Investigation of CO2 desorption kinetics in MDEA and MDEA+DEA rich amine solutions with thermo-gravimetric analysis method

International Journal of Greenhouse Gas Control 95 (2020) 102947

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Investigation of CO2 desorption kinetics in MDEA and MDEA+DEA rich amine solutions with thermo-gravimetric analysis method

T

Kang Shunjia,b, Shen Xizhoua,*, Yang Wenzec a

Key Laboratory of Green Chemical Process, Minstry of Education, Wuhan Institute of Technology, Wuhan, Hubei, China The College of Post and Telecommunication of Wuhan Institute of Technology, Wuhan, Hubei, China c Central Southern China Electric Power Design Institute, Wuhan, Hubei, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: MDEA DEA CO2 Desorption kinetics Thermo-gravimetric analysis

In this paper, by means of thermo-gravimetric analysis(TGA), the desorption kinetics of CO2 absorbed in two kinds of rich amine solutions, MDEA (3.25mol/L) and MDEA+DEA(3.25mol/L-0.3mol/L), were investigated under different heating rates(2.5℃/min, 5℃/min, 10℃/min and 20℃/min). The thermal analysis kinetics was applied to analyze the TG-DTG curves of two rich amine solutions so as to research CO2 desorption kinetics. In addition, the CO2 desorption kinetics parameters have been calculated with model-free method Flynn-WallOzawa (FWO) and model-fitting method Coats-Redfern (CR). The results indicated that CO2 desorption process could be divided into two stages. The CO2 and H2O were released with non-uniform speed in the first stage and MDEA or DEA with higher boiling points were evaporated in the second stage. For MDEA solution the average activation energy E was 50.36kJ/mol, the pre-exponential factor A was 1.68×107, and the most probable integral mechanism function was G (α ) = α3/2 . For MDEA+DEA solution the average activation energy E was 59.68kJ/mol, the pre-exponential factor A was 2.22×107, and the most probable integral mechanism function was G (α ) = [(1 + α )1/3 − 1]2 . The technical feasibility of CO2 desorption performance in rich amine solutions with thermo-gravimetric analysis method was demonstrated.

1. Introduction It is well known that CO2 is the main source of greenhouse gases. Undoubtedly, the capture and storage of CO2 have become a central issue of global warming research. (Jessica et al., 2018; Lin et al., 2019). Commonly, some alkanolamine solutions, such as monoethanolamine (MEA), diethanolamine (DEA), methyldiethanolamine (MDEA) or their blended amine solution, are used as absorbents to remove CO2 in plant. (Jiang et al., 2017; Mahdi et al., 2018; shahid et al., 2018). The CO2 capture process in industry generally includes two units, absorption and desorption. The energy consumption and cost are greatly influenced by regeneration section which suffers from high temperature (Díez et al., 2015; Casas et al., 2013; Zhang et al., 2017a,b). Therefore, desorption performance is also considered as an important factor for choosing efficient absorbent. Most studies traditionally focus on CO2 absorption performance, but the available literature referred to desorption is relatively scarce as compared to absorption research(Feng et al., 2013; Ghalib et al., 2016; Tang et al., 2016). A novel hemispherical device and a strict mathematical model for CO2 absorption-desorption in MEA, DEA, MDEA,

⁎

AMP and blends were investigated. The research showed that MEA has high reactivity with CO2, DEA was intermediate and MDEA has low reactivity. CO2 desorbed from MDEA solution faster than DEA and MEA solutions. The results predicted with rigorous mathematical model were consistent with experimental data (Jamal et al., 2006). The CO2 removal efficiency, reaction rate, and CO2 loading in aqueous blended solution MEA + AMP were researched. The results indicated that absorption efficiency of blended MEA + AMP solution was higher than MEA or AMP solution solely. MEA + AMP ratio in blended solution also affected CO2 desorption rate (Won-Joon et al., 2008). CO2 absorption and desorption in DEA and MDEA solutions have been investigated by means of 13C NMR spectroscopy. The change of CO2 content, formation time and temperature of reaction product could be monitored accurately with this method. The scope of this study was expanded for other alkanolamines so as to save the energy cost (Barzagli et al., 2011). The CO2 desorption rate for MDEA solutions was researched using a stirred cell with a flat gas-liquid interface and mass-transfer model was established. The prediction results of CO2 desorption rates were found to be in good agreement with experiment values under condition 298.15–313.15 K and 10–30wt% MDEA solution (Kierzkowska-Pawlak

