(123g) Vapor-Liquid Equilibrium Modeling for Carbon Dioxide + Difluoromethane (R-32) and Carbon Dioxide + Pentafluoroethane (R-125) Mixtures Using Helmholtz Energy Equations of State

Recently, pure carbon dioxide (CO2) has gotten a lot of attention as natural, nontoxic, and inflammable working fluid for heat pumps. However, this type of heat pump needs to be operated at a very high pressure (usually over 15 MPa). Refrigerant mixtures of CO2 with other refrigerants with lower boiling point can reduce the high operating pressure. On the other hand, difluoromethane (R-32) has high flammability. The global warming potentials (GWPs) of R-32 and pentafluoroethane (R-125) are far from negligible. Therefore, mixing these refrigerants with nonflammable and low-GWP substances such as CO2 is particularly promising.

This work deals with the modeling of the vapor-liquid equilibrium (VLE) of the CO2 + R-32 and CO2 + R-125 mixtures. This is done with a simple multi-fluid model based on Helmholtz energy equations of state. For pure CO2, R-32, and R-125, very accurate and wide-ranging Helmholtz energy equations of state are available. The multi-fluid model is the best approach to combine these pure-fluid equations for representations of mixture properties.

Although typical multi-fluid models developed for industrially important mixtures have many adjustable parameters, the model used here reduces the number of adjustable parameters so that it is applicable to fluids with limited experimental datasets. The model contains only three parameters that have to be optimized. Values for the parameters have been determined by fitting procedure to experimental VLE data of the CO2 + R-32 and CO2 + R-125 mixtures. With the parameters, the model can correlate the bubble-point pressures with the accuracy of 2%. Moreover, the model can predict the vapor densities within 0.8% for the CO2 + R-32 mixture and 0.2% for the CO2 + R-125 mixture. This model will be helpful for assessments of refrigeration systems using the mixtures.