Microsoft Word - 476hernandez.docx CHEMICAL ENGINEERING TRANSACTIONS VOL. 43, 2015 A publication of The Italian Association of Chemical Engineering Online at www.aidic.it/cet Chief Editors: Sauro Pierucci, Jiří J. Klemeš Copyright © 2015, AIDIC Servizi S.r.l., ISBN 978-88-95608-34-1; ISSN 2283-9216 Vapor-Liquid Equilibrium and Enthalpy of Absorption of the CO2-MEA-H2O System Stefano Langé*, Stefania Moioli, Laura A. Pellegrini Dipartimento di Chimica, Materiali e Ingegneria Chimica “G. Natta”, Politecnico di Milano, Piazza Leonardo da Vinci 32, I- 20133 Milano, Italy stefano.lange@polimi.it Amine scrubbing is widely used to remove acid gases, mainly CO2 and H2S, from various process gaseous streams, because of the enhancement due to chemical reactions occurring in the liquid phase. For designing the acid gas removal plant, information on the heats of absorption of carbon dioxide in amine aqueous solutions is of paramount importance. Due to the exothermicity of reactions, a temperature increase occurs in the absorber, affecting the equilibrium and the amount of absorbed acid gas, while in the regenerator the energy requirement at the reboiler strongly depends on the heat of desorption. This work is focused on the removal of CO2, a powerful greenhouse gas, from flue gases by means of aqueous solutions of monoethanolamine (MEA), an industrial technology which is part of carbon capture and sequestration, to limit global warming. The aim is the study of the CO2-MEA-water system, in particular as for thermodynamics. The Electrolyte- NRTL model is used for the description of Vapor-Liquid Equilibrium as well as for the computation of the heat of absorption, considering the influence exerted by thermodynamic parameters. The obtained parameters, checked against experimental data of VLE and of heat of absorption, can be implemented in the commercial software ASPEN Plus® and employed for simulations of the amine scrubbing scheme. 1. Introduction In the last years, increasing attention is being paid to the problem of global warming, due to the presence of greenhouse gases produced by anthropogenic activities (Damartzis et al., 2013). In order to reduce the amount of these gases in the atmosphere, in particular of carbon dioxide, processes for removal have been developed (Neveux et al., 2013; Puccini et al., 2013). Industrially, absorption by means of alkanolamine solutions is often preferred to other removal systems and different studies can be found in literature (Kohl and Nielsen, 1997). Chemical absorption of carbon dioxide in aqueous solutions is controlled and enhanced by chemical reactions that take place in the liquid phase (Molina and Bouallou, 2013). Monoethanolamine is one of the most used amine in industrial acid gas removal, in particular when dealing with emissions from power generation. MEA is characterized by high reactivity, low solvent cost, low molecular weight, reasonable thermal stability and low thermal degradation rate (McCann et al., 2008). However, some fundamental disadvantages of MEA include the fact that loaded solutions are corrosive and that the enthalpy of reaction is high, causing a high energy requirement at the reboiler of the solvent regenerator. For this reason, regeneration is the most expensive section of the plant. Information on heats of absorption for acid gases in amine solutions is of prime importance for designing unit operations of acid gas removal, so a reliable prediction of this quantity is desirable. The aim of the work is obtaining a model which properly describes both the vapor-liquid equilibrium and the heat of absorption of the system composed of CO2, MEA and water for several amine concentrations and for a wide range of temperatures and pressures, in order to use this tool to carry on the research in the reduction of the energy cost of this acid gas removal process. DOI: 10.3303/CET1543330 Please cite this article as: Lange S., Moioli S., Pellegrini L., 2015, Vapor-liquid equilibrium and enthalpy of absorption of the co2-mea-h2o