Research Note pubs.acs.org/IECR
Prospects of Using Room-Temperature Ionic Liquids as Corrosion Inhibitors in Aqueous Ethanolamine-Based CO2 Capture Solvents M. Hasib-ur-Rahman and F. Larachi* Department of Chemical Engineering, Université Laval, Québec, Canada G1 V 0A6 ABSTRACT: Corrosion is one of the major concerns being encountered in aqueous alkanolamine-based CO2 capture processes. The present work examines the capability of thermally stable and virtually nonvolatile room-temperature ionic liquids (RTILs) to curb corrosion in aqueous monoethanolamine solvents. Four imidazolium-based RTILs with ethyl side chains were chosen for this purpose: [emim][Otf], [emim][DCA], [emim][acetate], and [emim][tosylate]. Carbon steel 1020 has been used as a test material, since it is widely used as construction material in industrial installations. Electrochemical corrosion experiments were carried out using the linear polarization resistance (LPR) technique for measuring corrosion current thus enabling subsequent calculation of corrosion rate via the Tafel fit method. The outcomes illustrate that, out of the tested ionic liquids, [emim][acetate] is the most capable of rectifying the severe operational problem of corrosion in alkanolamine-based state-of-theart CO2 capture systems.
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INTRODUCTION Aqueous alkanolamine-based CO2 capture methodology is a well-established approach and has been in use since the 1930s, especially in natural gas sweetening plants.1 The process involves the reaction between CO2 and alkanolamines (primary/secondary amine) in a 1:2 molar ratio, thus forming carbamate species, which, in turn, are heated to release pure CO2, as shown by the following widely accepted mechanisms (see eqs 1−4):2−4 CO2 capture at lower temperature (∼40 °C): RR′NH + CO2 ↔ RR′NH+COO−
(1)
RR′NH+COO− + B ↔ RR′NCOO− + BH+
(2)
corrosion is one of the most important aspects of the aqueous amine-based chemical absorption systems.7,8,23 For corrosion prevention and control, various corrosion inhibitors such as compounds of arsenic, antimony, copper, vanadium, etc. are being used that not only add to the cost, but also pose risks to humans and the environment.24 Hence, there is an earnest need to search for stable and environmentally friendly alternatives that not only can conform to the demand but also can withstand the processing conditions. Here, we consider the use of RTILs as corrosion inhibitors in aqueous alkanolamine solvents and this approach can also address the concern about the viscous nature of room-temperature ionic liquids. Since room-temperature ionic liquids (RTILs), specifically based on their thermally stable and nonvolatile nature, are largely considered environmentally friendly,25 we have scrutinized their probable ability to suppress corrosion in a carbonated aqueous ethanolamine solvent. In case of CO2 absorption systems that involve aqueous alkanolamines, the major electrochemical corrosion reactions that tend to cause iron dissolution are summarized below (eqs 5−8):7,23
Regeneration at higher temperature (≥120 °C): RR′NCOO− + H 2O ↔ CO2 + RR′NH + OH−
(3)
BH+ + OH− ↔ B + H 2O
(4)
where R denotes an alkyl group or hydrogen and B is a base. However, there are several process hitches, such as equilibrium limitation, amine degradation/evaporation, high regeneration energy requirement, as well as equipment corrosion that require additional measures.5−8 Accordingly, in order to look for a more viable and efficient CO2 capture system, various options are being considered. In this context, thermally stable, generally nonvolatile (and with good CO2 solubility) highly tunable ionic liquids that do (or do not) possess amine functionality is a class of solvents that has attracted considerable attention of the scientific community.9−14 Some researchers are investigating amine-RTIL blends for CO2 capture.15−20 Nevertheless, ionic liquids, carrying an inherent drawback of higher viscosity, are prone to make process handling a difficulty.21,22 Although more efforts are focused on studying the CO2 capture and regeneration capabilities of the solvents, there is an equivalent necessity to probe the accompanying developments that also severely affect the process operations; in this regard, © 2013 American Chemical Society
RNHCOO− + H 2O → RNH 2 + HCO3−
(5)
2HCO3− + 2e− → 2CO3− + H 2
(6)
2H 2O + 2e− → 2OH− + H 2
(7)
Fe → Fe2 + + 2e−
(8)
In this respect, this work was intended to probe the viability of hydrophilic RTILs to diminish the occurrence of the abovestated reactions in CO2-loaded aqueous ethanolamine solvents, also taking into consideration the influence of anion type of the Received: Revised: Accepted: Published: 17682
