Estimation of Correlation between Electrical Conductivity and CO2

Article pubs.acs.org/jced

Estimation of Correlation between Electrical Conductivity and CO2 Absorption in a Monoethanolamine Solvent System 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 reports the ionic conductivities of carbamic acid (C2H5ONH3+) and carbamate (C2H5ONHCOO−) by analyzing the measured electrical conductivity (EC) variation according to the amount of CO2 absorbed in an aqueous monoethanolamine (MEA) solvent system. On the basis of these conductivities, the study also investigates the correlation between EC variation and the amount of CO2 absorbed in five MEA solvents with a concentration range of 0.1 M to 0.5 M. The study is underpinned by several assumptions, and a CO2 absorption experiment is carried out with the five MEA solvents. The ionic conductivity of C 2 H 5 ONH 3 + and C2H5ONHCOO− is calculated as 73.60 S·cm2·mol−1·z−1 and 16.40 S·cm2· mol−1·z−1, respectively. An average error between the estimated EC (ECc) variation calculated via the correlation equation and the measured EC (ECm) variation obtained from the experiments is 6.92 % in five MEA solvents. However, the ECc in the 0.5 M MEA system is 10.22 % lower than ECm. This difference may be ascribed to the substantial deviation from the assumptions of the 0.5 M solvent due to its relatively high concentration which decreases the ionic activity coefficient of existing ions in the solvent.

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their concentration, and the system temperature.17 Therefore, EC can be an important factor to characterize and control the system involved in chemical absorption with the solvent. In addition, many other valuable data can be directly or indirectly obtained from it. For example, EC was measured and directly used to calculate the ionic conductivity of ethylene glycol tetraacetic acid (EGTA), which is a chelating agent used to control the concentration of divalent ions in a biological situation.18 Therefore, measuring the EC variation during CO 2 absorption into MEA solvent can supply valuable data in terms of system operation. The most representative data may be the ionic conductivity of carbamic acid (RNH3+ (R; C2H5O)) and carbamate (RNHCOO−), which are generated or consumed when CO2 is chemically absorbed in MEA solvent. Although the ionic conductivity of these two ions may be very useful for intensive research on CO2 absorption using MEA solvent, this subject has not been the focus of any research in the literature. Therefore, the present paper reports the value of RNH3+ and RNHCOO− ionic conductivity calculated by analyzing the in situ measured EC (ECm) variation and the amount of CO2 absorbed in MEA solvent. On the basis of these values, this study also presents a correlation equation to directly estimate the amount of CO2 absorbed from the EC variation in an MEA solvent system. Several conditions had to be assumed, of which

INTRODUCTION The use of an alkanolamine-based solvent to chemically absorb CO2 leads to substantial corrosion of equipment and high energy consumption in solvent recovery.1,2 Nevertheless, the process is commercialized because the total cost is lower than any other CO2 capture technologies owing to the very high absorption capacity and readiness to be retrofitted.3,4 Among various alkanolamine-based solvents, monoethanolamine (MEA, H2NCH2CH2OH) aqueous solution, which is the elementary solvent in alkanolamine-based solvents, has received much attention due to its many advantageous features including fast reaction rate, low cost, and thermal stability.5−8 However, although many papers have investigated the CO2 absorption process using MEA solvent, most are related to the system performance, process design, solvent development, and regeneration.9−14 Besides these works, research on effective process control of the MEA system is also important. For example, if the amount of CO2 absorbed can be simply and directly estimated by measuring an easily obtainable process value, the system will be more stably operated and effectively controlled. Such a study may be helpful for the development of novel CO2 capture technologies. In CO2 absorption processes using solvents, the representative values that can be simply measured and easily obtained are temperature, pH, and electrical conductivity (EC). However, temperature and pH are not appropriate for use as significant control factors in the process because they are strongly influenced by many other variables.15,16 EC, however, is primarily sensitive to the kinds of ions present in the solvent, © 2013 American Chemical Society

