Effect of Various Parameters on the Thermal Stability and Corrosion Of

energies

Article Eﬀect of Various Parameters on the Thermal Stability and Corrosion of CO2-Loaded Tertiary Amine Blends

Usman Shoukat and Hanna Katariina Knuutila *

Department of Chemical Engineering, Norwegian University of Science and Technology (NTNU), 7491 Trondheim, Norway; [email protected] * Correspondence: [email protected]

Received: 26 April 2020; Accepted: 19 May 2020; Published: 21 May 2020

Abstract: In this study, the thermal stability and corrosivity of various CO2-loaded tertiary amine blends in both aqueous and non-aqueous form in stainless steel cylinders were studied for combined acid gas and water removal. The thermal stability was measured from the remaining amine concentration and the corrosivity was measured from the amount of various metals in blends using titration and inductively coupled plasma mass spectrometry (ICP-MS), respectively. The experimental data were used to calculate the rate constants of amine group loss. The developed model represented the experimental data very well. Solvent change from H2O to triethylene glycol (TEG) in blends decreased the thermal stability and vice versa for corrosivity. The amine stability was increased when contact with stainless steel was reduced. An increase in the amine concentration or CO2 loading at constant temperature decreased the thermal stability and vice versa for corrosivity.

Keywords: thermal stability; corrosivity; tertiary amines; amine concentration; CO2 loading; CO2 capture

1. Introduction

Acid gas impurities such as carbon dioxide (CO2) and hydrogen sulfide (H2S) are typically present in natural gas. Energy production from natural gas also produces large quantities of CO2 during combustion [1]. H2S is naturally present with methane, water vapor, CO2, and other impurities in many natural gas reservoirs [2]. Water vapor can cause corrosion in the pipelines and can also form hydrates in the presence of methane at suitable temperatures and pressures. These hydrates can plug manifolds, pipelines, and various accessories, such as valves and fittings [3]. Absorption-based acid gas capture processes are the most versatile processes for the removal of CO2 and H2S from natural gas [4]. Amine solutions are generally used for this type of process, and glycols such as monoethylene glycol (MEG) and triethylene glycol (TEG) are typically used to remove H2O and inhibit hydrate formation [5]. A process that can remove acid gases and water simultaneously from the natural gas stream can be beneficial and using this process for subsea applications will reduce the environmental footprint. Simultaneous acid gas and water removal was discussed as early as 1939 by Hutchinson [6] and later by McCartney [7] and Chapin [8]. An ethanolamine (MEA) and diethylene glycol blend was the first used blend, but it was discontinued due to higher amine degradation and severe corrosivity [4]. MEA is mostly used for CO2 absorption processes, but tertiary amines are alternatives for CO2 absorption due to their higher theoretical CO2 loading capacity and low energy requirement during the regeneration process [9]. Additionally, tertiary amine increases the thermal stability and reduces corrosivity [10].

Energies 2020, 13, 2626; doi:10.3390/en13102626 www.mdpi.com/journal/energies Energies 2020, 13, 2626 2 of 17

CO2 is absorbed in the presence of water in tertiary amines, and they form an unstable carbamate, subsequently producing bicarbonate ions, as shown in Equation (1) [9,11,12]:

1 2 3 1 2 3 + R R R N + CO +H O R R R NH +HCO− (1) 2 2 ↔ 3 The thermal stability and corrosivity of amine solutions are major problems in amine-based acid gas capture processes. High temperatures cause thermal degradation and the presence of oxygen can cause oxidative degradation [13,14]. Degradation can create various problems during the acid gas capture process, for example, solvent loss, by-product formation, fouling, foaming, corrosion, and a reduction in the absorption capacity [15–21]. Corrosion and thermal stability are interconnected since by-products can increase corrosion, while heat-stable amine salts, water vapor, organic acids, oxygen, and acid gas can also increase the corrosion rates [22–24]. Therefore, for a combined acid gas removal and water control process, it is essential to consider the corrosivity by checking the metal ion concentration in degraded blends. Corrosion can also be controlled by injecting film formation corrosion inhibitors (FFCIs) or pH stabilizers [25]. An example of a pH stabilizer used is N-methyl diethanolamine (MDEA), which is added to a lean MEG solution to lower the corrosion rate. The effect is based on the formation of a protective layer of FeCO3 on the pipeline wall in the presence of CO2 [25–27]. Corrosion inhibition by an FeCO3 protection layer has been observed on mild steel, carbon steel, and stainless steel surfaces [28–30]. Thermally degraded solutions (MDEA-MEG-FFCI) decrease the hydrate inhibition, which is still less than the reduction caused by pure thermally degraded MEG [31]. Therefore, combining amine with MEG/TEG can reduce corrosion in comparison to using pure MEG/TEG. The thermal stability of aqueous MDEA has been studied by various authors for concentrations ranging from 20 to 83.4 wt.% at temperatures from 100 to 230 ◦C[10,32–38], and it has been concluded that aqueous MDEA is more thermally stable in the absence of CO2. In the presence of CO2, minimal degradation was observed until 120 ◦C, but above this temperature, it started degrading rapidly. Moreover, an increase in the initial amine concentration and CO2 loading decreased the amine stability. The thermal stability of 2-(diethylamino)ethanol (DEEA) was studied in the presence of CO with 20–35.1 wt.% aqueous solutions at 135 C[35,37,39], and it was concluded that, 2 ≥ ◦ at temperatures >135 ◦C, the stability of aqueous DEEA was also reduced by an increase in the initial CO2 loading and amine concentration, like an aqueous MDEA solution. Only three published works are available for the thermal stability of non-aqueous amines. Al Harooni et al. [25] studied an MDEA-MEG-H2O solution (6.7 wt.% MDEA, 74.64 wt.% MEG, and 18.66 wt.% H2O) at 135–200 ◦C. The other two studies are our previously published works, where we studied MDEA blends (30 wt.% MDEA and 70 wt.% MEG/TEG) and both MDEA and DEEA blends (20 wt.% amine, 20 wt.% H2O, and 60 wt.% MEG/TEG) at 135 ◦C for seven weeks in the presence of CO2 [10,37]. It was concluded that the thermal stability of amine was decreased with the addition of MEG in aqueous MDEA and DEEA solutions. Generally, tertiary amines first degrade into primary and secondary amines, and later, these primary and secondary amines make other by-products [33]. This was confirmed by Gouedard et al. [16] by studying aqueous MDEA thermal degradation kinetics and mechanisms. No data are available for the thermal stability of both aqueous and non-aqueous blends of 3-dimethylamino-1-propanol (3DMA-1P) and 3-diethylamino-1-propanol (3DEA-1P). MEG and TEG remained thermally stable up to 157 and 210 ◦C, respectively, in the absence of oxygen [40,41]. Corrosivity by amine solutions at a high temperature was studied by Grimstvedt et al. [42] in metal cylinders. Solutions were analyzed for the total Fe, Cr, Ni, and Mo, and these metal concentrations were compared to pilot data. The higher the amount of metals in solution, the higher the corrosion rate, and vice versa. Later, a similar approach was used in numerous studies [10,28,35,37]. Eide-Haugmo [35] found that metal concentrations in aqueous MDEA solutions are higher than aqueous DEEA solutions at 135 ◦C after five weeks of an experiment in 30 wt.% solutions. A similar trend was also observed in our previous work [37] for all aqueous and non-aqueous MDEA and DEEA solutions. According to Energies 2020, 13, 2626 3 of 17

our other previous work, for 30 wt.% MDEA blends, the metal concentrations were in the order of MDEA.TEG > MDEA.H2O > MDEA.MEG [10]. The stability and corrosivity of amine are important factors to consider while selecting a blend

