Corrosion Behavior of Carbon Steel in CO2 Saturated Amine and

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Corrosion Behavior of Carbon Steel in CO2 Saturated Amine and Imidazolium-, Ammonium- and Phosphonium- Based Ionic Liquid Solutions Aida Rafat, Mert Atilhan, and Ramazan Kahraman Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b01794 • Publication Date (Web): 24 Dec 2015 Downloaded from http://pubs.acs.org on December 29, 2015

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Industrial & Engineering Chemistry Research

Corrosion Behavior of Carbon Steel in CO2 Saturated Amine and Imidazolium-, Ammonium- and Phosphonium- Based Ionic Liquid Solutions Aida Rafat1, Mert Atilhan*1 and Ramazan Kahraman*1 1 Department of Chemical Engineering, Qatar University, Doha, Qatar *Corresponding Authors: [email protected] and [email protected]

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Abstract. This work presents a recent investigation on corrosion behavior of carbon steel amine and ionic liquid based carbon dioxide absorbents. The first class focused on classical amine solutions; monoethnaolamine, diethanolamine, and methyldiethanolamine. Second class included activated amine blends using piperazine amine promoter. The third class included novel aqueous mixtures of alkanolamine/hydrophilic roomtemperature ionic liquids, namely; [BMIM][BF4], [BMIM][Otf], [P4441][Acetate], and [Choline][Acetate]. Electrochemical corrosion experiments were conducted using polarization techniques to determine corrosion rate of steel probing the effect of process temperature and CO2 loading. The findings of the investigation show that corrosivity of classical amines is governed by their characteristic CO2 absorption capacity whereas PZ-activated amines resulted in lower corrosion rates and higher CO2 absorption. The partial replacement of aqueous phase in MEA solution by RTILs showed to be effective in reducing steel corrosion rates with phosphonium and ammonium-based RTILs showing to be more effective than imidazolium-based RTILs. Key Words: Corrosion, Ionic Liquids, Amine Solutions, Piperazine, CO2 Capture

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1. Introduction The overwhelming scientific consensus about global warming caused by the anthropogenic carbon dioxide (CO2) emissions has driven the attention of scientists to explore technologies and materials to mitigate these emissions.1 Amine-based absorption process has been utilized in process industries since 1950’s due to its predictable chemical nature and its high capacity for CO2 capture. And most of the existing process units that are used for CO2 capture depends on the technology based on the amine solutions used mostly on post-combustion CO2 capture applications.2 However, there are several challenges of amine based CO2 capture process flue gas, such as chemical degradation, equilibrium limitations, high regeneration energy penalty, low gas loadings and corrosion of the pipes and the process equipment.3,4 In fact, corrosion plant experiences in gas sweetening facilities have been well documented in literature since the 1960’s.5 These experiences showed that corrosion occurs in the form of general corrosion as well as localized corrosion attacks at carbon steel infrastructure including valves, pumps and re-boiler bundles. In light of CO2 capture pilot plant experiences and corrosion studies, classical CO2-saturated amines solutions exhibit aggressive corrosion behavior with corrosion rates reaching up to several millimeters per year.6-8 Accordingly, it has been established that following factors are the main reasons for the the corrosion of steel in CO2 capture system: acid gas loading, process temperature, amine type and concentration, turbulence, process contaminates and amine degradation products. The electrochemical reactions (eq. 1-5) show that there are several oxidizing agents available that are responsible for iron dissolution, mainly the bicarbonate HCO , water H O , hydronium ion H O as well as protonated amines.9 Iron dissolution (Anodic reaction) ↔ + 2

(1)

Reduction of hydronium ion (Cathodic reactions) 2 + 2 ↔ + ()

(2)

Reduction of bicarbonate ion 2 + 2 ↔ 2 + ()

(3)

Reduction of un-dissociated water 2 + 2 ↔ 2 + ()

(4)

Reduction of protonated amine

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2 + 2 → 2 + ()

(5)

The corrosive nature of CO2-saturated amines along with their several drawbacks such as their high thermal regeneration cost, volatility and degradability were the main motivation behind the research and development of other CO2 absorbent alternatives.10,11 One of the proposed alternatives for amine solvent improvement considers the use of piperazine (PZ) activated amine and advanced formulated amines that consist of blend of different amines. These absorbents are expected to combine the merits of each amine and they are characterized with superior absorption capacity and kinetics due to the addition of piperazine activator.12,13 However, studies on corrosivity of piperazine activated amines are scarce and have been subject to investigation only recently.14 Another remarkable development in this area is the use of hydrophilic room temperature ionic liquids (RTILs) to partially replace the aqueous phase in amine solutions has gained a recent attention as a promising candidate CO2 absorbent. 10,15 The hybrid blend intends to merge the high CO2 solubility feature possessed by amines along with the eco-friendly behavior of RTILs due to their high non-volatile nature and thermal stability.16-20 Current research on RTILs for carbon capture purpose had focused mainly on imidazolium-based RTILs with fluorinated anions to enhance the physical CO2 absorption.20,21 Conversely, other RTILs classes such as phosphoniumbased or ammonium-based had received greater attention in electrochemistry application because of their high thermal and chemical stability.20 Not until recently, research groups started to investigate the use of those RTILs as new potential CO2 absorbents.23,24 For overall corrosion evaluation, valid comparisons of reported CO2 absorbents’ corrosivity are difficult to be made since researchers use different corrosion measurement techniques, different equipment models and different test environments. For this purpose, this work presents an evaluation on corrosion behavior of carbon steel CS1018 in the three CO2 absorbent systems; the industrially mature classical amines, the advanced PZ-activated amines and novel aqueous blends of MEA amine and imidazolium, phosphonium and ammonium-based RTILs.