Received: September 10, 2012 Accepted: July 29, 2013 Published: August 15, 2013 2381

dx.doi.org/10.1021/je400358f | J. Chem. Eng. Data 2013, 58, 2381−2388

Journal of Chemical & Engineering Data

Article

r is the ionic activity coefficient. Here, ko is calculated according to eq 9.21

the most important was that the solvent was considered to be an ideal solution with no interactions between ions and molecules. Therefore, very low concentration MEA solvents were used in the CO2 absorption and the data obtained from the experiments were used for the calculations.

ko =

■

RNH 2 + H 2O ↔

+ OH

−

(2)

OH− + CO2 (aq) ↔ HCO3−

(3)

2RNH 2 + CO2 (aq) ↔ RNH+3 + RNHCOO−

(4)

−

RNHCOO + CO2 (aq) + 2H 2O ↔

IS = 5·10−4 ∑ cizi 2

(1)

CO2 (g) ↔ CO2 (aq)

RNH+3

+

log r = −0.509z+z − IS

2HCO3−

log r = −0.5z+z −

Here, RNH2 denotes MEA. MEA-involved reactions such as eqs 1, 4, and 5 are all thermodynamically spontaneous. The overall reaction begins with a reaction between water and RNH2, resulting in RNH3+ and OH− generation, as listed in eq 1. At this time, instead of consumed RNH2, OH− absorbs CO2 according to eq 3 as many as the moles of RNH2 reacted with water, as listed in eq 1. Thereafter, when CO2(g) is injected into MEA solvent, gaseous CO2 is first converted into dissolved CO2(aq) which can be readily absorbed with higher reactivity. The primary reactions of CO2 absorption in the condition of high CO2 partial pressure and relatively low MEA concentration occur according to eqs 4 and 5. In the initial reaction period, most of CO2(aq) is directly absorbed by RNH2, which generates RNHCOO− as many as the moles of CO2 reacted, as listed in eq 4. Thereafter, CO2(aq) is additionally captured by generated RNHCOO− and water according to eq 5. Therefore, the net reaction can be expressed as listed in eq 6 and 1 mol of CO2 is theoretically absorbed by 1 mol of RNH2.

IS 1+

IS

for 0.01 ≤ IS ≤ 0.5

(11)

(12)

for IS > 0.5 (13)

where equation 11 is the Debye-Huckel limiting law, equation 12 is the Guntelberg approximation, and equation 13 is the Davies approximation. A in eq 13 is a constant that reflects the solvent property at a certain temperature and pressure. For example, the A of water at 25 °C and 1 atm is 0.509.23 However, eq 13 was not used in the present work because the IS of all five solvents were calculated to be less than 0.5. Therefore, the EC of the solution at a certain condition can be calculated via eqs 8 to 12 together with the concentration, ionic conductivity, and electric charge of each ion in solution. Assumption for Calculation. To calculate the ionic conductivity of RNH3+ and RNHCOO− as well as the EC of the solvent during the reaction, the following conditions are assumed: When absorbing CO2 in MEA solvent, four kinds of ions, namely RNH3+, RNHCOO−, HCO3−, and OH−, are solely generated or consumed in the solvent. The solvent is an ideal solution with no interactions between each ion in terms of EC. The chemical absorption of CO2 in the solvent is carried out consecutively from reactions 3 to 5 and the pK (negative logarithm of the dissociation constant K) value of reaction 1 is assumed to be 9.42.24 The amount of CO2 physically absorbed during the absorption cannot be measured in situ and is very small compared to the chemically absorbed amount. Therefore, the total amount of CO2 absorbed in the solvent is assumed to be solely due to chemical absorption. Calculation of Ionic Conductivity on the Basis of Error Analysis. To determine the ionic conductivity of RNH3+ and RNHCOO− via error analysis, first, their initial values should be estimated. The value should be positive and smaller than 349.8 S·cm2·mol−1·z−1, which is the largest ionic conductivity of H+ among all ions. Therefore, the initially estimated ionic conductivity was assumed to be between 0 S·cm2·mol−1·z−1 and 349.8 S·cm2·mol−1·z−1. With these initial and continuously

