Etude du transfert de masse réactif Gaz-Liquide le long de plans corrugués par simulation numérique avec suivi d'interface

Ce travail rentre dans le cadre du développement des procédés de traitement de gaz acides et de captage de CO2. L'objectif est d’étudier numériquement les phénomènes de transferts de masse réactif dans des configurations proches de celles rencontrées dans les contacteurs de type garnissage structuré. Les écoulements considérés sont du type « film ruisselant » le long d'une géométrie corruguée cisaillé par un flux gazeux chargé d’espèces acides. Les espèces acides de la phase gaz s'absorbent dans le film liquide où elles réagissent chimiquement. Des simulations numériques sont menées afin de comprendre l'impact des propriétés physiques et géométriques sur le transfert de masse réactif, pour des gammes proches des conditions industrielles. L'approche numérique développée dans le code JADIM pour traiter des problèmes d’absorption réactive dans des écoulements diphasique à interface déformable est basée sur la méthode VOF (Volume of Fluid). Dans cette approche, l'équation de conservation des espèces chimiques est résolue en étant couplée avec les équations de Navier-Stokes et l'équation de suivi d'interface. La prise en compte de l'équilibre thermodynamique des espèces chimiques à la traversée de l'interface gaz/liquide est résolue avec une modélisation originale, utilisant la méthode à un seul fluide et la loi de Henry avec un coefficient constant. Le premier axe d'étude abordé a été celui du transfert avec et sans réaction chimique dans un film liquide tombant sous l'effet de la gravité. Celui-ci s'est divisé principalement en trois parties. La première a porté sur le transfert de matière par absorption dans un film liquide tombant sous l'effet de la gravité. Les résultats obtenus montrent que le transfert dans un film liquide en écoulement laminaire se fait principalement en deux modes. Le premier mode se produit pour les temps de contact relativement courts où le mécanisme de transfert est piloté essentiellement par l'advection de l'interface qui transporte la concentration. Le second mode de transfert se produit pour les longs temps de contact. Le processus de transfert s'opère alors essentiellement par diffusion moléculaire dans un film saturé et le nombre de Sherwood est par conséquent constant (Sh=2). La deuxième partie a porté sur l'étude du transfert de masse réactif dans un film liquide tombant. Les résultats obtenus montrent que lorsque le transfert de masse est accompagné d’une réaction chimique irréversible du premier ordre et du deuxième ordre, les résultats des simulations numériques sont globalement en bon accord avec les solutions de Danckwerts (1970) et Brian et al. (1961) respectivement. Finalement, l'effet de la déstabilisation de l'interface sur le  transfert de masse dans un film liquide a été considéré. Dans cette partie nous avons montré l'influence de la formation d’onde sur le transfert de matière. Le second axe d’étude a concerné l'étude du transfert de masse réactif dans un écoulement de film liquide le long d'une paroi corruguée bidimensionnelle proche de celle rencontrée dans les contacteurs à garnissage structuré. Dans un premier temps, nous avons décrit l'hydrodynamique du film liquide. Cette étude nous a permis de comprendre l’impact de la géométrie et des propriétés physiques sur l'évolution du film liquide et sur la structure de l’écoulement. Dans un second temps nous avons étudié l’effet de la géométrie corruguée sur le transfert de masse sans réaction chimique. En s'appuyant sur la description locale du transfert, nous avons pu développer des coefficients de transfert globaux en reliant les paramètres de transfert à des grandeurs facilement maîtrisables en ingénieries tels que le nombre de Schmidt, le nombre de Reynolds et la longueur ou l'amplitude de la corrugation. Nous avons ainsi montré qu'une modélisation issue de la théorie de Higbie reste encore utilisable car l'espèce transférée à l'interface diffuse peu dans le film compte tenu de sa diffusivité. Enfin nous avons considéré un transfert réactif pour ce type de géométrie en considérant une réaction du second ordre. Nous avons montré que l'évolution du facteur d'accélération est peu sensible au garnissage et correspond à celle d'un film plan. La solution implicite de Brian et al. (1961) est par conséquent bien adaptée pour estimer le facteur d'accélération dans la configuration étudiée. ABSTRACT : This work is done within the framework of gas treatment and CO2 capture process development. The main objective of the present work is to fill the gap between classical experiments and industrial conditions by the use of Computational Fluid Dynamics (CFD). The physical problem considered corresponds to the liquid film flow down a corrugate surface under gravity in present of a gas phase. The chemical species in the gas phase absorb in the liquid phase and react. Numerical calculations are carried out in order to determine the impact of physical and geometrical properties on reactive mass transfer in industrial operating conditions. The computational approach developed in the JADIM code to study reactive mass transfer in two-phase flow with deforming interface is based on volume of fluid method. The chemical species concentration equation is solved coupled with the Navier-Stokes equations and volume fraction equation. The numerical difficulties arise in imposing jump discontinuity for chemical concentrations at the interface due to different solubility are solved by using a continuum mechanical modelling of two phases flow and Henry's law with constant coefficient. This new modelling allows interpreting jump conditions as continues effect only active in the interface zone, where diffusive mass flux across the interface remains continue. The first study performed focus on mass transfer with and without chemical reaction in falling liquid film flow on vertical wall. This first study is devised in three parts. In the first part we interest to mass transfer by absorption in falling liquid film under effect of gravity. The result shows that the transfer in laminar film flows occurs with two transfer mode. The first mode occurs for short contact time between chemical species and liquid film, the predominant mechanism of mass transfer in this case is the advection of the interface that transports the concentration. The second mode occurs for large contact time, the predominant mechanism of mass transfer in this case is molecular diffusion. The value of Sherwood number in this mode remains constant (Sh=2). The second parts focus on reactive mass transfer in falling liquid film. The results show that when the mass transfer is accompanied by first and second order irreversible chemical reaction, the numerical simulation results are in good agreement with Danckwerts (1970) and Brian et al. (1961) solution respectively. Finally the effects of interface destabilisation on mass transfer in liquid film flow are considered. In this part, we show the impact of wave formation on mass transfer. The second study deals with reactive mass transfer in laminar film flow along corrugate wall that correspond to 2D cross section of structured packing. Initially, we address how the corrugate wall geometry and physical properties of fluid affects the free surface profile and the structure of the film liquid flow. We next study the effect of the corrugate surface geometry on the mass transfer without chemical reaction. Furthermore, by using the local description of the mass transfer, we develop global mass transfer coefficients correlation in function of parameters easily computed such as the Schmidt number, the Reynolds number and the length or the amplitude of the corrugation. We also showed that the Higbie theory remains still usable in the case of liquid side mass transfer in liquid film flow on structured packing. Finally we consider a reactive mass transfer with second order chemical reaction for corrugate geometry. The result shows that the evolution of the enhancement factor is not very sensitive to the corrugate geometry and corresponds to that of a plane film. The implicit solution of Brian et al. (1961) is consequently well adapted to estimate the enhancement factor in the studied configuration

