Solubility of Carbon Dioxide in Aqueous Solutions of Three Secondary

Article pubs.acs.org/jced

Solubility of Carbon Dioxide in Aqueous Solutions of Three Secondary Amines: 2‑(Butylamino)ethanol, 2‑(Isopropylamino)ethanol, and 2‑(Ethylamino)ethanol Secondary Alkanolamine Solutions Sung June Hwang, Junghwan Kim, Huiyong Kim, and Kwang Soon Lee* Department of Chemical and Biomolecular Engineering, Sogang University, 35 Baekbeom-ro, Mapogu, Seoul 121-742, Korea ABSTRACT: Secondary amines have been proposed as potential solvents for CO2 capture. In this study, the CO2 solubility of three secondary alkanolamine aqueous solutions, 2-(butylamino)ethanol (BAE), 2-(isopropylamino)ethanol (IPAE), and 2-(ethylamino)ethanol (EAE), were measured with a pressure range of 0.02 to 395 kPa at 40, 80, and 120 °C. A static method based on an equilibrium cell unit was used for measurements in high CO2 equilibrium partial pressure regions at over 1 kPa, while the flow method based on a modified gas sparging reactor unit proposed by Kim et al.(2017) was employed for measurements in low pressure regions under 1 kPa. Two amine concentrations, 15 and 30 wt %, were considered for each amine, and their CO2 solubility was represented by the Kent-Eisenberg model. The cyclic CO2 absorption capacity and heat of the reaction for the three secondary alkanolamines were compared with those of monoethanolamine (MEA).

1. INTRODUCTION Climate change caused by global warming is the most critical environmental issue in the modern world, and CO2 emissions from coal-fired power plants are believed to be among the most serious causes of climate change. Various technologies have been researched and developed to effectively and economically capture CO2 from large point emission sources such as coal-fired power plants. Currently, aqueous amine-based absorption technology, which has a long history of industrial use, is considered the most promising.1 Aqueous monoethanolamine (MEA) solvent has been used as the benchmark solvent of amine-based absorption processes.2−5 MEA has many advantages such as fast CO2 absorption rate, high heat of reaction, low viscosity, and cheap price, but requires a rather large amount of energy for CO2 capture. Vast amounts of research have been carried out to explore energy thrifty solvents, but further exploration is still required.6−17 Secondary amines both with and without hindrance have been intensively researched as alternatives to MEA. Secondary amines possess a moderate absorption rate, absorption capacity, heat of reaction, and enhanced chemical stability compared to primary amines. Many researchers have proposed aqueous solvents that are composed of a secondary amine and a rate promoter as competent solvents for CO2 capture. Information regarding CO2 solubility in a solvent is essential for evaluating the solvent’s performance. Nevertheless, CO2 solubility data for limited amine and amine blend solvents are available in the open literature. This study investigates the CO2 solubility of three seemingly important aqueous secondary alkanolamine solvents: 2-(butylamino)ethanol (BAE, secondary amine), 2-(isopropylamino)ethanol (IPAE, hindered secondary amine), and © 2017 American Chemical Society

2-(ethylamino)ethanol (EAE, secondary amine). The CO2 solubility data for the three amines are partially available or unavailable in the open literature to the authors’ best knowledge, although those amines have attracted keen attention as new solvent candidates from some research groups. Yamada et al.18,19 measured CO2 solubility of aqueous IPAE and EAE solvents. Only 30 wt % solvents were considered, and measurements were made for the CO2 pressure range above 4 kPa and for limited temperatures. The measurements may have limited the usability of postcombustion CO2 capture solvents in the study, because the equilibrium CO2 pressure corresponding to a rich loading solvent from the absorber is normally 2 to 5 kPa. Kumar10 measured the CO2 solubility of aqueous EAE solvent with various concentrations at low temperatures of 30 to 50 °C. No solubility data of BAE were found in the open literature to the best of the author’s knowledge. In this study, the CO2 solubility of aqueous BAE, IPAE, and EAE solvents with amine concentrations of 15 and 30 wt % was measured for wide temperature and pressure ranges, 40 to 120 °C and 0.022 to 395 kPa, respectively. An equilibrium cell (EC) unit was used for high CO2 equilibrium partial pressure (EPP) measurements above 1 kPa, and a specially designed gas sparging reactor (GSR) by Kim et al.20 was used for solubility measurement in low CO2 EPP regions under 1 kPa at 40 °C. The vapor−liquid equilibrium (VLE) was expressed by the Kent−Eisenberg (K-E) model for the reaction equilibrium in the liquid phase and Henry’s law. The CO2 absorption capacity Received: April 19, 2017 Accepted: July 6, 2017 Published: July 20, 2017 2428

