Effect of number of amine groups in aqueous polyamine solution on carbon dioxide (CO2) capture activities

Accepted Manuscript Effect of Number of Amine Groups in Aqueous Polyamine Solution on Carbon Dioxide (CO2) Capture Activities Pailin Muchan, Jessica Narku-Tetteh, Chintana Saiwan, Raphael Idem, Teeradet Supap PII: DOI: Reference:

S1383-5866(17)30539-7 http://dx.doi.org/10.1016/j.seppur.2017.04.031 SEPPUR 13693

To appear in:

Separation and Purification Technology

Received Date: Revised Date: Accepted Date:

15 February 2017 6 April 2017 18 April 2017

Please cite this article as: P. Muchan, J. Narku-Tetteh, C. Saiwan, R. Idem, T. Supap, Effect of Number of Amine Groups in Aqueous Polyamine Solution on Carbon Dioxide (CO2) Capture Activities, Separation and Purification Technology (2017), doi: http://dx.doi.org/10.1016/j.seppur.2017.04.031

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Effect of Number of Amine Groups in Aqueous Polyamine Solution on Carbon Dioxide (CO2) Capture Activities

Pailin Muchan1, Jessica Narku-Tetteh2, Chintana Saiwan1*, Raphael Idem2*, Teeradet Supap2 1

Petroleum and Petrochemical College, Chulalongkorn University, Bangkok, 10330, Thailand

2

Clean Energy Technologies Research Institute (CETRI), University of Regina, Regina, Canada S42 0A2

*Corresponding authors E-mail address: [email protected]; Tel.: +662 218 4137; fax: +662 215 4459. E-mail address: [email protected] (R. Idem); Tel.: +1 306 585 4470; fax: +1 306 585 4855.

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Abstract The absorption and desorption of aqueous polyamine solutions with different number

of

amine

groups

(ethylenediamine

(EDA),

diethylenetriamine

(DETA),

triethylenetetramine (TETA), and tetraethylenepentamine (TEPA)) were determined for the equilibrium CO2 loading, initial absorption and desorption rates, heat duty for solvent regeneration, and heat of CO2 absorption. The absorption was carried out at 313 K using 15% CO2 in N2 balance while desorption was performed at 363 K. The results showed that the CO2 loading and the initial absorption rate were increased with increasing the number of amine groups because of the increase in the amine reactive sites for CO2 to form various species such as carbamate and dicarbamate at the equilibrium condition. The regeneration efficiency was also increasing with increasing the number of amine groups because there were the increase of the initial desorption rate and the decrease of heat duty. TEPA solution (2 mol/L) shows the best performance for all CO2 capture activities which are 2.12 molCO2/mol amine for the equilibrium CO2 loading, 0.74 × 10-2 molCO2/min for the initial absorption rate, 4.14 × 10-2 molCO2/min for the initial desorption rate, 41.13 kJ/mol for heat duty for solvent regeneration, and -70.18 kJ/mol for heat of absorption. Keywords: Polyamines; CO2 absorption; CO2 desorption; Heat of CO2 absorption; Heat duty

