Concentration Polarization Layer Research Articles

Modeling cross-flow ultrafiltration of permeable particle dispersions.

Cross-flow ultrafiltration is a pressure-driven separation and enrichment process of small colloidal particles where a colloidal feed dispersion is continuously pumped through a membrane pipe permeable to the solvent only. We present a semi-analytic modified boundary layer approximation (mBLA) method for calculating the inhomogeneous concentration-polarization (CP) layer of particles near the membrane and the dispersion flow in a cross-flow filtration setup with a hollow fiber membrane. Conditions are established for which unwarranted axial flow and permeate flow reversal are excluded, and non-monotonic CP profiles are observed. The permeate flux is linked to the particle concentration on the membrane wall using the Darcy-Starling expression invoking axially varying osmotic and trans-membrane pressures. Results are discussed for dispersions of hard spheres serving as a reference system and for solvent-permeable particles mimicking non-ionic microgels. Accurate analytic expressions are employed for the concentration and solvent permeability dependent dispersion viscosity and gradient diffusion coefficient entering into the effective Stokes flow and advection-diffusion equations. We show that the mBLA concentration and flow profiles are in quantitative agreement with results by a finite element method. The mBLA results are compared with predictions by an earlier CP layer similarity solution, showing the higher precision of the former method.

Roles of a mixed hydrophilic/hydrophobic interface in the regulation of nanofiltration membrane fouling in oily produced wastewater treatment: Performance and interfacial thermodynamic mechanisms

Nanofiltration (NF) membrane fouling during the treatment of oily produced wastewater remains the main technical challenge of crude oil (CO) exploitation. Here, a mixed hydrophilic/hydrophobic interface was introduced by adding anionic polyacrylamide (APAM) to CO solutions to mitigate NF membrane fouling. Interfacial free energies were applied to reveal how the mixed interface and coexisting metal ions regulate membrane fouling. Notably, in the initial stage of NF, both the flux decline rate and reversible fouling resistance (Rrev) were aggravated by adding APAM, and these changes were closely related to the higher osmotic pressure between the concentration polarization layer and permeation. With further addition of calcium ions (Ca2+), the strong coordination effects rendered the highest osmotic pressure in all test cases, and thus, the most serious Rrev and flux loss in the initial stage. Fortunately, in the subsequent pseudo-stable stage, the flux loss and irreversible fouling resistance (Rirr) were mitigated with the addition of APAM, which could be attributed to the formation of a mixed hydrophilic/hydrophobic interface (i.e., APAM/CO interface) with lower APAM–CO cohesive energies than the CO–CO/membrane free energies and the consequent retardation of irreversible fouling layer formation and growth. Moreover, the lower APAM–CO free energies for the mixed interface likely represented a more unstable thermodynamic system of the irreversible fouling layer and promoted the flux recovery of fouled membranes after chemical cleaning. In addition, the CO rejection of the NF membranes was also enhanced by the addition of APAM because the hydrophilic APAM molecules formed an energy barrier that inhibited the adsorption of hydrophobic CO.

Nitrate removal studies on polyurea membrane using nanofiltration system – membrane characterization and model development

Abstract Desalination of nitrates from brackish water is prominent in the coastal areas due to excessive disposal of pesticides by agricultural industries. Nowadays, membrane processes are growing tremendously for the desalination of brackish water. In this context, polyurea (PU) could be a useful membrane material for the treatment of brackish water. The present work deals with the removal of nitrates from synthetic water using PU membranes by nanofiltration (NF) process. Polyurea thin film composite (PU-TFC) membranes were prepared by interfacial polymerization followed by thermal crosslinking and characterized using Fourier transformed infrared spectral (FTIR), X-ray diffraction (XRD), scanning electron microscopy– energy dispersion X-ray spectroscopy (SEM–EDS), Atomic force microscopy (AFM), thermogravimetric (TGA), and universal testing machine (UTM) for structural analysis, crystallinity, morphological, compositional, thermal and mechanical properties, respectively. Experimental studies were conducted on an NF pilot plant by varying operating pressure from 2 to 10 bar and feed nitrate concentration from 60 to 200 mg/L for evaluating PU membrane performance. Experimental observations revealed a maximum water flux of 30.6 L/m2 h and nitrate rejection of 97.2% at a pressure of 10 bar for feed containing 140 mg/L of nitrate. A mass transfer model was developed on the basis of solution–diffusion mechanism for a semi-batch NF process by considering cake enhanced concentration polarization model, for laminar flow with feed recycle, using a plate and frame membrane module. A generic semi-batch NF process model was integrated taking into account concentration polarization and fouling layer resistance. The integrated model was successfully compared with existing data in literature and could be used for process scale-up. Due to the merits of hydrophilicity, negative charge, high thermal and mechanical resistance, the PU membrane can be termed as a low cost, commercially viable and ecofriendly barrier for separation of nitrates.