system, Chemical Engineering Transactions, 43, 1975-1980 DOI: 10.3303/CET1543330 DOI: 10.3303/CET1543330 Please cite this article as: Lange S., Moioli S., Pellegrini L., 2015, Vapor-liquid equilibrium and enthalpy of absorption of the co2-mea-h2o system, Chemical Engineering Transactions, 43, 1975-1980 DOI: 10.3303/CET1543330 1975 In this work, a new regression of significant thermodynamic parameters has been performed, on the basis of available vapor-liquid equilibrium data. The obtained parameters have been used to evaluate the enthalpy of absorption of carbon dioxide. Results show that the method proposed in this paper allows a representation of both VLE and ΔHabs in good agreement with the experimental evidence. 2. Thermodynamic framework VLE in amine solvents has been thoroughly modeled in the past (Austgen et al., 1989; Freguia, 2002; Liu et al., 1999; Zhang et al., 2011), based on VLE measurements and data. However, not all the authors have considered the influence of VLE parameters on the representation of the enthalpy of absorption of carbon dioxide in the solvent. Indeed, while many data on gas solubility for wide ranges of temperature, pressure and amine concentration are available in literature (Jones et al., 1959; Jou et al., 1995; Lee et al., 1974; Lee et al., 1976; Ma'mun et al., 2005; Maddox et al., 1987; Shen and Li, 1992; Wagner et al., 2013), very few thermal data, taken from direct measurements, can be found, at 298 K (Arcis et al., 2011) and at higher temperatures (Carson et al., 2000; Mathonat et al., 1998). From a thermodynamic point of view (Pellegrini et al., 2012), the system composed of CO2, amine and H2O is strongly non-ideal and needs to be properly represented (Langé et al., 2013; Moioli and Pellegrini, 2015b; Pellegrini et al., 2013): efforts to obtain a good description have been made firstly by Kent and Eisenberg (Kent and Eisenberg, 1976), who proposed a simplified model based on chemical reactions. Later, Chen and coworkers (Chen et al., 1979; Chen et al., 1982; Chen and Evans, 1986; Mock et al., 1986) developed a more rigorous model, taking into account the local composition of species, influenced by the presence of ions. Their model, Electolyte-NRTL, is widely used to model amine systems (Pellegrini et al., 2011) and has been taken as a basis for further developments, including the estimation of the vapor phase with different methods, as the PC-SAFT equation (Zhang et al., 2011). Absorption of carbon dioxide in amine solutions is enhanced by chemical reactions in the liquid phase (Moioli and Pellegrini, 2015a; Moioli et al., 2013). In particular, when the solvent is an aqueous solution of MEA, the formation of a carbammate is involved (Moioli and Pellegrini, 2013). The set of chemical equilibrium reactions can be found in previous works on MEA scrubbing (Moioli et al., 2014). The same references report also the expression used for Henry’s constant of carbon dioxide in water, while for vapor pressures of water and MEA the Extended Antoine equation (AspenTech, 2014) applies, with ASPEN Plus® default parameters. The carbammate formation is associated with a high enthalpy of reaction (McCann et al., 2008), so reversing this reaction in the stripper requires large quantities of energy, making the process energy intensive. In the aqueous phase CO2, MEA and H2O reacts and, by dissociation or protonation, produce ions. The description of the VLE system, then, is complicated by the presence of dissociated species, which are not interested by migration to the vapor phase, that is usually described by means of an Equation of State. The Electrolyte- NRTL model takes into account the presence of ions: it describes the strongly non-ideality of the system by calculating the excess Gibbs free energy as a sum of several contributions, due to short-range interactions, mainly due to molecule-molecule and/or ion-molecule interactions, and long-range interactions among ionic species. The nature of the force can be different according to the strength of ion and the