Simulation katalytischer Monolithreaktoren unter Verwendung detaillierter Modelle für Chemie und Transport

Gegenstand der vorliegenden Arbeit ist die mathematisch-numerische Modellierung katalytischer Monolithreaktoren unter Berücksichtigung detaillierter Transportmodelle in Gasphase und Festkörper sowie auf Elementarreaktionen aufbauenden Mechanismen für homogene und heterogene Reaktionen. Zuerst wird ein hierarchisches Modell der Prozesse, die in detaillierter Form für die Beschreibung des Monolithen herangezogen werden, entwickelt. Ausgehend vom Begriff der Spezies, werden die Transport- und die thermodynamischen Eigenschaften von Gasphasen- und Oberflächenensembles betrachtet. Zur Beschreibung der Stoffumwandlungen dienen Reaktionsmechanismen, die aus Elementarreaktionen aufgebaut sind. Darauf wiederum bauen Modelle der reaktiven Strömung auf. Die oberste Hierarchiestufe bildet das Modell des monolithischen Festkörpers. Eine zentrale Stellung in dieser Arbeit nimmt die numerische Implementation der Modelle ein. Dazu wurde das Programmpaket DETCHEM komplett neu strukturiert und um die Modelle der Kanalströmung sowie des Monolithen erweitert. Basis der detaillierten Simulation ist eine Programmbibliothek mit Funktionen zur Berechnung der thermodynamischen Größen und Transportkoeffizienten sowie der Reaktionsgeschwindigkeiten in der Gasphase und auf Oberflächen. Zur Simulation einer stationären laminaren Kanalströmung wird das Modell der Grenzschichtnäherung herangezogen. Unter der Annahme separierter Zeitskalen für die Durchströmung eines Einzelkanals und für die darauf erfolgende thermische Reaktion des Monolithen baut auf den Simualtionsergebnissen mehrerer unabhängiger Kanäle die Simulation des Gesamtmonolithen auf. Um den Zeitaufwand zu begrenzen, werden repräsentative Kanäle nach einem Cluster-Agglomerations-Algorithmus ausgewählt. Die Programmteile zur Simulation der Strömung bzw. des Gesamtmonolithen werden durch Vergleich mit Experimenten zur oxidativen Dehydrogenierung von Ethan bzw. zur wasserstoffunterstützten Verbrennung von Methan auf Platin validiert. Die experimentellen Ergebnisse werden mit guter Übereinstimmung reproduziert. Vorallem ist es durch die Simulation möglich, eine Erklärung der auf molekularer Ebene ablaufenden Prozesse zu geben. Mit dieser Simulation gelang es erstmalig, einen transienten Prozess des gesamten Monolithen auf Grundlage detaillierter Modelle sowohl für die Kanalströmung als auch für die Oberflächenreaktionen darzustellen. Vergleiche instationärer Prozesse in Experiment und Simulation erfolgen für die Zündung der vollständigen bzw. partiellen Oxidation von Methan in einem Platin- bzw. Rhodium-beschichten Monolithen. Es wird gezeigt, dass es für solche Systeme sinnvoll ist, ein über das Einkanalmodell hinausgehendes Modell des Monolithen heranzuziehen. Die vorhergesagten Zeitskalen werden anhand globaler Eigenschaften (Temperatur, Spezieszusammensetzung) überprüft. Die Simulation liefert Erkenntnisse über den inneren Ablauf der Zündung und dessen Ursachen. Die Vorteile des dargestellten Modells für den Monolithen werden sichtbar, wenn man räumlich veränderliche Eingangsbedingungen, wie sie in der technischen Anwendung aufgrund konstruktiver Randbedingungen vorkommen, betrachtet. Der Einfluss der Anströmung wird anhand der katalytischen Verbrennung von Methan untersucht