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1. Introduction Carbon dioxide (CO2) is the main anthropogenic greenhouse gas contributing to the global warming which can make a huge negative impact on environment and human lives. CO2 is released from various sources, such as refinery, oil and gas production sites, iron and steel factories, cement production, chemical plants, and fossil fuel fired power plant known specifically as the major CO2 emitter. In coal-based power plants, amine based absorption process used for the capture of CO2 from the flue gases is the most effective technology due to its maturity of process, efficiency, cost effectiveness, reusability of solvents, and capability of handling larger amount of exhaust stream [1–6]. Conventional amine, particularly monothanolamine (MEA) containing only one amine group in the structure, has been extensively used in the CO2 absorption process. Apart from being inexpensive compared to other amines in the market, MEA has a fast reaction kinetics which potentially allows its absorption tower to be more compact in size, thus helping in the saving of the capital cost. Despite this benefit, drawbacks of MEA still exist which include limited CO2 loading at only 0.5 molCO2/mol amine and high energy requirement for solvent regeneration due to highly stable carbamate formation [7,8]. To counteract to such deficiencies, blending MEA with other amines such as polyamines, possessing desired chemical characteristics that MEA lacks, is one way to improve the solvent’s CO2 absorption and regeneration efficiency. Polyamine is often used as an additive or activator to increase the CO2 absorption capacity and kinetics due to its structure that contains multiple reaction sites available for capturing the CO2 [9]. The reaction mechanism between CO2 and polyamine is quite complex compared to monoamine. Not only does a simple carbamate forms from the reaction similar to that between MEA and CO2 in Eq (1). Various polycarbamate species are also formed simultaneously from polyamine and CO2 reactions. As described in the work of Hartono et al. [10], 13C NMR spectroscopy was used

4

to investigate the formation of carbamate and polycarbamate in diethylenetriamine (DETA)CO2-H2O system. DETA with two groups of primary amines and one group of secondary amine within its structure, could form various carbamate species in the CO2 loaded solution. The carbamate of primary amine (i.e. DETACO2- (p)) was first formed at low CO2 loading. Other species of carbamates and polycarbamates including secondary carbamate (i.e. DETACO2- (s)), primary-primary dicarbamate (i.e. DETA(CO2)22- (pp)), and primary– secondary dicarbamate (i.e. DETA(CO2)22- (ps)) were formed preferentially at a higher CO2 loading. The reaction steps involving formations of these carbamates in DETA-CO2 solution are shown in Eqs (2) – (7). It was only tricarbamate that could not be confirmed by

13

C

NMR due to the absence of its spectrum peak from the analysis. These species have influence on CO2 capture efficiency by improving the absorption capacity [11].

MEA + CO2 + H2O

MEACO2- (p) + H3O+

(1)

DETA + CO2 + H2O

DETACO2- (p) + H3O+

(2)

DETA + CO2 + H2O

DETACO2- (s) + H3O+

(3)

DETACO2- (p) + CO2 + H2O

DETA(CO2)22- (pp) + H3O+

(4)

DETACO2- (s) + CO2 + H2O

DETA(CO2)22- (ps) + H3O+

(5)

DETA(CO2)22- (pp) + CO2 + H2O

DETA(CO2)33- + H3O+

(6)

DETA(CO2)22- (ps) + CO2 + H2O

DETA(CO2)33- + H3O+

(7)

Various polyamines having been used as a promoter in blended amine systems are also found in many literature works. As reported in the work of Aronu et al. [12], piperazine (PZ) added in 2-amino-2-methyl-1-propanol (AMP) could increase the CO2 absorption and desorption ability. Tetraethylenepentamine (TEPA) was also studied which showed very high absorption rate and cyclic capacity compared to MEA. Aronu et al. [13] also found that