polarity of the molecule. Then, the excess Gibbs free energy is calculated as: RT g RT g RT g RT g ExNRTL Ex Born Ex PDH E x ++= (1) Long-range electrostatic interactions are the sum of Pitzer, Debye and Hückel contribution and Born ion- reference-state-transfer from infinite dilution in mixed solvent to infinite dilution in water (Austgen, 1989). The contribution of short range interactions is represented using NRTL theory (Renon and Prausnitz, 1968) based on the local composition of the system, considering the local composition around the single molecule different from the mean composition of liquid mixture. In particular, interactions are modeled assuming local electroneutrality and repulsion of ions of the same charge. The NRTL contribution is described as function of non-random parameters and binary interaction parameters (Eq.(2)) between components of the system. These parameters are temperature dependent and have to be calculated from experimental vapor-liquid equilibrium data. Interactions can be between two different molecules, a molecule and a couple of anion and cation or between couples of cations and anions and are not symmetric. Only binary interaction parameters are required for the description of complex systems. Values of these parameters can be regressed from experimental data or found in literature. When the concentration of ionic species is high, ternary parameters may be required, but for systems of weak electrolytes binary parameters are enough to describe the system with good results. 1976 , , , , j = molec,ca,ac k = molec,ca,ac j k j k j k k j B A T τ τ= + ≠ (2) The calculation of enthalpies derives from the calculation of activity coefficients, since the equation used for the electrolyte-NRTL enthalpy is (AspenTech, 2014): * * *, *,l E m w w s s k k m s k H x H x H x H H∞= + + +  (3) where * wH is the pure water molar enthalpy, calculated from the Ideal Gas model and the ASME Steam Table; *,l sH is the enthalpy contribution from a non-water solvent, as MEA; kH ∞ is the aqueous infinite enthalpy calculated for any ion or molecular solute; *,E mH is the molar excess enthalpy, calculated from the activity coefficient model. Moreover, Zhang et al. in 2011 showed in their work that the enthalpy of absorption is dominated by the heat generated by MEAH+ ion and MEACOO- ion dissociation. The amount of the total heat released at chemical equilibrium is then strongly related to the amount of each species reacted or produced, which are influenced by the adopted thermodynamic model. A reliable thermodynamic tool, then, is needed not only for the description of the equilibrium between the vapor phase and the liquid one, but also for the determination of the amount of heat released by chemical reactions, and so the overall heat of absorption of carbon dioxide in the solvent. 3. Regression of VLE parameters In the first part of this work regressions have been performed in order to find values for H2O-(MEAH+- MEACOO-), (MEAH+-MEACOO-)- H2O, H2O-(MEAH+-HCO3-), (MEAH+-HCO3-)- H2O adaptive parameter Aj,k and Bj,k for binary interaction parameters in the NRTL contribution to the liquid phase activity coefficient. Experimental TPx VLE data have been taken from the work by Maddox et al. (1987), Shen and Li (1992), Lee et al. (1974) and Jou et al. (1995) and have been used to regress parameters with the Data Regression System in Aspen Plus®, using the Maximum Likelyhood method with the Britt-Luecke algorithm (Britt and Luecke, 1973). Values for standard deviation on measured variables are set equal to Aspen Plus® default: 0.1 [K] for temperature, 0.1% for pressure, 0.1% for liquid phase composition, 1% for vapor phase composition. When data in literature are not completely reported (yMEA), deviations of 100 % have been adopted for, taking into account the great uncertainty in the value of the calculated missing variable (Freguia, 2002). a) b) Figure 1: Parity plots of molar fraction of carbon dioxide a) in the liquid phase and b) in the vapor phase resulted by using the