Energiesfor a combined 2020, 13, x FOR acid PEER gas REVIEW and water removal process. In our previous work [43], we identified various3 of 18 potential blends for combined H2S and water removal. Among others, DEEA, 3DMA-1P, and 3DEA-1P 1Pshowed showed good good acid acid gas gas absorption, absorption, while while MDEA MDEA is currentlyis currently used used as as an an industrial industrial standard standard amine amine for forH2 HS2 removal.S removal. As As mentioned mentioned previously, previously, most most investigations investigations ofof thermal stability in in amine amine solutions solutions havehave been been conducted conducted for for aqueous aqueous and and water water lean lean solu solutions.tions. There There are are very very little little data data available available for for the the thermalthermal stability stability of of amine amine blends blends in in glycols glycols without without any any water water present present in in blends. blends. Similarly, Similarly, the the effects effects ofof the the initial initial amine amine concentration concentration and and CO CO2 2loadingloading were were previously previously only only investigated investigated for for the the thermal thermal stabilitystability of of amine, amine, and and no no data data are are available available for for their their effects effects on on corrosion. corrosion. Hence, Hence, in in the the current current work, work, fourfour tertiary tertiary amine amine blends blends in in water, water, MEG, MEG, and and TEG TEG were were studied studied to to investigate investigate their their thermal thermal stability stability andand corrosivity. corrosivity. The The effects effects of of solvent solvent type, type, amine amine concentration, concentration, and and CO CO2 2loadingloading were were investigated. investigated. Finally,Finally, the the impact impact of of metal metal on on MDEA MDEA blends’ blends’ thermal thermal stability stability was was investigated investigated by by running running thermal thermal degradationdegradation experiments experiments in glass tubestubes andand stainlessstainless steel steel cylinders. cylinders. The The chemical chemical structures structures of of all all the thechemicals chemicals used used in thisin this study study are are shown shown in Figurein Figure1. 1.

2-(Diethylamino)ethanol 3-Dimethylamino-1-propanol 3-Diethylamino-1-propanol (DEEA) (3DMA-1P) (3DEA-1P)

N-Methyldiethanolamine Monoethylene Glycol Triethylene Glycol (MDEA) (MEG) (TEG)

Water Carbon dioxide (H2O) (CO2) FigureFigure 1. 1. ChemicalChemical structures structures of of all all chemicals chemicals used used in in this this study. study. 2. Materials and Methods 2. Materials and Methods 2.1. Materials 2.1. Materials All chemicals used in this study were purchased from Sigma-Aldrich (Oslo, Norway), except for All chemicals used in this study were purchased from Sigma-Aldrich (Oslo, Norway), except for 3DEA1P and CO2. 3DEA1P was bought from TCI Europe (Zwijndrecht, Belgium) and CO2 was 3DEA1Pobtained and from CO AGA2. 3DEA1P AB (Oslo, was Norway). bought from All the TCI chemicals Europe used(Zwijndrecht, are listed inBelgium) Table1 andand wereCO2 usedwas obtainedwithout furtherfrom AGA puriﬁcation. AB (Oslo, Norway). All the chemicals used are listed in Table 1 and were used without further purification.

Table 1. Full name, CAS, and purity of all chemicals.

Chemical CAS Purity Molecular Weight (g/mol) N-Methyldiethanolamine (MDEA) 105-59-9 ≥99.0% 119.16 2-(Diethylamino)ethanol (DEEA) 100-37-8 ≥99.5% 117.19 3-Dimethylamino-1-propanol (3DMA-1P) 3179-63-3 ≥99.0% 103.16 3-Diethylamino-1-propanol (3DEA-1P) 622-93-5 ≥95.0% 131.22 Monoethylene glycol (MEG) 107-21-1 ≥99.5% 62.07 Triethylene glycol (TEG) 112-27-6 ≥99.8% 150.17 Carbon dioxide (CO2) 124-38-9 ≥99.9% 44.01

Amine solutions were prepared gravimetrically in flasks using the Mettler Toledo scale, model MS6002S/01 (Oslo, Norway) (±0.01 g). Solutions were loaded with CO2 by using a continuously

Energies 2020, 13, 2626 4 of 17

Table 1. Full name, CAS, and purity of all chemicals.

Chemical CAS Purity Molecular Weight (g/mol) N-Methyldiethanolamine (MDEA) 105-59-9 99.0% 119.16 ≥ 2-(Diethylamino)ethanol (DEEA) 100-37-8 99.5% 117.19 ≥ 3-Dimethylamino-1-propanol (3DMA-1P) 3179-63-3 99.0% 103.16 ≥ 3-Diethylamino-1-propanol (3DEA-1P) 622-93-5 95.0% 131.22 ≥ Monoethylene glycol (MEG) 107-21-1 99.5% 62.07 ≥ Triethylene glycol (TEG) 112-27-6 99.8% 150.17 ≥ Carbon dioxide (CO ) 124-38-9 99.9% 44.01 2 ≥

Amine solutions were prepared gravimetrically in ﬂasks using the Mettler Toledo scale, model MS6002S/01 (Oslo, Norway) ( 0.01 g). Solutions were loaded with CO by using a continuously ± 2 weighted gas wash bottle in an isolated environment to achieve the required CO2 concentration. All solutions were titrated for amine and CO2 concentration to verify the CO2 loading prior to the start of the experiment [10,44,45]. The unloaded amine wt.% and CO2 loading at the beginning of the experiment for all solutions are given in Table2.

Table 2. Rate constants (k) and correlation factor (R2) of all solutions, along with the experimental

duration (time), CO2 loading (α), and unloaded amine weight %.

4 Unloaded α (mol CO2/ Time k 10− 2 Name × 1 R Amine wt.% mol amine) (days) (day− )

DEEA.H2O 30 0.1 49 3.792 0.910 3DMA-1P.H2O 30 0.1 49 2.933 0.945 3DEA-1P.H2O 30 0.1 49 2.659 0.981 MDEA.H2O 30 0.1 49 3.960 0.908 MDEA.H2O (Glass) 30 0.1 35 4.546 0.904 DEEA.MEG 30 0.1 49 7.998 0.938 3DMA-1P.MEG 30 0.1 49 0.362 0.981 3DEA-1P.MEG 30 0.1 49 6.634 0.930 MDEA.MEG 30 0.1 49 22.910 0.919 MDEA.MEG (Glass) 30 0.1 49 17.390 0.900 DEEA.TEG 30 0.1 49 3.853 0.974 3DMA-1P.TEG 30 0.1 49 4.505 0.928 3DEA-1P.TEG 30 0.1 49 4.043 0.914 MDEA.TEG 30 0.1 49 19.450 0.988 MDEA.TEG (Glass) 30 0.1 49 11.920 0.939 DEEA.H2O 10 0.4 49 5.483 0.916 DEEA.H2O 30 0.4 49 9.473 0.941 DEEA.H2O 50 0.4 49 11.561 0.935 MDEA.H2O 10 0.4 49 3.058 0.974 MDEA.H2O 30 0.4 49 7.061 0.990 MDEA.H2O 50 0.4 49 16.520 0.998 MDEA.H2O 70 0.4 49 32.810 0.986

