Kinetics of carbon dioxide removal by aqueous diamines

Chemical Engineering Journal 169 (2011) 144–150

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Kinetics of carbon dioxide removal by aqueous diamines Ankush B. Bindwal a , Prakash D. Vaidya a,∗ , Eugeny Y. Kenig b a b

Department of Chemical Engineering, Institute of Chemical Technology, Mumbai 400 019, India Faculty of Mechanical Engineering, Chair of Fluid Process Engineering, University of Paderborn, D-33098 Paderborn, Germany

a r t i c l e

i n f o

Article history: Received 14 December 2010 Received in revised form 22 February 2011 Accepted 24 February 2011 Keywords: Absorption Gas purification Mass transfer Reaction kinetics Separation techniques

a b s t r a c t Because of the presence of one or more primary or secondary amino groups, solvents containing diamines have a good potential for CO2 capture. In the present work, the CO2 reactions with two diamines, viz. N-(2-aminoethyl)ethanolamine (AEEA) and piperazine (PZ), in aqueous solutions are investigated using a stirred-cell reactor. The reaction pathways are described using the zwitterion and the termolecular mechanism. Solution densities and viscosities are measured. The investigated reactions belong to the fast reaction regime systems. It is found that the CO2 reaction with AEEA is of the second order with respect to AEEA within the temperature range 298–308 K and amine concentration range 1.5–3 kmol/m3 . At 303 K, the reaction rate constant equals 8530 m6 /(kmol2 s). The value of the second-order rate constant for the CO2 reaction with PZ is found to be 25,800 m3 /(kmol s) at 303 K. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Carbon dioxide (CO2 ) has to be removed and recovered from various gaseous streams, which are either to be recycled to chemical processes (e.g., synthesis gas), or discharged into the atmosphere (e.g., flue gas, refinery off-gas), or used as fuels (e.g., natural gas, coke oven gas). Absorption performed with chemical solvents (e.g., alkanolamines, carbonate–bicarbonate buffers and amino acid salts) is an effective CO2 separation technology extensively used in the gas processing industry. Among alkanolamines, the primary amine, monoethanolamine (MEA), the secondary amine, diethanolamine (DEA), and the tertiary amine, triethanolamine (TEA), are industrially important [1]. Because of the presence of one or more primary or secondary amino groups in their structure, solvents containing diamines have a good potential for CO2 capture. In this work, the CO2 reactions with two diamines, viz. N-(2-aminoethyl)ethanolamine (AEEA) and piperazine (PZ), are studied. The performance of AEEA, an unhindered diamine comprising a primary and a secondary amine group, is superior to that of conventional solvents. For example, Ma’mun et al. [2] recently found that the absorption capacity, CO2 reactivity and energy

Abbreviations: AEEA, N-(2-aminoethyl)ethanolamine; DEA, diethanolamine; MDEA, methyldiethanolamine; MEA, monoethanolamine; PE, 2-piperidineethanol; PZ, piperazine. ∗ Corresponding author. Tel.: +91 22 3361 2014; fax: +91 22 3361 1020. E-mail addresses: [email protected], [email protected] (P.D. Vaidya). 1385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2011.02.074

efficiency of AEEA are higher than those of MEA. They also presented the physical and chemical reaction equilibria for the CO2 –AEEA–H2 O system [3]. In another study, Bonenfant et al. [4] reported a high CO2 loading value (1.35 mol of CO2 /mol of amine) of an aqueous 5 wt% AEEA solution at 296 K and atmospheric pressure. Moreover, they found that blends comprising AEEA and MDEA are attractive for the enhancement in CO2 capture, too [5]. At present, there is just scarce information available in the literature on the reaction kinetics of the CO2 –AEEA–H2 O system. Bouhamra and Alper [6] found that at 298 K the reaction is of the first order with respect to AEEA at low concentrations (0.015–0.05 kmol/m3 ). More recently, Ma’mun et al. [7] reported kinetic data at high concentrations (1.19–3.46 kmol/m3 ) and suggested that the reaction order with respect to amine is between one and two. Thus, it appears that the reaction order with respect to amine increases with rising amine concentration. It is obvious that this reactive system is complex and a detailed knowledge of the reaction mechanism and kinetics is essential. The cyclic diamine, PZ, which comprises two secondary amine groups in its structure, has high reactivity with CO2 . It is used as an efficient activator within the activated MDEA technology used by BASF SE. It is worthy of note that PZ is more effective than other absorption activators, e.g., MEA and DEA [8]. While the reaction between CO2 and PZ in aqueous solutions has been extensively studied, the agreement between the reaction rate constants estimated in earlier works is poor. In our recent paper [9], we compiled literature data on the reaction between CO2 and aqueous PZ at identical conditions and showed that the reaction rate constant, k2 , measured by different techniques at 303 K

