Incorporating Pitzer equations in a new thermodynamic model for the prediction of acid gases solubility in aqueous alkanolamine solutions Citation for published version (APA): Alhseinat, E., Mota Martinez, M., Peters, C. J., & Banat, F. (2014). Incorporating Pitzer equations in a new thermodynamic model for the prediction of acid gases solubility in aqueous alkanolamine solutions. Journal of Natural Gas Science and Engineering, 20, 241-249. https://doi.org/10.1016/j.jngse.2014.07.003 DOI: 10.1016/j.jngse.2014.07.003 Document status and date: Published: 01/01/2014 Document Version: Publisher’s PDF, also known as Version of Record (includes final page, issue and volume numbers) Please check the document version of this publication: • A submitted manuscript is the version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. 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Sep. 2021 Journal of Natural Gas Science and Engineering 20 (2014) 241e249 Contents lists available at ScienceDirect Journal of Natural Gas Science and Engineering journal homepage: www.elsevier.com/locate/jngse Incorporating Pitzer equations in a new thermodynamic model for the prediction of acid gases solubility in aqueous alkanolamine solutions * Emad Alhseinat, Maria Mota-Martinez, Cornelis Peters, Fawzi Banat Department of Chemical Engineering, The Petroleum Institute, PO Box 2533, Abu Dhabi, United Arab Emirates article info abstract Article history: In gas sweetening, acid gases such as CO2 and/or H2S are usually removed by “chemical” absorption Received 18 March 2014 through aqueous amine solutions such as N-Methyldiethanolamine (MDEA) solution. Reliable prediction Received in revised form of equilibrium properties (vaporeliquid equilibrium and species distribution) is needed for a rigorous 2 July 2014 design of such absorption processes. Information on energy requirements can also be obtained from a Accepted 3 July 2014 reliable vaporeliquid equilibrium thermodynamic model. The currently used methods for correlating/ Available online 26 July 2014 predicting the simultaneous solubility of H2S and CO2 in aqueous MDEA solutions require accurate experimental solubility data of single and mixed gases which, in general, confine their applicability in the Keywords: Solubility experimental region. The purpose of this paper is to develop a new theoretical thermodynamic model MDEA based on incorporating thermodynamic relationships that correlates the equilibrium and solubility Pitzer constants to the Gibbs free energy of reaction, leading to an enhanced predictive capability of the model. Acid gases In this work the Pitzer model is used to account for activity and specific ion interactive forces. This will Thermodynamic allow taking into account the effect of the presence of all cations and anions such as thermally stable Gibbs salts, dissolved organic species and amine degradation products that are usually encountered in ab- sorption units. The suggested model has been verified through comparison with literature data for CO2 and H2S absorption. The presented model can be a very powerful tool that could be of significant importance in the design of amine absorption processes as well as in simulations of the operating var- iables for optimization of gas sweetening systems. © 2014 Elsevier B.V. All rights reserved. 1. Introduction carbamate formation and protonation of the amines have been considered to be only temperature-dependent. The KE model is an Acid gases such as hydrogen sulfide and carbon dioxide are empirical model and, therefore, it cannot properly predict the sol- major impurities in natural, refinery, and synthesis gases. Chemical ubility of acid gases in aqueous amine solutions in a wide range of absorption by aqueous solutions of amines such as N-Methyl- temperature, pressure and amine concentrations. The NRTL model diethanolamine (MDEA) solution is one of the most effective in- (Chen and Evans, 1986) and the model of Deshmukh and Mather dustrial methods for the removal of these acid gases from gaseous (Deshmukh and Mather, 1981) are based on sound thermodynamic effluents (Speyer et al., 2012, 2009). Availability of accurate acid gas principles. Non-idealities of the solution are taken into consider- solubilities in the solvent is essential for any modeling of absorp- ation by considering long and short range interactions