2. Experimental 2.1 Materials. Monoethanolamine (MEA), diethanolamine (DEA), methyl diethanolamine (MDEA) and piperazine (PZ) were provided by Sigma-Aldrich (≥ 99% purity).The total amine concentration was kept constant as 30 wt% in all experiments. PZ concentration was fixed to 10 wt% in all activated amine solutions. For MEA/MDEA blend, the two amines were equally mixed. Imidazolium-based 4


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RTILs of 1-butyl-3methylimidazolium teteraflourborate [BMIM][BF4], 1-butyl 3methylimidazolium trifluoromethanesulfonate [BMIM][Otf] were provided from Sigma-Aldrich (≥ 99% purity). Choline acetate [Choline][Acetate] and tributyl(methyl)phosphonium acetate [P4441][Acetate] were purchased from Iolitec (≥ 99% purity). All RTILs were chosen properly such that they are hydrophilic and miscible in both MEA amine and water. Aqueous MEA/RTIL blends were prepared by mixing the desired weight of amine and desired weight of RTIL and then dilute it with deionized water. The blends were homogenized by keeping continuous stirring for 24 hours. Viscosities of aqueous MEA and MEA/RTIL solutions were measured using rolling ball microviscometer (Anton Paar Lovis 2000 ME). CO2 saturated solutions were prepared by bubbling CO2 (≥ 99.8% purity) into the solution at the test temperature to reach saturation. Saturation period has been pre-determined as 4 to 6 hours and it has also been monitored by the change in pH with time. CO2 loading, expressed as moles of CO2 absorbed per mole of amine, was determined by Chittick apparatus by adding excess known amount of standard HCl (1 M) to 5 mL sample solution.25 2.2 Experimental Setup. Electrochemical experiments were conducted in 100 cm3 jacketed micro cell supplied by Autolab. Experimental apparatus schematic is given in Figure 1. The microcell was equipped with a potentiostat (Autolab PGSTAT101) and data acquisition system (NOVEA1.7). The corrosion cell consisted of Ag/AgCl reference electrode (RE) filled with 3M KCl solution, working electrode (WE) and two 316 stainless steel counter electrodes (CE). Heated water circulator was connected to the outer cell jacket throughout experiments for temperature control. Water-cooled condenser was utilized to minimize vaporization losses of the test solution and thus avoiding any change in solution’s concentration. Gas supply streams and CO2 gas flow meter was connected to the cell. Corrosion experiments were conducted on circular specimens of 1018 carbon steel (0.186 wt% C, 0.214 wt% Si, 0.418 wt% Mn, 0.029 wt% P, 0.019 wt% S, balance Fe) with 1.93 cm2 surface area of specimen exposed to the medium. The specimen was prepared by wet grinding and polishing using 320, 600 and 1200 grit SiC papers. The specimen was then degreased by high purity acetone followed by rinsing with deionized water and drying with hot air. 2.3 Experimental Procedure. The sealed cell was first de-aerated by purging N2 gas through the gas phase of the cell to strip out oxygen presents above the solution prior to each experiment. Experimental temperature was controlled by an external circulator and isotherms were selected as 40 °C and 80 °C to mimic the process conditions that are used in industrial scale amine absorption process as they represent moderate and elevated temperatures in amine process. Once temperature reaches ± 0.1°C of the test temperature, CO2 stream is purged throughout the solution to reach saturation. After

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reaching the test environment, three solution samples were taken for CO2 loading analysis. The prepared specimen was then immediately immersed into the test solution to establish the open circuit potential (OCP) measurements against the reference electrode to reach steady state conditions (defined as ± 0.01 mV between successive voltage readings). The polarization curve was generated directly by the data acquisition system in the range of ± 250 mV vs OCP and a scan rate of 0.001 V/sec. Tafel extrapolation method was used to determine corrosion current (icorr) which was converted to corrosion rate by the following equation: CR =

. ! " #$%&& " '( )

(6)

Where, CR is corrosion rate in (mm/yr) , icorr is corrosion current in (A/cm2) , EW is the equivalent weight of the carbon steel specimen (g/equivalent) and D is the density of the specimen (g/cm3). 3. Results and Discussion. 3.1 Single amines. Experimental and electrochemical parameters for single amine systems are presented in Table 1. At 40 °C and zero CO2 loading, MEA, DEA and MDEA were virtually non-corrosive with corrosion rates of < 0.1 mm/yr with corrosivity order of MEA > DEA > MDEA (Figure 2-a). Aqueous MDEA solution had also resulted in a more positive steel potential of -0.49 V compared to MEA and DEA solutions which might indicate the spontaneous passivation of carbon steel specimen. In contrast, MEA and DEA solutions showed more active corrosion potentials of -0.95 V and -0.90 V, respectively indicating more active metal dissolution. At CO2 saturation, corrosion rates of steel increased dramatically in the three amine solutions with a corrosivity order of MDEA > MEA > DEA (Table 1). The higher corrosivity of MDEA solution compared to MEA and DEA is attributed mainly to its high CO2 absorption capacity at 40 °C temperature (i.e. absorption condition). MEA and DEA solutions have the same reaction chemistry with CO2 that leads to the formation of carbamate product.25 The carbamate formation reaction restricts the primary and secondary amines to maximum CO2 loading of 0.50molCO2/molamine as per the reactions: For primary amines For secondary amines

: 2 + → + : 2 + → +

(7) (8)

Where and are the protonated primary and secondary amines and and are their representative carbamate products respectively.

