Absorption of carbon dioxide in the aqueous mixtures of methyldiethanolamine with three types of imidazolium-based ionic liquids

Fluid Phase Equilibria 309 (2011) 76–82

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Absorption of carbon dioxide in the aqueous mixtures of methyldiethanolamine with three types of imidazolium-based ionic liquids Afshin Ahmady, Mohd. Ali Hashim, Mohamed Kheireddine Aroua ∗ Chemical Engineering Department, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia

a r t i c l e

i n f o

Article history: Received 20 February 2011 Received in revised form 5 June 2011 Accepted 19 June 2011 Available online 3 July 2011 Keywords: CO2 absorption MDEA Ionic liquids

a b s t r a c t The absorption of carbon dioxide in the 4 mol/L aqueous solution of methyldiethanolamine (MDEA) mixed with three types of ionic liquids, 1-butyl-3-methyl-imidazolium tetrafluoroborate ([bmim][BF4 ]), 1-butyl-3-methyl-imidazolium acetate ([bmim][Ac]) and 1-butyl-3-methyl-imidazolium dicyanamide ([bmim][DCA]) were measured as a function of temperature, CO2 partial pressure and concentration of ionic liquids in the solution. The data for aqueous MDEA + ILs solutions were obtained for temperature, CO2 partial pressure and ionic liquids concentrations ranging from 303 to 333 K, 100 to 700 kPa and 0 to 2 mol/L, respectively. The CO2 loading in all the studied mixtures decreases with an increase in temperature and increases with an increase in the CO2 partial pressure, at a given temperature. Also, it is found that the CO2 loading decreases significantly as the ionic liquid concentration increases, but this reduction in solutions contained [bmim][BF4 ] was less than other types of ionic liquids. © 2011 Elsevier B.V. All rights reserved.

1. Introduction The absorption of CO2 from gas mixtures is an important operation in many industries. Since CO2 is widely regarded as a major greenhouse gas, potentially contributing to global warming, there has been considerable interest in developing technologies for capturing and sequestration of large quantities of CO2 produced from industrial sources [1]. Various technologies have been developed for CO2 and H2 S removal from gas streams including absorption by chemical and physical solvents, cryogenic separation and membrane separation. Among these methods, gas absorption using chemical solvents such as aqueous solutions of alkanolamines is one of the most popular and effective methods [2,3]. MDEA has higher loading capacity and lower heat of reaction in comparison to secondary alkanolamines such as DEA. Recent works [4–10] on the applications of room-temperature ionic liquids (RTILs) as nonvolatile solvents suggest the possibility of utilizing them for CO2 capture. Imidazolium-based RTILs (Fig. 1) possess good solubility and selectivity for CO2 relative to N2 and CH4 [4]. These properties make RTILs useful media for gas separation and capture. Gas dissolution in RTILs is primarily a phys-

∗ Corresponding author. Tel.: +60 3 79675206; fax: +60 3 79675319. E-mail addresses: [email protected] (A. Ahmady), [email protected] (Mohd.A. Hashim), mk [email protected] (M.K. Aroua). 0378-3812/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.fluid.2011.06.029

ical phenomenon with no chemical reaction [4]. CO2 solubility in pure ionic liquids is affected much more by the nature of anion instead of cation. For example, Cadena et al. [5] have shown that bis(trifluoromethylsulfonyl)-imide anion has highest affinity for CO2 , while there is a small difference between CO2 solubility of ionic liquids having the tetrafluoroborate and hexafluorophosphate anion [11]. Generally, all tested RTILs show very low CO2 loading capacity in comparison to amine based solvents [9]. Thus, researchers are attempting to find other alternatives to have both the green properties of ionic liquids and the high productivity of amines, such as methods synthesizing task specific ionic liquids (TSILs) [7,12,13]. In fact, the performance of ionic liquid in absorbing CO2 can be dramatically improved through incorporating an amine function in the structure of the ionic liquid. However, the synthesis of amine-functionalized imidazolium salts requires several synthetic and purification steps and is not cost-competitive with commodity chemicals such as MEA (monoethanolamine) [7]. Another strategy to improve the performance of ionic liquids is to use their mixtures with amines [14]. Such mixtures retain the desired properties of TSILs for CO2 capture but without many of their inherent drawbacks such as high viscosity in their unreacted states, and intractable tars due to their corresponding CO2 adduct [4]. High solubility of CO2 in [bmim][Ac] is known in the literature [15]. Mixture of ionic liquids, [bmim][acetate], in aqueous solution of MDEA and MEA was studied for CO2 absorption in a gaseous stream in limited process conditions [8]. In a recent work, mixture of two types