5

blended 1 mol/L TEPA/5 mol/L MEA had the fastest CO2 absorption rate, while blended 1.5 mol/L bis-(3-dimethylaminopropyl) amine (TMBPA)/1.0 mol/L PZ showing cyclic capacity of 70% higher than that of 5 mol/L MEA. Sutar et al. [14] used PZ, N-(2aminoethyl), ethanolamine (AEEA), and 1,6-hexamethyl diamine (HMDA) as a promoter to enhance CO2 solubility and CO2 absorption rate of tertiary N,N-diethyldiethanolamine (DEEA). Xie et al. [15] found that the addition of PZ into AMP could decrease the heat of CO2 absorption while helping increase the CO2 loading capacity via the production of PZ dicarbamate and PZ carbamate. Hairul et al. [17] studied PZ and AMP blend whose mass transfer was found to be better than that of a single AMP system. Kim et al. [18] found that the addition of PZ, AEEA, or DETA into MEA could improve the CO2 capture capability of the MEA solvent by decreasing the activation energy of the reaction between CO2 and amine, thus enhanced the formation of carbamate. In addition, several works studied the relationship between structure of polyamine and CO2 capture activities. Singh et al. [19] studied the effect of number of amine groups in the amine structure (i.e. ethylenediamine (EDA), DETA, triethylenetetramine (TETA), and TEPA) on the CO2 absorption capacity and initial rate of absorption. Based on this work, the initial absorption rate and CO2 absorption capacity increased with an increase of number of amine groups. Khalili et al. [20] also reported that the CO2 absorption rate increased with increasing number of –NH groups in the amine. The same study also reported pseudo-first order kinetic rate constant of 2-(2-aminoethyl)-1,3-propane diamine measured using the stopped-flow technique. Kim et al. [11] credited formation of multi-dicarbamates for the increased CO2 absorption capacity with an increase of number of amine groups in their study of heat of absorption of CO2 in 30 wt% amine solution (N-methyl-1,3-propanediamine (MAPA), DETA, TETA, and TEPA), using differential reaction calorimeter. They also reported an increasing trend of heat of absorption with an increase of number of secondary amine groups in the structure.

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Most studies of polyamines reported heat of absorption, absorption capacity and initial absorption rate, which are important parameters for selection of amine for CO2 absorption. However, equally important parameters such as initial desorption rate and heat duty for solvent regeneration also need to be considered when selecting a potential amine for CO2 capture process. In this work, the initial desorption rate and the heat duty for solvent regeneration of polyamines (i.e. ethyleneamine (EA) series, i.e. EDA, DETA, TETA, and TEPA) were investigated. The effect of number of amine groups in the same polyamines was also evaluated on equilibrium CO2 loading, initial absorption rate, heat of absorption, and pKa. MEA was also included in all experiments for comparison. These parameters are useful for further selection of polyamine as an activator in blended amine system potentially to be used in the CO2 absorption process.

2. Experimental 2.1 Materials Polyamines with different number of amine groups consisted of ethylenediamine (EDA,

99%

purity,

triethylenetetramine

Merck), (TETA,

diethylenetriamine 60%

purity,

(DETA, ACROS

98%

purity,

ORGANICS),

Merck), and

tetraethylenepentamine (TEPA, 95% purity, Merck). Monoethanolamine (MEA, >99% purity, Merck) was used as a reference amine for validation and comparison with all polyamines studied in this work. Hydrochloric acid (HCl, 37% purity, Merck) was used in volumetric and CO2 displacement titration to determine the concentration of amine and the CO2 loading in all amine solutions. CO2 and N2 cylinders (99.9% purity, Praxair) were used for absorption experiments by mixing to appropriate percent concentration using high

7 precision mass flow controllers (ALLBORG, model GFC-17 with range of 0 − 200 mL/min ± 1.5% error). Deionized water was used to prepare all amine solutions.

2.2 CO2 absorption experiment The experimental setup for CO2 absorption was used to measure both equilibrium CO2 loading and absorption rate of all amines. Validation for the equilibrium CO2 loading measurement in this study was also carried out using the following conditions: 6 mol/L of MEA, 313 K, atmospheric pressure with 15 kPa CO2 (in N2 balance). MEA data obtained in this work was 0.52 molCO2/mol amine which was very close to 0.51 molCO2/mol amine from the work of Conway et al. [21]. The average absolute deviation (%AAD) was only 1.9 %AAD, thus validating the technique used in this study. The accuracy of the initial absorption rate obtained from this work was also confirmed by ± 1.66 % standard deviation of 3 repeated measurements. For equilibrium loading and initial absorption rate measurements, the absorption experiment was tested at the 313 K (± 0.1 K) and atmospheric pressure using the absorption setup as shown in Figure 1. A feed gas with 15% CO2 was obtained by pre-mixing CO2 with N2 at predetermined flow rates set via mass flow controllers. The total flow rate of mixed gas was kept constant at 160 mL/min throughout the experiment. The pre-mixed CO2 concentration was assured by a portable infrared (IR) CO2 gas analyzer (Model 906, ranging from 0.0 − 100.0 % CO2 with 0.1 % error, Quantek Instruments, Inc., Grafton, MA, USA). The water saturation cell and the absorption reactor filled respectively with deionized water and 75 mL of 2 mol/L aqueous amine solution were both placed in the temperature controlled water bath (± 0.1 K accuracy, GmbH + Co. KG Schwabach FRG, Germany) set at 313 K. Once, % CO2 in the premix had been confirmed and the temperature of aqueous