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On the other hand, the reaction mechanism of MDEA and CO2 is governed by a base catalyzed hydration of CO2. Since there is no hydrogen atom attached to the amine group in MDEA, direct reaction between MDEA and CO2 to form carbamate is not possible.26 Accordingly, reaction between MDEA and CO2 is not restricted by equilibrium reaction of carbamate formation and CO2 loading in MDEA solution can ideally reach to 1 molCO2/molMDEA as per the following reaction + + → +

(9)

The high CO2 absorption capacity in MDEA saturated solution resulted in higher concentrations of corroding agents (i.e. bicarbonate and protonated amine) and thus increased the steel corrosion rates. Corrosivity of CO2 saturated amine solutions at 40 °C was not reported earlier in literature. Furthermore, the obtained ranking is in different with the common ranking that primary amines are more corrosive than secondary amines, which in turns are more corrosive than tertiary amines. The corrosivity ranking of CO2 saturated amines at 40 °C highlights the significant impact of the amine’s CO2 absorption capacity such that tertiary amines can be more corrosive than primary and secondary amines since they absorb more CO2 at low temperatures. Increasing the test temperature from 40 to 80 °C has increased corrosion current densities in the three amine solutions due to the thermal activation of electrochemical reactions (Figure 2-b). At CO2 saturation, the corrosivity ranking changed to MEA > DEA > MDEA with corrosion rates approaching several millimeters per year (Table 1). This ranking for amine corrosiveness is in agreement with trend established by Kohl and Nielsen (1997), and Veawab (1999) with the same order of magnitude as compared with this work herein.5,27 Moreover, similar recent studies centered around the corrosion behavior investigation of the RTILs also studied MEA as a benchmarking fluid and the studies conducted by Hasib-ur-Rahman et al. (2013), Xenophon et al. (2014) and Acidi et al. (2014) showed MEA corrosion rate in the same order of magnitude of the values presented in this work.28,29,30 At 80 °C, MEA and DEA solutions were still rich with CO2 and the solutions’ CO2 loadings decreased by 14 % and 16 % respectively relative to their loadings at 40 °C. On the other hand, the bulk CO2 was stripped out of MDEA solution with CO2 loading reduction of 77 % when increasing the temperature to 80 °C (i.e. amine regeneration condition). In fact, one of the well-known key characteristics of MDEA amine is that its low heat of reaction with CO2 and ease of regeneration at relatively low temperatures compared to MEA and DEA.31 From the two set of experiments at 40 °C and 80 °C isotherms, it can be concluded that corrosivity ranking of CO2 saturated

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amines follows their CO2 absorption capacity ranking such that higher CO2 loading leads to higher corrosivity (Figure 3). 3.2 Activated Amines. Corrosion behavior of piperazine-activated amines was investigated at the highest test temperature (i.e. 80 °C) since corrosion is deemed to be more severe in the hottest parts of the process. Table 2 summarizes the experimental and electrochemical parameters for activated amines. Corrosion rate in PZ solution at zero CO2 loading was found to be 0.04 mm/yr. Accordingly, the corrosivity ranking of single amines at 80 °C: MEA > DEA > PZ > MDEA. The reason behind the high CO2 loading capacity of PZ solution of 0.34 molCO2/molPZ despite the low amine concentration of 10 wt% is largely attributed to the dual amino functional group in PZ structure. The two reactive amino groups in PZ undergo multiple reactions with CO2 such that each mole of PZ reacts with two moles of CO2 forming PZ-carbmate and PZdicarbamate products as per the following reactions32: *+ + + ↔ *+ + *+ + + ↔ *+( ) +

(10) (11)

The soft corrosion behavior of PZ solution might be also largely attributed to the dual amino groups featured in PZ structure. The amino functional group has a hetero nitrogen atom with lone electron pair that favors the adsorption into the metal surface by donating the lone electron creating metal-nitrogen bonding. In fact, the electronegativity from the nitrogen lone electron pair makes amine compounds in general good corrosion inhibitors that block the active metallic sites.33,34 The extent of adsorption and the type of adsorption is different from one amine to another depending on the number of alkyl substituents and the number of hetero atoms. Since PZ is a hetero-cyclic amine with two nitrogen atoms and two lone pairs, PZ has higher tendency to be adsorbed into the metal surface.33 For MEA/PZ solution, steel corrosion rate was 44% lower than in 30 wt % MEA solution at zero CO2 loading. At saturation condition, CO2 loading in MEA/PZ blend was 22 % higher than the loading in 30 wt % MEA solution while corrosion rate was lower by 46 % (Figure 4-b). This further highlights the role of PZ in inhibiting corrosion current while still enhancing CO2 absorption capacity. MDEA/PZ solution’s corrosivity seemed to be averaged of MDEA and PZ individual corrosivities at zero CO2 loading (Figure 4-c). At CO2 saturation, MDEA/PZ solution had slightly higher corrosivity compared to MDEA and PZ solutions. The reason behind this increase could be due to the increase of CO2 loading in MDEA/PZ blend compared to the low loading of MDEA solution at 80 °C (Table 2). However, the soft nature of PZ is still highlighted when considering the significant increase of CO2 loading in MDEA/PZ

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solution relative to MDEA solution (i.e. 59 %) compared to the minor corrosion rate increase (i.e. 4 %). For MEA/MDEA blend, anodic and cathodic branches of the polarization curve were similar to the polarization curve of MEA solution. This indicates that corrosion mechanism in MEA/MDEA blend is more similar to the corrosion mechanism in MEA solution in both conditions, zero CO2 loading and CO2 saturation (Figure 4-d). At CO2 saturation, the blend’s corrosion rate was in between corrosion rates in 30 wt% MEA and 30 wt% MDEA. Comparing the performance of MEA/PZ and MEA/MDEA blends, it can be indicated that even though MEA/PZ had higher concentration of MEA (i.e. 20 wt%) compared to its concentration in MEA/MDEA (i.e. 15 wt%), MEA/PZ achieved lower corrosivity and higher CO2 loading (Table 2). For the ternary blend of MEA/MDEA/PZ solution, corrosion potential of steel was more positive compared to that in MEA/MDEA solution at zero CO2 loading (Figure 4-e). This indicates that MEA has less dominant impact at such lower concentration (i.e. 10 %wt). This is also evident by the corrosion rate of steel in the mixture that has corresponded to 0.07 mm/yr, which is closer to corrosion rate in PZ solution (0.04 mm/yr) and 56% lower than corrosion rate in MEA/MDEA (0.16 mm/yr). At CO2 saturation, the blend had 31% higher CO2 loading compared to MEA/MDEA loading while steel corrosion rate was 38% lower. The soft behavior of the ternary blend can be attributed to two main reasons. First is the low concentration of the most corrosive amine; MEA that is 10 wt%. Second is attributed to the presence of PZ in the mixture. It seems that adding PZ to the amine mixture had the same effect in enhancing CO2 loading and reducing corrosion rate just as it reduced corrosion rate of MEA/PZ (1.83 mm/yr) compared to MEA solution (3.41 mm/yr). Therefore, the ternary mixture offers high CO2 absorption capacity which would be ever higher at the absorber condition (i.e. 40°C), yet at the same time it exhibits low corrosion rate at the most aggressive test conditions (i.e. CO2 saturation and 80°C) that is 46% lower than corrosion rate in 30 wt% MEA. Figure 5 shows the comparative performance of piperazine activated amines in terms of solutions’ CO2 loading and steel corrosion rate. 3.3 MEA/RTIL Blends. Corrosion rates in aqueous MEA/[BMIM][BF4] and MEA/[BMIM][Otf] blends were higher than corrosion rate in MEA solution at 40°C and zero CO2 loading condition (Figure 6-a & Table 3). This increase in corrosion rate is attributed to the increase in the ionic conductivity of the solution due to the addition of RTILs which are electrolytes in nature.35,36 However, although the ionic conductivity increased significantly in MEA/RTILs solutions, the increase in corrosion rates not as significant. This is might be attributed to the viscous nature of RTILs