DOI: 10.1021/acs.jced.7b00364 J. Chem. Eng. Data 2017, 62, 2428−2435

Journal of Chemical & Engineering Data

Article

Table 1. Information on Chemicals Used in This Study chemical name

source

2-(butylamino)ethanol (BAE) 2-(isopropylamino)ethanol (IPAE) 2-(ethylamino)ethanol (EAE) monoethanolamine (MEA) water carbon dioxide

Sigma-Aldrich Tokyo Chemical Industry Tokyo Chemical Industry Sigma-Aldrich SAMCHUN Chemical, Korea Dongwoo Gas Technology, Korea

initial mole fraction purity >0.98 >0.99 >0.99 >0.99 0.99999

purification method

CAS

none none none none none none

111-75-1 109-56-8 110-73-6 141-43-5 7732-18-5 124-38-9

and heat of reaction were estimated from the VLE model and discussed.

2. EXPERIMENTAL PART 2.1. Materials. Chemicals used in this study were listed in Table 1. MEA with a purity of 99.0+% and BAE with a purity of 98.0+% were purchased from Sigma-Aldrich, while IPAE and EAE with a purity of 99.0+% were supplied by Tokyo Chemical Industry. They were used without further purification. Aqueous amine solutions were prepared by mixing amines with HPLC reagent water supplied by SAMCHUN Chemical, Korea. CO2 gas with a purity of 99.999% was supplied by Dongwoo Gas Technology, Korea. The molecular structures of three secondary alkanolamines are shown in Figure 1.

Figure 3. Schematic diagram of the gas sparging reactor unit.

in a constant temperature convection air oven. The oven temperature was regulated with the accuracy within ±0.1 °C. The EC had a stirring bar and was placed on a magnetic stirrer. A one-touch connector was installed on the top cover of the EC so that a measuring glass cylinder could be easily connected to and detached from the EC for solvent transfer. A pressure transmitter (PT) with a range of 0 to 10 bara and a K-type thermocouple (K-TC) were installed in the CO2 reservoir. Two PTs, one with a range of 0 to 2 bara and the other with 0 to 6 bara, were installed in the EC. Only one could be selected using a three-way valve during the experiment. Two K-TCs were installed in the EC to separately measure the liquid and vapor temperatures. All PTs were products of Keller, Switzerland. The CO2 reservoir and EC were connected by a needle valve for the slow transfer of CO2 gas from the reservoir to the cell during the experiment. A vacuum pump was connected to both the reservoir and cell to evacuate the reactors prior to each experiment. All TCs and PTs were regularly recalibrated using the Fluke 7526A calibrator with an RTD reference sensor (Fluke 5622-10-M), the accuracy of which is ±0.04 °C, and a pressure module (Fluke-750PA06), the accuracy of which is 0.045% at 7 bara. Experimental measurements were taken in real time using a data acquisition system composed of NI (National Instrument) boards and the LABVIEW program. 2.2.2. Procedure and Data Analysis. The EC and reservoir were first evacuated by the vacuum pump, and approximately 70 mL of fresh aqueous amine solvent is transferred from the glass cylinder to the EC. The precise mass of the solvent in the EC was measured as the weight difference in the glass cylinder with the solvent before and after the transfer of solvent. Quintix3102-1S from Sartorius AG was used to weigh the mass with a ±0.01 g accuracy. Then, CO2 gas was injected from the gas bomb into the reservoir up to a pressure of approximately 6 bar. Upon confirming that all temperature and pressure measurements were stabilized, a predetermined amount of CO2 was transferred from the reservoir to the EC through the needle valve by monitoring the change in reservoir pressure. After 2 to 3 h, all measurements reached a new equilibrium state.

Figure 1. Molecular structures of (a) BAE, (b) IPAE, and (c) EAE.

