Kinetics of CO2 Absorption in an Ethylethanolamine Based Solution

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Kinetics of CO2 Absorption in an Ethylethanolamine Based Solution Siming Chen, Guoping Hu, Kathryn H. Smith, Kathryn A Mumford, Yongchun Zhang, and Geoffrey W Stevens Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02932 • Publication Date (Web): 28 Sep 2017 Downloaded from http://pubs.acs.org on September 30, 2017

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Industrial & Engineering Chemistry Research

Kinetics of CO2 Absorption in an Ethylethanolamine Based Solution Siming Chen a, b, Guoping Hu b, Kathryn H. Smith b, Kathryn A. Mumford b, Yongchun Zhang a,*, Geoffrey W. Stevens b,*

a

State Key Laboratory of Fine Chemistry, Department of Chemical Engineering, Dalian University of Technology, Dalian 116024, China

b

Peter Cook Centre for Carbon Capture and Storage Research (PCC), Particulate Fluids Processing Centre (PFPC), Department of Chemical Engineering, The University of Melbourne, Parkville, Victoria 3010, Australia

* Corresponding author. E-mail address: [email protected] (G.W Stevens). E-mail address: [email protected] (Y. Zhang).

Abstract The kinetics of CO2 absorption in ethylethanolamine based solutions were investigated using a wetted wall column (WWC) from (298 to 328) K. In the nonaqueous solutions, monoethylethanolamine (EMEA) was the reactant with the concentration range from (0.5 to 3.0) kmol m-3, and diethylethanolamine (DEEA) was the solvent. Water with volume ratio range from (0 to 150) % was added into 3.0 kmol m-3 EMEA + DEEA solution forming hydrated solutions. The overall reaction rate constants were obtained from the CO2 flux under the condition of a pseudo-first-order and fast reaction regime. The reaction rate of CO2 with nonaqueous solutions increased with either EMEA concentration or temperature, which can be represented by the

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termolecular mechanism. The reaction rate of CO2 with hydrated solutions increased with water volume ratio, which can be represented by both of the zwitterion and termolecular mechanisms. Keywords: Kinetics; CO2; Ethylethanolamine; Nonaqueous; Mechanism

1. Introduction Carbon dioxide (CO2) emissions to the atmosphere from fossil fuel consumption is a major driving force for global warming. Reactive absorption using aqueous alkanolamine solutions has been shown to be a suitable technology for CO2 capture due to its high efficiency.1-3 A range of amines including primary amines such as monoethanolamine (MEA) and 2-amino-2-methyl-1propanol (AMP), secondary amines such as diethanolamine (DEA) and di-isopropanolamine (DIPA), and tertiary amines such as N-methyl-diethanolamine (MDEA) and triethanolamine (TEA) have been investigated as carbon capture solvents.4-6 In spite of the clear advantages of aqueous alkanolamine solutions, high energy requirement and amine degradation during the solvent regeneration process are the main drawbacks that have limited further deployment.7-13 Therefore, the development of more efficient solvent processes and technologies is desirable. Recently, nonaqueous alkanolamine solutions have become very popular as they have the advantage of a low CO2 regeneration energy requirement due to the low heat capacity and evaporation enthalpy of the organic solvent.14 In addition, thermal degradation and equipment corrosion can be minimized due to their lower desorption temperatures (typically between 80 and 110 ℃) at atmospheric pressure.15-18 The kinetics of CO2 reacting with MEA and DEA in solvents including methanol, ethanol and 2-propano1 have been studied previously. In these investigations, the reaction order was found to be one with respect to CO2, and between one and two for MEA and DEA. This was explained via the zwitterion mechanism.19 It has also been


