Absorption of CO2 into novel aqueous bis(3-aminopropyl)amine and enhancement of CO2 absorption into its blends with N-methyldiethanolamine

International Journal of Greenhouse Gas Control 60 (2017) 172–185

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Absorption of CO2 into novel aqueous bis(3-aminopropyl)amine and enhancement of CO2 absorption into its blends with N-methyldiethanolamine Bisweswar Das, Binay Deogam, Bishnupada Mandal ∗ Department of Chemical Engineering, Indian Institute of Technology Guwahati, Guwahati 781039, Assam, India

a r t i c l e

i n f o

Article history: Received 15 August 2016 Received in revised form 14 February 2017 Accepted 21 March 2017 Keywords: CO2 capture Kinetic constant NMR analysis Reaction mechanism Bis(3-aminopropyl)amine activator N-Methyldiethanolamine

a b s t r a c t A study towards the kinetics of carbon dioxide (CO2 ) absorption into a novel aqueous solution of bis(3aminopropyle)amine (APA) was carried out at 303, 308, 313 and 323 K over a concentration range of 0.1–0.5 kmol m−3 and different CO2 partial pressure. The reaction mechanism of CO2 with primary and secondary amines (zwitterionic mechanism) is described and based on this, experimentally obtained kinetic data are interpreted. A qualitative nuclear magnetic resonance [NMR (1D and 2D)] spectroscopy method has been applied to develop the reaction scheme for novel aqueous APA with CO2. The kinetic rate parameters were investigated according to the pseudo-first-order condition. The values of secondorder rate constant, k2-APA and reaction rate with CO2 reported in this study were higher than piperazine (PZ), 2-(1-piperazinl)-ethylamine (PZEA), etc. Furthermore, enhancement factor increases significantly in comparison to aqueous MDEA solutions when the APA concentrations in the blends with MDEA varied from 0 to 0.5 kmol m−3 . © 2017 Elsevier Ltd. All rights reserved.

1. Introduction The CO2 emission abatement in the atmosphere is vitally important to mitigate the problem of global warming and climate change issues. While the coal-fired power generation sectors are the world’s major source of electricity, they significantly contribute to the runaway anthropogenic greenhouse gas like CO2 emissions (Richner et al., 2015). Besides these, to avoid corrosion of pipeline and equipment as well as to meet the fuel gas as a specification in the natural gas processing unit, CO2 removal is essential to a level less than 1%. Furthermore, to avoid freezing in cryogenic equipment in natural gas liquefaction plants, CO2 reduction is mandatory to a level of 50 ppm (Mandal and Bandyapadhyay, 2006; Muraleedharan et al., 2012). It is anticipated that in between 2010 and 2035, global energy demand will increase by 35%, out of which 59% will be carbon based fossil fuel resources (Richner et al., 2015). According to the various climate estimate models, by the year 2100, average earth temperature will increase by 1.4–5.8 ◦ C (Das et al., 2016). Hence, to control the earth’s surface temperature and enhance the sustainable development of society, CO2 emissions need to be reduced. In recent years, unwarranted growth in

∗ Corresponding author. E-mail address: [email protected] (B. Mandal). http://dx.doi.org/10.1016/j.ijggc.2017.03.018 1750-5836/© 2017 Elsevier Ltd. All rights reserved.

energy consumption and stringent environmental regulations has propelled extensive research and development in suitable technology to economically capture CO2 from large point sources (e.g., flue gas streams from power plants) of CO2 emissions. Currently, the post-combustion capture is the best-placed option to effectively separate CO2 from gaseous mixtures because it can be retrofitted to the existing process units. Among the various separation techniques, chemical absorption and physical adsorption are the most promising ones for capturing post-combustion CO2 . While CO2 capture by cryogenic means demand high energy, membrane separation as well as biological separation are still in their incipient stages (Khan et al., 2015). So, absorption using a chemical solvent is the present frontline commercial technology to chemically and reversibly bind the CO2 (Mondal et al., 2015). However, stainless steel absorber/stripping columns and energy associated with the regeneration of solvent from CO2 -rich amine solvent bring high capital cost, which are the key barriers to a large-scale industrial process. These factors demand significant improvements to facilitate the up-scaling (Barzagli et al., 2009; Conway et al., 2013). Out of various solvents, researchers primarily draw attention towards the use of amines and alkanolamines as a solvent due to its basicity, since CO2 is acidic in nature. A broad variety of conventional alkanolamines like monoethanolamine (MEA), diethanolamine (DEA), di-2-propanolamine (DIPA), Nmethyldiethanolamine (MDEA), triethanolamine (TEA), 2-amino-

