Experimental Study of Carbon Dioxide Solubility in Aqueous N

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

pubs.acs.org/jced

Experimental Study of Carbon Dioxide Solubility in Aqueous N‑Methyldiethanolamine Solution with 1‑Butylpyridinium Tetrafluoroborate Ionic Liquid Milad Damanafshan,† Babak Mokhtarani,*,‡ Mojtaba Mirzaei,‡ Morteza Mafi,‡ Ali Sharifi,‡ and Amir Hossein Jalili§ †

Department of Chemical Engineering, South Tehran Branch, Islamic Azad University, Tehran, Iran Chemistry and Chemical Engineering Research Center of Iran (CCERCI), P.O. Box 14335-186, Tehran, Iran § Gas Refining Technology Group, Gas Research Division, Research Institute of Petroleum Industry (RIPI), P.O. Box 14665-137, Tehran, Iran ‡

S Supporting Information *

ABSTRACT: New experimental data for CO2 solubility in the aqueous solutions of N-methyldiethanolamine (MDEA) and 1-butylpyridinium tetrafluoroborate ([Bpy][BF4]) ionic liquid (IL) are reported. The experiments were conducted in a static high pressure equilibrium cell. Solubility measurement of CO2 was performed in various solution compositions of IL and MDEA in a wide temperature range of 298.15−343.15 K and pressures up to 4 MPa. The experimental results revealed that, by increasing the weight percent of IL, the absorption capacity of aqueous mixtures diminishes. It was also observed that the main capacity of CO2 loading in the absorbent solutions belonged to MDEA through chemical absorption, whereas its concentration was more effective than IL in gas solubility. The experimental results are correlated using a semiempirical model with good accuracy.

distillation techniques.6 The most preferred and commonly used technology in the majority of refineries and gas treatment plants is chemical absorption by aqueous alkanolamine solutions. According to Astarita et al.,7 50−70% of the initial investment for an amine-based sweetening unit directly belongs to the magnitude of the solvent circulation rate and another 10−20% of the initial investment is dependent on the regeneration energy requirement. Therefore, choosing an appropriate solvent for absorption considering the feed composition, operational conditions, and specification of the requested gas is very important. However, use of the aqueous alkanolamines always has several unfavorable drawbacks such as corrosiveness, thermal instability particularly during the regeneration step, high energy consumption during regeneration due to the large enthalpies of chemical reactions for CO2 absorption with amines, and also volatility and significant losses during the process, which, in addition to having environmental disadvantages, will require the entry of more solvent into the system and hence it imposes high costs.8,9 Among the common alkanolamines, MDEA with a CO2 loading of about 1 mol of CO2/mol of amine bears a lower volatility, higher thermal stability, less alkalinity, and less regeneration cost than the rest. However, the absorption of CO2 in MDEA is quite slow. Therefore, the blend of MDEA

1. INTRODUCTION The indisputable and close correlation between escalating the CO2 concentration in the atmosphere and its unpleasant greenhouse effect represents a major challenge in the global arena and requires more effort and debate aimed at reducing CO2 emissions and other greenhouse gases.1,2 Hence, achieving new technologies in order to reduce environmental pollutants and eliminate greenhouse gases, in particular CO2, is one of the main concerns and priorities of many industries. At present, the power plants with 41% of the total CO2 emissions have the highest CO2 emissions, with a high share of this percentage being related to combustion of fuel for generating electricity or heat.3 To date, supplying global energy has been largely dependent on fossil fuels, such as coal, oil, and natural gas. These fuels provide more than 67% of the global electricity demand.4 On the other hand, the reduction of CO2 emissions is not only vital from the environmental point of view, but also its removal and separation are pretty remarkable in the process of natural gas sweetening, in order to increase the thermal value of the fuel and prevent unnecessary costs to pressure boost and gas transfer with low efficiency and also to reduce the devastating consequences of corrosion and clogging caused by the formation of dry ice or gas hydrates during the process of liquefaction of natural gas.5 So far, various technologies have been developed for the removal and separation of CO2 from gas stream mixtures like natural gas and flue gas, such as physical/chemical sorption, membrane separation, molecular sieves, carbonation, and cryogenic © XXXX American Chemical Society

Received: February 14, 2018 Accepted: May 25, 2018

A

DOI: 10.1021/acs.jced.8b00142 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Table 1. Literature Review of CO2 Solubility in Aqueous/Nonaqueous Alkanolamine + IL Hybrid Solutions absorbent

Pg (MPa)

Tg (K)

[N1111][Gly] + MDEA + H2O [N2222][Gly] + MDEA + H2O [N1111][Lys] + MDEA + H2O [N2222][Lys] + MDEA + H2O [Bmim][acetate] + MDEA + H2O [Bmim][acetate] + DEA + H2O [Bmim][acetate] + DIPA + H2O [Bmim][acetate] + AMP + H2O [C2OHmim][DCA] + MEA + H2O [Bmim][DCA] + MEA + H2O [Bmim][BF4] + MDEA + H2O [Bmim][AC] + MDEA + H2O [Bmim][DCA] + MDEA + H2O [N1111][Gly] + MDEA + H2O [Hmim][Tf2N] + AMP [Hmim][Tf2N] + DEA [Bmim][Tf2N] + DEA [Emim][Tf2N] + DEA [Hmim][Tf2N] + DEA [P66614][Bentriz] + MEA [P66614][Benzim] + MEA [P66614][123Triz] + MEA [P66614][124Triz] + MEA [P66614][Im] + MEA [Bmim][BF4] + MEA [Bmim][BF4] + MEA + DEA [Bmim][BF4] + MEA + MDEA [Bmim][BF4] + MEA + DEA + MDEA [Bmim][acetate] + MDEA + H2O [Bmim][acetate] + MDEA [Bmim][BF4] + MEA + H2O [Bmim][DCA] + MEA + H2O [Bpy][BF4] + MEA + H2O [Bmim][BF4] + MEA + H2O [Bmim][Gly] + MDEA + H2O [Bmim][BF4] + MEA + H2O [C2OHmim][Gly] + MEA + H2O [gua][OTf] + MDEA + H2O [gua][OTf] + MDEA [gua][OTf] + MDEA + H2O [Bmim][OAc] + MEA + H2O [Emim][OcSO4] + MEA + H2O [Bmpr][BF4] + MEA + H2O [Bmim][BF4] + MEA + H2O [Hmim][NTf2] + MEA [OHemim][NTf2] + MEA [Bmim][BF4] + MDEA + PZ + H2O [Bmim][NO3] + MDEA + PZ + H2O [Bmim][Cl] + MDEA + PZ + H2O [MEA][BF4] + MEA + H2O [MDEA][Cl] + MDEA + H2O [MDEA][Cl] + MDEA + H2O + PZ [Bmim][BF4] + MEA + H2O [Bheaa] + MEA + H2O

