Petr Mikulášek

The influence of pH and temperature on the rheology and stability of aqueous titanium dioxide dispersions

Abstract The effects of pH and temperature on the rheology and stability of aqueous dispersions of rutile (titanium dioxide) were investigated systematically. The dispersions exhibit shear-thinning behaviour, conforming to the Herschel-Bulkley or the Casson-type models in the shear rate range investigated. Extreme sensitivity with pH was observed, whereby the dispersion viscosity can be changed by as much as an order of magnitude with pH variation at constant volume fraction. The temperature effect study indicates that the relative contribution of Brownian motion and shear flow to the viscosity is dependent on the flocculation state of the dispersion. The rheological behaviour of the dispersions in this study has been explained in terms of hydrodynamic interactions between particles as well as non-hydrodynamic interactions. These observations have important implications in crossflow microfiltration of these dispersions.

Effects of backflushing conditions on permeate flux in membrane crossflow microfiltration of oil emulsion

The paper reports results of experiments with oil-in-water emulsion, which were carried out in a laboratory crossflow microfiltration unit equipped with backflushed zirconia ceramic membranes. The results demonstrate that the membrane backflushing can maintain the permeate flux at a level which is nearly three-fold over the long-term flux in the absence of membrane backflushing. It was observed that the effect of backflushing was the more pronounced when the backpulse duration was shorter, the transmembrane pressure difference was higher, and the retentate velocity was lower in forward filtration. An optimum backflushing frequency which maximized the average permeate flux was found to be in the range of 1 to 50 s depending on the operating conditions. The magnitude of the transmembrane pressure difference in the reverse flow had a relatively small effect. Attempt has also been made to explain the results in terms of a simple semiempirical model of the process. Parameters evaluated from dynamic and steady state experiments without membrane backflushing were shown to be useful in estimating the performance of the process with membrane backflushing. The influence of backflushing duration and frequency, transmembrane pressure difference, and retentate velocity on average permeate flux were well predicted using this model.

New monodisperse magnetic polymer microspheres biofunctionalized for enzyme catalysis and bioaffinity separations.

Magnetic macroporous PGMA and PHEMA microspheres containing carboxyl groups are synthesized by multi-step swelling and polymerization followed by precipitation of iron oxide inside the pores. The microspheres are characterized by SEM, IR spectroscopy, AAS, and zeta-potential measurements. Their functional groups enable bioactive ligands of various sizes and chemical structures to couple covalently. The applicability of these monodisperse magnetic microspheres in biospecific catalysis and bioaffinity separation is confirmed by coupling with the enzyme trypsin and huIgG. Trypsin-modified magnetic PGMA-COOH and PHEMA-COOH microspheres are investigated in terms of their enzyme activity, operational and storage stability. The presence of IgG molecules on microspheres is confirmed.

Characterization of Ceramic Tubular Membranes by Active Pore-Size Distribution∗

Abstract A description of the use of a liquid displacement (modified bubble-point) method to characterize tubular membrane microfilters in terms of pore sizes and poresize distributions is given. Following a theoretical analysis the method, some comments about pore-size distribution of tubular filters and experimental results for different ceramic microfilters are presented. The characterization studies reported in this paper are applicable to other ceramic membranes, and they represent a significant step in an understanding of ceramic membranes in various applications. ∗Presented during the 11th CHISA Congress, Prague, Czech Republic, 1993.

Crossflow microfiltration of mineral dispersions using ceramic membranes

Abstract Crossflow microflltration experiments were performed on aqueous dispersions of titanium dioxide through a 0.1 Urn pore size ceramic membrane at various operating parameters. The initial transient flux decline follows dead-end filtration theory, with the membrane resistance determined from the initial flux and the cake resistance determined from the rate of flux decline due to cake build-up. For long times, the observed fluxes reached steady- or nearly steady-state values, presumably as a result of the cake growth being arrested by the shear exerted at its surface. The steady-state fluxes increased with increasing inlet crossflow velocity and decreasing feed concentration. Rheological work has shown that the titania dispersions exhibit shear-thinning behaviour. Extreme sensitivity with pH was observed, whereby the dispersion viscosity can be changed by as much as an order of magnitude with pH variation at constant volume fraction. The steady-state permeate flux values were determined from the steady-state model based on the Kozeny—Carman equation for cake resistance and Darcys law applied over the filter area to relate filtration rate to average pressure difference between the feed and permeate sides of the filter. The model includes a cake resistance of the cake layer, which was determined for the titanium dioxide dispersions by fitting the experimental flux data to the model. The resulting fluxes obtained from the model using simple values for the membrane resistance, cake resistance, and rheological parameters for each data set are in good agreement with the measured fluxes.

