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Novel Green Solvents for CO2 Capture
Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 114 (2017) 2552 – 2560
13th International Conference on Greenhouse Gas Control Technologies, GHGT-13, 14-18 November 2016, Lausanne, Switzerland
Novel green solvents for CO2 capture Idowu Adeyemi a, Mohammad R. M. Abu-Zahra a, Inas Alnashef a * a
The Institute Center for Energy (iEnergy), Masdar Institute of Science and Technology, P. O. Box 54224, Masdar City, Abu Dhabi, United Arab Emirates
Abstract This study investigates the viability of novel green solvents for carbon capture. Three different types of amine based deep eutectic solvents were synthesized at three different molar ratio. The selected amines represent the primary (monoethanolamine), secondary (diethanolamine) and tertiary (methyldiethanolamine) amines, respectively. The CO2 absorption was conducted with a solvent screening set-up (SSS) and the CO2 loading was measured with an ‘Elementar’ total organic carbon (TOC) analyzer. Thereafter, FTIR of the samples was conducted in order to determine the qualitative analysis for tracking the appearance, disappearance and stability of different functional groups (400-1600 cm-1). The solubility experiments were performed based on the conditions of the absorber in the post-combustion capture process (PCO2 = 15kPa and T = 40oC). Results revealed that aminebased DESs have absorption capacity that is much higher than both 30wt% aqueous amine solutions and conventional DESs. The FTIR broadening of the O–H and N–H stretching of MEA and ChCl individual components, indicates the formation of hydrogen bonds between the two of them in the ChCl-MEA 1:6 before CO2 absorption. © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2017 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of GHGT-13. Peer-review under responsibility of the organizing committee of GHGT-13. Keywords: Deep eutectic solvent, Amines, Solubility of CO2, Choline chloride
1. Introduction Many scientists believe that the major cause of global warming is the emission of greenhouse gases, such as carbon dioxide (CO2). It has been estimated that CO2 is contributing to about 60% of the global warming effects [1]. The CO2 capture and sequestration from fossil-fueled power plants is drawing increasing attention as a potential method for controlling greenhouse gas emissions. However, several technological, economic and environmental issues as well as safety problems remain to be solved, such as (i) increasing the CO2 capture efficiency, (ii) reducing process costs, and (iii) verifying environmental sustainability of CO2 storage [2]. The most mature technology for
1876-6102 © 2017 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/4.0/). Peer-review under responsibility of the organizing committee of GHGT-13. doi:10.1016/j.egypro.2017.03.1413
Idowu Adeyemi et al. / Energy Procedia 114 (2017) 2552 – 2560
the CO2 post-combustion is the amine-based absorption due to its high affinity to CO2 [3]. However, this process demands intensive energy use to break the chemical bonds between the absorbents and the absorbed CO2 in the solvent regeneration step. Therefore, it is of benefit to find alternative solvents that compromise the high affinity for CO2 with the ease of solvent regeneration and reuse. Recently, a new group of green solvents called deep eutectic solvents (DESs) has attracted intensive interest in the research world. DESs are composed of a mixture of a salt and a hydrogen bond donor (HBD), which results in a liquid medium with a freezing point lower than the freezing points of the constituting compounds [4]. DESs are advantageous in comparison to ionic liquids (ILs) because they are easily synthesized and their component salts are much cheaper than those of ILs. In addition, their components can be selected to be biodegradable and nontoxic [5]. DESs have found applications in various fields as reported by many research groups [4]. In this study, three different types of amine based deep eutectic solvents were synthesized at three different molar ratio. The synthesized DESs were then tested for their carbon capture capability. 2. Experiment 2.1 Materials Monoethanolamine (MEA≥99%), diethanolamine(DEA=99%), methyldiethanolamine (MDEA≥99%), Choline chloride (2-hydroxyethyl-trimethylammonium) (≥98%) were all purchased from Sigma Aldrich and used without any further purification. Deionized water of purity more than 1 MΩ.cm was obtained using Purite’s PUREWATER 300. 2.2 Sample preparation In this work, the method of preparation reported by Abbott et al. [6] was used. The proper quantities of salt and HBD were put in a well-sealed vial. The vial was then placed on a hot plate and stirred at around 80qC for about a few hours until a homogeneous colourless liquid is formed. This is a very good method for producing DESs, easy to follow and less expensive compared to the synthesis of ionic liquids whereby reactors are needed to carry out the syntheses reactions and further purification is required to purify the resulting ionic liquid. 