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Kinetics of the Catalytic Desorption of CO2 from Monoethanolamine (MEA) and Monoethanolamine and Methyldiethanolamine (MEA-MDEA)
Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 114 (2017) 1495 – 1505
13th International Conference on Greenhouse Gas Control Technologies, GHGT-13, 14-18 November 2016, Lausanne, Switzerland
Kinetics of the Catalytic Desorption of CO2 from Monoethanolamine (MEA) and Monoethanolamine and Methyldiethanolamine (MEA-MDEA) Ananda Akachukua, Anima Oseib, Benjamin Decardi-Nelsonb, Wayuta Srisangb, Fatima Pouryousefib, Hussameldin Ibrahimb and Raphael Idem* a,b
Clean Energy Technologies Institute, Faculty of Engineering and Applied Science, University of Regina, Regina, Saskatchewan. S4S OA2, Canada
Abstract
The kinetics of the desorption of CO2 from CO2-loaded aqueous single 5M monoethanolamine and blended 5M/2M monoethanolamine-methyldiethanolamine solvents were studied over solid acid Ȗ-Al2O3 and HZSM-5 catalysts at three desorber temperatures (348, 358 and 368K) at CO2 loading ranging from 0.279-0.5mol/mol for different ratios of catalyst weight/flow rate of amine (W/FAo) using a bench-scale complete absorber–desorber CO2 capture pilot plant of 2-inch internal diameter and 3.5 ft total height. The kinetic performance, in terms of CO2 produced, activation energy and rate constants, showed that HZSM-5 catalyst provided faster kinetics, higher CO2 produced and lower activation energy than ZLWKȖ-Al2O3. © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2017 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of organizing the organizing committee of GHGT-13. Peer-review under responsibility of the committee of GHGT-13. Keywords: CO2 Desorption; solid acid catalyst; kinetics; Lewis acid site; Brønsted acid site.
* Corresponding author. Tel.: +1-306-585-4470; fax: +1-306-585-4855. E-mail address: [email protected]
1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of GHGT-13. doi:10.1016/j.egypro.2017.03.1274
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Nomenclature []ܣ AAD ܥ௪,ௗ d dp D EA, ܨ Kc k0 r ri R Rc T W Xi
Concentration of species, mol/dm3 average absolute deviation Weisz–Prater criterion for internal pore diffusion internal diameter of reactor, m diameter of particle, mm diffusivity coefficient, m2 sí1 activation energy, J molí1 Molar flow rate of species mol miní1 mass transfer coefficient, m2 sí1 pre-exponential or collision factor radius of the catalyst bed, m rate of reaction based on a particular species, mol gcatí1 miní1 (i =A,B,C, . . .) universal gas constant, kJ kmolí1 Kí1 radius of catalyst particle, m temperature, K weight of catalyst, g conversion of component i
Greek Letters οܪ Enthalpy of reaction, kJ kmolିଵ οܩ Gibbs free energy of reaction, kJ kmolିଵ οܵ Entropy of reaction, kJ kmolିଵ ߤ Viscosity ߩ, ߩ Density, kg mିଷ
1. Introduction The continuous increase in the rate of CO2 production resulting from human activities such as fossil fuel combustion and other industrial activities and its emission into the atmosphere has increased the importance placed on CO2 mitigation processes in light of fear of climate change [1]. Amongst the developed technologies used in CCS, Post Combustion CO2 Capture (PCCC) is the most developed and widely used due to its ability to be retrofitted into an existing power plant and capacity of handling large amount of exhaust stream. Absorption Operation and efficiency as well as reduction in thermal regeneration energy is needed to achieve a 90% efficiency with less than 35% increase in the post- combustion process [2]. However, the major challenge encountered in PCCC is the heat of regeneration associated with desorption of CO2 from loaded amine. This energy parasitic energy requirement can be minimized via solvent enhancement and process optimization. However, the practical achievable minimum energy estimated to be 0.72GJ/tonne [3] is far from reach. This has prompted various researchers to look for alternatives methods to lower the heat duty. Such methods include the use of alternative solvents such as amino acid [4] and sodium carbonate [5]. Recently, Idem et al. [6] introduced the addition of solid acid catalyst to the desorber unit. This method was aimed at reducing the regeneration temperature from 120-140oC to 90-95oC. This reduced temperature ensured that the heat of vaporization of water was eliminated by operating at a temperature lower than the boiling point of water [7]. Several authors have confirmed this to be true [8, 9] by performing similar experiments using the same solid acid catalyst recommended by Idem et al. [10]. Another approach is the use of enzyme such as carbonic anhydrase [11, 12]. This enzyme aids in the rapid hydrolysis of CO2 to form bicarbonate and hydrogen ion. However, the
