Measurement and Modeling of the PVT Behavior and Viscosity of Bis(2-Hydroxyethyl) Terephthalate and Mixtures with Ethylene Glycol Using PC-SAFT and Entropy Scaling

Bis(2-hydroxyethyl) terephthalate (BHET) and ethylene glycol (EG) are involved in both polyester synthesis and upcycling on a large industrial scale. However, there are little to no known thermophysical property data of BHET and mixtures of BHET and EG, especially at typical processing conditions. Here, the density and viscosity of molten BHET and its mixture with EG were measured between 393 to 443 K and pressures to 40 MPa. The density slightly increases linearly with pressure in these ranges, while the viscosity increases to a larger and more nonlinear extent. The PC-SAFT equation of state (EoS) was combined with Residual Entropy Scaling (RES) to simultaneously model the thermodynamic properties and viscosity. The measured density and viscosity data and literature vapor pressure data were used to obtain the pure component BHET PC-SAFT EoS parameters. The model correlates the vastly different data types very well with average absolute relative deviations (AARDs) of 0.11, 2.46, and 0.71% for density, viscosity, and vapor pressure, respectively. The density and viscosity of mixtures of BHET and EG were well predicted with AARDs of 0.38 and 4.9%, respectively, while keeping the equation of state binary interaction parameter ( k ij ) set to zero.