- Email: [email protected]
ionic-liquid emulsions
Separation and Purification Technology 118 (2013) 757–761
Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Short Communication
Kinetic behavior of carbon dioxide absorption in diethanolamine/ionic-liquid emulsions Muhammad Hasib-ur-Rahman, Faïçal Larachi ⇑ Department of Chemical Engineering, Laval University, Québec, QC G1V 0A6, Canada
a r t i c l e
i n f o
Article history: Received 17 February 2013 Received in revised form 14 August 2013 Accepted 16 August 2013 Available online 26 August 2013 Keywords: Carbon dioxide Absorption Diethanolamine Room-temperature ionic liquid Stirred cell
a b s t r a c t Room-temperature ionic liquids (RTILs) have been found to induce precipitation of CO2-captured carbamate product in case of amine–RTIL systems that may lead to an efficient carbon dioxide capture process. Here we have studied the kinetic behavior of CO2 absorption in (mutually immiscible) DEA–[hmim][Tf2N] blends in a laboratory scale stirred-cell reactor at ambient pressure (1 atm) to assess the effects of amine concentration (62 M DEA), CO2 partial pressure, agitation speed (1500–4500 rpm), and temperature variation (25–41 °C). A CO2 probe was used to monitor the change in gaseous CO2 volume ratio during the absorption experiments. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction To quench the anthropogenic CO2 emissions into the atmosphere and hence controlling the global warming phenomenon resulting from greenhouse gases buildup, efficient gas separation systems are needed [1]. In this regard, large point sources such as fossil-fueled power plants are the most convenient sites for CO2 capture. State-of-the-art aqueous alkanolamines are the most developed schemes being employed widely in natural gas purification installations. The major hindrance in large scale application of aqueous alkanolamine based CO2 capture processes is the unaffordably high regeneration energy requirement [2]. Equilibrium limitations, equipment corrosion, and amine degradation are some other drawbacks of the process, mainly inherited by the aqueous moiety [3–5]. Consequently, it may be a viable approach to replace aqueous phase wholly with more stable and secure solvent such as a room-temperature ionic liquid (RTIL). Being thermally stable, virtually non-volatile, as well as possessing lower heat capacities [6,7], RTILs may lead to an energy efficient pathway to CO2 capture and amine regeneration. Moreover, availability of numerous combinations of constituent ionic counterparts makes it quite feasible to tailor an ionic liquid in accordance with the required specifications. Typically imidazolium based ionic liquids either solely [8,9] or in combination with alkanolamines [10–13] are being investigated ⇑ Corresponding author. Tel.: +1 (418) 656 2131x3566; fax: +1 (418) 656 5993. E-mail address: [email protected] (F. Larachi). 1383-5866/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2013.08.027
as potential alternates for the current physical/chemical absorption processes. Among these, the most striking aspect of alkanolamine–RTIL combinations is the emergence of carbamate (CO2-captured product) precipitation that not only helps reach stoichiometric maximum gas loading capacity but also provides the opportunity to separate CO2-captured product, thereby offering likely reduction in regeneration energy. Also the suppression of corrosion occurrence particularly in case of gas absorption system comprising alkanolamine and hydrophobic ionic liquid adds further value to the process [11,12]. However, there have not been enough methodical efforts to assess the practicability of amine–RTIL based CO2 separation schemes. Accordingly, the objective of the current study was to scrutinize the kinetic aspects of such systems. To achieve this goal, CO2 absorption behavior was monitored using different amine concentrations and varying gas partial pressures. Moreover, the influence of agitation speed and temperature was also investigated. The exercise was conducted in a continuously stirred-cell reactor to probe the role of above stated experimental variables regarding CO2 capture in (immiscible) DEA–[hmim][Tf2N] blends. 2. Reaction mechanism in non-aqueous amines For chemical absorption of CO2 in alkanolamine based systems, the major reaction comprises the carbamate formation involving CO2 and amine interaction in 1:2 M ratio respectively. Considering primary/secondary alkanolamines, zwitterion mechanism is the most widely accepted model first proposed by Caplow in 1968 [14] and later reiterated by Danckwerts [15]. This mechanism
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M. Hasib-ur-Rahman, F. Larachi / Separation and Purification Technology 118 (2013) 757–761
involves the formation of an intermediate (zwitterion) in the first step that follows the abstraction of proton by a base.
