Thermodynamic Modeling of Hydrogen-Bonded Mixtures

Separation process design requires high quality phase equilibria predictions. Interest in bio-derived chemicals and fuels has inspired new pathways for chemical manufacture using polar raw materials, and thus almost all streams involve hydrogen-bonding components. Today’s engineers are faced with the challenge of quantitatively modeling hydrogen bonding (or association) effects when predicting the phase equilibria of polar solutions. Traditional gamma models such as NRTL[1] do not account explicitly for association but instead attribute deviations to strong dispersion forces, misrepresenting the physical characteristics of the hydrogen bond. As a result, predictions for dilute streams and multicomponent mixtures are often unsatisfactory.

Several thermodynamic models have been developed that incorporate association either through chemical theory or Wertheim’s perturbation theory. While the two approaches differ in the underlying assumptions, the apparent fugacities modeled by both approaches depend on the fraction of molecules that remain nonbonded — the ‘monomers’. The majority of modern association models are equations of state using the Wertheim method, such as SAFT[2], CPA[3] and ESD[4]. Equations of state association parameters are fitted to macroscopic pure component properties and subsequently used to predict mixture behavior. The model parameters depend on the quality/quantity of pure component data available. An equation of state requires accurate vapor pressure and excess Gibbs energy from the same model. A gamma model requires only the excess Gibbs energy from the mixture model. Activity coefficient (gamma) models have been the favored by industry for many applications (typically low or moderate pressures) because the pure-component vapor pressures are usually known to good accuracy and attention can be focused on the mixture nonideality. However, to date, the Wertheim method has not been widely applied in activity coefficient models.

This project is a collaboration between academic and industrial partners to merge the Wertheim association approach with an activity coefficient model. Microscopic tools including spectroscopy and quantum simulations are used to gain insight into association at the molecular level. The fundamental physical measurements and simulations provide additional insights into the hydrogen bonding.

References

[1] H. Renon, J.M. Prausnitz, On the thermodynamics of alcohol-hydrocarbon solutions, Chem. Eng. Sci. 22 (1967) 299–307. doi:10.1016/0009-2509(67)80116-7.

[2] W.G. Chapman, K.E. Gubbins, G. Jackson, M. Radosz, New reference equation of state for associating liquids, Ind. Eng. Chem. Res. 29 (1990) 1709–1721. doi:10.1021/ie00104a021.

[3] G.M. Kontogeorgis, E.C. Voutsas, I.V. Yakoumis, D.P. Tassios, An Equation of State for Associating Fluids, Ind. Eng. Chem. Res. 35 (1996) 4310–4318. doi:10.1021/ie9600203.

[4] J.R. Elliott, S.J. Suresh, M.D. Donohue, A simple equation of state for nonspherical and associating molecules, Ind. Eng. Chem. Res. 29 (1990) 1476–1485. doi:10.1021/ie00103a057.