Corresponding author. E-mail address: [email protected] (S. Xizhou).

https://doi.org/10.1016/j.ijggc.2019.102947 Received 9 July 2019; Received in revised form 21 November 2019; Accepted 16 December 2019 1750-5836/ © 2020 Elsevier Ltd. All rights reserved.

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and Chacuk, 2011). The chemical enhancement factors of absorption and desorption process in aqueous 2.0 MDEA at temperatures of 298.15 K, 313.15 K and 333.15 K in a batch-operated stirred tank reactor have been studied. The research introduced chemical enhancement factor so as to evaluate desorption performance in MDEA solution. (Espen and Geert, 2012). A novel membrane absorption-desorption tower regeneration apparatus was applied to achieve lower desorption energy consumption. The results showed that the tertiary amine (triethylenetetramine TETA and MDEA) have greater CO2 regeneration rate than ethylenediamine (EDA). The MDEA is mostly recommended in CO2 regeneration process (Wang et al., 2013). The absorption-desorption system were carried out to study CO2 absorption and desorption performance in aqueous MDEA + DMSO solutions in a bubbling absorption reactor and wetted wall desorption reactor. The result showed that 5 mol/L MDEA-2.5 mol/L DMSO mixture exhibited an optimal absorption efficiency of 91.04% with CO2 and optimal desorption temperature was 353 K (Luo et al., 2018). CO2 desorption from MDEA rich solutions in a highly efficient microchannel reactor was investigated experimentally. The research found that the microchannel reactor was a valid method and the empirical correlations were proposed to predict the mass transfer coefficient with good prediction performance. (Liu et al., 2017). As mentioned above there are mainly two approaches researching CO2 desorption performance. One method is heating rich amine solution in balloon flask or reactor to simulate desorption process experimentally. Another method is regenerating CO2 in pilot plant which consists of desorption tower, gas cylinder, heater, control valves instruments, etc. to simulate desorption process. (Zhang et al., 2017a,b; Li et al., 2016). Whatever apparatus used in experiment, CO2 desorption essence is heating the rich amine solution. The data are obtained by measuring the outlet CO2 concentration, partial pressure, analyzed online directly, and acidolysis of rich amine solutions, et al. However the deviation between experiment and theoretical results maybe resulted from some influences, such as device structure, operating conditions, analyzing approach, monitoring method and control instruments and so on. Moreover, kinetic parameters such as diffusion coefficient, absorption rate, enhancement factor and activation energy could be calculated with a known kinetic model. The model established with experiment results was restricted to equipment apparatus structure, operating conditions, absorbent concentration, and surface tension, et al. The mechanism of the TGA is similar to that of CO2 desorption with two approaches above which are both needed to heating. Furthermore the method of TGA has merits of easy operation, short process, high accuracy and little disturbance of external conditions. (Hu, 2008; Ren and Zhang, 2006). In this research, TGA was used to study CO2 desorption performance in MDEA and MDEA + DEA amine solutions at different heating rates. The feasibility of TGA method has been demonstrated by analysis of experimental results. The kinetic parameters (activation energy E, pre-exponential factor A, and mechanism function G(a)) and desorption kinetics model could be determined via thermal analysis kinetics research of CO2 desorption from MDEA and MDEA + DEA rich amine solutions. Moreover the research on determination for CO2 desorption kinetics parameters in MDEA and MDEA + DEA amine solutions with thermal analysis kinetics method has not been reported until now. As kinetics analysis of CO2 desorption played a significant role in absorbents selection, the proper absorbents could be chosen by comparison of desorption performance for different amine solutions, which provide theoretical basis for industry application.

