Physical Properties of Partially CO2 Loaded Aqueous

Article pubs.acs.org/jced

Physical Properties of Partially CO2 Loaded Aqueous Monoethanolamine (MEA) Ardi Hartono, Emmanuel Orji Mba, and Hallvard F. Svendsen* Department of Chemical Engineering, Norwegian University of Science and Technology, N-7491 Trondheim, Norway S Supporting Information *

ABSTRACT: Monoethanolamine (MEA) is commonly used as a benchmark for solvent development within postcombustion CO2 capture. The physical properties data for this system are scarce, especially for loaded solutions. In this paper new measurements of physical properties (density, viscosity, and physical N2O solubility) of unloaded and loaded solutions are presented. Available literature data were collected, and semiempirical models were developed and shown to represent all experimental data very well.

1. INTRODUCTION Monoethanolamine (MEA) is a benchmarking solvent for postcombustion CO2 capture and has made reactive absorption an established and proven technology for this purpose. As a benchmarking solvent, much research has been focused on the performance of the solvent, ranging from basic solvent characterization to demonstration and full-scale carbon capture and storage (CCS) plants. Solvent characterization is key information before further implementation in modeling or process simulation, pilot plant operation, and in the commercial plants. Good and reliable data are crucial for this process. Physical properties such as density, viscosity, and physical CO2 solubility are necessary in the design of a gas liquid contactor. Extensive studies on the density and viscosity of unloaded MEA solutions have been performed and are well-documented in the literature.1−6 For N2O the physical solubility of unloaded MEA solution can be found elsewhere.1,7−9 However, reported data on loaded MEA solutions are very limited: that is, Browning and Weiland10 performed measurements at 25 °C for (10, 20, and 30) mass % MEA, and some data are shown in Aronu et al.11 The reported density and/or viscosity of some works2,3,5,12,13 are, to the best of our knowledge, the only ones available. In this work the physical properties of aqueous MEA solutions (density, viscosity, and N2O solubility) were measured at different concentrations, loadings, and temperatures. The excess properties of the unloaded solution were correlated with a simplified Redlich−Kister14 equation to reduce the parameters. Also for the loading solutions, the concept of excess properties was implemented.

water. Partially CO2 loaded solutions were prepared by bubbling CO2 into aqueous unloaded MEA solutions. The CO2 loading of the solution was estimated from the weight change and then wet chemistry methods; that is, the BaCl2 method15 and acid−base titration16 were used to verify the CO2 and MEA concentrations. The chemicals used are presented in Table 1. 2.2. Equipment and Procedures. An Anton Paar Density meter DMA 4500 M with the XSample 452 for automatic filling, rinsing, and drying was used in the experiment. The standard calibration procedure was performed with air and water density measurements at 20 °C. The density of pure water was taken from Spieweck and Bettin.17 This calibration is valid for a wide temperature range as one of the benefits of this type of density meter. Internal procedures for the experiments were developed by starting with an air check (i.e., measuring the density of air), then water density measurements, sample density measurements, performing multiple measurements (at least twice) and, at the end, new water density measurements. A cleaning/drying procedure was performed for each sample. The main purpose of measuring the density of water both at the beginning and the end of the process was to ensure the quality of the density of the unknown solutions and as an additional cleaning of the system. The DMA 4500 M was operated up to 90 °C, and the repeatability was ± 0.01 °C for temperature and ± 0.01 kg·m−3 for density. No vapor phase is present in DMA 4500 M, and the pressure is atmospheric. An Anton Paar MCR 100 rheometer with a double gap measuring cell (DG-26.7) was used to measure the dynamic viscosity of solution. The MCR 100 rheometer is an atmospheric instrument, and the liquid sample is placed in a semiclosed cell

2. EXPERIMENTAL SECTION 2.1. Chemicals. Aqueous solutions of MEA were prepared on a mass basis (Precision balance model MS6002S with an accuracy of ± 10−5 kg) by dissolving MEA with deionized © 2014 American Chemical Society

Received: December 12, 2013 Accepted: May 6, 2014 Published: May 20, 2014 1808

dx.doi.org/10.1021/je401081e | J. Chem. Eng. Data 2014, 59, 1808−1816

Journal of Chemical & Engineering Data

Article

Table 1. Chemicals Used in This Work name

formula

CAS

purity

source

monoethanolamine carbon dioxide nitrous oxide

C2H7NO CO2 N2O

141-43-5 124-38-9 10024-97-2

≥ 99.0 % (GC); water ≤ 0.30 % 99.999 % 99.998 %

Sigma-Aldrich Yara-Praxair Yara-Praxair

2

with little exposure to the surrounding air. Close to vapor− liquid equilibrium can thus be assumed. Standard viscosity solutions (N1, D5, S3, and S60) from Paragon Scientific Ltd. were used to calibrate the rheometer. The standard viscosity solutions covered the range (0.6 to 140) mPa and were measured in the temperature range (20 to 80) °C. The calibration and measurements were performed by filling the solution into the Double Gap (DG 27.6) cell with a volume of ∼4.10−6 m3 and performing rotational tests with controlled shear rate and shear stress. The dynamic viscosities were determined from the slope between the shear rate (γ) and shear stress (τ). The repeatability of the measurements was observed as ± 0.03 °C for temperature and ± 2 % maximum for viscosity. Physical solubility measurements for N2O into unloaded and loaded MEA solutions were performed in a VLE apparatus as described by Hartono et al.18 The same procedure was applied to both unloaded and loaded solutions. However, for the loaded solutions the CO2 concentration may change during the initial degassing process. To ascertain that there was no significant change in loading in the solution after the degassing process, titration analyses for CO2 total for both initial (preloaded solution) and final solution were performed. The differences in loading before and after measurement were found to be less than 3 % maximum. Thus, assuming constant loading can be acceptable for the experiment.

