Computational Fluid Dynamics of Catalytic Reactors

Today, the challenge in chemical and material synthesis is not only the development of new catalysts and supports to synthesize a desired product, but also the understanding of the interaction of the catalyst with the surrounding flow field. Computational Fluid Dynamics or CFD is the analysis of fluid flow, heat and mass transfer and chemical reactions by means of computer-based numerical simulations. CFD has matured into a powerful tool with a wide range of applications in industry and academia. From a reaction engineering perspective, main advantages are reduction of time and costs for reactor design and optimization, and the ability to study systems where experiments can hardly be performed, e.g., hazardous conditions or beyond normal operation limits. However, the simulation results will always remain a reflection of the uncertainty in the underlying models and physicochemical parameters so that in general a careful experimental validation is required. This chapter introduces the application of CFD simulations in heterogeneous catalysis. Catalytic reactors can be classified by the geometrical design of the catalyst material (e.g. monoliths, particles, pellets, washcoats). Approaches for modeling and numerical simulation of the various catalyst types are presented. Focus is put on the principal concepts for coupling the physical and chemical processes on different levels of details, and on illustrative applications. Models for surface reaction kinetics and turbulence are described and an overview on available numerical methods and computational tools is provided

Modeling and simulation of fixed-bed reactors made of metal foam pellets

Offenzellige Metallschäume werden häufig als Katalysatorträger für katalytische Gasphasenreaktionen verwendet, da sie hervorragende Transporteigenschaften aufweisen. Aktuelle Fortschritte in den Herstellungstechniken haben zur Entwicklung von legierten Schäumen (z. B. NiCrAl, FeCrAl) mit verbesserter thermischer Stabilität geführt, die zu Drop-in Pellets für Festbettreaktoren geformt werden können. Die Metallschaum-Pellets gelten als vorteilhafte Alternative zu keramischen Katalysatorträgern, auch für den Einsatz in Festbettrohrreaktoren für großtechnische Prozesse wie die Dampfreformierung von Methan. Die gewundene Zellstruktur, die Strömungen innerhalb und zwischen den Partikeln in Verbindung mit den lokalen Effekten der Festbettstrukturen führen jedoch zu komplexeren Transportphänomenen bei Festbetten aus Metallschaumpellets im Vergleich zu Feststoffpellets. Daher ist es wichtig, ein grundlegendes Verständnis der zugrunde liegenden Transportprozesse zu haben, um die optimale Form der Metallschaumpellets für eine spezifische Betriebsbedingung zu bestimmen. In dieser Arbeit wird eine modifizierte Version des Partikelaufgelösten numerische Strömungsmechanik-Ansatzes präsentiert, um die Transportprozesse, insbesondere die Strömung und den radialen Wärmetransport, in schlanken Festbettreaktoren aus Metallschaumpellets zu untersuchen. Das synthetische Festbett wird mit der Rigid Body Dynamics (RBD)-Methode generiert, und die Transportgrößen werden in den Zwischenräumen vollständig dreidimensional aufgelöst. Die Strömung und der Wärmetransport im Inneren der Metallschaumpellets werden jedoch durch den Ansatz über ein poröses Medium unter Berücksichtigung geeigneter Submodelle behandelt. Für die Durchführung von Experimenten zum Druckverlust und der Wärmeübertragung wurden Pilotmaßstab-Reaktoren gebaut. Die CFD-Simulationen zeigen eine sehr gute Übereinstimmung mit den experimentellen Daten. Als Ergebnis wurde eine virtuelle Designplattform entwickelt, die es ermöglicht, den Einfluss verschiedener Formen und Morphologien von Metallschaumpellets sowie von Betriebsbedingungen wie Durchflussraten, Einlass- und Wandtemperaturen auf die Transportprozesse in solchen Festbettreaktoren zu untersuchen. Zur Optimierung der Metallschaumpellets wird die Gesamtleistung verschiedener Pelletkonfigurationen auf der Grundlage der wünschenswerten Eigenschaften eines Festbettreaktors, darunter niedriger Druckverlust, hoher Wärmeübergangskoeffizient, vergrößerter Oberfläche sowie hohe Katalysatorbeladung, analysiert. Darüber hinaus erfolgt eine umfassende Analyse der zugrunde liegenden Wärmeübertragungsmechanismen mithilfe von experimentellen Daten und Simulationen. Dies ermöglicht die Entwicklung von Korrelationen für kritische Wärmetransportparameter wie die effektive radiale Bettleitfähigkeit und die Wand-Fluid-Nusselt-Zahl. Abschließend wird ein vereinfachter CFD-Ansatz zur Modellierung katalytischer Schaumpellets vorgestellt, der auch die externen und internen Stoffübergangswiderstände in einem beschichteten Schaumpellet berücksichtigt.Open-cell metal foams have been widely used as catalyst supports for gas-phase catalytic reactions, as they exhibit excellent transport characteristics. Recent advancements in manufacturing techniques have led to the development of alloyed foams (e.g., NiCrAl, FeCrAl) with improved thermal stability, and these can be shaped into drop-in pellets for fixed-bed reactors. The metal foam pellets are regarded as a beneficial alternative to ceramic catalyst supports, also for the use in tubular fixed-bed reactors for large-scale processes like steam methane reforming. However, the tortuous cellular structure, intraparticle and inter-particle flows, combined with local bed structure effects, result in more complex transport phenomena for fixed-beds made of metal foam pellets, compared with solid pellets. Therefore, a thorough understanding of the underlying transport processes is important to find the optimal metal foam pellet shape relevant to a particular operating condition. This thesis presents a modified version of the particle-resolved Computational Fluid Dynamics (PRCFD) approach to investigate the transport processes, particularly flow and radial heat transport, in slender fixed-bed reactors made of metal foam pellets. The synthetic bed structure is generated using the Rigid Body Dynamics (RBD) method, and the transport quantities are fully resolved three-dimensionally in the interstitial spaces. The flow and heat transport inside the metal foam pellets are modeled, however, by the porous-media approach with appropriate sub-models. Pilot-scale reactors were built to conduct pressure drop and heat transfer experiments. The CFD simulations show very good agreement with experimental data. As a result, a virtual design platform has been realized for exploring the influence of different shapes and morphologies of metal foam pellets, as well as operating conditions, such as flow rates, inlet and wall temperatures, on transport processes in such fixed-bed reactors. To optimize the foam pellet shape, the overall performance of different pellet configurations is analyzed, based on the desirable properties of a fixed-bed reactor, such as low pressure drop, high heat transfer coefficient, increased surface area, and high catalyst inventory. Furthermore, a thorough analysis of the underlying heat transfer mechanisms is carried out with the aid of experimental data and simulations. This results in the development of correlations for critical heat transport parameters such as effective radial bed conductivity and wall-fluid Nusselt number. Finally, a simplified CFD approach to model catalytic foam pellets is illustrated, which also considers the external and internal mass transfer resistances in a washcoated foam pellet