(6)

(7) −1

where k is the electrical conductivity (S·m ), α is the degree of dissociation of solution, λ is the ionic conductivity (S·m2· mol−1), and c is the mole concentration of solution (mol·m3). However, eq 7 is valid for an ideal solution where concentration is almost equal to activity. Therefore, the activity coefficient should be considered in practical conditions, therefore eq 8 was used to calculate the EC in the present study.22

kcalc = koγ 2

for IS < 0.01

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

Meanwhile, if MEA is used as a dry absorbent, the amount of CO2 absorbed is theoretically half of that of the aqueous absorption process because reaction 5 is not carried out in a dry process. Calculation of Electrical Conductivity (EC). Basically, the equation for calculating the electrical conductivity (EC) of an electrolyte or solution is expressed as eq 7.21 k = (λ+ν+|z+| + λ−ν−|z −|)α c

(10)

Therefore, if there are two ions in solution and their respective electric charge is 1, the IS of the solution is equal to the solution concentration. After calculating IS, r can be estimated according to various IS ranges using eqs 11 to 13.22

(5)

RNH 2 + CO2 + H 2O ↔ RNH+3 + HCO−3

(9)

where zi is the bsolute value of electric charge of ion i, λi is the ionic conductivity of ion i, and ci is the concentration of ion i (mmol/L). The ionic activity coefficient in eq 8 is related to the ionic strength (IS) which can be estimated according to eq 10.21

THEORETICAL BACKGROUND Reaction of Absorbing CO2 in MEA Solvent. Many works3,4,6,12,19,20 have reported the mechanism of CO2 chemical absorption using MEA solvent. All reactions including MEA dissociation and CO2 absorption can be expressed as eqs 1 to 5. RNH+3

∑ z iλ i c i

(8)

where kcalc is the calculated EC of solution from the experiment (S·m−1), ko is the EC of infinitely diluted solution (S·m−1), and 2382



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renewed input values of ionic conductivity, the EC of the MEA solvent was iteratively calculated using eqs 8 to 13 by the MATLAB program until the minimum error was obtained. The error is the difference between the calculated EC (ECc) and ECm of the solvent via experiment, as listed in eq 14. error (%) =

|ECc − ECm| ·100 ECm

(14)

At this time, the ionic conductivity of two ions was finally determined. The calculation process is further detailed as follows. Before CO2 absorption in MEA solvent, four components are present in the solvent: RNH2, H2O, RNH3+, and OH−. At this time, the EC of the solvent is solely dependent on RNH3+ and OH− because the electric charge of RNH2 and water is zero. According to continuous CO2 absorption, reactions 4 and 5 were consecutively carried out and finalized to reach the CO2 saturated absorption state with only RNH3+ and HCO3− theoretically remaining as the products in the solvent. Therefore, with the already known ionic conductivity value and the concentration of OH− and HCO3−, the ionic conductivity of RNH3+ could be calculated by measuring the EC of the initial and final states of five MEA solvents with different concentrations. Basically, the ionic conductivity of RNHCOO− could be obtained by following the same procedure as that of the RNH3+ calculation, except that RNHCOO− cannot theoretically remain in MEA solvent following absorption completion, as listed in eq 5. It temporarily presents solely during the absorption by varying its concentration according to CO2 absorption. The ECm can be in situ measured and ECc can be in situ calculated based on the continuously measured amount of CO2 absorbed. Thereafter, ECc was repeatedly compared to ECm until the minimum error was obtained, as listed in eq 14. Finally, the ionic conductivities of RNH3+ and RNHCOO− could be determined.

