Kinetic and Thermodynamic Analysis of High-Pressure CO 2 Capture Using Ethylenediamine: Experimental Study and Modeling

: One of the alternatives to reduce CO 2 emissions from industrial sources (mainly the oil and gas industry) is CO 2 capture. Absorption with chemical solvents (alkanolamines in aqueous solutions) is the most widely used conventional technology for CO 2 capture. Despite the competitive advantages of chemical solvents, the technological challenge in improving the absorption process is to apply alternative solvents, reducing energy demand and increasing the CO 2 captured per unit of solvent mass. This work presents an experimental study related to the kinetic and thermodynamic analysis of high-pressure CO 2 capture using ethylenediamine (EDA) as a chemical solvent. EDA has two amine groups that can increase the CO 2 capture capacity per unit of solvent. A non-stirred experimental setup was installed and commissioned for CO 2 capture testing. Tests of the solubility of CO 2 in water were carried out to validate the experimental setup. CO 2 capture testing was accomplished using EDA in aqueous solutions (0, 5, 10, and 20 wt.% in amine). Finally, a kinetic model involving two steps was proposed, including a rapid absorption step and a slow diffusion step. EDA accelerated the CO 2 capture performance. Sudden temperature increases were observed during the initial minutes. The CO 2 capture was triggered after the absorption of a minimal amount of CO 2 (~10 mmol) into the liquid solutions, and could correspond to the “lean amine acid gas loading” in a typical sweetening process using alkanolamines. At equilibrium, there was a linear relationship between the CO 2 loading and the EDA concentration. The CO 2 capture behavior obtained adapts accurately (AAD < 1%) to the kinetic mechanism.


Introduction
CO 2 is a greenhouse gas that has a high impact on atmospheric pollution. In 2020, the global CO 2 emissions were estimated at 30.6 Gt [1], and sustained growth (approximately 0.43 Gt/y) is expected until 2040 [2]. Reducing CO 2 emissions is one of the Paris Agreement's goals [3] to offset the trend. Therefore, it is necessary to make technological and economic efforts worldwide to achieve this goal. The application of carbon capture and storage (CCS) technologies is an alternative for reducing CO 2 emissions from industrial sources-mainly the oil and gas industry [4].
Conventional technologies based on chemical absorption, physical absorption, and membranes are commonly used for CO 2 capture. In particular, "amine-based chemical absorption appears to be the most technologically mature and commercially viable method" [5], because it allows CO 2 capture at a large scale. Chemical absorption is a centralized process consisting of an absorption column where the CO 2 -containing gas (e.g., natural gas, flue gas, or fuel gas) comes into contact with a chemical solvent, whereupon resulting in the formation of 2-ammonioethylcarbamate [27,28]. A side reaction of 2CO 2 + 2H 2 N(CH 2 ) 2 NH 2 − OOCHN(CH 2 ) 2 NHCOO − + + H 3 N(CH 2 ) 2 NH + 3 (2) might take place, resulting in an intramolecular disalt. However, this reaction does not occur to a high extent, resulting in an insignificant contribution to the reaction rate [29]. The same behavior has been observed for CO 2 and EDA reactions under high-pressure conditions, mainly yielding 2-ammonioethylcarbamate [30]. According to the damping-film theory [31,32], the relationship between partial pressure of the gas and time can be described for the isothermal absorption of gas, as follows: where P represents the instantaneous CO 2 partial pressure, P o and P e denote partial pressure at the starting and at equilibrium conditions, respectively, u stands for the apparent absorption rate constant, and t is the time. By applying an equation of state for a real gas, the last equation can be written in terms of CO 2 mole numbers as follows: ln n o − n e n − n e = kt (4) where k is also the apparent absorption rate constant. The natural logarithm of the relationship between CO 2 mole numbers in the gas phase is directly proportional to the time passed during the absorption process. Thus, the constant rate k can be used to determine the apparent absorption rate performance of an aqueous EDA solution at different concentrations. CO 2 capture using chemical solvents under high-pressure conditions is a typical process in the oil and gas industry-specifically, in natural gas processing operations-to meet quality specifications or reduce operational problems due to the presence of CO 2 in pipelines and equipment [33]. Many studies of CO 2 capture at the laboratory scale are carried out to improve and identify new solvents; however, many of them are performed in low-pressure experimental systems, and their results could differ significantly from those obtained in high-pressure systems. To the best of our knowledge, an integrated study focused on the kinetics and thermodynamics of CO 2 capture using EDA in aqueous solutions has not been reported using high-pressure systems. In this work, an experimental study is presented that aims to delve into the kinetic and thermodynamic analysis of the high-pressure CO 2 capture process using EDA. A non-stirred experimental setup was installed and commissioned for CO 2 capture testing, with the main distinction being the pure CO 2 bubbling directly into the liquid phase to promote the initial mass transfer through the gas-liquid interface. In the first stage, tests of the solubility of CO 2 in water were carried out to validate both the experimental setup and the proposed methodology. Afterwards, CO 2 capture testing was performed using EDA in aqueous solutions at different concentrations. Some parameters-time-dependent and under equilibrium conditions-were defined in order to study the kinetic and thermodynamic behavior of the CO 2 capture process. Finally, a kinetic model that allows for the determination of the CO 2 loading vs. time and the apparent absorption rate performance was proposed.