R1 R2 NH þ CO2 $ R1 R2 NHþ COO R1 R2 NHþ COO þ B $ R1 R2 NCOO þ BHþ In aqueous amines the deprotonation species (B) include water, OH–, and amine itself but, contrary to aqueous amine systems, in non-aqueous media primary/secondary amine can be the only base available to deprotonate the zwitterion [16], and hence the gas loading capacity becomes limited to 0.5 mol CO2 per mole of amine (stoichiometric maximum). Thus the reaction can be specified as follows:
R1 R2 NH þ CO2 $ R1 R2 NHþ COO R1 R2 NHþ COO þ R1 R2 NH $ R1 R2 NCOO þ R1 R2 NHþ2 The same is pertinent to the amine–RTIL blends as the roomtemperature ionic liquid does not involve in any kind of chemical interaction either with CO2 or with amine [10–13]. 3. Experimental 3.1. Materials A secondary alkanolamine, diethanolamine (DEA: ACS reagent, P99.0%), was purchased from Sigma–Aldrich while the room-temperature ionic liquid, 1-hexyl-3-methylimidazoilium bis(trifluoromethylsulfonyl)imide ([hmim][Tf2N]: 99%), was provided by IoLiTec Inc. carbon dioxide and nitrogen (P99% purity) gases were obtained from Praxair Canada Inc. 3.2. Setup Gas absorption experiments were carried out in a double jacketed stirred-cell reactor as shown schematically in Fig. 1. An Omni homogenizer, fitted with rotor–stator generator, was immersed in the cell to agitate the liquid during absorption experiments whereas a CO2 probe (GMP221, Vaisala) was positioned in the headspace to monitor volumetrically the CO2 consumption rate. The reactor volume was 100 ml. The gaseous mixture was continuously circulated between the absorption cell and the reservoir (18.7 L vol) with the help of a peristaltic pump at a flow rate of 1 ± 0.01 L/min. The temperature of the stirred-cell reactor as well as of the headspace area was controlled by a thermostatic bath.
Fig. 1. Experimental set-up scheme: (A) gas inlet; (B) gas outlet (A and B connect to a gas reservoir via closed loop system); (C) CO2 probe; (D) injection port; (E) thermocouple; (F) rotor–stator homogeniser; (G) absorption cell; (Hi) heating bath inlet; and (Ho) heating bath outlet.
gaseous mixture. This allowed the calculation of CO2 absorption per unit time. 4. Results and discussion As has been observed during the earlier work [10–13] absorption of CO2 by primary/secondary alkanolamines, blended in room-temperature ionic liquids, results in precipitation of the CO2-captured product (carbamate) as shown in Fig. 2. Since there is no supplementary deprotonating species except amine in DEA–RTIL system, the maximum loading capacity does not exceed 50 mol% of CO2 as primary/secondary amine (DEA in this case) reacts with CO2 in 2:1 ratio obeying the following reaction:
DEA þ CO2 $ DEAHþ COO DEAHþ COO þ DEA $ DEACOO þ DEAHþ The current experiments were devised to peruse various parameters, i.e., amine concentration, CO2 gaseous ratio, agitation speed, and temperature, to study the CO2 uptake behavior in DEA– [hmim][Tf2N] mixtures using stirred-cell reactor. 4.1. Impact of variation in amine concentration The CO2 absorption mode shows two distinct regions in the curve, as shown in Fig. 3. The initial steeper part depicts an abrupt
3.3. Procedure Each time, prior to the absorption experiments, the setup was purged with nitrogen gas to remove any gaseous contaminant. Then the gas reservoir was filled with desired proportions of CO2 and nitrogen using Bronkhorst mass-flow controllers. After the introduction of a specified volume of pure RTIL into the cell through an inlet needle, the gaseous mixture was continuously recirculated for 120 min with the help of a peristaltic pump so that, under the specified conditions, the RTIL became saturated with CO2 (shown by the stable reading of the probe). Subsequently a known quantity of DEA (being immiscible with the ionic liquid) was injected into the RTIL containing cell reactor and the process was continued for 3 h. For each experiment, 12 ml of DEA–[hmim][Tf2N] fluid was used. During the experiment, the liquid was constantly stirred using Omni homogeniser fitted with rotor–stator generator. CO2 probe was linked to a computerized acquisition system, delivering data in terms of CO2 available as vol% in the
Fig. 2. CO2-captured product (carbamate) precipitation in DEA–[hmim][Tf2N]: (a) immediately after CO2 bubbling; (b) 24 h later.
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CO2 concentration
CO2 uptake (mole)
(a)
10.0 %
DEA in [hmim][Tf2N]
CO2 uptake (mole)
(a)
2.0 M
1.0 M
0.5 M
Time (min.)
