Interaction of dispersed phase with concentration polarization

The effect of a dispersed phase in reducing the concentration polarization in a membrane tube has been studied. The presence of a dispersed phase seems to have an effect in controlling the size of eddy formation and the rate of energy dissipation in the fluid medium. The role of eddy length and the energy dissipation rate on the mass transfer coefficient is discussed. Theoretical results obtained for the mass transfer coefficient and for the concentration polarization in the case of gelatin ultrafiltration are compared with the existing experimental results. The theoretical predictions seem to be in good agreement with the experimentally observed results.     

Non-equilibrium thermodynamics of concentration polarization

Ultrafiltration is a membrane separation process with many applications, including the treatment of industrial wastes and the processing of milk and juices. Academics are also interested in ultrafiltration as a possible tool for measuring empirical coefficients such as the diffusion coefficient and the permeability. One particular region of an ultrafiltration system that is not yet fully understood, and is related to a decline in the efficiency, is the concentration polarization layer, which develops as the macromolecules retained by the membrane form a highly concentrated layer that attempts to diffuse back toward the bulk of the solution. Using the postulates of classical non-equilibrium thermodynamics, a complete model, which accounts for the fact that a concentration polarization layer may have properties of both a porous medium and a region undergoing Brownian diffusion, has been derived and applied to several systems from the literature.     

Improvements in polarization spectroscopy based on high-frequency modulation

Improvements are reported for trace elemental analysis by polarization spectroscopy. By using higher frequency (150 kHz) modulation and setting the polarizers off-extinction to produce a dispersion-shaped signal, linear calibration curves and a limit of detection (S/N = 2) of 2 ng/l. sodium can be obtained for solutions aspirated into a conventional flame source.     

Modeling electrode polarization in dielectric spectroscopy: Ion mobility and mobile ion concentration of single-ion polymer electrolytes

A novel method is presented whereby the parameters quantifying the conductivity of an ionomer can be extracted from the phenomenon of electrode polarization in the dielectric loss and tan delta planes. Mobile ion concentrations and ion mobilities were determined for a poly(ethylene oxide)-based sulfonated ionomer with Li(+), Na(+), and Cs(+) cations. The validity of the model was confirmed by examining the effects of sample thickness and temperature. The Vogel-Fulcher-Tammann (VFT)-type temperature dependence of conductivity was found to arise from the Arrhenius dependence of ion concentration and VFT behavior of mobility. The ion concentration activation energy was found to be 25.2, 23.4, and 22.3+/-0.5 kJmol for ionomers containing Li(+), Na(+), and Cs(+), respectively. The theoretical binding energies were also calculated and found to be approximately 5 kJmol larger than the experimental activation energies, due to stabilization by coordination with polyethylene glycol segments. Surprisingly, the fraction of mobile ions was found to be very small, <0.004% of the cations in the Li(+) ionomer at 20 degrees C.     

Role of backpulsing in fouling minimization in crossflow filtration with ceramic membranes

Effect of backpulsing on crossflow filtration of different process streams was studied. Laboratory scale experiments were conducted with synthetic electroplating wastewater containing Cr(OH)₃ suspension. Porous ceramic membranes of various pore sizes (0.05–5.0 μm) were evaluated. Filtration experiments with and without backpulsing show that backpulsing is effective in minimizing membrane fouling. Up to five-fold increase in steady-state permeate flux and 100% flux recovery were observed. Theoretical aspects are reviewed to develop a better understanding of the critical parameters associated with high-pressure backpulsing.Pilot and commercial scale operating results on several industrial applications, such as yeast filtration, process slurry filtration and oily wastewater filtration are presented. Data analysis shows the critical importance of backpulsing in reducing long-term membrane fouling while allowing the realization of high product recovery. Optimization of process parameters with backpulsing typically results in higher flux and reduces the total capital cost required to achieve the desired production rate.     

Flux stabilization of silicon nitride microsieves by backpulsing and surface modification with PEG moieties

The influence of the surface properties of chemically modified silicon nitride microsieves on the filtration of protein solutions and defatted milk is described in this research. Prior to membrane filtrations, an antifouling polymer based on poly(ethylene glycol), poly(TMSMA-r-PEGMA) was synthesized and applied on silicon-based surfaces like silicon, silicon nitride, and glass. The ability of such coating to repel proteins like bovine serum albumin (BSA) was confirmed by ellipsometry and confocal fluorescence microscopy. In BSA and skimmed milk filtrations no differences could be seen between unmodified and PEG-coated membranes (decreasing permeability in time). On the other hand, reduced fouling was observed with PEG-modified microsieves in combination with backpulsing and air sparging.     

