Suction effects on the hydrodynamics in the vicinity of an ultrafiltration plane ceramic membrane

The wall shear stress is determined at the surface of a plane ceramic ultrafiltration membrane in a tangential ultrafiltration module. At first, the shear stress is determined at the surface of a plate of Plexiglas mimicking a membrane and at the plane membrane surface without fouling particles with the aim to investigate the influence of permeation. Then, ultrafiltration of a suspension of spherical rigid particles inducing a deposit at the membrane surface is done. In order to study the influence of the foulant, two concentrations of particles were used for the experiments. The values of the mean wall shear stress and its fluctuations (turbulent intensity rate) were measured by using an electrochemical method. Twenty microelectrodes, on which an electrochemical reaction occurs, are mounted flush to the plate of Plexiglas and to the surface of the membrane to determine the maps of shear stress and turbulent intensity rate for two inlet/outlet distributors’ configurations. This work emphasized the fact that, due to permeation across the membrane, the mean velocity gradient value is enhanced and, on the contrary, its local fluctuations are clearly damped, even in the turbulent flow regime. Thus, the effect of the shape of the inlet/outlet fluid distributors is also minimized compared with the results obtained without permeation at the surface of the Plexiglas plate.     

Comparative study of turbulent mass transfer in the viscous sublayer using electrochemical method and direct numerical simulations

In this study we present, analyze and compare the power spectral density of the wall shear stress in a turbulent plane channel flow obtained with different techniques. Experimentally the instantaneous wall shear stress was measured with the electrochemical technique using different probes, which give approximately the same results after applying the transfer function for correction of the probe??s inertia. Numerically, the time evolution of the wall shear stress has been determined using direct numerical simulations (DNS) and large eddy simulations (LES). The results of DNS are in a good agreement with the electrochemical flow measurements. However the power spectra of the wall shear stress obtained with LES shows deviations with respect to DNS at high frequencies because of the spatial filtering inherent to the LES technique.     

CFD modeling for flow and mass transfer in spacer-obstructed membrane feed channels

The effect of spacer geometry on fluid dynamics and mass transfer in feed channels of spiral wound membranes has been investigated. Three-dimensional computational fluid dynamics (CFD) simulations reveal significant influence of spacer geometric parameters such as filament spacing, thickness and flow attack angle on wall shear rates and mass transfer coefficients. The spacers with filaments in axial and transverse direction induce higher shear stresses at the top membrane surface when compared to the bottom; the mass transfer rates are almost equal. The distribution of mass transfer coefficients become uniform when the spacing between axial filaments is increased or transverse filament thickness is decreased. For spacers with filaments inclined to the channel axis, the flow structure depends on spacing and flow attack angle. The fluid follows a zigzag path when spacing is greater while it begins to line-up with the filaments when spacing is reduced or flow attack angle is increased. The flow when aligned with the filaments increases the wall shear stress but confines the region of higher mass transfer coefficient values to a narrower portion. The zigzag flow movement increases these values on a major portion of membrane surface which enhances the mass transfer rates.     

Solidification fouling of paraffin wax from hydrocarbons

Solidification fouling of paraffin wax from a solution in kerosene has been studied in a stirred system where a refrigerant was circulated through a surrounding jacket. The system is unique since it permits the study of the effects of varying shear stress on the deposit in one experiment and thus obviates the need of doing several experiments as in the case of flow through tubes. Effects of rotation speed and wall temperature have been studied. The results indicate that the shear stress has a profound influence on the deposit thickness. Based upon the experimental observations, the initial deposition is hypothesized to be diffusion controlled instead of particulate migration controlled.     

Dean vortices: comparison of numerical simulation of shear stress and improvement of mass transfer in membrane processes at low permeation fluxes

In order to rationalize the effect of Dean vortices on mass transfer improvement during membrane filtration, we present preliminary calculations of the wall shear stress in curved tube with non-porous walls. Previous experimental work has already shown strong positive effect of Dean vortices on mass transfer. In this paper, a numerical simulation of shear stress is proposed in order to determine the influence of the geometric parameters in four different tubes: straight, torus, helical and woven tube. The simulation results are tested against the analytical solutions which are available for velocity and pressure distributions in straight tubes. The simulation gives local values from which the location of Dean vortices in cross-section can be deduced and which depends on geometry and Reynolds number. Moreover, published results dealing with oxygenation of water by a membrane process and pervaporation of organic volatile compounds are considered using the present simulation results.     

