Research Article Rapid Prediction of CO 2 Solubility in Aqueous Solutions of Diethanolamine and Methyldiethanolamine In the present work, a simple-to-use correlation is developed to predict the solu- bility of CO 2 in aqueous solutions of DEA and MDEA as a function of the re- duced partial pressure and temperature. Using the interaction parameters gener- ated, the model is applied to correlate the CO 2 loading in different amine solu- tions. The results from the proposed correlation have been compared with the reported experimental data and it was found that there is a good agreement be- tween the observed data and the model predictions over a wide range of operating conditions in aqueous solutions of both diethanolamine (DEA) and methyl- diethanolamine (MDEA). Keywords: Carbon dioxide, Gas sweetening, Modeling, Solubility Received: July 22, 2007; revised: December 03, 2007; accepted: December 05, 2007 DOI: 10.1002/ceat.200700271 1 Introduction Some industrial processes, such as natural gas purification, re- quire the removal of carbon dioxide (CO 2 ). Alkanolamines are among several solvents that have been investigated and current research is focused on designing a chemically stable, less corro- sive solvent with fast reaction rates and low heats of absorption to minimize the energy requirements for regeneration of the solvent. These chemicals are broadly classified into primary, secondary and tertiary amines. A new class of amines, known as sterically hindered amines, has been introduced in recent years. Both primary and secondary amines generally exhibit low CO 2 loadings but possess a high rate of absorption. In contrast, tertiary amines show the opposite behavior. Recently, there has been an increased interest in the use of diethanola- mine (DEA) and methyldiethanolamine (MDEA) solvents in gas treatment processes. The industrial removal process requires knowledge of two important thermodynamic values, i.e., the limit of the solubili- ty and the enthalpy of absorption of the gas in the solvent. However, the technologies generally employed to remove these impurities are very often based on their absorption in chemical and/or physical solvents [1]. The absorption of CO 2 in aque- ous solutions of alkanoalamine couples physical absorption with chemical reactions, where both kinetics and thermody- namic equilibria may play important roles in determining the ultimate gas loading [2]. Several models are available to analyze the solubility of CO 2 in aqueous solutions of alkanolamines and to correlate the equilibrium CO 2 loading. Among the models that have been widely used, is the electrolyte-NRTL model of Chen and Evans [3], the model of Deshmukh and Mather [4] and the Kent and Eisenberg model [5]. The former model is the simplest among these models where the non-idealities that are present in the system are lumped together into the equilibrium constant or K values. This relatively simple model correlates fairly well with the experimental data. Haji-Sulaiman et al. [6] used this model to analyze the solubility data of CO 2 in aqueous solutions of DEA, MDEA, and their mixtures. However, they modified the expressions for the equilibrium constants for the protonation of the amine and carbamate formation for DEA as originally proposed by Kent and Eisenberg, to include the dependency on the free gas concentration in the solution and the amine concentration as evident from their experimental observations. Hu and Chakma [7] used this model to analyze solubility data of CO 2 and H 2 S in AMP solutions, where no carbamate ions exist in the system. Similarly, Kritpiphat and Tontiwachwuthi- kul [8] also fitted their experimental data on the equilibrium of CO 2 in AMP using the Kent and Eisenberg model. Both the Chen and Evans and Deshmukh and Mather mod- els were developed based on sound thermodynamic principles. Non-idealities of solutions are taken into consideration by considering long- and short-range interactions between the different species present. The Chen and Evans model used a combination of Debye-Huckel theory and the electrolyte- NRTL equation to calculate the activity coefficients. This © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim http://www.cet-journal.com Alireza Bahadori 1 Hari B. Vuthaluru 1 Saeid Mokhatab 2 1 Department of Chemical Engineering, Curtin University of Technology, Perth, Australia. 2 Process Technology Department, Tehran Raymand Consulting Engineers, Tehran, Iran. – Correspondence: Prof. A. Bahadori (Alireza.bahadori@student.curtin.- edu.au), Department of Chemical Engineering, Curtin University of Technology, GPO Box 1987, Perth, WA 6845, Australia. Chem. Eng. Technol. 2008, 31, No. 2, 245–248 245