Article pubs.acs.org/IECR
Nitrosamine Formation in the Desorber of Tertiary AlkanolamineBased Carbon Dioxide Capture Systems Kun Yu Department of Chemical and Biological Engineering, University at Buffalo, The State University of New York, Buffalo, New York 14260, United States
Mikayla C. Reichard Department of Environmental Sciences, Rutgers University, New Brunswick, New Jersey 08901, United States
Ning Dai* Department of Civil, Structural, and Environmental Engineering, University at Buffalo, The State University of New York, Buffalo, New York 14260, United States S Supporting Information *
ABSTRACT: Tertiary amines are being considered as absorption solvents for post-ombustion CO2 capture, but their potential to form harmful byproducts nitrosamines is yet to be evaluated. This study investigated the factors influencing the formation of nitrosamines from tertiary alkanolamines under simulated desorber conditions and the effects of amine structural characteristics. Total nitrosamine formation from tertiary alkanolamine was determined to be first-order with respect to nitrite concentration and the absorbed CO2, but was zero-order with respect to amine concentration in the range of 0.5−2.5 M. Tertiary alkanolamines formed less nitrosamine than their secondary amine analogues. For tertiary alkanolamines with the same number of 2-hydroxyethyl groups, smaller steric hindrance resulted in more nitrosamine formation and higher yields based on nitrite consumption. The analysis of specific nitrosamines revealed that the cleavage of 2-hydroxyethyl group was preferred over demethylation, but comparable to de-ethylation. Reaction pathways were proposed to account for the experimental observations.
1. INTRODUCTION Post-industrial human activities have led to more than 40% increase in atmospheric CO2 level since the mid-1800s.1 In 2013, electricity and heat generation contributed 40% of the global anthropogenic CO2 emissions.2 Therefore, it is imperative for fossil-fuel fired power plants, the largest stationary CO2 sources, to take actions to reduce their CO2 emissions. Amine scrubbing is the most mature technology for CO2 capture. It has been extensively used in gas or natural gas purification since the 1930s and can be used to retrofit existing power plants for post-combustion capture.3,4 An amine-based system has been operational since 2014 at Boundary Dam Power Station (Saskatchewan, Canada) to capture over one million metric tons of CO2 per year from a 139 MW coal-fired power plant post-combustion.5 In the United States, a demonstration plant that uses amine scrubbing to capture CO2 from 240 MW flue gas slipstream is under construction at the WA Parish Plant (Texas).6 © XXXX American Chemical Society
Despite the technical readiness of amine-based CO2 capture systems, concern has been raised regarding the formation of harmful byproducts nitrosamines during post-combustion CO2 capture. Nitrosamines form from the reactions between amines and nitrogen oxides (NOx) present in the flue gas and nitrite, the hydrolysis product of NOx, in the absorber and desorber, respectively.7,8 Nitrosamines have been detected in laboratoryand pilot-scale amine-based post-combustion CO2 capture systems.9−11 In a pilot-scale system treating a 1500 m3/h flue gas stream using the benchmark solvent monoethanolamine, emissions of 5−47 ng/m3 nitrosamines were observed.11 At least eight nitrosamines are listed as probable human Received: December 21, 2015 Revised: February 15, 2016 Accepted: February 17, 2016
A
DOI: 10.1021/acs.iecr.5b04858 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research
(≥98%), 2-(ethylamino)ethanol (≥98%), diethanolamine (≥99%), sodium bicarbonate (≥99.7%), sodium nitrite (≥99%), sulfamic acid (99.3%−100.3%), 4-methoxybenzen sulfonyl chloride (99%), N-nitrosomorpholine (5000 μg/mL in methanol), EPA 521 nitrosamine mix (2000 μg/mL of each Nnitrosodimethylamine, N-nitrosomethylethylamine, N-nitrosodiethylamine, N-nitrosodi-n-propylamine, 1-nitrosopyrrolidine, 1-nitrosopiperidine, and N-nitrosodi-n-butylamine), N-nitrosodiethanolamine, and N-nitrosodimethylamine. The following chemicals were obtained from Fisher Scientific: sodium carbonate, sodium hydroxide (99.1%), acetonitrile (HPLC grade, 99.9%), and methylene chloride (DCM, HPLC grade, 99.9%). Potassium iodide (≥99%) and iodine resublimed were purchased from Acros. Sulfuric acid (95−98%) and glacial acetic acid (99.9%) were purchased from J.T. Baker. Anhydrous sodium sulfate was purchased from Macron Fine Chemicals. The color reagent for nitrite determination (sulfanilamide/N(1-naphthyl)ethylenediamine dihydrochloride solution) was purchased from Ricca Chemical. Nitrogen gas (99.999%) was purchase from Jackson Welding and Gas Products. All chemicals were used as received, without further purification. The amines and N-nitrosamines investigated in this study and abbreviations are summarized in Table 1. Their structures are shown in Figure 1.