0.004−0.4

298−318

Feng et al.8,32,40

0.1−4

323

Shojaeian et al.33

0.01−0.8

293−333

Xu et al.34

0.1−0.7

303−343

Ahmady et al.35,41,42

0−0.3 0.101

298−318 308−383

Gao et al.36 Hasib-ur-Rahman et al.37

0.101

298

Hasib-ur-Rahman et al.38,45

0.1

295

McCrellis et al.39

0.05−1.5

303−323

Osman et al.43

0.2−2.5 0.1−3.9 0.101

323−348 303−343 473

Haghtalab et al.46 Haghtalab et al.47 Huang et al.48

0.101 0.101 0.015 N/A 0.1−1

323 303−323 303−333 303−333 293−333

Yang et al.49 Fu et al.50 Lu et al.51 Lv et al.52,53 Sairi et al.54

0.5−3 N/A

303−333 298−338

Sairi et al.55 Baj et al.56

0.101

313

Gonzalez-Miquel et al.57

0.1−1

313−373

Gao et al.58

0.5−2.5

303−358

Zhao et al.59,60

0.1−1.6

298−313

Taib et al.61

reference

solvents compared to conventional organic solvents. The negligible vapor pressure of RTILs and consequently no solvent loss and environmental contamination, no degradation through the absorption and desorption of acid gases, simple and frequent regeneration, diversity with unique properties of each one,

aqueous solution with activators of fast reactivity, such as monoethanolamine (MEA) or piperazine (PZ), is usually used in industrial applications.9,10 In recent years, room temperature ionic liquids (RTILs) have been considered as nonvolatile and environmentally friendly B



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Table 2. Purity and Supplier of the Materials Used in This Study chemical name

supplier

carbon dioxide MDEA

Hamta Gas Company Sigma-Aldrich Company synthesized in our lab

[Bpy][BF4]

mass fraction purity

molecular weight (kg·mol−1)

124-38-9 105-59-9

0.9999 >0.98

0.04401 0.11916

none none

203389-28-0

>0.98

0.22302

NMR, Karl Fischer titration, and potentiometric titration

CAS number

purification method

mixture in CO2 uptake, is scant. Thus, a pyridinium-based cation, along with a fluorine-containing anion, was selected as an IL to be blended with an amine solution. The positive effect of fluorine anion in the IL structure has already been proven for the CO2 solubility.5 Therefore, the objective of the present study is to explore the solubility of CO2 in the aqueous solutions of 30−50 wt % MDEA blended with 0−20 wt % [Bpy][BF4] at the wide temperature range from 298.15 to 343.15 K and pressures up to 4 MPa. Also, it can be noted that [Bpy][BF4] possesses a lower toxicity than other types of conventional ILs.44

nonflammability, tunable nature commensurate with the process, and wide liquidus ranges have made them a suitable alternative for classical solvents in gas treating processes.11−15 Among efficient hybrid solvents for absorbing and separating CO2, various mixtures of solvents including conventional RTILs,16−25 task specific ionic liquids (TSIL) with a particular functional group (like amine),26−31 and blends of the IL with the aqueous32−36 or nonaqueous37−39 alkanolamine solution have been examined in different processes. Nevertheless, there are some major drawbacks in utilizing pure ILs such as RTIL and TSIL for CO2 capturing. High viscosity at ambient temperature particularly for TSIL,8 lower CO2 loading capacity especially for RTIL,11 and also their high cost8 generally limit their applications in industrial scales. The introduction of a blend of alkanolamines and IL is a plan that has recently drawn the attention of researchers. Most studies in this field have sought to identify the appropriate combination with high CO2 absorption capacity. Given the need for both kinds of solvents mentioned above, the combination of IL with alkanolamines with the aim of integrating their desired properties to optimize an efficient solvent for CO2 uptake is proposed. In this regard, Feng et al.8,32,40 in a comprehensive study measured the CO2 absorption capacity and regeneration performance of aqueous solutions of MDEA with four amino acid based functionalized ILs, [N1111][Gly], [N2222][Gly], [N1111][Lys], and [N2222][Lys], and concluded that the addition of amino acid based IL to aqueous MDEA significantly enhances the absorption of CO2. Also, Ahmady et al.35,41,42 investigated the initial absorption rate, kinetics, and capacity of CO2 absorption in the MDEA aqueous solution with [Bmim][BF4], [Bmim][Ac], and [Bmim][DCA] and found that, by increasing the concentration of IL, the CO2 loading was considerably reduced, but this decrease was observed to be lower in [Bmim][BF4] solution than in other types of ILs. Osman et al.43 examined six nonaqueous solutions of [Bmim][BF4] with MEA, DEA, and MDEA in the forms of binary, ternary, and quaternary systems and reported the MEA + DEA + [Bmim][BF4] mixture with a mass composition of 100w = 31.8:12.1:56.1 as the solution with the highest absorption capacity. A number of studies which have been carried out related to the CO2 capture by alkanolamine + IL solutions are listed in Table 1. This table gives an overview of the available experimental data for the solubility of CO2 in hybrid solvents. Previous studies, which have been conducted on the solubility of CO2 in mixtures of alkanolamine + IL, mostly concern up to low pressure ranges. Among the various types of ILs, numerous studies have been performed on gas solubility measurement in pure or aqueous ILs (such as imidazolium,15−25 pyridinium,13 and ammonium59,60 based cations with different anions) or addition of imidazolium-based RTILs33−35,43,50 to amine solutions due to their considerable solubility of CO2.12 However, the detailed CO2 solubility measurement for a mixture of solvents containing pyridinium-based RTILs, which is helpful for deeply understanding the effect of adding this kind of IL to a

2. EXPERIMENTAL SECTION 2.1. Chemicals. The chemicals used in this study, along with their purity and suppliers, are given in Table 2, and the chemical structures of [Bpy][BF4] and MDEA are shown in Figure 1. CO2

Figure 1. Chemical structures of (a) N-methyldiethanolamine (MDEA) and (b) 1-butylpyridinium tetrafluoroborate ([Bpy][BF4]).

was supplied from Hamta Gas Company with a mass fraction purity of 0.9999. MDEA was purchased from Sigma-Aldrich Company with a mass fraction purity more than 0.98. All of these materials were used without any purification. To prepare the aqueous solutions of MDEA and IL, deionized water was used. Mass measurement for preparation of the solutions was carried out by a calibrated digital balance (Marshall Scientific, Mettler Toledo PM 6100, USA) with a standard uncertainty of 0.01 g. The ionic liquid [Bpy][BF4] was synthesized in our laboratory in the Chemistry and Chemical Engineering Research Center of Iran (CCERCI). [Bpy][BF4] was prepared from its corresponding [Bpy][Br] that was synthesized according to the method described in the literature.62 Utilizing pyridine and a little excess of 1bromobutane without addition of solvent, the alkylation stage was performed in an inert atmosphere at 338 K. The desired [Bpy][BF4] was obtained from a metathetic reaction of [Bpy][Br] with sodium tetrafluoroborate.62 Its water content mass fraction was determined to be less than 1 × 10−3 by a 684 Karl Fischer coulometer, and the mass fraction of bromide ion as an impurity was found to be about less than 1 × 10−4 in [Bpy][BF4] by the potentiometric titration method with silver nitrate (AgNO3). The nuclear magnetic resonance (NMR) spectrum of the [Bpy][BF4] is presented in the Supporting Information. Since ILs are good absorbents for moisture, they should be kept under inert gas and away from contact with air. Therefore, C