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integration of this catalytic system to an existing power plant requires the understanding of the fundamental kinetic phenomena which allows for an effective desorber design aimed at lowering the activation energy, EA. This study investigates the kinetics performance of two types of solid acid catalysts- Lewis acid: ߛ െ ݈ܣଶ ܱଷ and Brønsted acid: HZSM-5 in the desorption of CO2 from CO2-loaded aqueous solutions of primary amine monoethanolamine (MEA) and CO2-loaded primary-tertiary amine blend (MEA-MDEA). The overall objective of this research is to study and develop kinetic power law models that adequately describe the catalytic desorption of CO2 from CO2-loaded (MEA) and blend (MEA-MDEA)
2. Theory Kinetically controlled finite reactions are considered as fast reactions, the rate of which are influenced by the addition of a catalyst. These reactions will be used in the development of the power law model. A breakdown of the zwitterion reaction mechanism is needed to explain the role of the catalyst. This reaction consists of the formation of a zwitterion complex followed by the deprotonation of the zwitterion by a base. This same route is followed during solvent regeneration and is also characterized as a two-step mechanism involving carbamate breakdown and MEAH+ deprotonation as shown: MEAH ା deprotonation: MEAH ା + Hଶ O ՞ MEA + Hଷ Oା
οG ~ 78.2kJ/mol (90oC)
(1)
The high alkalinity of MEA makes it difficult for ܪܣܧܯା to release its proton to ܪଶ O thus resulting in the high heat of regeneration associated with loaded CO2-MEA solution. This energy requirement is reported to be 78.2 kJ/ mol at (90oC) [7]. Conversely, the presence of a stronger baseെ ܱܥܪଷି reduces the energy requirement to a value of 21.9 kJ/mol [7]. Carbamate breakdown: RNH െ COOି + Hଷ Oା ՞ Zwitterion ՞ MEA + COଶ ο~ ܩ14.7kJ/mol
(2)
Carbamate breakdown, though complex, requires less energy. It however requires large amounts of free protons and depends greatly on reaction 1, as most of the protons are attached to ܪܣܧܯା . The overall finite desorption mechanism of the CO2 + amine + H2O system used for the model development is described in the equation: RNHCOOି + ۰ ା ՞ RNHଶ + COଶ +B HCOଷି + B ା ՞ COଶ(ୟ୯) +Hଶ O + ۰ HCOଷି ՞ COଶ(ୟ୯) + OH ି RNHCOOି + HCOଷି + B ା ՞ RNHଶ + B+ 2COଶ + OH ି
(3) (4) (5) (6)
Where B is any base present in the system and ା is the corresponding conjugate acid. In this work the possible bases and conjugate acids are MEA , ܣܧܦܯ, ܱ ି ܪ, ܪଶ ܱ, ܱܥܪଷି , ܱܥଷଶି and ܪܣܧܯା , ܪܣܧܦܯା , ܪଶ O, ܪଷ ܱା , ܪଶ ܱܥଷ , ܽ݊݀ ܱܥܪଷି . Since it is difficult to know the amount of ܱܥܪଷି that contributes to deprotonation of ܴܰܪଶା and the amount that reacts with ܪଷ ܱ ା and also quite impossible to know ܴܰܪଶା that reacts with ܱ ି ܪand ܪଶ ܱ and the fraction that contributes to carbamate breakdown, the power law model will be written in terms of product formation which incorporates the formation of CO2 from ܱܥܪଷି and ܴܰ ି ܱܱܥܪ, (i.e. ݎை ଶ = ݎோேுைைష + ݎுைయష ).
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The CO2 production rate is written as: ݎைଶି = െܱ݇ܥ2 [ܱܥ2 ]
(7)
3. Experimental section The experiments were performed in complete absorber – desorber CO2 capture pilot plant unit. Both the absorber and desorber columns in the unit were of 2-inch internal diameter and 3.5 ft total height. The experimental set is as shown in Fig. 1. Experimental kinetic data for CO2 desorption were obtained in a catalytic packed bed tubular reactor (i.e. desorber) at three temperatures (348K, 358K, and 368K), with 5M MEA and 5/2M MEA-MDEA concentrations and CO2 loading ranging from 0.279-0.5 mol CO2/mol amine, for different ratios of catalyst weight/amine flow rate (W/FAo). Thermocouples and concentration measurements were located along the length of the absorber and desorber column and were used to obtain gas phase CO2 concentration and temperature profiles. Solvent losses were prevented by positioning a condenser at the top of the absorber and desorber columns. Several other auxiliary types of equipment were used in this study, namely, CO2 cylinder, gas flow meter, feed tank, and liquid flow pump. At the beginning of each run, N2 and CO2 were set to the desired partial pressures and concentrations before being introduced into the bottom of the absorber column through the gas flow meter which controlled the gas flows individually. Cooling water circulated through the condenser from the cold water bath. The MEA/ MEA-MDEA solution from the feed tank was let into the top of the column with the help of a constant liquid flow pump. This ensured that the column operated under contact counter-current flow, allowing CO2 to be absorbed by providing maximum driving force for mass transfer. The treated gas escaped through the top of the column while the rich solution exit at the bottom of the column after which it passed through the lean/rich heat exchanger and then a hot oil heater before flowing into the desorber column. Here, the temperature of the desorber is at elevated temperature, allowing desorption of CO2 to occur. After desorption, the desorbed CO2 escapes through the top of the column and the lean amine is recirculated back into the absorber.