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

measured every 5 s using an EC meter (Orion 4 Star, Thermo Scientific) during the absorption. All measured data were acquired and treated by an on line computer system.

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RESULTS AND DISCUSSION Amount of CO2 Absorbed and Reaction Rate. The methodology used to calculate the amount of CO2 absorbed in

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EXPERIMENTAL SECTION Five MEA (Samchun Chemical Co., 99.0 %) aqueous solutions with concentrations of (0.1, 0.2, 0.3, 0.4, and 0.5) M were prepared as the solvent for CO2 absorption and a schematic diagram of the experiment is shown in Figure 1. For CO2 absorption, first, a cylindrical Pyrex reactor (D, 110 mm; h, 80 mm) was filled with 0.5 L of the solvents. The reactor was maintained at 25 °C. The solvents were uniformly mixed at a stirring speed of 180 rpm in the reactor. Before absorption, the empty space including every line and fitting of the experimental apparatus was sufficiently flushed with N2 purging. The inlet gas for absorption was the mixture of CO2 and N2. They were supplied at a flow rate of 1 and 2 L· min−1, respectively, as controlled by a mass flow controller, to a gas mixer maintained at a temperature of 25 °C. To stabilize the inlet gas composition, the gas mixture was initially bypassed the reactor for 5 min, and then was injected through a sparger made by a glass filter. To remove the moisture in the outlet gas after passing through the reactor, a condenser with a coolant circulator was inserted between the reactor and the gas analyzer. The outlet gas flow rate was controlled for analysis by a sampling pump built in the gas analyzer and was maintained at 1.5 L·min−1. The CO2 composition in the outlet gas was measured every 1 s using a nondispersive infrared (NDIR) gas analyzer (maMos200, Madur Electronics). The variation of the solvent EC was

Figure 2. Amount of CO2 absorbed (a), and the variation of measured EC (ECm) (b), according to the reaction time in each MEA solvent.

the solvent using the CO2 outlet composition from the reactor was detailed in our previous works.25 Figure 2 shows the amount of CO2 absorbed and the variation of the ECm of the five solvents according to the reaction time. The reaction rate of each solvent was estimated by calculating its slope, as shown in Figure 2a. In all five solvents, the absorption rate of CO2 was relatively fast, in the initial 2383



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Table 1. Reaction Rate in the Initial Period and Total Absorption Time of Each Solvent concentration of MEA solvent (M) rate and time −1

−1

reaction rate in the initial period (mmol·L ·min ) absorption time (min)

0.1

0.2

0.3

0.4

0.5

17.8 21.0

22.0 31.7

28.4 43.0

30.1 48.0

30.5 50.0

already known ionic conductivities of OH− and HCO3− are summarized in Table 2.21,26 In addition, the ionic conductivities of RNH3+ and RNHCOO− as finally determined in the present work are listed in Table 2. In a conjugate acid−conjugate base system, the sum of the pK values of two ions is 14. Therefore, the pK value of RNH2 is 4.58 because that of RNH3+ is assumed to be 9.42 and together they form a conjugate acid−conjugate base pair. The dissociation constant of RNH2 was calculated to be 10−4.58. Therefore, initial concentrations of RNH3+ and OH− in the five solvents according to eq 1 can be theoretically calculated and are listed in Table 3. At the same time, ECm in the initial and final state in the solvent can be measured, and they are summarized in Table 3. The renewed ionic conductivity of RNH3+ was used to iteratively generate the ECc value to minimize the error in eq 14 via the calculation process, as aforementioned. As a result, the minimum error was calculated to be 1.752 and the ECc results value in the initial and final states of the solvent at this time could be obtained as listed in Table 3. Therefore, the ionic conductivity of RNH3+ was calculated to be 73.60 S·cm2·mol−1· z−1, as listed in Table 2. After measuring the amount of CO2 absorbed every 1 s and ECm in the solvents every 5 s, the ionic conductivity of RNHCOO− was calculated to be 16.40 S·cm2·mol−1·z−1 via the same procedure as aforementioned with a minimum error of 6.92 %. With the determined ionic conductivities of RNH3+ and RNHCOO−, the variation of ECm and ECc according to CO2 absorption in solvents is shown in Figure 3. Generally, the ECc and ECm variations were similar in all five solvents, as shown in Figure 3. In the initial reaction period, the slope and value of ECc were almost the same as those of ECm. However, ECc and ECm started to deviate from the point where the ECm slope is remarkably decreased, and the deviation decreased with increasing solvent concentration up to 0.4 M. Finally, ECc and ECm were almost the same in the 0.4 M solvent. Meanwhile, the absorption end-point based on the ECc variation, where the flat ECc line was first depicted in Figure 3, appeared earlier than that in the ECm line in the 0.1 to 0.4 M MEA solvent systems. In other words, the time to reach the end-point based on ECc was faster than that in ECm and the