Crossflow microfiltration of shear-thinning aqueous titanium dioxide dispersions

Crossflow microfiltration experiments were performed on aqueous dispersions of rutile (titanium dioxide) through a 0.1 μm pore size ceramic membrane at various operating parameters. The initial transient flux decline follows deadend filtration theory, with the membrane resistance determined from the initial flux and the cake resistance determined from the rate of flux decline due to cake build-up. For a long time, the observed fluxes reached steady or nearly steady-state values, presumably as a result of the cake growth being arrested by the shear exerted at its surface. The steady-state fluxes increase with increasing inlet crossflow velocity and decreasing feed concentration. The steady-state permeate flux values were determined from the steady-state model based on the Kozeny-Carman equation for cake resistance and Darcys law applied over the filter area to relate filtration rate to average pressure difference between the feed and permeate sides of the filter. The model includes a cake resistance of the cake layer, which was determined for the titanium dioxide dispersions by fitting the experimental flux data to the model. The apparent viscosity of the dispersion, which is an input parameter in the flux model, is also adjusted to account for a model of the Herschel-Bulkley type. The resulting fluxes obtained from the model using simple values for the membrane resistance, cake resistance, and rheological parameters for each data set are in good agreement with the measured fluxes.

The performance of a rotating filter 1. Theoretical analysis of the flow in an annulus with a rotating inner porous wall

Abstract The Navier-Stokes and continuity equations are solved together with Darcys law to analyze pure fluid flow through the membrane in a rotary filtration module. Generation of Taylor vortices is neglected limiting the analysis to flows below the critical Taylor number. The pressure differences across the annulus and the membrane are solved independently. By simplication of the mathematical model, two ordinary differential equations are obtained which are solved numerically using a Runge-Kutta method. The theoretical results are compared with the slit approximation of Belfort et al. [2] and with experimental data [3].

Flux and fouling in the crossflow ceramic membrane microfiltration of polymer colloids

Abstract The results of an experimental study of ceramic membrane microfiltration of synthetic polymer colloids differing greatly in average particle size and distribution broadness are presented. The effects of the polymer nature, pressure difference, feed velocity, and particle to membrane pore diameter ratio on the permeate flux and membrane fouling are discussed. Attempts have also been made to explain the results in terms of simple models of the operative mechanisms.

Gas-liquid two-phase flow in microfiltration mineral tubular membranes: relationship between flux enhancement and hydrodynamic parameters

Abstract An application of the gas—liquid two-phase flow for the flux enhancement during the microfiltration of aqueous titanium dioxide dispersions on an aluminium oxide tubular membrane has been studied. The results of experiments showed a positive effect of the constant gas—liquid two-phase flow on the flux. It might be concluded from the analysis of experimental results based on the dead-end filtration model that a two-phase flow seemed to expand the particle cake as it increased both the cake porosity and thickness, thus allowing higher fluxes. A mathematical model for the flux prediction during two-phase gas—liquid microfiltration has been developed. The results showed a good agreement between experimental data and model prediction.

USE OF A ROTATING FILTER TO ENHANCE CERAMIC MEMBRANE FILTRATION PERFORMANCE OF LATEX DISPERSIONS

Abstract The performances of both the tangential crossflow system and the rotating system with a tubular microfiltration ceramic membrance were investigated experimentally in the separation of latex dispersions. In addition, the shear rate at the membrane surface for azimuthal flow in an annulus with a porous wall on the rotating inner cylinder was solved for the narrow-gap approximation. In contrast to the tangential crossflow system, the rotary system was characterized by a significantly higher flux. The increase in the membrane flux was achieved at shear rates less than those for the crossflow system investigated.