Dépôt chimique CVD de silicium dopé in situ au phosphore. Partie 2: Analyse théorique et modélisation

The behaviour modelling of a CVD reactor is useful to better understand the involved complex mechanisms and determine the conditions required for the fabrication of depositions according to industrial standards. This approach was applied to the deposition of polycrystalline silicon doped in situ with phosphor. A model was developed to represent the deposition process which predicts correctly the excess of deposition thickness caused by silylene. The control of the deposition operation is mainly governed by the feed phosphine concentration and the inlet geometry where silylene is formed.     

Influence des principaux paramètres opératoires sur le dépôt de silicium à partir de silane par CVD en lit fluidisé en vue de limiter les problèmes d'agglomération

Chemical vapour deposition of silicon from silane on fluidized powders is an efficient and flexible surface modification technology; however, its main drawback is the agglomeration of the fluidized particles beyond a critical initial concentration of silane. To clarify this phenomenon, we have studied the influence of operating conditions covering temperatures lower than 550°C, both experimentally and theoretically using the simulation model of Kato and Wen (1969). The influence of the main operating parameters has been accurately determined not only on the process throughput but also on the thermal behaviour of the bed, which is directly linked to the solid phase motion and then to the possible agglomeration of particles.     

Dépôt chimique CVD de silicium dopé in situ au phosphore. Partie 1: Etude expérimentale

The fabrication of silicon layers doped in situ with phosphor using CVD technique is still poorly controlled by the microelectronics industry. Significant thickness heterogeneities are noticeable on circuit and load, which greatly limits the process utilization. An experimental study of this deposition was carried out. The influence of many parameters (temperature, pressure, feed composition) was studied in terms of deposition thickness and uniformity as well as crystalline structure and resistivity. The advantages and involved mechanisms are presented.     

Analyse par traitement d'images du dépôt de particules projetées par un jet gazeux sur une surface plane

The purpose of the present paper is the experimental study of the deposition of particles conveyed bya gaseous turbulent jet onto a flat and horizontal surface. The size distribution of settled particles is studied as a function of the distance from the jet stagnation point by image analysis. The measurements are made with several particle samples and under very different experimental conditions. The values of the settling diameters are gathered in a dimensionless representation dependant on a Stokes number, St₀. The results are finally correlated as a function of a dispersion parameter J.     

Modélisation d'un four de calcination du coke de pétrole: II. Simulation du procédé

A one-dimensional steady-state mathematical model has been developed to simulate the process of calcination of petroleum coke in a rotary kiln. A simulation is made to reproduce the operating conditions prevailing during a normal working day, in order to validate the model, and to study the effects of the main process control variables. It has been found that of the energy provided by combustion, 59.5 % comes from hydrogen, 14.6 % from the carbon of the coke bed, 13.8 % from coke dust, 11.3 % from methane and 0.8 % from tar. A comparison with existing models is made to identify eventual similarities between the various kilns under study. It appears that maximum coke temperature occurs at 45 to 55 % of kiln length, while volatile matters evolution takes place between 5 and 60 % of kiln length, measured from feed end.     

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.     

Récupération sélective du vanadium dans l'effluent d'un procédé de fabrication du blanc de titane

The selective recovery of vanadium from an effluent of a manufacturing process for titanium white (oxide) has been made. The process retained is based on the partial neutralization of the effluent by means of calcium carbonate and on the separation of metal hydroxides from calcium sulfate and the solution. The hydroxides are thereafter calcined in the presence of a sodium salt, so that sodium vanadate may then be extracted by washing with water. Vanadium is finally precipitated by the addition of ammonium chloride or pH adjustment. The experimental results obtained in the laboratory study of the effluent neutralization, the calcination and leaching of the hydroxides and the vanadium precipitation are presented and discussed. The yields obtained in the various stages and their interdependance are also discussed.     

Le rendement de deux réservoirs agités en série avec alimentation séparée,basé sur la cinétique de monod

The performance of a bioreactor composed of two chemostats in series can be affected significantly by the split of feed into two, one for each chemostat, for reactions following Monod kinetics. Under certain operating conditions, split of feed can be greatly beneficial to the reactor performance. Optimization of performance is discussed; a general criterion is presented, applicable to either the maximization of overall conversion for a given total residence time or the minimization of total residence time for a given overall conversion. The maximization of overall conversion has also been studied with respect to the feed-split ratio. The dimensionless Michaelis-Menten constant is a very important parameter in the selection of optimal operating conditions.     

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.     

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.     

Reaktorstabilität und sichere Reaktionsführung

Reactor stability and safe reaction engineering . Stable operating behaviour of chemical reactors and stability in the sense of stability theory of chemical processes are by no means synonymous concepts. Thus, appropriate control can permit reliable operation at a unstable operating point, while even global stability will not necessarily rule out a runaway reaction. The latter is the case when strongly exothermal reactions are characterized by a pronounced parametric sensitivity. In order to delineate the concepts, ?stability”? and ?parametric sensitivity”? are explained first for stirred tank and tubular reactors with strongly exothermal reaction. Both the conventional view of reaction engineering and the theory of heat explosion commonly used in reaction engineering are considered in detail. Practical problems of safe reaction engineering are subsequently discussed for the control of batch reactors, the control of semibatch reactors, and the behaviour of bundled-tube reactors.     