Experimental Setup
The experimental setup used for the high-pressure CO 2 capture experiments is schematically depicted in Figure 1; it consists of a non-stirred pressure vessel (Parr Instrument Co., Moline, USA, model 4763) made of alloy C-276 with an internal volume of 98.7 cm 3 . The vessel has a movable head equipped with: (1) a differential pressure gauge, (2) a 0-20,685 KPa pressure transducer with an accuracy of ±10 KPa, (3) a type J thermocouple accurate to within ±0.1 K, and (4) a valve series allowing gas release, liquid sampling, and gas injection into the liquid phase. A pressure regulator connected to a pure CO 2 store cylinder is used for the gas supply. A heating unit and a temperature controller (Parr Instrument Co., model 4838) are used to control the vessel temperature. The setup is connected to a data acquisition interface, which records temperature and pressure measurements on a computer using SpecView 32 SCADA software (SpecView Corp., Gig Harbor, WA, USA). The vessel pressurization was carried out in a pulse by bubbling the pure CO 2 directly into the liquid phase to guarantee an intimate contact between the gas and the liquid from the experiments' first instants. This action could represent an advantage in the initial gas mass transfer process through the gas-liquid interface [34,35], considering that it is a non-stirred system.

CO2 Solubility Testing Procedure
Testing of the solubility of CO2 in water was performed to validate the high-p experimental setup by measuring the high-pressure vessel's equilibrium pressu known amount of gas and water in a closed system (batch mode). Initia high-pressure vessel was charged with 30 ± 0.2 cm 3 of ultrapure water using a g pette. The vessel was coupled to the experimental setup, and the temperature w 303 K. Then, the vessel was pressurized with pure CO2 up to the desired initial p value (700, 2100, 3500 KPa), and the system was closed. Pressure and temperatu were recorded until pressure stabilization for 24 h.

CO2 Capture Testing Procedure Using EDA
The ethylenediamine aqueous solutions were prepared at different concen (0, 5, 10, and 20 wt.% in amine) by adding the appropriate amine mass to ultrapu while stirring it for 1 min. The experimental procedure defined for CO2 capture using EDA was similar to that developed for the CO2 solubility tests. The high-p vessel was loaded with 30 ± 0.2 cm 3 of the aqueous EDA solution to be tested. Th was coupled to the experimental setup, and the temperature was set to 303 K. Th was then pressurized with pure CO2 up to 3500 KPa, and the system was closed. tial pressure (3500 KPa) was established to maximize the driving force of the CO2 process [15]. Pressure and temperature data were recorded until pressure stab for 12 h.

Data Processing
The pressure and temperature data as a function of time obtained during periments were used for the kinetic and thermodynamic analysis of the high-p CO2 capture process. The analysis was based on quantifying the variability of phase moles. Two main parameters were defined:  The quantity of CO2 removed refers to the gaseous phase moles transferred to uid phase, with respect to the initial amount of gas loaded into the high-p vessel. This quantity was calculated using Equation (5): where is the CO2 mole number in the gas phase, is the vessel pressu The experimental setup is designed to work in extreme conditions, i.e., maximum allowable working pressure (MAWP) 20,685 KPa, and maximum temperature 500 K. For safety reasons, the maximum allowable operating pressure (MAOP) was reduced to 6900 KPa.

CO 2 Solubility Testing Procedure
Testing of the solubility of CO 2 in water was performed to validate the high-pressure experimental setup by measuring the high-pressure vessel's equilibrium pressure for a known amount of gas and water in a closed system (batch mode). Initially, the highpressure vessel was charged with 30 ± 0.2 cm 3 of ultrapure water using a glass pipette. The vessel was coupled to the experimental setup, and the temperature was set to 303 K. Then, the vessel was pressurized with pure CO 2 up to the desired initial pressure value (700, 2100, 3500 KPa), and the system was closed. Pressure and temperature data were recorded until pressure stabilization for 24 h.

CO 2 Capture Testing Procedure Using EDA
The ethylenediamine aqueous solutions were prepared at different concentrations (0, 5, 10, and 20 wt.% in amine) by adding the appropriate amine mass to ultrapure water while stirring it for 1 min. The experimental procedure defined for CO 2 capture testing using EDA was similar to that developed for the CO 2 solubility tests. The high-pressure vessel was loaded with 30 ± 0.2 cm 3 of the aqueous EDA solution to be tested. The vessel was coupled to the experimental setup, and the temperature was set to 303 K. The vessel was then pressurized with pure CO 2 up to 3500 KPa, and the system was closed. The initial pressure (3500 KPa) was established to maximize the driving force of the CO 2 capture process [15]. Pressure and temperature data were recorded until pressure stabilization for 12 h.

Data Processing
The pressure and temperature data as a function of time obtained during the experiments were used for the kinetic and thermodynamic analysis of the high-pressure CO 2 capture process. The analysis was based on quantifying the variability of the gas phase moles. Two main parameters were defined:

•
The quantity of CO 2 removed refers to the gaseous phase moles transferred to the liquid phase, with respect to the initial amount of gas loaded into the high-pressure vessel. This quantity was calculated using Equation (5): where n gas is the CO 2 mole number in the gas phase, P is the vessel pressure, V is the gas phase volume, z is the compressibility factor calculated by the Peng-Robinson equation of state [36], and T is the vessel temperature. Moreover, t o and t are the initial time and an instantaneous time of the experiment, respectively.

