Membrane Surface Concentration Research Articles

Effect of permeable spacer structure on energy loss and mass transfer in reverse osmosis membrane modules

This study used numerical simulation to investigate the effects of a novel permeable cylindrical-flow distributor on energy loss and mass transfer in reverse osmosis (RO) membrane modules. The analysis focused on variables, such as Reynolds number, inlet concentration and flow incidence angle, influencing the improvement of water production efficiency in RO membranes and the reduction of concentration polarisation on the membrane surface. The membrane surface shear stress, vorticity, permeate flux and concentration polarisation factor were used to describe these effects. To better explain the performance of the permeable flow distributor, the viscous resistance coefficient (Du) in the porous media model was used to measure the permeability of the flow distributor. Furthermore, a produced water energy consumption factor (JCP) was defined to represent the energy required for unit water production. The results indicate that the permeable flow distributor significantly reduces pressure loss in the membrane channels, avoiding the high energy consumption associated with traditional flow distributors in RO processes. The Du exhibits an optimal value in the range of 1e8 to 1e10, which maximises the JCP, leading to a 32.9 % improvement compared to traditional impermeable flow distributors. This achievement enables energy-efficient and effective desalination of seawater.

A facile method to estimate the effective membrane pore charge density through surface zeta potential measurement

In this work, an attempt was made to find a correlation between the effective membrane pore charge density (Xd), which is a non-measurable quantity and the measurable quantities like, the zeta potential of the membrane surface and initial feed concentration (C0). Effects of solution pH and feed electrolyte concentration were included in the measurements of membrane zeta (ζ) potential. Seven membranes were fabricated in the range of molecular weight cut-off between 1 and 5 kDa. Zeta potential, membrane permeability, molecular weight cut off, contact angle, average pore size and Fourier transform infrared spectroscopy were used for membrane characterization. A series of monovalent salt rejection experiments were done at three initial feed concentrations of 2 g/L, 5 g/L and 10 g/L, at pH of 3, 7 and 9, at 276 kPa transmembrane pressure and 50 L/h cross flow rate. A map between Xd, ζ-potential and C0 was generated by solving the Donnan Steric Pore Model (DSPM). The good agreement amid the theoretically calculated Xd and the Xd calculated from the developed correlation allows one to estimate the effective membrane pore charge density for the loose NF/low cut off UF membranes by only measuring the ζ-potential and feed concentration of the solute, eliminating the requirement to solve the entire DSPM model.

Enhanced treatment of perfluoroalkyl acids in groundwater by membrane separation and electrochemical oxidation

This work explores the treatment of poly- and perfluoroalkyl acids (PFAAs) in groundwater by coupling membrane separation and electrochemical oxidation (ELOX). A process system engineering approach based on modelling and empirical data was followed. Two nanofiltration (NF90) and reverse osmosis (BW30) membranes were characterized for treating an electrolyte (NaCl and CaSO4) mixture of perfluorocarboxylic acids (PFCAs) containing PFOA, PFHpA, PFHxA, PFPeA and PFBA with initial concentrations of 10 µg L−1 each. Membrane surface charge shielding and concentration polarization negatively influenced NF90 performance, and the BW30 membrane was selected. Electrochemical oxidation with boron doped diamond anodes treated the PFCAs mixture amended with PFOS and 6:2 FTSA, emulating previously pre-concentrated feed and non-preconcentrated feed conditions. Working at different current densities (J) between 20 and 350 A m−2, the removal of PFOA, PFOS and 6:2 FTSA followed first order apparent kinetics, although shorter chain PFCAs initially showed increasing trends because of their simultaneous electrogeneration and degradation. Overall, ΣPFAA electrolysis followed first order kinetics linearly correlated to J in the full range of testing. Unexpectedly, PFAAs electrolysis was faster for the low conductive non-preconcentrated feed, a result that was ascribed to the enhanced direct electron transfer mechanism resulting from the higher cell voltage. For 99.9% PFAAs removal, the total specific cost of treatment was minimized using a cascade of four RO stages and ELOX treatment of the concentrate, to reach ΣPFAA below the Health Advisory Levels recommended by the USEPA in drinking water (<70 ng L−1 sum of PFOA and PFOS).

