Solubilities of CO2 in, and Densities and Viscosities of, the

Feb 24, 2014 - ... as the increase of [Emim][Ac] concentration and decrease of PZ concentration with constant water content, and densities are vice ve...
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Solubilities of CO2 in, and Densities and Viscosities of, the Piperazine + 1‑Ethyl-3-methyl-imidazolium Acetate + H2O System Yun Li, Danxing Zheng,* Li Dong, Nan Nie, and Bin Xiong College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, China ABSTRACT: Densities and viscosities of hybrid absorbents piperazine (PZ) (1) + 1-ethyl-3-methyl-imidazolium acetate ([Emim][Ac]) (2) + H2O (3) were respectively measured with different mass compositions (w1/w2/w3 = 0.3/0/0.7, 0.3/0.1/0.6, 0.3/0.2/0.5, 0.3/0.3/0.4, 0.2/0.3/0.5, 0.1/0.3/0.6, 0/0.3/0.7) at temperatures ranging from 313.15 K to 343.15 K, and correlated with proper models. The density declines slightly while the viscosity decreases greatly with the increase of temperature. It is effective to lessen the viscosities of hybrid absorbents as the increase of [Emim][Ac] concentration and decrease of PZ concentration with constant water content, and densities are vice versa. Solubilities of CO2 in hybrid absorbents at T = 313.15 K to 343.15 K were measured by using an isothermal synthetic method under pressures of 0 MPa to 1.0 MPa. The results show that solubilities ascend with a decrease of temperature and increase of pressure. The addition of PZ highly improves the absorption ability of hybrid absorbent, while [Emim][Ac] has little effect on the CO2 absorption ability of a highly concentrated PZ aqueous solution. Furthermore, CO2 loading of 0.3 PZ + 0.1 [Emim][Ac] + 0.6 H2O is superior when it is compared with literature data, showing it is a potential CO2 absorbent deserving further research. The viscosity of the hybrid absorbent is much lower than that of pure [Emim][Ac], which improves the practical significance of hybrid absorbents.

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INTRODUCTION In recent years, great attention has been brought to CO2 capture and sequestration based on environmental and economic problems.1−3 CO2 is one of the most important greenhouse gases that are responsible for serious climate issues, such as global warming and rising of sea level. CO2 in natural gas lowers the fuel value of natural gas and increases corrosion of pipes and instruments of the natural gas process. CO2 in syngas poisons the catalyst of ammonia synthesis reaction and decreases the rate of production. On the other hand, CO2 gas itself is of great value that it can be used as an additive for the food industry and precursor to chemicals for chemical industry. Supercritical CO2 is innocuous and unpoisonous and can be used to enhance oil recovery and extract important components from mixtures. Solid CO2 can be used as refrigerant, fire extinguisher, and so on. There are three kinds of methods to capture CO2 such as precombustion, oxy-fuel combustion, and postcombustion. Presently, the way to treat a large amount of CO2 from mixed gases in huge gas separation equipment is commonly absorption by solvents, such as chemical, physical, and so on. Physical absorbents absorb acid gas based on their good solvency in liquids at low temperature and high pressure. Industrial physical absorbents are as follows: methanol (MeOH), N-methyl pyrrolidone (NMP), propylene carbonate (PC), and polyethylene glycol dimethyl ether (DEPG). They are noncorrosive and favored for absorbing high concentration acid gas with low energy consumption. However, the absorption rate of CO2 and the purity of the output gas are low for the physical method. Chemical absorbents capturing CO2 is based on a chemical © 2014 American Chemical Society

