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CO2 Capture Using Aqueous Potassium Carbonate Promoted by Ethylaminoethanol: A Kinetic Study Rahul Bhosale, Anand Kumar, Fares AlMomani, Ujjal Kumar Ghosh, Ahmed AlNouss, Jonathan Richard Scheffe, and Ram B. Gupta Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b04398 • Publication Date (Web): 18 Apr 2016 Downloaded from http://pubs.acs.org on April 23, 2016
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Industrial & Engineering Chemistry Research
CO2 Capture Using Aqueous Potassium Carbonate Promoted by Ethylaminoethanol: A Kinetic Study Rahul R. Bhosale*,1, Anand Kumar1, Fares AlMomani1, Ujjal Ghosh1, Ahmed AlNouss1, Jonathan Scheffe2, Ram B. Gupta3, 1
Department of Chemical Engineering, College of Engineering, Qatar University, P. O. Box 2713, Doha, Qatar.
2
Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, FL 32611-6250, USA.
3
Department of Chemical and Life Science Engineering, Virginia Commonwealth University, Richmond, VA 23284, USA
*Corresponding author. Dr. Rahul R. Bhosale Tel: (+974) 4403 4168; fax: (+974) 4403 4131
[email protected] [email protected] ACS Paragon Plus Environment
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Abstract Graphics / Table of Contents
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ABSTRACT
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Kinetics of absorption of CO2 in an aqueous potassium carbonate (K2CO3) promoted by
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ethylaminoethanol (EAE) solution (hereafter termed as APCE solvent) was investigated in a
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glass stirred cell reactor by employing a fall in pressure technique. The reaction chemistry
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associated with the CO2 – APCE solvent system was described by the zwitterion mechanism. The
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solubility and diffusivity of CO2 in the APCE solvent was experimentally determined at different
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experimental conditions. Furthermore, the effect of initial EAE concentration (0.6 to 2 kmol/m3)
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and reaction temperature (303 to 318 K) on the rate of absorption of CO2 was studied in detail.
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The experimental findings show that with the increase in the EAE concentration and reaction
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temperature, the rate of CO2 absorption in the APCE solvent also increases considerably. Kinetic
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measurements further confirm that the absorption of CO2 in the APCE solvent belongs to the fast
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reaction regime with first-order kinetics with respect to EAE and first-order kinetics with respect
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to CO2. Due to the addition of EAE as a promoter in an aqueous K2CO3, significant improvement
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in the rate of absorption of CO2 was realized. The rate constant ( ) for CO2 – APCE solvent
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system was observed to be higher as compared to monoethanolamine (MEA) promoted aqueous
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K2CO3. For instance, for the absorption of CO2 in the APCE solvent was observed to be equal
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to 45540 m3/kmol·s at 318 K. Furthermore, the activation energy for the CO2 – APCE solvent
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system was estimated to be 81.7 kJ/mol. The lumped parameter, · , where is
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solubility of CO2 and is diffusivity of CO2 in the APCE solvent, was calculated based on
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the kinetic measurements and observed to be equal to 1.2 × 10-6 kmol1/2/m1/2·s·kPa.
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Keywords: Kinetics, CO2 absorption, ethylaminoethanol, potassium carbonate, stirred cell
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reactor, fall in pressure, zwitterion and termolecular mechanism
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1.
Introduction
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Reactive absorption of CO2 from the industrial off gases by using chemical solvents is
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considered as one of the most common, efficient, and cost effective technologies utilized by the
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industry for CO2 capture. The captured CO2 can be stored by using the geological or oceanic
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sequestration approaches1. As an alternative to geological or oceanic sequestration, the captured
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CO2 can be re-energized into CO by using solar energy and combined with H2, which can be
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generated from different methods, to produce syngas2-9. The syngas produced can be further
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processed to liquid fuels such as methanol, gasoline, jet fuel, etc. via the catalytic Fischer-
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Tropsch process.
