(666e) Experimental Measurements and Predictions of the Properties of Refrigerant Fluids Using Ab-Initio, SAFT-VR Mie and COSMO Calculations

On account of the constraints imposed by the European and International legislations, the refrigerant industry must constantly find alternative refrigerant fluids that have lower impacts on the global warming of Earth and Ozone layer. The hydrofluoroolefins (HFO) have been proposed to replace the currently used hydro fluorocarbons (HFC), as they have a much lower global warming potential. In order to select the optimal mixture composition for the design and operation of a refrigeration process, it is necessary to know the phase diagram and thermodynamic properties of the mixtures. In particular, the vapor-liquid equilibria (VLE) and the location of azeotropes must be accurately known. Besides, the design of chemical processes and heat engines requires a good knowledge of both the ideal gas and the residual thermodynamic properties of the refrigerants. The aim of this work is to propose various thermodynamic approaches to predict these properties.

First, we performed ab initio calculations on isolated molecules to determine the ideal gas properties of refrigerant fluids. Ideal enthalpies of formation and heat capacities can be accurately predicted by using either DFT [1] or some specific semi-empirical methods. We then used several activity coefficient models based on the COSMO approach, to predict the VLE of refrigerant mixtures. VLE and azeotropes can be well predicted by the COSMO-RS model [2]. While the first versions of COSMO-SAC are very inaccurate for such systems, it is possible to obtain good predictions with the COSMO-SAC-dsp model [3] by readjusting some universal parameters. New experimental measurements of vapor-liquid equilibria on mixtures involving HFO have been performed with a static analytic equipment, to complete the literature data for mixtures of HFO with other refrigerants.

We then propose a modified version of the SAFT-VR-Mie [4] equation of state (EoS) that includes a polar contribution to explicitly take electrostatic interaction into account. The polar SAFT-VR-Mie EoS is used to describe the VLE and some derivative thermodynamic properties of refrigerant mixtures. A new association model that takes the formation of cyclic clusters is proposed for hydrogen fluoride (HF), which is one major compound of the refrigerant production process. The new models can be used in process simulation software, thanks to the cape-open and Simulis Thermodynamics® interfaces.

References

[1] A. Osmont et al., Combustion and Flame, 151, 262 (2007)

[2] F. Eckert, A. Klamt, Fluid Phase Equilib., 172, 73 (2010)

[3] C.-H. Hsieh et al., Fluid Phase Equilib., 367, 709 (2014)

[4] T. Lafitte et al., J. Chem. Phys., 139,154504 (2013).

Acknowledgements

The results presented in this work are part of the ongoing project "PREDIREF". We are grateful to the National French Research Agency (ANR-13-CDII-0008) for providing financial support to this project.