2.3 Carbon dioxide solubility measurements After the DES has been prepared, the CO2 solubility experiment was conducted with the solvent screening experimental set-up and the CO2 was analyzed with an ‘Elementar’ Total Organic Carbon (TOC) analyzer. The solvent screening set-up (S. S. S.) equipment is constituted of six glass reactors (V = 250mL±0.5) which can be operated independently in the temperature range from 298.15-423.15 K (±1K) and the pressure range of 0-6 bars (±0.01 bar). The tests were done at 313.15K with a mixing speed of 500 rpm. Moreover, to simulate the flue gas flow, a blend of CO2 and N2, with 15 vol% and 85 vol% respectively, is first fed to make-up vessel until a pressure of 2 bars is reached and then flown into the reactors at a flow rate of 15 L/h controlled by a mass flow controller. The pressure inside each reactors was kept at 1 bar during all the CO2 absorption experiment. The reaction of CO2 with the aqueous amine solution was complete when the equilibrium is reached i.e. when the flow of CO2 in is equal to the flow of CO2 out (Fig. 1). After the solvent has become saturated with CO2, the carbon loading in mole of CO2 per mole of solvent was obtained with the ‘Elementar’ total organic carbon (TOC) analyzer. In addition to the TOC analysis, FTIR was conducted to determine the qualitative analysis for tracking the appearance, disappearance and stability of different functional groups (400-1600 cm-1).
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Fig. 1 The absorption curve of CO2
3. Results and discussion 3.1 Validation of the Preparation and Solubility Method The validation of experimental methods and procedures is of prime importance in empirical studies before further estimation is made with the experimental set-up. The preparation and solubility methods for this study were validated with experimental data. The preparation method and CO2 loading for 30% weight of MEA obtained in this study agrees well with what is obtainable in the literature (Fig. 2-4). Hence, the solubility method was utilized for further study.
Density (g/cm3)
1.2 1.18 1.16 1.14 1.12 1.1 1.08 ChCl-EG This Study
ChCl-G Tang and Row [7]
Fig. 2 Validation of the density of Choline Chloride-Ethylene Glycol (ChCl-EG) and Choline Chloride-Glycerol (ChCl-G) DESs
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Viscosity (mPa.s)
300 250 200 150 100 50 0 ChCl-EG This Study
ChCl-G Tang and Row [7]
Fig. 3 Validation of the viscosity of Choline Chloride-Ethylene Glycol (ChCl-EG) and Choline Chloride-Glycerol (ChCl-G) DESs
Absorption Capacity (g/g)
0.14
0.12 0.1 0.08
0.06 0.04 0.02
0 This Study
Ali et al.
Cao et al. Shen and Li
Fig. 4 Validation of the solubility method
3.2 Effect of HBD and Molar Ratio on the Solubility of CO 2 One significant benefit of amine based DESs over conventional DESs (Choline Chloride- Urea (ChCl-U), Choline Chloride-Glycerol (ChCl-G), Choline Chloride-Ethylene Glycol (ChCl-EG) etc.) is that amine based DESs have far higher solubility of CO2 compared to their conventional DESs counterparts (Fig. 5). Moreover, the solubility of amine based DESs is much higher than that of aqueous solutions of amines. For instance, the solubility of CO2 in ChCl-MEA 1:8 is almost 265% of that of 30% wt, aqueous MEA as compared to 12.3% of ChCl-Glycerol.
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g of CO2/g of Solvent
0.3
0.25 0.2 0.15
0.1 0.05 0 30% wt MEA
ChCl-MEA 1:8
ChCl-Glycerol
Fig. 5 Amine based DESs compared to conventional DESs for CO2 absorption Absorption of CO2 in amine based DES can occur by both physical and chemical means. For the chemical absorption, the solubility of CO2 in primary, secondary and tertiary amines depends on the reaction steps in the equilibrium reaction mechanisms. For primary and secondary amines, the reaction proceeds in two steps without the need for water addition in order to obtain carbamate. However, for tertiary amines, the one step reaction would not proceed in the absence of water. The first step for the primary and secondary amines is bimolecular, second order and rate determining while the second step is instantaneous. The reaction steps for the mechanism are listed below for primary and secondary amine using R1R2NH to represent both [11]: First stage: Second stage:
Amine based DESs have similar solubility of CO2 when compared with stand-alone amines. ChCl-MEA DESs have solubility of CO2 of around 265% of the solubility of CO2 in 30% weight MEA (Fig. 6). This solubility comes from both physical absorption and chemical absorption. The physical absorption is attributed to the ability of the choline chloride to form hydrogen bonds with the amines. The solubility of CO2 increased with increasing molar ratio of amine in the DES. This is reasonable because the more the amine, the more the possibility of the formation of hydrogen bond network with the choline chloride.