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In past, a variety of chemical solvents (mostly aqueous amines and there derivatives) have
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been used for CO2 capture from different gaseous streams via reactive absorption10-16. Though
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the amines are attractive for the CO2 capture application, there are several disadvantages such as
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very strong corrosion to equipment and piping, high energy requirement during the stripping of
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CO2 and they are prone to oxidative and thermal degradation.
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Recently, use of aqueous potassium carbonate (K2CO3) as a solvent for the absorption of CO2
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has gained widespread attention. The usage of K2CO3 has been employed in a number of
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industries for the removal of CO2 and H2S. Due to its high chemical solubility of CO2, low
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toxicity and solvent loss, no thermal and oxidative degradation, low heat of absorption, and
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absence of formation of heat stable salts, K2CO3 seems to be more attractive compared to the
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conventional amines towards CO2 capture. However, K2CO3 solvent shows slow rate of reaction
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with CO2 and, consequently, low mass transfer in the liquid phase as compared to the amine
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solvents. Hence, several investigators are focused towards improving the rate of reaction of CO2
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in K2CO3 solvent with the help of different types of promoters. Inorganic salts, organics, and
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biological enzymes are the main three types of promoters have been examined towards the
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enhancement of rate of absorption of CO2 in K2CO3 solvent17-30.
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Kinetics of K2CO3 solution modified with different promoters was investigated by many
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researchers using a wetted wall column, a stirred cell reactor, a rapid mixing approach, a stopped
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flow technique, or a bubble column reactor. Savage et al.22 studied the reaction mechanisms for
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the absorption and desorption of CO2 in 25 wt% K2CO3 solution. The rate of absorption of CO2
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in diethanolamine (DEA)-promoted K2CO3 solution was estimated using a wetted wall column29.
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The same experimental technique was also used by Ghosh et al.19 to measure the rate of
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absorption of CO2 in 30wt % K2CO3 solution promoted by 1 to 5 wt% boric acid. Likewise, the
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reaction kinetics of absorption of CO2 into un-promoted and boric acid promoted K2CO3 solution
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under industrial conditions was explored by Thee et al.28. Behr et al.29 studied the kinetics of
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piperazine (PZ), monoethanolamine (MEA), methyl diethanolamine (MDEA), and some organic
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polymers promoted K2CO3 solution using a bubble reactor and a wetted wall column. Kinetics of
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absorption of CO2 in arginine promoted K2CO3 solution was analyzed by Shen et al.18 using a
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wetted wall column. Nicholas et al.30 researched the kinetics and mechanism of CO2 hydration
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by Pyrovanadate in K2CO3 solution. Tseng et al.25 used DEA as a promoter in K2CO3 solution
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and examine the effects of carbonate conversion and amine concentration on the CO2 capture
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kinetics. Pohorecki and Kucharski26 developed a method to determine the rate of absorption and
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desorption of CO2 in K2CO3 solution based on the industrial scale parameters. The kinetics of
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CO2 – K2CO3 – PZ system was rigorously modeled by Cullinane and Rochelle23-24. A series of
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disc apparatus was used by Knuutila et al.27 to study the kinetics of absorption of CO2 into the
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sodium promoted K2CO3 solution. The activation energy for the MEA promoted K2CO3 – CO2
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capture system was experimentally and thermodynamically determined by Thee et al.28.
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In 1969, Shrier and Danckwerts20 examined the CO2 absorption capacity of various amine-
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promoted K2CO3 solutions using a stirred cell reactor and reported that the ethylaminoethanol
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(EAE) seems to be the most effective promoter. However, to the best of our knowledge, there is
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no published data available towards a detailed investigation of kinetics of absorption of CO2
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using aqueous K2CO3 promoted by EAE. In addition to be an effective promoter for the K2CO3
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solution, EAE possesses several other advantages such as:
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•
The major raw material for the production of EAE is ethanol which can be produced from renewable resources such as agricultural products and/or residues15, 31-32.
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The rate of absorption of CO2 in aqueous EAE is higher to that of aqueous MEA31.
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•
The oxidative and thermal degradation of EAE is lower as compared to other amines32-33.