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shown that the CO2 absorption into DIPA mixed with methanol, ethanol, n-propanol, n-butanol, ethylene glycol, propylene glycol and propylene carbonate can be represented by the zwitterion mechanism.20 The kinetics of CO2 reaction with aniline (ANI) in solvents including acetonitrile, methyl ethyl ketone, toluene and m-xylene have also been studied. The reaction order with respect to aniline was found to be between -0.5 and 2.5 in these systems.21 However, there were still some limitations in these works, such as the high vapor pressure or the high viscosity of the solvents. Additionally, it is important to note that these experiments were conducted in a stirrer cell which is typically constrained by a low liquid mass transfer coefficient. In our previous work,22 we reported an efficient nonaqueous alkanolamine solution, in which a secondary sterically hindered alkanolamine of monoethylethanolamine (EMEA, Figure 1) was chosen as the reactant with CO2, and a tertiary alkanolamine of diethylethanolamine (DEEA, Figure 1) was used as the nonaqueous solvent. Results showed a higher CO2 absorption loading than other EMEA solutions using alcohols or glycols as solvents, and a higher CO2 desorption efficiency and better cyclic absorption loading than aqueous EMEA solutions. In this work, the kinetics of the CO2 absorption in nonaqueous EMEA + DEEA solution were investigated at (298, 313 and 328) K with EMEA concentrations of (0.5, 1.0, 2.0 and 3.0) kmol m-3. Furthermore, the kinetic characteristics of hydrated EMEA + DEEA solution for CO2 absorption were studied to simulate industrial CO2 capture conditions.23 The experimental technique used in this work was a wetted wall column (WWC) apparatus as it is simple and thus easy to model, and the contact time of gas and liquid is rather short (0.1-2 s).24

Figure 1. Structures of EMEA and DEEA.


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2. Theory 2.1 Reaction Mechanism In the nonaqueous solution, it is assumed that no reaction occurs between CO2 and tertiary alkanolamine of DEEA directly.25, 26 This is different from the reaction mechanism in aqueous solutions in which the reaction between CO2 and tertiary alkanolamine (Am) can be explained using a base-catalyzed reaction as following,27 CO + Am +H O ↔ AmH + HCO (1) This can be treated as a pseudo-first-order reaction if the replacement of amines on the surface of the solvents is fast enough. In this instance, the reaction rate is as follows, = , AmCO (2)

Where is the reaction rate of CO2 with tertiary amine.

Additionally, as water is not present, there are no reactions involving the hydration of CO2 (equation (3)) and bicarbonate formation (equation (4)). CO +H O ↔ H + HCO (3) The reaction rate is very slow and negligible.28 CO2 may also react with hydroxide as follows, ∗ !"

CO + OH $%& HCO (4)

And the reaction rate is,29 ∗ " OH CO (5) (" = (

∗ " can be calculated using the Where (" is the reaction rate of CO2 with OH , and (

following equation, ∗ " ) = 13.635 − log- ((

2895 (6) 3


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Here the OH value can be estimated by the following equation,30 OH = 4

58 56 Am + 4 AmH (7) 57, 57,(

Where Kw is the solubility product constant for water. The dissociation constants for DEEA (pKa,Am) and EMEA (pKa,AmH) at 313 K are 9.45 and 9.53, respectively.31 The reactions of CO2 with the reactant, EMEA, can be described either by the two-step zwitterion mechanism32, 33 or the single-step termolecular mechanism34, 35. 2.1.1 Zwitterion Mechanism CO2 reacts with EMEA (denoted as AmH) forming a zwitterion complex, followed by the deprotonation from zwitterion by a base (denoted as B). Reactions (8) and (9) may occur when CO2 reacts with nonaqueous or hydrated EMEA + DEEA solution. For a EMEA + DEEA solution with a low CO2-loading, the bases (B) are EMEA and DEEA. For a EMEA + DEEA + H2O solution with a low CO2-loading, the bases are EMEA, DEEA, OH or H2O. The formation of the zwitterion, : , ":

CO + AmH $%%%& AmH COO (8) The deprotonation of the zwitterion,
15 MX cm Ω‒1) was added to the EMEA + DEEA solution to investigate hydration characteristics of solutions for CO2 absorption. The H2O volume ratio (v/v%) in the hydrated solution was defined as the ratio between the volume of water added to the original solution volume. CO2/N2 gas mixture (10.2 vol% CO2 in N2) was obtained from BOC Gases Australia Limited, and was used for experiments and calibration of the CO2 concentration analyzer (Horiba VA-3000). This concentration was selected to simulate a


