Estimation of the Amount of CO2 Absorbed by Measuring the Variation

Article pubs.acs.org/jced

Estimation of the Amount of CO2 Absorbed by Measuring the Variation of Electrical Conductivity in Highly Concentrated Monoethanolamine Solvent Systems Sang-Jun Han and Jung-Ho Wee* Department of Environmental Engineering, The Catholic University of Korea, 43 Jibong-ro, Wonmi-gu, Bucheon-si, Gyeonggi-do 420-743, Republic of Korea ABSTRACT: The present paper investigates the various CO2 absorption behaviors in relatively highly concentrated monoethanolamine (MEA) solvent systems, containing 1−5 M MEA, based on the experimental results as well as the ionic conductivities of carbamic acid (RNH3+) and carbamate (RNHCOO−) reported in our previous study. Initial measured electrical conductivity (ECm) and effective RNH3+ concentration of the solvents decreased as the MEA concentration increased. The ratio of RNHCOO− hydrolyzed to free amine was also reduced according to MEA concentration, thus causing a decrease in the ratio of total CO2 absorbed to MEA concentration as the MEA concentration increased. The activity coefficient of the solvent also decreased with increase of the MEA concentration. These phenomena may be primarily ascribed to the lack of a sufficient amount of water molecules to surround and block the interaction between the two separated ions in the system, RNH3+ and RNHCOO−, as compared to low concentration solvent systems. Finally, the amount of CO2 absorbed in the system was almost proportional to ECm in each solvent, thus an equation for general estimation of the amount of CO2 absorbed by measurement of the ECm variation in a given highly concentrated MEA solvent was derived.

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INTRODUCTION Chemical absorption using an alkanolamine-based solvent is considered to be one of the most practical technologies for carbon capture and storage (CCS), and has already been partially commercialized.1−9 Many studies have focused on the use of monoethanolamine (MEA) aqueous solution in particular because of its many advantageous features such as high reactivity, low cost, and the lowest molecular weight among the various alkanolamine-based solvents.10−12 The development of technology regarding process control and operation of the CO2−MEA absorption system was highlighted in our previous work.13 Although there are many traditional options to calculate the amount of CO2 absorbed in the solvents such as the titration, BaCl2 method, and TOC analysis, they are all applied to the process discretely. The work,13 however, reported that the amount of CO2 absorbed in the process can be easily and systematically predicted through in situ measurement of the electrical conductivity (EC) variation of the solvent during absorption. For this estimation, the novel approach of calculating the ionic conductivities of RNH3+ and RNHCOO− from the experimental results was employed, after which an equation correlating the amount of CO2 absorbed and the EC was presented. However, in order for the ionic conductivities of the two ions to be determined, the concentration of MEA solvent used in the previous work was limited to lower than 0.5 M (3 wt %). Therefore, the developed equation correlating the amount of CO2 absorbed © XXXX American Chemical Society

and EC variation in the system may be not appropriate for application to highly concentrated MEA solvent systems of approximately 30 wt %, which is the generally accepted MEA concentration for practical application. Therefore, the previous results were regarded as providing fundamental information with which to further develop a correlation equation for practical use. The MEA concentration in the solvent is also a very important issue in terms of economy and technology. According to Badea and Dinca, although high concentration of MEA in the solvent reduces the energy consumption, it causes substantial corrosion and solvent degradation during the process.14 Therefore, they claimed that further more intensive research should be carried out on this issue to allow process optimization and commercialization. In addition, Veawab et al. concluded after examination of this issue that even though the use of highly concentrated solvent can reduce the operating cost, the associated corrosion should be addressed to improve the economic benefit of the process.15 Considering all these issues, the MEA concentration in the solvent was optimized to about 5 M (30 wt %), which has generally been accepted in commercial processes.5,16−19 However, informative data relating the amount of CO2 absorbed and the EC variation in highly concentrated MEA solvents during the absorption have not yet Received: February 26, 2015 Accepted: December 7, 2015

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DOI: 10.1021/acs.jced.5b00178 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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The HCO3− generated is the main cause of corrosion of the equipment due to its acidity.23 Therefore, if the solvent contains a high proportion of water, most of the RNHCOO− will become hydrolyzed, in which the overall reaction of eqs 3 and 4 can be expressed as shown in eq 5.