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which would retarding the diffusion of the oxidizing agents toward the metal surface and thereby nullify corrosion. Viscosity measurements showed that MEA/RTILs solutions were always almost twice viscous than MEA solution at the investigated temperature range (Table 4). At 80 ºC and zero CO2 loading, polarization curves of MEA/RTILs were unstable, especially with [BMIM][BF4] (Figure 6-b). The reason behind this instability might be due the hydrolysis of BF4 anion at such high temperature.37 At CO2 saturation condition, the polarization curves showed that MEA solution is more corrosive than both MEA/RTILs blends at the two test temperatures (Figure 6-a & 6-b) while CO2 loadings for both MEA/RTILs were higher than CO2 loadings in 30 wt % MEA solution (Table 3). Such results were expected since the mixed solution is comprised of 30 wt% RTIL which also contributes in CO2 absorption by physical absorption. Corrosion rate reduction in MEA/RTILs solutions is largely attributed to the reduced aqueous phase in solutions, which is the bulk provider of oxidizing agents (e.g. protons, protonated amines, undissociated H2O) and the chief responsible for corrosion occurrence in amine solutions since most of the cathodic reactions take place in the aqueous phase.38 Therefore, diminishing the aqueous phase and replacing it by the electrochemically stable RTILs have a major contribution in suppressing corrosion. Corrosion rate reduction in MEA/RTILs solutions can be due to the crystallization of carbamate, the major MEA-CO2 reaction product. By replacing the continuous aqueous phase with RTIL, the water-soluble carbamate product will crystalize out as solid moieties. By this, the solid carbamate can no longer participate indirectly in corrosion process or stimulate corrosion by acting as a heat stable salt.5 The appearance of fine solid particles that increased the turbidity in MEA/RTILs solutions was observed during CO2 saturation which might be attributed to carbamate crystallization. Similar behavior of carbamate crystallization has been reported in amine/RTILs mixtures studies.15,38 Since both RTILs have the same imidazolium-based cationic part, the difference in the solutions’ corrosive behavior is attributed to the anionic counter-part. At 40 °C, both MEA/RTILs solutions had similar CO2 loadings, yet MEA/[BMIM][BF4] solution exhibited a better performance compared to MEA/[BMIM][Otf]. At 80 °C, the behavior of MEA/[BMIM][BF4] changed such that it had lower CO2 loading and resulted in higher steel corrosion rate compared to MEA/[BMIM][Otf] (Figure 6-b). This could be interpreted due to the potential steric hindering of shielding effects at elevated temperatures on the studied MEA/RTIL system, which leads to lowering of anticorrosion inhibition behavior performance.37 Moreover, as discussed earlier by Hasib-ur-Rahman28, the main driving force regarding the reduction in the corrosion

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rate with the introduction of RTILs to the amine solutions can possibly be linked with the increase in the solution viscosity. The increase in the viscosity causes diffusion limitations for the oxidizing species in the solution, and thus reducing their movement toward the working electrode surface. Yet, since viscosity and diffusion coefficients are strong function of temperature and pressure, the depending on the operation condition, the hindrance effect of the oxidizing agents are alternated accordingly. Moreover, according to recent work of Atilhan et al.38 and Alcalde et al.39 on the viscosity of the RTILs, depending on the choice of cation and anion, viscosity is effected and thus it can be extended to the effect on the corrosion rate. From Figure 7 it is indicated that the tested aqueous mixtures of MEA and imidazolium-based RTILs show promising results by nullifying corrosion rates while increasing CO2 absorption capacity. However, the RTIL anionic group does not show a co-horrent inhibition trend with temperature. Corrosivity of phosphonium-based and ammonium-based RTILs blended with MEA solution was compared to the performance of MEA and MEA/[BMIM][BF4] solutions at 25 °C (Table 5). At zero CO2 loading, all MEA/RTILs exhibited more positive corrosion potential compared to 30 wt% MEA solution (Figure 8) with corrosion rates of ≤ 0.01 mm/yr. At saturation condition, MEA solution had the highest current density compared to all MEA/RTIL solutions with corrosivity ranking of MEA > MEA/[BMIM][BF4] > MEA/[Choline][Ac] > MEA/[P4441][Ac]. CO2 loadings for MEA/RTILs were similar to CO2 loading of MEA solution of 0.57 molCO2/molsoln with no significant change in CO2 absorption capacity. Since CO2 loadings were not much affected by the different RTILs used, the different inhibition performances must be largely attributed to the structure and properties of each RTIL. Corrosion inhibition performance showed that [P4441][Ac] with phosphorous functional group had a better inhibition performance compared to [Choline][Ac] or [BMIM][BF4] with nitrogen functional group. Similar corrosion inhibition behavior for magnesium alloys showed that phosphonium-based ionic liquids offer better corrosion protection compared to imidazolium-based.40,41,42 The results indicate that the use of these less explored ionic liquids in blends with of amines can offer great advantage in terms of corrosion protection and a viable alternative to the commonly used imidazolium-based RTILs. 4. Conclusions In this work, the corrosive behavior of current state of the art and emerging novel CO2 scrubbing solvents was investigated. Benchmarking analysis has been carried out for the selected amine solutions that are prepared by blending them with piperazine, imidazolium cation-based, phosphonium cation-based and ammonium cation-based ionic liquids. Figure 9 shows the comparative performance of CO2 absorbent classes in

