Flux In Crossflow Microfiltration Research Articles

Reducing the membrane fouling in cross-flow filtration using a facile fluidic oscillator

Membrane fouling is a main factor that hinders the wide spread adoption of membrane filtration process. In this study, we demonstrate that a passive fluidic oscillator can effectively reduce the fouling and enhance the permeate flux in cross-flow microfiltration. The oscillator produces a pulsating flow of frequency 56–115 Hz and pressure amplitude 500–700 kPa, which leads to transient high shear rates, surface scouring and back-flushing effects. As a result, it mitigates the concentration polarization, prevents micropores clogging, and inhibits the formation of fouling layer. Its applications for filtration of an inorganic particulate suspension and an organic macromolecular solution, as well as the effect of the operating pressure of the oscillator and the membrane material were investigated. In comparison with the static filtration, the membrane fouling was significantly reduced for both the feed solutions using the oscillator. The current method was further tested for recovery of bacterial capsular polysaccharide from 100 L of pneumococcus fermentation broth. The recovery ratio was improved from 2.0% to 39.3% using the conventional method to around 100% with the oscillator. The process time was also greatly reduced by 30%. Furthermore, though the oscillator provides dynamic filtration, it works under constant pressure without the need of additional motors and stirrers. Potentially, it could be applied to some currently existing filtration systems, to improve the flux performance but without increasing too much the system complexity.

Numerical simulations of lift force and drag force on a particle in cross-flow microfiltration of colloidal suspensions to understand limiting flux

The aim of this study was to understand the limiting flux in cross-flow microfiltration of a colloidal suspension in terms of the balance of hydrodynamic forces on a particle. Numerical simulations were carried out to estimate the lift force resulting from the cross-flow, drag force resulting from the convective flux, and net force. Assuming that the limiting flux is the flux when the net force on a particle is zero, we analyzed the simulation data to determine the correlation of two particle Reynolds numbers: Rep_Jlimit, calculated from the limiting flux, and Rep_uδ, calculated from the cross-flow rate. The results indicate that Rep_Jlimit is proportional to the square of Rep_uδ when a particle is at a dimensionless distance (distance divided by diameter) of less than 6.7 from the cake layer, in spite of the experimental fact that Rep_Jlimit is proportional to the 1.5th power of Rep_uδ. In contrast, Rep_Jlimit is proportional to the 1.5th power of Rep_uδ when a particle is at a dimensionless distance of 48–60. This means that the force balance of a particle at such a position, not at the membrane surface, determines the limiting flux in cross-flow microfiltration of a colloidal suspension.

Empirical modeling coupled with pore blocking for predicting cake formation of electric field effects on oily waste water cross-flow microfiltration

Cake formation is the most important phenomenon to reduce cross-membrane flux in cross-flow microfiltration although pore blocking and concentration polarization (CP) phenomena affect it. Electric field may be used to enhance process efficiency and decreased fouling phenomena in the process. An empirical model is developed here taking into consideration the cake formation effects on permeate flux reduction in the presence of AC electric field. A semi-empirical model is also developed in the presence and absence of electric field at two cases; with and without considering membrane structure in the modeling. It is the first time that electric field effects on microfiltration performance are modeled macroscopically and used with pore blocking phenomenon simultaneously. The predicted permeate fluxes by the models are compared against experimental permeate fluxes in absence and presence of electric field. Comparison of the model based predictions against experimental results indicate semi-empirical model precision with a maximum error of 8.9% for the case without and 3.6% for the case with pore blocking consideration and a maximum error for empirical model of 11.9%. Effects of different electric potentials and distances between the electrodes on CP layer thickness and oil concentration on membrane surface are also studied. The predicted results showed that the CP layer thickness is decreased by 60.9% through applying an electric potential of 500 V and also decreased by 65.2% through decreasing the distance between the electrodes from 2.3 to 1.3 mm at 200 V. The oil concentration on the membrane surface is also decreased by 72.3% as a result of applying the electric field while it is decreased by 19.2% when the electrodes distance is decreased from 2.3 to 1.3 mm at 200 V.

