Concentration Polarization Coefficient Research Articles

Effect of wavy corrugations on the performance enhancement of direct contact membrane distillation modules: A numerical study

This study employs three-dimensional (3D) computational fluid dynamics (CFD) simulations to investigate the influence of corrugations on direct contact membrane distillation (DCMD) modules. The accuracy of 3D CFD simulations is validated by the comparison of water flux and temperature polarization coefficient (TPC) against literature data for empty and spacer-filled DCMD modules. The dusty gas model is used to predict the permeate flux as a function of temperature and pressure over the surface of the membrane. We consider single and double sinusoidal corrugated membranes and examine the performance of DCMD membranes in terms of TPC, concentration polarization coefficient (CPC), power number (Pn), and Nusselt number (Nu) against various corrugation geometrical parameters. These parameters include the amplitude, streamwise, and spanwise frequency of corrugation waves. Our results show that corrugated membranes can successfully improve the membrane system by disrupting the boundary layer by an intense secondary flow. The mechanism works the best at high flow rates. Our results suggest that singly corrugated membranes improve the water flux and TPC by 32% and 25% at Re=1500. Also, doubly corrugated membranes provide 45% and 36% improvements in water flux and TPC at the same Reynolds number.

A novel braided structure with enhanced feed flow state for hollow fiber air-gap membrane distillation performance improvement

The feed flow state affects temperature polarization (TP), concentration polarization (CP), and membrane fouling during the membrane distillation process. A novel braided structure for AGMD was proposed in this paper, in which hollow fiber membranes and heat exchange tubes are braided together. The braided structure can alter feed flow direction and reduces the membrane's contact area. Theoretical calculations show that the braided structure reduces the critical Reynolds number for the feed's unstable flow, and improves the feed flow state by forming Dean vortices. Comparative experiments of the braided module and parallel module were carried out. The results show that the braided module performs better than the parallel module. The braided module has a higher flux (J) and gained output ratio (GOR) when treating the tap water with significant TP; a lower decline rate in J and GOR besides a smaller concentration polarization coefficient (CPC) when treating 3.5 wt% NaCl aqueous solution; a slower formation of membrane fouling and a lower reduction in J and GOR when treating a humic acid/calcium carbonate mixed solution. It is proved that the braided structure can improve the performance of AGMD by enhancing the feed flow state to mitigate the TP, CP, and membrane fouling.

Thermo-economical and environmental analyses of a Direct Contact Membrane Distillation (DCMD) performance

As a promising desalination technology, membrane distillation is expected to be developed continuously due to its wide application and low sensitivity to salt concentration in the feed stream. The performance of a direct contact membrane distillation (DCMD) has been investigated based on energy, exergy, economic and environmental analyses in the current study. Performance evaluation of a DCMD unit is comprehensively carried out in terms of the operating parameters, including gained output ratio (GOR), specific energy consumption (SEC), concentration polarization coefficient, temperature polarization coefficient, recovery ratio, exergy efficiency, specific exergy consumption, carbon cost, and cost of water. This study presents one of the most comprehensive investigations on DCMD behavior performed in the available literature by developing a robust computational code in MATLAB software. In addition to pervasive energy and exergy analyses, a complete economic study for freshwater production plants with different capacities based on heat extracted through geothermal energy and wasted heat, and investigation of the environmental effects of using heat recovery MD cycle to reduce greenhouse gas emissions, are presented. The results showed that the utilization of heat recovery heat exchangers significantly improves the system's operating conditions. The exergy efficiency of the DCMD module with and without heat recovery is 38.23% and 56.71%, respectively, indicating a 48.34% improvement in exergy efficiency. Finally, the economic and environmental analyses showed that for a 15000 m3/day desalination unit, recovering the wasted heat could reduce the distillate water production cost down to 1.31 $/m3 while CO2 emissions and energy consumption also decreased by 59%.

Modelling of the Electrical Membrane Potential for Concentration Polarization Conditions.