g of CO2/g of Solvent
0.35
0.3 0.25
0.2 0.15
0.1 0.05
0 MEA 30% ChCl-MEA ChCl-MEA ChCl-MEA wt 1:6 1:8 1:10
Fig. 6 Effect of molar ratio on the solubility of CO2 in ChCl-MEA
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g of CO2/g of Solvent
The solubility of CO2 in 30% weight DEA is lower than that of 30% weight MEA. The same is true for their amine based DESs. ChCl-DEA DES has a lower solubility of CO2 than that of ChCl-MEA DES. This can be attributed to the higher viscosity of ChCl-DEA as compared to ChCl-MEA. This results in an extensive hydrogen bond network between each component in the DES, which in turn results in lower mobility of free species within the DESs. Another cause of lower solubility of CO2 is the other forces of interaction like van der Waal’s and electrostatic interactions, and small void volume. The solubility of CO2 in ChCl-DEA is around 163% of the solubility of 30% weight DEA aqueous solution (Fig. 7). Moreover, the solubility of CO2 increased with the increase of the molar ratio of DEA in the DES. This is reasonable because more DEA addition will lead to increased physical and chemical absorption of CO2. 0.18 0.16 0.14 0.12 0.1 0.08 0.06 0.04 0.02 0 DEA 30% wt
ChCl-DEA ChCl-DEA ChCl-DEA 1:6 1:8 1:10
Fig. 7 Effect of molar ratio on the solubility of CO2 in ChCl-DEA For the chemical absorption of CO2, there is only one reaction step for the formation of carbamate with tertiary amines and that reaction step requires water [12]. The reaction would not proceed in the absence of water. This is why the solubility of CO2 in ChCl-MDEA is only around 16% of the solubility of CO2 in 30% weight MDEA (Fig. 8). Donaldson and Nguyen also suggested the mechanism in which tertiary alkanolamines cannot react directly with CO2. These amines have a base-catalytic effect in the hydration of CO2 [12]. This was also confirmed by Versteeg and Van Swaaij by the absorption of CO2 into a water free solution of MDEA and ethanol [13]. They concluded that CO2 was only physically absorbed and also agreed with the proposed reaction mechanism [11]. At higher pH values (pH =13), a direct reaction between CO2 and tertiary amine has been reported by Jørgensen and Faurholt [11]. However, the rate of this reaction can be neglected at lower pH values (pH < 11) [11], which is the case of MDEA. Versteeg and Swaaij [12] showed that the rate of absorption of CO2 into an MDEA-ethanol solution could be described as physical absorption, which was almost identical to absorption of N2O in the same solution. Hence, the 16% extra absorption of CO2 can be attributed to the physical absorption of CO2. The solubility of CO2 increased with the increase of MDEA molar ratio in the DES. This is reasonable because more MDEA addition will lead to increased physical absorption of CO2.