carcinogens in the U.S. Environmental Protection Agency (EPA)’s Integrated Risk Information System database (IRIS).12 The structure of amines plays a critical role in determining their reactivity toward nitrosamine formation in CO2 capture systems.13 Secondary amines exhibit the highest nitrosamine formation potential, followed by tertiary amines and far exceeding primary amines.13 Previous studies investigated the formation and decay of nitrosamines under desorber conditions when secondary amines were used as CO2 absorption solvents.8,14,15 At temperatures of 100−150 °C, stoichiometric conversion of nitrite to nitrosamine N-nitrosopiperazine (MNPZ) was observed for piperazine (PZ);14 the conversion was first-order, with respect to nitrite.8,14,16 Assuming stoichiometric conversion of nitrite to nitrosamines for other secondary amines, such as diethanolamine (DELA), methylethanolamine (MELA), and morpholine (MOR), the dependence of nitrosamine formation on amine concentration, solution pH, and CO 2 loading was reported.8,16 The decomposition of nitrosamine MNPZ was shown to be a function of solution pH, base strength, base concentration, and CO2 loading.15 Tertiary amines are being considered as alternatives to the benchmark solvent monoethanolamine for CO2 absorption. Compared to primary and secondary amines, tertiary amines have the potential advantages of featuring high theoretical absorption capacity,17 low heat of absorption,18−20 and relatively high stability in the desorber.21,23 Carbamate-forming amines are limited to a chemical absorption of 0.5 mol of CO2 per mole of amino functional group, while tertiary amines are capable of 1:1 (C/N) chemical absorption.24 Tertiary amines are commonly mixed with primary or secondary amines to enhance CO2 absorption performance. The mixture of 8.4 M methyldiethanolamine (MDELA) with activator PZ provided an energy savings of 15%−22%, compared with 7 M monoethanolamine (MEA).19 MDELA degradation in air at 120 °C was 10 times slower than DELA, and was 30 times slower than MELA.22 Using tertiary amine for CO2 capture is also likely to form less nitrosamines than their secondary amine counterparts,13 but the mechanism and structural effects of tertiary amines on nitrosamine formation have not been investigated. It is anticipated that the pathways to forming nitrosamines from tertiary amines are more complex than secondary amines, because of the additional dealkylation step. In this study, we investigated the formation and decay kinetics of nitrosamines and the effects of nitrite concentration, amine concentration, CO2 loading, and reaction temperature on the formation of nitrosamines using tertiary alkanolamine triethanolamine (TELA) as a model amine. Because of the uncertainty in the tertiary alkanolamine dealkylation step, total nitrosamines were measured to capture all products featuring N-nitroso functional groups. We then compared nitrosamine formation from six tertiary alkanolamines and three secondary alkanolamines with systematic structural variation, and analyzed specific nitrosamines formed from these model amines. Lastly, we proposed reaction pathways based on the experimental findings.
Table 1. Amines and Nitrosamines Investigated in This Study name 1 2 3 4 5 6 7 8 9 a b c d e f g h
Amines 2-(dimethylamino)ethanol 2-(diethylamino)ethanol N-methyldiethanolamine N-ethyldiethanolamine N-tert-butyldiethanolamine triethanolamine 2-(methylamino)ethanol 2-(ethylamino)ethanol diethanolamine Nitrosamines N-nitrosodiethanolamine N-nitrosomethylethanolamine N-nitrosoethylethanolamine N-nitrosodimethylamine N-nitrosomethylethylamine N-nitrosodiethylamine N-nitrosodi-n-butylamine N-nitrosomorpholine
abbreviation
CAS No.
