Kinetics of Carbon Dioxide Removal by n-Propyl- and n

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Kinetics of carbon dioxide removal by n-propyl- and n-butyl-monoethanolamine in aqueous solutions Ravindra B. Kanawade, Prakash D. Vaidya, Krishnan Subramanian, Vijay V Kulkarni, and Eugeny Y. Kenig Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b00527 • Publication Date (Web): 18 May 2016 Downloaded from http://pubs.acs.org on May 19, 2016

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Energy & Fuels

Kinetics of carbon dioxide removal by n-propyl- and n-butyl-monoethanolamine in aqueous solutions

Ravindra B. Kanawade1, Prakash D. Vaidya1,*, K. Subramanian2, Vijay V. Kulkarni2, Eugeny Y. Kenig3,4

1

Department of Chemical Engineering, Institute of Chemical Technology, Nathalal Parekh Marg, Matunga, Mumbai-400019, India

2

Amines and Plasticizers Limited, D 21/21A, TTC Industrial Area, Turbhe, Navi Mumbai-400075, India 3

Faculty of Mechanical Engineering, Chair of Fluid Process Engineering, University of Paderborn, D-33098 Paderborn, Germany

4

Gubkin Russian State University of Oil and Gas, Moscow, Russian Federation

(* Corresponding author, Fax: +9122 33611020; Tel.: +9122 33612014; Email: [email protected])

ACS Paragon Plus Environment

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Abstract Secondary alkanolamines associated with an alkyl group are potentially attractive postcombustion CO2-capturing solvents. Their reactivity with CO2 is influenced by the nature of the alkyl group that substitutes the hydrogen atom of the amino functional group. In the present work, we measured the rates of CO2 absorption into aqueous solutions of npropylmonoethanolamine (PMEA) and n-butylmonoethanolamine (BMEA) in a stirred cell reactor under fast pseudo-first order conditions. The concentrations of PMEA and BMEA in the water solutions were changed in the 2-3 kmol m-3 range. It was discovered that, unlike the usual amines, PMEA and BMEA followed overall third-order kinetics. We evaluated the reaction rate constants at different temperatures, viz. 303, 308 and 313 K and discovered that PMEA reacts with CO2 faster than BMEA. Furthermore, we gave an interpretation of the kinetic data using the zwitterion and termolecular mechanisms. Finally, a comparison of the efficacy of the investigated solvents with N-methylmonoethanolamine (MMEA) and N-ethylmonoethanolamine (EMEA) was accomplished. It was found that reactivity of amines diminished in the order EMEA>MMEA>PMEA>BMEA.


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1.

Introduction Carbon dioxide (CO2) separation from flue gas is a significant step in the operation of coal-

fired thermal power stations. Among the existing CO2-separation techniques, reactive absorption using alkanolamine-based absorbents and their blends is most widely used for flue gas treatment. Monoethanolamine (MEA), piperazine (PZ) and N-methyldiethanolamine (MDEA) are examples of the technologically significant amines for CO2 removal. Secondary alkanolamines associated with an alkyl group represent further potentially attractive post-combustion CO2-capturing solvents because they offer high reactivity and loading capacity.1 N-Methylmonoethanolamine (MMEA) and N-ethylmonoethanolamine (EMEA), illustrated in Fig. 1, are two such examples which show significant potential.2,3 Their regeneration is easier than that of the conventional primary amine MEA and they resist corrosion better than MEA even at high amine concentration.4,5 Recently, much effort was focused to probe the performance of EMEA. For example, Bhosale and Mahajani6 found that EMEA degrades slower than MEA and DEA even at high temperature. Yamada et al.7 provided detailed information on all ionic species formed during CO2 absorption in aqueous EMEA solution using nuclear magnetic resonance (NMR) spectroscopy. Chen et al.8 investigated the equilibrium and regeneration characteristics of EMEA in aqueous and organic solutions. The reactivity of this class of amines with CO2 is influenced by the nature of the alkyl group that swaps the hydrogen atom present in the amine functionality. For example, MMEA reacts faster than MEA,2,9 thus implying that the electron-donating methyl group strengthens the basic nature of the amine without notably intensifying the steric hindrance near the nitrogen atom. EMEA reacts faster than the traditional secondary alkanolamines DEA and diisopropanolamine (DIPA).3,10 Predictably, a large t-butyl group linked to the nitrogen atom decelerates the reaction with the significantly hindered amine t-butylmonoethanolamine.11


