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

CFD Simulation of Flow through Packed Beds using the Finite Volume Technique

When a disordered packed bed, or any heterogeneous media is studied using computational fluid dynamics, the tortuous task of generating a domain and creating a workable mesh presents a challenging issue to Engineers and Scientists. In this Thesis these challenges are addressed in the form of three studies in which both traditional and novel techniques are used to generate packed beds of spheres and cylinders for analysis using computational fluid dynamics, more specifically, the finite volume method. The first study uses a Monte-Carlo method to generate random particle locations for use with a traditional CADbased meshing approach. Computational studies are performed and compared in detail with experimental equivalent beds. In the second study, where there is a need for actual, physical beds to be studied, magnetic-resonance-imaging is used coupled with a novel approach known as image based meshing. In parallel experimental studies are performed on the experimental bed and compared with computational data. In the third study, to overcome fidelity issues with the previous approaches, a physical packed bed is manufactured which is 100% geometrically faithful to its computational counterpart to provide a direct comparison. All three computational studies have shown promising results in comparison with the experimental data described in this Thesis, with the data of Reichelt (1972) and the semi-empirical correlation of Eisfeld & Schnitzlein (2001). All experiments and computational models were carried out by the author unless otherwise stated

Thermophysical Phenomena in Metal Additive Manufacturing by Selective Laser Melting: Fundamentals, Modeling, Simulation and Experimentation

Among the many additive manufacturing (AM) processes for metallic materials, selective laser melting (SLM) is arguably the most versatile in terms of its potential to realize complex geometries along with tailored microstructure. However, the complexity of the SLM process, and the need for predictive relation of powder and process parameters to the part properties, demands further development of computational and experimental methods. This review addresses the fundamental physical phenomena of SLM, with a special emphasis on the associated thermal behavior. Simulation and experimental methods are discussed according to three primary categories. First, macroscopic approaches aim to answer questions at the component level and consider for example the determination of residual stresses or dimensional distortion effects prevalent in SLM. Second, mesoscopic approaches focus on the detection of defects such as excessive surface roughness, residual porosity or inclusions that occur at the mesoscopic length scale of individual powder particles. Third, microscopic approaches investigate the metallurgical microstructure evolution resulting from the high temperature gradients and extreme heating and cooling rates induced by the SLM process. Consideration of physical phenomena on all of these three length scales is mandatory to establish the understanding needed to realize high part quality in many applications, and to fully exploit the potential of SLM and related metal AM processes

Interface-Resolving Simulations of Gas-Liquid Two-Phase Flows in Solid Structures of Different Wettability

This PhD study is devoted to numerical investigations of two-phase flows on and through elementary and complex solid structures of varying wettability. The phase-field method is developed and implemented in OpenFOAM®. The numerical method/code is verified by a series of test cases of two-phase flows, and then applied to investigate: (1) droplet wetting on solid surfaces; (2) air bubble rising and interacting with cellular structures and (3) gas-liquid interfacial flows in foam structures