Performance of a forward osmosis mass exchanger based on detailed mass transfer boundary layer analysis

Forward osmosis-based processes are low energy separation technology that can help in the recovery of fresh water. The design of such systems is approximated either from the simple mass conservations or using the analogy of heat exchanger design correlations. Such relationship has serious drawbacks, particularly at high recoveries, when the impact of mass transfer boundary layer is paramount. Unlike the reverse osmosis, the permeate flux (hence the recovery) is reduced with the convection (Peclet number, Pe). The membrane surface concentration and the thickness of concentration boundary layer on the draw side decreases with Pe. Since, permeate flux increases with the surface concentration, the recovery is dependent on Pe. Besides the relative concentration and flow-rates, the membrane area is influenced by the mass transfer boundary layer, manifested through Pe. The results reported in the Mondal and Field (2018) is a special case when Pe ≪ 1 (negligible convection), leading to maximum recovery. We evaluate the required membrane area and the performance of the forward-osmosis system as a function of the operating conditions, including the Pe. We have identified the range of operating conditions, for which the sizing of FO system is underpredicted in the case of negligible convection and the log-mean concentration difference.

Renewable energy powered membrane technology: Computational fluid dynamics evaluation of system performance with variable module size and fluctuating energy

Renewable energy powered membrane systems operate with fluctuating energy. Such fluctuations affect pressure and feed flow and as such the hydrodynamic conditions in a membrane system. Hydrodynamic variations alter the membrane surface concentration and boundary layer thickness which in turn determines permeate water quality. In this work this is calculated using computational fluid dynamics (CFD) for the three most predominant energy levels obtained during such fluctuations and compared with experimental data.A 2D-CFD simulation was performed using OpenFOAM to calculate the wall concentration and boundary layer thickness over the length of a module. The influence of module type was investigated using two system configurations, namely three 2.5″ modules in series (BW30-2540 or NF270-2540) and one 4″ module (BW30-4040 and NF270-4040) with similar total membrane areas. Energy levels were extracted from experimental data at three solar irradiance, maximum intensity (1 kW/m2), light cloud (360 W/m2) and heavy cloud periods (190 W/m2).At the highest energy level, in the system with three 2.5″ modules the wall concentration was closer to the bulk concentration due to the higher flow velocity in a smaller channel. The resulting boundary layer thickness for BW30 was constant and almost zero. At the medium energy level, the simulation results show that the permeate flux decreased significantly due to the lower pressure and for the BW30 it was almost zero due to the low pressure. At the lowest energy level, the feed pressure was well below the osmotic pressure and no permeation was possible.Results from this study show that the model is able to describe the filtration process in spiral wound membrane modules under fluctuating energy conditions. Further investigations on the possibility to improve the boundary conditions of the model are required.

The role of nanofiltration membrane surface charge on the scale-prone ions concentration polarization for low or medium saline water softening

Based on the fact that when inlet bulk solution with low or medium concentration was involved, scaling was apt to appear on nanofiltration (NF) membrane surface in retentate side. Therefore, study on its surface charge performance under threshold concentration condition is necessary. Two polyamide composite NF membranes (DL and DK) were characterized with streaming potential (Es), zeta (ξ) potential and other measurements encountering with a wide ionic strength range (1.5–108 mol·m−3). Then, the interactional relations between membrane surface charge and concentration polarization (CP) degree, along with the relevant constituting factors were investigated. The results showed that even when the sparingly soluble salt concentration for CaSO4 exceeded its threshold point, both NF membranes at pressure drop of 180 kPa still exhibited low and medium-intensity surface electrokinetic phenomena (−5.94 ≤ Es ≤ −3.41 mV; −38.82 ≤ ξ-potential ≤ −36.34 mV). It turned out that solvent flux decreased almost linearly, but measured concentrations of the scalant ions in NF retentate streams increased slightly at first and then rapidly with the enlargement of ξ-potential, respectively. Meanwhile, mass transfer coefficient and boundary layer thickness had a negligible change in the scope of testing ξ-potential. Therefore, CPSO42− and CPCa2+ decreased apparently with ξ-potential becoming less negative for the adopted NF membranes.

Modeling of Computational Fluid Dynamics and Experimental Study of Electro-Catalytic Oxidation Enhanced Nanofiltration for Treating C.I. Acid Red 73 Wastewater

C.I. Acid Red 73 (AR73) wastewater was treated by cross-flow nanofiltration coupling electro-catalytic oxidation using an NF90 membrane and a Ti/SnO2–Sb anode prepared via electrodeposition. Experiments conducted for standard electrochemical degradation of AR73 studied the reaction rate of removing AR73 using the Ti/SnO2–Sb anode. A computational fluid dynamics (CFD) model was developed to predict the permeate flux under a laminar flow regime, including the effects of operating pressure, applied potential, initial concentration, and cross-flow velocity on this coupling process. The variations of the membrane surface concentration and permeate flux along the length of the channel were quantified. The experimental results were compared with those predicted by the model, and they agreed well.