Based on Kedem–Katchalsky formalism, the model equation of the membrane potential () generated in a membrane system was derived for the conditions of concentration polarization. In this system, a horizontally oriented electro-neutral biomembrane separates solutions of the same electrolytes at different concentrations. The consequence of concentration polarization is the creation, on both sides of the membrane, of concentration boundary layers. The basic equation of this model includes the unknown ratio of solution concentrations ( at the membrane/concentration boundary layers. We present the calculation procedure ( based on novel equations derived in the paper containing the transport parameters of the membrane (, , and ), solutions (, ), concentration boundary layer thicknesses (, ), concentration Raileigh number (), concentration polarization factor (), volume flux (), mechanical pressure difference (), and ratio of known solution concentrations (). From the resulting equation, was calculated for various combinations of the solution concentration ratio (), the Rayleigh concentration number (), the concentration polarization coefficient (), and the hydrostatic pressure difference ). Calculations were performed for a case where an aqueous NaCl solution with a fixed concentration of 1 mol m−3 () was on one side of the membrane and on the other side an aqueous NaCl solution with a concentration between 1 and 15 mol m−3 (). It is shown that () depends on the value of one of the factors (i.e., , , and ) at a fixed value of the other three.

Enhancing Absorption Performance of CO2 by Amine Solution through the Spiral Wired Channel in Concentric Circular Membrane Contactors.

The CO2 absorption rate by using a Monoethanolamide (MEA) solution through the spiral wired channel in concentric circular membrane contactors under both concurrent-flow and countercurrent-flow operations was investigated experimentally and theoretically. The one-dimensional mathematical modeling equation developed for predicting the absorption rate and concentration distributions was solved numerically using the fourth Runge–Kutta method under various absorbent flow rate, CO2 feed flow rate and inlet CO2 concentration in the gas feed. An economical viewpoint of the spiral wired module was examined by assessing both absorption flux improvement and power consumption increment. Meanwhile, the correlated average Sherwood number to predict the mass-transfer coefficient of the CO2 absorption mechanisms in a concentric circular membrane contactor with the spiral wired annulus channel is also obtained in a generalized and simplified expression. The theoretical predictions of absorption flux improvement were validated by experimental results in good agreements. The amine solution flowing through the annulus of a concentric circular tube, which was inserted in a tight-fitting spiral wire in a small annular spacing, could enhance the CO2 absorption flux improvement due to reduction of the concentration polarization effect. A larger concentration polarization coefficient (CPC) was achieved in the countercurrent-flow operations than that in concurrent-flow operations for various operations conditions and spiral-wire pitches. The absorption flux improvement for inserting spiral wire in the concentric circular module could provide the maximum relative increment up to 46.45%.

Systematic experimental assessment of concentration polarization and inhibition in Pd-based membranes for hydrogen purification

In this work, we show the application of a systematic general experimental analysis to estimate in a relatively easy way the paired effects of concentration polarization and inhibition in the hydrogen permeation through diverse supported Pd-based membranes as functions of temperature and mixture composition. For this purpose, three different membranes are fabricated and tested under pure‑hydrogen and binary-mixture of H2-N2 and H2-CO. The former mixture is used to quantify the concentration polarization influence only, whereas the latter is used to quantify the overall combined effect of polarization and inhibition. Manipulating the two effects, we finally obtain the influence of inhibition only. As main results, a maximum overall permeation reduction of around 35%, 55% and 45% is evaluated for Mem#1, for Mem#2, for Mem#3, respectively. Moreover, a maximum concentration polarization coefficient of around 2.5%, 19% and 12% and a maximum inhibition coefficient of 32%, 45% and 40% are respectively evaluated. These values state the importance of having a tool able to take into account these detrimental phenomena in designing ultra-thin-membrane equipment, remarking that the same approach can be applied to other types of metal membranes as well.

Evaluation of the Global S-Entropy Production in Membrane Transport of Aqueous Solutions of Hydrochloric Acid and Ammonia.

