Available online at www.sciencedirect.com ScienceDirect Energy Procedia 63 (2014 ) 1106 1114 GHGT-12 Temperature Dependent Enthalpy of CO 2 Absorption for Amines and Amino acids from theoretical calculations at Infinite dilution. Mayuri Gupta a, Hallvard F. Svendsen a a Department of Chemical Engineering, Norwegian University of Science and Technology, Sem Sælandsvei 4, Trondheim, 7491, Norway. Abstract The thermodynamics of carbamate formation from the reaction of amines or amino acids with CO 2 in post combustion CO 2 capture processes is studied in this work using computational chemistry calculations. Density functional theoretical calculations are coupled with the implicit and explicit solvation shell model presented by da Silva et al. [da Silva, E. F.; Svendsen, H. F.; Merz, K. M. J Phys Chem A 2009, 113, 6404] to calculate equilibrium constants for the reaction. These calculated temperature dependent equilibrium constants are used to derive temperature dependent heats of absorption for the carbamate formation reaction of amines and amino acids using the Gibbs Helmholtz equation. The computationally derived temperature dependent equilibrium constants and heats of absorption are compared against experimental results. 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license 2013 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and peer-review under responsibility of GHGT. Peer-review under responsibility of the Organizing Committee of GHGT-12 Keywords: Heat of absorption, equilibrium constants, Temperature, Amine, Amino Acid, PCM, SM8T, DFT. CO 2 capture. 1. Introduction Post combustion CO 2 capture (PCC) is one of the most mature technologies for removal of CO 2 from flue gas and the usage of amines and alkanolamines is one of the most researched and employed approaches. 1-4 Among all the solvents studied and used for PCC, monoethanolamine (MEA) is most commonly used due to its high selectivity for acid gases in flue gas, high solubility in water, reasonable volatility, high capacity and moderate heat of absorption to capture CO 2. Solvent degradation and corrosion are some of the drawbacks for using MEA as a solvent. 1,5 Several other solvents have been studied and in comparison to MEA some of the amine, alkanolamines and amino acids have shown better capacity and absorption rates. In PCC a solvent with a primary or secondary amino group functionality will react with CO 2 in the flue gas to form carbamate. 6,7 Large amounts of literature is available on the 1876-6102 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the Organizing Committee of GHGT-12 doi:10.1016/j.egypro.2014.11.119
Mayuri Gupta and Hallvard F. Svendsen / Energy Procedia 63 ( 2014 ) 1106 1114 1107 reaction chemistry of amines, alkanolamines and amino acids as solvent for CO 2 capture processes which highlight the importance of understanding the reaction chemistry and thermodynamics of the process to make the process economically viable. 8-11 Enthalpy of absorption is very important thermodynamic parameter in CO 2 removal using amine solvent solutions in post combustion temperature swing processes. The temperature dependency of the heat of absorption for such processes can be calculated from theoretical equilibrium constants 12 for each of the reactions involved in the absorption sequence. In the present work, the carbamate formation reaction of amines and amino acids with CO 2 was studied and temperature dependent heats of absorption for this reaction are given. 2. Method In understanding the behaviour of a PCC solvent, it is very important to understand the reaction chemistry of the system. Amines, alkanolamines and amino acids react with CO 2 in the solution to form bicarbonate or carbamate. 13 Primary and secondary amines have more tendencies to form carbamate whereas tertiary amines or alkanolamines form bicarbonate only. To understand the thermodynamics of the system it is very important to study the temperature dependency of the reaction enthalpy of the various reactions involved. Chemical absorption of CO 2 in amine solutions involves the following reactions. Similar reaction chemistry is observed in amino acid solvents. (1) (2) (3) (4) (5) Here reaction 1 corresponds to deprotonation of amine, reaction 2 corresponds to carbamate formation; reaction 3 is bicarbonate formation or dissociation of CO 2, reaction 4 refers to ionization of water and reaction 5 is dissociation of bicarbonate ion. In the present work, the temperature dependency of the equilibrium constant for the carbamate formation reaction (2), is calculated using DFT calculations coupled with the implicit and explicit solvation shell model 14. Equilibrium constants are calculated from overall Gibbs free energy obtained from adding gaseous and solution phase free energy using the following equation (6) The calculated free energy of solution at 298 K is shifted to the experimental free energy of solution for the same temperature and the difference between calculated and experimental free energy of solution is added at each temperature and termed as correction factor 2. More details of this correction factor can be found in Gupta et al. 12 These temperature dependent equilibrium constants for reaction 2 are added into the Gibbs Helmholtz equation 15 to obtain the temperature dependent enthalpy of absorption for the reaction as follows
1108 Mayuri Gupta and Hallvard F. Svendsen / Energy Procedia 63 ( 2014 ) 1106 1114 (7) According to Weiland et. al. the temperature dependency of the equilibrium constants can be represented as below 16 (8) According to equation 7, after differentiating equation 8 w.r.t temperature following equation 7, we obtain the temperature dependent enthalpy for carbamate formation, reaction 3. 3. Computational Details In this work, initial conformer search is carried out at HF/6-31+G* level and most stable conformer at this level were submitted at B3LYP/6-31+G(d, p) level. The conformer search is carried out in both gas phase and aq. Phase to ensure having minimum energy structures in both phases. Most stable conformer obtained in both gas phase and aqeous phase is submitted for optimization at B3LYP/6-311++G (d, p ) level of theory. Frequency calculations were also carried out, and absence of any imaginary frequency is confirmed. Conformer search is carried out in Spartan 08 17 software. Aqueous phase calculations are done on most stable conformer structure obtained in aq. phase as described above. Most stable conformer in aq. phase is submitted for optimization using SM8T model in Gamessplus 18. Aqueous phase geometries are also fully optimized at B3LYP level of theory using 6-311++G (d, p) functional in Spartan 08 for SM8T calculations using solvent model SM8. Single point energy calculations on the optimized geometry of the molecule obtained were used to study the solvation effects with the SM8T solvation models. SM8T calculations 19 were done in for the temperature range 273.15-373 K using the optimized structure obtained earlier. All SM8T calculations were done using Density Functional Theory at SM8T/B3LYP/6-311++G (d, p)//sm8/b3lyp-6-311++g (d, p) level. The solvation free energy of alkanolamine carbamate was also calculated by the explicit solvation shell model introduced by da Silva et. al. 14. Full details for extracting the solvation shell geometries from molecular simulations and further details of the model can be found in Gupta et al. 20 4. Results and Discussion In this work, temperature dependent enthalpies for the carbamate formation reaction for amines and amino acids are calculated. We have calculated heat of absorption based on correlations of temperature dependent lnk values obtained from computational chemistry results. 21 Experimental free energies of solution of carbamate formation reaction at 298 K were given as input in computational thermodynamic model 12. The correlations for carbamate formation reaction (2) corresponding to lnk values were used to calculate heat of absorption as function of temperature. The obtained parameters for MEA and PZ are given in Table 1.
