Selection of Mixed Amines in the CO2 Capture Process

In order to select the best mixed amines in the CO2 capture process, the absorption of CO2 in mixed amines was explored at the required concentrations by using monoethanolamine (MEA) as a basic solvent, mixed with diisopropanolamine (DIPA), triethanolamine (TEA), 2-amino-2-methyl1-propanol (AMP), and piperazine (PZ). Here, a bubble column was used as the scrubber, and a continuous operation was adopted. The Taguchi method was used for the experimental design. The conditional factors included the type of mixed amine (A), the ratio of the mixed amines (B), the liquid feed flow (C), the gas-flow rate (D), and the concentration of mixed amines (E). There were four levels, respectively, and a total of 16 experiments. The absorption efficiency (EF), absorption rate (RA), overall mass transfer coefficient (KGa), and scrubbing factor (φ) were used as indicators and were determined in a steady-state by the mass balance and two-film models. According to the Taguchi analysis, the importance of the parameters and the optimum conditions were obtained. In terms of the absorption efficiency (EF), the absorption rate (absorption factor) (RA/φ), and the overall mass transfer coefficient (KGa), the order of importance is D > E > A > B > C, D > E > C > B > A, and D > E > C > A > B, respectively, and the optimum conditions are A1B4C4D3E3, A1B3C4D4E2, A4B2C3D4E4, and A1B1C1D4E1. The optimum condition validation results showed that the optimal values of EF, RA, and KGa are 100%, 30.69 × 10−4 mol/s·L, 1.540 l/s, and 0.269, respectively. With regard to the selection of mixed amine, it was found that the mixed amine (MEA + AMP) performed the best in the CO2 capture process.


