Thermal and Oxidative Degradation of Aqueous N, N-Diethylethanolamine

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Energy Procedia 63 ( 2014 ) 1911 – 1918

GHGT-12 Thermal and oxidative degradation of aqueous N, N-Diethylethanolamine (DEEA) at stripping conditions for CO2 capture Hongxia Gaoa, Wichitpan Rongwonga, Chao Penga, Zhiwu Lianga*, Kaiyun Fua, Raphael Idema,b, Paitoon Tontiwachwuthikula,b

a Joint International Center for CO2 Capture and Storage (iCCS),College of Chemistry and Chemical Engineering, Hunan University, Changsha,410082, P.R. China b International Test Centre for CO2 Capture (ITC), Faculty of Engineering and Applied Science, University of Regina, Regina, Saskatchewan, S4S 0A2, Canada

Abstract

An exact and reliable estimation of amine degradation, which can lead to significant problems, such as solvent losses, degradation by-products, fouling, foaming, corrosion, and increasing solution viscosity, is very important for development of innovative amines with higher chemical stability. In this study, thermal and oxidative degradations of aqueous N, N-

Diethylethanolamine (DEEA) solution with CO2 were investigated in a 600 mL stainless steel autoclave and compared with aqueous MEA solution. The results showed that DEEA solutions demonstrated a higher resistance to thermal degradation. The

influence of key operating parameters, such as temperature, CO2 loading, and DEEA concentration, on DEEA loss (mol%) were also investigated. In addition, the possible mechanisms of DEEA thermal degradation have been developed on the basis of the identified products. © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ©(http://creativecommons.org/licenses/by-nc-nd/3.0/ 2013 The Authors. Published by Elsevier Ltd.). SelectionPeer-review and under peer-review responsibility under of theresponsibility Organizing Committee of GHGT. of GHGT-12

Keywords: degradation; N, N-Diethylethanolamine; autoclave; mechanism.

* Corresponding author. Tel.: 86-13618481627; fax: +86-731-88573033. E-mail address: [email protected] (Z. Liang).

1876-6102 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the Organizing Committee of GHGT-12 doi: 10.1016/j.egypro.2014.11.200 1912 Hongxia Gao et al. / Energy Procedia 63 ( 2014 ) 1911 – 1918

1. Introduction

Fossil fuel-fired power plants (i.e., coal, oil or natural gas) have made large contributions to the increase of atmospheric concentrations of carbon dioxide (CO2) which has been blamed as being responsible for the occurrence of climate change and global warming problems. Therefore, it is critical to remove large quantities of CO2 from large single point sources such as power plants that would otherwise be emitted into the atmosphere. CO2 absorption using aqueous amines solutions is the most mature post-combustion strategy due to its commercially maturity, cost efficiency, and convenience to retrofit into existing power plants [1-2]. Unfortunately, in the CO2 capture absorption-stripping processes, one of the major problem using amines is its degradation due to the presence of CO2, and/or oxygen (O2), NOx, and SOx leading to solvent loss, formation of amine by-products, foaming, fouling, corrosion, and an increase in the solvent’s viscosity [3-5]. Amine degradation can result in an increase of operation cost of CO2 capture through solvent make up and reduction of absorption capacity [1,6-8]. Therefore, understanding the mechanism of amine degradation is useful to improve the process performance and develop better solvent systems. Recently, N, N-Diethylethanolamine (DEEA), a new tertiary alkanolamine, has received considerable attention. DEEA is comprised of two ethyl groups that replace the hydrogen atoms of the amino group in MEA, a primary alkanolamine. It has the chemical structure which is very close to 4-diethyamino-2-butanol (DEAB) which, in turn, has been shown to possess high absorption capacity, good absorption rate, and low regeneration energy [10-12]. DEEA exhibits good performances in terms of absorption capacity (1.0 mol CO2/ mol amine), low absorption heat, low desorption energy and good absorption rate, and can be prepared from renewable and/or cheap resources (such as ethanol and ethylene). Although DEEA has a much higher rate constant for reaction with CO2 than MDEA, the kinetics of CO2 absorption is still slower than that of MEA. Therefore, in DEEA-based solvents, the addition of small amounts of other amines to DEEA aimed to enhance the CO2 absorption rate, appears to be a commercially attractive proposition for an absorbent for CO2 capture [13-15]. In this work, the thermal degradation of aqueous DEEA solution was studied and compared with that for MEA. The influence of operating parameters, such as temperature, CO2 loading, and DEEA concentration on the amine loss were investigated. The main purposes of this study are to focus on the quantification of the amine loss, and identification of the degradation products in order to establish a possible thermal degradation pathway of the degradation of DEEA.

