The modified Sanchez-Lacombe equation of state (MSL-EOS) is Neau's version of the Sanchez-Lacombe equation of state modified to include a Pe ´neloux-type volume translation. The purpose of this work is to report parameters for modeling the phase behavior of polyethylene solutions. The MSL equation is an empirical equation that contains four parameters to define each pure compound whether a solvent or a polydisperse polymer. The MSL equation uses conventional linear and quadratic mixing rules. A parametrization can be used to obtain the pure compound parameters from the molar mass, critical temperature, critical pressure, and acentric factor. These properties cannot be defined for polydisperse polymers, and parameters were determined from a combination of liquid density (PVT) data and polymer + solvent cloud point (xPT) data. Binary interaction parameters have been obtained for over 50 mixtures by correlating the fluid phase boundaries. Binary mixtures including ethylene, hexane, and/or cyclohexane are of particular interest to polyethylene production. A single binary interaction parameter is usually sufficient to represent vapor-liquid equilibrium, but temperature dependence is required to accurately represent liquid-liquid equilibrium. The MSL equation of state can be used to correlate the cloud points of polyethylene solutions at high pressures.

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Vapor-liquid equilibrium data have been measured for the ethylene + hexane system at temperatures of (390 to 510) K and pressures up to 10 MPa. The bubble and dew point pressures of ethylene + hexane samples with fixed compositions were measured using the Cailletet apparatus. The critical points of ethylene + hexane mixtures were also measured experimentally. The experimental data were correlated with the modified Sanchez-Lacombe (MSL) equation of state. The experimental data are also compared with the predictions of the Peng-Robinson (PR) and Soave-Redlich-Kwong (SRK) equations of state using the binary interaction parameter obtained in the MSL correlation. The three equations of state produce comparable qualitative representations of the experimental phase boundaries.