The electrolyte cubic square-well (eCSW) equation of state [Haghtalab, A.; Mazloumi, S. H. Fluid Phase Equilib. 2009, 285, 96-104] is applied to study the solubility of CO2 and H2S in aqueous Nmethyldiethanolamine (MDEA) solutions in the wide range of concentration, pressure, temperature, and acid gas loading. This electrolyte equation of state is composed of the mean spherical approximation (MSA) theory, the Born equation, and a cubic square-well equation of state (SWEOS). Using eCSW EOS, calculations of the vapor-liquid equilibria are performed for the binaries of water-alkanolamine and acid gas-water systems so that the binary molecular interaction parameters, kij in the combing rule of the EOS, are optimized for the CO2-H2O, H2S-H2O, and MDEA-H2O systems. Having the binary interaction parameters between the molecular species, the solubility of CO2 and H2S in the aqueous solution of MDEA is calculated by the use of the simultaneous vapor-liquid equilibria and chemical equilibria algorithm. Also, to improve the calculation of the solubility of the acid gases in the ternary CO2-MDEA-H2O, H2S-MDEA-H2O systems, some of the binary interaction parameters among ionic and molecular species are optimized using the same EOS approach. Influence of the binary interaction parameters on the performance of the SWEOS is investigated using a large number of experimental data points of the acid gas-aqueous MDEA solutions. Moreover, the solubilities of mixed H2S and CO2 gases in aqueous MDEA solution are predicted. The results demonstrate that the eCSW EOS is able to represent the solubility data of both ternary and quaternary aqueous alkanolamine systems with very good accuracy.

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