•
The CO 2 loading refers to the amount of CO 2 removed from the gas phase for each liquid (amine + water) mole initially introduced into the high-pressure vessel. The CO 2 loading was calculated using Equation (6): where n liquid is the initial number of moles in the liquid phase. Appendix A shows the number of moles in each solution for the different amine concentrations used in this study. For this calculation, the experimental density data of aqueous EDA solutions at 303 K reported by Egorov et al. [37] were used.
Furthermore, the following additional time-dependent parameters were defined to compare the CO 2 capture process kinetics: • t 25 , t 50 , and t 90 refer to the time required to reach 25%, 50%, and 90%, respectively, of the total amount of CO 2 removed from the gas phase at the end of each experiment. • dn/dt refers to the CO 2 capture rate. This was calculated directly on the curve of the removed amount of CO 2 from the gas as a function of time, and corresponds to the maximum value of gas consumption in the experiments' first instants. This value was obtained numerically using the initial slope method [38]. Figure 2 shows the typical curves obtained during the CO 2 solubility tests. Figure 2a represents the drop in pressure as a function of time, where the pressure stabilization was achieved in the first 3 h. The final pressure reached corresponds to the equilibrium pressure at the conditions of CO 2 saturation in water, since these values were close to the equilibrium pressure estimated by the semi-empirical model of CO 2 solubility proposed by Ricaurte et al. [39] applied to each initial pressure studied (700, 2100, 3500 KPa). Figure 2b shows the CO 2 removed from the gas phase as a function of time. The amount of CO 2 solubilized into the water was proportional to the initial pressure, due to a higher driving force. The CO 2 solubility tests indicated that it is possible in non-stirred systems to achieve pressure stabilization in short times (<4 h), up until equilibrium conditions are reached in the saturation of CO 2 into water. In contrast, Farajzadeh et al. [40] reported pressure stabilization for a time >48 h in high-pressure CO 2 solubilization tests in quiescent conditions. Our experimental system's main distinction was the initial bubbling of CO 2 into the liquid solution, significantly reducing the time required for liquid saturation in non-stirred experimental setups [41]. Table 1 summarizes the kinetic and thermodynamic data obtained from the CO 2 solubility tests. The CO 2 capture rate (dn/dt) was increased at a higher initial pressure. For the most suitable case (P o = 3500 KPa), the t 25 , t 50 , and t 90 values corresponded to 2.20 min, 23.31 min, and 145.30 min, respectively. For the other initial pressures, 90% of the CO 2 was captured in~180 min. The CO 2 loading in each experiment was consistent with the reference data of CO 2 's solubility in water [39]. Therefore, the CO 2 solubility tests showed that (a) the experimental setup was suitable for carrying out CO 2 capture studies where equilibrium conditions are reached in <4 h, and (b) at a higher initial pressure, a greater driving force was obtained for the CO 2 capture process, maximizing the CO 2 loading at the equilibrium conditions. For these reasons, the CO 2 capture tests using the EDA were carried out at the highest initial pressure.  Table 1 summarizes the kinetic and thermodynamic data obtained from the C solubility tests. The CO2 capture rate ( / ) was increased at a higher initial pressu For the most suitable case ( = 3500 ), the , , and values corresponded 2.20 min, 23.31 min, and 145.30 min, respectively. For the other initial pressures, 90% the CO2 was captured in ~180 min. The CO2 loading in each experiment was consiste with the reference data of CO2's solubility in water [39]. Therefore, the CO2 solubil tests showed that (a) the experimental setup was suitable for carrying out CO2 captu studies where equilibrium conditions are reached in <4 h, and (b) at a higher init pressure, a greater driving force was obtained for the CO2 capture process, maximizi the CO2 loading at the equilibrium conditions. For these reasons, the CO2 capture te using the EDA were carried out at the highest initial pressure.  