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g of CO2/g of Solvent
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0.1 0.08 0.06 0.04 0.02 0 MDEA 30% ChCl-MDEA ChCl-MDEA ChCl-MDEA wt 1:6 1:8 1:10
Fig. 8 Effect of molar ratio on the solubility of CO2 in ChCl-MDEA 3.3. Qualitative analysis with FTIR The FTIR provides the qualitative analysis for tracking the appearance, disappearance and stability of different functional groups (400-1600 cm-1). The main focus of the FTIR analysis is in the functional group region rather than the fingerprint region. This is because the relevant groups to this study like the amine, alkanol, saturated hydrocarbon groups etc. are located in the functional group region (1600-4000 cm-1). The amine N-H stretching bond for the stand-alone MEA is found between 3100 and 3500 cm-1 (medium and broad), and it has two peaks because it is a primary amine. The alkanol O-H stretching of the ChCl is located between 3200 and 3650 cm-1 (strong and broad). The saturated C-H stretching bond for ChCl, ChCl-MEA 1:6 before CO2 absorption and standalone MEA is between 2700 and 3000 cm-1 (Fig. 9). The broadening of the O–H and N–H stretching of MEA and ChCl, indicates the formation of hydrogen bonds between the two of them in the ChCl-MEA 1:6 before CO2 absorption [9]. Amine+Alkanol Group Alkanol Group
Amine Group
Saturated Hydrocarbon
•
ChCl-MEA 1:6 Before
•
Stand alone MEA
•
ChCl
Fingerprint region
Fig. 9 FTIR result of the pure ChCl, stand-alone MEA and ChCl-MEA 1:6 before absorption
The FTIR result of the ChCl-MEA 1:6 before and after CO2 absorption is presented in Fig. 10. The results showed that the presence of the amine group in the DES persists after CO2 absorption. This implies that there remains some
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amines that have not reacted even when the solvent is fully saturated with CO2. This implies that there is physical absorption in addition to the chemical absorption present in the process. The chemical absorption is confirmed with the formation of a carbamate between 1950 and 2200 cm-1. Amine+Alkanol Group
Saturated Hydrocarbon
•
ChCl-MEA 1:6 Before
•
ChCl-MEA 1:6 After
Fingerprint region
Amine+Alkanol Group
Carbamate
Fig. 10 FTIR result of the ChCl-MEA 1:6 before and after CO2 absorption 4. Conclusions In this study, three different types of amine based deep eutectic solvents were synthesized at three different molar ratio. The synthesized DESs were tested for their carbon capture ability. Results revealed that amine-based DESs have absorption capacity that is much higher than both 30wt% aqueous amine solutions and conventional DESs. The FTIR broadening of the O–H and N–H stretching of MEA and ChCl individual components, indicates the formation of hydrogen bonds between the two of them in the ChCl-MEA 1:6 before CO2 absorption. Acknowledgements The authors would like to acknowledge the support of Masdar Institute of Science and Technology, Abu Dhabi, UAE. References [1] Yamasaki, A. An overview of CO2 mitigation options for global warming—emphasizing CO2 sequestration options, J. Chem. Eng. Jpn., 36 (2003), 361–375. [2] Pires, J.C.M., Martins, F.G., Alvim-Ferraz, M.C.M., Simões, M., Recent developments on carbon capture and storage: an overview, Chem. Eng. Res. Des., 89 (2011), 1446–1460. [3] Abu-Zahra, M., Schneides, L.H., Niederer, J.P., Feron, P.H., Versteeg, G.F., CO2 capture from power plants part I, parametric study of the technical performance based on monoethanolamine, Int. J. Greenhouse Gas Control, 1 (2007), 37–46. [4] Zhang, Q., Vigier, K. D. O., Royer, S., & Jérôme, F. Deep eutectic solvents: syntheses, properties and applications. Chemical Society Reviews, (2012), 41(21), 7108-7146. [5] Hayyan, M., Hashim, M. A., Hayyan, A., Al-Saadi, M. A., AlNashef, I. M., Mirghani, M. S., Saheed, O. K. Are deep eutectic solvents benign or toxic? Chemosphere 90 (2013), 2193– 2195. [6] Abbott, A.P., Capper, G., Davies, D.L., Rasheed, R.K., Tambyrajah, V. Novel solvent properties of choline chloride/urea mixtures. Chemical Communications (2003); 1:70-71.
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[7] Tang, B., Row, K. H. (2013). Recent developments in deep eutectic solvents in chemical sciences. Monatshefte für Chemie-Chemical Monthly, 144(10), 1427-1454. [8] Ali E., Hadj-Kali M. K., Mulyono S., AlNashef I., Fakeeha A., Mjalli F., Hayyan A. Solubility of CO2 in deep eutectic solvents: Experiments and modelling using the Peng–Robinson equation of state. Chemical Engineering Research and Design, (2014), 92, 1898-1906. [9] Cao, L., Huang, J., Zhang, X., Zhang, S., Gao, J., & Zeng, S. Imidazole tailored deep eutectic solvents for CO 2 capture enhanced by hydrogen bonds. Physical Chemistry Chemical Physics, (2015), 17(41), 27306-27316. [10] Shen, K. P., & Li, M. H. Solubility of carbon dioxide in aqueous mixtures of monoethanolamine with methyldiethanolamine. Journal of chemical and Engineering Data, (1992), 37(1), 96-100. [11] Jensen, M. B., Jorgensen, E., Faurholt, C. Reactions between carbon dioxide and amino alcohols. Acta Chemica Scandinavica (1954); 8:1141-1144. [12] Versteeg, G.F., van Swaaij, W.B. On the kinetics between CO2 and alkanolamines both in aqueous and non-aqueous solutions—II. Tertiary amines. Chemical engineering science (1988); 43(3):587-591. [13] Versteeg, G.F. and Swaaij van, W.P.M. On the kinetics between CO2 and alkanolamines both in aqueous and non-aqueous solutions-I. Primary and secondary amines," Chemical engineering science (1988); 43(3): 573- 585.