The results of experimental studies of volume osmotic fluxes () and fluxes of dissolved substances () in a system containing a synthetic Nephrophan® membrane (Orwo VEB Filmfabrik, Wolfen, Germany) set in a horizontal plane are presented. The membrane separated water and aqueous HCl or ammonia solutions or aqueous ammonia and HCl solutions. It was found that for the homogeneity conditions of the solutions and depend only on the concentration and composition of the solutions. For concentration polarization conditions (where concentration boundary layers are created on both sides), and depend on both the concentration and composition of the solutions and the configuration of the membrane system. The obtained results of the and flux studies were used to assess the global production of entropy for the conditions of homogeneity of solutions (), while and —to assess the global production of entropy for concentration polarization conditions ). In addition, the diffusion-convective effects and the convection effect in the global source of entropy were calculated. The concentration polarization coefficient was related to modified concentration Rayleigh number, e.g., the parameter controlling the transition from non-convective (diffusive) to convective state. This number acts as a switch between two states of the concentration field: convective (with a higher entropy source value) and non-convective (with a lower entropy source value). The operation of this switch indicates the regulatory role of earthly gravity in relation to membrane transport.

Superhydrophobic-slip surface based heat and mass transfer mechanism in vacuum membrane distillation

Superhydrophobic membrane is favorable for the membrane distillation technology in desalination applications although the theoretic reasons are still unsettled. In this study, some novel theories were first proposed to establish the bridges between the surface parameters (including water contact angle and surface roughness) and the heat and mass transfer process. Based on the hydrophobic PVDF membrane (with a water contact angle of 124°), a kind of superhydrophobic PVDF membrane with a water contact angle of 162° was prepared through the surface modification. Then both membranes were tested in the vacuum membrane distillation (VMD) process, and simulation models based on both kind of membranes were constructed to compare the membrane performances and the heat and mass transfer characteristics. The results show that the slip flow exists on the superhydrophobic surface, and the superhydrophobic membrane has higher permeate flux than the original PVDF membrane. The insight investigations show that the heat transfer resistance (HTR) of the VMD process is focused on the membrane layers, and the variation of water contact angle or roughness has little influence on the heat transfer coefficient of the membrane layers. However, when the water contact angle varies from 124° to 165°, the total HTR value decreases from 3.156 × 10-3 (m2 K)/W to 2.599 × 10-3 (m2 K)/W, which indicates that the feed side HTR value is largely decreased by the superhydrophobic surface. The further polarization investigations found that the temperature polarization coefficient (TPC) changes slightly when the membrane contact angle varies from 124° to 165°. Instead, the concentration polarization coefficient (CPC) decreases from 0.1455 to 0.0071. The vapor polarization coefficient (VPC), which comprehensively considers the influences of TPC and CPC, has a value of above 0.96 when the water contact angle is higher than 155°, indicating that the performance of the superhydrophobic membrane is not limited by the polarization problems.

Impact of Operating Conditions on Measured and Predicted Concentration Polarization in Membrane Distillation.

Concentration polarization (CP) occurs in almost all membrane-based separation processes. In this study, the concentration profile of the dissolved salt has been accurately characterized using a previously developed laser-based spectrophotometric method which had a spatial resolution of 4.5 μm. The objective of the current work was to probe the concentration profile of the solute and analyze the impact of operating parameters, such as feed concentration, hydrodynamic conditions, and feed temperature, on the solute concentration profile in the boundary layer. This study also examined the validity of the conventional approach, where semi-empirical models are used to estimate the boundary layer thickness (BLT) and concentration polarization coefficient (CPC)-based on experimental results. Nusselt correlations were developed specifically for the membrane cell and validated through experimental observations at the operating conditions used in this study. A key finding of this study is that the conventional approach of estimating the effect of CP severely underpredicts the BLT and CPC. The results of this study highlight the need to develop new methods to estimate the BLT and CPC as the conventional approach of using semi-empirical Nusselt and Sherwood correlations does not agree with experimental observations obtained for a membrane distillation system employed in this study.

