Abstract. The pseudo-equilibrium model proposed by Kent and Eisenberg is used in this study to model the absorption of acid gases (H 2 S and CO 2 ) in aqueous diethanolamine solutions at equilibrium. A wide range of experimental data available in the literature is used to obtain the values of the pseudo-equilibrium constants of the amine reactions. These data cover amine con- centrations from 0.5 to 8 N, temperatures from 273 to 413 K, and acid gas loadings from zero to unity. New correlations for the pseudo-equilibrium constants and solution pH are obtained as a function of temperature, amine concentration, and acid gas loading for loading up to unity. The partial pressure of H 2 S is much better predicted by the new correlation compared to the work of Kent and Eisenberg. The predictions of the partial pressure of CO 2 are improved mainly at high temperatures and amine concentrations. 1 Introduction Alkanolamine aqueous solutions are frequently used for the removal of acid gases, such as CO 2 and H 2 S, from natural, refinery, and synthesis gas streams. Aqueous diethanol- amine (DEA) solutions have been used extensively because of their relatively high capacity for the absorption of acid gases. Many attempts have been made to model the alkanolamine ^ acid gas equilibrium. Most of these attempts postulate that certain reactions occur in the solution and propose a thermodynamic model for the reaction equilibrium. The first of these was made by Atwood et al (1957) who developed a model using the `mean ionic activity coefficient'. They assumed that the ionic strengths of all ionic species are equal. They applied their method to H 2 S in aqueous amines and found it to be suitable only for systems with low ionic strength. Klyamer et al (1973) generalised the model of Atwood et al for H 2 S ^ CO 2 ^ aqueous monoethanolamine solutions. They postulated a set of reactions similar to those given in equations (1)^(7) below and modelled the system accordingly. They used equilibrium constants from the literature and took the mean ionic activity coefficients and the ratio of activity of un-ionised amine to that of water from Atwood et al. Lee et al (1976a,1976b) compared the work of Klyamer et al with their experimental data. They found that this model did not agree with their data over the complete range within their data precision. Danckwerts and McNeil (1967) developed a model for carbon dioxide in aqueous amine solutions according to a proposed set of reactions. Equations for the pseudo- equilibrium constants corrected for the effects of ionic strength, Henry's law that relates acid gas partial pressure to its concentration in the solution, charge balance, and mole balances, were all used in the model. Kent and Eisenberg (1976) attempted this approach with published constants for all the reactions but they were unable to obtain a good match with the published experimental data. Therefore, they modified this model by Modelling and prediction of the solubility of acid gases in diethanolamine solutions High Temperatures ^ High Pressures, 2000, volume 32, pages 261 ^ 270 15 ECTP Proceedings pages 247 ^ 256 Mousa K Abu-Arabiô Department of Chemical Engineering, Jordan University of Science and Technology, Irbid, Jordan; fax: +962 2 295 018; email: mousa@just.edu.jo Shaheen A Al-Muhtaseb Department of Chemical Engineering, University of South Carolina, Columbia, SC 29208, USA Presented at the 15th European Conference on Thermophysical Properties, Wu« rzburg, Germany, 5 ^ 9 September 1999 ô Author to whom correspondence should be addressed, at The Middle East Desalination Research Centre, PO Box 21, Al-Kuwair PC 133, Oman. DOI:10.1068/htwu271