been presented and are required to allow further technological improvement of the process. Therefore, the present study was carried out for several reasons. First, analysis of the CO2 absorption upon variation of the concentration of MEA aqueous solution at relatively high concentrations of 1−5 M (approximately 6−30 MEA wt % in the solution) was carried out for the collection of data. The data collected included the amount of CO2 absorbed according to the measured electrical conductivity (ECm) variation in each solvent, the variation and calculation of the RNH3+ and RNHCOO− concentrations, the ratio of the amount of CO2 absorbed to MEA concentration, the hydrolysis ratio of RNHCOO− according to MEA concentration, and ionic activity coefficient of the solvent. Second, the reasons for the differences in the highly concentrated MEA solvent system compared to the lower concentrated MEA solvent system were explained. Third, using the results obtained herein as well as the values of the ionic conductivity of RNH3+ and RNHCOO− reported in the previous paper, an individual correlation equation between the amount of CO2 absorbed and the EC variation in each solvent was developed and presented. Finally, the correlation equation was generalized to allow direct estimation of the amount of CO2 absorbed in all MEA solvent systems of relatively high concentration using the MEA concentration and in situ measured EC variation as independent parameters. The newly developed correlation equation enables direct calculation of the amount of CO2 absorbed solely by measuring the EC variation of any highly concentrated MEA solvent in the process, and vice versa.

RNH 2 + CO2 + H 2O ↔ RNH3+ + HCO3−

On the basis of eq 5, the stoichiometric CO2 absorption capacity of the solvent can be calculated to be 1.0 mol CO2· mol−1 MEA, which can be obtained for very low concentration MEA solvents with relatively higher water composition. This phenomenon was confirmed in our previous paper. Although the overall capacity decreases as the MEA concentration of the solvent increases, the practical processes tend to maintain relatively high MEA concentrations which lead to the CO2 absorption capacity of about 0.25−0.45, hindering progression of eq 4 and preventing corrosion of the equipment.24 Calculation of Ionic Activity Coefficient via Measurement of Electrical Conductivity. The EC of an electrolyte can be calculated via eqs 6 to 11. ko =

OH− + CO2 ↔ HCO3−

(2)

RNH3+

(7)

IS = 500 ∑ Cizi 2

(8)

for IS < 0.01

IS 1+

IS

for 0.01 ≤ IS ≤ 0.1

⎛ ⎞ IS log γ = −Az+z −⎜ − 0.2IS⎟ ⎝ 1 + IS ⎠

(9)

(10)

for 0.1 < IS < 0.5 (11)

where ko is the electrical conductivity of infinitely diluted solution (S·m−1), zi is the absolute value of electric charge of ion i, Ci is the concentration of ion i (mol·m−3), λi is the ionic conductivity of ion i (S·m2 mol−1), IS is the ionic strength (mmol·m−3), γ is the ionic activity coefficient, and EC is the electrical conductivity of solution (S·m−1). ko can be readily calculated because it is the EC of the infinitely dilute solution, and is solely dependent on the three parameters of concentration, ionic conductivity, and electrical charge of the ions. In other words, eq 6 can be used only for the infinitely diluted solution systems, considered as the ideal solution in which the activity (effective concentration) is the same as the theoretical concentration. However, in highly concentrated solutions, that is, noninfinitely diluted solutions which display substantial deviation from ideality, the ionic activity coefficient (γ) should be considered when calculating the EC of the solution, as in eq 7, since the activity of the solution significantly differs from the theoretical concentration. To calculate the γ of highly concentrated solutions, the ionic strength (IS) of the solvent must first be obtained via eq 8. Thereafter, considering the range of the obtained IS values, the γ of each solvent is estimated by selecting one equation from among eqs 9−11. In our previous work, the γ values of the five MEA solvents were calculated via eqs 9−11 because all ISs were maintained below 0.5 throughout the CO2 absorption period, thus EC of all solvents used could be calculated via eq 7. However, in the

