1-hydroxyethy-3-methyl imidazolium glycinate solution

Chemical Engineering Journal 280 (2015) 695–702

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Mass transfer and kinetics of CO2 absorption into aqueous monoethanolamine/1-hydroxyethy-3-methyl imidazolium glycinate solution Bihong Lv a,⇑, Cheng Sun b, Nan Liu b, Wei Li b, Sujing Li b,⇑ a

Department of Environmental Science & Engineering, College of Chemical Engineering, Huaqiao University, Xiamen, Fujian 361021, China Key Laboratory of Biomass Chemical Engineering of Ministry of Education, Institute of Industrial Ecology and Environment, Department of Chemical and Biological Engineering, Zhejiang University (Yuquan Campus), Hangzhou 310027, China b

h i g h l i g h t s  Kinetics of the reaction of CO2 and MEA/[C2OHmim][Gly] was investigated.  k2,mix and E of this new system were all higher than those of MEA solution. 3

 k2,mix into MEA/[C2OHmim][Gly] solution was 6506.4 m kmol

a r t i c l e

i n f o

Article history: Received 23 April 2015 Received in revised form 30 May 2015 Accepted 1 June 2015 Available online 17 June 2015 Keywords: CO2 capture Monoethanolamine Amino acid ionic liquid Kinetic Mass transfer

1

s1 at 303.15 K.

a b s t r a c t The aqueous blend of monoethanolamine (MEA) and 1-hydroxyethy-3-methyl imidazolium glycinate ([C2OHmim][Gly]) was considered as a promising CO2-capturing solvent because of the advantages of fast absorption rate, high absorption capacity, great thermal stability and good resistance to O2. In the present work, kinetics of the reaction of CO2 and MEA/[C2OHmim][Gly] in aqueous solutions had been investigated using a double stirred-cell absorber with a defined gas/liquid interface. The zwitterion mechanism was used as the kinetic model to describe the absorption of CO2 in the mixed solution. The kinetic and mass transfer parameters, e.g., the reaction rate constant (k2,mix), the overall reaction rate constant (kov) and the enhancement factor (E), were evaluated at different absorbent concentrations and temperature. At 303.15 K, the values of k2 into MEA/[C2OHmim][Gly] and [C2OHmim][Gly] solution were respectively calculated as 6506.4 and 8980.6 m3 kmol1 s1. The relationship between k2 and the reaction temperature was determined by the Arrhenius Equation, and was presented as:

k2;mix ¼ 7:12  106 exp



 2126:3 T

. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction The concept of carbon dioxide (CO2) capture and sequestration (CCS) has attracted considerable interest over the past two decades, as a result of the emerging problem of climate change [1]. One of the most potent techniques widely used in capturing CO2 from low pressure flue gas streams in power plants is chemical absorption using aqueous amine-based absorbents [2]. ⇑ Corresponding authors. Tel.: +86 592 6166216; fax: +86 592 6162300 (B. Lv). Tel./fax: +86 571 87952513 (S. Li). E-mail addresses: [email protected] (B. Lv), [email protected] (S. Li). http://dx.doi.org/10.1016/j.cej.2015.06.004 1385-8947/Ó 2015 Elsevier B.V. All rights reserved.

Alkanolamines such as monoethanolamine (MEA), diethanolamine (DEA), N-methyldiethanolamine (MDEA) and 2-amino-2-methyl1-propanol (AMP) are the most commonly used absorbents in the past few years. The most advantage of MEA is high reactivity with CO2, but this process still suffers some disadvantages [3–6], e.g., low CO2 capacity (0.5 mol CO2/mol absorbent), amine degradation and oxidation and high energy consumption. Thus the mixed solvents of amines with various additives are developed to overcome these existing drawbacks. To date, ionic liquids (ILs) have received an advantage in various applications including in CO2 capture [7,8]. More interestingly, introducing some special groups to the anion or the cation of ILs

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B. Lv et al. / Chemical Engineering Journal 280 (2015) 695–702

Nomenclature CAbsorbent CB C CO2 Di,j E E1 G Hi, j Ha KG kG kL

concentration of the absorbent (kmol m3) concentration of base B in bulk liquid (kmol m3) concentration of CO2 (kmol m3) diffusivity of i in the solution of j (m2 s1) enhancement factor (–) the instantaneous enhancement factor (–) gas-phase 1 solubility of i in the solution of j (kPa m3 kmol ) Hatta number (–) overall gas phase mass transfer coefficient 1 (kmol m2 s1 kPa ) gas-phase mass transfer coefficient 1 (kmol m2 s1 kPa ) liquid-phase mass transfer coefficient (m s1)