Effect of Film Diffusion on the Ion-Exchange Kinetics of a Tracer Ion in Nafion-117 Membrane from a Mixture of Salt Solution.

The ion-exchange kinetics of a tracer ion (Cs+ and Ba2+) in presence of a bulk ion (Na+/H+) has been measured in Nafion-117 membrane for a range of concentrations of NaCl/HNO3 using the nonstationary radiotracer method. A systematic increase in the ion-exchange rate and decrease in the partition coefficients of the tracer ions between membrane and solution have been observed with the increase in bulk ion concentration. The sigmoidal nature of experimental profiles indicates film-diffusion-controlled kinetics even for well-stirred solutions. In the absence of an existing analytical or numerical solution, a simple empirical approach has been proposed to find the variable membrane surface concentration and has been used to solve the membrane diffusion equation by the finite difference method. The fitting of the experimental curves with a single diffusion coefficient for Cs+/Ba2+ has been achieved. The exchange rate has been found to be independent of the stirring speed beyond a limiting speed.

Modeling and Simulation of Rotating Disk-Membrane Module in Ultrafiltration of Bovine Serum Albumin

A dynamic mass transfer model, coupled with transient back transport flux for a Rotating Disk-Membrane (RDM) module has been developed in the present study. The simulation algorithm is capable of predicting the permeate flux (J), membrane surface concentration (cm), and permeate concentration (cp) under different parametric conditions of transmembrane pressure (TMP), feed concentration (c0), stirrer speed (Ω2), and membrane speed of rotation (Ω1). The key feature of the proposed model is the analytical solution of the governing component balance equation. Additionally, the well-known osmotic pressure model and the Spiengler-Kedem black box model were used to describe the solvent transport through membrane. The proposed model was validated with the experimental results obtained in ultrafiltration of bovine serum albumin (BSA)/water solution conducted in a standard RDM module, fitted with polyethersulphone (PES) membrane of 30 kDa molecular weight cut-off (MWCO). The maximum absolute deviation of the model predictions with respect to the experimental results was observed to be well within 5%.

Effect of the concentration polarization on the fouling driving force of UF membranes

The main objective of this work was to investigate the effect of foulant concentration prevailing at the membrane surface on the rate of flux decline. The significant role of foulant membrane concentration was highlighted by analysis of the well-known filtration blocking laws. All blocking mechanisms were found to predict that the initial fouling rate depends on the product of feed concentration and initial permeate flow rate. Humic acid fouling data were analyzed to provide values of the fouling rate dJv/dt vs. membrane surface concentration Cm. The data of all runs, covering a rather broad range of conditions, fell within a narrow band thus supporting the contention that foulant concentration on the membrane is one of the main parameters governing the fouling rate.

The Interfacial Tension of the Lipid Membrane Formed from Lipid–Amino Acid Systems

The interfacial tension of lipid membranes composed of phosphatidylcholine (lecithin, PC)–valine (Val), phosphatidylcholine–isoleucine (Ile), phosphatidylcholine–tyrosine (Tyr), and phosphatidylcholine–phenylalanine (Phe) has been studied. The membrane components formed 1:1 complexes. The interfacial tension measurements were used to determine the membrane surface concentration A 3−1, the membrane interfacial tension γ3, and the stability constant K.

Simulation of continuous stirred rotating disk-membrane module: An approach based on surface renewal theory

A rigorous surface renewal model has been developed describing the aspects of mass transfer in a rotating disk-membrane (RDM) ultrafiltration cell. The model takes into consideration of two distribution functions of random surface elements, one with respect to their point of origin and the other related to the corresponding residence time on the membrane surface. The back transport flux and the permeate flux are evaluated at the membrane surface in order to develop a surface component balance equation. The component balance equation and a flux-rejection relationship arising from irreversible thermodynamics are solved simultaneously to develop a dynamic simulation. The simulation elucidates on permeate flux, membrane surface concentration and the permeate concentration under various operating conditions of transmembrane pressure, bulk concentration, membrane and stirrer speeds. For validation of the proposed model, experiments were conducted with bovine serum albumin (BSA)/water as feed in a standard rotating disk membrane module fitted with polyethersulfone (PES) membrane of 30 kDa molecular weight cut-off (MWCO). The model predicted flux and permeate concentration was found to be in good agreement with the experimental data, and the maximum absolute deviation for both cases was found to be well within ±5%.

