Phase behavior and modeling of supercritical carbon dioxide-organic acid mixtures

Pressure-composition isotherms are obtained for binary mixtures of carbon dioxide with formic acid, acetic acid? butyric acid, valeric acid, caproic acid, and caprylic acid at temperatures of 35.0-120.0 degreesC and pressures up to 250 bar. The accuracy of the experimental apparatus was tested by comparing the measured phase equilibrium data of the carbon dioxide-acetic acid system at 40.0 and 60.0 degreesC with those of Laugier et al. These six carbon dioxide-polar solute systems exhibit type I phase behavior, which is characterized by an uninterrupted critical mixture curie that has a maximum in pressure. In each system, the mixture critical point increases as the temperatures increases, and also the mixture critical pressure increases as the molecular weight increases. On the contrary, the carbon dioxide-formic acid system shows a higher mixture critical pressure compared with those of the other systems. The experimental data are modeled using both the statistical associating fluid theory (SAFT) and the Peng-Robinson (P-R) equation of state. The SAFT equation of state reasonably models the pressure-composition isotherms for these six systems only if two temperature-independent mixture parameters are used for each system. The P-R equation of state calculated the phase behavior with one or two temperature-independent mixture parameters.