Prediction of Thermodynamic Derivative Properties of Pure Fluids through the Soft-SAFT Equation of State

Abstract

We present in this work the application of the soft-SAFT equation of state (EoS) to the calculation of some main derivative properties, including heat capacities, reduced bulk modulus, Joule-Thomson coefficient, and speed of sound. Calculations have been performed analytically through the derivation of a primary thermodynamic potential function. The application to the n-alkanes, n-alkenes, and 1-alkanols families has been done in a semipredictive manner, with the molecular parameters of the equation obtained from previous fitting to vapor-liquid equilibrium data of the same compounds. The equation is able to capture the typical extrema isothermal derivative properties exhibit with respect to density, providing quantitative agreement with experimental (or correlation) data in some cases. Results in the vicinity of the critical point are improved by adding a crossover treatment to take into account the long-range fluctuations present in this region. By taking advantage of the molecular nature of the equation, we have been able to separate and quantify the different contributions (reference fluid, chain, and association) to the total derivative properties. The association plays a predominant role in energetic properties, such as the heat capacities, while there is a competition between association and chain length as the chain length of the compound increases for volumetric properties, such as the isothermal compressibility. These results act in favor of the molecular-based equations, like soft-SAFT, as predictive tools for several applications.

Approach to Improve Speed of Sound Calculation within PC-SAFT Framework

“…They used a similar approach, as that proposed by Llovell et al, 38 to predict the second-derivative properties using the pure component parameters fitted to VLE data only. The results revealed that both models performed well, especially away from the critical region.…”

Group contribution methodology based on the statistical associating fluid theory for heteronuclear molecules formed from Mie segments

“…In a subsequent study different versions of the SAFT EoS were compared, and deviations of up to 20% from the experimental values of the derivative properties were found. 81 Llovell et al 82 have also presented a detailed analysis on the performance of the soft-SAFT EoS for the derivative properties of pure components and mixtures 83 concluding that the accuracy of the description can range from 1% to 20% depending on the compound and the property of interest, with the highest deviations typically obtained for the speed of sound of long-chain n-alkanes. In other work on smaller molecules where SW or LJ forms of the potential are implemented, the reported errors for the description of the derivative properties is found to be within 10%.…”

Direct calculation of interfacial properties of fluids close to the critical region by a molecular-based equation of state

“…In the last years we have been working on a systematic extension of the original soft-SAFT equation of state [38][39][40][41][42][43], expanding its capability to other regions of the phase space [30,31] and for the calculation of additional properties [44]. The purpose of the present work is to show that the crossover approach used to accurately represent the vapor-liquid critical region also allows the prediction of the interfacial tension in this region with great accuracy.…”