Build-up and relaxation of membrane fouling deposits produced during crossflow ultrafiltration of casein micelle dispersions at 12 °C and 42 °C probed by in situ SAXS

Skim milk filtration is performed either at low or high temperature. However, there is still a lack of knowledge concerning the influence of temperature on membrane fouling. We used in situ small-angle X-ray scattering (SAXS) to study external membrane fouling during crossflow ultrafiltration of milk protein (casein micelle) dispersions at 12 °C and 42 °C. Casein micelle concentration distribution was measured in the concentration polarization layer and the deposit in a three-step filtration experiment that consisted of (i) fouling development step (with applied pressure and average cross-flow velocity fixed at 110 kPa and 3.1 cm s−1, respectively), (ii) pressure relaxation step (with pressure reduced to 10 kPa), and (iii) deposit erosion step (with crossflow velocity increased to 15.6 cm s−1).Despite a higher average filtrate flux obtained at 42 °C, filtration at 42 °C resulted in the formation of a thicker and more concentrated deposit than filtration at 12 °C. At both temperatures studied, subsequent pressure relaxation resulted in deposit swelling, which was more intense in the less-compressed external part of the deposit. Deposit swelling rate was significantly higher at 42 °C than at 12 °C. The swelled deposit obtained at 42 °C was more eroded by the crossflow compared to the poorly swelled deposit obtained at 12°. This suggests that deposit formation and compression were more reversible at 42 °C than at 12 °C.

3D printed honeycomb-shaped feed channel spacer for membrane fouling mitigation in nanofiltration

The development of a superior feed channel spacer is one among the problems that have to be resolved to effectively mitigate foulant accumulation in the membrane system and improve filtration performance. In view of this, a novel honeycomb-shaped spacer, whose hexagonal form is the most stable and economical structure observed in nature, is proposed. 3D printing enables us to manufacture the honeycomb-shape spacer. Then, the performance of such spacer is demonstrated by comparing its filtration results with those of a standard diamond-shaped spacer in nanofiltration. The proposed structure is observed to have higher fouling mitigation performance under various fouling conditions (i.e., low and high fouling potentials and different organic foulants). Optical coherence tomography demonstrates that the foulant layer formed by utilizing honeycomb-shaped spacers (119.0 μm) is significantly thinner than that resulting from the use of standard spacers (175.5 μm); thereby, the permeate flux of the honeycomb-shaped spacers was 16.0% greater than that of the standard spacers. Hydraulic cleaning tests reveal that honeycomb-shaped spacers have a higher potential for mitigating fouling resistances driven by the concentration polarization layer (Rcp) and cake layer (Rc), leading to higher permeate production than that generated by filtration using standard spacers. The use of computational fluid dynamics simulation affords better insights into the hydrodynamic effects of these spacers on the feed channel. It is observed that honeycomb-shaped spacers have superior performance that is attributable to the generation of high-magnitude turbulent kinetic energy in the areas enclosed by spacer filaments.

Investigation on the Stability of Random Vortices in an Ion Concentration Polarization Layer with Imposed Normal Fluid Flow.

