Adsorption-desorption behavior in heterogeneous processes involving supercritical fluid solvents

Supercritical fluids are attractive solvents for heterogeneous processes including extraction, adsorptive separation and catalysis. Surface interactions in the near-critical region are poorly understood and there is little experimental data for industrially relevant materials. This research has focused on three closely related aspects of heterogeneous supercritical fluid processes. In the first part, the extraction of phenol from the surface of activated carbon was studied with an emphasis on determining the true effectiveness of the process based on an understanding of the adsorption conditions and the state of the adsorbed material. It was confirmed that the presence of oxygen during adsorption reduces the regenerability of the carbon. It was found that supercritical fluid was as effective for the regeneration as solvent extraction and that significant CO2 adsorption occurred during the regeneration. The second part of the study consisted of in depth analysis of the adsorption of the supercritical solvent on commonly used adsorbents. Near continuous adsorption and desorption isotherms for CO2 on Calgon F400 activated carbon and Zeolyst NaY zeolite were measured by a flow gravimetric apparatus over a wide pressure range (0--200 bar) near the critical temperature of CO2 (30 to 50°C). For both materials the excess adsorption increases sharply at low pressures, goes through a broad maximum followed by a sharp drop when the density of the bulk fluid increases sharply. A crossover is observed near the critical region beyond which the temperature dependence of excess is reversed. When analyzed as a function of solvent density the crossover disappears revealing an anomalous enhancement of adsorption near the critical point similar to enhanced local density or "charisma" observed in binary solute-supercritical fluid systems. A model using the two dimensional Peng-Robinson equation of state was able to qualitatively describe the adsorption behavior but the quantitative agreement was poor in the near critical region. The latter portion of the work was aimed at developing experimental and modeling procedures to measure the co-adsorption of the supercritical solvent and a solute. It was shown that high pressure adsorptive processes include the formation of a complex adsorbed phase comprising of tightly packed solvent and solute molecules.