Corresponding States Correlations for the Prediction of Thermodynamic Properties of Refrigerants
When process equipment such as separation units, heat pumps, and refrigeration systems are simulated or designed, physico-chemical properties of the fluids involved must be known. The outcome of the simulation or design depends on the accuracy of the correlations used in the prediction of physico-chemical data, so it is of the utmost importance that the correlations used correctly describe the properties of the fluids.
Owing to the phasing out of the CFCs, the need for reliable thermodynamic data for the replacement fluids has increased rapidly. Today there is a search for both experimental data and equations that correctly describe the thermodynamic properties of the possible replacement fluids. The purpose of this work was to develop accurate tools for the prediction of thermodynamic properties of refrigerants, especially for those refrigerants (HFCs) that are suggested as replacements for the ozone-depleting refrigerants (CFCs) previously used.
Compound-specific parameters have been determined in the Lee-Kesler equation of state for the refrigerants CFC12, HFC32, HFC125, and HFC134a. Experimental data used in the parameter estimation were: pressure-volume-temperature data at subcooled and superheated conditions, vapour pressure, saturated liquid and vapour molar volumes, speed of sound, second virial coefficient, and heat capacities. Errors in both dependent and independent variables were accounted for during the parameter estimation. Properties predicted with the compound-specific equations of state thus obtained agree well with experimental data.
The refrigerants have been used as reference fluids in the three-parameter corresponding states expression of Teja et al. Calculations have been performed for pure fluids and mixtures, and it has been shown that the predictions of thermodynamic properties are improved when the acentric factors of the reference fluids approach the acentric factor of the pure fluid or mixture for which the calculations are performed.
A new third corresponding states parameter based on the shape and size of a molecule, and a fourth parameter based on electrostatic properties, have been proposed. The parameters have been used in a general corresponding states expression, and vapour pressures for a number of fluids have been calculated. The results are comparable with the results from a three-parameter expression with the acentric factor as third parameter; it has thus been shown that the effects due to geometry and polarity, which are both included in the acentric factor, can be separated into two parts.
equation of state