04 Fakultät Energie-, Verfahrens- und Biotechnik
Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/5
Browse
71 results
Search Results
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 Development of hydrodynamic density functional theory for mixtures and application to droplet coalescence(Stuttgart : Universität Stuttgart, Institut für Technische Thermodynamik und Thermische Verfahrenstechnik, 2021) Stierle, Rolf; Groß, Joachim (Prof. Dr.-Ing.)Predicting accurately coalescence phenomena is critical to the accurate description of the hydrodynamics of fluids and their mixtures. A promising framework for the development of models for such phenomena is dynamic density functional theory. Dynamic density functional theory enables the analysis of dynamical processes in inhomogeneous systems of pure fluids and fluid mixtures at the molecular level. In this work, a hydrodynamic density functional theory model for mixtures in conjunction with Helmholtz energy functionals based on the PC-SAFT equation of state is proposed, that obeys the first and second law of thermodynamics and simplifies to the isothermal Navier-Stokes equation for homogeneous systems. The hydrodynamic density functional theory model is derived from a variational principle and accounts for both viscous forces and diffusive molecular transport. A Maxwell-Stefan model is applied for molecular transport. This work identifies a suitable expression for the driving force for molecular diffusion of inhomogeneous systems that captures the effect of interfacial tension. The proposed hydrodynamic density functional theory is a non-local theory that requires the computation of weighted (spatial averaged) densities around each considered spatial coordinate by convolution, which is computationally expensive. This work uses Fourier-type transforms to determine the weighted densities. A pedagogical derivation is presented for the efficient computation of the convolution integrals occurring in the Helmholtz energy functionals in Cartesian, cylindrical, and spherical coordinates on equidistant grids using fast Fourier and similar transforms. The applied off-the-shelf algorithms allow to reduce dimensionality and complexity of many practical problems. Furthermore, an algorithm for a fast first-order Hankel transform is proposed, allowing fast and easy density functional theory calculations in rotationally symmetric systems. Application of the hydrodynamic density functional theory model using a well-balanced finite-volume scheme to one-dimensional droplet and bubble coalescence of pure fluids and binary mixtures is presented. The required transport coefficients, shear viscosity and Maxwell-Stefan diffusion coefficients, are obtained by applying entropy scaling to inhomogeneous fluids. The considered systems show a qualitative difference in the coalescence characteristics of droplets compared to bubbles. This constitutes a first step towards predicting the phase rupture leading to coalescence using dynamic density functional theory.Item Open Access Acid catalyzed cross‐linking of polyvinyl alcohol for humidifier membranes(2021) Michele, Andre; Paschkowski, Patrick; Hänel, Christopher; Tovar, Günter E. M.; Schiestel, Thomas; Southan, AlexanderPolyvinyl alcohol (PVA) is a hydrophilic polymer well known for good film forming properties, high water vapor permeance JW, and low nitrogen permeance. However, depending on molar mass and temperature, PVA swells strongly in water until complete dissolution. This behavior affects the usability of PVA in aqueous environments and makes cross‐linking necessary if higher structural integrity is envisaged. In this work, PVA networks are formed by thermal cross‐linking in the presence of p‐toluenesulfonic acid (TSA) and investigated in a design of experiments approach. Experimental parameters are the cross‐linking period tc, temperature ϑ and the TSA mass fraction wTSA. Cross‐linking is found to proceed via ether bond formation at all reaction conditions. Degradation is promoted especially by a combination of high wTSA, tc and ϑ. Thermal stability of the networks after preparation is strongly improved by neutralizing residual TSA. Humidification membranes with a JW of 6423 ± 63.0 gas permeation units (GPU) are fabricated by coating PVA on polyvinyliden fluoride hollow fibers and cross‐linking with TSA. Summarizing, the present study contributes to a clearer insight into the cross‐linking of PVA in presence of TSA, the thermal stability of the resulting networks and the applicability as selective membrane layers for water vapor transfer.Item Open Access Integrated process design and optimisation for the production of cellobiose lipids from Ustilago sp.(2024) Oraby, Amira; Rupp, Steffen (Prof. Dr.)