Étude numérique des chargements et de l'hydrodynamique dans des réacteurs pilotes à lits fixes

Une étape clé du développement de nouveaux catalyseurs est l'évaluation de leurs performances. Les tests de catalyseurs sont généralement opérés dans des unités pilotes de "petite" taille dont les avantages sont une consommation moindre de réactif et un coût d'opération plus réduit. Les réacteurs d'unité pilote ont des diamètres de l'ordre de 3 à 10 fois la taille des catalyseurs qui sont généralement des sphères ou des cylindres dont la dimension typique est 2 à 3 mm. Des données expérimentales ont mis en évidence des difficultés de répétabilité des résultats sur certains réacteurs très courts. L’objectif de ce travail est de comprendre le lien entre la micro-structure de l’empilement, qui est aléatoire et localement hétérogène, l'hydrodynamique locale et la réactivité dans des réacteurs à lit fixe de petite taille, en vue de définir des critères de conception et des méthodologies de chargement permettant d'obtenir des performances chimiques répétables. La démarche retenue est purement numérique basée sur deux codes développés à IFPEN : Grains3D pour générer numériquement l'empilement et PeliGRIFF pour simuler l'écoulement réactif dans le lit fixe. Une analyse de l'état de l'art sur le sujet donne des informations abondantes sur les lits de particules sphériques, et très peu sur les cylindres, en particulier sur les couplages écoulements et transfert/réaction. En effet, ces travaux ne permettent pas de lier les performances réactives à la structure locale hétérogène de ces empilements. Dans ce travail, la caractérisation globale et locale d'empilements de sphères et de cylindres a permis de confirmer et compléter la littérature sur certains points : porosité au centre décrite de façon approximative par les corrélations, porosité axiale, orientations des cylindres... Des variations sont présentes à l’intérieur des lits de petite taille, variations qui ne s’atténuent pas avec l’augmentation du volume d’étude et qui sont équivalentes d’un chargement à l’autre. L'étude de l'hydrodynamique dans des lits de sphères et de cylindres mono- et polydisperses a également permis de quantifier l'effet de la répétition du chargement sur la perte de pression et les champs de vitesse. Les premiers résultats d'écoulement réactif ont montré que, qualitativement, le champ de concentration dans la particule réactive est sensible à l’écoulement en présence d'une limitation interne au transfert de matière. Ces résultats préliminaires ont permis de définir une méthodologie de travail qui pourra servir pour continuer l'étude. ABSTRACT : The evaluation of catalyst performances is needed for their development. Catalytic tests are generally operated in pilot units of "small" size. Their advantages are : the reduction of reactant consumption and the reduction of operating costs. The pilot plant reactors have diameters of about 3 to 10 times the size of the catalyst (mainly spheres and cylinders) and the results obtained should be representative of the same performance of the catalysts in industrial units whose characteristic scale is around 5 meters. Experimental data have shown unacceptable repeatability in some small size reactors. The objective of this work is to understand the link between the local micro-structure of the packed bed, which is random and locally heterogeneous, local hydrodynamics and chemical responses in fixed bed reactors of small size. This study aims to define design criteria, and packing methodologies in order to obtain repeatable results. The approach adopted for this work is essentially based on numerical simulations. Two IFPEN codes are used for this study: Grains3D which is used to generate numerically the packed bed, and PeliGRIFF to simulate the reactive flow within the reactor. Previous works give information on packing, hydrodynamics and reactive flow characterization, mostly on spheres, much less is available on cylinders. However, they do not link local structure to reactive performance of the bed. In our work, geometrical characterization of the randomly packed beds of spheres and cylinders confirm and complete literature on several points: porosity at the centre of the reactor for which the correlations are inaccurate, the effect of bed height on the axial bed porosity, cylinder orientations... Some variations are present within small size beds, variations that are not dampened with an increase of averaging volume, and are similar from one packing to another. Hydrodynamics study carried out in packed beds of spheres and cylinders of different sizes allowed to link the packing local structure to pressure drop and velocity field. Only preliminary simulations have been performed on reactive flow. First qualitative results show an evolution of the concentration field inside pellets with flow in case of internal mass transfer limitations. Continuing these preliminary results will yield to the definition of a methodology that can be used to link local structure to reactive performance in randomly fixed packed bed reactors of small size

Packing fixed bed reactors with cylinders: influence of particle length distribution

