A corresponding states thermodynamic model for pure and mixed refrigerants in terms of the Helmholtz energy

Abstract

For the alkane and halogenated alkane (HA) fluid families, several high accuracy dedicated equations of state (DEoS) have been recently published and models based on corresponding states (CS) format have been often used. These models aim at predicting the thermodynamic behavior of the fluids following different parameter contribution techniques, heedless of the “conformality” concept, which is nevertheless fundamental for a CS method application. Taking advantage of an extensive study on the thermodynamic “conformality”, and of the precise available DEoS, exact “scaling” parameters for the most significant state functions have been considered in a CS framework for HAs. A predictive volumetric model for pure fluids and mixtures, has been recently proposed and assumed as part of the present model. From a couple of standard liquid density and of vapor pressure values, as two inputs for each fluid of interest, a three parameters CS model for the residual Helmholtz free energy is reported, through which any thermodynamic residual property can be obtained in a semi-predictive mode. The model is then extended to mixtures, through both pseudocritical constants and scaling factors mixing rules. For pure fluids, the proposed model is validated on the main thermodynamic functions against DEoS of several HAs. For mixtures, the validation is developed on density, enthalpy and entropy against mixture DEoS and for VLE against data of non-azeotropic and azeotropic mixtures. In spite of the semi-predictive nature of this model, significant results are obtained for all the examined functions, assuring a very satisfactory representation of the thermodynamic properties of pure and blended HAs. The proposed models can be useful in view of the applications as refrigerants of the fluids studied.