Ingmar H. Huisman

Crossflow Membrane Filtration Enhanced by an External DC Electric Field: A Review

Techniques for fouling prevention in membrane filtration are needed. This paper reviews the process design, theory and some applications of crossflow membrane filtration enhanced by a DC electric field. An electric field can be applied across the membrane or the membrane itself can be the electrode. The performance of the filtration process is, in both cases, primarily improved due to electrophoresis. Electro-osmosis was found to be significant in some cases when an electric field was applied across the membrane. The theory of electrically enhanced crossflow membrane filtration is quite complex and there is no generally accepted model describing this process. Significant enhancement of the flux compared with the flux with no electric field has been found in the filtration of particles or colloids with high electrophoretic mobility using a pressure at which the cake was formed without an electric field and an electric field strength at which the net particle migration was away from the membrane. Apart from enhancement of the flux, the quality of the permeate has been found to improve by applying an electric field.

Determining the zeta-potential of ceramic microfiltration membranes using the electroviscous effect

Abstract The possibility of measuring the zeta-potentials of porous membranes using the electroviscous effect was investigated. The zeta-potential of Membralox® ceramic microfiltration membranes was determined both with the newly developed electroviscous technique and by streaming potential measurements. It was found that the electroviscous technique provided a simple means of obtaining accurate values of zeta-potential, especially for higher zeta-potentials. The streaming potential measurements were found to be more suitable for the determination of the iso-electric point, i.e. the pH at which the zeta-potential is zero. The iso-electric points of new α-alumina, zirconia, and titania membranes were found to be 8.5, 8.0, and 6.3, respectively. Upon using the membranes and cleaning them with a detergent, the iso-electric point of the α-alumina membrane decreased to 6.5, and that of the zirconia membrane decreased to 5.2, while the iso-electric point of the titania membrane stayed virtually constant. Cleaning these membranes with a strong acid or base could not reverse the observed decreases in iso-electric point.

Water permeability in ultrafiltration and microfiltration: Viscous and electroviscous effects

Abstract The applicability of Darcys law for explaining the water permeabilities of polymeric UF membranes and ceramic MF membranes was investigated at various temperatures, viscosities, salt concentrations, and trasmembrane pressures. It was found that Darcys law explains the permeabilities well if the viscosity is corrected for electroviscous effects, temperature, and solute concentration. For polymeric UF membranes the compaction of the membranes needs to be taken into account as well. Ceramic MF membranes do not seem compressible for TMPs up to 80 kPa. Increasing the salt concentration from 30 μM to 0.1 M resulted in increases in water fluxes of 2% to 8% both for MF and UF membranes. This apparent permeability increase was explained by electroviscous effects: increased salt concentrations lead to lower zeta-potentials and thinner double-layers, offering less resistance to water passage. From the apparent permeability change the zeta-potentials of the membranes at pH ≈ 7 were calculated. Realistic zeta-potential values were obtained. Water flux measurements at various salt concentrations are thus a simple method to study zeta-potentials of membranes. The resistance (Rm) of the UF membranes was independent of temperature in the range 4–20°C, but increased with increasing transmembrane pressure (TMP). The increase could be described by a power law ΔRm∼TMP0.8, typical for porous solids. The resistance of the ceramic MF membrane was independent of temperature and independent of TMP.

The influence of the membrane zeta potential on the critical flux for crossflow microfiltration of particle suspensions

Abstract It was investigated how the value of zeta potential of microfiltration membranes influenced the critical flux while filtering a suspension of silica particles. Ceramic membranes of three different materials were used and measurements were performed at two different values of pH for each material corresponding to two different values of zeta potential for each material. It was found that neither the zeta potential of the membrane nor the zeta potential of the particles influenced the observed critical flux. The critical flux increased linearly with the wall shear stress and decreased with increasing particle concentration. Expressions were obtained for the critical flux based on two different particle transport mechanisms: particle rolling (torque-balance model) and shear-induced diffusion. It was found that neither of the expressions could explain experimental critical fluxes adequately.

Particle transport in crossflow microfiltration – II. Effects of particle–particle interactions

Abstract A model previously developed for the calculation of limiting fluxes for crossflow microfiltration of non–interacting particles was extended to include the effect of physico-chemical particle–particle interactions. It was shown theoretically that the effect of particle–particle interactions on the microfiltration flux can be described by a diffusion-type equation with an effective interactioninduced diffusion coefficient. The microfiltration flux for a general situation, where particle transport is caused by convection, Brownian diffusion, shear-induced diffusion, and particle–particle interactions, was then calculated by adding the diffusion coefficients, and solving the governing convective-diffusion equation numerically. Results of these calculations agreed very well with experimental fluxes measured during crossflow microfiltration of model silica particle suspensions. The influence of wall shear stress, membrane length, particle size, and particle concentration on permeate flux was very well predicted. However, the effect of particle surface potential was quantitatively underpredicted. Shear-induced diffusion seems to be the main transport mechanism governing the flux in the microfiltration of suspensions of micron-sized particles, but charge effects can increase fluxes considerably.

