Solution Thermodynamics of Methyl, Ethyl, Propyl, and Butyl Lactates in High-Pressure CO₂

The demand for alkyl lactates (methyl, ethyl, propyl, and butyl) has risen significantly due to their unique properties and wide industrial applications. Accurate vapor-liquid equilibrium data are vital for the design and optimization of separation techniques involving these compounds, particularly in systems using supercritical carbon dioxide (SC-CO₂) as a solvent. In this study, the phase transition of binary systems of CO₂ + lactate (methyl, ethyl, propyl, butyl) was investigated at various temperatures (313.2, 333.2, 353.2, 373.2, and 393.2 K) and pressures up to 18.48 MPa, using elevated-pressure phase equilibrium equipment based on the synthetic method. Notably, comprehensive isothermal phase behavior data for these binary systems across a wide mole fraction range and high-pressure conditions are scarce. The pressure-temperature plots revealed that the critical mixture boundary connects the critical points of SC-CO₂ and the respective alkyl lactates. For all four systems, enhanced miscibility was observed as the lactate mole fraction increased under isothermal conditions, accompanied by a decrease in pressure. The phase transition observed aligns with Type-I, Van Konynenburg and Scott. Enhanced miscibility was observed across all systems as the mole fraction of lactate increased under isothermal conditions with a corresponding decrease in pressure. The experimental bubble point data were correlated using the Peng et al. equation of state (PR EoS) combined with van der Waals one-fluid mixing rules. The binary interaction parameters (k _ij and η _ij ) were optimized as follows: methyl lactate (k _ij = -0.030, η _ij = -0.090), ethyl lactate (k _ij = 0.000, η _ij = -0.085), propyl lactate (k _ij = 0.035, η _ij = -0.060), and butyl lactate (k _ij = 0.040, η _ij = -0.050). The model exhibited good agreement with experimental results, with root-mean-square deviation (RMSD) values of 3.92, 4.08, 6.46, and 6.79% for the CO₂ + methyl lactate, ethyl lactate, propyl lactate, and butyl lactate systems, respectively.