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post-combustion flue gas which is typically (7-15) vol% CO2, and decrease the CO2 partial pressure in order to satisfy the fast pseudo-first-order reaction condition.23 3.2 Density and Viscosity The densities (ρ, kg m-3) and viscosities (µ, mPa s) of the EMEA + DEEA solutions were measured at (298, 313 and 398) K using the pycnometer method and 1834 Ubbelohde viscometer, respectively, with the precisions of 0.2 kg m-3 and 0.01 mPa s. 3.3 CO2 Solubility As CO2 undergoes chemical reaction with EMEA + DEEA solutions, the physical solubility of CO2 in the reactive solution cannot be measured directly. In this study, the CO2 solubility (i , kPa m3 kmol-1) in EMEA + DEEA solutions was measured using a previously described technique,20 whereby it was assumed to be equal to that of the pure solvent of DEEA, because there was no reaction of CO2 with pure tertiary alkanolamine of DEEA.25 Here, the solubility of CO2 in pure DEEA solvent was obtained by the pressure differences of CO2 before and after equilibrium at (298, 313 and 398) K using the vapor-liquid equilibrium apparatus.42 Firstly, 25 ml of pure DEEA solvent (Vsolvent, ml) was introduced into a 100 ml cell (Vcell, ml); Secondly, the pure solvent was heated to the desired temperature (T, K); Thirdly, a certain amount of pure CO2 was introduced into the cell, and the initial pressure in the cell was recorded as P1 (kPa); Finally, the pressure in the cell after reaching equilibrium was recorded as P2 (kPa). The equilibrium concentration of CO2 (ceq., kmol m-3) in the solvent was calculated as follows, J. =

(x − x )(Zff − pqw ) (33) 3pfqw

And the Henry’s constant was calculated as following with an average deviation of 0.4 %, i =

x (34) J. ACS Paragon Plus Environment

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In the hydrated EMEA + DEEA solutions, the Henry’s constant (i,h ) was obtained by the following equation with the deviation range from +4.8 % to -1.3 %,43 i,h = C lh ih (35) Where mi is the mass fraction of water and EMEA + DEEA solution, and Hi is the corresponding Henry’s constant. The Henry’s constant for water can be calculated as following,44 i 87 = 2.8249 × 10 exp(−2044⁄3) (36) 3.4 Diffusivity The diffusivity of CO2 in the solutions (S , m2 s-1) was estimated by the method of Wilke which has been found to have an average deviation of 12 % for these systems.45 S

7.4 × 10 N 3oq q = × 10 (37) -. Pq ,7

where T is the temperature, α is the association coefficient, M is the molecular weight, µ is the viscosity, Vb is the molar volume at the normal boiling point. The subscripts of “sv” and “a” indicate the solvent and the solute, respectively. In current system, the “solvent” is nonaqueous or hydrated EMEA + DEEA solutions, the “solute” is CO2 gas. The association coefficient α is to define the effective molecular weight of the solvent. For water α = 2.6 and for nonassociated solvents α = 1. In this work, α was assumed to be 1 and 2.6 for nonaqueous and hydrated EMEA + DEEA solutions, respectively. The molar volume of CO2 at the normal boiling point is 34.0 cm3 mol-1. 3.5 CO2 Flux Reaction kinetics of CO2 with the different solutions were measured in the wetted wall column (WWC) (Figure 2).46-48 The apparatus consists of a stainless steel tube with the length of 116.9


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mm and the external diameter of 12.7 mm, the total external surface area of the column is 4840 mm2 including the longitudinal area of the tube (4590 mm2) and the area of the top of the tube (250 mm2). The column is enclosed by a glass cylinder with the diameter of 25 mm which provides a chamber for gas and liquid contact. The chamber was housed inside a second glass chamber which is filled with circulating temperature controlled water to maintain the isothermal condition. The detailed structure of the WWC is shown in Figure S1 in the supporting information. The experimental procedures are as follows: Firstly, CO2/N2 gas mixture was preheated to the desired temperature in the water bath and used to purge the system at a flow rate of 1.5 L min-1 (hwf ) which was controlled by the mass flow metre; Secondly, 0.5 L solution was loaded into the solvent tank and preheated to the desired temperature; Thirdly, the solution was pumped up from the solvent tank through the middle of the column with a flow rate of 0.15 L min-1 (L), the solution distributed on the surface of the column as a thin film, meanwhile, the outlet gas flow rate and CO2 concentration were recorded from 5 min to 20 min with the CO2 loading ranges of (0.04 to 0.05) kmol m-3; Finally, the liquid was collected from the sample port and pumped back to the solvent tank. The CO2 flux (^_` , kmol m-2 s-1) was calculated using the following equation with an average error of 0.32 × 10 kmol m-2 s-1, ^_` =

1000 x CO hwf − x ,pf pf CO pf (38) 60 ∙ 3 ∙ ,hwf hwf

Where x ,hwf and x ,pf (kPa) are the total inlet and outlet gas pressure, respectively,

which are both assumed to be equal to the atmospheric pressure; hwf , CO hwf , pf and

