103 research outputs found

    Nanofiltration de solutions de nitrate d'ammonium. Etude des paramĂštres influents

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    Cet article propose une Ă©tude de la rĂ©tention du nitrate d'ammonium par une membrane commerciale de nanofiltration (Nanomax 50). Les effets de la pression, de la concentration et de la vitesse d'Ă©coulement tangentiel ont Ă©tĂ© Ă©tudiĂ©s avec le souci d'une meilleure comprĂ©hension du mĂ©canisme de transport des ions nitrate et en vue d'une optimisation de la rĂ©tention. Le taux de rĂ©tention des ions nitrate augmente dans un premier temps avec la pression, atteint un maximum puis diminue. La rĂ©tention, pour des pressions Ă©levĂ©es, peut cependant ĂȘtre amĂ©liorĂ©e en augmentant la vitesse d'Ă©coulement tangentiel. La sĂ©paration rĂ©sulterait d'un rapport de diffĂ©rentes forces : une force d'entraĂźnement radial dans le pore (illustrĂ©e par l'effet de la pression transmembranaire), une force d'entraĂźnement tangentiel vers le rĂ©tentat (illustrĂ©e par l'effet de la vitesse d'Ă©coulement tangentiel) et une force de surface traduisant les interactions membrane-solutĂ© (illustrĂ©e par l'effet de la concentration). L'Ă©quation de Spiegler et Kedem est proposĂ©e en premiĂšre approche de modĂ©lisation pour une valeur limite de pression.Many water sources deal with the problem of increasing nitrate concentrations above authorised levels for drinking water. In order to minimise this amount of pollution and to achieve high quality of water and reused water in the distribution system, membrane processes are becoming a promising technology. Indeed, they present the major advantages of a small land area requirement, low temperature operation, continuous separation, better effluent quality, little or no sludge production and a large reduction in the quantities of chemical additives. Reverse osmosis has already been used to remove most of the nitrates together with the other solutes, but the disadvantage is that this technique induces a total demineralisation of the treated water. Another possible filtration process, nanofiltration, has been investigated in this study while no extensive research has been carried out on its nitrate removal potential. Theories cannot adequately predict the influence of operating parameters on membrane performance. Consequently, new membranes and modules must be experimentally evaluated for each new application. The main objective of this study was to provide fundamental data for designing an operation of nanofiltration under various operating conditions such as transmembrane pressure, cross-flow velocity and initial feed concentration for drinking water and water reuse purposes.The retention rate rises with an increase of the applied pressure, reaches a maximum and then decreases. Such a result is quite different from those usually mentioned in the literature where the retention increases and reaches a plateau when the pressure grows. The singular decrease of the retention rate observed in this study could be explained in terms of a concentration polarization phenomenon. However, since the volumetric flux increased linearly with the pressure and remained close to the pure water flux, it might be thought that such an assumption is not valid in the case of this work. Therefore, another hypothesis has to be provided to explain the variation of the retention with transmembrane pressure. As the size of NH4+ ion (ionic radius=0.148 nm) is lower than that of the pore of membrane (diameter=1 nm), cations can enter the pores where they are partially retained due to surface forces (electrostatic and friction forces). When the pressure increases, these forces remain constant while drag forces increase due to the flux in the pore. At low pressure (∆P < 5 bars), the surface forces are stronger than the drag forces. Therefore, the solute flux remains low while the solvent flux increases with the pressure, leading to an increase in the solute retention. Above a given pressure (≅ 5 bars), the drag forces become higher than the surface forces. Consequently, the retention rate decreases.As can be observed in the obtained results, the retention rate decreased when the feed concentration was increased regardless of the operating pressure. This effect is mainly attributed to the cation shielding of the effective charge of the membrane. This characteristic can be explained by the fact that the electric repulsion becomes less efficient at higher concentration. It has been recognized that the effective charge density of the membrane decreases with an increase in the feed concentration of an ionic solution. Consequently, the retention rate of the co-ion due to charge effect is reduced. It follows that a greater amount of nitrate ions could permeate when feed solutions of higher concentration are applied.The effect of cross-flow velocity on the fluxes is insignificant since the permeate flux depends only on transmembrane pressure. However, the retention performance increases with velocity. The lower the cross-flow velocity, the higher the interaction between the solute and the membrane. Therefore, at low cross-flow rate, the solute amount that enters the membrane pores is high. When the drag forces become stronger than the surface forces, as explained above, the retention sharply decreases. At high cross-flow velocity, the feed circulation transports a large solute amount and therefore, the solute amount that enters the pores is reduced and is less sensitive to operating pressure. In consequence, the sensitivity of the retention to transmembrane pressure is not so marked. It might be thought that for a very high cross-flow velocity, the retention increases and then remains constant.It was demonstrated in this work that nanofiltration can be successfully used to remove nitrates from water. The retention was shown to depend strongly on operating parameters such as feed solution concentration, applied pressure and circulation cross-flow rate. In fact, the retention is mainly determined by the intensity of the solute / membrane interaction. This interaction comes from two main forces: a tangential one due to the feed solute flow (illustrated by the cross-flow velocity effect) and a radial one in the pores due to drag forces (illustrated by the transmembrane pressure effect). Moreover, it was observed that the valence of the associated ions is an important factor that can affect nitrate retention. It can be expected that the optimization of the separation performance will result of the best combination of all these parameters. Therefore, with a view to a future industrial application, it will be necessary to take into consideration the chemical composition of the resource and to adapt the operating conditions to the desired objectives