Table 2. Value of Electric Charge and Ionic Conductivity of Each Ion ions

electric charge (z)

ionic conductivity (S·cm2·mol−1·z−1)

−1 +1 −1 −1

198.6 73.60a 16.40a 44.50

−

OH RNH3+ RNHCOO− HCO3− a

Calculated and finally determined in the present study.

period. Subsequently, from the end-point of the initial period, the slope was noticeably reduced and gradually decreased until the reaction completion point. The same trend was depicted in the EC variation slope with the same timing, as shown in Figure 2b. Moreover, EC maintained a constant value after the CO2 absorption completion point. These results confirmed that CO2 absorption in MEA solvent was conducted with two different rates throughout the reaction. First, reaction 4 occurred dominantly in the initial period, resulting in fast CO2 absorption. Subsequently, the relatively slower reaction 5 dominated in the latter period. Therefore, the assumption that reactions 4 and 5 occur consecutively can be reasonably accepted. The reaction rates in the initial period and total absorption times of the five solvents are summarized in Table 1. The reaction rate of solvents over the initial period was not calculated because the slope varied continuously up to the completion point. The absorption rate and time in the initial period were positively correlated with the MEA concentration. However, the reaction time of the 0.5 M solvent was slightly longer, and its rate was almost equal to that of the 0.4 M solvent. This was ascribed to the fact that although the absorption rate was proportional to the MEA concentration, if the concentration of the solvent was higher than a certain level such as 0.5 M, the absorption reaction was restricted in terms of rate and efficiency. Therefore, not all the MEA in the 0.5 M solvent seems to have reacted with CO2. This issue is further explained in the next section. Ionic Conductivity of RNH3+ and RNHCOO−. Before CO2 is absorbed, RNH3+ and OH− are present in the solvent. As CO2 absorption starts, three ions, namely, RNH3+, RNHCOO−, and HCO3−, are generated and consumed by reactions 4 and 5. Their values of electric charge and the

Table 3. Initial Concentration of RNH3+ (or OH−) and Measured (ECm) and Calculated (ECc) EC at Initial and Final State of Each Solvent final state of solvent (after CO2 absorption)

initial state of solvent (before CO2 absorption) concentration of MEA solvent (M)

concentration of RNH3+ (or OH−) (mM)

ECm (mS·cm−1)

ECc (mS·cm−1)

ECm (mS·cm−1)

ECc (mS·cm−1)

0.1 0.2 0.3 0.4 0.5

1.622 2.294 2.809 3.244 3.626

0.390 0.548 0.647 0.760 0.853

0.402 0.558 0.675 0.773 0.857

6.610 11.595 15.940 19.380 22.766

6.792 11.595 15.685 19.359 22.751

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Figure 3. Variation of measured (ECm) and calculated (ECc) electrical conductivity according to CO2 absorption in each MEA solvent: (a) 0.1 M, (b) 0.2 M, (c) 0.3 M, (d) 0.4 M, and (e) 0.5 M.

