Three and four phase predictions for polyethylene+hydrocarbon systems

• Published in: Fluid phase equilibria 2005-02, Vol.228-229 (Complete), p.427-437
• Main Authors: Krenz, R.A., Heidemann, R.A., de Loos, Th.W.
• Format: Article
• Language: English
• Subjects: Critical point; Equation of state; Four-phase boundary; Liquid–liquid equilibria; Polyethylene; Three-phase boundary
• ISSN: 0378-3812; 1879-0224
• DOI: 10.1016/j.fluid.2004.08.038

Experimental cloud-point isotherms for polyethylene+hydrocarbon systems can exhibit abrupt changes in slope when the polyethylene has a broad molar mass distribution. Equations of state with parameters correlated to qualitatively reproduce this behavior can result in cloud-point curves that fold, crossing themselves. The point of self-intersection is the terminus of a three-phase region, with three liquid-like phases in equilibrium. Teardrop-shaped three-phase boundaries have been calculated using the Modified Sanchez–Lacombe (MSL) equation of state for polyethylene+ethylene and polyethylene+n-hexane systems. We also show some examples of phase boundaries separating two-phase regions from three-phase regions, which exhibit folding and self-intersection, indicating the presence of a region with four liquid-like phases in equilibrium. A boundary isolating a four-phase region inside a three-phase region has been calculated for a polyethylene-co-octene-1+n-hexane system. It has been noted that the polyethylene molar mass distribution, especially the extension to high molar masses, determines whether three-phase regions and four-phase regions occur.