Dynamic modeling of the reactive twin-screw co-rotating extrusion process: experimental validation by using inlet glass fibers injection response and application to polymers degassing

International audienceIn this paper is described an original dynamic model of a reactive co-rotating twinscrew extrusion (TSE) process operated by the Rhodia company for the Nylon-66 degassing finishing step. In order to validate the model, dynamic experiments have been performed on a small-scale pilot plant. These experiments consist in a temporary injection of glass fibers at the inlet of the extruder after it has reached a given operating point. The outlet glass fibers mass fraction time variation is then measured. This experiment does not lead to the RTD measurement. As a matter of fact, due to the high quantity of glass fibers that is introduced, the behavior of the flow through the extruder is perturbed so that the glass fibers cannot be considered as an inert tracer. The dynamic model that we have published elsewhere (Choulak et al., Ind. Eng. Chem. Res., 2004, 43(23), 7373-7382) is adapted to take into account this nonlinear behavior of the extruder with respect to the glass fibers injection and is favorably compared to experimental results. The description of the degassing operation is also included in the model. The model allows simulations of the complete dynamic behavior of the process. When the steady state is reached, the good position of the degassing vent with respect to the partially and fully filled zones positions can also be checked, thus illustrating the way the model can be used for design purposes

Evolution de la porosite, structure et dynamique de l'eau dans les geopolymeres

National audienceLes geopolymeres sont des materiaux aluminosilicates mesoporeux1 a faible teneur en calcium. Ils sont envisages comme solution alternative aux ciments silico-calciques usuels, dans le cadre du conditionnement des dechets nucleaires de moyenne et faible activite. Les geopolymeres sont synthetises par activation alcaline d'une source aluminosilicatee, ici le metakaolin. La dissolution du metakaolin dans une solution de silicates alcaline, appelee solution d'activation, conduit a la formation d'oligomeres, puis, la polycondensation des oligomeres conduit a l'obtention d'une matrice solide tridimensionnelle, amorphe et mesoporeuse, constituee de tetraedres SiO4 et AlO4. Cette suite de reactions s'appelle la geopolymerisation.L'etude de la structure poreuse des geopolymeres et de la solution contenue dans les pores est essentielle pour repondre a des questions concernant leur durabilite. La structure poreuse des geopolymeres peut etre controlee par la formulation de la pate de geopolymeres, i.e. par la quantite d'eau et la concentration en silicates initialement introduits dans la solution d'activation. On note H2O/M2O et SiO2/M2O les rapports molaires en eau et en silice, ou M est un alcalin, Na ou K. Par ailleurs, on note M-H2O/M2O- SiO2/M2O une formulation de geopolymere designee par ses rapports molaires.Plus SiO2/M2O augmente, plus le volume poreux et la taille des pores diminuent, car la geopolymerisation conduit alors a la formation d'un reseau solide plus dense. Plus H2O/M2O augmente, plus le volume poreux et la taille des pores augmentent, car la polycondensation des oligomeres, lors de la formation du reseau solide des geopolymeres, libere de l'eau dont le volume est en grande partie responsable de la formation des pores. La nature du cation alcalin a aussi une influence sur la porosite des geopolymeres les geopolymeres synthetises a partir d'une solution de silicates de potassium (K-Geopolymeres) presentent un volume poreux plus important que les Na-Geopolymeres, ainsi qu'une surface specifique trois fois plus importante. Une precedente etude au LP2C a montre sur une formulation Na-Geopolymere et de K-Geopolymere que la structure poreuse des geopolymeres evolue dans le temps2. En effet, il a ete montre que le volume poreux determine par adsorption d'azote diminue entre 7 jours et 6 mois, signifiant que la porosite se ferme en partie quand le materiau vieillit. La presente etude confirme ce resultat sur un grand nombre de formulation de geopolymeres et sur une duree d'observation de 1 an. La porosite contient une solution confinee, appelee solution interstitielle, resultant des reactions de polycondensation. L'extraction, puis le dosage quantitatifs des elements chimiques presents dans cette solution sur des geopolymeres conserves a 100% HR pour eviter leur dessiccation, n'ont pas permis d'expliquer les resultats concernant l'evolution de la porosite des geopolymeres. Au contraire, la composition de la solution interstitielle est stable apres une periode de 7 jours. Soit les variations des concentrations sont trop faibles pour qu'une evolution soit detectee, soit ce n'est pas la precipitation des elements contenus dans la solution interstitielle qui entraine la fermeture de la porosite. Le mecanisme responsable de la fermeture de la porosite est toujours a l'etude.L'etude de la solution porale par calorimetrie differentielle a balayage a montre l'existence de trois types d'eau dans les geopolymeres un premier type d'eau que l'on attribue a de l'eau libre au centre des pores, un deuxieme type d'eau que l'on attribue a de l'eau proche des parois des pores, et une troisieme population d'eau, non gelable l'eau liee a la surface des pores. Bien que la litterature ait montre que la dynamique de l'eau n'est pas impactee lorsqu'elle est confinee dans des pores de plus de 4 nm de diametre a l'echelle de la nanoseconde, comme c'est le cas dans les geopolymeres, nous avons trouve que le coefficient de diffusion translationnel de l'eau est plus faible dans les geopolymeres que celui de l'eau libre. Ceci est vraisemblablement du aux ions contenus dans la solution interstitielle, plutot qu'a un effet de confinement

