33 research outputs found
Distributions of critical flux: modelling, experimental analysis and consequences for cross-flow membrane filtration
This paper discusses the distribution of critical flux (DCF) in cross flow filtration. These distributions are described here by a normal function with a mean critical flux and its standard deviation. The DCF model allows the description, through an analytical relationship, of the variation in steady state permeate flux with trans-membrane pressure.
Both strong and weak forms of critical flux, which can be observed on a membrane operating in cross-flow mode, are depicted. A simple graphical method to determine the mean critical flux and its standard deviation from experimental results is derived from the theoretical
model. The theoretical trends are compared to experimental data and show good agreement for cross flow filtration of latex and BSA suspensions. The distribution function parameters obtained by fitting the DCF model to experiments are compared to critical flux measured via a pressure step method. We thus propose a tool to analyse filtration results and to determine new global parameters for critical conditions (mean value and its standard deviation), which
appears to be a good way to account for fouling complexity
Filtration method characterizing the reversibility of colloidal fouling layers at a membrane surface: analysis through critical flux and osmotic pressure
A filtration procedure was developed to measure the reversibility of fouling
during cross-flow filtration based on the square wave of applied pressure. The
principle of this method, the apparatus required, and the associated
mathematical relationships are detailed. This method allows for differentiating
the reversible accumulation of matter on, and the irreversible fouling of, a
membrane surface. Distinguishing these two forms of attachment to a membrane
surface provides a means by which the critical flux may be determined. To
validate this method, experiments were performed with a latex suspension at
different degrees of destabilization (obtained by the addition of salt to the
suspension) and at different cross-flow velocities. The dependence of the
critical flux on these conditions is discussed and analysed through the osmotic
pressure of the colloidal dispersion.Comment: Journal of Colloid and Interface Science (2008) to be publishe
Numerical simulation of colloidal dispersion filtration: description of critical flux and comparison with experimental results
During filtration via membrane processes, colloids accumulate at the porous surface leading to fouling phenomena. In this study, a rigorous simulation of momentum and mass transfer using CFD modelling has been developed to describe such an accumulation during cross flow filtration. These simulations integrate detailed modeling of physicochemical properties specific to colloidal dispersions (because of the surface interactions (repulsive and attractive) occurring between the colloids particles). These interactions are accounted for via the experimental variation of the colloidal osmotic pressure with volume fraction (associated with a variation in the diffusion coefficient) which are fitted by a relationship integrated into the CFD code. It contains a description of the colloidal phase transition leading to the formation of a condensed phase (deposit or gel layer) from the accumulated dispersed phase (concentration polarization). It is then possible to determine the critical flux which separates filtration conditions below which mass accumulation is reversible (in the dispersed phase) and above which it is irreversible (in the condensed phase). The computed value of critical flux is compared with that determined experimentally for a dispersion of latex particles
Disruption of PF4/H multimolecular complex formation with a minimally anticoagulant heparin (ODSH)
Recent studies have shown that ultra-large complexes (ULCs) of platelet factor 4 (PF4) and heparin (H) play an essential role in the pathogenesis of heparin-induced thrombocytopenia (HIT), an immune-mediated disorder caused by PF4/H antibodies. Because antigenic PF4/H ULCs assemble through non-specific electrostatic interactions, we reasoned that disruption of charge-based interactions can modulate the immune response to antigen. We tested a minimally anticoagulant compound (2-O, 3-O desulfated heparin, ODSH) with preserved charge to disrupt PF4/H complex formation and immunogenicity. We show that ODSH disrupts complexes when added to pre-formed PF4/H ULCs and prevents ULC formation when incubated simultaneously with PF4 and UFH. In other studies, we show that excess ODSH reduces HIT antibody (Ab) binding in immunoassays and that PF4/ODSH complexes do not cross-react with HIT Abs. When ODSH and unfractionated heparin (UFH) are mixed at equimolar concentrations, we show that there is a negligible effect on amount of protamine required for heparin neutralisation and reduced immunogenicity of PF4/UFH in the presence of ODSH. Taken together, these studies suggest that ODSH can be used concurrently with UFH to disrupt PF4/H charge interactions and provides a novel strategy to reduce antibody mediated complications in HIT