8

amine solution reached 313 K ± 2 K which was monitored by the thermometer (± 0.5 K). The CO2 feed gas was passed sequentially into the water saturation cell and the absorption reactor. The absorption experiment started when the first bubble of the feed gas emerged into the aqueous amine solution. During the absorption experiment, 1 mL of the aqueous amine sample was withdrawn at 10 min intervals to determine the CO2 loading by titration technique using Chittick apparatus described by Horwitz [22]. The test was carried out until the CO2 absorption in the aqueous amine solution reached the equilibrium indicated by constant CO2 loading obtained from the titration. The initial absorption rate was determined by plotting CO2 loadings against their corresponding times which the slope from the linear section of the plot, typically from time 0 to 60 min, was measured and expressed as the initial rate of absorption.

Figure 1 Schematic diagram of absorption experimental setup

The same procedure used for equilibrium CO2 loading and absorption rate was also used to measure heat of absorption in this work by determining the equilibrium CO2 loading in aqueous amine solution at different temperature (298 K, 313 K, and 333 K) and CO2

9 partial pressure (PCO2) in the range of 3 – 100 kPa. The heat of absorption (ΔHabs) was estimated based on the Gibbs-Helmholtz given in equation (Eq. 8) [23],       (ln PCO2 )   H abs  R 1       T   

(8)

where ΔHabs is the heat of CO2 absorption (J/mol), PCO2 is CO2 partial pressure, T is temperature (K), and R is the universal gas constant (J/mol K). The ΔHabs can be obtained from the slope of the plot between lnPCO2 and 1/T using the set data with the same CO2 loading. The procedure was validated with 5 mol/L MEA and 2 mol/L MDEA which the results were close to the experimental calorimeter results of Kim et al. [24] and Jonassen et al. [25], respectively. The measured ΔHabs of 5 mol/L MEA by Gibbs-Helmholtz equation and calorimeter were -58.44 and -58.94 kJ/mol, respectively at the CO2 loading of 0.55 molCO2/mol amine. ΔHabs of 2 mol/L MDEA was -49.74 and -49.46 kJ/mol at the CO2 loading of 0.49 molCO2/mol amine of both methods. They were in a good agreement with those of the references indicated by only 0.85 and 0.56 %AAD.

2.3 CO2 desorption experiment The experimental setup for CO2 desorption was used to measure both initial desorption rate and heat duty for solvent regeneration of all amines. The schematic diagram of desorption experimental setup is shown in Figure 2. A 250 mL three-necked round bottom flask was immersed into a hot glycerine oil bath, which was heated to a desired temperature by an electrical stirring hotplate (heat output of 1500 W and the maximum temperature of 773 K ± 10 K capacity). A condenser connected to one of the flask openings was used to prevent amine and water vapor loss from evaporation. The others were fitted

10

with a thermometer (± 0.5 K error) for measuring the amine solution temperature and for amine solution sampling. The setup was well insulated by fiberglass to ensure a minimum heat loss. The desorption of CO2 began immediately when 50 mL of rich amine solution previously obtained from the equilibrium loading of CO2 experiment was transferred to the heated desorption flask and heated quickly to 363 K (+2 K accuracy) while being stirred continuously at a controlled stirring rate. During the desorption process, the amine solution was withdrawn to determine the CO2 loading at the time of 4, 8, 12, 16, 20, 30, 40, 50, and 60 min. The CO2 loading in the amine solution at the equilibrium was taken as the desorption loading at time zero. Similar to the initial absorption rate, slope taken between time 0 to 12 min of the CO2 loading - time plot was reported as the initial desorption rate. The heat duty was also calculated using Eqs. (9) and (10) proposed by Singto et al [26].