(b)
DEA in [hmim][Tf2N] 2.0 M
CO2 uptake (mole)
gas absorption phenomenon that seems to have occurred through mutual contribution of physically confined CO2 in the RTIL (solubilized prior to the injection of amine into the stirred-cell) and the additional CO2 approaching directly via continuous gas bubbling. While the other somewhat horizontal portion evolved after most of the unreacted amine accumulated over RTIL surface. As diluted gaseous mixture containing CO2 6 10 vol% was used to observe the gas absorption trends of DEA–[hmim][Tf2N] blends, higher amine content (DEA: 2 M) did not appear to be compatible with the experimental conditions as was evident from the slow absorption kinetics. However, decrease in amine content corresponded well to the low CO2 gaseous ratio. The immiscibility as well as the difference in the respective densities (1.09 g/cm3 and 1.37 g/cm3) of both the components, DEA and [hmim][Tf2N], did not let the amine droplets to stay dispersed long enough. Higher amine ratio further accelerated the coalescence of amine droplets thus resulting in fast accumulation of amine at the RTIL surface
1.0 M
0.5 M
5.0 %
Time (min.) 2.5 %
(c)
DEA in [hmim][Tf2N]
Time (min.) CO2 concentration
CO2 uptake (mole)
(b)
10.0 %
CO2 uptake (mole)
2.0 M
1.0 M 0.5 M
5.0 %
Time (min.)
2.5 %
Fig. 4. Influence of initial CO2 volume ratio (in gaseous mixture) on absorption rate w.r.t. [DEA], at 33 °C and 3000 rpm agitation speed: (a) 10 vol% CO2; (b) 5 vol% CO2; (c) 2.5 vol% CO2. Smoothed lines show trends.
Time (min.) Agitation speed
10.0 % 5.0 %
2.5 %
CO2 uptake (mole)
CO2 concentration
CO2 uptake (mole)
(c)
4500 rpm 3000 rpm 1500 rpm
Time (min.) Time (min.) Fig. 3. Influence of [DEA] molar concentration on absorption rate with respect to initial CO2 vol% in the gaseous mixture, at 33 °C and 3000 rpm agitation speed: (a) 2 M DEA in [hmim][Tf2N]; (b) 1 M DEA in [hmim][Tf2N]; (c) 0.5 M DEA in [hmim][Tf2N]. Smoothed lines show trends.
Fig. 5. Influence of agitation on CO2 absorption rate (2 M DEA in [hmim][Tf2N]; 10 vol% CO2; 33 °C). Smoothed lines show trends.
and consequently slowing down the CO2 uptake (mole of CO2 captured per unit time), as is obvious from Fig. 3.
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4.2. CO2 volume ratio in the gaseous mixture Influence of the variation of CO2 vol% (in the gaseous mixture) on absorption also corroborates the discussion in the previous section. The 0.5 M DEA appears well-suited to the gaseous mixture containing 10 vol% CO2 for quick gas absorption (Fig. 4). However, as the gaseous CO2 concentration was lowered the capture rate decreased accordingly. In case of DEA–RTIL blends with higher amine ratio (1–2 M DEA), even 10 vol% CO2 was not sufficient to drive the process quickly to maximum gas loading. 4.3. Influence of agitation speed Since diethanolamine and [hmim][Tf2N] are immiscible and there is significant density difference between the two (DEA: 1.09 g/cm3; [hmim][Tf2N]: 1.37 g/cm3), it is hard to keep DEA dispersed in [hmim][Tf2N] without the addition of a surfactant. Yet, proper agitation can help induce DEA dispersion for extended duration and thus can provide with increased surface area of DEA to interact with CO2. As has been shown in Fig. 5, increase in agitation speed from 1500 rpm to 4500 rpm (keeping other variables constant: 2 M DEA; 10 vol% CO2; 33 °C) caused faster CO2 absorption. This seems to be the outcome of greater residence time of dispersed amine inside the RTIL phase and/or smaller amine droplet size. Thus agitation speed can be optimized in accordance with the flue gas composition and the other experimental parameters (such as amine ratio, gas flow rate, and process temperature) to acquire a sufficiently high absorption rate. 4.4. Effect of temperature variation Three different temperatures (25, 33 and 41 °C) were chosen to assess the influence of temperature on CO2 absorption behavior. In spite of the fact that increase in temperature resulted in decreased liquid viscosity (Table 1) and hence gas transfer rate could have improved, the experimental outcome did not depict any systematic change in capture rate as shown in Fig. 6. Decrease in physical solubility of CO2 in RTIL at higher temperature might have undone the lower viscosity advantage if there was any. This behavior suggests that CO2-captured product (carbamate) precipitation is the dictat-
Table 1 Viscositiesa of the capture fluid components at three temperatures.
a
Component
25 °C
33 °C
41 °C
DEA [hmim][Tf2N]
470 cP 61 cP
241 cP 38 cP
139 cP 26 cP
Measured by AR-G2 rheometer (TA Instruments) with parallel plate geometry.