Although a slight amount of MEA is lost via eq 1 before the main CO2 absorption reaction, there would be little loss of solvent in terms of the total CO2 absorption capacity because the corresponding amount of CO2 would be absorbed by the OH− generated, according to eq 2. When CO2 injection begins for absorption, the OH− initially present becomes depleted quickly by CO2 absorption since OH− has high reactivity due to its strong basicity. Thereafter, the main absorption of the MEA solvent occurs as listed in eq 3. 2RNH 2 + CO2 ↔ RNH3+ + RNHCOO−

(6)

EC = koγ 2

log γ = − 0.5z+z −

THEORETICAL BACKGROUND Mechanism of CO2 Absorption into MEA Solvent and Absorption Capacity. Many papers including our previous work discussed the CO2 absorption mechanism in MEA solvent.20−22 The absorption begins with the dissociation of MEA, which reacts with water to produce OH− via eq 1, after which it can absorb CO2 according to eq 2. (1)

∑ ziCiλi

log γ = −0.509z+z − IS

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RNH 2 + H 2O ↔ RNH3+ + OH−

(5)

(3)

−

According to eq 3, and RNHCOO are generated by the absorption of CO2 into MEA, while the stoichiometric CO2 absorption capacity of the solvent is 0.5 mol CO2·mol−1 MEA. However, when the amount of water present in the solvent is higher than a certain level, some of the RNHCOO− become hydrolyzed to the free amine and HCO3− via the reaction presented in eq 4. RNHCOO− + H 2O ↔ RNH 2(free amine) + HCO3− (4) B



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infrared (NDIR) gas analyzer (maMos-200, Madur Electronics). N2 (99%) and CO2 (99.9%) gas mixtures were used as the feed gas for absorption. These two gases were mixed homogeneously in the gas mixer, which was maintained at 25 °C. The CO2 and N2 were supplied at the flow rates of 1 and 2 L·min−1, respectively, by a mass flow controller (MFC) to maintain the CO2 concentration of 33 vol % in the mixed gas, which is the simulated dry-based exhaust gas of the cement and steel industry. Before absorption, all parts of the apparatus, including the inside of the reactor and the gas line, were sufficiently flushed with N2 purging. The absorption process began with the injection of the stabilized gas mixture into the reactor through the sparger, made of a glass filter. The end of absorption was determined as the point at which the CO2 concentration of the outlet gas was equal to the initial CO2 concentration of the gas mixture.

highly concentrated solvent systems, the IS was higher than 0.5 because of the high concentration of ions produced by CO2 absorption. Therefore, eqs 9 to 11 could not be applied to this case. Furthermore, it is not easy to find a new equation which is reasonably applicable for calculation of the γ of solutions in which the ISs are higher than 0.5. Therefore, under such conditions, eq 7 is not applicable for determination of the calculated EC (ECc). However, the γ of such solutions can be directly estimated without first calculating the IS via a modified version of eq 7, as presented in eq 12. γhighly concentrated solution =

ECm k0

(12)

The measured EC (ECm) was substituted for EC in eq 7, which proved useful for calculating the actual values of γ for highly concentrated MEA solvent systems.

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EXPERIMENTAL SECTION MEA (Samchun Chemical Co., 99.0%) solvents of relatively high concentration, ranging from 1 to 5 M, were prepared for CO2 absorption experiments by mixing MEA with water. CO2 absorption was then carried out with the solvents using the apparatus illustrated in Figure 1.

RESULTS AND DISCUSSION Initial Electrical Conductivity and RNH3+ Concentration in Highly Concentrated MEA Solvents. The initial ECm of all MEA solvents, including those at low concentration, is shown in Figure 2a. The initial EC of the solvents was primarily influenced by the concentrations of RNH3+ and OH−, dissociated according to the equilibrium constant of eq 1, which were basically

Figure 1. Schematic diagram of CO2 chemical absorption using relatively high concentration MEA solvent: (1) N2 cylinder, (2) CO2 cylinder, (3) mass flow controller (MFC), (4) gas mixer, (5) sparger, (6) magnetic stirrer, (7) EC/pH sensor, (8) thermometer, (9) temperature controller, (10) EC/pH meter, (11) dehumidifier, (12) gas analyzer, and (13) computer for data acquisition.

For reaction, 0.5 L of MEA solvent at various concentrations were placed in cylindrical absorption reactors (D, 110 mm; h, 80 mm) equipped with water jackets to maintain a constant temperature of 25 °C. The solvents were uniformly mixed at the stirring speed of 180 rpm. EC/pH meters (Orion 4 Star, Thermo Scientific) were set inside the sealed reactors to allow continuous measurement of the EC and pH during the reaction. The EC and pH data were measured in situ, and data were obtained every five seconds throughout the reaction. The CO2 concentration in the outlet gas which passed into the reactor was measured every second using a nondispersive

Figure 2. Initial electrical conductivity (EC) (a), and concentration of RNH3+ (b) of all MEA solvents, including low concentration systems, calculated on the basis of pKa and on eq 13. (The low concentration values were taken from the previous paper.13) C



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and effective RNH3+ concentration may be intensified with an increase of the MEA concentration in the highly concentrated solvents. Amount of CO2 Absorbed, Reaction Rate, and Hydrolysis Ratio of Generated RNHCOO−. The amount of CO2 absorbed, the mole ratio of CO2 absorbed during the initial absorption period to the total CO2 absorbed throughout the reaction (CO2 initial absorption period/CO2 total) and the mole ratio of CO2 absorbed to the MEA concentration according to reaction time (CO2/MEA) are shown in Figure 3 for the five concentrations of MEA solvent.

determined by the pKa value (9.42) of the reaction irrespective of the MEA concentration range.13 Therefore, the initial calculated EC (ECc) of the low concentration MEA solvents (0.1−0.5 M) was determined in the previous study solely using the pKa of the reaction, which was a reasonable approach. Accordingly, the initial ECm was proportional to the MEA concentration, and these were almost equal to the initial ECc of the low concentration MEA solvent systems. However, in the highly concentrated MEA systems, the initial ECm did not display a linear increase according to the MEA concentration. In fact, the ratio of increase of the initial ECm between the solvents of 0.5 and 1 M concentration was substantially lower compared to the other even lower concentration ranges, while the ECm of the 2 M solvents displayed a slight increase. However, the initial ECm of the solvents at concentrations above 2 M was decreased according to the MEA concentration. This phenomenon was ascribed to nonideality due to the high concentrations of the solvents, and thus not allowing the ECc to be calculated on the basis of the pKa as in the low concentration solvents. Therefore, to determine the initial concentration of dissociated RNH3+ in the highly concentrated solvents, a correlation equation between ECm and RNH3+ concentration was first obtained by the least-squares method using the results of the low concentration MEA solvent systems. The equation is derived as listed in eq 13 with the regression coefficient of 0.998, and it was then applied for calculations in the high concentration systems. [RNH3+](mmol · L−1) = 4.363·10−3·ECm (μS· cm−1) − 0.071

(13)

RNH3+

In addition, the calculated concentrations of in the highly concentrated solvents were then used for the theoretical calculation of the ECc in the system by feedback. The initial ECc was estimated using eqs 6−11 with the concentrations and ionic conductivities of all ions in the solution including RNH3+ and OH−. The OH− concentration was assumed equal to that of RNH3+ according to eq 1. The initial RNH3+ concentrations in all solvents, including those of high concentration, as calculated by pKa and eq 13 (denoted as the effective [RNH3+]) are shown in Figure 2b. The RNH3+ concentrations estimated on the basis of the pKa and the ECc calculated on the basis of these concentrations were proportional to the concentrations of MEA in all solvents, as shown in Figure 2a and b, respectively. On the other hand, the ECc of the highly concentrated solvents, which was estimated on the basis of the RNH3+ concentrations calculated via eq 13, was almost identical to the ECm, as shown in Figure 2a. For example, for the 5 M solvent, the ECc calculated on the basis of eq 13 and the ECm were 713.39 uS·cm−1 and 699 uS· cm−1, respectively. The difference between these two values was only about 2%, while the average error in the other high concentration solvents examined was less than 0.5%. Finally, when the concentration of MEA solvent exceeded 2 M, further decreases of the initial ECm and effective RNH3+ concentration were observed. These phenomena may be attributed to the stronger attraction between RNH3+ and OH− in solvents exceeding MEA concentrations of 2 M, due to the relatively lower proportion of water molecules surrounding the two ions compared to that in the low concentration solvents. This could possibly increase the presence of RNH3+•OH−, for which the equivalent conductivity is relatively lower than the two separated ions. Therefore, a decrease of both the initial ECm

Figure 3. Amount of CO2 absorbed (a), mole ratio of CO2 absorbed in the initial absorption period to total CO2 absorbed (b), and mole ratio of CO2 absorbed to MEA concentration, according to reaction time (c).

The slopes in Figure 3a indicate the rates of CO2 absorption into the MEA solvents throughout the reaction. In the previous paper, the rate of the initial absorption period was relatively fast due to the dominant occurrence of the reaction illustrated in eq 3 for the low concentration systems. The slopes became reduced after the initial period, followed by gradual occurrence of a further decrease, during which only slight absorption of additional CO2 occurred until the reaction completion point. The CO2 initial absorption period/CO2 total was calculated to be approximately 0.5. However, in the highly concentrated MEA solvents, most of the CO2 was absorbed during the initial period, for which the end time of each solvent is denoted as a vertical line in Figure 3, while the subsequent reaction was almost terminated with little additional absorption observed. In addition, increase of the CO2 initial absorption period/CO2 total was observed according to the MEA concentration. The ratio of 0.615 was recorded for the 1 M solvent, which was the lowest value among the high concentration solvents, while the 5 M solvent was calculated to have the ratio of 0.906, as shown in Figure 3b. D



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RNHCOO− can theoretically be calculated as 15. Therefore, if eq 4 was not intentionally inhibited, most of the RNHCOO− generated in the low concentration MEA solvents could be hydrolyzed to free amine. Meanwhile the portion of hydrolyzed RNHCOO− present in the 1 M MEA solvent was calculated to be about 51%. Therefore, it was concluded that the H2O/RNHCOO− (mol· mol−1) value required to hydrolyze the RNHCOO− generated to achieve a ratio of more than 0.96 was estimated to be at least 270, which was the value of the 0.4 M solvent. Therefore, solvents which have H2O/RNHCOO− ratios lower than this value would not have active occurrence of RNHCOO− hydrolysis due to the relatively low portion of water molecules. For example, the hydrolysis ratio was reduced to 0.7 in the 0.5 M MEA solvent. An equation for calculation of the ratio of hydrolyzed RNHCOO− to RNHCOO− generated could be obtained by the exponential decay regression of these results, and is shown in eq 14, for which the regression coefficient was 0.9850.

The CO2/MEA in each solvent was increased through more absorption of CO2 as the reaction proceeded, as shown in Figure 3c. However, it was found that the rate of increase in this ratio was substantially reduced starting at the end point of the initial absorption period of each solvent. Thereafter, further decrease was observed until finally approaching a constant value at the absorption completion point. The final CO2/MEA ratios of the 1 and 2 M solvents were 0.805 and 0.601, respectively, while approximate convergence to 0.5 was observed for the 3− 5 M solvents. In the previous paper, the final CO2/MEA of the 0.5 M solvent was 0.857, which was the lowest value among the low concentration solvents, while the other four solvents of lower concentration displayed values close to 1. The reason for the decrease in the final CO2/MEA according to increasing MEA concentration increase can be found in the concentration of RNHCOO− generated via eq 3, or the water concentration in the solvent. In other words, this was ascribed to the little additional absorption of CO2 by free amine via eq 5 which occurred in the highly concentrated solvents, since the concentration of RNHCOO−, which could be hydrolyzed, was decreased due to a relatively lowered water concentration according to the MEA concentration increase. To explain this phenomenon in detail, the ratio of RNHCOO− hydrolysis for each solvent was calculated on the basis of the experimentally determined amount of CO2 absorbed in the reaction period, and the results were shown in Figure 4.