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Figure 2. Schematic diagram of the experimental setup: (1) water bath, (2) equilibrium cell, (3) water bath circulator, (4) vacuum pump, (5) gas container, (6) control panel, (7) personal computer, (8) pressure transducer, (9) temperature sensor, (10) mixer, and (11) rupture disc.

precise pressure transducer (Keller PPA-33X, Switzerland) with a standard uncertainty of 0.01% of the total pressure. The magnetic drive at a fixed speed ensured efficient mixing and stirring of the process media as well as excellent heat transfer, and it was adjusted manually to 500 rpm. CO2 gas was injected from the main gas cylinder to the middle cell, and the mass of CO2 was measured by a laboratory balance with a standard uncertainty of 0.01 g. To measure the density of hybrid solvents as well as pure components, a digital densitometer (Anton Paar, DMA-5000, Austria) was used and water was applied for calibration. The accuracy of the device is within 1 × 10−5 g·cm−3, and the standard uncertainty of the measurements was found to be less than 2 × 10−3 g·cm−3. 2.3. Experimental Procedure. In this study, six aqueous solutions containing MDEA + [Bpy][BF4] + H2O with different mass compositions were tested as new hybrid solvents. To check and validate the accuracy of the solubility measurement apparatus, a comparison was carried out with the previous works of the reference researchers before the start of the experiments. Also, to investigate and compare the absorption behavior of mixed solutions toward CO2 with corresponding IL free solutions, two other solutions including MDEA + H2O were tested. Table 3 shows the composition of each solution investigated in this study. Prior to the start of the experiment, a vacuum pump was applied for evacuation of gases from the equilibrium cell, which enables the pressure of the cell to be reduced below 1.0 kPa. To begin the experiment, in order to prepare about 100 g of the fresh mixture of {MDEA + [Bpy][BF4] + H2O}, each component was precisely weighed with the balance with an accuracy of 0.001 g, and after blending with each other, the single phase absorbent solution was made. By connecting one side of the hose to the

[Bpy][BF4] was dried and degassed for 24 h at 343.15 K under a vacuum and was kept under argon gas. 2.2. Apparatus. The experimental apparatus for gas solubility measurement, which operates on the basis of the constant volume method, is schematically represented in Figure 2. The apparatus consists of a high-pressure stirred autoclave as the equilibrium cell (Buchi, miniclave drive, material: stainless steel, Hastelloy C-22, Switzerland), a stainless steel middle cell with a volume of about 500 cm3 to introduce gases into the equilibrium cell, and a water bath circulator. The autoclave was inserted into a water bath for temperature control via a circulator (Julabo model FP50-HL, Germany). For this reason, the circulator was equipped with a Pt100 temperature sensor that was placed inside the water bath. Temperature stability was set and controlled in the water bath in the temperature range 273.15−373.15 K with a standard uncertainty of 0.1 K. In order to degas inside the equilibrium cell, a vacuum pump (Buchi, V700, Switzerland) was used. To measure the internal volume of the absorption cell, a certain mass of nitrogen was injected into a vacuum absorption cell, and by using the Wagner equation of state,63 the exact density of the gas was obtained at a specified temperature and pressure. The volume was calculated by dividing the mass of injected gas into the density. To be sure, this was repeated at constant temperatures and at several different pressures. In this way, the precise volume of the absorption cell was 303 ± 1 cm3. A pressure transducer (BD SENSORS, model 30.600 G, Germany) connected to a personal computer was used to determine and record the pressure changes inside the equilibrium cell. The operating range of this sensor is in the range from 0 to 6 MPa, and its accuracy is 1% of the full scale output. Due to the sensitivity of the measurement results to pressure variations, this sensor was calibrated by using a highly D



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pressure shown by the transducer reached the intended pressure. The middle cell was then detached from the system and was weighed to calculate the amount of injected gas in the equilibrium cell. The stirrer was switched on to work during the experiment at a constant speed. The injected gas is placed in direct contact with the solution inside the cell, and after the absorption process, vapor−liquid equilibrium is achieved in this container. The equilibrium point is obtained when no pressure change is observed for at least 30 min. To continue the work for higher pressures up to 4 MPa, a greater amount of CO2 is injected into the cell. The moles of CO2 in the gas phase under equilibrium conditions is calculated by applying the software Thermofluids.64 This software employs the most accurate equation of state for CO2Span and Wagner65at the corresponding temperature and pressure. The reference Span and Wagner65 equation of state for pure CO2 covers from the triple-point temperature up to very high pressures and temperatures with very high accuracy.66 Therefore, the mole number of CO2 dissolved in the liquid phase is calculated as

Table 3. Mass Composition (100w) of Hybrid Samples Used for CO2 Solubility Measurementa mass composition (100w)

a

sample

MDEA

[Bpy][BF4]

H2O

1 2 3 4 5 6 7 8 9

25.73 30 30 30 30 50 50 50 50

0 0 5 10 20 0 5 10 20

74.27 70 65 60 50 50 45 40 30

Standard uncertainties u are u(m) = 0.001 g.

equilibrium cell and placing the other side in the container containing the absorbent solution, all of the solution was sucked into the evacuated equilibrium cell. The mass of the solution inside the equilibrium cell was obtained by calculating the weight difference of the container containing the absorbent solution, before and after the solution was fed into the cell. The temperature of the cell was set inside the water bath at the desired temperature so that the solution was at equilibrium with its own vapor pressure. The vapor pressures of aqueous solutions (Pv) in the vacuum condition of the cell were precisely measured at different experimental temperatures by the same setup. However, another pressure transducer (Keller PPA-33X, 0−4.5 MPa, Switzerland) with an accuracy of 0.01% of the total pressure was used before the injection of CO2. The maximum vapor pressure was measured to be 26 kPa at 343.15 K. The partial pressure of CO2 in the gas phase is calculated by subtracting the pressure of the equilibrium cell and the vapor pressure of CO2free solvent at the experimental temperatures. The middle cell was filled with pure CO2 from a main CO2 cylinder and was weighed with ±0.01 g accuracy. The middle cell was then connected to the system, and CO2 entered into the equilibrium cell and maintained at this condition until the

g l nCO = nCO2 − nCO 2 2

(1)

ngCO2

where nCO2 and are the total moles of CO2 injected into the equilibrium cell and the moles of CO2 in the gas phase, respectively. CO2 solubility is defined as the molality of it in the liquid phase mCO2 that may be written as follows mCO2 =

l nCO 2

wsol

=

⎛ wCO2 ⎞ ⎛ PCO2V g ⎞ ⎜ ⎟ − ⎜ ⎟ ⎝ MWCO2 ⎠ ⎝ ZCO2RT ⎠ wsol

(2)

nlCO2

where and wsol are the number of moles of CO2 in the liquid phase and the mass of hybrid solvent in kg and wCO2 and MWCO2 are the total mass of CO2 injected into the equilibrium cell (kg) and the molar mass of CO2 (kg·mol−1). PCO2, ZCO2, R, and T are the partial pressure of CO2 (MPa), compressibility factor of CO2

Table 4. Density (kg·m−3) of Pure MDEA and [Bpy][BF4] Measured in This Work and Comparison with Literature Values at Different Temperatures and a Pressure of 86 kPaa 10−3ρ (kg·m−3) T (K) chemical MDEA

298.15

313.15

323.15

333.15

343.15

ref.