2.2. CO2 Solubility Measurement in High CO2 Partial Pressure Regions. 2.2.1. Apparatus. An EC unit as shown in Figure 2 was used for solubility measurement in high CO2

Figure 2. Equilibrium cell unit for CO2 solubility measurement. PT, TC, and EC mean pressure transmitter, thermocouple, and the equilibrium cell, respectively.

pressure regions. The setup consisted of a 2 L CO2 reservoir and a 0.375 L EC, both of which were made of SUS316 and contained 2429



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Table 2. Literature Data for Reaction Equilibrium Constants and Henry’s Constant values

a

parametera

eq

a

b

c

d

ref

K1 K2 K3 HCO2

1 2 3 6

−12431.7 −12092.1 −13445.9 −6789.04

−35.4819 −36.7816 −22.4773 −11.4519

0 0 0 −0.010454

220.067 235.482 140.932 94.4914

Edwards et al.21 Edwards et al.21 Edwards et al.21 Edwards et al.21

ln K (or H/L·atm·mol−1) = a/T + b ln T + cT + d. The basis is molarity.

unit, as shown in Figure 3, was used to measure the CO2 solubility in the lower CO2 EPP region of 0.02 to 1.0 kPa at 40 °C. Details of the experimental unit and procedure are described in Kim et al.,20 and only a brief description is given here. The ordinary GSR system for VLE measurement is typically operated by starting with fresh solvent and repeatedly increasing the inlet CO2 concentration in a stepwise manner. Each increase is made after complete equilibrium is reached for the previous change. This operation requires a very long period and is impractical when the CO2 concentration is low. Kim et al.20 proposed a modified GSR unit and an experimental method performed in unsteady states. In this method, CO2-loaded solvent with a known loading value is prepared in advance and a known amount is transferred to the reactor first. A gas mixture with a low CO2 concentration is fed to the reactor, and the outlet CO2 concentration is monitored. If the outlet CO2 concentration is higher (lower) than that of the inlet, it means that the CO2 EPP of the solvent is higher (lower) than the CO2 PP of the inlet gas mixture. Repeating the test several times at most by increasing and decreasing the inlet CO2 fraction and using interpolation permits the estimation of a quite accurate value for the CO2 EPP corresponding to the loaded solvent in a short time. Standard uncertainties for the GSR unit are u(T) = 0.02 °C, u(xamine) = 0.0001, u(xCO2) = 0.0001, and u(PCO2) = 0.01.

Figure 4. CO2 solubility data for the 30 wt % MEA solution from different sources.

The CO2 loading, αCO2, expressed by the moles of dissolved CO2 divided by the moles of amine in the solvent could be calculated based on the mass balance using the reservoir pressure difference before and after the CO2 transfer and the gas space volume, pressure, and solvent mass in the EC. The Redlich− Kwong equation of state was used for the CO2 gas’ P−V−T relationship. CO2 EPP, PCO2, can be estimated by subtracting the water vapor pressure from the EC pressure. Henry’s law was applied to the calculation of the water vapor pressure while the mole fraction of the water in the fresh solvent was assumed invariant during the experiment. Standard uncertainties for the equilibrium cell (EC) unit are u(T) = 0.025 °C, u(xamine) = 0.0001, u(xCO2) = 0.0001, and u(PCO2) = 0.22 kPa at 40 and 80 °C, 1.5 kPa at 120 °C where T is temperature, xamine and xCO2 are mole fraction of amine and CO2 in the aqueous solution respectively, and PCO2 is CO2 EPP. 2.3. CO2 Solubility Measurement in Low CO2 Partial Pressure Regions. The flow method using an in-house GSR

3. THERMODYNAMIC MODEL The chemical reaction equilibria of a secondary alkanolamine− H2O−CO2 system can be described by the following equations: K1

HCO−3 + H 2O ↔ H3O+ + CO32 − K2

CO2 + 2H 2O ↔ H3O+ + HCO−3 K3

2H 2O ↔ H3O+ + OH−

(1) (2) (3)

Figure 5. CO2 solubility data and model prediction for (a) 15 and (b) 30 wt % BAE solutions. 2430



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Figure 6. CO2 solubility data and model prediction for (a) 15 and (b) 30 wt % IPAE solutions.

Figure 7. CO2 solubility data and model prediction for (a) 15 and (b) 30 wt % EAE solutions.