parameters obtained in this work 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.00 0.05 0.10 0.15 0.20 x C O 2 c a lc xCO2 exp 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0.0 0.2 0.4 0.6 0.8 1.0 y C O 2 c a lc yCO2 exp 1977 Results obtained from the model are checked against experimental data for carbon dioxide-water-MEA systems and compared with other Electrolyte-NRTL studies found in literature for the same system: Austgen (1989), Liu et al. (1999), Freguia (2002), Aspen Plus® (2010), Zhang and Chen (2011). While the first three works use the same approach used here, the fourth work is a bit different, since the authors (Zhang and Chen, 2011) use the PC-SAFT equation of state to the vapor phase properties. Parity plots with results obtained with the proposed parameters are shown in Figure 1. The reliability of the model is estimated using the average value percent of the absolute values of relative errors, calculated according to Eq. (4). 100% 1 exp, exp,, ⋅ − =  NP i icalci i z zz AAD (4) 4. Model validation through heat of absorption In literature very few works on direct measurements of enthalpy of absorption can be found (Arcis et al., 2011; Carson et al., 2000; Mathonat et al., 1998), since many other authors derive values from experimental results of VLE data and do not directly measure them by a flow calorimeter or other ad hoc instrumentations. The commercial simulator ASPEN Plus® has been used as a tool for the evaluation of the enthalpy of absorption of carbon dioxide in the amine solution. All its default parameters can be changed according to the user’s preferences and it is provided with the Electrolyte-NRTL. A Separator unit with two feeds (a CO2 stream and a solvent stream) and two outputs (a gas stream and a liquid stream) has been simulated, with the aim of best reproducing the experimental equipment used in the literature measurements. Temperature and pressure of the two fed streams and of the separation unit, as well as the molar flow rate and the composition of the CO2 and the solvent stream are given as an input. Results are the amount of heat released during the operation, which consists in the heat of absorption of carbon dioxide in the solvent, and the characteristics of the streams coming out of the separation unit. From these results, the amount of carbon dioxide absorbed in the solvent and the heat of absorption per mole of amine or per mole of CO2 can be calculated and compared to the experimental data. Simulations with ASPEN Plus® default parameters, with literature models (Austgen, 1989; Freguia, 2002; Liu et al., 1999; Zhang et al., 2011) and with the proposed parameters have been performed and the obtained results have been compared to experimental values. 5. Results and discussion Figure 2 shows a comparison of results with experimental data as parity plots. Being evaluated tests a lot, to better clarify results a detailed plot (for a range of loading usually found in industrial gas removal plants) has been added to the overall one, with the aim of allowing the reader to clearly compare the obtained results with the experimental evidence (Figure 3). a) b) Figure 2: parity plot of results obtained with different methods of simulation of data by a) Mathonat et al. (1998) and b) Arcis et al.(2011) -200 -175 -150 -125 -100 -75 -50 -25 0 -200 -175 -150 -125 -100 -75 -50 -25 0 e n th a lp y o f s o lu b il it y [ k J /m o l C O 2 ] exp. enthalpy of solubility [kJ/mol CO2] ASPEN Plus® default parameters literature model (Austgen, 1989) literature model (Zhang et al., 2011) literature model (Freguia, 2002) literature model (Liu et al., 1999) proposed parameters y = x 0 20 40 60 80 100 120 140 160 180 200 0 50 100 150 200 e n th a lp y o f s o lu b il it y [ k J /m o l] exp. enthalpy of solubility [kJ/mol] ASPEN Plus® default parameters literature model (Austgen, 1989) literature model (Zhang et al., 2011) literature model (Freguia, 2002) literature model (Liu et al., 1999) proposed parameters y = x 1978 Figure 3: detailed results obtained with different methods of simulation of data by Mathonat et al. (1998) for loadings < 0.5. The obtained parameters, regressed on the vapor-liquid equilibrium measurements, well reproduce the available experimental data of heat of absorption. The model could be further improved if a simultaneous regression of both VLE and ΔHabs data could be performed, by using a tool external to ASPEN Plus®, which does not allow the user to consider ΔHabs in the objective function of the regression. 