4. Results and Discussion 4.1. Evaluation of possible heat and mass transfer effects Intrinsic kinetic data are obtained only in the absence of heat and mass transfer limitations. Since these transfer limitations tend to occur at higher temperature due to the increase in reaction rate, it is important to determine to what extent their existence affect the reaction rate. Using the correlations available in literature, the possible effect of heat and mass limitations on reaction rates were examined at the highest reaction temperature-368K as reported in the works of Ibrahim and Idem [11]. From the results obtained in tables 1 and 2, it can be seen that the effect of mass and heat transfer can be ignored as all requirements were met. 4.2. Kinetic Analysis The catalyst performance was evaluated in terms of carbamate and bicarbonate conversion to CO2 and the rate at which the conversion occurs. The kinetic performance of the catalyst was further expressed in terms of activation energy, EA, rate constants Kr and reaction order, n. The conversion is expressed as: ܺைଶ =(ܱܥଶ, െ ܱܥଶ,௨௧ )/ܱܥଶ,
(8)
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In the absence of heat and mass limitations, intrinsic experimental rates were calculated using the equal area differentiation technique which was obtained from the plots of conversion, X (i.e. CO2 desorbed) vs. weight of catalyst per amine flow rate (ܹ/ܨ ): ݎ = ݀ܺைଶ /݀(ܹ/ܨ )
(9)
Fig. 1. Experimental Setup
Table 1: Heat Transfer Limitations Catalyst
ɀ െ Alଶ Oଷ
HZSM-5
Solvents
ࡰࢇࢋ࢚ࢋ࢘ (m)
οࢀࢇ࢚࢘ࢉࢋ, ,ࢇ࢞(K)
οࢀࢌ,ࢇ࢞ (K)
Mears Criteria <0.15
MEA
0.003
3.45
7.8 ݔ10ିଷ
8.51 ݔ10ିସ
MEA-MDEA MEA
0.0025
4.36 5.19
8.1 ݔ10ିଷ 6.6 ݔ10ିଷ
8.32 ݔ10ି 6.31 ݔ10ିସ
6.32
5.72 ݔ10ି
5.47 ݔ10ିସ
MEA-MDEA
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Table 2. Mass Transfer Limitations Catalyst type
Film mass transfer resistance െ࢘ᇱ,࢈࢙ ࢊ࢈ /ࡷࢉ ࢙
Mass transfer limitation in the film െ࢘ᇱ,࢈࢙ ࣋࢈ ࡾࢉ /ࡷࢉ < 0.15
Internal Mass transfer limitations ᇱ ࢝,ࢊ = െۯܚ,ܛ܊ܗ ܀ ܋ܘ ܋/ࡰ ܍۱܁ۯ
ɀ െ Alଶ Oଷ
1.27 ݔ10ିଷ
3.44 ݔ10ିଷ
0.134
HZSM െ 5
8.7 ݔ10ିସ
1.56 ݔ10ିଷ
0.451
4.3. Role of catalyst ߛ െ ݈ܣଶ ܱଷ and HZSM-5 Catalyst in Amine Regeneration The partially uncoordinated metal cation and oxide anion that appear at the surface of Ȗ-݈ܣଶ ܱଷ catalyst enables it to act as an acid and base, respectively [12]. In the basic region, the lone pair electron density found in the N atom of the carbamate is attacked by the cus cation ion-Al3+ (Lewis acid). This weakens the N-C bond, resulting in the dissociative chemisorption of the carbamate ion. On the other hand, the oxide anion-ܱ݈ܣଶି attacks the ܪା found in ܪܣܧܯା . The presence of both Brønsted and Lewis acid site in HZSM-5 increases its role in the desorption of CO2. The Brønsted acid site donates proton to the carbamate ion. This proton donation converts the carbamate ion to a carboxylic acid (MEACOOH). This is then followed by a chemisorption on the Al site which results in the weakening of the N-C bond. According to Idem et al. [6] the HZSM-5 catalyst also transfers its available proton to bicarbonate ions, thus increasing the amount of desorbed CO2. The Lewis acid site attacks the free lone pair of electron density found in the N atom of the carbamate ion. 4.4. Effect of Varying Catalyst weights at constant ܨ Changing mass of catalyst, W, while keeping amine flow rate constant (ܨ ), showed that the reaction rate increases with catalyst weight. As described by the Maxwell Boltzmann theory, the increase in temperature and catalyst weight increases the average number of molecules with higher kinetic energy, thus allowing the reaction to occur at a faster rate and lower activation energy. This same effect was seen as the temperature and catalyst weight were varied as presented in Figs. 2 and 3. The presence of both Brønsted and Lewis acid sites in HZSM-5 explains the higher conversion and lower activation energy observed in CO2 desorption in the presence of HZSM-5 in comparison to Ȗ-Al2O3. The temperature effect can be seen at 0g catalyst weight for all temperatures (348, 358 and 368K). As earlier stated, increase in temperature increases the fraction of particles with higher energy. At 75oC (348K) the influence of the temperature effect on the reaction is minimal. Another effect to consider is the solvent effect. As mentioned by Shi et al [7] tertiary amine MDEA is a weaker base than MEA and is therefore easier to deprotonate ܪܣܧܦܯା than ܪܣܧܯା . Also ܪܣܧܯା would rather give its proton to MDEA than toܪଶ O. It can be seen that all reactions involving MDEA requires lesser energy than MEA as shown in equations 1-2 and 10-12. Consequently, the solvent effect plays an important role in increasing the conversion and reducing the activation energy alongside the catalyst contribution (Fig. 2 and 3).
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0.35
0.3 MEA-MDEA Al2O3 75oC
0.25
Conversion, X
MEA-MDEA Al2O3 85oC 0.2
MEA-MDEA Al2O3 95oC MEA 85oC Al2O3
0.15 MEA 75oC Al2O3 MEA 95oC Al2O3
0.1
0.05
0 0
1
2
3
4
5
6
W/FAo (g cat. min/mol) Fig. 2. Variation of Ȗ-݈ܣଶ ܱଷ catalyst weight (0-200g) and temperature (348K, 358K, and 368K) at constant amine flow rate 60 ml/min
0.35 0.3 MEA-MDEA HZSM 5 75oC
0.25
MEA-MDEA HZSM 5 85oC MEA-MDEA HZSM 5 95oC
0.15
MEA HZSM 5 75oC
Conversion, X
0.2
MEA HZSM 5 85oC
0.1
MEA HZSM 5 95oC 0.05 0 0
1
2
3
4
5
W/FAo (g cat. min/mol) Fig. 3. Variation of HZSM-5 catalyst weight (0-200g) and temperature (348K, 358K, and 368K) at constant amine flow rate 60 ml/min.