B. Das et al. / International Journal of Greenhouse Gas Control 60 (2017) 172–185

2-methyl-l-propanol (AMP) have been investigated for industrial gas treatment (Kohl and Nielsen, 1997). The solvent systems studied so far, has many drawbacks which include the inability to fulfil the desired physicochemical properties as well as the flexibility required for CO2 removal. In order to develop potential post-combustion carbon capture (PCC) using alkanolamine based solvent systems, significant advancements need to be made (Conway et al., 2015). Although there are many characteristics which determine the physical size and footprint of the absorber, reaction kinetics is the most important one. Due to carbamate formation by Lewis acid–base reaction, carbamate forming amine or fast-reacting amine is getting an advantage because of their reactivity with CO2 involve much faster kinetics compared to water/hydroxide with CO2 (Conway et al., 2013). Fast reaction results in significantly smaller size of the absorber column, and hence cost will be less. Similarly, as the pH of the solution increases, the rate of reaction increases and if it is greater than 8.5 then eventually it surpasses the reaction with water. So amines under the same conditions with high protonation will give higher rate of reaction. As we know Lewis acid-base interaction was observed during the reaction of CO2 and amine, therefore Lewis basicity is an important property of the amine which governs the rate as well as the equilibrium constant for the reaction with CO2. Generally, tertiary amine has the capacity to absorb the proton released from carbonic acid, which is formed by the reaction between CO2 and water but tertiary amines do not from carbamates in the reaction with CO2 . Compared to primary and secondary amines, tertiary amines have a higher absorption capacity (1 mol of CO2 /mole of amine) as well as higher degradation resistance (Das et al., 2016). While MDEA acts only as a base in solution and consequently being slower in reaction kinetics, the addition of a small amount of fast reacting amine like single molecule containing two or more amine groups is necessary for enhancing the overall absorption rate into the formulated amine solutions (Conway et al., 2015). However, the prospect of significant improvements towards current generation solvent which can provide faster reaction kinetics, high equilibrium loading capacity, smaller size of equipment, reduction in the overall energy penalty, higher degradation resistance, less corrosivity and ultimately, lower capital cost is one of the most pivotal point of PCC technology (Liu et al., 2014). In recent years, extensive research focused towards the finding of rate activators which are far superior to conventional rate accelerators. Sun et al. (2005) described the reaction kinetics of CO2 -PZ-H2 O system employing a wetted wall column absorber (303–313 K) and showed a pseudo first-order reaction rate constant at (303 K) to be 29185 m3 kmol−1 s−1 . However, Bindwal et al. (2011) reported the values of rate constant for the CO2 -PZ-H2 O system an identical condition at (303 K) to be 25,800 m3 kmol−1 s−1 . Paul et al. (2009a) studied absorption of CO2 in aqueous PZEA solutions and rate constant was 31867.6 m3 kmol−1 s−1 at 303 K. While, Dubois et al. (2010) experimentally estimated and clearly visualized the positive effect of blending of PZEA to MDEA and MEA solutions. PZ as well as PZEA are considered to be efficient activators than conventional rate accelerators and the rate constant of PZ is one order of magnitude higher than MEA (Samanta and Bandyopadhyay, 2011). CO2 capture technique using activated solvents such as blends of PZ or PZEA with MDEA has been reported to have a more CO2 absorption capacity due to the high reaction rate and high CO2 loading. Moreover, the cost of regeneration of such activated solvents is relatively low (Paul et al., 2009b; Appl et al., 1982). A bis(3-aminopropyle)amine (APA) which contains two primary amine groups and one secondary amine group is proposed as a novel activator for CO2 capture. Since APA has more pKa (10.85) value than PZ (9.73) (Das et al., 2016) and based on the relation of cyclic and noncyclic amines given by Conway et al. (2013); the reac-

173

tion rate of APA expected to be higher than PZ. Three amine groups are present in a single molecule of APA and hence it is expected to provide higher equilibrium loading capacity than PZ. Also, the high molecular weight of APA leads to less evaporation loss than PZ. Furthermore, according to our knowledge no such amine activator was found in the literature which occurs naturally in some species of plants, bacteria, and algae. Since APA occurs naturally in renewable resources, it is expected to be more promising rate activator than others (Das et al., 2016; Silva et al., 2013). As we know that the typical solvent regeneration temperature range is 353 K–393 K. So within this range, APA with loaded CO2 does not form any cyclic urea (Wu et al., 2010). The major objective of this study is to find out how bis(3aminopropyl)amine (APA) performs as an accelerator when blended with other amines like MDEA instead of an individual solvent and to develop a reaction mechanism of the CO2 -(MDEAAPA-H2 O) system. Hence, in this study, the rates of absorption in the CO2 -(APA-H2 O) system are determined. At the same time onedimensional (1D) and two-dimensional (2D) (1 H and 13 C) NMR spectroscopic methods have been applied to confirm the reaction scheme of CO2 -(APA-H2 O) system. Furthermore, the kinetics of CO2 absorption into (APA + MDEA + H2 O) which were not published in open literature so far, have been investigated. To observe the enhancement of overall CO2 absorption rate, APA concentration was varied from 0 to 0.5 kmol m−3 into aqueous MDEA whereas total concentration was kept at 3 kmol m−3. 2. Theory 2.1. Reaction scheme Absorption of CO2 into aqueous blends of tertiary amine, MDEA and rate activator, APA proceed via several reversible reactions. These reactions are discussed below. Base-catalysed hydration reaction Ka ,k

CO2 + MDEA + H2 O ↔2a MDEAH+ + HCO− 3

(1)

Monocarbamate formation by APA-H2 O: K ,k

CO2 +APA+H2 O b↔2b APACOO− +H3 O+

(2)

Monocarbamate formation by MDEA-APA: Kc ,k

CO2 +APA+MDEA ↔2c APACOO− +MDEAH+

(3)

Dicarbamate formation by APA-H2 O: K ,k



CO2 +APACOO− +H2 O d↔2d APA COO−



2

+H3 O+

(4)

Dicarbamate formation by MDEA-APACOO− : Ke ,k



CO2 +APACOO− +MDEA ↔2e APA COO−



2

+MDEAH+

(5)

Bicarbonate formation by direct reaction of CO2 with H2 O: K ,k

+ CO2 +2H2 O f↔2f HCO− 3 +H3 O

(6)

Formation of carbonate: Kg

− + HCO− 3 +H2 O↔CO3 +H3 O

(7)

Protonation of APA: Kh

APA+H3 O+ ↔APAH+ +H2 O

(8)

Protonation of mono-carbamate formed: Ki

APACOO− +H3 O+ ↔H+ APACOO− +H2 O

(9)

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Protonation of MDEA:

2.3. Reaction rate for CO2 absorption into (APA + H2 O) and (APA + MDEA + H2 O)

Kj

MDEA+H3 O+ ↔MDEAH+ +H2 O

(10)

Dissociation of water molecule: Km

2H2 O ↔H3 O+ +OH−

(11)

where, Kx(x = a,. . ..m), k2y(y = a,. . .f) denotes as equilibrium constant for reaction (x) and second-order forward rate coefficient for reaction (y), respectively. The structure of important reaction intermediates and product are shown in Fig. 1.