APPROCHE THEORIQUE ET EXPERIMENTALE DE LA FILTRATION TANGENTIELLE DE COLLOĂDES : FLUX CRITIQUE ET COLMATAGE
Though the problems raised by membrane fouling are old, prediction of fouling and the adaptation of filtration conditions to reduce fouling remain essential for a better control of the process. Because it represents the flux beyond which irreversible fouling appears on the membrane surface, the critical flux can be a key parameter in this control. In this study, we try to relate the osmotic pressure of a colloidal suspension to the experimental values of critical flux by modelling the process of tangential filtration. A filtration procedure was developed to determine critical flux in a precise and rigorous way. This method allows the reversibility of particle accumulation to be determined continuously by observing flux behaviour when pressure is suddenly reduced. This makes it possible in experiments to discriminate between the part of the flux reduction due to the osmotic pressure and that due to irreversible resistance. Characterization of the suspension at the microscopic level (zeta potential, size, ...) was supplemented by a more original, macroscopic measurement of particulate osmotic pressure by chemical compression. The osmotic pressure of the colloidal dispersion proves to be a parameter relevant to filtration because it is related to the way the particles resist being concentrated. A suitable two-dimensional model that calculates particle concentration profiles in a membrane separation process is used for theoretical confrontation of the experimental measurements of the osmotic pressure with the experimental values of the critical flux. Comparison of the experimental and calculated values of critical flux suggests that heterogeneities in the physical properties of both the membrane and the particles must be taken into account. Integration of the flow pattern into a descriptive model could make it possible to explain the differences between experiment and simulation of the filtration of colloidal suspensions.Si la problĂ©matique posĂ©e par le colmatage des membranes est ancienne, prĂ©voir et adapter les conditions de filtration pour rĂ©duire le colmatage reste essentiel pour un meilleur contrĂŽle du procĂ©dĂ©. Parce qu'il reprĂ©sente le flux au-delĂ duquel un colmatage irrĂ©versible apparaĂźt Ă la surface de la membrane, le flux critique peut ĂȘtre un paramĂštre clef pour ce contrĂŽle. Dans le cadre de cette Ă©tude,nous avons cherchĂ© Ă relier la pression osmotique d'une suspension colloĂŻdale aux valeurs expĂ©rimentales du flux critique par le biais de la modĂ©lisation du procĂ©dĂ© de filtration tangentielle. Une procĂ©dure de filtration a Ă©tĂ© dĂ©veloppĂ©e pour dĂ©terminer de façon prĂ©cise et rigoureuse le flux critique. Cette mĂ©thode permet une dĂ©termination continue de la rĂ©versibilitĂ© de l'accumulation de matiĂšre lors d'une diminution de flux permettant ainsi de dissocier expĂ©rimentalement la part de diminution de flux due Ă la pression osmotique et celle due Ă la rĂ©sistance irrĂ©versible. La caractĂ©risation de la suspension au niveau microscopique (potentiel zĂȘta, taille, etc.) a Ă©tĂ© complĂ©tĂ©e par une mesure macroscopique, plus originale, de la pression osmotique particulaire par compression chimique. La pression osmotique de la dispersion colloĂŻdale se rĂ©vĂšle ĂȘtre une caractĂ©risation pertinente par rapport Ă la filtration car relative Ă la rĂ©sistance des particules Ă la surconcentration. Un modĂšle bidimensionnel adaptĂ© calculant les profils de concentration en particules dans un procĂ©dĂ© de sĂ©paration par membranes est utilisĂ© pour confronter thĂ©oriquement les mesures expĂ©rimentales de pression osmotique aux valeurs de flux critique. La comparaison des valeurs expĂ©rimentales et modĂ©lisĂ©es de flux critique suggĂšrent que des hĂ©tĂ©rogĂ©nĂ©itĂ©s de propriĂ©tĂ©s physiques de la membrane et des particules doivent ĂȘtre prises en compte. IntĂ©grĂ©es dans un modĂšle descriptif, les distributions de flux permettent d'expliquer les diffĂ©rences entre expĂ©rience et simulation de la filtration d'une suspension colloĂŻdale
13. La pression osmotique
Les Ăgyptiens sont les premiers Ă avoir maĂźtrisĂ© les phĂ©nomĂšnes osmotiques pour la momification des corps, ainsi que pour la conservation des viandes par salaisons. Le principe de ces deux procĂ©dĂ©s est de dĂ©shydrater les cellules, par lâajout de sel en surface, qui fait migrer lâeau vers lâextĂ©rieur des cellules. La pression osmotique, qui signifie « poussĂ©e » en grec (osmos), fut dĂ©crite pour la premiĂšre fois au milieu du XVIIIe siĂšcle par un physicien français, lâabbĂ© NollĂ© et mesurĂ©e en 18..
Approche théorique et expérimentale de la filtration tangentielle de colloïdes (flux critique et colmatage)
TOULOUSE3-BU Sciences (315552104) / SudocSudocFranceF
DĂ©termination du flux critique : un outil pour la maĂźtrise du colmatage ?