Fig. 1. Rich amine solution preparation diagram.

supplied from Shanghai Aladdin biochemical technology Co. Ltd. (China). The thermogravimetric analyzer (PerkinElmer TGA 8000) was used to carry out the thermo-gravimetric analysis experiment. The concentration of MDEA solution 3.25 mol/L is consistent with absorbent of Sinopec Wuhan corporation 40 wt%. The MDEA/DEA ratio was 10:1 and it was converted as 0.3 mol/L. The amine solutions of 3.25 mol/L MDEA and 3.25 mol/L-0.3 mol/L MDEA + DEA were prepared to simulate absorbents in industry. 2.2. Experiment process (1) Two rich amine solutions MDEA (3.25 mol/L) and MDEA + DEA (3.25 mol/L-0.3 mol/L) were prepared to absorb CO2. In Fig. 1 the CO2 was fed into a packed tower for 6 h continuously, and then took samples from the bottom of the tower every half an hour. While the deviation of three consecutive samples were less than 1%, it could basically judge that the amine solutions were saturated with CO2 and the absorption was nearly in equilibrium. Then the rich amine solutions with high CO2loadings were prepared (Shen et al., 2010). (2) The solubility of CO2 in rich amine solution was measured by acid hydrolysis apparatus, and the content of CO2 could be calculated (Shen et al., 2010). (3) Samples were collected in thoroughly-rinsed glass bottles which were sealed tightly to prevent contamination. Take 29–33 mg sample into crucible and put the crucible in thermo-gravimetric analyzer. The atmosphere was the 40 ml/min nitrogen flow, heating rates were 2.5 ℃/min, 5 ℃/min, 10 ℃/min, 20 ℃/min and the temperature range was from 25 ℃ to 275 ℃. The TG and DTG curves of CO2 evaporation in rich amine solutions were obtained. (4) The kinetic parameters and model could be established through the research of thermal analysis kinetics of MDEA and MDEA + DEA rich amine solutions.

2. Experiment 3. Thermal analysis kinetics 2.1. Materials and instruments The thermo-gravimetric analysis was used to study the CO2 desorption performance in amine solutions in this work. Moreover CO2

The analytical grade MDEA and DEA with a purity of 99% were both 2

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determined through the slope and intercept respectively. The G(α) with good linear correlation is the most probable mechanism function according with Eq.(9) FWO and CR method had been selected to investigate CO2 desorption kinetics parameters. FWO method is chosen to determine the activation energy E firstly, and then calculate the activation energy values with different mechanism functions by CR method. Activation energy obtained by FWO method was used to verify the correctness of CR method. While the E calculated with a proper mechanism function by CR method was most closed to the value calculated with FWO method and the result has a good correlation, it was selected as the most probable mechanism function. Then the kinetics parameters and mechanism function can be determined.

kinetic parameters could be determined by thermal-analysis kinetics research with TG/DTG data (Kumar et al., 2019; Dong et al., 2018). 3.1. Model-Free Method——Flynn-Wall-Ozawa (FWO) The model-free method is an approach analyzing multiple thermal analysis curves measured under different heating rates conditions. This method can not only get more reliable activation energy E but also effectively avoid errors caused by improper selection of mechanism function (Gu et al., 2009). The dynamic equation of FWO method is Eq. (1):

AE ⎞ E lgβ = lg ⎛⎜ ⎟ − 2.315 − 0.4567 RG ( α ) RT ⎝ ⎠

(1)

where α is conversion rate; β is heating rate, ℃/min; G(α) is integral mechanism function; A is pre-exponential factor; E is activation energy. E

Take Z = lgβ , y = 1/ T , a = −0.4567 R and b = lg then the Eq. (1) can be reduced as Eq. (2)

Z = ay + b

(

AE RG (α )

4. Results and discussion

) − 2.315,

4.1. Thermo-gravimetric analysis of MDEA and MDEA + DEA rich amine solutions

(2)