between the tion processes. A number of models such as KenteEisenberg (KE), different species present in the solution. The NRTL model used a Modified KenteEisenberg (M-KE), Deshmukh and Mather model, combination of DebyeeHückel theory and the electrolyte-NRTL Electrolyte-NRTL, Extended DebyeeHückel (E-DH), Pitzer and equation to calculate the activity coefficients. The Deshmukh and LieMather models were proposed to correlate solubility data Mather model is much simpler to handle than the NRTL model. It (Benamor and Aroussi, 2013). Kent and Eisenberg (1976) modeled employs the Guggenheim equation to represent activity co- the solubility of acid gases and their mixtures in MEA and DEA efficients. The Kent and Eisenberg model is the simplest among all aqueous solutions. In this model equilibrium constants of mentioned models. The non-idealities present in the system are lumped together and represented by constant values (Benamor and Aroussi, 2013). Unfortunately, there are still significant limitations * Corresponding author. Tel.: þ971 2 6075099; fax: þ971 2 6075200. in models currently in use to predict acid gas solubilities in amine E-mail addresses: [email protected], [email protected] (F. Banat). solutions. These limitations are mainly caused by the use of http://dx.doi.org/10.1016/j.jngse.2014.07.003 1875-5100/© 2014 Elsevier B.V. All rights reserved. 242 E. Alhseinat et al. / Journal of Natural Gas Science and Engineering 20 (2014) 241e249 empirical approaches, in which crude assumptions have been À ⇔ þ þ 2À HCO3 H CO3 (R5) introduced. In the best scenario these models are mainly useful to KR5 correlate experimental data. To enhance the reliability of solubility predictions of acid gases þ þ MDEAH ⇔ H þ MDEA (R6) in aqueous amine solutions, the following can be done: Firstly, KR6 implementation of sound thermodynamic relations to account for e the equilibrium constants rather than just correlating the exper- The chemical reaction equilibrium constants (KR1 KR6) will be imental data at specific conditions. Application of thermody- calculated from the Gibbs free energy of reaction. Then the effect of namically reliable equations will account for the effect of temperature and pressure will be taken into account as will be temperature and pressure on equilibrium constants and solubil- explained in detail later. ities. Secondly, the activity coefficients of the species present in The condition for chemical equilibrium for a chemical reaction R ¼ … the aqueous phase and the fugacity coefficients of the species ( 1, , 6) is: present in the vapor phase must be considered. The Y e e ð ; Þ¼ Vi;R Debye Hückel and the extended Debye Hückel theory are the KR T P ai (1) basis of a number of successful semi-empirical equations for a i variety of thermodynamic properties (Loehe and Donohue, 1997). Pitzer and coworkers established very widely applicable equa- where KR (T, P) is the chemical reaction equilibrium constant for tions for calculating the activity coefficients of individual species reaction R, ai is the thermodynamic activity of species i in the liquid fi in aqueous media at high concentrations. In his publications phase, and ni,R is the stoichiometric coef cient of species i in reac- (Pitzer and Mayorga, 1973; Pitzer, 1991) the origin of these tion R. equations and their application are discussed in some detail. To Applying mass conservation equations on carbon dioxide, fi consider the deviation of real gas phase behavior from the ideal hydrogen sul de, MDEA, and water result in: gas state, the Virial (George Hayden and O'Connell, 1975), PR À ¼ þ þ 2À mCO2 mCO2 mHCO m (2) (Peng and Robinson, 1976)orSRK(Soave, 1972) equations of state 3 CO3 (EOS) can be used to calculate the fugacity-coefficients of the components in the gas phase. À m ¼ m þ m þ m 2À (3) In this work, a new theoretical thermodynamic model will be H2S H2S HS S developed based on incorporating theoretical thermodynamic re- lationships that correlates the equilibrium and solubility constants ¼ þ þ to the Gibbs free energy of reactions. For the first time, the effect of mMDEA mMDEA mMDEAH (4) temperature and pressure will be incorporated through funda- mental thermodynamic equations. The Pitzer model and the PR À À fi mw ¼ mw þ mHCO þ m 2À þ mOH (5) EOS will be used to account for activity coef cients and fugacity 3 CO3 coefficients, respectively.