ScienceDirect Energy Procedia 114 (2017) 2552 – 2560
13th International Conference on Greenhouse Gas Control Technologies, GHGT-13, 14-18 November 2016, Lausanne, Switzerland
Novel green solvents for CO2 capture Idowu Adeyemi a, Mohammad R. M. Abu-Zahra a, Inas Alnashef a * a
The Institute Center for Energy (iEnergy), Masdar Institute of Science and Technology, P. O. Box 54224, Masdar City, Abu Dhabi, United Arab Emirates
Abstract This study investigates the viability of novel green solvents for carbon capture. Three different types of amine based deep eutectic solvents were synthesized at three different molar ratio. The selected amines represent the primary (monoethanolamine), secondary (diethanolamine) and tertiary (methyldiethanolamine) amines, respectively. The CO2 absorption was conducted with a solvent screening set-up (SSS) and the CO2 loading was measured with an ‘Elementar’ total organic carbon (TOC) analyzer. Thereafter, FTIR of the samples was conducted in order to determine the qualitative analysis for tracking the appearance, disappearance and stability of different functional groups (400-1600 cm-1). The solubility experiments were performed based on the conditions of the absorber in the post-combustion capture process (PCO2 = 15kPa and T = 40oC). Results revealed that aminebased DESs have absorption capacity that is much higher than both 30wt% aqueous amine solutions and conventional DESs. The FTIR broadening of the O–H and N–H stretching of MEA and ChCl individual components, indicates the formation of hydrogen bonds between the two of them in the ChCl-MEA 1:6 before CO2 absorption. © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2017 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of GHGT-13. Peer-review under responsibility of the organizing committee of GHGT-13. Keywords: Deep eutectic solvent, Amines, Solubility of CO2, Choline chloride
1. Introduction Many scientists believe that the major cause of global warming is the emission of greenhouse gases, such as carbon dioxide (CO2). It has been estimated that CO2 is contributing to about 60% of the global warming effects [1]. The CO2 capture and sequestration from fossil-fueled power plants is drawing increasing attention as a potential method for controlling greenhouse gas emissions. However, several technological, economic and environmental issues as well as safety problems remain to be solved, such as (i) increasing the CO2 capture efficiency, (ii) reducing process costs, and (iii) verifying environmental sustainability of CO2 storage [2]. The most mature technology for
1876-6102 © 2017 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/4.0/). Peer-review under responsibility of the organizing committee of GHGT-13. doi:10.1016/j.egypro.2017.03.1413
Idowu Adeyemi et al. / Energy Procedia 114 (2017) 2552 – 2560
the CO2 post-combustion is the amine-based absorption due to its high affinity to CO2 [3]. However, this process demands intensive energy use to break the chemical bonds between the absorbents and the absorbed CO2 in the solvent regeneration step. Therefore, it is of benefit to find alternative solvents that compromise the high affinity for CO2 with the ease of solvent regeneration and reuse. Recently, a new group of green solvents called deep eutectic solvents (DESs) has attracted intensive interest in the research world. DESs are composed of a mixture of a salt and a hydrogen bond donor (HBD), which results in a liquid medium with a freezing point lower than the freezing points of the constituting compounds [4]. DESs are advantageous in comparison to ionic liquids (ILs) because they are easily synthesized and their component salts are much cheaper than those of ILs. In addition, their components can be selected to be biodegradable and nontoxic [5]. DESs have found applications in various fields as reported by many research groups [4]. In this study, three different types of amine based deep eutectic solvents were synthesized at three different molar ratio. The synthesized DESs were then tested for their carbon capture capability. 