Simulation of continuous stirred ultrafiltration process: an approach based on surface renewal theory

ABSTRACTUltrafiltration (UF) is an important operation in many processes, including foods and pharmaceuticals, and shear enhanced dead‐end systems are particularly suitable for small‐scale operations. Most industrial applications require a high permeate flux and maximum solute rejection, combined with long membrane life. A rigorous surface renewal model has been developed which describes the mass transfer processes occurring in a single stirred UF cell. The model takes into consideration two distribution functions of random surface elements, one with respect to their point of origin and the other related to the corresponding residence time on the membrane surface. The back transport flux and the permeate flux are evaluated at the membrane surface in order to develop a component balance equation. The component balance equation and a rejection–flux relationship arising from irreversible thermodynamics are solved simultaneously to allow a dynamic simulation. The dynamic simulation enables a more complete understanding of the permeate flux, membrane surface concentration and permeate concentration under various operating conditions. For validation of the proposed model, experiments were conducted on UF of polyethylene glycol 6000 (PEG 6000) solution in a continuous stirred UF cell comprising a cellulose acetate membrane of 5000 Da molecular weight cut‐off. Model predicted values are in excellent agreement with the experimental results. The effects of different stirring mechanisms on mass transfer with flat disk, propeller and turbine impellers were also characterized using the developed model. Copyright © 2010 Curtin University of Technology and John Wiley &amp; Sons, Ltd.

Incipient crystallization of sparingly soluble salts on membrane surfaces: The case of dead-end filtration with no agitation

This paper aims at contributing to development of reliable criteria for determining the onset of scaling on desalination membranes. A novel approach is proposed to investigate incipient crystallization on membranes through modeling and RO filtration experiments. The evolution of membrane surface concentration of rejected ionic species (e.g. Ca 2+, CO 3 2−) is theoretically determined, and coupled with a detailed kinetic model for nucleation and growth of particles on the membrane. Model results contribute to our understanding of this problem and are in general accord with experimental data on membrane surface crystal density and mean size. Experimental data and model results show that incipient crystallization on RO membrane surfaces, when the bulk supersaturation ratio is maintained near its critical value (S ~ 1), is controlled by fluid permeation, and that the “induction time” may be insignificant under typical RO conditions. This work also suggests that the ability to determine the onset of crystallization on membranes is dependent on the combined sensitivity and adequate spatial resolution of the experimental technique employed, which explains the significant uncertainty associated with the reported in literature “induction period” data of different origin. Directions for future research are discussed.

Transient solute adsorption incorporated modeling and simulation of unstirred dead-end ultrafiltration of macromolecules: An approach based on self-consistent field theory

An unsteady state mass transfer model of unstirred dead-end ultrafiltration module has been developed in the present study. The dynamic membrane surface concentration is evaluated using a modified self-consistent field theory, which enables the development of an algorithm to incorporate the contribution of solute adsorption in the membrane surface concentration dynamics. Knowing the corrected membrane surface concentration, permeate flux and permeate side concentration is determined using osmotic pressure model and flux–rejection relation as predicted by irreversible thermodynamics. The time evolution of all the different process variables is achieved by solving two component balance equations developed at the membrane surface and in the solution phase respectively. The basic feature of the model is the incorporation of adsorbed fraction in the unsteady state membrane surface concentration. For the validation of the proposed model, experiments were conducted with Bovine Serum Albumin (BSA)/water as feed in a standard unstirred batch ultrafiltration module fitted with Polyethersulfone (PES) membrane of 30 kDa molecular weight cut-off (MWCO). The model predicted flux and the permeate concentration were found to be in good agreement with the experimental data.

Prediction of permeate flux for turbulent flow in cross flow electric field assisted ultrafiltration

Separation of bovine serum albumin from aqueous electrolytic solution has been carried out by electric field assisted cross flow ultrafiltration using polyphenylene ethersulfone membrane of molecular weight cut-off (MWCO) 30 kDa in a rectangular channel. A theoretical model is developed under the turbulent flow regime including the effect suction for osmotic pressure controlled electric field assisted ultrafiltration. Similarity solution technique is applied to solve the governing partial differential equations of the concentration profile in the developing mass transfer boundary layer over the membrane surface. The proposed model is used to predict the permeate flux and membrane surface concentration. Effect of d.c. electric field on the variation of membrane surface concentration and permeate flux along the length of the channel is also quantified using this model. The expression of Sherwood number relation for estimation of mass transfer coefficient is derived. The analysis shows that there is a significant effect of electric field on the mass transfer coefficient. Parametric study has been carried out to observe the effect of feed concentration, electric field, cross flow velocity and pressure on the permeate flux and membrane surface concentration. The value of critical electric field has also been quantified from the knowledge of permeate flux. The predictions from theoretical analysis are successfully compared with the experimental results under a wide range of operating conditions.

