04 Fakultät Energie-, Verfahrens- und Biotechnik
Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/5
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Item Open Access Membrane electrode assembly for water electrolysis(2023) Nguyen, Thi Hai Van; Friedrich, K. Andreas (Prof. Dr. rer. nat.)Maintaining a sufficient energy supply while minimizing the impact on the environment and climate is one of the greatest social and scientific challenges of our times. There are various fields of research and technological developments in the context of global warming and limitless growing energy demand, and the focus of this PhD programme is on artificial photosynthesis, more specifically on the assembly of Membrane electrode assembly for water electrolyzer part. Mimicking photosynthesis in a scheme to trap solar energy in chemical bonds (fuels) is a scientific and technological challenge. Having a cost-effective and reliable process stays one of the main limitations in order to achieving the long-term goal of this approach. In this work, within the European eSCALED project, the elaboration of Membrane Electrode Assembly (MEA) for water electrolysis by introducing new materials and low-cost fabrication methods was investigated.Item Open Access Constitutive correlations for mass transport in fibrous media based on asymptotic homogenization(2023) Maier, Lukas; Kufferath-Sieberin, Lars; Pauly, Leon; Hopp-Hirschler, Manuel; Gresser, Götz T.; Nieken, UlrichMass transport in textiles is crucial. Knowledge of effective mass transport properties of textiles can be used to improve processes and applications where textiles are used. Mass transfer in knitted and woven fabrics strongly depends on the yarn used. In particular, the permeability and effective diffusion coefficient of yarns are of interest. Correlations are often used to estimate the mass transfer properties of yarns. These correlations commonly assume an ordered distribution, but here we demonstrate that an ordered distribution leads to an overestimation of mass transfer properties. We therefore address the impact of random ordering on the effective diffusivity and permeability of yarns and show that it is important to account for the random arrangement of fibers in order to predict mass transfer. To do this, Representative Volume Elements are randomly generated to represent the structure of yarns made from continuous filaments of synthetic materials. Furthermore, parallel, randomly arranged fibers with a circular cross-section are assumed. By solving the so-called cell problems on the Representative Volume Elements, transport coefficients can be calculated for given porosities. These transport coefficients, which are based on a digital reconstruction of the yarn and asymptotic homogenization, are then used to derive an improved correlation for the effective diffusivity and permeability as a function of porosity and fiber diameter. At porosities below 0.7, the predicted transport is significantly lower under the assumption of random ordering. The approach is not limited to circular fibers and may be extended to arbitrary fiber geometries.Item Open Access Thin organic‐inorganic anti‐fouling hybrid‐films for microreactor components(2022) Neßlinger, Vanessa; Welzel, Stefan; Rieker, Florian; Meinderink, Dennis; Nieken, Ulrich; Grundmeier, GuidoDeposit formation and fouling in reactors for polymer production and processing especially in microreactors is a well‐known phenomenon. Despite the flow and pressure loss optimized static mixers, fouling occurs on the surfaces of the mixer elements. To improve the performance of such parts even further, stainless steel substrates are coated with ultra‐thin films which have low surface energy, good adhesion, and high durability. Perfluorinated organosilane (FOTS) films deposited via chemical vapor deposition (CVD) are compared with FOTS containing zirconium oxide sol‐gel films regarding the prevention of deposit formation and fouling during polymerization processes in microreactors. Both film structures led to anti‐adhesive properties of microreactor component surfaces during aqueous poly(vinylpyrrolidone) (PVP) synthesis. To determine the morphology and surface chemistry of the coatings, different characterization methods such as X‐ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy as well as microscopic methods such as field‐emission scanning electron microscopy (FE‐SEM) and atomic force microscopy (AFM) are applied. The surface free energy and wetting properties are analyzed by means of contact angle measurements. The application of thin film‐coated mixing elements in a microreactor demonstrates a significant lowering in pressure increase caused by a reduced deposit formation.Item Open Access Acid-base flow battery, based on reverse electrodialysis with bi-polar membranes : stack experiments(2020) Xia, Jiabing; Eigenberger, Gerhart; Strathmann, Heinrich; Nieken, UlrichNeutralization of