CO pf are the inlet and outlet gas flow rate and CO2 concentration, respectively; A (m-2) is the gas-liquid contact area, which can be calculated as follows, K = \ ∙ (K + ) ∙ ℎ + \ ∙ H M (39) 2


12

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Where d (m) and h (m) are the diameter and height of the column, respectively. δ (m) is the film thickness, which is calculated as follows,

3 ∙ P] M (40) δ=H \[KR In this work, the kinetics of CO2 with aqueous MEA and EMEA at 298 K were investigated to validate the WWC apparatus. The apparent reaction order and rate constants of CO2 absorption into aqueous MEA and EMEA solutions at 298 K are shown in Table 1, when assuming / = pq CO and pq = MEA/EMEAn according

to

the

zwitterion

mechanism. It was found that the values obtained in this work compared favorably with those obtained in the literatures for the aqueous MEA and EMEA systems. Therefore, the experimental technique followed by this work was deemed valid.

Figure 2. The wetted-wall column apparatus.


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Table 1. Comparisons of Apparent Reaction Order and Rate Constants of CO2 Absorption into Aqueous MEA and EMEA Solutions at 298 K. Alkanolamine

[Alkanolamine] (kmol m-3)

n

k1 (m3 kmol-1 s-1)

References

MEA

0.5-2.5

1

3630

49

MEA

Not available

1

4090

50

MEA

0.5-3.0

0.9

3681

This work

EMEA

0.9-2.5

1

4170

49

EMEA

0.03-0.08

2

8000

51

EMEA

0.5-3.0

1.2

4731

This work

4 Results and Discussion 4.1 Physical Parameters The physical properties including density, viscosity, CO2 solubility and diffusivity of the nonaqueous and hydrated solutions of EMEA + DEEA at (298, 313 and 328) K are shown in Table S1 and Table S2 in the supporting information, respectively. In the nonaqueous solutions of EMEA + DEEA, the density and viscosity of the solution increased with EMEA concentration, and decreased with temperature. The Henry’s constant increased with temperature, which illustrates the decreased CO2 physical solubility in EMEA + DEEA solution with temperature. The variation of CO2 diffusivity was opposite to that of viscosity. In order to explore the kinetic characteristics of hydrated EMEA + DEEA solution for CO2 absorption, water was added to EMEA + DEEA solution with the original EMEA concentration of 3.0 kmol m-3 at 313 K. The EMEA and DEEA concentrations decreased with water addition. Figure S2 and Figure S3 show the variations of density and viscosity with H2O volume ratio. It can be seen that the density decreased slightly and then increased with H2O volume ratio, however, the viscosity increased significantly to the maximum value with H2O volume ratio of


14

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20 %, and then decreased with further water addition. The variation trend of diffusivity was opposite to that of viscosity, which is shown in Figure S4. It can be seen from Figure S5 that the Henry’s constant increased with water volume ratio, which illustrates the decreased CO2 physical solubility in the hydration process of EMEA + DEEA solution. 4.2 Kinetics of Nonaqueous Solution The reaction kinetics of CO2 with nonaqueous solutions of EMEA + DEEA with EMEA concentrations of (0.5, 1.0, 2.0 and 3.0) kmol m-3 were conducted at (298, 313 and 328) K using the WWC apparatus. The kinetic data obtained are listed in Table 2. It can be found that the infinite enhancement factor (Ei) is much larger than Ha under the same operating conditions. Therefore, the reaction occurs in the pseudo-first-order regime. The overall reaction rate constants (kov, s-1) were obtained using the regression analysis of the CO2 flux at different EMEA concentrations. The plots of kov vs [EMEA] are shown in Figure 3, which shows a non-linear exponential trend that is similar to the results observed by Vaidya et al. for the aqueous solutions of EMEA + DEEA.52 This trend is typical of temperature dependence following the Arrhenius law, which indicates that the reaction rate of CO2 with EMEA + DEEA solution increased with temperature. The reaction orders of EMEA at 298 K, 313 K and 328 K are 1.7, 1.3 and 1.3, respectively. The experimental CO2 flux as a function of EMEA concentration is shown in Figure 4. The CO2 flux increased with increasing EMEA concentration and also showed a temperature dependence of ^ , > ^ ,N > ^,N . This is due to the change in reaction kinetics and physical properties. Although the reaction kinetics between CO2 and amines increases with increasing temperature, the CO2 solubility (Table S1) decreases resulting in lower driving forces at higher temperatures.


