Solid–Liquid Equilibrium and UNIQUAC Modeling of Raffinose and Gallic Acid in Ethanol–Water Mixtures for Drowning-Out Crystallization

Abstract Solid–liquid equilibrium (SLE) data were measured for gallic acid (model polyphenol) and raffinose (model saccharide) in ethanol–water mixtures at temperatures from 278.15 to 303.15 K. The solubility of raffinose decreased 400-fold with increasing ethanol content, while gallic acid solubility increased 29-fold under identical conditions, demonstrating divergent solubility behavior consistent with the thermodynamic requirements for selective drowning-out crystallization. The modified Universal Quasi-Chemical (UNIQUAC) model with Larsen’s combinatorial term successfully correlated the experimental data (average absolute deviations < 5%). UNIQUAC Functional-group Activity Coefficients (UNIFAC) group contributions, including specialized saccharide groups (pyranose, furanose, OH ring ), were used to calculate molecular parameters. Thermodynamic properties of dissolution (Δ sol H °, Δ sol G °, Δ sol S °) were derived from van’t Hoff analysis. Favorable separation conditions were identified at ethanol mass fractions of 0.70–0.80 and temperatures of 283.15–293.15 K, achieving selectivity factors of 70–180. The thermodynamic framework provides a quantitative foundation for process design of polyphenol recovery from olive mill wastewater via drowning-out crystallization, as confirmed by preliminary separation experiments achieving 92.5% raffinose recovery at 96.3% purity.