1.0364

1.0251 1.0254 1.02652 1.02470 1.02670 1.02445 1.02490 1.02460 1.02500 1.2038 1.20300 1.20387 1.20260 1.20372 1.2032 1.20280

1.0175 1.0178 1.01888 1.01733 1.01940 1.01666 1.01740 1.01700 1.01740 1.1970 1.19700 1.19696 1.19600 1.19668 1.1964 1.19600

1.0099 1.0010 1.01143 1.00987 1.01230 1.00900 1.00980 1.00930 1.00960 1.1903 1.19000 1.19007 1.18890 1.18992 1.1896 1.18920

1.0022 1.0023 1.00332 1.00232

this work Shojaeian et al.33 Bernal-Garciá et al.69 DiGuilio et al.70 Al-Ghawas et al.71 Maham et al.72 Li et al.73 Wang et al.74 Rebolledo-Libreros et al.75 this work Blanchard et al.13 Guerrero et al.76 Tomida et al.77 Bandrés et al.78 Mokhtarani et al.79 Song et al.80

1.03786 1.03740 1.03590

[Bpy][BF4]

1.2139 1.21434 1.21370 1.2134 1.21310

a

1.00124 1.00230

1.1836

1.18200 1.1829 1.18250

Standard uncertainties u: u(ρ) = 2 kg·m−3, u(T) = 0.1 K, u(P) = 2 kPa. E



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Table 5. Density of MDEA (1) + [Bpy][BF4] (2) + H2O (3) at Various Temperatures and a Pressure of 86 kPa with Parameters (A0, A1) and Absolute Average Deviation (AAD%)a 10−3ρ (kg·m−3) T (K) wt1%

wt2%

25.7 30.0 30.0 30.0 30.0 50.0 50.0 50.0 50.0 a

0 0 5.0 10.0 20.0 0.0 5.0 10.0 20.0

wt3% 74.3 70.0 65.0 60.0 50.0 50.0 45.0 40.0 30.0

298.15 1.0205 1.0247 1.0346 1.0434 1.0612 1.0425 1.0514 1.0596 1.0755

313.15 1.0135 1.0171 1.0263 1.0347 1.0506 1.0328 1.0413 1.0490 1.0644

323.15

333.15

1.0096 1.0188 1.0281 1.0450 1.0260 1.0342 1.0417 1.0567

1.0027 1.0093 1.0216 1.0370 1.0187 1.0268 1.0340 1.0489

343.15 0.9889 0.9951 0.9995 1.0126 1.0264 1.0112 1.0191 1.0258 1.0409

b

A0

A1 × 104

AAD%

1.2132 1.2238 1.2702 1.2453 1.2862 1.2503 1.2661 1.2833 1.3049

−6.4278 −6.6405 −7.8374 −6.7441 −7.5181 −6.9548 −7.1880 −7.4922 −7.6854

0.104 0.074 0.143 0.084 0.112 0.036 0.030 0.036 0.018

Standard uncertainties u are u(ρ) = 2 kg·m−3, u(T) = 0.1 K, u(P) = 2 kPa, and u(wt) = 0.001. bAt T = 348.17 K.

gas, gas constant (m3·MPa·K−1·mol−1), and temperature (K), respectively. Vg is the volume of gas phase in the equilibrium cell (m3) that is calculated from the total volume of the equilibrium cell and the volume of solvent, which in turn is calculated using the density of the solvent at experimental temperature as follows w V g = V − sol ρsol (3)

reported data of this work for both MDEA and IL. These discrepancies can result from using different experimental techniques or apparatus, and also the water content and other impurities that may exist in IL. Experimental values in Table 5 reveal that the density of ternary solutions decreases with increasing temperature. Furthermore, it can be observed that the density of solution increases with the weight percent of [Bpy][BF4]. The densities of aqueous solutions were regressed linearly with temperature as follows

where V is the volume of the equilibrium cell (m3) and ρsol is the solvent density (kg·m−3). Furthermore, the CO2 loading in the liquid phase is defined as the moles of CO2 absorbed in the liquid phase over the moles of MDEA in the solution as follows α=

10−3ρ /(kg·m−3) = A 0 + A1(T /K)

The percent of absolute average deviation (AAD %) between the experimental and calculated values is obtained as follows

l nCO 2

nMDEA

(5)

AAD% = (4)

100 n

n

∑ |(ρi ,exp − ρi ,cal )/ρi ,exp | i=1

(6)

where ρ represents the density and the subscripts “exp” and “cal” stand for experimental and calculated values, respectively. The coefficients A0 and A1 represent regressed parameters that are calculated by linear fitting of the data of this study. Table 5 reports the densities of the aqueous solutions for different compositions in the temperature range 298.15−343.15 K and at ambient pressure with the values of A0, A1, and AAD%. 3.2. CO2 Solubility in Aqueous MDEA + IL Solutions. In the beginning, to check and validate the accuracy of the present apparatus and the procedure of the experiments, a comparison was made with the previous work in the literature. For this purpose, according to Sidi-Boumedine et al.,81 the CO2 solubility in the aqueous solution of 25.73 wt % MDEA at temperatures of 298.08, 313.17, and 348.17 K and pressures up to 4.1 MPa was measured, as shown in Figures 3. This group, in a comprehensive study, measured the CO2 solubility with three different setups, namely, computer-operated, manually operated, and analytic apparatus.81 It can be seen in Figure 3 that the experimental data obtained in this work are in good agreement with the data of SidiBoumedine et al.81 The quality of solubility measurement on this apparatus and validation of results can be discussed by calculating the average relative deviation (ARD%) and the maximum relative deviation (MRD%) for CO2 loading in this work (TW) and SidiBoumedine et al.81 (ref) according to eqs 7 and 8.23

Meanwhile, it is necessary to say that, because of the low vapor pressure of MDEA in the temperature range 293−413 K (0.00076−2.58567 kPa), as well as very low IL vapor pressure, the vapor phase is considered to be free of them.67 Therefore, the vapor phase only contains CO2 and H2O molecules and all ionic species are considered only in the liquid phase.68 In addition, changes in the liquid phase volume during the absorption of CO2 are ignored and liquid phase is assumed to be incompressible. This claim was already justified for pure [Bpy][BF4], and the effect of CO2 absorption in volume expansion of this IL was found to be negligible.13 Regarding the MDEA + IL mixtures, the incompressibility assumption of absorbent solutions was formerly considered in the literature.47 To verify the repeatability of the experiments, a number of solutions and some runs were retested. Repeated experiments indicated that the results were reproducible with a maximum error of 4%.