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Table 2. Kinetic Data Obtained for the CO2 Absorption in Nonaqueous EMEA + DEEA Solutions. T

[EMEA]

[DEEA]

(K)

(kmol m-3)

(kmol m-3)

298

0.5

298

x,

f-

kov

kEMEA

kDEEA

n

Ha

Ei

(105 m s-1)

(s-1)

(m6 kmol-2 s-1)

(m6 kmol-2 s-1)

5.21

2.13

42

94

12

1.7

7.5

27.9

4.04

5.08

2.07

179

94

12

1.7

15.4

58.2

7.80

5.99

4.97

1.99

517

94

12

1.7

25.8

125.4

5.3

7.31

7.28

4.79

1.88

1033

94

12

1.7

35.5

203.0

0.5

7.1

9.10

4.27

5.27

2.37

216

150

41

1.3

17.8

37.4

313

1.0

6.7

8.61

5.80

5.21

2.35

454

150

41

1.3

25.7

77.7

313

2.0

6.0

7.86

7.75

5.07

2.35

1113

150

41

1.3

37.7

170.4

313

3.0

5.3

7.31

9.09

4.92

2.24

2028

150

41

1.3

49.9

277.4

328

0.5

7.0

9.25

3.92

5.38

2.40

230

176

53

1.3

20.6

48.3

328

1.0

6.6

8.83

5.11

5.27

2.41

524

176

53

1.3

29.5

100.7

328

2.0

5.9

7.98

7.36

5.16

2.25

1320

176

53

1.3

47.9

222.9

328

3.0

5.1

7.39

8.67

4.96

2.16

2557

176

53

1.3

63.5

365.9

Flux

kg

(kPa)

(106 kmol m-2 s)

(108 kmol kPa-1 m-2 s-1)

7.2

8.99

2.16

1.0

6.8

8.49

298

2.0

6.0

298

3.0

313


16

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Figure 3. Experimental results of the overall reaction rate constant with EMEA concentration.

Figure 4. Experimental results of the CO2 flux with EMEA concentration. If the one-step termolecular mechanism is applied, equation (13) can be rewritten as follows, = EMEA + DEEA¢EMEACO (41)


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And the overall reaction rate constant of kov can be written as, pq = ( EMEA + DEEA)EMEA (42) Meaningful kEMEA and kDEEA values were obtained from the kov values using a linear regression procedure, which are shown in Table 2. Therefore, the one-step termolecular mechanism is applicable to the reaction of CO2 with EMEA + DEEA solution. This means that CO2 can react with EMEA and DEEA simultaneously forming a loosely bound encounter. However, the value of kEMEA was larger than kDEEA, which indicates that it is easier to form the bound encounter of CO ⋯ EMEA ⋯ EMEA than CO ⋯ EMEA ⋯ DEEA.

If the two-step zwitterion mechanism is applied, equation (10) can be rewritten as follows, =

EMEACO (43) 1+ EMEA + DEEA

And the overall reaction rate constant of kov can be written as, pq =

EMEA

1£ + 1 ¦¤( ⁄ )EMEA + ( ⁄ )DEEA¥

(44)

No meaningful values of 1⁄, ⁄ and ⁄ were obtained from the kov values using nonlinear regressions in Matlab R2017a. Therefore, the two-step zwitterion mechanism is not applicable to the reaction of CO2 with EMEA + DEEA solution. This means that CO2 cannot react with EMEA forming a zwitterion intermediate. 4.3 Kinetics of Hydrated Solution The CO2 reaction kinetics were investigated at 313 K with different water addition to EMEA + DEEA solution (original EMEA concentration: 3.0 kmol m-3) to investigate the hydration properties. The kinetic data obtained are listed in Table 3. It can be seen that the infinite enhancement factor (Ei) is much larger than Ha under the same operating conditions, which


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indicates that the reaction occurs in the pseudo-first-order regime. The plots of kov vs H2O volume ratio are shown in Figure 5. It can be seen that the kov values increased with H2O addition indicating an increased reaction rate of CO2 with the solution in the hydration process of EMEA + DEEA solution. The CO2 absorption flux decreased to a minimum value before increasing when water was added (0 to 150 %) (Figure 6). The reason for this phenomenon is the simultaneous action of reaction kinetics and physical properties on CO2 solubility and diffusivity (Table S2).


