Membrane Partition and Mass Transfer in Ultrafiltration

Abstract

Through ultrafiltration experiments with a macromolecular solution of dextran and a suspension of bentonite, it is shown how the understanding of mass transfer phenomena involved in ultrafiltration can be improved if several limiting mechanisms working simultaneously over different parts of a same membrane are taken into account. This partition is determined according to the operating conditions as well as the characteristics of the treated fluid. This approach is convenient to describe an entire range of experimental variations in solvent and solute transfer through hollow fiber membranes induced either by macromolecular concentration polarization or by particles fouling. It is shown that reversible fouling can have effects as dramatic as irreversible fouling (pore plugging) in some circumstances.     

Surface modification of polyethersulfone hollow-fiber membranes by γ-ray irradiation

The fouling of ultrafiltration membrane is often caused by gel formation on the membrane surface. This gel layer arises due to concentration polarization or macromolecular adsorption on the membrane surface. The gel layer affects both the hydraulic permeability and the rejection properties of the membrane. In this report, the adsorption of porcine albumin and the concentration polarization effect on modified and unmodified polyethersulfone (PES) hollow-fiber membrane is studied. PES ultrafiltration hollow-fiber membranes were modified by the grafting of polyethylene glycol (PEG) polymer on the internal surface using γ-ray irradiation method. The modified hollow fibers were less susceptible to fouling than were the unmodified fiber. The performance of both modified and unmodified hollow fibers was tested as a function of feed flow rates and protein concentrations. © 1994 John Wiley & Sons, Inc.     

FLUX ENHANCEMENT USING FLOW REVERSAL IN ULTRAFILTRATION

ABSTRACT

The effect of flow reversal on permeate flux in cross-flow ultrafiltration of bovine serum albumin (BSA) has been investigated experimentally. BSA is a well-studied model solute in membrane filtration known for its fouling and concentration polarization capabilities. Ultrafiltration experiments were performed with BSA feed solutions in a hollow-fiber membrane module. The BSA feed concentrations ranged from 0.01 to 5 wt% and were ultrafiltered at a transmembrane pressure of 20 psia. Permeate flux was determined both with and without the use of flow reversal for each concentration. The experimental results indicate that under flow reversal conditions, the permeate flux is enhanced significantly when compared with runs without flow reversal. The effect of flow reversal on flux enhancement is very pronounced for dilute BSA solutions.     

Studies of Air Slug Distributions and Preliminary Membrane Fouling by Optical Monitoring in a Side-Stream Membrane Module

Abstract

This article is aimed to propose an alternative method to characterize an air/water flow and new designs of rotating air distributors for membrane fouling control in a side-stream membrane module for a membrane bioreactor. A new optical sensor technique was experimentally demonstrated for the characterization of slug flows in a tubular ultrafiltration membrane module. The experimental results showed that the optical sensor system had high sensitivities for measurements of slug velocities, lengths, and frequencies in each membrane tube. The phenomena of slug coalescence/spilt were discussed. The rotating air distributor significantly improved the air slug distributions and had the potential to reduce the membrane fouling.     

Serum Protein Fractionation by Membrane Processes: Centrifugal Ultrafiltration,Osmosedimentation, and Multistage Ultrafiltration

Abstract

Whole sera proteins may be fractionated according to their molecular weights by using three membrane techniques: centrifugal ultrafiltration, osmosedimentation, and multistage ultrafiltration. Ultrafiltration or dialysis cells were mounted in the swinging baskets of a centrifuge in all three cases, with the membranes aligned parallel to the centrifugation radius. As a result, solute accumulated over the membrane was convectively removed from its surface, thus preventing membrane polarization and fouling. During these experiments, smaller proteins migrated across the membrane, leaving behind the larger ones. Multistage filtration experiments were performed using cells fitted with three different membranes of successively narrower pores. Four different fractions were thus obtained and analyzed by gel permeation chromatography; separation factors as high as 2000 were obtained.     