ρunloaded /kg·m−3 =

∑1 xi·Mi 2 xi·Mi ρi

V E + ∑1

where ρi, xi, Mi, and VE represent the density of pure solvent, mole fraction, molecular weights of MEA (i = 1) and H2O (i = 2), and an excess molar volume, respectively. An algebraic representation of thermodynamic properties (Redlich and Kister14) is widely used to represent the excess properties as a function of temperature and concentration. Obtaining a good fit of the excess property to the experimental data usually requires a high degree polynomial; hence a large number of parameters need to be fitted. Lower degree polynomials have been suggested1 to reduce the parameters without sacrificing the fit according to V E/m 3·mol−1 = (k1 + k 2t + k 3x1 + k4x12) ·x1x 2·10−6

Table 2. Densities of Unloaded and Loaded 6.2 Mass % MEAb,c in Water at Atmospheric Pressurea ρ/kg·m−3 loading t/°C

0.10 (0.0019)

0.20 (0.0037)

0.36 (0.0068)

0.47 (0.0089)

20 30 40 50 60 70 80

1000.88 998.06 994.42 990.09 985.12 978.86 973.57

1004.95 1002.16 998.47 994.07 989.10 983.58 977.56

1009.77 1006.80 1003.09 998.72 993.75 988.22 981.76

1017.47 1014.52 1010.79 1006.21 1001.33 995.87 989.89

1022.17 1019.00 1015.13 1010.67 1005.60 1000.03 993.98

(2)

where ki is the obtained parameter vector. Already reported1 excess property parameters for unloaded solutions were used in this work to correlate both measured and literature data. The fit is shown, together with the correlations reported by Han et al.2 and Cheng et al.,19 in Figure 1. As seen, the reported densities of water, given in Table A1, and for unloaded solutions from different sources1−3,6 agree well with each other. However, the correlation by Cheng et al.19 was found to overpredict the densities in the range 0.1 ≤ w1 ≤ 0.7 and at higher temperatures. The correlation by Han et al.2 was found to slightly under-predict density in the range 0 < w1 ≤ 0.4. The suggested correlation of this work is found to be significantly better as it reduces the number of parameters without sacrificing the accuracy of the fit. It is valid from (20 to 150) °C as seen in Table 4. When CO2 dissolves into aqueous MEA solutions (loaded solutions), the actual density is found to be slightly lower than the density that would be expected if no volume expansion occurs, that is, when CO2 is only taken as an addition to the weight. Thus, the density of loaded solutions can be modeled based on both the unloaded solution density and the added CO2:

3. RESULTS AND DISCUSSION 3.1. Density of the MEA (1) + Water (2) + CO2 (3) Mixtures. The measured densities of water and aqueous unloaded and loaded MEA solutions up to a loading 0.5 mol CO2/mol MEA and in the temperature range between (20 and 80) °C are listed in Tables 2, 3, and A1. For unloaded solutions, the excess molar volume is commonly used to correlate the density at different temperatures and concentrations as

0.00 (0.0000)

(1)

ρloaded /kg·m−3 =

ρunloaded 1 − wCO2added ·(1 − Φ3)

(3)

Here wCO2added is CO2 added to the solution on a mass basis and is calculated as 1 = wCO2added /kg CO added·kg −solution 2

αx1m3 x1m1 + (1 − x1 − αx1)·m2 + αx1m3

(4)

Here ρ, x1, M1, M2, M3, and α represent the density, MEA mole fraction, the molecular weight of MEA, H2O, CO2, and CO2 loading, respectively. Φ is the volume expansion caused by the CO2 addition and is proposed as a CO2 loading and MEA concentration-dependent parameter according to eq 5.

a

Loading is presented as (α = nCO2/nMEA (mol CO2/mol MEA)) and in parentheses in mole fraction, x3. bUncertainty: u(t) = ± 0.02 °C, u(ρ) = ± 0.02 kg·m−3, u(α) = ± 0.01 mol CO2/mol MEA, and u(w1) = ± 2·10−5 kg. cExpanded uncertainty: U(ρ) = ± 0.02 kg·m−3 (level of confidence = 0.95 where k = 2). 1809



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Table 3. Densities of Unloaded and Loaded 30 Mass % MEAb,c in Water at Atmospheric Pressurea ρ/kg·m−3 loading t/°C

0 (0.0000)

0.11 (0.0130)

0.19 (0.0215)

0.29 (0.0328)

0.39 (0.0431)

0.50 (0.0544)