to obtain functionalized ILs can achieve excellent performance in CO2 capture due to the following characteristics [9–14]: low volatility, low heat required for reaction, low thermal and oxidative degradation, high absorption capacity and easy to be regenerated. To achieve excellent performance, the complexities of amine and ILs or functionalized ILs are proposed as new solvents for CO2 capture [15–18]. It was found that CO2 solubilities in binary and ternary systems of guanidinium trifluoromethanesulfonate ([gua][OTf]) in MDEA and/or water were higher compared to other ILs, and decreased as the composition of [gua][OTf] in the systems increased [19]. Regeneration performance of [N1111][Gly] and MDEA aqueous solutions was performed, and It was found that the regeneration efficiency of 30 wt% MDEA + 15 wt% IL solution was a little higher than the other solution with different compositions [20]. In the other mixed solution, such as [N1111][Gly]/AMP system, the addition of [N1111][Gly] had greatly promoted the absorption of CO2 in AMP aqueous solution [21]. Besides, many works focus on kinetics analysis in order to investigate the mass transfer process. Our previous work found that [Bmim][BF4] had an active effect on the CO2 hydration and the values of enhancement factor (E) and the second-order reaction rate constant (k2;mix ) for CO2 absorption into MEA/[Bmim][BF4] solution were higher than that of MEA solution [22]. Kinetics of DEA dispersion in [Hmim][Tf2N] for efficient CO2 capture was investigated by Iliuta et al., and the kinetic model used to describe the chemical reaction was based on the zwitterion mechanism followed by the removal of a proton by the amine and formation of immiscible solid carbamate crystals in the ionic liquid phase [23]. The kinetics region of absorption CO2 into aqueous [N1111][Gly] + AMP solution was found to be the fast pseudo-first order reaction regime and the activation energy of CO2 capture into [N1111][Gly] + AMP aqueous solution found to be 40.7 kJ mol1 [21]. A summary of aqueous blend of amine and ILs or functionalized ILs for CO2 capture at low partial pressure were presented in Table 1, which indicated that the mixture seemed to be a suitable candidate for CO2 capture. In our previous work, a dual functionalized ILs ([C2OHmim] [Gly]) was designed and prepared based on the imidazolium ionic liquid with amino acid and hydroxyl group. Then the new hydrophilic amino acid ionic liquid was utilized to enhance the process of MEA for CO2 capture from low-pressure flue gas. The new blend of MEA and [C2OHmim][Gly] was proved to have an enhanced absorption capacity, a higher regeneration efficiency and a better resistance to O2 for the CO2 capture [26]. As a promising CO2-capturing solvent, although the absorption and desorption performance of CO2 capture into MEA/[C2OHmim][Gly] solution had been extensively studied, there is uncertainty about the mass

k2,

i

kov L n N PCO2 Qin Qout r R T

l z

second-order reaction rate constant in i solution (m3 kmol1 s1) overall reaction rate constant (s1) liquid-phase the stirring speed (rpm) absorption rate of CO2 (kmol m2 s1) partial pressure of CO2 (kPa) the inlet gas-flow rate (m3 s1) the outlet gas-flow rate (m3 s1) the reaction rate (kmol m2 s1) perfect gas constant (8.314 J mol1 K1) temperature (K) viscosity of solution (mPa s) stoichiometric coefficient (–)

transfer and the reaction kinetics of this new system. Herein, kinetics of the reaction of CO2 and MEA/[C2OHmim][Gly] in aqueous solutions was investigated using a double stirred-cell absorber with a defined gas/liquid interface. The kinetics model and parameters were measured in the presented work. 2. Materials and methods 2.1. Chemicals Monoethanolamine (MEA) with 99.0% purity was supplied by Shanghai Ling Feng Chemical Reagent Co. Ltd, China. The gas of CO2 (>99.99%), O2 (>99.99%) and N2 (>99.99%) supplied from the steel cylinder was purchased from Zhejiang Jin-gong Gas Co, China. The dual functionalized ionic liquid [C2OHmim][Gly] was prepared in our laboratory. No further purification was performed on the materials used. 2.2. Kinetic experiments The experiments were carried out in a double stirred-cell absorber [22], which had a defined gas/liquid interface as the mass transfer area and was convenient for kinetic investigation. The experimental setup is shown in Fig. 1. CO2 was mixed with N2 to simulate the flue gas. The total gas-flow rate was controlled at 1 L min1 by using a mass flow controller. The stirring speeds of the gas phase and liquid phase were kept at 250 rpm and 100 rpm, respectively. The absorption rate of CO2 into aqueous blend of MEA/[C2OHmim][Gly] were measured under different concentrations of initial mixed absorbent (1–3 kmol m3) and different temperatures (303.15–333.15 K), respectively. To establish pseudo-first order reaction conditions, the simulated gas and fresh aqueous solution were both continuously supplied to the absorber. The CO2 partial pressure of the gas phase remained unchanged under each condition during the experiments because of steady conditions. The inlet and outlet concentrations of CO2 were measured by using gas chromatography (GC-7890, Agilent, USA). All data shown in this paper were the mean values of duplicate or triplicate experiments. 3. Theoretical analysis 3.1. Reaction mechanisms The total reaction of CO2 absorption into aqueous blend of MEA/[C2OHmim][Gly] can be simplified as the sum of the reaction

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B. Lv et al. / Chemical Engineering Journal 280 (2015) 695–702 Table 1 Summary of aqueous blend for CO2 absorption at low partial pressure. Aqueous solution

mol CO2/mol absorbent

Experimental technique

k2 (L mol1 s1)

Reference

15 wt%[N1111][Gly] + 15 wt%MDEA 15% [N2222][Lys] + 15% MDEA 4 M MDEA + 1 M [gua][OTf] 0.95 M AMP + 0.3 M [N1111][Gly]

0.562 0.740 0.326 0.255

Absorption vessel Absorption vessel Double stirred cell reactor Double stirred-cell absorber

– – –

Zhang et al. [24] Zhang et al. [24] Sairi et al. [19] Zhou et al.[21]

4 M MDEA + 2 M [Bmim]BF4

–

Stirred cell reactor

Þ kam ¼ 3:55  102 expð1040:3 T

Ahmady et al.[25]

0.7 M MEA + 0.3 M [Bmim]BF4

0.638

Double stirred-cell absorber

k2;mix ¼ 2:98  107 expð2758:3 Þ T

Lv et al. [22]