Mass transfer and liquid hold-up determination in structured packing by CFD

Mass transfer and liquid hold-up in structured packing geometry are investigated using the volume of fluid method. Numerical simulations of two-dimensional co-current gas–liquid flow on structured packing with interfacial mass transfer are performed. The volume of fluid method is used to capture the gas–liquid interface motion. The mass transfer is computed by solving the concentration equation with an adapted modeling of the solubility (Haroun et al., 2010b). The liquid hold-up and the mass transfer are studied as function of liquid flow rate and structured packing geometry. Results show how the liquid flow rate and the complex geometry affect the liquid film flow topology and the interfacial mass transfer. For a specified packing geometry, it is demonstrated that for low liquid flow rate, the liquid film remains uniform and follow closely the profile of the structured wall. For uniform liquid film flow along packing wall, it is found that the liquid hold-up is in good agreement with the model proposed by Billet and Schultes (1999) and Raynal and Royon-Lebeaud (2007). When increasing the liquid flow rate, the liquid film does not follow the shape of the structured wall anymore, a static hold- up (recirculation zone) form in the cavities and grows as the Reynolds number increases until covering most of the packing cavities. The present work gives the liquid hold-up evolution for each liquid film flow regime according to the Reynolds number and the dimensionless amplitude of the corrugation. Concerning the liquid side mass transfer, it is found that the liquid side mass transfer is well predicted by the Higbie (1935) theory provided that adequate velocity and length scales are considered for exposure time determination. The exposure time of fluid element at the interface corresponds to the ratio between the curvilinear distance between two periodic corrugation contact point and the interface velocity. An exposure time model is proposed taking into the account physical and geometric parameters

