Phase equilibrium measurements and modeling of selected asymmetric ternary mixtures

Scope and method of study. The current state of the art indicates that a cubic equation-of-state (CEOS) model capable of precisely representing the vapor-liquid equilibrium (VLE) properties of asymmetric binary mixtures and providing reliable generalized predictions for such mixtures is predicated on (a) a modified covolume that accounts for molecular size asymmetry, (b) mixing rules reflective of the local-composition mixing, and (c) a determination that the model is able to describe asymmetric multicomponent mixtures based on pair-wise interactions.; The specific objectives of the study were to (a) evaluate the efficacy of the existing one-fluid and recently developed excess Gibbs/Helmoltz energy mixing rules in representing selected asymmetric binary and ternary mixtures; (b) develop improved excess free energy mixing rules; (c) modify the CEOS covolume utilizing the combinatorial contribution to excess Gibbs/Helmholtz energy formulation; (d) design and construct a mercury-free, high-pressure experimental apparatus to facilitate accurate solubility measurements for the asymmetric ternary mixtures of hydrogen/carbon dioxide and ethane/carbon dioxide in eicosane, octacosane, and hexatricontane at 323, 344, 373 and 473 K and pressures to 15.3 MPa; and (e) evaluate the correlative and predictive abilities of the new model in comparison with recent literature models.; Findings and conclusions. Internal and external consistency tests validate the viability of the newly-acquired ternary solubility measurements, which exhibit experimental uncertainties within 0.002 in mole fraction.; The results indicate that the one-fluid mixing rules equipped with two temperature-independent binary interaction parameters represent the asymmetric binary and ternary mixtures with average absolute deviation of 3.2% in bubble point pressure. Moreover, molecular and functional-group pair-wise interactions are effective in describing the asymmetric ternary mixtures considered in this study.; A new semi-theoretical mixing rule was developed for the Peng-Robinson EOS covolume, which accounts effectively for molecular size asymmetry in mixture phase behavior. In general, the new excess Helmholtz energy mixing rules yield predictions with average absolute deviation of about 4.7% in bubble point pressure for the systems studied. These results are comparable to those of Orbey and Sandler (1997) and better than those of Boukovalas et al. (1994). Further, the new mixing rules produce excellent results for the challenging hydrogen/n-paraffin binaries.