CO2 uptake (mole)
Temperature variation
41 °C 33 °C 25 °C
Time (min.) Fig. 6. Effect of temperature on CO2 capture rate (1 M DEA in [hmim][Tf2N]; 10 vol% CO2; 3000 rpm). Smoothed lines show trends.
ing factor that possibly has overshadowed the influence of temperature on CO2 absorption rate. As in the simulated gaseous mixture the CO2 ratio (opposed to the pure gas stream) was maintained within the concentration range of post-combustion flue gases (<15%), the CO2 solubility in the RTIL phase must had undergone a negative impact [17,18]. 5. Conclusion The results of this study reveal that though amine–RTIL blends are blessed with a unique advantage, i.e., CO2-captured product (carbamate) precipitation, an apposite dispersion of amine in the RTIL continuous phase is required for profiting from maximal absorption capabilities of the immiscible amine–RTIL systems. Agitation appeared to have significantly vivid influence as CO2 capture rate enhanced at higher homogenising speed. Also, with increase in CO2 volume ratio in the simulated gaseous mixture (CO2 + N2), the gas absorption rate was correspondingly improved. These experimental findings may help carve the way out towards designing a pertinent absorption column regarding immiscible amine–RTIL systems. Acknowledgements Financial support from FL Canada Research Chair ‘‘Green processes for cleaner and sustainable energy’’ and the Discovery Grant to F. Larachi from the Natural Sciences and Engineering Research Council (NSERC) are gratefully acknowledged. The authors are also thankful to Prof. Denis Rodrigue for help in viscosity measurements. References [1] B. Metz, O. Davidson, H. de Coninck, M. Loos, L. Meyer (Eds.), IPCC Special Report on Carbon Dioxide Capture and Storage, Prepared by Working Group III of the Intergovernmental Panel on Climate Change, Cambridge University Press, New York, 2005, pp. 51–74 (Chapter 1). [2] R. Idem, M. Wilson, P. Tontiwachwuthikul, A. Chakma, A. Veawab, A. Aroonwilas, D. Gelowitz, Pilot plant studies of the CO2 capture performance of aqueous MEA and mixed MEA/MDEA solvents at the University of Regina CO2 Capture Technology Development Plant and the Boundary Dam CO2 Capture Demonstration Plant, Ind. Eng. Chem. Res. 45 (2006) 2414–2420. [3] S. Bishnoi, G.T. Rochelle, Absorption of carbon dioxide into aqueous piperazine: reaction kinetics, mass transfer and solubility, Chem. Eng. Sci. 55 (2000) 5531– 5543. [4] M. Hasib-ur-Rahman, F. Larachi, Corrosion in amine systems – a review, Carbon Capture J. (September–October 2012) 22–24. [5] B.R. Strazisar, R.R. Anderson, C.M. White, Degradation pathways for monoethanolamine in a CO2 capture facility, Energy Fuels 17 (2003) 1034– 1039. [6] M. Hasib-ur-Rahman, M. Siaj, F. Larachi, Ionic liquids for CO2 capture – development and progress, Chem. Eng. Process 49 (2010) 313–322. [7] D. Waliszewski, I. Stepniak, H. Piekarski, A. Lewandowski, Heat capacities of ionic liquids and their heats of solution in molecular liquids, Thermochim. Acta 433 (2005) 149–152. [8] D. Camper, P. Scovazzo, C. Koval, R. Noble, Gas solubilities in roomtemperature ionic liquids, Ind. Eng. Chem. Res. 43 (2004) 3049–3054. [9] A. Yokozeki, M.B. Shiflett, Separation of carbon dioxide and sulfur dioxide gases using room-temperature ionic liquid [hmim][Tf2N], Energy Fuels 23 (2009) 4701–4708. [10] D. Camper, J.E. Bara, D.L. Gin, R.D. Noble, Room-temperature ionic liquid– amine solutions: tunable solvents for efficient and reversible capture of CO2, Ind. Eng. Chem. Res. 47 (2008) 8496–8498. [11] M. Hasib-ur-Rahman, M. Siaj, F. Larachi, CO2 capture in alkanolamine/roomtemperature ionic liquid emulsions: a viable approach with carbamate crystallization and curbed corrosion behavior, Int. J. Greenhouse Gas Control 6 (2012) 246–252. [12] M. Hasib-ur-Rahman, H. Bouteldja, P. Fongarland, M. Siaj, F. Larachi, Corrosion behavior of carbon steel in alkanolamine/room-temperature ionic liquid based CO2 capture systems, Ind. Eng. Chem. Res. 51 (2012) 8711–8718. [13] M. Hasib-ur-Rahman, F. Larachi, CO2 capture in alkanolamine–RTIL blends via carbamate crystallization: route to efficient regeneration, Environ. Sci. Technol. 46 (2012) 11443–11450. [14] M. Caplow, Kinetics of carbamate formation and breakdown, J. Am. Chem. Soc. 90 (1968) 6795–6803.
M. Hasib-ur-Rahman, F. Larachi / Separation and Purification Technology 118 (2013) 757–761 [15] P.V. Danckwerts, The reaction of CO2 with ethanolamines, Chem. Eng. Sci. 34 (1979) 443–446. [16] G.F. Versteeg, L.A.J. van Dijck, W.P.M. van Swaaij, On the kinetics between CO2 and alkanolamines both in aqueous and non-aqueous solutions. an overview, Chem. Eng. Commun. 144 (1996) 113–158.
