Universität Stuttgart
Permanent URI for this communityhttps://elib.uni-stuttgart.de/handle/11682/1
Browse
66 results
Search Results
Item Open Access Einfluss der Reaktionskinetik und Mischung auf die Selektivität in reaktiven Blasenströmungen(2022) Gast, Sebastian; Nieken, Ulrich (Prof. Dr.-Ing.)In dieser Arbeit wird das bisher noch unzureichend erforschte Wechselspiel zwischen Fluiddynamik, Stoffübergang und chemischer Reaktion in Blasenströmungen untersucht. Um die gegenseitigen Abhängigkeiten dieser Prozesse zu verstehen, müssen diese zuerst getrennt voneinander ohne die Beeinflussung der anderen Prozesse betrachtet werden. Um die Reaktionskinetik ohne Einfluss des Stofftransportes zu bestimmen, wurde ein neuer Kinetikreaktor entwickelt. Hierbei wird der Stoffübergang von der Gas- in die Flüssigphase räumlich von der Reaktion getrennt. Diese räumliche Entkopplung erlaubt die Untersuchung der Reaktionskinetik in homogener flüssiger Phase ohne jegliche Stofftransportlimitierung. Als Modellsystem wurde die Kinetik der unkatalysierten Toluoloxidation ermittelt und parametriert. Das selektive Reaktionsnetzwerk der Toluoloxidation, bestehend aus konkurrierenden Folge- und Parallelreaktionen bietet die notwendigen Voraussetzungen für die Studie der zuvor genannten Abhängigkeiten der Fluiddynamik, Stoffübergang und chemischer Reaktion in Blasenströmungen. Die ermittelte Reaktionskinetik erwies sich in numerischen Simulationen als zu langsam für die Interaktion mit der Blasenumströmung. Dies konnte experimentell in einer transparenten Hochdruckblasensäule technischer Größe bei industriellen Bedingungen von 30 bar und 190°C bestätigt werden. In weiterführenden Simulationen wurde die um einen Faktor KF beschleunigte Reaktionskinetik verwendet, um den Einfluss der nicht idealen Vermischung im Nachlauf einer Blase auf die Reaktion und das erzeugte Produktspektrum zu untersuchen. Es konnte gezeigt werden, dass nur Reaktionen durch die Blasenströmung beeinflusst werden, welche in einem Zeitbereich von 0.1 < Da_1 < 1000, dem sogenannten mischungsmaskierten Bereich, ablaufen. Langsamere oder schnellere Reaktionen laufen in der Bulkphase beziehungsweise ausschließlich an der Blasenoberfläche ab und werden nicht durch die unvollständige Vermischung im Nachlauf der Blase beeinflusst. Der größte Einfluss auf den Verlauf der Reaktion wird dabei von einer durch den stationären Blasenwirbel erzeugte Transportbarriere verursacht. Diese verhindert den Abtransport der erzeugten Produkte. Bei einem gleichzeitig konstanten Zustrom an Edukt werden Folgereaktionen gefördert. Dies führt zu einer starken Veränderung des Produktspektrums gegenüber des Reaktionsablaufes bei ideal vermischten Bedingungen. Darüber hinaus wurde ein Compartment Modell aufgestellt, um den Einfluss der nicht ideal vermischten Bedingungen einer Blasenumströmung auf die ablaufende Reaktion zu beschreiben. Das Compartment Modell basiert auf einem modifizierten Oberflächenerneuerungsmodell zur Darstellung der Abläufe an der Blasenoberfläche und einem Verweilzeitmodell zur Abbildung der unvollständigen Vermischung im Nachlauf der Blase. Es ist in der Lage, die identifizierte Abhängigkeit der Reaktion von Fluiddynamik und Stoffübergangs und -transport bei deutlich reduziertem Rechenaufwand zu reproduzieren und ist damit für den Einsatz in großskaligen Simulationen wie Euler-Euler und Euler-Lagrange geeignet.Item Open Access Modeling of porous polymer membrane formation(2017) Hopp-Hirschler, Manuel; Nieken, Ulrich (Prof. Dr.-Ing.)Porous polymer membranes are used in several separation processes, e.g. in dialysis or in water purification. The morphology of the membrane affects the quality of separation, e.g. selectivity, as well as the mechanical stability of the membrane. To control the morphology of the membrane during the preparation process we first need to understand the mechanism that leads to different pore structures. It is desirable to use a numerical model to predict the pore type and detailed structure. Wet-casting is a very common preparation process for porous polymer membranes where a liquid precipitation agent is used. Herein, a polymer solution and a coagulation bath is brought into contact. After contact the polymer solution is driven into a miscibility gap and starts to phase separate into a polymer lean and a polymer rich phase. Starting from the contact area between polymer solution and coagulation bath a pore structure grows where the polymer rich phase leads to the pore matrix. Although the process is used frequently in the last decades, its mechanism is still not fully understood. Therefore, the motivation in this thesis is to bridge experimental observations from membrane science to theoretical physics where concepts exist to understand the formation of pore structures in porous polymer membranes.Item Open Access Reaction mechanism development and numerical modeling of biomass gasification process(2021) Fernando, Niranjan; Riedel, Uwe (Prof. Dr. rer. nat.)Item Open Access On the mass transport phenomena in proton exchange membrane water electrolyzers(2020) García Navarro, Julio César; Friedrich, K. Andreas (Prof. Dr. rer. nat.)Proton exchange membrane (PEM) water electrolysis is a technology designed to produce hydrogen using only water and electricity as inputs; it has gained increased attention in industry and academia due to its advantages over incumbent hydrogen generation processes (of which the most widely used are steam reforming and coal gasification) namely, low temperature, carbon-neutral and intermittent operation. PEM electrolysis can be instrumental for creating a hydrogen economy, although still much research needs to be carried out before widespread industrial adoption is achieved. PEM water electrolyzers suffer energy losses associated with the chemical reactions and the transport of charge and mass; of these phenomena, mass transport in PEM electrolyzers is the least understood subject, given the complex nature of the interaction of multiphase flows (mainly consisting of liquid water and evolved gases) through micrometric pores. The subject of multiphase flow in water electrolysis and its relationship with the mass transport phenomena in PEM water electrolysis has been a prevalent subject in the literature. Despite numerous attempts at pinpointing the relationship between mass transport overpotential and the operating parameters, there is no clear consensus about which transport mechanisms dominate, nor about how the component design of PEM electrolyzers affects the mass transport. While the effect of temperature and current density on mass transport losses has been extensively studied and is well understood, there are significantly fewer studies that focus on the effect of water flow and pressure. Both water flow and pressure have a direct effect on mechanisms such as bubble nucleation and two-phase flows that occur in the porous structures within a PEM electrolyzer (electrodes and porous transport layers, PTLs). In this work, I studied the effect of water flow and pressure on the mass transport phenomena in PEM electrolyzers. Chapters 1 and 2 provide an introduction to the topic as well as a description of the materials and experimental setups used. Chapter 3 of this thesis depicts the visualization and modeling of bubble nucleation in an operating PEM electrolyzer. I discovered that bubble detachment radii are largely independent of water flow and I identified two types of bubbles: bubbles that detach after reaching a critical size, and bubbles that fill up the pores of a PTL before detaching. Chapter 3 consists of the measurements I carried out regarding the transport of evolved gas through the water-filled pores of a PTL, where I observed that water flow severely impedes the gas transport through the pores and that such impediment is related to a shear stress exerted by the water flow on the pores. Chapter 5 shows the measuring of mass transport losses using electrochemical impedance spectroscopy (EIS) on an operating PEM electrolyzer; the results indicate that pressure and water flow affect the diffusion of gas in the electrode and that the mass transport overpotential depends on design parameters of the PEM electrolyzer, such as electrode thickness and hydrophobicity. Overall, I derived a theoretical framework based on the assumption that the evolved gas in a PEM electrolyzer permeates through the PTL after diffusing from the active sites to the bubble nucleation sites. Such framework, constructed on the basis of the models regarding gas transport in porous media, can be used to explain the mass transport loses in a PEM electrolyzer that arise from operating with increased water flows and pressures. The model I derived can be used in future work as a guideline to optimize the components of a PEM electrolyzer, in particular regarding the hydrophobicity and pore size distribution of PTLs as well as the composition of the catalyst ink to produce the electrodes. Moreover, this work can also be used to further understand the mass transport losses and optimize the operation of PEM electrolyzers to decrease the energy consumption of hydrogen generation.Item Open Access CO2-Abtrennung aus Synthesegasen mit Hydrotalciten unter Hochtemperatur-Hochdruckbedingungen(2017) Bublinski, Martin; Seifert, Helmut (Prof. Dr.