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Article pubs.acs.org/jced

Oxygen Solubility Measurements in a MEA/H2O/CO2 Mixture Maxime H. Wang, Alain Ledoux,* and Lionel Estel INSA de Rouen Laboratoire de Sécurité des Procédés Chimiques/EA 4704 BP8, Normandie Université, Avenue de l’Université 76800 Saint Etienne du Rouvray, France ABSTRACT: This work measures oxygen concentration in the system MEA/H2O/CO2. A polarographic probe and the Winkler’s method have been used in this study. The influence of several parameters have been studied: MEA concentration in the range 100 w = 0 to 80, CO2 loading in the range 0 to 0.5, and temperature from 10 °C up to 60 °C. Measurements have been carried out at atmospheric pressure. We show that oxygen concentration decreases as temperature drops, but also as CO2 loading increases. MEA concentration does not affect oxygen solubility.

I. INTRODUCTION To achieve CO2 capture, flue gases from postcombustion processes are treated by an absorption/desorption loop with basic solutions.1 The best solvents for this kind of treatment should have simultaneously a good resistance to thermal and oxidative degradation, high absorption kinetic, and low regeneration energy. To fulfill these specific needs, solvents are often complex mixtures including specific chemicals to protect the molecule supporting capture. Amine solutions are often used for the CO2 capture process in postcombustion. In particular, a solution of monoethanolamine (MEA)100 w = 30 with water is used in most working units2−4 and as a benchmark to compare with new solvents.5,6 The absorption kinetic of this solutions is 2 mol·L−1·s−1 at 20 °C7−10 and the regeneration energy is 3.8 MJ/kg of CO2.11,12 Literature reports many ways of degradation for this molecule. The degradation of MEA decreases the performance of the solvent and forms degradation products which have a high corrosion power.13,14 Bibliographic studies show that degradation phenomena may be the source of more than 12 % of operating cost.15 One of them is degradation by oxidation16−18 which is driven by the presence of oxygen in treated flue gas. Moreover, the solubility of oxygen can appear as a key parameter for modeling CO2 capture in postcombustion. For example the oxygen solubility is used in the correlation to determine the degradation rate of MEA19 or to determine the corrosion power of the solution.13 Until now, the oxygen solubility data were used from the measurements carried out in MEA 100 w = 20 aqueous solution from Ronney.20 During the capture process, the solution of MEA 100 w = 30 is subjected to repeated heating/cooling cycles and carbon dioxide absorbed concentration in the solution is varying according to absorption and desorption loop. Consequently those parameters must be studied to understand their influence on oxygen solubility. In this paper, the measurements of oxygen’s solubility have been carried out in MEA/H2O/CO2 mixture. Oxygen saturation has been obtained using air as oxygen source. The bubbling system was kept a few minutes to © 2013 American Chemical Society

ensure saturation (water used for mixture was yet saturated). Three parameters have investigated: MEA concentration, temperature, and CO2 concentration. Measurements have been carried out using mainly a polarographic probe. The Winkler method has been used to validate results obtained by the polarographic probe.

II. CHEMICAL REACTION The absorption of CO2 inside MEA solution is a chemical absorption. Reactions take place between MEA and CO2 during the phenomena of absorption. Assuming a zwitterion mechanism,21 reactions 1 and 2 take place in the solution. The reaction between CO2 and MEA to give zwitterion specie: CO2 + RNH 2 ↔ RNH+2 COO−

(1)

Carbamate formation by deprotonation of the zwitterion: RNH+2 COO− + RNH 2 ↔ RNH+3 + RNHCOO−

(2)

The loading of solutions (α) is the ratio of CO2 and MEA concentrations in the solution. According to reactions 1 and 2, for one molecule of CO2 absorbed, two molecules of MEA are consumed. Therefore, the loading of the solution varies from 0, when no CO2 is absorbed by the solution, to 0.5, when all molecules of MEA are consumed. At the beginning, the solution is composed of H2O and MEA. This solution is a nonelectrolytic solution. After the absorption of CO2, the carbamate and the protonated MEA is produced. Consequently, the ionic strength of the solution increases. According to the data from Rooney (1998), the oxygen’s solubility in amine solution when no CO2 is absorbed is similar to the H2O solution. Those data are often used in important Received: October 2, 2012 Accepted: March 18, 2013 Published: April 4, 2013 1117

dx.doi.org/10.1021/je301077y | J. Chem. Eng. Data 2013, 58, 1117−1121

Journal of Chemical & Engineering Data

Article

solution. By adding sulfuric acid, a temperature increase is caused by an exothermic reaction between acid and base. To overcome this problem, we modified the Winkler method. The Winkler modified method is based on back-titration. To consume producted iodine, we add a specific quantity of thiosulfate before added sulfuric acid. After added acid, the solution is titrated with a solution of KIO3 to determine the excess of thiosulfate. Dissolved oxygen concentration is calculated by eq 6.

correlations but they do not represent the condition of the CO2 capture process. The aim of this paper is to show the influence of the increasing ionic strength, a direct consequence of absorbed CO2, on the oxygen solubility.

III. CHEMICAL PRODUCT AND PREPARATION OF CHARGED SOLUTIONS Two kinds of solution (MEA aqueous solution with and without CO2) are prepared using different methods. For the MEA/H2O solutions, the measurements are carried out using MEA 99 % purity from Fischer chemical and distilled water. For the MEA/H2O/CO2 solutions, the known quantity of CO2 (from Air Product 99 % purity) is injected inside a batch reactor containing the MEA/H2O solution. Several solutions are prepared as reagent for the Winkler method: The manganese sulfate solution is prepared using 182 g of manganese sulfate monohydrate (99 % purity from Acros Organics) in 500 mL of distilled water. The KI/NaOH solution is prepared using 150 g KI (99 % purity from Acros Organics) and 500 g NaOH pellets (from Carlo ERBA) in 1 L of distilled water. The concentrated sulfuric acid is purchased from Fisher chemical (> 95 % purity). The sodium thiosulfate solution is prepared using 6.205 g sodium thiosulfate pentahydrate (Acros organics) in 1 L of distilled water. The potassium iodate solution (0.025 mol·L−1) is prepared using potassium iodate (> 99.5 % purity Acros Organics). A starch solution is prepared using 2 g of starch from Acros Organics in 100 mL distilled water. It is used as an indicator.

[O2 ] =

(3)

MnO2 + 2I− + 4H+ → I 2 + Mn 2 + + 2H 2O

(4)

I 2 + 2S2 O32 − → S4 O6 2 − + 2I−

(5)

(6)

−1

where [O2] is the concentration of O2 (g·L ); V is the volume of the sample (L); M is the molar mass of O2 (g.mol−1); [Na2S2O3] is the concentration (mol·L−1) of thiosulfate solution; V(Na2S2O3) is the volume (L) of thiosulfate solution; [KIO3] is the concentration (mol·L−1) of KIO3 solution; and V(KIO3) is the volume (L) of KIO3 solution. In this work, the oxygen probe has been used to determine the oxygen amount in solution. These results have been validated by duplicating the measurements of some samples by the modified Winkler method.

V. RESULTS 1. H2O and MEA/H2O Solution. To calibrate the polarographic probe, the measurements are carried out in pure water for several temperature varying from 10 °C to 60 °C. The measurements are also carried out in MEA 100 w = 20 aqueous solution to compare with the literature. The results of this work are shown in Figure 1.

IV. MEASUREMENT METHODS Several methods exist to determine the quantity of dissolved oxygen in the solution (polarographic probe23 and the Winkler methods24 are mostly used). All of them are reported in ASTM D888-09.25 In this paper, we will use a polarographic probe “Inlab 600” from METTLER and the “Winkler modified” method. The polagraphic probe manufacturing by METTLER (Inlab600) has a membrane which is permeable to oxygen. Oxygen concentration is obtained by measuring the generated current between anode and cathode inside the probe which is proportional to the oxygen partial pressure. The oxygen probe is placed in the solution, and a final value is recorded when the signal is stabilized. The Winkler method is a colorimetric titration. It consists of a titration of iodine liberation caused by the presence of divalent manganese by thiosulfate. The mechanism of this method is described below. 2Mn 2 + + O2 + 4HO− → 2MnO2 + 2H 2O

M ([Na 2S2O3]V(Na 2S2O3) − 6[KIO3]V(KIO3)) 4V

Figure 1. Experimental oxygen mass fraction vs temperature (°C) in 100 w = 20 MEA solution compared to the literature at atmospheric pressure: □, oxygen mass fraction for 100 w = 20 MEA from Rooney et al.;20 ×, oxygen mass fraction in distilled water from Tromans et al.;22 Δ, measurement in 100 w = 20 MEA solution; ◊, measurement in H2O.

