Unsteady state flux response: a method to determine the nature of the solute and gel layer in membrane filtration

The unsteady-state permeate flux response to a step change in transmembrane pressure is shown to result in unique flux–pressure profiles for the three types of solutes common in membrane ultrafiltration (UF): (a) solutes which exert an osmotic pressure but do not form a ‘gel’; (b) solutes which do not exert an osmotic pressure but form a ‘gel’ and (c) solutes which exert an osmotic pressure and also form a ‘gel’. It is also shown that for stirred cell UF, changes in the bulk feed solution properties (concentration, volume) are negligible on the time scale needed to attain a stable permeate flux. Unsteady-state permeate flux measurements could therefore be made at short filtration times so that the results would not be masked by changes in bulk properties.     

A mass transfer model for the prediction of rejection and flux during ultrafiltration of PEG-6000

A mass transfer model in case of ultrafiltration is proposed in the present study which is capable of predicting the permeate volumetric flux and rejection at different pressure, concentration and stirrer speed. The model is based on the steady state mass balance over the boundary layer, coupled with the results from irreversible thermodynamics. It first predicts the membrane surface and permeate concentrations — which are then utilized to calculate rejection. Permeate flux is then predicted using the result obtained from filtration theory. The model utilizes four parameters, namely, solvent permeability, solute permeability, reflection coefficient and specific cake resistance. These parameters along with the known values of the operating conditions and solution properties enable one to predict the flux as a function of time and rejection. The computed results are found to be in good agreement with the previously published data of Bhattacharjee and Bhattacharya during ultrafiltration of PEG-6000 by cellulose acetate membrane.     

A hydrodynamic study of the retention of polyethylene glycols by cellulose acetate membranes in the absence and presence of proteins

The ultrafiltration behaviour of polyethylene glycols has been investigated with respect to their partial retention by YM5 and YM10 membranes. Retention coefficients were found to follow a log-normal distribution except at low molecular weights, which exhibited higher than expected values. Increasing the applied pressure resulted in increased retention of PEG, although linear flux-pressure relationships were observed in all cases. Increasing hydrodynamic activity by applying higher stirrer speeds led to reduced permeate concentrations. The mean molecular weights of the permeates were higher under hydrodynamic conditions resulting in lower retention coefficients, which suggests increased permeabilities for the larger PEG component molecules.

The retention behaviour of solutions of PEG with varying concentration of bovine serum albumin (BSA) as the flux rate was altered paralleled the results for pure PEG solutions. However, when the flux rate was kept constant and the concentration varied, there was an initial increase of retention at a BSA concentration of 0.2% w/v and thereafter a reduction up to a tested bulk concentration of 10%. Various proposals were made to explain this behaviour.     

Pulsed-electric-field crossflow ultrafiltration of bovine serum albumin

A conventional crossflow ultrafiltration (CUF) apparatus was modified by the inclusion of electrodes which permitted a pulsed electric field to be produced across the ultrafiltration membrane (PEF-UF process). Using this apparatus, a discontinuous electrophoretic velocity was imposed upon the proteins being concentrated, opposing their convective movement toward the CUF membrane. This resulted in a lower concentration of rejected solute protein in the fluid boundary layer adjacent to the high-pressure side of the membrane and, hence, in a lower solute-related filtration resistance than in the case of conventional ultrafiltration (zero electric field). Studies of the PEF-UF process with bovine serum albumin (BSA) in the range of 0.5–5% w/v demonstrated a 25–40% decrease in the solute-related resistance to the permeate flux compared to the case of a zero electric field. Accordingly, higher permeate fluxes and, therefore, higher rates of concentration of the protein solution were obtained than for conventional crossflow ultrafiltration. When the electric field was reimposed following a period of operation under conventional CUF conditions, the permeate flux could be restored to nearly the same higher value observed initially for the PEF-UF process.     