2. Experiment 2.1 Materials Monoethanolamine (MEA≥99%), diethanolamine(DEA=99%), methyldiethanolamine (MDEA≥99%), Choline chloride (2-hydroxyethyl-trimethylammonium) (≥98%) were all purchased from Sigma Aldrich and used without any further purification. Deionized water of purity more than 1 MΩ.cm was obtained using Purite’s PUREWATER 300. 2.2 Sample preparation In this work, the method of preparation reported by Abbott et al. [6] was used. The proper quantities of salt and HBD were put in a well-sealed vial. The vial was then placed on a hot plate and stirred at around 80qC for about a few hours until a homogeneous colourless liquid is formed. This is a very good method for producing DESs, easy to follow and less expensive compared to the synthesis of ionic liquids whereby reactors are needed to carry out the syntheses reactions and further purification is required to purify the resulting ionic liquid. 2.3 Carbon dioxide solubility measurements After the DES has been prepared, the CO2 solubility experiment was conducted with the solvent screening experimental set-up and the CO2 was analyzed with an ‘Elementar’ Total Organic Carbon (TOC) analyzer. The solvent screening set-up (S. S. S.) equipment is constituted of six glass reactors (V = 250mL±0.5) which can be operated independently in the temperature range from 298.15-423.15 K (±1K) and the pressure range of 0-6 bars (±0.01 bar). The tests were done at 313.15K with a mixing speed of 500 rpm. Moreover, to simulate the flue gas flow, a blend of CO2 and N2, with 15 vol% and 85 vol% respectively, is first fed to make-up vessel until a pressure of 2 bars is reached and then flown into the reactors at a flow rate of 15 L/h controlled by a mass flow controller. The pressure inside each reactors was kept at 1 bar during all the CO2 absorption experiment. The reaction of CO2 with the aqueous amine solution was complete when the equilibrium is reached i.e. when the flow of CO2 in is equal to the flow of CO2 out (Fig. 1). After the solvent has become saturated with CO2, the carbon loading in mole of CO2 per mole of solvent was obtained with the ‘Elementar’ total organic carbon (TOC) analyzer. In addition to the TOC analysis, FTIR was conducted to determine the qualitative analysis for tracking the appearance, disappearance and stability of different functional groups (400-1600 cm-1).
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Fig. 1 The absorption curve of CO2
3. Results and discussion 3.1 Validation of the Preparation and Solubility Method The validation of experimental methods and procedures is of prime importance in empirical studies before further estimation is made with the experimental set-up. The preparation and solubility methods for this study were validated with experimental data. The preparation method and CO2 loading for 30% weight of MEA obtained in this study agrees well with what is obtainable in the literature (Fig. 2-4). Hence, the solubility method was utilized for further study.
Density (g/cm3)
1.2 1.18 1.16 1.14 1.12 1.1 1.08 ChCl-EG This Study
ChCl-G Tang and Row [7]
Fig. 2 Validation of the density of Choline Chloride-Ethylene Glycol (ChCl-EG) and Choline Chloride-Glycerol (ChCl-G) DESs
Idowu Adeyemi et al. / Energy Procedia 114 (2017) 2552 – 2560
Viscosity (mPa.s)
300 250 200 150 100 50 0 ChCl-EG This Study
ChCl-G Tang and Row [7]
Fig. 3 Validation of the viscosity of Choline Chloride-Ethylene Glycol (ChCl-EG) and Choline Chloride-Glycerol (ChCl-G) DESs
Absorption Capacity (g/g)
0.14
0.12 0.1 0.08
0.06 0.04 0.02
0 This Study
Ali et al.
Cao et al. Shen and Li
Fig. 4 Validation of the solubility method
3.2 Effect of HBD and Molar Ratio on the Solubility of CO 2 One significant benefit of amine based DESs over conventional DESs (Choline Chloride- Urea (ChCl-U), Choline Chloride-Glycerol (ChCl-G), Choline Chloride-Ethylene Glycol (ChCl-EG) etc.) is that amine based DESs have far higher solubility of CO2 compared to their conventional DESs counterparts (Fig. 5). Moreover, the solubility of amine based DESs is much higher than that of aqueous solutions of amines. For instance, the solubility of CO2 in ChCl-MEA 1:8 is almost 265% of that of 30% wt, aqueous MEA as compared to 12.3% of ChCl-Glycerol.
