A Molecular Simulation Study Of Thermodynamics Properties Of CO₂ Expanded Liquids/Supercritical Fluids

It is known that supercritical fluids (SCFs), room-temperature ionic liquids (ILs), CO₂ expanded liquids (CXLs), water are currently regarded as green solvents. The thermodynamics properties of green solvents are the key for reaction and separation. This thesis studied equilibrium and transport properties of fluids in CXLs, SCFs adsorption by molecular simulation.In the first part, the densities, expansion coefficients, diffusion coefficients and viscosities of CO₂ expanded methanol and CO₂ expanded ethanol, which contain p-chloronitrobenzene (p-CNB) and benzonitrile, are discussed by Molecular Dynamic (MD) method. With the amount of CO₂ in the liquids increase, the expansions of liquids are increased, the diffusion coefficients of solutes and solvents are enhanced, and the viscosities are decreased. The equilibrium and transport properties of CO₂/[bmim][PF₆] and CO₂/[bmim][NO₃] are investigated by MD method. Although ILs dissolve large amount of CO₂, the volume expansions of ILs are negligible. The simulation of diffusion coefficients indicate that both CO₂ and ILs diffusion coefficients lower than CXLs. However, with the amount of CO₂ increase, the diffusion coefficients of ILs increase, and the viscosities decrease. CO₂ can be used to improve the diffusions and viscosities of ILs.In the second part, SCFs adsorption in confined space is discussed. The adsorption isotherms, densities and diffusion coefficients are investigated by three methods, including Simplified Local Density (SLD), Grand Canonical Monte Carlo (GCMC) and MD. SLD can be used to calculate the excess adsorption isotherms of SCFs adsorption on active carbon. However, SLD theory makes error fitting high pressure excess adsorption. In this thesis, adding an adjust parameter to modify adsorption volume can fit data exactly including high pressure. To extend the SLD theory, use LJ12-6 potential model to displace 10-4 potential model. New SLD equation can be used in SCFs adsorption on active carbon and silica gel.GCMC method is used to simulate CO₂ and CO₂/toluene adsorption on graphite. The adsorption may occur on the graphite when the slit width is the same as CO₂ diameter in the low pressure. When the pressure is very high, the absolute adsorption amount of CO₂ is liner with the slit width. CO₂ can adsorb in the deep potential well firstly. With the slit width increase, more adsorption layers appear as the concentration of CO₂ at higher pressure. The angle between CO₂ and graphite layer show that adsorption is favorable at small angle. The CO₂/toluene adsorption show that the distribution coefficient of toluene decrease significantly as pressure increase. The pore size distributions (PSD) for F30/470 and DeGussa carbon are determined. Prediction isotherms of CO₂ adsorption on two carbons by PSD are good with experiment.MD method is used to investigate the transport properties of CO₂ and CO₂/toluene adsorption on graphite slits. The radial distribution function (RDF) of CO₂ shows that CO₂ is a liquid like in the graphite slit. The diffusion coefficient of adsorbed CO₂ is lower than normal CO₂. In the vertical graphite layer, the diffusion coefficient of toluene is lower than CO₂, which means the graphite has stronger interaction with toluene.