Transport phenomena in an agitated vessel with an eccentrically located impeller

The paper presents results of an experimental analysis of the transport phenomena at the vicinity of the wall of an unbaffled agitated vessel with an eccentrically located impeller. Distributions of the transport coefficients were experimentally studied using an electrochemical method within the turbulent regime of the Newtonian liquid flow. Measurements were carried out in an agitated vessel with the inner diameter T = 0.3 m. Liquid height in the vessel was equal to the inner diameter, H = T. The agitated vessel was equipped with a Rushton or a Smith turbine or an A 315 impeller. Eccentricity of the impeller shaft was varied from 0 to 0.53. Local values of the dimensionless shear rate, shear stress, dynamic velocity and friction coefficient were integrated numerically for the whole surface area of the cylindrical wall of the vessel. Averaged values of these quantities were correlated with the impeller eccentricity and modified Reynolds number. The proposed Eqs. (5)–(8), with the coefficients given in Table 2, have no equivalent in open literature concerning this subject. Distributions of the shear rate, γ/n, and friction coefficient, f, at the vicinity of the cylindrical wall of the unbaffled vessel equipped with eccentric Rushton or Smith turbine or A 315 impeller are very uneven and they depend significantly on the impeller eccentricity, e/R. Maximum local values of these variables are located on the wall section closest to the impeller blades. From among the tested impellers, the greatest effects of the impeller eccentricity, e/R, and the liquid turbulence (described by the modified Reynolds number Re _P,M) on the averaged dimensionless shear rate (γ/n)_m and friction coefficient, f _m, are found for the radial-flow Rushton turbine located eccentrically in an unbaffled agitated vessel.     

CFD simulations of net-type turbulence promoters in a narrow channel

The most common spacers or turbulence promoters for membrane processes are net-like materials which enhance mass transfer as well as provide passage for feed solutions. The enhanced membrane performance of spacer-filled channels is determined by the fluid flow patterns induced by the spacer filaments. Insight into the effect of spacer characteristics can be obtained by computational fluid dynamics. In this research, the commercial finite volume package FLUENT was used to visualise the flow pattern in a rectangular membrane channel. Three transverse filament arrangements were simulated. The results show that both high shear stress regions and eddies are present in the channel due to the spacer cylinders. The mass transfer enhancement on the wall/membrane surface is directly related to the high shear stress value, velocity fluctuation, and eddy formation. The peak shear stress and velocity fluctuation are repeated after each spacer cylinder, while the eddies are generally found before and after each cylinder. The CFD simulation also suggests that reducing the transverse filament distance will reduce the distance between shear stress peaks and consequently introduce larger shear stress regions near the wall region and increase the number of eddies, which will benefit membrane mass transfer. However, the penalty for this is that energy losses will also be significantly increased. The selection of optimum spacer geometry design involves a trade-off between these competing effects.     

Characterization and Transport Properties of Surfactant-Templated Silica Layers for Membrane Applications

An intermediate surfactant-templated silica (STS) layer is applied between the supporting mesoporous γ-Al₂O₃ and the amorphous microporous silica overlayer resulting in dual-layered microporous silica membranes for gas separation applications that show improved values for both hydrogen flux and selectivity. Determination of thickness and porosity of as-deposited membrane layers by spectroscopic ellipsometry reveals that the STS layer is present as a distinctive layer of ～20 nm thickness, with penetration up to a depth of ～70 nm into the underlying γ-Al₂O₃support layer, whose thickness and porosity are determined to be 1.3 μm and 50%, respectively.     