In this work, we are interested in a better understanding of the local packing structure of fixed bed reactors made of cylindrical pellets packed in cylindrical tubes. Packing of cylindrical particles is simulated using Grains3D DEM code and results are analyzed in terms of void fraction using 3D analysis tools. With tube diameter as a secondary parameter, we investigate the porous structure depending on the pellet length distribution: average porosity, radial porosity, orientation of the pellets and variability of those indicators when repeating the numerical experiments

Numerical study of packing and hydrodynamics in fixed bed pilot reactors

Une étape clé du développement de nouveaux catalyseurs est l'évaluation de leurs performances. Les tests de catalyseurs sont généralement opérés dans des unités pilotes de "petite" taille dont les avantages sont une consommation moindre de réactif et un coût d'opération plus réduit. Les réacteurs d'unité pilote ont des diamètres de l'ordre de 3 à 10 fois la taille des catalyseurs qui sont généralement des sphères ou des cylindres dont la dimension typique est 2 à 3 mm. Des données expérimentales ont mis en évidence des difficultés de répétabilité des résultats sur certains réacteurs très courts. L’objectif de ce travail est de comprendre le lien entre la micro-structure de l’empilement, qui est aléatoire et localement hétérogène, l'hydrodynamique locale et la réactivité dans des réacteurs à lit fixe de petite taille, en vue de définir des critères de conception et des méthodologies de chargement permettant d'obtenir des performances chimiques répétables. La démarche retenue est purement numérique basée sur deux codes développés à IFPEN : Grains3D pour générer numériquement l'empilement et PeliGRIFF pour simuler l'écoulement réactif dans le lit fixe. Une analyse de l'état de l'art sur le sujet donne des informations abondantes sur les lits de particules sphériques, et très peu sur les cylindres, en particulier sur les couplages écoulements et transfert/réaction. En effet, ces travaux ne permettent pas de lier les performances réactives à la structure locale hétérogène de ces empilements. Dans ce travail, la caractérisation globale et locale d'empilements de sphères et de cylindres a permis de confirmer et compléter la littérature sur certains points : porosité au centre décrite de façon approximative par les corrélations, porosité axiale, orientations des cylindres... Des variations sont présentes à l’intérieur des lits de petite taille, variations qui ne s’atténuent pas avec l’augmentation du volume d’étude et qui sont équivalentes d’un chargement à l’autre. L'étude de l'hydrodynamique dans des lits de sphères et de cylindres mono- et polydisperses a également permis de quantifier l'effet de la répétition du chargement sur la perte de pression et les champs de vitesse. Les premiers résultats d'écoulement réactif ont montré que, qualitativement, le champ de concentration dans la particule réactive est sensible à l’écoulement en présence d'une limitation interne au transfert de matière. Ces résultats préliminaires ont permis de définir une méthodologie de travail qui pourra servir pour continuer l'étude.The evaluation of catalyst performances is needed for their development. Catalytic tests are generally operated in pilot units of "small" size. Their advantages are : the reduction of reactant consumption and the reduction of operating costs. The pilot plant reactors have diameters of about 3 to 10 times the size of the catalyst (mainly spheres and cylinders) and the results obtained should be representative of the same performance of the catalysts in industrial units whose characteristic scale is around 5 meters. Experimental data have shown unacceptable repeatability in some small size reactors. The objective of this work is to understand the link between the local micro-structure of the packed bed, which is random and locally heterogeneous, local hydrodynamics and chemical responses in fixed bed reactors of small size. This study aims to define design criteria, and packing methodologies in order to obtain repeatable results. The approach adopted for this work is essentially based on numerical simulations. Two IFPEN codes are used for this study: Grains3D which is used to generate numerically the packed bed, and PeliGRIFF to simulate the reactive flow within the reactor. Previous works give information on packing, hydrodynamics and reactive flow characterization, mostly on spheres, much less is available on cylinders. However, they do not link local structure to reactive performance of the bed. In our work, geometrical characterization of the randomly packed beds of spheres and cylinders confirm and complete literature on several points: porosity at the centre of the reactor for which the correlations are inaccurate, the effect of bed height on the axial bed porosity, cylinder orientations... Some variations are present within small size beds, variations that are not dampened with an increase of averaging volume, and are similar from one packing to another. Hydrodynamics study carried out in packed beds of spheres and cylinders of different sizes allowed to link the packing local structure to pressure drop and velocity field. Only preliminary simulations have been performed on reactive flow. First qualitative results show an evolution of the concentration field inside pellets with flow in case of internal mass transfer limitations. Continuing these preliminary results will yield to the definition of a methodology that can be used to link local structure to reactive performance in randomly fixed packed bed reactors of small size