A.B. Bindwal et al. / Chemical Engineering Journal 169 (2011) 144–150

Nomenclature (AEEA)0 AmH (AmH) B (B) (CO2 ) DAmH DCO2 DN2 O E Ei HCO2 Ha (H2 O) kˆ k2 k−1 kAmH kˆ B kH2 O kG kL kmn kobs kOH− kPZ m n (OH− ) PCO2 (PZ) (PZ)0 r RCO2 R (NH)2 roverall t z

initial AEEA concentration (kmol/m3 ) alkanolamine alkanolamine concentration (kmol/m3 ) base assisting in zwitterion deprotonation concentration of base B in bulk liquid (kmol/m3 ) concentration of CO2 (kmol/m3 ) diffusivity of amine in liquid phase (m2 /s) diffusivity of CO2 in liquid phase (m2 /s) diffusivity of N2 O in liquid phase (m2 /s) enhancement factor due to chemical reaction enhancement factor for an instantaneous reaction Henry’s law constant, kmol/(m3 kPa) dimensionless number defined by Eq. (16) concentration of water (kmol/m3 ) rate constant in Eq. (10) forward reaction rate constant in Eq. (1) (m3 /(kmol s)) backward reaction rate constant in Eq. (1) (1/s) deprotonation constant for amine reaction rate constant in Eq. (3) forward reaction rate constant for the CO2 reaction with H2 O gas-side mass transfer coefficient (kmol/(m2 s atm)) liquid-side mass transfer coefficient (m/s) reaction rate constant for m,nth order reaction observed reaction rate constant (1/s) forward reaction rate constant for the CO2 reaction with OH− deprotonation constant for PZ reaction order with respect to CO2 reaction order with respect to amine hydroxyl ion concentration (kmol/m3 ) partial pressure of CO2 in bulk gas phase (kPa) piperazine concentration (kmol/m3 ) initial piperazine concentration (kmol/m3 ) rate of reaction specific rate of absorption of CO2 (kmol/(m2 s)) piperazine overall reaction rate in aqueous salt solutions (kmol/(m3 s)) time (s) stoichiometric coefficient

Greek symbols  density (kg/m3 )  viscosity (kg/(m s))

varied from 29185 to 60766 m3 /(kmol s). Furthermore, we studied CO2 absorption into activated N,N-diethylethanolamine (DEEA) solutions, viz. PZ + DEEA + H2 O system, and reported a k2 value of 24450 m3 /(kmol s). In this work, we investigated CO2 absorption kinetics in the PZ + H2 O system. 2. Theory In an aqueous amine solution, CO2 may simultaneously react with the amine, OH− and H2 O. In our recent papers, we presented comprehensive overviews on the kinetics of the CO2 reaction with amines [10,11]. The reaction kinetics can be described either by the two-step zwitterion mechanism (originally proposed by Caplow [12] and later reintroduced by Danckwerts [13]) or by the singlestep termolecular mechanism (originally proposed by Crooks and

145

Donnellan [14] and recently revisited by da Silva and Svendsen [15]). 2.1. Zwitterion mechanism There are a few comprehensive reviews on the zwitterion mechanism available [16,17]. Assuming this mechanism, the reaction between CO2 and the amine (here denoted as AmH) proceeds through the formation of a zwitterion as an intermediate: k ,k

2 −1 AmH+ COO− CO2 + AmH ←→

(1)

This zwitterion undergoes deprotonation by a base (or bases) B, thereby resulting in carbamate formation: kˆ B

AmH+ COO− + B−→AmCOO− + BH+

(2)

Applying the steady-state principle to the intermediate zwitterion in Eq. (1), the overall rate of reaction of CO2 in aqueous amine solutions can be expressed in general as: roverall =

k2 (CO2 )(AmH) 1 + (k−1 /kˆ B (B))