2.2. Methodology Both stirred cell reactors [32,33,36,39] and metal cylinders [10,21,28,37] have been used to study amine solutions’ thermal stability. In this work, metal cylinders were selected, as various amine solutions at different concentrations could be tested in multiple cylinders at the same time, consequently reducing the overall experimental duration. The corrosivity of the blends can be studied by analyzing the solutions for the total Fe, Cr, Ni, and Mo. The metal concentrations in the solution can be used as an indicator of corrosivity [42]. Therefore, multiple cylinders were made by using 10 cm long and 1.3 cm Energies 2020, 13, 2626 5 of 17 diameter 316 stainless steel tubes, with both ends closed with Swagelok® end caps (Stavanger, Norway). Pre-loaded CO2 amine solutions were added in cylinders, leaving around 1 cm gaps between the solution surface and cap. Additionally, glass tubes were used inside metal cylinders to study the effect of metal on the thermal stability of amine solutions for 30 wt.% MDEA solutions (both aqueous and non-aqueous). A replica of each cylinder was made to attain more precise results. Cylinders were stored vertically in a forced convection oven at 135 ◦C, in order to enhance the degradation and shorten the experimental time. Since several studies using these conditions have been performed previously, the current data can be easily compared with literature data [10,28,35,37]. The experiment was run for seven weeks, and one cylinder, along with a replica of each solution, were taken out from the oven after 1, 3, 5, and 7 weeks at the same time of day. Cylinders were cooled down to room temperature before opening in the fume cabinet and were stored in the fridge to prevent any further decomposition. Cylinders were weighed before and after the experiment to identify any leakages. All cylinders, except for aqueous MDEA solutions in glass tubes for week 3 and week 5, showed less than a 2% difference in weight. The cylinders that leaked, including aqueous MDEA solutions in glass tubes for week 3 and week 5, were not used in the calculations of results. All solutions were titrated with 0.1 M H2SO4 to measure the amine group in the solutions [37,44]. For all samples, the average difference in amine concentrations was less than 2% between the solution ± and its replica in all non-leaking cylinders. In this paper, the average concentrations between the replicates are reported. At the end of the experiment, cation ionized chromatography (IC) was also used to determine the amine concentration of 30 wt.% solutions (α = 0.1) and the difference between the amine group concentration calculated by titration and cation IC was less than 2.5 percentage points. ± The average of the total amine group concentration measured by titration was used in Equation (2) to calculate the experimental values of the remaining amine group (%). In this equation, Ci is the amine group concentration at time = t (days), and Co is the amine group concentration in fresh solution at the start of the experiment. Metal concentrations (Fe, Cr, Ni, and Mo) were measured using inductively coupled plasma mass spectrometry (ICP-MS) for only week five solutions and used as an indicator of the relative corrosivity caused by amine solutions on the stainless steel [10,28,37,42]. The primary focus of the current work was to study the effect of TEG/MEG in the absence of water, amine concentration, and CO2 loading on the stability and corrosivity of amine. Therefore, by-products produced due to amine loss were not studied. C Remaining amine group (%) = 100 i (2) × Co

Modeling Rate constants of amine group loss can be calculated with a linear regression model by using Equations (3) and (4) while assuming ﬁrst-order reactions concerning amine and integrated over time “t” [37,46]. dC = r = kC (3) dt C ln o = k.t (4) Ci C 1 i = (5) Co ek.t

Plotting ln (Co/Ci) as a function of time from Equation (4) provided the ﬁrst-order rate constants (k) as the slope in linear regression, while the correlation coeﬃcient (R2) gave the accuracy of the model. Rate constants were used in Equation (5) to calculate (Ci/Co), which were subsequently used to predict the remaining amine group (%) from Equation (2). First-order rate constants (k), the experimental duration (time “t”), and R2 are given in Table2. Energies 2020, 13, 2626 6 of 17

3. Results and Discussion

3.1. Comparison with Literature Data

3.1.1. Thermal Stability EnergiesThe validation2020, 13, x FOR of PEER degradation REVIEW results is not simple. In the laboratory, the degradation is accelerated6 of 18 to shorten the experimental time and small differences in solvent compositions or experimental The validation of degradation results is not simple. In the laboratory, the degradation is conditions can give large differences in the results. Of the solutions studied in this work, the thermal accelerated to shorten the experimental time and small differences in solvent compositions or degradationexperimental of aqueous conditions DEEA can give and MDEAlarge differences has previously in the results. been studied Of the andsolutions there isstudied adequate in this agreement work, foundthe thermal between degradation the current of study aqueous and DEEA published and MD data,EA as has shown previously in Figure been2 .studied For aqueous and there DEEA is solutions,adequate Gao agreement et al. [39 found] (35 wt.%between DEEA the withcurrentα =study0.5) and showed published 4 percentage data, as pointsshown andin Figure 7 percentage 2. For pointsaqueous higher DEEA amine solutions, loss after Gao 12.5et al. and[39] 20(35 days,wt.% respectively,DEEA with α probably= 0.5) showed due 4 to percentage the higher points amine concentrationand 7 percentage and higherpoints higher CO2 loading. amine loss Eide-Haugmo after 12.5 and [ 3520] days, (30 wt.% respectively, DEEA with probablyα = 0.5)due showedto the 11higher percentage amine points concentration higher amine and higher loss afterCO2 loading. five weeks Eide-Haugmo at 135 ◦C in [35] comparison (30 wt.% DEEA to our with current α = 0.5) data, whichshowed can 11 be percentage partly explained points higher by the amine higher loss loading. after five Finally, weeks at the 135 new °C in data comparison are in good to our agreement current withdata, our which previous can data,be partly showing explained a good by reproducibility the higher loading. [10]. Figure Finally,2 also the presents new data a comparisonare in good of thisagreement work (30 with wt.% our MDEA previous in H data,2O, MEG, showing and a TEGgood with reproducibilityα = 0.1) and [10]. our Figure previous 2 also work presents (34 wt.% a MDEAcomparison in H2O of with this αwork= 0.19, (30 30wt.% wt.% MDEA MDEA in H in2O, MEG MEG, with andα TEG= 0.19, with and α = 300.1) wt.% and our MDEA previous in TEG work with α =(340.11) wt.% [10 MDEA]. The agreementin H2O with is α good = 0.19, and 30 bothwt.% studies MDEA showin MEG the with same α trend= 0.19, in and the 30 thermal wt.% MDEA stability in of TEG with α = 0.11) [10]. The agreement is good and both studies show the same trend in the thermal MDEA blends after seven weeks, which is in the order of MDEA.H2O > MDEA.TEG > MDEA.MEG. It canstability be concluded of MDEA that blends there after is a seven good weeks, overall which agreement is in betweenthe order theof MDEA.H published2O literature > MDEA.TEG data and> degradationMDEA.MEG. data It reportedcan be concluded in this paper. that there is a good overall agreement between the published literature data and degradation data reported in this paper.

100

Remaining amine group % 70

60 0 5 10 15 20 25 30 35 Days DEEA.H₂O (This work, α=0.4) DEEA.H₂O (Eide-Haugmo 2011, α=0.5) DEEA.H₂O (Gao et al. 2015, α=0.5) MDEA.H₂O (This work, α=0.1) MDEA.H₂O (This work, α=0.4) MDEA.H₂O (Eide-Haugmo 2011, α=0.5) MDEA.H₂O (Shoukat et al. 2016, α=0.2) MDEA.MEG (This work, α=0.1) MDEA.MEG (Shoukat et al. 2016, α=0.2) MDEA.TEG (Shoukat et al. 2016, α=0.1) MDEA.TEG (This work, α=0.1)

FigureFigure 2. 2.Comparison Comparison withwith the literature in in terms terms of of the the remaining remaining amine amine group group concentration concentration in 30 in 30wt.% amine–H amine–H2O2O solutions solutions (Eide-Haugmo (Eide-Haugmo 2011 2011 [35], [35 Gao], Gao et al. et 2015 al. 2015 [39], [ 39Shoukat], Shoukat et al. et2016 al. [10]). 2016 [10]).

