A statistical mechanical group contribution method for calculating thermodynamic properties of fluids

The estimation of thermodynamic properties for process design using fundamental molecular theories is limited by their necessary assumptions about intermolecular forces and approximations in statistical mechanics as well as by their extensive computational requirements. At the same time, the alternative of strictly empirical correlations is often unreliable because their fundamental basis is inadequate. The present work establishes the actual statistical mechanical approximations for models of the solution-of-groups (SOG) form and proposes a more rigorous, flexible and calculable formulation using a site-based concentric shell intermolecular potential.First, a statistical mechanical analysis of the SOG method has been performed to expose its molecular level assumptions. Besides localization of electronic and electrostatic effects, it appears necessary to have composition-independent conformational effects, symmetry of site locations and conformality of site interactions with scaling parameters independent of molecular structure in order to satisfy the SOG formulae.Second, an alternative model using site-site interactions on concentric shells has been proposed. This formulation is a less restrictive group-contribution method with the interactions being explicitly treated while the sites of the same type need not be thermodynamically equivalent. Further, site symmetry is retained, providing a match with the SOG form, along with showing three-parameter corresponding states' behavior for nonspherical and globular normal fluids.Applications have been made of this model through group-contribution equations of state (GC-EOS) of both the van der Waals and virial types. The former yields accurate pressure-volume-temperature (PVT) behavior of several fluids and confirms the transferability of the site-site parameters. Molecular dynamics simulations with the concentric shell model have been made to demonstrate its agreement with more rigorous simulations of diatomic molecules. Both methods yield good results for limited comparisons with dense fluid properties.The fundamental level of formulation and preliminary success of the concentric shell model suggest its further development.