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terms of CO2 absorption capacity and steel corrosion rate at 80°C, from that it can be concluded: (1) Classical amine solutions had a variable performance depending on the amine type and its characteristic CO2 absorption capacity at the test temperature such that higher CO2 loading lead to higher steel corrosion rates. (2) Blending the primary amine MEA and the tertiary amine MDEA with equal proportions averaged the individual performances of the two amines. Activating the amines with PZ resulted in an obvious increase in CO2 absorption capacity and a decrease in steel corrosion rates which is mainly attributed to the PZ structure compared to the classical amines. (3) The partial replacement of aqueous phase in MEA solution with hydrophilic imidazolium-based RTILs seemed to be effective in reducing corrosion and increasing CO2 loading. However, MEA/RTIL solutions didn’t show a coherent trend that is attributed to the different anion stability and CO2 absorption affinity. (4) The two most attractive solvents in terms of high CO2 absorption capacity and relatively low corrosion rates are identified as aqueous mixtures of MEA/[BMIM][Otf] and MEA/PZ. Besides, the preliminary investigation on non-fluorinated phosphoniumbased and ammonium-based RTILs showed promising results in terms of corrosion inhibition compared to imidazloium-based RTILs. Comparison analysis with recently published work also showed some level of similarities for studied MEA/RTIL systems. For [EMIM][BF4]+MEA+CO2 system corrosion rate was reported as 0.22 mm/yr at 25 °C.30 Whereas in this work for [BMIM][BF4]+MEA+CO2 system corrosion rate is reported as 0.25 mm/yr at slightly higher temperatures at 40 °C. On the other hand, in the same study for [EMIM][Otf]+MEA+CO2 system the corrosion rate at room temperature is reported as 2.65 mm/yr, however, in this work for [BMIM][Otf]+MEA+CO2 is being reported as 0.52 mm/yr at 40 °C. Both these works showed that the presence of imidazolium based RTILs aqueous solutions decreases the current density and that leads to reduction of the corrosion rate. On the other hand, both in this work and in the work published by Acidi et al. (2014) also shows that the change in anion has significant effect on the recorded corrosion rate more than the effect observed in the alternation of the cation30. By changing the anion in [BMIM][Anion]+MEA+CO2 system from [BF4] to [Otf], the change in the corrosion rate is doubled at 40 °C. Similar behavior is also observed by Acidi et al. (2014) when the anion is changed in [EMIM][Anion]+MEA+CO2 system; corrosion rate is almost one order of magnitude greater with [Otf] when compared with [BF4] case.30 These results shows that there is a strong dependency on the anion alternation and more in-depth and systematic studies are required to establish a larger scale database of RTILs and their corrosion behavior in the case of their blends with amine solutions for the use of CO2 capture process.

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Acknowledgements. This paper was made possible by the support of NPRP grants (6-330-2-140) and (4662-2-249) from the Qatar National Research Fund. The statements made herein are solely the responsibility of the authors.

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References (1) Metz, B.; Davidson, O.; de Coninck, H.; Loos, M.; Meyer, L. IPCC Special Report on Carbon Dioxide Capture and Storage, Prepared by Working Group III of the Intergovernmental Panel on Climate Change; Cambridge University Press: New York, 2005 (2) Wang, M.; Lawal, A.; Stephenson, P.; Sidders, J.; Ramshaw, C. Post-combustion CO2 capture with chemical absorption: a state-of-the-art review. Chem. Eng. Res. Des. 2011 89, 1609 (3) Soosaiprakasam, I. R.; Veawab, A. Corrosion and polarization behavior of carbon steel in MEA-based CO2 capture process. Int. J. Greenhouse Gas Control. 2008, 2, 553−562 (4) Nainar, M.; Veawab, A. Corrosion in CO2 capture unit using MEA-piperazine blends. Energy Procedia 2009, 1, 231 (5) Kohl, A. L.; Nielsen, R. B. Gas Purification; Gulf Publishing Company; TX, 1997 (6) Kittel, J.; Idem, R.; Gelowitz, D.; Tontiwachwuthikul, P.; Parrain, G.; Bonneau, A. Corrosion in MEA units for CO2 capture: pilot plant studies. Energy Procedia 2009,1, 791 (7) Dupart, M. S.; Bacon, T. R.; Edwards, D.J. Understanding corrosion in alkanolamine gas treating plants, Part 1 & 2. Hydrocarb. Process. 1993, 93, 75 (8) Kladkaew, N.; Idem, R.; Tontiwachwuthikul, P.; Saiwan, C. Corrosion behavior of carbon steel in the monoethanolamine – H2O – CO2 – O2 – SO2 system: Products, Reaction Pathways, and Kinetics. Ind. Eng. Chem. Res. 2009, 48, 10169 (9) Veawab, A.; Aroonwilas, A. Identification of oxidizing agents in aqueous amine–CO2 systems using a mechanistic corrosion model. Corros. Sci. 2002, 44, 967 (10) Kumar, S.; Cho, J.H.; Moon, I; Ionic liquid-amine blends and CO2BOLs: Prospective solvents for natural gas sweetening and CO2 capture technology—A review. Int. J. Greenhouse Gas Control 2014, 20, 87 (11) Karadas, F.; Atilhan, M.; Aparicio, S. Review on the Use of Ionic Liquids (ILs) as Alternative Fluids for CO2 Capture and Natural Gas Sweetening. Energy Fuels 2010, 24,5817 (12) Samanta, A.; Bandyopadhyay; S. S. Kinetics and modeling of carbon dioxide absorption into aqueous solutions of piperazine. Chem. Eng. Sci. 2007, 62, 7312 (13) Samanta, A.; Bandyopadhyay, S. S. Absorption of carbon dioxide into aqueous solutions of piperazine activated 2-amino-2-methyl-1-propanol.Chem. Eng. Sci. 2009, 64, 1185 (14) Gunasekaran, P.; Veawab, A.; Aroonwilas, A. Corrosivity of Single and Blended Amines in CO2 Capture Process. Energy Procedia 2013, 37, 2094 (15) Camper, D.; Bara,J.; Gin,D.; Noble,R. Room-Temperature Ionic Liquid Amine Solutions : Tunable Solvents for Efficient and Reversible Capture of CO2. Ind. & Eng. Chem. Res. 2008, 47, 8496 (16) 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. Greenh. Gas Control 2012, 6, 246