acid and base to produce electricity in the process of reverse electrodialysis with bipolar membranes (REDBP) presents an interesting but until now fairly overlooked flow battery concept. Previously, we presented single-cell experiments, which explain the principle and discuss the potential of this process. In this contribution, we discuss experiments with REDBP stacks at lab scale, consisting of 5 to 20 repeating cell units. They demonstrate that the single-cell results can be extrapolated to respective stacks, although additional losses have to be considered. As in other flow battery stacks, losses by shunt currents through the parallel electrolyte feed/exit lines increases with the number of connected cell units, whereas the relative importance of electrode losses decreases with increasing cell number. Experimental results are presented with 1 mole L^-1 acid (HCl) and base (NaOH) for open circuit as well as for charge and discharge with up to 18 mA/cm2 current density. Measures to further increase the effciency of this novel flow battery concept are discussed.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 Synthese, Charakterisierung und Degradation ionisch vernetzter Blendmembranen für den Brennstoffzellen-Einsatz(2013) Chromik, Andreas; Roduner, Emil (Prof. Dr.)Hier sollen die relevanten Ergebnisse dieser Arbeit zusammengefasst werden. Diese stellen wichtige Resultate für die Polymersynthese mittels Polykondensation, das Verhalten und die Kriterien für die Eignung von Polymeren für Blends, deren Degradationsverhalten, ihre BZ-Leistung und die Eignung von verschiedenen Analysetechniken zur Polymer- und Membrancharakterisierung bereit. Speziell bei der Polykondensation von Arylpolymeren können sehr viel höhere Molekularmassen erzielt werden als bisher. Es zeigte sich, dass die unterschiedlichen Molekularmassen einen starken Einfluss auf die Topografie, Morphologie und darüber auf die BZ-Leistung der Blends ausüben. Es wurde ebenfalls gezeigt, dass die GPC eine gute Methode darstellt, um Degradationsvorgänge in der Membran zu detektieren und zu charakterisieren. Zudem kann die GPC als ein wichtiges Instrument zur Qualitätssicherung betrachtet werden. Die hier gewonnenen Daten zeigen, dass man die theoretische Betrachtung von Degradationsmechanismen und deren Auswirkung auf die MWD auch in die Praxis übertragen kann. Des Weiteren konnte nachgewiesen werden, wie die Degradation der Membranen mit der Molekularmasse zusammenhängt und die Molekularmasse sich auf die Topographie, Morphologie und darüber auf die Brennstoffzellenleistung und Stabilität auswirkt. Zudem konnten als wichtige Analysemethoden die GPC- und AFM-Technik implementiert werden, um zum einen das Degradationsverhalten zu untersuchen und zum anderen ein tieferes Verständnis für die mikroskopischen Zusammenhänge von Morphologie und Brennstoffzellenleistung zu gewinnen.Item Open Access Novel anion exchange membrane based on poly(pentafluorostyrene) substituted with mercaptotetrazole pendant groups and its blend with polybenzimidazole for vanadium redox flow battery applications(2020) Cho, Hyeongrae; Atanasov, Vladimir; Krieg, Henning M.; Kerres, Jochen A.In order to evaluate the performance of the anion exchange membranes in a vanadium redox flow battery, a novel anion exchange polymer was synthesized via a three step process. Firstly, 1-(2-dimethylaminoethyl)-5-mercaptotetrazole was grafted onto poly(pentafluorostyrene) by nucleophilic F/S exchange. Secondly, the tertiary amino groups were quaternized by using iodomethane to provide anion exchange sites. Finally, the synthesized polymer was blended with polybenzimidazole to be applied in vanadium redox flow battery. The blend membranes exhibited better single cell battery performance in terms of efficiencies, open circuit voltage test and charge-discharge cycling test than that of a Nafion 212 membrane. The battery performance results of synthesized blend membranes suggest that those novel anion exchange membranes are promising candidates for vanadium redox flow batteries.Item Open Access Fouling during solution polymerization in continuously operated reactors(2021) Zander, Christian; Nieken, Ulrich (Prof. Dr.-Ing.)Specialty polymers are mostly produced in discontinuous processes in tank reactors due to the need for flexibility in the production of this product class. Milli-structured, continuously operated reactors are promising alternatives for process intensification to increase energy efficiency and space-time-yield, reduce time-to-market for new products and maintain flexibility. A major obstacle for the transfer of batch processes to such reactor systems is the formation of fouling deposits, which grow and block the reactor. To overcome this obstacle, knowledge of the mechanisms of the formation of fouling deposits is essential. In this thesis, fouling during the polymerization of N-Vinylpyrrolidone (NVP) in aqueous solution is studied both experimentally