DMELA DEELA MDELA EDELA BDELA TELA MELA EELA DELA
108-01-0 100-37-8 105-59-9 139-87-7 2160-93-2 102-71-6 109-83-1 110-73-6 111-42-2
NDELA NMELA NEELA NDMA NMEA NDEA NDBA NMOR
1116-54-7 26921-68-6 13147-25-6 62-75-9 10595-95-6 55-18-5 924-16-3 59-89-2
2.2. Sample Preparation. Amine solutions were prepared volumetrically to achieve amine concentrations ranging from 0.5 M to 2.5 M. Sodium bicarbonate (NaHCO3) was added to the solution to achieve C/N molar ratios between 0 and 0.2. A nitrite stock solution (200 mM) was prepared by dissolving sodium nitrite in deionized water. Various amount of the stock solution was subsequently spiked into the samples to achieve the target nitrite concentrations (0−100 mM). Freshly prepared 10 mL reaction solutions were placed in Swagelok stainless steel tubes (6 in. in length, 0.5 in. in outer diameter (OD)), leaving 10 mL of headspace. The tubes were placed in a convection oven at set temperatures. The tubes were removed from the oven at preselected time, cooled immediately in a
2. EXPERIMENTAL SECTION 2.1. Materials. The following chemicals were obtained from Sigma−Aldrich: N-methyldiethanolamine (≥99%), N-ethyldiethanolamine (98%), N-tert-butyldiethanolamine (97%), 2(dimethylamino)ethanol (≥99.5%), 2-(diethylamino)ethanol (≥99.5%), triethanolamine (≥99%), 2-(methylamino)ethanol B
DOI: 10.1021/acs.iecr.5b04858 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research
Figure 1. Amines and N-nitrosamines investigated in this study.
7890B GC-240 MS) was used. The GC-MS method was modified from EPA Method 521.27 First, 100 μL of sample solution was diluted into 8 mL of Milli-Q water, spiked with 5 μL of d6-NDMA as the internal standard, and extracted with 2 mL dichloromethane (DCM). The ratio of the responses determined by MS between specific nitrosamines and d6NDMA in the samples were compared against those in the standard curve for quantification. The DCM extract (5 μL) was injected using PTV Solvent Vent mode. The initial column temperature was set at 37 °C and held for 0.2 min, and then increased to 40 °C at a rate of 5 °C min−1 and held for 3.5 min, and then increased to 60 °C at a rate of 3 °C min−1, and then increased to 100 °C at a rate of 5 °C min−1, and then increased to 130 °C at a rate of 10 °C min−1 and held for 1 min; finally, the column temperature was raised to 250 °C at a rate of 60 °C min−1 and held for 1 min. The transfer line temperature was 230 °C. Methanol was used as the reagent for chemical ionization in the ion trap mass spectrometer. Secondary amines were derived using 4-methoxybenzen sulfonyl chloride and analyzed via HPLC. Briefly, 5 μL samples were added into 1.5 mL of Milli-Q water buffered by 20 mM Na2CO3 and 20 mM NaHCO3. The solution was spiked with 20 μL of the derivatizing agent (200 g/L 4-methoxybenzenesulfonyl chloride in acetonitrile), and then shaken for 10 min. The derivatized samples were analyzed by HPLC and detected by UV spectroscopy at 250 nm. An Agilent Poroshell 120 ECC18 column (4.6 mm × 50 mm, 2.7 μm) was used. The eluent was 80% Milli-Q water and 20% acetonitrile with a flow rate of 1.5 mL/min. The standard curves prepared in deionized water and in tertiary alkanolamine solutions showed the same response, indicating that the presence of large amount of tertiary alkanolamines did not interfere with the secondary amine analysis. The only exception was DMELA. MELA signal of 0.22 M was observed for 2.5 M DMELA solution without reaction, in contrast to the 99.5% purity provided by the vendor and the high purity confirmed by 1H NMR spectroscopy, using a Varian Inova-500 spectrometer (see Figure S1 in the