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MEAH ା + Hଶ O ՞ MEA + Hଷ Oା ~ 78.2kJ/mol MEAH ା + HCOି ଷ ՞ MEA + COଶ(ୟ୯) + Hଶ O ~ 21.9 kJ/mol
(1) (2)
MDEAH ା + Hଶ O ՞ MDEA + Hଷ Oା ~ 63.53kJ/mol MEAH ା + MDEA ՞ MDEAH ା +MEA ~ 14.7kJ/mol MDEAH ା + HCOଷି ՞ MDEA + COଶ(ୟ୯) + Hଶ O ~ 7.2kJ/mol
(10) (11) (12)
4.5. Parameter Estimations An empirical power law model in terms of CO2 desorption rate was based on this overall reaction with respect to CO2 and is represented in the form: ݎଶି = െ݇ ݁ି( ݔாಲ /ோ்) [ܱܥଶ ]
(13)
Equation (13) shows the general dependence of desorption rate on the concentration of reacting species expressed in the Arrhenius form. The Power law model parameters were obtained from Arrhenius equation by plotting lnk vs. (1/T) as is shown in Fig. 4 and 5. The obtained parameters are presented in Table 3. ߛ െ ݈ܣଶ ܱଷ and HZSM-5 catalyst increases the rate of desorption of CO2 either by increasing the frequency of collision between the reacting molecules which is represented by the pre exponential factor, ݇ , or by providing an alternative pathway with lower activation energy, ܧ . The order of reaction, n, which shows to what extent the rate of reaction depends on reactant concentration was 1 overall for both catalysts and solvent type. The pre exponential factor, ݇ , of Ȗ-Al2O3 was seen to be higher than that of HZSM-5.
-10.8 0.00268 0.00272 0.00276
0.0028
0.00284 0.00288 0.00292
-11.2
-11.6
ln K
HZSM 5 Al2O3
-12
-12.4
-12.8
Fig. 4. Arrhenius plot (CO2+MEA system)
1/T (K)
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-10.6 0.0027
0.00275
0.0028
0.00285
0.0029
-10.8 -11 -11.2
ln k
HZSM 5 Al2O3
-11.4 -11.6 -11.8 -12
1/T (K)
Fig. 5: Arrhenius plot (CO2 + MEA + MDEA system)
Table 3. Parameter estimation for CO2+ MEA + H2O and CO2 + MEA+ MDEA+ H22V\VWHPIRUȖ-Al2O3 and HZSM-5
Parameters
ࢽ െ Al2O3
HZSM-5
ࢽ-Al2O3
HZSM-5
n
0.98~ 1
0.6 ~ 1
0.6~ 1
0.49 ~ 1
3.4 × 10
2.2 ݔ10ହ
1.7 ݔ10ଷ
3.4 ݔ10ଶ
ܧ (J/mol)
8.02 ݔ10ସ
7.1 ݔ10ସ
6.7 ݔ10ସ
6.1 ݔ10ସ
AAD
5.1%
4.0%
2.9%
3.3%
݇ dm3.sec-1.g.catalyst-1 dm6.݈݉.sec-1.g.catalyst-1
All the models were validated by determining the absolute average deviation (AAD %) between the experimental rate and predicted rate obtained from the proposed models. Also a parity chart which depicts how well the predicted rates fit the experimental rate data was plotted as shown in Fig. 6.
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Predicted rate (mol/g.catalyst.sec)
0.00003
0.000025
0.00002 Power Law MEA-Al2O3
0.000015
0.00001
Power Law model MEA HZSM-5
0.000005
Power Law MEA-MDEA Al2O3
0
Power Law model MEAMDEA HZSM 5
0
0.000005
0.00001
0.000015
0.00002
0.000025
0.00003
Predicted rate (mol/g.catalyst.sec) Fig. 6. Parity Chart
4. Conclusions x
The desorption kinetics of CO2 from loaded aqueous MEA and MEA-MDEA solutions were studied over Ȗ-Al2O3 and HZSM-5 catalysts as a function amine flow rates, catalyst weight and reaction temperature in a tubular packed-bed reactor. The power law models were used to describe the experimental kinetic data for CO2 desorption.
x
The kinetic performance was evaluated in terms of conversion (i.e. %CO2 desorbed), activation energy and rate constants. The optimum operating conditions to obtain maximum conversion over HZSM- DQG ȖAl2O3 was achieved at 85oC at 60ml/min.
x
HZSM-5 catalyst gave lower activation energies in comparisRQ WR Ȗ-Al2O3 for both solvents and is presented in the Arrhenius form in the power law models with AAD lower than 15% and are given as:
ݎைమିொି ఊమைయ = 3.4 ݔ10 ݁(ݔെ8.02 ݔ10ସ /ܴܶ)ܱܥ[ ݔଶ ].ଽ଼
(13)
ݎைమିொିுௌெ ହ = 2.2 ݔ10ହ ݁(ݔെ7.09 ݔ10ସ /ܴܶ)ܱܥ[ ݔଶ ].ଷ
(14)
ݎைమିொିொି ఊమைయ = 1.7 ݔ10ଷ ݁(ݔെ6.7 ݔ10ସ /ܴܶ)ܱܥ[ ݔଶ ].
(15)
ݎைమିொିொିுௌெ ହ = 3.36 ݔ10ଶ ݁(ݔെ6.1 ݔ10ସ /ܴܶ)ܱܥ[ ݔଶ ].ସଽ
(16)
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Acknowledgements The authors would like to thank Clean Energy Technologies Research Institute (CETRI), Natural Sciences and Engineering Research Council of Canada (NSERC) and Faculty of Graduate Studies and Research (FGSR), University of Regina for their financial support.