CFD simulations of flow in random packed beds of spheres and cylinders: analysis of the velocity field

In this work, we aim to better understand the flow patterns in a random arrangement of particles that might affect local mass transfer and effective reactor performance. Using the DEM code Grains3D, spherical or cylindrical particles are randomly inserted inside a horizontally biperiodic container and fall under gravity. Hydrodynamic simulations are performed with PeliGRIFF, a Fictitious Domain/Finite Volume numerical model. Simulations parameters are the bed height and particulate Reynolds number. Effect of random packing on the flow field is analysed in terms of the probability distribution function (PDF) of the normalized vertical velocity. A higher Reynolds number makes more backward flow zones and changes the PDF curves that we interpret as thinner boundary layers. Unexpectedly, internal variability is independent of bed height. We propose that the probability of occurrence of random structures increases with bed volume in opposition with volume averaging effects. Internal and external variability are similar for beds of spheres and cylinders of aspect ratio (< 2). However, for longer cylinders (higher aspect ratio), subdomains with same thickness are statistically different from one bed to another. We propose that the subdomain thickness required to average out sources of variability increases with high particle aspect ratio

CFD simulations of flow in random packed beds of spheres and cylinders: analysis of the velocity field

In this work, we aim to better understand the flow patterns in a random arrangement of particles that might affect local mass transfer and effective reactor performance. Using the DEM code Grains3D, spherical or cylindrical particles are randomly inserted inside a horizontally biperiodic container and fall under gravity. Hydrodynamic simulations are performed with PeliGRIFF, a Fictitious Domain/Finite Volume numerical model. Simulations parameters are the bed height and particulate Reynolds number. Effect of random packing on the flow field is analysed in terms of the probability distribution function (PDF) of the normalized vertical velocity. A higher Reynolds number makes more backward flow zones and changes the PDF curves that we interpret as thinner boundary layers. Unexpectedly, internal variability is independent of bed height. We propose that the probability of occurrence of random structures increases with bed volume in opposition with volume averaging effects. Internal and external variability are similar for beds of spheres and cylinders of aspect ratio (< 2). However, for longer cylinders (higher aspect ratio), subdomains with same thickness are statistically different from one bed to another. We propose that the subdomain thickness required to average out sources of variability increases with high particle aspect ratio.publishedVersio

Fully-resolved simulations of the flow through a packed bed of cylinders : effect of size distribution.

International audienceFully resolved simulations of the flow through a fixed bed of pellets are performed to better understand the effect of the local microstructure on the energy loss, i.e., pressure drop through the bed. Both mono-disperse and poly-disperse systems as well as spherical and cylindrical pellets (solid particles) are investigated. Using a DEM-DLM/FD simulation method inspired by “Wachs, A. (2009). A DEM-DLM/FD method for direct numerical simulation of particulate flows: sedimentation of polygonal isometric particles in a Newtonian fluid with collisions. Comput. Fluids 38(8), 1608–1628” and implemented here in a Finite Volume context with second order reconstruction of the particle boundary as in “Rahmani, M., Wachs, A. (2014). Free falling and rising of spherical and angular particles. Phys. Fluids 26, 083301”, we evidence that the computed solution converges nicely with mesh refinement and provide guidelines on the grid size to guarantee a satisfactory level of accuracy. Based on these trustworthy simulation results, we investigate the impact of the particle shape as well as the degree of poly-dispersity in the system on the pressure drop over the fixed bed in the viscous regime. Unprecedented simulation results on the flow through a bed of poly-disperse cylinders indicate that the correlation for poly-disperse spheres suggested in “Van der Hoef, M.A., Beetstra, R., Kuipers, J.A.M. (2005). Lattice-Boltzmann simulations of low-Reynolds-number flow past mono- and bidisperse arrays of spheres: results for the permeability and drag force. J. Fluid Mech. 528, 233–254” may still be valid for cylinders of moderate aspect ratio

Experimental and numerical study of the salt dissolution in porous media

Dissolution in porous media is a complex phenomenon. In most of the approaches, density variations are ignored but they generate important convection structures like those presented in [5]. In this chapter, our aim is to develop an experimental approach in order to validate our numerical tool. To do so, we will make use of local measurments using microtomography in order to get the surface evolution of the dissolved medium. We will also get some results about the weight losses of salt dissolved and then try to compare to those predicted by numerical simulations