Since bis(3-aminopropyle)amine contains two primary amine groups and one secondary amine group it is expected to be faster than other rate promoters like N-(2-Aminoethyl) ethanolamine, PZ and PZEA. It was established in the literature that at a small loading of CO2 , the primary amine group reacts faster than the secondary amine group (Barzagli et al., 2009; Kierzkowska-Pawlak, 2015). Therefore, using similar pathways the reaction of CO2 with both amine groups of APA can be described since carbamate are formed by both amine groups. The reactions are given as follows. + CO2 + APA +B ↔ APACOO− p + BH

2.2. Zwitterion mechanism

CO2 + APA +B ↔

Caplow (1968) and Danckwerts (1979) proposed the two-step zwitterion mechanism which is the widely accepted reaction mechanism for the reaction between CO2 and amines (primary and secondary). Presuming this zwitterion mechanism for APA, although it is not an alkanolamine, the formation of an intermediate zwitterion in the reaction of CO2 and APA should occur as follows: k2,APA ,k−1

↔

CO2 +APA

APAH+ COO−

(12)

where, k-1 is the reverse reaction rate constant, m3 kmol−1 s−1 . In Eq. (12) the intermediate zwitterion formed in the reaction undergoes deprotonation by either a base or a number of bases, such as H2 O, OH− or amine, APA to form carbamate: 

kB APAH+ COO− +B→APACOO− + BH+

(13)

The overall rate of the reaction of CO2 in aqueous amine solutions using the steady-state principle via intermediate zwitterion as given in Eq. (12) can be written as follows: rCO2 -APA =

[APA][CO2 ] 1 k2,APA

+

k−1

=



[APA][CO2 ] 1 k2,APA

+

1 kB (B)

(14)

A combination of bases which is represented by the kinetic con

stant kB (B), where kB = k2,APA kB ⁄k−1 . When the reverse reaction is very slow in comparison to deprotonation in Eq. (12), (k-1 << 

kB (B)) and the rate determining step is zwitterion formation, Eq. (14) is given by rCO2 -APA = k2,APA [CO2 ][APA]

(15)

As given by Eq. (15), the reaction with respect to both amine and carbon dioxide has a first order and hence the overall reaction has a second order. If zwitterion deprotonation is rate-determining 

(k−1 >> kB (B)), Eq. (14) can be written as 

rCO2 -APA

+ BH+

k2,APA kB (B) = [CO2 ][APA] k−1

(16)

The reaction rate depends on the concentration of each base and its strength. However, the contribution of the hydroxide ion is very less and hence it doesn’t affect the accuracy much because [OH− ] has a low concentration in amines’ reaction with CO2 (Paul et al., 2009a). Laddha and Danckwerts (1981) also considered only the amines as the base for aqueous alkanolamine solutions in their studies.

(17) (18)

− where, APACOO− p and APACOOs denotes primary and secondary carbamates, respectively. But finding the actual concentration of APA which contributes to a particular reaction is troublesome. Therefore, to estimate the comparative donation of each reaction towards the overall reaction rate is also difficult (Paul et al., 2009a). Hence, the reaction was assumed to be of first order w.r.t. both CO2 and APA. In later discussions, the justifiability of this assumption is substantiated experimentally. Therefore, the reaction rate of CO2 with APA can be expressed based on Eq. (15) as:

rCO2 -APA = k2,APA [CO2 ] [APA] = kov-APA [CO2 ]

(19)

Here kov-APA and k2,APA denotes the overall rate constant and the second order reaction rate constant, respectively. The tertiary alkanolamine cannot react directly with CO2 to form carbamates because of lack of hydrogen atom attached to its nitrogen atom. In an aqueous environment, tertiary amines act as a base to promote CO2 hydrolysis reaction and forms bicarbonate ions and protonated amine. It was reported in most of the literature that the reaction between CO2 and tertiary alkanolamine (such as MDEA and DEEA) is a second-order reaction (Liu et al., 2014; Paul et al., 2009b; Kierzkowska-Pawlak, 2015). Therefore, absorption rate of CO2 with MDEA can be expressed as rCO2 -MDEA = k2,MDEA [CO2 ] [MDEA] = kov-MDEA [CO2 ]

kB (B)k2,APA



APACOO− s

(20)

So the overall absorption rate of CO2 for the absorption into (APA + MDEA + H2 O) is presented as follows rCO2

= rCO2 -APA +rCO2 -MDEA = k2,APA [CO2 ] [APA] + k2,MDEA [CO2 ] [MDEA]

(21)

= kov [CO2 ] where Kov is in terms of overall rate constant and expressed as kov = k2,APA [APA] + k2,MEDA [MEDA] The reactions of CO2 with amine in the presence of water include the hydration of CO2 and the reaction of CO2 with amines. The reaction rates of CO2 with aqueous amine solutions are mainly dependent on below discussed reactions. If APA had any impurity, the reaction occurred due to this was assumed to be negligible and hence didn’t exhibit a significant contribution to the experimental data. The following reactions are involved in hydration of CO2 and formation of bicarbonate: + CO2 + H2 O ↔ HCO− 3 +H

(22)

CO2 + OH− ↔ HCO− 3

(23)

The Reaction (22) contributes very less to the overall reaction rate since it has a negligible rate constant (k = 0.026 s−1 at 298 K)

B. Das et al. / International Journal of Greenhouse Gas Control 60 (2017) 172–185

175

Fig. 1. The structure of important reaction intermediates and product of CO2 -(MDEA-APA-H2 O) system.