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Table 3. Kinetic Data Obtained for the CO2 Absorption in Hydrated EMEA + DEEA Solutions. kov

OH

kapp

(105 m s-1)

(s-1)

(kmol m-3)

(s-1)

4.86

2.16

2067

0.038

8.08

4.72

2.11

2685

7.68

7.55

4.41

1.84

3.9

7.64

7.21

4.17

2.1

2.6

7.88

6.93

70

1.8

1.6

7.98

100

1.5

0.8

120

1.4

150

1.2

H2O volume ratio

[EMEA]

[DEEA]

P,

Flux

kg

(%)

(kmol m-3)

(kmol m-3)

(kPa)

(106 kmol m-2 s)

(108 kmol kPa-1 m-2 s-1)

1

3.0

5.2

7.33

8.59

5

2.9

5.0

7.56

10

2.8

4.5

20

2.5

50

f-

Ha

Ei

1141

50.8

289.8

0.037

1776

54.9

977.5

4505

0.036

3629

68.0

1083.2

1.58

10263

0.034

9441

98.6

1277.6

4.19

1.54

26301

0.029

25592

164.6

1556.0

6.52

4.34

1.64

30137

0.025

29531

185.3

1402.1

8.04

6.60

4.46

1.73

46058

0.020

45562

239.1

1268.8

0.4

7.96

7.23

4.59

1.84

63783

0.018

63355

292.6

1121.2

0.2

8.01

7.41

4.69

1.91

86037

0.015

85672

350.4

1053.4


20

Page 21 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. Experimental results of the overall reaction rate constant with H2O volume ratio.

Figure 6. The variations of CO2 flux with water loading. For the CO2 reaction in EMEA + DEEA + H2O solution, the overall reaction rate (Rov) can be expressed as,


21


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Page 22 of 36

rs = + + (" (45) If the one-step termolecular mechanism is applied, equation (13) can be rewritten as follows, = EMEA + DEEA + ( H O + ( OH EMEACO (46) The overall reaction rate constant of kov can be written as, pq = EMEA + DEEA + ( H O + ( OH EMEA + , DEEA ∗ " OH (47) + (

And the apparent reaction rate constant (kapp, s-1) can be defined as, ∗ " OH = 7¨¨ = pq − (

EMEA + DEEA + ( i © + ( OH EMEA + , DEEA (48)

The values of , , ( , ( and , were calculated to be (0.3, 5.4 × 10 ª , 3.2 × 10 ª , 4.6 × 10 ) m6 kmol-2 s-1 and 33.7 m3 kmol-1 s-1, respectively, which were obtained

from the kapp values using a linear regression procedure. The values of , (, ( were so

small that they were negligible. As such, the contribution of DEEA, H2O and OH- as acting as bases can be neglected in the reaction of CO2 with EMEA. The value of , was the largest, which indicates that the base-catalyzed reaction of CO2 with tertiary alkanolamine of DEEA contributed significantly to the CO2 reaction with the hydrated EMEA + DEEA solution. Therefore, the one-step termolecular mechanism is applicable to the reaction of CO2 with hydrated EMEA + DEEA solution. If the two-step zwitterion mechanism is applied, equation (10) can be rewritten as follows,

=

EMEACO (49) 1+ EMEA + DEEA + ( H O + ( OH

And the overall reaction rate constant of kov can be written as,


22

Page 23 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


pq =

EMEA 1£ + 1 ¦«( ⁄ )EMEA + ( ⁄ )DEEA + ( ⁄ H O + ( ⁄ OH ¬

∗ " OH (50) + , DEEA + (

And the apparent reaction rate constant (kapp, s-1) is as, ∗ " OH 7¨¨ = pq − (

=

EMEA 1£ + 1 ¦«( ⁄ )EMEA + ( ⁄ )DEEA + ( ⁄ H O + ( ⁄ OH ¬

+ , DEEA (51)

The values of 1⁄ , ( ⁄ ) , ( ⁄ ), ( ⁄ , ( ⁄ and , were

16.3 kmol m-3 s, (5.5 × 10 , 1.6, 0.5, 18.3) m6 kmol-2 s-1, and 1.7 × 10 m3 kmol-1 s-1, respectively, which were obtained from the kapp values using nonlinear regression in Matlab