RNHCOO− reacted = 1.117 exp( −0.675[MEA]) RNHCOO− formed

(14)

Ionic Activity Coefficient of MEA Solvent According to Absorption Time. For calculation of the γ of the five MEA solvents, ko was first estimated via eq 6 using the concentrations of all ions in the solvents, which varied due to CO2 absorption, as well as the ionic conductivity values of RNH3+ and RNHCOO−, reported in the previous paper.13 Second, by applying eq 12, which was derived from eq 7, the γ values of the five MEA solvents were obtained. The values according to reaction time are presented in Figure 5.

Figure 4. Ratio of hydrolysis of the RNHCOO− generated in each solvent. (The low concentration values were taken from the previous paper.13)

Most of the RNHCOO− generated by eq 3 was hydrolyzed to free amine in the low concentration solvents via eq 4, which resulted in substantial absorption of additional CO2 via eq 5. However, the ratio of RNHCOO− hydrolysis in the highly concentrated solvents was significantly decreased as the MEA concentration increased. For example, in the 4 M solvent, only 10% of the RNHCOO− generated participated in eq 4 to become free amine, which in turn allowed only slight absorption of CO2. For the hydrolysis of RNHCOO− to occur, it needs to be sufficiently surrounded by water molecules.25−27 However, the concentration of water surrounding one molecule of RNHCOO− was decreased as the MEA concentration increased. In the 5 M MEA solvent, the number of water molecules present around one molecule of

Figure 5. Variation of ionic activity coefficient (γ) in MEA solvents according to reaction time.

As aforementioned, the initial concentrations of RNH3+ and OH− of the five highly concentrated MEA solvents before CO2 absorption were directly calculated by eq 13, derived based on the results obtained from the low concentration MEA solvent systems, and thus could be assumed to be equal to the initial effective concentrations of RNH3+ and OH−. Therefore, the initial γ of all solvents was calculated to be approximately 1, as shown in Figure 5. Thereafter, the γ of all solvents was decreased with the absorption of CO2. When the CO2 absorption was completed in the 1 M solvent, that is, when the concentration of all generated ions was the highest, the γ of E



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the solvent was 0.62. Lower values of γ were measured as the MEA concentration increased, with the lowest value of 0.39 obtained in the 5 M solvent. As mentioned above, while there are low concentrations of water around ions such as RNH3+ and RNHCOO− in the highly concentrated solvents, the concentrations of generated ions are high. Therefore, in highly concentrated solvents, the relatively low amount of water molecules make it more likely that the RNH3+ and RNHCOO− will be present as the electrically neutralized form, RNH3+•RNHCOO−, than in their separated form. This zwitterion has relatively little influence on ECm, which reduces the γ of the solvent. In the previous paper, the γ of the solvents could be calculated according to CO2 absorption because the ionic conductivity values of RNH3+ and RNHCOO− were obtained and the ionic strength was low. Therefore, the ECc of the low concentration MEA solvents could be calculated and compared to the ECm. However, ECc could not be calculated according to the CO2 absorption in the high concentration solvent system, since the equation for γ estimation in highly concentrated solvents was unknown. Therefore, it was impossible to compare ECm to ECc, thus only the variation of ECm according to absorption time was shown in Figure 6.

increased up to a maximum in the 3 M solvent, after which decreases occurred in the 4 and 5 M solvents due to their relatively higher concentrations of all ions with lower concentrations of water. In other words, even though more ions were generated in the solvents of higher concentration than 3 M through absorption of more CO2, the activity of the effective conductive ions in the solvent was lower because of the relative shortage of water molecules, which caused a reduction in the γ of the solvent. Empirical Correlation Equation for the Amount of CO2 Absorbed and ECm. As mentioned earlier, the amount of CO2 absorbed and the ECm increased according to reaction time. However, unlike the amount of CO2 absorbed, the ECm was not proportional to the MEA concentration. The relation among the three variables, including absorption time, the amount of CO2 absorbed, and ECm of the five solvents, using MEA concentration as the parameter, was depicted in the center space of a three-dimensional graph, as shown in Figure 7.

Figure 7. Three-dimensional diagram relating absorption time, amount of CO2 absorbed, and ECm of the solvents using the MEA concentration as the parameter.

Figure 6. Variation of measured electrical conductivity (ECm) in each solvent according to reaction time.