Analysis of temperature and concentration polarizations for performance improvement in direct contact membrane distillation

Regarding numerical simulation of the direct contact membrane distillation (DCMD) based on computational fluid dynamics, the variation of mass flowrate and concentration along the flow direction has merely been considered. To relieve such issues, a refined model coupling Navier-Stokes equations and species transportation equations is employed to illustrate the flow, heat and mass transfer characteristics in the flat sheet DCMD process. The potential of attaching baffles to the feed/permeate channel shell to improve the DCMD performance has been investigated under the laminar and turbulent flow, respectively. Modified channels were designed to have regularly distanced baffles with various shapes at different characteristic lengths. The heat transfer coefficient, mass transfer coefficient, temperature polarization coefficient, concentration polarization coefficient, mass flux, thermal efficiency and power consumption of the original and modified modules are calculated and compared. Results reveal that a structural modification could promote the temperature polarization phenomenon and restrain the concentration polarization phenomenon, thus to improve the module water production. However, the presence of baffles contributes to extra power consumption. At a feed flow Reynolds number of 358, for module with semi-circle shaped baffle at the characteristic length of 1 mm, the water production is increased by 28.3%, meanwhile the hydraulic energy consumption is increased by 3.32-fold. Under the laminar flow, the water production can be significantly improved by attaching baffles in the channel shells, which is not appealing under the turbulent flow due to the huge augment in hydrodynamic loss.

Computational fluid dynamics simulations of polarization phenomena in direct contact membrane distillation

Direct Contact Membrane Distillation (DCMD) has emerged as a promising means of concentrating brines to their saturation limit. To date, computational fluid dynamics (CFD) studies of DCMD focus primarily on temperature polarization. However, when treating hypersaline brines, concentration polarization is an additional challenge that reduces system efficiency and leads to mineral scaling. To study coupled temperature and concentration polarization in the DCMD treatment of hypersaline brines, we develop an experimentally validated two-dimensional CFD code that simulates heat and mass transport in plate-and-frame DCMD systems. We then perform a comprehensive parametric study of polarization phenomena for a wide range of feed and distillate operating conditions, system length, and co-current versus counter-current operation. We also investigate the system-level performance by measuring the average permeate flux, single-pass water recovery, maximum concentration polarization coefficient, and gained output ratio of DCMD systems with heat recovery. Though the transmembrane vapor flux is small, we observe dramatic increases in solute concentration at the membrane surface, exceeding 1.6 times the feed value. The temperatures, concentration, and vapor flux vary considerably in the downstream direction, and are poorly approximated by common Nusselt and Sherwood correlations.

Ethanol mass transfer during pervaporation with PDMS membrane based on solution-diffusion model considering concentration polarization

A mass transfer model of ethanol recovery by pervaporation with polydimethylsiloxane (PDMS) membrane was developed based on solution-diffusion theory considering concentration polarization. The water/polymer interaction parameter, ethanol/polymer interaction parameter, the limiting diffusivity of ethanol and plasticization coefficient of ethanol were determined. The concentration polarization coefficient was calculated based on the convective mass transfer on the upstream of the membrane. The effect of convective mass transfer coefficient, partition coefficient and ethanol diffusion mass transfer in membrane on ethanol flux and concentration polarization coefficient were explored. Partition coefficient of 2.98 and ethanol diffusion mass transfer in membrane of 7.05 × 10−7 m2 s−1 were determined. The concentration polarization coefficient of 0.15 and the ethanol flux of 318 g m−2 h−1 were obtained under the condition of 5 wt% of ethanol feed concentration and convective mass transfer coefficient being 1.23 × 10−5 m2 s−1. Higher convective mass transfer coefficient, higher partition coefficient and higher ethanol diffusion mass transfer in membrane could improve the membrane flux. However, the bad effect of concentration polarization coefficient on ethanol flux was severe if the partition coefficient and ethanol diffusion mass transfer in membrane are higher. The developed model could accurately predict membrane separation, especially under the condition of higher concentration polarization coefficient.

