Methods for the characterization and comparison of mixing efficiency of different confined opposing jet mixing devices.

International audienceThe work reported here describes the methods to characterize and compare mixingefficiency of three high-intensity mixers of the same family : T-shaped tube, Y-shaped tube with a 90°angle and Hartridge-Roughton mixing device. Mixing efficiency is investigated using two chemicalmethods, the “iodide-iodate” micromixing test reaction and an acid-base neutralization. Experimentsare carried out under the same operating conditions, with Reynolds numbers from 15,000 to 40,000.Micromixing time constant values determined by both methods are very close and lead to the samemixer classification. Under the same operating conditions, the Y-tube mixer has the lowest mixingefficiency, while the Hartridge-Roughton mixing device is much more efficient than the two othermixers

Effects of the stirred tank's design on power consumption and mixing time in liquid phase

International audienceThe influence of the stirrer type and of the geometrical parameters of both tank and agitator (clearance of an impeller from tank bottom, impeller diameter, draft tube and geometry of the tank bottom) on power consumption and mixing time in liquid phase under turbulent regime conditions (Re > 10(4)) have been studied. Different types of agitators have been used, namely Rushton turbine, 45 degrees pitched-blade turbine, MIXEL TT and TTP propellers and 1-stage or 2-stage EKATO-INTERMIG propellers. All these stirrers were tested with the same power consumption per unit mass of liquid. On the basis of measured power consumption per unit mass, which is required to achieve the same degree of mixing, the results obtained in the present work show that the TTP propeller is the most efficient in liquid phase. Recommendations on the optimum geometric configuration have been made for each type of stirrer

Modelling of the agglomeration in suspension process with multidimensional kernels

International audienceThis paper deals with the modelling and the simulation of an agglomeration in suspension process. It is shown how classical kernel models (constant, sum or product kernel) based only on the agglomerate size are inappropriate to represent such a complex process. A methodology for the elaboration of a phenomenological multidimensional kernel considering both liquid binding composition and size agglomerate is proposed. This kind of kernel is far more realistic than usual ones. The model parameters may be identified from experimental study. Preliminary results are presented and discussed

Method for the prediction of nuclear waste solution decontamination by coprecipitation of strontium ions with barium sulphate using the experimental data obtained in non-radioactive environment

International audienceThe theoretical and experimental prediction of the decontamination factor during the treatment of nuclear waste solutions is generally a cumbersome problem due to their complex composition and extremely dangerous environment. Here, a methodology is proposed and experimentally validated allowing the prediction of the decontamination factor by carrying out experiments in small pilot installations in non-radioactive environment. These experiments provide the population density and the crystal growth data of the coprecipitating agent which serve for calculations according to the general relation presented in this paper

Modeling and Comparison of Continuous and Semi-Continuous Processes for Simulating Decontamination of Liquid Nuclear Wastes by the Coprecipitation of Strontium Ions with Barium Sulphate

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Comparison of the method of classes and the quadrature of moment for the modelling of Neodymium Oxalate Precipitation

International audienceOxalic precipitation is generally used in the nuclear industry to deal with radioactive waste and recover the actinides from a multicomponent solution. To facilitate the development of experimental methods and data acquisitions, actinides are often simulated using lanthanides, gaining experience more easily.The purpose of this article is to compare the results achieved by two methods for solving the population balance during neodymium oxalate precipitation in a continuous MSMPR (Mixed Suspension Mixed Product Removal). The method of classes, also called discretized population balance, used in this study is based on the method of Litster. Whereas, the Quadrature Method of Moment (QMOM) is written in terms of the transport equations of the moments of the number density function, all the integrals are solved through a quadrature approximation thanks to the product-difference algorithm or the Chebyshev algorithm.Primary nucleation, crystal growth and agglomeration are taken into account. Agglomeration phenomena are been found to be represented by a loose agglomerates model. Thermodynamic effects are modeled by activity coefficients which are calculated using the Bromley model. The sizes of particles predicted by the two methods are in good agreement with experimental measurements

A simultaneous determination of nucleation and growth rates from batch spontaneous precipitation

