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

Pressure-composition isotherms are obtained for binary mixtures of carbon d ioxide with formic acid, acetic acid? butyric acid, valeric acid, caproic a cid, 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 co mparing the measured phase equilibrium data of the carbon dioxide-acetic ac id 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 maximu m 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 o f the other systems. The experimental data are modeled using both the stati stical associating fluid theory (SAFT) and the Peng-Robinson (P-R) equation of state. The SAFT equation of state reasonably models the pressure-compos ition isotherms for these six systems only if two temperature-independent m ixture parameters are used for each system. The P-R equation of state calcu lated the phase behavior with one or two temperature-independent mixture pa rameters.