Predicting limiting flux of skim milk in crossflow microfiltration

Four models for back-transport mechanisms in crossflow microfiltration have been investigated concerning their ability to predict the limiting permeate flux for skim milk. A tubular, ceramic membrane was used to measure the limiting fluxes for a series of crossflow velocities at two temperatures. One of the models — the shear-induced diffusion model — predicts values of the limiting flux close to our experimental values both at 55 and 15°C. The best prediction of the limiting flux is obtained by the empirical relation: flux=Re · 6.94×10−10 m/s.

Particle transport in crossflow microfiltration – I. Effects of hydrodynamics and diffusion

The limiting flux in the crossflow microfiltration of particle suspensions was calculated by numerically solving the convective-diffusion equation governing concentration polarisation. It was assumed that a cake layer is formed at the membrane surface, and that particles are transported towards the membrane by convection, and transported away from the membrane by Brownian diffusion or shear-induced diffusion, or a combination of both these mechanisms. The numerical results for Brownian diffusion and for shear-induced diffusion could be summarised by approximate equations which related the predicted flux to parameters such as wall shear stress, bulk concentration, and membrane length. Both the approximate equations and the exact numerical results compared well with fluxes measured in crossflow microfiltration of non-interacting spherical silica particles. It was shown theoretically that turbulent diffusion contributes only marginally to particle transport in the concentration polarisation layer. Microfiltration fluxes for turbulent flow conditions were therefore predicted accurately by the numerical calculations, although turbulent diffusion was neglected.

Properties of the cake layer formed during crossflow microfiltration

Abstract The conditions necessary for the formation of a reversible cake layer during crossflow microfiltration were studied both experimentally and theoretically. Crossflow microfiltration experiments were performed with suspensions of silica particles with a narrow size distribution. The steady-state flux was first measured at a low transmembrane pressure (TMP), then at increased TMP, and again at the original low TMP. The cake-layer thickness was measured indirectly using a light absorbance technique. The thickness of the cake layer increased with increased TMP. Upon decreasing the TMP, the cake-layer thickness either decreased (reversible cake), or stayed constant (irreversible cake). It was shown that irreversible cakes are formed when the silica particles have a relatively low charge, whereas reversible cakes are formed when the silica particles have a relatively high charge. The occurrence of irreversible cakes is unexpected, since approaching silica particles are reported to always repel each other. The irreversibility of the cakes was explained by the assumption that bridging between the particles can occur, causing the interparticle interaction to be attractive when the particles retreat. To explain the reversibility results quantitatively, a model was developed which links the physicochemical interaction forces of the silica particles to the permeate flux through the cake layer. A detailed description of the interaction forces of silica particles was given in order to feed this model with accurate parameters. A reversibility index was introduced which quantifies the amount of reversibility. Model calculations of the reversibility index were in excellent agreement with measurements.

Determining the zeta potential of ultrafiltration membranes using their salt retention

Abstract A theory was developed that relates the salt retention of porous membranes to the membrane zeta potential. This theory predicts that the salt retention of charged ultrafiltration and microfiltration membranes may differ from zero, because of the existence of a double layer of ions near the charged pore surfaces. Calculated values of salt retention were found to increase with decreasing ionic strength (i.e. increasing double layer thickness) and with increasing zeta potential. Based on this theory, the zeta potential of cellulose acetate and ceramic ultrafiltration membranes were calculated from experimentally obtained salt retention values. Realistic values of zeta potential were obtained.

Design of a Crossflow Microfiltration Unit for Studies of Flux and Particle Transport

This paper describes the design, construction, and performance of a crossflow microfiltration rig that allows accurate measurement and control of transmembrane pressure (TMP), crossflow velocity, permeate flux, and the amount of particle deposition. The rig uses the uniform TMP concept, which allows the TMP to be independent of the position along the membrane. The performance of the rig filtering a well defined particle suspension is reported. It was found that both flux and amount of matter deposited reached constant values after about an hour. The level of the steady-state flux was found to depend on the pH of the suspension, and to increase with increasing pH. The correlation observed between flux and amount of matter deposited suggested that the void fraction of the cake is not dependent, or only slightly dependent on pH; the observed differences in flux being caused by differences in thickness of the cake, not by differences in void fraction.