20 25

1012.68 1010.55 1010.92 1010.633 1013.05 1008.31 1003.45 1003.52 1003.43 998.07 998.12 998.13 992.23 992.32 985.96 986.12 985.83 979.27 979.42

1030.99 1028.87 1033.32 1021.03 1033.05 1026.65 1021.88 1025.32 1021.03 1016.60 1019.62 1016.03 1010.90 1013.82 1004.80 1007.62 1004.83 998.31 1000.22

1046.92 1044.80 1041.03

1068.90 1066.76 1070.03

1086.89 1084.70 1095.73

1108.20 1105.88 1121.13

1054.05 1042.58 1037.84

1073.05 1064.53 1059.81

1095.05 1082.44 1077.75

1117.05 1103.51 1098.66

1041.03 1032.62

1062.93 1054.64

1088.53 1072.60

1114.03 1093.45

1035.53 1027.00

1058.03 1049.12

1083.03 1067.11

1108.03 1087.88

1021.01

1043.23

1061.28

1082.00

1024.03 1014.66

1046.43 1037.04

1071.93 1055.16

1075.85

30 40

50

60 70

80

a Loading is presented as (α = nCO2/nMEA (mol CO2/mol MEA)) and in parentheses in mole fraction, x3. bUncertainty: u(t) = ± 0.02 °C, u(ρ) = ± 0.02 kg·m−3, u(α) = ± 0.01 mol CO2/mol MEA, and u(w1) = ± 2·10−5 kg. cExpanded uncertainty: U(ρ) = ± 0.02 kg·m−3 (level of confidence = 0.95 where k = 2).

Table 4. Average Absolute Relative Deviations of Different Suggested Correlations for Unloaded MEA Solutions [MEA] (mass %) 0 to 100

a

a1x1α + a 2x1 a3 + x1

20 to 150

4 8 4

634

source(s)

AARDa (%)

this work Han et al. (2012) Cheng et al. (1996)

0.04 0.06 0.12

AARD (%) = [(100/N)·∑Ni |((ρcalculated − ρmeasured)/ρmeasured)|].

CO2 (eq 5). At a loading 0.5 mol CO2/ mol MEA, the volume increase is approximately ∼25 %. The representation of the model for different concentrations, loadings, and concentrations is summarized in Table 5 together with the average absolute relative deviations (AARD). The developed model was successful in predicting all of the reported data with less than 1 % in AARD. For (30 and 40) mass % MEA, the results are shown in Figures 2 and 3 together with parity plots given in the Supporting Information (Figures S1 and S2). The reported data for 30 mass % MEA of this work agree very well with the reported data.3,5 The reported data by Han et al.2 tend to deviate at higher concentrations, and the density ratios between the loaded and unloaded solutions show a nonlinear trend. The experimental discrepancies increase the AARD to 0.4 %. At 40 mass % MEA solution the same trend is observed between the reported data by Amundsen et al.3 and Han et al.2 The disagreement seen at the higher loadings and temperatures in this case leads to an AARD between model and experimental data of 0.5 %. At higher MEA concentrations, (50, 60, and 80) mass % MEA, the suggested model shows higher AARD values (up to 0.7 %) for the reported data by Han et al.2 3.2. Viscosity of MEA (1) + Water (2) + CO2 (3) Mixtures. Calibration results from the used rheometer (MCR 100) are shown in figure S3 and given in Table A2. The calibration

Figure 1. Density of unloaded MEA solutions at different concentrations and temperatures (20, 40, 60, 80, 100, 120, 140, and 150) °C. Model compared with data. Data: ○, ref 1; □, ref 2; △, ref 3; ∗, ref 6, ●, this work. Model: dash−dotted lines, ref 19; dashed lines, ref 2; solid lines, ref 1.

Φ=

temperature no. no. (°C) parameters points

(5)

Here x1 is the MEA mole fraction, and α is the CO 2 loading. All reported MEA loaded solutions were used to fit the three parameters in eq 5, and their values are a1 = 0.29 ± 0.05, a2 = 0.18 ± 0.02, and a3 = 0.66 ± 0.03, respectively. The CO2 added per kg solution at a certain loading increases with increasing MEA concentration (eq 4) and leads to a volume expansion approximately proportional to the added 1810



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Table 5. Average Absolute Relative Deviations of the Model for All Reported Loaded MEA Solutions temperature [MEA] loading (α) (°C) (mass %) (mol CO2/mol MEA) 6.2 (∼1 M) 10 20

0 to 0.50 0 to 0.50 0 to 0.50

30 (∼5 M)

0 0 0 0 0 0 0 0 0 0

40

50 60 80 a

to to to to to to to to to to

0.50 0.50 0.50 0.56 0.50 0.50 0.45 0.47 0.48 0.51

20 25 25 25 20 25 25 25 25 25 25 25 25 40

to 80

to 80 to 80 to 80 to 150 to to to to to

80 150 150 150 70

source(s)

AARDa (%)

this work Weiland et al. (1998) Weiland et al. (1998) Amundsen et al. (2009) this work Weiland et al. (1998) Amundsen et al. (2009) Han et al. (2012) Weiland et al. (1998) Amundsen et al. (2009) Han et al. (2012) Han et al. (2012) Han et al. (2012) Jayarathna et al. (2013)

0.04 0.18 0.93 0.14 0.13 0.09 0.28 0.43 0.23 0.41 0.50 0.44 0.66 0.30

Figure 3. Density for 40 mass % MEA solution at different loadings and temperatures (25, 40, 60, 80, 100, 120, and 140) °C. Model compared to data. Data: □ from (25 to 140)°C (ref 2); ∗ from (25 to 80) °C, (ref 3); △, 25 °C (ref 5). Model: dashed lines, ref 5; solid lines, this work.

