Petr Doleček

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.

Permeate flow in hexagonal 19-channel inorganic membrane under filtration and backflush operating modes

Numerical simulations and experimental study of incompressible Newtonian permeate flow in porous support of hexagonal 19-channel inorganic membrane are presented both for filtration and backflush operating modes. Under several simplifying assumptions the problem could be treated as two-dimensional potential flow. The mathematical model was solved using finite element method. The results of numerical simulations show that the contributions of particular channels to the total permeate flux are not equal and depend on the ratio of skin layer to porous support permeability as well as on the distance of a channel from the membrane outer surface. For membranes with high permeability of skin layer there is an area of nearly constant pressure around inner channels and their contribution to total flux is negligible. This effect will probably be more pronounced in backflush operating mode while in filtration mode possible dynamic membrane adds a resistance to that of skin layer which leads to more uniform permeate flux distribution. Qualitative trends of the numerical simulations were verified by experiments with ceramic 19-channel membranes of Membralox® type in backflush operating mode.

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.

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].

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.

Drag and fall velocity of a spherical particle in generalized newtonian and viscoplastic fluids

Abstract An approach to the calculation of drag and fall velocity of a spherical particle in generalized Newtonian and viscoplastic fluids is suggested. It is based on the application of the modified Rabinowitsch–Mooney equation together with the corresponding relations for consistency variables. The usefulness of this approach has been verified for the Ellis and Bingham fluids. The solution has been used to suggest the corresponding relationships for the calculation of pressure drop in the flow of a viscoplastic fluid through a random fixed bed of particles.

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.

Creeping flow of viscoelastic fluid through fixed beds of particles

Abstract A method is presented for the pressure drop calculation during the viscoelastic fluid (VEF) flow through fixed beds of particles. It is based on the application of the modified Rabinowitsch–Mooney equation together with the corresponding relations for consistency variables. The course of dependence of dimensionless quantity coming from the momentum balance and expressing the influence of elastic effects on the suitably defined elasticity number is determined experimentally. The satisfactory validity of the approach suggested has been verified for pseudoplastic viscoelastic fluids characterized by the power-law flow model.

Poiseuille flow of purely viscous non-Newtonian fluids through ducts of non-circular cross section

Abstract In this paper, the relationships are presented for an approximate calculation of pressure drop–flow rate dependence and maximum fluid velocity in Poiseuille flow of power-law and Robertson–Stiff fluids through ducts of noncircular cross section. The suitability of the relationships derived is demonstrated by the comparison of calculated data of pressure drop and maximum fluid velocity with those taken from literature for various duct geometries.