While nanoscale electrokinetic studies based on ion concentration polarization has been actively researched recently, random vortices naturally occur, leading to significantly destabilize in laboratory experiments or practical applications. These random vortices agitate the fluid inside microchannels and let the sample molecules seriously leak out preventing them from being controlled. Therefore, several trials have been reported to regulate those uninvited fluctuations by fluid flow tangential to a nanoporous membrane. Indeed, the influence of normal flow should be studied since the mass transport happens in the normal direction to the membrane. Thus, in this work, the nonlinear influence of normal flow to the instability near ion-selective surface was investigated by fully-coupled direct numerical simulation using COMSOL Multiphysics. The investigation on the effect of normal flow revealed that a space charge layer plays a significant role in the onset and growth of instability. The normal flow from the reservoir into the ion-selective surface pushed the space charge layer and decreased the size of vortices. However, there existed a maximum point for the growth of instability. The squeeze of the space charge layer increased the gradient of ion concentration in the layer, which resulted in escalating the velocity of vortices. On the other hand, the normal flow from the ion-selective surface into the reservoir suppressed the instability by spreading ions in the expanding space charge layer, leading to the reduction of ion concentration delayed the onset of instability. These two different mechanisms rendered asymmetric transition of stability as a function of the Peclet number and applied voltage. Therefore, this investigation would help understand the growth of instability and control the inevitable random vortices for the inhibition of fluid-agitation and leakage.

Numerical study on the moisture transfer characteristics of membrane-based liquid desiccant dehumidifiers: Resistance distribution and concentration polarization

Membrane-based liquid desiccant dehumidifiers have received increasing attentions due to their main advantage of preventing liquid carryover in air handling process, but the low dehumidification efficiency limits their further applications. The underlying mechanism of their moisture transfer deterioration is not entirely clear yet, and further efforts are needed to effectively identify and reduce the mass transfer resistances in the dehumidification process. In this work, considering impressionable transmembrane permeation flux, the resistance distribution of moisture transfer in the quasi-counter flow flat-plate membrane-based liquid desiccant dehumidifier is studied by a three-dimensional conjugate heat and mass transfer model. The velocity, temperature and concentration fields in dehumidifying process are analyzed in baseline condition, especially on the membrane surfaces. Analogy to a thermal circuit, the normalized moisture transfer resistances through gaseous, solid and liquid phases are defined to describe their relative importance. Causes of these normalized resistances in variable conditions are analyzed in terms of concentration polarization and boundary layer theory. Furthermore, the resistance variation is compared with the latent effectiveness, to reveal their inherent relation. The results suggest that moisture transfer resistances are mostly originated from the membrane and air side, and thus dehumidifying enhancement is more likely to emerge from the air-side concentration polarization alleviation in synergy with the appropriate improvement of membrane porosity. Among all operating parameters, the air inlet temperature and solution flow rate have relatively limited effect on the total resistance, while increasing solution concentration, followed by decreasing solution temperature, can significantly reduce the moisture transfer resistances at a nondecreasing latent effectiveness. In addition, the latent effectiveness shows a similar variation tendency to that of the air-side moisture transfer resistance, but it cannot reflect the total resistance variation. These results can help researchers and designers make decisions on where and how to improve the performance of membrane-based liquid desiccant dehumidifiers.

Computation of Global and Local Mass Transfer in Hollow Fiber Membrane Modules

Computational fluid dynamics (CFD) provides a flexible tool for investigation of separation processes within membrane hollow fiber modules. By enabling a three-dimensional and time dependent description of the corresponding transport phenomena, very detailed information about mass transfer or geometrical influences can be provided. The high level of detail comes with high computational costs, especially since species transport simulations must discretize and resolve steep gradients in the concentration polarization layer at the membrane. In contrast, flow simulations are not required to resolve these gradients. Hence, there is a large gap in the scale and complexity of computationally feasible geometries when comparing flow and species transport simulations. A method, which tries to cover the mentioned gap, is presented in the present article. It allows upscaling of the findings of species transport simulations, conducted for reduced geometries, on the geometrical scales of flow simulations. Consequently, total transmembrane transport of complete modules can be numerically predicted. The upscaling method does not require any empirical correlation to incorporate geometrical characteristics but solely depends on results acquired by CFD flow simulations. In the scope of this research, the proposed method is explained, conducted, and validated. This is done by the example of CO2 removal in a prototype hollow fiber membrane oxygenator.