Þ k2 ¼ 2:4589  109 expð4:8927 T

Assuming the reaction of CO2 with [C2OHmim][Gly] follows a second order kinetic, the reaction rate in CO2–[C2OHmim][Gly]– H2O system can be given as:

GC Gas outlet

rCO2 AAILs ¼

N2+CO2

1 k2

C AAILs C CO2 þ Pk2 k2

¼

k C i Bi Bi

k2;AAILs C AAILs C CO2 1 þ Pk2

ð6Þ

k C i Bi Bi

Since the value of kBiCBi is so large that Pkk2 C can be neglected [30], i Bi

Absorbent

Bi

Eq. (6) can be simplified as follows:

Circulating water Liquid outlet

rCO2 AAILs ¼ k2;AAILs C AAILs C CO2

ð7Þ

Meanwhile, many other reactions may also take place in the aqueous solution:

Fig. 1. Schematic diagram for CO2 capture.

H2 ¢ OHþ þ OH into CO2–MEA–H2O system and the reaction into CO2–[C2OHmim] [Gly]–H2O system. The chemical reaction of CO2 absorption into MEA solution was based on the formation of a zwitterion and follows by the removal of a proton by a base [27,28], Bi. The reaction of CO2–MEA–H2O system follows a second order kinetic and is described by the overall reaction:

ks

CO2 þ H2 O ¢ HCO3 þ Hþ kOH

CO2 þ OH ¢ HCO3

ð8Þ ð9Þ ð10Þ

Since the reaction of CO2–MEA–H2O system follows a second order kinetic, the overall forward reaction rate can be expressed as follows:

The reaction of Eq. (8) is rather instantaneous and is not the limiting step of the whole reaction. The reactions of Eqs. (9) and (10) can be neglected in lean absorbent [30]. The mechanism of the whole CO2 reaction into MEA + [Bmim]BF4 + H2O system is shown in Fig. 2. Therefore, the overall reaction rate of CO2 capture into aqueous blend of MEA/[C2OHmim][Gly] is given by the sum of the reaction rates expressed by Eqs. (2) and (7):

r CO2 MEA ¼ k2;MEA C MEA C CO2

roverall ¼ ðk2;MEA C MEA þ k2;AAIL C AAIL ÞC CO2 ¼ kov C CO2

k2;MEA

 CO2 þ 2R1 R2 NH !  R1 R2 NCOO þ R1 R2 NHþ2

ð1Þ

k1;MEA

ð2Þ

The chemical reaction into CO2–[C2OHmim][Gly]–H2O system is more complex than MEA solution. Based on our previous work and used for reference on the reaction of CO2 capture into other amino acid functionalized ionic liquids [24,29], the reaction into CO2–[C2OHmim][Gly]–H2O system can be concluded. In aqueous solution, [C2OHmim][Gly] could dissociate to [C2OHmim]+ and [H2NCH2COO]. The latter exhibits a similar reactivity to CO2 as the amino acid salt and it had been proved to follow the zwitterion mechanism. Therefore, the reaction into CO2–[C2OHmim][Gly]– H2O system can be expressed as follows (here [C2OHmim]+ was denoted as IL, then [C2OHmim][Gly] was represented as IL-CH2COONH2): k

2;AAILs þ  IL-CH2 COONH2 þ CO2 !  IL-CH2 COONH2 COO

ð3Þ

k2

The zwitterion is simultaneously neutralized by any base, Bi , such as [C2OHmim][Gly], OH, or H2O in solution.

IL-CH2 COONHþ2 COO

kBi  þ Bi !  IL  CH2 COONHCOO þ Bi Hþ kBi

ð4Þ



2IL  CH2 COONH2 þ CO2 !  IL  CH2 COONHCOO þ IL k1



CH2 COONHþ3

Here, kov represents the overall reaction rate constant and can be presented as:

kov ¼ k2;MEA C MEA þ k2;AAIL C AAIL

ð12Þ

3.2. Mass transfer The double stirred-cell absorber used in this work had a defined gas/liquid interface as the mass transfer area. According to the mass transfer theory and the film model approach [31], the CO2 mass transfer rate into MEA/[C2OHmim][Gly] solution can be expressed as:

N ¼ kG ðpCO2  pCO2 ;i Þ 0

N ¼ kL ðC CO2 ;i  C CO2 ;L Þ

ð13Þ ð14Þ

0

Here kL is the liquid-phase mass transfer coefficient with chemical reaction, and the enhancement factor (E) is given by: 0

In this AAILs solution, [C2OHmim][Gly] is the main Bi, the overall reaction into CO2–[C2OHmim][Gly]–H2O system can be simplified by the sum of Eqs. (3) and (4): k2;AAILs

ð11Þ

ð5Þ

E¼

kL

ð15Þ

0

kL

According to Eqs. (13) and (14), the concentration of CO2 on the interface can be calculated as:

C CO2 ;i ¼

  1 N  pCO2  HCO2 kG

ð16Þ

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B. Lv et al. / Chemical Engineering Journal 280 (2015) 695–702

As mentioned in Section 2.2, lean aqueous blend of MEA/[C2OHmim][Gly] is continuously supplied to the absorber and the fast reaction only takes place on the interface of the absorber. Therefore, CO2 concentration in the bulk liquid (PCO2 ;bulk ) is close to zero. Eq. (14) can be simplified as:

N¼H

pCO2

CO2 Ek0L

þ

ð17Þ

1 kG

The overall mass transfer coefficient (KG) can be written as follows:

HCO2 1 1 ¼ þ K G kG E  k0L In fact, the value of

ð18Þ

1 kG

is much smaller than that of

HCO

2

EkL

N¼

¼

0 EkL

, which had

PCO2 HCO2

3 < Ha << E1  1

0 kL

z  C amine Damine C CO2 DCO2

¼

PCO2

ð22Þ

HCO2 ;mix

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi DCO2 ;mix kov qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi DCO2 ;mix ðk2;MEA C MEA þ k2;AAILs C AAILs Þ

C AAILs C MEA

kov ¼ k2;mix C MEA

ð23Þ

ð24Þ ð25Þ

Then Eq. (23) can be rearranged as:

N¼

PCO2 HCO2 ;mix

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi DCO2 ;mix k2;mix C MEA

ð26Þ

Meanwhile, the CO2 absorption rate into MEA/[C2OHmim][Gly] solution can also be measured from the kinetic experiments.

N¼

DN PðQ in  Q out Þ ¼ AT ART

ð29Þ

DN2 O;water ¼ 5:07  106 exp

  2371 T

ð30Þ

DCO2 ;a min e 1l0:82

ð31Þ

ð32Þ

The viscosity ratio between MEA/[C2OHmim][Gly] and MEA solution at the same mole concentration of MEA was measured by using an Ubbelohde viscometer [22]. Meanwhile, on the use of reference correlation for the diffusivity of MDEA in MDEA solutions by Snijder et al. [32], the diffusivity of amine into the mixed solution can be calculated as follows:

lnðDamine Þ ¼ 13:088 

2360:7  24:727  105 C amine T

ð33Þ

Similarly by the N2O analogy, H of CO2 in MEA solution could be calculated as follows [34]:

As reported in our previous work [26], the optimum proportion of MEA/[C2OHmim][Gly] solution is 0.7 kmol m3 MEA and 0.3 kmol m3 [C2OHmim][Gly]. The chemical reaction of MEA-CO2 is still the main reaction in this new system. Here, k2,mix is defined as the reaction rate constant of CO2 capture into the mixed solution and can be calculated by

k2;mix ¼ k2;MEA þ k2;AAILs

DCO2 ;water

In CO2–MEA–[C2OHmim][Gly] system, the mixed solution owns a higher viscosity than that of the MEA solution. The value of DCO2 ;mixed can be obtained by the following empirical formula [34]:

Base on Eqs. (12), (19) and (21), the CO2 absorption rate into MEA/[C2OHmim][Gly] solution can be measured as follows:

PCO2 HCO2 ;mix

ð28Þ

ð21Þ

3.3. Kinetics

N¼

 DCO2 ;water DN2 O;water   2119 ¼ 2:35  106 exp T

DCO2 ;MEA ¼ DN2 O;MEA

ð20Þ

And E1 can be measured based on the penetration theory:

E1 ¼ 1 þ

The diffusion coefficient (D) and solubility (H) of CO2 in amine solution have been estimated or measured in other existing researches. These parameters of the new system are not easy to measure directly and are frequently estimated by the N2O analogy. The values of D and H of CO2 in MEA solution had been estimated in our previous work. D of CO2 in MEA solution could be calculated as follows [33–35]:

DN2 O;MEA ¼ ð5:07  106 þ 8:65  107 CMEA þ 2:78   2371  93:4C MEA  107 C2MEA Þ  exp T

Here Ha is the Hatta number and can be obtained by

Ha ¼ E ¼

4.1. Physicochemical properties

ð19Þ

The chemical reaction of CO2 capture into MEA/[C2OHmim][Gly] solution is assumed to follow the fast pseudo-first-order regime [31], which requires that:

pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi DCO2 kov

4. Results and Discussion



been estimated in our previous work [22]. Thus, Eq. (17) can be rearranged as: 0 EkL C CO2 ;i

where P is the pressure in the gas phase (1:01  105 Pa) and Q is the gas-flow rate of CO2.

ð27Þ

 HCO2 ;water HN2 O;water   2044 ¼ 2:8249  106 exp T

HCO2 ;MEA ¼ HN2 O;MEA HCO2 ;water



HN2 O;water ¼ 8:5470  106 exp

  2284 T

HN2 O;MEA ¼ ð5:52 þ 0:7CÞ  106 exp

  2166 T

ð34Þ

ð35Þ

ð36Þ

ð37Þ

Use for reference on solubility calculation of the [N1111][Gly] + AMP solution [21] and our measurement, the value of HCO2 ;mixed in MEA/[C2OHmim][Gly] solution could be given as:

HCO2 ;mix ¼ 509:77C AAIL þ HCO2 ;MEA

ð38Þ

The physicochemical properties of MEA/[C2OHmim][Gly] and MEA solution are given in Table 2. The value of DCO2 ;mixed was decreased as the mole concentration of [C2OHmim][Gly] increased in the mixed solution because of the increasing viscosity. The value of DCO2 ;mixed was increased as the mole concentration of [C2OHmim] [Gly] increased.

