Eintrag in der Universitätsbibliographie der TU Chemnitz
Volltext zugänglich unter
URN: urn:nbn:de:bsz:ch1-qucosa2-978636
Bernardini, Luca
Richter, Markus (Prof. Dr.-Ing.) ; May, Eric F. (Prof. Dr.) (Gutachter)
Dew-Point Density Measurements of Gas Mixtures with Low Uncertainty
Kurzfassung in englisch
The development of models for predicting thermodynamic properties is essential for designing technical systems that reduce energy consumption and minimize environmental impact. A key requirement for advancing these models is high-quality experimental data with low uncertainties. Among such data, dew-point properties of gas mixtures are particularly important for progress in thermodynamics and process engineering. However, measuring properties along the saturation line presents challenges due to effects arising during phase transitions that can distort results. These challenges require advanced experimental techniques, a robust dataset, and a sound uncertainty analysis model.The current state-of-the-art approach for density and dew-point measurements relies on gravimetric instruments using the Magnetic Suspension Balance (MSB). Although widely used, these instruments have limitations with pure gases and mixtures due to sorption effects. Two major issues near the dew line include: (1) adsorption on the object immersed in the fluid, affecting weight and calculated densities, and (2) selective adsorption on internal surfaces, altering fluid composition. These effects lead to higher uncertainties in characterizing densities and fluid composition. Addressing them is critical for improving data quality and advancing modeling.
An improved gravimetric apparatus for dew-point density measurements, the Four-Sinker Densimeter (FSD), was designed by Moritz et al. (2017). This instrument simultaneously determines fluid density and adsorption loads, partially correcting the distorting effects. Achieving low uncertainties requires accurate calibrations and precise determination of key parameters. Among these, the low-uncertainty determination of sinker volumes is especially critical for accurate buoyancy force estimation—one of the main calibration challenges.
Given the FSD’s complexity and the limitations of existing uncertainty analysis methods, a more accessible method for uncertainty estimation was developed to assess the quality of measured densities and adsorption data.
Since gravimetric systems alone cannot provide detailed information about the adsorbate, gravimetric data analysis was combined with Molecular Dynamics Simulation (MDS). MDS offers insights into selective adsorption of mixture components and the density of the adsorbed phase.
The challenge of measuring dew-point densities of gas mixtures is interdisciplinary. It involves advancing gravimetric techniques, implementing high-accuracy calibrations, developing an accessible uncertainty analysis method, and using MDS for deeper insight.
This work focuses on the setup and commissioning of the FSD. Instrument calibrations and low-uncertainty parameter determinations were performed. A hydrostatic comparator, used to determine the volume of sinkers, was evaluated by measuring 14 objects with varying densities. A novel measurement sequence reduced the time required for volume determination. The comparator showed high precision, with average expanded uncertainties (k = 2) of 0.000041 cm³. Additionally, a simpler comparator suitable for academic or industrial research was developed, though with higher uncertainties.
Subsequently, first measurements were performed using the FSD. Densities of He, Ar, and N₂ were measured from (5.68 to 180.7) kg·m⁻³ and compared with densities from equations of state and literature datasets. Adsorption on gold sinker surfaces was studied up to saturation pressure for CO₂, C₂H₆, and C₃H₈, with observed adsorption loads ϒ between (0.10 and 2.2) µg·cm⁻². Results were compared with literature data and isotherms from the previous state-of-the-art instrument, the tandem-sinker densimeter. Measurements of a binary gas mixture (0.75 CO₂ + 0.25 C₃H₈) were also conducted.
To evaluate data quality, a new methodology for uncertainty analysis was developed. This numerical approach replaces partial derivatives from the Guide to the Expression of Uncertainty in Measurement (GUM), enabling analysis of complex systems. It uses numerically evaluated sensitivity coefficients, hence 'sensitivity analysis method', to assess system behavior. Tested on three case studies—including volume determination and FSD measurements—the method confirmed the FSD’s performance, establishing it as the new state-of-the-art. Where possible, uncertainties were compared to GUM-based or literature values. The method has proven reliable and practical and could become a standard in thermodynamic property research, accessible to researchers of varying expertise.
Adsorption isotherms of pure fluids were used to validate corresponding MDS models, forming a robust foundation for integrating gravimetric analysis and MDS. This integrated method shows promise. Using simulation-derived data, an empirical model was developed to estimate adsorbate density via symbolic regression. Future work will extend these simulations to mixtures, enabling improved correction of sorption effects in gravimetric measurements.
This dissertation thus provides a solid foundation for advancing research on mixture systems. The proposed methodology enables correction of selective adsorption, supporting dew-point density measurements of gas mixtures with low uncertainty.
| Universität: | Technische Universität Chemnitz | |
| Institut: | Professur Technische Thermodynamik | |
| Fakultät: | Fakultät für Maschinenbau | |
| Dokumentart: | Dissertation | |
| Betreuer: | Richter, Markus (Prof. Dr.-Ing.) | |
| DOI: | doi:10.60687/2025-0133 | |
| SWD-Schlagwörter: | Gasgemisch , Thermodynamik , Taupunkt , Experimentelle Versuchsforschung | |
| Freie Schlagwörter (Englisch): | Thermodynamic Properties , Dew-point Density , Experimental Investigation , Gas Mixtures | |
| DDC-Sachgruppe: | 621.4021 | |
| Sprache: | englisch | |
| Tag der mündlichen Prüfung | 08.05.2025 |