Mayuri Gupta and Hallvard F. Svendsen / Energy Procedia 63 ( 2014 ) 1106 1114 1109 Table 1: Coefficients for the reaction equilibrium constants using following equation, Reaction Parameter A B C D E No. 2 K c (MEA) -69.35 7501.459 8.518298-0.00015-2.85E-06 2 K c (PZ) -760.608 19823.69 143.7794-0.51585 2.95E-04 Fig.1 shows the thermodynamic cycle used to calculate the carbamate stability constants in this work. The free energy of the gaseous and solution phases are added together to obtain the free energy of solution for reaction 2 following the thermodynamic cycle given in Fig. 1. Fig 1: Thermodynamic cycle employed for calculating free energy of carbamate formation reaction (Reaction 2). Fig. 2 describes the thermodynamic cycle used for calculation of free energy of solvation using the explicit shell model 14 Details of the calculation of the free energy of solvation using this thermodynamic cycle can be found in literature. 14,20 For calculating the free energy of solvation, the explicit solvation shell model at 298 K is used as described above. ESS is shown to give good results for solvation free energies of charged species. 14,20. Fig. 3 shows the ESS clusters of the carbamates of monoethanolamine (MEA), Piperazine (PZ) and Glycine (Gly). Explicit water molecules in these clusters tend to stabilize charged groups in the molecule and hence the ESS describes better the interaction between molecule and solvent (i.e. water in this case) than do continuum solvation models. Fig. 2: Thermodynamic Cycle for Computing Solvation Free Energies with Explicit Solvation Shell Model.
1110 Mayuri Gupta and Hallvard F. Svendsen / Energy Procedia 63 ( 2014 ) 1106 1114 Fig 3: Optimized clusters of monoethanol carbamate, piperazine carbamate and glycine carbamate. (Dotted lines show Hydrogen bonds and bond lengths of Hydrogen bonds are given in Angstrom). Figure 4 shows the temperature dependent reaction equilibrium constants and enthalpies of the carbamate formation reaction (2) for MEA, Piperazine and Glycine. The calculated reaction equilibrium constants were used in equation 7 to give enthalpies of carbamate formation. In Fig. 4, Fig. 1 (a), 2 (a) and 3 (a) show the temperature dependency of lnk values for MEA, PZ and Gly respectively. Solid lines show the results for the temperature dependency of lnk values using the SM8T continuum solvation model. In case of MEA and PZ we have some experimental and modelling work available in the literature for the temperature dependency of ln K and these are plotted in Fig. 1 (a) and 2 (a). 22-36 We can see From Fig, 1 (a) that the temperature dependency of ln K is calculated within experimental error bars for MEA. In case of PZ, the only experimental data available for ln K of PZ were from the NMR work of Bishnoi et al 37-39 and Ermatchkov et al. 40 Bishnoi 39 gives estimates based on limited and speciation data. Ermatchkov et al. 40 presents constants regressed from a large amount of 1H NMR data using the Pitzer Debye Hückel model. There is also some literature which calculates concentration-based equilibrium constants from VLE data. Aroua and Salleh 41 regressed the concentration-based equilibrium constants from VLE data. In Fig. 3 (a) the temperature dependency of ln K of glycine is plotted. We could not find any experimental data for ln K for this reaction in the literature. Fig. 1 (b), 2 (b) and 3 (b ) in Fig. 4 show temperature dependent enthalpy of absorption for carbamate formation reaction of MEA, PZ and Gly. One of the problems with experimental measurement of these equilibrium constants and enthalpies of absorption is the uncertainty in their predictions becuase of current measurement devices. From this figure we can see that not only data for at 298 K are scattered based on different literature sources, but also the temperature dependency is uncertain. A computational chemistry model can be considered as a valuable tool for calculating temperature effects on equilibrium constants of solvents of post
Mayuri Gupta and Hallvard F. Svendsen / Energy Procedia 63 ( 2014 ) 1106 1114 1111 combustion CO 2 capture where experimental determination of these constants through NMR spectroscopy or VLE is relatively uncertain and challenging. Figure 4: (a) and - H (b) for MEA, PZ and Gly Carbamate formation, reaction 2, as a function of temperature.
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