Nanofiltration membranes for toxic lead removal: contribution of various mass transfer mechanisms on membrane performance

ABSTRACT The performance of nanofiltration membrane process for lead retention has been modelled by the modified Donnan steric pore-flow model (DSPM). The model predictions have been used to analyse the contribution of diffusive, convective and electrical flux on ion retention. A good agreement was found between the experimental and modelling data at various operating conditions. The influence of various process parameters, such as the permeation flux, feed solution concentration and cross-velocity, on lead (Pb) ion retention, was investigated. Maximum lead ion rejection was 93.8% at permeate flux of 1.25 * 10−5 m/s. It was found that the lead retention increases initially with permeate flux and then decreases with further increase in permeate flux due to the developed concentration polarisation layer. Furthermore, the rejection drops by ∼ 80% when the uncharged membrane is used, revealing the important role of electrical potential on the performance of the nanofiltration membrane. Finally, it is confirmed that the diffusion phenomenon is the dominant mechanism for ion transfer through the membrane.

Quantifying Concentration Polarization \u2013 Raman Microspectroscopy for In-Situ Measurement in a Flat Sheet Cross-flow Nanofiltration Membrane Unit

In this work, the concentration polarization layer (CPL) of sulphate in a cross-flow membrane system was measured in-situ using Raman microspectroscopy (RM). The focus of this work is to introduce RM as a new tool for the study of mass transfer inside membrane channels in reverse osmosis (RO) and nanofiltration (NF) generally. Specifically, this work demonstrates how to use RM for locally resolved measurement of sulphate concentration in a cross-flow flat-sheet NF membrane flow cell with channel dimensions similar to commonly applied RO/NF spiral wound modules (channel height about 0.7 mm). Concentration polarization profiles of an aqueous magnesium sulphate solution of 10 gsulphate·L−1 were obtained at operating pressure of 10 bar and cross-flow velocities of 0.04 and 0.2 m·s−1. The ability of RM to provide accurate concentration profiles is discussed thoroughly. Optical effects due to refraction present one of the main challenges of the method by substantially affecting signal intensity and depth resolution. The concentration profiles obtained in this concept study are consistent with theory and show reduced CPL thickness and membrane wall concentration with increasing cross-flow velocity. The severity of CP was quantified to reach almost double the bulk concentration at the lower velocity.

Multi-phenomenal macroscopic investigation of cross-flow membrane flux in microfiltration of oil-in-water emulsion, experimental & computational

The primary reasons for reduction of permeate flux in membrane separation processes are cake layer formation and concentration polarization phenomenon. Although pore blocking affects cross membrane flux, but its influence has been neglected in the previous numerical studies. In present study, macroscopic 2D CFD modeling and experimental survey in treatment of oil-in-water emulsion with cellulose acetate (CA) membrane in a cross-flow microfiltration are presented. In order to investigate the simultaneous effects of both phenomena of cake formation and pore blocking, an oil droplet size distribution based on experimental measurements is considered in the modeling. The pore blocking is simulated macroscopically in the microfiltration process. As a main result, by considering membrane governing equations into a 2D domain, i.e. pore blocking phenomenon, the simulation prediction can improve about 15%. Effects of various operational conditions such as trans-membrane pressure (TMP), cross-flow velocity (CFV) and feed oil concentration on permeate flux and process performance are also evaluated. By increasing feed oil concentration from 1000 to 10000 mg/L, the maximum value of oil concentration on membrane surface increases 9.68 times and by increasing CFV from 0.5 to 1.1 m/s, the maximum thickness of concentration polarization (CP) layer decreases 14%. Comparison of model predictions against experimental data for permeate flux indicates acceptable accuracies with a maximum error of 4.62%.

Layered organization of anisometric cellulose nanocrystals and beidellite clay particles accumulated near the membrane surface during cross-flow ultrafiltration: In situ SAXS and ex situ SEM/WAXD characterization