Scalability of multitube membrane modules for hydrogen separation: Technical considerations, issues and solutions

Palladium membrane technology has shown promising features for the development of a sustainable hydrogen economy. Nonetheless, the contribution of a palladium membrane technology to economic and societal development requires its commercialization, diffusion and utilization. To generate enough incentives for commercialization, it is necessary to demonstrate the scalability and robustness of the membranes in industrial settings. Consequently, this work utilizes pilot-scale experimental data generated under industrial conditions to validate a Computational Fluid Dynamics (CFD) model, which was up-scaled and utilized to determine the intrinsic phenomena of palladium membrane scale up. This study reveals the technical/engineering requirements for the effective design of large scale multitube membrane modules. Mass transfer limitations and concentration polarization effects were studied quantitatively with the developed model using the defined parameters Concentration Polarization Coefficient (CPC) and Effective Average CPC (EAC). Methods for diminishing the concentration polarization effect were proposed and tested through the simulations such as i) increasing convective forces and ii) designing baffles to create gas recirculation. For scaled-up membrane modules, mass transfer limitation is an important parameter to consider as large modules showed severe concentration polarization effects. Certainly, this work shows for the first time the main features required when designing large scale membrane reactor modules.

Study on the heat and mass transfer in ultrasonic assisting vacuum membrane distillation

An ultrasonic assisting vacuum membrane distillation (VMD) system was designed to promote the heat and mass transfer in membrane distillation (MD) process. Both the effects of operating conditions and ultrasonic parameters to permeation flux in this process were investigated; the heat and mass transfer mechanism was also being discussed in this paper. The results showed that the performance of VMD process was improved significantly by ultrasonic assisting. The permeation flux was boosted at a certain feed solution temperature, pressure at permeate side and feed solution velocity whether or not to PP and PTFE. The results also indicated that ultrasonic power and frequency also was the key factor affecting the mass and transfer efficiencies. The feed side transfer coefficient (Kf), corresponding to ultrasonic power (Kf=4.406−0.026xP+7.824x10-5xP2) and ultrasonic frequency (Kf=0.941+0.598xf−0.012xf2+6.283x10-5f3), was obtained and employed in the modeling of ultrasonic assisting VMD process. The modeling results showed that the calculated value of Kf aligned with experimental results well. Both variations of temperature polarization coefficient (TPC) and concentration polarization coefficient (CPC) were studied based on the obtained data. The results showed that both TPC and CPC were improved obviously by the ultrasonic parameters.

A review on membrane applications and transport mechanisms in vacuum membrane distillation

AbstractThe main aim of this article is to provide a state-of-the-art review of the experimental studies on vacuum membrane distillation (VMD) process. An introduction to the history of VMD is carried out along with the other membrane distillation configurations. Recent developments in process, characterization of membrane, module design, transport phenomena, and effect of operating parameters on permeate flux are discussed for VMD in detail. Several heat and mass transfer correlations obtained by various researchers for different VMD modules have been discussed. The impact of membrane fouling with its control in VMD is discussed in detail. In this paper, temperature polarization coefficient and concentration polarization coefficient are elaborated in detail. Integration of VMD with other membrane separation processes/industrial processes have been explained to improve the performance of the system and make it more energy efficient. A critical evaluation of the VMD literature is incorporated throughout this review.

Simulation of heat and mass transfer with cross-flow hollow fiber vacuum membrane distillation: The influence of fiber arrangement

Two factors (fiber row space and intersection angle) of fiber arrangement in hollow fiber vacuum membrane distillation (VMD) module design were studied through simulation works for optimizing the configuration design. The flow field in the module and the permeate fluxes were simulated while the feed inlet velocity was varied from 0.02m/s to 0.08m/s. The temperature polarization coefficient (TPC), concentration polarization coefficient (CPC) and the vapor polarization coefficient (VPC) in the modules were discussed based on the average values of the whole configuration. The experimental validation of the simulation models were carried out and good agreement was achieved. The results show that the fiber arrangement can significantly influence the performance of the modules. High permeate flux and VPC values can be observed in modules with row space 2.5 times of the fiber outside diameter and intersection angle 60° or 90°. The TPC and CPC values first increase but then decrease when the row space or the intersection angle enlarges. The enhancement of feed inlet velocity increased the value of TPC and VPC as well as the permeate flux in all simulated modules.

