A generalisation of the SAFT-γ group contribution method for groups comprising multiple spherical segments

Abstract A new group contribution (GC) approach based on the statistical associating fluid theory (SAFT- γ ) has recently been proposed [A. Lymperiadis, C. S. Adjiman, A. Galindo, G. Jackson, J. Chem. Phys. 127 (2007) 234903]. In this continuum equation of state GC approach, the molecules are formed from fused heteronuclear spherical segments each of which represents a distinct chemical functional group. The different segments are characterised by size and attractive energy (well-depth and range) parameters, and a shape-factor parameter which describes the contribution that each segment makes to the overall molecular properties. In addition a number of bonding sites are included on a given segment to deal with association interactions where appropriate; the association between sites introduces two additional energy and range parameters. Our heteronuclear molecular models are thus fundamentally different from the homonuclear models employed with other GC versions of SAFT in which the GC concept is introduced to obtain average molecular parameters. In the current work, we generalise the SAFT- γ equation of state to treat chemical groups which are represented by more than a single spherical segment. This allows for a good description of the properties of large functional groups such as carboxyl and carbonyl groups. The original parameter table for the CH3, CH2, CH3CH, ACH (where AC denotes an aromatic carbon), ACCH2, CH2 , CH and OH groups is now extended to include the C O, COOH, and NH2 groups by examining the vapour–liquid equilibria (VLE) of pure 2-ketones, carboxylic acids, and primary amines. It is demonstrated that the proposed theory provides an excellent description of the vapour–liquid equilibria for all of the chemical families considered, and that the new group parameters can be used in a predictive fashion to model the phase behaviour of larger compounds not included in the estimation database. One of the principal advantages of the SAFT- γ formalism is that the binary interaction parameters between groups of different types can be estimated from pure component data alone. This is particularly useful in describing the properties of fluid mixtures. The adequacy of the method in predicting the VLE is assessed for selected binary mixtures of associating and non-associating compounds. A strict test of any GC method is its ability to capture both vapour–liquid and liquid–liquid equilibria (LLE) with the same set of group parameters. It is very gratifying to find that with SAFT- γ one is able to reproduce the VLE and LLE behaviour for several mixtures simultaneously, for example, n- hexane + propanone , and n- pentane + polyethylene .