6. Conclusions Absorption by means of amine solution is widely industrially used to remove CO2 to accomplish environmental regulations. Information on heats of absorption of CO2 in amine solutions is of prime importance for designing unit operations because, due to the high enthalpy of reaction, the energy requirement at the reboiler of the regeneration section is high. In this work a method able to describe both the VLE and the ΔHabs of the CO2-MEA-H2O system has been used, based on thermodynamic parameters obtained by regression of experimental VLE data. The obtained results are considered in good agreement with the available literature data, allowing a reliable description of the absorption system. References Arcis, H., Ballerat-Busserolles, K., Rodier, L., Coxam, J.Y., 2011, Enthalpy of Solution of Carbon Dioxide in Aqueous Solutions of Monoethanolamine at Temperatures of 322.5 K and 372.9 K and Pressures up to 5 MPa, J. Chem. Eng. Data 56, 3351-3362. AspenTech, 2014, ASPEN Plus® Guidelines. AspenTech, Burlington, MA. Austgen, D.M., Rochelle, G.T., Peng, X., Chen, C.C., 1989, Model of Vapor Liquid Equilibria for Aqueous Acid Gas Alkanolamine Systems Using the Electrolyte Nrtl Equation, Ind. Eng. Chem. Res. 28, 1060-1073. Austgen, D.M.A., 1989, Model of Vapor-Liquid Equilibria for Acid Gas-Alkanolamine–H2O Systems. University of Texas, Austin, Texas. Britt, H.I., Luecke, R.H., 1973, The Estimation of Parameters in Nonlinear, Implicit Models, Technometrics 15, 233-247. Carson, J.K., Marsh, K.N., Mather, A.E., 2000, Enthalpy of solution of carbon dioxide in (water + monoethanolamine, or diethanolamine, or N-methyldiethanolamine) and (water + monoethanolamine + N- methyldiethanolamine) atT = 298.15 K, J. Chem. Thermodyn. 32, 1285-1296. Chen, C.C., Britt, H.I., Boston, J.F., Evans, L.B., 1979, Extension and application of the Pitzer equation for vapor-liquid equilibrium of aqueous electrolyte systems with molecular solutes, AIChE J. 25, 820-831. Chen, C.C., Britt, H.I., Boston, J.F., Evans, L.B., 1982, Local composition model for excess Gibbs energy of electrolyte systems. Part I: single solvent, single completely dissociated electrolyte systems, AIChE J. 28, 588-596. Chen, C.C., Evans, L.B., 1986, A local composition model for the excess Gibbs energy of aqueous electrolyte systems, AIChE J. 32, 444-454. Damartzis, T., Papadopoulos, A.I., Seferlis, P., 2013, Generalized framework for the optimal design of solvent- based post-combustion CO2 capture flowsheets, Chem. Eng. Trans., pp. 1177-1182. Freguia, S., 2002, Modeling of CO2 Removal from Flue Gase with Monoethanolamine. The University of Texas, Austin, Texas. -140 -120 -100 -80 -60 -40 -20 0 0.0 0.1 0.2 0.3 0.4 0.5 e n th a lp y o f s o lu b il it y [ k J /m o l C O 2 ] mol CO2 IN / mol solv proposed parameters exp. data (Arcis et al., 2011) 1979 Jones, J.H., Froning, H.R., Claytor, E.E., 1959, Solubility of Acidic Gases in Aqueous Monoethanolamine, J. Chem. Eng. Data 4, 85-92. Jou, F.Y., Mather, A.E., Otto, F.D., 1995, The Solubility of CO2 in a 30-Mass-Percent Monoethanolamine Solution, Can. J. Chem. Eng. 73, 140-147. Kent, R.L., Eisenberg, B., 1976, Better data for amine treating, Hydrocarb. Process. 55, 87-90. Kohl, A.L., Nielsen, R., 1997, Gas Purification, 5th ed. Gulf Publishing Company, Book Division, Houston, Texas, USA. Langé, S., Pellegrini, L.A., Moioli, S., Picutti, B., Vergani, P., 2013, Influence of Gas Impurities on Thermodynamics of Amine Solutions. 