Multiphase Equilibrium in a CO₂-Based Ternary System with Oleic Acid and a Renewable Cosolvent

A detailed understanding of phase equilibrium in pressurized CO₂-containing systems is crucial for designing supercritical processes with potential future applications in the enzymatic synthesis of sugar esters. In this context, this work presents the experimental determination and thermodynamic modeling of the ternary system CO₂ + oleic acid + 2-methyltetrahydrofuran-3-one (2-MeTHF-3-one), at temperatures between 313 and 343 K and pressures ranging from 5.24 to 24.00 MPa. The experimental data allowed the construction of phase diagrams and the identification of single-phase, two-phase, and three-phase regions, highlighting the strong influence of composition on the phase behavior of the system. The addition of the cosolvent 2-MeTHF-3-one increased the miscibility between the components, promoting the reduction of phase transition pressures and the expansion of single-phase regions, while the increase in the molar fraction of oleic acid favored the formation of three-phase liquid-liquid-vapor (LLV) regions. The experimental data were correlated using the Peng–Robinson equation of state combined with the Wong–Sandler mixing rule (PR-WS). The binary system CO₂ + oleic acid showed the largest correlation deviations (MAD = 6.52 MPa; RMSD = 7.71 MPa), while the ternary system containing 2-MeTHF-3-one at a molar ratio of 0.35:1 showed the smallest deviations (MAD = 4.27 MPa; RMSD = 5.38 MPa), indicating better model performance under these conditions. The system with a ratio of 0.71:1 presented a higher kij value (9.5591), suggesting greater incompatibility between the components. The results provide fundamental thermodynamic information for the simulation, optimization, and intensification of sustainable supercritical processes aimed at producing bioactive surfactants and functional ingredients from renewable sources.