Microfiltration Flux Research Articles

Microfiltration-Rate Increase of O/W Emulsion by Use of Rotating Membrane Filter with Additional Suspended Solids.

The high-flux dynamic microfiltration of an oil-in-water emulsion was studied under constant pressure by using a rotating cylinder type membrane filter with a suspending particle bed in the filter chamber. Using this dynamic filtration, much smaller hydraulic cake-resistance can be obtained than that encountered in conventional operation. It was quickly discovered that, due to the powerful sweeping force acting on the fouling cake-layer, the dynamic effects of the additional suspended solids yield remarkable improvements in the microfiltration flux of poorly filterable O/W systems.

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.

Steady-state permeate flux of cross-flow microfiltration

A membrane filtration cell was employed in this study to investigate the steady-state permeate flux of cross-flow microfiltration under various operating conditions, including wall shear rate, particle size, membrane pore size, and electrolyte concentration. Spherical polystyrene latex particles were used as the suspension particles in the experiments. The flow of the suspension in the filtration channel was controlled under the laminar flow region. It was observed in the experiments that a true steady state was not reached due to a small amount of impurities in deionized water. Consequently, a quasi-steady state was assumed, and it was obtained experimentally for each operating condition. The quasi-steady-state permeate flux increased with increasing wall shear rate or membrane pore size, but decreased with an increase in electrolyte concentration in the suspension. Furthermore, a mathematical model based on a moment balance of the hydrodynamic and interparticle forces exerted on a single spherical particle retained on the surface of the membrane was developed and used to predict the quasi-steady-state permeate flux satisfactorily.

Clarification of Green Tea Extract by Microfiltration and Ultrafiltration

Membrane clarification of green tea extract was studied as a treatment to reduce sediments in packaged drinks and as a pretreatment for concentration processes. The flux and variation of components were examined in dead-end and crossflow filtration with several types of membranes. In dead-end ultrafiltration, the flux reduction rate was small, although the initial flux was similar to the final flux in microfiltration. Prefiltration was effective in decreasing the reduction rate of flux. As the pore size of microfiltration membranes became smaller, the dry weight decreased gradually and the optical transmission at 660 nm increased. By ultrafiltration, 30–50% pectin, 3–11% catechins and, 7–20% caffeine were rejected. Crossflow filtration was effective in keeping the flux high. The ultrafiltration spiral membrane (pore size: 0.008 μm) was selected for repeated batch clarification of prefiltered green tea crude extract and showed reproducible performance.

Influence of shear breakage of microbial cells on cross-flow microfiltration flux

In the cross-flow filtration of fermentation broth, cross-flow pumping applies shear stress onto bacterial cells. The resulting shear field leads to breakage of bacterial cells. In this study, the influence of cell breakage due to shear stress on filtration flux was examined. Shear stress broke baker's yeast cells and induced discharge of granulated matter such as glycogen and cell wall fragments. These smaller particles contributed to the increase of the mean specific filtration resistance. In the cross-flow filtration, cell breakage due to shear stress reduced the filtration flux because of the increase of the hydraulic resistance of the particle-packed layer, R c, which was formed on the membrane during filtration. This flux reduction was explained by considering the lift velocities, which back-transported the particles from the membrane surface. The lift velocity was expressed as follows: V L = K· u 1.0· C p −0.5· d p 0.67· η −1.0, where V L, K, u, C p, d p and η are the lift velocity, filtration constant, feed velocity, particle concentration, particle size and viscosity, respectively. In the mixture of differently sized particles such as a shear-broken cell suspension, the steady-state flux was given as a value equivalent to the smallest V L, which was derived from the calculation of the V L values for each particle representing particles grouped according to size, using the above equation.

The transversal flow microfiltration module. Theory, design, realization and experiments

This study shows that the permeate flux in microfiltration can be inscreased without increasing energy consumption, by using the hollow fiber membranes themselves as turbulence promoters. This is realized by using shell-side fed hollow fibers, which are placed perpendicular to the feed stream. This module type is called the transversal flow module. The theoretical benefits of such a module have been calculated by applying the model proposed by Zydney and Colton to predict the permeate flux. Furthermore the technical realization of a module with a filtering area of 0.5 m 2 as well as the first preliminary microfiltration results will be discussed.

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.

Crossflow microfiltration of animal cells

Laminar shear is the primary mechanism of cell damage, limiting flow rate (and hence flux) in crossflow microfiltration of animal cells. Sensitivity to hydrodynamic and interfacial stress is reduced by the addition of 0.1% Pluronic polyol. A critical average wall shear rate of 3000 s(-1) (above which damage occurs) is found for several cell types, including mammalian and insect cells. Hydrodynamic stress also limits the maximum tip speed in a rotary lobe pump to less than 350 cm/s. Turbulent flow in the recirculation loop piping at Reynolds numbers of up to 71,000 does not cause cell damage. Maximum sustainable flux decreases with cell concentration and increases with cell size (in qualitative agreement with the hydrodynamic lift model). A flux of 30 to 75 L/m(2) h (depending on cell size) can be sustained during 20-fold concentration from 2.5 x 10(6) cells/ml, while maintaining high cell viability.

Yeast cell harvesting from cider using microfiltration

AbstractYeast cell harvesting from cider was performed with a 0.2 μm pore size hydrophilic polyvinylidene fluoride membrane. The microfiltration (MF) membrane exhibited a large flux decline during the first 20 min of operation when challenged with a fermentation broth. This may be attributed to rapid particle polarization initiating membrane fouling. Pore plugging may be more likely to have reduced the flux throughout the experiments than cake formation on top of the membrane, as cross‐flow controls cell build‐up on the membrane and removes some of the cells from the layer.A small proportion of yeast cells were not retained by the membrane; this was attributed to a proportion (1 in 10 000) of the membrane pores being oversize. Aeration during fermentation affected subsequent fouling during MF, probably through alteration of the ability of the microorganism to attach on to the membrane. A high proportion of nonviable cells typical of an aged broth also improved the flux performance, suggesting that any fermentation change having an effect on the yeast cell surface could influence the MF flux.

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.