The thermodynamics of (carbon dioxide + methanol) mixtures from 25C to 250C and at pressures up to 20 MPa

Aylward, Wanda Marie (2002) The thermodynamics of (carbon dioxide + methanol) mixtures from 25C to 250C and at pressures up to 20 MPa. Masters thesis, Memorial University of Newfoundland.

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Abstract

The presence of carbon dioxide in natural gas, synthetic gas and liquid streams has prompted the industrial community to examine the properties of carbon dioxide mixtures with the intent of finding novel methods for removing this acidic impurity. Carbon dioxide is also of interest to researchers because it possesses a readily accessible critical point. This has stimulated studies of the phase behaviour and thermodynamic properties of supercritical fluid mixtures, particularly in systems used in supercritical fluid chromatography (SFC) and supercritical fluid extraction (SFE). For both of these applications, it is important to obtain information pertaining to the vapour-liquid phase boundaries for the carbon dioxide mixtures and their thermodynamic properties over a wide range of pressure and temperature. The (CO: + MeOH) system is of interest to applied research on fluid properties because the pure liquids have widely differing critical points. -- There are many experimental issues associated with supercritical fluids, and laboratory equipment must be specially designed to handle these high pressure, high temperature fluids. A high precision isothermal microcalorimeter has been commissioned to measure the excess molar enthalpy of mixing Hm for the mixture at 298.15 K and pressures of 7.5 and 10.0 MPa. A platinum vibrating U-tube densitometer with a specially designed injection system has been used to measure the densities of (CO: + MeOH) mixtures at temperatures from 298.15 K to 523.15 K and pressures up to 20.0 MPa, from which excess molar volumes VEm were calculated. -- The HEmcurves display sigmoidal shapes and negative minima in the methanol-rich regions and were observed at both pressures. A positive maximum at high CO: molefraction, xco2, was obtained at the higher pressure. These results, along with those obtained for (CO2 + MeOH) mixtures from Christensen et al. (1987) at higher temperatures were interpreted in terms of the states of the mixtures and pure components. The volumetric results obtained from the density data showed negative VE results that became much more negative as the temperature was increased. Reduced excess molar volumes VRm = VEm/ [x(l-x)] obtained at high xco2 showed very large negative deviations from ideal behaviour, indicating strong interactions between supercritical CO2 and molecular methanol. The VEm data were used to determine the apparent molar volumes and the standard partial molar volumes of CO: and methanol in the mixtures. Once again, the results were interpreted in terms of phase behaviour of the mixture relative to the pure components.

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