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[17] C.W. Jones, CO2 capture from dilute gases as a component of modern global carbon management, Annu. Rev. Chem. Biomol. Eng. 2 (2011) 31–52. [18] R.E. Baltus, B.H. Culbertson, S. Dai, H. Luo, D.W. DePaoli, Low-pressure solubility of carbon dioxide in room-temperature ionic liquids measured with a quartz crystal microbalance, J. Phys. Chem. B 108 (2004) 721–727.
Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Short Communication
Kinetic behavior of carbon dioxide absorption in diethanolamine/ionic-liquid emulsions Muhammad Hasib-ur-Rahman, Faïçal Larachi ⇑ Department of Chemical Engineering, Laval University, Québec, QC G1V 0A6, Canada
a r t i c l e
i n f o
Article history: Received 17 February 2013 Received in revised form 14 August 2013 Accepted 16 August 2013 Available online 26 August 2013 Keywords: Carbon dioxide Absorption Diethanolamine Room-temperature ionic liquid Stirred cell
a b s t r a c t Room-temperature ionic liquids (RTILs) have been found to induce precipitation of CO2-captured carbamate product in case of amine–RTIL systems that may lead to an efficient carbon dioxide capture process. Here we have studied the kinetic behavior of CO2 absorption in (mutually immiscible) DEA–[hmim][Tf2N] blends in a laboratory scale stirred-cell reactor at ambient pressure (1 atm) to assess the effects of amine concentration (62 M DEA), CO2 partial pressure, agitation speed (1500–4500 rpm), and temperature variation (25–41 °C). A CO2 probe was used to monitor the change in gaseous CO2 volume ratio during the absorption experiments. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction To quench the anthropogenic CO2 emissions into the atmosphere and hence controlling the global warming phenomenon resulting from greenhouse gases buildup, efficient gas separation systems are needed [1]. In this regard, large point sources such as fossil-fueled power plants are the most convenient sites for CO2 capture. State-of-the-art aqueous alkanolamines are the most developed schemes being employed widely in natural gas purification installations. The major hindrance in large scale application of aqueous alkanolamine based CO2 capture processes is the unaffordably high regeneration energy requirement [2]. Equilibrium limitations, equipment corrosion, and amine degradation are some other drawbacks of the process, mainly inherited by the aqueous moiety [3–5]. Consequently, it may be a viable approach to replace aqueous phase wholly with more stable and secure solvent such as a room-temperature ionic liquid (RTIL). Being thermally stable, virtually non-volatile, as well as possessing lower heat capacities [6,7], RTILs may lead to an energy efficient pathway to CO2 capture and amine regeneration. Moreover, availability of numerous combinations of constituent ionic counterparts makes it quite feasible to tailor an ionic liquid in accordance with the required specifications. Typically imidazolium based ionic liquids either solely [8,9] or in combination with alkanolamines [10–13] are being investigated ⇑ Corresponding author. Tel.: +1 (418) 656 2131x3566; fax: +1 (418) 656 5993. E-mail address: [email protected] (F. Larachi). 1383-5866/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2013.08.027
as potential alternates for the current physical/chemical absorption processes. Among these, the most striking aspect of alkanolamine–RTIL combinations is the emergence of carbamate (CO2-captured product) precipitation that not only helps reach stoichiometric maximum gas loading capacity but also provides the opportunity to separate CO2-captured product, thereby offering likely reduction in regeneration energy. Also the suppression of corrosion occurrence particularly in case of gas absorption system comprising alkanolamine and hydrophobic ionic liquid adds further value to the process [11,12]. However, there have not been enough methodical efforts to assess the practicability of amine–RTIL based CO2 separation schemes. Accordingly, the objective of the current study was to scrutinize the kinetic aspects of such systems. To achieve this goal, CO2 absorption behavior was monitored using different amine concentrations and varying gas partial pressures. Moreover, the influence of agitation speed and temperature was also investigated. The exercise was conducted in a continuously stirred-cell reactor to probe the role of above stated experimental variables regarding CO2 capture in (immiscible) DEA–[hmim][Tf2N] blends. 2. Reaction mechanism in non-aqueous amines For chemical absorption of CO2 in alkanolamine based systems, the major reaction comprises the carbamate formation involving CO2 and amine interaction in 1:2 M ratio respectively. Considering primary/secondary alkanolamines, zwitterion mechanism is the most widely accepted model first proposed by Caplow in 1968 [14] and later reiterated by Danckwerts [15]. This mechanism
758
M. Hasib-ur-Rahman, F. Larachi / Separation and Purification Technology 118 (2013) 757–761
involves the formation of an intermediate (zwitterion) in the first step that follows the abstraction of proton by a base.