A unified model for flux prediction during batch cell ultrafiltration

An analysis of the flux decline encountered during ultrafiltration (UF) in a batch cell is presented by including the combined influence of the osmotic pressure and the gel layer. A predictive model for the flux decline in unstirred and stirred batch cell UF processes is developed by unifying the osmotic pressure and gel layer models. UF experiments were performed in a batch cell with polymeric solutes (PEG, dextran and PVA) and a protein (BSA), ranging widely in molecular weights and physico-chemical properties, under various operating conditions (pressure, solution pH, and stirrer speed). The present unified model predictions match closely with the experimental flux behaviour for all cases, while individual osmotic pressure and gel layer models are found to be inadequate.     

The influence of protein aggregates on the fouling of microfiltration membranes during stirred cell filtration

Although macromolecular fouling of microfiltration membranes is one of the critical factors governing the performance of these filtration processes, there is still little fundamental understanding of the underlying phenomena that influence the initiation, rate, and extent of fouling. We have obtained experimental data for the flux decline during the stirred cell filtration of different commercial preparations of bovine serum albumin (BSA) through asymmetric polyethersulfone microfiltration membranes. The fouling characteristics of these commercial solutions varied substantially, with the flux decline directly related to the technique utilized to initially precipitate and prepare the BSA. Prefiltration of BSA solutions prior to microfiltration substantially reduced their fouling tendency, with the degree of improvement increasing as the prefiltration was performed through smaller molecular weight cut-off membranes. The protein solutions were also characterized using gel permeation chromatography (GPC), with the fouling tendency of the different BSA preparations highly correlated with the concentration of BSA dimers and other high molecular weight species present in these BSA solutions. These results suggest that BSA fouling of these microfiltration membranes is associated with the deposition of trace quantities of aggregated and/or denatured BSA, with these fouling species serving as initiation sites for the continued deposition of bulk protein.     

Influence of protein–protein interactions on bulk mass transport during ultrafiltration

Bulk mass transfer limitations can have a significant effect on the flux and selectivity during membrane ultrafiltration. Most previous studies of these phenomena have employed the simple stagnant film analysis, but this model is unable to account for the effects of solute–solute interactions on mass transport. We have developed a generalized framework for multicomponent mass transfer that includes both thermodynamic and hydrodynamic (frictional) interactions. Thermodynamic (virial) coefficients were evaluated from osmotic pressure data for albumin (BSA) and immunoglobulins (IgG), while hydrodynamic interaction parameters were determined from filtrate flux data obtained in a stirred cell using fully retentive membranes. The protein concentration profiles in the bulk solution were evaluated by numerical solution of the governing continuity equations incorporating the multicomponent diffusive flux. This model was used to analyze flux and protein transmission data obtained for the filtration of BSA and IgG mixtures through partially permeable membranes. The model accurately predicted the large reduction in flux and BSA transmission upon addition of IgG. These effects were due to the coupling between BSA and IgG mass transfer caused by protein–protein interactions.     

Tubular ultrafiltration flux prediction for oil-in-water emulsions: analysis of series resistances

Using the resistance-in-series (RIS) approach to permeate flux modeling, a general relationship between permeate flux, transmembrane pressure, cross-flow velocity, and feed kinematic viscosity was developed for the tubular ultrafiltration (UF) of synthetic oil-in-water emulsions. The fouling layer resistance, R_f, was 63% of the total membrane resistance, R_m′; however, concentration polarization was the predominant factor controlling resistance in the tubular UF system. An explicit form of the resistance index, Φ, was postulated based on the observed interactions between Φ, cross-flow velocity and feed kinematic viscosity and the RIS model was modified to further describe the interactions between permeate flux and operational parameters. The modified model adequately predicted flux–pressure data over the range of experimental variables examined in this study. Additionally, a set point operating pressure was determined as a function of cross-flow velocity and feed viscosity to achieve a balance between polarization and total membrane resistance.     