Evaluation of the "DSPM" model on a titania membrane: measurements of charged and uncharged solute retention, electrokinetic charge, pore size, and water permeability

The DSPM (Donnan steric partitioning pore model) was evaluated in the case of a titania membrane with "nanofiltration properties" by measuring the electrokinetic charge, pore size, and water permeability of the membrane, along with charged and uncharged solute retention. The zeta potential values (zeta) were determined from measurements of the electrophoretic mobility (EM) of titania powder forming the filtering layer of the membrane. Zeta potential values were converted into membrane volume charge (X) by assuming two limiting cases: a constant surface charge (sigma(s)(cst)) and a constant surface potential (psi(s)(cst)). The mean pore radius and thickness/porosity ratio of the membrane were determined by permporometry and from water permeability measurements, respectively. Retention measurements were carried out as a function of the permeate volume flux for both neutral solutes (polyethylene glycol PEG of different size) and salts (KCl, MgSO4, K2SO4, and MgCl2) at various pH values. Ionic retentions showed minimum values near the IEP of the membrane. Retention data were analyzed using the DSPM. Very good agreement was found between the pore radius calculated by the model and that determined by permporometry. X values calculated from fitting retention data using the DSPM were also in satisfactorily agreement with X values calculated from EM measurements assuming a constant surface potential for a large pH range. Furthermore, the DSPM leads to X values (X(DSPM)) between those calculated from EM (X(EM)) using the two limiting bounds. In other words, X(DSPM) was higher than X(EM) assuming psi(s)(cst) at pH values far from the isoelectric point (IEP) and lower than X(EM) assuming sigma(s)(cst). These results show that the DSPM is in qualitative agreement with the charge regulation theory (increase of the pore surface potential and decrease of the pore surface charge density with decreasing the pore size). On the other hand, the thickness/porosity ratio of the membrane calculated from solute retention data differed significantly from that determined from water permeability measurements. Moreover, a single value of Deltax/Ak could not be determined from PEG and salt retention data. This means that the Deltax/Ak parameter loses its physical meaning and includes physical phenomena which are not taken into account by the DSPM. Nevertheless, the model satisfactorily predicted the limiting retention, as this is not influenced by the Deltax/Ak parameter.     

Electrochemistry of zeolites on thickness shear mode oscillators

This paper describes electrochemical studies of thickness shear mode (TSM) acoustic wave oscillators coated with zeolites. The frequency response of gold on AT-cut 9 MHz quartz oscillators of silver-ion-exchanged zeolite-modified electrodes (ZMEs) under an electrochemical bias is interpreted. This is achieved using a combination of cyclic voltammetry, double-potential-step chronocoulometry (DPSC), and the frequency and resistance responses of the quartz crystal oscillators. Three ZMEs were investigated including fully exchanged Ag(12)A plus partially exchanged Ag(6.4)A and Ag(3.5)A. In all cases, the frequency response of the quartz crystal nanobalance (QCN) could only be interpreted when motional resistance changes were considered. This determines the importance of energy storage and energy dissipation of the shear wave produced by the oscillator in the zeolite film, which was affected by the deposition of silver at the zeolite-electrode-solution interface. The silver deposit formed via the reduction of silver ions originally within the zeolite phase mechanically couples the zeolite film to the underlying substrate. The resistance changes occurring during redox are thus linked to an inner interfacial slip between the zeolite film and the underlying oscillating surface. The data presented are consistent with an extrazeolite redox mechanism.     

Length dependency of flux and protein permeation in crossflow microfiltration of skimmed milk

Crossflow microfiltration of skimmed milk to fractionate casein micelles and whey protein was investigated regarding length dependency of flux and whey protein permeation using a 1.2 m long, 0.1 μm tubular ceramic membrane. A special module consisting of four sections was constructed allowing to assess the effects of membrane length online by measuring flux and permeation of the whey protein β-lactoglobulin as a function of local processing conditions. It was found that under the applied filtration parameters (mean transmembrane pressure Δp_TM,m = 0.5 bar; temperature ? = 55 °C; wall shear stress τ_w = 115 Pa) main parts of the membrane were controlled by a deposit layer. In consequence, the transmission of the whey protein β-lactoglobulin increases from 38% to 87% from membrane inlet to membrane outlet. Results show that a local optimum for protein fractionation exists regarding membrane resistance and process conditions.     