Liquid spreading in trickle-bed reactors: Experiments and numerical simulations using Eulerian--Eulerian two-fluid approach

Liquid spreading in gas-liquid concurrent trickle-bed reactors is simulated using an Eulerian twofluid CFD approach. In order to propose a model that describes exhaustively all interaction forces acting on each fluid phase with an emphasis on dispersion mechanisms, a discussion of closure laws available in the literature is proposed. Liquid dispersion is recognized to result from two main mechanisms: capillary and mechanical (Attou and Ferschneider, 2000; Lappalainen et al., 2009- The proposed model is then implemented in two trickle-bed configurations matching with two experimental set ups: In the first configuration, simulations on a 2D axisymmetric geometry are considered and the model is validated upon a new set of experimental data. Overall pressure drop and liquid distribution obtained from $\gamma$-ray tomography are provided for different geometrical and operating conditions. In the second configuration, a 3D simulation is considered and the model is compared to experimental liquid flux patterns at the bed outlet. A sensitivity analysis of liquid spreading to bed geometrical characteristics (void-fraction and particles diameter) as well as to gas and liquid flow rates is proposed. The model is shown to achieve very good agreement with experimental data and to predict, accurately, tendencies of liquid spreading for various geometrical bed characteristics and/or phases flow-rates

Usinage d'un modèle reconstruit de l'articulation du genou et d'une prothèse totale de genou sur mesure sur un centre d'usinage quatre axes.

La fabrication d'une prothèse totale de genou (PTG) est un travail très difficile en raison des formes gauches qui sont nécessaires pour imiter la forme complexe. En outre, la PTG doit offrir différentes tailles selon l'âge du patient. L'objectif de ce travail est de répondre à ces exigences en se basant sur des modèles reconstruits de l'articulation de genou pour la conception d'une PTG sur mesure en faisant appel à différents démarches et méthodologies dans des environnements CAO et FAO.Des stratégies d'usinages sont adoptées pour le choix d'une méthode de réalisation optimisée des PT

Pore-network modeling of trickle bed reactors: Pressure drop analysis

A pore network model (PNM) has been developed to simulate gas–liquid trickle flows inside fixed beds of spherical particles. The geometry has been previously built from X-ray micro-tomography experiments, and the flow in the throats between pores is modeled as a pure viscous Poiseuille two-phase flow. The flow distribution between pores and throats is obtained by solving mass and momentum balance equations. As a first application of this simple but powerful meso-scale model, a focus is proposed on the ability of PNM to estimate pressure drop and liquid saturation in co-current gas–liquid flows. PNM results are compared to the classical 1D pressure drop models of Attou et al. (1999), Holub et al. (1992) and Larachi et al. (1991). Agreement and discrepancies are discussed, and, finally, it has been found that the actual PNM approach produces realistic pressure drops as far as inertial contributions to friction are negligible. Concerning liquid saturation, the PNM only estimates its value in the throats between pores. As a consequence, liquid saturations are overestimated, but they can be easily corrected by an ad hoc empirical model

PORE-SCALE SIMULATION OF FLUID FLOW IN PACKED-BED REACTORS VIA RIGID-BODY SIMULATIONS AND CFD

The problem of fluid flow in porous media is of paramount importance in the process, oil and metallurgical industries, since it is involved in the extraction of minerals and oil, in aquifer dynamics, as well as chemical reactions carried out in fixed bed catalytic reactors. Its CFD simulation is particularly interesting, as it offers the possibility of reducing the extent of costly experimental investigations, but presents a number of technical challenges. One of the main issues is the generation of a geometrical model that realistically represents the porous medium/particle packing. Its derivation from experiments (i.e. micro-computer tomography) is complicated and packing codes are often limited to simple convex (mainly spherical) objects. In this work a computational tool developed in computer graphics, and integrated with the Bullet Physic Library, is used to generate realistic packings of polydisperse catalytic spheres and trilobes. The geometrical model is then meshed with SnappyHexMesh and then simulated with Ansys Fluent. Results show excellent agreement with experiments, demonstrating the great potentiality of the approac