Extended space charge in concentration polarization

This paper is concerned with ionic currents from an electrolyte solution into a charge-selective solid, such as, an electrode, an ion-exchange membrane or an array of nano-channels in a micro-fluidic system, and the related viscous fluid flows on the length scales varying from nanometers to millimeters. All systems of this kind have characteristic voltage-current curves with segments in which current nearly saturates at some plateau values due to concentration polarization — formation of solute concentration gradients under the passage of a DC current. A number of seemingly different phenomena occurring in that range, such as anomalous rectification in cathodic copper deposition from a copper sulfate solution, super-fast vortexes near an ion-exchange granule, overlimiting conductance in electrodialysis and the recently observed non-equilibrium electroosmotic instability, result from the formation of an additional extended space charge layer next to that of a classical electrical double layer at the solid/liquid interface. In this paper we review the peculiar features of the non-equilibrium electric double layer and extended space charge and the possibility of their direct probing by harmonic voltage/current perturbations through a linear and non-linear system's response, by the methods of electrical impedance spectroscopy and via the anomalous rectification effect. On the relevant microscopic scales the ionic transport in the direction normal to the interface is dominated by drift-diffusion; hence, the extended space charge related viscous flows remain beyond the scope of this paper.     

Factors affecting membrane fouling reduction by surface modification and backpulsing

Several factors affecting microfiltration membrane fouling and cleaning, including backpulsing, crossflushing, backwashing, particle size, membrane surface chemistry, and ionic strength, were investigated with suspensions of latex beads. Approximately two-fold permeate volume enhancements over 1 h of filtration were obtained by using water or gas backpulsing, and 50% enhancement was obtained with crossflushing, for filtration of 1.0 μm diameter carboxylate modified latex (CML) particles using unmodified polypropylene (PP) membranes of 0.3 μm nominal pore diameter. When 0.2 μm diameter CML particles or mixtures of 1.0 and 0.2 μm CML particles were used, however, the average flux decreased 60% compared with using 1.0 μm CML particles for experiments with or without backpulsing.PP membranes were rendered hydrophilic with neutral or positively on negatively charged surfaces by grafting monomers of poly(ethylene glycol 200) monomethacrylate (PEG200MA), dimethyl aminoethyl methacrylate (DMAEMA), or acrylic acid (AA), respectively, to the base PP membranes. Filtration experiments show that fouling is not strongly dependent on membrane surface chemistry for filtration of 1.0 μm CML particles without backpulsing. With backpulsing, however, a 10% increase and a 20% decrease of permeate volumes collected in 1 h were observed when the CML particles and the membranes had like charges and opposite charges, respectively, compared to the permeate collected with the unmodified membrane. Using the PP membranes modified with AA, permeate volumes with backpulsing decreased 30 and 40% when NaCl concentrations of 0.01 and 0.1 M, respectively, were added to the feed. However, the permeate volumes did not vary significantly with changing ionic strength for filtration without backpulsing.     

Modeling of the mutual molecular polarization with an electronegativity equalization approach

A model for the mutual polarization of two approaching molecules is proposed, exploiting the principle of electronegativity equalization. The deformation of the electronic density of one molecule is the response to the perturbation of its chemical potential due to the electrostatic potential of the other molecule. The electronic densities, the density deformations, and the electrostatic potentials of both molecules are described with a previously developed asymptotic density model (ADM ). The ADM model allows a partitioning of all relevant properties in terms of atomic quantities. The perturbation of the chemical potential is given in atomic resolution, and the change of the electronic density is represented in terms of atomic charges. A hardness tensor, which determines the changes of the atomic chemical potentials due to the changes of the atomic charges, is modeled consistently with the ADM and earlier approaches. The results of the model, the changes of atomic charges within the molecules due to their mutual interaction, are compared with the changes of atomic charges obtained from population analysis of ab initio calculations. © 1995 John Wiley & Sons, Inc.     