699

B. Lv et al. / Chemical Engineering Journal 280 (2015) 695–702 Table 2 Physicochemical properties of CO2 in MEA + [C2OHmim][Gly] + H2O. TK

303.15

313.15 323.15 333.15

CMEA kmol  m3

CAAILs kmol  m3

lmix/ lMEA

DCO2,mix 109 m2 s1

HCO2,mix

1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.7 0.7 0.7

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.3 0.3 0.3

1.00 1.05 1.08 1.11 1.14 1.18 1.21 1.09 1.08 1.08

1.95 1.89 1.86 1.84 1.81 1.78 1.76 2.35 2.92 3.58

3579.2 3589.9 3600.6 3611.2 3621.9 3632.6 3643.3 4389.1 5277.2 6281.1

1

CO2

CO2

Gas phase Liquid phase

kPa m3 kmol

Carbamate

[C2OHmim][Gly]

MEA

[Gly][Gly]H [C2OHmim]+

[MEA]H

[CH2COONHCOO-]-

Fig. 2. Mechanism of CO2 capture into MEA + [C2OHmim][Gly] + H2O solution.

4.2. Reaction kinetics 0

The liquid-phase mass transfer coefficient (kL ) of CO2 in water was measured by using the plotting method, and the gas-phase mass transfer coefficient (kG) of CO2 was also estimated by the 0

SO2 analogy in our previous work. The values of kL and kG were cal1

culated to be 4.19  105 m s1 and 8:99  106 kmol m2 s1 kPa when the stirring speeds of gas and liquid phase were respectively adjusted at 250 rpm and 100 rpm. Kinetic data for CO2 adsorption into aqueous blend of MEA/[C2OHmim][Gly] + H2O at 303.15 K was given in Table 3. The value of k2,mix and kov in MEA/[C2OHmim] [Gly] solution at 303.15 K is presented in Fig. 3. The total concentration of the mixed absorbent was kept at 1 kmol m3. As mentioned above, the viscosity of the mixed solution was higher than MEA solution and was unfavourable for the mass transfer. But on the other hand, the absorption rate (N), the reaction rate constant (k2,mix) and the overall reaction rate constant (kov) of CO2 into mixed solution were all greatly enhanced as the concentration of [C2OHmim][Gly] increased. The results indicated that [C2OHmim][Gly] would act as an activator in the mixed solution and enhance the absorption capacity of CO2 in the new system, which was similar with the reported results about other glycinate functionalization ILs. When the concentration of [C2OHmim][Gly] was higher than 0.3 kmol m3, the values of N, kov and the enhancement factor (E) was not significantly increased with the increasing of [C2OHmim][Gly] concentration. The results indicated that the activation effect was tended to be stable when the concentration above 0.3 kmol m3. These results agreed with our previous work [26], the optimum mole ratio of the mixed solution was choose as 0.7 kmol m3 MEA and 0.3 kmol m3 [C2OHmim][Gly]. The value of E1 is often between 1000 and 1600, which is much higher than Ha in this system. These results proved the hypothesis of CO2 capture into MEA/[C2OHmim][Gly] solution followed a fast pseudo-first-order regime assumed in Section 3.2. Kinetic data for CO2 absorption in MEA/[C2OHmim][Gly] solution at different absorbent concentration and temperature is shown in Table 4. At the same proportion of the mixed solution, the values of N, kov and E were all increased as the concentration and temperature increased. As analysed above, k2,mix is related to

the specific chemical reaction, and its value basically unchanges at a certain proportion of the mixed solution. The average value of k2,mix into MEA/[C2OHmim][Gly] solution (mole ratio, 7:3) at 303.15 K was 6506.4 m3 kmol1 s1, and that of k2;MEA was 1936.7 m3 kmol1 s1. From Eq. (24), the value of k2;AAILs could be calculated as 8980.6 m3 kmol1 s1. Based on the Eqs. (25) and (25), it could be found that the value of E was effect by the composition of the solvent. Thus, E of different MEA/[C2OHmim][Gly] solutions were compared in Fig. 4. The results presented that the value of E increased as the concentration of MEA increased. For a given mole ratio of MEA/[C2OHmim][Gly] solution, the relationship between E and MEA concentration could be calculated as follows (R2 = 0.9997).

pffiffiffiffiffiffiffiffiffiffiffi E ¼ 58:204 C MEA þ 20:463

ð39Þ

Beside, compared with the value of E into MEA solution investigated in our previous work, E into MEA/[C2OHmim][Gly] solution were all higher than that of MEA solution under the same MEA concentration. The enhancement was attributed to the chemical reaction of CO2 capture by [C2OHmim][Gly]. As shown in Fig. 5, E into MEA/[C2OHmim][Gly] solution was compared with that of MEA solution at different temperature. The values of E into these two solutions were both increased as the temperature increased. The reaction of Eq. (3) is an exothermic process, and higher temperature is unfavorable to the reaction but still accelerated the reaction rate. As mentioned in Section 2.2, the fresh aqueous solution was continuously supplied to the absorber, and the unfavourable effect of temperature would be waning. The value of E into MEA/[C2OHmim][Gly] solution was higher than that of MEA solution at the same temperature. This result was different from the MEA/[Bmim][BF4] system, in which the value of E was in accord with MEA solution at high temperature [22]. The results indicated that the chemical reaction of CO2 capture into [C2OHmim][Gly] solution was also enhanced as temperature increased. The relationship between k2 and the reaction temperature can be determined by the Arrhenius Equation. The relationship

Table 3 Kinetic data for CO2 in MEA + [C2OHmim][Gly] + H2O at 303.15 K. P kPa

CMEA kmol m3

CAAILs kmol m3

N  106 kmol m2 s1

kov s1

k2,mix m3 kmol1 s1

Ha

E

11.40 11.30 11.23 11.16 11.15 11.15 11.18

1.0 0.9 0.8 0.7 0.6 0.5 0.4

0.0 0.1 0.2 0.3 0.4 0.5 0.6

8.38 ± 0.12 8.61 ± 0.08 8.77 ± 0.08 8.94 ± 0.10 8.96 ± 0.09 8.96 ± 0.12 8.89 ± 0.10