Although it was observed in the previous paper that the final ECm at the end-point of the low concentration MEA solvent system was increased proportionally to the MEA concentration, this was not observed for the highly concentrated solvents. Conversely, the value of the 5 M solvent was found to be even lower than that of the 2 M solvent. The ECm difference between the 1 and 2 M solvents was 8.8 mS·cm−1, which is a larger difference than that between the 2 and 3 M solvents, 2.3 mS·cm−1. In addition, the final ECm of 3 M, 42.9 mS·cm−1, was the highest among all the solvents tested. The large amount of CO2 absorbed indicates that there was a large amount of ions generated. Therefore, basically, the generated ions would be most abundant in the 5 M solvent; thus, the ECm for this solvent should be larger than that of any of the others. However, the results presented in Figure 6 did not follow this assumption. This discrepancy may be ascribed to the influence of MEA (or water) concentration on the concentration of ions effectively conducting electricity among all the various ions generated in the solvents. These ions were

Two-dimensional graphs projected in three planes were also shown in the figure. The graphs depicted in plane 1 and 2 are the same as those in Figure 3a and Figure 6, respectively. In the highly concentrated MEA solvent system, the amount of CO2 absorbed in each solvent was almost proportional to the ECm, as shown in plane 3 of Figure 7, and the correlations between them could be fitted by the least-squares method, as shown in Figure 8. The results of the low concentration solvents reported in the previous paper were depicted in the same Figure. As reported in the previous paper, though the amount of CO2 absorbed was proportional to the ECm in the low concentration solvents, the difference according to MEA concentration was too small to clarify a distinction. Moreover, when the correlation equations were indicated in Figure 8, almost a straight line was obtained for the low concentrations when compared to the high concentration MEA solvents. The slope, y-intercept, and regression coefficient of the correlation equations, which relates F



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slope = 13.1 ·C + 7.79

(15)

intercept = −79.3·C + 75.5

(16)

where C is the MEA concentration higher than 1 M in the solvents (mol MEA·L−1). If the MEA concentration in the solvents, as the parameter, were coupled with these two equations, the final equation, which can generally be used to estimate the amount of CO2 absorbed by measuring the EC m variation in highly concentrated MEA solvent, was derived as eq 17. CO2 = (13.1 ·C + 7.79) ·ECm − 79.3 ·C + 75.5

(17)

Here, CO2 is the amount of CO2 absorbed in the highly concentrated MEA solvent (mmol CO2·L−1) and EC is the electrical conductivity (mS·cm−1). Finally, when the concentration of MEA solvent is given and the ECm variation is monitored during the CO2 absorption process, the amount of CO2 absorbed can be calculated in situ via eq 17.

Figure 8. Correlation between the amount of CO2 absorbed and the measured electrical conductivity (ECm) in all MEA solvent systems. (The low concentration values were taken from the previous paper.13)

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CONCLUSION CO2 absorption experiments were conducted using relatively high concentrations of MEA aqueous solutions, at approximately 6−30 MEA wt %, as the solvent, and many data were obtained. On the basis of the results, as well as the values of the ionic conductivity of RNH3+ and RNHCOO− reported in the previous paper, this work investigated the absorption behaviors of the solvent, including ECm variation, the amount of CO2 absorbed/MEA concentration, hydrolysis ratio of RNHCOOaccording to MEA concentration, and ionic activity coefficient of the solvent. In particular, the correlation equation relating the amount of CO2 absorbed to the ECm of all MEA solvent systems of relatively high concentration was generalized herein. From the results, the following conclusions were obtained. (i) Initial ECm and effective RNH3+ concentration of solvents with MEA concentrations higher than 2 M before carbonation were decreased as the MEA concentration increased. This may be ascribed to a stronger attraction between RNH3+ and OH− in the highly concentrated MEA solvent due to the relative shortage of water molecules surrounding the two ions compared to that in low concentration solvent systems, which leads to a preference of RNH3+•OH−, of which the equivalent conductivity is relatively lower than the two separated ions. (ii) The lower water concentration (or cases where the H2O/ RNHCOO− (mol·mol−1) value is below 270) in MEA solvents of higher concentration caused further reduction of the hydrolysis ratio of RNHCOO-, which hindered free amine production, thus causing little additional CO2 to be absorbed in the later absorption period. Therefore, the ratio of total amount of CO2 absorbed to MEA concentration was decreased according to the increase in MEA concentration for the high concentration MEA solvent systems.