An improved model to evaluate the oxidation kinetics of uranium dioxide at temperature below 400DC

International audienceDuring dry air storage, the oxidation of the spent fuel in case of cladding and container failure (accidental scenario) could be detrimental for further handling of the spent fuel rod and for the safety of the facilities. Recently, the phase transition sequence during the first step of parabolic oxidation kinetic has been challenged again and two well-distinguished intermediate products, U4O9 and U3O7 have been identified. Moreover, these observations have shown that the three phases (UO2, U4O9 and U3O7) coexist together. Starting from a previous model of grain oxidation based on finite difference approach, a new model, describing the parabolic oxidation kinetic, has been developed based on the oxygen vacancies diffusion. This model allows in one hand to take into account the coexistence of the three phases and in another hand to describe accurately the plateau behaviour. A confrontation between the model and literature data obtained on non irradiated powders has been carried out and shows that this model can describe the weight gain evolution as a function of time for different temperatures. The diffusion coefficients of oxygen in the two phases (U4O9 and U3O7) were obtained by fitting the model results to experimental data. The comparison with the values given in literature is quite good

Visualisation and Analysis of the Flow along the Kneading Block of a Twin-screw Extruder

Prova tipográfica (In Press).The flow of a silicone oil of high viscosity along a block of staggered kneading discs of a co-rotating twin screw extruder was investigated with the use of an acrylic transparent barrel. A digital video camera recorded the flow path of a red dye, dispersed in the silicone oil, along the screws and in the intermeshing zone. Different geometrical configuration were studied, namely various staggering angles (+30º, 90º and -30º) and thickness (7.5, 15.0 and 22.5 mm) of kneading blocks. The effects of these geometrical parameters on the pressure generation and filling ratio were investigated. It was observed that, even for negative staggering, the filling ratio of the kneading block was lower than one. Based upon these experiments, analytical expressions describing the filling ratio as function of geometry (staggering angle, thickness and number of discs) and operating conditions (feed rate and screw speed) were derived

Direct 3D-printing of a Geopolymer-based Membrane Dedicated to Cesium Decontamination

International audienceIn the context of the dismantling of industrial sites, the search for efficient and economically competitive solutions for the remediation of installations is a key objective to be achieved, in particular the depollution of industrial effluents containing polluting ions. Compared to other methods, solid / liquid extraction exhibits decisive advantages such as a possible selectivity of the targeted pollutant and the use the solid support as a pollutant containment matrix thus minimizing the volume of secondary wastes generated. In the literature, among many materials used as metal adsorbents (zeolites, apatites, ion exchange resins ), the use of geopolymer as exchange materials is also reported. The term geopolymer usually denotes alumino-silicate inorganic polymers synthesized at low temperature (typically less than 95 DC.) by alkaline activation of alumino-silicate materials. The material, which is initially fluid and has no yield stress, set and hardened gradually forming a monolithic material with an amorphous structure and a local order similar to zeolite. This work considers the fabrication and characterization of geopolymer-based architected membrane dedicated to the treatment of cesium-containing solutions. The key aspects of this study rely on the design of a viscoelastic material with requisite rheological characteristics for direct 3D-printing while controlling the microstructural properties and the ion-exchange properties of the geopolymer ink. This direct writing technic enables the control of the multiscale porous structure macroporosity can be designed by 3D direct printing while meso and microporosity are controlled by the chemical formulation of the geopolymer ink

