Experimental and Theoretical Investigation of Equilibrium

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Experimental and Theoretical Investigation of Equilibrium Absorption Performance: The Effect of Alkyl amines as Promoters on the CO2 Loading of 2-amino-2-methyl-1-propanol at 313 K Rahele Mahmoodi, Masoud Mofarahi, Amir Abbas Izadpanah, Morteza Afkhamipour, and Abdollah Hajizadeh Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b01957 • Publication Date (Web): 29 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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Experimental and Theoretical Investigation of Equilibrium Absorption Performance: The Effect of Alkyl amines as Promoters on the CO2 Loading of 2-amino-2-methyl-1-propanol at 313 K Rahele Mahmoodi, Masoud Mofarahi1, Amir Abbas Izadpanah, Morteza Afkhamipour, and Abdollah Hajizadeh

Department of Chemical Engineering, Faculty of Petroleum, Gas and Petrochemical Engineering, Persian Gulf University, Bushehr,75169, Iran

Abstract In this study, the effect of adding various activators (Alkyl amines) such as tetraethylenepentamine (TEPA), triethylenetetramine (TETA), diethanolamine (DEA), and monoethanolamine (MEA) on the CO2 loading of 2-amino-2-methyl-1-propanol (AMP) was investigated experimentally. With the aim of evaluating the performance of each activator on the CO2 loading, a vapor-liquid equilibrium (VLE) apparatus was used. The CO2 solubility data into TEPA+AMP+H2O, TETA+AMP+H2O, DEA+AMP+H2O, and MEA+AMP+H2O systems were measured in the range of 0.281-186.3 kPa. The experiments carried out under fixed absorption temperature of 313 K and different molar ratios of activators to AMP. The measured solubility data were modeled using the modified Kent-Eisenberg and the Deshmukh–Mather as thermodynamic models. The adjustable parameters of thermodynamic models were obtained using the simplex method as an optimization algorithm. The results of Deshmukh–Mather model showed a better prediction for experimental data than the modified Kent-Eisenberg model. By comparing the loading results in different mixing systems under constant temperature and at a constant concentration, the TEPA showed the highest absorption capacity among other activators (TETA, DEA, and MEA) assessed in this study. The superiority of TEPA is due to the presence of more amine groups in TEPA compared to other activators in combination with AMP.

Keywords: Alkyl amines, AMP, CO2 loading, Deshmukh–Mather, Modified Kent-Eisenberg, CO2 absorption

1

Corresponding author. E-mail: [email protected]. Fax: +98 7733441495. 1 ACS Paragon Plus Environment

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1. Introduction 1.1. Background Global warming caused by the emission of greenhouse gases (GHG) surrounding the earth is the issue which is taken into account by scholars in the last decades. This issue has exposed the globe at the threshold of a significant human and environmental disaster which is mainly because of CO2 emission from fossil and mineral fuels of industrial counties [1, 2]. Over the years, production and vent of CO2 to the atmosphere has turned into an environmental problem. There is considerable scientific evidence that CO2 emissions are causing irreversible and potentially disastrous environmental changes, such as the constant melting of ice in the Arctic. The global warming phenomenon is believed that resulted from the greenhouse gases emissions. While all greenhouse gases such as water vapor, carbon dioxide, methane, and nitrous oxide, are trapping the heat in the atmosphere, the carbon dioxide is more responsible for global warming [3]. The data on CO2 emissions originated from fossil fuels consumption shows that it will rise to 37.2 Gt in 2035 [4], from its value of 33.2 Gt at 2018 [5]. Although it is tried to use renewable energy sources, it is believed that fossil fuels remain the main energy source in the upcoming decades. Thus, CO2 capture seems crucial to decrease the rate of CO2 emission. It is estimated that the concentration of CO2 in the Earth’s atmosphere would become double in 2100 without performing CO2 capture activities [6]. Therefore, the theoretical and practical study on the separation of this gas which in itself includes 82% of GHG is rapidly increasing [7, 8]. Concerning high volume of industrial activities of oil, gas, and petrochemical in the world, the investigation, and application of conventional methods and new methods for separation of CO2 from different gas flows such as flue gas is necessary [9]. There are various methods for separating this type of gas, including adsorption and membrane, etc. 2 ACS Paragon Plus Environment

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Among all methods, chemical absorption is known as the most common method. The main reason for using this method is its cost-effectiveness and its industrialization, as a more practical option than other methods [10]. One of the main points about this technology is hiring an effective solvent. High loading capacity, greater stability, high absorption and low regeneration energy are the most effective factors in choosing the right solvent [11]. In this regard, primary amines, e.g., monoethanolamine (MEA), are most promising due to their strong alkalinity property, high absorption rate, and the low-cost of CO2 absorption. However, this amine shows some disadvantages including thermal and chemical degradation, corrosion, foam production, the high energy requirement for regeneration, and high capacity of the process due to high amine flow rate (to overcome its absorption capacity) [12, 13]. Tertiary amines and amines with steric hindrance are currently good options for CO2 absorption in the industry [14]. The 2-amino-2-methyl-1propanol (AMP) is one of these amines, while its application is limited because it has a lower reaction rate to CO2 compared to primary and secondary amines [15]. Thus, the researchers continuously seek to explore the most advantageous amines to be replaced for common amines. In this regard, alkyl amines have been highly considered among the introduced ones. The alkyl amines have multiple amino groups in their structure. One of the deficiencies of amines with steric hindrance is low absorption rate in the middle CO2 loading. The problem of low absorption rate could be improved using another amine with a higher absorption rate beside this type of amine [16].

1.2. Literature review Much research has been done on the absorption of CO2 from the gas streams using various mixtures of amines [17]. The use of a mixture of AMP with other amines has been taken into consideration 3 ACS Paragon Plus Environment

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since the last few years [15]. Mixing solvent is a good way to combine their advantages into a new product. Mixing can match solvents with different physical and chemical properties [18]. Mixed amines, a combination of primary/secondary amines with tertiary/steric hindrance amines, which creates a combination of higher equilibrium capacity of tertiary/steric hindrance amines with a higher reaction rate of primary/secondary amines. This combination could make considerable improvement in gas absorption, high saving in required energy for regeneration (which constitutes 70% of operational cost), less tendency to corrosion and need to lower circulation rate to reach the desired removal degree [19]. Park et al., have measured the CO2 solubility in DEA+AMP and MEA+AMP in 313, 333 and 353 K while keeping the total amine concentration fixed (30% wt.) in moderate CO2 partial pressures (0.69-344 kPa) [20]. Their results indicate that in MEA+AMP system in the loadings range of 0-0.5 (mol CO2/mol solution), CO2 loading decreases with an increase in AMP concentration and decrease by MEA concentration which is because of high rate of MEA reaction with CO2. In the loadings range of 0.5-1 (mol CO2/mol solution), an increase in AMP concentration and decrease in MEA concentration have a reverse effect and leads to increase loading due to the reduction of sustainable carbamate rate. For the DEA+AMP system, in the loadings range of 0-0.5 (mol CO2/mol solution), the concentration of amines does no effect on the loading; however, in the loadings range of 0.5-1 (mol CO2/mol solution), increasing AMP concentration and decreasing DEA concentration will lead to increase of loading. Kundu and Bandyopadhyay have investigated the performance of MEA+AMP+H2O, DEA+AMP+H2O, MDEA+AMP+H2O, and MDEA+MEA+H2O on the CO2 absorption rate [17]. They showed that the mixture of AMP with DEA and MEA have better performance compared to the mix of MDEA+AMP and MDEA+MEA. They reported new experimental data for the CO2 solubility into the DEA+AMP amine solution in the temperature range of 303-323 K and the pressure range of 1