Table 3. CO2 Solubility Data for 15 and 30 wt % Aqueous BAE Solution at 40, 80, and 120°Ca 15 wt % aqueous solution xamine

xCO2

αCO2 mol/mol

30 wt % aqueous solution PCO2 kPa

xamine

xCO2

αCO2 mol/mol

PCO2 kPa

0.0612 0.0611 0.0605 0.0600 0.0596 0.0594 0.0590 0.0587

0.0106 0.0118 0.0216 0.0301 0.0355 0.0395 0.0457 0.0504

0.174 0.193 0.357 0.502 0.596 0.666 0.774 0.859

0.07b 0.08b 0.51b 2.8 7.82 16.05 43.6 97.35

0.0614 0.0610 0.0609 0.0606 0.0602 0.0600 0.0596 0.0595 0.0594

0.0074 0.0125 0.0152 0.0203 0.0262 0.0294 0.0351 0.0375 0.0392

0.120 0.205 0.249 0.336 0.435 0.490 0.588 0.631 0.660

1.06 3.08 4.63 10.22 23.67 36.78 80.80 109.80 133.36

0.0615 0.0613 0.0611 0.0609 0.0608 0.0606 0.0605 0.0604

0.0047 0.0079 0.0114 0.0147 0.0167 0.0188 0.0212 0.0231

0.077 0.129 0.187 0.241 0.279 0.310 0.350 0.383

10.81 24.41 46.66 76.14 102.46 124.55 161.61 192.22

T = 40 °C 0.04b 0.32b 3.09 11.09 32.12 63.42

0.0263 0.0261 0.0260 0.0259 0.0258 0.0257

0.0052 0.0103 0.0167 0.0205 0.0236 0.0256

0.198 0.393 0.644 0.794 0.917 0.992

0.0263 0.0262 0.0261 0.0260 0.0259 0.0259 0.0258

0.0040 0.0074 0.0110 0.0144 0.0175 0.0197 0.0214

0.152 0.268 0.422 0.555 0.674 0.762 0.826

1.32 3.57 9.63 23.93 52.61 96.41 152.81

0.0263 0.0262 0.0262 0.0261 0.0261 0.0260 0.0260 0.0260

0.0038 0.0074 0.0097 0.0110 0.0133 0.0147 0.0164 0.0171

0.145 0.284 0.369 0.423 0.509 0.563 0.631 0.658

11.79 43.71 76.28 103.76 167.58 222.89 313.14 348.24

T = 80 °C

T = 120 °C

Standard uncertainties for the EC unit are u(T) = 0.025 °C, u(xamine) = 0.0001, u(xCO2) = 0.0001, and u(PCO2) = 0.22 kPa at 40 and 80 °C, 1.5 kPa at 120 °C. bMeasured using the GSR unit. Standard uncertainties for the GSR unit are u(T) = 0.02 °C, u(xamine) = 0.0001, u(xCO2) = 0.0001, and u(PCO2) = 0.01 kPa. a

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Table 4. CO2 Solubility Data for 15 and 30 wt % Aqueous IPAE Solution at 40, 80, and 120°Ca 15 wt % aqueous solution xamine

xCO2

αCO2 mol/mol

30 wt % aqueous solution PCO2 kPa

xamine

xCO2

αCO2 mol/mol

PCO2 kPa

0.0691 0.0684 0.0672 0.0669 0.0665 0.0658 0.0655

0.0079 0.0181 0.0348 0.0393 0.0453 0.0544 0.0588

0.115 0.265 0.518 0.587 0.682 0.826 0.898

0.05b 0.29b 2.58 4.35 8.73 29.31 62.84

0.0692 0.0688 0.0687 0.0684 0.0682 0.0679 0.0674 0.0671 0.0667

0.0062 0.0114 0.0140 0.0170 0.0205 0.0256 0.0316 0.0359 0.0423

0.089 0.166 0.204 0.249 0.301 0.377 0.469 0.534 0.635

1.27 3.31 4.97 7.54 10.62 15.27 25.57 36.36 63.01

0.0692 0.0688 0.0686 0.0683 0.0681 0.0678 0.0676

0.0062 0.0113 0.0150 0.0185 0.0221 0.0267 0.0298

0.090 0.164 0.219 0.271 0.324 0.394 0.440

17.11 47.58 79.42 114.78 156.42 224.69 272.87

T = 40 °C 0.0298 0.0296 0.0293 0.0292 0.0291 0.0290

0.0038 0.0100 0.0184 0.0241 0.0280 0.0287

0.126 0.337 0.627 0.827 0.962 0.989

0.02b 0.23b 2.70 12.50 79.95 156.10

0.0297 0.0295 0.0294 0.0293 0.0292 0.0292 0.0291

0.0079 0.0123 0.0155 0.0191 0.0220 0.0240 0.0257

0.265 0.417 0.525 0.651 0.751 0.822 0.883

3.69 9.56 17.19 34.28 61.38 96.93 154.08

0.0298 0.0297 0.0296 0.0295 0.0295 0.0294 0.0293 0.0292

0.0046 0.0080 0.0108 0.0127 0.0149 0.0168 0.0196 0.0209

0.154 0.269 0.364 0.431 0.505 0.572 0.670 0.715

15.34 44.00 77.45 108.08 154.33 209.07 327.76 394.96

T = 80 °C

T = 120 °C


K4

AmH+ + H 2O ↔ Am + H3O+ Am +

K5 HCO−3 ↔

AmCOO− + H 2O

(4)

