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Institute: search.filters.UBSInstitut.Institut für Technische Thermodynamik und Thermische VerfahrenstechnikSubject: search.filters.subject.660

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Now showing 1 - 7 of 7
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    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.
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    A new dispersion contribution based on the PCP-SAFT equation of state in the framework of classical density functional theory
    (Stuttgart : Universität Stuttgart, Institut für Technische Thermodynamik und Thermische Verfahrenstechnik, 2019) Sauer, Elmar; Groß, Joachim (Prof. Dr.-Ing.)
    This dissertation presents the development and evaluation of a dispersion contribution model of a Helmholtz energy functional in the framework of classical density functional theory. The model is based on the PCP-SAFT equation of state and was applied to fluid-liquid interfaces, confined systems, and sessile droplet systems.
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    Calculation of pure substance and mixture viscosities using PCP-SAFT and entropy scaling
    (Stuttgart : Universität Stuttgart, Institut für Technische Thermodynamik und Thermische Verfahrenstechnik, 2020) Lötgering-Lin, Oliver; Gross, Joachim (Prof. Dr.-Ing.)
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    Predicting and rationalizing the Soret coefficient of binary Lennard‐Jones mixtures in the liquid state
    (2022) Zimmermann, Nils E. R.; Guevara‐Carrion, Gabriela; Vrabec, Jadran; Hansen, Niels
    The thermodiffusion behavior of binary Lennard‐Jones mixtures in the liquid state is investigated by combining the individual strengths of non‐equilibrium molecular dynamics (NEMD) and equilibrium molecular dynamics (EMD) simulations. On the one hand, boundary‐driven NEMD simulations are useful to quickly predict Soret coefficients because they are easy to set up and straightforward to analyze. However, careful interpolation is required because the mean temperature in the measurement region does not exactly reach the target temperature. On the other hand, EMD simulations attain the target temperature precisely and yield a multitude of properties that clarify the microscopic origins of Soret coefficient trends. An analysis of the Soret coefficient suggests a straightforward dependence on the thermodynamic properties, whereas its dependence on dynamic properties is far more complex. Furthermore, a limit of applicability of a popular theoretical model, which mainly relies on thermodynamic data, was identified by virtue of an uncertainty analysis in conjunction with efficient empirical Soret coefficient predictions, which rely on model parameters instead of simulation output. Finally, the present study underscores that a combination of predictive models and EMD and NEMD simulations is a powerful approach to shed light onto the thermodiffusion behavior of liquid mixtures.
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    Interfacial properties using classical density functional theory : curved interfaces and surfactants
    (Stuttgart : Universität Stuttgart, Institut für Technische Thermodynamik und Thermische Verfahrenstechnik, 2021) Rehner, Philipp; Groß, Joachim (Prof. Dr.-Ing.)
    Interfaces play an important role in natural and industrial processes. Classical density functional theory (DFT) has been established as a tool capable of predicting interfacial properties, but also of providing insight in the structure of fluids at interfaces. Compared to other statistical mechanical methods, particularly molecular simulation, an efficient implementation of DFT offers a significant reduction in computation time. This advantage comes with the cost of an increased modeling effort. In this work, the calculation of interfacial properties using DFT is discussed and applied to different aspects of interfaces. First, the properties of highly curved interfaces, as they appear in nucleation processes, are studied. This is done first by directly calculating the properties of nanodroplets using DFT in spherical coordinates and afterwards in an expansion around a flat interface. Because for some applications, the calculation time of DFT is a limiting factor, a new method to predict surface tensions from equation of state parameters is introduced. This is achieved by using a Taylor expansion of the full DFT Helmholtz energy functional around a local density. The resulting functional is identical to that used in density gradient theory except for an explicit, temperature and density dependent expression for the influence matrix. The method is subsequently used to examine in detail the parametrization of associating components, particularly water and alcohols, that pose difficulties with respect to the simultaneous description of bulk phase equilibria and interfacial properties. A multiobjective optimization approach is used to assess different models and to quantify their capabilities and limitations. The so obtained water model presents the foundation for the last segment of this work, that studies the interfacial properties of water/surfactant and water/alkane/surfactant systems. The amphiphilic surfactant molecules are modeled using a heteronuclear DFT approach that resolves the distributions of individual segments. The parameters of this group contribution method are obtained by fitting to properties of small surfactant molecules and can then be used to predict properties of larger molecules for which less or no experimental data is available. The model is used to study the adsorption and orientation of surfactant molecules at interfaces and the corresponding reduction in interfacial tension.
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    An atomistic view on the uptake of aromatic compounds by cyclodextrin immobilized on mesoporous silica
    (2022) Kraus, Hamzeh; Hansen, Niels
    The effect of immobilized β-cyclodextrin (bCD) molecules inside a mesoporous silica support on the uptake of benzene and p -nitrophenol from aqueous solution was investigated using all-atom molecular dynamics (MD) simulations. The calculated adsorption isotherms are discussed with respect to the free energies of binding for a 1:1 complex of bCD and the aromatic guest molecule. The adsorption capacity of the bCD-containing material significantly exceeds the amount corresponding to a 1:1 binding scenario, in agreement with experimental observations. Beside the formation of 1:2 and, to a lesser extent, 1:3 host:guest complexes, also host-host interactions on the surface as well as more unspecific host-guest interactions govern the adsorption process. The demonstrated feasibility of classical all-atom MD simulations to calculate liquid phase adsorption isotherms paves the way to a molecular interpretation of experimental data that are too complex to be described by empirical models.
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    Process-based screening of porous materials for vacuum swing adsorption based on 1D classical density functional theory and PC-SAFT
    (2025) Mayer, Fabian; Buhk, Benedikt; Schilling, Johannes; Rehner, Philipp; Gross, Joachim; Bardow, André
    Adsorption-based processes are showing substantial potential for carbon capture. Due to the vast space of potential solid adsorbents and their influence on the process performance, the choice of the material is not trivial but requires systematic approaches. In particular, the material choice should be based on the performance of the resulting process. In this work, we present a method for the process-based screening of porous materials for pressure and vacuum swing adsorption. The method is based on an equilibrium process model that incorporates one-dimensional classical density functional theory (1D-DFT) and the PC-SAFT equation of state. Thereby, the presented method can efficiently screen databases of potential adsorbents and identify the best-performing materials as well as the corresponding optimized process conditions for a specific carbon capture application. We apply our method to a point-source carbon capture application at a cement plant. The results show that the process model is crucial to evaluating the performance of adsorbents instead of relying solely on material heuristics. Furthermore, we enhance our approach through multi-objective optimization and demonstrate for materials with high performance that our method is able to capture the trade-offs between two process objectives, such as specific work and purity. The presented method thus provides an efficient screening tool for adsorbents to maximize process performance.