Modeling of flux enhancement in presence of concentration polarization by pressure pulsation during laminar cross flow ultrafiltration

A theoretical study for the flux enhancement by pulsation of transmembrane pressure is presented for osmotic pressure controlled ultrafiltration under laminar flow regime. The transient velocity profile is solved analytically using Green`s function method. Time dependent convective diffusive equation is solved to quantify the membrane surface concentration and the permeate flux, numerically. The effects of the amplitude and frequency of pulsation on flux, surface concentration and observed retention are studied.

Modeling the performance of a standard single stirred ultrafiltration cell using variable velocity back transport flux

A semi-analytical model based on unsteady state mass transfer has been developed for the prediction of permeate flux, membrane surface concentration and permeate side concentration during ultrafiltration in a standard single stirred cell. Starting from the basic fluid mechanical analysis of the system, stirrer induced back transport flux was evaluated considering the actual non-uniform velocity field and the final expression was incorporated in the fundamental mass balance equation. As a result the obtained partial differential equation (PDE) became associated with variable coefficients. The PDE after taking Laplace transform was reduced to standard confluent hypergeometric equation via a series of appropriate substitutions, which was finally mapped again in the time domain considering only the first four terms of the resulting series solution. Once the analytical form of a concentration field was obtained, permeate flux and the membrane surface concentration were evaluated using polymer solution theory and irreversible thermodynamics. Finally an iterative scheme has been designed to simulate the permeate flux and the membrane surface concentration under specified set of operating parameters. The prediction from this model was found to be in good agreement with experimental data obtained from PEG-6000/water system using cellulose acetate membrane of 5000 Da molecular weight cut-off.

Modeling and simulation of unstirred dead end ultrafiltration of macromolecules

A mass transfer model based on unsteady state component balance over the concentration boundary layer has been developed in the present study. The model uses semi-infinite consideration in order to solve the governing partial differential equation by Laplace transform that gives an analytical expression for the concentration profile. During the solution procedure, pseudo steady state assumption has been used, as a consequence of which permeate flux and membrane surface concentration have been taken constant over any small time interval. Once the expression for concentration profile is known, a time evolution loop has been developed to simulate the permeate flux, membrane surface concentration and permeate concentration under specified operating conditions of transmembrane pressure drop and bulk concentration. The prediction of the proposed model was found to be in good agreement with experimental results obtained during unstirred dead end ultrafiltration of polyethylene glycol (of average molecular weight 6000) solution using a cellulose acetate membrane of 5000 Da molecular weight cut-off.

PERFORMANCE PREDICTION OF MEMBRANE MODULES INCORPORATING THE EFFECTS OF SUCTION IN THE MASS TRANSFER COEFFICIENT UNDER LAMINAR AND TURBULENT FLOW CONDITIONS FOR NON‐NEWTONIAN FLUIDS

ABSTRACT Design equations for spiral wound membrane modules in case of power law fluid are derived for the first time. The case is solved for both laminar and turbulent flow regimes considering orange juice as the model system. The governing equations are solved numerically and the profiles of transmembrane pressure drop, bulk velocity, concentration, permeate flux, membrane surface concentration and permeate concentration along the membrane module are obtained. A detailed parametric study is carried out to observe the effects of operating conditions on the permeate flux and permeate concentration. The present analysis is helpful to design and simulate the performance of the membrane modules adequately. For the model system, it is observed that the axial pressure drop is about 2 to 3% of the inlet transmembrane pressure drop for 1.0 m length of the module. The mass transfer coefficient increases by twofold when the flow regime becomes turbulent. In case of laminar flow, at 7,000‐kPa inlet transmembrane pressure drop, the average flux is about 22 L/m2·h and the flux value under turbulent flow condition becomes about 30 L/m2·h at the same pressure drop. For a slightly leaky membrane, at 7,000‐kPa pressure drop and 130 kg/m3 of feed soluble sugar concentration, the permeate stream contains about 10 kg/m3 of total sugar. PRACTICAL APPLICATIONThe application of membrane technology is one of the emerging areas in food industry. The major application includes fruit juice clarification and concentration. Therefore, appropriate design of membrane modules is of utmost importance. Most of the fruit juices are non‐Newtonian in nature and they obey, in general, power law rheology. The derivation of detailed design equations of a spiral wound module is carried out considering a power law fluid, both under laminar and turbulent flow regimes. The presented method provides a way to design the membrane modules in great detail (i.e., incorporating effects of suction in mass transfer coefficient, developing concentration boundary layer, non‐Newtonian fluid, etc.). This will be helpful to design a spiral wound module to meet a target productivity (i.e., permeate flux) and permeate quality (i.e., permeate concentration).