Statistical Mechanics Theory Of Physical Essential For Carbon Dioxide Separation

Separation of carbon dioxide in flue gases is of great significance in environmental science and technology. Nowadays separation of CO2using physical methodology has shown good prospective. In this thesis, we focus on the micro-mechanism of two physical methods, including ionic liquid adsorption and hydrate encagement. Under the framework of statistical mechanics, we combine the three-dimensional integral equation theory with the three-dimensional density functional theory to deal with the essential issues for CO2adsorption and encagement. The main investigations are as follows:(1) A free energy expression based on density functional theory was integrated into three-dimensional reference interaction site model integral equation to improve non self-consistence caused by asymmetrical structure, when the integral equation described the interactions between CO2and ionic liquids. This theory was then used to describe the three-dimensional density distributions of CO2surrounding different ion pairs in ionic liquids. The influences of spatial structure of ionic liquids and the properties of functional groups on anions and cations on the solubility were analyzed to give the mechanism explanations for adsorption from the molecular level. Based on the spatial density distributions, the adsorbing capacities were obtained to evaluate and predict the solubilities of CO2in these ionic liquids. The calculated data agreed well with available experimental data, suggesting that the descriptions of structures were accurate and structure-solubility correlation was reliable. The direct correlation of solubility with structure provided theoretical basis for the molecular design of new ionic liquids. Compared with classical methods, the present theoretical model provided an efficient and useful tool to estimate dissolving capabilities of ionic liquids and the results directly reflected the microscopic essential features of the dissolving process.(2) As a highly crystalline material, CO2hydrate encages CO2molecules in crystal cavities formed by hydrogen bonded networks. To explore mechanism of hydrate encagement, it is necessary to take into account the properties of crystal structure. However, the crystallization theory at the molecular level is still immature. As such, to study the formation and characters of CO2hydrate, we started our research from the crystallization of Lennard-Jones fluid. A three-dimensional density functional approach was presented. To construct the functional theory, the modified fundamental measure theory was used to describe the free energy contribution arising from hard sphere repulsion, and the weighted density method based on first order mean spherical approximation was applied to describe the free energy functional of the attractive interaction. The special morphologies of face-centered-cubic and body-centered-cubic crystals and of equilibrium crystal-liquid interfaces were fully considered. The crystal-liquid phase equilibria and interfacial tensions for different orientations at various temperatures were correctly predicted by restricted and free minimization. The results were coincided with simulation data, proving that the current theoretical model could quantitatively describe nucleation thermodynamics of Lennard-Jones fluid. On this basis, the nucleation free energy variations for two types of crystals were calculated. The energy barriers and critical nucleus radii for different crystal structures at different orientations were discussed and compared.(3) CO2hydrate is a special crystal, in which CO2molecules are encaged within a crystal structure formed by water. Empty hydrate crystal can be seen as unstable ice constructed by the strong hydrogen bonds. Thus, based on the theoretical research of the nucleation of Lennard-Jones fluid, the crystallization of supercooled water was further studied to determine the role of hydrogen bond. Under fully illuminating the effects of ver der Waals interactions, hydrogen bonding associations, and steric configuration of water molecules, the three-dimensional density functional theory was then extended to real freezing water systems with the direct correlation function of oxygen-oxygen in the equilibrium water derived from integral equation theory in consideration of the interactions of hydrogen-hydrogen, hydrogen-oxygen, and oxygen-oxygen. Local structure of hexagonal ice crystal and interfacial structure of water-ice were shown. The vapor-liquid and crystal-liquid phase equilibria and interfacial tension of water-ice were predicted and the results were consistent with the experimental data. A dynamic density functional theory approach was applied to investigate ice nucleation process in supercooled water. The variations of free energy, nucleus radii, and crystal structures versus time step were analyzed. The calculated free energy barrier and critical nucleus radius corresponded to simulation data, demonstrating that it was capable to describe ice crystallization by the current approach. In particular, the process provided theoretical basis for the research on CO2hydrate crystallization.(4) At low temperature and high pressure, CO2hydrate is formed by CO2in contact with water. CO2molecules encaged in CO2hydrate interact with clathrate hydrate by van der Waals force. Accordingly, we presented a hybrid three-dimensional-density functional theory-reference interaction site model to theoretically predict the thermodynamic properties for CO2and CH4hydrates. To study mechanism of hydrate encagement, the three-dimensional reference interaction site model integral equation was extended to establish the structure-solubility correlation for CO2hydrate, in which the interactions between CO2molecules and clathrate hydrate were considered. The spatial density distribution of CO2in hydrate was shown. A coarse crystal model was applied to describe the structure of CO2hydrate. The three-dimensional density functional theory was presented to investigate properties of CO2hydrate crystal. To quantitatively calculate phase equilibria of CO2hydrate, an iterative method combining three-dimensional integral equation and density functional theory was constructed. The equilibrium pressures, average densities and potential parameters were obtained after the iteration. On this basis, the new three-dimensional density functional theory was then used to describe the density distributions of local crystal and crystal-liquid interface under the condition of phase equilibrium. The crystal-liquid phase equilibria and interfacial tension of CO2hydrate-water were calculated. It was found that the results were consistent with experimental data, indicating that it was reasonable to calculate the properties of hydrate using this theoretical approach. By comparison, the phase equilibria for CH4hydrate was studied with the same method. The density distributions of local crystal and crystal-liquid interface and the interfacial tension were also calculated. The nucleation barriers and the critical nucleus radii for both CH4hydrate and CO2hydrate were discussed by the dynamic density functional theory approach to analyze the feasibility of CO2replacing CH4gas from its hydrate. It has been shown that the nucleation of CH4hydrate is more difficult than CO2hydrate, and the storage of CO2from replacement of CH4hydrate is possible.