reaction between the gas and liquid. Commercial chemical absorbents are amine aqueous solutions such as methylethanolamine (MEA), methyldiethanolamine (MDEA), diethanolamine (DEA), and so on.4 In particular, piperazine (PZ) aqueous solution is considered as a promising absorbent of CO2, because absorption rate and capacity are double those of an MEA aqueous solution, and less energy is required for PZ regeneration.5 Chemical methods are efficient, the output is of high purity, and they are suitable for low concentration of CO2 in the gas treatment. But during the absorbent regeneration, high cost, corrosion, degradation, and loss of amine are serious problems.6,7 Ionic liquids (ILs) also have been studied for CO2 capture because they are nonvolatile, nonflammable, have high thermal stability, and desorbed without loss of solvent.8,9 ILs that are used for CO2 capture can be divided as task-specific and conventional. Task-specific ILs containing amine groups absorb a large amount of CO2 because of a chemical reaction. However, they are not suitable for industrial CO2 capture, because the high viscosities are adverse to mass transfer between gas (solute) and liquid (solvent) molecules in the liquid phase.10 Conventional ILs such as 1-butyl-3-methyl-imidazolium hexafluorophosphate ([Bmim][PF 6 ]) and 1-hexyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide ([Hmim][Tf2N]) have high solubility and selectivity for high partial CO2 pressure.11,12 And most particularly, Yokozeki13 found 1-ethyl-3-methyl-imidazolium Received: May 21, 2013 Accepted: February 12, 2014 Published: February 24, 2014 618

dx.doi.org/10.1021/je400491p | J. Chem. Eng. Data 2014, 59, 618−625

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CO2 absorption capacities are measured under temperatures from 313.15 K to 343.15 K and pressures up to 1.0 MPa. Effects of temperature, pressure, and different compositions of hybrid absorbents on absorption ability and CO2 solubilities are assessed.

acetate ([Emim][Ac]) and 1-butyl-3-methyl-imidazolium acetate ([Bmim][Ac]) with an acetate group are less viscous and more soluble for CO2 especially at low pressure when they are compared with other conventional ILs. Zhang14 quoted much literature and compared the absorption capacity of 31 kinds of ionic liquids when there were only physical interactions between IL and CO2. It turned out that [Emim][Ac] showed good dissolving ability for CO2 when per mol absorbents were compared. What is more, per gram, the dissolving ability of [Emim][Ac] for CO2 is outstanding because of its small molecular weight. Baj15 selected seven conventional imidazoles ILs and added an IL to an MEA solution. The results showed that hybrid absorbents added with acetate anionic IL are the best for CO2 absorption. However, their absorption capacity is much smaller than task-specific ILs or alkanolamine solutions.14 Amines, ILs, and water were recently mixed for use as a prospective absorbent to capture more CO2 than ILs alone. The existence of water in these prospective absorbents is essential for lowering viscosity and enhancing mass transfer, making them useful for industry production.16 They are energy-saving especially when compared with traditional amine aqueous solutions. High energy consumption is needed for thermal regeneration of amine aqueous solutions because heat evaporation and the heat capacity of water are extremely high. However, heat capacities of ILs are about one-third that of water. There is less of an energy cost for solvent regeneration, because specific heat capacity and the evaporation heat of the absorbents are less than those of the amine aqueous solution as water is partly replaced by ILs.17 Besides, corrosion of instruments and pipes lessens with decreasing water. Moreover, the hybrid absorbent has a low loss of absorbent due to the zero vapor pressure of the ILs. There are various studies suggesting that using hybrid absorbents saves construction costs, improves gas output, reduces the dimension of the unit, and moderates corrosion. As a result, the ILs−amine−water absorbent is much closer to industrial applications.18 Many researchers are devoted to the study of basic thermophysical and thermodynamic properties and of the CO2 absorption behavior of hybrid absorbents, etc. Ahmady19 measured CO2 solubility and initial absorption rate at low partial pressures in MDEA + [Bmim][BF4] + H2O hybrid absorbents with various IL concentrations at T = 303 K to 333 K. The results showed that the addition of a low concentration of [Bmim][BF4] had no obvious effect on absorption capacity but raised the absorption rate. Furthermore, Ahmady20 determined the density and viscosity data of various concentrations of MDEA + [Bmim][BF4] + H2O solutions. Physical solubility and diffusivity of N2O in hybrid absorbents were determined, and all of data developed corrections for prediction. Zhang21 studied the absorption behavior of CO2 in highly concentrated MDEA and amino acid based IL at 298 K with the constant volume method. That work concluded that loading capacity increased dramatically with the increase of amine concentration, and was invariable with the addition of IL. This solution was suggested as an excellent absorbent for high regeneration efficiency and absorption capacity. Therefore, hybrid absorbents PZ (1)/[Emim][Ac] (2)/H2O (3) of different mass concentrations (w1/w2/w3 = 0.3/0/0.7, 0.3/0.1/0.6, 0.3/0.2/0.5, 0.3/0.3/0.4, 0.2/0.3/0.5, 0.1/0.3/0.6, 0/0.3/0.7) are selected in this paper as a comprehensive study. Thermophysical properties as densities and viscosities of hybrid absorbents at temperatures from 313.15 K to 343.15 K are determined. Effects of temperature and different compositions of hybrid absorbents on density and viscosity are discussed.