3549.2 3962.9 4252.1 4554.5 4671.2 4785.4 4760.9

3549.2 4403.3 5315.1 6506.4 7785.3 9570.8 11902.2

62.8 ± 1.3 65.3 ± 0.8 67.1 ± 0.8 69.0 ± 1.0 69.5 ± 0.9 69.7 ± 1.0 69.1 ± 1.1

62.8 ± 1.3 65.3 ± 0.8 67.1 ± 0.8 69.0 ± 1.0 69.5 ± 0.9 69.7 ± 1.0 69.1 ± 1.1

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B. Lv et al. / Chemical Engineering Journal 280 (2015) 695–702

Table 4 Kinetic data for CO2 absorption in MEA + [C2OHmim][Gly] + H2O at different temperature. TK

P kPa

CMEA kmol m3

CAAILs kmol m3

N  106 kmol m2 s1

DCO2,mix 109 m2 s1

HCO2,mix kPam3 kmol1

kov s1

k2,mix m3 kmol1 s1

E

303.15

11.16 10.72 10.56 11.00 10.81 10.59

0.7 1.4 2.1 0.7 0.7 0.7

0.3 0.6 0.9 0.3 0.3 0.3

8.94 ± 0.10 9.96 ± 0.08 10.33 ± 0.10 9.01 ± 0.12 9.15 ± 0.13 9.34 ± 0.12

1.84 1.60 1.42 2.35 2.92 3.58

3611.2 4046.1 4481.0 4389.1 5277.2 6281.1

4554.5 8859.4 13566.8 5512.7 6820.0 8569.1

6506.4 6328.1 6460.4 7875.3 9742.9 12241.5

69.0 ± 1.0 89.7 ± 0.9 104.6 ± 1.2 85.8 ± 1.3 106.6 ± 1.4 132.2 ± 1.3

313.15 323.15 333.15

14000

135 120

4600

k2,mix

10000

-1

kov/(s )

4400 8000

3

4200 4000

6000

3800

Enhancement factor

12000

-1

kov

-1

4800

k2,mix/( m kmol s )

5000

105 90 75 60

4000 3600

45 2000

3400 0.4

0.5

0.6

0.7

0.8

0.9

300

1.0

305

310

315

CMEA/(kmol m ) Fig. 3. k2,mix and kov in MEA + [C2OHmim][Gly] + H2O with different proportions at 303.15 K.

between k2;AAILs and the reaction temperature was estimated from Eq. (24). Arrhenius plots of CO2 capture into MEA/[C2OHmim][Gly] and [C2OHmim][Gly] solutions were shown in Fig. 6. Then k2,mix and k2;AAILs can be obtained as follows:

k2;mix ¼ 7:12  106 exp

  2126:3 T

ð40Þ

  929:2 T

ð41Þ

k2;AAILs ¼ 1:92  105 exp

320

325

330

335

T/K

-3

Then the value of k2,mix into MEA/[C2OHmim][Gly] solution estimated from Eq. (39) was compared with the experimental results (Fig. 7). The predicted results are identical with the experimental

Fig. 5. Comparison of enhancement factors at different temperature. j MEA + [C2OHmim][Gly] + H2O; MEA + H2O {CMEA:C[C2OHmim][Gly] = 7:3}.

results at different temperature. The model predication of k2,mix helped to provide approaches and methods for obtaining more information on the reaction of CO2 capture into MEA/[C2OHmim] [Gly] solution. There are two types of ILs frequently used to enhance the CO2 capture ability in aqueous blend of amine/ILs solution, one is the conventional ILs and the other is the functionalized ILs. The comparison of kinetic data for CO2 absorption into different aqueous blend of MEA and ILs is given in Table 5. Since CO2 dissolution in conventional ILs was a pure physical phenomenon with low adsorption rate, the enhancement of [Bmim][BF4] on CO2 capture is limited [22]. The values of k2,mix and E in MEA/[Bmim][BF4] solution was slightly higher than that of MEA solution [22]. The ILs used in this work was prepared with dual functionalized group

110

9.5 9.4

90

9.3 9.2

80

lnk2

Enhancement factor

100

70

9.1 9.0

k2,mix

8.9

k2,[C2OHmim][Gly]

60 8.8

0.6

0.8

1.0

1.2

1.4

1.6

CMEA

8.7 3.00

3.05

3.10

3.15

3.20

3.25

3.30

1000/T(1/K) Fig. 4. Enhancement factors into MEA + [C2OHmim][Gly] + H2O with function of MEA concentration. {In the first stage, CMEA + C[C2OHmim][Gly] = 1 kmol m3; in the second stage, CMEA:C[C2OHmim][Gly] = 7:3}.

Fig. 6. Arrhenius plot of CO2 absorption into MEA + [C2OHmim][Gly] + H2O and [C2OHmim][Gly] + H2O.