(Pinsent et al., 1956) and hence can be avoided for the experimental condition in the present study. The Reaction (23) doesn’t show a significant contribution to the overall reaction. Therefore the reaction between the CO2 and OH− ion is neglected as well. 2.4. Mass transfer effect When the absorption experiments performed in a gas–liquid contactor, the absorption flux is enhanced by the chemical reaction in the liquid phase and is presented by Eq. (24) NCO2 = EA kL [CO2 ]i

(24)

The above equation is valid when the liquid bulk does not contain any CO2 and the gas-side mass transfer coefficient is completely negligible. The enhancement factor EA describes the mass transfer rate which will be influenced by a chemical reaction. Since there is an identical concentration difference between the interface and the liquid bulk of the absorbing gas so the enhancement factor, defined as the ratio of the flux of CO2 in the presence of chemical reactions to the flux of CO2 in the absence of chemical reactions, is considered. It is necessary because this single term incorporates the possible effects of all chemical reactions on the absorption rate. For certain reaction regimes, to predict the enhancement factor based on the different mass transfer models, several approximate solutions are available in the literature (Kierzkowska-Pawlak, 2015). Doraiswamy and Sharma (1984) presented the specific rate of mass transfer of CO2 as: NCO2 = −DCO2

d

[CO2 ] dw



 w=0

= [CO2 ]i

tanh

DCO2 kov



DCO2 kov /kL



(25)

In Eq. (25), the interfacial (gas-liquid interface) concentrations of CO2 were denoted by [CO2 ]i , kL is defined as liquid phase mass transfer coefficient and DCO2 is the diffusivity of CO2 . To ensure that absorption has been carried out in the pseudofirst-order reaction regime, the absorption conditions of the

CO2 -(APA-H2 O) and CO2 -(APA-MDEA-H2 O) systems were selected accordingly. The linear relationship of the flux versus CO2 partial pressure is expected in this regime. When the Hatta number is greater than 3, then tanh Ha in above equation is close to one and it is assumed that enhancement factor is equal to dimensionless Hatta number (EA = Ha) with Ha =



kov DCO2 /kL

(26)

Also, the defining condition in the fast-pseudo-first order regime is 3 < Ha « E∞ . The infinite enhancement factor (E∞ ) can defined as follows:

 E∞ =

DCO2 DA min e

+

[A min e] Z[CO2 ]i



DA min e DCO2

(27)

where Z is stoichiometric coefficient and DAmine is Diffusivity of amine. The determination of the exact value of infinite enhancements factor is very difficult. However, independency of contact time on flux and linear relationship of flux versus CO2 partial pressure satisfies the fast-pseudo-first order regime (Singh and van Swaaij, 2011). Using Eqs. (24) and (26), the specific rate of mass transfer of CO2 becomes NCO2 = [CO2 ]i



DCO2 kov

(28)

From the above equation, it can be concluded that rate of absorption does not depend on liquid side mass transfer coefficient, kL , and therefore, independent of the hydrodynamic conditions. The interfacial concentrations of CO2 were denoted by [CO2 ]i and obtained from Henry’s law: [CO2 ]i =

pCO2 HCO2

(29)

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After incorporation of Eq. (29), Eq. (28) is transformed to: NCO2 =

pCO2 HCO2



DCO2 kov

(30)

Hence, the absorption kinetics measurements were interpreted on the basis of Eq. (30). On the other hand, to find kov , (in the fast-pseudo-first-order regime) determination of HCO2 , DCO2 and the absorption rate for CO2 into the amine is essential. 3. Experimental section 3.1. Preparation of the aqueous amine solvent In this study, an aqueous solution of APA and an aqueous blend of APA and MDEA were used as absorbents for removal of CO2 from flue gas. The purity of the both the solvents were 98% and solvents were used as received without any further purification. The solutions required for all the experiments were prepared using Millipore water of surface tension and conductivity of the magnitude 72 mN/m and 1 × 10−7 −1 cm−1 at 298 K, respectively (Rahaman et al., 2010). For the preparation of each amine solutions, a Sartorious balance (BSA 224S-CW) with an accuracy of 0.001% was used. Prior to preparation of amine solutions, the water degassing was conducted using prolonged boiling followed by cooling under vacuum. To determine the actual concentration of amine solutions, a quality control step, acid-base titration against standard HCL (Hydrochloric acid) was carried out using an auto-titrator (DL 50, Mettler Toledo). Thus the uncertainty in the determination of amine concentrations was found out to be within 0.01%. For the experimental findings, three different APA concentration (0.1–0.5 kmol m−3 ) and four amine blends of MDEA, APA i.e (3 kmol m−3 MDEA), (2.9 kmol m−3 MDEA + 0.1 kmol m−3 APA), (2.7 kmol m−3 MDEA + 0.3 kmol m−3 APA), (2.5 kmol m−3 MDEA + 0.5 kmol m−3 APA) were used with total concentration of 3 kmol m−3 for absorption of CO2 .

amine solutions and flow rates of liquid and gas were monitored using calibrated rotameters. The gas phase enclosure was made up with a glass shroud. It has a gas inlet at the top and three gas outlets at the bottom. From the bottom of absorption column CO2 loaded liquid solutions were collected. Three samples were collected at different time intervals and the average value of these was reported for each data. The CO2 content was immediately analysed by acidulating a known volume of liquid sample into a gas cell which was placed in a thermostated bath and measured the evolved gas with a monometric device (Mandal et al., 2004; Mandal and Bandyapadhyay, 2006). Hence the rate of CO2 absorption was estimated merely upon liquid phase analysis. 4. Results and discussion 4.1. Physicochemical properties To interpret the absorption rate data as well as the determination of the overall reaction kinetic rate constant, knowledge of several physicochemical and transport properties of the gas-liquid system such as density, viscosity of the aqueous amine solutions and physical solubility, diffusivity of CO2 into the aqueous solutions were essential. Furthermore, to find out the liquid film thickness (w) and contact time (␪) in the wetted wall column contactor, value of density and viscosity of the aqueous amine solutions were necessary. Similarly, determination of the liquid side mass transfer coefficient, kL , that is attainable in the experimental apparatus, require the corresponding values of ␪ along with the values of diffusivity of CO2 into the aqueous amine solutions. For this purpose, density and viscosity of aqueous bis(3-aminopropyl)amine (APA) and an aqueous blend of N-methyldiethanolamine (MDEA) and APA solutions as well as solubility and diffusivity of CO2 into these binary and ternary solutions were measured and published elsewhere (Das et al., 2016). For convenience, the required data are included in the Supporting information (Tables S2–S4).