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g of CO2/g of Solvent
0.3
0.25 0.2 0.15
0.1 0.05 0 30% wt MEA
ChCl-MEA 1:8
ChCl-Glycerol
Fig. 5 Amine based DESs compared to conventional DESs for CO2 absorption Absorption of CO2 in amine based DES can occur by both physical and chemical means. For the chemical absorption, the solubility of CO2 in primary, secondary and tertiary amines depends on the reaction steps in the equilibrium reaction mechanisms. For primary and secondary amines, the reaction proceeds in two steps without the need for water addition in order to obtain carbamate. However, for tertiary amines, the one step reaction would not proceed in the absence of water. The first step for the primary and secondary amines is bimolecular, second order and rate determining while the second step is instantaneous. The reaction steps for the mechanism are listed below for primary and secondary amine using R1R2NH to represent both [11]: First stage: Second stage:
Amine based DESs have similar solubility of CO2 when compared with stand-alone amines. ChCl-MEA DESs have solubility of CO2 of around 265% of the solubility of CO2 in 30% weight MEA (Fig. 6). This solubility comes from both physical absorption and chemical absorption. The physical absorption is attributed to the ability of the choline chloride to form hydrogen bonds with the amines. The solubility of CO2 increased with increasing molar ratio of amine in the DES. This is reasonable because the more the amine, the more the possibility of the formation of hydrogen bond network with the choline chloride.
g of CO2/g of Solvent
0.35
0.3 0.25
0.2 0.15
0.1 0.05
0 MEA 30% ChCl-MEA ChCl-MEA ChCl-MEA wt 1:6 1:8 1:10
Fig. 6 Effect of molar ratio on the solubility of CO2 in ChCl-MEA
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g of CO2/g of Solvent
The solubility of CO2 in 30% weight DEA is lower than that of 30% weight MEA. The same is true for their amine based DESs. ChCl-DEA DES has a lower solubility of CO2 than that of ChCl-MEA DES. This can be attributed to the higher viscosity of ChCl-DEA as compared to ChCl-MEA. This results in an extensive hydrogen bond network between each component in the DES, which in turn results in lower mobility of free species within the DESs. Another cause of lower solubility of CO2 is the other forces of interaction like van der Waal’s and electrostatic interactions, and small void volume. The solubility of CO2 in ChCl-DEA is around 163% of the solubility of 30% weight DEA aqueous solution (Fig. 7). Moreover, the solubility of CO2 increased with the increase of the molar ratio of DEA in the DES. This is reasonable because more DEA addition will lead to increased physical and chemical absorption of CO2. 0.18 0.16 0.14 0.12 0.1 0.08 0.06 0.04 0.02 0 DEA 30% wt
ChCl-DEA ChCl-DEA ChCl-DEA 1:6 1:8 1:10
Fig. 7 Effect of molar ratio on the solubility of CO2 in ChCl-DEA For the chemical absorption of CO2, there is only one reaction step for the formation of carbamate with tertiary amines and that reaction step requires water [12]. The reaction would not proceed in the absence of water. This is why the solubility of CO2 in ChCl-MDEA is only around 16% of the solubility of CO2 in 30% weight MDEA (Fig. 8). Donaldson and Nguyen also suggested the mechanism in which tertiary alkanolamines cannot react directly with CO2. These amines have a base-catalytic effect in the hydration of CO2 [12]. This was also confirmed by Versteeg and Van Swaaij by the absorption of CO2 into a water free solution of MDEA and ethanol [13]. They concluded that CO2 was only physically absorbed and also agreed with the proposed reaction mechanism [11]. At higher pH values (pH =13), a direct reaction between CO2 and tertiary amine has been reported by Jørgensen and Faurholt [11]. However, the rate of this reaction can be neglected at lower pH values (pH < 11) [11], which is the case of MDEA. Versteeg and Swaaij [12] showed that the rate of absorption of CO2 into an MDEA-ethanol solution could be described as physical absorption, which was almost identical to absorption of N2O in the same solution. Hence, the 16% extra absorption of CO2 can be attributed to the physical absorption of CO2. The solubility of CO2 increased with the increase of MDEA molar ratio in the DES. This is reasonable because more MDEA addition will lead to increased physical absorption of CO2.
