2-(Methylamino)ethanol for CO2 Absorption in a Bubble Reactor

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2‑(Methylamino)ethanol for CO2 Absorption in a Bubble Reactor I. Folgueira,† I. Teijido,† A. García-Abuín,‡ D. Gómez-Díaz,*,‡ and A. Rumbo† †

Department of Organic Chemistry, Faculty of Sciences, and ‡Department of Chemical Engineering, School of Engineering (ETSE), University of Santiago de Compostela (USC), Santiago de Compostela E-15782, Galicia, Spain ABSTRACT: This paper deals with the study of carbon dioxide capture using 2-(methylamino)ethanol (MAE) aqueous solutions. The reaction was studied in a bubble column reactor (BCR), and the most important aspects of the process, such as the hydrodynamic behavior (bubble size distribution, gas holdup, and interfacial area) and absorption phenomena (load of carbon dioxide and mass-transfer coefficient on the liquid phase), were discussed. The interfacial area was determined using a photographic method. Furthermore, the reaction mechanism was determined using nuclear magnetic resonance (1H and 13C NMR), thereby the species in the medium were controlled throughout time. These results agreed with carbon dioxide loading values, observing the formation of carbamate at the first part of the experiments and bicarbonate at high carbon dioxide loadings. The carbamate hydrolysis is the bicarbonate production pathway. This wide work has enabled us to perform a global analysis of process and to understand how MAE absorbs the gas phase. MAE aqueous solutions show an attractive behavior with a high carbon dioxide loading and high reaction rate. This information could be used in the near future to select the best conditions to capture carbon dioxide and to improve amine regeneration processes.

1. INTRODUCTION In the past decade, international and national programs and organizations [e.g., International Energy Agency Greenhouse Gas (IEGHG)] have supported intensive research on carbon dioxide separation processes to avoid direct CO2 emission to the atmosphere. These kinds of studies have focused their interest in different carbon-dioxide-confining processes. Different new strategies have been developed on the basis of chemical looping,1 membrane separation,2 and gas−solid adsorption,3,4 but one of the most successful studies is centered on the development of new solvents used on physical or chemical processes. These new solvents can be included in different groups, such as ionic liquids5−7 or amine blends,8,9 or the development and use of new amines.10−12 All of these fields of research have focused on the improvement of conventional processes, such as increasing carbon dioxide loading, enhancing the absorption rate, decreasing solvent degradation (which produces an increase in the cost), decreasing chemical risks, etc. Some of these aims are affected by solvent characteristics, such as physicochemical properties, contact devices, chemical reaction rate, or stability of reaction products. Last, lustrum has shown increasing research in carbon dioxide chemical absorption by different kinds of amines. Our group has proposed that tertiary amines could be an interesting reagent for carbon dioxide capture, in agreement with previous studies using this kind of amine as a co-solvent.13 These amines, such as triethanolamine (TEA), have two valuable feature characteristics. The first characteristic is that the global reaction shows a CO2/amine stoichiometry of 1:1. This fact allows for a significant improvement in carbon dioxide loading. This suitable stoichiometry is due to the lack of carbamate production because the mechanism is different and the main reaction product is bicarbonate/carbonate.14 The second feature is also related to the absence of carbamate as the reaction product during the carbon dioxide chemical absorption. These raise the cost reduction associated with the © 2014 American Chemical Society

solvent regeneration operation. The energy associated with regeneration could be lower because the energy needed to break the carbamate bonds is higher than that needed to break the bicarbonate bonds.15 However, the use of tertiary amines for carbon dioxide capture by chemical absorption could be limited by their lower reaction rate or mass-transfer rate. A lower mass-transfer rate of tertiary amine solutions could be due to the high viscosity of these aqueous solutions. This fact could produce a reduction in carbon dioxide and amine diffusivity in the liquid phase. Taking into account the previous discussion, we focused our aim on secondary amines, trying to find a reagent that combines the positive characteristics of tertiary (suitable stoichiometry) and secondary (higher reaction rate) amines, avoiding the mass-transfer reduction caused by the generally high liquid-phase viscosity using tertiary amines.

2. EXPERIMENTAL SECTION 2.1. Materials. Aqueous solutions of 2-(methylamino)ethanol (MAE) to be used as an absorbent phase in the bubble reactor were supplied by Aldrich (CAS registry number 109-83-1) with a purity of >98%. Bi-distilled water was used to prepare the liquid phase in the range of 0−1 mol L−1. 2.2. Experimental Setup. The different studies carried out in this work (hydrodynamic and absorption studies) were performed in a bubble column reactor (BCR) with a square geometry (side length, 4 cm; height, 65 cm) made in methacrylate and with a working volume of liquid phase of 0.9 L at room temperature (∼22 °C). The gas phase was fed in bubble shape using a five-hole sparger built in Teflon. This phase was pure carbon dioxide supplied by Carburos Metálicos, and it was saturated with water to avoid other mass-transfer phenomena that can influence the carbon dioxide mass-transfer rate. The gas flow rate range used in this work was 18−40 L h−1. The experimental setup and procedures have been described widely in a previous work.16 Received: April 22, 2014 Revised: June 4, 2014 Published: June 5, 2014 4737

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2.3. Hydrodynamic Studies. These kinds of studies involve the determination of the gas−liquid-specific interfacial area, analyzing the influence of liquid-phase composition, gas flow rate, and experimental time on bubble size distribution and gas holdup. The first parameter (bubble size distribution) was determined using a photographic method based on the analysis of several photographs of bubbles under all of the experimental conditions along the bubble reactor. A video camera (Sony DCR-PC330E) was used to capture the images, and a minimum number of 100 well-defined bubbles were used to evaluate the size distribution of the bubbles in the liquid phase. The Image Tool, version 3.0, software package was employed to perform the geometrical measurements corresponding to the bubbles. The images taken for the gas dispersion in the bubble column show that the bubbles obtained have an ellipsoid shape. Then, major (E) and minor (e) axes of the projected ellipsoid (in two dimensions) were determined. The diameter of the equivalent sphere (eq 1) was used as the representative bubble dimension.

d=

3

E2e

amine solution were taken from the middle zone of the reactor system. Deuterated water (D2O) was used as an internal reference for the processing of 1H NMR, and tetradeuterated methanol (CD3OD) was used as an internal reference for the processing of 13C NMR.

3. RESULTS AND DISCUSSION 3.1. Hydrodynamic Studies. The geometrical characteristics corresponding to bubbles were determined using the photographs taken from the BCR. This information was obtained and analyzed under the different experimental conditions and during the absorption experiments. Figure 1

(1)

Different authors recommend the use of the Sauter mean diameter (d32), which can be determined17 using the data calculated for the equivalent diameter by means of eq 2

d32 =

∑i (nidi 3) ∑i (nidi 2)

(2)

where ni is the number of bubbles that have an equivalent diameter (di). The overall gas holdup (εG) is a very important parameter in interfacial area determination. The overall gas holdup was measured using the volume expansion method (eq 3). The calculation of total volume change in the bubble column was based on the change observed on the liquid level and on the increase of this value after gassing in the bubble contactor

ΔV εG = ΔV + VL

Figure 1. Influence of time upon bubble size distribution obtained from photographs. CB = 0.42 mol L−1. Qg = 18 L h−1. (○) t = 1 min, (●) t = 10 min, (□) t = 20 min, (■) t = 40 min, and (△) t = 60 min.

shows an example of bubble size distributions determined during an absorption experiment. These data indicate the important changes produced in bubble size when the reagent in the liquid phase decreases because of the chemical absorption. At the beginning of the experiment, the bubble size distribution varies from 0.5 to 4 mm. This low size range is due to the important influence of gas mass-transfer rate caused by the chemical absorption because the MAE concentration near the gas−liquid interface is high. When the experimental time increases, the bubble size distribution moves to the right side, increasing the diameter for all of the bubbles. The changes detected in these distributions are higher in the first part of the experiment. For large times, the changes decrease their magnitude, and at the end of the absorption experiment, the bubble size distribution shows slight modifications. The behavior shown in Figure 1 about the influence of time upon bubble size distribution is the opposite of the corresponding behavior about the influence of the amine initial concentration upon this kind of distribution, because when experiment time increases, the amine concentration decreases. Then, when the amine concentration is higher in the liquid phase, the bubble size distribution shows small bubbles. Figure 2 shows the influence of the amine concentration and gas flow rate upon the bubble size distribution obtained in the BCR. In relation to the influence of the amine concentration, a decrease in the bubble size is observed. This fact can be produced by two influences: (i) an increase in the reaction intensity because the reaction rate increases too and (ii) a decrease in surface tension of the liquid phase because of the higher presence of amine.19 The first influence causes a decrease in bubble diameter because it increases the masstransfer rate, and the second influence produces the decrease in bubble size in the formation at the gas sparger. On the other

(3)

where VL is the ungassed liquid volume and ΔV is the volume expansion after gas dispersion, calculated from the liquid level change and cross-sectional area. The previously calculated values for the Sauter mean diameter (eq 2) and the gas holdup (eq 3) were used to determine the specific interfacial area by means of eq 4.18 The interfacial area was calculated using the specific interfacial area and the liquid volume used in the bubble contactor with an uncertainty of 5%.

a=

6εG d32(1 − εG)

(4)

2.4. Carbon Dioxide Absorption Studies. Carbon dioxide chemical absorption studies were performed analyzing the amount of gas absorbed throughout time using the MAE aqueous solutions and different gas flow rates. The absorption rate was determined on the basis of inlet and outlet gas flow rates that were controlled and measured with two mass flow controllers (Alicat Scientific MC5SLMP-D). These mass flow controllers/flow meters were calibrated by the supplier for the gas employed in the processes and for the flow rate and pressure ranges. Also, the Flow Vision SC software package (Alicat Scientific) was used to record the carbon dioxide flow rate during the experiments. The pressure drop was measured between the column inlet and outlet, using a Testo 512 digital manometer. The working regime was continuous in relation to the gas phase and batch regarding the absorbent liquid phase. 2.5. Speciation Studies. 1H and 13C nuclear magnetic resonance (NMR) spectroscopy was applied to investigate qualitatively the solutions of MAE [concentration of solution of 0.42 M (mol L−1)] loaded with carbon dioxide in a process of capture by means of chemical absorption. The MestrReC 4.7 software developed by MestreLab Research was used for spectra processing. Spectra were acquired on a 300 MHz Varian Mercury spectroscope. The samples of 4738

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reactor. This figure shows that an increase in experimental time also causes an increase in interfacial area, tending to a constant value. This last constant value is reached in a lower time when low amine concentrations are used in the liquid phase. This fact is due to the chemical absorption finishes in a lower time. Also, at the beginning of the experiments, the amine aqueous solutions with a higher concentration take lower values for interfacial area because of the high absorption intensity caused by the high amine concentration in the liquid phase. Then, the bubble volume decreases quickly as well as the gas holdup. The global behavior consists of a decrease in interfacial area. An important operation variable in hydrodynamic studies is the gas flow rate, and in this system, an example of the obtained behavior is shown in Figure 4. These data show that an Figure 2. Influence of the gas flow rate fed to a bubbling reactor upon bubble size distribution. t = 20 min. CB = 0.42 mol L−1: (○) Qg = 18 L h−1 and (●) Qg = 40 L h−1. CB = 1 mol L−1: (□) Qg = 18 L h−1 and (■) surface tension of MAE aqueous solutions.21

hand, the effect caused by the gas flow rate upon the bubble size distribution present in the reactor is shown in Figure 2 for a middle amine concentration. The experimental data indicate that an increase in the gas flow rate produces an important decrease in the bubble size distribution, but this behavior was not commonly observed in previous studies using other configurations.20,21 Generally, an increase in the gas flow rate also causes an increase in the size of bubbles because of coalescence processes. The accumulation of a certain amount of amine in the gas−liquid interface generally produces a decrease in coalescence processes, and it favors the generation of a bubble size distribution with low diameter values. Also, an increase in liquid-phase viscosity can produce a rupture of the bubble in the gas sparger during the bubble formation. Both effects produce the behavior shown in Figure 2. On the basis of bubble size distributions obtained for all of the experimental conditions and the values corresponding to gas holdup, the gas−liquid interfacial area was determined for all of the experiments. The interfacial area data can be used to calculate the liquid-side mass-transfer coefficient for this system. Figure 3 shows an example of the calculated values for specific interfacial area. This information allows us to analyze the influence of experimental time (and amine concentration) upon the amount of area produced in the

Figure 4. Influence of the gas flow rate fed to the reactor upon a specific interfacial area. CB = 1 mol L−1. (○) Qg = 18 L h−1 and (●) Qg = 40 L h−1.

important change in interfacial area is produced when the gas flow rate is increased. An enhancement in interfacial area is clearly observed. This behavior is observed for all of the liquid phases used in the present work. An increase in the gas flow rate causes an important increase in the gas holdup in bubbling equipment. This parameter generally has a higher effect upon interfacial area than the bubble diameter.22,23 Previous studies concluded that an increase in the gas flow rate causes an increase in gas holdup and also bubble diameter24 (the last influence has a negative effect upon interfacial area). However, in the present work, the effect of the gas flow rate upon the bubble diameter is the opposite (see Figure 2), then the change produced in gas holdup and bubble diameter has a positive effect upon interfacial area (see eq 4), and an increase in this parameter is observed. 3.2. Chemical Absorption Mechanism. Our group has assumed that NMR spectroscopy is a very useful tool to study the mechanism reaction to allow for the determination of the different species in amine solution. Generally, we have started with 1H and 13C NMR spectra of the amine−H2O system, in this case MAE (Figures 5 and 6). Deuterium oxide (D2O) was used as a NMR solvent and as an internal reference in 1H NMR, while a drop of deuterated methanol was a 13C NMR internal reference (chemical shift of 49.05 ppm). 1H NMR of aqueous MAE solution shows three groups of signals: a singlet to 2.24 ppm corresponding to protons of CH3N− (A in Figure 5), a triplet to 2.58 ppm corresponding to N−CH2− (B in Figure 5), and the last triplet to 3.58 ppm of −CH2OH (C in Figure 5). The 13C NMR shows the same group of signals: 35.4

Figure 3. Influence of the amine concentration upon a specific interfacial area. Qg = 40 L h−1. (○) CB = 0.084 mol L−1, (●) CB = 0.42 mol L−1, and (□) CB = 1 mol L−1. 4739

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Figure 5. 1H NMR spectra corresponding to the liquid phase during the carbon dioxide absorption. CB = 0.42 mol L−1. Qg = 40 L h−1.

ppm of CH3N− (A in Figure 6), 52.7 ppm of N−CH2− (B in Figure 6), and 61.0 ppm corresponding to −CH2OH (C in Figure 6). At 2 min time, it is possible that the spectra show three new groups of the signal probe for the existence in the media of the carmabate species. The signals of carbamate are at 3.57 ppm corresponding to −CH2OH (F in Figure 5), 3.24 ppm corresponding to N−CH2− (E in Figure 5), and 2.75 ppm of CH3N− (D in Figure 5) as a singlet. The carbamate species was present in the aqueous solution until the 20 min spectra, while the original signal of amine moved to lower fields because of the amine−amine protonated. The same behavior is shown, of course, at 13C NMR. The presence of carbamate was shown by four signals, where the most significant is the one at 165.3 ppm of absorbed CO2, which generates the carbamate. It was also possible to find a signal at 61.2 ppm of −CH2OH (F in Figure

6), 51.3 ppm of N−CH2− (E in Figure 6), and 34.5 ppm of CH3N− (D in Figure 6). As in 1H NMR, the carbamate signals were present until the 20 min spectra. At 40−50 min, the absorption process has finished and, in the media, there was present only the bicarbomate anion (161.3 ppm, Figure 6) and the amine protonated, which showed the three groups of signals for 1H and 13C NMR: 3.73 and 57.5 ppm for −CH2OH, 3.06 and 51.3 ppm for N−CH2−, and 2.64 and 33.6 ppm for CH3N−. In relation to the species present in the liquid phase during the chemical absorption of carbon dioxide using MAE, previous studies25 are in agreement with the simultaneous presence of different reaction products: carbamate, protonated amine, and bicarbonate, but in our study, the presence of carbamate at equilibrium was not observed. This discrepancy could be related to the amine concentration range because a higher 4740

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Figure 6. 13C NMR spectra corresponding to the liquid phase during the carbon dioxide absorption. CB = 0.42 mol L−1. Qg = 40 L h−1.

amount of amine makes the presence of both reaction products at the end of experiments easy. The experimental results obtained using the NMR technique allow us to reach a first approximation to the reaction mechanism corresponding to carbon dioxide chemical absorption in MAE aqueous solutions. As in previous studies that analyze the chemical absorption of carbon dioxide in amine aqueous solutions, two main reactions can take place: (i) carbamate production and (ii) bicarbonate/carbonate production. The last reaction has a higher importance when the tertiary of sterically hindered amines is used. Figures 5 and 6 show that both products are present in the liquid phase. Bicarbonate/carbonate can be produced by different ways: (i) reaction between amine and solvent or (ii) carbamate hydrolysis. Because of the nature of the liquid phases used in present work (MAE aqueous solutions), the second mechanism is considered as the bicarbonate production pathway (see Figure 7), because in the first part of absorption experiments,

Figure 7. Pathway of the CO2 absorption reaction using MAE aqueous solutions.

only the formation of carbamate is observed. Only when the concentration of this product increases, the bicarbonate/ carbonate is detected in the liquid phase. In the spectra shown in Figure 6, it is possible to see that the chemical absorption finishes at 30 min, because the chemical shifts corresponding to protonate amine maintain their values. During the absorption process, it is possible to observed the presence of both products: carbamate and bicarbonate. On the basis of these conclusions corresponding to the reaction mechanism, it is possible to conclude that one of the aims of the present work is reached because the behavior of this 4741

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solvent (aqueous solutions of MAE) is similar to the objective search using amine blends.8,9 On one hand, MAE shows a behavior similar to primary and secondary amines with a high reaction rate26 (because of the production of carbamate), but on the other hand, MAE also reached an increase of the carbon dioxide loading as tertiary amines (because of the formation of bicarbonate).14 Then, this solvent allows us to reach a suitable behavior with the use of only one reagent because of its sterically hindered structure. The conclusions reached by means of the analysis of NMR data previously described (Figures 5 and 6) are in agreement with the study about the carbon dioxide loading obtained for each MAE aqueous solution. Figure 8 shows the evolution of Figure 9. Influence of the amine concentration upon absorption kinetics. (○) CB = 0 mol L−1, (●) CB = 0.084 mol L−1, (□) CB = 0.42 mol L−1, and (■) CB = 1 mol L−1. Qg = 18 L h−1.

different initial concentrations. The presence of amine in the liquid phase causes a very important increase in the absorption rate in comparison to water. However, this increase at the beginning of the experiments is the same for all of the amine aqueous solutions (with different concentrations) used in the present work. An increase in the MAE concentration does not increase the mass-transfer rate at the beginning of the experiment (in comparison to low concentrated solutions), but it shows a behavior similar to a pseudo-first-order system. Using this high concentrated solution, a first zone (z1 for CB = 1 mol L−1) of a constant absorption rate is observed. This constant absorption rate is observed because the limiting step is the mass transfer, and then carbon dioxide absorbed reacts in the interface. When the amine concentration decreases in the liquid phase, a decrease in the absorption rate is also observed. This zone can be divided into two decreasing zones (z2 and z3 for CB = 1 mol L−1) with different trends, reducing the absorption rate with time. The different slopes showed that the decrease in the absorption rate is due to changes in the reaction mechanism. The first part (z1) corresponds to the carbamate formation that is a fast-rate reaction, and it maintains a high driving force by the carbon dioxide removing near the gas−liquid interface. The first reduction in the absorption rate (z2) is due to the formation of bicarbonate/carbonate and also because of the decrease in the amine concentration. The last change (z3) is associated with the non-existence of chemical absorption, and only the physical process takes place until the saturation is reached. Figure 9 analyzes the influence of the amine concentration upon the absorption rate and the shape of this curve during the experiment. On the other hand, Figure 10 allow us to analyze the influence of the gas flow rate fed to the bubbling contactor upon absorption kinetics. This figure compares the use of different carbon dioxide flow rates using different amine concentrations in the liquid phase. In both cases, an increase in the amount of gas absorption is produced when the carbon dioxide flow rate is increased. This increase is mainly due to an increase in the gas−liquid interfacial area (see Figure 4) because this parameter has an important influence in bubbling devices. Also, an increase in the gas flow rate produces an enhancement in turbulence into the contactor that has a positive influence in mass-transfer processes (see Figure 10).

Figure 8. Carbon dioxide loading for MAE aqueous solutions. (○) CB = 0.084 mol L−1, (●) CB = 0.42 mol L−1, and (□) CB = 1 mol L−1. Qg = 40 L h−1.

this parameter during the absorption experiments using different amine concentrations in the liquid phase. The experimental results have shown that, when the amine concentration is higher, the capture capacity decreases from ∼1.25 to ∼0.9 mol of CO2 mol−1 of amine. This behavior (a value of carbon dioxide loading higher than 1 mol−1 of amine for diluted amine aqueous solutions) is due to the higher importance of physical absorption in this kind of aqueous solution in comparison to concentrated solutions. When the amine concentration increases, the importance of the physically absorbed amount of carbon dioxide decreases, and then carbon dioxide loading decreases too. At equilibrium in carbon dioxide absorption in amine aqueous solutions, a loading value near 1 indicates that the main reaction product is the bicarbonate ion (reaction stoichiometry of 1:1). Then, under certain experimental conditions, it is difficult to obtain a specific stoichiometry during all of the absorption processes and the importance of each reaction is different in every moment. The experimental values of carbon dioxide loading are in agreement with the previous analysis of NMR because the signal corresponding to the bicarbonate ion is the main product observed at the end of the chemical absorption process. 3.3. Mass-Transfer Studies. Carbon dioxide absorption studies in MAE aqueous solutions were carried out using the experimental procedure described in the Experimental Section, and it was based in the difference between inlet and outlet gas flow rates in the bubbling reactor. Figure 9 shows a comparison between the obtained behaviors for the carbon dioxide absorption flow rate (Fa) using MAE aqueous solutions with 4742

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a part of the chemical reaction between CO2 and amine is carried out at the interface and the other part is carried out in the liquid bulk. The surface renewal theory developed by Dankwerts32 contributed to the expression shown in eq 6. N = CA*a DA k 2C Bbulk + kL 2

(6)

Then, using the experimental values of absorbed flow rate and eq 6, the liquid-side mass-transfer coefficient was calculated for each experimental condition (carbon dioxide flow rate and amine concentration). Figure 11 shows the behavior of eq 6

Figure 10. Influence of the gas flow rate and amine concentration upon absorption kinetics. CB = 0.42 mol L−1: (○) Qg = 18 L h−1 and (●) Qg = 40 L h−1. CB = 1 mol L−1: (□) Qg = 18 L h−1 and (■) Qg = 40 L h−1.

Both influences produce a significant reduction in saturation time. A previous study centered on the kinetic equation determination for carbon dioxide chemical absorption in MAE aqueous solutions26 has concluded that the reaction regime is instantaneous. Taking into account this information and the experimental data for absorption experiments, it is possible to conclude that the limiting step is the mass-transfer process. Under certain experimental conditions, the chemical reaction between these compounds can be produced at the interface. In this work, this behavior is produced in the absorption rate zone shown in Figure 9 for MAE highconcentrated solutions. In this part of the experiment, eq 5 can be used, taken into account that the amine concentration can be considered constant N = CA*a DA k 2C Bbulk

Figure 11. Mass-transfer coefficient determination using eq 6. (○) CB = 0.42 mol L−1 and (□) CB = 1 mol L−1. Qg = 18 L h−1.

fitting the experimental data, and the corresponding value for the intercept is low. The intercept value is used to calculate the mass-transfer coefficient. The fits shown in Figure 11 indicate that the use of different amine concentrations in the liquid phase does not change the intercept value very much but clear differences in the slope value are observed. This increase in the value of slope when the amine concentration increases is caused by the different importances of each chemical reaction (production of carbamate or production of bicarbonate) in the global process. When the amine concentration increases, the presence of carbamate is observed during all of the processes, and this chemical reaction has a more favorable rate than the production of bicarbonate. Then, the second-order kinetic constant is higher for rich amine solutions. The importance of the mechanism upon the slope in Figure 11 is high because the other parameter (diffusivity) decreases with the amine concentration. Figure 12 shows a summary of the calculated values for the mass-transfer coefficient. This parameter is highly influenced by the presence of amine because a small addition of this substance to the liquid phase produces an important decrease in the value of this parameter. This reduction in the value of the liquid-side mass-transfer coefficient is generally caused by the increase produced in the liquid viscosity that increases the mass-transfer resistance. For instance, previous studies using glucosamine (GA) aqueous solutions concluded that low increments in liquid-phase viscosity can produce important decreases in the mass-transfer coefficient,33 in agreement with results shown in Figure 12. Higher amine concentrations do not produce significant changes in the value of the masstransfer coefficient. This behavior is in agreement with the experimental values shown in Figures 9 and 10. In these figures, an increase in the amine concentration did not produce an enhancement in the maximum absorption rate. Then, the mass-

(5)

where N is the absorption rate of CO2, CA* and DA are the solubility and diffusivity of carbon dioxide in the aqueous phase, respectively, a is the interface area, k2 is the rate constant for the reaction between CO2 and hydroxyl ions, and Cbulk B is the amine concentration in the bulk of the aqueous phase. The use of this equation allows us to determine the specific interfacial area using the chemical absorption of carbon dioxide in MAE aqueous solutions as a chemical method for this aim.27,28 This method can only be applied under certain conditions (constant absorption rate). A comparison between the specific interfacial area values obtained using both methods (chemical and photographic) shows certain differences. These differences can be due to certain error in gas holdup determination in the first part of the experiments. The chemical method can be applied only in this first part, and the calculation of gas holdup is more difficult in this zone. However, in general, the chemical method implies other additional problems related to overestimation because of interfacial turbulence by surface tension gradients,29,30 and for this reason, this methodology has not been used. On the other hand, under the main part of experiments, the necessary conditions to use eq 5 are not satisfied. Also, the determination of the liquid-side mass-transfer coefficient implies the use of eq 6. The use of this expression needs the concentration of amine to remain practically constant throughout time,31 but if this condition is not satisfied, then 4743

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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Figure 12. Influence of the amine concentration and gas flow rate upon mass-transfer coefficient. MAE aqueous solutions: (○) Qg = 18 L h−1 and (●) Qg = 40 L h−1. GA aqueous solutions:33 (□) Qg = 18 L h−1 and (■) viscosity of MAE aqueous solutions.34

transfer coefficient is the limiting step in the global process. Then, an agreement between the experimental results of absorption kinetics and mass-transfer coefficient is observed. In relation to the influence of the gas flow rate upon this coefficient, Figure 12 also shows this information. No differences in the mass-transfer coefficient were observed under different values of gas flow rates, and these results are in agreement with conclusions previously reached.35,36 This lack of influence is mainly due to the type of reactor used in the present work because an increase in the gas flow rate (in the studied range) does not produce a significant increase in liquidphase turbulence.

4. CONCLUSION This paper is a complete and thorough study about carbon dioxide absorption in MAE aqueous solutions. This work summaries a hydrodynamic study in a bubbling reactor, gas/ liquid mass-transfer rate, and reaction mechanism analysis of the chemical absorption process using MAE aqueous solutions. This novel study confirms the crucial importance of interface area observing significant changes in the evolution of this magnitude associated with an increase in carbon dioxide loading during the experiments. Furthermore, an increase in the flow rate of feeding gas to the reactor causes a decrease in bubble size associated with breakage processes, because of the low surface tension of aqueous amine solution. As known, carbon dioxide loading is related to reagent and product amount in the global chemical reaction mechanism. At low values of carbon dioxide loading, the carbamate production has a significant importance, although bicarbonate production becomes more and more important when carbon dioxide loading is increased in the liquid phase (confirmed by NMR spectra), because of hydrolysis of carbamate. Absorption kinetics presents a planar absorption rate zone associated with high concentrated amine solution and fast chemical reaction (carbamate production). This behavior could be reflected as a pseudo-first-order regime in this experimental period. Under different experimental conditions, a liquid-side mass-transfer coefficient was determined and shows a significant decrease in its value when the amine concentration was increased until it reached a constant value. Generally, this behavior is assigned to an increase in liquid-phase viscosity, which is affected by the amine concentration. 4744

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