The mass fraction of CO2 in rich amine solutions of MDEA(3.25 mol/L) and MDEA + DEA (3.25 mol/L + 0.3 mol/L) were measured at the condition of 101.3 kPa and 298.15 K with step 2 in experiment part and the content of other components could be calculated, which was present in Table 1. The TG/DTG curves of MDEA and MDEA + DEA rich amine solutions which were measured at different heating rates of 2.5℃/min, 5℃/ min, 10℃/min, and 20℃/min were shown in Figs. 2 and 3 respectively. In Fig. 2 and Fig. 3, the trend of TG and DTG curves for both MDEA and MDEA + DEA amine solutions were almost the same at four heating rates. The thermal behavior indicated that the CO2 desorption process could be divided to two stages. The weight cut down to nearly 60% quickly with temperature increasing in the first stage; the weight decreased a little gently for a while and then reduced to zero rapidly in the second stage. According to Fig. 2, CO2 and H2O evaporation temperature range is 85℃-135℃. The TG curve was on the far left at 2.5℃/ min and on the far right at 20℃/min. Moreover the TG curve moved to the right with the heating rate increasing. The hysteresis caused by thermo-gravimetric device is inevitable at high heating rate. The higher of the heating rate, the more serious the temperature hysteresis. The desorption temperature range is wider as heating rate inceasing. Thus the difference among TG curves at four heating rate is mainly caused by thermo-gravimetric experiment device. Although the heating rate had an effect on weight loss rate, the TG trends for the two rich amine solutions at four heating rates were the same basically. It suggested the CO2 desorption process from rich amine solutions by thermo-gravimetric method has good repeatability. In Fig. 3, there were two peaks on the DTG curves at 10℃/min, which were correspond with TG curves correspondingly. It also indicated that there were two stages in CO2 desorption process and it was feasible to study the CO2 desorption process in rich solutions with thermo-gravimetric method. The weight loss data of the first stage at different heating rates were presented in Table 2. The average weight losses at four heating rates

For a temperature T and conversion rate α on the TG curve, the G(α) is a constant C and then Eq. (2) can be transformed as Eq. (3):

lgβ = −0.4567

E +C RT

(3)

Eq. (3) showed a linear relationship of lgβ-1/T. The average activation energy E could be acquired according with the slope. 3.2. The model-fitting method——coats-redfern (CR) The CR method was used to analyze kinetic parameters of the TG/ DTG curves so as to determine pre-exponential factor A, activation energy E, and proper mechanism function. (Simone G. et al., 2015). The traditional kinetic model is Eq. (4).

G (α ) =

∫0

α

dα / f (α )

(4)

Where f(α) is differential mechanism function of G(α). Since the research was carried out at different heating rates, the non-isothermal method was commonly used to analyze TG/DTG curves. Then the differential equation was as following:

1 dα ⎛ E ⎞ = ⎜⎛ ⎟⎞ A exp ⎜− ⎟ f (α ) dT ⎝β⎠ ⎝ RT ⎠

(5)

And integral equation is,

G (α ) =

∫T

T

0

⎛⎜ 1 ⎞⎟ A exp ⎛− E ⎞ dT ≈ ⎜ RT ⎟ ⎝β⎠ ⎝ ⎠

∫0

T

⎛⎜ 1 ⎞⎟ A exp ⎛− E ⎞ dT = AE p (u) ⎜ RT ⎟ βR ⎝β⎠ ⎝ ⎠ (6)

Where p(u) is temperature integral and the Frank-Kameneskii approximation formula is

p (u) =

∫0

u

e−u e−u −⎜⎛ 2 ⎞⎟ du ≈ 2 u u ⎝ ⎠

(7)

Table 1 Mass fraction of each component in MDEA and MDEA + DEA rich amine solutions.

Where u = E / RT . Substitute Eq. (7) into Eq. (6),

Component

G (α ) AR − E = ⋅e RT T2 βE

(8)

Take logarithm to Eq.(8), then

AR ⎞ E ⎡ ⎤ ln ⎢G (α )/ T 2⎥ = ln ⎛⎜ ⎟ − βE RT ⎝ ⎠ ⎣ ⎦ A line of

ln[G (α )/ T 2]

MDEA, wt% DEA, wt% H2O, wt% CO2, wt% H2O + CO2, wt%

(9)