L'électrolyseur dans l'optique de la dynamique des procédés

This article written for the non-specialists reviews the various methods for modeling electrolyzer behavior. Deterministic and stochastic models are discussed and the adaptation of process dynamic to electrolysers is emphasized.     

Dispositif de déposition de couches de suspensions de céramiques appliqué à la stéréolithographie

A system to coat uniform layers of ceramic suspensions has been developed for rapid prototyping applications. The metering system is made of a gas injection nozzle which height, angle of attack and pressure can be adjusted. The coated layer homogeneity and the process reproducibility have been studied. The effect of each parameter on the coated layer thickness has been characterized using a design of experiments approach and a reliable mathematical model is proposed to optimze the coating system.     

Evaluation des dépôts d'aérosols par thermo et diffusiophorèse lors de l'ecoulement dans une conduite cylindrique — applications aux essais tuba diffusiophorèse

In the hypothesis of a severe accident on a pressurized water nuclear reactor, the aerosols created and carried by a gas flow consist of steam and incondensable gases which can settle by different mechanisms on the walls of the primary circuit. In particular, in the tubular bundle of steam generator, on contact with the wall cooled by the water flowing at the outside of the bundle, the aerosols are transferred and fixed on the wall by thermo and diffusiophoresis. A model of the deposition on a vertical cylindrical tube based on the mass and energy balances allows us to show that the efficiency of the deposit depends only on two thermohydraulic parameters. The theoretical relations of the efficiency applied to the Institut de Protection et de Sûrete Nucleaire experimental program “Tuba diffusiophoresis” agreed with experimental results and confirm that the efficiency of deposition by thermo and diffusiophoresis must be evaluated simultaneously rather than independently.     

Etude cinétique de la déshydrtation du sulfate de cuivre par thermogravimétrie et thermogradientimétrie

A new type of thermobalance has been built, Sample temperatures have been determined by a new technique called “thermogradientimetry”. A special crucible assembly allows control of the atmosphere surrounding the sample. Kinetic results for the three successive stages in the dehydration of copper sulphate indicate that the reactions involved are of zero order. The apparent activation energy for each stage of the dehydration has been avaluated under isobaric conditions for different water vapor pressures.     

Traitement par une méthode d'éléments finis de modèles de colonnes de rectification discontinue à garnissage

Mathematicals models representing several operating modes of a packed batch distillation column are detailed. In the first part, the authors consider steady-state operation with total reflux, where the axial and radial dispersion phenomena are superimposed on the plug flow of the liquid and vapour phases. The equations are solved by means of a finite element method based on the Galerkin criterion, and also by using a finite difference procedure. The numerical study of the steady-state operating mode, including both axial and radial dispersion phenomena, allows us to note that the last term can be, in most cases, neglected. On the other hand, the dynamic model analysis has shown that the axial dispersion phenomenon is always indispensable for accurately representing the column operation.     

Étude de la déshydratation des ethers de glycol comme une nouvelle voie de synthèse des vinyl-ethers

A study has been made to determine the basic conditions to develop an economical manufacturing process for vinyl glycol ethers. To achieve the aforesaid purpose, the synthesis of the above-mentioned products by means of glycol ether dehydration has been experimentally investigated; the most suitable way to achieve it has been found to be an indirect dehydration (pyrolysis of the corresponding acetates). After having found the best way of synthetizing the vinyl glycol ethers, a determination has been made of the parameters which have a significant influence on the reaction carried out, in the gaseous phase, in a glass tubular reactor under normal pressure. An optimization study has enabled to develop a model from which the maximum response area has been found, selectivity being the objective function.     

Cinétique de dégagement des matières volatiles lors de la pyrolyse d'électrodes de carbone industrielles

In aluminium plants, the anode baking process is associated with an important release of volatile combustible matter. It is the purpose of this study to investigate the kinetics of the evolution of these volatiles. A large thermogravimetry set-up has been designed in such a way that it could simulate real plant conditions. Samples of two distinct masses have been subjected to pyrolysis at different heating regimes. The loss of weight together with the concentrations of the released gases were recorded on a continuous basis during the temperature rise. Kinetic results were found for methane, hydrogen and tar. No variation of the order of reaction with the heating rate was observed, whereas the activation energy and the pre-exponential factor increased. Furthermore, for the range of sample dimensions studied, it was found that the mass of the solid had no significant influence on the kinetic parameters.     

Modélisation du traitement de purification de l'acide phosphorique de voie humide par la méthylisobutylcétone

The present paper reports results for the purification of phosphoric acid by liquid—liquid extraction with methylisobutylketone. The simultaneous effect of some operating parameters, namely concentration of the feed acid, massic ratio of solvent to feed, temperature and number of theoretical stages, on the performances of the extraction step has been examined using a factorial design 2⁴. The established models allowed us to forecast the experimental conditions required to obtain a given yield of P₂O₅ and/or a well‐defined quality of the stripped acid. The optimal conditions have been determined by using a desirability function.