3D printing design of turbulence promoters in a cross-flow microfiltration system for fine particles removal

In this study, we designed three turbulence promoters using three-dimensional (3D) printing technology for cross-flow microfiltration. Designing turbulence promoters with optimal geometry for microfiltration systems remains a key challenge. The effects of the operating conditions, such as cross-flow velocity, transmembrane pressure and geometry of the turbulence promoter on the filtration flux and cake properties were investigated. The distribution of the fluid velocity was examined using computational fluid dynamics (CFD). For normal microfiltration tests without promoters, the filtration flux merely increased by 10% when the cross-flow velocity increased from 0.1 to 0.5 m/s under a transmembrane pressure of 20 kPa. Therefore, the turbulence promoters were individually added into the channel of the microfiltration device to induce vortices in the flow stream and increase cross-flow velocity. Three turbulence promoters with different configurations (circular, diamond and elliptic) were made with a 3D printer. The elliptic type of promoter with a hydraulic angle of 90° displays the flux enhancement by approximately 30–64% under a transmembrane pressure of 20 kPa compared to the normal type of microfiltration, whereas the diamond type of promoter with a hydraulic angle of 60° shows a lower flux enhancement by approximately 7–16% under the same transmembrane pressure. Overall, adding the turbulence promoter to the microfiltration module shows a great potential to significantly mitigate membrane fouling and enhance filtration flux in cross-flow microfiltration.

A comparison of cake properties in traditional and turbulence promoter assisted microfiltration of particulate suspensions

The use of turbulence promoter can effectively enhance the permeate flux in crossflow microfiltration (CFMF) of particulate suspensions. Flux enhancement which is generally attributed to the reduction in cake thickness, however, has still not been clearly understood. In this study, the effects of turbulence promoter on cake properties (thickness, porosity and particle size) were investigated during CFMF of calcium carbonate suspension. It indicates that turbulence promoter has important effects on cake properties that directly affect the cake resistance. The significant reduction in thickness and slight increase in porosity are positive to reduce the cake resistance. The remarkable decrease in particle size is the negative effect due to the increased specific resistance of a cake. As a whole, the overall cake resistance is still diminished by turbulence promoter and therefore permeate flux is improved. The theoretical calculation of cake resistance shows a good consistence with the experimental result. The cake properties in both cases (using a turbulence promoter or not) almost exhibit the similar trends under various operating conditions. Differently, the use of turbulence promoter can greatly alleviate the effects of transmembrane pressure or feed concentration on growth of cake layer and intensify the effects of inlet velocity on diminishing the particle deposition.

Effects of operating parameters on fouling mechanism and membrane flux in cross-flow microfiltration of whey

In this paper, the dependency of permeate flux decline on operating parameters in cross-flow microfiltration of whey at constant operating pressure was studied. The governing fouling mechanisms and the effect of operating conditions on these mechanisms were investigated using linear forms of classic models as well as two-phase combined fouling models. A microfiltration set-up with flat module and Polyethersulfone (PES) membrane with 0.45 μm mean pore size was used. The results showed the positive effect of Reynolds number and operating pressure toward an optimum value of permeate flux. The optimum condition in this study was the highest Reynolds number (Re = 2500) and operating pressure (P = 1.5 bar). By classic models, for almost all experiments, flux decline was consistent with the “Standard blocking” model during the earlier stages and with the “Cake filtration” model during the later stages of filtration. Agreement with the “Cake filtration” model improved with increasing the operating pressure. In combined models, “Complete blocking–Cake filtration” and “Intermediate blocking–Cake filtration” were the best governing models in this system, but only at low Reynolds numbers and high operating pressures.

Impact of colloidal interactions on the flux in cross-flow microfiltration of milk at different pH values: A surface energy approach

The effect of colloidal interactions between casein micelles on the flux of a tubular ceramic membrane at cross-flow microfiltration (MF) of skim milk was studied. Filtration experiments were performed at a pH range of 6.8–5.9. Compared to filtration of milk at its native pH (6.8), the flux was reduced and membrane fouling proceeded faster when acidified milk was filtered. To explain the observed flux behavior, a new interaction model for casein micelles was developed, which incorporates on the basis of the extended DLVO (xDLVO) theory hydrophilic and hydrophobic interactions in the form of Lewis acid–base forces, which were derived from protein surface energies. It could be shown that deposit layer build-up is strongly influenced by the charge-dependent protein surface hydrophilicity, whereas electrostatic interactions between proteins can be neglected in high ionic strength fluids like milk.

The convective model of flux prediction in a hollow-fiber module for a steady-state cross-flow microfiltration system

A simple semi-theoretical steady-state model for the convective flux in cross-flow microfiltration has been developed based on the mass balance. The model, which has two coefficients, A and K, can describes the cake increase on membrane surfaces in a hollow-fiber membrane module and predicts its membrane flux decline. Then this model was validated by using yeast suspensions in a 0.1 μm hydrophilic polyvinylidene fluoride (PVDF) hollowfiber membrane module with a 0.3 m 2 filtration area. The experimental and analysis results showed that the two coefficients, A and K, actually varied in a small range, but when the overall average values of A and K were taken into the model as two constants under the different operating conditions, the experimental observation is in good accordance with the model prediction and the absolute value of relative error is less than 7%. So it was advisable to consider the model coefficients, A and K, as two constants in the model to predict the steady-state flux decline in industrial applications.