Separation and Fractionation of Macromolecular Solutions by Ultrafiltration

Abstract

The present state-of-the-art of membrane ultrafiltration with reference to macromolecular fractionations is reviewed. Ultrafiltration is now a widely used technique, both in the laboratory and industrial applications, which stems from the development of asymmetric membranes followed by the recognition of the importance of fluid mechanical and mass transfer processes and their management through equipment design and fluid-flow practices. However, large-scale fractionation of macromolecular mixtures or solutions such as proteins has not yet been feasible. This inability is attributable to a number of factors, viz., concentration polarization and fouling processes which may also be coupled with limitations imposed by nonuniform pore size as well as protein–protein (solute) interactions, the latter being determined by the solution chemistry. It is now well recognized that boundary-layer and interfacial effects, in general, are extremely important in membrane applications, as evidenced by a number of manifestations. Several models have been put forward to explain the effects of concentration polarization, whereas membrane-fouling owing to solute-membrane interactions and membrane pore-obstruction or secondary membrane formation via macrosolute deposition, thus causing major changes in effective pore size distribution and therefore effecting inevitable changes in membrane characteristics, have hardly been considered in detail in ultrafiltration transport modeling. Nevertheless, the recognition of the importance of surface and colloid chemical phenomena in governing membrane performance has focused attention upon techniques of membrane modification and feed solution properties control as the key to ultrafiltration applications. These are particularly important for macro-molecular fractionations which depend upon a reasonably clear understanding of the mechanisms of the various processes and which emerge from a good deal of basic or fundamental research.     

膜形态、膜污染和浓差极化对流动电位的影响

引　言膜与水溶液接触后 ,由于自身所具有的固定荷电基团的静电吸附或膜表面ｖａｎｄｅｒＷａａｌｓ力对溶液中某些反离子的不等量吸附作用 ,使被吸附的离子紧贴在固体表面 ,形成一个固定的吸附层 ,叫Ｓｔｅｒｎ层 ,同时从Ｓｔｅｒｎ层到液体内部又形成了扩散层 ,带电膜的表面与电解质溶液相对移动时在Ｓｔｅｒｎ层和扩散层之间形成一个剪切面 ,剪切面与溶液体相的电位差即 ζ电位 ,其大小与膜表面吸附离子的性质与数量有关 ,反映出膜表面荷电性能 .多年来 ,许多关于膜荷电性能的研究工作表明膜表面的荷电会影响膜的通量和对溶液中带电微粒…     

Fouling of Thin-Channel and Tubular Membrane Modules by Dilute Suspensions

Abstract

In this paper, fouling of thin-channel and tubular ultrafiltration (UF) membrane modules by dilute suspensions have been studied theoretically. A hydrodynamic analysis of fluid-particle system is presented to describe the role of dilute suspensions in fouling such membrane modules. The present analysis assumes that for very dilute suspensions, only inertial effects-are-important for particulate fouling. Particle trajectory history and hence the fouling is computed from equations of motion for the particles, where the fluid-flow is given by the full solution of Navier-Stokes equation. To simulate the flux decline due to build up of foulant layer on the membrane walls, it is assumed that the deposition of particles on the membrane surface at discrete time interval is a steady state event and thus formulating the fouling problem as an infinite series of successive steady state events. Present simulation results indicate that inertial effects are important and under positive wall permeation flux conditions, particles are encouraged to migrate towards the membrane wall causing so-called ‘membrane fouling’ by the particulars.     

Ultrafiltration Characteristics of Oil-Detergent-Water Systems: Membrane Fouling Mechanisms

Abstract

The ultrafiltration characteristics of oil (bilge oil and synthetic based lubricating oil)-nonionic detergent-water (river water and distilled water) systems are evaluated with noncellulosic, tubular membranes. The water flux behavior (membrane fouling) is dictated by the membrane resistance increase due to detergent-membrane interaction and due to surface fouling in the presence of oil-detergent emulsions and suspended solids. Membrane fouling and cleaning requirements depend on the type of oily water systems. Flux drop can be minimized by operating at temperatures above 35°C and/or with short-term membrane depressurization. In all cases the steady-state water flux is a function of the initial membrane water flux. Depending on the oil water systems, water fluxes of 8 to 52 ± 10^?4 cm/sec are obtained. Excellent oil rejections are observed in all cases: even with oil-detergent systems, an ultrafiltrate oil concentration of less than 10 mg/1 can be achieved.     

Influence of pH and Ionic Strength during Food Protein Ultrafiltration: Elucidation of Permeate Flux Behavior,Fouling Resistance,and Mechanism

Ultrafiltration (UF) has found wide application in the food industry. In this work, gelatin, as a model food protein, was used to study the influence of pH and ionic strength on fouling during ultrafiltration using 30 kDa polyethersulfone (PES) membrane. The results showed significant water flux reductions after filtration for all feed solution conditions. The maximum fouling rate was obtained near gelatin's IEP, suggesting corresponding electrostatically driven fouling. Addition of salt increased the flux at pH near the gelatin's IEP but it had a negative effect at pH above or below gelatin's IEP. Analysis by the resistance-in-series model suggested that concentration polarization, reversible and irreversible fouling has contributed to the overall fouling. Cake formation was identified as the dominant mechanism for flux decline.     