3. RESULTS AND DISCUSSION 3.1. The Density of the Aqueous MDEA + IL Solutions. The density of absorbent solutions, pure MDEA, and [Bpy][BF4] were measured at different temperatures, and their amounts are reported in Tables 4 and 5. To be sure and validate the results, the densities of pure compounds were compared with the values given in the literature, as shown in Table 4. Comparison of the results shows a good agreement between the values of this work and literature data at different temperatures. However, there are some discrepancies in the values of the density between all of the literature values and

ARD% = F

100 N

N

∑ i=1

αTW (T , p) − αRef (T , p) αRef (T , p)

(7)



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⎛ α ( T , p ) − α ( T , p) ⎞ Ref ·100⎟⎟ MRD% = max⎜⎜ TW αRef (T , p) ⎝ ⎠

(8)

The calculated values of ARD% and MRD% from the above equations for CO2 loading in the aqueous 25.73 wt % MDEA solution were obtained as 0.848 and 4.465, respectively. Therefore, the results of solubility comparison with a maximum error of less than 5% indicated good consistency between the equilibrium data of this work and reliable literature values reported by Sidi-Boumedine et al.,81 which proved the production of reliable data. In this research, the experiments for solubility measurements of CO2 in {MDEA + [Bpy][BF4] + H2O} were performed at different temperatures, pressures, and component concentrations. The MDEA concentration was kept constant at 30 and 50 wt %, which was close to the typical concentration used in a gas treatment plant. The concentrations of IL in each aqueous solution were 5, 10, and 20 wt %. The results of CO2 solubility in the absorbent solutions have been presented through the two conventional scalesCO2 loading in a liquid mixture (mol of CO2/mol of MDEA) and CO2 molality based on an absolute scale (mol of CO2/kg of hybrid solvent)and reported in Tables 6 and 7. The solubility data presented as the partial pressure of CO2 versus the CO2 loading are shown in Figures 4 and 5. These figures illustrate the partial pressure of CO2 against the CO2 loading at various concentrations of MDEA + IL systems. Furthermore, the solubility results in aqueous MDEA + IL solutions shown in Figures 4 and 5 have been fitted by a semiempirical model according to eq 982 ln PCO2 = a + b/T + cα + dα /T + eα 2

(9)

where PCO2, T, and α denote the equilibrium partial pressure of CO2 (kPa), temperature (K), and CO2 loading (mol of CO2/mol of MDEA), respectively. In fact, PCO2 is only assumed as a function of temperature and CO2 loading. As one can observe from Figures 4 and 5 and also the values of AAD% in Table 8, the calculated values from the semiempirical model indicated good consistency with the experimental values over a temperature range from 298.15 to 343.15 K. The percent of absolute average deviation (AAD %) between the experimental and calculated values is obtained as follows AAD% =

100 n

n

∑ i=1

|P exp − P cal|i Piexp

(10)

Table 8 demonstrates the regressed parameters of the model and also the values of AAD% for different absorbent solutions used in this study. 3.3. Effect of Temperature and Pressure on the CO2 Solubility. Temperature and pressure play a key role in determining the absorption capacity of the dissolved gas. According to Figures 4 and 5 for CO2 solubility in absorbent solutions, by increasing temperature or decreasing pressure, the solubility of CO2 in absorbent solutions is reduced, which can be generalized to all studied systems in this research. The reason for the existence of such a relationship between the solubility of CO2 gas and temperature can be found in the chemistry of dissolution. In fact, more dissolved gas is present in a solution at a lower temperature than a higher temperature solution. The attractive intermolecular interactions in the gas phase for most of the materials are basically zero. When a gas is dissolved, this is due to

Figure 3. Comparison of experimental and literature data for solubility of CO2 in the aqueous 25.73 wt % MDEA solution at different pressures: (a) T = 298.08 K; (b) T = 313.17 K; (c) T = 348.17 K. ●, this work; ■, Sidi-Boumedine et al.81 (computer-operated apparatus); ▲, SidiBoumedine et al.81 (manually operated apparatus); ◆, Sidi-Boumedine et al.81 (analytic apparatus). G


H

a

0.927 0.990 1.040 1.072 1.096 1.124 1.152 1.177

0.925 1.000 1.040 1.070 1.096 1.116 1.143 1.178

0.952 1.029 1.062 1.088 1.106 1.123 1.156 1.162

1.026 1.085 1.123 1.141 1.165 1.186 1.211 1.225

αCO2

c

0.35 0.74 1.25 1.70 2.13 2.55 3.00 3.43

0.30 0.73 1.27 1.76 2.25 2.68 3.13 3.58

0.33 0.84 1.40 1.81 2.16 2.56 3.00 3.34

0.80 1.36 1.82 2.16 2.46 2.81 3.09 3.48

PCO2 (MPa)

−1

2.228 2.413 2.517 2.612 2.671 2.741 2.793 2.879

2.241 2.445 2.545 2.620 2.677 2.743 2.794 2.871

2.310 2.516 2.610 2.669 2.707 2.748 2.815 2.840

2.542 2.656 2.759 2.796 2.847 2.885 2.949 3.002

mCO2 (mol·kg )

b

T = 313.15 K

0.885 0.958 1.000 1.037 1.061 1.089 1.109 1.143

0.890 0.971 1.011 1.041 1.063 1.089 1.110 1.140

0.918 0.999 1.037 1.060 1.075 1.092 1.118 1.128

1.010 1.055 1.096 1.111 1.131 1.146 1.171 1.193

αCO2 c

mCO2 (mol·kg )

−1

αCO2 c

30.0 wt % MDEA + 0 wt % [Bpy][BF4] 0.89 2.493 0.990 1.47 2.609 1.036 1.95 2.711 1.077 2.29 2.762 1.097 2.62 2.799 1.112 2.97 2.855 1.134 3.28 2.904 1.154 3.70 2.952 1.172 30.0 wt % MDEA + 5.0 wt % [Bpy][BF4] 0.43 2.241 0.890 0.94 2.460 0.977 1.51 2.563 1.018 1.94 2.620 1.041 2.30 2.663 1.058 2.73 2.695 1.070 3.20 2.755 1.094 3.55 2.790 1.108 30.0 wt % MDEA + 10.0 wt % [Bpy][BF4] 0.40 2.170 0.862 0.84 2.378 0.944 1.38 2.493 0.990 1.89 2.568 1.020 2.40 2.627 1.043 2.86 2.684 1.066 3.32 2.750 1.092 3.80 2.825 1.122 30.0 wt % MDEA + 20 0.0 wt % [Bpy][BF4] 0.47 2.142 0.851 0.87 2.328 0.925 1.37 2.455 0.975 1.84 2.547 1.012 2.29 2.605 1.035 2.73 2.675 1.062 3.21 2.719 1.080 3.66 2.809 1.116

PCO2 (MPa)

b

T = 323.15 K

0.64 1.04 1.54 2.01 2.47 2.93 3.41 3.89

0.53 0.99 1.53 2.05 2.57 3.04 3.53 4.05

0.56 1.07 1.66 2.10 2.47 2.90 3.39 3.77

1.02 1.60 2.10 2.45 2.80 3.16 3.47 3.90

PCO2 (MPa)