L'Ă©tude prĂ©sentĂ©e ici repose sur la dĂ©termination expĂ©rimentale du flux critique en ultrafiltration de suspension colloĂŻdales, la caractĂ©risation macroscopique des suspensions colloĂŻdales et l'utilisation d'un modĂšle alimentĂ© par ces caractĂ©ristiques. Une mĂ©thode prĂ©cise de mesure de flux critique est prĂ©sentĂ©e permettant de savoir si une diminution de flux est due Ă une accumulation de matiĂšre rĂ©versible ou irrĂ©versible. La pression osmotique est la mesure qui a Ă©tĂ© privilĂ©giĂ©e pour caractĂ©riser une suspension de latex PVC qui a servi de base Ă cette Ă©tude. En effet, la variation de pression osmotique avec la fraction volumique reflĂšte bien la rĂ©sistance d'une suspension de particules Ă la compression ; les mesures expĂ©rimentales mettent en Ă©vidence une pression osmotique critique autour de 60% volumique en particules traduisant l'agrĂ©gation des particules qui sera responsable de la formation d'un dĂ©pĂŽt irrĂ©versible lors de la filtration. Le modĂšle de calcul de flux utilisĂ© est basĂ© sur un bilan de matiĂšre et est alimentĂ© avec des mesures expĂ©rimentales de pression osmotique et de viscositĂ©. L'hydrodynamique du systĂšme est intĂ©grĂ©e par le biais du facteur de friction. De cette façon, le modĂšle est entiĂšrement basĂ© sur des donnĂ©es acquises par des mĂ©thodes indĂ©pendantes. Les calculs de flux critiques sont comparĂ©s avec les rĂ©sultats du banc de dĂ©tection de flux critique. Le modĂšle donne des profils de concentrations Ă la surface de la membrane ce qui permet de dĂ©terminer le flux critique. Il apparaĂźt que la simulation donne des rĂ©sultats du mĂȘme ordre de grandeur que l'expĂ©rience. La surestimation par le modĂšle est discutĂ©e
Fouling effects and critical flux in relation with module design and aeration conditions for a side stream outside/in filtration system
International audienc
Le plan-relief informatique, outil de représentation des transformations urbaines
International audienceIn the Campus Martius in Rome, the Piazza Navona takes out the exact shape of the stadium built by Domitian in 81. The Ecole française de Rome startedthe archaeological study of the monument. Jean-François Bernard is at the head of this project financed by the French National Agency for Research (ANR). Understanding the ancient period of the monument required considering the topographical environment: how was the site for an urban program chosen to permit the creation of the âludus capitolinusâ.For each monument or each layout, great care is taken in the scientific justification of the choices. The survey of the archaeological knowledge has shown a development achieved in four phases. The 3D visualization of the site at four different periods requires the making of a virtual relief plan. It is based on the modern tradition of representing the city. This making has been entrusted to the Archeovision Platform situated in the ArchĂ©opĂŽle in Bordeaux. At the end of the project, this work will result in the creation of a set of superimposable plans having in common limits, scale, style and coding to show in perspective the architectural and town-planning evolution of the area, from Antiquity to the XXth century. The difficulty of the bias is to create a basic model positively homogeneous but flexible enough to adapt to the various sights of the area. The drawing development process is another problem. The sector with the stadium and the Odeon is represented with very precise details. As for the rest, the incomplete documentation commands a working drawing. This method allows to bring continuous changes linked with the progress of research and immediate reaction to the discussion.Au cĆur du Champ de Mars romain, lâactuelle place Navone reprend la forme du stade construit en 81 par lâempereur Domitien. Une Ă©tude archĂ©ologique de lâĂ©difice de spectacle a Ă©tĂ© lancĂ©e par lâEcole française de Rome (EfR). Ce projet dirigĂ© par Jean-François Bernard est financĂ© par lâAgence Nationale de la Recherche (ANR). La comprĂ©hension du monument antique nĂ©cessite la prise en considĂ©ration de son environnement topographique, qui permet dâexpliquer le choix du site dâimplantation et les relations entretenues avec les constructions voisines. En amont, du travail de reprĂ©sentation, un grand soin est apportĂ© Ă la mise Ă jour des donnĂ©es scientifiques. Lâinventaire des connaissances archĂ©ologiques a fait apparaĂźtre un processus dâurbanisation que lâon peut diviser en quatre phases principales. Le problĂšme posĂ© par sa visualisation a nĂ©cessitĂ© la mise au point dâune sĂ©rie de plans-reliefs, directement inspirĂ©s par la tradition moderne de reprĂ©sentation de Rome. Cette opĂ©ration a Ă©tĂ© confiĂ©e Ă la Plateforme ArchĂ©ovision de lâArchĂ©opĂŽle dâAusonius (Bordeaux). Au terme du projet, ce travail aboutira Ă la rĂ©alisation dâune sĂ©rie de plans superposables, ayant en commun les limites, lâĂ©chelle, le style et la codification graphique, afin de mettre en perspective lâĂ©volution architecturale et urbanistique du quartier, de lâAntiquitĂ© au xxe siĂšcle. La difficultĂ© de ce parti-pris rĂ©side dans lâĂ©laboration prĂ©alable dâun modĂšle de base suffisamment souple pour sâadapter Ă des configurations trĂšs diverses. Le secteur correspondant Ă lâemprise du stade et de lâodĂ©on est reprĂ©sentĂ© avec un niveau de dĂ©tail Ă©levĂ©. Pour le reste, les lacunes imposent une Ă©pure. La facilitĂ© de lecture et lâĂ©volutivitĂ© constituent deux qualitĂ©s majeures du modĂšle