AALE =

N m ̂ ∑ |log10 PCO , n − log10 PCO , n| 2

n=1

2

(7)

̂ where Pm CO2 and PCO2 are the measured and estimated CO2 EPPs, respectively, and N denotes the number of total data points for each secondary alkanolamine.

(5)

Equations 1 to 3 respectively represent the dissociation of bicarbonate, carbon dioxide, and water for the H2O−CO2 binary system, while eqs 4 are 5 respectively describe amine protonation and carbamate formation. The equilibrium relationship between the CO2 partial pressure in the vapor phase and the CO2 mole fraction in the liquid phase are expressed by Henry’s law. PCO2 = HCO2xCO2

1 N

4. RESULTS AND DISCUSSION 4.1. Validation of the VLE Measurement. The CO2 solubility in the 30 wt % MEA solution was first measured to verify the reliability of the experimental methods. The measured values were compared with the literature data by Aronu et al.,23 Shen and Li,24 and Jou et al.,25 as shown in Figure 4. The experimental data from this study agree with the literature data quite closely, which confirms the reliability of the experimental methods used in this study. 4.2. CO2 Solubility in Aqueous BAE, IPAE, and EAE Solutions. The CO2 solubility in 15 and 30 wt % BAE solutions was measured over the CO2 EPP range of 0.04 to 348.24 kPa at 40, 80, and 120 °C and presented in Figure 5 with estimates based on the proposed thermodynamic model. It shows that the proposed simple thermodynamic model can represent the CO2 solubility satisfactorily for two different concentrations over a wide temperature range. The AALE for BAE was computed as 0.069. There were similar results in Figure 6 to those in Figure 5, but aqueous IPAE solutions were exhibited with model predictions.

(6)

The chemical reaction equilibrium constants for eqs 1 to 3 and Henry’s constant used in this study are shown in Table 2. The reaction equilibrium was represented by the K-E model. This model assumes that the activity coefficients of all species in the liquid phase are unity, and that the vapor phase follows the ideal gas equation of state. Hwang et al.22 described the details of the K-E model for various aqueous alkanolamines and showed that the K-E model can predict the CO2 solubility of various single alkanolamines and their blends quite satisfactorily despite its simplicity. The following average absolute logarithmic error (AALE) was used to assess the accuracy of the model prediction. 2432



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Table 5. CO2 Solubility Data for 15 and 30 wt % Aqueous EAE Solution at 40, 80, and 120°Ca 15 wt % aqueous solution xamine

