Martine Meireles

A contribution to the translation of retention curves into pore size distributions for sieving membranes

Abstract The analysis of solute and solvent transport through capillaries according to sieving and Poiseuille flow models allows the calculation of retention curves corresponding to a given pore size distribution. A log-normal distribution is assumed with the average radius and the geometric standard deviation as parameters. A method is described to retrieve the parameters of the apparent pore size distribution of a clean or a fouled membrane when rejection measurements are available for test molecules. The method is applied to experimental data obtained with dextran filtered on polysulfone membranes. The curves calculated using the parameters given by the method are in good agreement with experimental data.

Investigations of surface properties of polymeric membranes by near field microscopy

Abstract Atomic force and scanning tunneling microscopes have been used to investigate the surface of polymeric ultrafiltration and microfiltration membranes. The atomic force microscope has been shown to resolve the features of the selective skin layer of the membranes providing information on both size and shape of pores as well as surface roughness of the skin. When applied to the study of ultrafiltration and microfiltration membrane for the determination of pore size characteristics, the scanning tunneling microscope has been shown to give similar results with those obtained using polyethylene glycols transport measurements. The microscopic techniques have discerned pore diameters in the range of 11 to 114 nm for 40 to 200 kDa MWCO SPS ultrafiltration membranes and confirmed a mean pore diameter of 96 nm for the 0.1 μm PVDF microfiltration membrane.

Use of the log-normal probability density function to analyze membrane pore size distributions: functional forms and discrepancies

Abstract The log-normal probability density function has been used extensively in the characterization of membrane pore size distributions and in the theoretical analysis of the effect of these distributions on membrane transport. There has, however, been considerable discrepancy regarding the proper functional form for this probability distribution as well as the proper interpretation of the parameters used to define the distribution. This short communication examines the different forms that have been presented in the literature for the log-normal distribution, properly interprets the parameters that appear in these functions, and provides the appropriate equations required to transform between these different distributions and properly evaluate the appropriate statistical parameters.

Effects of protein fouling on the apparent pore size distribution of sieving membranes

Abstract A characterisation method giving the average radius and the geometric standard deviation is used to study the influence of fouling on the apparent porous structure of ultrafiltration membranes. A way of estimating the pore density is proposed. Polysulphone membranes of different molar weight cut-offs and globular proteins of different size are used. It is shown how the effects of fouling on solvent flux and on retention depend on the size of both pores and foulant molecules. The results suggest that fouling involves two mechanisms: internal pore fouling and build-up of a layer depending on both porous structure and solute size.

An appropriate molecular size parameter for porous membranes calibration

Abstract Data for the transport of different series of solutes (polyethylene glycol, dextrans and proteins) are measured through asymmetric sulfonated polysulfone ultrafiltration membranes of various molecular weight cut-off and a polyvinyldifluoride microfiltration membrane of nominal pore size 0.1 μm. When sieving coefficients are plotted as a function of molecular weight, a curve is obtained for each series of solute while when data are plotted versus the hydrodynamic volume, sieving coefficients fall close to a single curve for each membrane. The hydrodynamic volume is therefore more appropriate than the molecular weight for a standard characterisation of membranes. Results are in good agreement with available hydrodynamics models for the hindrance factor of rigid spherical solutes through cylindrical pores, with the effective solute size evaluated from viscosimetric measurements and assuming a log-normal pore size distribution for the membrane.

Modelling of filtration: from the polarised layer to deposit formation and compaction☆

A theoretical study is developed for the modelling of mass accumulation from the polarised layer to the formation of a deposit. Concentrated suspension properties are accounted for through a solid pressure corresponding to osmotic pressure in the suspension and to compression resistance in the deposit. Mass and solvent transfer are depicted through balanced permeation/diffusion transport in the polarised layer and balanced friction/compaction forces in the deposit. The modelling applied to transient state for dead end filtration gives information about the coupling between mass transfer in the polarised layer and in the deposit. Consequences on specific cake resistance are presented and discussed.