Effect of ionic strength and protein concentration on the transport of proteins through chitosan/polystyrene sulfonate multilayer membrane

The influence of ionic strength and protein concentration on the transport of bovine serum albumin (BSA), ovalbumin and lysozyme through chitosan (CHI)/polystyrenesulfonate (PSS) multilayers on polyether sulfone supports are investigated under ultrafiltration conditions. The percentage transmission and flux of BSA, ovalbumin and lysozyme were found to increase with increase in salt concentration in the protein. The percentage transmission of BSA through 9 bilayer membrane was found to increase from 5.3 to 115.6 when the salt concentration was varied from 0 to 1 M. It was observed that 0.1 M NaCl in BSA solution is capable of permeating all the BSA. When the salt concentration in BSA was further increased, a negative solute rejection (solute enrichment in permeate) was found to take place. With 9 bilayer membrane, the percentage transmission of ovalbumin was found to increase from 23.3 to 125.8 when the salt concentration in protein was increased from 0 to 0.05 M. The effect of protein concentration on protein transport is studied taking BSA as a model protein. BSA was rejected by the multilayer membrane at all the studied concentrations (0.25, 0.5, 1 and 2 mg/ml). With increase in feed concentration, maximum rejection of protein occurred at higher number of CHI/PSS bilayers. BSA solution flux was found to decrease with an increase in BSA concentration. This study indicates that it is possible to fine tune the transport properties of proteins through multilayer membranes by varying the concentration and ionic strength of protein solutions.     

Protein type effects on steady-state crossflow membrane ultrafiltration fluxes and protein transmission

The permeate fluxes and percent protein transmission were evaluated for steady-state crossflow ultrafiltration of two proteins of different composition: bovine serum albumin (BSA), containing fatty acid, and “fatty-acid-poor” BSA, from which most of the fatty acids had been removed (BSA/FAP). The influences of protein concentration up to 6.5 percent w/v, transmembrane pressure, ionic environment and membrane type (i.e. nominal molecular weight cut-off) were investigated. For both BSA and BSA/FAP, the fluxes and the protein transmission were dependent on the amount of salt present. The higher fatty acid content in the BSA apparently enhanced protein-protein interaction, resulting in a more cohesive and resistant fouling layer; permeate fluxes were lower with BSA/FAP than with BSA at otherwise corresponding operating conditions. A hysteresis behaviour of the flux (J)-transmembrane pressure (TMP) relationship was observed whenever the ultrafiltration unit was operated at a TMP less than some higher value to which the membrane previously had been exposed.     

Prediction of permeate flux during osmotic pressure-controlled electric field-enhanced cross-flow ultrafiltration

Electric field-enhanced cross-flow ultrafiltration has been carried out to separate protein, bovine serum albumin, from aqueous solution using a 30,000 molecular weight cutoff membrane. A theoretical model is developed to predict permeate flux under a laminar flow regime including the effects of external d.c. electric field and suction through the membrane for osmotic pressure-controlled ultrafiltration. The governing equations of the concentration profile in the developing mass transfer boundary layer in a rectangular channel are solved using a similarity solution method. The effect of d.c. electric field on the variation of membrane surface concentration and permeate flux along the length of the channel is quantified using this model. The expression of Sherwood number relation for estimation of mass transfer coefficient is derived. The analysis revealed that there is a significant effect of electric field on the mass transfer coefficient. A detailed parametric study has been carried out to observe the effect of feed concentration, electric field, cross-flow velocity, and pressure on the permeate flux. For 1 kg/m3 BSA solution, by applying a d.c. electric field of 1000 V/m, the permeate flux increases from 42 to 98 L/m2 h compared to that with zero electric field. The experimental results are successfully compared with the model predicted results.     