3.1.2. Corrosion The corrosion results are in agreement with previously published data from various authors, as shown in Figure 3. Shoukat et al. [10] also found similar total metal concentrations and a similar order for MDEA blends as the current work. Furthermore, the new aqueous MDEA and DEEA data are also in acceptable agreement with Eide-Haugmo [35]. Individual metal ion concentrations display very small quantities (<67 mg/L), both in our current data set and previously published data sets. It can be concluded that the current data set is in good agreement with the literature data.

Energies 2020, 13, 2626 7 of 17

3.1.2. Corrosion The corrosion results are in agreement with previously published data from various authors, as shown in Figure3. Shoukat et al. [ 10] also found similar total metal concentrations and a similar order for MDEA blends as the current work. Furthermore, the new aqueous MDEA and DEEA data are also in acceptable agreement with Eide-Haugmo [35]. Individual metal ion concentrations display very small quantities (<67 mg/L), both in our current data set and previously published data sets.

It canEnergies be concluded2020, 13, x FOR that PEER the REVIEW current data set is in good agreement with the literature data. 7 of 18

DEEA-H₂O (This work, α=0.4) DEEA.H₂O (Eide-Haugmo 2011, α=0.5) MDEA.H₂O (This work, α=0.4) MDEA.H₂O (Eide-Haugmo 2011, α=0.5) MDEA.H₂O (This work, α=0.1) MDEA.H₂O (Shoukat et al. 2016, α=0.2) MDEA.MEG (This work, α=0.1) MDEA.MEG (Shoukat et al. 2016, α=0.2) MDEA.TEG (This work, α=0.1) MDEA.TEG (Shoukat et al. 2016, α=0.1)

0306090120150 METAL CONCENTRATON (MG/L) Fe Ni Cr Mo

FigureFigure 3. 3.Total Total metal metal concentrations concentrations inin 30 wt.% amine amine blends blends (Eide-Haugmo (Eide-Haugmo 2011 2011 [35], [35 Shoukat], Shoukat et al. et al. 20162016 [10 [10]).]).

3.2.3.2. Thermal Thermal Stability Stability TheThe thermal thermal stability stability of of amines amines isis presentedpresented by the remaining remaining amine amine group group (%) (%) (total (total remaining remaining alkalinity)alkalinity) as as a functiona function of of experimental experimental days.days. The remaining experimental experimental amine amine group group % %for for each each solutionsolution is is shown shown as as markers, markers, while while thethe solidsolid lineslines are are the the values values calculated calculated from from linear linear regression. regression. AllAll the the experimental experimental data data of of amine amine groupgroup concentrationsconcentrations is is given given in in Table Table A1. A1 .Rate Rate constants constants were were computed for all solutions until 49 days, except MDEA.H2O (Glass) solutions, where the solutions computed for all solutions until 49 days, except MDEA.H2O (Glass) solutions, where the solutions leaked after week one. The week one results of 50 wt.% DEEA.H2O were also not included in linear leaked after week one. The week one results of 50 wt.% DEEA.H2O were also not included in linear regression. Unloaded amine wt.% and CO2 loading at the start of an experiment for all solutions, regression. Unloaded amine wt.% and CO2 loading at the start of an experiment for all solutions, 2 alongalong with with the the experimental experimental duration, duration, rate rate constants constants (k), and(k), and correlation correlation coeffi coefficientscients (R2) for(R ) all for blends, all blends, are given in Table 2. The rate constant increased with the increase in amine concentration and are given in Table2. The rate constant increased with the increase in amine concentration and CO 2 CO2 loading in aqueous solutions. The change of solvent had a different effect on each amine solution loading in aqueous solutions. The change of solvent had a different effect on each amine solution rate constant. The correlation factor (R2) value was higher than 0.9 in all solutions, showing the good rate constant. The correlation factor (R2) value was higher than 0.9 in all solutions, showing the good accuracy of the model, and is visible in the thermal stability results (Figures 4–6 & 10–12). accuracy of the model, and is visible in the thermal stability results (Figures 4–6 & 10–12). Figure 4 shows the thermal stability results of 30 wt.% aqueous amine solutions with CO2 loadingFigure of4 0.1.shows After the seven thermal weeks, stability the maximum results of thermal 30 wt.% stability aqueous was amine observed solutions in 3DMA-1P. with CO Overall,2 loading of 0.1.the Afterthermal seven stability weeks, was the quite maximum high, as thermal less than stability 2 percentage was observed points in of 3DMA-1P.Overall, amine were lost during the thermal the stabilitywhole wasset of quite experiments. high, as less No thansignificant 2 percentage difference points in thermal of amine stability were lost was during observed the wholewhen the set of experiments.methyl group No was significant swapped di withfference an ethyl in thermal group in stability solutions, was i.e., observed 3DMA-1P when to 3DEA-1P. the methyl group was swapped with an ethyl group in solutions, i.e., 3DMA-1P to 3DEA-1P.

Energies 2020, 13, 2626 8 of 17 Energies 2020,, 13,, xx FORFOR PEERPEER REVIEWREVIEW 8 of 18

100% DEEA

98%

DMA-1P 96%

94% 3DEA-1P

92%

MDEA Remaining amine group (%) Remaining amine group (%) 90%

88% 0 1020304050 Days

Figure 4.4. RemainingRemaining amine amine group group concentration concentration in in aqu aqueouseous amine solutions, with solid lineslines representing the model results; unloaded amine wt.% = 30; CO loading 0.1. representing the model results; unloadedunloaded amineamine wt.%wt.% == 30;30; COCO22 loadingloading ≈ 0.1.0.1.

Figures5 and6 represent the 30 wt.% amine blends in MEG and TEG with CO 2 loading 0.1, Figures 5 and 6 represent the 30 wt.% amine blends in MEG and TEG with CO22 loadingloading ≈≈ 0.1,0.1, respectively.respectively. InIn bothboth cases,cases, MDEAMDEA blendsblends showedshowed the least thermal stability compared to to others. others. InstallingInstalling aa glassglass glass tubetube tube inin in betweenbetween between thethe the metalmetal metal cylincylin cylinderder and and blend blend increased increased the the thermal thermal stability stability of ofMDEA MDEA blends. blends. In both In both MEG MEG and TEG and blends TEG blends of MDEA, of MDEA, the glass the tube glass increased tube increased the thermal the stability thermal stabilityby 2 percentage by 2 percentage points after points seven after weeks, seven weeks,which whichmight mightbe due be to due the to absence the absence of metal of metal ions. ions. An Anincreaseincrease increase inin thethe inthe alkylalkyl alkyl groupgroup group inin inamineamine amine fromfrom from 3DMA3DMA 3DMA-1P-1P-1P toto to 3DEA-1P3DEA-1P 3DEA-1P decreaseddecreased decreased thethe the thermalthermal thermal stabilitystability in amine–MEG blendsblends andand thethe opposite trendtrend waswas seenseen withwith amine–TEG blends.blends. Eide-Haugmo Eide-Haugmo [[35][35]35] and and Shoukat et al. [[37]37] alsoalso foundfound thatthat anan increaseincrease inin thethethe carboncarboncarbon numbernumbernumber oror alkylalkyl groupgroup lengthlengthlength reducedreduced thethe thermalthermal stabilitystability inin aqueousaqueous amineamine solutions, solutions, and and vice vice versa, versa, for for amine–water–glycolamine–water–glycol solutions.solutions. 3DMA-1P andand DEEA DEEA showed showed the highestthe highest thermal thermal stability stability among MEGamong and MEG TEG blends,and TEG respectively, blends, afterrespectively, 49 days. after 49 days.