-Ing.)Membranverfahren trennen Gasgemische kontinuierlich, selektiv und energieeffizient. Damit stellt dieses Trennverfahren eine interessante Alternative zu herkömmlichen, energieintensiven Gasreinigungsverfahren mittels Wäschern dar. Allerdings steigen insbesondere für Hochtempera-turanwendungen die material- und prozesstechnischen Anforderungen an das Membransystem, wodurch die Materialauswahl stark eingegrenzt wird. Bisher existiert im kommerziellen Maßstab noch kein hochtemperaturtaugliches, kontinuierliches Abscheideverfahren für die selektive Abscheidung von CO2 aus Synthesegas. Der Einsatz von Membranen aus Hydrotalcit stellt für die CO2-Abscheidung aus vorgereinigten Synthesegasen einen vielversprechenden Lösungsansatz dar. In dieser Arbeit wird die systematische Entwicklung von anorganischen mehrschichtigen Hydrotalcit-Membranen vorgestellt, mit denen CO2 selektiv aus dem Gasstrom unter Hochtemperatur- und Hochdruckbedingungen (T > 350 °C, p ≤ 80 bar) abgeschiedenen werden kann. Die Prozesse CO2-Sorption bzw. CO2-Desorption auf der Membranoberfläche sowie die Diffusionseigenschaf-ten der Membran wurden dabei getrennt voneinander untersucht. Zuerst wurden an reinen und mit Kaliumcarbonat dotierten Hydrotalciten mit einem Sorptions-/Druckreaktor CO2-Sorptionsgleichgewichtsdaten im Temperatur- und Druckbereich zwischen 200-500 °C bzw. 20-80 bar aufgenommen. Dabei wurden maximale CO2-Kapazitäten von 1,2 mol/kg für reines Hydrotalcit und 2,0 mol/kg für dotiertes Hydrotalcit mit trockenem, bzw. 1,95 mol/kg für reines Hydrotalcit und 5,70 mol/kg für K-dotiertes Hydrotalcit mit feuchtem Gas ermittelt. Die Desorptionseigenschaften wurden mittels zyklischen CO2-Sorptionsexperimenten bestimmt. Dabei stellte sich sowohl für reines als auch für K-dotiertes Hydrotalcit für trockenes und feuchtes Gas eine konstante Arbeitskapazität nach mehreren Sorptionszyklen auf dem Niveau von zwei Drittel der ursprünglichen CO2-Sorptionskapazität ein. Die Hydrotalcit-Membransynthese erfolgte auf Al2O3-Substraten mit einem Harnstoff-Hydrolyse-Verfahren. Dadurch konnte ein Wachstum der Hydrotalcit-Kristalle direkt auf der Substratoberfläche erreicht werden und eine homogene Hydrotalcit-Membran synthetisiert werden. Durch eine weitere Hydrotalcit-Schicht gelang es, die Defekte der Membran weiter zu verringern. Die CO2-Permeanzen wurden mit einem Hochtemperatur-Membranmodul bei 200 °C auf 3,03·10-7 mol/(m2·s·Pa) und 500 °C auf 1,06·10-6 mol/(m2·s·Pa) bestimmt. Neben der Knudsen-Diffusion wurde die Lösungs-Diffusion von CO2 als weiterer Transportmechanismus durch die Hydrotalcit-Membran identifiziert. Die idealen CO2-Selektivitäten bezüglich N2, H2 und CO lagen demnach meist leicht über den Knudsen-Selektivitäten. Gegenüber N2 konnte bei Tempera-turen von 350 °C eine Selektivität von 1,31 ermittelt und folglich eine partielle CO2-selektive Trennung mit den synthetisierten Hydrotalcit-Membranen erzielt werden.Item Open Access Herstellung von Acrylnitril aus biobasierter Milchsäure und Propionsäure(2019) Mack, Daniel; Klemm, Elias (Prof. Dr.-Ing.)Item Open Access Thermal characterization, multi-scale thermal modeling and experimental validation of lithium-ion batteries for automobile application(2016) Tahir, Muhammad Wasim; Merten, Clemens (Prof. Dr.-Ing. habil.)This work focuses on studying the thermal aspects of automotive battery systems that includes developing a detailed thermal model for lithium-ion battery systems comprising an electrochemical heat generation model and a heat transfer model dynamically coupled together to form a full 3D thermal model. And finally validating the model by experimental findings.Item Open Access Tar reforming over low-cost active materials for gasification derived syngas(2021) Chen, Yen-Hau; Scheffknecht, Günter (Univ-Prof. Dr. techn.)The present work investigated the potential of in-situ biochar mixture from gasification process (straw char containing fly ash), pyrolyzed biochars by self-production (wood char, straw char, and palm shell char), and metal impregnated biochars (potassium and iron-loaded palm shell char) to be reused as tar-reforming catalysts in gasification processes. The catalytic activity and reforming selectivity of different materials were evaluated with toluene and naphthalene as tar model compounds in the presence of steam and hydrogen (major composition in the syngas from sorption enhanced gasification and steam-oxygen gasification) in a lab-scale fixed bed reactor at high temperature up to 900 °C. Straw char containing fly ash derived from the steam-oxygen gasification process was verified to be able to apply as a tar-reforming catalyst for enhancing the overall performance of the steam-oxygen gasification process. It was found that the significant effect of hydrogen on toluene reforming was demonstrated by the formation of benzene through hydrodealkylation reaction. The volumetric ratio of H2O to H2 was an essential parameter that decided the selectivity of toluene reforming. Although the coexist of toluene and hydrogen would inhibit the gasification, straw char containing fly ash was proved to be gasified during toluene reforming. The surface migration and agglomeration of inorganics due to gasification also resulted in the change of catalytic activity and reforming selectivity through time. With regard to pyrolyzed biochars, the low-cost material, wood char, straw char, and palm shell char, can successfully be used as the tar-reforming catalyst after sorption enhanced gasification process. A theoretical space time to reach the