Ternary Solid-liquid Equilibria Of Xylene Isomers

The new suspension crystallization technology for the separation p-xylene from its isomer-mixture, because of its low energy consumption, is becoming an important alternative to the traditional adsorptive separation method. The thermodynamic phase equilibrium model and the phase diagram is the underline for any separation process. The subject of this thesis will focus on the obtain of the phase equilibrium model and phase diagram of ternary xylene isomers system via different methods.Xylene isomer ternary solid-liquid thermodynamic equilibrium model and the phase diagram is extrapolated from the previous binary model. However, the model do not agree very well with the experimental data from the literatures, much poor prediction in the temperature range of 220-240K.In order to obtain the necessary experimental data of xylene ternary system, a crystallization system is set up. The solid-liquid phase equilibrium of xylene ternary system in the range of 240-280K is determined. Concentration of the liquid phase is determined by GC. It may concluded from the experimental results that, the ternary system could not be considered as an ideal system.Based on the principle of CALPHAD, using Thermo-Calc software, xylene isomer ternary solid-liuqid thermodynamic equilibrium model was assesed with the experimental results obtained by this study and from the literatures. In the calculation, regular solution model is adopted for the excess Gibbs energy of the system. Parameters of the nonideal mixing term of binary and ternary are obtained. The predicted termary phase diagram fits all the experimental data very well.Based on the parameters of the nonideal mixing term, an analytical expression for the solid-liquid equilibrium is derived. The phase behavier of the binary and ternary system could be predicted based either on the regular solution model or the ideal solution model.