Separation and Purification Technology 118 (2013) 757–761

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Separation and Purification Technology

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Short Communication

Kinetic behavior of carbon dioxide absorption indiethanolamine/ionic-liquid emulsions

1383-5866/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.seppur.2013.08.027

⇑ Corresponding author. Tel.: +1 (418) 656 2131x3566; fax: +1 (418) 656 5993.E-mail address: faical.larachi@gch.ulaval.ca (F. Larachi).

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 2013Received in revised form 14 August 2013Accepted 16 August 2013Available online 26 August 2013

Keywords:Carbon dioxideAbsorptionDiethanolamineRoom-temperature ionic liquidStirred cell

a b s t r a c t

Room-temperature ionic liquids (RTILs) have been found to induce precipitation of CO2-captured carba-mate 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 ofamine concentration (62 M DEA), CO2 partial pressure, agitation speed (1500–4500 rpm), and tempera-ture variation (25–41 �C). A CO2 probe was used to monitor the change in gaseous CO2 volume ratio dur-ing the absorption experiments.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

To quench the anthropogenic CO2 emissions into the atmo-sphere and hence controlling the global warming phenomenonresulting from greenhouse gases buildup, efficient gas separationsystems are needed [1]. In this regard, large point sources suchas fossil-fueled power plants are the most convenient sites forCO2 capture. State-of-the-art aqueous alkanolamines are the mostdeveloped schemes being employed widely in natural gas purifica-tion installations. The major hindrance in large scale application ofaqueous alkanolamine based CO2 capture processes is the unaffor-dably high regeneration energy requirement [2]. Equilibrium limi-tations, equipment corrosion, and amine degradation are someother drawbacks of the process, mainly inherited by the aqueousmoiety [3–5].

Consequently, it may be a viable approach to replace aqueousphase wholly with more stable and secure solvent such as aroom-temperature ionic liquid (RTIL). Being thermally stable, vir-tually non-volatile, as well as possessing lower heat capacities[6,7], RTILs may lead to an energy efficient pathway to CO2 captureand amine regeneration. Moreover, availability of numerous com-binations of constituent ionic counterparts makes it quite feasibleto tailor an ionic liquid in accordance with the requiredspecifications.

Typically imidazolium based ionic liquids either solely [8,9] orin combination with alkanolamines [10–13] are being investigated

as potential alternates for the current physical/chemical absorp-tion processes. Among these, the most striking aspect ofalkanolamine–RTIL combinations is the emergence of carbamate(CO2-captured product) precipitation that not only helps reachstoichiometric maximum gas loading capacity but also providesthe opportunity to separate CO2-captured product, thereby offeringlikely reduction in regeneration energy. Also the suppression ofcorrosion occurrence particularly in case of gas absorption systemcomprising alkanolamine and hydrophobic ionic liquid addsfurther value to the process [11,12].

However, there have not been enough methodical efforts to as-sess the practicability of amine–RTIL based CO2 separationschemes. Accordingly, the objective of the current study was toscrutinize the kinetic aspects of such systems. To achieve this goal,CO2 absorption behavior was monitored using different amine con-centrations and varying gas partial pressures. Moreover, the influ-ence of agitation speed and temperature was also investigated. Theexercise was conducted in a continuously stirred-cell reactor toprobe the role of above stated experimental variables regardingCO2 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 involvingCO2 and amine interaction in 1:2 M ratio respectively. Consideringprimary/secondary alkanolamines, zwitterion mechanism is themost widely accepted model first proposed by Caplow in 1968[14] and later reiterated by Danckwerts [15]. This mechanism

Fig. 1. Experimental set-up scheme: (A) gas inlet; (B) gas outlet (A and B connect toa gas reservoir via closed loop system); (C) CO2 probe; (D) injection port; (E)thermocouple; (F) rotor–stator homogeniser; (G) absorption cell; (Hi) heating bathinlet; and (Ho) heating bath outlet.

758 M. Hasib-ur-Rahman, F. Larachi / Separation and Purification Technology 118 (2013) 757–761

involves the formation of an intermediate (zwitterion) in the firststep that follows the abstraction of proton by a base.

R1R2NHþ CO2 $ R1R2NHþCOO�

R1R2NHþCOO� þ B$ R1R2NCOO� þ BHþ

In aqueous amines the deprotonation species (B) include water,OH–, and amine itself but, contrary to aqueous amine systems, innon-aqueous media primary/secondary amine can be the only baseavailable to deprotonate the zwitterion [16], and hence the gasloading capacity becomes limited to 0.5 mol CO2 per mole of amine(stoichiometric maximum). Thus the reaction can be specified asfollows:

R1R2NHþ CO2 $ R1R2NHþCOO�

R1R2NHþCOO� þ R1R2NH $ R1R2NCOO� þ R1R2NHþ2

The same is pertinent to the amine–RTIL blends as the room-temperature ionic liquid does not involve in any kind of chemicalinteraction either with CO2 or with amine [10–13].