A novel approach for modeling concentration polarization in crossflow membrane filtration based on the equivalence of osmotic pressure model and filtration theory

A theoretical model for prediction of permeate flux during crossflow membrane filtration of rigid hard spherical solute particles is developed. The model utilizes the equivalence of the hydrodynamic and thermodynamic principles governing the equilibrium in a concentration polarization layer. A combination of the two approaches yields an analytical expression for the permeate flux. The model predicts the local variation of permeate flux in a filtration channel, as well as provides a simple expression for the channel-averaged flux. A criterion for the formation of a filter cake is presented and is used to predict the downstream position in the filtration channel where cake layer build-up initiates. The predictions of permeate flux using the model compare remarkably well with a detailed numerical solution of the convective diffusion equation coupled with the osmotic pressure model. Based on the model, a novel graphical technique for prediction of the local permeate flux in a crossflow filtration channel has also been presented.     

Theoretical analysis of the influence of the axial variation of the transmembrane pressure in cross-flow filtration of rigid spheres

A model of the axial and the radial transmembrane pressure drop in a cylindrical cross-flow filtration module was developed by performing a hydrodynamic analysis of the fluid flow based on the momentum and the continuity equations. Use of this expression for the transmembrane pressure drop together with the resistance model and the concept of shear induced diffusion of the particles at the membrane surface resulted in an expression of the permeate flux. The predictions of the transmembrane pressure drop, the permeate flux and the particles near the membrane surface are discussed for cases with and without the formation of a stagnant layer. The importance of the cylindrical membrane fiber dimensions on the permeate flux is also discussed.     

Separation of serum proteins with uncoupled microsphere particles in a stirred cell

Submicron microspheres were used directly without ligand coupling for the batch and continuous separations of proteins. In the batch experiments for separating BSA (bovine serum albumin) from BHb (bovine hemoglobin), introducing both hydrophobic effects for BSA and electrostatic repulsion for BHb (and vice versa) was required for high selectivity, and microspheres with low number density of surface groups were advantageous. For the continuous experiments, the utilization of a stirred cell was successful, where the microspheres were in the form of latex with good dispersion of particles. The flow rate without a pump was 0.5–1.3 ml min⁻¹, and the ratio of BSA and BHb was varied. In the experiments for eliminating BHb from BSA, elution curves of BHb corresponded to the single component breakthrough curves, while those for BSA did not. The latter is believed to be due to the interference by BHb in the adsorption of BSA.     

Modeling of flux decline during crossflow ultrafiltration of colloidal suspensions

Mass transfer during crossflow ultrafiltration is mathematically expressed using the two-dimensional convective–diffusion equation. Numerical simulations showed that mass transfer in crossflow filtration quickly reaches a steady-state for constant boundary conditions. Hence, the unsteady nature of the permeate flux decline must be caused by changes in the hydraulic boundary condition at the membrane surface due to cake formation during filtration. A step-wise pseudo steady-state model was developed to predict the flux decline due to concentration polarization during crossflow ultrafiltration. An iterative algorithm was employed to predict the amount of flux decline for each finite time interval until the true steady-state permeate flux is established. For model verification, crossflow filtration of monodisperse polystyrene latex suspensions ranging from 0.064 to 2.16 μm in diameter was studied under constant transmembrane pressure mode. Besides the crossflow filtration tests, dead-end filtration tests were also carried out to independently determine a model parameter, the specific cake resistance. Another model parameter, the effective diffusion coefficient, is defined as the sum of molecular and shear-induced hydrodynamic diffusion coefficients. The step-wise pseudo steady-state model predictions are in good agreement with experimental results of flux decline during crossflow ultrafiltration of colloidal suspensions. Experimental variations in particle size, feed concentration, and crossflow velocity were also effectively modeled.     

Microfiltration of protein-cell mixtures with crossflushing or backflushing

Crossflow microfiltration experiments were performed with and without crossflushing or backflushing using yeast suspensions, bovine serum albumin (BSA) solutions, and mixtures of yeast and BSA. Scanning electron microscope (SEM) photographs of the membrane surface were taken before and after crossflushing and backflushing. Backflushing is highly effective, while crossflushing is partially effective, in removing the external cake formed during filtration of yeast suspensions. Crossflushing is completely ineffective and backflushing is only partially effective for removal of internal foulants during filtration of BSA solutions. During backflushing or crossflushing of yeast–BSA mixtures, complete or partial removal of the yeast cake reduces the hydraulic resistance to permeate flow. However, this removal also exposes the primary membrane to internal fouling by BSA, against which neither crossflushing nor backflushing is very effective.     