AARD (%) = [(100/N)·∑Ni |((ρcalculated − ρmeasured)/ρmeasured)|].

Table 6. Viscosity of Unloaded and Loaded 6.2 mass % MEAb,c in Water at Atmospheric Pressurea η/mPa·s loading

Figure 2. Density of 30 mass % MEA solution at different loadings and temperatures of (20, 25, 40, 60, 80, 100, 120, and 140) °C. Model compared with data. Data: □ from (25 to 140) °C (ref 2); ∗ from (25 to 80) °C, (ref 3); △, 25 °C (ref 5); ○ from (20 to 80) °C (this work). Model: dashed lines, ref 5; solid lines, this work.

1

(6)

14

A Redlich−Kister type correlation could be used to represent the viscosity deviation as a function of temperature and concentration. In this work a simpler correlation is suggested as ln(ηγ )/Pa·s = (l1 + l 2t + l3t 2 + l4x1) ·x1x 2

0.36 (0.0068)

0.48 (0.0091)

20 20 30 30 40 40 50 50 60 60

1.228 1.226 0.960 0.960 0.790 0.790 0.668 0.668 0.560 0.560

1.236 1.236 0.975 0.975 0.799 0.799 0.669 0.669 0.560 0.560

1.286 1.285 1.016 1.016 0.822 0.822 0.697 0.697 0.581 0.581

(°C). l1, l2, l3, and l4 are the parameters obtained. All reported unloaded viscosity data and the viscosities of pure water and MEA from DIPPR20 were used to fit the viscosity deviation according to eq 7. The obtained parameters are shown in Table 8. Most of the reported viscosity data are consistent with each other as seen in Figure 4. However, the reported data by Weiland et al.5 at 10 mass % MEA are found to disagree with other data (shown by arrows) and were not used in the parameter fitting. The developed model is able to represent the experimental data from this work and from different sources3,4,21 (with an AARD within 4.2 % (Figure S4)). Reviewing the empirical correlation by Cheng et al.,19 it is clear that this model tends to deviate at higher temperatures and at higher concentrations, while the suggested correlation by Weiland et al.5 works well only up to 40 mass % MEA as expected since the correlation was validated up this concentration.

2

∑ xi ln(ηi)

0.00 (0.0000)

a Loading is presented as α = nCO2/nMEA (mol CO2/mol MEA) and in parentheses in mole fraction, x3. bUncertainty: u(t) = ± 0.03 °C, u(η) = ± 0.007 mPa·s at η ≤ 10 mPa·s, u(α) = ± 0.01 mol CO2/mol MEA, and u(w1) = ± 2·10−5 kg. cExpanded uncertainty: U(η) = ± 0.007 mPa·s at η ≤ 10 mPa·s (level of confidence = 0.95 where k = 2).

viscosities cover a range from (0.6 to 140) mPa·s and (20 to 80) °C. The AARD for the calibration measurements was 0.8 %. Resulting experimental viscosities for the unloaded and loaded aqueous MEA solutions up to loading 0.5 mol CO2/mol MEA and in the temperature range (20 to 80) °C are listed in Tables 6 and 7. The unloaded viscosity is calculated as the sum of an “ideal” viscosity based on the weighted sum of the solution component’s pure viscosities and a viscosity deviation (ηγ). It is given as ln(ηunloaded /Pa·s) = ln(ηγ ) +

t/°C

(7)

In eqs 6 and 7 ηi, ηγ, xi, and t represent respectively the viscosity of pure solvent, viscosity deviation, mole fraction, and temperature 1811



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Table 7. Viscosities of Unloaded and Loaded 30 mass % MEAb,c in Water at Atmospheric Pressurea η/mPa·s loading t/°C 20 25

30 40

50

60 70

80

0 (0.0000)

0.11 (0.0130)

0.19 (0.0215)

0.29 (0.0328)

0.39 (0.0431)

0.50 (0.0544)

2.874 2.879 2.450 2.457 2.483 2.525

3.112 3.107 2.662 2.648 2.603 2.725 2.5113 2.290 2.284 1.765 1.757 1.703 1.405 1.400 1.403 1.148 1.148 0.967 0.962 0.903 0.818 0.816 0.803

3.309 3.291 2.820 2.816 2.903 2.925 2.8213 2.425 2.420 1.875 1.874 2.003 1.486 1.482 1.603 1.235 1.224 1.031 1.027 1.103 0.879 0.873 0.903

3.597 3.581 3.086 3.077 3.103 3.215 3.1813 2.702 2.696 2.101 2.100 2.003 1.669 1.668 1.603 1.369 1.366 1.144 1.154 1.103 0.987 0.986 0.903