(3)

where the kinetic constant kˆ B (B) represents deprotonation of the zwitterion by any base, such as H2 O, OH− or AmH, or by a combination of bases. Eq. (3) does not account for the CO2 reactions with OH− and H2 O, whose contributions to the overall rate are assumed to be negligible. The reaction rate represented by Eq. (3) exhibits a fractional order between one and two with respect to the amine concentration. When deprotonation is almost instantaneous as compared to the reverse reaction in Eq. (1) (k−1  kˆ B (B)) and zwitterion formation is rate-determining, Eq. (3) takes the form: roverall = k2 (CO2 )(AmH)

(4)

thereby suggesting that the reaction is of the first order with respect to both CO2 and amine, and hence, overall of the second order. When zwitterion deprotonation is rate-determining (k−1  kˆ B (B)), Eq. (3) becomes: roverall =

k2 kˆ B (B) (CO2 )(AmH) k−1

(5)

Similar to Eq. (3), the latter expression suggests a fractional reaction order between one and two with respect to the amine concentration. In the limiting case when the contribution of AmH to zwitterion deprotonation is much more significant than that of other bases, such as H2 O and OH− , the overall reaction is of the second order with respect to AmH. 2.2. Termolecular mechanism The termolecular mechanism assumes that the amine reacts simultaneously with one molecule of CO2 and one molecule of a base. The reaction proceeds in a single step via a loosely bound encounter complex as the intermediate (rather than a zwitterion). This can be represented as: CO2 + AmH· · ·B ↔ AmCOO− · · ·BH+

(6)

This complex breaks up to form reactant molecules (CO2 and amine), while its small fraction reacts with a second molecule of the amine or a water molecule to give ionic products (carbamates). The forward reaction rate for this mechanism, for the case where H2 O, OH− and AmH are the dominating bases, is given by: r = [kH2 O (H2 O) + kOH− (OH− ) + kAmH (AmH)](AmH)(CO2 )

(7)

or r = kobs (CO2 )

(8)

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where kobs is expressed by

3.3. Experimental procedure −

kobs = [kH2 O (H2 O) + kOH− (OH ) + kAmH (AmH)](AmH)

(9)

Eq. (7) suggests that H2 O, OH− , AmH, and other bases, if any, can influence the reaction in parallel. Its form is similar to that of the limiting case of the zwitterion mechanism represented by Eq. (5), and it can describe fractional and higher-order kinetics, too [15]. In aqueous solutions, deprotonation proceeds mainly via water and the alkanolamine [18]. When the solvent (water) is the dominant base, the reaction is of the first order with respect to the amine, and the rate is given by ˆ r = kH2 O (H2 O)(CO2 )(AmH) = k(CO 2 )(AmH)

(10)

where kˆ = kH2 O (H2 O). When AmH is the most dominant base, the reaction is of the second order with respect to the amine and the rate is given by r = kAmH (CO2 )(AmH)2

(11)

For the intermediate case when the contribution of water is comparable to that of AmH, Eq. (9) can be rewritten as follows: kobs = [kH2 O (H2 O) + kAmH (AmH)](AmH)

(12)

