Michael J. Clifton

A unifying model for concentration polarization, gel-layer formation and particle deposition in cross-flow membrane filtration of colloidal suspensions

Abstract A model is proposed to describe the cross-flow filtration of colloidal particles and molecules. This two-dimensional model depicts both concentration polarization and gel or cake formation in a tubular filtration device. A description of transport phenomena in a concentrated colloidal suspension is the core of the model. Surface and hydrodynamic interactions are used to predict the variation of the osmotic pressure and diffusion coefficient with the volume fraction of the suspension. The mathematical development leads to an analytical equation used for calculating the stationary permeate flux from integral calculations. The two-dimensional concentration profile along the membrane, together with the corresponding permeate flux is obtained. This paper illustrates how mass transfer equations coupled with a realistic description of the fluid can describe both concentration polarization and gel or cake formation. The paper includes a discussion on the differences between limiting and critical fluxes, and between particles and macromolecular cross-flow filtrations.

Growth of the polarization layer in ultrafiltration with hollow-fibre membranes☆

Abstract A mathematical model is proposed which represents the concentration polarization phenomenon in ultrafiltration. A hollow-fibre membrane geometry has been chosen because it allows well-defined hydrodynamic conditions to be obtained and simplifies the measurement of local membrane permeation rates, and also because its mathematical description, which requires a number of special considerations, has rarely been attempted. Hollow-fibre modules have been built which are divided, on the low-pressure side, into a number of compartments; this allows the measurement of local permeation rates at different positions along the length of the fibre bundle. Experimental values of the local permeation rates are compared with the values predicted by the mathematical model.

Mass transfer improvement by secondary flows: Dean vortices in coiled tubular membranes

A helically wound hollow-fibre (or tubular) membrane module was studied in an oxygenation operation with water flowing in the laminar regime inside the tube. Data are compared with a conventional module where straight hollow-fibre membranes are in parallel alignment. In the former design, the results are analyzed taking into account the formation of secondary flow (Dean vortices) and a new mass transfer correlation is presented. In the latter design, results are consistent with the Leveque equation. It is shown that the presence of vortices gives better performance in terms of oxygen transfer. Improvement factors were in the range of 2 to 4.

From ultrafiltration to nanofiltration hollow fiber membranes: a continuous UV-photografting process☆

A new way to prepare nanofiltration membranes, consisting of in-line external modification of the skin of a polysulfone ultrafiltration hollow fibre skin, is described. The paper presents a continuous process (dip-coating followed by photografting) and the influence of the operating conditions on the membrane characteristics. This study is focused on a simplified model based on the evaluation of the two sources of monomer available for grafting: one is monomer contained in the thin film drawn from the dip bath, second is contained in the pores of the membrane. The results show that two extreme types of modification can be produced, depending on the experimental conditions: a high rate coupled with a short dip contact time leads essentially to an external grafting whereas a low rate coupled with a long dip residence time leads essentially to an “in pore” grafting.

Use of air sparging to improve backwash efficiency in hollow-fiber modules

The use of air in backwash of hollow-fiber modules was investigated experimentally from bench to full scale. Modules operated in a dead-end and outside-in mode: they were fouled by either a bentonite suspension or a raw river water and then backwashed in presence of air. The air was injected into the retentate compartment either in combination with a reversed permeate flux or together with feed water after a brief permeate back flow. Results indicate that the cake layer is instantaneously lifted off by the reversed permeate flux and is concentrated in the free volume of the module. To remove it from the module and recover the feed concentration, this volume has to be rinsed with a volume at least three times as big. The air, by its piston-like action, improves material removal and reduces the volume of concentrated foulant to be flushed. So the backwash time is reduced and its efficiency is improved. An optimum air flow rate can be found that is independent of the water flow rate used to flush the module free-volume.

Membrane fouling in the ultrafiltration of polyelectrolyte solutions: polyacrylic acid and bovine serum albumin

Abstract Batch ultrafiltration was used to study the reduction in membrane permeability due to fouling by polyelectrolyte solutes. The flexibility of the polymer chain was found to be an important factor in determining the degree of fouling in the ultrafiltration of poly(acrylic acid) (PAA) solutions, as the effects observed were dependent on the pH and the cation concentration. These parameters were found to have a less pronounced effect in the ultrafiltration of bovine serum albumin (BSA) solutions. An observation of the effects of temperature and applied pressure led to the conclusion that membrane fouling by PAA was due to a pore-plugging effect while that caused by BSA was attributed to hydrophobic adsorption.

Dead-end ultrafiltration in hollow fiber modules: Module design and process simulation

Abstract Ultrafiltration in a hollow-fiber module operating with outside-in and dead-end flow at a constant flow rate was simulated using a model that takes into account the longitudinal pressure drops inside the fibers and within the fiber bundle. The model considers both the filtration phase during which the membrane is fouled by the formation of a filter cake and the backwash phase in which it is cleaned, so as to predict the net rate of production of the module during an operating cycle. The results show that there is a combination of packing density and fiber diameter that gives a maximum net flow rate. Furthermore, this model allows the influence of operating conditions and feed properties on the module performance to be estimated. This can be used to determine how operating parameters must be modified when there is a change in the feed properties.

Flux decline in batch ultrafiltration: Concentration polarization and cake formation

Abstract Flux decline was measured in batch ultrafiltration both with solutions of a polysaccharide polymer (Dextran T40, T500) and with colloidal suspensions (bentonite). Results are analysed in terms of an osmotic pressure polarization model for dextran and in terms of a traditional compressible filter-cake model for bentonite. Values of the mass-transfer coefficient in the stirred cell were determined in the case of the results obtained with dextran.

Hollow-fibre membrane module design: comparison of different curved geometries with Dean vortices

Abstract The performance of several designs of curved membrane modules with Dean vortices was compared through experiments using a colloidal bentonite suspension and cellulose acetate hollow-fibre ultrafiltration (UF) membranes. The different module geometries were: straight, helically coiled, twisted and sinusoidal, or meander-shaped. The experiments show a remarkable increase in mass transfer in curved modules as compared to conventional straight ones. Comparisons were made for modules equipped with the same hollow fibres and the same Dean number (De) for a given Reynolds number (Re). At the same Dean number, all the curved geometries gave the same limiting permeate flux. A mass transfer correlation relating limiting UF flux with the mean wall shear stress has been obtained.

Flux improvement by Dean vortices: ultrafiltration of colloidal suspensions and macromolecular solutions

Coiled and straight hollow-fibre modules have been built and tested; the permeate flux obtained in ultrafiltration with these two geometries is compared for two feeds: a colloidal bentonite suspension and a dextran solution. In the case of colloidal suspensions, the secondary flows induced by the coiled geometry allow fouling to be reduced and the permeate flux is multiplied by a factor of up to 2. An empirical relationship is proposed to express the limiting flux of permeate as a function of both the velocity and some geometrical parameters of the coiled modules. Analogous results are obtained during the ultrafiltration of dextran. It is also shown that under certain conditions almost no deposit was formed; the permeate flux under these conditions is three times higher for coiled modules than for straight ones. For a given energy expenditure and ultrafiltration process, the gain in permeate flux can reach a factor of 1.8.