Introduction
Currently, carbon dioxide (CO 2 ) emissions are a major issue worldwide, especially in industry, and coal-fired power plants are the most concerning source. To reduce CO 2 emissions, several studies on the capture, storage, and reuse of CO 2 have been explored. They have focused mainly on post-combustion and used the absorption method [1][2][3], which uses an alkaline solution to capture the CO 2 . The absorption of CO 2 in an alkaline solution is also an effective method that is used for the removal of CO 2 from flue gas; it adopts a lot of solvents, including amines, amino salts, sodium hydroxide solution, and bicarbonate solutions [4]. To date, monoethanolamine (MEA) has been used as a wilder solvent to capture CO 2 and hydrogen sulfide, due to its lower cost and higher boiling point, compared to other solvents [5,6]. The important industrial alkanolamines are MEA, diethanolamine (DEA), aminomethyl propanol (TEA), and methyldiethanolamine (MDEA). However, 2-amino-2-methyl-1-propanol (AMP) and piperazine (PZ) [7][8][9][10] are always used as an absorption solvent and promoter, respectively. Here, amines can be divided into four classes, namely: primary amines (such as MEA and diglycolamine (DGA)), secondary amines (such as DEA and diisopropylamine (DIPA)), tertiary amines (such as TEA and MDEA), and steric hindrance amines (such as AMP). The properties and drawbacks of each class of amine are listed in Table 1. In addition, the loading of MEA is limited by stoichiometry to 0.5 moles CO 2 per mole amine. However, the loading is higher for C 2021, 7, 25 2 of 14 secondary and tertiary amines, which have higher loadings of up to 1 mole of CO 2 per mole of amine. The primary and secondary amines react rapidly with CO 2 to form carbamates, which results in higher solvent regeneration costs. On the other hand, tertiary amines catalyze the CO 2 hydrolysis reduction, forming bicarbonate ions and protonated amines. The heat of the reaction in bicarbonate formation is lower than that in carbamate formation, thus reducing the solvent regeneration costs. However, tertiary amines react more slowly with CO 2 than primary and secondary amines. In addition, PZ, which is a cyclic diamine, can be used as an activator [11,12].
Promote reaction rate • Water and CO 2 absorption in air [8] The absorption of acid gases, by using mixed amines, has an advantage over the use of single amines [9, 15,17]. Therefore, the rapidly-rising energy demands have led to the development of more efficient solvents for the absorption of CO 2 gas [11,14,[18][19][20][21][22][23][24]. The absorption rate of CO 2 into the mixed amines of MEA + MDEA and MEA + TEA has been studied by several authors [13,25,26]. The use of AMP + MEA as an effective solvent was reported in the literature [15]. The mixed amine systems, which combine the higher equilibrium capacity of the tertiary amine (MDEA) with the higher reaction rate of the primary (MEA) and secondary amine (DEA), can bring about a considerable improvement in the gas absorption rate. The addition of piperazine to potassium carbonate, MEA, tertiary amine, and steric hindrance amine, to enhance the reaction rate, has been explored [16,19]. Most of these studies have reported the kinetics of the reaction of CO 2 with different amine blends [9, [15][16][17]19].
In MEA aqueous solutions, two mechanisms for the formation of carbamate have been proposed: one is the zwitterions mechanism, and the other is the termolecular mechanism [6,27]. This termolecular mechanism assumes that an amine reacts simultaneously with one molecule of CO 2 and one molecule of a base, and forms an intermediate looselybound complex. The reaction between CO 2 and the amine can be described by a two-step C 2021, 7, 25 3 of 14 zwitterions mechanism [6,14]. The chemistry equations can be reduced to three important steps, i.e., carbamate formation, bicarbonate formation, and carbamate reversion: Carbamate formation: Bicarbonate formation: Carbamate reversion: The total reaction rate of all CO 2 reactions in an aqueous solution is shown below: where, k obs is the observed reaction rate constant, which is defined as follows: where k 1 is the forward reaction rate constant in Equation (1), k −1 is the backward reaction rate constant in Equation (1), k B is the rate constant in Equation (2), and k OH − is the reaction constant of CO 2 with OH − . The overall reaction rate depends on the concentration of hydroxyl ions, which increases with the pK a value of the amine [6]. At a high concentration of RNH 2 , the absorption rate of CO 2 is greater, but more RNHCOO − is produced and, hence, more bicarbonate, as shown in Equations (1) and (2). Therefore, the method of controlling the chemical species in the CO 2 /MEA/H 2 O system is significant in the CO 2 capture process. During the absorption of CO 2 with mixed amines, the reaction rate of the bicarbonate ion is very slow, and thus r HCO − 3 is neglected. Therefore, the kinetics of individual amine for the case of k −1 /(∑ (k b [B])<< 1, which is second-order kinetics, can be expressed as follows [12]: and steric hindrance amine can be expressed as follows [14]: The kinetics of piperazine [28] can be expressed as: For the case of k −1 /(∑ (k b [B])››1, the reaction rate included the item ∑ k B B/k −1 . However, the overall reaction rate for the absorption of CO 2 into mixed amines can be expressed as: r CO 2 ,OV = r CO 2 ,MEA + r CO 2 ,Add (11) where, r CO 2 ,Add is the addition of a secondary amine, a tertiary amine, a steric hindrance amine, or piperazine into MEA solution.
In the absorption experiment, the packed column [29][30][31], the wetted wall absorber [10,14,[32][33][34], the Rotating Packed Bed [4], the bubble column [11,[35][36][37][38], and other scrubbers [28,39] are often used. The operation of the packed column is complicated, and its operating costs are high, while the bubble column is characterized by its merits, as it has a controllable pH value, a high mass transfer coefficient, a high absorption factor, and it is easy to operate. Because of this, some researchers have adopted a bubble-column scrubber, as reported in the literature [11,[35][36][37][38]. Therefore, the absorption experiment in this study is performed by using the bubble column with a mixed amine aqueous solution as the absorbent.
In this study, by using various mixed amines as solvents, a continuous bubble-column scrubber was adopted to study the effects of the type of mixed amines (A), the ratio of mixed amines (B), the liquid-flow rate (C), the gas-flow rate (D), and the concentration of total amines (E) on the removal efficiency (E F ), the absorption rate (R A ), the overall masstransfer coefficients (K G a), and the scrubbing factor (φ). This was because the E F , the R A , K G a, and φ can be used to evaluate the performance of amines in the scrubbers and, hence, the best amines are selected. To reach the purpose, this could be accomplished effectively by using the Taguchi method [40], which is popularly known as the experimental design for industrial processes and covers many applications. For verification, the data reported here were used to evaluate the optimum data through the signal and noise (S/N) ratio analysis, which was used as a basis for the selection of mixed amines. Therefore, original research was carried out in this work. The schematic of the research is shown in Figure 1.
amine, or piperazine into MEA solution.
In the absorption experiment, the packed column [29][30][31], the wetted wall abs [10,14,[32][33][34], the Rotating Packed Bed [4], the bubble column [11,[35][36][37][38], and other s bers [28,39] are often used. The operation of the packed column is complicated, an operating costs are high, while the bubble column is characterized by its merits, as a controllable pH value, a high mass transfer coefficient, a high absorption factor, a is easy to operate. Because of this, some researchers have adopted a bubble-column s ber, as reported in the literature [11,[35][36][37][38]. Therefore, the absorption experiment i study is performed by using the bubble column with a mixed amine aqueous soluti the absorbent.
In this study, by using various mixed amines as solvents, a continuous bubbl umn scrubber was adopted to study the effects of the type of mixed amines (A), the of mixed amines (B), the liquid-flow rate (C), the gas-flow rate (D), and the concentr of total amines (E) on the removal efficiency (EF), the absorption rate (RA), the overall m transfer coefficients (KGa), and the scrubbing factor ( ). This was because the EF, th KGa, and  can be used to evaluate the performance of amines in the scrubbers and, h the best amines are selected. To reach the purpose, this could be accomplished effec by using the Taguchi method [40], which is popularly known as the experimental d for industrial processes and covers many applications. For verification, the data rep here were used to evaluate the optimum data through the signal and noise (S/N) analysis, which was used as a basis for the selection of mixed amines. Therefore, or research was carried out in this work. The schematic of the research is shown in Figu

Absorption Experiment Design
The aim of the experiment was to absorb CO 2 by using mixed amines in a bubble column. The results were expected to be applied to the absorption of CO 2 emitted by coal-fired power plants. Therefore, the CO 2 of flue gas in a coal-fired power plant at a 15% concentration of and at 50 • C was simulated to enter the column. According to previous works [41,42], the absorption of CO 2 in the bubble-column scrubber is affected by the gas flow rate, the liquid flow rate, the concentration of the solvent, as well as the temperature and pH. In order to select the mixed amines, the type of mixed amines (A), the ratio of mixed amines (B), the liquid-flow rate (C), the gas-flow rate (D), and the concentration of mixed amines (E) were considered as the condition factors, and four levels for each condition factor were taken respectively, i.e., the ratio of mixed amine (5-20 wt%), gas-flow rate, Q g , (3-12 L/min), concentration of the mixed amines (1-2.5 M), and the liquid-flow rate, Q L , C 2021, 7, 25 5 of 14 (0.15-0.3 L/min). Theoretically, a total of 1024 (=4 5 ) experiments needed to be conducted; however, to save time and experimental costs, we used the Taguchi experimental design to reduce the groups of the experiment to L 16 (4 5 ) = 16. Here, the degree of freedom was found to be as follows [40]: where N is the degree of freedom, L i is the level of the ith variable, and NV is the number of the variable. Using this equation, N = 1 + (3 + 3 + 3 + 3 + 3) is found to be 16, which is also the number of experiments. The value obtained in the steady-state for each experiment was adopted to obtain the absorption rate, the absorption efficiency, the scrubbing factor, and the overall volumetric mass transfer coefficient, and then the sequence of significance and optimum condition were obtained by using statistical software. Table 2 shows the conditions and levels, and Table 3 shows the orthogonal arrays. Finally, 16 groups of experiments needed to be conducted under different conditions.  1  1  1  1  1  1  2  1  2  2  2  2  3  1  3  3  3  3  4  1  4  4  4  4  5  2  1  2  3  4  6  2  2  1  4  3  7  2  3  4  1  2  8  2  4  3  2  1  9  3  1  3  4  2  10  3  2  4  3  1  11  3  3  1  2  4  12  3  4  2  1  3  13  4  1  4  2  3  14  4  2  3  1  4  15  4  3  2  4  1  16  4  4  1  3  2 The Taguchi method uses the signal and noise ratio (S/N) as the process optimization function [40,41]. The S/N value for larger-the-better is as follows: Here, n is the experimental number and z i is the experimental data. Using this equation and experimental data, the optimum condition and parameter importance could be determined.

Calculation of Experimental Data
Experimental data, including the absorption efficiency, the absorption rate, the scrubbing factor, and the overall mass-transfer coefficient, were all evaluated in a steady-state condition. Based on the two-film model and materials balance, the absorption rate and overall volumetric mass-transfer coefficient could be derived [43]. The equations for this are as follows: And In Equation (15), it is assumed that the concentration of CO 2 gas in the liquid phase is extremely small and that it can be ignored (C A HC AL ) [43,44]. Therefore, the overall mass transfer coefficient can be derived. The result equation shows that K G a can be calculated from the known inlet and outlet conditions. In the above equations, y A , V L , Q g , P A , and T represent the mole fraction of CO 2 , the final solution volume, the gas volumetric flow rate, the CO 2 gas partial-pressure, and the gas temperature, respectively. The notations of 1 and 2 represent the inlet and outlet, respectively.
In addition, the absorption efficiency and scrubbing factor [45] are shown as: And where F A and F L are the molar flow rate of CO 2 -gas and the aqueous amine molar flow rate, respectively.

Experimental Devices and Procedures
The devices required for this experiment are shown in Figure 2 and include a bubble column, a gas-flow feed system (Bronkhorst, F-201CB DMFC), a liquid-flow system (Easy-Load, 7518-00), a pH-meter (Suntex, PC-310), a CO 2 m (Guardian Plus, D600), a gas heating system (5020 Data Acquisition System), and a liquid cooling system (Deng Yng, D-620). The range of a CO 2 m is 0-100%, while the confidence of a CO 2 m is ±1 − 2%. In this experiment, the mixed amine was used as a test absorbent, which accounted for 5-20 wt % of the ratio in total amine concentration, which was in the range of 1-2.5 M. The required mixed amine concentration was prepared by using distilled water. Secondly, the flow rate of CO 2 and nitrogen input was based on the proportion of 15% of CO 2 , and the gas inlet temperature was maintained at 50 • C. The experiment was started after the mixed amine was added to the column. During the experiment, the pH of the solution, the liquid temperature, the gas inlet temperature, the gas outlet temperature, the pressure, and the CO 2 concentration were recorded every five minutes. The liquid at the outlet was also withdrawn for titration to observe the concentration of carbonate in the scrubbed solution. At the end of the one-hour operation, the liquid input was closed and the solution was withdrawn by using a tubing pump to measure the volume of liquid (V L ) presented in the column. temperature, the gas inlet temperature, the gas outlet temperature, the pressure, and the CO2 concentration were recorded every five minutes. The liquid at the outlet was also withdrawn for titration to observe the concentration of carbonate in the scrubbed solution. At the end of the one-hour operation, the liquid input was closed and the solution was withdrawn by using a tubing pump to measure the volume of liquid (VL) presented in the column.

Operation and Data Calculation of the Steady-State Condition
Using No. 1 as an example, the mixed amine was MEA + AMP (A), with the condition of 5% (B), 150 mL/min (C), 3 L/min (D), and 1 M (E). To understand the relationship of the outlet concentration of the gas, the gas temperature, the liquid temperature, the pH, and the inlet pressure against the duration of the operation, the measured value were divided by the initial value or setting value (such as the liquid temperature and gas inlet temperature) was defined as the Y (-) value, which was taken as the y-axis, and the time (t) was

Operation and Data Calculation of the Steady-State Condition
Using No. 1 as an example, the mixed amine was MEA + AMP (A), with the condition of 5% (B), 150 mL/min (C), 3 L/min (D), and 1 M (E). To understand the relationship of the outlet concentration of the gas, the gas temperature, the liquid temperature, the pH, and the inlet pressure against the duration of the operation, the measured value were divided by the initial value or setting value (such as the liquid temperature and gas inlet temperature) was defined as the Y (-) value, which was taken as the y-axis, and the time (t) was taken as the x-axis, as shown in Figure 3. After one hour, the Y-values were constant after 20 min, showing that the system had reached a steady state. Therefore, all data could be evaluated in a steady-state condition, i.e., after 20 min of operation. The CO 2 concentration dropped very low, indicating that the mixed amine had a high absorption efficiency. The analysis of the experimental results may be divided into two parts. Part 1 used the Taguchi method to design 16 groups of experiments that calculated the data, namely, the absorption rate, the absorption efficiency, the overall volumetric mass-transfer coefficient, and the scrubbing factor, as shown in Table 4. Part 2 searched for and confirmed the optimum condition and the sequence of significance, based on the results of the data that were calculated in Part 1 in the Taguchi analysis. Table 4 presents the values calculated by Equations (14)- (17), where the range of E is 61.33-100%, R A is 4.02-26.48 × 10 −4 (mol/s·L), K G a is 0.229-0.798 (s −1 ), and φ is 0.034-0.263 (mole/mole·L). These data are comparable with those reported earlier [42,45,46]. Table 5 showed that the data obtained in this work, such as K G a in the range of 0.229-0.789 s −1 , were higher than that of the sodium glycinate solution (0.051-0.189 s −1 ), the ammonia solution (0.0136-0.3302 s −1 ), the NaOH solution (0.015-0.246 s −1 ), and the MEA solution (0.0377-0.8881 s −1 ). In addition, the R A and E F were also higher than the other systems listed in Table 5. tions (14)- (17), where the range of E is 61.33-100%, RA is 4.02-26.48 × 10 (mol/s.L), KGa is 0.229-0.798(s −1 ), and  is 0.034-0.263 (mole/mole·L). These data are comparable with those reported earlier [42,45,46]. Table 5 showed that the data obtained in this work, such as KGa in the range of 0.229-0.789 s −1 , were higher than that of the sodium glycinate solution (0.051-0.189 s −1 ), the ammonia solution (0.0136-0.3302 s −1 ), the NaOH solution (0.015-0.246 s −1 ), and the MEA solution (0.0377-0.8881 s −1 ). In addition, the RA and EF were also higher than the other systems listed in Table 5.    Sodium glycinate solution(pH-stat) pH = 9.5-11 Q g = 3-9 L/min T = 25-40 • C C L = 3-6 M y A1 = 15% [42] 17.5-97.

Taguchi Analysis
The values of the absorption efficiency (E F ), the absorption rate (R A ), the overall volumetric mass-transfer coefficient (K G a), and the scrubbing factor (φ) were substituted into Equation (12) for a Taguchi S/N ratio analysis, and the results are shown in Figure 4 and Table 6. The parameters that significantly affected the E F , R A (or φ), and K G a were in the order of D > E > A > B > C, D > E > C > B > A, and D > E > C > A > B, respectively. The importance of each parameter can be quantified and can be expressed in a radar chart, as shown in Figure 5. The scope was in the range 1 to 5, such as the importance of parameters in E F , (D(5) > E(4) > A(3) > B(2) > C(1)), which are shown in the form of dotted lines. While R A (A1B3C4D4E2) and φ (A1B1C1D4E1) appeared to have the same radar chart, they had different optimum conditions. The data showed that D (the gas-flow rate), and E (the concentration of mixed amine) were the primary factors, while A (the type of mixed amines), and B (the percentage of mixed amines) were the minor factors. The optimum conditions for E F and K G a were A1B4C4D1E4 and A4B2C3E4D4, respectively. It also revealed the type of mixed amine (A1, MEA + AMP) that was dominant for E F , R A , and φ, and the type of mixed amine (A4, MEA + PZ) that was better for K G a. However, the optimum conditions still need to be confirmed. Table 6. Optimum conditions and importance of parameters.

Outcome Data
Optimum Conditions Importance of Parameters

Confirmation of the Optimum Condition
Four experiments were carried out, based on the optimum conditions, and the results are presented in Table 7. The values obtained were 100%, 30.69 mol/s·L, 1.54 s −1 , and 0.297 mol/mol·L for E F , R A , K G a, and φ, respectively. These are the largest values, and the larger-the-better, indicating that the optimum conditions could be confirmed here. This also demonstrated that the Taguchi method that was used was efficient. of mixed amine (A4, MEA + PZ) that was better for KGa. However, the optimum conditions still need to be confirmed.

Confirmation of the Optimum Condition
Four experiments were carried out, based on the optimum conditions, and the re are presented in Table 7. The values obtained were 100%, 30.69 mol/s·L, 1.54 s −1 , and mol/mol·L for EF, RA, KGa, and , respectively. These are the largest values, and the la

Comparisons of Mixed Amines
To compare the outcome data obtained in Table 4, the bar graphs were plotted for E F , R A , K G a, and φ, respectively, as shown in Figure 6. From the bar charts, the mixed amine MEA + AMP appears to be better, compared to the others, except for φ. In order to obtain a quantitative comparison, the average value of each group, i.e., AMP, DIPA, TEA, and PZ, was required. Using E F as an example, the average values for AMP, DIPA, TEA, and PZ were 86.1, 81.4, 83.4, and 83.3, respectively. The sequence of the value was expressed as excellent ( ), good ( ), fair ( ), and poor (×), respectively. The R A , K G a, and φ can also be estimated by using the same method. The results of the estimation of each item are listed in Table 8. Cumulatively, the significant sequence of mixed amine was A1(MEA + AMP) > A3(MEA + TEA) > A4(MEA + PZ) > A2 (MEA + DIPA). This shows that the mixed amine (MEA + AMP) was the best.

Conclusions
A continuous bubble-column scrubber was used successfully to select mixed amines in the CO2 capture process, by using the absorption efficiency (EF), the absorption rate (RA), the overall mass transfer coefficient (KGa), and the scrubbing factor ( ) as indicators.

Conclusions
A continuous bubble-column scrubber was used successfully to select mixed amines in the CO 2 capture process, by using the absorption efficiency (E F ), the absorption rate (R A ), the overall mass transfer coefficient (K G a), and the scrubbing factor (φ) as indicators. A total of 16 runs were carried out, using Taguchi's experimental design, which greatly reduced the number of experiments required. According to the Taguchi analysis, the mixed amine (MEA + AMP) had a better E F , R A , and φ, while the mixed amine (MEA + PZ) had a better K G a. The major factors affecting the outcome data were found to be the gas-flow rate (D) and the concentration of mixed amines (E). A confirmation of the optimum conditions showed that the most optimum values of E F (100%), R A (30.69 × 10 −3 mol/s·L), K G a (1.540 s −1 ), and φ (0.297), fitted into the larger-the-better category. From the radar chart and bar chart analyses, the significant sequence of mixed amines was found to be A1 (MEA + AMP) > A3 (MEA + TEA) > A4 (MEA + PZ) > A2 (MEA + DIPA). This result indicated that the mixed amine (MEA + AMP) performed the best in the CO 2 capture process under the operating conditions of this study. In this case, A1B4C4D4E4 is the best for K G a and R A , while A1B1C1D1E1 is the best for E F and φ. In future, mixed amine (MEA + AMP) can be improved further by adding TEA, to improve the R A, and the addition of PZ, to improve φ.