Influence of foam morphology on flow and heat transport in a random packed bed with metallic foam pellets: an investigation using CFD

Open-cell metallic foams used as catalyst supports exhibit excellent transport properties. In this work, a unique application of metallic foam, as pelletized catalyst in a packed bed reactor, is examined. By using a wall-segment Computational Fluid Dynamics (CFD) setup, parametric analyses are carried out to investigate the influence of foam morphologies (cell size φ = 0.45 –3 mm and porosity ε = 0.55–0.95) and intrinsic conductivity on flow and heat transport characteristics in a slender packed bed (N = D/dp = 6.78) made of cylindrical metallic foam pellets. The transport processes have been modeled using an extended version of conventional particle-resolved CFD, i.e., flow and energy in inter-particle spaces are fully resolved, whereas the porous-media model is used for the effective transport processes inside highly-porous foam pellets. Simulation inputs include the processing parameters relevant to Steam Methane Reforming (SMR), analyzed for low ( Rep ∼ 100) and high ( Rep ∼ 5000) flow regimes. The effect of foam morphologies on packed beds has shown that the desired requirements contradict each other, i.e., an increase in cell size and porosity favors the reduction in pressure drop, but, it reduces the heat transfer efficiency. A design study is also conducted to find the optimum foam morphology of a cylindrical foam pellet at a higher Rep ∼ 5000, which yields φ = 0.45, ε = 0.8. Suitable correlations to predict the friction factor and the overall heat transfer coefficient in a foam-packed bed have been presented, which consider the effect of different foam morphologies over a range of particle Reynolds number, 100 ≤ Rep ≤ 5000

Mass transfer towards a reactive particle in a fluid flow: Numerical simulations and modeling

We study mass transfer towards a solid spherical catalyst particle experiencing a first order irreversible reaction coupled to an external laminar flow. Internal chemical reaction and convective-diffusive mass transfer in the surrounding fluid flow are coupled by concentration and flux boundary conditions at the particle surface. Through this coupling, the mean particle surface and volume concentrations are predicted and the internal/external Sherwood numbers are obtained. We investigate the interplay between convection, diffusion, and reaction by computational fluid dynamics and establish a model for the mass transfer coefficient accounting for diffusion and internal first-order chemical reaction. We obtain a prediction of the mass transfer coefficient through mass balance or using the classical additivity rule. The model is numerically validated by fully resolved numerical simulations over a wide range of Reynolds number, Schmidt number and Thiele modulus which shows that assuming decoupled treatment of external and internal mass transfer gives very accurate predictions. Finally, we test the unsteady response of the model. The model predicts the evolution of the mean volume concentration for a particle placed in a steady convective-diffusive stream. Predictions of the unsteady model are in very good agreement with computed results

Computational Fluid Dynamics of Reacting Flows at Surfaces: Methodologies and Applications

This review presents the numerical algorithms and speed-up strategies developed to couple continuum macroscopic simulations and detailed microkinetic models in the context of multiscale approaches to chemical reactions engineering. CFD simulations and hierarchical approaches are discussed both for fixed and fluidized systems. The foundations of the methodologies are reviewed together with specific examples to show the applicability of the methods. These concepts play a pivotal role to enable the first-principles multiscale approach to systems of technological relevance