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EXPERIMENTAL SECTION Materials. CO2 was supplied by Beijing Zhaoge Sepecial Gas Co., Ltd. with a purity of better than 0.99999. PZ was obtained from Tokyo Chemical Industry Co., Ltd. with a purity of better than 0.98. [Emim][Ac] was purchased from Shanghai Chengjie Chemical Co., Ltd. The purity is better than 0.97 in mass fraction. The deionized water was obtained from Beijing University of Chemical Technology. Density Measurement. Densities of hybrid absorbents were determined using the pycnometer test method. Temperature of the pycnometer was kept constant by a water bath with temperature controller (Beijing Changliu Scientific Instrument Co., model A2) within an accuracy of ± 0.1 K. Water temperature was detected by a mercury-in-glass thermometer with a precision of 0.05 K. The mass of solvent was weighed by a Mettler Toledo AL 204 balance with an error of 10−4 g. The pycnometer with specification 5 mL was calibrated for volumes ranging from 313.15 K to 343.15 K for three times according to the density of deionized water. Also density determinations at temperatures were repeated three times for each measurement. The uncertainties of the pycnometer and densities were respectively 0.0001 cm3 and 0.0001 g·cm−3. To check the reliability of density determination, the density of ethylene glycol was measured and compared with the literature values. The maximum absolute relative deviations are 0.05 %, 0.15 %, 0.04 % and 0.08 % corresponding to temperatures from 313.15 K to 343.15 K, and the average absolute relative deviation are correspondingly 0.04 %, 0.07 %, 0.04 % and 0.08 %. Good correspondence with the literature data was observed as shown in Table 1. Table 1. Density Comparison of the Experimental Data and Literature Values22−24 for Ethylene Glycol at T = (313.15 to 343.15) K and p = 0.1 MPaa T/K

ρexp/g·cm−3

ρlit/g·cm−3

|ρexp − ρlit|/ρexp (%)

313.15

1.0989

1.098722 1.099323 1.098424 1.091722 1.092223 1.093324 1.084622 1.077322

0.02 0.04 0.05 0.00 0.05 0.15 0.04 0.08

323.15 1.0917 333.15 343.15 a

1.0842 1.0781

Standard uncertainties u are u(T) = 0.05 K and u(ρ) = 0.0001 g·cm−3.

