The role of cross-association between carbon dioxide and hydrogen sulfide using the SAFT-VR Mie equation of state

Abstract

One of the advantages of the contemporary, statistical-mechanical-based equations of state is the possibility of connecting molecular information to macroscopic behavior through an analytical mathematical model. The Statistical Associating Fluid Theory (SAFT) is the basis of a family of such equations of state, which are able, in principle, to represent the thermodynamic behavior of associating fluids such as water, alcohols, among others. In this work, we study the role of cross-association between carbon dioxide and hydrogen sulfide using the SAFT-VR Mie equation of state, which is one of the most recent and robust versions within the SAFT family. Two possibilities of cross-association are investigated: a Lewis acid-base interaction between the sulfur atom from hydrogen sulfide and the carbon atom from carbon dioxide, and a hydrogen bond between the hydrogen atoms from hydrogen sulfide and the oxygen atoms from carbon dioxide. Two cross-association parameters are fitted to vapor-liquid equilibrium data of the binary mixture for each hypothesis: the specific energy and the bonding volume. We show that taking into account cross-association leads to improved vapor-liquid equilibrium description. We also compare the different modeling strategies for the calculation of species distribution in a gravitational field for a ternary mixture of methane-carbon dioxide-hydrogen sulfide, exploring how sensitive the consideration of cross-association is in describing such a phenomenon. Both vapor-liquid equilibrium and gravitational segregation results lead us to the conclusion that the cross-interaction effects should be considered to describe the thermodynamic behavior of mixtures involving hydrogen sulfide and carbon dioxide. Performing classical equilibrium molecular dynamics simulations, we were able to show that the Lewis acid-base interaction between the sulfur atom from hydrogen sulfide and the carbon atom from carbon dioxide seems to be more predominant than the hydrogen bond between the hydrogen atoms from hydrogen sulfide and the oxygen atoms from carbon dioxide.