the amount of CO2 absorbed to the ECm of the five highly concentrated MEA solvents, are listed in Table 1. The slope indicates the amount of CO2 absorbed when the ECm is increased by 1 mS·cm−1 due to CO2 absorption in the solvent. The slopes were increased according to the MEA concentration. This means that if the same increase of ECm were detected in the 1 and 5 M solvents, the amount of CO2 absorbed in the 5 M solvent would be larger than that in the 1 M solvent. As mentioned previously, this strong sensitivity of the 5 M solvents in terms of the amount of CO2 absorbed is due to the lower proportions of primary conducting ions, such as separated RNH3+ and RNHCOO−, which are not proportional to the amount of CO2 absorbed in highly concentrated solvent due to insufficient amounts of water. This instead supports the preference for the two ions to exist as the electrically neutralized form, RNH3+·RNHCOO− in the highly concentrated solvent. The y-intercept, that is, the amount of CO2 absorbed at the point where the EC is zero, can be theoretically estimated from the correlation equation of each solvent, and was found to be inversely proportional to the MEA concentration. In addition, its negative value is ascribed to the initial EC (in the state without CO2 absorption) exhibiting the positive value in highly MEA concentrated solvents because of their already ionized substances. Therefore, the y-intercept is a hypothetical value and used solely to derive the final correlation equation. Because the slopes and y-intercepts were linearly and inversely proportional to the MEA concentration, respectively, new correlation equations between the slope and concentration, as well as between the y-intercept and concentration, could be obtained by the least-squares method, and are listed as eqs 15 and 16 with the regression coefficient of 0.9374 and 0.9871, respectively.

Table 1. Slope, y-Intercept, and Regression Coefficient of Correlation Equations between the Amount of CO2 Absorbed and the ECm of the Five Highly Concentrated MEA Solvents concentration of MEA solvents (M) factors examined

1

2

3

4

5

slope (mmol CO2·cm·L−1·mS−1 of solvent) y-intercept (mmol of CO2·L−1 of solvent) regression coefficient, r2

24.44 −15.6 0.999

34.67 −82.8 0.998

41.76 −139 0.997

54.78 −244 0.984

79.93 −332 0.980

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(iii) In the highly concentrated MEA solvent systems, the ionic activity coefficient, γ, was directly estimated from the ECm value. The RNH3+ and RNHCOO− prefer to exist as the electrically neutralized RNH3+·RNHCOO− than in their original form in highly concentrated MEA solvent systems because of the lack of sufficient water molecules to surround and block interaction of the two separated ions. Therefore, the γ decreased in highly concentrated MEA solvents with an increase of the MEA concentration and amount of CO2 absorbed. (iv) The amount of CO2 absorbed in the highly concentrated MEA solvents was almost proportional to the ECm, and their correlation equation was obtained. However, the correlation between the amount of CO2 absorbed and the ECm becomes more sensitive according to MEA concentration in the solvent. From the correlation equations of each MEA solvent, the final equation was determined for general estimation of the amount of CO2 absorbed as follows. CO2 = (13.1 ·C + 7.79) ·EC − 79.3 ·C + 75.5

The calculation could be accomplished by measuring the ECm variation in the given highly concentrated MEA solvent and vice versa. The work supports that the estimated values of the ionic conductivity of RNH3+ and RNHCOO− reported in the previous paper were valid, and the correlation equation is expected to contribute to the process development of the CO2−MEA absorption system. In reality, the EC variation might be affected by many factors such as flue gas composition, temperature, gas−liquid ratio, and other components in the solution. Therefore, further studies considering these factors are required and will be conducted in near future.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +82-2-2164-4866. Fax: +82-2-2164-4765. E-mail: [email protected], [email protected]. Funding

This research was supported by Basic Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2013R1A1A2A10010414) as well as supported by the Catholic University of Korea, Research Fund, 2014. Notes

The authors declare no competing financial interest.

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Estimation of the Amount of CO2 Absorbed by Measuring the Variation

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