Václav Vinš

Relative density measurements of International Association for the Physical Sciences of the Oceans (IAPSO) Standard Seawater (P161, Sp = 34.995) were carried out with the recently developed dual-capillary method. Isobaric measurements were performed from 298.15 K down to the supercooled liquid region, ∼8–13 K below the equilibrium freezing line, at pressures from atmospheric up to 110 MPa. Densities relative to the density at the reference temperature of 293.15 K are presented together with derived thermal expansion coefficients for individual isobars. Expanded uncertainties (k = 2) of the relative densities range from 0.000 020 to 0.000 031. A particular advantage of the present approach is that the thermal expansion coefficients close to the equilibrium freezing line are based on measurements from both of its sides—in the stable and metastable regions. Besides the dual-capillary data, results of complementary density measurements with IAPSO Standard Seawater using a vibrating tube densimeter (VTD) from 275.15 to 343.15 K at ambient pressure are presented. The dual-capillary and VTD results are compared with the Thermodynamic Equation of Seawater 2010 (TEOS-10). The new data agree with TEOS-10 within its expanded uncertainties from 273.15 to 313.15 K and they deviate significantly outside this temperature range. A simple correlation based on scaling the equation of state for supercooled ordinary water was developed to represent standard seawater densities in the studied region. The correlation represents measured densities within 10 ppm up to 70 MPa and within 25 ppm up to 110 MPa.

This work introduces an extension of an existing thermodynamic model for gas hydrates based on the van der Waals and Platteeuw (vdWP) approach for structure H (sH) hydrates. A model for the hydrate volume is adapted to the hexagonal unit cell with a universal correlation for the thermal expansion, guest-specific lattice parameters at reference conditions, a compressibility relation given in terms of the Murnaghan equation of state (EOS), and a multi-layered description of the hydrate cavities. Available experimental data for the lattice parameters are represented within ±0.05 Å, while molecular simulation predictions show larger deviations. The results allow for the determination of lattice parameters for 10 help gases and 15 large-guest molecule substances (LGMSs), which enables the application to a wide range of hydrate-forming mixtures. In combination with multiparameter EOS for fluid phases, i.e., the IAPWS-95 for water, the proposed vdWP-type model is validated by comparing predicted hydrate formation conditions with experimental data for methylcyclohexane or 2-methylbutane containing mixtures, demonstrating good agreement. The fluid EOS for methylcyclohexane systems with water, carbon dioxide, and methane and 2-methylbutane with water is revised.

This study reviews available van der Waals- and Platteeuw-based hydrate models considering multiple occupancy of cavities. Small guest molecules, such as hydrogen and nitrogen, are known to occupy lattice cavities multiple times. This phenomenon has a significant impact on hydrate stability and thermodynamic properties of the hydrate phase. The objective of this work is to provide a comprehensive overview and required correlations for the implementation of a computationally sufficient cluster model that considers up to five guest molecules per cavity. Two methodologies for cluster size estimation are evaluated by existing nitrogen hydrate models showing accurate results for phase equilibria calculations. Furthermore, a preliminary hydrogen hydrate model is introduced and compared with the results of other theoretical studies, indicating that double occupancy of small sII cavities is improbable and four-molecule clusters are predominant in large sII cavities for pressures above 300 MPa. This work lays the foundation for further exploration and optimization of hydrate-based technologies for small guest molecules, e.g., storage and transportation, emphasizing their role in the future landscape of sustainable energy solutions.

Vibrating tube densimeters (VTDs) have become relatively popular instruments for measuring the density of a large variety of gases and liquids since their development about fifty years ago. The apparatuses measure the characteristic frequencies of vibrating U-shaped tube filled with a fluid sample, when the fluid fill shifts the dynamic characteristics of the U-tube. This study describes design and testing of the in-house developed supporting setup for a commercial high-pressure VTD Anton Paar DMA HP. The instrument covers density range from 0 to 3000 kg/m3 at pressures up to 700 bar and temperatures from -10 to 200 °C. Whereas the temperature of density measurement is fully controlled by the apparatus using a thermal block with Peltier unit, pressure has to be generated and monitored externally. Therefore, a high-pressure sampling system was designed and assembled. Special care is taken when measuring temperature with an external resistance thermometer connected to precise thermometry bridge. In order to avoid unfavorable condensation of air moisture during the measurement, VTD apparatus is protected by a dry box of own design. Technical details together with the calibrations of pressure and temperature probes are described in detail. A preliminary data for temperature dependence of density of the selected liquids (water, ethanol or ethylene glycol) obtained at the barometric pressure are also provided and compared to the reference density correlations.