The structural organization of the concentration polarization layer (CPL) during the cross-flow membrane separation process of anisometric aqueous suspensions of colloidal cellulose nanocrystals and beidellite clay particles has been characterized by in situ time-resolved small-angle X-ray scattering (SAXS). Dedicated cross-flow filtration cells were implemented on the ID02 TRUSAXS beamline at the European Synchrotron Radiation Facility (Grenoble, France). From the analysis of the scattered intensities and structure factors of particles in the CPL, both the concentration profiles ɸ(Δz,Δt) and anisotropic structural organization have been characterized as a function of filtration time (Δt) and distance from the membrane surface (Δz). Remarkably, a coupling between concentration and anisotropy was revealed and modeled using either a simple or stretched exponential trend for rod- or disk-like systems, respectively. Using a simple filtration model, the time evolution of the deposit thickness, membrane resistance and specific resistance of the deposit, deduced from an analysis of the normalized concentration profiles, allowed directly predicting the rapid decay of permeate flux associated to the exponential growth of concentration and anisotropic organization inside the CPL. Ex situ scanning electron microscopy (SEM) observations and wide-angle X-ray diffraction (WAXD) analyses performed on dried deposits parallel and perpendicular to the membrane surface revealed well-defined layered structures from nanometer to micrometer length scales.

Role of concentration polarization in cross flow micellar enhanced ultrafiltration of cadmium with low surfactant concentration

Concentration polarization is an important issue in micellar enhanced ultrafiltration (MEUF) of wastewater containing heavy metal ions at low surfactant concentrations. In this paper, we studied removal of Cd(Ⅱ) by cross flow MEUF at low sodium dodecyl sulfate (SDS) concentration levels, and the role of concentration polarization in flux decline and Cd(Ⅱ) rejection was emphasized. Concentration polarization resistance and SDS concentration near membrane were calculated to characterize concentration polarization. The results showed that SDS concentration near membrane was 13 mM when feed concentration was merely 0.8 mM. By combining phase diagram of SDS, structures of SDS micelles in concentration polarization layer were deduced and thin layer structure transformed to porous structure formed by accumulated globular micelles when SDS concentration increased. Although micelles formed in concentration polarization layer was responsible for flux decline, they also provided adsorption sites for Cd(Ⅱ).

CFD modeling and simulation of concentration polarization reduction by gas sparging cross-flow nanofiltration

In this study, computational fluid dynamics (CFD) modeling of gas–liquid cross-flow spacer-filled nanofiltration was developed to simulate the process in a vertical flat-sheet module. Experiments were carried out using MgSO4 solution as the feed to validate the developed model. As the novelty of this research, the Eulerian based multiphase method was implemented to assess the effect of air sparging on the concentration polarization and to compute the Sherwood number. It was shown that in most cases air sparging led to a higher permeate flux since it could reduce the concentration polarization layer on the membrane surface up to 73% with the lowest occurring at the air flowrate of 0.4 l min−1. It was also proved that the bubbly flow (0.2, 0.4, and 0.6 l min−1) had no effect on the permeate flux while moving the flow regime to slug flow (0.8 and 1 l min−1) increased the permeate flux. Finally, a good agreement between CFD simulation results and experimental data was seen.

Structure, Rheological Behavior, and in Situ Local Flow Fields of Cellulose Nanocrystal Dispersions during Cross-Flow Ultrafiltration

This study focuses on investigating the phenomena involved in the formation of the concentration polarization and fouling layer during the cross-flow ultrafiltration process of cellulose nanocrystal (CNC) suspensions. Thanks to new cross-flow ultrafiltration cells specially designed, experiments of first time-resolved in situ Small Angle X-ray Scattering (SAXS) and second time-resolved in situ micro Particle Image Velocimetry (micro-PIV) at the vicinity of the membrane surface during filtration were performed. These two methods have given access to the concentration profiles and the velocity field as a function of the distance z from the membrane surface within the concentration polarization layers. The results obtained show an increase in particles concentration related to a decrease in the velocity within the layers formed during the process. This information linked to the rheological behavior of the suspensions permitted access to the calculated stress field within the concentrated layers during the cross-flow ultrafiltration process. Three different areas of shear stress/shear rate behaviors near the membrane surface have been emphasized. The important role of the rheological behavior and interparticle interaction in the dynamic evolution in space and time of the accumulated layers has been highlighted.

A Mathematical Modeling of the Reverse Osmosis Concentration Process of a Glucose Solution

A mathematical modeling of glucose–water separation through a reverse osmosis (RO) membrane was developed to research the membrane’s performance during the mass transfer process. The model was developed by coupling the concentration–polarization (CP) model, which uses one-dimensional flow assumption, with the irreversible thermodynamic Spiegler–Kedem model. A nonlinear parameter estimation technique was used to determine the model parameters Lp (hydraulic permeability constant), σ (reflection coefficient), and Bs (solute transport coefficient). Experimental data were obtained from the treatment of a pre-treated glucose solution using a laboratory-scale RO system, and studies on the validation of the model using experimental results are presented. The calculated results are consistent with the experimental data. The proposed model describes the RO membrane concentration process and deduces the expression of k (mass transfer coefficient in the CP layer). The verification shows that the expression of k well-describes the reverse osmosis mass transfer of a glucose solution.