− 1/ T could be plotted, and then E and A can be 3

Solution MDEA

MDEA + DEA

34.58 0 54.13 11.29 65.42

31.85 2.84 54.11 11.20 65.31

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Fig. 3. TG-DTG curves of rich amine solutions at 10℃/min (a) MDEA; (b) MDEA + DEA. Fig. 2. TG curves of rich amine solutions (a) MDEA; (b) MDEA + DEA. Table 2 Weight losses of MDEA and MDEA + DEA amine solutions at different heating rates.

were 64.05% in MDEA and 64.80% in MDEA + DEA rich amine solutions and the values were agree well with mass fraction theoretical values of CO2 and H2O 65.42% and 65.31% in Table 1. It further suggested that the CO2 and H2O with lower boiling point were desorbed from the rich amine solutions in the first stage. The average weight losses in the second stage at four heating rates were 35.95% in MDEA and 35.2% in MDEA + DEA, and it indicated MDEA or DEA with high boiling points were evaporated gradually in the second stage.

Heating rate, ℃/min

2.5 5 10 20 Ave theoretical value

4.2. Desorption activation energy for rich amine solutions Based on the aforementioned results, it concluded that CO2 and H2O mainly desorbed in the first stage. Thus the study on thermal analysis kinetics of CO2 desorption was mainly aimed at the first stage. The model-free method, FWO method was selected so as to avoid the error caused by choosing improper mechanism function. The TG data were substituted in Eq. (3), and the activity energy E could be calculated. The results of MDEA and MDEA + DEA solutions were presented in Fig. 4. It was interesting to find that activation energy E of MDEA was overall smaller than MDEA + DEA at any conversion rate in Fig. 4. As mentioned in literature CO2 reacted with MDEA to form bicarbonate, and form carbamate with DEA directly (Jamal et al., 2006). Since the bicarbonate was easier to be decomposed than carbamate, the activation energy E for MDEA was smaller than DEA which was obviously agree well with the trend in Fig. 4. Moreover the trends of activation energy for both MDEA and MDEA + DEA solutions were almost the same in Fig. 4. The activation energy E for both MDEA and MDEA + DEA rich amine solutions

Weight loss, wt% MDEA

MDEA + DEA

65.5 64.7 63.8 62.2 64.05 65.42

65.7 65.5 64.6 63.4 64.80 65.31

decreased with the increasing of conversion rate under the same operation condition. CO2 desorption is a reverse process of absorption. The CO2 desorbing result from chemical bonds breaking of bicarbonate or carbamate slowly at the beginning of desorption. It was a chemical reaction so the activation energy E was larger at preliminary stage. The decomposed CO2 and H2O were transferred from liquid to gas-liquid interface as heating continually. The mass transfer was a physical process, thus it was faster than chemical reaction and the activation energy E was smaller lately. In generally desorption rate will be slower and energy consumption will be larger as absorption rate and the CO2 loading increasing. Thus absorption and desorption should be both considered to select proper absorbents on the view of the whole CO2 capture process. The researches have shown that adsorption rates order of different amine solutions was primary amine > secondary amine > tertiary amine and the desorption rates order was primary amine < secondary 4

