Modeling of the thermophysical properties of CO2-lubricant oil mixtures

G. Raabe1 and J. Koehler1

1Institut fuer Thermodynamik, TU Braunschweig, Germany

Keywords: modeling, PC-SAFT
property: phase equilibria, viscosity
material: CO2, lubricants

The introduction of carbon dioxide as alternative natural refrigerant has the potential to significantly reduce CO2 equivalent greenhouse gas emissions in many application fields of refrigeration, air conditioning, and heat pumping. In closed refrigeration cycles, a portion of the lubricant oil of the compressor circulates with the refrigerant flow and influences the behaviour and the energy-efficiency of all components of the circuit. Therefore, the design of highly energy-efficient systems requires a fundamental knowledge of the thermophysical properties of CO2-lubricant oil mixtures in order to accurately predict their behavior at various working conditions. Though CO2-lubricant oil mixtures are characterized by strong molecular asymmetries that result in complex phase behaviors and also effect the transport properties of the mixtures. This requires sound physical models that allow for reliable predictions of the mixtures phase equilibria properties and viscosities. The PC-SAFT equation of state, which is based on statistical thermodynamics, has already successfully been applied to model various complex systems [1]. Thus, we have investigated the performance of the PC-SAFT equation of state regarding its ability to accurately predict the phase equilibria and densities of mixtures of CO2 with different polyol ester lubricant oils, such as POE68 and pentaerythritol (PEC, PEB) lubricants. Different models have been proposed in literature to model the viscosities of pure components as function of the temperature and pressure. In this work, we applied the free volume [2] and the friction theory viscosity model [3] for the PC-SAFT EOS to model the viscosities of CO2 and the lubricants oils. We then investigated different predictive mixing rules and tested their ability to reliably estimate the mixtures viscosities.

  1. F. Tumakaka, J. Gross, G. Sadowski, FPE 228, 89 (2005)

  2. A. Allal, M. Moha-Ouchane, C. Boned, Phys. Chem. Liq. 39, 1 (2001)

  3. S. E. Quiňones-Cisneros and C. K. Zéberg-Mikkelsen, FPE. 178, 1 (2001).

     Official Sponsors

uni Anter_logo Tziolas_logo_ linseis netzsch