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It would be interesting to investigate the kinetic characteristics of other secondary amines belonging to this group (see Fig. 1). In the present work, we investigated the kinetics of CO2 removal by n-propylmonoethanolamine (PMEA) and n-butylmonoethanolamine (BMEA) in aqueous solutions using the stirred cell reactor method. Moreover, we performed a comparison of the reactivity of the investigated solvents with that of MMEA and EMEA. For the first time, we showed that the reactivity of secondary amines linked to an alkyl group follows the trend EMEA>MMEA>PMEA>BMEA. We also proved that, unlike the traditional amines, PMEA and BMEA follow overall third-order kinetics. So far, there exists no information on the reaction kinetics of the CO2-PMEA system. Further, the work done by Mimura et al.9 and Ali et al.11 is the only available data on CO2-BMEA kinetics. Currently, PMEA is used as an intermediate in the pharmaceutical and agro-chemical industry, while BMEA is applied in corrosion inhibitor formulation, oil industry and as stabilizer. It is expected that this work will stimulate further interest in the usage of these linear alkyl group-bearing secondary alkanolamines as solvents for post-combustion CO2 capture.

2.

Experimental Section

2.1. Materials. MMEA and EMEA (purity 98%) were procured from a local vendor (S. D. Fine Chemicals Pvt. Ltd., Mumbai). n-Propylmonoethanolamine (PMEA, molecular weight 103) and n-butylmonoethanolamine (BMEA, molecular weight 117), supplied by Amines and Plasticizers Ltd., Mumbai, were 99% pure. CO2, nitrogen (N2) and nitrous oxide (N2O) were used from cylinders (purity 99.95%), which were bought from Inox Air Products Ltd., Mumbai.

2.2. Measurement of Absorption Rates and Validation of the Reaction Regime. The experimental apparatus and procedure were described in detail in our previous works.2,3,12 We used a stirred cell reactor (inner diameter 97 mm, height 187 mm) with an undisturbed,


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horizontal gas-liquid interface. To justify the experimental method, we analyzed the reaction between CO2 and aqueous MEA in this reactor. We found that our value of the rate constant (kMEA=7311 m3 kmol-1 s-1 at 303 K) was in line with the works of Hikita et al.13 (7721 m3 kmol-1 s-1), Sada et al.14 (7740 m3 kmol-1 s-1) and Ali15 (7530 m3 kmol-1 s-1). In each experiment performed at near atmospheric pressure, the reactor was filled with 0.45 dm3 of the solvent (i.e. PMEA or BMEA). The concentrations of PMEA and BMEA in the aqueous phase were changed in the 2-3 kmol m-3 range. After the desired temperature (303, 308 or 313 K) was attained, CO2 was charged inside the reactor and the reduction in pressure due to the reactive absorption process was noted (accuracy=1 mbar). With the known values of the pressure gradient, the values of the CO2 absorption rate were found (error=3%). Reaction kinetic parameters could be effortlessly found by this drop-in-pressure technique even without analysing the liquid phase. The existence of the fast pseudo-first order reaction regime was established by studying the influence of the agitation speed, and hence the liquid-side mass transfer coefficient kL, on the CO2 absorption rates. Because the reaction rates at 303 K did not depend on the mixing speed in the 40-100 rpm range, we concluded that the CO2-PMEA and CO2-BMEA reactions occur in the liquid film and the concentration of amine inside this film is not depleted. We performed all experiments at a rotation speed of 60 rpm. The rate did not vary linearly with the reactant concentration in the liquid, thereby proving that the instantaneous reaction regime was unlikely.