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(17) Yang, J.; Yu, X.; Yan, J.; Tu, S. T. CO2 capture using amine solution mixed with ionic liquid. Ind. Eng. Chem. Res. 2014, 53, 2790 (18) Rogers, R. D.; Seddon, K. R. Ionic Liquids-Solvents of the Future? Science 2003, 302, 792 (19) 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 (20) Ramdin, M.; de Loos, T. W.; Vlugt, T. J. State-of-the-art of CO2 capture with ionic liquids. Ind. Eng. Chem. Res. 2012, 51, 8149 (21) Bara, J. E.; Carlisle, T. K.; Gabriel, C. J.; Camper, D.; Finotello, A.; Gin, D. L.; Noble, R. D. Guide to CO2 separations in imidazolium-based room-temperature ionic liquids. Ind. Eng. Chem. Res. 2009, 48, 2739 (22) Fraser, K. J.; & MacFarlane, D. R. Phosphonium-based ionic liquids: An overview. Aust. J. Chem. 2009, 62, 309 (23) Ramdin, M.; Olasagasti,T.; Vlugt, T.; J.H ,Loos; T. W. High pressure solubility of CO2 in non-fluorinated phosphonium-based ionic liquids. J. Supercrit. Fluids 2013, 82, 41 (24) Jia, X.; Xie, Y.; Zhang,Y.; Lu,X. CO2 capture/separation using choline chloride-based ionic liquids, Proceedings of 13th international conference on Properties and Phase Equilibria, Argentia, Brazil, 2013 (25) Dreimanis, A. Quantitative Gasometric Determination of Calcite and Dolomite by Using Chittick Apparatus. J. Sediment. Petrol. 1962, 32, 520 (26) Vaidya, P. D.; Kenig, E. Y. CO2‐Alkanolamine Reaction Kinetics: A Review of Recent Studies. Chem. Eng. Technol. 2007, 30, 1467 (27) Veawab, A.; Tontiwachwuthikul, P.; Chakma, A. Corrosion Behavior of Carbon Steel in the CO2 Absorption Process Using Aqueous Amine Solutions. Ind. Eng. Chem. Res. 1999, 38, 3917 (28) Hasibur-Rahman, M.; Larachi, F. Prospects of using room-temperature ionic liquids as corrosion inhibitors in aqueous ethanolamine-based CO2 capture solvents. Ind. Eng. Chem. Res. 2013, 52, 17682 (29) Xenophon, L. P.; Nikolaos, S. H.; Igor, S. M.; Lawien, F. Z.; Nathan, D. B.; Michalis, K. A.; Athanassios, G. K.; Vlassis, L.; Boyan, I.; George, Em. R.; Polycarpos, F.; Kostas, S.; Konstantinos, G. B.; Maaike, C. K.; George, E. T.; Jessica, K.; Thomas, J. S. S. CO2 Capture Efficiency, Corrosion Properties, and Ecotoxicity Evaluation of Amine Solutions Involving Newly Synthesized Ionic Liquids. Ind. Eng. Chem. Res. 2014, 53, 12083 (30) Acidi, A.; Hasib-ur-Rahman, M.; Larachi, F.; Abbaci, A. Ionic liquids [EMIM][BF4], [EMIM][Otf] and [BMIM][Otf] as corrosion inhibitors for CO2 capture applications. Korean J. Chem. Eng, 2014, 31 (6), 1043 (31) Sakwattanapong, R.; Aroonwilas, A.; Veawab, A. Behavior of reboiler heat duty for CO2 capture plants using regenerable single and blended alkanolamines. Ind. Eng. Chem. Res. 2005, 44, 4465. (32) Bishnoi, S.; Rochelle, G. T. Absorption of carbon dioxide into aqueous piperazine: reaction kinetics, mass transfer and solubility. Chem. Eng. Sci. 2000, 55, 5531