非対称多孔質ガラス膜の調製と精密ろ過における膜構造の効果

In order to improve the large membrane resistance of symmetric conventional porous glass membrane in microfiltration, new asymmetric type of porous glass membrane was prepared by use of two kinds of NaO-CaO-Al2O3-B2O3-ZrO2-SiO2 glass with different growth rate of phase separation, and was characterized for separative membrane.Growth rate of separated phase from the primary glass composition decreased with increasing content of Al2O3. Therefore, Al2O3-rich glass and Al2O3-poor glass were laminated in two layers to be formed into a tube. The primary glass tube was thermal-treated to cause the phase separation, and the separated phase was leached out with an acid. Both skin layer and support layer of the asymmetric porous glass membrane had the uniformly controlled pore size. Skin layer with 15 μm in thickness was formed inside the membrane.The pure water flux through the asymmetric porous glass membrane were much higher about 10 times of that through conventional symmetric porous glass membrane, because of decreasing of membrane resistance. The flux in crossflow microfiltration of acrylic resin suspension were much higher about 2 times of those through symmetric membrane. High permeability of the asymmetric porous glass membrane was confirmed through these measurements.

Experimental determination of critical flux in cross-flow microfiltration

In this study, two definitions of critical flux were introduced based on the cross-flow microfiltration (CFMF) experiments conducted under an operational mode of constant permeate flux. The critical flux based on mass balance was calculated from the rate of particle deposition at a certain permeate flux. Below the critical flux, there will be no particle deposition on the membranes. The other was defined on the basis of the increase in transmembrane pressure (TMP). The critical flux based on the TMP increase is the flux below which membrane fouling does not occur. The critical flux based on both definitions was evaluated from experiments with different particle size, membrane pore size, influent concentration and ionic strength of the influent suspension.

An enhancement of critical flux in crossflow microfiltration with a pretreatment of floating medium flocculator/prefilter

In this study, use of a combined system of flocculation-microfiltration was assessed for its capability in removing solids and organics. The improvement of flux by the preflocculation was also investigated. The experimental set-up consisted of (i) Millipore flat plate microfiltration module with constant permeate withdrawal arrangement and (ii) a floating medium flocculator/filter consisting of polypropylene beads as a pretreatment. Commercial ferric chloride(FeCl3) was used for flocculation. The purpose of floating medium filter was primarily to produce filterable flocs and also to achieve partial solids and organics removal. The critical flux of kaolin clay suspension of 10–100mg/L was measured for membrane of pore size of 0.2μm in the presence of 0–4mg/L of fulvic acid in the suspension without any pretreatment. The critical flux was found to decrease with the increase in kaolin clay concentration. The results also indicated that there was no significant effect of organic (fulvic acid) concentration on critical flux. The pretreatment of floating medium filter alone without flocculant addition removed 30–40% of solids and resulted in a significant improvement of critical flux in crossflow microfiltration. Degree of solids and organic removal and the variation in critical flux were then studied in detail for the preflocculated/filtered kaolin clay suspension in the presence of organics. The pretreatment of flocculation/prefiltration removed approximately 50% of organics while producing uniform microflocs of 13–16 μm size. In addition, it enhanced the critical flux by 70% and resulted in 30–70% of the remaining organic removal in the crossflow microfiltration.

An enhancement of critical flux in crossflow microfiltration with a pretreatment of floating medium flocculator/prefilter

In this study, use of a combined system of flocculation-microfiltration was assessed for its capability in removing solids and organics. The improvement of flux by the preflocculation was also investigated. The experimental set-up consisted of (i) Millipore flat plate microfiltration module with constant permeate withdrawal arrangement and (ii) a floating medium flocculator/filter consisting of polypropylene beads as a pretreatment. Commercial ferric chloride(FeCl3) was used for flocculation. The purpose of floating medium filter was primarily to produce filterable flocs and also to achieve partial solids and organics removal. The critical flux of kaolin clay suspension of 10 – 100mg/L was measured for membrane of pore size of 0.2μm in the presence of 0 – 4mg/L of fulvic acid in the suspension without any pretreatment. The critical flux was found to decrease with the increase in kaolin clay concentration. The results also indicated that there was no significant effect of organic (fulvic acid) concentration on critical flux. The pretreatment of floating medium filter alone without flocculant addition removed 30 – 40% of solids and resulted in a significant improvement of critical flux in crossflow microfiltration. Degree of solids and organic removal and the variation in critical flux were then studied in detail for the preflocculated/filtered kaolin clay suspension in the presence of organics. The pretreatment of flocculation/prefiltration removed approximately 50% of organics while producing uniform microflocs of 13 – 16μm size. In addition, it enhanced the critical flux by 70% and resulted in 30 – 70% of the remaining organic removal in the crossflow microfiltration.