xCO2

30 wt % aqueous solution

αCO2 mol/mol

PCO2 kPa

xamine

xCO2

αCO2 mol/mol

PCO2 kPa

0.0786 0.0778 0.0761 0.0759 0.0755 0.0752 0.0749 0.0746 0.0743

0.0145 0.0237 0.0448 0.0482 0.0531 0.0561 0.0604 0.0636 0.0675

0.184 0.304 0.589 0.636 0.704 0.746 0.806 0.852 0.908

0.04b 0.17b 4.74 7.21 13.42 19.98 34.36 52.85 92.57

0.0787 0.0783 0.0779 0.0776 0.0772 0.0769 0.0765 0.0762 0.0757

0.0129 0.0178 0.0226 0.0266 0.0317 0.0354 0.0402 0.0443 0.0507

0.164 0.227 0.290 0.343 0.411 0.460 0.526 0.582 0.670

1.51 2.64 4.20 6.28 11.58 17.90 32.70 54.70 120.50

0.0793 0.0790 0.0787 0.0784 0.0780 0.0778 0.0776 0.0774 0.0770 0.0768

0.0049 0.0089 0.0131 0.0165 0.0214 0.0246 0.0269 0.0295 0.0338 0.0360

0.062 0.113 0.166 0.211 0.274 0.316 0.347 0.382 0.439 0.468

3.37 10.15 18.75 27.32 45.03 61.53 80.48 105.39 152.60 194.55

T = 40 °C 0.0341 0.0340 0.0337 0.0336 0.0335 0.0334 0.0333 0.0332 0.0332

0.0088 0.0121 0.0213 0.0245 0.0272 0.0299 0.0321 0.0347 0.0353

0.258 0.356 0.632 0.730 0.811 0.896 0.964 1.044 1.063

0.06b 0.19b 2.94 6.42 12.37 26.12 51.66 146.24 197.95

0.0342 0.0341 0.0340 0.0338 0.0337 0.0337 0.0336 0.0335

0.0072 0.0105 0.0135 0.0174 0.0200 0.0225 0.0254 0.0275

0.211 0.307 0.398 0.514 0.594 0.668 0.757 0.822

1.64 3.49 6.45 15.04 27.19 46.96 90.81 153.16

0.0343 0.0342 0.0341 0.0339 0.0339 0.0338 0.0337 0.0337

0.0042 0.0072 0.0103 0.0142 0.0162 0.0181 0.0201 0.0215

0.123 0.212 0.302 0.418 0.477 0.534 0.595 0.638

4.84 13.69 31.71 72.98 103.20 141.48 200.60 251.63

T = 80 °C

T = 120 °C


Table 6. Estimated Reaction Equilibrium Constants of BAE, IPAE, and EAE BAE

a

IPAE

EAE

parametera

a

b

a

b

a

b

K4 K5

−3.46 ± 0.02 −12.46 ± 0.09

−5997 ± 128 3997 ± 83

−4.02 ± 0.03 −29.84 ± 0.32

−5872 ± 143 8936 ± 247

−4.86 ± 0.04 −9.67 ± 0.17

−5734 ± 135 3099 ± 120

ln K = a + b/T. The basis is molarity.

The data produced by Yamada et al.18 using a GSR system were plotted together for a 30 wt % IPAE solution in Figure 6b. The data were in good agreement with ours at 120 °C, but were larger than ours at 40 °C for high loading regions. However, the CO2 solubility measurements at 40 °C from this study fit the model prediction accurately over the entire CO2 loading region. It is generally accepted that the flow method is more prone to experimental errors than the static method in equilibrium measurement. As the temperature grew, the mode prediction tended to deviate from the data, but not to a serious degree. The AALE for IPAE was calculated as 0.099. Figure 7 displays the experimental results for 15 and 30 wt % EAE solutions together with the literature data Kumar,10 Yamada et al.,19 and the model prediction based on our measurements. The data in Kumar10 at 40 °C for 30 wt % solution showed nice

accordance with ours, but those by Yamada et al.19 again showed larger values than ours, as in Figure 6. The model prediction was quite satisfactory and results in a 0.069 AALE. The raw experimental data for BAE, IPAE, and EAE solutions are given in Tables 3−5. The estimated reaction equilibrium constants for BAE, IPAE, and EAE in eqs 4 and 5 are given in Table 6. 4.3. Solvent Performances. The three major performance items of an amine solvent are the cyclic absorption capacity, heat of reaction, and the absorption rate except for chemical stability. Among them, the first two can be estimated from the CO2 solubility information on a solvent. In Figure 8, the CO2 solubilities at 40 °C for the six investigated amine solutions plus the 30 wt % MEA solution are compared with the unit of the x-axis (CO2 loading) converted to g-CO2/kg-solvent. 2433



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ingredients for advanced solvents, although other properties such as the absorption rate, viscosity, and thermal and oxidative stability should be further verified.

5. CONCLUSIONS The CO2 solubility values of the three aqueous secondary alkanolamine solutions BAE, IPAE, and EAE were measured using the EC and the GSR unit. Low CO2 EPP data within 0.02 to 0.51 kPa at 40 °C were measured using the GSR, while high CO2 EPPs in the range 1.06 to 395.0 kPa at 40, 80, and 120 °C were measured using the EC. Aqueous solutions of two different amine concentrations, 15 and 30 wt % were prepared and tested for each amine. The K-E model and Henry’s law were employed to correlate the measured CO2 EPP data with the solution state. The cyclic absorption capacity and heat of reaction of the three amines were compared with those of the 30 wt % MEA solvent using the developed solubility model. All investigated alkanolamine solvents showed larger cyclic absorption capacities and smaller heat of reaction than the MEA solvent of the same amine weight fraction, and are considered promising candidates for advanced solvents.

