| As temperature increases, the properties of water are changing greatly. Water's density and dielectric constant are greatly reduced, resembling most organic solvents. Reactions in high-temperature aqueous solution can be proceeded very fast with high percent conversion and almost zero-pollution. High-temperature aqueous solutions are widely used in many areas, such as hydrothermal synthesis and supercritical water oxidation. But experiments at high temperature and pressure can hardly be carried out and the properties of high-temperature and -pressure aqueous solutions are much more complex and much less certain than those at 298K. Therefore, both experimental data and theoretical studies are quite scarce. With developments of computer science and simulation methods, molecular simulation (Molecular Dynamics simulation, Monte Carlo simulation and Quantum Chemistry calculation) becomes more and more effective in thermodynamic studies on aqueous solution. Many successful applications, such as complementing experimental data and explaining microscopic mechanism, have been reported. In this paper, a "bottom-up" idea based on molecular simulation is proposed to solve macroscopic thermodynamic problems from electronic and atomic scale. In this work, the gas-liquid equilibrium of high-temperature and -pressure N2-O2-H2O and the activity coefficient model for high-temperature and -pressure MgCl2 and CaCl2 solution are studied. N2-O2-H2O is a model system for humid air. Having high heat capacity, humid air is used as working medium in power cycle to increase the efficiency. But the experimental and theoretical studies on humid air are limited to room temperature conditions and can not meet the requirements of industry processes. Gibbs Ensemble Monte Carlo simulation is used to study the equilibrium data and microstructure of gas and liquid phase. Then a model capable of calculating saturated humidity and enthalpy is established. First, GEMC study on N2-H2O and O2-H2O systems is carried out, respectively. The values of equilibrium properties have a deviation more than 10% while reflecting the strong increase of humidity and gas solubility in water with the increase of temperature, which agrees with experimental data. The analysis on radial distribution functions explains why the miscibility of two phases increased. This result is also an evidence that the non-ideal properties of this system can be neglected when establishing thermodynamic models for it.Combined with the simulation results, a model for saturated humidity and enthalpy is established based on Helgeson model. The saturated composition and enthalpy calculated by this model are in good agreement with experimental data and results from other models. Then this model is used to predict the saturated humidity and enthalpy of humid air in a temperature range from 473 to 573K and a pressure range form 5 to 15MPa. The calculations for operation of humidifier in humid air turbine cycle are also carried out at 423.15~573.15K, 5~15MPa. The results suggest that to produce high-enthalpy moist air, high-temperature and large-flow water is needed. The comparison between this model and ideal gas model used in industy shows that the ideal gas model can be used in simulation to simplify the calculation when the temperature is lower than 473K and pressure is lower than 5MPa.MgCl2 and CaCl2 solutions are widely distributed and very crucial to many processes such as sea water desalination and hydrated gas preparing. It is interesting that the behaviors of these two solutions are much more complex than NaCl solution at high temperature and pressure, and the establishment of activity coefficient model for them could be a good guide to thermodynamic study on high-temperature and -pressure aqueous solution. Quantum chemistry calculations are performed to investigate the ionic hydration and association as the basis of an activity coefficient model for MgCl2 and CaCl2 solutions.Quantum chemistry calculation is performed firstly to study the ionic hydration and associat...
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