2.3. Determination of the Mass Transfer Coefficient and Physical Properties. Littel et al.16 described a method for estimating kL in a stirred-cell. By using this procedure, we established that the value of kL is equal to 0.0033 cm s-1, which is corresponding to the usual values for stirred cell reactors.3 Solution densities (ρ) and viscosities (µ) were measured (see Table 1). CO2 diffusivity and solubility in PMEA and BMEA solutions were estimated by the N2O analogy


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method.17 Details of the estimation procedure are given in a previous work2, whereas the results at 2.5 kmol m-3 amine concentration are represented in Table 1.

3.

Results and Discussion

3.1. Study of Reaction Kinetics. If the mass transfer resistance is only on the liquid side and the bulk liquid has very low CO2 concentration, the rate of absorption of CO2 in an aqueous solution of the alkanolamine (AmH) is given by R = k CO E

(1)

where the enhancement factor E reflects the mass transfer acceleration by chemical reactions. The value of E equals the Hatta number (Ha) when the reaction conforms to the fast pseudo-first order regime.18,19 If CO2 and amine react irreversibly and m and n denote their respective reaction orders, the following relation is used to find Ha18: Ha =

D , CO AmH k

(2)

where km,n is the rate constant. When the fast regime holds, the inequality 10 < Ha ≪ (E" − 1) is satisfied. Here, Ei denotes the enhancement factor for a reaction occurring instantaneously and is represented as: E" = 1 + %

AmH D'() * (3) & CO D

Eq. 3 is valid when the film theory is employed. From Eqs. 1 and 2, it follows that:

R = , D , CO AmH

(4)

The CO2-AmH reaction is of the first order with respect to CO2.20 Thus, Eq. 4 can be written as: R = CO ,D , AmH = CO -D k ./0


(5)

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where the observed reaction rate constant k ./0 = , AmH and CO = H P . Eq. 5 can be expressed as follows: 678

log 5

-978

: = ; log , < + ; log AmH <

678

From a plot of log 5

-978

(6)

: vs. log [AmH], the values of , and n can be found.3,21,22

3.2. Evaluation of Kinetic Parameters. To investigate the dependency of the absorption rate on CO2 partial pressure, experiments were done in the 5-20 kPa range at T=303 K. The concentration of amine (i.e. PMEA or BMEA) in solutions was 2.5 kmol m-3. The results are represented in Fig. 2. Predictably, the reactions with both amines exhibited first-order kinetics for CO2 concentration, which is consistent with the kinetic behaviour of CO2 with other alkanolamines. Further, the effect of amine concentration in solution on the absorption rate was studied. The CO2 pressure was approximately 5 kPa. The results for both amines are shown in Table 2. As the PMEA concentration increased from 2 to 3 kmol m-3, the absorption rate at T=303 K increased from 4.2×10-6 to 5.3×10-6 kmol m-2 s-1. Similar enhancement in rate with rising values of (PMEA) was also observed at 308 and 313 K (see Table 2). Moreover, the rise in temperature enhanced the rate of absorption. For example, the rate in 3 kmol m-3 solution increased from 5.3×10-6 (at 303 K) to 6.5×10-6 kmol m-2 s-1 (at 313 K). The results for the case of BMEA were analogous; however, the mass transfer rate for this amine was lower (see Table 2). For instance, the absorption rate at T=303 K increased from 2.6×10-6 to 3.6×10-6 kmol m-2 s-1 when BMEA concentration increased from 2 to 3 kmol m-3. Ali et al.11 attributed the relatively low rates to the bulky n-butyl substituent group on the nitrogen atom in BMEA. 678

Plots of log 5

-978

: vs. log [amine] at 303, 308 and 313 K are represented in Figs. 3 and

4. The slope equals unity, which suggests second-order kinetics with PMEA and BMEA (i.e.


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n=2), and hence, overall third order. Such behavior is in line with our previous work on EMEA.3 Until now, there is no reported data on PMEA kinetics, which our results can be compared with. Curiously, both Mimura et al.9 and Ali et al.11 reported first-order kinetics with BMEA. When the molarity of amine in solution was 2 kmol m-3, the values of the rate constant at T=303 K were found to be 1443 and 1211 m-6 kmol-2 s-1 for the reactions with PMEA and BMEA, respectively (see Table 3). Mimura et al.9 and Ali et al.11 reported that the second-order rate constant for the CO2-BMEA reaction was equal to 4760 and 2000 m-3 kmol-1 s-1 at 298 K. The difference in the values of the kinetic parameters, viz. reaction order and rate constant can probably be attributed to the lack of clarity in physical properties and the variation in the kinetics measurement technique. Using the values of D and H , the values of Ha and Ei were determined (see Table 3). The inequality 10 < Ha ≪ (E" − 1) was satisfied, and thus, our assumption of the fast