3.2. Materials and NMR measurement 4.2. One-dimensional and two-dimensional NMR study The solvents APA and MDEA were supplied from Sigma-Aldrich and E. Merck Germany, respectively. CO2 gas had a purity of 99.995%. Similarly, Vadilal Gases, India provided calibrated CO2 and N2 gas mixtures for the experiment where absorption was carried out. The aqueous solution of 2 kmol m−3 MDEA as reference absorbent was used to validate the experimental set-up. Deuterium oxide (D2 O) was used for NMR measurement while concentrated HCL acid was utilized for desorption cell as well as titration purpose. A batch solution was prepared using APA into Millipore water (unloaded solution) for the present NMR measurement. CO2 /N2 gas mixture (15% CO2 ) was bubbled into the amine solution and it was continued in the measurements to ensure complete equilibration of the reaction (Barzagli et al., 2010; Conway et al., 2013; Lim et al., 2012). In equilibrium condition, the inlet and outlet CO2 concentration are same which was checked by GC analysis. This equilibrium solution and unloaded solutions were collected and studied using 600 MHz NMR spectrometer (AscendTM aeon 600 NMR spectrometer, Bruker). D2 O was used to get a signal lock for NMR measurement. 1 H and 13 C NMR measurements of 1D and 2D study were performed in details. NMR analysis parameters at room temperature are given in Supplementary information (Table S1). 3.3. Measurement of CO2 absorption rate To determine the reaction kinetics of the CO2 -APA-H2 O system as well as CO2 -APA-MDEA-H2 O for absorption, a wetted-wall contactor whose absorption surface length maintained at 7 × 10−2 m was used. An overhead vessel was employed to store the aqueous

Figs. 2–4 show the 1 H and 13 C NMR spectra of aqueous APA solution before and after the reaction with CO2 . In 1 H NMR spectra (Fig. 2) of the reaction mixture, several overlapping regions were observed that indicate the presence of CH2 groups, which correspond to different CH2 groups of mono- and di-carbamates. As we know Total Correlation Spectroscopy (TCOSY) represents a separate spin system of a compound depending on the mixing time. In our TOCSY (1 H–1 H) spectra as shown in Fig. 3, many separate regions were visualized which represent a different spin system of different compounds. So from that spectrum, we can say mono-carbamate and di-carbamate corresponding to primary and secondary amine groups were present. But no clear evidence of the presence of tricarbamate was found. The presence of different CH2 groups in the reaction mixture was confirmed by Distortionless Enhancement by Polarization Transfer (DEPT-135) experiment (shown in Fig. S1, Supplementary information). From the 13 C NMR spectra of CO2 loaded APA solution (Fig. 4), four peaks were observed at different positions in the low field range (163–165 ppm) corresponding to carbonyl carbon, which represent the different carbamate groups. In the higher field range (26–45 ppm), different CH2 carbons were present, which corresponds to different CH2 carbon from a different set of products. These were also confirmed by Heteronuclear Multiple Bond Coherence (HMBC), Heteronuclear Single Quantum Coherence (HSQC) experiments (Figs. S2 and S3). The enlarged version of 1 H spectra as well as the HMBC (1 H–13 C) of CO2 -loaded APA and HSQC (1 H–13 C) spectra of starting APA solutions is given in Supplementary information (Figs. S4–S6). Hence, the formulation of

B. Das et al. / International Journal of Greenhouse Gas Control 60 (2017) 172–185

Fig. 2.

1

177

H NMR spectra for (a) aqueous APA solution (b) CO2 -loaded aqueous APA solution.

mono and di-carbamates corresponding to primary and secondary amine groups were visualized based on the desired peaks in the NMR spectra.

4.3. Carbamate formation during absorption The chemistry of the CO2 -APA-H2 O reaction system is very complex since APA contains multiple amine functional groups. This

leads to a large number of possible chemical reactions and formation of liquid phase species (e.g., carbamate and dicarbamate corresponding to primary and secondary amine groups). Hence it is tough to find out the most important reaction(s), and the effects of these reactions on an overall absorption rate are not easily predictable. However, by utilizing Brønsted dependency of the reactivity (pKa value), an initial prediction on these reactions can be made. This is because of the existence of a linear relationship

178

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Fig. 3. TOCSY NMR spectra for CO2 -loaded aqueous APA solution.

between amine’s pKa value with the logarithm of a forward rate constant (Conway et al., 2013; Singh and van Swaaij, 2011; Paul et al., 2009a). If Brønsted relation to APA is assumed, then the formation of carbamates (primary and secondary) due to the reaction of CO2 with amine groups (primary and secondary) should be taken into account. Now, through the concentration of the major reactants APA and carbamates (primary and secondary), the possible formulation of di-carbamate can be assessed. To ensure the absorption regime, the absorption conditions (amine concentrations and partial pressures of CO2 ) were selected accordingly. Owing to the reaction with CO2, the interfacial concentration of APA in this regime is not noticeably decreasing. Therefore, reaction products of the mono-carbamates concentrations are small, even near to the gas-liquid interface. In comparison with the remaining APA concentrations, mono-carbamates concentrations are small and this gave a small contribution to the formulation of di-carbamates. Hence, the concentrations of di-carbamates were negligible. 4.4. Absorption of CO2 in wetted wall column contactor 4.4.1. Experimental validation The experimental setup validation for kinetics study was carried out using 2, 3 kmol m−3 MDEA solutions. Numerous studies of kinetics data is available in the literature for absorption of CO2 into (MDEA + H2 O) solution. So in the present study aqueous MDEA are used for the purpose of validation. The experimental flux of CO2 absorption for 3 kmol m−3 MDEA solutions was found to be 2.6 × 10−7 m3 kmol−1 s−1 at 313 K with a CO2 partial pressure of 4.7 kPa. There is a good agreement between experimental and published data (Kierzkowska-Pawlak and Chacuk, 2010; Paul et al., 2009b). A graphical representation of flux versus CO2 partial pressure for 2 kmol m−3 MDEA solution at 303–323 K gave a straight line as shown in Fig. S7, which confirms the fast-pseudo-first-order regime. This regime was also verified