Composition Modulation over Three-Way Catalysts

Compared to conventional internal combustion engines, modern hybrid electric vehicles (HEVs) can save fuel under urban driving conditions, which results in lower CO2 emissions. However, frequent stops and restart of the HEV engine go along with periods of low exhaust gas temperatures and therefore cause a decline in pollutant conversion over the three-way catalyst (TWC) system that is typically used for exhaust gas after-treatment [1]. Composition modulation is reported to increase pollutant conversion over the TWC at low temperatures and to be beneficial for cold start performance [2]. In this regard, dithering frequency, amplitude, temperature, and space velocity are the most important parameters influencing the rate enhancement from composition modulation. A synthetic gas bench with fast switching valves is used to conduct a comprehensive parameter study on the influence of dithering parameters on the TWC performance. For this, monolithic catalysts with Pd/Al2O3 and a commercially relevant Ce-based Pd catalyst are prepared by incipient wetness impregnation and subsequent dip coating. The application of a square wave signal to the catalytic converter remains a challenging task due to axial dispersion in the setup periphery. To further examine the phenomena and predict the behaviour of the TWC under periodic conditions, a detailed kinetic model is under development. Literature suggests that the dithering effect can be described by kinetic models with detailed chemistry considering the interaction among species adsorbed on surface sites [3]. Under the assumption of an ideally backmixed reactor, initial modelling results exploiting a detailed microkinetic model for CO oxidation [4] show an increased average CO conversion for all frequencies around the light-off and a decrease for higher temperature (Figure 1). Furthermore, an increase of optimal frequency with increasing temperature and amplitude was observed for constant amplitude and temperature respectively, which is in line with experimental data from literature [2]. Using transient data for model development will provide valuable insights on surface phenomena that are responsible for the dithering effect on three-way catalysts and will ultimately allow for reducing pollutant emissions from HEVs. [1] Y. Huang, N. Surawski, B. Organ, J. Zhou, O. Tang and E. Chan, "Fuel consumption and emission performance under real driving: Comparison between hybrid and conventional vehicles", Sci. Total Environ. 659, 275-282 (2019). [2] P. Silveston, "Automotive exhaust catalysis: Is periodic operation beneficial?", Chem. Eng. Sci. 51, 2419-2426 (1996). [3] P. Kočí, M. Kubíček, M. Marek, "Multifunctional aspects of Three-Way Catalyst: Effects of Complex Washcoat Composition", Chem. Eng. Res. Des. 82, 284-292 (2004). [4] D. Chan, S. Tischer, J. Heck, C. Diehm, O. Deutschmann, "Correlation between catalytic activity and catalytic surface area of a Pt/Al2O3 DOC: An experimental and microkinetic modelling study", Appl. Catal. B 156-157, 153-165 (2014)

Numerical Simulation of Methane and Propane Reforming Over a Porous Rh/Al2_{2}O3_{3} Catalyst in Stagnation-Flows: Impact of Internal and External Mass Transfer Limitations on Species Profiles

Hydrogen production by catalytic partial oxidation and steam reforming of methane and propane towards synthesis gas are numerically investigated in stagnation-flow over a disc coated with a porous Rh/Al$_{2}$O$_{3}$ layer. A one-dimensional flow field is coupled with three models for internal diffusion and with a 62-step surface reaction mechanism. Numerical simulations are conducted with the recently developed computer code DETCHEM$^{STAG}$. Dusty-Gas model, a reaction-diffusion model and a simple effectiveness factor model, are alternatively used in simulations to study the internal mass transfer inside the 100 µm thick washcoat layer. Numerically predicted species profiles in the external boundary layer agree well with the recently published experimental data. All three models for internal diffusion exhibit strong species concentration gradients in the catalyst layer. In partial oxidation conditions, a thin total oxidation zone occurs close to the gas-washcoat interface, followed by a zone of steam and dry reforming of methane. Increasing the reactor pressure and decreasing the inlet flow velocity increases/decreases the external/internal mass transfer limitations. The comparison of reaction-diffusion and Dusty-Gas model results reveal the insignificance of convective flow on species transport inside the washcoat. Simulations, which additionally solve a heat transport equation, do not show any temperature gradients inside the washcoat