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 Unravelling parameter interactions in calcium alginate/polyacrylamide double network hydrogels using a design of experiments approach for the optimization of mechanical properties(2024) Gorke, Oliver; Stuhlmüller, Marc; Tovar, Günter E. M.; Southan, AlexanderCalcium alginate/polyacrylamide double network hydrogels were reported to be exceptionally tough. However, literature reports so far varied the sample compositions mainly by one parameter at a time approaches, thus only drawing an incomplete picture of achievable material properties. In this contribution, sample compositions are varied according to a face-centered central composite experimental design taking into account the four parameters of alginate concentration cAlg, high/low molar mass alginate mixing ratio RP, acrylamide concentration cAAm, and N,N′-methylenebisacrylamide concentration cMBA. Each sample composition is investigated in triplicate. Thus, 75 samples were investigated by tensile testing, and a detailed analysis of the significant parameters and parameter interactions influencing the mechanical properties is conducted. The data shows that two parameter interactions, involving all four tested parameters, have a large effect on the Young's modulus, the strength, the toughness and the strain at material failure. As a consequence, it becomes evident that the experimental procedure from previous studies did not always result in optimum sample compositions. The results allow optimization of the mechanical properties within the studied parameter space, and a new maximum value of the strength of 710 kPa is reported. The data also give rise to the assumption that other parameters and parameter interactions ignored also in this study may allow further tailoring of mechanical properties.Item Open Access Optimizing mass transfer in multiphase fermentation : the role of drag models and physical conditions(2023) Mast, Yannic; Wild, Moritz; Takors, RalfDetailed knowledge of the flow characteristics, bubble movement, and mass transfer is a prerequisite for the proper design of multiphase bioreactors. Often, mechanistic spatiotemporal models and computational fluid dynamics, which intrinsically require computationally demanding analysis of local interfacial forces, are applied. Typically, such approaches use volumetric mass-transfer coefficient (kLa) models, which have demonstrated their predictive power in water systems. However, are the related results transferrable to multiphase fermentations with different physicochemical properties? This is crucial for the proper design of biotechnological processes. Accordingly, this study investigated a given set of mass transfer data to characterize the fermentation conditions. To prevent time-consuming simulations, computational efforts were reduced using a force balance stationary 0-dimension model. Therefore, a competing set of drag models covering different mechanistic assumptions could be evaluated. The simplified approach of disregarding fluid movement provided reliable results and outlined the need to identify the liquid diffusion coefficients in fermentation media. To predict the rising bubble velocities uB, the models considering the Morton number (Mo) showed superiority. The mass transfer coefficient kL was best described using the well-known Higbie approach. Taken together, the gas hold-up, specific surface area, and integral mass transfer could be accurately predicted.Item Open Access Perovskite chromite-based fuel electrode for solid oxide cells (SOCs): towards the understanding of the electrochemical performance(2023) Amaya Dueñas, Diana María; Friedrich, K. Andreas (Prof. Dr. rer. nat.)The current energy transition is a key driver for the continuous development of fuel cells and electrolyzers due to the rapid growth of the clean energy demand and the need to overcome the intermittency of the power supply of renewable energy sources, such as wind and solar energy. In this regard, solid oxide cells (SOC) are promising systems that allow to overcome such fluctuations: they convert renewable electrical energy into chemical energy in the form of hydrogen and valuable fuels and chemicals, while they can also repower the grid by converting fuels and hydrogen into electrical power. This feature in reversibility has attracted the interest among Power-to-X technologies, which can be exploited by operating SOCs in fuel cell (SOFC), electrolysis (SOEL) and reversible (rSOC) modes. Nevertheless, SOCs are not yet a mature technology due to limitations on the performance of their electrolyte and electrodes. Typical fuel electrodes made of Ni-based cermets are in contact not only with hydrogen, but also with reactants such as natural gas, biogas, steam and carbon dioxide, leading to important operation issues related to high temperatures and poisoning tolerance, which significantly detriment the performance of these systems. Due to the urgent need for the development of sustainable SOC systems in clean energy scenarios, this thesis aims to cover the Ni cermets issues related to SOCs operation, such as nickel agglomeration, nickel migration, structural cell damage and carbon deposition. Therefore, with the motivation to propose alternative fuel electrode