(9)

(10)

where Qreg is the heat duty for solvent regeneration (kJ/mol), q is heat transfer (W), CO2 produced (mol CO2/mol amine/s) is CO2 loading difference between 0 and 60 min, K is a thermal conductivity of desorption flask (i.e. 1.14 W/mK for pyrex glass), A is surface area of spherical sector of sample solution (18.36 × 10-4 m2), dt is the temperature difference between the amine solution and the glycerine oil in the oil bath (6 K), and dx is the thickness of desorption flask (1.8 × 10-3 m). The accuracy of the desorption test in this work confirmed by 3 repeated measurement of 2 mol/L MEA initial desorption rate was 1.66 %SD.

11

Figure 2 Schematic diagram of desorption experimental setup

2.4 Amine dissociation constant (pKa) determination A pKa value indicates the basicity of amine which is related to the kinetics and mechanism between CO2 and amines [27, 28]. Thus, it must also be considered in the amine selection process for CO2 absorption application. In this work, pKa value of the amines was obtained by Eq. (11) modified from Handerson-Hasselbalch equation and proposed by Tissue [29].

pH = pKa + log[V/ (E-V)]

(11)

where E is the titrant volume at the endpoint and V is the variable volume for titrant acid added. The pH of the amine solution was measured through an acid-base titration using 100

12

mL of 0.05 mol/L amine solution. 0.5 mL of 1 mol/L HCl was added at a time and the pH of the solution was measured. This procedure was repeated until the endpoint was reached. The pKa was obtained from the y-intercept of the plot between pH and log[V/(E-V)].

3. Results and discussion 3.1 Equilibrium CO2 loading and initial CO2 absorption rate in amine solution The equilibrium CO2 loadings of all aqueous amine solutions are shown in Table 1. It is increased in the order of 0.55, 0.95, 1.33, 1.66, and 2.12 mol CO2/mol amine for MEA, EDA, DETA, TETA, and TEPA, respectively. Such an increasing trend in equilibrium CO2 loading was observed with an increase of the amount of amine groups in the amine structure. MEA reacts with CO2 through zwitterion mechanism which then, needs another MEA molecule to deprotonate the zwitterion complex to form a stable carbamate (Eq 12 and 13) [30 32]. Based on this mechanism, two MEA molecules are required to react with one molecule of CO2, thus limits the loading only to 0.5 molCO2/mol amine. Extra loading beyond 0.5 mol CO2/mol amine was reached by the formation of bicarbonate via hydrolysis reaction as in Eq. 14 [33, 34].

R1R2NH+COO- (zwitterion)

R1R2NH + CO2(aq) R1R2NH + R1R2NH+COOCO2 + H2O

R1R2NH2+ + R1R2NCOO- (carbamate)

HCO3-(bicarbonate) + H3O+

(12) (13) (14)

For EDA, this diamine has two primary amine groups within its structure available to react with CO2 to form primary carbamate and primary-primary dicarbamate. This same scenario can be used to explain why DETA had even higher loading than the previous

