Influence Of Concentration Polarization Research Articles

IMPACT OF CONCENTRATION POLARIZATION ON THE EFFICIENCY OF ELECTROMEMBRANE DEMINERALIZATION OF SUGAR SYRUP

The article presents the results of investigation the phenomenon of concentration polarization.
 The technological parameters of electrodialysis and electrodeionization processes at various applied voltages are presented. The results about the influence of concentration polarization on the efficiency of the electromembrane demineralization process of sugar syrup were concluded.

Experimental Investigation on the Concentration Polarization and Power Output of a Reverse Electrodialysis Stack With Permselective Spacers for Different Factors

Abstract The use of permselective spacers in reverse electrodialysis (RED) stack introduces a more severe concentration polarization phenomenon at membrane–solution interfaces in spite of effectively suppressing the spacer shadow effect. In this paper, the concentration polarization phenomenon in RED with permselective spacers is analyzed for different membrane types, numbers of cell pairs, solution concentrations, and temperatures by measuring non-ohmic resistance based on direct and alternating current methods combined with theoretical formula. Experiments on direct current method are further made to determine the influence of concentration polarization on power output and power generation performance of stack for different factors. The results show that the non-ohmic resistance accounts for less than 40% of the total stack resistance. Increasing the flowrate and solution temperature effectively suppresses the concentration polarization in stack and thus is beneficial to improving the power output. However, the membrane type, number of cell pairs, and solution concentration hardly affect concentration polarization. In spite of this, these factors all have a considerable impact on the power generation performance of the stack. Among them, increasing the concentration of concentrated and diluted solutions is most beneficial to improving the power density.

REGULARITIES OF EXTRACTION AND SEPARATION OF TECHNOLOGICAL SYSTEMS COMPONENTS ON INDUSTRIAL ENTERPRISES

The development of analysis and synthesis of energy-saving technics of industrial enterprises on the basis of technological processes properties is of special interest nowadays. This article aiming at treat-ment facilities design methods is an example of scientific approach to this problem. The main design blocks are: innovative technologies and resource-saving equipment block; solvent and dissolved com- 134 ponent of quality-control system; environmental and economic efficiency of evaluation system. On the basis of liquid systems separation by capillary-porous walls mechanism studying by technological calcu-lation of industrial wastes separation was considered. Diaphragmal methods accompanied by gel layer formation are studied. This evaluation is likely to allow effective technological regime choice and effec-tive technological process regime determination. In the article, technological calculation and compara-tive analysis of different types of polymeric anisotropic diaphragm application are given for concentra-tion, separation and purification of industrial wastes. The influence of concentration polarization on wa-ter-ingredient system data change and diaphragm permeability change depending on solved substance concentration and pressure on the diaphragm is shown.

Non-ideal modelling of polymeric hollow-fibre membrane systems: Pre-combustion CO2 capture case study

Some membrane gas separation applications operate at high temperature and pressure. However, the majority of membrane gas separation models employ simplifying assumptions which are not realistic under these conditions. In this study, a rigorous model is developed for polymeric hollow-fibre membrane modules incorporating non-isothermal separation (the Joule-Thomson effect), real gas behavior and concentration polarization. The model also accounts for temperature-dependent permeability, friction-based pressure loss on both feed and permeate sides and variable physical and transport properties. The rigorous model is applied for pre-combustion CO2 capture, i.e. CO2/H2 separation, and compared with a simplistic model for various polymeric membranes through changing temperature-independent activation energy of permeation and pre-exponential factor. Two types of H2- and CO2-selective membranes are then chosen for further analysis. As feed conditions change, the deviation between the rigorous and simplistic models ranges approximately from 2 to 12% for stage-cut and 2–20% for the permeate composition. The difference is mostly because of real gas behavior at low stage-cuts, while the Joule-Thomson effect adds to this behavior at high stage-cuts (≥40%) resulting in the greater deviation. The influence of concentration polarization, however, is found negligible even at high stage-cuts.

Influence of concentration polarization and thermodynamic non-ideality on salt transport in reverse osmosis membranes

The classic Merten and Lonsdale transport model for reverse osmosis membranes was reformulated to explicitly demonstrate the effects of concentration polarization and solution phase thermodynamic non-idealities on salt transport. A framework presented here accounts for the concentration dependence of ion activity coefficients in salt solutions, which was not explicitly included in the classic model. This approach was applied to four salt solutions, NaCl, MgCl2, CaCl2, and Na2SO4, tested in cross-flow conditions for a commercial RO membrane, Dow Filmtec™ BW30XFR. Salt transport coefficients corrected for concentration polarization and non-ideal thermodynamic effects, Ba, were calculated as a function of permeate flux and compared with apparent salt transport coefficients, B. These corrections were significant, resulting in Ba values greater than B values by a factor of 1.3–2.1 for 2:1 and 1:2 salts (i.e., MgCl2, CaCl2, and Na2SO4). Ba values for NaCl (a 1:1 salt), however, were similar to or somewhat smaller than B values.