701

B. Lv et al. / Chemical Engineering Journal 280 (2015) 695–702 Table 5 Comparison of kinetic data for CO2 absorption into mixed solution {T = 303 K; C = 1 kmol m3; CMEA:C[IL] = 7:3}. k2,mix m3 kmol1 s1

Absorbent

k2,IL m3 kmol1 s1

E

Arrhenius Relationship k2;MEA ¼ 1:18  108 expð3214:3 Þ T

MEA

2657.6

–

46.0 ± 1.6

MEA/[Bmim][BF4]

3487.6

1962.3

61.6 ± 1.2

Þ k2;mix ¼ 2:98  107 expð2758:3 T

MEA/[C2OHmim][Gly]

6506.4

8980.6

69.0 ± 1.0

Þ k2;mix ¼ 7:12  106 expð2126:3 T

15000

model predication

12000

-1

-1

k2,mix/(m kmol s )

13500

3

10500 9000 7500 6000 4500 290

300

310

320

330

340

350

T/(K) Fig. 7. The predicted value of k2;mix at different temperature.

of hydroxyl and glycinate, and could greatly enhance the CO2 capture ability. The values of k2,mix and E in MEA/[C2OHmim][Gly] solution was much higher than that of MEA solution.

5. Conclusions An aqueous blend of MEA/[C2OHmim][Gly] was proposed for CO2 capture into a double stirred-cell absorber. The reaction mechanism of CO2 capture into this new system was simplified as the sum of the CO2–MEA and CO2–[C2OHmim][Gly] reaction referring to the Zwitterion mechanism. Some important kinetic parameters, such as k2,mix, kov and E were measured and estimated in this work. The values of k2,mix and E into MEA/[C2OHmim][Gly] solution were all higher than that of MEA solution. They were increased as the concentration of [C2OHmim][Gly] and temperature increased. Moreover, the relationship between k2,mix and temperature followed Arrhenius Equation was obtained. The results demonstrated that [C2OHmim][Gly] would act as an activator in the mixed solution and greatly enhance the absorption capacity of CO2 in the new system. Acknowledgments The work was sponsored by the National Natural Science Foundation of China (Nos. 21476203, 20976157), and supported by the Scientific Research Funds of Huaqiao University (15BS106). References [1] B. Metz, O.R. Davidson, P. Bosch, R. Dave, L.A. Meyer, IPCC climate change 2007: mitigation of climate change, in: Working Group III Contribution to the Fourth Assessment Report of the IPCC, Cambridge University Press, Cambridge, New York, 2007, p. 97. [2] M.E. Boot-Handford, J.C. Abanades, E.J. Anthony, M.J. Blunt, S. Brandani, N. Mac Dowell, J.R. Fernandez, M.C. Ferrari, R. Gross, J.P. Hallet, R.S. Haszeldine, P. Heptonstall, A. Lyngfelt, Z. Makuch, E. Mangano, R.T.J. Porter, M. Pourkashanian, G.T. Rochelle, N. Shah, J.G. Yao, P.S. Fennel, Carbon capture and storage update, Energy Environ. Sci. 7 (2014) 130–189.