A. Ahmady et al. / Fluid Phase Equilibria 309 (2011) 76–82

77

Fig. 1. Molecular structure of 1-butyl-3-methyl-imidazolium.

of ionic liquid and MDEA was used by Feng et al. [16] to absorb CO2 . The objective of the present work presented here is to measure the total CO2 loading in aqueous MDEA solutions mixed with three types of commonly used ionic liquids over a wide range of temperature, CO2 partial pressure and ionic liquids concentration. Three types of ionic liquids, 1-butyl-3methyl-imidazolium tetrafluoroborate ([bmim][BF4 ]), 1-butyl-3methyl-imidazolium acetate (bmim][Ac]) and 1-butyl-3-methylimidazolium dicyanamide ([bmim][DCA]) with concentrations ranging from 0 to 2 mol/L in 4 mol/L MDEA were investigated. Miscibility in water and good potential of imidazolium based ionic liquids to absorb CO2 were important to select these ionic liquids [5]. 2. Experimental apparatus and procedure

Fig. 2. Schematic diagram of the experimental set up for CO2 absorption study.

2.1. Materials The carbon dioxide used was from Mox-Linde with a mole fraction purity of 99.9%. Methyldiethanolamine 1-n-buthyl-3-methylimidazolium (MDEA ≥ 98.5 wt.%), tetrafluoroborate ([bmim][BF4 ] ≥ 98 wt.%) and 1-butyl-3methyl-imidazolium dicyanamide ([bmim][DCA] ≥ 98 wt.%) purchased from Merck. The 1-butyl-3-methyl-imidazolium acetate ([bmim][Ac] ≥ 97 wt.%) was purchased from Sigma Aldrich. Distilled water used as diluter solvent for all the experiments. 2.2. Experimental set-up The experimental set-up with a stainless steel equilibrium cell was a modification to the one used by Camper et al. [17]. This apparatus which was used to measure CO2 loading in mixture of aqueous MDEA and three types of RITLs at temperature range 303–333 K is shown in Fig. 2. It designed and assembled to have high accuracy of absorption measurement with low consumption of solvents. The capacity of the gas reservoir and equilibrium cell was measured by filling them with water and then measuring the volume of the water using a graduated cylinder. The volume of the equilibrium cell (41 mL) and gas reservoir (915.2 mL) was measured to within 0.1 mL. To confirm the accuracy of measured volume with connected tubes, a portable reference volume has been used. The capacity of this reference volume measured using same method as above and then it connected to upper section, to measure both upper and lower section volume with their tubing using following process: isolation valve between portable reference volume and apparatus closed and it pressurized up to 2 bar. Then, the apparatus evacuated by connected vacuum pump while it was isolated from gas cylinder and then both upper and lower parts isolated by closing valve between them. The upper part was pressurized by opening the valve of portable reference volume and allowed to temperature equilibrate. The volume of the upper part, using the ideal gas law, was calculated from the pressure drop observed in portable reference volume when the valve was opened. Finally, valve between portable reference volume closed and valve between

