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Ind. Eng. Chem. Res. 2009, 48, 9068–9075
New Amines for CO2 Capture. II. Oxidative Degradation Mechanisms Helene Lepaumier,† Dominique Picq,† and Pierre-Louis Carrette*,‡ LMOPS, UMR 5041 CNRS/UniVersite´ de SaVoie, chemin du canal, 69360 Solaize, France, and IFP, e´changeur de Solaize, BP 3, 69360 Solaize, France
This study examines oxidative degradation of 12 ethanolamines and ethylenediamines. They were chosen to establish structure-property relationships: the role of replacement of the alcohol function by one second amine function, amine nature, steric hindrance, and cyclic structure were studied. Degradation of aqueous amine solutions was evaluated at 140 °C under air pressure (2 MPa) in stainless steel reactors for 15 days. At the end of the run, most degradation products were identified by gas chromatography (GC)/mass spectrometry (MS); amounts of remaining amine and its degradation products were determined with the quantitative GC method. Main degradation mechanisms are proposed, and some relationships between structure and chemical stability are given. 1. Introduction One of the main human CO2 emissions sources is flue gases (coal-fired power plants, cement manufacturing plants, refineries), which contribute to the increasing greenhouse effect. CO2 capture in postcombustion is currently the most advanced technology and can be applied to existing plants. In the case of diluted and low-pressure steams, absorption based on chemical reaction with aqueous alkanolamine solutions is the most attractive technology. Among all known solvents, ethanolamine (MEA) is the benchmark molecule because of its properties toward CO2 capture (high absorption capacity, fast kinetics, high water solubility, low price, etc.). However, a major problem associated with chemical absorption is solvent degradation. Our previous study on amine degradation with CO21 showed the impact of this gas on the chemical stability of different amines. Moreover, the impact of O2 also has to be taken into account. Flue gases from coal-fired power plants contain indeed a significant concentration of O2 (usually approximately 5% vol) leading among others to carboxylic acids formation, species highly suspected to be responsible for corrosion and fouling in industrial units.2-4 Ethanolamines’ sensitivity to O2 is thus often attributed to the oxidation of alcohol function into acid. For this reason, polyamines without alcohol function could be good candidates for CO2 capture in flue gas. Oxidative degradation of alkanolamines has been reported in the literature, especially for N-methyldiethanolamine5 (MDEA) and diethanolamine6 (DEA), commonly used in natural gas treatment, and MEA,7,8 the benchmark solvent for postcombustion application. Oxidative degradation of MEA was studied by several authors: Idem,7,8 Rochelle,9,10 Goff,11 and Petryaev.12 The main degradation products are volatile compounds, amines (methylamine, ammoniac), aldehydes (formaldehyde and acetaldehyde), and also carboxylic acids (formic, acetic, oxalic, and glycolic acids). The formation of these degradation products is explained by two different radical mechanisms: electron transfer9-11,13,14 or hydrogen abstraction.12 Rooney15 explained carboxylic acids formation from MEA oxidation or, in a similar way, from DEA oxidation. * To whom correspondence should be addressed. E-mail:
[email protected]. † Universite´ de Savoie. ‡ IFP, e´changeur de Solaize.
Thermal degradation of MDEA, reported by Chakma and Meisen,16 showed its relative stability. In the presence of O2, main degradation compounds were amines (DEA and DMAE: N,N-dimethylethanolamine), amino-acids derivatives and carboxylic acids.3-5 Ye and Zhang17 proposed a different way to explain acids formation: ethanolamine dealkylation leads to ethylene oxide readily hydrolyzed into ethylene glycol which is oxidized upon contact with air into the designed carboxylic acids. To our knowledge, polyamines degradation in CO2 capture conditions has not yet been studied. However, some articles dealt with amine stability in the presence of air. Two kinds of reactions arose: radical reactions and N-oxide rearrangements.18,19 This work was based on stability comparison between ethanolamines and ethylenediamines in order to identify the impact of the replacement of one alcohol function by one second amine function. Our results were compared with MEA, the benchmark molecule for CO2 capture. Other parameters were investigated: the role of amine function nature (primary, secondary, and tertiary), steric hindrance (AMP: 2-amino-2methylpropan-1-ol), and cyclic structure (DMP: N,N’-dimethylpiperazine). All studied amines are summarized in Table 1. 2. Experimental Section 2.1. Equipment and Chemicals. All amines (reagent grade) were commercially available. Aqueous solutions were prepared using deionized water. Experiments were performed in 100 mL stainless steel batch reactors equipped with magnetic stirrers. Pressure and temperature were monitored respectively with an electronic captor (accuracy 0.2% of full scale) and a thermocouple (accuracy 0.2%). Reactors were equipped with fittings for introducing gas. Analytical-grade synthetic air supplied by Messer (O2 21%, N2 79%, CO + CO2 < 0.5 vpm, CnHm < 0.1 vpm, H2O < 2 vpm, NOx < 0.1 vpm) was used. 2.2. Typical Experimental Run. Experimental conditions were chosen in order to obtain high enough amounts of amine losses to show significant differences between amines. Degradation experiments were conducted using a 4 mol · kg-1 amine concentration. This is a medium value between the different benchmark amines concentrations used for CO2 capture or gas treatment: MEA 30% weight (5 mol · kg-1), DEA 40% weight (3.8 mol · kg-1), MDEA 50% weight (4.2 mol · kg-1).
10.1021/ie9004749 CCC: $40.75 2009 American Chemical Society Published on Web 09/28/2009
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Table 1. Studied Amines Classified by Family (Ethanolamines and Ethylenediamines) and by Amine Nature (Primary I, Secondary II, Tertiary III)
So, all aqueous amine solutions were degassed before degradation in order to strip CO2. Degassed solutions were then loaded into the reactor (40 mL). Each solution was stirred at 250 rpm and heated at 140 °C. This temperature is used for gas treatment application: it is close to reboiler temperature in the case of aqueous solution of MDEA 50% weight. To compare the stability of selected amines, we used the same temperature for all of them, even if the stripper temperature would be different from an amine to another because of bubble points differences. When temperature was stabilized, a 2 MPa air pressure (i.e., 0.42 MPa O2 pressure) was added to the initial vapor pressure. Higher oxygen pressure than in industrial application was used in order to reduce the length of experiments to 15 days: amine degradation in a postcombustion CO2 capture process is a very slow phenomenon. The use of high air pressure increased degradation rates without changing reaction mechanisms because we noticed the formation of the same degradation products at different air pressures. Throughout the reaction time (15 days), extra gas supply was added as needed to maintain a constant pressure. Some runs were conducted twice to check the repeatability of the experiments. 2.3. Analyses. Gas Chromatography. Gas chromatography (GC) was used to determine the remaining amount of starting amine and the amount of identified degradation products. In order to provide the optimal separation of all components, two methods with different capillary columns were carried out: one high polar (CARBOWAX-Amines) and one nonpolar (CPSIL8CB-Amines) column, respectively, on Agilent HP6890 and Varian CP3800 chromatographs. Two different columns were used because some degradation products were well-separated with one column but not with the