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13th International Conference on Greenhouse Gas Control Technologies, GHGT-13, 14-18 November 2016, Lausanne, Switzerland
Kinetics of the Catalytic Desorption of CO2 from Monoethanolamine (MEA) and Monoethanolamine and Methyldiethanolamine (MEA-MDEA) Ananda Akachukua, Anima Oseib, Benjamin Decardi-Nelsonb, Wayuta Srisangb, Fatima Pouryousefib, Hussameldin Ibrahimb and Raphael Idem* a,b
Clean Energy Technologies Institute, Faculty of Engineering and Applied Science, University of Regina, Regina, Saskatchewan. S4S OA2, Canada
Abstract
The kinetics of the desorption of CO2 from CO2-loaded aqueous single 5M monoethanolamine and blended 5M/2M monoethanolamine-methyldiethanolamine solvents were studied over solid acid Ȗ-Al2O3 and HZSM-5 catalysts at three desorber temperatures (348, 358 and 368K) at CO2 loading ranging from 0.279-0.5mol/mol for different ratios of catalyst weight/flow rate of amine (W/FAo) using a bench-scale complete absorber–desorber CO2 capture pilot plant of 2-inch internal diameter and 3.5 ft total height. The kinetic performance, in terms of CO2 produced, activation energy and rate constants, showed that HZSM-5 catalyst provided faster kinetics, higher CO2 produced and lower activation energy than ZLWKȖ-Al2O3. © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2017 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of organizing the organizing committee of GHGT-13. Peer-review under responsibility of the committee of GHGT-13. Keywords: CO2 Desorption; solid acid catalyst; kinetics; Lewis acid site; Brønsted acid site.
* Corresponding author. Tel.: +1-306-585-4470; fax: +1-306-585-4855. E-mail address: [email protected]
1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of GHGT-13. doi:10.1016/j.egypro.2017.03.1274
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Nomenclature []ܣ AAD ܥ௪,ௗ d dp D EA, ܨ Kc k0 r ri R Rc T W Xi
Concentration of species, mol/dm3 average absolute deviation Weisz–Prater criterion for internal pore diffusion internal diameter of reactor, m diameter of particle, mm diffusivity coefficient, m2 sí1 activation energy, J molí1 Molar flow rate of species mol miní1 mass transfer coefficient, m2 sí1 pre-exponential or collision factor radius of the catalyst bed, m rate of reaction based on a particular species, mol gcatí1 miní1 (i =A,B,C, . . .) universal gas constant, kJ kmolí1 Kí1 radius of catalyst particle, m temperature, K weight of catalyst, g conversion of component i
Greek Letters οܪ Enthalpy of reaction, kJ kmolିଵ οܩ Gibbs free energy of reaction, kJ kmolିଵ οܵ Entropy of reaction, kJ kmolିଵ ߤ Viscosity ߩ, ߩ Density, kg mିଷ
1. Introduction The continuous increase in the rate of CO2 production resulting from human activities such as fossil fuel combustion and other industrial activities and its emission into the atmosphere has increased the importance placed on CO2 mitigation processes in light of fear of climate change [1]. Amongst the developed technologies used in CCS, Post Combustion CO2 Capture (PCCC) is the most developed and widely used due to its ability to be retrofitted into an existing power plant and capacity of handling large amount of exhaust stream. Absorption Operation and efficiency as well as reduction in thermal regeneration energy is needed to achieve a 90% efficiency with less than 35% increase in the post- combustion process [2]. However, the major challenge encountered in PCCC is the heat of regeneration associated with desorption of CO2 from loaded amine. This energy parasitic energy requirement can be minimized via solvent enhancement and process optimization. However, the practical achievable minimum energy estimated to be 0.72GJ/tonne [3] is far from reach. This has prompted various researchers to look for alternatives methods to lower the heat duty. Such methods include the use of alternative solvents such as amino acid [4] and sodium carbonate [5]. Recently, Idem et al. [6] introduced the addition of solid acid catalyst to the desorber unit. This method was aimed at reducing the regeneration temperature from 120-140oC to 90-95oC. This reduced temperature ensured that the heat of vaporization of water was eliminated by operating at a temperature lower than the boiling point of water [7]. Several authors have confirmed this to be true [8, 9] by performing similar experiments using the same solid acid catalyst recommended by Idem et al. [10]. Another approach is the use of enzyme such as carbonic anhydrase [11, 12]. This enzyme aids in the rapid hydrolysis of CO2 to form bicarbonate and hydrogen ion. However, the