    Enhancement of Heat Transfer by Ultrasound: Review and Recent Advances

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    This paper summarizes some applications of ultrasonic vibrations regarding heat transfer enhancement techniques. Research literature is reviewed, with special attention to examples for which ultrasonic technology was used alongside a conventional heat transfer process in order to enhance it. In several industrial applications, the use of ultrasound is often a way to increase productivity in the process itself, but also to take advantage of various subsequent phenomena. The relevant example brought forward here concerns heat exchangers, where it was found that ultrasound not only increases heat transfer rates, but might also be a solution to fouling reduction

    Effet des ultrasons basse frĂ©quence sur l’hydrodynamique d’un rĂ©acteur annulaire continu : approche expĂ©rimentale en Distribution des Temps de SĂ©jour (DTS)

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    Ultrasound (US) are particularly interesting for their mechanical effects enabling transfers activation, in particular by generating mixing. However, this phenomenon has not yet been quantified in a continuous reactor, which is nevertheless a key point for the intensification of such processes. For this purpose, this work characterized the hydrodynamics within a continuous annular reactor under low frequency ultrasound via a Residence Time Distribution experimental approach (RTD). Reliable and reproducible experimental protocol and data processing method were developed. The experiments under silent conditions showed that,due to its geometry, the studied reactor had dead zones that are not negligible. The comparison of these results with those obtained under US had clearly demonstrated the action of US in the flow rate range investigated (laminar flow). The comparison of the RTD curves, as well as the average residence time values obtained, confirmed the US effect on the mixing within the reactor. By creating micro-mixing, ultrasound also reduced dead zones. The study of US power influence showed a threshold beyond which its contribution on hydrodynamics is less marked. This point is encouraging for the scale up of reactors under ultrasound

    Transitions sol-gel de colloïdes anisotropes sous champs de cisaillement, pression et ondes ultrasonores, caractérisées par diffusion de rayons x aux petits angles in-situ

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    L'objectif de ce travail est de caractériser aux échelles mésoscopiques, l'effet combiné des champs de pression, hydrodynamiques et ultrasonores sur les mécanismes de transition sol-gel de colloïdes anisotropes d'argiles lors de l'ultrafiltration tangentielle. Pour cela, des cellules de filtration ont été développées en intégrant une lame vibrante sollicitée à 20kHz par un générateur ultrasonore. Ces cellules de filtration permettent l'observation in-situ aux échelles nanométriques par diffusion de rayons X aux petits angles (SAXS). Différentes suspensions aqueuses d'argiles ont été étudiées : des argiles naturelles de montmorillonite Wyoming-Na et des argiles synthétiques de Laponite en présence ou non d'un peptisant le tetrasodium diphosphate (Na4P2O7). Par ailleurs l'effet des ultrasons sur le comportement rhéologique de suspensions a aussi été étudié.  L'effet du pré-cisaillement induit par la pompe du circuit de filtration et l'effet des ultrasons, sur les contraintes de cisaillement des suspensions de Laponite ont été mises en évidence. Les deux sollicitations réduisent les niveaux de contrainte et l'effet est plus marqué sur les suspensions avec peptisant (à interaction répulsive dominante) que sur les suspensions sans peptisant (à interaction attractive dominante). Les évolutions temporelles de la structure et de la concentration en colloïdes en fonction de la distance à la membrane ont ainsi été caractérisées sous différentes conditions de filtration et de sollicitations ultrasonores. Deux mécanismes principaux ont été mis en évidence lors de l'application des ultrasons : soit un mécanisme de fracturation ou d'intensification locale de l'écoulement lorsque les colloïdes forment un réseau dense trÚs anisotrope (cas des suspensions de Montmorillonite et de Laponite sans peptisant), soit un mécanisme d'érosion des couches concentrées pour les colloïdes assemblés en structures ouvertes (cas des suspensions de Laponite avec peptisant)

    Characterisation of mixing rate due to high power ultrasound

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    For meaningful assessment of results from laboratory and pilot plant trials, it is often necessary to know the mixing characteristics within the ultrasonic reactors. Previous workers have used conductivity measurements in an attempt to characterize the residence time distribution in ultrasonic reactors, but these results do not provide direct data on the mixing within the high power region adjacent to the ultrasonic probe tip. We overcome this difficulty though direct visualization of the mixing process within the high energy region close to the tip of the ultrasonic probe. Our analysis proceeds by determining an approximate turbulent diffusivity in a batch reactor arrangement for different values of ultrasonic energy input. For input electrical power levels between 70 and 120 W and a processing volume of 30 ml, the effective turbulent diffusivity varied from about 0.2e-3 m^2/s to 0.7e-3 m^2/s. We demonstrate that such results can be coupled to a suitable dispersion model to estimate the actual residence time distribution in flow-though arrangements when the through-put adds little additional mixing energy. Therefore, coupling the effective turbulent diffusivity identified in a batch reactor with a suitable dispersion model for the reactor therefore offers an alternative approach to the deduction of RTD when determining the actual RTD in the high intensity zone of steady flow sonochemical reactors is problematic

    Method for etching a material in the presence of a solid particles

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    Procédé de gravure d'une structure (1) comprenant au moins un matériau à graver (4), comprenant : - choisir au moins une espÚce chimique apte à réagir avec le matériau à graver (4), - choisir au moins un composé soluble apte à libérer l'espÚce chimique précitée, - réaliser une solution (11) contenant ledit composé et contenant une poudre de particules ou grains solides (13) en suspension, - placer le matériau à graver en présence de la solution, - et produire dans la solution des ultrasons à haute fréquence, à au moins une fréquence, apte à générer des bulles actives de cavitation telles que l'espÚce chimique est générée et réagit avec le matériau à graver en produisant un composé soluble ou un précipité
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