reaction regime was confirmed. From the Arrhenius plots (see Fig. 5), the activation energy values for the reactions with PMEA and BMEA were obtained (98.8 and 102.8 kJ mol-1). Patil et al.2 reported a predictably lower value for the more reactive amine MMEA (i.e. 53 kJ mol-1). Plots of kobs vs. [AmH] are shown in Figs. 6 and 7; as expected, kobs values increase with rising temperature and amine concentration.

3.3. Reaction Pathways. The two-step zwitterion mechanism23,24 can elucidate the reaction pathway of the CO2-PMEA and CO2-BMEA systems in aqueous solutions. According to this mechanism, CO2 reacts with the secondary amine (AmH) to form an intermediary zwitterion: = ,=>?

CO + AmH @AAB AmH COO

(7)

Any base (or bases) B deprotonates this zwitterion to form carbamate: DE =

AmH COO + B FB AmCOO + BH

(8)

If the steady-state rule is applied to the intermediary zwitterion in Eq. 7, the rate of the CO2AmH reaction is given by:


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r'() =

k CO AmH =>?

1 + =D

E H

(9)

Here, the kinetic constant kJ H B signifies zwitterion deprotonation by any single base (e.g., H2O, OH or AmH) or by all bases together.1,20,25

If deprotonation occurs instantaneously in contrast to the reverse reaction in Eq. 7 (k ≪

kJ H B ) and formation of the transitional zwitterion is rate-limiting, Eq. 9 is transformed into r'() = k CO AmH

(10)

thus proposing that the reaction order with respect to both CO2 and AmH is unity and the overall reaction order is two. For example, Ali et al.11 found that zwitterion formation limits the rate and water predominantly deprotonates the zwitterion at low BMEA concentration (≤0.1 kmol m-3). If zwitterion deprotonation is slow, and hence, rate-limiting(k ≫ kJ H B ), Eq. 9 becomes: r'() =

D H B k k CO AmH (11) k

It is evident from Eq. 11 that the reaction order with respect to AmH is between one and two. If the amine contributes more substantially to zwitterion deprotonation than others (H2O and OH), the reaction exhibits second order with respect to the amine concentration. Thus, we attributed our observation of second-order kinetics with PMEA and BMEA at high amine concentration to the fact that deprotonation of the intermediate zwitterion is slow and both amines exclusively contribute to deprotonation. The termolecular mechanism also appropriately interprets the pathway of the CO2 reaction with PMEA and BMEA in aqueous solutions. It proposes a one-step concurrent reaction of the amine with one molecule of CO2 and, simultaneously, with one molecule of the base, which progresses through a loose transitional complex (instead of the zwitterion). Eq. 12 elucidates this pathway: CO 2

+

AmH ⋅ ⋅ ⋅ B ←→

AmCOO− ⋅ ⋅ ⋅ BH +


(12)

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The reactant molecules, viz. CO2 and amine, are formed when the complex splits. Carbamates are built in the reaction between a small fraction of the complex and another molecule of amine or water. If the bases H2O, OH- and AmH are controlling, the rate of the forward reaction, according to this mechanism, is represented by Eq. 13: r'() = Lk ) H O + k )> OH + k '() AmH M CO AmH (13) As evident from Eq. 13, any base (e.g., H2O, OH- or AmH) can affect the reaction. Eq. 13 is analogous to one extreme case of the zwitterion pathway (Eq. 11) and it ably portrays reaction orders between 1 and 2.26 The prevailing role of water and amine in deprotonation in aqueous solutions is renowned.27 If water is the foremost base (as observed by Ali et al.11 at low amine concentration), the rate exhibits first-order dependence on the amine concentration as shown in Eq. 14: r'() = k ) H O CO AmH = kJ CO AmH (14)

where kJ = k ) H O . If AmH is governing (as anticipated in this work at high amine concentration), the dependence of the rate on the amine concentration is of the second-order as shown below: r'() = k '() CO AmH (15)