Figure 3 shows the characteristic curves obtained during the CO2 capture testi using EDA. Figure 3a depicts the drop in pressure as a function of time. The drop pressure was proportional to the amine concentration. For the 20 wt.% amine soluti the gas consumption represented a pressure drop of ~50% from the initial pressure. F ure 3b shows the temperature profile in the first 5 min of the CO2 capture process, wh a "sudden" temperature increase was observed in the initial moments of the CO2-liqu solution contact. The highest temperature points were reached approximately one m nute after the CO2 pressurization, being proportional to the amine concentration. T  3.2. CO 2 Capture Testing Using EDA Figure 3 shows the characteristic curves obtained during the CO 2 capture testing using EDA. Figure 3a depicts the drop in pressure as a function of time. The drop in pressure was proportional to the amine concentration. For the 20 wt.% amine solution, the gas consumption represented a pressure drop of~50% from the initial pressure. Figure 3b shows the temperature profile in the first 5 min of the CO 2 capture process, where a "sudden" temperature increase was observed in the initial moments of the CO 2 -liquid solution contact. The highest temperature points were reached approximately one minute after the CO 2 pressurization, being proportional to the amine concentration. The maximum value reached was~324 K for the 20 wt.% amine solution, representing a ∆T = 21 K. The temperature increments were related to the exothermic reaction between the CO 2 and the aqueous EDA solutions. Similar behavior was observed in the absorption processes of CO 2 [42] and other gases [43] in aqueous amine solutions. This exothermic phenomenon must be considered in the design of absorption towers for CO 2 capture using amines, since "one of the most important considerations involved in designing gas absorption towers is to determine whether temperatures will vary along with the height of the tower due to heat effects; note that the solubility usually depends strongly on temperature" [44]. A more detailed analysis of the exothermicity effect is presented in Section 3.3. process by increasing the solubility capacity of CO2 in the amine aqueous solutions (chemical solvents). Snapshots of the aqueous amine solutions were taken at the end of the experiments (see Figure 3d). An increase in the intensity of a yellowish-brown color (amber color) could be observed with the naked eye in the aqueous amine solutions. The variability in the color intensity was proportional to the amine concentration, which correlates with the amount of CO2 captured.  Table 2 summarizes the kinetic and thermodynamic data obtained from the CO2 capture testing using EDA. The CO2 capture rate ( / ) varied with the EDA concentration, obtaining the highest rate at 10 wt.%. Surprisingly, at the 20 wt.% amine concentration, the CO2 capture rate was the lowest, but the CO2 removed from the gas phase at equilibrium was the highest. A higher amine concentration increases the viscosity of aqueous amine solutions, unfavorably affecting the CO2 mass transfer rates [20,45]. Moreover, at 20 wt.% amine concentration, there was a more significant temperature increase in the liquid solution (see Figure 3b), which might directly affect the CO2 capture kinetics. Fan et al. [46] reported a decrease in the CO2 capture rate using alkanolamine aqueous solutions (e.g., MEA and diethanolamine (DEA)) with temperature increases. In all of the EDA aqueous solutions studied, the parameter was less than 300 min (<5 h). The time required for CO2 capture can be reduced in stirred experimental setups or continuous processes at different scales, i.e., pilot-plant-scale [47][48][49] or large-scale [50][51][52].  