Modeling the solubility of CO2 and H2S in DEA–MDEA alkanolamine solutions using the electrolyte–UNIQUAC model

A thermodynamic model was developed to represent the solubility of CO2 and H2S in single and blend of aqueous solutions of DEA and MDEA. The model accounts for chemical equilibria in the liquid phase and physical equilibria between the liquid and vapor phases. The Soave–Redlich–Kwong (SRK) equation was used to represent the fugacity coefficient in the vapor phase. The liquid phase was treated as an electrolyte solution and activity coefficients were represented by the electrolyte–UNIQUAC equation. The electrolyte–UNIQUAC equation is composed of two parts, one to account for long range interactions and the other to account for short interactions. Contribution from long range ion–ion interactions were represented by a Debye–Huckel formula suitable for mixed solvent, water and alkanolamines. Adjustable parameters of the electrolyte–UNIQUAC equation, representing short range interactions, were determined by orthogonal distance regression technique using a large and wide range of experimental VLE data. Earlier extensions of UNIQUAC to handle electrolyte solutions are ion-specific. In this work, the adjustable parameters were setup to use the ion-pair interaction approach and satisfy both the principle of like-ion repulsion and local electroneutrality. Eight different systems were considered in this work. The model predictions were in excellent agreement with the experimental measurements and the speciation results are useful to be used in mass transfer models to represent the CO2 and H2S absorption process using alkanolamine solutions.

► Vapor–liquid equilibrium by electrolyte–UNIQUAC model.
► Ion-pair approach is used to extend UNIQUAC equation for electrolyte solutions.
► Modeling the solubility of CO2 and H2S in DEA–MDEA solutions.
► Model predictions are in excellent agreement with experimental measurement.
► Concentration and activity coefficient of ionic and molecular species are obtained.