A molecular-based group contribution equation of state for the description of fluid phase behaviour and thermodynamic derivative properties of mixtures (SAFT-γ Mie)
Creators: name: Papaioannou, Vasileios
Publisher: name: Imperial College London
Year: 2012

description: An accurate knowledge of the thermophysical properties and phase behaviour of fluid mixtures is essential for the reliable design of products and processes across a wide range of chemical engineering applications, varying from the processing of petroleum fluids to the manufacturing of pharmaceuticals. Thermodynamic tools and, in the context of this work, group contribution (GC) methods are predictive approaches that are expected to play an important role in meeting these industrial needs. The principal focus of the work presented in this thesis is the development of a novel GC method based on the statistical associating fluid theory (SAFT): the SAFT-γ Mie approach. The method is developed based on a detailed molecular model and a realistic intermolecular potential, the Mie potential with variable attractive and repulsive ranges, for the description of interactions at a molecular level. Over the past decade, an increasing research effort has been devoted to developing formalisms that couple the accuracy of the SAFT equation of state (EoS) with the predictive capabilities of group contribution approaches. In the development of such methods one aims to overcome the limitations inherent to GC approaches based on activity coefficient models, such as in the well-established universal quasi-chemical functional group activity coefficient (UNIFAC) approach. A more recent landmark has been the development of heteronuclear methods within SAFT. The SAFT-γ EoS based on the square-well (SW) potential has been shown to describe accurately the phase behaviour of a wide variety of fluids. In the work presented in this thesis, SAFT-γ SW is applied to the study of the fluid phase behaviour of aqueous solutions of hydrocarbons. These mixtures are of high industrial relevance, and the accurate representation of their highly non-ideal nature is very challenging from a theoretical perspective. The SAFT-γ method is shown to perform comparatively well in predicting the behaviour of the systems examined. Nonetheless, some challenges are identified, such as the description of thermodynamic derivative properties and the description of near-critical fluid phase behaviour, where the performance of the method is shown to be less accurate. These challenges partially arise from the simplistic intermolecular square-well potential employed within SAFT-γ SW, which allows for a rigorous theoretical development, but fails to reproduce accurately finer aspects of the thermophysical behaviour of fluids, such as second-order derivative thermodynamic properties. These challenges are tackled here with the development of the SAFT-γ Mie GC approach, based on the versatile Mie intermolecular potential and a third-order treatment of the thermodynamics of the monomer segments. The SAFT-γ Mie method is applied to the study of the properties of two chemical families, n-alkanes and 2- ketones, and it is shown that a significant improvement over existing SAFT-based group contribution approaches can be achieved in the description of the pure component phase behaviour of the compounds studied. Moreover, the application of a realistic intermolecular potential is shown to allow for an excellent description of second-order derivative thermodynamic properties, and the accurate treatment of the intersegment interactions is shown to improve the performance of the method in the description of the near-critical fluid phase behaviour. The predictive capability of the method is demonstrated in the description of mixture fluid phase behaviour and excess thermodynamic properties in a predictive manner. Given the promising performance of the SAFT-γ Mie EoS, the method is applied to the case study of the solubility of two active pharmaceutical ingredients in organic solvents. The method is shown to satisfactorily predict the solubilities of the mixtures considered, based on limited experimental data for simple systems. Given the complexity of the mixtures studied, the performance of the SAFT-γ Mie is considered very encouraging and shows that there is great potential in the application of the method to this challenging field.