Numerical method for three-dimensional macroscale simulations of two-phase flows with moving contact lines

Un modèle d'angle de contact dynamique est développé avec une méthode level-set pour simuler des écoulements macroscopiques tridimensionnels avec lignes de contact mobiles. Le code est validé à partir de simulations numériques directes et résultats expérimentaux d'étalement de gouttelette en régimes visqueux et inertiel. Le code permet de simuler des écoulements tridimensionnels avec ligne de contact mobile en tenant compte de l'hystérésis de l'angle de contact

Étude du transfert de masse réactif gaz-liquide le long de plans corrugués par simulation numérique avec suivi d'interface

Ce travail rentre dans le cadre du développement des procédés de traitement de gaz acides et de captage de CO2. L'objectif est d'étudier numériquement les phénomènes de transferts de masse réactif dans des configurations proches de celles rencontrées dans les contacteurs de type garnissage structuré. Les écoulements considérés sont du type film ruisselant le long d'une géométrie corruguée cisaillé par un flux gazeux chargé d'espèces acides. Les espèces acides de la phase gaz s'absorbent dans le film liquide où elles réagissent chimiquement. Des simulations numériques sont menées afin de comprendre l'impact des propriétés physiques et géométriques sur le transfert de masse réactif, pour des gammes proches des conditions industrielles.This work is done within the framework of gas treatment and CO2 capture process development. The main objective of the present work is to fill the gap between classical experiments and industrial conditions by the use of Computational Fluid Dynamics (CFD). The physical problem considered corresponds to the liquid film flow down a corrugate surface under gravity in present of a gas phase. The chemical species in the gas phase absorb in the liquid phase and react. Numerical calculations are carried out in order to determine the impact of physical and geometrical properties on reactive mass transfer in industrial operating conditions.TOULOUSE-ENSEEIHT (315552331) / SudocSudocFranceF

Use of Computational Fluid Dynamics for Absorption Packed Column Design

Computational Fluid Dynamics (CFD) is today commonly used in a wide variety of process industries and disciplines for the development of innovative technologies. The present article aims to show how CFD can be used as an effective analysis and design tool for the development and design of packed gas/liquid absorption columns. It is first shown how CFD can be used for the characterisation of packings. The different hydrodynamic and mass transfer design parameters are investigated and adapted CFD methods are suggested. Secondly, column distribution internal development is discussed to show how CFD simulations should be performed to improve the design of gas and liquid distributors. An example of the development of new distribution technologies for floating installation of a reactive absorption column is also presented

Prediction of effective area and liquid hold-up in structured packings by CFD.

International audienceInterfacial effective area and liquid hold-up in structured packing geometries are investigated using the Volume Of Fluid method. Tree-dimensional numerical simulations of gas-liquid flow on inclined plane plate and structured packing are performed. The VOF method is used to capture the gas-liquid interface motion. After a first validation case on the wetting phenomena prediction in inclined plane plate, the effective interfaciale area, the liquid hold-up and the degree of wetting of packing are studied as function of liquid flow rate and wall surface characteristic (adherence contact angle). Results show that the liquid flow-rate and the contact angle play a significant role. It is found that the interfacial effective area and the degree of wetting of packing increase as the liquid flow rate increases and as the contact angle decreases. Moreover, under the influence of the contact angle, different liquid film shapes are observed. The simulations results are compared to experimental data available in literature. This work shows how CFD can be used as an effective tool to investigate performance characteristics of structured packing and provide information on fluid behaviour as the determination of minimum flow rate for which the entire surface of packing is fully wetted by the liquid phase