−1

2.023 2.217 2.358 2.464 2.529 2.598 2.665 2.750

2.081 2.283 2.412 2.494 2.564 2.631 2.691 2.752

2.153 2.385 2.488 2.551 2.599 2.649 2.713 2.739

2.417 2.548 2.649 2.704 2.736 2.799 2.864 2.926

mCO2 (mol·kg )

b

T = 333.15 K

0.804 0.881 0.936 0.979 1.005 1.032 1.059 1.092

0.827 0.907 0.958 0.991 1.018 1.045 1.069 1.093

0.855 0.947 0.988 1.013 1.032 1.052 1.078 1.088

0.960 1.012 1.052 1.074 1.087 1.112 1.138 1.162

αCO2 c

0.83 1.26 1.76 2.24 2.69 3.17 3.66 4.16

0.71 1.17 1.71 2.24 2.76 3.25 3.76 4.28

0.72 1.26 1.83 2.28 2.65 3.10 3.60 4.00

1.18 1.77 2.28 2.63 2.96 3.35 3.68 4.13

PCO2 (MPa)

1.897 2.076 2.228 2.339 2.427 2.495 2.573 2.663

1.961 2.172 2.315 2.404 2.491 2.559 2.623 2.712

2.049 2.269 2.404 2.472 2.533 2.582 2.659 2.685

2.323 2.461 2.568 2.636 2.698 2.750 2.811 2.876

mCO2b (mol·kg−1)

T = 343.15 K

Standard uncertainties u are u(T) = 0.1 K, u(P) = 0.01 MPa, u(mCO2) = 0.005 mol·kg−1, and u(wt) = 0.001. bαCO2 = mol of CO2/mol of MDEA. cmCO2 = mol of CO2/kg of hybrid solvent.

2.333 2.493 2.619 2.698 2.760 2.831 2.900 2.964

0.21 0.61 1.07 1.51 1.91 2.30 2.70 3.11

2.398 2.591 2.675 2.739 2.784 2.826 2.911 2.925

0.21 0.71 1.25 1.63 1.95 2.32 2.71 3.03

2.330 2.517 2.617 2.693 2.758 2.811 2.877 2.966

2.583 2.733 2.828 2.873 2.933 2.987 3.048 3.084

0.71 1.20 1.64 1.95 2.22 2.53 2.79 3.16

0.18 0.61 1.12 1.58 2.03 2.44 2.84 3.24

mCO2 (mol·kg )

−1

PCO2 (MPa)

b

T = 298.15 K

Table 6. Solubility of CO2 in Aqueous Solutions of 30 wt % MDEA + 0−20 wt % [Bpy][BF4] at Various Temperaturesa

0.753 0.825 0.885 0.929 0.964 0.991 1.022 1.058

0.779 0.863 0.920 0.955 0.989 1.017 1.042 1.077

0.814 0.901 0.955 0.982 1.006 1.026 1.056 1.067

0.923 0.978 1.020 1.047 1.072 1.092 1.117 1.142

αCO2c

Journal of Chemical & Engineering Data Article


I

a

0.810 0.898 0.962 0.996 1.023 1.038 1.050 1.064

0.838 0.929 0.986 1.017 1.036 1.048 1.059 1.071

0.880 0.955 0.995 1.012 1.026 1.038 1.052 1.059

0.914 0.983 1.010 1.027 1.037 1.050 1.061 1.072

αCO2

c

0.56 0.89 1.35 1.75 2.13 2.47 2.71 2.97

0.41 0.69 1.21 1.77 2.28 2.62 2.96 3.22

0.44 0.82 1.35 1.76 2.16 2.53 2.98 3.30

0.52 0.98 1.47 1.80 2.16 2.50 2.78 3.19

PCO2 (MPa)

−1

3.211 3.536 3.793 3.942 4.074 4.137 4.201 4.243

3.397 3.727 3.959 4.107 4.195 4.258 4.298 4.347

3.558 3.850 4.021 4.112 4.178 4.231 4.292 4.318

3.718 3.972 4.108 4.178 4.234 4.290 4.338 4.384

mCO2 (mol·kg )

b

T = 313.15 K

0.765 0.843 0.904 0.939 0.971 0.986 1.001 1.011

0.810 0.888 0.944 0.979 1.000 1.015 1.024 1.036

0.848 0.918 0.958 0.980 0.996 1.008 1.023 1.029

0.886 0.947 0.979 0.996 1.009 1.022 1.034 1.045

αCO2 c

mCO2 (mol·kg )

−1

αCO2 c

50.0 wt % MDEA + 0 wt % [Bpy][BF4] 0.69 3.596 0.857 1.18 3.835 0.914 1.68 3.975 0.947 2.00 4.064 0.968 2.36 4.133 0.985 2.73 4.173 0.995 3.00 4.244 1.011 3.42 4.302 1.025 50.0 wt % MDEA + 5.0 wt % [Bpy][BF4] 0.62 3.426 0.816 1.03 3.701 0.882 1.55 3.894 0.928 1.97 3.988 0.950 2.37 4.067 0.969 2.75 4.126 0.983 3.22 4.188 0.998 3.54 4.232 1.009 50.0 wt % MDEA + 10.0 wt % [Bpy][BF4] 0.59 3.265 0.778 0.92 3.558 0.848 1.44 3.801 0.906 2.00 3.964 0.945 2.52 4.060 0.968 2.87 4.126 0.983 3.21 4.183 0.997 3.49 4.224 1.007 50.0 wt % MDEA + 20.0 wt % [Bpy][BF4] 0.80 3.028 0.722 1.16 3.333 0.794 1.65 3.573 0.851 2.05 3.730 0.889 2.44 3.862 0.920 2.77 3.946 0.940 3.03 3.999 0.953 3.28 4.061 0.968

PCO2 (MPa)

b

T = 323.15 K

1.10 1.50 2.00 2.41 2.82 3.15 3.39 3.65

0.84 1.21 1.73 2.29 2.81 3.18 3.51 3.80

0.86 1.29 1.82 2.23 2.64 3.03 3.50 3.84

0.91 1.42 1.91 2.25 2.63 2.98 3.26 3.70

PCO2 (MPa)

−1

2.807 3.083 3.322 3.479 3.601 3.693 3.774 3.836

3.085 3.352 3.604 3.780 3.891 3.950 4.029 4.073

3.255 3.521 3.716 3.829 3.911 3.974 4.055 4.094

3.442 3.675 3.835 3.915 3.977 4.049 4.121 4.178

mCO2 (mol·kg )

b

T = 333.15 K

0.669 0.735 0.792 0.829 0.858 0.880 0.899 0.914

0.735 0.799 0.859 0.901 0.927 0.941 0.960 0.971

0.776 0.839 0.886 0.913 0.932 0.947 0.966 0.976

0.820 0.876 0.914 0.933 0.948 0.965 0.982 0.996

αCO2 c

1.46 1.90 2.43 2.84 3.26 3.59 3.84 4.11

1.14 1.54 2.08 2.65 3.16 3.52 3.88 4.16

1.15 1.60 2.14 2.56 2.97 3.35 3.83 4.17

1.21 1.74 2.23 2.57 2.94 3.30 3.57 4.01

PCO2 (MPa)

2.550 2.798 3.019 3.182 3.304 3.404 3.482 3.541

2.877 3.126 3.371 3.549 3.683 3.762 3.825 3.889

3.055 3.313 3.509 3.623 3.716 3.799 3.888 3.941

3.235 3.461 3.630 3.718 3.799 3.873 3.964 4.039

mCO2b (mol·kg−1)

T = 343.15 K

Standard uncertainties u are u(T) = 0.1 K, u(P) = 0.01 MPa, u(mCO2) = 0.005 mol·kg−1, and u(wt) = 0.001. bαCO2 = mol of CO2/mol of MDEA. cmCO2 = mol of CO2/kg of hybrid solvent.