微滤和超滤过程中浓差极化和膜污染控制方法研究

本文较全面的介绍了微滤和超滤过程中浓差极化和膜污染的各种控制方法,并比较了各种方法的优缺点。     

Temporal evolution of resistances in the ultrafiltration of humic substances

Many ultrafiltration-related studies have emphasized fouling mechanisms, but few works have been done on resistance modeling. This study investigated the temporal variation of different resistances including membrane intrinsic resistance, fouling resistance, and concentration polarization resistance, based on a well established resistance--in-series model. The various resistances were determined at an early stage of ultrafiltration operation. During the initial operation period, the total filtration resistance for ultrafiltration of humic substance solution ranged from 1.9×10⁹ to 2.2×10⁹Pa·/m. The principal resistance of ultrafiltration is from intrinsic membrane resistance, accounting for more than half of the total resistance (56-85%).     

Potential Applications of Pulsed Flow for Minimizing Concentration Polarization in Ultrafiltration

Abstract

Various methods have been proposed to control or minimize concentration polarization and membrane fouling. A brief review of these alternatives is presented in this paper. Flow pulsation as a means of improving transmembrane flux has been studied experimentally by a few investigators. A mathematical model is developed to evaluate the performance of a tubular membrane module under oscillatory flow conditions. Besides the effect of osmotic pressure and axial pressure variation, the model considers the convective-diffusive mass transport without decoupling the momentum equation from the solute continuity equation. Model equations are solved by a finite difference method as part of an iterative solution. Model predictions of transmembrane flux with experimental data are found to be in good agreement. By flow pulsing, it is possible to improve the transmembrane flux by more than 68% at pulsing frequency of 60 cycles · min^?1. An analysis of extra power requirement for flow oscillation shows that the gain in transmembrane flux outweighs the cost of extra power, which is a minute fraction of the power required to maintain steady flow.     

Membrane Characteristics and Fouling Study in MEUF for the Removal of Chromate Anions from Aqueous Streams

Abstract

Micellar‐Enhanced Ultrafiltration (MEUF) of the chromate anions from aqueous solutions has been studied at room temperature (28±2°C) using cationic surfactants, cetyltrimethylammonium bromide (CTAB), and cetylpyridinium chloride (CPC), micelles of which adsorb the chromate ions by electrostatic interactions. The solution is processed by ultrafiltration, using a membrane with a pore size small enough to block the passage of the micelles and the adsorbed ions. The process is highly efficient in removing the chromate ions. In the absence of other electrolytes, chromate ion rejections up to 99% were observed at optimal conditions of pH, pressure, temperature, feed chromate, and surfactant concentrations. The presence of added NaCl reduces the chromate rejection, but it was still considerable (up to 82%), even in the presence of 100 mM NaCl. The rejection rate of chromate was found to be highly dependent on the pH of the feed solution. The influence of membrane characteristics on the chromate ion removal was also studied. Various resistances like fouling resistance, concentration polarization resistance, and membrane resistance were also estimated to quantify their effects on the removal efficiency and on the flux behavior.     

Fouling Control in a Submerged Flat Sheet Membrane System: Part I – Bubbling and Hydrodynamic Effects

Abstract

Submerged flat sheet membranes are mostly used in membrane bioreactors for wastewater treatment. The major problems for these modules are concentration polarization and subsequent fouling. By using gas‐liquid two‐phase flow, these problems can be ameliorated. This paper describes a study of the use of gas‐liquid two‐phase flow as a fouling control mechanism for submerged flat sheet membrane bioreactors. The effect of various hydrodynamic factors such as airflow rate, nozzle size, intermittent filtration, channel gap width, feed concentration, imposed flux, and the use of membrane baffles were investigated. Experiments conducted on model feeds showed that fouling reduction increased with air flow rate up to a given value and beyond this flowrate no further enhancement was achieved. The effect of bubbling was also found to increase with nozzle size at constant airflow. Using intermittent filtration as an operating strategy was found to be more effective than continuous filtration and it also reduced energy requirements. The study showed the importance of the size of the gap between the submerged flat sheet membranes. As the gap was increased from 7 mm to 14 mm, the fouling became worse and the degree of fouling reduction by two‐phase flow decreased by at least 40% based on suction pressure rise (dTMP/dt). This is the first study which has reported the effects of baffles in improving air distribution across a flat sheet submerged membrane. It was found that baffles could decrease the rate of fouling by at least a factor of 2.0 based on the dTMP/dt data, and significantly increase critical flux.     

