Semiempirical Thermodynamic Modeling of Liquid-Liquid Phase Equilibria: Coal Tar Dissolution in Water-Miscible Solvents

This work investigates thermodynamic modeling of phase equilibria of mixtures of coal tar, solvent, and water using the nonrandom, two-liquid (NRTL) equation, a semiempirical excess free energy equation. Coal tar, a complex mixture of polycyclic aromatic hydrocarbons (PAHs), was represented as a pseudocomponent, permitting coal tar/solvent/water systems to be modeled as ternary systems. Experimental ternary liquid-liquid equilibria (LLE) and coal tar/water mutual solubility data, along with literature data for solvent/water vapor-liquid equilibria (VLE), were used in a simultaneous regression procedure to optimize the representation of ternary LLE. Estimated model parameters characterize the collective chemical thermodynamic behavior of the coal tar pseudocomponent, and the calibrated liquid-phase activity coefficient equations can be used to predict solvent extraction process effectiveness. Application to a ternary system of pure components, acetonitrile/benzene/n-heptane, identified minimum data requirements. Good representation of ternary LLE was obtained with only one tie line in the regression when three binary data sets were included. When one of the binary VLE data sets was left out of the regression, the inclusion of three tie lines produced good results.