Evolution axiale du coefficient de transfert de matière en colonne à bulles et en lit fluidisé triphasique

Liquid phase volumetric mass transfer coefficients for oxygen are determined in a three-phase fluidized bed and in a bubble column. The concept of exponential decreasing axial variation of volumetric mass transfer coefficient leads to a better representation of oxygen concentration profiles inside the column. Compared to the bubble column, k_la axial variations are more important in the lower part of the fluidized bed column, where solid particles increase the coalescence phenomenum, particularly with viscous liquids.     

Modélisation de l'efficacité de filtration d'un échangeur cyclone

This work is a modelling of the Aireco® filtration system. This heat exchanger, with particle catching capabilities, is made of a cyclonic body which contains refrigerated coils. The water vapour contained in the gas to be treated condensed on the coils surface. Our modelling considers two main capture surfaces: the entry zone in which particles are collected by inertial impaction and the cyclonic body in which particles diffused by centrifugal force is the main phenomena. We found that these considerations explain our experimental results with an accuracy more than 90%, when the Aireco® is on a normal point of working.     

Méthodologie d'étude et de modélisation de la déphosphatation d'effluents aqueux

Due to phosphorus impact on water pollution, a P‐recovery process by calcium phosphate precipitation was developed. The reactor process is a fluidized bed in which phosphate precipitates continuously and coats sand grains. The pilot unit can treat synthetic wastewater flow rates ([P] = 50 mg/l) of about 0.1 m3/h. A two‐stage methodology is proposed to understand the physicochemical phenomena. Fluidized bed precipitation modeling involves thermodynamical concepts taking into account the various calcium phosphates that can precipitate vs. operating conditions (PH, [P], [Ca], T). Coating modeling is performed by a first‐order kinetics reaction between fines and sand grains, combined with a network of ideal reactors representing system hydrodynamics.     

Une nouvelle technologie d'appareils de dépôt chimique en phase vapeur. Partie 2 : Modélisation du dépôt de silicium pur à partir de silane et de silicium dopé in situ au phosphore

A better understanding of Chemical Vapour Deposition (CVD) process can be obtained by modelling the phenomena involved in CVD reactors. The established model is then a useful tool for industrial equipment design and for the optimization of operating conditions. The research for optimal operating conditions is done by a more rational and efficient way using a model. The specific case of the polycristalline silicon deposition from silane in a new technology of LPCVD reactor is used to illustrate the approach. Hypotheses and methods necessary for the development of the model are presented and discussed first, then results are described and compared to experimental data. Finally, the more complex case of in situ phophorus doped polysilicon is discussed.     

Cinétique de la calcination du coke de pétrole en atmosphères neutre et oxydante

A devolatilization kinetic study of oil coke samples under conditions close to those of industrial calcination furnace is presented. The effects of the surrounding conditions (neutral or oxydizing medium), the heating rate and the percentage of oxygen in an oxydizing environment are analyzed. Experiments are carried out in an induction oven and involved thermogravimetric and chromatographic analyses. Results show that, under certain conditions, the nature of the environment influence the devolatilization process. A slow heating rate and a low content in volatiles promote the coke degradation by oxygen in an oxydizing medium. The reactive schemes are derived by tracking the concentration of CH₄, H₂, CO and CO₂. In regions where the coke is not degraded, a kinetic model is used to compare the parameters describing the develotilization phenomena in the two media (neutral and oxydizing), i.e.: the reaction order, the activation energy and the pre-exponential factor. It is shown that the oxydizing medium promotes an increase in the activation energy.     

Optimisation de l'hydrolyse des protéines du lactosérum en utilisant des détergents commerciaux enzymatiques et non-enzymatiques

The main aim of this study is to characterize the effect of some commercial detergents on whey proteins in order to select an efficient detergent to clean inorganic membranes fouled during the process of whey proteins fractionation. The influence of such variables such as time, pH, temperature, type and detergent concentration on the rate of protein hydrolysis has been established. Also, the nature and size of produced peptides have been determined. The most efficient values of cleaning parameters have been obtained by using the enzymatic detergent P3-Ultrasil 62.     

Etude numérique et paramétrique de la combustion de particules d'aluminium

Several numerical models have been investigated to study the reactivity of aluminium particles coming from the combustion of solid propellants in spatial launchers. The Law model has been rapidly selected as it yields a continuous model from an aluminium drop to an oxide drop. The model also gives the residue size at the end of the combustion process. This could explain the scaling observed in the rocket engines. In order to study the coupling between the flow and the combustion, the combusion of aluminium drops has been widely analyzed with 0D computation. The main mechanisms of the combustion process have been identified.     