Evaluation of fouling mechanisms in nanofiltration as a polishing step of yeast MBR-treated landfill leachate

ABSTRACTThe aim of this study was to evaluate the nanofiltration process as a polishing step of a membrane bioreactor inoculated with commercial baker yeast (Saccharomyces cerevisiae) used to treat sanitary landfill leachate. The contaminants retention and influence of concentration polarization and fouling phenomena on the permeate flux decline (FD) at different operating pressures were analysed. The greatest total flux reductions of 63.57% and 70.83% were observed for the lowest and the highest pressures, respectively, being this reduction attributed mainly to the concentration polarization. Membrane itself and concentration polarization phenomena were the main resistances to the nanofiltration process. Hermia model adjustment to the experimental data revealed that cake formation was the main mechanism that explained the FD at pressures of 8, 10 and 12 bar. At recovery rates above 40%, there was a significant decrease in permeate quality, so this value was chosen as the viable value for the proposed system. Integrated MBR-nanofiltration system led to the high removal of pollutants and made the treated effluent feasible for reuse in the landfill itself.

Influence of concentration polarization on DNA translocation through a nanopore.

Concentration polarization can be induced by the unique ion-perm selectivity of small nanopores, leading to a salt concentration gradient across nanopores. This concentration gradient can create diffusio-osmosis and induce an electric field, affecting ionic currents on DNA that translocates through a nanopore. Here this influence is theoretically investigated by solving the continuum Poisson-Nernst-Planck model for different salt concentrations, DNA surface charge densities, and pore properties. By implementing the perturbation method, we can explicitly compute the contribution of concentration polarization to the ionic current. The induced electric field by concentration polarization is opposite to the imposed electric field and decreases the migration current, and the induced diffusio-osmosis can decrease the convection current as well. Our studies suggest that the importance of the concentration polarization can be determined by the parameter λ/G where λ is the double-layer thickness and G is the gap size. When λ/G is larger than a critical value, the influence of concentration polarization becomes more prominent. This conclusion is supported by the studies on the dependence of the ionic current on salt concentration and pore properties, showing that the difference between two models with and without accounting for concentration polarization is larger for low salts and small pores, which correspond to larger λ/G.

Impact of biofilm accumulation on transmembrane and feed channel pressure drop: Effects of crossflow velocity, feed spacer and biodegradable nutrient

Biofilm formation causes performance loss in spiral-wound membrane systems. In this study a microfiltration membrane was used in experiments to simulate fouling in spiral-wound reverse osmosis (RO) and nanofiltration (NF) membrane modules without the influence of concentration polarization. The resistance of a microfiltration membrane is much lower than the intrinsic biofilm resistance, enabling the detection of biofilm accumulation in an early stage. The impact of biofilm accumulation on the transmembrane (biofilm) resistance and feed channel pressure drop as a function of the crossflow velocity (0.05 and 0.20 m s−1) and feed spacer presence was studied in transparent membrane biofouling monitors operated at a permeate flux of 20 L m−2 h−1. As biodegradable nutrient, acetate was dosed to the feed water (1.0 and 0.25 mg L−1 carbon) to enhance biofilm accumulation in the monitors. The studies showed that biofilm formation caused an increased transmembrane resistance and feed channel pressure drop. The effect was strongest at the highest crossflow velocity (0.2 m s−1) and in the presence of a feed spacer. Simulating conditions as currently applied in nanofiltration and reverse osmosis installations (crossflow velocity 0.2 m s−1 and standard feed spacer) showed that the impact of biofilm formation on performance, in terms of transmembrane and feed channel pressure drop, was strong. This emphasized the importance of hydrodynamics and feed spacer design. Biomass accumulation was related to the nutrient load (nutrient concentration and linear flow velocity). Reducing the nutrient concentration of the feed water enabled the application of higher crossflow velocities. Pretreatment to remove biodegradable nutrient and removal of biomass from the membrane elements played an important part to prevent or restrict biofouling.

