Density Functional Theoretical Study For Adsorption Behaviors Of Complex Fluids Within Porous Media

In recent years, the molecular-level study of adsorption behaviors for the complex fluid mixture within porous solid media has been a very active and interesting field. Free energy density functional theory (DFT) is based on the concept of the inhomogeneous statistical mechanics, in which the macroscopic property is determined by the molecular structure and interactions. Due to its relatively rigorous theoretical foundation, this approach plays an important role in the study of solid wetting,phase transitions of confined space,solid surface adsorption,membrane separation etc. The objective of this paper is to carry out a comprehensive study for the adsorption of complex fluid mixtures on porous media by using the DFT approach as follows: 1. In chapter 1, the relevant research progress in literatures, such as free energy density functional theory, supercritical fluid adsorption and the characteristics of activated carbon is briefly reviewed. 2. In chapter 2, a free energy density functional theoretical model based on statistical mechanics is developed for studying the adsorption of binary complex fluid mixture systems consisting of supercritical carbon dioxide and benzene onto solid media. The effect of pore size, temperature, pressure (density), mole composition and the different adsorbed ability on the adsorption equilibrium is systematically studied. By applying the adsorption integral equation theory to correlate the experimental data for benzene adsorption onto activated carbon in the mixture, the pore size distribution (PSD) of activated carbon can be obtained, which indicates that the solute adsorption of benzene, in such a supercritical condition, is mainly adsorbed in micropore ranges. In addition, the deduced adsorption mechanism and the valid evaluation of the related influence factor will provide the theoretical basis for designing the reasonable and effective adsorbent, and optimizing the experimental operation condition. 3. In chapter 3, combining the adsorption integral equation theory, the required PSDs were determined from fitting the experimental adsorption data of two pure components simultaneously in the supercritical condition, and then a DFT approach is employed to predict the adsorption equilibrium of the corresponding binary supercritical gas mixtures. The calculated results suggest that this current method can satisfactorily predict the equilibrium adsorption behavior of three supercritical mixture systems of CH4/ N2, CH4/ CO2, and CO2/ N2 with various mole compositions. In comparison to the ideal adsorbed theory, the predictive advantage is obvious. It is also suggested that the consideration of the characteristic of the adsorbed molecules and the structure property of the adsorbent will offer an essential improvement of prediction ability. 4. In chapter 4, considering that the conventional DFT approach predicts the weakly adsorbed component unsatisfactorily, we propose an improved DFT method, in which a correction for the inadequacy in the mean field approximation of attractive part of conventional NDFT treatment is included, and the traditional infinite thickness wall is replaced by a two-layer wall potential. It was found that this improved approach is better than the conventional DFT by compared with the experimental data. 5. In chapter 5, the summary and outlook of the whole paper are presented.