two parts opened and same method was used to calculate lower part volume. During all experiments, the equilibrium cell held the MDEA + ILs solution, with a stir bar, and the apparatus was kept in a temperature-controlled insulated water bath that was regulated at set temperature with an accuracy of ±0.05 K. Ten milliliters of the MDEA + ILs solution was used for absorption measurements, and the solvent was constantly stirred throughout the experiment. In every experiment, at first 10 mL of MDEA + ILs solution was transferred to the equilibrium cell. Then evacuated using vacuum pump, so the liquid exists under its own vapor pressure and allowed to reach the desired temperature. Experimentally is found the pressure of gas reservoir should be about 1 bar more than proposed pressure of equilibrium cell, in order to make up required CO2 for absorption as well as having low pressure to use ideal gas law. Total pressure of CO2 at equilibrium cell was regulated using PCV (pressure controller valve). The upper part was pressurized via CO2 cylinder to considered pressure and allowed to pressure and temperature equilibrates. Since the measurement of CO2 loading in MDEA + ILs solution was performed using a pressure decay method by two pressure transducer with data acquisition system, both upper and lower transducer started to record pressure with 0.1 s time interval and accuracy of ±0.5 kPa. In the next step, valve between upper and lower section opened and equilibrium cell pressure reached to set pressure. Next the stirrer was turned on so that the time needed to reach equilibrium significantly reduced. During the CO2 absorption operation, CO2 gas was fed from the gas reservoir vessel to the reactor via pressure controller valve automatically. After remaining pressure at constant value for 30 min, stirrer turned off and initial and final pressure of CO2 at reservoir and equilibrium cell used to measure the absorbed CO2 in the aqueous MDEA + ILs solution. Equilibrium cell pressure and temperature were reported as equilibrium conditions for CO2 loading data. Different pressure of gas reservoir and equilibrium cell (max = 1 bar) caused to ignore Joule-Thomson effect in the CO2 loading calculations.

78

A. Ahmady et al. / Fluid Phase Equilibria 309 (2011) 76–82 Table 1 Comparison of the measured CO2 loading in 48.9 mass% MDEA with literature values. PCO2 (kPa)

Absolute deviationa from literature × 100 (%)

Reference

10–600 100 100

9.1 1.5 1.4

Jou et al. [23] Austgen et al. [22] Benamor [23]

a

 ˛−˛  lit  . ˛

Relative absolute deviation = 

lit

Fig. 3. CO2 loading in 48.9 mass% aqueous MDEA solution at 313 K. () this work; (䊉) Benamor et al. [18]; () Austgen et al. [22]; () Jou et al. [23]; () Ermatchkov et al. [25]; () Benamor et al. [30].

3. Results and discussion The quantity of absorbed CO2 in the solvents, nCO2 , was calculated using following equations [19,20]. Volume of existing gas part in the equilibrium cell, VEC was obtained from VEC = VEC,t − VL

(1)

with an accuracy of ±0.1 mL. VEC,t is the total volume of equilibrium cell and VL is the volume of existing solvent in the cell with an accuracy of ± 0.1 mL, nCO2 =

VGR RT



PGR,1 PGR,2 − zR,1 zR,2



VEC + RT



PEC,1 PEC,2 − zC,1 zC2



(2)

where VGR , PGR,1 and PGR,2 are the volume, initial and final pressure of gas reservoir part, zR,1 and zR,2 are the compressibility factors corresponding to the initial and final pressure in the gas reservoir. PEC,1 and PEC,2 are initial and final pressure of the equilibrium cell, zC,1 and zC,2 are the compressibility factors corresponding to the initial and final pressure in the cell. The CO2 loading in the liquid phase, mCO2 is defined as mCO2 =

nCO2 mL

Fig. 4. CO2 loading in mixture of aqueous 4 M MDEA solution and ILs at 303 K and PCO2 = 700 kPa. (䊉) [bmim][BF4 ]; () [bmim][Ac]; () [bmim][DCA].

maximum values are reported in Table 1 at same conditions using following equation:

   ˛ − ˛lit   ˛

Relative absolute deviation = 

(5)

lit

where ˛ and ˛lit are CO2 loading obtained from this work and from literatures at same conditions. Benamor [18] used the BaCO3 precipitation method to determine the CO2 loading. CO2 loading data for mixture of [bmim][BF4 ] and aqueous MDEA ranging 303–333 K and CO2 partial pressure, 100–700 kPa are listed in Table 2. At least three measurements were done at each temperature and CO2 partial pressure, and the tabulated loading mCO2 corresponds to their arithmetic mean.

(3)

where mL is the mass of solvent that has been charged into the cell. Reported CO2 partial pressure, PCO2 , was calculated from PCO2 = PEC,2 − PV

(4)

where PV is the solution vapor pressure at equilibrium temperature. To verify the applicability of the experimental setup and the procedure used in this study, the CO2 loading in 48.9 mass% MDEA aqueous solutions at 40 K was measured at the partial pressure of CO2 ranging from 10 to 600 kPa and compared with the previously reported data in the literature [18,22,23,25,29] and comparison has been shown in Fig. 3. Good agreement with the previously reported values showed the experimental apparatus and the procedure used in this work was precise enough for performing such measurements (Table 1). Relative absolute deviation (RAD) for results of this work and results obtained from other literatures has been calculated and

Fig. 5. CO2 loading in mixture of aqueous 4 M MDEA solution and ILs at 333 K and PCO2 = 700 kPa. (䊉) [bmim][BF4 ]; () [bmim][Ac]; () [bmim][DCA].