It is clear from Eq. (9) that the number of fitting parameters in the termolecular mechanism is fewer than that in the zwitterion mechanism. Ma’mun et al. [7] suggested that the termolecular mechanism is best-suited to describe the nature of reactions occurring in the CO2 –AEEA–H2 O system. 3. Experimental 3.1. Materials 2-((2-Aminoethyl)amino)-ethanol (purity 98%) and piperazine (purity 98%) used in experiments were purchased from S. D. Fine Chemicals Ltd., Mumbai. Carbon dioxide, nitrous oxide and nitrogen cylinders, with a given purity of 99.95%, were purchased from Inox Air Products Ltd., Mumbai. 3.2. Experimental setup A glass stirred-cell reactor with a plane, horizontal gas–liquid interface was used for the absorption studies (see Vaidya and Mahajani [19]). The main advantage of the stirred cell is that the rates of absorption can be measured using a liquid with a single, known composition. This easy-to-use experimental device (inner diameter 97 mm, height 187 mm) was operated batchwise. The total volume of the reactor was 1.45 dm3 and the interfacial surface area was 7.5 × 10−3 m2 . The reactor was equipped with a flange made of stainless steel (Sharad Autoclave Engineers, Mumbai). A pressure transducer (Trans Instruments, UK, 0–1 bar), mounted on this flange and coupled with a data acquisition system, enabled measurement of the total pressure inside the reactor, the uncertainty in this measurement being ±1 mbar. The reactor was also equipped with inlet and outlet ports for the gas and liquid phases. The entire assembly was proven to have no leak. The setup was supplied by a variable speed magnetic drive (Premex Instruments, Switzerland). The gas and liquid were stirred by two impellers, mounted on the same shaft. The speed of stirring could be adjusted to the desired value with an accuracy of ±1 rpm. The impeller speed during kinetic measurements was limited to 90 rpm, in order to ensure that the gas–liquid interface was undisturbed. The reactor was immersed in a water bath to guarantee isothermal conditions. The temperature was adjusted to the desired value with an accuracy of ±0.1 ◦ C. The solute gas passed through a coil, also kept in the water bath, before being charged inside the reactor.

A series of experiments were conducted over a wide range of temperatures and amine concentrations. In each experiment, the reactor was charged with 0.4 dm3 of the fresh amine solution. The gas inside the reactor was then purged with nitrogen to ensure an inert atmosphere. Thereafter, nitrogen was released through the gas outlet port. All the lines were closed and the reactor content attained the desired temperature. CO2 from the gas cylinder was then charged inside the reactor, this was considered to be the starting point for the reaction. The reactor content was stirred at the desired speed of agitation. The decrease in system pressure due to reaction was monitored by the pressure transducer and the “PCO2 vs. t” data were recorded during 30 s using the data acquisition system. These data were plotted for the time interval between t = 5 s and t = 25 s and fitted to a third degree polynomial using the least-square regression. The absorption rates were calculated from the values of the slope −dPCO2 /dt. This measurement method based on the fall-in-pressure technique enabled a simple and straightforward estimation of the absorption rates. Furthermore, no analysis of the liquid phase was required and the pressure decrease was the only factor necessary for the evaluation of the kinetic parameters. We found that, in the range of agitation speeds studied, the mass transfer rate is independent of the gas-side mass transfer coefficient, kG . Therefore, we concluded that the CO2 absorption process was liquid-phase-controlled. The reproducibility of experiments was checked and the error in all experimental measurements was found to be less than 3%. 4. Results and discussion 4.1. Estimation of physical properties Knowledge of physical properties is essential for the estimation of kinetic parameters. We measured the density () and viscosity () of aqueous AEEA and PZ solutions at 298, 303 and 308 K and these values are represented in the Supplementary material. The diffusion coefficients of N2 O and CO2 in water, viz. 2.03 × 10−9 and 2.15 × 10−9 m2 /s at 303 K, were earlier reported by Versteeg and van Swaaij [20]. From our viscosity measurements, we estimated the values of the N2 O diffusivity in the aqueous amine solutions by using the modified Stokes–Einstein correlation (DN2 O 0.80 )Amine = const = (DN2 O 0.80 )Water

(13)

We measured the solubility of N2 O in aqueous AEEA and PZ solutions, too (see Supplementary material). The values of DCO2 and HCO2 in solutions were found using the N2 O analogy [20]. The physical absorption of CO2 in water at 303 K was studied and it was found that HCO2 (2.75 × 10−4 kmol/(m3 kPa)) agrees well with the published value [20]. Littel et al. [21] earlier outlined a procedure for the estimation of the liquid-side mass transfer coefficient, kL , in a stirred cell reactor. We used this technique and found that the value of kL (0.0037 cm/s) is in line with those typical for stirred cell reactors. 4.2. Study of reaction kinetics When CO2 concentration in the bulk liquid is negligible and the resistance to mass transfer is entirely in the liquid phase, it can be shown, based on the two-film theory of mass transfer [22], that the following relation holds RCO2 = kL (CO2 )E

(14)

where the enhancement factor E is used to describe the enhancing effect of chemical reactions on mass transport. To study the reaction

A.B. Bindwal et al. / Chemical Engineering Journal 169 (2011) 144–150

kinetics, it is essential that the system belongs to the fast reaction regime, where E equals the Hatta number [22,23]. The necessary conditions for the fast reaction regime are: 10 < Ha  (Ei − 1)