Figure 3c shows the CO 2 removed as a function of time. The amount of CO 2 captured was proportional to the concentration of the amine solution due to the EDA-CO 2 chemical affinity. Kumar et al. [22] proposed the use of EDA as an activator in the CO 2 capture process by increasing the solubility capacity of CO 2 in the amine aqueous solutions (chemical solvents). Snapshots of the aqueous amine solutions were taken at the end of the experiments (see Figure 3d). An increase in the intensity of a yellowish-brown color (amber color) could be observed with the naked eye in the aqueous amine solutions. The variability in the color intensity was proportional to the amine concentration, which correlates with the amount of CO 2 captured. Table 2 summarizes the kinetic and thermodynamic data obtained from the CO 2 capture testing using EDA. The CO 2 capture rate (dn/dt) varied with the EDA concentration, obtaining the highest rate at 10 wt.%. Surprisingly, at the 20 wt.% amine concentration, the CO 2 capture rate was the lowest, but the CO 2 removed from the gas phase at equilibrium was the highest. A higher amine concentration increases the viscosity of aqueous amine solutions, unfavorably affecting the CO 2 mass transfer rates [20,45]. Moreover, at 20 wt.% amine concentration, there was a more significant temperature increase in the liquid solution (see Figure 3b), which might directly affect the CO 2 capture kinetics. Fan et al. [46] reported a decrease in the CO 2 capture rate using alkanolamine aqueous solutions (e.g., MEA and diethanolamine (DEA)) with temperature increases. In all of the EDA aqueous solutions studied, the t 90 parameter was less than 300 min (<5 h). The time required for CO 2 capture can be reduced in stirred experimental setups or continuous processes at different scales, i.e., pilot-plant-scale [47][48][49] or large-scale [50][51][52]. The CO 2 removed and the CO 2 loading were proportional to the amine concentration at equilibrium conditions, so with the increase in the concentration of EDA in the liquid solution, there was a corresponding increase in the CO 2 solubility and, subsequently, the EDA-CO 2 chemical reaction took place, increasing the CO 2 loading.  Figure 4 shows the pressure behavior (pressure-temperature diagram and ln P vs. 1/T) and the variation in the amounts of CO 2 removed as a function of temperature, so as to study the reactivity and exothermicity effects in CO 2 capture testing using EDA. The temperature increased suddenly from the initial temperature (303 K) to point A (see Figure 4a). As previously discussed, the initial ∆T was due to the exothermicity of the CO 2 capture process using chemical solvents [6]. The combined effect of gas solubilization into the liquid phase and the chemical reaction between the solubilized gas and the liquid solution should increase the temperature in a CO 2 capture process using EDA. After point A, the heat released gradually dissipated in the same liquid solution and the reactor walls, producing continuous and gradual temperature decreases (segments AB and BC, see Figure 4b) until it stabilized again at the initial temperature. From that moment on, the reactor heating system maintained a constant temperature (segment CD). The reaction's thermal shock at the first moments of gas-liquid contact occurred without significant CO 2 consumption. The CO 2 capture was triggered after a minimal amount of CO 2 was absorbed into the liquid solutions (point B, see Figure 4c). Li et al. [53] proposed a reaction mechanism between EDA and CO 2 , wherein the amino groups (-NH 2 ) react with CO 2 under the sufficient CO 2 conditions. Furthermore, it seems that this minimal amount of CO 2 (~10 mmol) was independent of the amine concentration, and could correspond to the "lean amine acid gas loading" in a typical sweetening process using alkanolamines as chemical solvents [32]. Momeni and Riahi [54] established that amines' chemical structure and nature are the most important parameters for CO 2 absorption purposes.