A. Ahmady et al. / Fluid Phase Equilibria 309 (2011) 76–82

79

Table 2 CO2 loading in 4 M MDEA (1) mixed with various concentration of [bmim][BF4 ] (2). C2 (mol/L) T = 303.15 K 0.0 0.5 1.0 1.5 2.0 0.0 0.5 1.0 1.5 2.0 T = 313.15 K 0.0 0.5 1.0 1.5 2.0 0.0 0.5 1.0 1.5 2.0 T = 323.15 K 0.0 0.5 1.0 1.5 2.0 0.0 0.5 1.0 1.5 2.0 T = 333.15 K 0.0 0.5 1.0 1.5 2.0 0.0 0.5 1.0 1.5 2.0

100

3.26 3.13 2.95 2.55 2.03 3.80 3.72 3.57 3.52 2.90

± ± ± ± ± ± ± ± ± ±

0.10 0.02 0.07 0.05 0.06 0.09 0.08 0.05 0.03 0.04

0.086 0.096 0.108 0.116 0.121 0.099 0.112 0.128 0.153 0.164

± ± ± ± ± ± ± ± ± ±

2.60 2.60 2.33 1.78 1.26 3.45 3.37 3.15 3.00 2.54

± ± ± ± ± ± ± ± ± ±

0.02 0.03 0.05 0.05 0.06 0.05 0.04 0.02 0.06 0.04

0.070 0.081 0.087 0.084 0.079 0.091 0.102 0.114 0.133 0.147

2.20 1.77 1.45 1.30 1.00 3.34 3.01 2.77 2.44 1.82

± ± ± ± ± ± ± ± ± ±

0.01 0.03 0.09 0.09 0.05 0.04 0.07 0.07 0.06 0.05

1.25 1.03 0.94 0.71 0.52 2.72 2.43 2.00 1.54 1.13

± ± ± ± ± ± ± ± ± ±

0.01 0.04 0.05 0.03 0.02 0.05 0.04 0.07 0.04 0.07

100

500

100

500

100

500

n

SD =

mCO2 ± SDa (mol/kg)

500

 

a

PCO2 (kPa)

i

2

¯ CO )2 (mCO ,i −m 2 2



XCO2

PCO2 (kPa)

mCO2 ± SD (mol/kg)