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to 100 kPa. They also concluded that the CO2 loading decreases by increasing the DEA weight percent in the solution. Li and Chang have investigated the CO2 solubility in the MEA+AMP mixture [21]. They showed that the higher AMP weight percent in the solution results to the higher CO2 solubility. Kunihiko et al., have presented a method for CO2 absorption which includes mixed AMP with PZ [22]. It was specified that the 2.5 molar solution of AMP has less absorption rate than MEA; however, adding 0.5 molar PZ to AMP leads to a considerable increase in the absorption rate and improved loading. Aronu et al., have performed their studies on tetraethylenepentamine (TEPA) which is a solvent of high absorption capacity and high reaction rate [23]. They focused on the rate and absorption capacity of different alkanolamine compounds with various concentrations. The compounds included different concentrations of MEA (e.g., 1, 2.5, 5 and 10 molar) to be compared with TEPA compound and the loading curves were obtained in 313 K for different systems. It was observed that 1 and 2 molar of TEPA solvent has the highest loading rates compared to other systems, respectively. Although it was observed that the reaction rate of 2 molar of TEPA was less than 1 molar and the absorption rate in 1 molar of TEPA was almost three times more than 1 molar of MEA. Singh et al., have carried out a study on the structure and reaction rate of various amines in the CO2 absorption process and observed that increasing the number of amine groups increases the absorption rate from 1.83 to 3.03 (mol CO2/mol solution) [24]. Al-Marzouqi et al., have concluded that TEPA acts better than other amines in the absorption process [25]. Their results showed that the CO2 absorption rate increases with an increase in the number of nitrogen atoms: Diethylenetriamine (DETA)< Triethylenetetramine (TETA)< TEPA. Ramazani et al., have investigated the effect of adding various activators in the molar ratio of 0.25, 0.66, and 1 on the MEA [26]. Their experiments results showed that the MEA+TEPA has the highest effect on the 5 ACS Paragon Plus Environment

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CO2 loading than other mixed amines. Amann and Bouallou investigated the CO2 absorption rate and CO2 solubility into MDEA/TETA blends at different concentrations [27]. They considered two mass concentrations for each amine including 17.5 and 40 wt.% for MDEA, while 3 and 6 wt.% for TETA. They concluded that adding a small amount of TETA to the MDEA solution significantly improves the CO2 absorption capacity. Ouimet studied the CO2 absorption using different polyamines [28]. He investigated the performance of diethylenetriamine (DETA), triethylenetetramine (TETA), and tetraethylenepentamine (TEPA) and their mixture. He concluded that TEPA has the highest CO2 absorption capacity comparing to the TETA, DETA, and EDA. Choi et al. implemented the alkylamines as an activator to enhance the performance of MDEA for CO2 absorption [29]. They used mixtures of 20 wt.% MDEA and 10 wt.% of alkylamines including MAPA, DETA, TETA, and TEPA. The performance of each amine blends are evaluated based on the experimental results of CO2 absorption rate, CO2 absorption capacity, and heat of absorption. The MDEA/TEPA blend showed the highest CO2 absorption capacity with the CO2 loading amount of 0.753 (mol CO2/mol solution) at 313 K. Muchan et al. investigated the effect of number of amine groups in the polyamine structure [30]. They used EDA, DETA, TETA, and TEPA to experimentally evaluate the effect of increasing number of amine groups on the CO2 absorption rate. The results show that the CO2 absorption rate and capacity increase with the number of amine groups.

1.3. Study objectives Since the application of any new amine solvent for the absorption of CO2 requires initial studies, to investigate and achieve solvents of better efficiency, AMP is taken as the primary amine in this study and the activators of alkyl amines including TEPA and TETA are proposed for CO2 6 ACS Paragon Plus Environment

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absorption in combination with this type of amine. This group of alkyl amine includes a high loading rate in addition to a high absorption rate. Concerning the positive effects of this group of amines and as far as there is no data equilibrium of the combined systems in references. Moreover, for investigation and comparison of loading and solubility of various systems, 4 compound amine systems have been selected, and the role of activator and positive effects of each one on loading have been investigated in fixed temperature and with various concentrations. In this work, the MEA and DEA solvents were selected as a benchmark for measuring the data obtained from the experiment, and also given that there were few data in the references, along with the TETA and TEPA alkylamines. This also shows us the role of these alkyl amines on the absorption capacity and absorption rate in combined systems. The studied systems in this work include MEA+AMP, DEA+AMP, TETA+AMP, TEPA+AMP and for these systems, CO2 equilibrium data will be obtained and compared in 313 K in various molar ratios and partial pressure of 0.281-186.3 kPa. In this regard, the measurement of CO2 solubility was performed by static-synthetic apparatus, and the isotherm equilibrium data for proposed systems was obtained. After extraction and collection of experimental data, for accuracy and prediction of the obtained data, the results were analyzed using two different thermodynamic modeling: modified Kent-Eisenberg and Deshmukh–Mather models.

2. Materials and methods 2.1. Materials AMP (95% purity), TEPA (97% purity), TETA (99% purity), DEA (98% purity), MEA (99% purity) were purchased from Merck Company, and CO2 (99.99% purity) was prepared from Technical Gas Services Co. by Lian Oxygen Co of Bushehr. The physical properties of them, 7 ACS Paragon Plus Environment

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including molecular formula, purity percentage, boiling point, vapor pressure, etc. are presented in Table 1. Table 1

2.2. The experimental method for measuring CO2 solubility The samples of mixed solvents were prepared by deionized water. The used equipment in our apparatus for measuring of CO2 solubility in the mixed solvents includes the following: water circulation bath, buffer cell for storing of CO2, vapor-liquid equilibrium (VLE) cell, magnetic mixer, vacuum pump, monitor, two thermocouples, and two pressure transmitters. The apparatus used in the present study is shown in Fig. 1. In our apparatus, the equilibrium cell contains a certain volume of amine solvent and buffer cell which are placed inside a water bath. VLE cell volume plus connections (from top of VLE cell to valve 4) is 360 mm, and the volume of buffer cell plus connections is 1470 ml. The temperature variations measured by a thermocouple which was placed inside both cells, VLE, and buffer. The monitor is ATRON; model SL-45 made in Iran with four simultaneous input channels. This monitor could register and display various rates of pressure and temperature during the experiment. The equilibrium temperature for both cells was adjusted using Ta water bath (LAUDA Alpha, Model: RA) with the reliability of 0.05 K. To increase the mass transfer inside the VLE cell, a magnetic stirrer made by MTOPS Company (HS180) was used. In the apparatus, the pressure sensors manufactured by Sensys Company of South Korea, (PSCH0025BCIJ) model, was used and both sensors used to measure pressure in 0-25 bar range with the precision of 0.01 mbar. Moreover, two temperature sensors (LM35) with a precision of 0.01 C were utilized. Before performing the experiment, 50 ml of solvent was placed inside the VLE cell, then the valves between VLE and buffer cells were closed and VLE cell was kept for 10 8 ACS Paragon Plus Environment

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to 20 minutes in an isolated state. Therefore, the liquid was kept in VLE cell under its vapor pressure. In this study, the method performed by Afkhamipour and Mofarahi [31] and Pakzad et al., [32] was used to calculate the CO2 loading. The procedure for measuring the CO2 loading is as follows: first, CO2 was transferred from buffer cell to VLE cell, then the buffer cell was separated from the VLE cell. To determine the number of CO2 molecules injected to VLE cell from the buffer cell, the following equation was used.