DEEA 100%

98% 3DMA-1P

96% 3DEA-1P

94% MDEA 92% Remaining amine group (%) Remaining amine group (%) 90% MDEA (Glass)(Glass) 88% 0 1020304050 Days

Figure 5.5. RemainingRemaining amine amine group group concentration concentration in aminein amin monoethylenee monoethylene glycol glycol (MEG) (MEG) blends, blends, with solidwith lines representing the model results; unloaded amine wt.% = 30; CO loading 0.1. solid lines representing the model results; unloaded amine wt.% = 30;2 CO22 loadingloading≈ ≈ 0.1.0.1.

Energies 2020, 13, 2626 9 of 17 Energies 2020, 13, x FOR PEER REVIEW 9 of 18

DEEA 100%

98% 3DMA- 1P 96% 3DEA-1P 94% MDEA 92%

90% MDEA

Remaining amine group (%) MDEA Remaining amine group (%) (Glass) 88% 0 1020304050 0 1020304050Days Days

Figure 6. Remaining amine group concentration in amine triethylene glycol (TEG) blends, with solid lines representing thethe modelmodel results;results; unloadedunloaded amineamine wt.%wt.% == 30; CO2 loadingloading ≈ 0.1.0.1. lines representing the model results; unloaded amine wt.% = 30; CO2 loading ≈ 0.1. Figure7 presents the e ffect of a change of solvent from water to MEG on the thermal stability of Figure 7 presents the effect of a change of solvent from water to MEG on the thermal stability of amine for 30 wt.% amine blends at 0.1 CO2 loadings. MEG usage instead of water replacement decreased amine for 30 wt.% amine blends at 0.1 CO2 loadings. MEG usage instead of water replacement the thermal stability of all solutions, except 3DMA-1P. The thermal stability of MDEA was decreased decreased the thermal stability of all solutions, except 3DMA-1P. The thermal stability of MDEA was by 8 percentage points, which was more than three times that of other blends. Similarly, Figure8 decreased by 8 percentage points, which was more than three times that of other blends. Similarly, shows the results for the change of solvent from water to TEG. TEG decreased the thermal stability Figure 8 shows the results for the change of solvent from water to TEG. TEG decreased the thermal of all blends. Again, like the effect of MEG, the maximum thermal stability decrease was observed stability of all blends. Again, like the effect of MEG, the maximum thermal stability decrease was in the MDEA.TEG blend. Figure9 represents the results of changing the solvent from MEG to TEG. observed in the MDEA.TEG blend. Figure 9 represents the results of changing the solvent from MEG Apart from 3DMA-1P, the thermal stability was increased for all blends. The maximum increase to TEG. Apart from 3DMA-1P, the thermal stability was increased for all blends. The maximum (3 percentage points) in thermal stability was observed in the MDEA (Glass) week five blend. increase (3 percentage points) in thermal stability was observed in the MDEA (Glass) week five blend.

12% DEEA

8% 3DMA-1P 4%

3DEA-1P 0%

MDEA -4% MDEA (Percentage Points) (Percentage Points)

-8% MDEA (Glass) Difference in remaining amine group in remaining Difference Difference in remaining amine group in remaining Difference -12% 0 7 14 21 28 35 42 49 0 7 14 21Days 28 35 42 49 Days Figure 7. Diﬀerence between the remaining amine group (percentage points) of aqueous amine Figure 7. Difference between the remaining amine group (percentage points) of aqueous amine solutions vs. the amine–MEG blend; unloaded amine wt.% = 30; CO2 loading 0.1. solutions vs. the amine–MEG blend; unloaded amine wt.% = 30; CO2 loading ≈≈ 0.1.

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12% DEEA 12% DEEA 8% 8% 3DMA-1P 3DMA-1P 4% 4% 3DEA-1P 0% 3DEA-1P 0% -4% MDEA MDEA (Percentage Points) -4% (Percentage Points) -8% MDEA -8% (Glass)MDEA Difference in remaining amine group in remaining Difference -12% (Glass) Difference in remaining amine group in remaining Difference -12% 0 7 14 21 28 35 42 49 0 7 14 21Days 28 35 42 49 Days Figure 8. Difference between the remaining amine group (percentage points) of aqueous amine Figure 8. Diﬀerence between the remaining amine group (percentage points) of aqueous amine solution solutionFigure 8. vs. Difference the amine–TEG between blends the ; remainingunloaded amineamine wt.% grou =p 30;(percentage CO2 loading points) ≈ 0.1. of aqueous amine vs. the amine–TEG blends; unloaded amine wt.% = 30; CO2 loading 0.1. solution vs. the amine–TEG blends; unloaded amine wt.% = 30; CO2 ≈loading ≈ 0.1. 12% 12% DEEA DEEA 8% 8% 3DMA-1P 4% 3DMA-1P 4% 3DEA-1P 0% 3DEA-1P 0% -4% MDEA