0.003 0.001 0.002 0.002 0.003 0.003 0.004 0.002 0.001 0.002

300

3.64 3.66 3.33 3.12 2.74 4.06 4.03 4.00 4.07 3.47

± ± ± ± ± ± ± ± ± ±

0.05 0.04 0.03 0.07 0.04 0.07 0.09 0.05 0.06 0.08

0.095 0.110 0.120 0.138 0.157 0.105 0.120 0.141 0.172 0.190

± ± ± ± ± ± ± ± ± ±

0.001 0.001 0.001 0.003 0.002 0.002 0.001 0.001 0.002 0.003

± ± ± ± ± ± ± ± ± ±

0.001 0.001 0.002 0.001 0.001 0.002 0.001 0.001 0.002 0.002

300

3.33 3.21 2.90 2.49 2.14 3.84 3.83 3.78 3.49 3.07

± ± ± ± ± ± ± ± ± ±

0.04 0.08 0.06 0.03 0.02 0.04 0.08 0.03 0.05 0.03

0.088 0.098 0.106 0.113 0.127 0.100 0.115 0.134 0.152 0.172

± ± ± ± ± ± ± ± ± ±

0.001 0.003 0.002 0.001 0.001 0.001 0.001 0.001 0.002 0.001

0.060 0.056 0.056 0.063 0.062 0.088 0.092 0.102 0.111 0.110

± ± ± ± ± ± ± ± ± ±

0.001 0.001 0.003 0.004 0.002 0.001 0.002 0.002 0.002 0.002

300

3.03 2.77 2.49 2.07 1.59 3.72 3.60 3.43 3.08 2.34

± ± ± ± ± ± ± ± ± ±

0.02 0.05 0.05 0.02 0.02 0.04 0.06 0.04 0.03 0.03

0.081 0.086 0.093 0.096 0.096 0.097 0.109 0.123 0.136 0.137

± ± ± ± ± ± ± ± ± ±

0.001 0.001 0.002 0.001 0.001 0.001 0.001 0.001 0.001 0.001

0.035 0.033 0.037 0.036 0.035 0.073 0.076 0.076 0.074 0.071

± ± ± ± ± ± ± ± ± ±

0.001 0.001 0.002 0.001 0.001 0.001 0.001 0.002 0.001 0.003

300

2.32 2.09 1.79 1.32 0.91 3.26 3.08 2.57 2.40 1.74

± ± ± ± ± ± ± ± ± ±

0.07 0.06 0.04 0.05 0.08 0.07 0.08 0.07 0.07 0.09

0.063 0.066 0.068 0.064 0.059 0.086 0.094 0.095 0.110 0.106

± ± ± ± ± ± ± ± ± ±

0.002 0.002 0.001 0.002 0.003 0.002 0.002 0.002 0.003 0.004

SD =

2

700

700

700

n−1

The standard deviation (SD) of the CO2 loading at each condition was calculated with Eq. (6), where n is the number of measurements at each temperature and CO2 partial pressure.

 

700

XCO2

n (mCO2 ,i i

¯ CO2 ) −m

2

H2 O(l) ↔ H+ (aq) + OH− (aq)

(7)

• formation of bicarbonate (HCO3 − ) and carbonate (CO3 2− ):



n−1

• autoprotolysis of water:

(6)

Same method was used to calculate CO2 loading and SD in mixture of [bmim][Ac] and aqueous MDEA and mixture of [bmim][DCA] and aqueous MDEA ranging 303–333 K and CO2 partial pressure, 100–700 kPa as reported in Tables 3 and 4.

CO2 (aq) + H2 O(l) ↔ HCO3 − (aq) + H+ (aq) −

HCO3 (aq) ↔ C3

2−

+

(aq) + H (aq)

(8) (9)

• protonation of MDEA: MDEA(aq) + H+ (aq) ↔ MDEAH+ (aq)

(10)

In these reactions which can be described as a base catalyzed hydration, CO2 cannot be reacted with MDEA directly and water is main species of CO2 absorption mechanism [24].

3.1. CO2 loading in aqueous MDEA solution CO2 is absorbed in aqueous MDEA as chemical and physical absorption [24]. Following mechanism is suggested for chemical reactions in aqueous MDEA [24]:

3.2. Effect of ionic liquids on CO2 loading In this work, CO2 loading data were measured in 4.0 mol/L MDEA solutions, and ionic liquid concentration ranged from

80

A. Ahmady et al. / Fluid Phase Equilibria 309 (2011) 76–82

Table 3 CO2 loading in 4 M MDEA (1) mixed with various concentration of [bmim][Ac] (3). C3 (mol/L) T = 303.15 K 0.5 1.0 1.5 2.0 0.5 1.0 1.5 2.0 T = 313.15 K 0.5 1.0 1.5 2.0 0.5 1.0 1.5 2.0 T = 323.15 K 0.5 1.0 1.5 2.0 0.5 1.0 1.5 2.0 T = 333.15 K 0.5 1.0 1.5 2.0 0.5 1.0 1.5 2.0

PCO2 (kPa)

mCO2 ± SD (mol/kg)

100

3.02 2.65 2.27 1.63 3.66 3.51 3.25 2.80

± ± ± ± ± ± ± ±

0.04 0.02 0.03 0.09 0.09 0.07 0.07 0.06

0.092 0.099 0.106 0.108 0.109 0.127 0.145 0.172

± ± ± ± ± ± ± ±

2.37 2.08 1.65 1.15 3.39 3.16 2.70 2.06

± ± ± ± ± ± ± ±

0.04 0.06 0.09 0.05 0.02 0.05 0.08 0.03

0.074 0.079 0.079 0.079 0.102 0.116 0.123 0.132

1.93 1.42 1.06 0.70 3.00 2.67 1.92 1.28

± ± ± ± ± ± ± ±

0.02 0.07 0.04 0.08 0.03 0.07 0.04 0.05

1.09 0.89 0.62 0.36 2.32 2.03 1.30 0.84

± ± ± ± ± ± ± ±

0.05 0.04 0.06 0.03 0.03 0.02 0.03 0.04

500

100

500

100

500

100

500

XCO2

PCO2 (kPa)

mCO2 ± SD (mol/kg)