water bath, and analyzed for nitrite, total nitrosamine, specific nitrosamines, and secondary amines. 2.3. Sample Analysis. Nitrite was measured by the N-(1naphthyl)ethylenediamine dihydrochloride colorimetric method.25 Samples were diluted 1000 times prior to the addition of color reagent in order to achieve nitrite concentrations within the measurement range and to minimize the interference from amines. Total nitrosamine was determined as previously described.9,26 Briefly, samples were pretreated with 40 g/L sulfamic acid at pH 2 to remove the interference of nitrite; the acidic pH was achieved by sulfuric acid after the addition of sulfamic acid. The pretreated samples were injected into a 60mL reaction chamber (80 °C). The reaction solution consisted of 40 mL of glacial acetic acid, mixed with 4 mL of freshly prepared aqueous solution containing 540 g/L potassium iodide and 114 g/L iodine. The triiodide solution liberates nitric oxide from nitrosamines by cleaving the N−N bond. The nitric oxide was purged from the reaction chamber by nitrogen gas and then flowed through a base trap into nitric oxide analyzer (EcoPhysics 88Yet1283). An NDELA standard solution was used to establish the calibration curves and was verified to produce the same response as nitrite and NDMA, which was used as the standard in previous studies.9,13,26 The specific nitrosamines NDELA, NMELA, and NEELA derived from alkanolamines are relatively hydrophilic, and therefore were analyzed using high-performance liquid chromatography (HPLC) with a diode array detector (DAD) (Agilent 1260 Infinity). An Agilent Poroshell 120 EC-C18 column (4.6 mm × 50 mm, 2.7 μm) was used with isocratic elution composed of 96% Milli-Q water and 4% methanol. The eluent flow rate was 1.5 mL min−1 and sample injection volume was 5 μL. The column compartment temperature was set at 30 °C. Nitrosamines were detected by ultraviolet (UV) absorbance at 234 nm. For the more-volatile nitrosamines (the seven nitrosamines included in the EPA Method 521 and NMOR), a gas chromatography−mass spectrometry system (Agilent Model C
DOI: 10.1021/acs.iecr.5b04858 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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of kN for DELA solution at 120 °C and TELA solutions at 120 and 150 °C are shown in Table 2. At 120 °C, nitrite loss was
Supporting Information). Certain tertiary amines, such as N,Ndimethylaniline, have been reported to interfere the benzenesulfonyl chloride derivatization analysis by generating false signals.28 2.4. Nitrosamine Synthesis and Characterization. NMELA and NEELA were synthesized according to a procedure modified from the literature:29 500 μL of MELA or EELA was transferred into a 40 mL vial, using a 500 μL gastight syringe, and then slowly titrated with 5 mL of glacial acetic acid. The solution was then placed in an ice bath, and 2.5 mL of freshly prepared 5 M sodium nitrite aqueous solution was added dropwise with rapid stirring. The reaction was allowed to proceed for 90 min, after which point 7.5 mL of Milli-Q water was added to the vial while the stirring continued for another 30 min. For the extraction of product nitrosamine compounds, 40 mL of saturated sodium bicarbonate solution was added to neutralize the solution in a 125 mL separatory funnel; the neutralized solution was then extracted with 10 mL of DCM 10 times. The DCM extract was dried over 10 g of anhydrous sodium sulfate for 30 min with stirring. The supernatant of the DCM extract was then transferred to conical vials and blown down under a gentle stream of N2 gas to retrieve pure nitrosamines. The structure of synthesized NMELA and NEELA was confirmed by 1H NMR spectroscopy, using a Varian Inova-500 spectrometer; the NMR spectra are shown in Figures S2 and S3 in the Supporting Information. Two isomers of NEELA were likely to be in our synthesized sample, which induced peak multiplicities of proton in the same group. The synthesized NMELA and NEELA generated 72.8% and 72.6% response, compared to NDELA standards in total nitrosamine analysis.