by changing the contact time at a particular partial pressure. As depicted in Fig. S8, the flux of CO2 does not change with variation of contact times, which confirms the desired regime of the reaction. Hence, all other kinetics experiments are conducted following this protocol. 4.4.2. Determination of overall reaction rate constant and second order rate constant for aqueous APA system The absorption experiments of the CO2 -APA-H2 O system were undertaken at four different temperatures (303, 308, 313 and 323 K) over the amine concentration range of 0.1–0.5 kmol m−3 . All experiments were conducted at an initial CO2 -free amine solution under atmospheric pressure. Throughout the experimental run, a constant aqueous amine flow rate was maintained at 2 × 10−6 m3 s−1 . As we know the turbulence appears above 400 (Reynolds’s number) and hence, throughout the experiment the liquid phase Reynolds’s number (NRe ) was maintained at about 20 which lie within the laminar region. All absorption experimental measurements were conducted at a gas flow rate of 180 × 10−6 m3 s−1 which is above 140 × 10−6 m3 s−1 so that the contribution of gas phase resistance becomes insignificant (Mandal et al., 2004). It has also been verified earlier that the change in contact time (t) does not affect the rate of absorption of CO2 . So in our work, the contact time is maintained nearly constant throughout the experiment. The specific rate of absorption of the CO2 -APA-H2 O system is shown in Fig. 5 and also given in Table 1. This figure clearly demonstrates that specific rate of absorption of CO2 at a particular partial pressure of CO2 increases when the reaction temperature and concentration of APA is increased. The calculated values of kov using Eq. (29) at various APA concentrations and at four different temperatures of 303, 308, 313 and 323 K are given in Table 2. Applying the least squares method, experimentally obtained kinetics data were fitted to an empirical power law equation. Therefore, the reaction orders were found to

B. Das et al. / International Journal of Greenhouse Gas Control 60 (2017) 172–185

Fig. 4.

13

179

C NMR spectra for (a) aqueous APA solution (b) CO2 -loaded aqueous APA solution.

be in the range of (0.95–0.99) for four different temperatures mentioned above. Hence, the reaction order falls close to first order with respect to APA. Utilizing kov values, the values of k2,APA were estimated by Eq. (19) as discussed subsequently. The enhancement factor values (EA ) have been calculated at different flux values of CO2 . Since the val-

ues of the calculated Ha numbers were greater than 3 in every case, the values of enhancement factor are equal to those of Ha numbers, respectively, as given in Table 1. However, in fast-pseudofirst-order regime determination of exact E∞ value for reversible reaction at low amine concentration and high partial pressure of CO2 is very difficult as mentioned earlier. For this aqueous APA

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Fig. 5. Specific absorption rates of CO2 into aqueous solutions of APA at different temperatures for gas compositions (a) 5% CO2 and 95% N2 (b) 10% CO2 and 90% N2 (c) 15% CO2 and 85% N2 .

Table 1 Kinetic data for the absorption of CO2 into (APA + H2 O). T (K)

[APA] (kmol m−3 )

PCO2 (kPa)

 (s−1 )

kL (105 m s−1 )

NCO2 (106 kmol m−2 s−1 )

Ha

E

303

0.1 0.3 0.5 0.1 0.3 0.5 0.1 0.3 0.5

4.85

0.350 0.368 0.383 0.352 0.368 0.396 0.354 0.365 0.389

8.33 7.71 7.18 8.31 7.72 7.06 8.29 7.75 7.13

4.05 5.80 7.10 8.15 11.42 14.15 12.12 17.10 21.15

34.2 54.7 74.1 34.4 53.8 75.0 34.2 53.5 74.1

34.2 54.7 74.1 34.4 53.8 75.0 34.2 53.5 74.1

0.1 0.3 0.5 0.1 0.3 0.5 0.1 0.3 0.5

4.78

0.347 0.346 0.377 0.341 0.347 0.368 0.348 0.356 0.359

9.32 8.71 7.92 9.41 8.70 8.02 9.32 8.58 8.12

4.82 6.70 8.10 9.60 13.00 15.91 14.40 19.54 23.95

39.7 61.3 84.1 39.1 59.6 81.6 39.5 60.5 80.8

39.7 61.3 84.1 39.1 59.6 81.6 39.5 60.5 80.8

0.1 0.3 0.5 0.1 0.3 0.5 0.1 0.3 0.5

4.70

0.321 0.340 0.364 0.331 0.337 0.357 0.328 0.332 0.354

10.40 9.70 8.93 10.20 9.74 9.01 10.30 9.81 9.06

5.65 7.78 9.55 11.30 15.59 19.05 16.95 23.32 28.65

44.4 67.4 92.8 45.1 67.3 91.8 44.9 66.6 91.5

44.4 67.4 92.8 45.1 67.3 91.8 44.9 66.6 91.5

0.1 0.3 0.5 0.1 0.3 0.5 0.1 0.3 0.5

4.45

0.298 0.318 0.343 0.307 0.323 0.330 0.305 0.321 0.333

11.90 11.10 10.30 11.70 11.00 10.50 11.80 11.00 10.50

6.95 9.50 11.00 13.78 18.87 22.05 20.75 28.88 33.82

58.9 88.8 113.3 59.3 88.9 111.4 58.0 88.4 111.9

58.9 88.8 113.3 59.3 88.9 111.4 58.0 88.4 111.9

308

313

323

9.71

14.60

9.55

14.30

9.39

14.10

8.90

13.65

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181

Table 2 Kinetic constant for the absorption of CO2 into (APA + H2 O). T (K)

[APA] (kmol m−3 )

PCO2 (kPa)

E∞

kov-APA (s−1 )

k2,APA (m3 kmol−1 s−1 )