3.400 3.768 4.038 4.180 4.293 4.358 4.406 4.463

0.32 0.59 1.02 1.41 1.79 2.11 2.35 2.58

3.694 4.008 4.175 4.245 4.307 4.355 4.413 4.444

0.26 0.60 1.11 1.52 1.90 2.25 2.67 2.96

3.517 3.898 4.138 4.269 4.348 4.397 4.444 4.493

3.834 4.123 4.237 4.308 4.353 4.405 4.453 4.497

0.36 0.76 1.25 1.56 1.91 2.23 2.49 2.87

0.25 0.46 0.95 1.50 1.99 2.32 2.63 2.87

mCO2 (mol·kg )

−1

PCO2 (MPa)

b

T = 298.15 K

Table 7. Solubility of CO2 in Aqueous Solutions of 50 wt % MDEA + 0−20 wt % [Bpy][BF4] at Various Temperaturesa

0.608 0.667 0.719 0.758 0.787 0.811 0.830 0.844

0.686 0.745 0.803 0.846 0.878 0.897 0.911 0.927

0.728 0.790 0.836 0.863 0.886 0.905 0.927 0.939

0.771 0.825 0.865 0.886 0.905 0.923 0.945 0.963

αCO2c

Journal of Chemical & Engineering Data Article



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Figure 4. Partial pressure of CO2 above liquid mixtures of (CO2 + MDEA + [Bpy][BF4] + H2O) with a composition of 30 wt % MDEA + 0−20 wt % [Bpy][BF4] based on CO2 loading. Scattering points: experimental data at a temperature of (■) T = 298.15 K, (◆) T = 313.15 K, (▲) T = 323.15 K, (★) T = 333.15 K, and (●) T = 343.15 K. Solid lines: model predictions.

solutions at a temperature of 313.15 K. As one can observe from Figure 6, adding [Bpy][BF4] to both 30 and 50 wt % aqueous MDEA solution leads to a reduction in CO2 loading and, if the concentration of IL is increased from 5 to 20 wt %, the falling trend of CO2 absorption becomes more considerable. This absorption capacity drop has been reported by other researchers for [Bmim][BF4], [Bmim][Ac], and [Bmim][DCA] in MDEA aqueous solution.33,35 The reason for such behavior may be found in the components contributed in the chemical reactions of the absorption process. MDEA as a tertiary alkanolamine is not capable of having reaction directly with CO2 so that water plays the main role with a high impact on CO2 chemical absorption. The main reaction for tertiary amine in the CO2 absorption process is described as84

interactions between gas and solvent molecules. Since the formation of such new attractive interactions is associated with the release of heat, the solubility of most of the gases in the liquids is an exothermic process (ΔHsoln < 0). Conversely, adding heat to the solution provides thermal energy that overcomes the attractive forces between gas and solvent molecules and, consequently, decreases the solubility of the gas.83 This behavior is quite the opposite in some gases such as argon, and due to the endothermic process of dissolution of argon in liquids, the solubility of this gas increases through enhancement of temperature.17 Furthermore, as was expected, by enhancing the CO2 partial pressure, the solubility of the gas increases. This phenomenon may be explained by this fact that, in the case of increasing pressure, the gas molecules are forced into the solution, since this will best relieve the pressure that has been applied, so that the number of gas molecules is diminished and the number of gas molecules dissolved in solution is boosted.83 3.4. Effect of the IL’s Concentration on the CO2 Solubility. Figure 6 illustrates the partial pressure of CO2 against the CO2 loading in the case of adding 0, 5, 10, and 20 wt % of [Bpy][BF4] to the aqueous 30 and 50 wt % MDEA

KMDEA − CO2

R1R 2R3 N: + H 2O + CO2 ←⎯⎯⎯⎯⎯⎯⎯⎯⎯→ R1R 2R 3NH+ + HCO3− (11)

The second order reaction rate constant has been presented by Versteeg et al.85 for the reaction of CO2 with aqueous MDEA in the form of the following correlation: J



Article

Figure 5. Partial pressure of CO2 above liquid mixtures of (CO2 + MDEA + [Bpy][BF4] + H2O) with a composition of 50 wt % MDEA + 0−20 wt % [Bpy][BF4] based on CO2 loading. Scattering points: experimental data at temperature of (■) T = 298.15 K, (◆) T = 313.15 K, (▲) T = 323.15 K, (★) T = 333.15 K, and (●) T = 343.15 K. Solid lines: model predictions.

Table 8. Regressed Values of Parameters for the Semiempirical Model (eq 9) for Different Weight Percent (MDEA (1) + [Bpy][BF4] (2) + H2O (3)) Solutions absorbent wt1%

wt2%

wt3%

a

b

c

d

e

AAD%

30 30 30 30 50 50 50 50

0 5 10 20 0 5 10 20

70 65 60 50 50 45 40 30

32.20 45.07 54.67 57.80 54.92 55.10 55.40 50.94

−10795.61 −15596.77 −20443.10 −20673.02 −17181.45 −17198.32 −17052.17 −16821.82

−14.69 −25.33 −29.11 −32.79 −39.42 −38.83 −38.96 −28.12

7883.56 11859.58 16843.03 16917.20 13565.32 13380.25 13256.53 13650.90

−1.43 −1.00 −7.07 −5.96 3.62 3.64 3.41 −4.82

0.66 0.86 0.86 0.78 0.45 0.44 0.56 0.55

⎛ 5080 3 ⎞ kam = 3.1 × 105 exp⎜ − [m mol−1 s−1]⎟ ⎝ T ⎠

a physical absorbent in the solution and consequently diminishes the effect of water, which contributed to the chemical absorption of CO2. In other words, at a fixed concentration of MDEA, a greater amount of water in the MDEA aqueous solution facilitates the chemical absorption of CO2. Therefore, the CO2 absorption capacity of a mixture containing IL decreases in comparison to MDEA aqueous solution at the same concentration. However, this reason cannot be generalized for all of the

(12)

Furthermore, in this research, according to Table 3, by adding [Bpy][BF4] to the MDEA aqueous solution, the amount of water in the hybrid solution diminishes and IL precisely replaces the water in the solution. Due to the low absorption capacity of physical absorbents compared to chemical absorbents in low pressures, by replacing [Bpy][BF4] instead of water, the IL acts as K



Article

Figure 6. Effect of addition 0, 5, 10, and 20 wt % of [Bpy][BF4] to the aqueous 30 and 50 wt % MDEA solution on the CO2 loading at a temperature of 313.15 K and comparison with literature values. (a) ◆, 30% MDEA + 0% IL; ■, 30% MDEA + 5% IL; ▲, 30% MDEA + 10% IL; ●, 30% MDEA + 20% IL; ★, 20.9% MDEA + 30.5% sulfolane.87 (b) ◇, 50% MDEA + 0% IL; □, 50% MDEA + 5% IL; △, 50% MDEA + 10% IL; ○, 50% MDEA + 20% IL; ☆, 47% MDEA + 3% DIPA.88 Lines are used only to join the experimental data points.

