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| Book/Dissertation / PhD Thesis | FZJ-2024-06947 |
2024
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
ISBN: 978-3-95806-784-4
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Please use a persistent id in citations: doi:10.34734/FZJ-2024-06947
Abstract: Gas separation membranes made of ceramic oxygen ion and electron conducting [MIEC] materials can be used for energy-efficient oxygen production or in a membrane reactor for the homogeneous supply of oxygen to a chemical reaction. To increase efficiency, the membranes are often manufactured in an asymmetrical design. They consist of a very thin membrane and a porous support to provide mechanical stability. This structure results in complex, interdependent transport processes. The aim of this work is to describe all relevant gas transport steps separately and with high resolution using suitable models that are based on the most fundamental physical properties possible. Membrane transport is calculated either using the Wagner equation including surface exchange in the form of the characteristic thickness or using the model of Zhu et al. The transport in the support is described using the binary friction model, which takes into account binary diffusion, Knudsen diffusion and viscous flow in the pores of the support. An iterative numerical root finding algorithm enables the calculation of the partial pressure at the interface between the membrane and the support. This can then be used to calculate the oxygen flux through the asymmetric membrane. Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) is used in this work as the individual transport steps can be easily analysed experimentally due to the high permeability of this material. The transport processes in the gas phases on both sides of the membrane are described using computational fluid dynamics (CFD) in the Ansys Fluent software. The use of a 3D geometry which replicates the test cell used in the experiments as accurately as possible enables the flow condi -tions and partial pressures during the experiment to be modelled precisely. The transport model for the membrane is integrated into the CFD software by means of a user-defined function (UDF). The agreement between experiment and simulation is analysed by comparing the state variables at the inlets and outlets, by this the transport model is validated. The suitability and accuracy of the methods for determining the material parameters used are discussed and evaluated with regard to their range of applicability. This work thus provides important information on the methodology for determining the model parameters and shows the limits of usability. By considering the influence of the individual parameters within a sensitivity analysis, a dedicated optimisation of the microstructure of asymmetric membranes is facilitated. The integration of the operating parameters and the mapping of the test cell as a 3D geometry contribute to the optimisation of the experimental setup. In several cases, the simulations achieve good agreement with the experimental data. In cases where significant deviations occur, the results show, for example, that only the consideration of the partial pressure-dependent surface exchange can provide agreement with all experimental results. The modular design of the presented framework and the use of fundamental physical quantities allow the transferability to other materials.
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