Processing of pharmaceutical effluent condensate by nanofiltration and reverse osmosis membrane techniques

Nanofiltration (NF) and reverse osmosis (RO) are well-established processes for desalination of sea and ground water besides treatment of industrial effluents. Pharmaceutical effluents usually contain organic and inorganic waste materials that contribute to total dissolved solids (TDS), chemical oxygen demand (COD) and biochemical oxygen demand (BOD), rendering the mode of treatment highly challenging. In the present investigation, performance of hydrophilized polyamide (HPA-250) NF and thin film composite (TFC) polyamide RO membranes has been studied for the treatment of pharmaceutical effluent condensate. Effect of applied pressure on separation performance on NF/RO membranes was investigated for both non-aerated (Feed A) and aerated (Feed B) condensates. Influence of concentration polarization and fouling on membrane performance parameters such as flux, water recovery and rejection was evaluated as a function of time. A comparative study on treatment of aerated and non-aerated feed samples was carried out with respect to flux and rejection of TDS, COD and BOD. At a constant feed pressure of 10.34bar, %rejection of TDS was found to be 78.14 for Feed B and 75.24 for Feed A in NF process whereas the corresponding values for RO were 95.30% and 91.76%. COD removal efficiencies in the same order were observed to be 94.30% and 93.09% for NF and 99.50% and 98.33% for RO. Design and economic estimation of commercial NF/RO systems for processing of 1000L/h pharmaceutical condensate was carried out. NF process was found to more economical than RO due to advantage of greater flux leading to requirement of lower operating pressures.

Influence of concentration polarization on flux behavior in forward osmosis during desalination using ammonium bicarbonate

Forward osmosis is recognized as one of the promising membrane based desalination process and viable alternative to reverse osmosis as a lower cost and more environmentally friendly desalination technology. The solution of ammonium bicarbonate was used as a draw solution to extract water from a feed containing 0.5M sodium chloride. The water was transported from feed to draw solution and it can be recovered upon moderate heating of the draw solution, which is decomposed into ammonia and carbon dioxide gases producing pure water. The mechanism of water transport in the case of feed containing pure water or sodium chloride has been elucidated depending upon the orientation of the membrane. The concentrative and dilutive internal concentration polarizations played a major role in a situation when feed was towards support layer and feed was towards active layer, respectively. For the desalination applications, the feed towards support layer was found to be best mode to achieve higher flux. An increase in draw solution concentration from 1.0 to 3.6M, resulted in an increase in flux values from 0.58 to 1.39×10−6m3m−2s−1 at 30°C. Further, an increase in temperature of the draw solution from 30 to 45°C resulted in increase in transmembrane flux from 1.39 to 2.11×10−6m3m−2s−1 at the highest draw solution concentration. It was concluded that forward osmosis can prove to be a feasible technique for the recovery of water from saline water.

Concentration Polarization BP Model of Nanofiltration Separation

In order to effectively control the influence of Concentration Polarization (CP) during the nanofiltration separating wastewater process, this study applied parameters characterization of membrane flux attenuation coefficient (mwt) and the Back-propagation (BP) neural network algorithm to simulate the development rules of CP and membrane pollution, set up CP BP Model of Nanofiltration Separation, based on the tested data of NF90. The correlation coefficient between simulation and test of the simulation BP model was over 0.99, with the absoluteness error below 1.5%. According to the model’s prediction, the separation effect of nanofiltration technology become attenuate with running time increasing in nanofiltration separating wastewater process. mwtstart raised obviously within first 0.5h in operation and stay stable after 1h. It was advised to appropriately maintain u>0.2m/s for NF90 membrane effectively controlling mwt<0.1.

POWER GENERATION FROM “PRO” PROCESS USING FLAT SHEET TFC–ULP KOCH MEMBRANES

The production of power using the process of pressure–retarded osmosis (PRO) has been studied both experimentally and theoretically for simulated sea water vs. river water and deionized water under two cases: the first is for simulated real conditions of sea water and river water and second under low brine solution concentration to examine the full profile of the power- pressure. The influence of concentration polarization (CP) on water flux has been examined as well.

Concentration polarization analysis in self-supported Pd-based membranes

In this work, the concentration polarization phenomenon in hydrogen permeation through self-supported Pd-alloy membranes is evaluated by an opportune coefficient expressed as a function of the ratio of the flux calculated by means of a validated complex model considering all the elementary steps involved in the permeation [A. Caravella, G. Barbieri, E. Drioli, Modelling and Simulation of Hydrogen Permeation through Supported Pd-based membranes with a multicomponent approach, Chemical Engineering Science 63 (8) (2008) 2149–2160] and the one obtained by the Sieverts’ law utilizing the bulk driving force and hydrogen permeance. The polarization coefficient was evaluated as a function of several operating conditions: upstream hydrogen molar fraction ([0, …, 1]), total pressure of upstream ([200, …, 1000] kPa), total pressure of down-stream ([100, …, 800] kPa), temperature ([300, …, 500] °C), membrane thickness ([1, …, 150] μm), permeance ([0.1, …, 20] mmol m −2 s −1 Pa −0.5) and upstream fluid-dynamic conditions (Reynolds’ number). The analysis shows that the polarization effect can be relevant not only when using very thin membranes (1–5 μm ca.), but also when thicker ones (100 μm ca.) are operated in specific conditions. A validation of the analysis is provided by means of some experimental data from literature, finding a good agreement with them. The overall result of the paper is the development of so-called “polarization maps”, on which the influence of concentration polarization can be evaluated quantitatively in different conditions, providing a useful tool to reduce the uncertainties in the hydrogen purification equipment design.

Quantitative evaluation of concentration polarization in the permeation of VOCs/water mixtures through PDMS membrane using model equation

The influence of concentration polarization on the pervaporation and separation of VOCs/water mixtures through the PDMS membrane was investigated, using model equation based on the resistance-in-series concept in the boundary layer in feed adjacent to the membrane surface. Dichloromethane, chloroform, 1,2-dichloroethane, and 1,1,2-trichloroethane were used as VOCs. From the permeation data of the mixtures, the overall mass transfer coefficient, mass transfer coefficient of VOCs component across the boundary layer and concentration polarization modulus as a measure of extent of concentration polarization were determined quantitatively with help of the model equations. From the analysis on the determined model parameters, the boundary layer resistance due to the concentration polarization of VOCs component contributed more to the overall permeation process than the membrane resistance did.

Simulation of binary gas separation in hollow fiber membrane-acetylene dehydration

A mathematical model is established to describe a hollow fiber membrane separator by binary gases which including water vapor under low feed gas pressure and vacuum operation. The permeability of water vapor is described by resistance model in each differential unit and the boundary resistance caused by concentration polarization is taken into consideration. Industrial gas acetylene dehydration by PSF hollow fiber composite membrane is used to test the model The simulated results show good agreement with the experimental data. The calculated results will bring 50% error of the water vapor composition of the retentate gas when concentration polarization is ignored. The permeability of water vapor is decreased slowly along the fiber from the entrance of the feed gas to the exit, mainly because of the water vapor partial pressure changes with the length of the fiber. The influence of concentration polarization on separation is also discussed by this model. The actual separation factor α H 2O/C 2H 2 is less than 1/5 to the intrinsic separation factor of the composite membrane due to the concentration polarization. The concentration polarization of the feed side boundary has little influence on the membrane separation process. The permeate side boundary resistance controls the whole mass transfer in the separator.

Reactor Configuration and Concentration Polarization in Methane Steam Reforming by a Membrane Reactor with a Highly Hydrogen-Permeable Membrane

To examine the effect of so-called “concentration polarization” on the performance of a membrane reactor with a highly hydrogen-permeable membrane, methane steam reforming was conducted, using a Pd/Ag membrane with a thickness of a few micrometers. First, the relation between the methane conversion and the hydrogen recovery was experimentally examined, and the relation was compared with that predicted by a rather simple simulation that assumes the instant achievement of equilibrium. When the hydrogen recovery was smallest, the experimental results agreed well with the simulation results. With increasing reaction pressure, the experimental methane conversion became lower than the simulated conversion. These results suggest that the reaction is limited by reduced hydrogen removal, because of the concentration polarization. The influence of concentration polarization was confirmed by the comparison of the experimental results of hydrogen permeation from a mixture of H2 and N2 with the simulation results based on the plug-flow model. It then was experimentally attempted to reduce the concentration polarization by changing the configuration of the reactor. The methane conversion was successfully improved using reactors that had narrower inner diameters and baffle plates, probably because of the reduction in concentration polarization. It was concluded that the reactor configuration was quite essential to make the best use of a membrane reactor with a highly permeable membrane.

On the Mechanism of Flexible Chain Polymer Ultrafiltration

The regularities of ultrafiltration (UF) of flexible chain polymers (FCP) differ from that of globular proteins (GP) which is manifested, in particular, in opposite dependences of retention coefficient (ϕ) on the applied pressure. According to one of the existing points of view this difference is caused by different influence of concentration polarization on these polymers and according to another by the deformation of polymer coils of FCP in longitudinal velocity gradients at the pore entrance. To settle this question we examined the behavior of globular proteins and dextrans under absolutely identical conditions, which was reached by UF of the mixture of these polymers in one experiment with separate registration of FCP and GP concentrations in permeate and bulk. As it follows from the experiments, one can make a conclusion that, at low pressures and also at low molecular masses (M), FCP molecules keep themselves as globular proteins i.e., as rigid spherical particles. However, with the increase in pressure and M, the transport of flexible chain molecules through pores of UF membrane begins to be determined by the mechanism of polymer chain uncoiling in longitudinal gradients.