Empirical modeling coupled with pore blocking for predicting cake formation of electric field effects on oily waste water cross-flow microfiltration

Cake formation is the most important phenomenon to reduce cross-membrane flux in cross-flow microfiltration although pore blocking and concentration polarization (CP) phenomena affect it. Electric field may be used to enhance process efficiency and decreased fouling phenomena in the process. An empirical model is developed here taking into consideration the cake formation effects on permeate flux reduction in the presence of AC electric field. A semi-empirical model is also developed in the presence and absence of electric field at two cases; with and without considering membrane structure in the modeling. It is the first time that electric field effects on microfiltration performance are modeled macroscopically and used with pore blocking phenomenon simultaneously. The predicted permeate fluxes by the models are compared against experimental permeate fluxes in absence and presence of electric field. Comparison of the model based predictions against experimental results indicate semi-empirical model precision with a maximum error of 8.9% for the case without and 3.6% for the case with pore blocking consideration and a maximum error for empirical model of 11.9%. Effects of different electric potentials and distances between the electrodes on CP layer thickness and oil concentration on membrane surface are also studied. The predicted results showed that the CP layer thickness is decreased by 60.9% through applying an electric potential of 500 V and also decreased by 65.2% through decreasing the distance between the electrodes from 2.3 to 1.3 mm at 200 V. The oil concentration on the membrane surface is also decreased by 72.3% as a result of applying the electric field while it is decreased by 19.2% when the electrodes distance is decreased from 2.3 to 1.3 mm at 200 V.

Concentration polarization in membrane distillation: I. Development of a laser-based spectrophotometric method for in-situ characterization

Membrane-based processes have gained prominence for environmental and industrial separations, especially in water purification and desalination. In spite of these developments, a fundamental knowledge gap exists with regards to fully understanding the processes that occur at or near the membrane surface. Concentration polarization is a phenomenon that may have a significant impact on mass transfer and fouling of the membrane. While attempts have been made to experimentally characterize this phenomenon for pressure-driven filtration processes, most studies employ semi-empirical methods to estimate the extent of concentration polarization. This study presents a novel non-intrusive spectrophotometric method developed to measure the concentration profile of solute near the membrane surface in a direct contact membrane distillation system. A custom membrane cell was designed and fabricated for this study to allow accurate in-situ characterization of the concentration polarization layer with spatial resolution of 4.5 μm. The spectrophotometric method was developed and validated using transition metal salt (i.e., nickel chloride) in the feed solution to probe the concentration profile near the membrane surface. The method developed in this study can be extended to other membrane-based separation processes and other solutes with known absorption spectra.

Influence of operating pressure on concentration polarization layer resistance in revers osmosis

Influence of operating pressure on concentration polarization layer resistance in revers osmosis

Modelling ultrafiltration performance by integrating local (critical) fluxes along the membrane length

Despite the vast number of studies on the understanding and estimation of the permeate flux in ultrafiltration, most of them base their estimations on either one or another mechanism, without pointing out a clear ‘bridge’ between them. The aim of this paper is to assess these mechanisms on the determination of the permeate flux, using as feed a multicomponent mixture of BSA, NaCl and H2O.Maxwell-Stefan Equations expressed as function of the components' volume fractions were used for an easier consideration of the non-idealities of the system. These non-idealities (hydration, adsorption, electrical interactions and volume exclusion) were critical in the local fluxes calculation, for which an increase in the thickness of the boundary layer along the filtration channel was considered.The developed model proved to be suitable for the estimation of fluxes lower than the limiting flux. Since the non-idealities of the system can be calculated along the concentration polarization layer, no extra information on the protein diffusivity was needed. Additionally, the fact that the model includes all the components from the solution offers the possibility of including the rejection of the accompanying ions in the calculations.