The mathematical model of concentration polarization coefficient in membrane transport and volume flows

In this paper, the authors investigate the membrane transport of aqueous non-electrolyte solutions in a single-membrane system with the membrane mounted horizontally. The purpose of the research is to analyze the influence of volume flows on the process of forming concentration boundary layers (CBLs). A mathematical model is provided to calculate dependences of a concentration polarization coefficient (ζs) on a volume flux (Jvm), an osmotic force (Δπ) and a hydrostatic force (ΔP) of different values. Property ζs = f(Jvm) for Jvm > 0 and for Jvm ≈ 0 and property ζs = f(ΔC1) are calculated. Moreover, results of a simultaneous influence of ΔP and Δπ on a value of coefficient ζs when Jvm = 0 and Jvm ≠ 0 are investigated and a graphical representation of the dependences obtained in the research is provided. Also, mathematical relationships between the coefficient ζs and a concentration Rayleigh number (RC) were studied providing a relevant graphical representation. In an experimental test, aqueous solutions of glucose and ethanol were used.

Correct evaluation of the effective concentration polarization influence in membrane-assisted devices. Case study: H2 production by Water Gas Shift in Pd-membrane reactors

In this paper, a non-conventional approach is introduced to correctly measure the overall effective influence of concentration polarization in membrane-assisted devices. The basis of this study starts from considering that the Concentration Polarization Coefficient (CPC) tends to become high in those device zones where permeation driving force is relatively small (i.e., small flux). Therefore, these zones have to provide a small contribution to the overall polarization level, as membrane is not influential to the process. However, if calculating the average CPC along the device using the standard method, an unrealistic over-estimation of the overall polarization level results, which is not representative of the real behavior of device. To avoid such an issue, a novel coefficient, here-named Effective Average CPC (EAC), is introduced and used. This coefficient, representing a more useful method to determine the real polarization extent in a membrane-assisted device, is then investigated as a function of key-engineering working parameters, like temperature, total feed and permeate pressure and Gas Hourly Space Velocity (GHSV), determining the conditions allowing the overall polarization level to be minimized.

Concentration polarization distribution along Pd-based membrane reactors: A modelling approach applied to Water-Gas Shift

In this work, an improved membrane characterization, computational fluid dynamic (CFD) simulations of inert particle beds, and simulations of a membrane reactor for Water Gas Shift (WGS) are coupled together to evaluate the concentration polarization distribution along Pd-based membrane reactors. To perform this investigation, first a 3.6 μm-thick Pd75Ag25-membrane is characterized using experimental data from the literature as input to an improved approach allowing the non-ideality of internal diffusion to be quantitatively evaluated as a function of hydrogen partial pressure. Then, CFD simulations of a particle bed composed of regular arrays of mono-disperse spherical particles are carried out, showing that the velocity field between particles and membranes contributes to enhance the mass transfer towards the membrane surface. Finally, after validating a 1D-1D reactor model by literature experimental data in terms of both conversion and recovery index vs. feed pressure, simulations of concentration polarization coefficient (CPC) profiles along the reactor are performed. These profiles allow a clearly identification of the reactor zones where polarization is higher, providing useful information to minimize the polarization influence on reactor performances.

Economic Evaluation of a Hybrid Desalination System Combining Forward and Reverse Osmosis

This study seeks to evaluate the performance and economic feasibility of the forward osmosis (FO)–reverse osmosis (RO) hybrid process; to propose a guideline by which this hybrid process might be more price-competitive in the field. A solution-diffusion model modified with film theory was applied to analyze the effects of concentration polarization, water, and salt transport coefficient on flux, recovery, seawater concentration, and treated wastewater of the FO process of an FO-RO hybrid system. A simple cost model was applied to analyze the effects of flux; recovery of the FO process; energy; and membrane cost on the FO-RO hybrid process. The simulation results showed that the water transport coefficient and internal concentration polarization resistance are very important factors that affect performance in the FO process; however; the effect of the salt transport coefficient does not seem to be large. It was also found that the flux and recovery of the FO process, the FO membrane, and the electricity cost are very important factors that influence the water cost of an FO-RO hybrid system. This hybrid system can be price-competitive with RO systems when its recovery rate is very high, the flux and the membrane cost of the FO are similar to those of the RO, and the electricity cost is expensive. The most important thing in commercializing the FO process is enhancing performance (e.g.; flux and the recovery of FO membranes).