13

amines. Compared to MEA and EDA, DETA has more amine groups consisting of two primary amines and one secondary amine, readily available for CO2 absorption to occur via formation of various species of DETA carbamate, thus higher equilibrium loading, as shown previously in Eqs (2) – (7). The loading continued to increase with the increase of amine groups from 3 in DETA to 4 and 5 in TETA and TEPA, respectively. The ability of multiamine reactive sites in TETA and TEPA to induce the formation of various carbamate and dicarbamate species with CO2, similar to DETA was responsible for such a high equilibrium CO2 loading. The initial absorption rates of all amines used in this study were estimated from the linear portion of the slope of plot between CO2 loading and time as described previously in Section 2.2. Similar to the equilibrium CO2 loading, the initial absorption rate showed an increasing trend with an increase of number of amine groups in the polyamine structure. The initial absorption rate increased from 0.57 × 10-2 molCO2/min for EDA to 0.59 × 10-2, 0.67 × 10-2 and 0.74 × 10-2 molCO2/min, respectively for DETA, TETA, and TEPA. Such an increase was due to the fact that the secondary amine groups in these amines help provide an extra reaction site to react with CO2, thus increase the CO2 absorption rate of the amine. The increase of amine groups affecting on the increase of absorption rate was also reported by Singh et al. [19] and Khaili et al. [20]. In case of EDA and MEA, MEA showed higher initial absorption rate (0.67 × 10-2 molCO2/min) than EDA whose structure composed of two amine groups. Based on a literature [26], OH in a certain location in an amine structure was described as having the ability to help form a stronger hydrogen bond with water. Since water is a major ingredient in initiating the CO2 absorption reactions, having the OH in the structure of amine like MEA could potentially help the amine to react with CO2 better than the amine of comparable size such as EDA.

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3.2 Amine dissociation constant (pKa) The basicity (pKa) values of polyamines were experimentally measured at 298 K and also reported in Table 1. The pKa increased in the order of 8.91, 9.43, 9.45, 9.69, and 9.99 for TEPA, TETA, DETA, MEA, and EDA, respectively. Such data of pKa obtained for the polyamines could be explained using the basic structure of ethylamine (EA) whose pKa also shown in Table 1, was 10.81 [35]. Electronic activity of functional group attached to βcarbon atom was also used to evaluate structure effect to the amine’s pKa value. Once, hydroxyl group was introduced at β-carbon of EA to be MEA. The pKa was reduced to 9.65 possibly due to the strong effect of hydroxyl electron withdrawing group reducing the electron cloud density of the nitrogen atom. Though not to the same extent seen from OH in MEA, a less effective electron withdrawing primary amine group in EDA also reduced the pKa of the amine to 9.99 [36, 37]. For DETA, TETA, and TEPA whose part of structure resembled that of EDA, their multi secondary amine groups could have the same electron withdrawing effect that lowered their basicity from EDA. In addition, two amine groups in polyamines could form intramolecular hydrogen bonding between themselves which further decreased the basicity [37]. These reasons could explain the decreasing trend of pKa with an increase of amine groups in the polyamines studied in this work.

15

Table 1 Equilibrium CO2 loading, pKa value, initial absorption/desorption rate, heat duty for solvent regeneration, and heat of CO2 absorption of 2 mol/L single amine, MEA, EDA, DETA, TETA, and TEPA.

Initial

Initial

absorption

desorption

rate

rate

(10-2

10-2

mol/min)

mol/min)

Equilibrium Amine

CO2 loading Structure

(2 mol/L)

Qreg pKa

(molCO2/mol

ΔHabs

(kJ/mol) (kJ/mol)

amine)

EA

-

10.81*

-

-

-

-

MEA

0.55

9.69

0.67

1.23

175.72

-55.94

EDA

0.95

9.99

0.57

1.42

127.55

-48.58

DETA

1.33

9.45

0.59

2.35

67.01

-56.49

TETA

1.66

9.43

0.67

2.63

63.14

-56.64

TEPA

2.12

8.91

0.74

4.14

41.13

-70.18

* Perrin [35].