(Percentage Points) -4% MDEA

(Percentage Points) -8% MDEA -8% MDEA

Difference in remaining amine group in remaining Difference (Glass) -12%

Difference in remaining amine group in remaining Difference (Glass) -12% 0 7 14 21 28 35 42 49 0 7 14 21Days 28 35 42 49 Days Figure 9. Figure 9. DiDifferenceﬀerence betweenbetween thethe remaining remaining amine amine group group (percentage (percentage points) points) of of amine–MEG amine–MEG blends blends vs. amine–TEG blends; unloaded amine wt.% = 30; CO2 loading 0.1. vs.Figure amine–TEG 9. Difference blends; between unloaded the remainingamine wt.% amine = 30; COgroup2 loading (percentage≈ ≈ 0.1. points) of amine–MEG blends 3.2.1.vs. Eﬀ amine–TEGect of the Initial blends; Amine unloaded Concentration amine wt.% = 30; CO2 loading ≈ 0.1. 3.2.1. Effect of the Initial Amine Concentration 3.2.1.Figures Effect of 10 the and Initial 11 presentAmine Concentration the results of 10–70 wt.% MDEA.H2O solutions and 10–50 wt.% Figures 10 and 11 present the results of 10–70 wt.% MDEA.H2O solutions and 10–50 wt.% DEEA.H2O solutions, respectively,with initial CO2 loading of 0.4. An increase in the amine concentration Figures 10 and 11 present the results of 10–70 wt.% MDEA.H2O solutions and 10–50 wt.% decreasedDEEA.H2O the solutions, thermal stabilityrespectively, of amine with in allinitial solutions. CO2 Theloading thermal of 0.4. stability An ofincrease amine wasin the decreased amine DEEA.H2O solutions, respectively, with initial CO2 loading of 0.4. An increase in the amine moreconcentration rapidly decreased in MDEA the solutions thermal with stability an increase of amine in in the all solutions. amine concentration The thermal instability comparison of amine to concentration decreased the thermal stability of amine in all solutions. The thermal stability of amine DEEAwas decreased solutions. more Every rapidly increase in MDEA of 20 wt.%solutions MDEA with concentration an increase in in the aqueous amine solution concentration doubled in was decreased more rapidly in MDEA solutions with an increase in the amine concentration in thecomparison amine loss to DEEA in each solutions. step. The Every results increase are in of good 20 wt.% agreement MDEA withconcentration the literature. in aqueous Chakma solution and comparison to DEEA solutions. Every increase of 20 wt.% MDEA concentration in aqueous solution Meisendoubled [ 32the] foundamine thatloss anin increaseeach step. in The the initialresults MDEA are in good concentration agreement increased with the the literature. amine loss Chakma rates doubled the amine loss in each step. The results are in good agreement with the literature. Chakma dueand toMeisen the higher [32] found solubility that ofan CO increasein the in solutions the initial achieving MDEA concentration constant CO increasedloading at the higher amine amine loss and Meisen [32] found that an increase2 in the initial MDEA concentration2 increased the amine loss concentrations.rates due to the Similarly, higher solubility Gao et al. of [39 CO] concluded2 in the solutions that an increase achieving in theconstant initial concentrationCO2 loading at of higher DEEA aminerates due concentrations. to the higher Similarly, solubility Gao of COet al.2 in [39] the conc solutionsluded thatachieving an increase constant in the CO initial2 loading concentration at higher ofamine DEEA concentrations. significantly decreasedSimilarly, theGao stability et al. [39] of concamineluded due thatto the an availability increase in of the a moreinitial reactive concentration DEEA moleculeof DEEA significantlyfor CO2 absorption. decreased the stability of amine due to the availability of a more reactive DEEA molecule for CO2 absorption.

Energies 2020, 13, 2626 11 of 17 signiﬁcantlyEnergies 2020, 13 decreased, x FOR PEER the REVIEW stability of amine due to the availability of a more reactive DEEA molecule11 of 18 Energies 2020, 13, x FOR PEER REVIEW 11 of 18 for CO2 absorption.

100% 100% 97% 97% 94% 94% 91% 91% 88% 88%

Remaining amine group (%) 85% Remaining amine group (%) 85% 0 1020304050 0 1020304050Days Days 10%MDEA 30%MDEA 50%MDEA 70%MDEA 10%MDEA 30%MDEA 50%MDEA 70%MDEA Figure 10. Remaining amine group (%) in various aqueous N-methyl diethanolamine (MDEA) Figure 10. Remaining amine group (%) in various aqueous N-methyl diethanolamine (MDEA) solutions, Figuresolutions, 10. with Remaining solid lines amine repr esentinggroup (%) the inmodel various results aqueous at CO2 loadingN-methyl ≈ 0.4.diethanolamine (MDEA) with solid lines representing the model results at CO2 loading 0.4. solutions, with solid lines representing the model results at CO≈2 loading ≈ 0.4.

100% 100% 97% 97% 94% 94% 91% 91% 88% Remaining amine group (%) 88% Remaining amine group (%) 85% 85% 0 1020304050 Days 0 1020304050 10%DEEA 30%DEEADays 50%DEEA 10%DEEA 30%DEEA 50%DEEA Figure 11.11.Remaining Remaining amine amine group group (%) in(%) various in various aqueous aqueo 2-(diethylamino)us 2-(diethylamino) ethanol (DEEA)ethanol solutions, (DEEA) with solid lines representing the model results at CO loading 0.4. Figuresolutions, 11. with Remaining solid lines amine representing group (%)the modelin various results2 aqueo at CO≈us2 loading 2-(diethylamino) ≈ 0.4. ethanol (DEEA) solutions, with solid lines representing the model results at CO2 loading ≈ 0.4. 3.2.2. Eﬀect of Initial CO2 Loading 3.2.2. Effect of Initial CO2 Loading 3.2.2.The Effect eﬀ ectof Initial of CO 2COloading2 Loading on the thermal stability in the initial 30 wt.% MDEA and DEEA aqueous solutionsThe effect is presented of CO2in loading Figure on12. the After thermal 49 experimental stability in days, the initial the thermal 30 wt.% stability MDEA was and reduced DEEA aqueousThe solutionseffect of COis presented2 loading inon Figure the thermal 12. Afte stabilityr 49 experimental in the initial days, 30 thewt.% thermal MDEA stability and DEEA was with an increase in CO2 loading from 0.1 to 0.4 for both amines. Aqueous DEEA solution exhibited a greateraqueousreduced reduction withsolutions an increase inis thepresented thermal in CO in2stability loading Figure compared12.from Afte 0.1r to49 to 0.4experimental aqueous for both MDEA. amines. days, Again, theAqueous thermal our dataDEEA stability agree solution withwas thereducedexhibited published with a greater an literature. increase reduction Chakmain CO in 2the loading and thermal Meisen from stabil [0.132]ity to and compared0.4 Gao for etboth al. to [amines.aqueous39] found Aqueous MDEA. that an Again,DEEA increase oursolution in data the exhibitedagree with a the greater published reduction literature. in the Chakma thermal andstabil Meisenity compared [32] and toGao aqueous et al. [39] MDEA. found Again, that an our increase data CO2 concentration in solutions decreased the thermal stability of aqueous MDEA and DEEA solutions, agreein the withCO2 theconcentration published literature. in solutions Chakma decreased and Meisenthe thermal [32] and stability Gao et of al. aqueous [39] found MDEA that anand increase DEEA respectively. Lepaumier et al. [33] and Eide-Haugmo [35] also concluded that the addition of CO2 reducedinsolutions, the CO the 2respectively. concentration thermal stability Lepaumier in solutions of amines. et al. decreased [33] and theEide-Haugmo thermal stability [35] also of aqueousconcluded MDEA that the and addition DEEA solutions,of CO2 reduced respectively. the thermal Lepaumier stability et ofal. amines. [33] and Eide-Haugmo [35] also concluded that the addition of CO2 reduced the thermal stability of amines.

