Adsorption of mixtures near their critical points: Thermodynamic measurements and application to a new process separation technology

Many separation and reaction schemes in the chemical industry are based on the process of adsorption at normal fluid conditions. Operating in the neighborhood of the critical point of the solvent fluids can sometimes extend the range of applicability of these processes and enhance the feasibility of processes which would otherwise not work. To do this however, requires knowledge of the adsorption behavior of fluids in their most compressible state. The unique adsorption characteristics of pure and mixed supercritical solvents were investigated using ethane, carbon dioxide and their mixtures as representative solvents. Beginning with the acquisition of chromatographic capacity factor data, partial molar volumes, partial molar enthalpies, Henry's constant and isosteric heats of adsorption data for some non-volatile solutes were extracted using an approach based on both classical and non-classical analysis of fluids in their critical region. The partial molar volume and enthalpy data are the first of its kind in near-critical mixed solvents. Mixtures were studied because the ability to vary their composition and to modify solute-solvent interactions by choice of diluents, allows for flexibility in the design of solvents for specific applications.; The capacity factor data were also used to develop an adsorption model based on the Bragg-Williams approximation for molecular interactions on the stationary phase. The model was successfully used to represent adsorption isotherms at both high and low pressures in pure and mixed solvents. Surface concentrations, which are of practical relevance but rather difficult to measure directly, were obtained with this model and compared favorably to experimental data. Another model based on Henry's constants representation for the surface phase was used to investigate the effects of different solvents and solutes on the adsorption equilibrium.; The final part of the thesis involved the development of a new separation process based on adsorption of supercritical mixtures on inorganic membranes. The process was based on the experimental results, obtained from the above chromatographic data, that distribution coefficient of adsorbate solutes could be significantly enhanced near the critical point of the solvent. This provides a means of enriching mixed products of supercritical fluid extraction prior to depressurization.