R2017a. It follows k-1>>kB [B] indicating that most of the zwitterions were reverted to EMEA and CO2, and only a small part of the zwitterions were converted to carbamate. The value of , obtained was much smaller compared with other values, which indicates that the base-

catalyzed reaction of CO2 with tertiary alkanolamine of DEEA was very weak in the CO2 reaction with the hydrated EMEA + DEEA solution. Therefore, the two-step zwitterion mechanism is also applicable to the reaction of CO2 with EMEA + DEEA solution. The products in the CO2-saturated solution were analyzed using

13

C NMR spectroscopy

(Varian INOVA) operated at 400 MHz. Figure 7 and Figure 8 show the 13C NMR spectra of the CO2-saturated solutions at low field and high field, respectively. The interpretations of the peaks are shown in Figure 9. It can be seen from Figure 7 that CO2 reacts with nonaqueous EMEA + DEEA solution forming carbamate (peak 1) only, however, the products are carbamate (peak 1),


23


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Page 24 of 36

carbonate/bicarbonate (peak 2) and alky-carbonate (peak 3) by reacting with hydrated EMEA + DEEA solution. Figure 8 shows that the chemical shifts of DEEA/DEEAH+ (peaks A, B, C and D) in the hydrated EMEA + DEEA solutions shifted upfield significantly compared with nonaqueous EMEA + DEEA solutions after CO2 absorption,53 which indicates that much more protonated DEEA is formed in the hydrated EMEA + DEEA solution after CO2 absorption. However, there were no significant changes of the chemical shifts of EMEA/EMEAH+ in the hydrated EMEA + DEEA solutions comparing with the nonaqueous EMEA + DEEA solutions after CO2 absorption. According to the kinetics and 13C NMR spectroscopic analysis results, the addition of water to the nonaqueous solution of EMEA + DEEA can improve the chemical reaction process.

Figure 7. 13C NMR spectra of the CO2-loaded solutions at low field.


24

Page 25 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 8. 13C NMR spectra of the CO2-loaded solutions at high field.

Figure 9. Interpretations of the peaks in Figure 7 and 8. 4.4 Comparisons


25


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Page 26 of 36

The kinetics of CO2 absorption in ethylethanolamine based solutions were compared with some common aqueous and nonaqueous alkanolamine solutions reported in the literatures, which are shown in Table 4. It can be seen that the CO2 reaction rates with the nonaqueous EMEA + DEEA solution with EMEA concentrations of (1.0, 2.0 and 3.0) kmol m-3 at 298 K are higher than the nonaqueous DIPA and DEA solutions as well as the aqueous MDEA solution with similar amine concentrations at 298 K. The CO2 reaction rate with the hydrated EMEA + DEEA solution with water volume ratio of 150 % at 313 K is much higher than the aqueous MEA solution. Table 4. Comparisons of the Kinetics of CO2 Absorption in Ethylethanolamine Based Solutions with Some Common Aqueous and Nonaqueous Alkanolamine Solutions. Amine

Solvent

[Amine]0/(kmol m-3)

T/K

kov (s-1)

Reference

MEA

H2O

1.0

313

11643

54

2.0

313

31155

54

3.0

313

55409

54

0.9

298

3267

49

2.0

298

7260

49

0.9

298

3753

49

2.0

298

8740

49

0.9

298

16

49

2.0

298

36

49

1.03

298

519

20

2.01

298

1069

20

3.04

298

1687

20

1.01

298

51

20

2.03

298

121

20

3.01

298

199

20

MEA

EMEA

MDEA

DIPA

DIPA

H2O

H2O

H2O

H2O

Methanol


26

Page 27 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


DIPA

DEA

EMEA

EMEA

1.02

298

28

20

1.99

298

49

20

3.03

298

117

20

Ethanol

1.0

298

96

55

Polyethylene glycol

1.0

298

102

56

DEEA

1.0

298

179

This work

2.0

298

517

This work

3.0

298

1033

This work

1.2

313

85672

This work

Ethanol

DEEA + 150 % H2O

5. Conclusion In this work, the kinetics of CO2 reaction with EMEA + DEEA solutions were investigated using a wetted wall column (WWC) at (298, 313 and 328) K. The chemical reaction rate of CO2 with EMEA + DEEA solution increased with temperature, however, the CO2 flux showed a temperature dependence of ^ , > ^ ,N > ^ ,N . This is due to the change in reaction kinetics and physical properties. Although the reaction kinetics between CO2 and amines increased with increasing temperature, the CO2 solubility decreases resulting in a lower driving force at higher temperature. The reaction pathway of CO2 with EMEA + DEEA solution can be represented by the termolecular mechanism. Due to the presence of water in industrial gas streams from coal-fired power plants, the kinetic characteristics of hydrated EMEA + DEEA solution for CO2 absorption were also investigated. The CO2 absorption flux decreased to the minimum value and then increased with water addition (0 to 150) %. However, the addition of water can improve the chemical reaction of CO2 with the solution. The reaction can be well explained by both of the zwitterion and termolecular