𝑛𝐶𝑂2 =

𝑉𝐵𝑢𝑓𝑓𝑒𝑟 𝑃1 𝑃2 ( ― ) 𝑅𝑇𝑏𝑎𝑡ℎ 𝑍1 𝑍2

(1)

where VBuffer is the volume of the buffer container, 𝑍1and 𝑍2 are respectively compressibility factor in the initial pressure (P1) and final pressure (P2) in buffer cell before and after transferring of CO2. Tbath is the temperature of the water bath and R is the universal gas constant. The compressibility factor was calculated using the Peng–Robinson equation of state. After transferring of CO2 to VLE cell, the magnetic stirrer is turned on with 360 rpm to create a mixture and reduce the time to achieve equilibrium. The equilibrium will be achieved when the pressure of VLE cell remains unchanged at least for 30 minutes. After reaching equilibrium condition, the partial pressure of CO2, the number of absorbed moles and at the end the loading of CO2 absorption will be calculated. After the end of the first stage, the valve between the VLE cell and buffer cell is again opened and let CO2 transfer from the buffer to the VLE cell. In the end, all previous stages will be repeated to calculate CO2 loading. Therefore, CO2 solubility will be achieved in partial pressure higher than the previous stage. In this way, CO2 loading in higher partial pressures will be measured. For more information on the equations and details, refer to Afkhamipour and Mofarahi [31] and Pakzad et al., [32] studies.


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Fig. 1

2.3. The validation of experimental apparatus and procedure First of all, the validation of the experimental method and the apparatus used in this study is presented. To do that, the CO2 solubility (in term of CO2 partial pressure against CO2 loading) measured using the experimental apparatus. The validation experiments are carried out for CO2 solubility in 2 M DEA aqueous solution and 2.8 M AMP aqueous solution. The results are compared with published data in the literature [33, 34]. Fig. 2 shows the measured CO2 solubility data in 2 M DEA aqueous solution in a wide range of CO2 loading at 313 K. As is seen, the experimental CO2 solubility data measured in the present work are very close to the published data reported by Haji-Sulaiman et al., [33]. To validate the experimental method and apparatus for the AMP aqueous solution, the CO2 solubility in the 2.8 M AMP aqueous solution investigated at 313 K. The measured solubility data are compared to those of published data by Kundu et al., [34] in Fig. 3. It is observed that the experimental data attained in the present work are in good agreement with the published data.

Fig. 2 Fig. 3

In addition to the AMP and DEA amines, the CO2 solubility into the MEA+AMP and DEA+AMP amines are measured in the present study and compared to those of experimental data on the open literature [21, 35]. The comparison is shown in Fig. 4, where the CO2 solubility data is shown 10 ACS Paragon Plus Environment

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versus CO2 loading. As is seen in Fig. 4, the experimental CO2 solubility data obtained is this work is in a good agreement with the works of Li and Chang [21] and Seo and Hong [35]. However, the slight difference between the data is due to the different concentrations and partial pressures that the experiments were carried out. Fig. 4

3. Theory Absorption of CO2 in aqueous amine solutions consists of both phase equilibrium and chemical equilibrium. The phase equilibrium accounts for physical absorption of CO2 in amine solution which is represented by Henry’s law. The chemical equilibrium considers chemical reactions where the equilibrium constants denote the chemical equilibrium. This section provides the theory of CO2 absorption using amine solutions. Also, the basics of two thermodynamic models including Deshmukh-Mather and modified Kent-Eisenberg are presented.

3.1. Phase equilibrium equations To investigate the absorption of CO2 in amine solutions, the amine is considered as a solute and water is the solvent. In phase equilibrium of these systems, both physical and chemical equilibrium occurs. In the vapor-liquid equilibrium, the fugacity of each component in the liquid phase and the vapor phase need to be equal. 𝑓𝑣 = 𝑓𝑙

(2)


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The solubility of CO2 in the amine solutions is dependent on the chemical reaction is shown by Equation (3). 𝐶𝑂2 (𝑔)

𝐻𝐶𝑂2

(3)

𝐶𝑂2(𝑎𝑞)

In the equilibrium absorption of CO2 in aqueous amine solution, the vapor-liquid equilibrium is only accounted for the volatile molecules. Because the vapor pressure of amine is lower than water and CO2, the presence of an amine in the gas phase can be neglected. Thus, CO2 and water are the only components in the gas phase. For calculation of physical solubility of CO2, Henry’s law is applied. 𝑃𝐶𝑂2𝜑𝐶𝑂2 = 𝛾𝐶𝑂2𝐻𝐶𝑂2[𝐶𝑂2]

(4)

where 𝑃𝐶𝑂2 is the partial pressure of CO2 and 𝜑𝐶𝑂2 is the fugacity coefficient of CO2. 𝐻𝐶𝑂2 is termed for Henry’s constant of CO2 in water.

3.2. Chemical equilibrium The chemical equilibrium of CO2 absorption into aqueous solutions can be explained using some reactions. As is known, AMP has a connected carbon atom of type three to the amine group; its carbamate ion is very unstable and converts to amine and bicarbonate easily [32]. Chakraborty et al. reported that in AMP solutions, the carbamate ions cannot be recognized and thus, only the carbonate and bicarbonate are the main species in the absorption of CO2 in AMP solution [36]. It should be noted that in the following equations AM1 stands for AMP and AM2 introduces the 12 ACS Paragon Plus Environment

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second amine (i.e., MEA, DEA, TETA, and TEPA). Due to the presence of primary and secondary amines in the chemical structure of alkyl amines and referring to the following articles, the same reaction mechanisms are considered in the present study for MEA, DEA, TETA, and TEPA [29, 37]. 

Protonated amine dissociation: 𝐾1

(5)

𝐾2

(6)

𝐴𝑀1𝐻 + 𝐴𝑀1 + 𝐻 + 𝐴𝑀2𝐻 + 𝐴𝑀2 + 𝐻 +



Carbamate hydrolysis (for amine types 1 and 2 and Alkyl amines because of amino groups of primary and secondary amines they have) 𝐾3

(7)

𝐴𝑀2𝐶𝑂𝑂 ― + 𝐻2𝑂 𝐴𝑀2 + 𝐻𝐶𝑂3―



CO2 dissociation 𝐾4

(8)

𝐶𝑂2 + 𝐻2𝑂 𝐻𝐶𝑂3― + 𝐻 +



Bicarbonate dissociation 𝐾5

(9)

𝐻𝐶𝑂3― 𝐶𝑂23 ― + 𝐻 +



Water ionization 13 ACS Paragon Plus Environment

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𝐾6

(10)

𝐻2𝑂 𝐻 + + 𝑂𝐻 ―

The equilibrium constant of reactions are calculated based on the activity of the species: 𝐾𝑗 =

∏𝑎

𝑣𝑖 𝑖𝑖

=

∏(𝛾 × 𝑐 )

