Author's personal copy Fluid Phase Equilibria 277 (2009) 131–144 Contents lists available at ScienceDirect Fluid Phase Equilibria journal homepage: www.elsevier.com/locate/fluid Developing a predictive group-contribution-based SAFT-VR equation of state Yun Peng, Kimberly D. Goff, M. Carolina dos Ramos, Clare McCabe ∗ Department of Chemical and Biomolecular Engineering, Vanderbilt University, Nashville, TN 37215-1604, United States article info Article history: Received 22 August 2008 Received in revised form 10 November 2008 Accepted 10 November 2008 Available online 19 November 2008 Keywords: Phase equilibria Pure fluids Mixtures Group contribution Heteronuclear SAFT GC-SAFT-VR abstract The hetero-segmented version of the statistical associating fluid theory for potentials of variable range (hetero-SAFT-VR) is used to develop a predictive molecular-based group-contribution SAFT-VR equation of state (GC-SAFT-VR). The hetero-SAFT-VR approach models molecules composed of segments of dif- ferent size and/or energies of interaction enabling an accurate description of real molecules composed of different functional groups. The differences in interactions between functional groups are maintained throughout the theory in contrast to other approaches in which the parameters for functional groups are averaged in order to model a molecule as a homonuclear chain with “group-averaged” parameters. Through the GC-SAFT-VR approach we can study the effect of molecular functionality and topology on the thermodynamic properties of real fluid systems, since parameters are determined for each func- tional group and chain connectivity is explicitly specified. In this initial study GC-SAFT-VR parameters are developed for key organic functional groups (CH 3 , CH 2 , CH 2 CH, C O, C 6 H 5 , OCH 3 and OCH 2 ) by fit- ting to experimental vapor pressure and saturated liquid density data for a selected group of compounds that contain these functional groups. The transferability of the parameters obtained is tested by com- paring theoretical predictions with experimental data for pure fluids not included in the fitting process and binary mixtures. Using the GC-SAFT-VR approach good agreement is obtained between experimen- tal data and the theoretical predictions for pure substances, including isomers, and their mixtures. The GC-SAFT-VR approach is able to accurately predict the effect of molecular functionality on mixture phase behavior without fitting to any experimental data for the system being studied. © 2008 Elsevier B.V. All rights reserved. 1. Introduction The study of fluid phase behavior and the development of accurate theoretical approaches to predict physical properties and phase diagrams remains an enduring challenge and an area of great fundamental and applied significance. Reliable methods for the theoretical prediction of thermophysical properties and phase equilibria are essential to the chemical, biochemical and energy industries, and are made all the more critical as energy costs require more frequent re-analysis of existing chemical process [1] as well as the design of new novel energy efficient processes. While the goals of fluid property prediction have not changed – i.e., understanding and accurately modeling their thermodynamics and phase behavior – the systems of interest are of increasing com- plexity, such as heavy hydrocarbons, branched and hyperbranched polymers, and, more generally, molecules with multiple functional groups. In such systems, molecular architecture (e.g., branching) can have an important effect on the thermodynamic properties. For example, it is well known that for hydrocarbons with a given number of carbon atoms branching typically leads to a decrease ∗ Corresponding author. Tel.: +1 615 322 6853; fax: +1 615 343 7951. E-mail address: c.mccabe@vanderbilt.edu (C. McCabe). in the boiling point and critical temperature, and that at a given temperature the cloud point pressure of polymers, such as poly- olefins, generally decreases as the degree of polymer branching increases [2]. Similarly the nature of the functional groups in small molecules, and the incorporation of different functional groups and their arrangement into a polymer backbone, can also strongly influ- ence thermodynamic properties and phase behavior. For example, poly(vinyl acetate) (PVAc) is more CO 2 -soluble than poly(methyl acrylate) (PMA) due to the position of the carbonyl group in the polymer repeat unit; the PMA and PVAc repeat units both have the same number of carbon, hydrogen, and oxygen atoms, they differ only in the position of the carbonyl group [3]. While experimental thermodynamics and phase behavior data is essential to the development of accurate theoretical tools, exper- iments alone are unable to quantify the effects of molecular structure and composition on the phase behavior of pure flu- ids and their mixtures, due to the sheer number of experiments that would be needed for a systematic study. While equations of state (EoS) are today routinely used to calculate thermody- namic properties, cubic equations, which dominate in the study of simple fluids, tend to give poor predictions for complex flu- ids and their mixtures, such as polymers and associating systems, as they do not explicitly take into account the architecture of the molecules and association interactions. This often results in 0378-3812/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.fluid.2008.11.008