Viscosity and density measurements for carbon dioxide + pentaerythritol ester lubricant mixtures at low lubricant concentration

This paper reports density and viscosity data for mixtures containing carbon dioxide (CO2), a refrigerant fluid, and a lubricant (pentaerythritol tetrapentanoate (PEC5), pentaerythritol tetraheptanoate (PEC7) and pentaerythritol tetranonanoate (PEC9)) at low lubricant concentration, at temperatures from 303 to 353 K and pressures up to 60 MPa. Viscosity and density were measured simultaneously using a vibrating-wire sensor, in an apparatus specifically built to study gas–liquid mixtures. The density data at each composition were correlated with a Tait equation and the excess volumes were determined. The viscosity of the mixtures increases drastically with the addition of small quantities of lubricant to the pure refrigerant. The influence of the density in the viscosity of the mixtures was analyzed using a Doolittle-type equation. The values of the excess activation energy of viscous flow, ΔEa, for the mixtures are positive and their values increase with the lubricant concentration.