2. Experimental Section

2.1 Chemicals and Experimental apparatus

Reagent grade DEEA and MEA each with purity of% 98 were purchased from Tianjin Kermel Chemical Reagent Co. Ltd., China. Commercial grade CO2 and air were supplied by Changsha Jingxiang Gas Co. LTD., China, each with a purity of Thermal degradation experiments were performed in a 600 mL stainless steel autoclave (Model 4768, Parr Instrument Co., Moline, IL) which consisted of a gas feed port, an extra feed port, a gas product port, a liquid sampling valve, a thermowell, and a pressure gauge. A K-type thermocouple placed in the thermowell was used to measure the solution temperature, while an electric furnace using a proportional-integral-derivative (PID) temperature controller was used to supply the heat. The accuracy of the temperature control was within ±2 K.

2.2 Typical Degradation Experimental Run

The amine solutions were prepared to a desired concentration using deionized water in a volumetric flask. The concentration was then determined by GC-MS. The CO2 loaded amine solutions were prepared by bubbling CO2 gas through the solution until the desired concentration was obtained.In addition, the amount of CO2 dissolved in the solution or CO2 loading was detemined from the evolved gas collected in a precision gas burette containing a displacement solution in which an excess 1 mol/L HCl was added. For a typical thermal degradation experimental run, 350 mL of aqueous amine solution of the desired Hongxia Gao et al. / Energy Procedia 63 ( 2014 ) 1911 – 1918 1913 concentration was loaded into the reactor. Then, the reactor was put into a heating mantle and the solvent was stirred at 500 rpm. In the experiment, the samples were taken from the reactor through the liquid sampling port at appropriate predetermined intervals and then quickly moved to a refrigerator. The liquid samples were analyzed using a GC-MS technique for determinations of amine loss and degradation products. Figure 1 shows the schematic of the experimental setup in this study.

F igure 1. Schematic representation of the experimental setup.

2.3 Analyses

The amine concentration and its degradation products were determined using GC-MS (model 6890N/5973N, Agilent Technologies). The column used was a DB-wax (high-SRODULW\ FROXPQ ȝPWKLFNQHVVȝPLGDQG 60 m length, Agilent Technologies) The products were identified by matching their mass spectra with commercial mass spectra of the National Institute of Standards and Technology (NIST) database (1998 version), and its retention time was compared with the pure standard (for those commercially available). Each sample was diluted using deionized water in the ratio of 1:5 prior to the analysis to avoid contamination of the system and to have a higher sensitivity. The samples were analyzed twice to check for repeatability. The error in repeatability was 2%. The percentage of amine loss compared with the initial amine concentration is defined as C C Ĳ 0 u100% C0

Where C is the remaining concentration of amine and C0 is the initial amine concentration.

3. Results and discussion

Since amine degradations take months to proceed, the amine degradation experiments were operated at an elevated temperature and pressure higher than normal stripper conditions in order to accelerate the reactions. The DEEA thermal degradation products were also tracked during the experiments. However, it should be noted that some small volatility molecules (e.g. ammonia), ionic compounds, or polymers, cannot be found in the GC spectra due to the limitation of our analytical method. A summary of major DEEA thermal degradation products identified by GC-MS based on a large library of mass spectra (NIST) are listed in Table 1. 1914 Hongxia Gao et al. / Energy Procedia 63 ( 2014 ) 1911 – 1918

Table 1. Thermal degradation products of aqueous DEEA solution

Number Degradation products Abb. Structure Mw (g/mol)

1N-Ethylethanolamine EEA 89

2Triethylamine 101

3Ethylene Glycol EG 62

4Diethylamine 73

5 Ethylene Oxide EO 44

6N-Ethyoxazolidinone EOZD 115

7 N,N-ethyl-N’-(2-hydroxyethyl)ethylenediamine DEHEED 160

8 N,N,N’-ethyl-Ethylenediamine TEED 144

9 N,N-Diethylpiperazine DEP 142

10 3-Ethyl-Oxazolidine 101

11 Dimethylol ethylene urea 146

12 N,N,N’,N’- tetraethyl-1,2-Ethanediamine 172

13 N-Ethylmorpholine EM 115

14 1,3,5-Triazine-1,3,5(2H,4H,6H)-triethanol 219

15 N,N,N’,N’’ ,N’’-Pentaethyldiethylenetriamine 243 Hongxia Gao et al. / Energy Procedia 63 ( 2014 ) 1911 – 1918 1915

3.1 Thermal degradation in the DEEA-CO2-H2O system

3.1.1 Effect of temperature

The thermal degradation of DEEA was observed at four temperatures: 120 oC (normal stripper operating o o o condition), 135 C, 150 C and 175 C using 3mol/L DEEA with CO2 loading of 0.5 mol CO2/mol DEEA. The plots of DEEA percentage loss versus time at different temperatures are shown in Figure 2. As expected, the reduction of DEEA concentration is significantly influenced by the solution temperature and the slope of the DEEA loss were quite linear during the experiments. After 300 h, there were 49.0 mol% and 97.0 mol% of the DEEA remaining in the solution at 175 oC and 120 oC, respectively. The increase of solution temperature resulted in the larger decrease of DEEA concentration because it increased the degradation rate constant (k) of the DEEA-CO2-H2Osystem. The thermal degradation of 5M MEA has been investigated to evaluate the stability performance of DEEA, as shown in Figure 2. In comparison, at the same experimental conditions, DEEA was found to be more stable than MEA. This is due to the fact that DEEA needs a preliminary deethylation reaction step that is necessary to initiate significant degradation [17].