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integration of this catalytic system to an existing power plant requires the understanding of the fundamental kinetic phenomena which allows for an effective desorber design aimed at lowering the activation energy, EA. This study investigates the kinetics performance of two types of solid acid catalysts- Lewis acid: ߛ െ ݈ܣଶ ܱଷ and Brønsted acid: HZSM-5 in the desorption of CO2 from CO2-loaded aqueous solutions of primary amine monoethanolamine (MEA) and CO2-loaded primary-tertiary amine blend (MEA-MDEA). The overall objective of this research is to study and develop kinetic power law models that adequately describe the catalytic desorption of CO2 from CO2-loaded (MEA) and blend (MEA-MDEA)
2. Theory Kinetically controlled finite reactions are considered as fast reactions, the rate of which are influenced by the addition of a catalyst. These reactions will be used in the development of the power law model. A breakdown of the zwitterion reaction mechanism is needed to explain the role of the catalyst. This reaction consists of the formation of a zwitterion complex followed by the deprotonation of the zwitterion by a base. This same route is followed during solvent regeneration and is also characterized as a two-step mechanism involving carbamate breakdown and MEAH+ deprotonation as shown: MEAH ା deprotonation: MEAH ା + Hଶ O ՞ MEA + Hଷ Oା
οG ~ 78.2kJ/mol (90oC)
(1)
The high alkalinity of MEA makes it difficult for ܪܣܧܯା to release its proton to ܪଶ O thus resulting in the high heat of regeneration associated with loaded CO2-MEA solution. This energy requirement is reported to be 78.2 kJ/ mol at (90oC) [7]. Conversely, the presence of a stronger baseെ ܱܥܪଷି reduces the energy requirement to a value of 21.9 kJ/mol [7]. Carbamate breakdown: RNH െ COOି + Hଷ Oା ՞ Zwitterion ՞ MEA + COଶ ο~ ܩ14.7kJ/mol
(2)
Carbamate breakdown, though complex, requires less energy. It however requires large amounts of free protons and depends greatly on reaction 1, as most of the protons are attached to ܪܣܧܯା . The overall finite desorption mechanism of the CO2 + amine + H2O system used for the model development is described in the equation: RNHCOOି + ۰ ା ՞ RNHଶ + COଶ +B HCOଷି + B ା ՞ COଶ(ୟ୯) +Hଶ O + ۰ HCOଷି ՞ COଶ(ୟ୯) + OH ି RNHCOOି + HCOଷି + B ା ՞ RNHଶ + B+ 2COଶ + OH ି
(3) (4) (5) (6)
Where B is any base present in the system and ା is the corresponding conjugate acid. In this work the possible bases and conjugate acids are MEA , ܣܧܦܯ, ܱ ି ܪ, ܪଶ ܱ, ܱܥܪଷି , ܱܥଷଶି and ܪܣܧܯା , ܪܣܧܦܯା , ܪଶ O, ܪଷ ܱା , ܪଶ ܱܥଷ , ܽ݊݀ ܱܥܪଷି . Since it is difficult to know the amount of ܱܥܪଷି that contributes to deprotonation of ܴܰܪଶା and the amount that reacts with ܪଷ ܱ ା and also quite impossible to know ܴܰܪଶା that reacts with ܱ ି ܪand ܪଶ ܱ and the fraction that contributes to carbamate breakdown, the power law model will be written in terms of product formation which incorporates the formation of CO2 from ܱܥܪଷି and ܴܰ ି ܱܱܥܪ, (i.e. ݎை ଶ = ݎோேுைைష + ݎுைయష ).
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The CO2 production rate is written as: ݎைଶି = െܱ݇ܥ2 [ܱܥ2 ]
(7)
3. Experimental section The experiments were performed in complete absorber – desorber CO2 capture pilot plant unit. Both the absorber and desorber columns in the unit were of 2-inch internal diameter and 3.5 ft total height. The experimental set is as shown in Fig. 1. Experimental kinetic data for CO2 desorption were obtained in a catalytic packed bed tubular reactor (i.e. desorber) at three temperatures (348K, 358K, and 368K), with 5M MEA and 5/2M MEA-MDEA concentrations and CO2 loading ranging from 0.279-0.5 mol CO2/mol amine, for different ratios of catalyst weight/amine flow rate (W/FAo). Thermocouples and concentration measurements were located along the length of the absorber and desorber column and were used to obtain gas phase CO2 concentration and temperature profiles. Solvent losses were prevented by positioning a condenser at the top of the absorber and desorber columns. Several other auxiliary types of equipment were used in this study, namely, CO2 cylinder, gas flow meter, feed tank, and liquid flow pump. At the beginning of each run, N2 and CO2 were set to the desired partial pressures and concentrations before being introduced into the bottom of the absorber column through the gas flow meter which controlled the gas flows individually. Cooling water circulated through the condenser from the cold water bath. The MEA/ MEA-MDEA solution from the feed tank was let into the top of the column with the help of a constant liquid flow pump. This ensured that the column operated under contact counter-current flow, allowing CO2 to be absorbed by providing maximum driving force for mass transfer. The treated gas escaped through the top of the column while the rich solution exit at the bottom of the column after which it passed through the lean/rich heat exchanger and then a hot oil heater before flowing into the desorber column. Here, the temperature of the desorber is at elevated temperature, allowing desorption of CO2 to occur. After desorption, the desorbed CO2 escapes through the top of the column and the lean amine is recirculated back into the absorber.