3.4. Comparison with MMEA and EMEA. We compared the performance of PMEA and BMEA with other secondary amines, namely MMEA and EMEA. By this way, the reactivity of secondary amines linked to different linear alkyl groups could be evaluated. The results of the comparison are presented in Table 4. From our measurements at 303 K in 2.5 kmol m-3 aqueous solutions, it is clear that the CO2 absorption rate in MMEA and EMEA is greater than the corresponding values in PMEA and BMEA. Further, reactivity decreases in the order EMEA>MMEA>PMEA>BMEA.


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4. Conclusions The secondary amines PMEA and BMEA are attractive substitute solvents for CO2 capture. In this work, chemical kinetics of the CO2 reactions with PMEA and BMEA in an aqueous medium was investigated at 303, 308 and 318 K in the 2-3 kmol m-3 concentration range using the stirredcell reactor method. It was found that PMEA reacts faster than BMEA. Both reaction systems exhibited overall third-order kinetics. The dependence of the pseudo-first order reaction rate constants on temperature and amine concentration was studied. Both of the two renowned mechanisms, viz. zwitterion and termolecular correctly represented the reaction pathway. Finally, the CO2-capturing performance of PMEA and BMEA was compared to those of other secondary amines linked to the methyl and ethyl group. It was found that reactivity decreases in the order EMEA>MMEA>PMEA>BMEA.

Acknowledgements Ravindra Kanawade thanks the University Grants Commission, New Delhi, for the financial support (UGC-SAP fellowship).

Nomenclature AmH

alkanolamine

[AmH]

alkanolamine concentration (kmol m-3)

B

base contributing to deprotonation of the zwitterion

[B]

concentration of base B in liquid phase (kmol m-3)

[BMEA]

initial BMEA concentration (kmol m-3)

[CO2]

concentration of CO2 (kmol m-3)

DAmH

diffusivity of amine in liquid phase (m2 s-1)

D

diffusivity of CO2 in liquid phase (m2 s-1)


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E

enhancement factor due to chemical reaction

Ei

enhancement factor for instantaneous reaction

H

Henry’s law constant (kmol m-3 kPa-1)

Ha

Hatta number

[H2O]

concentration of water (kmol m-3)

kJ

rate constant in Eq. 14

k2

forward reaction rate constant in Eq. 7

k-1

reverse reaction rate constant in Eq. 7

kAmH

deprotonation constant for amine

kJ H

reaction rate constant in Eq. 8

k )

deprotonation constant for H2O

kL

liquid-side mass transfer coefficient (m s-1)

,

rate constant for an m,nth order reaction

kobs

observed reaction rate constant (s-1)

k )>

deprotonation constant for OH

m

reaction order with respect to CO2

n

reaction order with respect to amine

OH

hydroxyl ion concentration (kmol m-3)

P

partial pressure of CO2 in bulk gas phase (kPa)

[PMEA]

initial PMEA concentration (kmol m-3)

rAmH

rate of reaction

R

specific rate of absorption of CO2 (kmol m-2 s-1)

z

stoichiometric coefficient

Greek letters


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ρ

density (kg m-3)

µ

viscosity (kg m-1 s-1)

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(9)