First, 1 mL of manganese sulfate solution is added to the sample (volume of the sample = 200 mL). Reaction 3 takes place after adding 1 mL of KI/NaOH solution to form the dioxide of manganese. The iodine is formed after adding a strong acid like concentrated sulfuric acid (reaction 4). After the formation of iodine, the solution is quickly titrated by the thiosulfate solution because the iodine is volatile specie. The method is often used on the water where the pH is neutral. Problems appear when we use this method in basic

The dissolved oxygen amount decreases with the temperature. The dissolved oxygen mass fraction begins from 11·10−6 at 10 °C to 5·10−6 at 60 °C. Altogether, a downward trend is noted when temperature increases. The presence of MEA in water does not modify the concentration of dissolved oxygen. Our measurements fit well to the result reported by Rooney (1998) and Tromans (1998) for pure water and an MEA aqueous solution of 100 w = 20. A small difference appears for 1118



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Table 1. Comparison between the Electrochemical (Dissolved Oxygen Probe) and Chemical (Winkler Modified) Methodsa

a

temp

pressure

polarographic probe

Winkler modified method

% deviation

solution

°C

mbar

xP·106

xW·106

100|xP − xW|/xP

H2O MEA 100 w = 30 MEA 100 w = 30 MEA 100 w = 30

18.2 19.5 19.5 18.6

1006 1007 1007 1006

9.00 9.76 9.30 8.76

9.3 9.8 8.3 8.3

3 0 11 6

Standard uncertainties u are u(T) = 0.01 °C, u(P) = 10−4 Pa, u(xP) = 0.01·10−6, u(xW) = 0.1·10−6.

the highest value which may result from uncertainty of measurement. To verify the measure carried out with the probe, the Winkler modified method is applied to several samples. Table 1 shows a comparison between the probe and Winkler modified method. In the ASTM D888-09 method, an average gap between both methods of 15 % is reported. In this work, we observed only 11 % of gap between polarographic and Winkler methods. The measurements carried out with the Winkler modified method confirm the results which are done with the probe. After that, all measurement are carried out with the polarographic probe. To observe the influence of concentration of MEA, various solutions are prepared. The measurements are carried out for the following concentrations of MEA: 100 w = (20, 30, 50 and 80). All data from the measurements are in Table 2. Figure 2 shows these data.

Figure 2. Experimental oxygen mass fraction vs temperature (°C) for different MEA solutions at atmospheric pressure: □, 100 w = 20; ∗, 100 w = 30; +, 100 w = 50; ×, 100 w = 80.

For each concentration, the dissolved oxygen decreases with the temperature. Whatever the concentration of MEA, the data are similar for each temperature. The concentration of MEA does not influence the concentration of oxygen. 2. - MEA/H2O/CO2 Mixture Solution. The absorption of carbon dioxide leads to ionic species (carbamate and protonned MEA). To investigate the oxygen solubility during the capture process, the carbon dioxide absorbed effect should be taken into account. Based on MEA 100 w = 30 solutions in water, several solutions were prepared with different concentrations of CO2 absorbed. For each concentration of CO2 absorbed and temperature, repeatability was assumed by tripling measurements. Figure 3 and Table 3 show the results of those measurements. Whatever the solution, the concentration of dissolved oxygen decreases with the temperature. As we observe before, when no CO2 is absorbed, the concentration of dissolved oxygen is similar to results obtain in pure water. But here, the increase of the loading of the solution, that is, the quantity of CO2 absorbed, leads to a decrease in the concentration of dissolved

Table 2. Dissolved Oxygen in Various Solution of MEA at Different Concentrations at Atmospheric Pressureb MEA concentration [MEA]

CO2 Absorbed [CO2]

loading

temp

dissolved oxygen mass fraction

100 w

mol·L−1

mol·kg−1

mol of CO2/ mol of MEA

°C

x (·106)

20 20 20 20 20 20 30 30 30 30 30 30 50 50 50 50 50 50 80 80 80 80 80 80

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

10 20 30 40 50 60 10 23 32 40 50 60 10 23 32 40 50 59.3 11 24 32.5 42 52 59.1

10.91 9.51 7.87 6.82 5.57 4.66 10.11 9.24 7.96 7.13 5.60 3.65 9.34 8.10 7.13 6.42 5.48 4.83 9.85 8.09 7.01 5.82 4.78 4.68

Figure 3. Experimental oxygen mass fraction vs temperatures (°C) for 100 w = 30 MEA solutions with different concentrations of CO2 absorbed at atmospheric pressure: ◊, α = 0; +, α = 0.1; ○, α = 0.2; □, α = 0.3; ∗, α = 0.4; -, α = 0.5. Values extracted from Table 3.

b

Standard uncertainties u are u([MEA]) = 0.1 %, u([CO2 absorbed]) = 0.1 mol·L−1, u(T) = 0.01 °C, u(x) = 0.2 × 10−6. 1119



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Table 3. Dissolved Oxygen in MEA 100 w = 30 with Various Loadings at Atmospheric Pressurec MEA concentration [MEA]

CO2 absorbed [CO2]

loading

temp

dissolved oxygen mass fraction

°C

x (·106)

10 20 30 40 50 60 10 20 30 40 50 60 10 20 30 40 50 60 10 20 30 40 50 60 10 20 30 40 50 60