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(33) Kabanda, M. M.; Murulana, L. C.; Ozcan, M., Karadag, F.; Dehri, I.; Obot, I. B.; Ebenso, E. E. Quantum chemical studies on the corrosion inhibition of mild steel by some triazoles and benzimidazole derivatives in acidic medium. Int. J. Electrochem. Sci. 2012, 7, 5035 (34) Eustaquio-Rincón, R.; Rebolledo-Libreros, M. E.; Trejo, A.; Molnar, R. Corrosion in aqueous solution of two alkanolamines with CO2 and H2S: NMethyldiethanolamine+ Diethanolamine at 393 K. Ind. Eng. Chem. Res. 2008, 47, 4726 (35) Bereket, G.; Öğretir, C.; Özşahin, Ç. Quantum chemical studies on the inhibition efficiencies of some piperazine derivatives for the corrosion of steel in acidic medium. J. Mol. Struc-THEOCHEM. 2003, 663, 39 (36) Galiński, M.; Lewandowski, A.; Stępniak, I. Ionic liquids as electrolytes. Electrochim. Acta. 2006, 51, 5567 (37) Endres, F.’; MacFarlane, D.; Abbott, A. (Eds.). Electrodeposition from ionic liquids; John Wiley & Sons, 2008 (38) Atilhan, M.; Jacquemin, J.; David, R.; Majeda, K.; Santiago, A. Viscous Behavior of Imidazolium-Based Ionic Liquids, Ind. Eng. Chem. Res., 2013, 52, 16774 (39) Alcalde, R.; Garcia, G.; Atilhan, M.; Aparicio, S. Systematic Study on the Viscosity of Ionic Liquids: Measurement and Prediction. Ind. Eng. Chem. Res., 2015, 54, 10918 (40) Hasib-ur-rahman, M.; Bouteldja, H.; Fongarland, P.; Siaj, M.; Larachi, F. Corrosion Behavior of Carbon Steel in Alkanolamine / Room-Temperature Ionic Liquid Based CO2 Capture Systems. Ind. Eng. Chem. Res. 2012, 51,8711 (41) Sun, J.; Howlett, P.C.; MacFarlane D.R.; Lin, J.; Forsyth, M. Synthesis and physical property characterisation of phosphonium ionic liquids based on P(O)2(OR)2− and P(O)2(R)2− anions with potential application for corrosion mitigation of magnesium alloys. Electrochim. Acta. 2008, 54, 254 (42) Huang, P.; Latham, A. J.; MacFarlane, D. A review of ionic liquid surface film formation on Mg and its alloys for improved corrosion performance. Electrochim. Acta. 2013, 110,501

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List of Tables. Table 1. Summary of experimental and electrochemical parameters for single amines Table 2. Summary of experimental and electrochemical parameters for activated amines at 80 °C Table3. Summary of experimental and electrochemical parameters in MEA and imidazolium-based RTILs blends Table 4. Viscosities of MEA and MEA/imidazolium-based RTILs Table 5. Summary of experimental and electrochemical experiments in MEA, MEA/[P4441][Ac], MEA/[Choline][Ac] and MEA/[BMIM][BF4] at 25 °C

17



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Table 1. Summary of experimental and electrochemical parameters for single amines Medium (wt %)

MEA (30)

DEA (30)

Temperature (°C) 40 80 40 80 40

MDEA (30)

80

CO2 loading (molCO2/molamine)

Ecorr (V)

icorr (./)

Corrosion rate (mm/yr)

0 0 = 0.54 0 0 = 0.48 0 0 = 0.50 0 0 = 0.42 0 0 = 0.73

-0.95 -0.75 -0.98 -0.79 -0.90 -0.74 -0.92 -0.79 -0.49 -0.76

5.06 288.5 27.0 580.2 4.4 177.2 29.1 411.9 1.9 200.8

0.05 0.97 0.18 3.41 0.04 0.76 0.09 2.45 0.01 1.21

0

-0.59

2.3

0.02

0 = 0.17

-0.80

210.1

1.25

18


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Table 2. Summary of experimental and electrochemical parameters for activated amines at 80 °C Medium (wt %)


Ecorr (V)

icorr (./)


MEA/PZ (20/10)

0

-0.96

23.1

0.10

0 =0.62

-0.80

228.6

1.83

0

-0.94

31.4

0.16

0= 0.38

-0.80

382.7

2.30

0

-0.59

4.8

0.03

0 =0.42

-0.80

212.1

1.30

MEA/MDEA (15/15) MDEA/PZ (20/10) MEA/MDEA/PZ (10/10/10)

0

-0.64

13.2

0.07

0 = 0.50

-0.79

230.3

1.42

PZ (10)

0

-0.57

7.1

0.04

0 =0.34

-0.78

140.9

0.80

19



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Page 20 of 30

Table3. Summary of experimental and electrochemical parameters in MEA and imidazolium-based RTILs blends Medium (wt%)

Temperature (°C ) 40

MEA

(30)

MEA/[BMIM][BF4] (30/30)

MEA/[BMIM][Otf] (30/30)

80 40 80 40 80


Ecorr (V)

icorr (12)


0 0 = 0.54 0

-0.95 -0.75 -0.98

5.06 288 27.01

0.05 0.97 0.18

0 = 0.48 0 0.63 0 0.53 0 0.65 0 0.60

-0.79 -0.66 -0.75 -0.68 -0.75 -0.65 -0.76 -0.96 -0.76

580.20 15.21 44.32 29.91 420.03 17.65 89.90 33.78 240.61

3.41 0.10 0.25 0.18 2.50 0.10 0.52 0.20 1.41

20


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Table 4. Viscosities of MEA and MEA/imidazolium-based RTILs Viscosity (mP.s) Temperature (°C)

MEA (30 wt%)

MEA/[BMIM][Otf] (30 wt% / 30 wt%)

MEA/[BMIM][BF4] (30 wt% / 30 wt%)

20

2.95

6.73

6.84

40

1.66

3.41

3.44

60

1.07

2.01

2.04

80

0.75

1.35

1.41

21



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Page 22 of 30

Table 5. Summary of experimental and electrochemical experiments in MEA, MEA/[P4441][Ac], MEA/[Choline][Ac] and MEA/[BMIM][BF4] at 25 °C Medium (wt%) MEA MEA/[P4441][Ac] (30/5) MEA/[Choline][Ac] (30/5) MA/[BMIM][BF4] (30 /5)

CO2 loading (molCO2/molsoln)

Ecorr (V)

icorr (./)


0

-0.89

1.1

0.006

0 = 0.57 0

-0.70

90.5

0.54

-0.83

1.1

0.006

0 = 0.57 0

-0.74 -0.84

44. 2 2.8

0.26 0.01

0 = 0.57

-0.74

58.6

0.39

0 0 = 0.57

-0.50 -0.74

2.1 78.0

0.01 0.47

22


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List of Figures. Figure 1. Experimental set-up for electrochemical corrosion test Figure 2. Polarization curves of carbon steel in single amines: (a) 40 °C (b) at 80 °C Figure 3. Comparison of corrosion rates and CO2 loadings in single amines at 40 °C and 80 °C Figure 4. Polarization curves of carbon steel in activated amines and their parent amine at 80 °C: (a) PZ solutions (b) MEA & MEA/PZ solutions (c) MDEA & MDEA/PZ solutions (d) MEA/MDEA & MEA/MDEA solutions (e) MEA/MDEA, MEA/MDEA/PZ & PZ solutions Figure 5. Comparison of corrosion rates and CO2 loadings in piperazine activated amines at 80 °C Figure 6. Polarization curves of carbon steel in MEA, MEA/[BMIM][BF4] and [BMIM][Otf]: (a) at 40 °C (b) at 80 °C Figure

7.