[3] M.Y. Le, M. Kanniche, Screening of flowsheet modifications for an efficient monoethanolamine (MEA) based post-combustion CO2 capture, Int. J. Greenhouse Gas Control 5 (2011) 727–740. [4] A.K. Voice, T. Gary, G.T. Rochelle, Products and process variables in oxidation of monoethanolamine for CO2 capture, Int. J. Greenhouse Gas Control 12 (2013) 472–477. [5] Y.S. Choi, J. Im, J.K. Jeong, S.Y. Hong, H.G. Jang, M. Cheong, J.S. Lee, H.S. Kim, CO2 absorption and desorption in an aqueous solution of heavily hindered alkanolamine: structural elucidation of CO2-containing species, Environ. Sci. Technol. 48 (2014) 4163–4170. [6] P. Jackson, M. Attalla, Environmental impacts of post-combustion capture-new insights, Energy Proc. 4 (2010) 2277–2284. [7] L.H. Yang, H.M. Wang, Recent advances in carbon dioxide capture, fixation, and activation by using N-heterocyclic carbenes, Chem. Sus. Chem. 7 (2014) 962– 998. [8] S.J. Zhang, J. Sun, X.C. Zhang, J.Y. Xin, Q.Q. Miao, J.J. Wang, Ionic liquid-based green processes for energy production, Chem. Soc. Rev. 43 (2014) 7838–7869. [9] X.Y. Luo, F. Ding, W.J. Lin, Y.Q. Qi, H.R. Li, C.M. Wang, Efficient and energysaving CO2 capture through the entropic effect induced by the intermolecular hydrogen bonding in anion-functionalized ionic liquids, J. Phys. Chem. Lett. 5 (2014) 381–386. [10] B.S. Guo, G.H. Jing, Z.M. Zhou, Regeneration performance and absorption/ desorption mechanism of tetramethylammonium glycinate aqueous solution for carbon dioxidecapture, Int. J. Greenhouse Gas Control 34 (2015) 31–38. [11] H. Hu, F. Li, Q. Xia, X.D. Li, L. Liao, M.H. Fan, Research on influencing factors and mechanism of CO2 absorption by poly-amino-based ionic liquids, Int. J. Greenhouse Gas Control 31 (2014) 33–40. [12] S. Saravanamurugan, A.J. Kunov-Kruse, R. Fehrmann, A. Riisager, Aminefunctionalized amino acid-based ionic liquids as efficient and high-capacity absorbents for CO2, Chem. Sus. Chem. 7 (2014) 897–902. [13] Z.M. Xue, Z.F. Zhang, J. Han, Y. Chen, T.C. Mu, Carbon dioxide capture by a dual amino ionic liquid with amino-functionalized imidazolium cation and taurine anion, Int. J. Greenhouse Gas Control 5 (2011) 628–633. [14] C.M. Wang, X.Y. Luo, H.M. Luo, D. Jiang, H.R. Li, S. Dai, Tuning the basicity of ionic liquids for equimolar CO2 capture, Angew. Chem. 123 (2011) 5020–5024. [15] A. Ahmady, M.A. Hashim, M.K. Aroua, Absorption of carbon dioxide in the aqueous mixtures of methyldiethanolamine with three types of imidazoliumbased ionic liquids, Fluid Phase Equilib. 309 (2011) 76–82. [16] Y. Gao, F. Zhang, K. Huang, J.W. Ma, Y.T. Wu, Z.B. Zhang, Absorption of CO2 in amino acid ionic liquid (AAIL) activated MDEA solutions, Int. J. Greenhouse Gas Control 19 (2013) 379–386. [17] M.M. Taib, T. Murugesan, Solubilities of CO2 in aqueous solutions of ionic liquids (ILs) and monoethanolamine (MEA) at pressures from 100 to 1600 kPa, Chem. Eng. J. 181–182 (2012) 56–62. [18] Q. Huang, Y. Li, X.B. Jin, D. Zhao, G.Z. Chen, Chloride ion enhanced thermal stability of carbon dioxide captured by monoethanolamine in hydroxyl imidazolium based ionic liquids, Energy Environ. Sci. 4 (2011) 2125–2133. [19] N.A. Sairi, N. Abd Ghani, M.K. Aroua, R. Yusoff, Y. Alias, Low pressure solubilities of CO2 in guanidinium trifluoromethanesulfonate-MDEA systems, Fluid Phase Equilib. 385 (2015) 79–91. [20] F. Zhang, Y. Gao, X.K. Wu, J.W. Ma, Y.T. Wu, Z.B. Zhang, Regeneration performance of amino acid ionic liquid (AAIL) activated MDEA solutions for CO2 capture, Chem. Eng. J. 223 (2013) 371–378. [21] Z.M. Zhou, G.H. Jing, L. Zhou, Characterization and absorption of carbon dioxide into aqueous solution of amino acid ionic liquid [N1111][Gly] and 2amino-2-methyl-1-propanol, Chem. Eng. J. 204 (2012) 235–243. [22] B.H. Lu, X.Q. Wang, Y.F. Xia, N. Liu, S.J. Li, W. Li, Kinetics of carbon dioxide absorption into mixed aqueous solutions of MEA + [Bmim]BF4 using a double stirred cell, Energy Fuels 27 (2013) 6002–6009. [23] I. Iliuta, M. Hasib-ur-Rahman, F. Larachi, CO2 absorption in diethanolamine/ ionic liquid emulsions – chemical kinetics and mass transfer study, Chem. Eng. J. 240 (2014) 16–23. [24] F. Zhang, C.G. Fang, Y.T. Wu, Y.T. Wang, A.M. Li, Z.B. Zhang, Absorption of CO2 in the aqueous solutions of functionalized ionic liquids and MDEA, Chem. Eng. J. 160 (2010) 691–697. [25] A. Ahmady, M.A. Hashim, M.K. Aroua, Kinetics of carbon dioxide absorption into aqueous MDEA + [bmim][BF4] solutions from 303 to 333 K, Chem. Eng. J. 200–202 (2012) 317–328. [26] B.H. Lv, Y. Shi, C. Sun, N. Liu, W. Li, S.J. Li, CO2 capture by a highly-efficient aqueous blend of monoethanolamine and a hydrophilic amino acid ionic liquid [C2OHmim][Gly], Chem. Eng. J. 270 (2015) 372–377. [27] M. Caplow, Kinetics of carbamate formation and breakdown, J. Am. Chem. Soc. 90 (1968) 6795–6803.

702

B. Lv et al. / Chemical Engineering Journal 280 (2015) 695–702

[28] P.V. Danckwerts, The reaction of CO2 with ethanolamines, Chem. Eng. Sci. 34 (1979) 443–446. [29] G.H. Jing, L. Zhou, Z.M. Zhou, Characterization and kinetics of carbon dioxide absorption into aqueous tetramethylammonium glycinate solution, Chem. Eng. J. 181 (2012) 85–92. [30] P.D. Vaidya, E.Y. Kenig, A study on CO2 absorption kinetics by aqueous solutions of N,N-diethylethanolamine and N-ethylethanolamine, Chem. Eng. Technol. 32 (2009) 556–563. [31] M.M. Sharma, P.V. Danckwerts, Chemical methods of measuring interfacial area and mass transfer coefficients in two-fluid systems, Br. Chem. Eng. 15 (1970) 522.

[32] E.D. Snijder, M.J.M. Riele, G.F. Versteeg, W.P.M. Van Swaaij, The diffusion coefficient of several aqueous alkanolamine solutions, J. Chem. Eng. Data 3 (1993) 475. [33] J.J. Ko, T.C. Tsai, C.Y. Lin, H.M. Wang, M.H. Li, Diffusivity of nitrous oxide in aqueous alkanolamine solutions, J. Chem. Eng. Data 46 (2001) 160–165. [34] A.K. Saha, S.S. Bandyopadhyay, P. Saju, A.K. Biswas, Solubility and diffusivity of N2O and CO2 in aqueous solutions of. 2-amino-2-methyl-1-propanol, J. Chem. Eng. Data. 38 (1993) 78–82. [35] G.F. Versteeg, W.P.M. van Swaaij, Solubility and diffusivity of acid gases (carbon dioxide, nitrous oxide) in aqueous alkanolamine solutions, J. Chem. Eng. Data 33 (1988) 29–34.