303

0.1 0.3 0.5 0.1 0.3 0.5 0.1 0.3 0.5

4.85

40.3 121.4 207.0 21.1 61.5 104.3 14.7 41.7 70.2

4242.7 10354.9 18258.7 4286.4 10015.4 18093.0 4221.8 10000.9 18002.0

35775

0.1 0.3 0.5 0.1 0.3 0.5 0.1 0.3 0.5

4.78

43.8 132.8 227.1 22.8 67.3 114.5 15.8 45.5 76.9

5768.6 13844.3 23870.0 5720.8 13028.1 23014.7 5716.8 13079.9 23171.2

46414

0.1 0.3 0.5 0.1 0.3 0.5 0.1 0.3 0.5

4.70

46.4 140.0 239.4 24.2 71.0 120.7 16.7 47.9 81.0

7814.0 17041.3 30099.3 7830.6 17143.5 30005.8 7814.0 17012.1 30099.3

59816

0.1 0.3 0.5 0.1 0.3 0.5 0.1 0.3 0.5

4.45

57.2 172.1 293.0 29.5 87.0 147.0 19.9 57.0 96.8

14819.9 31563.0 47673.0 14565.0 31132.6 47889.9 14039.9 31001.3 47894.9

99,220

308

313

323

9.71

14.60

9.55

14.30

9.39

14.10

8.90

13.65

solution, some E∞ value at a different flux of CO2 is less than Ha number as given in Table 2. However, the independency of contact time on flux and linear relationship between experimental flux and CO2 partial pressure indicates that Eq. (26) is fulfilled (Singh and van Swaaij, 2011). A comparison of kov values of APA, PZ (Sun et al., 2005) and PZEA (Paul et al., 2009a) has been made at different temperatures for the absorption of CO2 and is presented in Fig. 6. As given by the Arrhenius plot (the discussion was made later), the values of rate constants for the CO2 -APA-H2 O system were larger than the rate constants of the CO2 -PZ-H2 O and CO2 -PZEA-H2 O system which is clearly shown in Fig. 7. The kinetic parameter, k2,APA is evaluated from the values of ®

kov-APA (as given in Table 2) using MATLAB (The Math Works, Natick, MA; Version: 2013b) by a nonlinear regression method and are also included in Table 2. This nonlinear regression gave an average deviation of ±9.76% for 36 data points. Over the experimental temperature range, a correlation of k2,APA with varying temperature is developed, and it is represented by the following equation:

 41833 

k2,APA = 5.77 × 1011 exp −

RT

(32)

where the molar gas constant (R) value is 8.314 J mol−1 K−1 . From the above correlation, the average deviation is found to be about ±0.45%. For k2,APA , the activation energy is estimated to be 41.8 kJ mol−1 . The measured and estimated rate constants using Eq. (32) are compared in a parity plot (Fig. 8). As observed from this plot, these values are in good agreement with each other. In this study, the experimental and calculated values of k2,APA for APA as well as other literature are compared over four different

temperatures as shown in an Arrhenius plot (Fig. 7). Also, the solid line for absorption of CO2 in APA is obtained by evaluating k2,APA using Eq. (32). As per this study, the observed k2,APA for the CO2 APA-H2 O system is found to be more than that of the CO2 -PZ-H2 O (Sun et al., 2005) and CO2 -PZEA-H2 O (Paul et al., 2009a) systems. 4.4.3. Determination of overall reaction rate constant and second order rate constant for aqueous (APA + MDEA) A novel blended amine system such as (APA + MDEA + H2 O) has been considered in this work for CO2 absorption. It was pointed out in the introduction section that MDEA can be promoted by amines with stronger reactivity. So, formulated solvent i.e. aqueous blend of MDEA and so-called activators like APA can be considered as a promising absorbent for bulk CO2 removal. In this work, a series of experiments consisting of (MDEA + APA) blend were conducted at three different temperatures. 3 kmol m−3 of MDEA is used as initial concentration and then 0.1, 0.3 and 0.5 kmol m−3 of promoter concentration were applied into the formulated solutions. Several experiments with different temperature and concentration were performed and absorption rate of CO2 in the formulated amine solution at a particular partial pressure was measured in a manner similar to a single amine solution. The enhancement factor can be estimated based on the measured rate under fast-pseudo-firstorder regime using Eq. (26). For the aqueous blend of (APA + MDEA), the variation of kov with concentrations is shown in Fig. 9 at three different temperatures of 303, 313 and 323 K. To evaluate the values of overall reaction rate constants (k2 ), Eq. (21) was used and the values are shown in Table 3. Since the calculated enhancement factors at different fluxes of CO2 were greater than 3 in every case as shown in Table 3, the values of Ha numbers are equal to those of

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Fig. 6. Overall reaction rate constants for the reaction of CO2 with aqueous APA solutions as a function of amine concentration.

Fig. 7. Arrhenius plots of second order reaction rate constants for APA, PZ and PZEA aqueous solutions. Table 3 Kinetic data for the absorption of CO2 into (APA + MDEA + H2 O). T (K)

APA/MDEA (kmol m−3 )

PCO2 (kPa)

 (s−1 )

kL (105 m s−1 )

NCO2 (106 kmol m−2 s−1 )

Ha

E

E∞

kov (s−1 )

303

0/3 0.1/2.9 0.3/2.7 0.5/2.5

9.71 4.85 4.85 4.85

0.573 0.588 0.604 0.617

4.81 4.63 4.43 4.21

0.38 4.21 6.06 7.40

3.3 76.1 115.2 148.9

3.3 76.1 115.2 148.9

683.6 1377.0 1385.5 1395.0

23.9 12539.6 27999.5 45770.7

0/3 0.1/2.9 0.3/2.7 0.5/2.5

9.39 4.70 4.70 4.70

0.507 0.519 0.537 0.548

5.89 5.69 5.45 5.26

0.52 5.85 8.25 9.60

4.2 97.3 144.2 174.8

4.2 97.3 144.2 174.8

774.5 1556.9 1566.0 1575.8

43.3 23220.5 49339.4 71057.6

0/3 0.1/2.9 0.3/2.7 0.5/2.5

8.90 4.45 4.45 4.45

0.458 0.472 0.487 0.497

6.96 6.71 6.49 6.30

0.72 7.30 9.70 11.75

5.9 124.6 171.8 215.6

5.9 124.64 171.84 215.57

937.1 1885.4 1890.9 1902.0

96.5 41929.5 77242.9 119114.2

313

323

k2,APA (m3 kmol−1 s−1 )

k2,MDEA (m3 kmol−1 s−1 )

99370

7.98

165736

14.45

272290

32.20

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183

Fig. 9. Overall reaction rate constants for the reaction of CO2 with aqueous (APA + MDEA) solutions as a function of amine concentration.