June 8, 2013 October 29, 2013 November 22, 2013 November 22, 2013 dx.doi.org/10.1021/ie401816w | Ind. Eng. Chem. Res. 2013, 52, 17682−17685
Industrial & Engineering Chemistry Research
Research Note
100 cm3 volume corrosion cell. The mass flow meters were used to control the gas supply, while the temperature of the test fluid was maintained using an oil bath. A water-cooled condenser was installed at the outlet of the gas, to minimize evaporation of the solvent. Procedure. Prior to each electrochemical run, CO2 gas was bubbled at a flow rate of 100 mL/min through the aqueous solution of MEA (30% w/w) and RTIL (10% or 30% w/w) to reach the required gas loading (0.35 mol of CO2 per mole of amine). A Chittick apparatus was used to quantify the CO2 loading in the solvent according to the procedure as stated in our previous work.18 The working electrode surface was first polished by wet grinding with 600 grit SiC paper and then degreased with acetone, followed by rinsing with deionized water. After drying, the specimen electrode was mounted in the Teflon cap. Subsequently, the three electrodes (reference, counter, and working electrodes) were immersed in the test solution in order to establish a steady-state open-circuit potential. After reaching the specified conditions of temperature and gas loading under ambient pressure, the electrochemical polarization run was commenced. Potentiodynamic linear polarization was initiated at a scan rate of 0.16 mV/s in the anodic direction between the potential limits of ±250 mV (versus open-circuit potential). A rotational speed of 500 rpm was maintained for the working electrode during the electrochemical experimentation. To ensure data reproducibility, each experiment was repeated at least once. The Tafel extrapolation method was used to determine the corrosion current (icorr), which was converted to the corrosion rate of the carbon steel specimen by the following equation:26
RTILs. For this purpose, ionic liquids with a common 1-ethyl3-methylimidazolium cation and four different anions (trifluoromethanesulfonate, dicyanamide, acetate, tosylate) were chosen. The RTILs were chosen as these are thermally stable and unlikely to undergo evaporation loss during the regeneration step. Since, by and large, imidazolium-based RTILs are stable up to ∼200 °C, and if proven to be effective in controlling corrosion, these can be reused again and again without the risk of any negative impact on the environment.
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EXPERIMENTAL SECTION Materials. Hydrophillic room-temperature ionic liquids (1ethyl-3-methylimidazolium trifluoromethanesulfonate [emim][Otf], 1-ethyl-3-methylimidazolium dicyanamide [emim][DCA], 1-ethyl-3-methylimidazolium acetate [emim][acetate], and 1-ethyl-3-methylimidazolium tosylate [emim][tosylate]) were purchased from IoLiTec, Inc. Monoethanolamine (MEA) was obtained from Sigma−Aldrich Canada, Ltd., whereas Praxair Canada, Inc., provided carbon dioxide and nitrogen gases (≥99.8% purity). All the chemicals were used without further purification. Electrochemical Corrosion Tests. Equipment. A BioLogic VSP potentiostat was employed to determine the relationship between applied electrochemical potential and current generated, using linear polarization resistance (LPR) technique, and the resulting data was used to calculate the corrosion rate by applying the Tafel extrapolation method. An experimental setup consisting of a three-electrode configurationa platinum counter-electrode, a (Ag/AgCl/sat. KCl) silver/silver chloride reference electrode, and a working electrode (rotating disk electrode)was used to perform the electrochemical tests (Figure 1).17,18 A disk-shaped working electrode made of carbon steel 1020 was selected to study the corrosion phenomenon. [Note: Carbon steel 1020 has the following chemical composition: C, 0.20%; Mn, 0.50%; P, 0.04%; S, 0.05%; and Fe, balance.] The working electrode, which had a surface area of 0.196 cm2, was mounted in a Teflon cap. For each experiment, 80 cm3 of test liquid was placed in a
⎛i W ⎞ CR = 1.29 × 105⎜ corr ⎟ ⎝ ρA ⎠
where CR is the corrosion rate (in units of milli-inches per year, mpy), icorr the corrosion current (in amperes, A), W the equivalent weight of the metal specimen (in gram per equivalent, g/equiv), ρ the density of metal (in units of g/ cm3); and A the area (in contact with the experimental fluid) of the rotating disk working electrode (given in units of cm2).
