Phase Equilibria of the Ternary System of Lithium Sulfate plus Polyethylene Glycol (PEG3000) + Water at Different pH: Experiment Determination, Correlation, and Thermodynamic Modeling

The phase equilibrium data for the ternary systems [lithium sulfate (Li2SO4) + polyethylene glycol (PEG3000) + water] were measured at 298.15 K and at various pH values (2.94, 4.90, and 6.80). The solid-liquid equilibrium data including the solubility, density, and refractive index were determined using the isothermal dissolution equilibrium method. The liquid-liquid equilibrium (LLE) data for the aqueous two-phase system were obtained experimentally by measuring the biphasic physical properties, including the density and refractive index at 298.15 K. Subsequently, the binodal curves were correlated by using the Pirdashti and Merchuk equations, the effective excluded volumes were calculated from the binodal curve data, and the tie-line compositions were adjusted to the Othmer-Tobias and Bancroft equations, respectively. Results show that the solubility of Li2SO4 in the aqueous polymeric solution mitigates as the content of PEG increases, eventually. It was found that the more acidic solution induces the salting-out effect, and the LLE formed easily at the lower pH of studied systems. Finally, the extended UNIQUAC, the modified UNIQUAC-FV, and the modified UNIFAC-NRF models were selected to correlate the LLE data of the studied systems, and the modified UNIQUAC-FV model gives a better correlation for tie-lines data.