Using a Volume Shift in Perturbed-Chain Statistical Associating Fluid Theory To Improve the Description of Speed of Sound and Other Derivative Properties

Volume shifts are commonly applied to cubic equations of state (EoS) to improve the description of densities, since they do not affect the phase equilibria. This concept can in principle be extended to any other EoS, but studies using this approach are scarce for EoS based on perturbation theory, mainly because most of these equations already provide accurate liquid density results. However, as with cubic EoS, most statistical associating fluid theory (SAFT) type EoS have difficulties in describing derivative properties, more noticeably the speed of sound. In this work, we study the use of a volume shift in perturbed-chain statistical associating fluid theory (PC-SAFT). A new parametrization procedure is proposed that involves the use of vapor pressure, the pressure derivative with respect to density obtained from isothermal compressibility, and molar volume data. This last property is only applied to fit a constant Peneloux-type volume shift. It is shown that this approach provides a good description of saturation pressures and liquid densities, as well as an improved description of the speed of sound. The impact of these changes on the performance of PC-SAFT in the description of properties and phase equilibria is further investigated by comparing the results of this methodology with those obtained with the available PC-SAFT parameter sets.