10.19 8.71 7.15 4.51 2.28 0.24 8.32 7.71 6.32 2.16 0.61 0.05 8.04 6.86 4.65 2.02 0.34 0.02 7.77 6.64 4.06 2.17 0.44 0.15 3.35 2.56 0.83 0.42 0.14 0.00

100 w

mol·L−1

mol·kg−1

mol of CO2/ mol of MEA

30

0.49

0.47

0.1

30

0.98

0.93

0.2

30

1.47

1.37

0.3

30

1.96

1.78

0.4

30

2.45

2.18

0.5

of ionic strength of the solution when CO2 reacts with MEA. During a classical process of carbon dioxide capture such as the absorption/desorption cycle, variations of temperature and CO2 loading of the solution will lead to a very low solubility of oxygen. This information should be taken into account for further modeling of MEA oxidation for example.

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AUTHOR INFORMATION

Corresponding Author

* E-mail: [email protected]. Funding

This project was supported by Région Haute-Normandie, The French State, Europe ERDF (European Regional Development Fund), and Veolia Environnement. Notes

The authors declare no competing financial interest.

■

REFERENCES

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c Standard uncertainties u are u([MEA]) = 0.1 %, u([CO2 absorbed]) = 0.001 mol·L−1, u(T) = 0.01 °C, u(x) = 0.5 × 10−6.

oxygen. As we explain above, the mechanism of chemical absorption of CO2 in MEA solutions leads to carbamate and protonated MEA species, which increase the ionic strength of the solution. By increasing the ionic strength, the concentration of dissolved oxygen decreases. Several studies have reported this observation. For example Benson et al.26 have carried out research about ocean salinity. They propose a correlation which shows clearly that the effect of salt coupling with temperature leads to a high nonlinear effect.

VI. CONCLUSION Two methods have been used to determine the concentration of dissolved oxygen. One of them (Winkler method) is modified to be adapted to our chemical system. The Winkler modified method confirms the results obtained with the polarographic probe. The measurements of oxygen’s solubility are carried in the MEA/H2O/CO2 solution and complete a preceding work on the MEA/H2O system done by Rooney (1998). Without CO2 in the solution, oxygen solubility is not influenced by MEA concentration, but in the system MEA/H2O/CO2, the oxygen solubility is strongly dependent on temperature and CO2 loading. This influence can be explained by a large increase 1120



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(14) Rooney, P. C.; DuPart, M. S.; Bacon, T. R. Effect of heat stable salts on MDEA solution corrosivity. Hydrocarbon Process. 1997, 65− 71. (15) 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. (16) Bello, A.; Idem, R. O. Pathways for the formation of products of the oxidative degradation of CO2-loaded concentrated aqueous monoethanolamine solutions during CO2 absorption from flue gases. Ind. Eng. Chem. Res. 2005, 44, 945−969. (17) Sexton, A. J.; Rochelle, G. T. Catalysts and inhibitors for MEA oxidation. Energy Procedia 2009, 1179−1185. (18) Chi, S.; Rochelle, G. T. Oxidative degradation of monoethanolamine. Ind. Eng. Chem. Res. 2002, 41, 4178−4186. (19) Supap, T.; Idem, R.; Veawab, A.; Aroonwilas, A.; Tontiwachwuthikul, P.; Chakma, A.; Kybett, B. D. Kinetics of the oxidative degradation of aqueous monoethanolamine in a flue gas treating unit. Ind. Eng. Chem. Res. 2001, 40, 3445−3450. (20) Rooney, P. C.; DuPart, M. S.; Bacon, T. R. Oxygen's role in alkanolamine degradation. Dow Chem. Co. 1998, 77, 109−113. (21) Danckwerts, P. V. The reaction of CO2 with ethanolamines. Chem. Eng. Sci. 1979, 34, 443−446. (22) Tromans, D. Oxygen solubility modeling in inorganic solutions: Concentration, temperature and pressure effects. Hydrometallurgy 1998, 50, 279−296. (23) Linek, V.; Moucha, T.; Kordač, M.; Dubcová, M.; Hovorka, F.; Rejl, J. Liquid film effect on dynamics of optical oxygen probe. Comparison with polarographic oxygen probes. Diffusion coefficients measuring technique. Chem. Eng. Sci. 2009, 64, 4005−4015. (24) Wong, G. T. F.; Li, K.-Y. Winkler's method overestimates dissolved oxygen in seawater: Iodate interference and its oceanographic implications. Mar. Chem. 2009, 115, 86−91. (25) D19 Committee Test Methods for Dissolved Oxygen in Water; ASTM International, 2012. (26) Benson, B.; Krause, D.; Peterson, M. The solubility and isotopic fractionation of gases in dilute aqueous solution. I. Oxygen. J. Solution Chem. 1979, 8, 655−690.

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