Fig. 2. CO2-captured product (carbamate) precipitation in DEA–[hmim][Tf2N]: (a)immediately after CO2 bubbling; (b) 24 h later.

3. Experimental

3.1. Materials

A secondary alkanolamine, diethanolamine (DEA: ACS reagent,P99.0%), was purchased from Sigma–Aldrich while the room-tem-perature ionic liquid, 1-hexyl-3-methylimidazoilium bis(trifluoro-methylsulfonyl)imide ([hmim][Tf2N]: 99%), was provided byIoLiTec Inc. carbon dioxide and nitrogen (P99% purity) gases wereobtained from Praxair Canada Inc.

3.2. Setup

Gas absorption experiments were carried out in a double jack-eted stirred-cell reactor as shown schematically in Fig. 1. An Omnihomogenizer, fitted with rotor–stator generator, was immersed inthe cell to agitate the liquid during absorption experimentswhereas a CO2 probe (GMP221, Vaisala) was positioned in theheadspace to monitor volumetrically the CO2 consumption rate.The reactor volume was 100 ml. The gaseous mixture was contin-uously circulated between the absorption cell and the reservoir(18.7 L vol) with the help of a peristaltic pump at a flow rate of1 ± 0.01 L/min. The temperature of the stirred-cell reactor as wellas of the headspace area was controlled by a thermostatic bath.

3.3. Procedure

Each time, prior to the absorption experiments, the setup waspurged 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 theintroduction of a specified volume of pure RTIL into the cellthrough an inlet needle, the gaseous mixture was continuouslyrecirculated 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 knownquantity of DEA (being immiscible with the ionic liquid) was in-jected into the RTIL containing cell reactor and the process wascontinued for 3 h. For each experiment, 12 ml of DEA–[hmim][Tf2-

N] fluid was used. During the experiment, the liquid was con-stantly stirred using Omni homogeniser fitted with rotor–statorgenerator. CO2 probe was linked to a computerized acquisitionsystem, delivering data in terms of CO2 available as vol% in the

gaseous mixture. This allowed the calculation of CO2 absorptionper unit time.

4. Results and discussion

As has been observed during the earlier work [10–13] absorp-tion of CO2 by primary/secondary alkanolamines, blended inroom-temperature ionic liquids, results in precipitation of theCO2-captured product (carbamate) as shown in Fig. 2.

Since there is no supplementary deprotonating species exceptamine in DEA–RTIL system, the maximum loading capacity doesnot exceed 50 mol% of CO2 as primary/secondary amine (DEA inthis case) reacts with CO2 in 2:1 ratio obeying the followingreaction:

DEAþ CO2 $ DEAHþCOO�

DEAHþCOO� þ DEA$ DEACOO� þ DEAHþ

The current experiments were devised to peruse various param-eters, 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 thecurve, as shown in Fig. 3. The initial steeper part depicts an abrupt

CO

2up

take

(m

ole)

Time (min.)

(a) DEA in [hmim][Tf2N]

2.0 M

1.0 M

0.5 M

(b) DEA in [hmim][Tf2N]

M. Hasib-ur-Rahman, F. Larachi / Separation and Purification Technology 118 (2013) 757–761 759

gas absorption phenomenon that seems to have occurred throughmutual contribution of physically confined CO2 in the RTIL (solubi-lized prior to the injection of amine into the stirred-cell) and theadditional CO2 approaching directly via continuous gas bubbling.While the other somewhat horizontal portion evolved after mostof the unreacted amine accumulated over RTIL surface. As dilutedgaseous mixture containing CO2 6 10 vol% was used to observethe gas absorption trends of DEA–[hmim][Tf2N] blends, higheramine content (DEA: 2 M) did not appear to be compatible withthe experimental conditions as was evident from the slowabsorption kinetics. However, decrease in amine content corre-sponded well to the low CO2 gaseous ratio. The immiscibility aswell as the difference in the respective densities (1.09 g/cm3 and1.37 g/cm3) of both the components, DEA and [hmim][Tf2N], didnot let the amine droplets to stay dispersed long enough. Higheramine ratio further accelerated the coalescence of amine dropletsthus resulting in fast accumulation of amine at the RTIL surface

CO

2up

take

(m

ole)

Time (min.)