Viscosity Measurement. Viscosities of hybrid absorbents at temperatures from 313.15 K to 343.15 K were measured by capillary viscometers which were put into the water bath as previously discussed.25 Time of sample flowing was recorded by a digital stopwatch with an accuracy of 0.01 s. Viscosity (η) can be calculated as follows:

ν = Ct η = νρ

(1) (2)

where ν is the kinetic viscosity, C is a viscometer constant which is calibrated with oil by the manufacturer, t is the flowing time, and ρ is the density. 619

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Initially PZ and [Emim][Ac] were placed into equilibrium cell. Then the whole system was evacuated by the vacuum pump, and water was injected by buret. Pure CO2 was charged into the gas chamber. The gas chamber and equilibrium cell were set to the desired temperature. Then a certain amount of CO2 was injected into the equilibrium cell. The magnetic stirrer was used to shorten the absorption time. As the pressure was constant for 2 h, the equilibrium was assumed. The solubility of CO2 in hybrid absorbents was calculated by the following equations:16

To validate the reliability of the experimental apparatus, the viscosities of ethylene glycol at T = 313.15 K to 343.15 K were compared with some literature data, and the results were listed in Table 2. The maximum absolute relative deviations are 1.44 %, Table 2. Viscosity Comparison of the Experimental Data and Literature Value23,26,27 for Ethylene Glycol at T = (313.15 to 343.15) K and p = 0.1 MPaa

a

T/K

ηexp/mPa·s

ηlit/mPa·s

|ηexp − ηlit|/ηexp (%)

313.15

9.309

323.15

6.818

333.15

5.027

343.15

3.915

9.44326 9.40423 9.40727 6.99226 6.77423 5.06026 5.03027 3.98726

1.44 1.02 1.05 2.55 0.65 0.66 0.06 1.84

nG − nE ntotal ⎡ VG pG1 − ⎣⎢ RT ZG1

α=

(

=

mCO2 =

Standard uncertainties u are u(T) = 0.05 K and u(η) = 0.005 mPa·s.

pG2 ZG2

)⎤⎦⎥ − ⎡⎣⎢

(VE − VL) RT

(

pE2 Z E2

−

pE1 Z E1

)⎤⎦⎥

mPZ /MPZ + mEmimAc /MEmimAc

(3)

nG − nE nG − nE = msolvent mH2O + mPZ + mEmimAc

(4)

where α and mCO2 are CO2 loading and nCO2 per kilogram of solvent that expressed the composition, nG and nE are CO2 moles change of gas chamber, VG is volume of the gas chamber = 1131.21 mL, VE is volume of the equilibrium cell = 332.89 mL, VL is volume of hybrid absorbent, pG1 and pG2 are initial and final pressure of gas chamber, ZG1 and ZG2 are the compressibility factors corresponding to the initial and final pressure in the gas chamber, pE1 and pE2 are initial and final pressure of the equilibrium cell, ZC1 and ZC2 are the compressibility factors corresponding to the initial and final pressure in equilibrium cell, msolvent, mH2O, mPZ and m[Emim][Ac] are mass of hybrid solvent, water, PZ, and [Emim][Ac], and MPZ and M[Emim][Ac] are the molecular weight of PZ and [Emim][Ac]. The partial CO2 pressure in equilibrium cell was calculated as follows:

2.55 %, 0.66 %, and 1.84 % corresponding to temperatures from 313.15 K to 343.15 K, and the average absolute relative deviation are correspondingly 1.17 %, 1.60 %, 0.36 % and 1.84 %. Good correspondence with the literature data were observed as shown in Table 2. Solubility Measurement. The experimental apparatus and procedures adopted in this work were discussed in our previous study with some modifications.28 A schematic diagram of the device was shown in Figure 1. The device using an isothermal

pCO2 = pE2 − pv

(5)

where pV is vapor pressure of the solution at equilibrium temperature. The uncertainties of CO2 solubility can be calculated by the same method on the basis of the literature.28 For example, the uncertainties of CO2 loading can be calculated as followed: α = f (nG , nE , ntotal)

(6)

⎛ ∂α ⎞2 ⎛ ∂α ⎞2 ⎛ ∂α ⎞2 2 2 (δα) = ⎜ ⎟ (δntotal)2 ⎟ (δnG) + ⎜ ⎟ (δnE) + ⎜ ⎝ ