. To solve the problem numerically, the equation of motion was simplified to represent permeation of incompressible, Newtonian fluid with changing viscosity through a micro-tube. Even after considerable simplification, the accuracy of the results was the same as that obtained by previously reported results for some specific cases using rigorous formulation. The algorithm developed in the present work is found to be numerically robust and simple so that it can be easily integrated with other simulations.

Concentration polarization phenomenon during the nanofiltration of multi-ionic solutions: influence of the filtrated solution and operating conditions

One of the major difficulties for the prediction of separation performances in the case of multi-ionic mixtures nanofiltration lies in the description of the concentration polarization phenomenon. Usual models available in literature do not take account of the polarization phenomenon or only describe it cursorily. Very few studies dedicated to the understanding and the specific description of the concentration polarization phenomenon are available in literature and a 2-D multi-ionic model describing the layer heterogeneity along the membrane length has never been proposed yet. The model used in the present work, called Pore and Polarization Transport Model (PPTM), allows an accurate description of the concentration polarization layer occurring during the filtration of multi-ionic solutions by taking account of the radial electromigrative transport in the layer, the turbulence, as well as the axial heterogeneity. In this context, the present paper aims at proposing a numerical investigation of the influence of operating conditions on the behavior of the polarization layer occurring at the membrane vicinity. The input parameters governing the transport through the membrane have been assessed in a previous study in the same experimental conditions so that only the polarization layer is investigated here. The proposed model which was previously validated on experimental observed rejection curves is then used to understand how operating conditions, such as applied pressure, feed flow-rate, or divalent ion proportion, govern the polarization phenomenon. For this purpose, concentration and thickness axial profiles along the membrane length and radial profiles within the polarization layer are investigated for various conditions. Finally, the impact of the type of divalent ion and the number of ions is also studied on various mixtures.

Recent advances in membrane bioreactors (MBRs): membrane fouling and membrane material

Membrane bioreactors (MBRs) have been actively employed for municipal and industrial wastewater treatments. So far, membrane fouling and the high cost of membranes are main obstacles for wider application of MBRs. Over the past few years, considerable investigations have been performed to understand MBR fouling in detail and to develop high-flux or low-cost membranes. This review attempted to address the recent and current developments in MBRs on the basis of reported literature in order to provide more detailed information about MBRs. In this paper, the fouling behaviour, fouling factors and fouling control strategies were discussed. Recent developments in membrane materials including low-cost filters, membrane modification and dynamic membranes were also reviewed. Lastly, the future trends in membrane fouling research and membrane material development in the coming years were addressed.