Figure 2. Effect of temperature on amine loss.

3.1.2 Effect of CO2 loading

CO2 loading is considered to be a vital important parameter in the CO2 separation process because it reflects the available amine left in the solution. In the present work, the CO2 loading in the DEEA solution was varied in the range of 0.3-0.7 CO2/mol DEEA. The effect of CO2 loading on the DEEA loss is shown in Figure 3. The results show that the DEEA loss increased as the CO2 loading in the DEEA solution was increased from 0.3-0.7. This indicates that the degradation rate of DEEA increases with the increase in CO2 loading since more active species, protonated DEEA (DEEAH+), increases in the solution [18]. 1916 Hongxia Gao et al. / Energy Procedia 63 ( 2014 ) 1911 – 1918

Figure 3. Effect of CO2 loading on amine loss.

3.1.3 Effects of initial DEEA concentration

o Figure 4 shows the influence of DEEA concentration on the DEEA loss at 150 Cand CO2 loading of 0.5. The results show that the increase in percentage DEEA loss with the degradation time was quicker as the initial DEEA concentration was increased. The higher the DEEA concentration, the more reactive DEEA molecules are available + - for absorbing CO2 to generate larger amounts of DEEAH and HCO3 , and reacting with other products. This observation corresponded well with the works of Davis [16,19], who studied the thermal degradation of CO2 loaded aqueous solutions of MEA and PZ.

Figure 4. Effect of DEEA concentration on amine loss. Hongxia Gao et al. / Energy Procedia 63 ( 2014 ) 1911 – 1918 1917

3.2 Products and possible pathway of DEEA

DEEA is considered to be a tertiary amine due to the attachment of three carbon atoms to the nitrogen atom. The possible route for thermal degradation of tertiary amines has been discussed by many researchers [17,19-21]. In this case, DEEA cannot react directly with CO2 to form carbamate but acts as a base that catalyzes the hydration of CO2 - + + to give HCO3 and DEEAH . Later, DEEA molecule can attack the protonated DEEA (DEEAH ) to generate one EEA (number 1 in Table 1) and a quaternary amine. The quaternary amine then releases the triethylamine (2). In the presence of CO2, EEA (1) reacts with CO2 to form EEA carbamate and protonated EEA, and then the EEA carbamate can give EOZD (6) through the irreversible dehydration reaction. Additionally, the diethylamine (4) can also react with the EOZD (6) to form TEED (8) which was detected by the mass spectroscopy. DEP (9) is formed through TEED (8) by the ring closure reaction and is then accumulated in the solution. The proposed pathway of DEEA thermal degradation is presented in Figure 5. N H OH 4 N

N DEEA

deethylation ethylation

H N N O N HO N HO H NN

1 6 8 9

Figure 5. Proposed reaction pathway for thermal degradation of DEEA

4. Conclusions

The thermal degradation performance of aqueous DEEA solutions was experimentally measured in a 600 mL stainless steel autoclave. In experimental studies, it was clearly that an increase in temperature, CO2 loading and DEEA concentration all resulted in an increase in the DEEA degradation rate, as expected. Also, DEEA has a high resistance to thermal degradation compared to MEA, the benchmark molecule. Additionally, DEEA degradation products were tracked during the experiments and identified by GC-MS based on the large library of mass spectra (NIST). The possible general pathway of DEEA thermal degradation was also proposed based on the experimental results. Oxidative degradation of amine solutions, which is the more significant cause of amine solvent degradation and more complex compared to thermal degradation, generally occur in the presence of dissolved O2. Since amine degradation is a very slow process, it takes months to proceed. Thus, our future studies will look at oxidative DEEA degradation.These studies will examine the oxidative DEEA degradation rate, products, and mechanism.

Acknowledgements

The financial supports from the National Natural Science Foundation of China (Nos. 21276068, 21376067 and U1362112), Doctoral Program Foundation (20130161110025), National Key Technology R&D Program (Nos. 2012BAC26B01 and 2014BAC18B04), Technology Development contract of Shaanxi Yanchang Petroleum (Group) Co., LTD (No. Shanyan 12-34), Innovative Research Team Development Plan-Ministry of Education of China (No. IRT1238), and China’s State“Project 985”in Hunan University Novel Technology Research & Development for CO2 Capture are gratefully acknowledged. 1918 Hongxia Gao et al. / Energy Procedia 63 ( 2014 ) 1911 – 1918

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