(a) CO2 concentration

10.0 %

5.0 %

2.5 %

CO

2up

take

(m

ole)

Time (min.)

(b) CO2 concentration

10.0 %

5.0 %

2.5 %

CO

2up

take

(m

ole)

Time (min.)

(c) CO2 concentration

10.0 %

5.0 %

2.5 %

Fig. 3. Influence of [DEA] molar concentration on absorption rate with respect toinitial 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.

Time (min.)

2.0 M

1.0 M

0.5 M

Time (min.)

(c) DEA in [hmim][Tf2N]

2.0 M

1.0 M

0.5 M

CO

2up

take

(m

ole)

CO

2up

take

(m

ole)

Fig. 4. Influence of initial CO2 volume ratio (in gaseous mixture) on absorption ratew.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.

CO

2up

take

(m

ole)

Time (min.)

Agitation speed

4500 rpm

3000 rpm

1500 rpm

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 cap-tured per unit time), as is obvious from Fig. 3.

760 M. Hasib-ur-Rahman, F. Larachi / Separation and Purification Technology 118 (2013) 757–761

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 sec-tion. The 0.5 M DEA appears well-suited to the gaseous mixturecontaining 10 vol% CO2 for quick gas absorption (Fig. 4). However,as the gaseous CO2 concentration was lowered the capture rate de-creased accordingly. In case of DEA–RTIL blends with higher amineratio (1–2 M DEA), even 10 vol% CO2 was not sufficient to drive theprocess quickly to maximum gas loading.

4.3. Influence of agitation speed

Since diethanolamine and [hmim][Tf2N] are immiscible andthere is significant density difference between the two (DEA:1.09 g/cm3; [hmim][Tf2N]: 1.37 g/cm3), it is hard to keep DEA dis-persed in [hmim][Tf2N] without the addition of a surfactant. Yet,proper agitation can help induce DEA dispersion for extendedduration and thus can provide with increased surface area ofDEA to interact with CO2. As has been shown in Fig. 5, increasein agitation speed from 1500 rpm to 4500 rpm (keeping other vari-ables constant: 2 M DEA; 10 vol% CO2; 33 �C) caused faster CO2

absorption. This seems to be the outcome of greater residence timeof dispersed amine inside the RTIL phase and/or smaller aminedroplet size. Thus agitation speed can be optimized in accordancewith the flue gas composition and the other experimental param-eters (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 toassess the influence of temperature on CO2 absorption behavior. Inspite of the fact that increase in temperature resulted in decreasedliquid viscosity (Table 1) and hence gas transfer rate could haveimproved, the experimental outcome did not depict any systematicchange in capture rate as shown in Fig. 6. Decrease in physical sol-ubility of CO2 in RTIL at higher temperature might have undone thelower viscosity advantage if there was any. This behavior suggeststhat CO2-captured product (carbamate) precipitation is the dictat-

Table 1Viscositiesa of the capture fluid components at three temperatures.

Component 25 �C 33 �C 41 �C

DEA 470 cP 241 cP 139 cP[hmim][Tf2N] 61 cP 38 cP 26 cP

a Measured by AR-G2 rheometer (TA Instruments) with parallel plate geometry.

CO

2up

take

(m

ole)

Time (min.)

Temperature variation

41 °C

33 °C

25 °C

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 temper-ature on CO2 absorption rate.

As in the simulated gaseous mixture the CO2 ratio (opposed tothe pure gas stream) was maintained within the concentrationrange of post-combustion flue gases (<15%), the CO2 solubility inthe RTIL phase must had undergone a negative impact [17,18].

5. Conclusion

The results of this study reveal that though amine–RTIL blendsare blessed with a unique advantage, i.e., CO2-captured product(carbamate) precipitation, an apposite dispersion of amine in theRTIL continuous phase is required for profiting from maximalabsorption capabilities of the immiscible amine–RTIL systems. Agi-tation appeared to have significantly vivid influence as CO2 capturerate enhanced at higher homogenising speed. Also, with increase inCO2 volume ratio in the simulated gaseous mixture (CO2 + N2), thegas absorption rate was correspondingly improved. These experi-mental findings may help carve the way out towards designing apertinent absorption column regarding immiscible amine–RTILsystems.

Acknowledgements

Financial support from FL Canada Research Chair ‘‘Green pro-cesses for cleaner and sustainable energy’’ and the Discovery Grantto F. Larachi from the Natural Sciences and Engineering ResearchCouncil (NSERC) are gratefully acknowledged. The authors are alsothankful to Prof. Denis Rodrigue for help in viscositymeasurements.

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