Simulation bidimensionnelle de phénomènes de convection naturelle et forcée en formulation primitive

The air circulation, and the temperature and pressure profiles were simulated in two dimensions using a hybrid finite difference method which combines the MAC method and the stream function/vorticity approach. Three different flow problems were solved: (i) forced convection in a closed cavity; (ii) natural convection in a closed cavity; and (iii) forced and natural convection in a cavity with localised openings and hot sources. The results for the first two problems were compared to the literature and the results for the third problem were compared to data obtained from an experimental apparatus.     

Analyse Expérimentale et Numérique du Comportement de Membranes Thermoplastiques en ABS et en HIPS dans le Procédé de Thermoformage

In this work, we are interested, on the one hand in the characterization of circular polymeric ABS and HIPS membrane under biaxial deformation using the bubble inflation technique, on the other hand in modelling and numerical simulation of the thermoforming of ABS and HIPS materials using the dynamic finite element method. Hyperelastic models (Mooney‐Rivlin, Ogden) are considered. First, the governing equations for the inflation of a flat circular membrane are solved using a variable‐step‐size‐finite difference method and a modified Levenberg‐Marquardt algorithm to minimize the difference between the calculated and measured inflation pressure. This will determine the material constants embedded within the models used. For numerical simulation, the lagrangian formulation together with the assumption of the membrane theory is used. Moreover, the influence of the hyperalastic model on the thickness and on the stress distribution in the thermoforming sheet are analysed for ABS and HIPS materials.     

Propriétés rédox de fullerènes fonctionnalisés

The C₆₀ physical chemistry properties make it possible to use it an elemental base for the synthesis of new materials. As the functionalization of fullerenes modify these properties, the modifications of their physical chemistry properties, in particular their electrochemical properties have been studied for a series of highly functionalized fullerenes. This article presents the results obtained with the electrochemical studies of fullerenes C₆₀ mono- and polyfunctionalized covalently. Our objective was to analyze the possible correlations between the redox properties of fullerenes and the degree, shape and natue of the functionalization. A series of functionalized fullerenes on positions [6, 6] was synthetised to carry out the investigation. This unique series of mono- and poly-functionalized fullernes provides an effective study of the modifications of the physical chemistry and electrochemical properties vs. the degee, shape and nature of the functionalization. The results obtained have been compared with literature data.     

Modélisation Mathématique de la Décantation de la Boue Rouge

A good performance of the solid‐liquid unit operation is required for the economical exploitation of the Bayer process. A computational fluid dynamics (CFD) model simulating the operation of the last washing stage mud thickener of a large Canadian alumina plant is presented. The parametric study of the impact of changes in four parameters shows that the diameter of flocculated red mud particles, the feed flow rate and the radius of the feed well are critical parameters for the operation of the thickener.     

Étude hydrodynamique des lits fluidisés sous vide et sous haute température

After having analyzed the few literature results concerning fluidized beds hydrodynamics under reduced pressure, new theoretical elements are proposed which provide an estimation of the beginning and finishing fluidization velocities. Numerous experimental results(U_mfapp, U_mb, U_mp, ?_mf, ?_mb, ?_mp), obtained at 20 and 500°C, are then presented for several sub-atmospheric pressures. Finally, f& the first time, at leastto our knowledge, by high frequency recording pressure drops through the bed, the influence of pressure decreases on the hydrodynamics is accurately analyzed.     

Etude de l'influence des paramètres de fonctionnement d'un réacteui électrochimique à lit fixe conique

The recovery of metallic ions from dilute solutions has been carried out with a three-dimensional electrode. This work follows a study of electrochemical reactors with transported beds enclosed in a conical vessel. It deals more precisely with the influence of flow direction (upwards or downwards) and the cone angle when a reactor is working at the limiting diffusion current. The results of this study permit to define the geometric parameters determining the most efficient operation of the reactor.     

Contrôle hydrodynamique de l'evaporation d'un film liquide ruisselant sur une paroi isotherme à symétrie de révolution dans un courant d'air humide

A thin-film evaporation from a surface of a axisymmetrical body in a humid air stream was studied using a finite difference method associated with Thomas' algorithm. The surface of the body is supposed isothermal. The variability of physical properties is taken into account, and a steady-state and laminar regime is supposed. The influence of the main parameters of the system on the evaporation is determined, especially the wall profile. It is shown that, for certain conditions of calculation, it is possible to find a flat ellipsoid of revolution offering a uniformly accessible wall between the pole and the equator.     

