Prediction of the liquid–liquid phase equilibria for polydisperse polyethylene solutions under conditions of high temperature and pressure

Abstract

In the present study, the liquid–liquid (LL) phase equilibria for the hexane + polydisperse polyethylene (PE) and the ethylene + hexane + polydisperse PE systems were simulated using the Sanchez–Lacombe equation of state, in order to investigate the effects of the addition of ethylene and the polydispersity of PE on the LL phase equilibria of the PE solution. In the calculation, the polydispersity of PE was represented as a mixture of 16 types of monodisperse PE. The interaction parameter between hexane and PE was determined by fitting to the LL phase boundary curve of the hexane + polydisperse PE binary system in the previous work, and it depended on the molecular weight of PE. The interaction parameter between ethylene and PE was used, as also determined from the LL phase boundaries for the ethylene + hexane + polydisperse PE system in the present work. And, the parameter of the ethylene–hexane pair that was based on the value reported in the literature was used.The simulated results indicated that the critical PE weight fraction increased as the feed ethylene content increased. On the other hand, although the addition of ethylene greatly increased the LL phase boundary, the ethylene content had only a small influence on the PE content of both separated LL phases and their molecular weight distributions for the LL phase equilibria. The effect of the polydispersity of PE was notable on the PE weight fractions in the separated phases.