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CO2 in rich amine solution 4.3. Mechanism function and pre-exponential factor Mechanism function is a mathematical model established by different reaction mechanisms. Mechanism functions commonly used in thermal analysis dynamics were selected for data analysis of TG curves so as to determine the most probable mechanism function and CO2 desorption kinetic parameters in amine solutions. Mechanism functions are usually divided into four types: nucleation, geometric contraction, diffusion and series reaction. 20 kinetics mechanism functions include these four types were selected in Table 3 and the activation energy E and correlation coefficient R2 were calculated with CR method at four heating rates of 2.5℃/min, 5℃/min, 10℃/min and 20℃/min. The mechanism function with high R and the E value which was highly closed with the value of FWO method would be selected as the most probable mechanism function of CO2 desorption kinetics equation. The kinetic parameters of CO2 desorption in MDEA and MDEA + DEA amine-rich solutions calculated with CR method by 20 mechanism functions were shown in Tables 4 and 5 respectively. The data were analyzed with FWO and CR method and then desorption kinetic parameters (activation energy, pre-exponential factor and mechanism function) for the two rich amine solutions were determined. In Table 4 the average activation energy of No.13 mechanism function calculated with CR method was 50.36 kJ/mol and it was apparently closed to 48.52 kJ/mol with FWO method. In Fig. 5(a), the relative error of average activation energy between CR and FWO was 3.8%. Furthermore, the average correlation coefficient R was 0.9661 for No.13 mechanism function in Fig. 6 (a). Hence, the No.13 was chosen as the most probable mechanism function for MDEA rich amine solution. The kinetic parameters for MDEA + DEA solution were determined in the same way of MDEA. In Table 5 the activation energy values of No. 4 & 9 mechanism function were respectively 59.68 kJ/mol and 55.76 kJ/mol which were both closed to result of FWO method 57.34 kJ/mol. Meanwhile the average correlation coefficients were 0.9582 and 0.9775 for MDEA and MDEA + DEA respectively in Fig. 6(b), which showed linear correlation well for these two mechanism functions. However the relative error for No.4 was 2.9% which was smaller than 3.9% for No.9 Fig. 5(b). Therefore No.4 was selected as the most probable mechanism function for MDEA + DEA rich amine solution.

Fig. 4. Comparison of activation energy E for MDEA and MDEA + DEA rich amine solutions at different conversion rates with FWO method.

Table 3 Kinetics mechanism functions. No.

mechanism function, G(α)

No.

mechanism function, G(α)

1 2 3 4 5 6 7 8 9 10

α+(1-α)ln(1-α) [1-(1-α)1/2]2 [1-(1-α)1/3]2 [(1+α)1/3-1]2 [-ln(1-α)]1/4 [-ln(1-α)]2/5 [-ln(1-α)]2/3 -ln(1-α) [-ln(1-α)]3/2 [-ln(1-α)]4

11 12 13 14 15 16 17 18 19 20

α1/3 α α3/2 α2 1-(1-α)1/3 1-(1-α)1/2 1-(1-α)2 1-(1-α)4 (1-α)−1 2[1-(1-α）1/2]

amine < tertiary amine (Aqil et al., 2006, Wang et al., 2013). In this paper since activation energy E of MDEA was overall smaller than MDEA + DEA, it was inferred that CO2 was easier to be desorbed from MDEA than MDEA + DEA rich amine solutions. Moreover the desorption efficiency of MDEA + DEA was lower than MDEA, although absorption rate of MDEA + DEA was higher than MDEA. This conclusion was consistent with the view in literatures. It further proved that thermo-gravimetric analysis was suitable for studying the desorption of

Table 4 The activation energy E of MDEA rich amine solution with Coats-Redfern method. NO.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

2.5℃/min

5℃/min 2

10℃/min 2

20℃/min 2

AVE

E

R

E

R

E

R

E

R

E

R2

87.72 90.34 92.91 77.25 7.42 15.2 29.06 46.34 72.28 202.02 9.19 38.75 60.89 83.05 43.67 42.39 32.34 22.58 10.84 42.39

0.94951 0.95456 0.95896 0.92933 0.90525 0.94267 0.95707 0.96243 0.96551 0.9689 0.86515 0.93058 0.93694 0.93982 0.95315 0.948 0.88711 0.77289 0.88324 0.948

73.27 75.49 77.68 64.38 5.16 11.77 23.52 38.18 60.2 170.28 6.66 31.72 50.52 69.29 35.92 34.82 26.28 18.01 8.13 34.83

0.93039 0.93591 0.94073 0.90895 0.8025 0.89844 0.92994 0.94081 0.94683 0.95325 0.77378 0.90414 0.91509 0.91994 0.93002 0.92407 0.85502 0.7271 0.8447 0.92407

68.22 70.29 72.33 59.93 4.27 10.47 21.5 35.25 55.9 159.16 5.69 29.24 46.88 64.52 33.14 32.12 24.15 16.43 6.95 32.12

0.94648 0.95157 0.95598 0.92621 0.81214 0.91749 0.94701 0.95654 0.96166 0.96699 0.78256 0.92202 0.93227 0.92672 0.94658 0.94102 0.87369 0.74157 0.81462 0.94102

