Salt-Dependent Liquid–Liquid Equilibria and Pyridine Partitioning in Water + Pyridine + 2-Ethyl-1-hexanol Systems with KCl and CaCl2

Abstract To investigate salt-type and salt-concentration effects on pyridine extraction performance, liquid–liquid equilibrium data for the aqueous pyridine +2-ethyl-1-hexanol system with KCl or CaCl2 were measured at 298.15 K and 101.3 kPa. Aqueous phases containing 5 and 10 wt % KCl and CaCl2 were systematically studied. Experimental results show that both inorganic salts expand the two-phase region, promote pyridine partitioning into the rich-organic phase, and enhance separation efficiency. The pyridine distribution coefficient and separation factor generally increase with the salt concentration. Relative to the salt-free system, the average separation factor rises by approximately 42.01% and 111.10% for 5 and 10 wt % KCl, respectively, and by 19.17% and 74.27% for the corresponding CaCl2 concentrations, respectively. Experimental LLE data were correlated using NRTL and UNIQUAC activity coefficient models, with RMSD below 1.02%. Thermodynamic consistency tests and Gibbs stability criteria further validate the reliability of the regressed model parameters. This work provides credible LLE data sets and thermodynamic model parameters for the design and optimization of pyridine extraction and separation processes in saline aqueous systems.