changes in CO2 solubility in aqueous MDEA + IL mixtures, because different types of ILs have various effects on CO2 absorption with the same concentration of water and MDEA. Furthermore, according to Figure 6, it can be seen that by increasing the partial pressure of CO2 both chemical and physical absorption take place together, which causes a reduction in the deviation for all mixtures containing IL. A precise investigation of the solutions’ solubility trend in Figure 6 shows that the addition of IL exhibits better effects at high pressures, which can pertain to the high absorption capacity of physical solvents compared to chemical solvents at high pressures.86 Therefore, using this concept, it may be possible that, in the case of increasing the partial pressure of CO2 to higher pressures than this, the absorption capacity of MDEA aqueous solutions will be enhanced by adding [Bpy][BF4]. Last but not least, the addition of [Bpy][BF4], despite a reduction in the absorption capacity at low pressures, might have a positive effect on gas solubility at high partial pressures of CO2, which is suitable for the natural gas sweetening process at the industrial scale. The solubility of CO2 in the present research is compared with two other aqueous solutions reported in the literature87,88 containing 20.9 wt % MDEA + 30.5 wt % sulfolane + 48.6 wt % water and 47 wt % MDEA + 3 wt % diisopropanol amine (DIPA) + 50 wt % water at a temperature of 313.15 K in Figure 6. As one can observe, the addition of sulfolane as a physical solvent to MDEA aqueous solution leads to a notable increment in CO2 loading from pressures higher than 0.9 MPa compared to 30 wt % MDEA aqueous solution, which indicates the considerable impact of physical solvents at high pressures.87 By substituting 3 wt % DIPA instead of MDEA at a fixed 50 wt % MDEA + DIPA composition at a temperature of 313.15 K, a smooth diminution of CO2 loading is observed.88 This reduction of CO2 loading may arise from the lower capacity of the secondary amine (DIPA) in CO2 absorption relative to the tertiary amine (MDEA), as was already reported by Jenab et al.89 and Liang et al.90 The solubility of CO2 in pure [Bpy][BF4] and also absorbent solution containing 30 wt % MDEA + 10 wt % [Bmim][acetate] is illustrated in Figure 7 and compared with the solubility data of this work at a temperature of 323.15 K based on the CO2

Figure 7. Comparison of the CO2 solubility results in absorbent solutions used in this work with literature values based on the CO2 molality at a temperature of 323.15 K. ◆, 30% MDEA + 0% [Bpy][BF4]; ■, 30% MDEA + 5% [Bpy][BF4]; ▲, 30% MDEA + 10% [Bpy][BF4]; ●, 30% MDEA + 20% [Bpy][BF4]; ☆, 30% MDEA + 10% [Bmim][acetate];33 ★, [Bpy][BF4].13 Lines are used only to join the experimental data points.

molality. This comparison is performed to display the CO2 absorption capacity in pure IL relative to hybrid solvents and also to show the effect of addition of another type of IL to MDEA aqueous solution. As shown in Figure 7, the CO2 absorption capacity of the [Bpy][BF4] is much lower than the hybrid solvents used in the present work. This result reveals that the CO2 solubility in MDEA + IL + H2O solutions is lower than MDEA aqueous solutions, but they are far more than pure IL. This trend can be seen up to pressures of almost 4.5 MPa. However, due to the incremental trend of the solubility curve for pure IL, compared to hybrid solvents, it can be expected that, at higher pressures above this value, the pure [Bpy][BF4] possesses a higher amount of CO2 uptake. Furthermore, it can be seen that L



Article

Also, the results shows that addition of [Bpy][BF4] to MDEA aqueous solutions leads to a reduction in CO2 loading with respect to the free IL of MDEA solutions and this downtrend continues if the concentration of IL is enhanced. All in all, the addition of IL to aqueous MDEA solution may demonstrate a different behavior in CO2 absorption depending on the chemical structure of IL and alkanolamine. In this study, the comparison of MDEA aqueous solutions in the presence of [Bpy][BF4] has been mainly investigated in terms of variation of the CO2 absorption capacity. However, the addition of IL might enhance the rate of CO2 absorption in MDEA aqueous systems, or it can greatly facilitate the regeneration of absorbent solutions. Thus, accurate measurement in other characteristics of solvents such as CO2 absorption rate and regeneration performance is imperative to ascertain the possibility of utilizing such aqueous solutions of alkanolamine + IL in CO2 absorption at larger scales which may form a good priority for future works.

[Bmim][acetate] has a better effect from the CO2 solubility point of view compared to [Bpy][BF4] for addition to 30 wt % MDEA aqueous solution because the deviation of 30 wt % MDEA + 10 wt % [Bmim][acetate] solution from its corresponding IL free solution is somewhat lower than the solution containing 10 wt % [Bpy][BF4]. 3.5. Effect of Substitution of IL Instead of MDEA in Aqueous Solutions. The molality of CO2 at 50 wt % MDEA + IL and 323.15 K is illustrated in Figure 8 to demonstrate the

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.8b00142. NMR spectra of [BPy][BF4] (Figure S1) (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +98 2144787770. Fax: +98 2144787781. ORCID

Babak Mokhtarani: 0000-0002-8230-5646

Figure 8. Effect of substitution of [Bpy][BF4] instead of MDEA based on CO2 molality at a fixed 50 wt % MDEA + [Bpy][BF4] composition at a temperature of 323.15 K. ◆, 50% MDEA + 0% IL; ■, 30% MDEA + 20% IL. Lines are used only to join the experimental data points.

Funding

The authors are grateful to the National Iranian Gas Company (NIGC) for their financial support of this research. Notes

The authors declare no competing financial interest.

effect of replacement of [Bpy][BF4] instead of MDEA in absorbent solutions. As one can see, by keeping the total weight percent of both alkanolamine and IL constant at 50 wt %, the weight percent of [Bpy][BF4] changes from 0 to 20 wt % and the uptake amount of CO2 is diminished dramatically by replacing IL instead of MDEA. This behavior is due to the fact that [Bpy][BF4] presents very poor alkalinity compared with MDEA. A similar result in the reduction of absorption capacity in the case of replacing [Bmim][acetate] instead of MDEA has also been reported previously by Shojaeian et al.33

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4. CONCLUSION The new set of experimental data for the solubility of CO2 in the aqueous solutions of {30−50 wt % MDEA + 0−20 wt % [Bpy][BF4]} as new hybrid solvents was reported at temperatures from 298.15 to 343.15 K and pressures up to 4 MPa by using an isochoric saturation technique. Furthermore, the densities of these new hybrid solvents were measured at the experimental temperatures and regressed linearly with temperature. The results of the solubility measurements were represented as the CO2 partial pressure versus CO2 loading and the molality of gas. The experimental data revealed that the absorption capacity in all MDEA + [Bpy][BF4] aqueous solutions increases with increasing CO2 partial pressure or decreasing temperature. It had been found that the CO2 molality significantly diminishes in the case of substitution of [Bpy][BF4] instead of MDEA at fixed 50 wt % MDEA + IL composition. M



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