Phase Equilibria of Supercritical Carbon Dioxide and Hydrocarbon Mixtures.

The phase equilibria of supercritical carbon dioxide and hydrocarbon mixtures were studied. Correlation techniques for vapor-liquid and solid-fluid equilibria were studied for binary CO$\sb2$ and hydrocarbon mixtures using existing experimental data. Experimental investigation and correlation of the experimental data were carried out for solid-liquid equilibria of light and heavy hydrocarbon mixtures, and CO$\sb2,$ light and heavy hydrocarbon mixtures. A new technique of vapor-liquid equilibrium calculation by the activity coefficient method was developed and applied to binary CO$\sb2$ and hydrocarbon mixtures. The extrapolated vapor pressure of CO$\sb2$ was used for calculation of the fugacity of pure hypothetical liquid CO$\sb2$ and for the pressure correction of the activity coefficient. The results were improved, especially in the high temperature and pressure region. Correlation techniques of solid-fluid phase equilibrium using the Soave-Redlich-Kwong equation of state were suggested based on various amounts of pure component data. Using available pure component data, experimental solubility data were successfully correlated for an example mixture of supercritical CO$\sb2$ and octacosane. This mixture is difficult to deal with by conventional methods. The activity coefficient method used in the vapor-liquid equilibrium calculation was also applied, and good agreement was obtained. Pressure effects on the solid-liquid equilibrium for light and heavy hydrocarbon mixtures were determined experimentally. The effects of temperature and pressure on the activity coefficient were studied theoretically. Prediction based on low pressure solubility data and correlation by the Flory-Huggins plus regular solution equation, Wilson, Heil and NRTL equations gave good agreement, when a simple pressure correction term was used. The effect of CO$\sb2$ on the phase behavior of hydrocarbon mixtures which simulate residual oil was investigated experimentally. At low CO$\sb2$ content, the solubility remained almost constant with increasing CO$\sb2$ content, but at high CO$\sb2$ content, the effect of CO$\sb2$ became significant. The data were correlated successfully at low CO$\sb2$ content by a similar model to that for light and heavy hydrocarbon mixtures. For the whole range of CO$\sb2$ content, a satisfactory correlation was possible by interpolating the parameters of light and heavy hydrocarbon mixtures and those of CO$\sb2$ and heavy hydrocarbon mixtures