High-Pressure Phase Behavior of α-Olefin + n-Hexane + Ethylene/1-Octene Copolymer Systems: Experimental Study and Modeling

Accurate knowledge of phase behavior in polyolefin–solvent mixtures is critical for ensuring stable operation and safe scale-up of industrial solution polymerization processes. The binary (n-hexane + ethylene/1-octene copolymer, POE96k-10) and ternary (α-olefin +n-hexane + POE96k-10) phase behaviors were investigated via a visual high-pressure cell (POE96k-10:Mw= 96 kg·mol–1,Mw/Mn= 3.87, 1-octene mole fraction = 10.31 mol%) at temperatures of 380~480 K and pressures as high as 14 MPa. To systematically analyze the effects of α-olefin mass fraction and type on phase transition, four industrially relevant α-olefins (ethylene, 1-butene, 1-hexene, and 1-octene) were investigated. The results show that the phase transition temperature and pressure for liquid–liquid and liquid–vapor transitions show an approximately linear dependence on α-olefin mass fraction. Ethylene, 1-butene, and 1-hexene lower the phase transition temperature, whereas 1-octene increases it. Ethylene exhibits a strong anti-solvent effect, significantly lowering the transition temperature while increasing the phase transition pressure. The modified Sanchez-Lacombe equation of state (MSL EOS) effectively correlates and reproduces the phase equilibrium data of the α-olefin +n-hexane + POE96k-10 ternary systems, though its accuracy decreases with increasing α-olefin chain length.