Fig. 8. Comparison of the calculated rates to the experimental rates of absorption for CO2 into aqueous solutions of APA.

Fig. 10. Comparison of the calculated rates to the experimental rates of absorption for CO2 into aqueous solutions of (APA + MDEA).

enhancement factors, respectively. Here in all the cases, E∞ value for this reversible reaction is greater than Ha number. Hence Eq. (26) is fulfilled. ® A nonlinear regression method using MATLAB (2013b) was introduced to evaluate the second order reaction rate constants (k2,APA, k2,MDEA ) from the values of kov and are added in Table 3. This nonlinear regression gave an average deviation of ± 8.1% for 12 data points. Over the experimental temperature range, the estimated k2,APA , and k2,MDEA , values for CO2 -(APA + MDEA + H2 O) system have been correlated according to the following Arrhenius equations.

systems were compared and given in Table S5. The experimental results and calculated model values are compared by a parity plot and these are in good agreement with each other as shown in Fig. 10.

 41188 

k2,APA = 1.246 × 1012 exp −

RT

 61669 

k2,MDEA = 3.01 × 1011 exp −

RT

(33) (34)

Both the correlated equations depicted above gave the average deviation of about ±0.40% and ±6.31% and also the activation energy is evaluated from these equations to be 41.2 and 61.7 kJ mol−1 , respectively. The activation energies for different

4.4.4. Enhancement of CO2 absorption using APA activated MDEA The measured absorption rates of CO2 and enhancement factors into (MDEA + APA + H2 O) are presented in Table 3. The addition of small amount of APA into the MDEA solutions results in significant increase in absorption rate as shown in Fig. 11. For instance, at 4.7 kPa partial pressure of CO2 and T = 313 K, the enhancement factor for aqueous solutions of (3 M MDEA), (2.9 M MDEA + 0.1 M APA), (2.7 M MDEA + 0.3 M APA) and (2.5 M MDEA + 0.5 M APA) are about 4.2, 97.2, 144.2 and 174.8, respectively. Therefore, replacing 0.1 M MDEA with an equal amount of 0.1 M APA, the enhancement factor was increased by about 2214%. Replacing an additional 0.2 M amount of MDEA with APA, an additional 48% enhancement was observed. A further replacement of 0.2 M amount of MDEA with APA, the additional enhancement was 21%. So the substantial

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of +23% to −29% was observed with +50% to −50% deviations in the rate constant values. 5. Conclusions Kinetics experiments of CO2 absorption into aqueous APA are determined here using a wetted-wall model contactor at temperatures of 303–323 K. To estimate the overall pseudo- first-order reaction rate constants from the present experiments, the kinetic measurements are carried out in fast pseudo-first-order reaction regime. Beside this, a qualitative NMR analysis aided us about the formation of mono and di-carbamates which correspond to primary and secondary amine groups. Since the relative contribution of APA to the formulation of carbamates (primary and secondary) by the reactions of CO2 with APA was very difficult to determine, hence an overall second order rate mechanism was assumed. This assumption’s validity was justified by the reaction order with respect to APA (0.95–0.99). The values of k2,APA were obtained as 35775, 46414, 59816 and 99220 m3 kmol−1 s−1 at 303, 308, 313 and 323 K, respectively. It was found that the k2,APA for APA was more than those of PZ. It can be concluded from the kinetic rate constants of APA that it has a great potential as an activator with AMP, MDEA etc. for CO2 removal. Furthermore, the addition of a small amount of fast reacting APA to a much larger amount of MDEA significantly enhances the absorption rate over conventional single amine. Acknowledgements Fig. 11. Enhancement of MDEA using APA promoter at different temperature.

increase in the enhancement factor by replacing a small amount of MDEA with an equal amount of APA may be due to the increase in the basicity of blended amine solutions, which intern enhances the base catalysis reaction between MDEA and CO2 in aqueous solutions. The initial enhancement factor is very high. Then it is very low and above (2.5 M MDEA + 0.5 M APA) concentration the change in enhancement factor does not follow the same trend. The reduced diffusivity of CO2 in the solvent with the higher concentration of APA may be one of the reasons. Furthermore, a negligible increase in the equilibrium CO2 capacity of the solvent with the higher concentration of APA also puts a limit on the absorption rate of CO2. Similar improvement in the absorption rate and enhancement factor has been observed by Samanta and Bandyopadhyay (2011) with the addition of small amount of PZ in aqueous MDEA for the absorption of CO2 into (MDEA + PZ + H2 O) solutions. 4.4.5. Effects of pertinent parameters on the rate of absorption of CO2 A parametric sensitivity analysis was carried out to understand the effects of various physicochemical and kinetic parameters on the specific rate of absorption of CO2 into the aqueous APA solutions. The parameters considered for this study were Henry’s law constant and diffusivity of CO2 into (APA + H2 O) and kinetic constant of CO2 -APA reaction in aqueous solutions. The +50% to −50% deviations were introduced in the values of all the parameters and the results are generated. Fig. S9 depicts the effects of deviations of Henry’s law constant and diffusivity of CO2 on the rate of absorption. As depicted from this figure, the corresponding deviation in the specific rate of absorption of CO2 is in the range of −33% to +100% for Henry’s law constant and +23% to −29% for diffusivity. Thus, the Henry’s law constant values are much more sensitive compared to the diffusivity of CO2 . Fig. S10 shows the deviation calculated rate of absorption of CO2 for the variation of kinetic constant values. The deviation for the rate of absorption in the range

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