4. Results and Discussion 4.1. Evaluation of possible heat and mass transfer effects Intrinsic kinetic data are obtained only in the absence of heat and mass transfer limitations. Since these transfer limitations tend to occur at higher temperature due to the increase in reaction rate, it is important to determine to what extent their existence affect the reaction rate. Using the correlations available in literature, the possible effect of heat and mass limitations on reaction rates were examined at the highest reaction temperature-368K as reported in the works of Ibrahim and Idem [11]. From the results obtained in tables 1 and 2, it can be seen that the effect of mass and heat transfer can be ignored as all requirements were met. 4.2. Kinetic Analysis The catalyst performance was evaluated in terms of carbamate and bicarbonate conversion to CO2 and the rate at which the conversion occurs. The kinetic performance of the catalyst was further expressed in terms of activation energy, EA, rate constants Kr and reaction order, n. The conversion is expressed as: ܺைଶ =(ܱܥଶ, െ ܱܥଶ,௨௧ )/ܱܥଶ,
(8)
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In the absence of heat and mass limitations, intrinsic experimental rates were calculated using the equal area differentiation technique which was obtained from the plots of conversion, X (i.e. CO2 desorbed) vs. weight of catalyst per amine flow rate (ܹ/ܨ ): ݎ = ݀ܺைଶ /݀(ܹ/ܨ )
(9)
Fig. 1. Experimental Setup
Table 1: Heat Transfer Limitations Catalyst
ɀ െ Alଶ Oଷ
HZSM-5
Solvents
ࡰࢇࢋ࢚ࢋ࢘ (m)
οࢀࢇ࢚࢘ࢉࢋ, ,ࢇ࢞(K)
οࢀࢌ,ࢇ࢞ (K)
Mears Criteria <0.15
MEA
0.003
3.45
7.8 ݔ10ିଷ
8.51 ݔ10ିସ
MEA-MDEA MEA
0.0025
4.36 5.19
8.1 ݔ10ିଷ 6.6 ݔ10ିଷ
8.32 ݔ10ି 6.31 ݔ10ିସ
6.32
5.72 ݔ10ି
5.47 ݔ10ିସ
MEA-MDEA
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Table 2. Mass Transfer Limitations Catalyst type
Film mass transfer resistance െ࢘ᇱ,࢈࢙ ࢊ࢈ /ࡷࢉ ࢙
Mass transfer limitation in the film െ࢘ᇱ,࢈࢙ ࣋࢈ ࡾࢉ /ࡷࢉ < 0.15
Internal Mass transfer limitations ᇱ ࢝,ࢊ = െۯܚ,ܛ܊ܗ ܀ ܋ܘ ܋/ࡰ ܍۱܁ۯ
ɀ െ Alଶ Oଷ
1.27 ݔ10ିଷ
3.44 ݔ10ିଷ
0.134
HZSM െ 5
8.7 ݔ10ିସ
1.56 ݔ10ିଷ
0.451
4.3. Role of catalyst ߛ െ ݈ܣଶ ܱଷ and HZSM-5 Catalyst in Amine Regeneration The partially uncoordinated metal cation and oxide anion that appear at the surface of Ȗ-݈ܣଶ ܱଷ catalyst enables it to act as an acid and base, respectively [12]. In the basic region, the lone pair electron density found in the N atom of the carbamate is attacked by the cus cation ion-Al3+ (Lewis acid). This weakens the N-C bond, resulting in the dissociative chemisorption of the carbamate ion. On the other hand, the oxide anion-ܱ݈ܣଶି attacks the ܪା found in ܪܣܧܯା . The presence of both Brønsted and Lewis acid site in HZSM-5 increases its role in the desorption of CO2. The Brønsted acid site donates proton to the carbamate ion. This proton donation converts the carbamate ion to a carboxylic acid (MEACOOH). This is then followed by a chemisorption on the Al site which results in the weakening of the N-C bond. According to Idem et al. [6] the HZSM-5 catalyst also transfers its available proton to bicarbonate ions, thus increasing the amount of desorbed CO2. The Lewis acid site attacks the free lone pair of electron density found in the N atom of the carbamate ion. 4.4. Effect of Varying Catalyst weights at constant ܨ Changing mass of catalyst, W, while keeping amine flow rate constant (ܨ ), showed that the reaction rate increases with catalyst weight. As described by the Maxwell Boltzmann theory, the increase in temperature and catalyst weight increases the average number of molecules with higher kinetic energy, thus allowing the reaction to occur at a faster rate and lower activation energy. This same effect was seen as the temperature and catalyst weight were varied as presented in Figs. 2 and 3. The presence of both Brønsted and Lewis acid sites in HZSM-5 explains the higher conversion and lower activation energy observed in CO2 desorption in the presence of HZSM-5 in comparison to Ȗ-Al2O3. The temperature effect can be seen at 0g catalyst weight for all temperatures (348, 358 and 368K). As earlier stated, increase in temperature increases the fraction of particles with higher energy. At 75oC (348K) the influence of the temperature effect on the reaction is minimal. Another effect to consider is the solvent effect. As mentioned by Shi et al [7] tertiary amine MDEA is a weaker base than MEA and is therefore easier to deprotonate ܪܣܧܦܯା than ܪܣܧܯା . Also ܪܣܧܯା would rather give its proton to MDEA than toܪଶ O. It can be seen that all reactions involving MDEA requires lesser energy than MEA as shown in equations 1-2 and 10-12. Consequently, the solvent effect plays an important role in increasing the conversion and reducing the activation energy alongside the catalyst contribution (Fig. 2 and 3).
Ananda Akachuku et al. / Energy Procedia 114 (2017) 1495 – 1505
0.35
0.3 MEA-MDEA Al2O3 75oC
0.25
Conversion, X
MEA-MDEA Al2O3 85oC 0.2
MEA-MDEA Al2O3 95oC MEA 85oC Al2O3
0.15 MEA 75oC Al2O3 MEA 95oC Al2O3
0.1
0.05
0 0
1
2
3
4
5
6
W/FAo (g cat. min/mol) Fig. 2. Variation of Ȗ-݈ܣଶ ܱଷ catalyst weight (0-200g) and temperature (348K, 358K, and 368K) at constant amine flow rate 60 ml/min
0.35 0.3 MEA-MDEA HZSM 5 75oC
0.25
MEA-MDEA HZSM 5 85oC MEA-MDEA HZSM 5 95oC
0.15
MEA HZSM 5 75oC
Conversion, X
0.2
MEA HZSM 5 85oC
0.1
MEA HZSM 5 95oC 0.05 0 0
1
2
3
4
5
W/FAo (g cat. min/mol) Fig. 3. Variation of HZSM-5 catalyst weight (0-200g) and temperature (348K, 358K, and 368K) at constant amine flow rate 60 ml/min.