0.001 0.001 0.001 0.004 0.003 0.002 0.003 0.002

300

3.35 3.37 3.00 2.16 4.00 3.89 3.81 3.46

± ± ± ± ± ± ± ±

0.08 0.06 0.07 0.08 0.06 0.02 0.04 0.08

0.101 0.123 0.135 0.138 0.118 0.139 0.166 0.204

± ± ± ± ± ± ± ±

0.002 0.002 0.003 0.003 0.002 0.001 0.001 0.003

± ± ± ± ± ± ± ±

0.001 0.002 0.004 0.002 0.001 0.001 0.003 0.001

300

3.05 2.70 2.56 1.71 3.74 3.39 3.12 2.36

± ± ± ± ± ± ± ±

0.09 0.03 0.04 0.09 0.05 0.08 0.07 0.03

0.093 0.101 0.118 0.112 0.111 0.123 0.140 0.149

± ± ± ± ± ± ± ±

0.003 0.001 0.002 0.004 0.001 0.002 0.002 0.001

0.061 0.056 0.052 0.049 0.091 0.100 0.091 0.087

± ± ± ± ± ± ± ±

0.001 0.002 0.001 0.003 0.001 0.002 0.001 0.002

300

2.72 2.45 1.73 1.12 3.44 3.11 2.44 1.98

± ± ± ± ± ± ± ±

0.07 0.05 0.03 0.04 0.03 0.02 0.03 0.05

0.084 0.092 0.083 0.077 0.103 0.114 0.113 0.128

± ± ± ± ± ± ± ±

0.002 0.001 0.001 0.001 0.001 0.001 0.001 0.002

0.035 0.036 0.031 0.026 0.072 0.078 0.064 0.059

± ± ± ± ± ± ± ±

0.001 0.001 0.002 0.001 0.001 0.001 0.001 0.001

300

2.00 1.66 1.05 0.69 2.72 2.47 1.72 1.34

± ± ± ± ± ± ± ±

0.03 0.04 0.05 0.02 0.05 0.04 0.05 0.02

0.063 0.064 0.052 0.049 0.084 0.093 0.082 0.090

± ± ± ± ± ± ± ±

0.001 0.001 0.002 0.001 0.001 0.001 0.002 0.001

0 to 2.0 mol/L and results are listed in Tables 2–4. Prior to the absorption experiments, the miscibility of the used ionic liquids in aqueous MDEA was evaluated and they were completely miscible. Aki et al. [21] have shown that the CO2 loading in pure [bmim] cation based ILs increases in the following order: [NO3 ] < [DCA] < [BF4 ] [PF6 ] < [TfO] < [Tf2 N] < [methide] at 298 K. Based on their report, the enthalpy for CO2 dissolution in [bmim][BF4 ] is significantly greater than [bmim][DCA] so that at 333 K the order of increasing CO2 solubility has changed to [BF4 ] < [DCA]. In contrast, in this work, CO2 loading in mixture of [bmim][BF4 ] and aqueous MDEA is more than mixture of [bmim][DCA] and aqueous MDEA throughout studied range of temperature, 303–333 K and increasing in temperature has not changed the order of CO2 loading as illustrated in following figures (see Figs. 4 and 5). In general, CO2 loading decreases with increasing of IL concentration, especially at high concentration of ILs and at high temperature. This behavior can be explained by CO2 absorption study in aqueous MDEA solution, have carried out by many researches using various concentration of MDEA [25–30]. For instance, Rho et al. [29] presented the CO2 loading in aqueous solutions of 5, 20.5, 50 and 75 mass% MDEA at various temperature as well as various CO2 partial pressures. Presented CO2 loading results shows that increasing in MDEA concentration in solvent decreased CO2 loading because of reduced water (see reactions (7)–(10)). With considering to the gas dissolution in RTILs is primarily a physical phenomenon with no chemical reaction [4] and presented CO2 loading data for various ILs [21], the CO2 loading in ILs at CO2 pressure ranging below 1 MPa is negligible in comparison

