Ultrafiltration Of Macromolecules Research Articles

Analysis and optimization of the influence of operating conditions in the ultrafiltration of macromolecules using a response surface methodological approach

In this work, the ultrafiltration of macromolecules was analysed using a response surface methodological approach. The behaviour of two different inorganic membranes was investigated. The membranes selected were a Carbosep M2 membrane (Orelis, France) with a molecular weight cut-off (MWCO) of 15 kDa and a Tami MSKT membrane (Tami Industries, France) with a MWCO of 5 kDa. The solute employed was polyethylene glycol of 35 kDa molecular weight. The influence of transmembrane pressure (0.1, 0.2, 0.3, 0.4 and 0.5 MPa), crossflow velocity (1, 2 and 3 m/s) and feed concentration (5, 10 and 15 g/L) on permeate flux and permeate flux decline was investigated. Analysis of variance was proved to be a useful tool to determine the effect of operating variables on both parameters. The method used demonstrated the presence of coupled effects between factors as well as squared effects that are relevant to the ultrafiltration process. The surface contours obtained from fitted models were used for the optimization of the operating conditions. The goal was to simultaneously maximize the average permeate flux and minimize the flux decline. The optimal operating conditions for the Carbosep M2 membrane were a transmembrane pressure of 0.38 MPa and a crossflow velocity of 3 m/s. The optimal operating conditions for the Tami MSKT membrane could not be determined by means of multiple response optimization due to the low accuracy of the regression model obtained for the cumulative permeate flux decline (SFD) response variable.

Validation of dynamic models to predict flux decline in the ultrafiltration of macromolecules

The aim of this work was to validate the dynamic model for the average permeate flux that considers membrane fouling as a dynamic process from non-equilibrium to equilibrium to predict permeate flux decline with time in the ultrafiltration of macromolecules. This is one of the most accepted ultrafiltration dynamic models and it considers cake formation as the main fouling mechanism. For this purpose, several experiments were performed at pilot plant scale and the results were compared with the ones predicted by the model. A ceramic TiO 2–Al 2O 3 monotubular membrane (Tami, S.A., France) of 5 kDa molecular weight cut-off was used. Aqueous solutions with different concentrations of polyethylene glycol were used as feed. The flow rate and the transmembrane pressure were varied while the temperature was maintained constant at 25°C. As the model does not consider cake compaction, the best results were obtained for low transmembrane pressures, as expected. Crossflow velocity was observed to be one of the main factors affecting fouling. The best agreement between the experimental results and those predicted by the model was obtained for low crossflow velocities as cake formation can be considered as the main fouling mechanism in this case.

Prediction of flux decline in the ultrafiltration of macromolecules

In this work a non-steady state model for cross flow ultrafiltration was tested and the results estimated by the model were compared with those experimentally obtained. Crossflow ultrafiltration experiments were performed in a pilot plant using tubular ZrO 2-TiO 2 ceramic membranes with a MWCO of 15 kDa (Orelis, France). The membranes were fouled with a 5 g/L PEG (35000 Da molecular weight) aqueous solution at different transmembrane pressures and crossflow velocities. The main goal of this work was to predict flux decline with a model whose input parameters are dependent on the operating conditions. Therefore all the model parameters were theoretically calculated, avoiding the utilization of empirical parameters. The validity of the model tested was observed to be restricted to the case of low crossflow velocities where gel layer formation is the main fouling mechanism.

Crossflow ultrafiltration of cake forming solutes: a non-steady state model

The aim of this work was to check the validity of a non-empirical non-steady state model, which was initially developed to predict permeate flux for dead-end ultrafiltration, in the crossflow ultrafiltration of macromolecules. All the model parameters were theoretically calculated. Consequently, the model is capable of making predictions without the need of experimentation. The results estimated by the model were compared with those experimentally obtained in an ultrafiltration pilot plant. Tests were performed with Carbosep M2 ceramic membranes (Orelis, France) at different transmembrane pressures (0.1, 0.2, 0.3 and 0.4 MPa) and crossflow velocities (1, 2 and 3 m/s). Polyethylene glycol of 35,000 g/mol was used as a standard macromolecule in the fouling experiments. The validity of the model is restricted to the case when cake layer formation is the fouling mechanism. Model predictions are in agreement with experimental results for a crossflow velocity of 1 m/s, when the fouling mechanism is more likely to be cake formation. For other fouling mechanisms the predictions are not so good.