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RESULTS AND DISCUSSION Corrosion Behavior of Carbonated Aqueous Monoethanolamine Solvent. Linear polarization behavior of carbon steel 1020 in 30% (w/w) aqueous solution of MEA, containing 0.35 mol of CO2 per mole of amine at 25 °C, has been shown in Figure 2 while the corresponding corrosion parameters have been provided in Table 1. The outcomes show that, under the tested conditions, the amine solvent is reasonably corrosive as the carbon steel specimen immersed in the solvent appears to be in an active state and the iron dissolution occurs at the rate of ∼19 mpy. This was also evident from the appearance, as the clean specimen (working electrode) surface became dull and covered by a grayish brown material after the test. The most significant electrochemical reactions in this regard comprise the reduction of bicarbonate ions/nondissociated water and the oxidation of surface iron (see eqs 6−8) at the metal/solution interface.23,27 Corrosion Behavior in the Presence of Ionic Liquids. Figures 2a−d illustrate the influence of the ionic liquids (at two different concentrations: 10% w/w and 30% w/w) on the polarization behavior of carbon steel. Table 1 illustrates the
Figure 1. Schematic of the electrochemical corrosion setup. 17683
dx.doi.org/10.1021/ie401816w | Ind. Eng. Chem. Res. 2013, 52, 17682−17685
Industrial & Engineering Chemistry Research
Research Note
Figure 2. Linear polarization curves of carbon steel 1020 at 25 °C in (a) aqueous ethanolamine (30% w/w) + [emim][Otf]; (b) aqueous ethanolamine (30% w/w) + [emim][DCA]; (c) aqueous ethanolamine (30% w/w) + [emim][acetate]; and (d) aqueous ethanolamine (30% w/w) + [emim][tosylate]. The red curve presents the linear polarization behavior of carbon steel 1020 at 25 °C in aqueous ethanolamine (30% w/w).
Although the tested ionic liquids exhibited quite promising results, they do not appear efficient enough to check the corrosion phenomenon wholly. The RTILs demonstrated a wide range of inhibition performances, varying from 44% (in case of [emim][Otf]) to 88% (in case of [emim][acetate]), when used in higher concentration (i.e., 30 wt % RTIL concentration). However, 10 wt % [emim][DCA] did not show any improvement regarding corrosion inhibition. As is clear from Table 1, the shift in corrosion potentials is not positively inclined and this behavior eludes any good possibility of the ionic liquid adsorption onto the metal surface that would have acted as a barrier against the active agents (oxidants) to reach the cathodic sites.24 Moreover, since the hydrophilic RTILs have been used, aqueous media would not have allowed the RTIL moieties to cling onto the metal surface. The outcomes suggest that the main driving force regarding the decrease in the corrosion rate upon the addition of RTILs can possibly be the increase in the solution viscosity (Table 2), which essentially causes the diffusion of the oxidizing species to slow within the fluid, thus retarding their movement toward the working electrode surface. The trend of increase in solvent viscosity (with RTIL from [emim][DCA] to [emim][acetate]) and decrease in the corresponding corrosion rate also complements this assumption. The RTIL anion type does not reveal any coherent trend. Apparently, RTIL with acetate ion seems more effective in curbing corrosion but, as has been stated earlier, this can be the impact of higher viscosity rather than the role of acetate ion itself.