Comparison

of

corrosion

rates

and

CO2

loadings

in

MEA,

MEA/[BMIM][BF4] and MEA/[BMIM][Otf] at at 40 °C and 80 °C Figure 8. Polarization curves of carbon steel in MEA and MEA/RTILs blends at 25 °C Figure 9. Comparison of corrosion rates and CO2 loadings in single amines, piperazine activated amines and amine/RTILs solutions at 80 °C

23



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Figure 1

24


Page 24 of 30

Page 25 of 30

1.E-02 Current Density (A/cm2)

1.E-02

1.E-03 1.E-04 1.E-05 1.E-06 1.E-07

MEA MEA+CO2 DEA DEA+CO2 MDEA MDEA+CO2

1.E-03 1.E-04 1.E-05 1.E-06 1.E-07 1.E-08

1.E-08 -1.5

-1 -0.5 Potential (V vs Ag/AgCl)

MEA MEA+CO2 DEA DEA+CO2 MDEA MDEA+CO2

-1.5

0

-1 -0.5 Potential (V vs Ag/AgCl)

(a)

(b) Figure 2.


Current Density (A/cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


MEA DEA MDEA

4 3 2 1 0

80 0.16 0.24

0.32

0.4

0.48

0.56

40 0.64

CO2 Loading (molCO2/molamine)

Figure 3.

25


0.72

0


1.E-02 Current Density (A/cm2)


1.E-02

1.E-04

1.E-06 PZ PZ+CO2

1.E-08

1.E-04

MEA MEA+CO2 MEA/PZ MEA/PZ+CO2

1.E-06

1.E-08 -1.2

-0.7 -0.2 Potential ( V vs Ag/AgCl)

-1.4

-1 -0.6 Potential (V vs Ag/AgCl) (b)

(a)


1.E-02

1.E-04

1.E-06

MDEA MDEA+CO2 MDEA/PZ MDEA/PZ+CO2

1.E-08 -1

-0.6

-0.2

1.E-04 MEA MEA+CO2 MDEA MDEA/PZ+CO2 MEA/MDEA MEA/MDEA+CO2

1.E-06

-1.8

Potential (V vs Ag/AgCl) (c)

-1.4 -1 -0.6 -0.2 Potential (V vs Ag/AgCl) (d)

1.E-02

1.E-04

1.E-06

1.E-08

MEA/MDEA MEA/MDEA+CO2 PZ PZ+CO2 MEA/MDEA/PZ MEA/MDEA/PZ+CO2

-1.8

-1.4 -1 -0.6 Potential (V vs Ag/AgCl) (e)

Figure 4.

26

-0.2

1.E-02

1.E-08 -1.4



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 30


-0.2

PZ MEA/MDEA MDEA/PZ MEA MEA/MDEA/PZ MEA/PZ

4 3 2 1 0 0.34

0.38

0.42

0.46

0.5

0.54

0.58

0.62


Figure 5.

1.E-02 1.E-03 Current Density (A/cm2)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



Page 27 of 30

1.E-04

1.E-06

1.E-08

MEA MEA/[BMIM][BF4] MEA/[BMIM][Otf] MEA+CO2 MEA/[BMIM][BF4]+CO2 MEA/[BMIM][Otf]+CO2

-1.7

-1.4 -1.1 -0.8 Potential (V vs Ag/AgCl)

1.E-05

1.E-07

1.E-09

MEA MEA/[BMIM][BF4] MEA/[BMIM][Otf] MEA+CO2 MEA/[BMIM][BF4]+CO2 MEA/[BBMIM][Otf]+CO2

-1.4

-0.5

(a)

-1.2 -1 -0.8 -0.6 Potential (V vs Ag/AgCl)

(b) Figure 6.

27


-0.4

-0.2

Page 28 of 30

MEA MEA/[BMIM][BF4] MEA/[BMIM][Otf]

4 3 2 1 0

80 0.48 0.5

0.52 0.54 0.56

0.58

0.6

40 0.62


Figure 7.

1.E-03

Current Density

(A/cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



1.E-05 MEA MEA+CO2 MEA/[Ch][Ac] MEA/[Ch][Ac]+CO2 MEA/[BMIM][BF4] MEA/[BMIM][BF4]+CO2 MEA/[P4441][Ac] MEA/[P4441][Ac]+CO2

1.E-07

1.E-09 -1.6

-1.3

-1

-0.7

Potential (V vs Ag/AgCl)

Figure 8.

28


-0.4

0.64

Page 29 of 30

3.5 3

0 0.16

0.22

0.28

0.34

0.40

0.42

0.46

0.50

0.52


Figure 9.

29


DEA

MEA/PZ

MEA/[BMIM][Otf]

0.5

MEA/[BMIM][BF4 ]

PZ

1

MDEA/PZ

1.5

MEA/MDEA

2

MEA/MDEA/PZ

DEA

MEA

2.5

MDEA


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.6

0.62


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

For Table of Contents Only

Ionic Liquids Amines

Piperazine Solvent Feed

Corrosion Tests

Acid Gas Feed

Separated Gas Discharge CO2 Rich Effluent

30


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Corrosion Behavior of Carbon Steel in CO2 Saturated Amine and

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