New decline pattern of filtrate flux in cross-flow microfiltration of dilute suspension

The cross-flow microfiltration flux is assumed to fall smoothly with a decreasing slope from startup to the steady state. Under actual operating conditions beyond a critical level, however, an anomalous decline pattern arises; initially cross-flow medium filtration occurs and then thin-cake filtration. At the earlier stage, fine particles in polydispersed suspensoids are filtered out on the surface of the membrane and/or captured within the latter, clogging the membrane pores gradually. After the membrane surface is covered by the deposit, further filtration occurs by the buildup of filter cake on the initial deposit. Among the many operating variables, three major controlling factors for the earlier stage are confirmed: solids concentration, relative size of solids and pores, and relative force of sweeping and capturing of suspensoids. Because suspended solids are separated by medium- and cake-law filtration in a series, a method for evaluating the time dependence of flux with the new pattern is derived. Reliable flux analysis is achieved for the transient flux of the dilute suspension forming a low compressibility fouling layer, which helps understand efficient flux performance.

Filtrate flux in crossflow microfiltration of dilute suspension forming a highly compressible fouling cake-layer

The crossflow microfiltration characteristics of dilute suspension, which produces a highly compressible fouling cake-layer and/or a highly viscous filtrate, is studied under constant pressure. With the filtration conditions within a critical level, medium filtration at an earlier stage ceases, and crossflow microfiltration follows the principle of thin-cake filtration after operation begins. An equation for flux, which relates the cake resistance to a number of operational variables, is determined for the thin-cake filtration process by introducing a controlling parameter composed of the shear stress exerted at the cake surface and the coefficient of constant-pressure filtration. On the basis of an empirical correlation of the flux-controlling parameters, the effects of both cake-compressibility and filtrate-viscosity on the flux are evaluated.

Unsteady-State Permeate Flux of Crossflow Microfiltration: Effect of Particle Size Distribution

Abstract A mathematical model based on a hydrodynamic theory and mass balance was developed for the prediction of the unsteady-state permeate flux in crossflow microfiltration under the influence of particle size distribution. Experiments were also conducted in a membrane filtration cell to verify this model. Spherical polystyrene latex particles of 0.303, 0.606, and 1.020 μm were used to make suspensions of various particle size distributions. The flow of the suspension in the channel of the filtration cell was controlled under the laminar flow region. It was found that the unsteady-state permeate flux increased as the mean particle size of the suspension was increased. Moreover, the model predicted satisfactorily the unsteady-state permeate flux under the effect of particle size distribution.

A comprehensive model for cross-flow filtration incorporating polydispersity of the influent

A semi-empirical model is presented for the prediction of foulant deposit thickness and concentration, and thereby, predicting the permeate flux in cross-flow microfiltration and ultrafiltration. The scope of the model is limited to colloidal suspensions (i.e. non-diffusive solutes) and its hallmark is its applicability to polydispersed influents. The theoretical model was verified with experimental observations and found to yield a fairly good agreement with an error less than 62% of the average flux over 100 min of filtration. However, evaluation of model coefficients a and b is necessary through laboratory studies. The model was successfully used to predict the effect of particle size distribution ( PSD) on permeate flux. The simulation results corroborate the contention that the long-term flux decline is due to the reduction is foulant deposit permeability via infiltration of fines into the cake, thereby demonstrating the importance of influent particle size distribution ( PSD) on foulant deposit permeability. The proposed model is a first step towards the modeling of polydispersed influents. As such there is a lot of scope for further refinements which is briefly discussed.

Hydraulic resistance of red cell beds in an unstirred filtration cell

A stagnant batch filtration system, in which saline is filtered through an unstirred bed of red cells at constant pressure, was employed to study the hydraulic permeability of red cell beds as well as the extent of cellular blockage of membrane pores. Since red cells are highly deformable, the compressibility of the cell bed was characterized by porosity measurements in centrifuged cell beds. Red cell deformation was also examined by microscopic observation of the cell bed fixed during filtration. Photomicrographs provided evidence that the red blood cells deform into a hexagonal close-packed structure. A simple two-dimensional model for cell bed compressibility developed on the basis of this observation was in good agreement with porosity data taken in the centrifugal system. Filtration data were analyzed using an integrated form of Darcy's law which permits the effects of cell bed hydraulic resistance of pore blockage to be evaluated separately. Functional relationships between hydraulic permeability, porosity, and compressive pressure were developed which can be incorporated into a detailed model for the filtrate flux in cross-flow microfiltration of red cell suspensions.