Mécanisme d'oxydation / dissolution du dioxyde d'uranium

Conférence du 02 au 06 juillet 2007. Communication par affiche

Alpha radiolysis induced by plutonium dioxide

International audienceUp to now, plutonium dioxide is the major end product of nuclear fuel reprocessing. Due to the radioactive properties of plutonium, handling and management both of plutonium dioxide and of contaminated wastes involve to take into account the radiolysis phenomena plutonium can induce. For instance, plutonium is known to be a highly hygroscopic compound. Kept under a humid atmosphere, up to 3% wt. water can be sorbed on the surface of plutonium dioxide. The sorbed water then is going to undergo a radiolytic decomposition and this latter will lead to hydrogen evolution. It has been shown through our studies and other reported results that hydrogen formation kinetic depends on the specific surface area of plutonium dioxide, the isotopic composition of plutonium and the relative humidity in the atmosphere. Unfortunately no clear dependences between the rate of hydrogen formation and these parameters can be established. However, it has been shown that in many cases hydrogen amount in the atmosphere reaches a steady-state. Such behaviour is probably linked to surface evolution of plutonium dioxide in presence of moist air and under radiolytic decomposition of water. Studies are still in progress to identify surface changes due to these phenomena. Hydrogen can be produced under radiolysis of some plastic waste contaminated by plutonium dioxide too. Whereas $\gamma$-radiolysis of plastic materials is quite well described, $\alpha$-radiolysis of such compounds, especially the ones in close contact with actinides, is very poorly known. It can be demonstrated that only a small part of the whole energy emitted by plutonium dioxide is going to be deposited in the material and leads to a radiolytic decomposition with hydrogen and others hazardous gaseous products (CH$_4$ for instance) formation. Taking into account the amount of energy transferred to the plastic material, $\alpha$-radiolytic yield of hydrogen can be calculated and then used for waste repository scale-up. It should be noticed that plutonium dioxide can exhibit specific behaviour towards some radiolytic products. For instance, it has been shown that chlorine arising from PVC radiolytic decomposition is going to be retained mainly on plutonium dioxide surface, whereas without plutonium dioxide chlorine is evolved as gaseous HCl. In the same way, $\alpha$-radiolysis induced by plutonium dioxide on several other materials is now studied, such as hydrogen, carbon oxides and short-length alkane production from some lubricating compounds used in fuel pellet manufacturing or from specific waste embedding materials (bitumen, mineral polymers). As the present trend is still to increase fuel burn-up in one hand, and, in the other hand to manufacture new fuel with increasing amount of plutonium, it appears necessary to look closer at the radiolytic phenomena induced by actinide oxide and especially plutonium dioxide to actualize the safety analysis for handling and management of future nuclear fuels and wastes

Rheological behavior of a NA-geopolymer

International audienc

Dosage de traces d'uranium(VI) dans des eaux d'environnement

Conférence du 02 au 06 juillet 2007. Communication par affiche

Context Sensitive Links Add to Marked List (0) Send to: Ions effects on sol-gel transition and rheological behavior in alumina slurries

International audienceAqueous slurries made of silica or alumina particles may behave like sols or like gels depending on the particle concentration, but also the pH or the ion force of the medium. The present study aimed at investigating the different sol-gel transitions observed in systems between a diluted state and concentrated formulations. For this purpose zeta potential and G', G '' measurements, reflecting elastic and viscous behaviors, were selected as the tools of choice to get insights into the mechanisms accounting for the sol-gel transition. We discuss about the relevancy of these simple measurements and affirm that they are sufficient to explain the evolutions in an alumina based system. Indeed, particles can adopt a "repulsive" organization at low salt concentration values (i.e. high surface potential values) or form an attractive percolating network, when the ion strength reaches a critical level. Moreover, the addition of NaOH can lead to very complex moduli variations as both the electrostatic shielding and the pH change; yet, these variations are easily predictable using our reasoning. (C) 2013 Elsevier B.V. All rights reserved