Energies 2020, 13, 2626 12 of 17 Energies 2020, 13, x FOR PEER REVIEW 12 of 18

Energies 2020, 13, x FOR PEER REVIEW 12 of 18

100%

100%98%

96%98%

96% 94%

94% 92% 92% 90% Remaining amine group (%) 90%

Remaining amine group (%) 88% 0 1020304050 88% Days 0 1020304050 DEEA(α=0.1) DEEA(α=0.4) Days MDEA(α=0.1) MDEA(α=0.4) DEEA(α=0.1) DEEA(α=0.4) MDEA(α=0.1) MDEA(α=0.4) FigureFigure 12. 12.Remaining Remaining amine amine group group (%) in (%) various in various aqueous aqueo amineus solutions, amine solutions, with solid with lines solid representing lines therepresenting modelFigure results;12. Remainingthe unloadedmodel results;amine amine unloadedgroup wt.% (%)= amine30. in various wt.% =aqueo 30. us amine solutions, with solid lines 3.3. Corrosionrepresenting the model results; unloaded amine wt.% = 30. 3.3. Corrosion 3.3. Corrosion WeekWeek 5 5 blendsblends were were analyzed analyzed for individual for individual metal components metal components (iron (Fe), (iron nickel (Fe), (Ni), chromium nickel (Ni), chromium(Cr), andWeek (Cr),molybdenum 5 blends and molybdenumwere (Mo)) analyzed using for (Mo))IC P-MS,individual using and themeta ICP-MS, resultsl components andas a sum the (iron of results all (Fe), of these asnickel a summetals (Ni), of chromium for all 30 of wt.% these metalsamine(Cr), for and blends 30 molybdenum wt.% with amineinitial (Mo)) 0.1 blends CO using2 loadings with ICP-MS, initial are and given 0.1 the CO resultsin Figure2 loadings as a13. sum A arehigherof all given of corrosivity these in metals Figure of for a13 blend 30. Awt.% higherwas corrosivityindicatedamine blends of by a blenda with higher wasinitial metal indicated 0.1 COconcentration.2 loadings by a higher are Each given metal solution in concentration. Figure and 13. Aits higher Eachreplica corrosivity solution were analyzed and of a its blend replica and was the were analyzedaverageindicated and was by the used a averagehigher for result metal was calculat usedconcentration. forions. result The averageEach calculations. solution deviation Theand in averageits the replica replicas deviation were was analyzed ±0.1 in themg/L. replicasand Various the was 0.1metalaverage mg/ concentrationsL. Variouswas used metal for (mg/L)result concentrations calculat in all solutionsions.(mg The / afteraverageL) in five all deviation solutionsweeks are in after thegiven replicas ﬁve in weeksTable was A2. ±0.1 are Iron givenmg/L. contributed inVarious Table A2 . ± Ironmostmetal contributed to concentrations the total most metal to (mg/L) the concentration total in metalall solutions concentration in all after amine five in weeksblends all amine are in givenMEG blends inand Table in MEGTEG. A2. andIn Iron aqueous TEG. contributed In aqueousamine aminesolutions,most solutions, to theMo total Moshowed showedmetal the concentration thehighest highest contribution, contribution,in all amine except blends except for in foraqueous MEG aqueous and MDEA MDEATEG. solutions,In solutions,aqueous where amine where Cr Cr contributedcontributedsolutions, the Mo the most. showedmost. In In all theall amine aminehighest blends blends contribution, exceptexcept thethe except DEEA for blend, aqueous the the totalMDEA total metal metal solutions, concentration concentration where Crwas was contributed the most. In all amine blends except the DEEA blend, the total metal concentration was highesthighest in in amine–TEG amine–TEG blends, blends, followed followed by by amine–H amine–H22O blends. The The lowest lowest metal metal concentrations concentrations were were highest in amine–TEG blends, followed by amine–H2O blends. The lowest metal concentrations were foundfound in in amine–MEG amine–MEG blends. blends. TheThe totaltotal metalmetal concentrationconcentration in in DEEA DEEA blends blends was was in in the the order order of of found in amine–MEG blends. The total metal concentration in DEEA blends was in the order of DEEA.H2O < DEEA.MEG < DEEA.TEG. DEEA.H2O < DEEA.MEG < DEEA.TEG. DEEA.H2O < DEEA.MEG < DEEA.TEG.

DEEA.H O DEEA.H2 O DEEA.MEG2 DEEA.MEG DEEA.TEG DEEA.TEG 3DMA-1P.H O 3DMA-1P.H2 O 3DMA-1P.MEG2 3DMA-1P.MEG 3DMA-1P.TEG 3DMA-1P.TEG 3DEA-1P.H2O 3DEA-1P.H2O 3DEA-1P.MEG3DEA-1P.MEG 3DEA-1P.TEG3DEA-1P.TEG MDEA.H2O MDEA.H2O MDEA.MEGMDEA.MEG MDEA.TEGMDEA.TEG 00 20406080100 Metal Concentration (mg/L) (mg/L) Fe Ni Cr Mo Fe Ni Cr Mo

Figure 13. Metal concentrations of amine solutions after ﬁve weeks; unloaded amine wt.% = 30;

CO2 loading 0.1. ≈ Energies 2020Energies, 13 ,2020 2626, 13, x FOR PEER REVIEW 13 of 18 13 of 17

Figure 13. Metal concentrations of amine solutions after five weeks; unloaded amine wt.% = 30; CO2 3.3.1. Eﬀectloading of the ≈ initial 0.1. amine concentration

The3.3.1. results Effect for of the the einitialﬀects amine of the concentration amine concentration on corrosivity at a constant initial CO2 loading of 0.4 are presentedThe results in for Figure the effects 14. Anof the increase amine concentration in the amine on concentration corrosivity at a increased constant initial the corrosivity,CO2 mainlyloading due to of higher 0.4 are amine presented loss. in TheFigure maximum 14. An increase metal in concentration the amine concentration (534 mg/L) increased was found the with 70 wt.%corrosivity, MDEA aqueous mainly due solution, to higher whereas amine loss. the The minimum maximum value metal (1 concentration mg/L) was (534 found mg/L) with was 10 wt.% DEEA aqueousfound with solution. 70 wt.% MDEA The total aqueous metal solution, concentration whereas the in minimum aqueous value MDEA (1 mg/L) increased was found exponentially, with whereas10 in wt.% aqueous DEEA DEEA aqueous it increased solution. quiteThe total linearly. metal concentration in aqueous MDEA increased exponentially, whereas in aqueous DEEA it increased quite linearly.

10%DEEA

30%DEEA

50%DEEA

10%MDEA

30%MDEA

50%MDEA

70%MDEA

0 100 200 300 400 500 600 Metal Concentration (mg/L) Fe Ni Cr Mo

Figure 14.FigureMetal 14. Metal concentrations concentrations of variousof various aqueous aqueous MDEA and and DEEA DEEA solutions solutions after afterfive weeks ﬁve weeksat at CO loadingCO2 loading0.4. ≈ 0.4. 2 ≈

3.3.2. Eﬀ3.3.2.ect ofEffect Initial of Initial CO2 COLoading2 Loading

CO2 loadingCO2 loading aﬀected affected the the corrosivity corrosivity caused caused byby amine solutions. solutions. The The total total metal metal concentrations concentrations for 30 wt.% aqueous MDEA and DEEA solutions with CO2 loadings of 0.1 and 0.4 after week five are for 30 wt.% aqueous MDEA and DEEA solutions with CO2 loadings of 0.1 and 0.4 after week ﬁve shown in Figure 15. An increase in CO2 loading increased the total metal concentration in both are shown in Figure 15. An increase in CO2 loading increased the total metal concentration in both solutions. The aqueous MDEA solution with 0.4 CO2 loading contained the highest amount of metals solutions. The aqueous MDEA solution with 0.4 CO loading contained the highest amount of metals compared to other solutions. Eide-Haugmo [35]2 also found that CO2-loaded amine solutions comparedcorroded to other more solutions. in comparison Eide-Haugmo to their unloaded [35] also counterpart. found that CO2-loaded amine solutions corroded more inEnergies comparison 2020, 13, x FOR to their PEER REVIEW unloaded counterpart. 14 of 18

DEEA (α=0.1)

DEEA (α=0.4)

MDEA (α=0.1)

MDEA (α=0.4)

0 20406080100 Metal Concentration (mg/L) Fe Ni Cr Mo

FigureFigure 15. Metal 15. Metal concentrations concentrations of of 30 30 wt.% wt.% aqueous aqueous MDEA MDEA and and DEEA DEEA solutions solutions after afterweek weekfive at ﬁve at variousvarious CO2 loadings. CO2 loadings.