materials to the state-of-the-art Ni cermets, formulations of perovskite chromite-based fuel electrodes were investigated in different SOC operating conditions. Firstly, different perovskite compositions were investigated by X-ray diffraction (XRD) to ensure the desired phase. With these crystal structure characterizations, the lanthanum-chromite perovskite with Ni doping (LSCrN) was selected as candidate fuel electrode material with the compositions La0.7Sr0.3Cr0.85Ni0.15O3-δ (L70SCrN) and La0.65Sr0.3Cr0.85Ni0.15O3-δ (L65SCrN). These materials were synthetized by the glycine-nitrate combustion method and ceramic powder morphology was characterized by scanning electron microscopy (SEM). An experimental protocol for the cell manufacturing process was designed and the electrolyte-supported-cells (ESCs) were produced by screen-printing, drying and sintering processes. ESCs were tested in different operating SOC modes: fuel cell (SOFC), steam electrolysis (SOEL), steam and carbon dioxide co-electrolysis (co-SOEL), as well as in reversible mode (rSOC) and even in dry carbon dioxide electrolysis operation. In situ electrochemical characterizations were performed by evaluating the voltage - current response and the electrochemical impedance spectroscopy (EIS). In parallel, the exsolution of nickel particles from the produced LSCrN ceramic powders was investigated by means of temperature programmed reduction (TPR), X-ray spectroscopy (XPS) and XRD techniques. It was shown that the introduction of A-site deficiency promoted the reduction of metallic nickel particles on the perovskite surface. The particle distribution was found to be dependent on the temperature, the atmosphere and the overpotential. In co-SOEL operation, cells with the developed L65SCrN electrode showed a comparable performance to the ones with state-of-the-art Ni cermets, e.g. - 0.8 A·cm-2 at 1.32 V and 860 °C. The long-term stability (~ 1000 hours) suggested that under strongly reducing atmospheres, such as in SOEL at 860 °C, the L65SCrN electrode suffered from accelerated performance degradation due to an alteration of the transport properties. Nonetheless, it was found that a decrease in operating temperature (below 830 °C) could be a suitable strategy to mitigate this durability issue. These findings are related to a gain in performance of the perovskite electrodes against the state-of-the-art Ni electrodes at temperatures between 770 °C and 830 °C, possibly due to lower reaction energy barriers. These outcomes were used as basis for a scale-up analysis from the cell level up to the system level, i.e. up to the MW scale, by analyzing a real case application of SOEL-based systems for hydrogen production. This analysis suggested that the implementation of perovskite electrodes in SOEL systems, together with a decrease of the system operating temperature, would lead to a significant reduction of the number of cells in the stacks and hence of the system components, simplifying the system layout. Additionally, the required amount of Ni raw material would also be significantly decreased, which would mitigate future supply chain issues that the mineral market may experience in the upcoming years. This study paves the way for future alternative electrode development for SOC applications while suggesting potential benefits at the system scale.Item Open Access Parametric study on the adjustability of the syngas composition by sorption-enhanced gasification in a dual-fluidized bed pilot plant(2021) Hafner, Selina; Schmid, Max; Scheffknecht, GünterFinding a way for mitigating climate change is one of the main challenges of our generation. Sorption-enhanced gasification (SEG) is a process by which syngas as an important intermediate for the synthesis of e.g., dimethyl ether (DME), bio-synthetic natural gas (SNG) and Fischer-Tropsch (FT) products or hydrogen can be produced by using biomass as feedstock. It can, therefore, contribute to a replacement for fossil fuels to reduce greenhouse gas (GHG) emissions. SEG is an indirect gasification process that is operated in a dual-fluidized bed (DFB) reactor. By the use of a CO2-active sorbent as bed material, CO2 that is produced during gasification is directly captured. The resulting enhancement of the water-gas shift reaction enables the production of a syngas with high hydrogen content and adjustable H2/CO/CO2-ratio. Tests were conducted in a 200 kW DFB pilot-scale facility under industrially relevant conditions to analyze the influence of gasification temperature, steam to carbon (S/C) ratio and weight hourly space velocity (WHSV) on the syngas production, using wood pellets as feedstock and limestone as bed material. Results revealed a strong dependency of the syngas composition on the gasification temperature in terms of permanent gases, light hydrocarbons and tars. Also, S/C ratio and WHSV are parameters that can contribute to adjusting the syngas properties in such a way that it is optimized for a specific downstream synthesis process.