Solubility of Dimethyl Terephthalate and Monomethyl Terephthalate in the Methanol Aqueous Solution and Its Application To Recycle Monomethyl Terephthalate from Crude Dimethyl Terephthalate

The solubilities of dimethyl terephthalate (DMT) and monomethyl terephthalate (MMT) in methanol aqueous solutions were determined in the temperature range of 280–336 K. The solute-free mass fractions of methanol extended from 0.3 to 1.0. Both the solubilities of DMT and MMT in aqueous methanol solution increase with the temperature and the addition of methanol. The experimental data were correlated with the Wilson and the NRTL models. The root-mean-square deviations for the system of (DMT + methanol + water) were 1.30 and 1.33 K calculated by the Wilson and the NRTL models, respectively, while for the system of (MMT + methanol + water), the root-mean-square deviations were 0.55 and 0.82 K. The melting points and the enthalpy of fusion of DMT and MMT were detected by the differential scanning calorimetry (DSC). A simple purification and recycling process was designed to separate MMT from the esterification reaction product according to the solubility difference between DMT and MMT.