3.899 3.872 3.339 3.321 3.503 3.525 3.4413 2.887 2.886 2.246 2.230 2.403 1.804 1.800 1.903 1.492 1.487 1.262 1.255 1.303 1.075 1.071 1.103

4.251 4.250 3.663 3.665 3.903 3.825 3.9713 3.184 3.183 2.470 2.452 2.703 1.988 1.999 2.103 1.630 1.635 1.343 1.351 1.503 1.143 1.136 1.303

2.133 2.130 1.628 1.638 1.673 1.305 1.318 1.333 1.055 1.067 0.878 0.874 0.923 0.742 0.740 0.773

Figure 4. Viscosity of unloaded MEA solutions at different concentrations and temperatures of (20, 25, 30, 40, 50, 60, 80, and 100) °C. Data: □, ref 3; ○, ref 21; △, ref 4; ◊, ref 5; ∗, this work; dashed lines, ref 19. Model: dash−dotted lines, ref 5; solid lines, this work.

Loading is presented as α = nCO2/nMEA (mol CO2/mol MEA) and in parentheses in mole fraction, x3. bUncertainty: u(t) = ± 0.03 °C, u(η) = ± 0.007 mPa·s at η ≤ 10 mPa·s, u(α) = ± 0.01 mol CO2/mol MEA, and u(w1) = ± 2·10−5 kg. cExpanded uncertainty: U(η) = ± 0.007 mPa·s at η ≤ 10 mPa·s (level of confidence = 0.95 where k = 2). a

Table 8. Parameters for the Viscosity Deviation [MEA] (mass %)

temperature (°C)

no. points

0 to 100

20 to 100

204

parameters l1/(−) = 8.36 ± 0.12 l2/(°C)−1 = (−4.664 ± 0.5)·10−2 l3/(°C)−2 = (1.6 ± 0.6)·10−4 l4/(−) = (−4.14 ± 0.08)

Figure 5. Viscosity of 30 mass % MEA solution at different loadings and temperatures of (20, 25, 30, 40, 50, 60, 70, and 80) °C. Data: ∗, this work; +, ref 13;○, ref 3; △, ref 5. Model: dashed lines, ref 5; solid lines, this work.

For loaded MEA solutions a new viscosity deviation (η*γ ), representing the deviation from unloaded solutions, is suggested as ln(ηloaded /Pa·s) = x3 ln(ηγ*) + (1 − x3) ln(ηunloaded)

ln(103·ηγ*/Pa ·s) =

3.3. N2O Solubility into MEA (1) + Water (2) + CO2 (3) Mixtures. Physical solubilities of N2O, given as apparent Henry’s law constants, into aqueous unloaded and loaded MEA solutions were measured at two MEA concentrations and at different loadings and temperatures as shown in Tables 10 and 11 (Complete solubility data are given in Tables S1 and S2 in the Supporting Information). Some experiments were randomly done twice to check the repeatability. It is clearly seen that the difference between repeats is less than 1 %. Like for viscosity, the unloaded apparent Henry’s constant was modeled as the sum of contributions, one being the weighted sum of apparent Henry’s constants in the pure solvents, the other being an apparent Henry’s constant deviation (ΔkHunloaded ). The Henry’s N2O constants for the mixtures (kHunloaded ) are given as N2O

(8)

(6.98 ± 0.48) ·x1 + (10.48 ± 1.0) ·αx1 (0.049 ± 0.008) + x1 (9)

The addition of CO2 was taken into account as its mole fraction in solution. The suggested model was tested for (30 and 40) mass % MEA solutions as seen in Figures 5 and 6 and in parity plots (Figures S5 and S6). The model predicts well the loaded viscosities for both (30 and 40) mass % MEA solutions5,13 with a 3.9 % maximum AARD. The representation of the model for different temperatures, loadings, and MEA concentrations is very satisfactory, and the results are summarized in Table 9. Accuracies are indicated by the average absolute relative deviation values (AARD).

2

ln(k H Nunloaded /kPa·m 3·mol−1) = ln Δk H unloaded + O N O 2

1812

2

∑ φi ln(k Hi) 1

(10)



Article

Table 10. Measured Henry’s Constant for Unloaded and Loaded 6.2 mass % MEAb,c in Watera kHN2O/kPa·m3·mol−1 loading t/°C

0.00 (0.0000)

0.10 (0.0019)

0.20 (0.0038)

0.36 (0.0068)

0.47 (0.0088)

4.16 5.36 6.61

4.33 5.54 6.85 8.20 8.21 9.95 9.97 11.66

4.58 5.84 7.24 7.20 8.68 8.70 10.21

4.69 5.99 7.34

8.03

4.51 5.74 7.10 7.08 8.55

25 35 45 45 55 55 65 65 75 75

9.59 11.07

10.07 10.03 11.68 11.67

8.89 8.85 10.45 10.46 12.29

12.00

Loading is presented as α = nCO2/nMEA (mol CO2/mol MEA) and in parentheses in mole fraction, x3). bUncertainty: u(t) = ± 0.05 °C, u(P) = ± 0.05 kPa, u(α) = ± 0.01 mol CO2/mol MEA, and u(w1) = ± 2·10−5 kg. cExpanded uncertainty: U(kH) = ± 0.03 kPa·m3/mol (level of confidence = 0.95 where k = 2). a

Figure 6. Viscosity of 40 mass % MEA solution at different loadings and temperatures of (25, 40, 50, 70, and 80) °C. Model compared to data. Data: +, ref 13; ○, ref 3; △, ref 5. Model: dashed lines, ref 5; solid lines, this work.

