Thermodynamics of supersaturated steam: Molecular simulation results (original) (raw)
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Thermodynamics of supersaturated steam: Towards an equation of state
Fluid Phase Equilibria, 2018
Thermodynamics of supersaturated (supercooled) steam at conditions occurring in steam turbines, and inaccessible thus to experiments, has been studied by a number of theoretical and semi-theoretical methods with the goal to assess available theoretical tools for developing an equation of state. Extended virial expansions with different reference systems have been used along with two, qualitatively different equations of state. According to the proposed strategy, all equations were first used for the vapor-liquid coexistence curve and then extended to the metastable region and tested against computer simulation data. Despite the fact that no specificities of supersaturated steam have been accounted for, all the considered methods provide results with deviations from (pseudo)experimental data up to about 5% over the entire range of thermodynamic conditions except the critical region. It turns out that the perturbed virial expansion with a suitably chosen reference system outperforms other methods, including equations of state originally developed for liquid water.
Journal of Quantitative Spectroscopy and Radiative Transfer, 2006
Recent computational findings of temperature increase of clustering degree in several quite different saturated vapors are analyzed further. A thermodynamic proof is presented, showing that this event should be rather common, if not general. Illustrations are based on the saturated steam and consequences for the atmosphere are discussed with both homo-and hetero-clustering. MP2 ¼ FC=6-311G ÃÃ results for the H 2 O Á N 2 (the best stabilization energy À1:62 kcal=mol, which corresponds to À1:44 kcal=mol with the G3 theory) and H 2 O Á O 2 (the best stabilization energy À0:96 kcal=mol) hetero-dimers, and the G1-theory ðO 2 Þ 2 data (the best stabilization energy À0:62 kcal=mol) are reported. The water-dimer thermodynamics is recomputed in an anharmonic regime and a remarkable agreement with experiment is found. The results have some significance for the atmospheric greenhouse effect. r
Prediction of thermodynamic properties of heavy hydrocarbons by Monte Carlo simulation
Fluid Phase Equilibria, 2005
In this study, Monte Carlo simulation techniques based on the anisotropic united atom (AUA) potential have been used to predict thermodynamic properties, comprising saturation pressures, vaporization enthalpies and liquid densities, at different temperatures for several isoalkanes (2,3-dimethylpentane, 2,4-dimethylpentane), alkylbenzenes (propylbenzene and hexylbenzene), alkyl-substituted cycloalkanes (propylcyclohexane and propylcyclopentane), polycyclic alkanes (trans-decalin), and naphtenoaromatics (tetralin and indan), representing gasoil range fractions of hydrocarbons. This variety of hydrocarbons with different molecular structures served to demonstrate the transferability of the AUA potential parameters. For this purpose, two types of Monte Carlo algorithm were used: the Gibbs ensemble algorithm to predict equilibrium properties at high temperatures, and the NPT algorithm followed by the thermodynamic integration to extend the prediction to lower temperatures. Techniques increasing the efficiency of the algorithms such as configuration bias, reservoir bias, and parallel tempering were also implemented in the algorithms. Based on available experimental information, good predictions of equilibrium properties were obtained for all the hydrocarbon families studied, and small differences between isomers were represented with a good accuracy.
Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy, 2014
The number of nanoparticles available as nucleation sites for heterogeneous nucleation of droplets in the steam turbine is an important parameter for the modelling of the non-equilibrium condensation process. In this paper, measurements of particles of sizes down to 3 nm are reported. Superheated steam was sampled at Power Plant Prunéřov II using a special sampling device avoiding sample condensation. Samples were taken at two pressure and temperature levels: 0.133 MPa, 174 C and 0.27 MPa, 250 C. The number of particles was determined as 1.5 Â 10 9 per kg of steam at the lower pressure and as 2.5 Â 10 9 at the higher pressure. A diffusion battery was used to determine the size distribution of particles instead of the previously used scanning mobility particle sizer. The size distribution was obtained by deconvolution of particle concentrations obtained for various numbers of screens. Compared to the scanning mobility particle sizer results, the diffusion battery revealed a smaller geometric mean particle size.
