Salt Effect on the Liquid-Liquid Equilibria for the Ternary System Water plus N,N-Dimethylformamide (DMF) plus Chloroform

Wastewater containing DMF (N,N-dimethylformamide) and salt is generated in production of many chemicals and medicines. The composition of phase equilibrium should be known primarily for the recovery of DMF from salt-containing wastewater by solvent extraction. In this study, liquid-liquid equilibrium (LLE) and solubility data of water + DMF + chloroform in the presence of NaCl with various concentrations of 0, 2.5, 5.0, 7.5, 10.0, and 13.3 wt % and Na2SO4 with concentrations of 1.5 and 2.5 wt % were determined at T = 298.2 K and atmospheric pressure. Besides, the salt effect on the LLE data was investigated. Compared with the salt-free system, the salt led to an enhancement in the maximum DMF distribution coefficient by 13.4-102.6% for 2.5-13.3 wt % NaCl and 16.2-31.7% for 1.5-2.5 wt % Na2SO4, respectively. Since the distribution coefficient of water increased concurrently, the separation factor barely changed and the two-phase region expanded outward slightly. At the same salt mass fraction, the order of salting-out effect strength on extraction was Na2SO4 > NaCl, which was in good agreement with the Hofmeister series. The reliability of the experimental values was verified through the Eisen Joffe equation. The NRTL thermodynamic model was applied to correlate the experimental data with a root-mean-square deviation less than 0.0062. The coherent consistency of the model parameters for the salt-free system was checked by topological analysis based on the Gibbs tangent plane test.