2. Mercaptans, Ind. Eng. Chem. Res. 52, 2025-2031. Lee, J.I., Otto, F.D., Mather, A.E., 1974, The solubility of H2S and CO2 in aqueous monoethanolamine solutions, Can. J. Chem. Eng. 52, 803-805. Lee, J.I., Otto, F.D., Mather, A.E., 1976, Equilibrium Between Carbon Dioxide and aqueous Monoethanolamine Solutions, J. Appl. Chem. Biotech. 26, 541-549. Liu, Y.D., Zhang, L.Z., Watanasiri, S., 1999, Representing vapor-liquid equilibrium for an aqueous MEA-CO2 system using the electrolyte nonrandom-two-liquid model, Ind. Eng. Chem. Res. 38, 2080-2090. Ma'mun, S., Nilsen, R., Svendsen, H.F., Juliussen, O., 2005, Solubility of carbon dioxide in 30 mass % monoethanolamine and 50 mass % methyldiethanolamine solutions, J. Chem. Eng. Data 50, 630-634. Maddox, R.N., Bhairi, A.H., Diers, J.R., Thomas, P.A., 1987, Equilibrium Solubility of Carbon Dioxide or Hydrogen Sulfide in Aqueous Solutiions of Monoethanolamine, Diglycolamine, Diethanolamine and Methyldiethanolamine, Tulsa, Oklahoma. Mathonat, C., Majer, V., Mather, A.E., Grolier, J.P.E., 1998, Use of flow calorimetry for determining enthalpies of absorption and the solubility of CO2 in aqueous monoethanolamine solutions, Ind. Eng. Chem. Res. 37, 4136-4141. McCann, N., Maeder, M., Attalla, M., 2008, Simulation of Enthalpy and Capacity of CO2 Absorption by Aqueous Amine Systems, Ind. Eng. Chem. Res. 47, 2002-2009. Mock, B., Evans, L.B., Chen, C.C., 1986, Thermodynamic Representation of Phase Equilibria of Mixed- Solvent Electrolyte Systems, AIChE J. 32, 1655-1664. Moioli, S., Pellegrini, L.A., 2013, Regeneration Section of CO2 Capture Plant by MEA Scrubbing with a Rate- Based Model, Chem. Eng. Trans. 32, 1849-1854. Moioli, S., Pellegrini, L.A., 2015a, Improved rate-based modeling of the process of CO2 capture with PZ solution, Chem. Eng. Res. Des. 93, 611-620. Moioli, S., Pellegrini, L.A., 2015b, Physical properties of PZ solution used as a solvent for CO2 removal, Chem. Eng. Res. Des. 93, 720-726. Moioli, S., Pellegrini, L.A., Gamba, S., Li, B., 2014, Improved rate-based modeling of carbon dioxide absorption with aqueous monoethanolamine solution, Front. Chem. Sci. Eng. 8, 123-131. Moioli, S., Pellegrini, L.A., Picutti, B., Vergani, P., 2013, Improved rate-based modeling of H2S and CO2 removal by MDEA scrubbing, Ind. Eng. Chem. Res. 52, 2056-2065. Molina, C.T., Bouallou, C., 2013, Kinetics Study and Simulation of CO2 Absorption into Mixed Aqueous Solutions of Methyldiethanolamine and Diethanolamine, Chem. Eng. Trans. 35, 319-324. Neveux, T., Le Moullec, Y., Corriou, J.P., Favre, E., 2013, Energy performance of CO2 capture processes: interaction between process design and solvent, Chem. Eng. Trans. 35, 337-342. Pellegrini, L.A., Langé, S., Moioli, S., Picutti, B., Vergani, P., 2013, Influence of Gas Impurities on Thermodynamics of Amine Solutions. 1. Aromatics, Ind. Eng. Chem. Res. 52, 2018-2024. Pellegrini, L.A., Moioli, S., Gamba, S., Ceragioli, P., 2012, Prediction of vapor–liquid equilibrium for reservoir mixtures with cubic equations of state: binary interaction parameters for acidic gases, Fluid Phase Equilib. 326, 45-49. Pellegrini, L.A., Moioli, S., Picutti, B., Vergani, P., Gamba, S., 2011, Design of an Acidic Natural Gas Purification Plant by means of a Process Simulator, Chem. Eng. Trans. 24, 271-276. Puccini, M., Seggiani, M., Vitolo, S., 2013, Lithium silicate pellets for CO2 capture at high temperature, Chem. Eng. Trans., pp. 373-378. Renon, H., Prausnitz, J.M., 1968, Local compositions in thermodynamic excess functions for liquid mixtures, AIChE J. 14, 135-144. Shen, K.P., Li, M.H., 1992, Solubility of Carbon-Dioxide in Aqueous Mixtures of Monoethanolamine with Methyldiethanolamine, J. Chem. Eng. Data 37, 96-100. Wagner, M., von Harbou, I., Kim, J., Ermatchkova, I., Maurer, G., Hasse, H., 2013, Solubility of Carbon Dioxide in Aqueous Solutions of Monoethanolamine in the Low and High Gas Loading Regions, J. Chem. Eng. Data 58, 883-895. Zhang, Y., Que, H.L., Chen, C.C., 2011, Thermodynamic modeling for CO2 absorption in aqueous MEA solution with electrolyte NRTL model, Fluid Phase Equilib. 311, 67-75. 1980