Hartono et al.1 reported that the apparent Henry’s constant deviations can be correlated with the excess molar volume. The free void gives space for a gas to dissolve into a liquid phase. They suggested that an excess molar volume could be defined proportional to the deviation in apparent Henry’s constant as ln(Δk H unloaded /kPa ·m 3·mol−1) = −106 ·V E· 1 − φ12 N O 2

Table 11. Measured Henry’s Constants for Unloaded and Loaded 30 mass % MEAb,c in Watera kHN2O/kPa·m3·mol−1 loading

(11)

Here VE is obtained from eq 2, and φ1 = [(x1·M1/ρ1)/(x1·M1/ρ1 + (1 − x1)·M2/ρ2)] is the MEA volume fraction. The apparent Henry’s constant increases with temperature and loading as expected. When CO2 is added to the MEA solution, the ionic strength increases and leads to less vacant spaces for N2O. Using the N2O analogy apparent Henry’s constants for CO2 can be inferred. The ratio between measured apparent Henry’s constant for loaded and unloaded MEA solutions and the Henry’s law constant for CO2 in water at infinite dilution can give a measure for the activity coefficient of CO2 in the solution.11 A simple correlation for the apparent

t/°C

0.00 (0.0000)

0.20 (0.0226)

0.40 (0.0440)

0.50 (0.0544)

25.2

4.67 4.7910 6.34 6.34 7.57 9.01 10.43 13.47

5.64

7.02

7.94

7.84

9.48

10.90

9.68 11.46

11.05 13.95

17.33

20.98

12.83 16.15 17.98 25.32

39.5 39.5 49.1 58.6 68.1 87.1

Loading is presented as α = [(nCO2/nMEA)(mol CO2/mol MEA)] and in parentheses in mole fraction, x3). bUncertainty: u(t) = ± 0.05 °C, u(P) = ± 0.05 kPa, u(α) = ± 0.01 mol CO2/mol MEA, and u(w1) = ± 2·10−5 kg. cExpanded uncertainty: U(kH) = ± 0.03 kPa·m3/mol (level of confidence = 0.95 where k = 2). a

Table 9. Average Absolute Relative Deviations of the Model for All Reported Loaded MEA Solutions [MEA] (mass %)

a

loading (α) (mol CO2/mol MEA)

temperature (°C)

source(s)

AARDa (%)

25 to 80 25 to 80 30, 50, 75, 100 25 to 80 25 25 to 80

Maham et al. (2002) Amundsen et al. (2009) Dow (2003) this work Weiland et al. (1998) Amundsen et al. (2009) Fu et al. (2012) this work Weiland et al. (1998) Amundsen et al. (2009) Fu et al. (2012) Weiland et al. (1998) Amundsen et al. (2009) Fu et al. (2012)

2.3 2.8 4.2 1.6 1.8 2.8 2.8 2.0 2.5 3.6 3.5 3.9 2.8 3.2

0 to 100 15 to 100 0 to 100 6.2 (∼1 M) 20

0

30 (∼5 M)

0 to 0.5

20 to 80 25 25 to 80

40

0 to 0.5

25 25 to 80

0 to 0.5 0 to 0.5

AARD (%) = [(100/N)·∑Ni |((ηcalculated − ηmeasured)/ηmeasured)|]. 1813



Article

Figures 7 and 8 show the measured apparent Henry’s constants compared to the developed model. The slopes of the curves indicate the apparent activity coefficients for N2O, and it is seen to increase with temperature for both MEA concentrations. The apparent activity coefficients of N2O in are more sensitive to loading at high concentration and high temperature. At the highest temperature, the suggested correlation is seen to slightly under-predict the reported data. The uncertainty in the unloaded correlation predictions will propagate into uncertainties also for the loaded correlation. The suggested correlation follows well the trends of the data in this work as shown by the absolute average relative deviation values (AARD) of about 1.8 % and 3.8 %, respectively (Figures S7 and S8). When compared to literature data, for example, Browning and Weiland,10 it gives an AARD within 5 %, a value which is deemed very satisfactory.

Henry’s constant deviation for loaded solutions (ΔkHloaded ) is N2O suggested as ln(Δk H loaded /kPa ·m 3·mol−1) = b1x1 + b2x1αT N O

(12)

2

ln(k H loaded /kPa ·m 3·mol−1) = ln(Δk H Nloaded ) + ln(k H Nunloaded ) N O O O 2

2

2

(13)

where b1, b2, x1, and α represent the parameters, MEA mole fraction, and CO2 loading, respectively. Reported data10 and from this work were used to fit the two parameters in eq 12, and their values are b1 = 0.77 ± 0.12 and b2 = 0.033 ± 0.001. The agreement between the model developed in this work and the reported data for unloaded and loaded (6.2 and 30) mass % MEA data is very good as shown in Figures 7 and 8.