other one. For example, MDEA and DEA are not separated with CPSIL8-CB-Amines but with CARBOWAX-Amines. On the other hand, CPSIL8-CB-Amines is more suitable than CARBOWAX-Amines for high molar mass degradation products. All liquid samples were analyzed by GC with the conditions given in Table 2. The vapor phase was not analyzed, but some volatile amines (dimethylamine or trimethylamine for example), also present in liquid phase, were identified by GC/MS analysis. At the end of the degradation run, an internal standard, triethylene glycol (TEG), was added to the degraded sample at 1% w/w to perform quantification. GC response factors were determined with commercially available standards or estimated from similar structure compounds. So, the degradation rate τ of starting amine is defined by τ)
C × 100 C0
(1)
where C is the remaining concentration of starting amine and C0 is the initial concentration of starting amine. On the other hand, the formation rates τf,i of identified degradation products are defined by τf,i )
Ci × 100 C0
(2)
where Ci is the concentration of the degradation product i and C0 is the initial concentration of starting amine. Mass Spectrometry. Usually, degradation compounds were identified by GC/MS. In some cases, synthesis of standards was necessary. GC/MS analyses were carried out with a Finnigan (TOF) with electronic impact (EI). For chemical ionization (CI),
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Table 2. Specifications of the GC and GC/MS Systems GC
column length internal diameter thickness
GC/MS
method A
method B
EI
CI
CARBOWAX-Amines 15 m 530 µm 1.0 µm
CPSIL8-CB-Amines 25 m 320 µm 1.2 µm
CPSIL8-CB-Amines 30 m 250 µm 1.2 µm
HP5-MS 30 m 250 µm 0.25 µm
35 °C / 3 °C · min-1 / / 230 °C 30 min 0.9 mL · min-1 275 °C
35 °C / 3 °C · min-1 / / 230 °C 30 min 0.9 mL · min-1 280 °C
parameters initial temp initial hold time oven ramp (1) intermediate temp oven ramp (2) final temp final hold time flow rate (constant) injector temp detector (FID) temp
60 °C 5 min 5 °C · min-1 75 °C 8 °C · min-1 200 °C 36.4 min 3.9 mL · min-1 280 °C 250 °C
35 °C / 3 °C · min-1 / / 230 °C 30 min 1.6 mL · min-1 280 °C 300 °C
methane was used as a reagent gas and HP5-MS as nonpolar column. This method permitted to obtain the molecular weight of each compound. The highest molecular weight compounds were identified with a high resolution mass spectrometry technique (FT-ICR/MS) with electrospray ionization (ESI). Table 2 provides a summary of GC and GC/MS methods used to identify and to quantify starting amine and its degradation products. Nuclear Magnetic Resonance. 1H and 13C NMR spectra were carried out on a 250 MHz Bruker spectrometer. Deuterated solvents (D2O, MeOD, or CDCl3) were used and the internal standard was sodium salt of 2,2,3,3-tetradeutero-3-trimethylsilyl propionic acid (TSPd4) or tetramethylsilane (TMS). Ionic Chromatography. The samples were diluted with distilled water for the determination of glycolate, formate, acetate, oxalate, nitrite, and nitrate ions. A 25 µL portion of the solution was injected twice on a Dionex ICS-3000 ion chromatograph. The separator column was AS-15 (4 × 250 mm). The eluent was aqueous KOH solution with a concentration gradient (from 8 to 60 mM) at a flow rate of 1 mL · min-1.
A 60 min run time permitted an optimal separation and quantification to be obtained with an uncertainty of (10%. 3. Results and Discussion Thermal and oxidative degradation rates are represented for each amine in Figure 1. Thermal degradation was done in the same conditions as oxidative degradation without introducing air. Thermal degradation was studied to know the contribution of degradation only due to temperature without any gas (CO2 or O2). The hatched frame plots evaluate temperature contribution at 140 °C for 15 days in the degradation due to O2. As it could be noticed, thermal degradation is quite low and can be considered negligible except for DEA, N-methylethanolamine (MAE), MEA, and N,N,N’trimethylethylenediamine (N,N,N’-triMEDA). However, for these four amines, byproducts due to temperature are similar to those due to O2 and involve the same radical mechanisms as we will see below. Regarding oxidative degradation rates, few differences were noticed between ethanolamines and ethylenediamines. Deter-
Figure 1. Degradation rates (τ%) of the different amines with temperature and air.
Ind. Eng. Chem. Res., Vol. 48, No. 20, 2009 Table 3. Carboxylic Acids Amount (ppm) after Oxidative Degradation (4 mol · kg-1, 140 °C, Pp_air ) 2 MPa, 15 days) amine
τ%
glycolate
acetate
formate
oxalate
DMAE MDEA AMP MEA MAE DEA DMP TMEDA N,N,N′-triMEDA N,N-diMEDA N,N′-diMEDA
11 14 8.6 21 17 22 13 16 17 18 18
980 1300