An Effective Continuum Model for the Gas Evolution in Internal Steam Drives
SPE Journal, 2003
Summary We derive an effective continuum model to describe the nucleation and subsequent growth of a gas phase from a supersaturated liquid in a porous medium, driven by heat transfer. The evolution of the gas results from the reduction of the system pressure at a constant rate. The model addresses two stages before the onset of bulk gas flow, nucleation, and gas-phase growth. The problem arises in internal steam drive - for example, of the type recently discussed in blowdown experiments in carbonate rocks (Dehghani et al.,1 Dehghani and Kamath2). Important quantities, such as the fraction of pores that host activated sites, the deviation from thermodynamic equilibrium, the maximum supersaturation, and the critical gas saturation depend crucially on the nucleation characteristics of the medium. We use heterogeneous nucleation models in the form of pre-existing gas, trapped in hydrophobic cavities, to investigate the nucleation behavior. Using scaling analysis and a simpler analytica...
Computer molecular modeling of catalytic steam gasification of low rank coals
A molecular model of low rank coal was constructed with properties similar to brown coal; 10- 20wt% water was hydrogen bonded to coal oxygen groups and the remainder was bulk water. Single point self-consistent field (1scf) computations of coal models provided octahedral mono-, and di-nuclear complexes of Cr, Fe, Co, and Ni, but SE computations often provided distorted structures. Models of char were developed by transforming the coal model containing multi-nuclear metal species into char according to pyrolysis chemistry; the composition of char models containing iron oxides was similar to char samples obtained over 250-800 °C. SE and DFT calculations of char models containing metal clusters were conducted for mechanisms for H2 and CO formation from pyrolysis and iron catalysed steam gasification; the active site for gasification was (Fe-C) and its accessibility to H2O was related to the configuration of the char model. Catalytic steam gasification at 900 °C after 15 min, consumed 2...
Experimental research of heterogeneous nuclei in superheated steam
EPJ Web of Conferences, 2016
A mobile steam expansion chamber has been developed to investigate experimentally homogeneous and heterogeneous nucleation processes in steam, both in the laboratory and at power plants using the steam withdrawn from the steam turbine. The purpose of the device is to provide new insight into the physics of nonequilibrium wet steam formation, which is one of the factors limiting the efficiency and reliability of steam turbines. The expanded steam or a mixture of steam with a non-condensable gas rapidly expands in the expansion chamber. Due to adiabatic cooling, the temperature drops below the dew point of the steam at a given pressure. When reaching a sufficiently high supersaturation, droplets are nucleated. By tuning the supersaturation in the so-called nucleation pulse, particles of various size ranges can be activated. This fact is used in the present study to measure the aerosol particles present in the air. Homogeneous nucleation was negligible in this case. The experiment demonstrates the functionality of the device, data acquisition system and data evaluation methods. This is an Open Access article distributed under the terms of the Creative Commons Attribution License 4.0, which permits distribution, and reproduction in any medium, provided the original work is properly cited.
Journal of Molecular Liquids, 2005
The Gibbs Ensemble Monte Carlo (GEMC) simulation technique was used to study the vapor-liquid equilibrium (VLE) of pure methane in a wide range of thermodynamic state points. The properties of the pure fluid were also studied at supercritical (SC) conditions by performing NVT and NPT molecular dynamics (MD) simulations. Previously developed intermolecular potential models were employed to model the fluid and their properties were obtained and compared with available experimental data. The simulations have shown that a simple one site Lennard-Jones (LJ) potential model [B.
Chemical Physics, 2008
Molecular dynamics (MD) simulation is applied to the condensation process of supersaturated vapors of methane, ethane, and carbon dioxide. Simulations of systems with up to a million particles were conducted with a massively parallel MD program. This leads to reliable statistics and makes nucleation rates down to the order of 10 30 m −3 s −1 accessible to the direct simulation approach. Simulation results are compared to the classical nucleation theory (CNT) as well as the theory of Laaksonen, Ford, and Kulmala (LFK) which introduces a size dependence of the specific surface energy. CNT describes the nucleation of ethane and carbon dioxide excellently over the entire studied temperature range, whereas LFK provides a better approach to methane at low temperatures.