Effects of ethanol concentration on flux and gel formation in dead end ultrafiltration of PEG and dextran*1

The phenomenon of concentration polarization and gel formation plays an important role in ultrafiltration of macromolecules from water. However, there is lack of studies on similar phenomena for macromolecule ultrafiltration from blends of aqueous and non-aqueous solvents. This paper reports the effect of ethanol concentration on flux in ultrafiltration of polyethylene glycol and dextran with a solvent resistant polymeric membrane. Transient filtration data, collected after steady state, was utilized to determine actual filtration resistance and contribution of polarization using the osmotic pressure model. It was observed that both steady state flux and gel formation were significantly influenced in the ultrafiltration of both PEG and dextran from blended solvents. It was also observed that the onset of gel formation for PEG and concentration polarization for dextran in blended solvents occurred at significantly higher bulk solute concentrations than in water alone.

Effects of ethanol concentration on flux and gel formation in dead end ultrafiltration of PEG and dextran

The phenomenon of concentration polarization and gel formation plays an important role in ultrafiltration of macromolecules from water. However, there is lack of studies on similar phenomena for macromolecule ultrafiltration from blends of aqueous and non-aqueous solvents. This paper reports the effect of ethanol concentration on flux in ultrafiltration of polyethylene glycol and dextran with a solvent resistant polymeric membrane. Transient filtration data, collected after steady state, was utilized to determine actual filtration resistance and contribution of polarization using the osmotic pressure model. It was observed that both steady state flux and gel formation were significantly influenced in the ultrafiltration of both PEG and dextran from blended solvents. It was also observed that the onset of gel formation for PEG and concentration polarization for dextran in blended solvents occurred at significantly higher bulk solute concentrations than in water alone.

Ultrafiltration of high-viscosity solutions: Theoretical developments and experimental findings

The theory of the existence of limiting fluxes and concentrations in the ultrafiltration of macromolecules is reviewed and extended to non-Newtonian power-law fluids. Recent work by Pritchard et al. (1995, J. Membrane Sci. 102, 223–235) has shown the existence of situations where the ultrafiltration flux can increase with concentration. The criterion for this occurring has been developed and evaluated using rheological data as a function of concentration for a variety of fluids including xanthan gum, dextran, pectin and reconstituted whey. These were then concentrated to high concentrations by ultrafiltration to verify the validity of the criterion. Higher fluxes with increased concentration were only found for xanthan gum.

Ideal limiting fluxes in ultrafiltration: comparison of various theoretical relationships

The ultrafiltration of macromolecules is characterised by a limiting flux at high transmembrane pressures. There is also some evidence that at high pressures and low crossflow velocities the flux decreases slightly with increasing pressure. It is confirmed from a theoretical viewpoint that this can only be caused by a decrease in the average mass-transfer coefficient due to concentration increases in the boundary layer. At the practical level, we propose an expression which, for a given system, enables the ideal flux to be estimated a priori as a function of the transmembrane pressure. The ideal flux is defined as that flux which would occur in the absence of fouling and gelation. The model includes the influence of both osmotic pressure and the variation in viscosity due to concentration polarisation. Thus for predictive purposes knowledge of osmotic pressure and viscosity as a function of concentration is required. The only membrane parameter that has to be experimentally determined is the membrane permeability. In the absence of adsorption (which is the ideal case) this is the permeability to the pure solvent. The model has been tested against Jonsson's data for the ultrafiltration of dextran solutions. The results are most encouraging.

A novel approach to transfer-limiting phenomena during ultrafiltration of macromolecules

A novel approach to transfer-limiting phenomena during ultrafiltration of macromolecules