Table 1. Corrosion Rates of Carbon Steel Immersed in CO2Loaded Aqueous Solutions Containing 30% (w/w) MEA and RTIL RTIL concentration (% w/w)
corrosion potential (mV)
corrosion current (μA)
corrosion rate (mpy)
0
−774.31
8.24
19.23
MEA-[emim] [Otf] MEA-[emim] [Otf]
10
−824.74
4.81
11.24
30
−823.77
4.63
10.80
MEA-[emim] [DCA] MEA-[emim] [DCA]
10
−740.60
8.50
19.85
30
−748.80
4.60
10.74
MEA-[emim] [acetate] MEA-[emim] [acetate]
10
−824.56
3.22
7.51
30
−870.10
0.98
2.29
MEA-[emim] [tosylate] MEA-[emim] [tosylate]
10
−791.67
5.92
30
−853.45
3.47
solvent (aqueous) MEA
13.8 8.11
variation of the corrosion rates, depending on the type/ concentration of the RTIL in the CO2-loaded MEA−RTIL aqueous solutions. 17684
dx.doi.org/10.1021/ie401816w | Ind. Eng. Chem. Res. 2013, 52, 17682−17685
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(10) Hasib-ur-Rahman, M.; Siaj, M.; Larachi, F. Ionic liquids for CO2 captureDevelopment and progress. Chem. Eng. Process. Process Intensification 2010, 49, 313−322. (11) Anthony, J.; Anderson, J.; Maginn, E.; Brennecke, J. Anion Effects on Gas Solubility in Ionic Liquids. J. Phys. Chem. B 2005, 109, 6366−6374. (12) Anderson, J.; Dixon, J.; Brennecke, J. Solubility of CO2, CH4, C2H6, C2H4, O2, and N2 in 1-Hexyl-3-methylpyridinium Bis(trifluoromethylsulfonyl)imide: Comparison to Other Ionic Liquids. Acc. Chem. Res. 2007, 40, 1208−1216. (13) Bara, J.; Carlisle, T.; Gabriel, C.; Camper, D.; Finotello, A.; Gin, D.; Noble, R. Guide to CO2 Separations in Imidazolium-Based RoomTemperature Ionic Liquids. Ind. Eng. Chem. Res. 2009, 48, 2739−2751. (14) Yokozeki, A.; Shiflett, M. Separation of Carbon Dioxide and Sulfur Dioxide Gases Using Room-Temperature Ionic Liquid [hmim][Tf2N]. Energy Fuels 2009, 23, 4701−4708. (15) Camper, D.; Bara, J. E.; Gin, D. L.; Noble, R. D. Roomtemperature ionic liquid-amine solutions: Tunable solvents for efficient and reversible capture of CO2. Ind. Eng. Chem. Res. 2008, 47, 8496−8498. (16) Bara, J.; Camper, D.; Gin, D.; Noble, R. Room-Temperature Ionic Liquids and Composite Materials: Platform Technologies for CO2 Capture. Acc. Chem. Res. 2010, 43, 152−159. (17) Hasib-ur-Rahman, M.; Siaj, M.; Larachi, F. CO2 Capture in Alkanolamine/Room-Temperature Ionic Liquid Emulsions: A Viable Approach with Carbamate Crystallization and Curbed Corrosion Behavior. Int. J. Greenhouse Gas Control 2012, 6, 246−252. (18) Hasib-ur-Rahman, M.; Bouteldja, H.; Fongarland, P.; Siaj, M.; Larachi, F. Corrosion behavior of carbon steel in alkanolamine/roomtemperature ionic liquid based CO2 capture systems. Ind. Eng. Chem. Res. 2012, 51, 8711−8718. (19) Hasib-ur-Rahman, M.; Larachi, F. CO2 Capture in Alkanolamine−RTIL Blends via Carbamate Crystallization: Route to Efficient Regeneration. Environ. Sci. Technol. 