concentrations of each ion and ECc were calculated on the basis of this assumption. Therefore, the end-point in the ECc variation was earlier than that of ECm, which was solely influenced by the chemically absorbed CO2. The difference between ECc and ECm caused by the physical absorption consideration was greater in the lower concentration solvents because its impact was larger than in the higher concentration solvents. In the 0.4 M solvent, the difference was minimized. On the other hand, in 0.5 M solvent, ECc was lower

end-point appearance in the ECc line became delayed according to the MEA concentration. This phenomenon was ascribed to the treatment of the physically absorbed CO2 as chemically absorbed CO2 in the calculations. Small amounts of CO2 were physically absorbed into the solvent during the chemical absorption. However, EC was not strongly influenced by the physically absorbed CO2. Nevertheless, when the ECc was calculated, all of the CO2 absorbed, including by physical absorption, was assumed to be solely consumed by the chemical absorption. In other words, the 2385


intercept (mmol of CO2·L−1 of solvent)

0.994 0.995 0.996 0.997 0.995 2.415 3.166 3.523 3.164 2.317 14.57 17.11 18.98 20.55 21.94 0.990 0.996 0.997 0.997 0.997 5.112 7.635 1.817 −2.260 6.882

than ECm, contrary to in the other solvents, and this difference increased with increasing reaction time. The reaction end-point in the 0.5 M solvent was determined by confirming the first point at which the feed concentration of CO2 is recovered, as with different solvent systems. However, the amount of CO2 absorbed at that point was calculated to be 10.34 % less than the theoretical value based on eq 6. In practice, the chemical behavior of the 0.5 M solvent may deviate further from the theoretical behavior and the assumptions of the present paper due to its relatively high concentration which decreases the ionic activity coefficient of the existing ions in the solvent. Therefore, the absorption rate in the solvent may not be as high as expected from the MEA concentration, and may induce a substantial deviation from the assumption that reactions 4 and 5 are consecutively carried out. This possibility was supported by the fact that the amount of CO2 absorbed was lower than the theoretical amount. Therefore, the real compositions of RNH3+, RNHCOO−, and HCO3− in the solvent were different from the values calculated solely based on the amount of CO2 absorbed. When the assumption to the 0.5 M solvent system that reactions 4 and 5 were consecutively carried out with less CO2 absorbed than the theoretical value is applied, reaction 4 should be considered to have been completed with an absorption efficiency of 100 % and reaction 5 to have been terminated with very low efficiency.

of

intercept (mmol of CO2·L solvent)

Figure 4. Correlation between (a) measured electrical conductivity (EC), and (b) calculated EC variation and the amount of CO2 absorbed in each MEA solvent.

slope (mmol of CO2·cm·L ·mS solvent)

−1

18.14 18.18 20.02 20.65 19.48 0.1 0.2 0.3 0.4 0.5

concentration of MEA solvent (M)

ECm

−1

of

deviations; r2

−1

slope (mmol of CO2·cm·L ·mS solvent)

−1

of

ECc

Article

−1

Table 4. Results of Least Square Fitting to Estimate Correlation Equations between EC and Amount of CO2 Absorbed in Each Solvent

deviations, r2


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measuring the EC variation of the solvents and vice versa. (ii) Because of linearity, the ionic conductivity of RNH3+ and RNHCOO− could be calculated, and they were finally determined as 73.60 and 16.40 S·cm2·mol−1·z−1, respectively. (iii) On the basis of the determined ionic conductivity of the two ions, ECc can be accurately represented by ECm in the 0.4 M solvent. (iv) The practical amount of CO2 absorbed in the 0.5 M solvent was lower than the theoretical value, in contrast to the other solvents of lower concentration. This was ascribed to the relatively low ionic activity coefficient of the 0.5 M solvent. Therefore, ECc in the 0.5 M solvent was lower than ECm, in contrast to the 0.1−0.4 M solvent systems in which the amount of CO2 absorbed was similar to or greater than the theoretical value. (v) The range of MEA concentration considered here is significantly lower than the 3 M to 5 M solutions of MEA that are used for CO2 capture in industry, and the solutions are for relatively pure and undegraded MEA solutions. Therefore, it is difficult to apply the results to industrial CO2 capture process using MEA. To address the problems, a further experiment is in progress and the results will be reported in near future.