A Study on Concentration Polarization in Ultrafiltration

Abstract

A finite-difference solution of coupled transport equations for momentum and solute continuity is presented to model the concentration polarization in a tubular ultrafiltration (UF) system. The model includes the effects of solute osmotic pressure and solute rejection at the membrane surface, axial pressure drop and resistance of the gel layer. This provides a fundamental understanding of the dynamics of various operating parameters on concentration polarization and transmembrane flux. Simulation results are presented for a wide range of operating variables to show their effects on local variation of solute concentration and transmembrane flux. The numerical results were also compared with previously published experimental data, which shows that a concentration polarization model based on constant membrane permeability (usually obtained from pure water flux data) grossly overestimates the flux behavior. If the effect of gel polarization is included, the model can predict the actual permeate flux very closely. Thus, in modeling ultrafiltration, one needs to be careful in using the appropriate membrane permeability terms. The commonly used intrinsic membrane permeability which is usually a constant, may not describe the true flux behavior in ultrafiltration. Actually the nature of the feed, solute-surface interaction and gel layer formation control the effective permeability, which varies axially along the membrane length.     

Resistance Analyses for Ultrafiltration in Tubular Membrane Module

Abstract

The resistance analyses for ultrafiltration of macromolecular solutions by the resistances-in-series model and the modified gel-polarization model, respectively, have been extended to the turbulent ultrafiltration system in this study. The experiments are carried out by ultrafiltrating dextran T500 solutions in a tubular membrane module with membrane material of ZrO₂-TiO₂/carbon. It is found that the permeate fluxes are predicted very well by these models for both laminar and turbulent systems, and the resistance caused by the concentration polarization/gel layer (R _p) can be reduced by increasing the crossflow velocity on the membrane surface. Analysis by the resistances-in-series model showed that, R _p decreases with flow velocity with exponents of 0.49 and 0.99 for the laminar system and the turbulent system, respectively.     

Concentration polarization and fouling

Concentration polarization is often the reason for the serious limitation of the membrane processes due to its negative influence on the transmembrane flux. Many theoretical studies on this polarization phenomena have resulted in mathematical models for concentration polarization. In most of them, solutions are sought for the coupled nonlinear system of the equations of continuity and motion. Each of these solutions makes use of some assumptions in order to simplify the equations which represent the phenomenon.Different kinds of flow systems have been constructed in order to reduce the concentration polarization. The aim of these flow systems has mainly been to improve the mass transport from the membrane surface back to the bulk solution.Fouling often is a result of concentration polarization, but can also have other reasons.The attention that was drawn to fouling ten to fifteen years ago mostly consisted of recognizing the fact as such and to roughly identify the foulant. Later, efforts to avoid the negative effects have basically followed the paths of altering the composition of the feed solution by a pretreatment to change or get rid of the foulant, to change the hydrodynamics of the membrane module or to alter the membrane itself.However, unveiling the basic mechanisms of fouling attracted little attention until the late seventies when for example fouling of an RO sea water desalination membrane could be subdivided into four consecutive steps.Also, studies of the chemistry and physics of fouling have being performed on whey lately and revealed in closer detail some of the responsible phenomena.     

Effect of Viscosity on Membrane Fluxes in Cross-Flow Ultrafiltration

Abstract

For practical applications of ultrafiltration (UF), an estimation of membrane fluxes under various operational conditions is very important. This study analyzed concentration polarization (CP) as a coupled transport problem with concentration-dependent solute viscosity. Besides the effects of variable viscosity, the model includes the effects of solute osmotic pressure, solute rejection at the membrane surface, and the axial pressure drop. This provides a fundamental understanding of the effects of various operating parameters on concentration polarization and transmembrane flux. A finite-difference solution of the transport equations is presented to model the concentration polarization in a thin-channel UF system. Simulation results for ultrafiltration of Dextran T-70 show that concentration-dependent solute viscosity adversely affects the transmembrane flux and needs to be carefully considered in modeling concentration polarization in membrane filtration.