27


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Page 28 of 36

mechanisms. CO2 was converted to three forms in the hydrated solution: carbamate, carbonate/bicarbonate and alky-carbonate. Nomenclature A = gas-liquid interfacial area, m2 AmH = alkanolamine of monoethylethanolamine (EMEA) [AmH] = EMEA concentration, kmol m-3 α = association coefficient B = base [B] = base concentration, kmol m-3 [CO2] = CO2 concentration, kmol m-3 DAmH = diffusivity of EMEA in liquid phase, m2 s-1 DCO2 = diffusivity of CO2 in liquid phase, m2 s-1 dh = hydraulic diameter, m E = enhancement factor due to chemical reaction Ei = enhancement factor for an instantaneous reaction HCO2 = Henry’s law constant of CO2, kPa m3 kmol-1 Ha = Hatta number h = height of the annulus, m k-1 = backward reaction rate constant, s-1 k1 = forward reaction rate constant, m3 kmol-1 s-1 kov = overall reaction rate constant, s-1 kB = deprontonation constant for amine kg = gas mass transfer coefficient, kmol kPa-1 m-2 s-1

e- = liquid phase mass transfer coefficient, m s-1 L = liquid flow rate, L min-1

M = molecular weight, g mol-1


28

Page 29 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


^ = CO2 flux, kmol m-2 s-1

x = CO2 pressure, kPa

pKa = dissociation constant of amine pKw = dissociation constant of water R = ideal gas constant, 8.314 J K-1 mol-1 RCO2-AmH = reaction rate of CO2, kmol m-2 s-1 Re = Reynolds number Sc = Schmidt number Sh = Sherwood number T = temperature, K tc = the contact time of gas and liquid in WWC, s z = stoichiometric coefficient ρ = density, kg m-3 µ = viscosity, mPa s Vb = molar volume at the normal boiling point, m3 V = gas flow rate, L min-1

v = linear velocity of the gas, m s-1

δ = film thickness, m

Notes The authors declare no competing financial interest. Acknowledgements The authors acknowledge the infrastructure support from the Particulate Fluids Processing Center (PFPC), a Special Research Center of the Australian Research Council and Peter Cook


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Page 30 of 36

Centre (PCC) for Carbon Capture and Storage (CCS) Research. China Scholarship Council (CSC) is also gratefully acknowledged for financial support to Siming Chen.

Supporting Information Table S1. The Physical Properties of the Nonaqueous EMEA + DEEA Solutions. Table S2. The Physical Properties of the Hydrated EMEA + DEEA Solutions at 313 K. Figure S1. The structure of the wetted-wall column. Figure S2. Variations of density with H2O volume ration at 313 K for hydrated EMEA + DEEA solution. Figure S3. Variations of viscosity with H2O volume ration at 313 K for hydrated EMEA + DEEA solution. Figure S4. Variations of diffusivity with H2O volume ration at 313 K for hydrated EMEA + DEEA solution. Figure S5. Variations of Henry’s constant with H2O volume ration at 313 K for hydrated EMEA + DEEA solution. References (1) Ali, S. H.; Al-Rashed, O.; Merchant, S. Q. Opportunities for faster carbon dioxide removal: A kinetic study on the blending of methyl monoethanolamine and morpholine with 2-amino-2methyl-1-propanol. Sep. Purif. Technol. 2010, 74, 64. (2) And, A. A.; Veawab, A. Characterization and Comparison of the CO2 Absorption Performance into Single and Blended Alkanolamines in a Packed Column. Ind. Eng. Chem. Res. 2004, 43, 2228. (3) Meisen, A.; Shuai, X. Research and development issues in CO2 capture. Energ. Convers. Manage. 1997, 38, S37. (4) Chowdhury, F. A.; Yamada, H.; Higashii, T.; Goto, K.; Onoda, M. CO2 Capture by Tertiary Amine Absorbents: A Performance Comparison Study. Ind. Eng. Chem. Res. 2013, 52, 8323.


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Kinetics of CO2 Absorption in an Ethylethanolamine Based Solution

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