63.01 64.93 66.81 55.33 3.37 9.14 19.41 32.22 51.46 147.65 4.71 26.67 43.14 59.59 30.28 29.34 21.97 14.8 5.73 29.34

0.96197 0.96646 0.9703 0.94345 0.8208 0.93759 0.96366 0.97137 0.97536 0.97939 0.79065 0.94017 0.94931 0.95319 0.96263 0.95764 0.89363 0.758 0.75593 0.95764

73.055 75.2625 77.4325 64.2225 5.055 11.645 23.3725 37.9975 59.96 169.7775 6.5625 31.595 50.3575 69.1125 35.7525 34.6675 26.185 17.955 7.9125 34.67

0.947088 0.952125 0.956493 0.926985 0.835173 0.924048 0.94942 0.957788 0.96234 0.967133 0.803035 0.924228 0.933403 0.934918 0.948095 0.942683 0.877363 0.74989 0.824623 0.942683

5

2

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Table 5 The activation energy E of MDEA + DEA rich amine solution with Coats-Redfern method. NO.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

2.5℃/min

5℃/min 2

10℃/min 2

20℃/min 2

AVE 2

E

R

E

R

E

R

E

R

E

R2

80.28 82.74 85.16 70.5 6.34 13.52 26.31 42.26 66.22 185.97 79.36 35.13 55.52 75.9 39.7 38.56 29.12 20.04 98.02 38.5

0.90954 0.91596 0.92162 0.88489 0.79529 0.87907 0.91077 0.92243 0.92908 0.93635 0.76066 0.88082 0.89231 0.8975 0.90992 0.90314 0.82777 0.69698 0.89793 0.90314

68.63 70.72 72.77 60.28 4.6 10.81 21.86 35.64 56.33 159.78 6.02 29.58 47.24 64.9 33.51 32.49 24.47 16.69 7.35 32.49

0.93794 0.94336 0.94807 0.91656 0.80867 0.90789 0.93836 0.94857 0.95416 0.96005 0.77806 0.91227 0.92293 0.92761 0.93798 0.9321 0.86276 0.73112 0.83966 0.9321

64.27 66.23 68.16 56.41 37.77 9.64 20.07 33.09 52.62 150.32 5.12 27.38 44.07 64.27 31.09 30.12 22.57 15.25 6.33 30.12

0.94441 0.94967 0.95424 0.92337 0.78184 0.90986 0.9437 0.95435 0.96001 0.96584 0.75676 0.91815 0.92943 0.94441 0.94389 0.93805 0.86751 0.7289 0.81001 0.93805

58.7 60.48 62.24 51.53 2.84 8.24 17.86 29.85 47.86 137.9 4.1 24.67 40.09 55.51 28.04 27.16 20.29 13.59 5.02 27.16

0.96627 0.97055 0.97419 0.94833 0.80871 0.9418 0.96789 0.97524 0.97897 0.98268 0.78206 0.94504 0.95406 0.95784 0.9669 0.96209 0.89872 0.75991 0.72009 0.96209

67.97 70.0425 72.0825 59.68 12.8875 10.5525 21.525 35.21 55.7575 158.4925 23.65 29.19 46.73 65.145 33.085 32.0825 24.1125 16.3925 29.18 32.0675

0.93954 0.944885 0.94953 0.9182875 0.7986275 0.909655 0.94018 0.9501475 0.955555 0.96123 0.769385 0.91407 0.9246825 0.93184 0.9396725 0.933845 0.86419 0.7292275 0.8169225 0.933845

Fig. 6. The correlation coefficient for two rich amine solutions. (a) MDEA; (b) MEDA + DEA.

Fig. 5. The activation energy E and relative error for two rich amine solutions. (a) MDEA; (b) MDEA + DEA.

Table 6 Kinetics parameters of NO.13 mechanism function for MDEA rich amine solution.

