Modeling CO2 and H2S solubilities in aqueous alkanolamine solutions via an extension of the Cubic-Two-State equation of state

An extension of the Cubic-Two-State (CTS) equation of state was employed to evaluate the fugacities of ionic species in solution. The equation contains three terms relating to the various intermolecular interactions occurring in electrolyte solutions: a short range non-specific term represented by the Soave–Redlich–Kwong equation of state, a specific association term described by the two-state association model and a Debye–Hückel primitive model term for long range ion–ion interactions. The resulting equation, named eCTS, has six adjustable parameters: three for the non-specific part, two for the association term and one for the ionic term. Then, the eCTS EoS was used to describe both chemical and phase equilibria for systems including acid gases (H2S and CO2), alkanolamines (monoethanolamine, diethanolamine and methyldiethanolamine) and water. Measured saturation pressures, liquid densities and binary and ternary vapor–liquid equilibria data were used to estimate the model's pure and binary parameters. Low deviations from experimental data were observed for pure, binary and ternary systems. Finally, the eCTS was employed to predict the behavior of quaternary VLE with mixtures containing CO2–H2S and alkanolamine blends. Except for the prediction of CO2 partial pressures over aqueous alkanolamine blends, the eCTS gave good predictions of experimental quaternary data over a broad range of temperatures and alkanolamine concentrations.

► The CTS EoS was extended by including the Debye–Hückel electrolytic contribution. ► The VLE data of mixtures containing water/alkanolamines were correlated with eCTS. ► The VLE data of mixtures of water–alkanolamines–acid gas were correlated with eCTS. ► The eCTS parameters for protonated alkanolamines and carbamate ions were determined. ► The VLE behavior of quaternary systems were predicted with very good results.