𝑖

𝑖

𝑖

𝑣𝑖

(11)

𝑖

where 𝐾𝑗 is the equilibrium constant of the reaction 𝑗, 𝑎𝑖 is the activity of species 𝑖, 𝑣𝑖 is the stoichiometric coefficient of specie 𝑖 in the reaction 𝑗, 𝛾𝑖 is the activity coefficient of species 𝑖, and 𝑐𝑖 is the concentration of species 𝑖. Hence, the equilibrium constants of the reactions are as follows:

𝐾1 =

𝐾2 =

𝐾3 =

[𝐴𝑀1][𝐻 + ]𝛾𝐴𝑀1 𝛾𝐻 +

[𝐴𝑀1𝐻 + ]

(12)

𝛾𝐴𝑀1𝐻 +

[𝐴𝑀2][𝐻 + ]𝛾𝐴𝑀2 𝛾𝐻 +

[𝐴𝑀2𝐻 + ]

(13)

𝛾𝐴𝑀2𝐻 +

[𝐴𝑀2][𝐻𝐶𝑂3― ]

𝛾𝐴𝑀2 𝛾𝐻𝐶𝑂3―

[𝐴𝑀2𝐶𝑂𝑂 ― ][𝐻2𝑂]𝛾𝐴𝑀 𝐶𝑂𝑂 2

𝐾4 =

𝐾5 =

―

(14)

𝛾𝐻2𝑂

[𝐻𝐶𝑂3― ][𝐻 + ]𝛾𝐻𝐶𝑂3― 𝛾𝐻 +

(15)

[𝐶𝑂2][𝐻2𝑂] 𝛾𝐶𝑂2 𝛾𝐻2𝑂

[𝐶𝑂23 ― ][𝐻 + ]𝛾𝐶𝑂23 ― 𝛾𝐻 + [𝐻𝐶𝑂3― ] 𝛾𝐻𝐶𝑂3―

(16)


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𝐾6 =

[𝐻 + ][𝑂𝐻 ― ]𝛾𝐻 + 𝛾𝑂𝐻 ― [𝐻2𝑂]

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(17)

𝛾𝐻2𝑂

Based on the mentioned reactions in the aqueous phase, the following ions and molecules can be formed: 𝐴𝑀1𝐻 + , 𝐴𝑀1, 𝐴𝑀2𝐻 + , 𝐴𝑀2, 𝐴𝑀2𝐶𝑂𝑂 ― , 𝐶𝑂2, 𝐻𝐶𝑂3― , 𝐶𝑂23 ― , 𝐻2𝑂, 𝐻3𝑂 + , and OH ― . The equilibrium constants (𝐾𝑖) and the Henry’s constant are calculated using the below temperature-dependent equation: 𝐴 𝐾𝑖 ( 𝑜𝑟 𝐻𝐶𝑂2/(𝑀𝑃𝑎.𝑘𝑔.𝑚𝑜𝑙 ―1)) = 𝑒𝑥𝑝 ( + 𝐵𝑙𝑛 𝑇 + 𝐶𝑇 + 𝐷) 𝑇

(18)

where 𝐴, 𝐵, 𝐶, and 𝐷 are constant parameters which are presented in Table 2. Table 2 The constant parameters for equilibrium constant calculation of TEPA and TETA were not available in the literature. Thus, by regression of experimental data and optimization in 313 K, the constants are calculated and presented in Table 3. Table 3 In addition to the phase equilibrium and chemical equilibrium equation, the mass balance, charge balance, and the equation of neutrality of electrolyte solution need to be evaluated. The mass balance equations are presented in Equations (19) to (22). 

The charge balance equation:


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[𝐻3𝑂 + ] + [𝐴𝑀1𝐻 + ] + [𝐴𝑀2𝐻 + ] = [𝐴𝑀2𝐶𝑂𝑂 ― ] + [𝐻𝐶𝑂3― ] + [𝑂𝐻 ― ] + 2 × [𝐶𝑂23 ― ]

(19 )



The CO2 balance equations:

𝛼 × ([𝐴𝑀1]0 + [𝐴𝑀2]0) = [𝐶𝑂2] + [𝐻𝐶𝑂3― ] + [𝐶𝑂23 ― ] + [𝐴𝑀2𝐶𝑂𝑂 ― ]



(20)

The amine balance equations:

[𝐴𝑀1]0 = [𝐴𝑀1] + [𝐴𝑀1𝐻 + ]

(21)

[𝐴𝑀2]0 = [𝐴𝑀2] + [𝐴𝑀2𝐻 + ] + [𝐴𝑀2𝐶𝑂𝑂 ― ]

(22)

3.3. Thermodynamic modeling 3.3.1. Deshmukh-Mather model The activity coefficients of species in the liquid phase are calculated using a thermodynamic model, the Deshmukh-Mather model [38]. In this model, the activity coefficient is calculated based on the Debye-Huckel equation for electrolyte systems [39] (Equation (23)). Equation (23) comprises two terms. The first term shows the electrostatic forces and the second term is used to calculate the short-range Van-der-Waals forces.

𝑙𝑛 𝛾𝑖 = ―

𝐴𝑍2𝑖 𝐼 1+𝐵 𝐼

∑𝛽 𝑚

+2

𝑖𝑗

(23)

𝑗

𝑖


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where 𝛾𝑖 is the activity coefficient of species 𝑖, 𝑍𝑖 is the charge of ion 𝑖 (it is equal to zero for molecules), 𝐼 is the ionic strength of the solution, 𝛽𝑖𝑗 is the interaction parameter between species 𝑖 and 𝑗, and 𝑚𝑗 is the molality of species 𝑗. The ionic strength term in Equation (23) is defined as Equation (24): 𝐼=

1 2

∑𝑚𝑍 𝑖

2 𝑗 𝑗

(24)

In this study, 𝛽𝑖𝑗 is considered as a temperature-dependent equation presented as Equation (25). (25)

𝛽𝑖𝑗 = 𝑎𝑖𝑗 + 𝑏𝑖𝑗𝑇

where 𝑎𝑖𝑗 and 𝑏𝑖𝑗 are adjustable parameters. In Equation (23), 𝐵 equals to 1.2 [40, 41] and 𝐴 is dependent on temperature and is calculated using Equation (26). 𝐴 = 1.313 + 1.335 × 10 ―3 × (𝑇 ― 273.15) + 1.164 × 10 ―5 × (𝑇 ― 273.15)2

(26)

There are 11 species in the liquid phase for amine systems contributed to this work, including 𝐴𝑀1 𝐻 + , 𝐴𝑀1, 𝐴𝑀2𝐻 + , 𝐴𝑀2, 𝐴𝑀2𝐶𝑂𝑂 ― , 𝐶𝑂2, 𝐻𝐶𝑂3― , 𝐶𝑂23 ― , 𝐻2𝑂, 𝐻3𝑂 + , and 𝑂𝐻 ― . Thus, 121 interaction parameters should be evaluated for all species. This high number of interaction parameters leads to difficulty in the optimization process. However, the parameters can be reduced to lower numbers using some assumptions: (1) the interactions between the low concentration species such as 𝐻3𝑂 + and 𝑂𝐻 ― assumed to be negligible, (2) the interactions between the species with the same charges (e.g., 𝑀𝐸𝐴𝐻 + ― 𝐴𝑀𝑃𝐻

+

) assumed negligible, and (3) the interactions


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between the same species assumed to be negligible [31, 42]. Based on the mentioned assumptions, 12 interaction parameters are considered for the optimization process. It should be noted that the reactions, conditions, solvent properties, and equilibrium balances of the systems are different for each system investigated in this study. Therefore, generalized parameters couldn’t be used in the D-M model for all systems as published papers show this in the field of the CO2 removal processes by amine solutions [40, 43-45].