Fouling and cleaning of membranes in the ultrafiltration of the aqueous extract of soy flour

The aqueous extract of soy flour is an emulsion/suspension of proteins, lipids and carbohydrates. The foulant deposit formed on the surface of polysulfone membranes in the ultrafiltration of this complex extract was investigated from several aspects including thickness, physical structure, chemical analysis and rheological behaviour. SEM studies showed the thickness of the foulant deposit was approximately 0.2 μm for 50000 MWCO membrane and 0.4 μm for 100000 MWCO membrane. The structure of the foulant deposit consisted of lipids in a globular form of 0.2 to 1 μm diameter adhered to, and supported by, a protein-polysaccharide matrix. Rheological measurements were conducted on a sample of the foulant deposit collected from the 100000 MWCO membrane. This foulant deposit exhibited pseudoplastic and viscoelastic properties which totally resisted the surface shear stresses in the flat-plate module. Recovery of the water flux of the fouled membranes was achieved by a four-stage cleaning procedure comprising successive stages of washing with sodium hydroxide, protease detergent, sodium hypochlorite and flushing with water.     

Evaluation of the main processing parameters influencing the performance of poly(vinylidene fluoride–trifluoroethylene) lithium-ion battery separators

Poly(vinylidene fluoride–trifluoroethylene) (PVDF–TrFE) membranes are evaluated for lithium-ion battery separator applications. Some of the main parameters affecting separator performance such as porosity, dehydration of lithium ions, and processing technique (Li-ion uptake versus composite formation) are investigated. The polymer characteristics, as determined by infrared spectroscopy, do not change as a function of porosity, dehydration of lithium ions in the electrolyte solution, or processing technique. The electrochemical impedance spectroscopy represented through the Nyquist plot, Bode plot, and the ionic conductivity as a function of temperature strongly depends on the aforementioned parameters. The membrane that exhibits the highest ionic conductivity is a porous membrane without dehydration of lithium ions and prepared by the uptake technique. The performance of the membrane for battery applications are, therefore, strongly influenced both by porosity and processing technique.     

Effects of High Salt Concentration and Residue on Copper and Aluminum Corrosion

Traditional researches on metal corrosion under salt solutions deposit conditions are usually carried out by visual, electron microscopic observations and simple electrochemical measurement via a traditional one-piece electrode. These techniques have difficulties in measuring localized corrosion that frequently occur in inhomogeneous media. This paper reports the results from the experiments using specially shaped coupons and a relatively new method of measuring heterogeneous electrochemical processes, namely, the wire beam electrode(WBE). Preliminary results from copper and aluminum corrosion in highly concentrated sodium chloride solutions with and without solid deposits show that the method is useful in simulating and studying corrosion especially localized corrosion in pipelines.     

Influence of polypropylene membrane surface porosity on the performance of membrane distillation process

Two kinds of polypropylene capillary membranes were used in the membrane distillation (MD). These membranes exhibited a similar morphology, but one of them has an additional low porosity layer on the internal surface of capillaries. The changes of membrane performance during MD process of tap water were investigated. The presence of low porosity layer (thickness below 1 μm) caused that the air permeability was reduced from 1.365 to 0.863 dm³/m² s kPa, whereas the MD permeate flux was decreased only by 15%. A significantly larger decline of the flux was caused by CaCO₃ deposit formed during distillation of tap water. This deposit was removed every 30–70 h by rinsing the modules with a 2–5 wt.% HCl. Unfortunately, a repetition of this operation several times resulted in a gradual decline of the maximum permeate flux (distilled water as a feed). However, the module efficiency with the membranes covered by a surface layer of low porosity was found to decreases twice as slowly. The investigations revealed that a low surface porosity does not limit the possibility of surface wetting of polypropylene membranes, but hindered the scale formation inside the pores.     

Thickness decrease of a grafted polyelectrolyte membrane exposed to shear flow

The effect of the shear flow on the thickness change of a polyelectrolyte membrane grafted onto a glass substrate was directly investigated with a flow cell combined with a confocal laser scanning microscope. The membrane thickness decreased proportionally to an increase in the shear stress of the flow when the shear rate exceeded a critical value of 1 s^?1. The higher the ionic strength was of the fluid, the greater the thinning effect was. The correlation between the critical shear rate and the relaxation of the polymer in the gel membrane was examined. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2808–2815, 2003     

Analysis of rheology and wall depletion of microfibrillated cellulose suspension using optical coherence tomography