Table 2. Rate Constants for Nitrite Loss, Total Nitrosamine Formation, and Total Nitrosamine Decay in DELA and TELA Solutions (2.5 M Amine with 0.2 Loading (C/N)) amine
temperature (°C)
kN (× 10−2 h−1)
kTF (× 10−3 h−1)
kTD (× 10−3 h−1)
DELA TELA TELA
120 120 150
7.5 ± 0.17 1.1 ± 0.04 5.4 ± 0.29
3.7 ± 0.11b 2.5 ± 0.13a 6.6 ± 0.12b
7.0 ± 1.33 3.0 ± 0.37 5.9 ± 0.75
kTF for TELA (120 °C) was calculated from the slope of total nitrosamine concentration vs initial nitrite concentration in Figure 4 by eq 4. bkTF for DELA (120 °C) and TELA (150 °C) was calculated from fitting of experimental data in Figure 3 with eq 4 by applying the Levenberg−Marquardt algorithm.
a
seven times faster in DELA than in TELA. When the temperature increased from 120 °C to 150 °C, kN in TELA increased 5-fold, but was still smaller than that in DELA at 120 °C. The time profiles of total nitrosamine accumulation and decay are shown in Figure 3. With 50 mM initial nitrite
3. RESULTS AND DISCUSSION 3.1. Formation and Decay of Nitrosamines. DELA and TELA were studied as representative secondary and tertiary alkanolamines, respectively. Because nitrite is responsible for total nitrosamine formation in the desorber of CO2 capture systems,14 nitrite loss was monitored in all experiments to allow the construction of a kinetic model and to provide mechanistic insight into the reaction pathways. First-order kinetics was obtained (Figure 2): [NO−2 ] = [NO−2 ]0 exp(−kNt )
[NO2−]
(1)
[NO2−]0
where and are the concentrations (in units of M) of nitrite at time t and time zero, respectively; kN is the nitrite loss rate constant (expressed in units of h−1). The values
Figure 3. Formation and decay of total nitrosamine in 2.5 M amine with 0.2 loading (C/N) and (a) 50 mM initial nitrite or (b) 10 mM initial nitrite. The solid lines represent modeled data and dash lines represent fitted data. Experimental data are shown in Table S1.
concentration, 2.5 M amine, and 0.2 loading (C/N), total nitrosamine concentration in DELA (120 °C) and TELA (150 °C) solutions initially increased and then decreased, with the highest concentration observed at ∼50 h (Figure 3a). This corresponded to the time by which nitrite was exhausted in the two solutions: 96% nitrite loss (2 mM residual) was observed at 45 and 65 h for DELA (120 °C) and TELA (150 °C), respectively (Figure 2). In contrast, at 120 °C, total nitrosamine continued to accumulate in TELA solution up to 140 h, by
Figure 2. Nitrite loss in 2.5 M amine with 0.2 loading (C/N) and 50 mM initial nitrite concentration. Experimental data are shown in Table S1 in the Supporting Information. D
DOI: 10.1021/acs.iecr.5b04858 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research
h) should be proportional to the initial nitrite concentration according to eq 4. Indeed, when we varied the initial nitrite concentration (ranging from 10 mM to 100 mM), the total nitrosamine concentration at 21.1 h increased proportionally (Figure 4). This suggests that our initial assumption that total
which 18% of the initial nitrite (9 mM residual) still remained in the solution. Total nitrosamine accumulation in TELA solutions was slower at 120 °C than at 150 °C initially; however, after 150 h, the total nitrosamine concentration was 2.5 times higher in the 120 °C solution, as a result of the slower nitrite loss and potentially slower total nitrosamine decay at this temperature. To capture the total nitrosamine decay kinetics in TELA solutions at 120 °C, another experiment with 10 mM initial nitrite was conducted (Figure 3b). Maximal total nitrosamine concentration was observed after ∼150 h, when the residual nitrite was 2.3 mM. The initial total nitrosamine accumulation was faster in DELA than in TELA regardless of temperature, but the subsequent total nitrosamine decay was also faster in DELA. At 120 °C, total nitrosamine concentration in DELA was four times higher than that in TELA at 50 h, when total nitrosamine maximum was observed in DELA; the difference, however, quickly diminished as total nitrosamine concentration started to decline: by 125 h, total nitrosamine concentration in DELA was only two times higher than that in TELA. Attempt was made to model the formation and decay of total nitrosamines under simulated desorber conditions. First, we observed that the decay of total nitrosamines followed firstorder kinetics after nitrite was consumed (