4. Conclusions In this work, the thermal stability and corrosivity of four tertiary amine blends in water, MEG, and TEG, loaded with CO2, were studied in stainless steel cylinders at 135 °C for seven weeks. The effects of direct metal contact with the blend, the initial amine concentration, and the initial CO2 loading were discussed. An increase in the initial amine concentration, CO2 loading, and direct contact between the cylinder and blends decreased the thermal stability, and vice versa, for corrosivity. Amine blends with TEG displayed a higher thermal stability, except for 3DMA-1P, and higher corrosion, in comparison to MEG. 3DEA-1P blends showed a good combination of low corrosion and a higher thermal stability in comparison to other solutions.

Author Contributions: Conceptualization, H.K.K. and U.S.; methodology, U.S.; writing—original draft preparation, U.S.; writing—review and editing, H.K.K.; supervision, H.K.K.

Funding: This work was carried out under funding provided by the NTNU-SINTEF Gas Technology Centre (GTS) and the Faculty of Natural Sciences, Norwegian University of Science and Technology (NTNU), Trondheim, Norway.

Acknowledgments: We would like to thank the Department of Chemical Engineering (IKP) at the Faculty of Natural Sciences and Technology, Norwegian University of Science and Technology (NTNU), Trondheim, Norway, for their support.

Conflicts of Interest: The authors declare no conflict of interest.

Appendixes

Table A1: Amine group concentrations in all solutions.

CO2 Remaining Amine Group Concentration Name Loading (mole/L) after Time (days) (α) 0 7 21 35 49

DEEA.H2O 0.1 2.5005 2.4999 2.4886 2.4594 2.4561

3DMA-1P.H2O 0.1 2.9098 2.8992 2.8935 2.8755 2.8716

3DEA-1P.H2O 0.1 2.1932 2.1871 2.1833 2.1730 2.1641

MDEA.H2O 0.1 2.4178 2.4051 2.3942 2.3796 2.3772

Energies 2020, 13, 2626 14 of 17

4. Conclusions In this work, the thermal stability and corrosivity of four tertiary amine blends in water, MEG, and TEG, loaded with CO2, were studied in stainless steel cylinders at 135 ◦C for seven weeks. The eﬀects of direct metal contact with the blend, the initial amine concentration, and the initial CO2 loading were discussed. An increase in the initial amine concentration, CO2 loading, and direct contact between the cylinder and blends decreased the thermal stability, and vice versa, for corrosivity. Amine blends with TEG displayed a higher thermal stability, except for 3DMA-1P, and higher corrosion, in comparison to MEG. 3DEA-1P blends showed a good combination of low corrosion and a higher thermal stability in comparison to other solutions.

Author Contributions: Conceptualization, H.K.K. and U.S.; methodology, U.S.; writing—original draft preparation, U.S.; writing—review and editing, H.K.K.; supervision, H.K.K. All authors have read and agreed to the published version of the manuscript. Funding: This work was carried out under funding provided by the NTNU-SINTEF Gas Technology Centre (GTS) and the Faculty of Natural Sciences, Norwegian University of Science and Technology (NTNU), Trondheim, Norway. Acknowledgments: We would like to thank the Department of Chemical Engineering (IKP) at the Faculty of Natural Sciences and Technology, Norwegian University of Science and Technology (NTNU), Trondheim, Norway, for their support. Conﬂicts of Interest: The authors declare no conﬂict of interest.

Appendix A

Table A1. Amine group concentrations in all solutions.

CO Loading Remaining Amine Group Concentration (mole/L) after Time (days) Name 2 (α) 0 7 21 35 49

DEEA.H2O 0.1 2.5005 2.4999 2.4886 2.4594 2.4561

3DMA-1P.H2O 0.1 2.9098 2.8992 2.8935 2.8755 2.8716

3DEA-1P.H2O 0.1 2.1932 2.1871 2.1833 2.1730 2.1641

MDEA.H2O 0.1 2.4178 2.4051 2.3942 2.3796 2.3772

MDEA.H2O(Glass) 0.1 2.4178 2.4022 - 2.3812 - DEEA.MEG 0.1 2.5044 2.4871 2.4761 2.4212 2.4130 3DMA-1P.MEG 0.1 2.9063 2.9052 2.9038 2.9025 2.9014 3DEA-1P.MEG 0.1 2.2519 2.2318 2.2209 2.1922 2.1869 MDEA.MEG 0.1 2.4167 2.3426 2.2768 2.2162 2.1854 MDEA.MEG(Glass) 0.1 2.4167 2.3497 2.3161 2.2639 2.2371 DEEA.TEG 0.1 2.4816 2.4714 2.4628 2.4447 2.4378 3DMA-1P.TEG 0.1 2.8995 2.8876 2.8856 2.8562 2.8295 3DEA-1P.TEG 0.1 2.2387 2.2254 2.2153 2.2058 2.1987 MDEA.TEG 0.1 2.4297 2.3973 2.3367 2.2537 2.2183 MDEA.TEG(Glass) 0.1 2.4297 2.4121 2.3697 2.3543 2.2748

DEEA.H2O 0.4 0.8141 0.8068 0.8031 0.7992 0.7933 Energies 2020, 13, 2626 15 of 17

Table A1. Cont.

CO Loading Remaining Amine Group Concentration (mole/L) after Time (days) Name 2 (α) 0 7 21 35 49

DEEA.H2O 0.4 2.4062 2.3737 2.3574 2.3390 2.2919

DEEA.H2O 0.4 3.9907 - 3.9392 3.8226 3.7596

MDEA.H2O 0.4 0.7779 0.7756 0.7743 0.7697 0.7658

MDEA.H2O 0.4 2.3081 2.2970 2.2802 2.2522 2.2266

MDEA.H2O 0.4 3.8348 3.8001 3.7076 3.6184 3.5348

MDEA.H2O 0.4 5.3424 5.2131 4.9166 4.7757 4.5688

Table A2. Various metal concentrations (mg/L) in all solutions after ﬁve weeks.

Unloaded CO Name Amine 2 Fe (mg/L) Ni (mg/L) Cr (mg/L) Mo (mg/L) Loading (α) Weight (%)

DEEA.H2O 30 0.1 1.55 0.62 0.62 4.17 3DMA-1P.H2O 30 0.1 1.13 0.66 0.07 12.36 3DEA-1P.H2O 30 0.1 1.15 0.06 0.11 3.15 MDEA.H2O 30 0.1 3.24 8.08 10.53 1.49 DEEA.MEG 30 0.1 15.11 0.44 0.44 0.56 3DMA-1P.MEG 30 0.1 3.06 0.53 0.14 0.74 3DEA-1P.MEG 30 0.1 3.70 0.22 0.03 0.36 MDEA.MEG 30 0.1 3.99 0.16 0.51 0.10 DEEA.TEG 30 0.1 36.21 2.48 2.48 0.76 3DMA-1P.TEG 30 0.1 27.28 0.98 0.08 0.22 3DEA-1P.TEG 30 0.1 8.18 0.46 0.07 0.12 MDEA.TEG 30 0.1 66.64 11.25 17.53 2.12 DEEA.H2O 10 0.4 0.33 0.01 0.26 0.37 DEEA.H2O 30 0.4 5.06 0.91 5.69 0.64 DEEA.H2O 50 0.4 9.11 1.46 6.91 0.84 MDEA.H2O 10 0.4 1.13 3.54 4.83 0.82 MDEA.H2O 30 0.4 5.42 33.24 52.27 6.31 MDEA.H2O 50 0.4 14.10 79.80 130.50 16.07 MDEA.H2O 70 0.4 34.48 174.17 290.49 34.39

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