Microfiltration of amphoteric surfactant using ceramic membranes

The microfiltration of commercially available amphoteric surfactant using ceramic membranes has been investigated. Various combinations operating conditions such as pH, electrolyte and surfactant concentrations were employed. Zeta potential and adsorption isotherms were obtained for the components of membrane surfactant system as functions of pH using surfactant or indifferent electrolyte (KCl). The shift in the membrane isoelectric point induced by the surfactant is linked to the carboxylic groups present on the surfactant which are believed to play a dominant role in the net surface charge of the membrane. A minimum in the permeate flux was found at the pH corresponding to the isoelectric point of the zwitterionic surfactant. This behaviour is ascribed to the interactions occurring between the surfactant–surfactant molecules and the surfactant–membrane. The higher fluxes obtained at low pH as compared to high pH arise from different fouling mechanisms and ionic strengths. Lower fluxes were found when inorganic electrolytes were used in conjunction with surfactant. However, as the valency of the salt increases, flux behaviour of the zwitterionic surfactant (close to isoelectric point) does not vary whilst the cationic and anionic state of the surfactants are much more affected. Interactions between surfactant molecules as a result of the charge screening effects by the larger valence ions are encouraged. The permeate flux declines with an increasing surfactant concentration even though some concentrations fall under the critical micelle concentration (c.m.c.). This is attributed to concentration polarisation in which the accumulated surfactant concentration at the membrane surface could form a stable viscous phase which is resistant to permeate flow in the secondary layer next to the membrane surface. This paper demonstrates the role interactions such as surfactant–surfactant and surfactant–membrane play in influencing the filterability of surfactant solutions using ceramic membranes.     

Concentration Polarization of Interacting Solute Particles in Cross-Flow Membrane Filtration

A theoretical approach for predicting the influence of interparticle interactions on concentration polarization and the ensuing permeate flux decline during cross-flow membrane filtration of charged solute particles is presented. The Ornstein-Zernike integral equation is solved using appropriate closures corresponding to hard-spherical and long-range solute-solute interactions to predict the radial distribution function of the solute particles in a concentrated solution (dispersion). Two properties of the solution, namely the osmotic pressure and the diffusion coefficient, are determined on the basis of the radial distribution function at different solute concentrations. Incorporation of the concentration dependence of these two properties in the concentration polarization model comprising the convective-diffusion equation and the osmotic-pressure governed permeate flux equation leads to the coupled prediction of the solute concentration profile and the local permeate flux. The approach leads to a direct quantitative incorporation of solute-solute interactions in the framework of a standard theory of concentration polarization. The developed model is used to study the effects of ionic strength and electrostatic potential on the variations of solute diffusivity and osmotic pressure. Finally, the combined influence of these two properties on the permeate flux decline behavior during cross-flow membrane filtration of charged solute particles is predicted. Copyright 1999 Academic Press.     

Downstream processing of lactic acid-whey permeate fermentation broths by hollow fiber ultrafiltration

Membrane filtration is a suitable method for cell harvesting and clarification of fermentation broths. Hollow fiber ultrafilters gave essentially 100% rejection ofL. bulgaricus cells from a whey permeate fermentation broth. A combination of low pressures and high velocity generally gave the best permeate flux. Fermentation media components (in this case, from the whey permeate) contributed significantly to fouling. Considering the pressure limitations of the current generation of asymmetric hollow fiber modules and the changes in physical properties of the fermentation broths, a cell concentration of 100–150 g/L could be obtained with the flux still relatively high (above 20LMH), although the chemical compatibility of the membrane module itself under long-term exposure to high acid conditions should be considered.