Phase behavior for the supercritical ethylene + hexane + polyethylene systems

The present work investigated the phase behavior for the ethylene + hexane + polyethylene (PE) systems at temperatures from 373 to 473 K, PE weight fractions from 0.01 to 0.13, and ethylene weight fractions from 0 to 0.1 by using a synthetic-type apparatus. Two nearly-monodisperse PE samples having weight-average molecular weights of 15.5 × 103 (PE15k) and 108 × 103 (PE108k) were used in this work. All the binary hexane + PE and ternary mixtures exhibited lower critical solution temperature-type liquid–liquid phase behavior in the temperature range examined. The phase separation pressures increased with increasing temperature and PE molecular weight. The addition of supercritical ethylene to the hexane + PE solutions dramatically shifted the liquid–liquid phase boundary to lower temperatures and higher pressures, because it acted as a poor solvent for PE. The Sanchez–Lacombe equation of state was used to correlate and predict the phase boundaries. The experimental liquid–liquid phase boundaries for the hexane + PE systems could be correlated satisfactorily with a temperature-dependent binary parameter. By adjusting the binary parameter for ethylene + PE pair, the ternary phase boundaries could also be correlated and predicted satisfactorily.