3.3 Heat of absorption CO2 loading data in aqueous amine solution measured at 298, 313, and 333K and CO2 partial pressure of 2.9 to 81.4 kPa used for heat of absorption calculation are shown in Table 2. These experimental data was used to calculate the Habs using the Gibbs-Helmholtz equation [20]. For MEA, two plots of lnPCO2 and 1/T at the CO2 loading of 0.53 and 0.55

16

molCO2/mol amine are shown in Figure 3a. The averaged slope found from the 2 plots was 6728.5 K which was correspondent to -55.94 kJ/mol heat of CO2 absorption. lnPCO2 and 1/T plots for EDA, DETA, TETA, and TEPA similar to those of MEA are also shown in Figure 3b - 3e, respectively. The averaged slopes were -5842.7, -6794.0, -6812.5, and -8441.3 corresponding to -48.58, -56.49, -56.64, and -70.18 kJ/mol of heat of absorption for EDA, DETA, TETA, and TEPA, respectively. The negative values indicated the exothermic reaction of CO2 absorption in the aqueous amine solutions. The trend observed from the test could be explained in terms of heat of carbamate and dicarbamate formations. EDA had lower ΔHabs than that of MEA due to the formation of EDA-dicarbamate contributing to a lower heat of reaction than carbamate of MEA. The heat of absorption of EDA was normally obtained by the sum of heat generated via the formation of carbamate, dicarbamate, and protonated carbamate. Generally, heat released from the formation of dicarbamate was lower than that of carbamate and protonated carbamate, thus resulting in the lower of overall heat of absorption. The explanation was supported by a similar study done by Ermatchkov et al.[16] for determining ΔHabs of cyclic diamine (PZ) from heats of formation of PZ-carbamate (30.07 kJ/mol), PZ-dicarbamate (10.99 kJ/mol), and protonated PZ-carbamate (29.04 kJ/mol). In case of DETA, TETA, and TEPA, they also produced dicarbamate species with low heat of absorption. However, effect of various secondary amine groups in the structure could be dominant and increased the overall ΔHabs due to the increase in possibility of the secondary carbamate formation [11].

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Figure 3 Plots of lnPCO2 and 1/T of 2 mol/L amine. (a) MEA, (b) EDA, (c) DETA, (d) TETA, and (e) TEPA. The data obtained by measuring the equilibrium CO2 loading at 298, 313, and 333 K with various PCO2 in the range of 2.9 – 81.4 kPa.

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Table 2 Equilibrium CO2 loading of MEA, EDA, DETA, TETA, and TEPA at 2 mol/L concentration at temperature range of 298 - 333 K and CO2 partial pressure range of 2.9 81.4 kPa for heat of absorption calculation.

Equilibrium CO2 loading (molCO2/mol amine )

Temp.

PCO2

(K)

(kPa)

