High-pressure viscosity and density of carbon dioxide + pentaerythritol ester mixtures: Measurements and modeling

This article reports densities and viscosities up to 60 MPa and from 303.15 to 353.15 K on two binary mixtures of CO₂ and lubricant pentaerythritol tetra-2-ethylhexanoate with 7.8% and 14.4% mass fraction of lubricant. Because CO₂ and the lubricant are not in the same phase at atmospheric pressure and room temperature, a specially-designed vibrating-wire apparatus has been used. The experimental data, together with our previous data for six mixtures of CO₂ and three polyolesters, have been used to test the predictive and correlation ability of several models: simple mixing laws, the self-referencing method, and the hard-sphere and free volume models. In general, it is not possible to predict the viscosity with deviations smaller than 10%. The hard-sphere model and the self-referencing model are the more adequate to represent the viscosity of these mixtures. Most of the models studied underestimate the viscosities over the investigated (T,p) range. © 2008 American Institute of Chemical Engineers AIChE J, 2008