Incorporating Pitzer equations in a new thermodynamic model for the prediction of acid gases solubility in aqueous alkanolamine solutions

In gas sweetening, acid gases such as CO₂ and/or H₂S are usually removed by "chemical" absorption through aqueous amine solutions such as N-Methyldiethanolamine (MDEA) solution. Reliable prediction of equilibrium properties (vapor-liquid equilibrium and species distribution) is needed for a rigorous design of such absorption processes. Information on energy requirements can also be obtained from a reliable vapor-liquid equilibrium thermodynamic model. The currently used methods for correlating/predicting the simultaneous solubility of H₂S and CO₂ in aqueous MDEA solutions require accurate experimental solubility data of single and mixed gases which, in general, confine their applicability in the experimental region. The purpose of this paper is to develop a new theoretical thermodynamic model based on incorporating thermodynamic relationships that correlates the equilibrium and solubility constants to the Gibbs free energy of reaction, leading to an enhanced predictive capability of the model. In this work the Pitzer model is used to account for activity and specific ion interactive forces. This will allow taking into account the effect of the presence of all cations and anions such as thermally stable salts, dissolved organic species and amine degradation products that are usually encountered in absorption units. The suggested model has been verified through comparison with literature data for CO₂ and H₂S absorption. The presented model can be a very powerful tool that could be of significant importance in the design of amine absorption processes as well as in simulations of the operating variables for optimization of gas sweetening systems.