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MEAH ା + Hଶ O ՞ MEA + Hଷ Oା ~ 78.2kJ/mol MEAH ା + HCOି ଷ ՞ MEA + COଶ(ୟ୯) + Hଶ O ~ 21.9 kJ/mol
(1) (2)
MDEAH ା + Hଶ O ՞ MDEA + Hଷ Oା ~ 63.53kJ/mol MEAH ା + MDEA ՞ MDEAH ା +MEA ~ 14.7kJ/mol MDEAH ା + HCOଷି ՞ MDEA + COଶ(ୟ୯) + Hଶ O ~ 7.2kJ/mol
(10) (11) (12)
4.5. Parameter Estimations An empirical power law model in terms of CO2 desorption rate was based on this overall reaction with respect to CO2 and is represented in the form: ݎଶି = െ݇ ݁ି( ݔாಲ /ோ்) [ܱܥଶ ]
(13)
Equation (13) shows the general dependence of desorption rate on the concentration of reacting species expressed in the Arrhenius form. The Power law model parameters were obtained from Arrhenius equation by plotting lnk vs. (1/T) as is shown in Fig. 4 and 5. The obtained parameters are presented in Table 3. ߛ െ ݈ܣଶ ܱଷ and HZSM-5 catalyst increases the rate of desorption of CO2 either by increasing the frequency of collision between the reacting molecules which is represented by the pre exponential factor, ݇ , or by providing an alternative pathway with lower activation energy, ܧ . The order of reaction, n, which shows to what extent the rate of reaction depends on reactant concentration was 1 overall for both catalysts and solvent type. The pre exponential factor, ݇ , of Ȗ-Al2O3 was seen to be higher than that of HZSM-5.
-10.8 0.00268 0.00272 0.00276
0.0028
0.00284 0.00288 0.00292
-11.2
-11.6
ln K
HZSM 5 Al2O3
-12
-12.4
-12.8
Fig. 4. Arrhenius plot (CO2+MEA system)
1/T (K)
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-10.6 0.0027
0.00275
0.0028
0.00285
0.0029
-10.8 -11 -11.2
ln k
HZSM 5 Al2O3
-11.4 -11.6 -11.8 -12
1/T (K)
Fig. 5: Arrhenius plot (CO2 + MEA + MDEA system)
Table 3. Parameter estimation for CO2+ MEA + H2O and CO2 + MEA+ MDEA+ H22V\VWHPIRUȖ-Al2O3 and HZSM-5
Parameters
ࢽ െ Al2O3
HZSM-5
ࢽ-Al2O3
HZSM-5
n
0.98~ 1
0.6 ~ 1
0.6~ 1
0.49 ~ 1
3.4 × 10
2.2 ݔ10ହ
1.7 ݔ10ଷ
3.4 ݔ10ଶ
ܧ (J/mol)
8.02 ݔ10ସ
7.1 ݔ10ସ
6.7 ݔ10ସ
6.1 ݔ10ସ
AAD
5.1%
4.0%
2.9%
3.3%
݇ dm3.sec-1.g.catalyst-1 dm6.݈݉.sec-1.g.catalyst-1
All the models were validated by determining the absolute average deviation (AAD %) between the experimental rate and predicted rate obtained from the proposed models. Also a parity chart which depicts how well the predicted rates fit the experimental rate data was plotted as shown in Fig. 6.
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Predicted rate (mol/g.catalyst.sec)
0.00003
0.000025
0.00002 Power Law MEA-Al2O3
0.000015
0.00001
Power Law model MEA HZSM-5
0.000005
Power Law MEA-MDEA Al2O3
0
Power Law model MEAMDEA HZSM 5
0
0.000005
0.00001
0.000015
0.00002
0.000025
0.00003
Predicted rate (mol/g.catalyst.sec) Fig. 6. Parity Chart
4. Conclusions x
The desorption kinetics of CO2 from loaded aqueous MEA and MEA-MDEA solutions were studied over Ȗ-Al2O3 and HZSM-5 catalysts as a function amine flow rates, catalyst weight and reaction temperature in a tubular packed-bed reactor. The power law models were used to describe the experimental kinetic data for CO2 desorption.
x
The kinetic performance was evaluated in terms of conversion (i.e. %CO2 desorbed), activation energy and rate constants. The optimum operating conditions to obtain maximum conversion over HZSM- DQG ȖAl2O3 was achieved at 85oC at 60ml/min.
x
HZSM-5 catalyst gave lower activation energies in comparisRQ WR Ȗ-Al2O3 for both solvents and is presented in the Arrhenius form in the power law models with AAD lower than 15% and are given as:
ݎைమିொି ఊమைయ = 3.4 ݔ10 ݁(ݔെ8.02 ݔ10ସ /ܴܶ)ܱܥ[ ݔଶ ].ଽ଼
(13)
ݎைమିொିுௌெ ହ = 2.2 ݔ10ହ ݁(ݔെ7.09 ݔ10ସ /ܴܶ)ܱܥ[ ݔଶ ].ଷ
(14)
ݎைమିொିொି ఊమைయ = 1.7 ݔ10ଷ ݁(ݔെ6.7 ݔ10ସ /ܴܶ)ܱܥ[ ݔଶ ].
(15)
ݎைమିொିொିுௌெ ହ = 3.36 ݔ10ଶ ݁(ݔെ6.1 ݔ10ସ /ܴܶ)ܱܥ[ ݔଶ ].ସଽ
(16)
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Acknowledgements The authors would like to thank Clean Energy Technologies Research Institute (CETRI), Natural Sciences and Engineering Research Council of Canada (NSERC) and Faculty of Graduate Studies and Research (FGSR), University of Regina for their financial support.
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