Figure 8. Comparison of the CO2 solubility prediction between BAE, IPAE, EAE, and MEA solutions at 40 °C: (solid lines) 30 wt % solutions, (dashed lines) 15 wt % solutions.

Table 7. Calculated Cyclic Capacity and Heat of Reaction for Seven Solutions concentration

cyclic capacity

heat of reaction

amine

wt %

g-CO2/kg-solvent

−kJ/mol-CO2

BAE

15 30 15 30 15 30 30

26 45 35 60 34 58 38

70 72 68 69 68 70 84

IPAE EAE MEA

■

*Tel.: +82 2 705 8477. E-mail: [email protected]. ORCID

Kwang Soon Lee: 0000-0002-6730-0839 Funding

The cyclic absorption capacity is defined as the difference between rich and lean loadings. Rich and lean loadings represent the solvents’ states after absorption in the absorber and desorption in the stripper, respectively. In a real process, the rich and lean loadings will vary depending on the operating conditions. The nominal values of rich and lean loadings were defined such that the corresponding respective CO2 EPPs are 0.05 and 5 kPa when 40 °C of the absorber temperature is assumed. Therefore, the cyclic absorption capacity can be drawn from the CO2 solubility curve at 40 °C alone, as demonstrated in Figure 8 for the MEA case. The cyclic absorption capacities for the seven solutions were obtained and listed in Table 7 on the basis of this definition. The capacity obviously increases as the amine concentration increases. Secondary alkanolamines tend to form bicarbonate and show weaker carbamate stability than primary amines. In addition, IPAE has a steric hindrance, and EAE has a relatively small molecular weight. All of these enable IPAE and EAE to have larger absorption capacities than MEA. However, BAE shows an only marginally larger capacity than MEA. The primary cause for this is considered to be the 2-fold larger molecular weight of BAE compared to MEA. Heat of reaction of CO2, ΔHabs, was estimated by applying the following Gibbs−Helmholtz equation to the CO2 solubility model for each solution where R is a gas constant: −

⎡ ∂ ln PCO ⎤ ΔHabs 2 =⎢ ⎥ R ⎣ ∂(1/T ) ⎦α

CO2

AUTHOR INFORMATION

Corresponding Author

This work was supported by the Korea CCS R&D Center (Korea CCS 2020 Project) grant funded by the Korea government (Ministry of Science, ICT & Future Planning) in 2017 (KCRC2014M1A8A1049261). Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Rochelle, G. T. Amine scrubbing for CO2 capture. Science 2009, 325, 1652−1654. (2) Oexmann, J.; Kather, A.; Linnenberg, S.; Liebenthal, U. Postcombustion CO2 capture: chemical absorption processes in coal-fired steam power plants. Greenhouse Gases: Sci. Technol. 2012, 2, 80−98. (3) Pellegrini, L. A.; Moioli, S.; Gamba, S. Energy saving in a CO2 capture plant by MEA scrubbing. Chem. Eng. Res. Des. 2011, 89, 1676− 1683. (4) Rao, A. B.; Rubin, E. S. A technical, economic, and environmental assessment of amine-based CO2 capture technology for power plant greenhouse gas control. Environ. Sci. Technol. 2002, 36, 4467−4475. (5) Rochelle, G.; Chen, E.; Freeman, S.; Van Wagener, D.; Xu, Q.; Voice, A. Aqueous piperazine as the new standard for CO2 capture technology. Chem. Eng. J. 2011, 171, 725−733. (6) Chowdhury, F. A.; Yamada, H.; Higashii, T.; Goto, K.; Onoda, M. CO2 capture by tertiary amine absorbents: a performance comparison study. Ind. Eng. Chem. Res. 2013, 52, 8323−8331. (7) Fernandes, D.; Conway, W.; Wang, X.; Burns, R.; Lawrance, G.; Maeder, M.; Puxty, G. Protonation constants and thermodynamic properties of amines for post combustion capture of CO2. J. Chem. Thermodyn. 2012, 51, 97−102. (8) Goto, K.; Okabe, H.; Chowdhury, F. A.; Shimizu, S.; Fujioka, Y.; Onoda, M. Development of novel absorbents for CO2 capture from blast furnace gas. Int. J. Greenhouse Gas Control 2011, 5, 1214−1219. (9) Kim, I.; Svendsen, H. F. Heat of absorption of carbon dioxide (CO2) in monoethanolamine (MEA) and 2-(aminoethyl) ethanolamine (AEEA) solutions. Ind. Eng. Chem. Res. 2007, 46, 5803−5809.

