Correlation of CO 2 solubility in N-methyldiethanolamine + piperazine aqueous solutions using extended Debye–Hückel model Mehdi Vahidi a, * , Naser Seyed Matin a , Mahdi Goharrokhi b , Masih Hosseini Jenab a , Majid Abedinzadegan Abdi a,1 , Seyed Hesam Najibi c,d a Research Institute of Petroleum Industry (R.I.P.I.), National Iranian Oil Company (N.I.O.C.), P.O. Box 14665-1998, Tehran, Iran b Department of Chemical Engineering, Tehran South Campus, Islamic Azad University, Tehran, Iran c Department of Petroleum Engineering, Petroleum University of Technology, Ahwaz, Iran d R&D Department, National Iranian Gas Co. (N.I.G.C.), Tehran, Iran article info Article history: Received 4 December 2008 Received in revised form 12 May 2009 Accepted 28 May 2009 Available online 6 June 2009 Keywords: Solubility MDEA Piperazine Carbon dioxide Extended Debye–Hückel abstract Solubility data of CO 2 in aqueous N-methyldiethanolamine (MDEA) solutions of concentration (2.52, 3.36, and 4.28) kmol/m 3 were obtained at temperatures (313, 323, and 343) K and partial pressures ranging from about (30 to 5000) kPa. A thermodynamic model based on extended Debye–Hückel theory was applied to predict and correlate of CO 2 solubility in various aqueous amine solutions. The effect of piper- azine (PZ) concentration on CO 2 loading in MDEA solutions was determined at PZ concentration (0.36, 0.86, and 1.36) kmol/m 3 . Using experimental data in various temperatures the interaction parameters of activity coefficient model for these systems were determined. The results show the model consistency with experimental and literature data and PZ is beneficial to the CO 2 loading. The comparison of results of this study with previous data work shows the wide range of CO 2 loading considered in this work and the better agreement of model with experimental data. The average absolute relative deviation percent (d AAD ) for all data points were 8.11%. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Aqueous alkanolamine solutions are widely used as the solvent for removal of the acid gases (H 2 S, CO 2 ) from natural gas, synthesis gas, and refinery gas streams. The solubility of acid gases in amine aqueous solutions, i.e., (vapor + liquid) equilibrium data, must be known to design gas units. Due to chemical reactions in the (ami- ne + acid gases + H 2 O) systems, a strong deviation from ideality has been observed in the liquid phase. Therefore, the thermodynamic study of these systems is a difficult task. The equilibrium solubility of the acid gases in alkanolamine solutions depends on the temper- ature, concentration of the solution and also acid gas loading. A large body of experimental VLE data has been reported in the liter- ature, in which prediction of these data with an appropriate model is the objective. There are two various approaches for prediction of gas solubility in alkanolamine solutions. First, which presented by Kent and Eisenberg [1], employed a simple model to correlate VLE data for the absorption of acidic gases in amine solutions. They tackled the system’s non-ideality by employing the apparent equi- librium constants and applied the model to monoethanolamine (MEA) and diethanolamine (DEA) systems. This model was very simple, but it cannot be complete for real conditions. Second model uses the activity coefficients and excess Gibbs free energy for cal- culation of solubility of gases [2]. Tontiwachwuthikul et al. [3] used a modified Kent–Eisenberg model for interpretation of the solubility of CO 2 in solutions of 2- amino-2-methyl-1-propanol (AMP) and the results were compared with that of MEA. Posey et al. [4] have used a simple model for pre- diction of (vapor + liquid) equilibrium of acid gas in alkanolamine solutions. Their approach is similar to the Kent–Eisenberg model, but they assumed a single combined reaction instead of the five or six reactions in the amine solution. Their work can be used for the systems containing alkanolamine and sour gases such as H 2 S and CO 2 but the group interaction parameters should be deter- mined. Lal et al. [5] studied the solubility of H 2 S, CO 2 , and their mixtures in an aqueous solution of DEA through employing the simplified model for amine equilibria. They assumed all the chem- ically combined hydrogen sulfide exists only in the form of bisul- fide ions, while carbon dioxide is assumed to exist only as the carbamate. This assumption is valid at low partial pressures. Leung et al. [6] studied (CO 2 + alkanolamine + water) systems. They con- sidered the AMP and DEA solutions with the apparent equilibrium constants that incorporate the activity coefficients of the relevant species involved in the reactions. 0021-9614/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jct.2009.05.017 * Corresponding author. Tel./fax: +98 21 44739716. E-mail addresses: vahidim@ripi.ir, vahidimehdi@yahoo.com (M. Vahidi). 1 Present address: Faculty of Engineering and Applied Science, Memorial University of Newfoundland (MUN), St. John’s, NL, Canada A1B 3X5. J. Chem. Thermodynamics 41 (2009) 1272–1278 Contents lists available at ScienceDirect J. Chem. Thermodynamics journal homepage: www.elsevier.com/locate/jct