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g of CO2/g of Solvent
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0.1 0.08 0.06 0.04 0.02 0 MDEA 30% ChCl-MDEA ChCl-MDEA ChCl-MDEA wt 1:6 1:8 1:10
Fig. 8 Effect of molar ratio on the solubility of CO2 in ChCl-MDEA 3.3. Qualitative analysis with FTIR The FTIR provides the qualitative analysis for tracking the appearance, disappearance and stability of different functional groups (400-1600 cm-1). The main focus of the FTIR analysis is in the functional group region rather than the fingerprint region. This is because the relevant groups to this study like the amine, alkanol, saturated hydrocarbon groups etc. are located in the functional group region (1600-4000 cm-1). The amine N-H stretching bond for the stand-alone MEA is found between 3100 and 3500 cm-1 (medium and broad), and it has two peaks because it is a primary amine. The alkanol O-H stretching of the ChCl is located between 3200 and 3650 cm-1 (strong and broad). The saturated C-H stretching bond for ChCl, ChCl-MEA 1:6 before CO2 absorption and standalone MEA is between 2700 and 3000 cm-1 (Fig. 9). The broadening of the O–H and N–H stretching of MEA and ChCl, indicates the formation of hydrogen bonds between the two of them in the ChCl-MEA 1:6 before CO2 absorption [9]. Amine+Alkanol Group Alkanol Group
Amine Group
Saturated Hydrocarbon
•
ChCl-MEA 1:6 Before
•
Stand alone MEA
•
ChCl
Fingerprint region
Fig. 9 FTIR result of the pure ChCl, stand-alone MEA and ChCl-MEA 1:6 before absorption
The FTIR result of the ChCl-MEA 1:6 before and after CO2 absorption is presented in Fig. 10. The results showed that the presence of the amine group in the DES persists after CO2 absorption. This implies that there remains some
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amines that have not reacted even when the solvent is fully saturated with CO2. This implies that there is physical absorption in addition to the chemical absorption present in the process. The chemical absorption is confirmed with the formation of a carbamate between 1950 and 2200 cm-1. Amine+Alkanol Group
Saturated Hydrocarbon
•
ChCl-MEA 1:6 Before
•
ChCl-MEA 1:6 After
Fingerprint region
Amine+Alkanol Group
Carbamate
Fig. 10 FTIR result of the ChCl-MEA 1:6 before and after CO2 absorption 4. Conclusions In this study, three different types of amine based deep eutectic solvents were synthesized at three different molar ratio. The synthesized DESs were tested for their carbon capture ability. Results revealed that amine-based DESs have absorption capacity that is much higher than both 30wt% aqueous amine solutions and conventional DESs. The FTIR broadening of the O–H and N–H stretching of MEA and ChCl individual components, indicates the formation of hydrogen bonds between the two of them in the ChCl-MEA 1:6 before CO2 absorption. Acknowledgements The authors would like to acknowledge the support of Masdar Institute of Science and Technology, Abu Dhabi, UAE. References [1] Yamasaki, A. An overview of CO2 mitigation options for global warming—emphasizing CO2 sequestration options, J. Chem. Eng. Jpn., 36 (2003), 361–375. [2] Pires, J.C.M., Martins, F.G., Alvim-Ferraz, M.C.M., Simões, M., Recent developments on carbon capture and storage: an overview, Chem. Eng. Res. Des., 89 (2011), 1446–1460. [3] Abu-Zahra, M., Schneides, L.H., Niederer, J.P., Feron, P.H., Versteeg, G.F., CO2 capture from power plants part I, parametric study of the technical performance based on monoethanolamine, Int. J. Greenhouse Gas Control, 1 (2007), 37–46. [4] Zhang, Q., Vigier, K. D. O., Royer, S., & Jérôme, F. Deep eutectic solvents: syntheses, properties and applications. Chemical Society Reviews, (2012), 41(21), 7108-7146. [5] Hayyan, M., Hashim, M. A., Hayyan, A., Al-Saadi, M. A., AlNashef, I. M., Mirghani, M. S., Saheed, O. K. Are deep eutectic solvents benign or toxic? Chemosphere 90 (2013), 2193– 2195. [6] Abbott, A.P., Capper, G., Davies, D.L., Rasheed, R.K., Tambyrajah, V. Novel solvent properties of choline chloride/urea mixtures. Chemical Communications (2003); 1:70-71.
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