R1 R2 NH þ CO2 $ R1 R2 NHþ COO R1 R2 NHþ COO þ B $ R1 R2 NCOO þ BHþ In aqueous amines the deprotonation species (B) include water, OH–, and amine itself but, contrary to aqueous amine systems, in non-aqueous media primary/secondary amine can be the only base available to deprotonate the zwitterion [16], and hence the gas loading capacity becomes limited to 0.5 mol CO2 per mole of amine (stoichiometric maximum). Thus the reaction can be specified as follows:
R1 R2 NH þ CO2 $ R1 R2 NHþ COO R1 R2 NHþ COO þ R1 R2 NH $ R1 R2 NCOO þ R1 R2 NHþ2 The same is pertinent to the amine–RTIL blends as the roomtemperature ionic liquid does not involve in any kind of chemical interaction either with CO2 or with amine [10–13]. 3. Experimental 3.1. Materials A secondary alkanolamine, diethanolamine (DEA: ACS reagent, P99.0%), was purchased from Sigma–Aldrich while the room-temperature ionic liquid, 1-hexyl-3-methylimidazoilium bis(trifluoromethylsulfonyl)imide ([hmim][Tf2N]: 99%), was provided by IoLiTec Inc. carbon dioxide and nitrogen (P99% purity) gases were obtained from Praxair Canada Inc. 3.2. Setup Gas absorption experiments were carried out in a double jacketed stirred-cell reactor as shown schematically in Fig. 1. An Omni homogenizer, fitted with rotor–stator generator, was immersed in the cell to agitate the liquid during absorption experiments whereas a CO2 probe (GMP221, Vaisala) was positioned in the headspace to monitor volumetrically the CO2 consumption rate. The reactor volume was 100 ml. The gaseous mixture was continuously circulated between the absorption cell and the reservoir (18.7 L vol) with the help of a peristaltic pump at a flow rate of 1 ± 0.01 L/min. The temperature of the stirred-cell reactor as well as of the headspace area was controlled by a thermostatic bath.
Fig. 1. Experimental set-up scheme: (A) gas inlet; (B) gas outlet (A and B connect to a gas reservoir via closed loop system); (C) CO2 probe; (D) injection port; (E) thermocouple; (F) rotor–stator homogeniser; (G) absorption cell; (Hi) heating bath inlet; and (Ho) heating bath outlet.
gaseous mixture. This allowed the calculation of CO2 absorption per unit time. 4. Results and discussion As has been observed during the earlier work [10–13] absorption of CO2 by primary/secondary alkanolamines, blended in room-temperature ionic liquids, results in precipitation of the CO2-captured product (carbamate) as shown in Fig. 2. Since there is no supplementary deprotonating species except amine in DEA–RTIL system, the maximum loading capacity does not exceed 50 mol% of CO2 as primary/secondary amine (DEA in this case) reacts with CO2 in 2:1 ratio obeying the following reaction:
DEA þ CO2 $ DEAHþ COO DEAHþ COO þ DEA $ DEACOO þ DEAHþ The current experiments were devised to peruse various parameters, i.e., amine concentration, CO2 gaseous ratio, agitation speed, and temperature, to study the CO2 uptake behavior in DEA– [hmim][Tf2N] mixtures using stirred-cell reactor. 4.1. Impact of variation in amine concentration The CO2 absorption mode shows two distinct regions in the curve, as shown in Fig. 3. The initial steeper part depicts an abrupt
3.3. Procedure Each time, prior to the absorption experiments, the setup was purged with nitrogen gas to remove any gaseous contaminant. Then the gas reservoir was filled with desired proportions of CO2 and nitrogen using Bronkhorst mass-flow controllers. After the introduction of a specified volume of pure RTIL into the cell through an inlet needle, the gaseous mixture was continuously recirculated for 120 min with the help of a peristaltic pump so that, under the specified conditions, the RTIL became saturated with CO2 (shown by the stable reading of the probe). Subsequently a known quantity of DEA (being immiscible with the ionic liquid) was injected into the RTIL containing cell reactor and the process was continued for 3 h. For each experiment, 12 ml of DEA–[hmim][Tf2N] fluid was used. During the experiment, the liquid was constantly stirred using Omni homogeniser fitted with rotor–stator generator. CO2 probe was linked to a computerized acquisition system, delivering data in terms of CO2 available as vol% in the
Fig. 2. CO2-captured product (carbamate) precipitation in DEA–[hmim][Tf2N]: (a) immediately after CO2 bubbling; (b) 24 h later.
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M. Hasib-ur-Rahman, F. Larachi / Separation and Purification Technology 118 (2013) 757–761
CO2 concentration
CO2 uptake (mole)
(a)
10.0 %
DEA in [hmim][Tf2N]
CO2 uptake (mole)
(a)
2.0 M
1.0 M
0.5 M
Time (min.)