Thermodynamics and reaction mechanism of urea decomposition

Selective catalytic reduction (SCR) for automotive applications depends on ammonia production from a urea-water solution by thermolysis and hydrolysis. In this process, undesired liquid and solid by-products are formed in the exhaust pipe. The formation and decomposition of these by-products have been studied by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Based on a previously published reaction mechanism by Brack et al. [1], a new reaction scheme is proposed that emphasizes the role of thermodynamic equilibrium of the reactants in liquid and solid phases [2]. The observed phenomenon of liquefaction and re-solidification of biuret in the temperature range 193–230 °C can be explained by formation of a eutectic mixture with urea. According to DSC data, the direct decomposition of urea to ammonia and isocyanic acid can be ruled out. The dominant route is a self-polymerisation of urea to biuret and triuret. Biuret and triuret decomposition are dominated by thermodynamic equilibria with gaseous isocyanic acid. For this, thermodynamic data of triuret have been refined. The apparent melting point of biuret at 193 °C is explained by the formation of a eutectic mixture within the urea-biuret-triuret-cyanuric acid ensemble. Furthermore, DSC data shows that cyanuric acid sublimates without decomposition at temperatures above 300 °C. Numerical simulations of the TGA and DSC experiments are performed by a multi-phase tank reactor model (DETCHEMMPTR [3]). The new reaction mechanism describes well the main features (decomposition steps and calorimetry) and dependencies (on heating rate and surface area) of the decompositions of urea, biuret, triuret and cyanuric acid. [1] W. Brack, B. Heine, F. Birkhold, M. Kruse, G. Schoch, S. Tischer and O. Deutschmann, “Kinetic modeling of urea decomposition based on systematic thermogravimetric analyses of urea and its most important by-products”, CES 106, 1–8 (2014). [2] S. Tischer, M. Börnhorst, J. Amsler, G. Schoch and O. Deutschmann, “Thermodynamics and reaction mechanism of urea decomposition”, PCCP, in press, DOI: 10.1039/C9CP01529A (2019). [3] www.detchem.co

Thermodynamics and reaction mechanism of urea decomposition

The selective catalytic reduction technique for automotive applications depends on ammonia production from a urea-water solution by thermolysis and hydrolysis. In this process, undesired liquid and solid by-products are formed in the exhaust pipe. The formation and decomposition of these by-products have been studied by thermogravimetric analysis and differential scanning calorimetry. A new reaction scheme is proposed that emphasizes the role of thermodynamic equilibrium of the reactants in liquid and solid phases. Thermodynamic data for triuret have been refined. The observed phenomenon of liquefaction and re-solidification of biuret in the temperature range 193–230 °C is explained by formation of a eutectic mixture with urea

Multiscale microkinetic modelling of carbon monoxide and methane oxidation over Pt/γ-Al2O3 catalyst

Although compared to conventional diesel and gasoline engines gas engines running on methane-based fuels emit less pollutants, slip of unburnt methane is a hurdle to be overcome. In this regard, particularly noble metal-based catalysts allow for an efficient methane conversion even at low temperatures. Since these catalysts can undergo modifications under the highly dynamic operation [1] affecting activity and stability, the present work aims at creating a multiscale microkinetic model that has a strong link to the structure of the active sites, which change according to the chemical environment they are exposed. A detailed surface reaction mechanism for platinum-catalysed abatement of exhaust gases by Koop et al. [2] was used as a basis for the further development. The model is validated using light-off experiments with a monolithic Pt/Al2O3 catalyst in stoichiometric model gas mixtures. Simulations were carried out using the DETCHEMCHANNEL software [3] and show a remarkable difference, especially regarding the predicted ignition temperature. This different behaviour could be associated to the activation energies of the key reactive steps that need further investigation, i.e. dissociative adsorption of CH4. Along with theoretical considerations, spatially resolved information from experiments are used to improve the model. [1] P. Lott, O. Deutschmann, “Lean-Burn Natural Gas Engines: Challenges and Concepts for an Efficient Exhaust Gas Aftertreatment System” Emiss. Control Sci. Technol. 7, 1-6 (2021). [2] J. Koop, O. Deutschmann, “Detailed surface reaction mechanism for Pt-catalyzed abatement of automotive exhaust gases”, Appl. Cat. B 91, 1 (2009) [3] O. Deutschmann, S. Tischer, C. Correa, D. Chatterjee, S. Kleditzsch, V.M. Janardhanan, N. Mladenov, H. D. Minh, H. Karadeniz, M. Hettel, V. Menon, A. Banerjee, H. Goßler, E. Daymo, DETCHEM Software package, 2.8 ed., www.detchem.com, Karlsruhe 2020

Methane Oxidation over PdO: Towards a Better Understanding of the Influence of the Support Material

The presence of water vapor during the oxidation of the strong greenhouse gas methane over PdO-based catalysts is known to result in severe inhibition and catalyst deactivation. In this context, our current study elucidates the role of the support material for different water concentrations in the reaction gas mixture. Compared to a reference PdO/Al2O3 catalyst, the catalytic activity can be significantly enhanced when using SnO2 and ZrO2 as support materials and remains stable during 24 h of operation at 823 K in the presence of 12 % H2O, whereas under identical conditions CH4 conversion drops by 68 % over PdO/Al2O3. The interplay between Pd species and catalyst support was systematically characterized by thermogravimetric analysis, temperature-programmed reduction experiments and TEM measurements. Finally, a kinetic scheme was derived based on the experimental data