Therefore, when estimating the ECc variation according to time in the 0.5 M solvent, the RNHCOO− concentration, with a relatively low ionic conductivity, was calculated to be high and the HCO3− concentration, with a high ionic conductivity, was calculated to be low. However, in a practical 0.5 M solvent, the absorption efficiency according to reaction 4 was lower than 100 % and the absorption efficiency according to reaction 5 might be higher than that of the reaction based on the previously ECc calculated. As a result, the practical concentrations of RNHCOO− and HCO3− were lower and higher, respectively, than their corresponding ECc-based calculated values. Therefore, ECm was higher than ECc in the 0.5 M MEA solvent system. However, this deviation from the study assumptions did not arise in the 0.1−0.4 M MEA solvent systems because the amount of CO2 absorbed was the same or slightly larger than the theoretical value. Correlation between Electrical Conductivity (EC) and Amount of CO2 Absorbed. For all five solvents, EC was almost proportional to the amount of CO2 absorbed, as shown in Figure 2. Therefore, the equations relating EC and the amount of CO2 absorbed can be derived by least-squares fitting. The slope, y-intercept, and deviations (r2) in all five solvents were calculated and are listed in Table 4. In practice, although the slope of the plot of ECm versus amount of CO2 absorbed was increased with increasing MEA concentration, their correlation was not clear and even the slope of the 0.5 M solvent was lower than that of the 0.4 M solvent. However, in ECc, the slope was linearly proportional to the MEA concentration, at 14.57 and 21.94 mmol of CO2·cm· L−1·mS−1 for the 0.1M and 0.5 M solvents, respectively. The clear ECc linearity was ascribed to the accurate assumptions applied to the system in this study. On the other hand, the application of these assumptions to a real solvent system is not appropriate. In particular, the 0.5 M solvent can deviate the most from the assumptions due to its relatively high concentration, which leads to a slightly lower slope than that of the 0.4 M solvent. The amount of CO2 absorbed according to the EC variation in all five solvents can be estimated by expanding the results of the least-squares fitting, and the results are shown in Figure 4. In other words, if ECm can be measured or ECc can be calculated in an MEA solvent of a certain concentration, the amount of CO2 absorbed in the solvent can be calculated. For example, when ECm was measured as 15.94 mS·cm−1 in the 0.3 M solvent, the maximum capacity of the amount of CO2 absorbed was 0.3298 mol·L−1 of solvent, as shown in Figure 4a. On the other hand, ECc could be estimated as 15.685 mS·cm−1 for the same CO2 capacity, as shown in Figure 4b. This small difference between ECm and ECc may have resulted from the ideality of the solvents.

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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) the Ministry of Education (2013R1A1A2A10010414). Notes

The authors declare no competing financial interest.

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REFERENCES

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CONCLUSION The present paper has reported the ionic conductivity of generated or consumed RNH3+ and RNHCOO− ions when CO2 is absorbed in water-diluted MEA solvents. In addition, the correlation between the EC variation and the amount of CO2 absorbed in the system was investigated. Under certain study assumptions, CO2 absorption experiments were carried out with five MEA solvents. From the results, the following conclusions were obtained. (i) The amount of CO2 absorbed in the low concentration MEA solvent was linearly proportional to the EC variation. Therefore, the amount of CO2 absorbed in an MEA solvent of a certain concentration could be calculated by 2387



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Estimation of Correlation between Electrical Conductivity and CO2

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