A rheometric method based on velocity profiling by optical coherence tomography (OCT) was used in the analysis of rheological and boundary layer flow properties of a 0.5% microfibrillated cellulose (MFC) suspension. The suspension showed typical shear thinning behaviour of MFC in the interior part of the tube, but the measured shear viscosities followed interestingly two successive power laws with an identical flow index (exponent) and a different consistency index. This kind of viscous behaviour, which has not been reported earlier for MFC, is likely related to a sudden structural change of the suspension. The near-wall flow showed existence of a slip layer of 2–12 μm thickness depending on the flow rate. Both the velocity profile measurement and the amplitude data obtained with OCT indicated that the slip layer was related to a concentration gradient appearing near the tube wall. Close to the wall the fluid appeared nearly Newtonian with high shear rates, and the viscosity approached almost that of pure water with decreasing distance from the wall. The flow rates given by a simple model that included the measured yield stress, viscous behavior, and slip behavior, was found to give the measured flow rates with a good accuracy.     

Performance of partially permeable microfiltration membranes under low fouling conditions

Controlling the onset of fouling and concentration polarization is critical in many membrane operations, particularly in the bioseparation area. By using stepping and constant flux experiments, the fouling threshold or `incipient fouling' region was studied for various microfiltration membranes, pH's, and bulk concentrations using bovine serum albumin. Experiments were conducted to try to decouple effects such are porosity and pore size on incipient fouling by using a combination of tracked etched and polyvinylidene difluoride membranes. Changes in protein transmission and wall concentrations near the fouling threshold were also compared across these membranes. While porosity determined the fouling rate after the exceeding the fouling threshold, pore size appear to be an dominant factor in determining level of the fouling threshold itself. The effect of pH also supports the hypothesis that the rejections are initially dominated by membrane–solute interactions but are subsequently modified by protein adsorption to the surface as the wall concentration increases. Repulsive forces between membrane and solute allow greater rejection (greater wall concentration) to be maintained without fouling but did not increase the critical flux substantially. Attractive electrostatic forces allow greater passage of solute (lower wall concentration), but the protein adsorption soon dominated and the onset of fouling occurred much more quickly. Using a conventional concentration polarization model, analysis of the results indicates that the onset of fouling is occurring at a relatively low wall concentrations.     

Shear and NMR experiments of materials with flow behaviour depending on the deformation history

Oxide ceramic masses react to simple shearing with hardening (peptisation: increase in the shear stress with the shear deformation). In the present study the correlation between the increase in the shear stress and the porosity, agglomeration processes and the type of flow are analysed. For this purpose oxide ceramic masses are tested in a shear device especially developed for pastes and analysed by rheometric experiments, NMR methods and particle size analysis. The results support the hypothesis that structural changes (hardening, increase in the mean porosity) of the material during the peptisation mainly depend on the magnitude and not on the kind of the energy input and thus of the type of flow. The fraction of bound (more generally, the immobilised) water increases with the shear displacement. Also crushing of primary particles could be observed. Both the crushing of solid particles causing an increased solid surface and the formation of a three-dimensional gel structure are microscopic effects capable of resulting in the binding or retaining water. On a macroscopic scale these phenomena cause hardening. Magnetic resonance imaging visualises flow-induced agglomerates, which form owing to the shear flow and increase the porosity averaged over the whole sample. After the shear experiment rolls of paste can be seen which indicate that the general assumption of a plane shear flow in the shear device is not warrantable. Received: 19 July 2001 Accepted: 25 October 2001     

Yield Stress Measurement of Dense Pastes with Pipe Transport and Vane

Yield stress is a key rheological parameter of dense paste. Considering practical utility and to estimate the possible slipping flow, yield stress measurements were carried out using the curve extrapolation method and vane method with rheometer in both controlled shear stress (CSS) and controlled shear rate (CSR) model. All measured yield stresses show to increase exponentially with concentration in different scale. The vane method yield stresses are both higher than the extrapolation curve yield stress. For the coal slurry used in this paper, the extrapolation curve yield stress is lower by 17–30% than the dynamic yield stress that obtained under test model CSR. It indicates that wall slip exists in pipeline transportation; meanwhile wall slip can reduce transportation resistance of dense paste to some extent.