(15)

Table 1 CO2 absorption rates into aqueous AEEA solutions at 298, 303 and 308 K. Temperature (K)

(AEEA)0 (kmol/m3 )

PCO2 (kPa)

RCO2 × 106 (kmol/(m2 s))

298

1.5 2.0 2.5 3.0 1.5 1.5 1.5 1.5 2.0 2.0 2.0 2.0 2.5 2.5 2.5 2.5 3.0 3.0 3.0 3.0 1.5 2.0 2.5 3.0

7.3 5.7 5.9 5.9 6.1 10.7 17.5 25.0 5.9 12.0 16.5 23.1 6.2 10.8 17.7 22.6 6.5 13.1 15.6 22.7 6.9 6.0 6.3 6.5

8.67 9.21 10.33 10.71 9.25 15.38 18.22 21.23 10.62 18.37 20.27 24.14 11.92 20.06 24.00 27.28 13.06 23.10 25.74 31.17 11.79 12.14 13.35 14.92

For an irreversible reaction of mth-order with respect to CO2 and nth-order with respect to the amine, the Hatta number is given by



Ha =

303

(2/m + 1)DCO2 km,n (CO2 )m−1 (AmH)n

(16)

kL

where km,n denotes the reaction rate constant. The enhancement factor for an instantaneous reaction is given by:



Ei = 1 +

(AmH) DAmH z(CO2 ) DCO2



(17)

It is worthy of note that Eq. (17) is valid only if the film theory is used. Using Eqs. (14) and (16), the rate of absorption in aqueous amine solutions can be expressed as:



RCO2 =

2 DCO2 km,n (CO2 )m+1 (AmH)n m+1

(18)

There is unanimous agreement among all researchers that the reaction order with respect to CO2 is one. Therefore, Eq. (18) can be rewritten as



RCO2 = (CO2 )

DCO2 k1,n (AmH)n

(19)

where (CO2 ) = HCO2 PCO2 . Eq. (19) can be expressed in the following form:



log



RCO2



(CO2 )

DCO2

=

1 2

n

log(k1,n )

+

2

log(AmH)

(20)

If the variation in RCO2 with (AmH) is studied, a plot of



log{RCO2 /[(CO2 ) DCO2 ]} vs. log(AmH) enables the estimation of the value of k1,n and n. Shen et al. [24] used such plots to investigate the kinetics of the reaction between CO2 and 2-piperidineethanol (PE) in aqueous solutions. 4.3. CO2 –AEEA–H2 O system The chemical behavior of this system is complex and several ionic species are formed during the reaction. For example, Ma’mun et al. [3] considered the possible formation of primary and secondary carbamates and dicarbamates in aqueous AEEA solutions. In a recent study, they found that the primary carbamate is the major product in the solution [7]. This is due to the fact that the primary amine group in AEEA reacts faster than the secondary amine group. We studied the CO2 reaction with AEEA over the range of temperatures, 298–308 K, and amine concentrations, 1.5–3 M. The CO2 absorption rates into aqueous AEEA solutions at various temperatures are represented in Table 1. In the fast reaction regime, the rate of absorption is independent of the liquid-side mass transfer coefficient and hence it should not depend on the agitation speed. We studied this effect experimentally and found practically no change in the absorption rate, while varying the stirring speed in the range 50–90 rpm at 308 K. Hence, it can be concluded that the CO2 –AEEA–H2 O system belongs to the fast reaction regime systems. All further experiments  were conducted at a speed of 90 rpm. A plot of log{RCO2 /[(CO2 ) DCO2 ]} vs. log(AEEA)0 at 303 K is shown in Fig. 1. The slope equals unity, thereby suggesting that the reaction order with respect to AEEA (n) is two. Thus, the reaction is overall of the third order. The second-order dependence at high values of the amine concentration is similar to the behavior of aqueous DEA [25,26]. The value of the rate constant was found to