700

700

700

700

XCO2

with MDEA (e.g. the CO2 loading at 100 kPa and 323 K in pure [bmim][BF4 ] is 0.054 mol/kg [11] and for 4 mol/L MDEA at same condition is 2.2 mol/kg). Therefore, main reason of reducing of CO2 loading at high concentration of ILs in MDEA + ILs mixture, can be explained due to reduced water which replaced by ILs. But, this reason cannot explain all changing in CO2 loading in MDEA + ILs mixtures, whereas different types of ILs, has different influence on CO2 loading at same percentage of water and MDEA. For example, mixture of 2 mol/L [bmim][BF4 ] and 4 mol/L MDEA contained about 14 wt.% water and mixture of 2 mol/L [bmim][DCA] and 4 mol/L MDEA contained about 18 wt.% water, but CO2 loading in all studied conditions in mixture of [bmim][BF4 ] + MDEA is more than mixture contained [bmim][DCA] as shown in presented figures and tables (Figs. 4 and 5 and Tables 2–4). Thus, chemical reaction between CO2 and existing species in aqueous [bmim][BF4 ] + MDEA mixture is not improbable. However at the highest range of concentration of ionic liquid in the studied solutions which leaded to minimum CO2 loading capacity, the loading is still much more than pure ionic liquids. (CO2 loading is 1 mol/kg for aqueous 4 M MDEA + 2 M [bmim][BF4 ] in comparison with 0.056 mol/kg for pure [bmim][BF4 ] at 100 kPa and 323 K [11]). As expected, the CO2 loading increases with increasing of CO2 partial pressure for all studied solvents (Tables 2–4). According to the CO2 loading results, the mixed solvent of [bmim][BF4 ] + MDEA + water is the best one in these three mixed solvents and the feasibility of these mixed solvents in the absorption of CO2 will finally be justified from the kinetic data measured and published later.

A. Ahmady et al. / Fluid Phase Equilibria 309 (2011) 76–82

81

Table 4 CO2 loading in 4 M MDEA (1) mixed with various concentration of [bmim][DCA] (4). C4 (mol/L) T = 303.15 K 0.5 1.0 1.5 2.0 0.5 1.0 1.5 2.0 T = 313.15 K 0.5 1.0 1.5 2.0 0.5 1.0 1.5 2.0 T = 323.15 K 0.5 1.0 1.5 2.0 0.5 1.0 1.5 2.0 T = 333.15 K 0.5 1.0 1.5 2.0 0.5 1.0 1.5 2.0

PCO2 (kPa)

mCO2 ± SD (mol/kg)

100

3.03 2.82 2.29 1.49 3.68 3.22 3.10 2.49

± ± ± ± ± ± ± ±

0.07 0.05 0.03 0.06 0.03 0.04 0.05 0.02

0.091 0.107 0.111 0.100 0.108 0.121 0.145 0.157

± ± ± ± ± ± ± ±

2.46 2.08 1.60 0.97 3.23 3.08 2.33 1.95

± ± ± ± ± ± ± ±

0.05 0.08 0.04 0.03 0.03 0.06 0.04 0.02

0.075 0.082 0.080 0.068 0.096 0.116 0.113 0.127

1.83 1.32 1.01 0.58 2.92 2.57 2.04 1.40

± ± ± ± ± ± ± ±

0.04 0.03 0.03 0.07 0.06 0.03 0.02 0.02

1.19 0.91 0.52 0.23 2.24 1.95 1.36 0.87

± ± ± ± ± ± ± ±

0.02 0.03 0.02 0.02 0.04 0.04 0.03 0.01

500

100

500

100

500

100

500

XCO2

PCO2 (kPa)

mCO2 ± SD (mol/kg)