International audienceA review of the literature shows that many methods for determining the nucleation and growth rates from batch precipitation experiments are inaccurate. Therefore, a new method has been developed to derive these two rates based on the simultaneous measurement of the supersaturation and the crystal size distribution during a batch spontaneous precipitation. All experimental data are used without any previous numerical treatment and the differential kinetic equations are replaced by integral ones Id avoid numeric derivation. The kinetic parameters of the two postulated laws of nucleation and crystal growth rates in kinetic equations are separately determined by non-linear optimization. This method was applied to barium sulphate precipitation. Experiments were performed in a 201 stirred reactor. Effects of reagent adding order and stirring speed were noticed. The influence of initial supersaturation was determined. The nucleation rate was found to obey an exponential law with a primary heterogeneous dominating process. The linear rate of crystal growth follows a 2.1 power law and the process seems to be limited by a surface integration step. The kinetic results were compared to those previously reported

CFD simulation of barium carbonate precipitation in a fluidized bed reactor

International audienceCFD techniques are used to study the precipitation of barium carbonate in a solid–liquid fluidized bed reactor. Experimental analysis of the hydrodynamic behaviour for a neutralization reaction in the fluidized bed column, followed by CFD simulations is carried out using different reaction models. The Eddy Dissipation model, the Eddy Dissipation model-MTS and the Eddy Dissipation Concept micro-mixing models are tested in order to simulate the acid–base instantaneous reaction. The modelling of the precipitation in a fluidized bed reactor is coupled with the Eddy Dissipation model-MTS micro-mixing model and the k- standard turbulence model. Barium carbonate is chosen as the model substance. The liquid phase reaction rate as well as the nucleation, growth and aggregation kinetics is included in the precipitation model. The discrete method is chosen to solve the population balance equation. The activity coefficient needed to calculate the supersaturation in the kinetic equations is determined using the Bromley's method. A good agreement is obtained between the experimental results and the CFD results

Modélisation de la précipitation de l'oxalate de néodyme dans un réacteur parfaitement mélangé

Voir aussi: Récents Progrès en Génie des Procédés, (CDrom) Numéro 107 - 2015ISSN: 1775-335X ; ISBN: 978-2-910239-81-7, Ed. SFGP, Paris, FranceNational audienceLes réactions de précipitation étant très sensibles aux paramètres de fonctionnement, la modélisationapparaît comme un outil très utile pour prédire les évolutions d’un système soumis à des conditionsvariables. L’objectif de cet article est de présenter la démarche de modélisation appliquée à laprécipitation de l’oxalate de néodyme dans un réacteur parfaitement mélangé. Le modèle prend en compteles lois cinétiques de nucléation primaire, de croissance cristalline et d’agglomération. Les effetsthermodynamiques sont pris en compte au travers des coefficients d'activité calculés à partir du modèle deBromley. Pour l'étude de la nucléation, les expériences ont été réalisées dans un appareil spécifique quipermet d’obtenir un temps de micromélange très efficace, inférieur à la milliseconde. La vitesse denucléation primaire homogène suit l'équation de Volmer-Weber. La vitesse de croissance cristalline est dupremier ordre par rapport à la sursaturation. Quant au noyau d'agglomération, il ressort qu’il estindépendant de la taille des particules et correspond à la formation d’agglomérats de type ouvert.Le bilan de population est résolu par la méthode des classes à l’aide d’un schéma d’ordre 3 pour le termede croissance (Koren) et d’une discrétisation de type Hounslow, améliorée par Lister puis Wynn, pour lestermes d’agglomération. Les tailles de particules prédites pour différentes conditions expérimentales deprécipitation sont en bon accord avec les mesures expérimentales, ce qui valide le modèle ainsi quel'approche phénoménologique utilisée dans le cadre de cette étude

Determination of residual sodium content in silver and silver oxide composite materials

The presence of silver ions deforms strongly the linear response for sodium determination by flame spectrophotometry. Relative errors up to 60% can be obtained if linear instead of non-linear relations are used. A correction function is defined and a method based on mathematical considerations is proposed for elimination of interference in determination of sodium concentration