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The solid–liquid equilibrium (SLE) for ternary system of caprolactam + cyclohexanone oxime + methyl tert-butyl ether were determined at three temperatures of (278.15, 293.15 and 308.15) K under pressure of 101.2 kPa. Three isothermal phase diagrams of the system were constructed based on the measured mutual solubility data. There were two pure solids formed in the ternary phase diagram, including pure caprolactam and pure cyclohexanone oxime, which were identified by the method of Schreinemakers' wet residue. At each temperature, the phase diagram contained three crystallization regions (which corresponded to caprolactam, cyclohexanone oxime, and a mixture of caprolactam and cyclohexanone oxime), two crystallization curves, and one co-saturated point. The crystallization field of caprolactam was larger than that of cyclohexanone oxime. The NRTL model was employed to correlate and calculate the ternary phase diagram. The calculated ternary phase diagram with the NRTL model agreed well with the experimental ones. The solid–liquid phase equilibrium and phase diagram for the ternary system of caprolactam + cyclohexanone oxime + methyl tert-butyl ether could provide basis for the purification process of caprolactam.

Solid–liquid phase equilibrium and solubility data for quaternary system of terephthalic acid + isophthalic acid + phthalic acid + N-methyl-2-pyrrolidone at 303.15 K and 313.15 K were determined by Schreinemakers’ method of wet residue under atmosphere pressure. Based on the measured solubility, the quaternary phase diagrams were constructed at the two studied temperatures according to the Janeck method. At each temperature, the quaternary phase diagram includes three crystallization regions of pure solid, three co-saturated curves and one eutectic point. The three pure solids are phthalic acid, adduct of isophthalic acid with N-methyl-2-pyrrolidone (the mole ratio of isophthalic acid to N-methyl-2-pyrrolidone is 1:2), adduct of terephthalic acid with N-methyl-2-pyrrolidone (the mole ratio of terephthalic acid to N-methyl-2-pyrrolidone is 1:2), which are confirmed by the method of Schreinemakers’ wet residue. The crystallization region of adduct of terephthalic acid with N-methyl-2-pyrrolidone is larger than those of phthalic acid and adduct of isophthalic acid with N-methyl-2-pyrrolidone. Furthermore, the quaternary solid–liquid phase equilibrium were predicted by NRTL model. The calculated quaternary phase diagrams agreed well with experimental data.