4. CONCLUSION Densities, viscosities, and N2O solubilities for unloaded and loaded MEA solution were measured for CO2 loadings up to 0.5 (mol CO2/mol MEA) and temperatures in the range (20 to 90) °C. The measured densities increase with increasing loading but decrease with temperature. The measured unloaded densities Table A1. Measured Densities of Water before and after Measurementa,b ρ/kg·m−3

Figure 7. Apparent Henry’s law constant for N2O in 6.2 mass % MEA solutions at different loadings and temperatures. Model compared with data. Data: ○, 25 °C; □, 35 °C; ◊, 45 °C; ∗, 55 °C; +, 65 °C; ◁, 75 °C. Model: solid lines, this work eqs 12 and 13.

Figure 8. Apparent Henry’s law constant for N2O in 30 mass % MEA solution at different loadings and temperatures. Model compared to data. Data: ●, 25.0 °C, ref 10; ○, 25.2 °C; □, 39.5 °C; ◊, 49.1 °C; ∗, 58.6 °C; +, 68.1 °C; ◁, 87.1 °C. Model: solid lines, this work, eqs 12 and 13.

t/°C

measured

ref 17

20.01 20.01 20.01 20.01 25.01 25.01 25.01 25.01 29.99 29.99 29.99 30.00 39.99 39.99 39.99 40.00 49.99 50.00 49.99 50.00 59.99 59.99 59.99 59.99 69.99 69.99 69.99 70.00 79.99 79.99 79.99 79.99

998.20 998.21 998.21 998.21 997.05 997.05 997.05 997.05 995.67 995.66 995.67 995.66 992.24 992.23 992.24 992.24 988.05 988.05 988.06 988.06 983.20 983.19 983.21 983.21 977.74 977.74 977.78 977.78 971.74 971.74 971.79 971.79

998.203 998.203 998.203 998.203 997.040 997.040 997.040 997.040 995.650 995.650 995.650 995.650 992.212 992.212 992.212 992.212 988.030 988.030 988.030 988.030 983.191 983.191 983.191 983.191 977.759 977.759 977.759 977.759 971.785 971.785 971.785 971.785

Uncertainty: u(t) = ± 0.02 °C, u(ρ) = ± 0.02 kg·m−3. bExpanded uncertainty: U(ρ) = ± 0.02 kg·m−3 (level of confidence = 0.95 where k = 2).

a

1814



■

were consistent with the literature data found, but small discrepancies were found for loaded data. The model developed, based on an excess volume approach for unloaded solutions and a volume expansion for the addition of CO2, was able to represent the measured densities very well (AARD ∼ 1 %). The measured unloaded viscosity data were found to agree well with available literature data. For CO2 loaded solution discrepancies in literature date were observed and their own data agreed with some of the literature data. The viscosities were found to increase rapidly with loading at low temperatures, but this effect is less strong at higher temperatures. The developed model, covering both unloaded and loaded solutions, represents available data well (AARD ∼4 %). The measured physical N2O solubilities of the loaded MEA solutions agree well with the single literature datum found for 30 mass % MEA, 25 °C, and a loading of 0.35 (mol CO2/mol MEA). Less N2O gas dissolves into the liquid phase with increasing loading and temperature. At low MEA concentrations a weaker effect of loading on the N2O solubility was observed compared to high MEA concentrations and temperatures. The correlation developed was able to represent both unloaded and loaded N2O solubilities very well within an AARD of 4 %.

Supporting figures, including density and viscosity correlations and plots of the N2O solubility model, and tables of measured physical N2O solubility. This material is available free of charge via the Internet at http://pubs.acs.org.

■

*Phone: +47-73594100. Fax: +47-73594080. E-mail: hallvard. [email protected]. Funding

Authors highly acknowledge the financial support from the CCERT project. The CCERT project is supported by the Research Council of Norway (NFR 182607), Shell Technology Norway AS, Metso Automation, Det Norske Veritas AS, and Statoil AS. Notes

The authors declare no competing financial interest.

■

η/mPa·s referencec

D5

20 20 25 40 50 20 25 40 50 20 20 25 40 50 50 50 80 80 20 20 25 40 50

5.772 5.768 4.976 3.371 2.698 3.731 3.281 2.337 1.922 135.74 135.77 100.88 46.46 30.01 30.03 29.99 10.76 10.77 0.994 0.998 0.920 0.753 0.667

5.784 5.784 4.978 3.358 2.684 3.709 3.261 2.316 1.903 136.40 136.40 101.00 45.98 29.57 29.57 30.57 10.58 10.58 1.001 1.001 0.928 0.753 0.664