3.3.2. Modified Kent-Eisenberg model The Kent-Eisenberg model is known as one of the simplest thermodynamic models where the activity coefficients assumed to be unity. In this model, the relation between the experimental and theoretical data are calculated considering equilibrium constants of main reactions. Haji-Sulaiman et al used modified Kent-Eisenberg model to correlate vapor-liquid equilibrium data of CO2 solubility in the DEA, MDEA, and their blends in the temperature range from 303 to 332 K and the pressure range from 0.09 to 100 kPa [33]. They defined apparent equilibrium constants which are calculated by multiplying a factor named 𝐹𝑖 in equilibrium constants of main reactions. In this study, by using the same method to the work of Haji-Sulaiman, et al., [33], the solubility of CO2 in 4 aqueous solutions are evaluated. The factor 𝐹𝑖 is considered as a function of the partial pressure of CO2 and amine concentration. (27)

𝐾′𝑖 = 1,2 = 𝐾𝑖 = 1,2 × 𝐹𝑖 = 1,2


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(

𝑓𝑖

𝐹𝑖 = 𝑒𝑥𝑝 𝑃𝐶𝑂2

𝑗𝑖

+ 𝑔𝑖 × 𝑙𝑛 𝑃𝐶𝑂2 + ℎ𝑖 × 𝑃𝐶𝑂2 +

[𝑅𝑁𝐻2] 𝑘𝑖 × 𝑙𝑛 [𝑅𝑁𝐻2] + 𝑚𝑖 × [𝑅𝑁𝐻2]

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+

)

(28)

where 𝑃𝐶𝑂2 is CO2 partial pressure, [𝑅𝑁𝐻2] is the concentration of the solution, and 𝑓𝑖, 𝑔𝑖, ℎ𝑖, 𝑗𝑖, 𝑘𝑖, and 𝑚𝑖 are interaction parameters.

3.4. Regression of experimental data The adjustable parameters of the apparent equilibrium constant in modified Kent-Eisenberg model (𝐾′𝑖), and interaction parameters of Deshmukh-Mather model (𝛽𝑖𝑗) were found by using the experimental data presented in this study. To do this, an objective function (𝑂𝐹) is defined based on Equation (29). 1 𝑂𝐹 = 𝑁

𝑛

∑

𝐸𝑥𝑝 |𝛼𝐶𝑎𝑙𝑐 𝐶𝑂 ― 𝛼𝐶𝑂 |

𝑖=1

2

2

(29)

𝛼𝐸𝑥𝑝 𝐶𝑂2

𝐸𝑥𝑝 where 𝛼𝐶𝑎𝑙𝑐 𝐶𝑂2 is theoretical CO2 loading, 𝛼𝐶𝑂2 is experimental CO2 loading, and 𝑁 is the number of

data points. The 𝑂𝐹 was minimized using the simplex method (i.e., fminsearch function) in MATLAB software.

4. Results and discussion 4.1. The CO2 solubility in activator + AMP aqueous solutions


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The CO2 solubility data is one of the most important required data to obtain the different parameters for the design and improvement of a post-combustion CO2 capture plant. Thus, in this section, the effect of using different activators to the AMP aqueous solution on the CO2 absorption capacity are experimentally measured. The CO2 solubility in aqueous amines solutions of MEA+AMP, DEA+AMP, TETA+AMP, and TEPA+AMP are measured. The experiments are performed at a constant temperature of 313 K (i.e., the absorber condition) and CO2 partial pressure up to 186.3 kPa. On the other hand, the experiments were carried out for three different molar ratio of activators to AMP aqueous solutions, including 1 M Activator + 3 M AMP (i.e., molar ratio=0.33), 2 M Activator + 3 M AMP (i.e., molar ratio=0.66), and 3 M Activator + 3 M AMP (i.e., molar ratio=1). The experimental data points for CO2 absorption in aqueous amine solutions are presented in Table 4. Table 4

Figs. 5-7 compare the performance of different activators on CO2 absorption capacity. Fig. 5 shows the CO2 solubility in 1 M Activator + 3 M AMP aqueous solution. As illustrated in this figure, the CO2 loading increases with CO2 partial pressure for all activators. The highest CO2 loading obtained using TEPA activator, and the lowest CO2 loading belongs to MEA activator. In other words, the TETA+AMP and TEPA+AMP have the highest capacity for CO2 absorption compare to DEA+AMP and MEA+AMP. The reason is that the TEPA and TETA are alkyl amines with multiple amine groups [36]. Fig. 6 shows the CO2 solubility in the 2 M Activator + 3 M AMP aqueous solution. As exhibited in this figure, the same trend in CO2 loading capacity occurred when using 2 M activator. Similar to Fig. 5, the CO2 partial pressure range for TEPA is the lowest among the other activators. However, the range of CO2 loading in 2 M TEPA is broader than using 20 ACS Paragon Plus Environment

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1 M TEPA. Fig. 7 exhibits the CO2 solubility in 3 M Activator + 3 M AMP aqueous solution. It can be observed that TEPA and TETA have more capacity for CO2 absorption. Fig. 5-7

As is seen in the Figs. 5-7, in the mixture of MEA+AMP, the variation of CO2 loading is not very much; however, by increasing the molar ratio of MEA in the mixture, the CO2 loading slightly decreases. In the mixture of DEA+AMP, at a constant temperature, the CO2 loading increases by increasing CO2 partial pressure. However, by increasing the ratio of DEA in the mixture of DEA+AMP, the CO2 loading decreases. In the system of TETA+AMP+H2O+CO2, it can be seen that the behavior of CO2 loading and CO2 partial pressure is the same as the MEA+AMP+H2O+CO2 and DEA+AMP+H2O+CO2. This means that the CO2 loading increases by increasing CO2 partial pressure. The range of CO2 loading is wider compared to MEA+AMP and DEA+AMP solutions. Regarding the TETA+AMP and TEPA+AMP amines, the high CO2 loading is observed even in the low pressures. This is due to the molecular structure of TEPA and TETA which leads to a higher capacity for CO2 absorption and faster reaction rate. On the other hand, by increasing the molar ratio of TEPA and TETA in their mixture with AMP, the CO2 loading almost remained constant, especially in the case of 3 M TEPA + 3 M AMP and 3 M TETA + 3 M AMP, the CO2 loading decreased compared to the 1 M TEPA+ 3 M AMP and 1 M TETA + 3 M AMP cases. It can be interpreted by the high viscosity of TEPA and TETA. When the molar ratio of TEPA and TETA increases in the system, the viscosity of amine mixture rises, and it causes the lower reaction rate. Thus, the CO2 absorption capacity can be suffered by the high viscosity of TEPA and TETA, if their molar ratio increases significantly. However, it should be noted that the


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enhancement in CO2 absorption continues until the viscosity of the mixture does not exceed a specific value [29].