147

308

be 8530 m6 /(kmol2 s). Its dependence on temperature was studied, and the activation energy was found to be 34 kJ/mol. Using the values of DCO2 and HCO2 (see Table 2), the parameters Ha and Ei were estimated. The conditions given by Eq. (15) were satisfied, thereby confirming that this system belongs to the fast reaction regime. A comparison of the rate constant with other values earlier reported in the literature is given in Table 3. The values of kobs (= k1,n (AEEA)2 ) at 303 K were measured and compared with those reported by Ma’mun [27] (see Fig. 2). It is probable that such reactions can be represented equally well by the zwitterion and the termolecular mechanisms, whereas the kinetic data can be fitted to either of those. If the two-step zwitterion mechanism were to be the case, zwitterion deprotonation would be rate-determining and contribution of AEEA to the deprotonation step significant. If the termolecular model were appropriate, it would be obvious that AEEA is the most dominant base (cf. Eq. (11)). Eq. (12) was processed by plotting [kobs /(AmH)] vs. (AmH) and this plot yielded a satisfactory relation-



Fig. 1. A plot of log{RCO2 /[(CO2 )

DCO2 ]} vs. log(AEEA)0 at 303 K.

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A.B. Bindwal et al. / Chemical Engineering Journal 169 (2011) 144–150

Table 2 Equilibrium and kinetic characteristics of the CO2 –AEEA–H2 O system at 303 K. (AEEA)0 (kmol/m3 )

RCO2 × 106 (kmol/(m2 s))

HCO2 × 104 (kmol/(m3 kPa))

DCO2 × 1010 (m2 /s)

k2 (m6 /(kmol2 s))

Ha

Ei

1.5 2.0 2.5 3.0

9.25 10.62 11.92 13.06

3.07 2.93 2.78 2.69

12.55 10.44 8.59 6.80

8640 9038 8909 9116

173 237 294 357

801 1157 1450 1716

Table 3 Reaction rate constant for the CO2 –AEEA–H2 O system at 303 K. Temperature (K)

(AEEA)0 (kmol/m3 )

Rate constant

Reaction system

Experimental technique

Reference

298 298 303

0.015–0.05 1.2–3.5 1.5–3.0

3717 12,100 8530

CO2 –AEEA–H2 O CO2 –AEEA–H2 O CO2 –AEEA–H2 O

Stopped-flow Disk contactor Stirred-cell

Bouhamra and Alper [6] Ma’mun et al. [7] This work

10

ship (see Fig. 3). We used the initial amine concentration values for this plot. From the slope, the value of kAmH was found to be 9050 m6 /(kmol2 s).

Present work Ref. 27

k obs x 10

-4

(1/s)

8

4.4. CO2 –PZ reaction mechanism PZ reacts with CO2 rapidly to form carbamates [8]. At low CO2 loading, the monocarbamate of PZ is the major product. The reaction that describes dicarbamate formation in an aqueous solution containing PZ (here denoted by R (NH)2 ) can be represented as

6

4

R (NH)2 + 2CO2 ↔ R (NHCOO)2

(21)

Besides, the following reaction may occur: 2

R (NHCOO)2 + 2H2 O → R (NH2 )2 + + 2HCO3 −

0 0

1

2

3

4

(AEEA) 0 (kmol/m 3 ) Fig. 2. A plot of kobs vs. (AEEA)0 at 303 K and a comparison with the kobs values reported by Ma’mun [27].

(22)

The zwitterion mechanism is commonly used to describe reaction kinetics in aqueous PZ solutions. When the contributions of the CO2 reactions with OH− and H2 O to the overall rate are negligible, and PZ, H2 O and OH− are the dominant bases that contribute to zwitterion deprotonation, the overall rate of all CO2 reactions is given by: roverall =

k2 (CO2 )(PZ) 1 + (k−1 /[kPZ (PZ) + kOH− (OH− ) + kH2 O (H2 O)])

where (PZ) denotes the amine concentration. When zwitterion formation is rate-determining, Eq. (23) takes the form:

4

roverall = k2 (CO2 )(PZ)

k obs x 10 -4 /(AEEA) 0 (m 3 /(kmol s))

(23)

(24)

which suggests that the reaction is of the first order with respect to both CO2 and PZ, and hence, overall of the second order.

3

Table 4 CO2 absorption rates in aqueous PZ solutions at 298, 303 and 308 K.