complete naphthalene conversion was 0.07 kg h m-3 at 850 °C for wood char, demonstrating a promising catalytic activity. It was also found that potassium and iron-loaded palm shell chars exhibited much better catalytic activity than palm shell char, while the parallel reaction of gasification of K-loaded palm shell char influenced the conversion with its drastic mass loss. Lastly, the spontaneous gasification and catalytic activity of wood char were thoroughly evaluated in simulated sorption enhanced gasification environments with toluene and naphthalene as tar model compounds. Besides, CaO was used as a reference catalyst for the comparison purpose since CaO is a commonly used sorbent in the sorption enhanced gasification process. A model of gasification reactivity during reforming of tar model compounds over wood char was developed in this work based on the reactivity at 20 % carbon conversion adopting the Extended Random Pore Model, where the pre-exponential factor, activation energy, and structural parameter were calculated to be 1.65·1010 min-1, 265.8 kJ mol-1, and 127, respectively. During the catalytic activity investigation, hydrogen was found to inhibit tar-reforming performance over CaO while the impact of hydrogen was insignificant over wood char. The results showed that spontaneous gasification during tar surrogates reforming led to mass loss, pore collapse, and inorganics agglomeration, which contributed to the catalytic deactivation of wood char. By considering gasification-caused deactivation, the carbon conversion was then used as a variable to modify the kinetic equations of the tar surrogates reforming. The activation energy and pre-exponential factor of naphthalene reforming (and toluene reforming) over wood char were calculated with the values of 422.5 kJ mol-1 and 2.92·1022 m3 kg-1 h-1 (284.8 kJ mol-1 and 1.90·1015 m3 kg-1 h-1), respectively, whereas the values for CaO were 126.9 kJ mol-1 and 6.79·104 m3 kg-1 h-1 (254.5 kJ mol-1 and 6.73·1011 m3 kg-1 h-1), respectively. The kinetic models developed in this study were later used for designing a tar reformer integrated with the sorption enhanced gasification process.Item Open Access Multistep reactions of molten nitrate salts and gas atmospheres(2024) Steinbrecher, Julian; Thess, André (Prof. Dr.)Dissertation zur Untersuchung der Stabilität von Nitratsalzschmelzen unter verschiedenen atmosphärischen Bedingungen und Temperaturen.Item Open Access The lithium-sulfur battery : design, characterization, and physically-based modeling(2015) Fronczek, David Norman; Bessler, Wolfgang G. (Prof. Dr.)The lithium-sulfur (Li/S) battery is a promising candidate for next-generation electrochemical energy storage. Its unique combination of electrochemical performance, cost effectiveness, and environmental sustainability are unprecedented among battery materials. As of today, however, Li/S batteries are only used for few niche applications; a broader adoption of this technology is impeded by the yet unsatisfactory energy efficiency, self discharge, and limited lifetime. This work contributes to the advancement of Li/S technology in two respects: First, a novel kind of positive electrode, based on coated lithium sulfide, was prepared, tested and optimized. Second, the understanding of the complex chemical and physical processes in the cell was improved by creating and utilizing a computational model of the Li/S battery. For the experimental part of this work, a chemical vapor deposition process was developed to apply a carbon coating to lithium sulfide particles. The focus of this work was on the optimization of the process chain from commercially available chemicals to the final coin cell in general and on the characterization of the materials and electrodes during various processing steps in particular. For the modeling part, an existing multiscale electrochemical modeling framework was extended to enable full-cell simulations of Li/S batteries. The model includes a detailed description of electrochemistry, transport, and the evolution of solid phases in the cell, but also accounts for the electrical double layer and, in a generic fashion, the microstructure of the electrodes. Finally, a phenomenological description of the shuttle effect and associated cell degradation was implemented and analyzed. The parametrization and partial validation of the model makes use of original data collected for this purpose, but also data from literature. Simulation results comprise charge/discharge profiles, cyclic voltammetry, impedance spectra, and the evolution of the chemical composition of both the electrolyte and the electrodes over time. The analysis of these results reveals limiting factors and suggests improved operating conditions. The apt combination of theoretical and empirical methods enabled an improvement of the performance and cycle life of the novel cathode material, but also contributed to a more profound understanding of the Li/S battery.