CO 2 Capture Using EDA: Kinetic and Thermodynamic Analysis
When analyzing the CO 2 loading behavior as a function of time (see Figure 3c), it was observed that the kinetic mechanism involved two steps: a rapid absorption step, and a slow diffusion step ( Figure 5). This kinetic mechanism is similar to that reported for CO 2 adsorption processes [55]. Da Silva and Svendsen [56] commented that the two-step mechanism applies to the reaction between CO 2 and primary and secondary amines. Furthermore, Wai et al. [57] performed a kinetic and thermodynamic analysis of CO 2 capture for combustion gases using AMP-DETA (2-amino-2-methyl-1-propanol-diethylenetriamine) mixtures, and the trend was similar to what was observed in this work.

CO2 Capture Using EDA: Kinetic and Thermodynamic Analysis
When analyzing the CO2 loading behavior as a function of time (see Figure 3c), it was observed that the kinetic mechanism involved two steps: a rapid absorption step, and a slow diffusion step ( Figure 5). This kinetic mechanism is similar to that reported for CO2 adsorption processes [55]. Da Silva and Svendsen [56] commented that the two-step mechanism applies to the reaction between CO2 and primary and secondary amines. Furthermore, Wai et al. [57] performed a kinetic and thermodynamic analysis of CO2 capture for combustion gases using AMP-DETA (2-amino-2-methyl-1-propanoldiethylenetriamine) mixtures, and the trend was similar to what was observed in this work. Equation (7) is the mathematical expression for the kinetic mechanism propo first term corresponds to the CO2 loading at equilibrium. The par ( , , , ) of the time-dependent terms were determined by the least squares sion from the experimental data (CO2 loading vs. time, Figure 3c). Equation (7) is the mathematical expression for the kinetic mechanism proposal. The first term corresponds to the CO 2 loading at equilibrium. The parameters (A 1 , k 1 , A 2 , k 2 ) Energies 2021, 14, 6822 10 of 15 of the time-dependent terms were determined by the least squares regression from the experimental data (CO 2 loading vs. time, Figure 3c). n CO 2 loading t=t = n CO 2 loading equil. Figure 6a shows the linear correspondence between CO 2 loading at equilibrium as a function of the EDA solution concentration. When EDA was not present in the liquid phase, the value of CO 2 loading at equilibrium coincided with the solubility of CO 2 in pure water. Increases in the EDA concentration produced an increased CO 2 loading. Muchan et al. [58] worked at atmospheric pressure with 15 KPa CO 2 (in nitrogen balance) using an aqueous EDA solution, while Singh [59] performed the evaluation of different amines for CO 2 capture using high-pressure systems. In both cases-at atmospheric pressure and at high pressure-the results were consistent with the linear tendency obtained in this work. This behavior suggests that the CO 2 loading at equilibrium conditions in aqueous EDA solutions is not a pressure-dependent parameter.  Table 3 summarizes the kinetic model parameters for Equation (7). Figure 6b shows the experimental data fit according to the proposed equation. The average absolute deviation (AAD) was <1% for each EDA aqueous solution, showing that the CO2 capture behavior obtained during the experimental testing adapted accurately to the kinetic mechanism, which involves two steps: a rapid absorption step, and a slow diffusion step. Li et al. [53] conducted CO2 capture tests in a glass reactor, and obtained similar trends to our proposed mechanism. Table 3. CO2 capture using EDA: kinetic model parameters for Equation (7). EDA -This work EDA -data from [58] EDA -data from [59] MEA -data from [46] DEA -data from [46]  Moreover, Figure 6a shows the data on CO 2 loading for other amines (MEA, DEA) taken from [46]. Similarly, there was a linear relationship between the CO 2 loading and the amine concentration, as obtained for EDA in this work. Bernhardsen and Knuutila [60] reviewed potential amine solvents for the CO 2 absorption process, showing the linear dependence of CO 2 loading at equilibrium on the MEA concentration. MEA has a greater Energies 2021, 14, 6822 11 of 15 absorption capacity than DEA, but EDA surpasses both in the concentration range studied. EDA has two amino groups that promote affinity and reactivity towards CO 2 ; however, this trend is not consistent with the results reported by Gomes et al. [61], where DEA and MEA achieved greater absorption capacity compared to EDA, possibly because the equilibrium conditions were not reached under their experimental setup. Table 3 summarizes the kinetic model parameters for Equation (7). Figure 6b shows the experimental data fit according to the proposed equation. The average absolute deviation (AAD) was <1% for each EDA aqueous solution, showing that the CO 2 capture behavior obtained during the experimental testing adapted accurately to the kinetic mechanism, which involves two steps: a rapid absorption step, and a slow diffusion step. Li et al. [53] conducted CO 2 capture tests in a glass reactor, and obtained similar trends to our proposed mechanism. Finally, the damping-film theory model was applied to investigate the apparent absorption rate performance of an aqueous EDA solution using Equation (4) at the rapid absorption step. Slopes of the constant rate (k) of aqueous solutions of EDA are shown in Figure 6c. The apparent absorption rate constant of aqueous EDA solutions was much higher than that of pure water, with a value of 0.0019 min −1 . It was also observed that EDA significantly intensified the CO 2 absorption performance of aqueous solutions, resulting in k values of 0.0040 min −1 , 0.0049 min −1 , and 0.0060 min −1 for EDA concentrations of 5, 10, and 20 wt.%, respectively. Note that in the initial minutes (<10 min), the CO 2 absorption rate was higher for 10 wt.% EDA solutions, consistent with the initial CO 2 capture rate values (see dn/dt data in Table 2). The addition of EDA accelerated the absorption performance of the CO 2 -trapping chemical solvent within the investigated timeframe. The increased absorption performance in aqueous EDA solutions was due to the chemical absorption of CO 2 into aqueous solutions of diamine at high pressure taking less time to absorb more CO 2 molecules than pure water.