Gasification reactions of carbon anodes; multi scale reaction model

La réactivité des anodes de carbone avec le CO₂ est l'une des principales préoccupations des alumineries utilisant le procédé Hall-Héroult. Une telle réactivité n'est pas souhaitable car elle augmente la consommation nette de carbone et raccourcit ainsi la durée de vie des anodes. La surconsommation d'anode est affectée par la réactivité intrinsèque de l'anode et les phénomènes de transport de masse. Différents modèles mathématiques du processus de gazéification ont été développés pour différentes géométries et techniques : La première partie de ce travail se concentre sur la gazéification d'une seule particule d'anode de carbone avec du CO₂, en utilisant un modèle de réaction-transport détaillé, basé sur la cinétique intrinsèque de la réaction et le transport des espèces gazeuses. Le modèle comprend les équations de conservation de la masse pour les composants gazeux et les particules solides de carbone, ce qui donne un ensemble d'équations différentielles partielles non linéaires, résolues à l'aide de techniques numériques. Le modèle peut prédire le taux de génération de gaz, les compositions de gaz et le taux de consommation de carbone pendant la gazéification d'une particule de carbone. Différents modèles cinétiques ont été comparés pour décrire le comportement de gazéification des particules de carbone. Il a été constaté que le modèle de pores aléatoires (RPM) fournissait la meilleure description de la réactivité des particules d'anode. Le modèle a également prédit le retrait des particules pendant le processus de gazéification. Le modèle a été validé à l'aide de résultats expérimentaux obtenus avec différentes gammes de tailles de particules. Un bon accord entre les résultats du modèle et les données expérimentales a montré que cette approche pouvait quantifier avec succès la cinétique de gazéification et la distribution du gaz au sein de la particule anodique. De plus, le modèle Langmuir-Hinshelwood (L-H) est utilisé afin de capturer l'effet d'inhibition du monoxyde de carbone sur la réaction de gazéification. Dans la deuxième partie, la simulation du processus de gazéification de l'anode avec du CO₂, en tant que lit de particules d'anode a été considérée. Le modèle numérique de la méthode des éléments discrets CFD multi-échelles (DEM) a été développé sur la base d'un concept eulérien-lagrangien. Le modèle comprend une méthode des éléments finis eulériens (FEM) pour le gaz et les particules solides, et un DEM lagrangien pour la phase particulaire, cette dernière visant à capturer l'effet de retrait des particules (mouvement des particules lors de la gazéification). Les propriétés physiques des particules, telles que la porosité et la surface spécifique, et les propriétés thermochimiques des particules, telles que la chaleur de réaction, sont finalement suivies. Les changements géométriques des particules, le transfert de chaleur et de masse, le retrait des particules et les réactions chimiques sont pris en compte lors de la gazéification de l'anode avec du CO₂. Les profils dynamiques de concentration et de température du réactif et des gaz produits ainsi que la conversion solide ont été modélisés à la fois dans les vides entre les particules et les pores à l'intérieur de chaque particule. Pour valider le modèle, des tests expérimentaux ont été réalisés à l'aide d'un lit de particules anodiques. Dans la dernière partie, une simulation d'une dalle d'anode a été réalisée. Le modèle contient la masse et les équations de transfert de chaleur pour les composants gazeux et les particules solides de carbone, ce qui donne un ensemble d'équations différentielles partielles non linéaires, résolues à l'aide de techniques numériques. Le modèle peut prédire le taux de génération de gaz, les compositions de gaz et le taux de consommation de carbone, la chute de pression et la distribution de température pendant la gazéification d'une particule de carbone.The reactivity of carbon anodes with CO₂ is one of the main concerns in aluminum smelters using the Hall-Héroult process. Such reactivity is not desirable because it increases the net carbon consumption and thus shortens the lifetime of the anodes. Anode overconsumption is affected by anode intrinsic reactivity and mass transport phenomena. Different mathematic models of the gasification process were developed for different geometries and technics: The first part of this work focuses on the gasification of a single carbon-anode particle with CO₂, using a detailed reaction-transport model, based on the reaction intrinsic kinetics and transport of gaseous species. The model includes the mass conservation equations for the gas components and solid carbon particles, resulting in a set of nonlinear partial differential equations, being solved using numerical techniques. The model may predict the gas generation rate, the gas composition, and the carbon consumption rate during the gasification of a carbon particle. Various kinetic models were compared to describe the gasification behavior of carbon particles. It was found that the Random pore model (RPM) provided the best description of the reactivity of anode particles. The model also predicted the particle shrinkage during the gasification process. The model was validated using experimental results obtained with different particle size ranges. Good agreement between the model results and the experimental data showed that this approach could quantify with success the gasification kinetics and the gas distribution within the anode particle. In addition, the Langmuir-Hinshelwood (L-H) model is used in order to capture the inhibition effect of carbon monoxide on the gasification reaction. In the second part, the simulation of the gasification process of anode with CO₂, as an anode particle bed, was considered. Numerical multiscale CFD-discrete element method (DEM) model was developed based on an Eulerian-Lagrangian concept. The model includes an Eulerian finite element method (FEM) for the gas and solid particles, and a Lagrangian DEM for the particle phase, the latter intending to capture the particle shrinkage effect (movement of particles during gasification). The physical properties of particles, such as porosity and specific surface area, and the thermochemical properties of particles, such as the heat of reaction, are ultimately tracked. Geometric changes in particles, heat and mass transfer, particle shrinkage and chemical reactions are considered during anode gasification with CO₂. The dynamic concentration and temperature profiles of the reactant and product gases as well as the solid conversion were modeled both in the voids between the particles and the pores inside each particle. To validate the model, experimental tests were performed using a bed of anode particles. In the last part, a simulation of the anode slab was carried out. The model contains the mass, and heat transfer equations for the gas components and solid carbon particles, resulting in a set of nonlinear partial differential equations, which are solved using numerical techniques. The model can predict the gas generation rate, gas compositions, and carbon consumption rate, pressure drop, and temperature distribution during the gasification of an anode slab

High-temperature pyrolysis modeling of a thermally thick biomass particle based on an MD-derived tar cracking model

Biomass pyrolysis in the thermally thick regime is an important thermochemical phenomenon encountered in many different types of reactors. In this paper, a particle-resolved algorithm for thermally thick biomass particle during high-temperature pyrolysis is established by using reactive molecular dynamics (MD) and computational fluid dynamics (CFD) methods. The temperature gradient inside the particle is computed with a heat transfer equation, and a multiphase flow algorithm is used to simulate the advection/diffusion both inside and outside the particle. Besides, to simulate the influence of intraparticle temperature gradient on the primary pyrolysis yields, a multistep kinetic scheme is used. Moreover, a new tar decomposition model is developed by reactive molecular dynamic simulations where every primary tar species in the multistep kinetic scheme cracks under high temperature. The integrated pyrolysis model is evaluated against a pyrolysis experiment of a centimeter-sized beech wood particle at 800 to 1050 \ub0C. The simulation results show a remarkable improvement in both light gas and tar yields compared with a simplified tar cracking model. Meanwhile, the MD tar cracking model also gives a more reasonable prediction of the species yield history, which avoids the appearance of unrealistically high peak values at the initial stage of pyrolysis. Based on the new results, the different roles of secondary tar cracking inside and outside the particle is studied. Finally, the model is also used to assess the influence of tar residence time and several other factors impacting the pyrolysis