Analyse des signaux temporels de pression dans un lit fluidisé turbulent de FCC

The fluidized bed turbulent regime is an intermediate regime between bubbling fluidization and pneumatic transport, which can be characterized by the study of wall pressure fluctuations with time. Our experimental results confirm that the statistical analysis of the signal enables to determine the regime boundaries. An original frequency analysis shows that there is no systematic change in dominant frequencies in the turbulent fluidization regime. However, an enrichment of frequency spectra occurs. The signal stochastic analysis by Hurst's method leads to the same conclusion.     

Le cyclone: Un réacteur chimique. Application à la réaction de décarbonatation de NaHCO3

The purpose of the present paper is the experimental and theoretical study of the behaviour of the cyclone working as a chemical reactor. The model reaction is the decarbonation of NaHCO₃. Thanks to the use of scale-up relationships previously determined, it is possible to derive kinetic constants in good agreement with other published values obtained under quite different conditions. The measurements are performed in a wide range of experimental conditions (particle size, residence time of particles, cyclone wall temperatures, solid flow rate). The paper emphasizes two possible fields of utilization for the cyclone: a device for the measurement of kinetic constants, a processing reactor operating with high throughput of solids.     

Simulation de la distillation catalytique par un nouveau modèle de transfert – Application à la production d'acétate de méthyle

A catalytic distillation model, based on Maxwel‐Stefan Equations, is presented. In this model, the effective diffusion in porous catalyst is taken into account. Since this model is developed in a commercial environment (ProSim Plus process simulator), particular attention is paid to the solution strategy: robustness and adaptability are of particular importance for the user. The influence of catalyst characteristic, column design, mixture non – ideality and liquid/vapour interface are discussed. We conclude that an optimum design exists. Reflux ratio and catalyst location are key‐parameters.     

Écoulement de Taylor‐Couette en géométrie finie et à surface libre

The reactors of Taylor‐Couette type are relatively recent application in the engineering processes field. The stability and the flow structure in this device are examined under the effect combined of the free surface and the fluid's height in annular space. The exploitation of the flow regimes is carried out according to the number of Taylor for various values of aspect ration Γ. By means of ultrasonic velocimetry (UPV) we determine axial average velocity profile and the axial wavelength. We also carry out the spectral analysis by Fourier's fast transform of the fluctuations associated with the average velocity values in order to analyze the flow structure. We particularly aim for checking the presence or the absence of the azimuth wave according to the filling height and the rotational velocity imposed. We highlight the effects of free surface and aspect ratio on the conditions of appearance of the second instability ( wavy mode). The analysis of the fluctuation shows that the appearance of the azimuth wave regime (Wavy mode) is delayed when the aspect ration decreases. Below a critical value noted Γc = 10, the azimuth wave regime is not observed any more.     

Analyse des interactions fluide-paroi dans un système ouvert a partir de la connaissance de la distribution des temps de sejour. Application aux dépôts d'aérosols par thermophorèse

In an open material system where the residence time distribution of a fluid is known, it is possible, using micromixing Zwietering' model to predict the results of linear interaction between the fluid and the wall of system but also for the aerosol particles transported by the fluid flow. This methodology is applied for aerosol deposition by thermophoresis in cylindrical pipe with constant wall temperature. The interpretation of experimental results in laminar flow shows that, for Knudsen numbers between 0.2 and 1.0, the thermophoresis coefficient must be calculated by Talbot' correlation. Finally, in transition flow, the aerosol mass deposition by thermophoresis and turbulent diffusion becomes probably minimal.     

Comportement thermique et chimique de particules solides subissant une réaction de décomposition endothermique sous ľaction ďun flux de chaleur externe

The purpose of the present paper is an experimental and theoretical study of the behaviour of solid particles undergoing an endothermic reaction under the effect of an external heat flux. It is shown that after a pure heating period, the reaction occurs under quasi isothermal conditions as soon as the particles have reached a certain temperature T_R. T_R is normally much lower than the external heat source temperature. The variations of T_R and the effect of temperature stabilization are studied as a function of solid properties, chemical reaction (kinetic and thermodynamic), and heat transfer conditions. Data obtained from measurements of the decarbonation reaction of NaHCO₃ agree well with predictions of the theoretical study.