Measurement and Calculation of the Liquid-Liquid Phase Boundaries and Phase Equilibria for the Hexane plus Polyethylene System at High Temperatures

The liquid-liquid phase equilibria for the hexane + polydisperse polyethylene (PE) systems were measured at 473 K and at feed PE weight fractions ranging from 0.075 to 0.20. An apparatus was developed for the measurement, which consisted of a variable-volume optical cell and two sampling tubes for correcting both the top (hexane-rich) and bottom (PE-rich) phases. The PE weight fraction of the hexane-rich phase decreased with decreases in the feed weight fraction of PE and the phase equilibrium pressure. On the other hand, that of the PE-rich phase increased as the feed PE weight fraction and pressure decreased. Furthermore, the experimental data were compared with the predicted results obtained using the Sanchez-Lacombe equation of state. The predicted results approximately reproduced the experimental results for the PE weight fractions of the hexane-rich phase. Although some deviations were found in the experimental results of the PE-rich phase, the predicted phase equilibrium lines qualitatively reproduced the experimental results, which showed that the PE weight fraction increased with decreases in the feed PE weight fraction and pressure. Moreover, both the experimental and the predicted results also indicated that the influences of the feed PE weight fraction on the weight average molecular weight and on the polydispersity index of PE in the PE-rich phase were larger than those in the hexane-rich phase at low pressure.