4.2. Thermodynamic modeling results The measured CO2 solubility data from the experiments of the present work are used to optimize the adjustable parameters of two thermodynamic models. In this study, two thermodynamic models including Deshmukh-Mather and modified Kent-Eisenberg are used to correlate the experimental CO2 solubility data. First, the interaction parameters of Deshmukh-Mather model for all species, considering the mentioned assumptions in section 3, in the liquid phase are calculated. These parameters are regressed using the experimental data. The regressed parameters of Deshmukh-Mather model are summarized in Table 5. Then, the apparent equilibrium constant in modified Kent-Eisenberg model is calculated for different amines. The parameters of the modified Kent-Eisenberg model are also regressed using the experimental data and presented in Table 6. Table 5 Table 6

The experimental data for CO2 absorption are measured for the molar ratios of 0.33, 0.66, and 1 of all amine mixtures including MEA+AMP, DEA+AMP, TETA+AMP, and TEPA+AMP at 313 K. The Deshmukh-Mather model with 24 adjustable parameters predicts the experimental data more accurate than modified Kent-Eisenberg with 9 adjustable parameters. In lower amounts of CO2 partial pressure (low CO2 loading), both Deshmukh-Mather and modified Kent-Eisenberg models show a relatively high deviation from the experimental data. This reveals that these two models are not suitable for low CO2 partial pressures [46]. To find the reliability of the 22 ACS Paragon Plus Environment

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thermodynamic models in prediction of CO2 solubility (i.e., prediction of CO2 loading), the average relative deviation percent (%ARD) is calculated for each set of experiments. The %ARD is defined as Equation (30) and were highlighted in the following figures. 100 %𝐴𝑅𝐷 = × 𝑁

𝑁

∑

|𝛼𝑒𝑥𝑝. | ― 𝛼𝑐𝑎𝑙𝑐. 𝑖 𝑖

𝑖=1

(30)

𝛼𝑒𝑥𝑝. 𝑖

where 𝑁 denotes the total number of data points, 𝛼𝑒𝑥𝑝. is the experimental CO2 loading, and 𝛼𝑐𝑎𝑙𝑐. 𝑖 𝑖 stands for calculated CO2 loading using thermodynamic models. Table 7 presents the accuracy of thermodynamic models in prediction of CO2 solubility. As is observed, the Deshmukh-Mather model predicts the CO2 loading more accurate than the modified Kent-Eisenberg model. However, for CO2 solubility in 3 M DEA+3 M AMP and 2 M TEPA+3 M AMP aqueous solutions, the modified Kent-Eisenberg model predicts CO2 solubility more accurate. The %ARD of the Deshmukh-Mather model and the modified Kent-Eisenberg model are 2.91 and 5.79, respectively. Table 7 To investigate the performance of adding each Activator + AMP, the plots of CO2 partial pressure against CO2 loading for the different molar ratio of Activator + AMP are presented in Figs. 8-10. As is seen, the performance of CO2 loading in MEA+AMP and DEA+AMP show the same trend. The highest amount of CO2 loading in the MEA+AMP and DEA+AMP systems are 0.844 and 0.865 (mol CO2/mol solution), respectively. The increase in activator concentration in its mixture with AMP, have a slightly negative effect on CO2 loading. Thus, the highest amount of CO2 loading is obtained in a molar ratio of 0.33 for MEA+AMP and DEA+AMP (1 M Activator + 3 M 23 ACS Paragon Plus Environment

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AMP). For TETA+AMP and TEPA+AMP systems, the amount of CO2 loading is much higher than MEA+AMP and DEA+AMP systems at the same molar ratio of Activator/AMP. This shows that these two activators have a much more positive effect on CO2 absorption compare to MEA and DEA. The highest amount of CO2 loading in the TETA+AMP and TEPA+AMP systems belongs to TEPA+AMP in the molar ratio of 0.66 (2 M TEPA+3 M AMP) which equals to 1.22 (mol CO2/mol solution). The superiority of TEPA is due to the structure of TEPA. The TEPA is an alkyl amine with two sites of amine type I and three sites of amine type II [28]. This structure helps to enhance CO2 loading when it added to AMP. The TETA+AMP shows higher CO2 loading compare to MEA+AMP and DEA+AMP while the CO2 loading for TETA+AMP is lower than TEPA+AMP. The highest amount of CO2 loading for the TETA+AMP is about 1.15 (mol CO2/mol solution). The TETA is an alkyl amine with two sites of amine type I and two sites of amine type II. Thus, TETA has high reactivity to CO2. As a general result for this section, it can be inferred that the alkyl amines can be successfully used as an activator for CO2 absorption. They are a good substitution for MEA and DEA. Also, by increasing the number of amine groups in an alkyl amine, the CO2 loading and reaction rate increases. Details of modeling results for the CO2 solubility predictions from Deshmukh-Mather and modified Kent-Eisenberg models compared to the experimental data are presented in supplementary information Figs. 1.S - 12.S.

Figs. 8-10

4.3. The CO2 absorption rate of activator + AMP aqueous solutions


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In this section, the effect of using different Activator + AMP aqueous solutions on the CO2 absorption rate at the concentration (1 M Activator + 3 M AMP) was investigated and compared them with 3 M AMP solution without any activator. The CO2 absorption rate is highly relevant because of the higher CO2 absorption rate results in the lower size and cost of CO2 capture equipment [47]. Thus, the CO2 absorption rate in different Activator + AMP aqueous solutions is measured in terms of decreasing CO2 partial pressure versus time. Figure 11 shows the absorption rate of CO2 from the start of the experiment to a one-time step. As is seen in Fig. 11, the TEPA+AMP system shows the highest rate of CO2 absorption while the lowest absorption rate goes to AMP. The reaction rate for the case of AMP is lower than the case of using AMP with activators. It shows that the addition of activators to the AMP has a positive effect on the reaction rate and demonstrates the performance of the combination systems and the role of activators on the reaction rate and the absorption performance. The higher rate of CO2 absorption with TEPA+AMP is due to the existence of amine groups in the structure of TEPA. After TEPA, the TETA shows the highest reactivity with CO2. . Fig. 11

5. Conclusion In this work, we have evaluated the performance of CO2 absorption capacity by adding the activators to the AMP solvent using a VLE apparatus. For this aim, four additives were selected to enhance the CO2 absorption rate and CO2 loading capacity of AMP as the main solvent. For better investigation, all the experiments were performed at an absorption temperature (313 K) and 25 ACS Paragon Plus Environment

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at concentrations of 1, 2, 3 M. We selected alkyl amines containing multiple amino groups (TETA, and TEPA) were used as activators to improve the CO2 absorption performances of aqueous AMP solutions. We have obtained that in the MEA+AMP and DEA+AMP aqueous solutions, the CO2 loading decreases by increasing the concentration of MEA and DEA in the mixture. This is because of the structure of MEA and DEA where they have lower CO2 absorption capacity, and higher CO2 absorption rate compare to AMP. In the TETA+AMP and TEPA+AMP aqueous solutions, the CO2 loading increases by increasing the concentration of TETA and TEPA in the mixture. However, in the high concentration of TEPA and TETA (e.g., 3 M TEPA + 3 M AMP and 3 M TETA + 3 M AMP), the absorption rate of CO2 reduces because the viscosity of the solution increases. Considering the activators, TEPA shows the highest CO2 absorption capacity compare to TETA, DEA, and MEA. The higher absorption capacity of TEPA is due to the structure of TEPA which has more amine groups. The results of thermodynamic modeling showed that both Deshmukh-Mather and modified Kent-Eisenberg models predict the CO2 solubility data accurately. However, the Deshmukh-Mather model with %ARD equal to 2.91 is more accurate than the modified Kent-Eisenberg model with %ARD equal to 5.79.