The results of kinetic parameters for MDEA rich amine solution calculated with NO.13 mechanism function at four heating rates were shown in Table 6. The average activation energy E was 50.36 kJ/mol, the pre-exponential factor A was 1.68 × 107, and the most probable integral mechanism function wasG (α ) = a3/2 . The reaction rate constant can be expressed according with Arrhenius equation. For MDEA rich amine solution:

Heating rate β (℃/min) 2.5 5 10 20 Ave

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E (kJ/mol) 60.89 50.52 46.88 43.14 50.36

A

R 7

6.6 × 10 9.96 × 105 2.77 × 105 8.25 × 104 1.68 × 107

0.9680 0.9566 0.9655 0.9743 0.9661

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K. Shunji, et al.

intellectual content; I have approved the final version to be published; I agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. The authors declare no conflict of interest.

Table 7 Kinetics parameters of NO.4 mechanism function for MDEA + DEA rich amine solution. Heating rate β (℃/min)

E (kJ/mol)

A

R

2.5 5 10 20 Ave

70.5 60.28 56.41 51.53 59.68

8.62 × 107 2.29 × 106 4.45 × 105 8.48 × 104 2.22 × 107

0.9407 0.9574 0.9609 0.9738 0.9582

−6057 ⎞ k = 1.68 × 107 exp ⎜⎛ ⎟ ⎝ T ⎠

CRediT authorship contribution statement Kang Shunji: Methodology, Data curation, Project administration, Writing - original draft, Writing - review & editing. Shen Xizhou: Conceptualization, Methodology, Funding acquisition, Supervision. Yang Wenze: Investigation, Formal analysis, Validation, Software, Resources.

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In the same way the results of kinetic parameters for MDEA + DEA rich amine solution calculated with NO.4 mechanism function at four heating rates were listed in Table 7. The average activation energy E was 59.68 kJ/mol, the pre-exponential factor A was 2.22 × 107, and the most probable integral mechanism function wasG (α ) = [(1 + α )1/3 − 1]2 . The reaction rate constant for MDEA + DEA rich amine solution:

k = 2.22 × 107 exp ⎜⎛ ⎝

−7178 ⎞ ⎟ T ⎠

Declaration of Competing Interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of the manuscript entitled.

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Acknowledgements The authors greatly appreciate the financial support for this research provided by Sinopec Wuhan Corporation (102049).

5. Conclusions The TGA was firstly used to research the CO2 desorption kinetics in MDEA and MDEA + DEA rich amine solutions. The kinetic parameters were acquired by analyzing TG and DTG curves with FWO and CR methods. The results indicated that the evaporation process could be divided into two stages. The CO2 and H2O were released in the first stage, and MDEA and DEA evaporated gradually in the second stage. The thermal analysis kinetics was applied to CO2 desorption in the first stage, and the kinetic parameters (pre-exponential factor A, activation energy E, and proper mechanism function) were determined. CO2 desorption dynamic equations could be acquired through most probable mechanism functions and CO2 desorption performance could be predicted. In addition activate agent DEA could not only promote CO2 absorption in MDEA amine solution, but also it made CO2 more difficult to be desorbed. The results demonstrated CO2 desorption performance in MDEA and MDEA + DEA amine solutions by means of thermodynamic method was consistent with the literature. Absorbents should be selected by taking consideration of desorption performance. Moreover the essence of CO2 evaporation from amine solution is the same with other device like packed reactor or tower and the TGA method has advantages of simple, accurate, reproducible and operable. It was feasible to research CO2 desorption in rich amine solution with TGA method although it has limitations on the respect of mass transfer.

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Author statement In this paper, we have made some research on CO2 desorption performance. The CO2 desorption performance in MDEA and MDEA + DEA rich amine solutions were studied with thermo-gravimetric analysis (TGA). Moreover CO2 desorption kinetics parameters (pre-exponential factor A, activation energy E, and proper mechanism function) were determined with thermal dynamic analysis method. The Desorption reaction rate constant for both MDEA and MDEA + DEA rich amine solutions were acquired. The comparation has been done between these two systems so as to study desorption performance of MDEA and MDEA + DEA rich amine solutions. I have made substantial contributions to the conception or design of the work; or the acquisition, analysis, or interpretation of data for the work; I have drafted the work or revised it critically for important 7

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