0.002 0.001 0.001 0.002 0.001 0.001 0.002 0.001

300

3.56 3.27 2.86 2.07 4.06 3.74 3.39 3.02

± ± ± ± ± ± ± ±

0.04 0.02 0.05 0.04 0.03 0.07 0.06 0.04

0.105 0.122 0.135 0.134 0.118 0.138 0.156 0.185

± ± ± ± ± ± ± ±

0.001 0.001 0.001 0.001 0.001 0.002 0.002 0.001

± ± ± ± ± ± ± ±

0.001 0.002 0.001 0.001 0.001 0.002 0.001 0.001

300

3.05 2.78 2.16 1.71 3.54 3.37 3.03 2.43

± ± ± ± ± ± ± ±

0.03 0.04 0.03 0.05 0.07 0.07 0.05 0.02

0.092 0.106 0.106 0.114 0.105 0.126 0.142 0.154

± ± ± ± ± ± ± ±

0.001 0.001 0.00 0.00 0.002 0.002 0.002 0.001

0.057 0.053 0.052 0.042 0.088 0.099 0.100 0.095

± ± ± ± ± ± ± ±

0.001 0.001 0.001 0.003 0.002 0.001 0.001 0.001

300

2.66 2.13 1.67 1.05 3.25 2.98 2.59 1.90

± ± ± ± ± ± ± ±

0.03 0.05 0.04 0.03 0.02 0.07 0.05 0.02

0.081 0.083 0.084 0.073 0.097 0.113 0.124 0.125

± ± ± ± ± ± ± ±

0.001 0.001 0.001 0.001 0.001 0.002 0.002 0.001

0.038 0.037 0.028 0.017 0.069 0.077 0.069 0.061

± ± ± ± ± ± ± ±

0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001

300

2.05 1.60 1.06 0.62 2.89 2.06 1.80 1.13

± ± ± ± ± ± ± ±

0.07 0.03 0.02 0.02 0.05 0.06 0.03 0.02

0.063 0.064 0.055 0.044 0.087 0.081 0.090 0.078

± ± ± ± ± ± ± ±

0.002 0.001 0.001 0.001 0.001 0.002 0.001 0.001

4. Conclusions In this work, data for CO2 loading in mixture of 4.0 mol/L aqueous MDEA and three types of imidazolium based ionic liquids, 1-butyl-3-methyl-imidazolium tetrafluoroborate ([bmim][BF4 ]), 1-butyl-3-methyl-imidazolium acetate (bmim][Ac]) and 1-butyl3-methyl-imidazolium dicyanamide ([bmim][DCA]) as a new type of absorbents for the CO2 capture were obtained at temperature ranging from 303 to 333 K and CO2 partial pressure ranging from 100 to 700 kPa. The effect of ionic liquids on CO2 loading in aqueous MDEA solutions was determined at ionic concentrations ranging from 0 to 2.0 mol/L. The results show that the CO2 loading in all IL + MDEA mixtures increases with increasing CO2 partial pressure and decreases with increasing temperature. Also, it had been found that all used ionic liquids reduced the CO2 loading in aqueous MDEA solutions. Moreover, the CO2 loading performances of the [bmim][BF4 ] in aqueous MDEA solutions, specially at high CO2 partial pressure and low temperature was better than [bmim][Ac] and [bmim][DCA]. List of symbols [bmim][Ac] 1-butyl-3-methyl-imidazolium acetate [bmim][BF4 ] 1-butyl-3-methyl-imidazolium tetrafluoroborate [bmim][DCA] 1-butyl-3-methyl-imidazolium dicyanamide C concentration carbonate CO3 2− HCO3 − bicarbonate ILs ionic liquids methyldiethanolamine MDEA MDEAH+ protonated methyldiethanolamine

700

700

700

700

XCO2

[methide] tris(trifluoromethylsulfonyl)methide anion M molarities mCO2 CO2 loading per mass of solvent mass of solvent in the cell mL n number of CO2 loading measurements at same conditions mole of CO2 nCO2 [NO3 ] nitrate anion PGR,1 initial pressure of gas reservoir final pressure of gas reservoir PGR,2 PEC,1 initial pressure of the equilibrium cell PEC,2 final pressure of the equilibrium cell CO2 partial pressure PCO2 PV solution vapor pressure at equilibrium temperature [PF6 ] hexafluorophosphate anion R gas constant Relative absolute deviation RAD SD standard deviation [TfO] trifluoromethanesulfonate anion [Tf2 N] bis(trifluoromethanesulfonyl)imide anion total volume of equilibrium cell VEC,t VGR volume of gas reservoir VL volume of solvent in the cell compressibility factors corresponding to the initial preszC1 sure in the cell zC2 compressibility factors corresponding to the final pressure in the cell zR1 compressibility factors corresponding to the initial pressure in the gas reservoir compressibility factors corresponding to the final preszR2 sure in the gas reservoir

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