A corresponding-states Helmholtz-free-energy model based on a conformality analysis

Abstract

In recent times, for fluids belonging to the alkane and halogenated-alkane families, several high accuracy dedicated equations of state (DEOS) have been published and models based on the corresponding-states (CS) format have often been utilised to calculate thermodynamic properties. Such models aim at predicting the thermodynamic behaviour of the fluids following different parameter contribution techniques, heedless of the conformality' concept, which is nevertheless fundamental for a CS method application. An extensive study on the thermodynamic conformality is here proposed for fluids belonging to families of alkanes and halogenated alkanes. By taking advantage of the precise available DEOS, the most significant state functions have been considered, and exact scaling parameters for each of them have been defined in a CS framework. A volumetric model for the liquid phase is at first proposed in which, following some conformality study suggestions, an improvement is introduced substituting the scaling factor ω of Pitzer with a new constant one. The model is predictive for both saturated and compressed liquid density, reaching high accuracy. From saturated vapour density and vapour pressure correlations, as input for each fluid of interest, a three-parameter CS model for the residual Helmholtz free energy is furthermore provided, through which any thermodynamic residual property can be obtained in a semipredictive mode. The proposed model is validated on the main thermodynamic functions against DEOS of several fluids of the cited conformal families. In spite of the semipredictive nature of this model significant results are obtained for all the functions examined. Because the fluids considered are refrigerants, the proposed models can furthermore be useful in view of technological applications.