2012, 46, 11443−11450. (20) Huang, Q.; Li, Y.; Jin, X.; Zhao, D.; Chen, G. Z. Chloride ion enhanced thermal stability of carbon dioxide captured by monoethanolamine in hydroxyl imidazolium based ionic liquids. Energy Environ. Sci. 2011, 4, 2125−2133. (21) Liu, Z.; Wu, W.; Han, B.; Dong, Z.; Zhao, G.; Wang, J.; Jiang, T.; Yang, G. Study on the Phase Behaviors, Viscosities, and Thermodynamic Properties of CO2/[C4mim][PF6]/Methanol System at Elevated Pressures. Chem.Eur. J. 2003, 9, 3897−3903. (22) Gutowski, K. E.; Maginn, E. J. Amine-functionalized task-specific ionic liquids: A mechanistic explanation for the dramatic increase in viscosity upon complexation with CO2 from molecular simulation. J. Am. Chem. Soc. 2008, 130, 14690−14704. (23) Veawab, A.; Aroonwilas, A. Identification of oxidizing agents in aqueous amine−CO2 systems using a mechanistic corrosion model. Corros. Sci. 2002, 44, 967−987. (24) Veawab, A.; Tontiwachwuthikul, P. Investigation of Low-Toxic Organic Corrosion Inhibitors for CO2 Separation Process Using Aqueous MEA Solvent. Ind. Eng. Chem. Res. 2001, 40, 4771−4777. (25) Rogers, R. D.; Seddon, K. R. Ionic LiquidsSolvents of the Future? Science 2003, 302, 792−793. (26) Thompson, N. G.; Payer, J. H. DC Electrochemical Test Methods; NACE International: Houston, TX, 1998. (27) Tanthapanichakoon, W.; Veawab, A.; McGarvey, B. Electrochemical Investigation on the Effect of Heat-stable Salts on Corrosion in CO2 Capture Plants Using Aqueous Solution of MEA. Ind. Eng. Chem. Res. 2006, 45, 2586−2593.
Table 2. Viscosities (η) of Pure RTILs and Their Respective Mixtures with Aqueous MEA, Measured at 25 °C material [emim][Otf] [emim][DCA] [emim][acetate] [emim][tosylate] aqueous MEA (30% aqueous MEA (30% aqueous MEA (30% aqueous MEA (30%
w/w) w/w) w/w) w/w)
+ + + +
[emim][Otf] (30% w/w) [emim][DCA] (30% w/w) [emim][acetate] (30% w/w) [emim][tosylate] (30% w/w)
η (cP) 43.1 14.4 106.5 solid 8.5 7.2 18.3 12.3
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CONCLUSION This work reveals that the corrosion inhibition performance of the room-temperature ionic liquids (RTILs) varies with concentration. Out of the tested ionic liquids, [emim][acetate] appears quite effective and can reduce corrosion as much as 88%. These RTILs, because of their higher viscosity, seem to check corrosion phenomenon preferably by retarding the movement of the oxidants toward the metal surface. However, a more systematic scrutiny is required by considering the ionic liquids that have a better affinity for carbon steel. Besides, there is a need to evaluate, in detail, the hydrolytic decay, if there is any, of these species under the gas capture and regeneration conditions.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: (418) 656-2131, Ext. 3566. Fax: (418) 656-5993. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from FL Canada Research Chair “Green Processes for Cleaner and Sustainable Energy” and the Discovery Grant to F.L. from the Natural Sciences and Engineering Research Council (NSERC).
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REFERENCES
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