High-pressure viscosity and density of carbon dioxide plus 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. (C) 2008 American Institute of Chemical Engineers.