Colloidal surface interactions and membrane fouling: Investigations at pore scale

In this paper, we examine the contributions of colloidal surface interaction in filtration processes. In a first part, we describe the way surface interactions affect the transport of colloidal particles or macromolecules towards a membrane, and its theoretical description. The concept of critical flux is introduced and linked to particle-membrane wall and particle-particle surface interactions. From this review, it seems important to consider how surface interactions occur at pore scale and control the development of fouling layers. In this context, we report in a second part experiments where the capture of micron-sized particles is observed in a poly-dimethylsiloxane (PDMS) microfluidic filtration device. Direct observations of the filtering part by video-microscopy allow to investigate the way the fouling of the microchannels by the particles is taking place. The experimental results underline the important role played by the particle-wall interactions on the way particles are captured during filtration. A small change in surface properties of the PDMS has important consequences in the way pore clogging occurs: in more hydrophobic conditions the particles first form arches at the microchannels entrance, then leading to the growth of a filtration cake, whereas in more hydrophilic conditions the particles are captured on the walls between the microchannels, then leading to the progressive formation of dendrites. To conclude, both experimental and theoretical approaches show the important role played by surface interactions in filtration processes. The complex interplay between multi-body surface interactions and hydrodynamics at nanometric scale leads to clogging phenomena observed experimentally in microfluidic systems that have not been predicted by numerical simulations. In the future, the two way coupling between simulation and experimental approaches at the pore scale have to progress in order to reach a full understanding of the contribution of colloid science in membrane processes.

VISCOSITY OF CONCENTRATED SUSPENSIONS: INFLUENCE OF CLUSTER FORMATION

Dispersed particles can form clusters even at low concentrations. Colloidal and hydrodynamic forces are responsible for this phenomenon and these forces determine both structure and size of clusters. We assume that the viscosity of a concentrated suspension is completely determined by cluster size distribution, regardless if clusters form under the action of colloidal, hydrodynamic interactions or applied shear rates. Based on this assumption an equation, which describes dependency of viscosity on a concentration of dispersed particles taking into account cluster formation, is deduced. Under special restrictions the deduced dependency coincides with the well-known Dougherty-Kriegers equation except for a clear physical meaning of parameters entered. Our consideration shows that Dougherty-Kriegers equation has deeper physical background than it has been supposed earlier. Experimental verification of the suggested model shows a good agreement with the theory predictions and proves a presence of clusters even at low concentrations of dispersed particles.

Structural Characterisation of Deposits Formed During Frontal Filtration

Understanding the mechanisms that control the filtration of a complex media is a major challenge for the development of membrane-based processes in bioindustries, agro-industries or sludge treatment, where the complexity of the fluids is seen in terms of composition (liquid-liquid or liquid solid mixtures), or physico-chemical characteristics (rheology, stability,..). This complexity is likely to induce different material organisation within the fluid depending on concentration and hydrodynamics fields. One of the aims of this study is to determine a relation between structural mechanisms and macroscopic properties of cakes formed on filtration of a colloidal suspension. Our investigations were carried out on clays suspensions. Filterability of those suspensions was appreciated throught simple dead en filtration tests and structural characteristics of the deposits formed during filtration were determined by using small-angle neutron scattering (SANS), static light scattering (SLS) and local birefringence techniques, associated with rheometric studies. These led to the conclusion that in the cakes formed from laponite suspensions (volumic fraction= 0.48 %, transmembrane pressure = 0.5 bar), the particles are packed in an anisotropical arrangement parallel to the membrane. Moreover, we show that upon filtration, an aggregation mechanism is promoted , leading to the formation of a porous fractal structure. Adding a peptiser to the suspension changes the structural characteristics of the cake in the sense of a higher fractal dimension in the arrangement. Macroscopic measurements of permeation velocity through the cakes formed with an without a peptiser showed that the initial aggregated state promoted by addition of a peptiser led in this case to a greater permeability.

Ultrafiltration of an olive oil emulsion stabilized by an anionic surfactant

Abstract In this paper we report results on limiting flux in ultrafiltration of an oil in water emulsion stabilized by an anionic surfactant (sodium dodecyl sulfate). Whereas most of previous investigations report variations of filtration flux as a function of pressure at different flow rates, here we also consider the influence of surfactant/oil ratio and emulsion’s ionic strength. The present study clearly suggests that filtration of stabilized emulsions exhibits paradoxical experimental similarities with filtration of macromolecules. However, classical film models used for the latter case did not yield proper values for mass transfer coefficients. When using flux models to account for shear enhanced diffusion and pseudo-plastic behavior of the concentrated oil in water emulsions a good agreement between calculated and experimental mass transfer coefficient is obtained.