Computational Fluid Dynamics Studies in Heat and Mass Transfer Phenomena in Packed Bed Extraction and Reaction Equipment: Special Attention to Supercritical Fluids Technology

El entendimiento de los fenómenos de transferencia de calor y de masa en medios porosos implica el estudio de modelos de transporte de fluidos en la fracción vacía del medio; este hecho es de fundamental importancia en muchos sistemas de Ingeniería Química, tal como en procesos de extracción o en reactores catalíticos. Los estudios de flujo realizados hasta ahora (teóricos y experimentales) usualmente tratan al medio poroso como un medio efectivo y homogéneo, y toman como válidas las propiedades medias del fluido. Este tipo de aproximación no tiene en cuenta la complejidad del flujo a través del espacio vacío del medio poroso, reduciendo la descripción del problema a promedios macroscópicos y propiedades efectivas. Sin embargo, estos detalles de los procesos locales de flujo pueden llegar a ser factores importantes que influencien el comportamiento de un proceso físico determinado que ocurre dentro del sistema, y son cruciales para entender el mecanismo detallado de, por ejemplo, fenómenos como la dispersión de calor, la dispersión de masa o el transporte entre interfaces.La Dinámica de Fluidos Computacional (CFD) como herramienta de modelado numérico permite obtener una visión mas aproximada y realista de los fenómenos de flujo de fluidos y los mecanismos de transferencia de calor y masa en lechos empacados, a través de la resolución de las ecuaciones de Navier - Stokes acopladas con los balances de materia y energía y con un modelo de turbulencia si es necesario. De esta forma, esta herramienta permite obtener los valores medios y/o fluctuantes de variables como la velocidad del fluido, la temperatura o la concentración de una especie en cualquier punto de la geometría del lecho empacado.El objetivo de este proyecto es el de utilizar programas comerciales de simulación CFD para resolver el flujo de fluidos y la transferencia de calor y de masa en modelos bi/tri dimensionales de lechos empacados, desarrollando una estrategia de modelado aplicable al diseño de equipos para procesos de extracción o de reacción catalítica. Como referencia se tomaran procesos de tecnología supercrítica debido a la complejidad de los fenómenos de transporte involucrados en estas condiciones, así como a la disponibilidad de datos experimentales obtenidos previamente en nuestro grupo de investigación. Estos datos experimentales se utilizan como herramienta de validación de los modelos numéricos generados, y de las estrategias de simulación adoptadas y realizadas durante el desarrollo de este proyecto.An understanding of the heat and mass transfer phenomena in a porous media implies the study of the fluid transport model within the void space; this fact is of fundamental importance to many chemical engineering systems such as packed bed extraction or catalytic reaction equipment. Experimental and theoretical studies of flow through such systems often treat the porous medium as an effectively homogeneous system and concentrate on the bulk properties of the flow. Such an approach neglects completely the complexities of the flow within the void space of the porous medium, reducing the description of the problem to macroscopic average or effective quantities. The details of this local flow process may, however, be the most important factor influencing the behavior of a given physical process occurring within the system, and are crucial to understanding the detailed mechanisms of, for example, heat and mass dispersion and interface transport.Computational Fluid Dynamics as a simulation tool allows obtaining a more approached view of the fluid flow and heat and mass transfer mechanisms in fixed bed equipment, through the resolution of 3D Reynolds averaged transport equations, together with a turbulence model when needed. In this way, this tool permit to obtain mean and fluctuating flow and temperature values in any point of the bed. The goal of this project is to use commercial available CFD codes for solving fluid flow and heat and mass transfer phenomena in two and three dimensional models of packed beds, developing a modeling strategy applicable to the design of packed bed chemical reaction and extraction equipment. Supercritical extraction and supercritical catalytic reaction processes will be taken as reference processes due to the complexity of the transport phenomena involved within this processes, and to the availability of experimental data in this field, obtained in the supercritical fluids research group of this university. The experimental data priory obtained by our research group will be used as validation data for the numerical models and strategies dopted and followed during the developing of the project