(b)
DEA in [hmim][Tf2N] 2.0 M
CO2 uptake (mole)
gas absorption phenomenon that seems to have occurred through mutual contribution of physically confined CO2 in the RTIL (solubilized prior to the injection of amine into the stirred-cell) and the additional CO2 approaching directly via continuous gas bubbling. While the other somewhat horizontal portion evolved after most of the unreacted amine accumulated over RTIL surface. As diluted gaseous mixture containing CO2 6 10 vol% was used to observe the gas absorption trends of DEA–[hmim][Tf2N] blends, higher amine content (DEA: 2 M) did not appear to be compatible with the experimental conditions as was evident from the slow absorption kinetics. However, decrease in amine content corresponded well to the low CO2 gaseous ratio. The immiscibility as well as the difference in the respective densities (1.09 g/cm3 and 1.37 g/cm3) of both the components, DEA and [hmim][Tf2N], did not let the amine droplets to stay dispersed long enough. Higher amine ratio further accelerated the coalescence of amine droplets thus resulting in fast accumulation of amine at the RTIL surface
1.0 M
0.5 M
5.0 %
Time (min.) 2.5 %
(c)
DEA in [hmim][Tf2N]
Time (min.) CO2 concentration
CO2 uptake (mole)
(b)
10.0 %
CO2 uptake (mole)
2.0 M
1.0 M 0.5 M
5.0 %
Time (min.)
2.5 %
Fig. 4. Influence of initial CO2 volume ratio (in gaseous mixture) on absorption rate w.r.t. [DEA], at 33 °C and 3000 rpm agitation speed: (a) 10 vol% CO2; (b) 5 vol% CO2; (c) 2.5 vol% CO2. Smoothed lines show trends.
Time (min.) Agitation speed
10.0 % 5.0 %
2.5 %
CO2 uptake (mole)
CO2 concentration
CO2 uptake (mole)
(c)
4500 rpm 3000 rpm 1500 rpm
Time (min.) Time (min.) Fig. 3. Influence of [DEA] molar concentration on absorption rate with respect to initial CO2 vol% in the gaseous mixture, at 33 °C and 3000 rpm agitation speed: (a) 2 M DEA in [hmim][Tf2N]; (b) 1 M DEA in [hmim][Tf2N]; (c) 0.5 M DEA in [hmim][Tf2N]. Smoothed lines show trends.
Fig. 5. Influence of agitation on CO2 absorption rate (2 M DEA in [hmim][Tf2N]; 10 vol% CO2; 33 °C). Smoothed lines show trends.
and consequently slowing down the CO2 uptake (mole of CO2 captured per unit time), as is obvious from Fig. 3.
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4.2. CO2 volume ratio in the gaseous mixture Influence of the variation of CO2 vol% (in the gaseous mixture) on absorption also corroborates the discussion in the previous section. The 0.5 M DEA appears well-suited to the gaseous mixture containing 10 vol% CO2 for quick gas absorption (Fig. 4). However, as the gaseous CO2 concentration was lowered the capture rate decreased accordingly. In case of DEA–RTIL blends with higher amine ratio (1–2 M DEA), even 10 vol% CO2 was not sufficient to drive the process quickly to maximum gas loading. 4.3. Influence of agitation speed Since diethanolamine and [hmim][Tf2N] are immiscible and there is significant density difference between the two (DEA: 1.09 g/cm3; [hmim][Tf2N]: 1.37 g/cm3), it is hard to keep DEA dispersed in [hmim][Tf2N] without the addition of a surfactant. Yet, proper agitation can help induce DEA dispersion for extended duration and thus can provide with increased surface area of DEA to interact with CO2. As has been shown in Fig. 5, increase in agitation speed from 1500 rpm to 4500 rpm (keeping other variables constant: 2 M DEA; 10 vol% CO2; 33 °C) caused faster CO2 absorption. This seems to be the outcome of greater residence time of dispersed amine inside the RTIL phase and/or smaller amine droplet size. Thus agitation speed can be optimized in accordance with the flue gas composition and the other experimental parameters (such as amine ratio, gas flow rate, and process temperature) to acquire a sufficiently high absorption rate. 4.4. Effect of temperature variation Three different temperatures (25, 33 and 41 °C) were chosen to assess the influence of temperature on CO2 absorption behavior. In spite of the fact that increase in temperature resulted in decreased liquid viscosity (Table 1) and hence gas transfer rate could have improved, the experimental outcome did not depict any systematic change in capture rate as shown in Fig. 6. Decrease in physical solubility of CO2 in RTIL at higher temperature might have undone the lower viscosity advantage if there was any. This behavior suggests that CO2-captured product (carbamate) precipitation is the dictat-
Table 1 Viscositiesa of the capture fluid components at three temperatures.
a
Component
25 °C
33 °C
41 °C
DEA [hmim][Tf2N]
470 cP 61 cP
241 cP 38 cP
139 cP 26 cP
Measured by AR-G2 rheometer (TA Instruments) with parallel plate geometry.