2

Temperature (K)

(PZ) (kmol/m3 )

PCO2 (kPa)

RCO2 × 106 (kmol/(m2 s))

298

0.025 0.050 0.075 0.1 0.025 0.050 0.075 0.1 0.1 0.1 0.1 0.025 0.050 0.075 0.1

2.1 2.4 2.9 2.7 2.7 2.4 2.7 2.8 5.6 7.5 9.2 2.9 2.9 2.4 2.5

0.60 1.01 1.41 1.70 0.85 1.08 1.54 1.88 2.56 3.10 3.34 0.98 1.40 1.58 1.94

1 303

0 0

1

2

(AEEA) 0 (kmol/m 3 )

3

Fig. 3. A plot of [kobs /(AEEA)0 ] vs. (AEEA)0 at 303 K.

4 308

A.B. Bindwal et al. / Chemical Engineering Journal 169 (2011) 144–150

149

Table 5 Equilibrium and kinetic characteristics of the CO2 –PZ–H2 O system at 303 K. (PZ) (kmol/m3 )

HCO2 × 104 (kmol/(m3 kPa))

DCO2 × 1010 (m2 /s)

k2 (m3 /(kmol s))

Ha

Ei

0.025 0.05 0.075 0.1

2.96 2.85 2.76 2.64

21.23 21.08 20.96 20.82

21311 23643 27149 31061

29 43 56 69

63 146 201 271

4

5. Conclusions The kinetics of the CO2 reactions with two diamines, N-(2aminoethyl)ethanolamine and piperazine, was investigated using a stirred-cell reactor and the fall-in-pressure technique. The possible reaction mechanisms (e.g., zwitterion and termolecular) were described in detail. The CO2 solubility and diffusivity in solutions were estimated using the N2 O analogy. The reaction with N-(2aminoethyl)ethanolamine is of the second order with respect to the diamine, and hence, overall of the third order. At 303 K, the reaction rate constant equals 8530 m6 /(kmol2 s). The value of the secondorder rate constant for the CO2 reaction with PZ was found to be 25,800 m3 /(kmol s) at 303 K. Finally, it was found that the value of kL (0.0037 cm/s) estimated in this work is in line with those typical for stirred cell reactors.

k obs x 10 -3 (1/s)

3

2

1

0 0

0.04

0.08

0.12

Acknowledgement

(PZ) 0 (kmol/m 3 )

Ankush B. Bindwal is grateful to University Grants Commission, New Delhi, for the financial assistance.

Fig. 4. A plot of kobs vs. (PZ)0 at 303 K.

Appendix A. Supplementary data In this work, the CO2 reaction with PZ was studied over the ranges of temperatures, 298–308 K, and amine concentrations, 0.025–0.1 M. The CO2 absorption rates in aqueous PZ solutions are represented in Table 4. The values of density, viscosity, DN2 O and HN2 O for aqueous PZ solutions are given in the Supplementary material. We found that the CO2 –PZ–H2 O system belongs to the fast pseudo-first order reaction systems. Therefore, the specific rate of absorption was expressed as:



RCO2 = (CO2 )

DCO2 k2 (PZ)0

(25)

Using the values of DCO2 and HCO2 (see Table 5), k2 was measured. The average value of the rate constant was found to be 25,800 m3 /(kmol s), which is in good agreement with that reported by Konduru et al. [9] (24,450 m3 /(kmol s)). Since CO2 loading in solution is low, it is expected that its dicarbamate concentration is negligible; thus, the rate constant describes monocarbamate formation only. CO2 capture using significantly more concentrated PZ solutions was earlier considered, e.g., in [28–30]; as expected, the rate constants were much higher. For example, Bishnoi and Rochelle [28] reported a value of 53,700 m3 /(kmol s) at 298 K using high amine concentrations (0.2–0.6 M). However, we excluded absorption measurements at high PZ concentrations from the scope of this work. We further studied the rate constant dependence on temperature and found the activation energy to be 34.6 kJ/mol. A plot of kobs vs. (PZ)0 at 303 K is shown in Fig. 4. If the termolecular model were appropriate, it would be obvious that H2 O represents the most dominant base (cf. Eq. (10)). However, it has recently been suggested that the contribution of H2 O to carbamate formation in the CO2 –PZ–H2 O system is low [31]. Thus, in its present form, the termolecular mechanism fails to explain the reaction kinetics.

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