Perspective on concentration polarization effects in electrochromatographic separations

This work illustrates the appearance and electrohydrodynamic consequences of concentration polarization in the particulate and monolithic fixed beds used in capillary electrochromatography and related electrical-field assisted processes. Key property of most porous materials is the co-existence of bulk, quasi-electroneutral macroporous regions and mesoporous compartments which are ion-permselective (due to electrical double-layer overlap) causing different transport numbers for co-ionic and counterionic species, e.g., background electrolyte components, or the analytes. For a cathodic electroosmotic flow the (cation) permselectivity, together with diffusive and electrokinetic transport induces depleted and enriched concentration polarization zones at the anodic and cathodic interfaces, respectively, in dependence of the mobile phase ionic strength and applied electrical fields. At high field strength a secondary, nonequilibrium electrical double layer may be created in the depleted concentration polarization zones of a material stimulating electroosmosis of the second kind. The potential of this induced-charge electroosmosis with respect to nonlinear flow velocities and electrokinetic instability mixing (basically destroying the concentration polarization zones) is analyzed in view of the pore space morphology in random-close packings of spherical-shaped, porous particles and hierarchically structured monoliths. Possible applications based on a fine-tuning of the illustrated effects emerge for microfluidic pumping and mixing, or the intensification of sample recovery in adsorption processes. With this perspective we want to focus the attention on concentration polarization in electrochromatographic systems by presenting and discussing original data acquired on relevant microscopic as well as macroscopic scales, and point towards the importance of related effects in colloid and membrane science.*     

Hydrodynamic resistance of concentration polarization boundary layers in ultrafiltration

The influence of concentration polarization on the permeate flux in the ultrafiltration of aqueous Dextran T70 solutions can be described by (i) the osmotic pressure model and (ii) the boundary layer resistance model. In the latter model the hydrodynamic resistance of the non-gelled boundary layer is computed using permeability data of the Dextran molecules obtained by sedimentation experiments. It is shown both in theory and experiment that the two models are equivalent.     

Temperature and concentration polarization in membrane distillation of aqueous salt solutions

In this paper we have studied water transport in membrane distillation using a flat PTFE membrane. Experiments have been carried out with water and aqueous solutions of NaCl as feed. The effects of temperature and concentration polarization on the reduction of vapour pressure differences across the membrane with regard to the vapour pressure differences corresponding to the bulk phases which are separated by the membrane, are evaluated. A coefficient which measures this reduction has been introduced. This coefficient and the temperature polarization coefficient coincide when water is used as feed, but they are more and more different when the salt concentration of feed increases.The measured flux results and the calculated polarization results are discussed for different temperatures, recirculation rates and solution concentrations.     

Reduction of concentration polarization in pervaporation using vibrating membrane module

A vibrating membrane module currently marketed for filtration applications was evaluated for the separation of volatile organic compounds (VOCs) from aqueous solutions by pervaporation. Preliminary screening experiments with three VOCs, three silicone membranes, and in the presence and absence of a surfactant were performed to determine if further consideration of the vibrating module for a field demonstration project was warranted. The primary process variables studied were vibrational amplitude and liquid flow rate. The vibrations greatly reduced concentration polarization in the system as inferred from an order of magnitude increase in the overall mass transport coefficient. Mass transfer coefficients for the vibrating module compared favorably with those for traditional spiral wound modules.     

A concentration polarization model for the ultrafiltration of nonionic surfactants

A theoretical model has been developed that describes ultrafiltration of nonionic surfactants. The model takes into account the fact that surfactants start to aggregate and form micelles at the critical micelle concentration. The model can be used to predict the performance of the membrane if the transport properties inside and at the membrane surface as well as the surfactant association behavior, are known. Three hydrophilic ultrafiltration membranes, made of regenerated cellulose, were used in the investigation. The cut-offs of the membranes were 10,000, 20,000, and 30,000 Da. The surfactant used in the investigation was the nonionic surfactant Triton X-100. The influence of the concentration of surfactant, transmembrane pressure and pure water flux were studied theoretically and experimentally. From the results presented in this work it can be concluded that the calculated values are in good agreement with experimental data.     