Summary and Outlook
In this work, an experimental setup was installed and commissioned for CO 2 capture testing, using EDA as a chemical solvent, with the main distinction being the pure CO 2 bubbling directly into the liquid phase so as to guarantee intimate contact between the gas and the liquid from the first instants of the experiments. Initial testing of the solubility of CO 2 in water allowed the validation of the experimental setup and the proposed methodology, demonstrating that it was possible to reach equilibrium conditions in short times (<4 h) using a non-stirred system. CO 2 capture testing was carried out using EDA aqueous solutions at different concentrations (0, 5, 10, and 20 wt.% in amine). The addition of EDA accelerated the CO 2 capture performance, and the drop in pressure was proportional to the amine concentration. For the 20 wt.% amine solution, the gas consumption represented a drop in pressure of~50% from the initial pressure. During the initial minutes of the CO 2 capture process, sudden temperature increases were observed as a result of the exothermic reaction between the CO 2 and the EDA aqueous solutions. The maximum value reached was~324 K for the 20 wt.% amine solution, representing a ∆T = 21 K.
The CO 2 capture was triggered after the absorption of a minimal amount of CO 2 (~10 mmol) into the liquid solutions. There was a linear relationship between the CO 2 loading and the EDA concentration at equilibrium conditions. Compared with other alkanolamines commonly used as chemical solvents, EDA had a higher CO 2 absorption capacity than MEA and DEA in the concentration range studied. EDA has two amino groups that promote affinity and reactivity towards CO 2 . A kinetic model involving two steps (a rapid absorption step and a slow diffusion step) was proposed to ascertain the CO 2 loading at equilibrium, the CO 2 loading as a function of time, and the apparent absorption rate. The apparent absorption rate constants of aqueous EDA solutions were higher than those for pure water, resulting in a k of 0.0060 min −1 for 20 wt.% EDA solutions.
Subsequent characterization studies of saturated amines will allow us to identify and quantify reaction products for the CO 2 -EDA-water system. Additionally, amine regeneration testing will be performed through absorption/desorption cycles in order to ascertain the energy demands of the CO 2 capture process using EDA, as well as the limit operating temperature in the stripping process. CO 2 capture at different temperatures will allow us to conduct absorption enthalpy analyses using EDA as a chemical solvent.  Acknowledgments: The support of Daniela Navas (laboratory technician at Yachay Tech University) is appreciated.

Conflicts of Interest:
The authors declare no conflict of interest.

Appendix A. Mole Number in EDA Aqueous Solutions
A polynomial adjustment ( Figure A1) of the experimental data reported by Egorov et al. [37] was carried out to estimate the density of aqueous EDA solutions at different concentrations (<30 wt.% in amine).

Energies 2021, 14, x FOR PEER REVIEW
Acknowledgments: The support of Daniela Navas (laboratory technician at Yachay Tech sity) is appreciated.

Conflicts of Interest:
The authors declare no conflict of interest.

Appendix A. Mole Number in EDA Aqueous Solutions
A polynomial adjustment ( Figure A1) of the experimental data reported by et al. [37] was carried out to estimate the density of aqueous EDA solutions at d concentrations (<30 wt.% in amine). Then, the mole numbers of amine ( ), water , and total moles in uid phase initially present in each of the EDA aqueous solutions (Ta were calculated from the solutions' density and the molecular weight of EDA an A fixed volume (30 ± 0.2 cm 3 ) of the amine solutions was used in the CO2 capture  Then, the mole numbers of amine (n EDA ), water n H 2 O , and total moles in the liquid phase n liquid initially present in each of the EDA aqueous solutions (Table A1) were