S3

S60

N1

REFERENCES

(1) Hartono, A.; Mba, E. O.; Svendsen, H. F. Prediction of N2O solubility in alkanolamine solutions from the excess volume property. Energy Procedia 2013, 37, 1744−1750. (2) Han, J.; Jin, J.; Eimer, D. A.; Melaaen, M. C. Density of water (1) + Monoethanolamine (2) + CO2 (3) from (298.15 to 413.15) K and surface tension of water (1) + Monoethanolamine (2) from (303.15 to 333.15) K. J. Chem. Eng. Data 2012, 57, 1095−1103. (3) Amundsen, T. G.; Øi, L. E.; Eimer, D. A. Density and viscosity of Monoethanolamine + Water + Carbon Dioxide from (25 to 80) °C. J. Chem. Eng. Data 2009, 54 (11), 3096−3100. (4) Ethanolamines; Dow Chemical Company: Midland, MI, 2003. (5) Weiland, R. H.; Dingman, J. C.; Cronin, D. B.; Browning, G. J. Density and viscosity of some partially carbonated aqueous alkanolamine solutions and their blends. J. Chem. Eng. Data 1998, 43 (3), 378−382. (6) Maham, Y.; Teng, T. T.; Hepler, L. G.; Mather, A. E. Densities, excess molar volume, and partial molar columes for binary mixtures of water with Monoethanolamine, Diethanolamine, and Triethanolamine from 25 to 80 °C. J. Solution Chem. 1994, 23 (2), 195−205. (7) Yaghi, B.; Houache, O. Solubility of Nitrous Oxide in Amine Aqueous Solutions. J. Eng. Comput. Arch. 2008, 2 (1), 1−14. (8) Tsai, T. C.; Ko, J. J.; Wang, H. M.; Lin, C. Y.; Li, M. H. Solubility of Nitrous Oxide in Alkanolamine Aqueous Solutions. J. Chem. Eng. Data 2000, 45, 341−347. (9) Wang, Y. W.; Xu, S.; Otto, F. D.; Mather, A. E. Solubility of N2O in Alkanolamines and in Mixed Solvents. Chem. Eng. J. 1992, 48, 3−40. (10) Browning, G. J.; Weiland, R. H. Physical solubility of Carbon Dioxide in aqueous alkanolamines via Nitrous Oxide analogy. J. Chem. Eng. Data 1994, 39, 817−822. (11) 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% MEA from 40 to 120°C and model representation using the extended UNIQUAC framework. Chem. Eng. Sci. 2011, 66, 6393− 6406. (12) Jayarathna, S. A.; Jayarathna, C. K.; Kottage, D.; Dayarathna, S.; Eimer, D. A.; Melaaen, M. C. Density and surface tension measurements of partially carbonated aqueous monoethanolamine solutions. J. Chem. Eng. Data 2013, 58, 343−348. (13) Fu, D.; Chen, L.; Qin, L. Experiment and model for the viscosity of carbonated MDEA-MEA aqueous solutions. J. Chem. Eng. Data 2012, 319, 42−47. (14) Redlich, O.; Kister, A. T. Algebraic Representation of Thermodynamic Properties and the Classification of Solutions. Ind. Eng. Chem. 1948, 40, 345−348.

Table A2. Measured Viscosities of Standard Solutiona,b measured

AUTHOR INFORMATION

Corresponding Author

APPENDIX A Tables A1 and A2 show the measured densities of water before and after measurement and measured viscosities of standard solution.

t/°C

ASSOCIATED CONTENT

S Supporting Information *

■

standard solution

Article

Uncertainty: u(t) = ± 0.05 °C, u(η) = ± 0.007 mPa·s at η ≤ 10 mPa· s, and u(η) = ± 0.2 mPa·s at 10 ≤ η ≤ 200 mPa·s. bExpanded uncertainty: U(η) = ± 0.007 mPa·s at η ≤ 10 mPa·s and U(η) = ± 0.2 mPa·s at 10 ≤ η ≤ 200 mPa·s (level of confidence = 0.95 where k = 2). c Paragon Scientific Ltd. a

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(15) Ma’mun, S.; Jakobsen, J. P.; Svendsen, H. F. Experimental and modeling study of the solubility of Carbon dioxide in aqueous 30 mass % 2-((2-Aminoethyl)amino)ethanol solution. Ind. Eng. Chem. Res. 2006, 45 (8), 2505−2512. (16) Hartono, A.; Saleem, F.; Arshad, M. W.; Usman, M.; Svendsen, H. F. Binary and ternary VLE of the 2-(diethylamino)-ethanol (DEEA)/3-methylamino)-propylamine (MAPA) /water system. Chem. Eng. Sci. 2013, 101, 401−411. (17) Spieweck, F.; Bettin, H. Review: Solid and liquid density determination. Techn. Messen 1992, 59, 285−292. (18) Hartono, A.; Juliussen, O.; Svendsen, H. F. Solubility of N2O in aqueous solution of Dietylenetriamine. J. Chem. Eng. Data 2008, 53, 2696−2700. (19) Cheng, S.; Meisen, A.; Chakma, A. Predict amine solution properties accurately. Hydrocarbon Process. 1996, 75, 81−84. (20) DIPPR 801 Information and Data Evaluation Manager for the Design Institute for Physical Properties, Version 4.1.0; DIPPR: Provo, UT, 2004. (21) Maham, Y.; Liew, C.-N.; Mather, A. E. Viscosities and excess properties of aqueous solutions of Ethanolamine from 25 to 80 °C. J. Solution Chem. 2002, 31 (9), 743−756.

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Physical Properties of Partially CO2 Loaded Aqueous

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