Recent advances in paper-based preconcentrators by utilizing ion concentration polarization

One of the most cited limitations of biochemical detection is its poor sensitivity, owing to the relatively high complexity of micro-samples. Moreover, some samples cannot be easily self-replicated and their abundance cannot be increased through traditional technologies. Therefore, the preconcentration of low-abundance samples is a key requirement for microfluidic biological analysis. In recent years, the ion-concentration polarization phenomenon has aroused widespread interest in the application of microfluidic technology. In addition, paper-based materials are readily available, easy to modify, and exhibit good hydrophilicity. The study of the ion-concentration polarization preconcentration of micro-samples in paper-based microfluidic chips is of considerable significance. In this review, we discuss the development and applications of ion-concentration polarization paper-based preconcentrator in the past 5 years, with emphasis on key progresses in chip fabrication and performance optimization under different conditions. The current needs and development prospects in this field have also been discussed.     

Theoretical study on concentration polarization in gas separation membrane processes

A mathematical model was established successfully to analyze the gas separation concentration polarization which becomes an important problem due to the rapid development of membranes, especially the increase of permeation rate. The influences of membrane performance and operation parameters on concentration polarization were studied in terms of permeation fluxes of the more and the less permeable gases and separation factor. Sample calculations were presented for the two typical gas separation applications, hydrogen recovery and air separation, with shell side feed in hollow fiber module. The permeation rate was found to be a dominating factor in affecting concentration polarization, while the influences of separation factor to be significant initially and to level off gradually. Increasing feed gas velocity leads to a decrease in the concentration polarization. Operation pressures' effect is limited and the composition of feed gas shows no effect. The range in which concentration polarization is significant has been identified by studying the combined effects of the permeation rate, separation factor and feed gas velocity. Concentration polarization is important for process analysis and design when the permeation rate of the more permeable gas is larger than 1×10⁻⁴ cm³ (STP) cm⁻² s⁻¹ cmHg⁻¹ (100 GPU).     

Predicting ion concentration polarization and analyte stacking/focusing at nanofluidic interfaces

We report on the investigation of electropreconcentration phenomena in micro-/nanofluidic devices integrating 100 μm long nanochannels using 2D COMSOL simulations based on the coupled Poisson–Nernst–Planck and Navier–Stokes system of equations. Our numerical model is used to demonstrate the influence of key governing parameters such as electrolyte concentration, surface charge density, and applied axial electric field on ion concentration polarization (ICP) dynamics in our system. Under sufficiently extreme surface-charge-governed transport conditions, ICP propagation is shown to enable various transient and stationary stacking and counter-flow gradient focusing mechanisms of anionic analytes. We resolve these spatiotemporal dynamics of analytes and electrolyte ICP over disparate time and length scales, and confirm previous findings that the greatest enhancement is observed when a system is tuned for analyte focusing at the charge, excluding microchannel, nanochannel electrical double layer (EDL) interface. Moreover, we demonstrate that such tuning can readily be achieved by including additional nanochannels oriented parallel to the electric field between two microchannels, effectively increasing the overall perm-selectivity and leading to enhanced focusing at the EDL interfaces. This approach shows promise in providing added control over the extent of ICP in electrokinetic systems, particularly under circumstances in which relatively weak ICP effects are observed using only a single channel.     

Analysis of concentration polarization phenomenon in ultrafiltration under turbulent flow conditions

An ultrafiltration data set previously reported by Gekas and Olund is used to test three concentration polarization models. The data consist of testing eight ultrafiltration membranes representing different polymer materials and molecular weight cut-offs with 0.5 wt% Dextran T10 at 25°C and 0.5 MPa under turbulent flow conditions. The tested concentration polarization models include the modified film theory model, the original film theory model and the Sherwood correlation model. The Chilton-Colburn analogy is the common foundation of the these models.     

Ion concentration polarization near microchannel-nanochannel interfaces: effect of pH value

This study investigates the effect of the pH value on the ion concentration polarization phenomenon and the nonlinear current-voltage characteristics of a hybrid soda-lime glass micro/nanochannel for a constant KCl salt concentration of about 1 mM. The experimental results show that the electrical conductance of the nanochannel in the Ohmic regime and the critical threshold voltage of the limiting current are both dependent on the pH value of the salt solution when the electrical double layer thickness is considerable in the nanochannel. Specifically, the nanochannel conductance increases and the critical threshold voltage for the limiting current decreases as the pH value is increased. It also suggests that a higher pH value induces a higher surface charge density on the nanochannel walls, and therefore increases both the ionic conductance and the counter-ion flux within the nanochannel.