Irreversible thermodynamics and overall mass transport in ion-exchange membrane electrodialysis

An ion-exchange membrane electrodialysis phenomenon was presented by using phenomenological equation based on the irreversible thermodynamics. The overall mass transport equation was introduced based on the electrodialysis of seawater. The overall mass transport equation was joined to the phenomenological equation by setting both equations are equivalent each other. As the result of this process, the overall membrane characteristics appeared in the overall mass transport equation (overall transport number λ, overall solute permeability μ, overall electro-osmotic permeability ϕ and overall hydraulic conductivity ρ) were expressed by the function of the irreversible membrane pair characteristics appeared in the phenomenological equation (transport number t, solute permeability ω, electro-osmotic permeability β and hydraulic conductivity L P). λ, μ, ϕ, r (electric resistance) and W (water content) were expressed by the empirical function of ρ. Reflection coefficient was discussed by defining the pressure reflection coefficient σ and concentration reflection coefficient σ′. From the pressure dialysis of a KCl solution, σ of an ion-membrane was generally assumed to be 1. σ′ corresponded with the permselectivity between ions and water molecules across an ion-exchange membrane at just after electric current interruption (electric current switching off concept), and was expressed by the function of μ, ρ and logarithmic mean concentration C *. t, ω, β, L P, L PD (osmotic volume flow coefficient), L DP (ultrafiltration coefficient) and L D (exchange flow parameter) included in the phenomenological equation were computed by applying the overall mass transport equation to electrodialysis experiments. The influence of concentration polarization upon the overall membrane characteristics measurement was negligibly small.

A characterization of concentration polarization in a boundary layer in the permeation of VOCs/N 2 mixtures through PDMS membrane

Influence of concentration polarization in the boundary layer in feed adjacent to the membrane surface has been investigated on the vapor permeation and separation of volatile organic compounds (VOCs)/N 2 mixtures through poly(dimethylsiloxane) (PDMS) membrane. The chlorinated hydrocarbons that were part of the homologous series of chloromethanes and chloroethanes were used as organic vapor. The vapor permeation experiments were carried out at various VOCs concentrations in feed, operating temperatures and feed flow rates. With decreasing feed flow rate, the membrane performances, that is, permeation rate and selectivity were reduced in the permeation of the VOCs/N 2 mixture by reducing VOC solubility in the membrane. Especially the reducing of the membrane performance was found to be more significant when the condensability of VOC was greater, VOC content in the feed mixture was higher or operating temperature was lower. These observations were discussed in terms of the effect of concentration polarization on the permeation of VOCs/N 2 mixture through the PDMS membrane.

Theoretical and Experimental Study of the Voltage–Current Characteristic of an Asymmetric Cellulose Acetate Membrane

The voltage–current characteristic of an asymmetric cellulose acetate (KP96) membrane in NaCl solutions (0.005–0.1 M) has been studied both experimentally and theoretically. The theoretical analysis was carried out within the context of a bilayer model, thereby taking into account ion transport across the interfaces of the active layer and the effect of concentration polarization at the selective layer/nonselective layer boundary. The transport process was treated mathematically by applying the Nernst–Planck–Poisson equations. Image forces affecting ion transport across the surfaces of the active layer have been incorporated into the theoretical description by means of a mathematical simulation of the profile of the image force potential. Most of the parameters characterizing the model membrane were estimated from literature data, from the known value of the final rejection coefficient of an asymmetric KP96 cellulose acetate membrane, and from our experimental results on the NaCl concentration dependency of the small-current resistance of an asymmetric KP96 membrane. The thickness of the active layer, however, was considered as an adjustable parameter in our analysis. The theoretical predictions, obtained by numerically solving the Nernst–Planck–Poisson equations, have been compared with the experimental results to obtain insight into the factors underlying the observed nonlinear and asymmetric features of the voltage–current characteristic. For the case of a relatively thick active layer, the nonequilibrium state of the interfaces was also investigated theoretically. Our analysis indicates that ion transport across the interfacial regions of the active layer plays a crucial role in the explanation of the observed phenomena, whereas the influence of concentration polarization in the porous support layer is of minor importance.