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between carbon dioxide and sterically hindered amines for carbon dioxide recovery from power plant flue gases. Chem. Eng. Commun. 1998, 170, 245-260. (10) Li, J.; Henni, A.; Tontiwachwuthikul, P. Reaction kinetics of CO2 in aqueous ethylenediamine, ethyl ethanolamine, and diethylmonoethanolamine solutions in the temperature range of 298–313 K, using the stopped-flow technique Ind. Eng. Chem. Res. 2007, 46, 44264434. (11) Ali, S. H.; Merchant, S. Q.; Fahim, A. Reaction kinetics of some secondary alkanolamines with carbon dioxide in aqueous solutions by stopped flow technique Sep. Purif. Technol. 2002, 27, 121-136. (12) Vaidya, P. D.; Mahajani, V. V. Kinetics of the reaction of CO2 with aqueous formulated solution containing monoethanolamine, N-methyl-2-pyrrolidone and diethylene glycol. Ind. Eng. Chem. Res. 2005, 44, 1868-1873. (13) Hikita, H.; Asai, S.; Ishikawa, H.; Honda, M. The kinetics of reactions of carbon dioxide with monoethanolamine, diethanolamine and triethanolamine by a rapid mixing method. Chem. Eng. J. 1977, 13, 7-12. (14) Sada, E.; Kumazawa, H.; Han, Z. Q.; Matsuyama, H. Chemical kinetics of the reaction of carbon dioxide with ethanolamines in nonaqueous solvents. AIChE J. 1985, 31, 1297-1303. (15) Ali, S. H. Kinetics of the reaction of carbon dioxide with blends of amines in aqueous media using the stopped-flow technique. Int. J. Chem. Kinet. 2005, 37, 391-405. (16) Littel, R. J.; Versteeg, G. F.; van Swaaij, W. P. M. Physical absorption into nonaqueous solutions in a stirred cell reactor. Chem. Eng. Sci. 1991, 46, 3308-3313. (17) Versteeg, G. F.; van Swaaij, W. P. M. Solubility and diffusivity of acid gases (CO2, N2O) in aqueous alkanolamine solutions. J. Chem. Eng. Data 1988, 33, 29-34.


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(18) Doraiswamy, L. K.; Sharma, M. M. Heterogeneous reactions: Analysis, examples and reactor design, vol. 2; John Wiley and Sons: New York, 1984. (19) Danckwerts, P. V. Gas-liquid reactions; McGraw-Hill: New York, 1970. (20) Mahajani, V. V.; Joshi, J. B. Kinetics of reactions between carbon dioxide and alkanolamines. Gas Sep. Purif. 1988, 2, 50-64. (21) Shen, K. P.; Li, M. H.; Yih, S. M. Kinetics of carbon dioxide reaction with sterically hindered 2-piperidineethanol aqueous solutions. Ind. Eng. Chem. Res. 1991, 30, 1811-1813. (22) Bindwal, A. B.; Vaidya, P. D.; Kenig, E. Y. Kinetics of carbon dioxide removal by aqueous diamines. Chem. Eng. Sci. 2011, 169, 144-150. (23) Caplow, M. Kinetics of carbamate formation and breakdown. J. Am. Chem. Soc. 1968, 90, 6795-6803. (24) Danckwerts, P. V. The reaction of CO2 with ethanolamines. Chem. Eng. Sci. 1979, 34, 443-446. (25) Versteeg, G. F.; van Dijck, L. A. J.; van Swaaij, W. P. M. On the kinetics between CO2 and alkanolamines both in aqueous and non-aqueous solutions. An overview. Chem. Eng. Commun. 1996, 144, 113-158. (26) da Silva, E. F.; Svendsen, H. F. Ab initio study of the reaction of carbamate formation from CO2 and alkanolamines Ind. Eng. Chem. Res. 2004, 43, 3413-3418. (27) Versteeg, G. F.; van Swaaij, W. P. M. On the kinetic between CO2 and alkanolamines both in aqueous and non-aqueous solutions. I. Primary and secondary amines. Chem. Eng. Sci. 1988, 43, 573-585.

List of Figures Fig. 1. Secondary alkanolamines linked to an alkyl group used in the present work. Fig. 2. Plots of R vs. P in 2.5 kmol m-3 aqueous PMEA and BMEA solutions at T=303 K.


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678

: vs. log [PMEA] at 303, 308 and 313 K.

678

: vs. log [BMEA] at 303, 308 and 313 K.

Fig. 3. A plot of log 5 Fig. 4. A plot of log 5

-978 -978

Fig. 5. Arrhenius plots for the CO2-PMEA and CO2-BMEA systems. Fig. 6. A plot of kobs vs. [PMEA] at 303, 308 and 313 K. Fig. 7. A plot of kobs vs. [BMEA] at 303, 308 and 313 K.