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In this paper, the kinetics of absorption of CO2 into an aqueous K2CO3 (20wt %)
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promoted by EAE solution (hereafter termed as APCE solvent) was studied in a glass stirred cell
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reactor using a fall in pressure method. The physico-chemical properties such as viscosity,
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density, solubility and diffusivity of CO2 into APCE solvent were experimentally estimated.
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Effects of initial EAE concentration and reaction temperature on kinetics of absorption of CO2
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into APCE solvent were examined in detail. Also, the reaction chemistry was systematically
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explained by zwitterion mechanism.
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2.
Reaction Chemistry
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2.1
Reaction of CO2 with aqueous K2CO3
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The reactions involved in the CO2 – aqueous K2CO3 system can be expressed as follows:
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+
(1)
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+ +
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In case of basic solution ( ˃ 8), formation via reaction (2) can be neglected as the rate
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of reaction is faster by two order of magnitude and hence reaction (1) is the predominant and the
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rate-limiting step18. The rate of forward reaction represented by Eq. (1) is as follows:
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=
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2.2
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(2)
(3)
Reaction of CO2 with aqueous EAE In the past, several investigators have attempted to explain the complex reaction
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mechanism associated with the CO2 – aqueous alkanolamine system14,
21, 33-34
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aqueous EAE system can be described by a two-step zwitterion mechanism as explained by
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Vaidya and Kenig15, and Bhosale and Mahajani31. In following steps, the EAE is represented as
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, where = , and = , respectively In the reaction chemistry of
. The CO2 –
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CO2 – aqueous EAE system, initially reacts with via formation of a zwitterion as
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an intermediate. The zwitterion formed can undergo deprotonation by a base of different bases
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(!), thereby resulting into formation of carbamate. If the formation of carbamate is achieved by
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using the as the base, then the overall reaction for the CO2 – aqueous system
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can be represented as:
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+ 2 → +
(4)
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If the zwitterion reacts more easily with H2O rather than , formation of bicarbonate is
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possible. The overall reaction for the bicarbonate formation can be expressed as:
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O + + ↔ +
(5)
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In case of , as it has moderate carbamate stability, bicarbonate formation is possible
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even at lower CO2 partial pressures. Hence, the number of bicarbonate ions will increase in
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comparison with the carbamate ions due to the reaction of the free molecule with CO2.
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By applying the steady state principle to the intermediate zwitterion, the overall rate of
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absorption of CO2 in an aqueous can be expressed as:
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=
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& ' (')& ) *(
(6)
+ , &
+- '-(
In the above Eq. (6), ./ '!( represents zwitterion deprotonation by base B ( , , or
). Furthermore, this reaction rate equation does not take into account the reaction of CO2
with or . This rate expression exhibits a fractional order between one and two with respect to the concentration and can be expressed as:
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= 012 ' (
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2.3
(7)
Reaction of CO2 with APCE solvent
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Similar to our previous study related to absorption of CO2 using an aqueous blend of
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EAE and N-methyl-2-Pyrollidone (NMP)31, the overall reaction between CO2 and APCE solvent
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can be considered as the reaction between CO2 and in parallel with the reaction of CO2
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with K2CO3. Therefore, similar to the CO2-EAE-NMP system31, the specific rate of absorption of
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CO2 in APCE solvent can be expressed as:
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= ' (3 4)&)* ' ( + ' (5
(8)
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= ' (3 ' N(
(9)
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where,
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= )& )* +
' ( ')& ) *(
(10)
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Kinetic measurements
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Kinetic measurements for the CO2-APCE system can be carried out by employing the similar
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approach used by Bhosale and Mahajani31. The expressions required for the determination of 78 ,
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Hatta number (9 ),and the necessary conditions to satisfy the fast reaction regime are taken from
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the previous studies15, 31. There is an undisputed agreement among all the investigators that, in
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case of absorption of CO2 using aqueous amines, the reaction order with respect to CO2 is
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unity14, 21. E in the fast reaction regime is equal to 9 ⁄:;