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[20]

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Sang Hyun Park, Ki Bong Lee, Jae Chun Hyun, and Sung Hyun Kim, "Correlation and prediction of the solubility of carbon dioxide in aqueous alkanolamine and mixed alkanolamine solutions," Industrial & engineering chemistry research, vol. 41, no. 6, pp. 1658-1665, 2002. Meng-Hui Li and Bei-Chia Chang, "Solubilities of carbon dioxide in water+ monoethanolamine+ 2-amino-2-methyl-1-propanol," Journal of Chemical and Engineering Data, vol. 39, no. 3, pp. 448-452, 1994. Kunihiko Yoshida, Tomio Mimura, Shigeru Shimojo, Mutsunori Karasaki, Masaki Iijima, Touru Seto, and Shigeaki Mitsuoka, "Method for removing carbon dioxide from combustion exhaust gas," 2002. Ugochukwu E Aronu, Hallvard F Svendsen, Karl Anders Hoff, and Olav Juliussen, "Solvent selection for carbon dioxide absorption," Energy Procedia, vol. 1, no. 1, pp. 10511057, 2009. Prachi Singh, John PM Niederer, and Geert F Versteeg, "Structure and activity relationships for amine-based CO2 absorbents-II," Chemical Engineering Research and Design, vol. 87, no. 2, pp. 135-144, 2009. Mohamed H Al-Marzouqi, Sayed AM Marzouk, Muftah H El-Naas, and Nadia Abdullatif, "CO2 removal from CO2− CH4 gas mixture using different solvents and hollow fiber membranes," Industrial & Engineering Chemistry Research, vol. 48, no. 7, pp. 3600-3605, 2009. R Ramazani, Saeed Mazinani, A Jahanmiri, Siavash Darvishmanesh, and Bart Van der Bruggen, "Investigation of different additives to monoethanolamine (MEA) as a solvent for CO2 capture," Journal of the Taiwan Institute of Chemical Engineers, vol. 65, pp. 341349, 2016. Jean-Marc G Amann and Chakib Bouallou, "CO2 capture from power stations running with natural gas (NGCC) and pulverized coal (PC): Assessment of a new chemical solvent based on aqueous solutions of N-methyldiethanolamine+ triethylene tetramine," Energy Procedia, vol. 1, no. 1, pp. 909-916, 2009. Michel A Ouimet, "Process for the recovery of carbon dioxide from a gas stream," 2009. Song Yi Choi, Sung Chan Nam, Yeo Il Yoon, Ki Tae Park, and So-Jin Park, "Carbon dioxide absorption into aqueous blends of methyldiethanolamine (MDEA) and alkyl amines containing multiple amino groups," Industrial & Engineering Chemistry Research, vol. 53, no. 37, pp. 14451-14461, 2014. Pailin Muchan, Jessica Narku-Tetteh, Chintana Saiwan, Raphael Idem, and Teeradet Supap, "Effect of number of amine groups in aqueous polyamine solution on carbon dioxide (CO2) capture activities," Separation and Purification Technology, vol. 184, pp. 128-134, 2017. Morteza Afkhamipour and Masoud Mofarahi, "Experimental measurement and modeling study on CO2 equilibrium solubility, density and viscosity for 1-dimethylamino-2propanol (1DMA2P) solution," Fluid Phase Equilibria, vol. 457, pp. 38-51, 2018. Peyman Pakzad, Masoud Mofarahi, Amir Abbas Izadpanah, Morteza Afkhamipour, and Chang-Ha Lee, "An experimental and modeling study of CO2 solubility in a 2-amino-2methyl-1-propanol (AMP)+ N-methyl-2-pyrrolidone (NMP) solution," Chemical Engineering Science, vol. 175, pp. 365-376, 2018. MZ Haji-Sulaiman, MK Aroua, and A Benamor, "Analysis of equilibrium data of CO2 in aqueous solutions of diethanolamine (DEA), methyldiethanolamine (MDEA) and their 28 ACS Paragon Plus Environment

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mixtures using the modified Kent Eisenberg model," Chemical Engineering Research and Design, vol. 76, no. 8, pp. 961-968, 1998. Madhusree Kundu, Bishnu P Mandal, and Syamalendu S Bandyopadhyay, "Vapor− Liquid Equilibrium of CO2 in Aqueous Solutions of 2-Amino-2-methyl-1-propanol," Journal of Chemical & Engineering Data, vol. 48, no. 4, pp. 789-796, 2003. Dong-Joo Seo and Won-Hi Hong, "Solubilities of carbon dioxide in aqueous mixtures of diethanolamine and 2-amino-2-methyl-1-propanol," Journal of Chemical & Engineering Data, vol. 41, no. 2, pp. 258-260, 1996. AK Chakraborty, G Astarita, and KB Bischoff, "CO2 absorption in aqueous solutions of hindered amines," Chemical Engineering Science, vol. 41, no. 4, pp. 997-1003, 1986. Young Eun Kim, Sung Jun Moon, Yeo Il Yoon, Soon Kwan Jeong, Ki Tae Park, Shin Tae Bae, and Sung Chan Nam, "Heat of absorption and absorption capacity of CO2 in aqueous solutions of amine containing multiple amino groups," Separation and Purification Technology, vol. 122, pp. 112-118, 2014. RD Deshmukh and AE Mather, "A mathematical model for equilibrium solubility of hydrogen sulfide and carbon dioxide in aqueous alkanolamine solutions," Chemical Engineering Science, vol. 36, no. 2, pp. 355-362, 1981. P Debye and E Hückel, "De la theorie des electrolytes. I. abaissement du point de congelation et phenomenes associes," Physikalische Zeitschrift, vol. 24, no. 9, pp. 185-206, 1923. Mohammad Ehsan Hamzehie and Hesam Najibi, "Experimental and theoretical study of carbon dioxide solubility in aqueous solution of potassium glycinate blended with piperazine as new absorbents," Journal of CO2 Utilization, vol. 16, pp. 64-77, 2016. Ali T Zoghi, Farzaneh Feyzi, and Saeed Zarrinpashneh, "Equilibrium solubility of carbon dioxide in a 30 wt.% aqueous solution of 2-((2-aminoethyl) amino) ethanol at pressures between atmospheric and 4400 kPa: An experimental and modelling study," The Journal of Chemical Thermodynamics, vol. 44, no. 1, pp. 66-74, 2012. Mohammad Ehsan Hamzehie and Hesam Najibi, "CO2 solubility in aqueous solutions of potassium prolinate and (potassium prolinate+ 2-amino-2-methyl-1-propanol) as new absorbents," Journal of Natural Gas Science and Engineering, vol. 34, pp. 356-365, 2016. Morteza Afkhamipour and Masoud Mofarahi, "A modeling-optimization framework for assessment of CO2 absorption capacity by novel amine solutions: 1DMA2P, 1DEA2P, DEEA, and DEAB," Journal of cleaner production, vol. 171, pp. 234-249, 2018. A Benamor and MK Aroua, "Modeling of CO2 solubility and carbamate concentration in DEA, MDEA and their mixtures using the Deshmukh–Mather model," Fluid Phase Equilibria, vol. 231, no. 2, pp. 150-162, 2005. Mehdi Vahidi, Ali T Zoghi, Behnoosh Moshtari, and Behrouz Nonahal, "Equilibrium solubility of carbon dioxide in an aqueous mixture of n-methyldiethanolamine and diisopropanolamine: an experimental and modeling study," Journal of Chemical & Engineering Data, vol. 58, no. 7, pp. 1963-1968, 2013. Humbul Suleman, Abdulhalim Shah Maulud, and Zakaria Man, "Review and selection criteria of classical thermodynamic models for acid gas absorption in aqueous alkanolamines," Reviews in Chemical Engineering, vol. 31, no. 6, pp. 599-639, 2015. Ida M Bernhardsen and Hanna K Knuutila, "A review of potential amine solvents for CO2 absorption process: Absorption capacity, cyclic capacity and pKa," International Journal of Greenhouse Gas Control, vol. 61, pp. 27-48, 2017. 29 ACS Paragon Plus Environment