CO2 uptake (mole)
Temperature variation
41 °C 33 °C 25 °C
Time (min.) Fig. 6. Effect of temperature on CO2 capture rate (1 M DEA in [hmim][Tf2N]; 10 vol% CO2; 3000 rpm). Smoothed lines show trends.
ing factor that possibly has overshadowed the influence of temperature on CO2 absorption rate. As in the simulated gaseous mixture the CO2 ratio (opposed to the pure gas stream) was maintained within the concentration range of post-combustion flue gases (<15%), the CO2 solubility in the RTIL phase must had undergone a negative impact [17,18]. 5. Conclusion The results of this study reveal that though amine–RTIL blends are blessed with a unique advantage, i.e., CO2-captured product (carbamate) precipitation, an apposite dispersion of amine in the RTIL continuous phase is required for profiting from maximal absorption capabilities of the immiscible amine–RTIL systems. Agitation appeared to have significantly vivid influence as CO2 capture rate enhanced at higher homogenising speed. Also, with increase in CO2 volume ratio in the simulated gaseous mixture (CO2 + N2), the gas absorption rate was correspondingly improved. These experimental findings may help carve the way out towards designing a pertinent absorption column regarding immiscible amine–RTIL systems. Acknowledgements Financial support from FL Canada Research Chair ‘‘Green processes for cleaner and sustainable energy’’ and the Discovery Grant to F. Larachi from the Natural Sciences and Engineering Research Council (NSERC) are gratefully acknowledged. The authors are also thankful to Prof. Denis Rodrigue for help in viscosity measurements. References [1] B. Metz, O. Davidson, H. de Coninck, M. Loos, L. Meyer (Eds.), IPCC Special Report on Carbon Dioxide Capture and Storage, Prepared by Working Group III of the Intergovernmental Panel on Climate Change, Cambridge University Press, New York, 2005, pp. 51–74 (Chapter 1). [2] R. Idem, M. Wilson, P. Tontiwachwuthikul, A. Chakma, A. Veawab, A. Aroonwilas, D. Gelowitz, Pilot plant studies of the CO2 capture performance of aqueous MEA and mixed MEA/MDEA solvents at the University of Regina CO2 Capture Technology Development Plant and the Boundary Dam CO2 Capture Demonstration Plant, Ind. Eng. Chem. Res. 45 (2006) 2414–2420. [3] S. Bishnoi, G.T. Rochelle, Absorption of carbon dioxide into aqueous piperazine: reaction kinetics, mass transfer and solubility, Chem. Eng. Sci. 55 (2000) 5531– 5543. [4] M. Hasib-ur-Rahman, F. Larachi, Corrosion in amine systems – a review, Carbon Capture J. (September–October 2012) 22–24. [5] B.R. Strazisar, R.R. Anderson, C.M. White, Degradation pathways for monoethanolamine in a CO2 capture facility, Energy Fuels 17 (2003) 1034– 1039. [6] M. Hasib-ur-Rahman, M. Siaj, F. Larachi, Ionic liquids for CO2 capture – development and progress, Chem. Eng. Process 49 (2010) 313–322. [7] D. Waliszewski, I. Stepniak, H. Piekarski, A. Lewandowski, Heat capacities of ionic liquids and their heats of solution in molecular liquids, Thermochim. Acta 433 (2005) 149–152. [8] D. Camper, P. Scovazzo, C. Koval, R. Noble, Gas solubilities in roomtemperature ionic liquids, Ind. Eng. Chem. Res. 43 (2004) 3049–3054. [9] A. Yokozeki, M.B. Shiflett, Separation of carbon dioxide and sulfur dioxide gases using room-temperature ionic liquid [hmim][Tf2N], Energy Fuels 23 (2009) 4701–4708. [10] D. Camper, J.E. Bara, D.L. Gin, R.D. Noble, Room-temperature ionic liquid– amine solutions: tunable solvents for efficient and reversible capture of CO2, Ind. Eng. Chem. Res. 47 (2008) 8496–8498. [11] M. Hasib-ur-Rahman, M. Siaj, F. Larachi, CO2 capture in alkanolamine/roomtemperature ionic liquid emulsions: a viable approach with carbamate crystallization and curbed corrosion behavior, Int. J. Greenhouse Gas Control 6 (2012) 246–252. [12] M. Hasib-ur-Rahman, H. Bouteldja, P. Fongarland, M. Siaj, F. Larachi, Corrosion behavior of carbon steel in alkanolamine/room-temperature ionic liquid based CO2 capture systems, Ind. Eng. Chem. Res. 51 (2012) 8711–8718. [13] M. Hasib-ur-Rahman, F. Larachi, CO2 capture in alkanolamine–RTIL blends via carbamate crystallization: route to efficient regeneration, Environ. Sci. Technol. 46 (2012) 11443–11450. [14] M. Caplow, Kinetics of carbamate formation and breakdown, J. Am. Chem. Soc. 90 (1968) 6795–6803.
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