The assessment of hydrofluorocarbons as novel media for supercritical processing

• Main Author: Eardley, Christopher Alan
• Published: University of Leicester 1999
• Subjects: 547
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.696610

The solvent properties of 1,1,1,2-tetrafluoroethane (HFC 134a), difluoromethane (HFC 32) and pentafluoroethane (HFC 125) are measured in both the liquid and supercritical phases. The relative permittivity, , of these fluids is presented as a function of pressure and temperature, and is fitted to the reduced density using the function ( - 1)/(2 + 1). The dipole moments of the hydrofluorocarbons in the condensed phase are calculated. The dipolarity/polarizability parameter, *, is also reported as a function of temperature and pressure for the three hydrofluorocarbons. The change in dipolarity/polarizability with reduced density is explained in terms of local density augmentation in the supercritical phase. A mean sphere approximation model (MSA) is applied to the * data and is found to produce an excellent fit for each liquid and supercritical solvent. The electrical conductivity of a tetrabutylammonium tetrafluoroborate electrolyte in both liquid HFC 134a and liquid and supercritical HFC 32 is reported as a function of pressure. The relative permittivity of the solvent is found to be the major factor affecting the conductivity, through its effect on ion association. The unprecedented wide potential window of HFC 134a is demonstrated and utilized in a variety of novel oxidation processes. Voltammetry is used to study the oxidation of ferrocene at a platinum microelectrode surface in both liquid and supercritical HFC 32. Deviations from Nernstian behaviour are observed in the supercritical phase at pressures close to that of the critical point. These deviations are ascribed to a change in the electrical double layer structure, as confirmed by double layer capacitance measurements. The electroreduction of carbon dioxide in supercritical CO2/HFC 134a mixtures is studied at both platinum and lead electrodes. It is demonstrated that oxalate can be produced at a platinum electrode at higher yields than commonly observed in liquid media.