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Stephen A Lawrence, Amines: synthesis, properties and applications. Cambridge University Press, 2004. I-Shien Jane and Meng-Hui Li, "Solubilities of mixtures of carbon dioxide and hydrogen sulfide in water+ diethanolamine+ 2-amino-2-methyl-1-propanol," Journal of Chemical & Engineering Data, vol. 42, no. 1, pp. 98-105, 1997. Sung June Hwang, Huiyong Kim, and Kwang Soon Lee, "Prediction of VLE for aqueous blended amines using VLE models of single amines," International Journal of Greenhouse Gas Control, vol. 49, pp. 250-258, 2016. TJ Edwards, Gerd Maurer, John Newman, and JM Prausnitz, "Vapor‐liquid equilibria in multicomponent aqueous solutions of volatile weak electrolytes," AIChE Journal, vol. 24, no. 6, pp. 966-976, 1978.

Acknowledgment We thank the Persian Gulf University for financial support, for providing various facilities, and for necessary approval.

Nomenclature Acronyms AMP

2-amino-2-methyl-1-propanol

ARD

Average relative deviation

DEA

Diethanolamine

DETA

Diethylenetriamine

GHG

Greenhouse gas

MEA

Monoethanolamine

OF

Objective function

PZ

Piperazine

TETA

Triethylenetetramine

TEPA

Tetraethylenepentamine

VLE

Vapor-liquid equilibrium

Variables\Letters 30 ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

A

Activity

C

Concentration

F

Fugacity

F

Factor used in the Kent-Eisenberg model

H

Henry’s constant

I

Ionic strength of solution

K

Equilibrium constant of reaction

K’

Apparent equilibrium constant

M

Molality

M

Molarity

N

Number of points

P

Pressure

T

Temperature

Z

Charge of ion

Greek Letters 𝛼

CO2 loading

𝛽

Interaction parameter

𝜑

Fugacity coefficient

𝛾

Activity coefficient

𝑣

Stoichiometric coefficient

Subscripts i, j

Component

j

Reaction

0

Initial concentration

Superscripts +

Cation

-

Anion


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Page 33 of 45

Fig. 1. Schematic of chemical absorption measurement

1000 2 M DEA, Haji-Sulaiman et al., 1998 2 M DEA, This work

CO2 partial pressure (kPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

100

10

1

0.1

0.01 0

0.1

0.2 0.3 0.4 0.5 0.6 0.7 CO2 loading (mol CO2/mol DEA)

0.8

0.9

Fig. 2. Comparison of experimental CO2 solubility in 2 M DEA aqueous solution at 313 K with the work of Haji-Sulaiman, et al. [33].


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100

2.8 M AMP, Kundu et al., 2003 2.8 M AMP, This work


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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10

1 0.4

0.5

0.6 0.7 0.8 0.9 CO2 loading (mol CO2/mol AMP)

1

Fig. 3. Comparison of experimental CO2 solubility in 2.8 M AMP aqueous solution at 313 K with the work of Kundu et al., [34].


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1000


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

100

10

AMP+MEA, Li and Chang, 1994

1

AMP+MEA, This work AMP+DEA, Seo and Hong, 1996 AMP+DEA, This work

0.1 0

0.2 0.4 0.6 0.8 1 CO2 loading (mol CO2/mol (AMP+MEA/DEA))

1.2

Fig. 4. Comparison of experimental CO2 solubility in MEA+AMP and DEA+AMP aqueous solution at 313 K with the works of Li and Chang [21] and Seo and Hong [35].


Energy & Fuels

1000

1 M MEA/3 M AMP 1 M DEA/3 M AMP


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1 M TETA/3 M AMP

100

1 M TEPA/3 M AMP

10

1

0.1 0.1

0.2

0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 CO2 loading (mol CO2/mol (Activator/AMP))

1.2

Fig. 5. The CO2 solubility in the 1 M Activator/3 M AMP aqueous solution at 313 K.


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100


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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10

1

2 M MEA/3 M AMP 2 M DEA/3 M AMP 2 M TETA/3 M AMP 2 M TEPA/3 M AMP

0.1 0.3

0.4

0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 CO2 loading (mol CO2/mol (Activator/AMP))

1.3



Energy & Fuels

1000


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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100

10

3 M MEA/3 M AMP

1

3 M DEA/3 M AMP 3 M TETA/3 M AMP 3 M TEPA/3 M AMP

0.1 0.2

0.3

0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 CO2 loading (mol CO2/mol (Activator/AMP))

1.3



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200

Expt. Data, 1 M MEA/3 M AMP Expt. Data, 1 M DEA/3 M AMP Expt. Data, 1 M TETA/3 M AMP Expt. Data, 1 M TEPA/3 M AMP Deshmukh-Mather model

180 160


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

140 120 100 80 60 40 20 0 0

0.2 0.4 0.6 0.8 1 CO2 loading (mol CO2/mol (Activator/AMP))

1.2

Fig. 8. Plot of CO2 partial pressure vs. CO2 loading for the 1 M Activator/3 M AMP solution.


Energy & Fuels

140



120 100 80 60 40 20 0 0

0.2 0.4 0.6 0.8 1 1.2 CO2 loading (mol CO2/mol (Activator/AMP))

1.4


200


160


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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120

80

40

0 0

0.2 0.4 0.6 0.8 1 1.2 CO2 loading (mol CO2/mol (Activator/AMP))

1.4



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1400 MEA/AMP 1200


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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DEA/AMP TETA/AMP

1000

TEPA/AMP AMP

800 600 400 200 0 0

5

10

15

20 25 Time (s)

30

35

40

Fig. 11. The CO2 absorption rate in Activator/AMP aqueous solutions (1 M Activator/3 M AMP) and 3 M AMP aqueous solution


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Table1. Physical properties of amines [48] Properties

AMP

DEA

MEA

TETA

TEPA

Chemical formula

C4H11NO

C4H11NO2

C2H7NO

C6H18N4

C8H23N5

Molecular weight (gr/mol)

89.14

105.14

61.08

146.23

189.3

Purity

99%

98%

99%

99%

97%

Density (gr/cm3)

0.934

1.092

1.011

0.982

0.994

Boiling point (°C)

165

268

171

266

320

Vapor pressure (mmHg)

Experimental and Theoretical Investigation of Equilibrium

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