MEA

EDA

DETA

TETA

TEPA

2.9

-

-

-

-

2.12

4.9

0.53

0.92

1.35

1.76

2.22

6.9

0.55

0.93

1.38

1.82

-

9.8

0.58

0.96

1.40

1.90

2.29

9.4

0.53

0.92

1.30

1.64

2.06

14.0

0.55

0.94

1.34

1.75

2.14

18.7

0.57

0.95

1.36

1.86

2.23

23.5

0.58

0.97

1.38

1.88

2.31

57.0

0.54

0.93

1.34

1.74

2.02

65.1

0.55

0.95

1.36

-

-

69.2

-

-

-

1.79

2.12

81.4

0.56

0.97

1.38

1.83

2.24

298

313

333

19

3.3 Initial desorption rate and heat duty for solvent regeneration The initial CO2 desorption rate was also estimated in the same way as used in the initial CO2 absorption rate by determining the slope of the linear portion of the plot between CO2 loading and time. The results are also summarized in Table 1. The initial CO2 desorption rates of aqueous polyamine solutions increased with increasing the number of amine groups in the order of 1.42 × 10-2, 2.35 × 10-2, 2.63 × 10-2 and 4.14 × 10-2 molCO2/min for EDA, DETA, TETA, and TEPA, respectively. MEA desorption rate was also at 1.23 × 10-2 molCO2/min The desorption rate was found to be the lowest among all amines which was due to highly stable primary carbamate formed as the dominant product, thus difficult to regenerate at 363 K [11]. According to the work of Choi et al. [38], based on the bond strength of ionic species formed from CO2-amine reactions, primary carbamate was the most difficult ion to be regenerated followed by primary-primary or primarysecondary or secondary-secondary dicarbamates, secondary carbamate, and bicarbonate or carbonate. The initial desorption rate results obtained from this work were also in a good agreement with the literature finding. A higher desorption rate observed in EDA when compared to MEA, could possibly derive from a formation of EDA primary-primary dicarbamate whose bond strength was weaker than that of primary carbamate formed by MEA.. Similar explanation was applied to DETA, TETA, and TEPA. These polyamines all had the desorption rate higher than that of MEA and EDA due to their ability in generating easier to regenerate ionic products formed from CO2 reaction (e.g. secondary carbamate and various species of dicarbamate). Heat duty (Qreg) was calculated based on Eqs. (9) and (10). The results were well aligned with the desorption rates discussed earlier. From Table 1, Qreg decreased with the increasing number of amine groups in the order of 175.72, 127.55, 67.01, 63.14 and 41.13 kJ/mol for MEA, EDA, DETA, TETA, and TEPA, respectively. In this case, the increase of

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the number of amine groups required lower regeneration energy due the presence of more species of secondary carbamate and dicarbamates. These ions could break down at a faster rate to release CO2 compared to primary carbamate and primary-primary dicarbamate in MEA and EDA solutions.

4. Conclusion Polyamines containing two primary and various secondary amine reactive sites can be used to capture CO2 effectively. The increase of amine groups increased the equilibrium CO2 loading and the initial absorption rate due to a presence of more reactive amine groups available for CO2 capture. The increase of number of amine group also produced more secondary carbamate and dicarbamate that enhanced the initial rate of desorption. The heat of absorption was exothermic and found to increase with the increase of number of amine groups, On the other hand, the regeneration energy required to desorb CO2 showed a decreasing trend with the increase of number of amine groups. TEPA containing the most secondary amine groups, was found to have the highest CO2 loading of 2.12 molCO2/mol amine, initial absorption rate of 0.74 × 10-2 molCO2/min, and heat of absorption of -70.18 kJ/mol. This polyamine also had the best desorption characteristics confirmed by the highest initial desorption rate and the lowest heat duty for regeneration of 4.14 × 10-2 molCO2/min and 41.13 kJ/mol, respectively. The pKa value of polyamines could not be used to help predict the reactivity of amine to CO2. However, pKa reduced when the polyamines increased its amine groups in the structure. The results from this study suggested that TEPA could be used potentially as an activator or a blend component with other amines such as MEA, to improve the overall absorption/desorption performance of the capture solvent. However, this blending concept has to be investigated in our future work.

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Acknowledgements This work was supported by the Royal Golden Jubilee Ph.D Program (RGJ), the sustainable Petroleum and Petrochemical Research Unit Center for Petroleum, Petrochemicals, and Advanced Materials, Rachadaphiseksomphot Endowment Fund Part of the “Strengthen CU’s Researcher’s Project”, Chulalongkorn University Thailand, and the Clean Energy Technologies Research Institute (CETRI), University of Regina, Canada.

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Highlights • Polyamines show potentials in being used as an activator in amine blend for CO2 capture process.

• An increase in CO2 capture performance occurs when the number of amine groups in the polyamine increases.

• Activities for CO2 absorption and desorption in polyamine solutions are investigated as a function of the polyamine structure.

• Tetraethylenepentamine (TEPA) is proposed to be a potential activator for amine blend system.