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

A new solution for closure problem in crystallization modeling using moments method

Modeling of crystallization processes taking simultaneously into account nucleation, crystal growth, agglomeration and breakage gives complex equations very difficult to solve numerically. Furthermore, in most cases, the prediction of mean particle size and coefficient of variation are enough for practical use. For these two reasons, the resolution of crystallization equations is not directly realized from the population density but rather from the moments of the first one. For example, crystallization in non-stationary regime taking into account nucleation, crystal growth and agglomeration is modeled using the following equations where k =0, 1, 2, . . .: dmk dt ¼ rNILk 0 þ kGmk1 þ Z V 0 Z V 0 1 2 L3 þ k3 k=3 Lk Ib ðL; kÞInðL; tÞInðk; tÞdLdk For certain agglomeration kernels b(L,k), differential equations system can be obtained explicitly when k /3 is a whole number. However, intermediate moments take part in crystallization models and must be also determined. For example, for moments of order 0, 3 and 6, we can obtain three differential equations where moments of order 2 and 5 also appear. Consequently, this system is not closed, because it contains 5 unknowns. To close it, it is necessary to express moments 2 and 5 as a function of moments 0, 3 and 6. An original idea for closuring the system consists in determining these moments by interpolation from the moments of order 0, 3, 6, . . .. Because of large variation values between moments 0, 3, 6, it is necessary to use very stiff interpolation functions. These interpolation functions can be used to express moments of any order (even rational moments in the case of crystal breakage) according to moments m0, m3 and m6. Simple relations are given to illustrate this method which allows to simplify and solve complex prediction models for mean size and coefficient of variation of particle size distributions obtained in crystallization processes

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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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

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

Experimental study on mixing and precipitation in a fluidized bed reactor

Visualization experiments using an acid–base decolourization technique with phenolphthalein as indicator are performed in order to investigate the influence of operating parameters and feed tube position on the mixing efficiency of reacting fluids in the absence and presence of the fluidized solid phase composed of glass micro balls of different diameters. The precipitation of calcium carbonate as calcite is then experimentally investigated in the same cylindrical fluidized bed reactor where nucleation, crystal growth and agglomeration take place simultaneously. In all experiment narrow particle size distributions are obtained. An increase of fluid velocity increases the agglomerate particle size principally due to the intensification of the agglomeration process. At the same time, the carryover particle size is increased. The maximum capacity of the fluidized bed reactor is experimentally obtained at the intersection of agglomerate and carryover particle size variations