List of Tables Table 1. Physical properties of PMEA and BMEA (2.5 kmol m-3) at 303, 308 and 313 K. Table 2. CO2 absorption rates into aqueous PMEA and BMEA solutions. Table 3. CO2 absorption rates, solution properties and results of the interpretation of the experimental kinetic data for PMEA and BMEA at 303 K. Table 4. Comparison of the efficacy of secondary amines (2.5 kmol m-3) at 303 K.


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MMEA

EMEA

PMEA

BMEA

Fig. 1


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20

Absorption rate × 10 6 , kmol m-2 s-1

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

16

12

8

PMEA

4

BMEA 0 0

4

8

12

16

CO2 partial pressure, kPa

Fig. 2


20

Page 19 of 27

Energy & Fuels

log P

R

CO -D

Q

19

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log [PMEA]

Fig. 3


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Page 20 of 27

log P

R

CO -D

Q

20

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log [BMEA]

Fig. 4


Page 21 of 27

Energy & Fuels

ln k1,2

21

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1000/T, K-1

Fig. 5


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Page 22 of 27

kobs, s-1

22

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[PMEA], kmol m-3

Fig. 6


Page 23 of 27

Energy & Fuels

kobs, s-1

23

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[BMEA], kmol m-3

Fig. 7


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Table 1. Physical properties of PMEA and BMEA (2.5 kmol m-3) at 303, 308 and 313 K.

Amine

PMEA

BMEA

Temp.

ρ

µ

D × 10S

H × 10T

K

kg m-3

mPa s

m2 s-1

kmol m-3 kPa-1

303

986.8

0.81

20.1

2.24

308

985.4

0.73

22.2

1.62

313

971.9

0.66

24.7

1.31

303

972.1

0.81

20.6

1.58

308

968.3

0.72

23.3

1.48

313

964.9

0.66

25.1

1.22


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Table 2. CO2 absorption rates into aqueous PMEA and BMEA solutions.

Amine

PMEA

Temp.

[AmH]

P

R × 10U

K

kmol m-3

kPa

kmol m-2 s-1

303

2

5

4.17

2.5

5

5.01

3

5.1

5.32

2

5

4.92

2.5

5.1

5.45

3

5

5.75

2

5.1

5.82

2.5

5.2

6.1

3

5

6.5

2

4.4

2.6

2.5

5

3.4

3

5.2

3.56

2

5.1

3.84

2.5

4.6

3.65

3

5

4.55

2

5.1

4.8

2.5

5

5.01

3

4

4.87

308

313

BMEA

303

308

313


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Table 3. CO2 absorption rates, solution properties and results of the interpretation of the experimental kinetic data for PMEA and BMEA at 303 K.

Amine

[AmH]

P

R × 10U

kmol

kPa

kmol m-2 s-1

m-3

H × 104

D × 1010

kmol m-3 kPa-1

m2 s-1

k2

E=Ha

Ei

m-6 kmol-2 s-1

PMEA

BMEA

2

5

4.17

2.42

20.5

1443

104

825

2.5

5

5.01

2.24

20.1

1599

136

1119

3

5.1

5.32

1.99

19.9

1565

160

1493

2

4.4

2.6

1.84

21

1211

97

1227

2.5

5

3.4

1.58

20.6

1422

130

1573

3

5.2

3.56

1.44

20.3

1263

146

2023


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Table 4. Comparison of the efficacy of secondary amines (2.5 kmol m-3) at 303 K.

Amine

MMEA

EMEA

PMEA

BMEA

P

R × 10U

kPa

kmol m-2 s-1

5.1

9.6

10.1

11.5

15.1

15.5

18.5

18

5

11.8

10.2

14.5

15.1

17.7

18.5

19.3

5

5

10.3

10.1

15.2

13.9

20

17.2

5

3.4

9.4

7.9

15.2

12.5

19.9

16.1


Kinetics of Carbon Dioxide Removal by n-Propyl- and n

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