and in simulations to gain insight into the underlying mechanism, find a model-based description of this mechanism and make suggestions how to prevent or at least decrease the formation of fouling deposits. First, results from experiments in different kinds of tank and tubular reactors are presented. In all these reactor systems, fouling deposits are formed by an insoluble polymer gel, which adheres strongly to metal surfaces. Initially, the polymer gel is formed in regions with increased local residence time, e.g. in dead-water zones of static mixer elements, at baffles of tank reactors or at walls of tubular reactors without mixer elements. Once fouling deposits have been formed, they grow by reaction and lead to clogging of tubular reactors systems. Since a polymer gel is formed, side reactions that lead to high-molecular and branched polymer chains must play an important role for the formation of deposits. Kinetic models that are based on a recently suggested reaction mechanism and predict microstructural property distribution are presented and validated using continuously stirred tank reactor (CSTR) experiments. The results confirm the suggested reaction mechanism in which creation and propagation of terminal double bonds lead to branched or crosslinked polymer chains. Although gelation of the bulk phase does not occur, fouling deposits are formed at the baffles of the tank reactor and in other poorly mixed regions of the reactor. This observation emphasizes the importance of the flow pattern and diffusive mass transport for the formation of fouling deposits. To demonstrate the interplay of the flow pattern, the reaction and diffusive mass transport, simulations using a transient CFD solver including a reduced version of the reaction kinetics model together with a model for diffusive mass transport are presented. The mass transport model is able to describe diffusive transport of statistical moments and is, therefore, consistent with the reaction kinetics model. Simulations in different two-dimensional geometries confirm that regions with increased local residence time lead to the formation of polymer gels. These regions, e.g. regions close to reactor walls or dead-water zones, cause concentration gradients, which induce mass transport between such regions and the bulk phase. Due to their lower diffusion coefficients in comparison to low molecular species, polymer molecules accumulate in these regions, which increases the viscosity locally. Because of the viscosity gradients, the flow pattern is distorted and the size of regions with increased residence time expands. The combination of an increased residence time, high polymer and low monomer contents promotes the formation of polymer gels by side reactions. Together with the adhesion of macromolecules on metal surfaces, this seems to be the relevant mechanism for the formation of fouling deposits. Therefore, strategies to decrease fouling should focus on surface modifications, which reduce adhesion of macromolecules, as well as the elimination of dead-water zones and viscosity gradients.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 Coupled electrohydrodynamic and thermocapillary instability of multi-phase flows using an incompressible smoothed particle hydrodynamics method(2022) Almasi, Fatemeh; Hopp-Hirschler, Manuel; Hadjadj, Abdellah; Nieken, Ulrich; Safdari Shadloo, MostafaThis paper concerns the study of coupled effects of electrohydrodynamic (EHD) and thermocapillary (TC) on the dynamic behaviour of a single liquid droplet. An incompressible Smoothed Particle Hydrodynamic (ISPH) multiphase model is used to simulate EHD-TC driven flows. The complex hydrodynamic interactions are modeled using the continuum surface force (CSF) method, in which the gradient of the interfacial tension and the Marangoni forces are calculated with an approximated error or 0.014% in the calculation of Marangoni force compared to the analytical solutions which is a significant improvement in comparison with previous SPH simulation studies, under the assumption that the thermocapillarity generates sufficiently large stress to allow droplet migration, while the electrohydrodynamic phenomena influences the droplet morphology depending on the electrical and thermal ratios of the droplet and the ambient fluid. This study shows that, when applying a vertical electric field and thermal gradient, the droplet starts to stretch horizontally towards a break-up condition at a high rate of electrical permitivity. The combined effect of thermal gradient and electric field tends to push further the droplet towards the break-up regime. When the thermal gradient and the electric field vector are orthogonal, results show that the droplet deformation would take place more slowly and the Marangoni forces cause the droplet to migrate, while the stretching in the direction of the electric field is not seen to be as strong as in the first case.