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The estimated values are listed in Table 7. It can be noted that the heat of reaction of secondary alkanolamines is smaller than that of MEA due to the weak formation of carbamate. The three secondary alkanolamines investigated in this study show larger cyclic absorption capacities and moderate heat of reaction compared to MEA. They might be promising candidate 2434



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(10) Kumar, G. Vapour-liquid Equilibrium of Carbon Dioxide in Newly Proposed Blends of Alkanolamines. Ph.D. Thesis, NIT Rourkela, 2013. (11) Kundu, M.; Bandyopadhyay, S. Solubility of CO2 in water +diethanolamine+N-methyldiethanolamine. Fluid Phase Equilib. 2006, 248, 158−167. (12) Mondal, M. K. Solubility of carbon dioxide in an aqueous blend of diethanolamine and piperazine. J. Chem. Eng. Data 2009, 54, 2381− 2385. (13) Porcheron, F.; Gibert, A.; Mougin, P.; Wender, A. High throughput screening of CO2 solubility in aqueous monoamine solutions. Environ. Sci. Technol. 2011, 45, 2486−2492. (14) Puxty, G.; Rowland, R.; Allport, A.; Yang, Q.; Bown, M.; Burns, R.; Maeder, M.; Attalla, M. Carbon dioxide postcombustion capture: a novel screening study of the carbon dioxide absorption performance of 76 amines. Environ. Sci. Technol. 2009, 43, 6427−6433. (15) Sanchez, I. C.; Svendsen, H. F. Carbon dioxide solubility and mass transfer in aqueous amines for carbon capture. Ph.D. Dissertation, The University of Texas at Austin, 2015. (16) Sema, T.; Naami, A.; Idem, R.; Tontiwachwuthikul, P. Correlations for equilibrium solubility of carbon dioxide in aqueous 4(diethylamino)-2-butanol solutions. Ind. Eng. Chem. Res. 2011, 50, 14008−14015. (17) Singh, P.; Brilman, D. W.; Groeneveld, M. J. Evaluation of CO2 solubility in potential aqueous amine-based solvents at low CO2 partial pressure. Int. J. Greenhouse Gas Control 2011, 5, 61−68. (18) Yamada, H.; Chowdhury, F. A.; Goto, K.; Higashii, T. CO2 solubility and species distribution in aqueous solutions of 2(isopropylamino) ethanol and its structural isomers. Int. J. Greenhouse Gas Control 2013, 17, 99−105. (19) Yamada, H.; Higashii, T.; Chowdhury, F.; Goto, K.; Kazama, S. Experimental study into carbon dioxide solubility and species distribution in aqueous alkanolamine solutions. WIT Trans. Ecol. Environ. 2012, 157, 515−523. (20) Kim, J.; Hwang, S. J.; Kim, H.; Lee, K. S., Experimental method for measuring CO2 solubility in a carbon capture solvent in the low pressure region. Int. J. Greenhouse Gas Control 2017, Submitted. (21) Edwards, T.; Maurer, G.; Newman, J.; Prausnitz, J. Vapor-liquid equilibria in multicomponent aqueous solutions of volatile weak electrolytes. AIChE J. 1978, 24, 966−976. (22) Hwang, S. J.; Kim, H.; Lee, K. S. Prediction of VLE for aqueous blended amines using VLE models of single amines. Int. J. Greenhouse Gas Control 2016, 49, 250−258. (23) Aronu, U. E.; Gondal, S.; Hessen, E. T.; Haug-Warberg, T.; Hartono, A.; Hoff, K. A.; Svendsen, H. F. Solubility of CO2 in 15, 30, 45 and 60 mass% MEA from 40 to 120°C and model representation using the extended UNIQUAC framework. Chem. Eng. Sci. 2011, 66, 6393− 6406. (24) Shen, K.-P.; Li, M.-H. Solubility of carbon dioxide in aqueous mixtures of monoethanolamine with methyldiethanolamine. J. Chem. Eng. Data 1992, 37, 96−100. (25) Jou, F. Y.; Mather, A. E.; Otto, F. D. The solubility of CO2 in a 30 mass percent monoethanolamine solution. Can. J. Chem. Eng. 1995, 73, 140−147.

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Solubility of Carbon Dioxide in Aqueous Solutions of Three Secondary

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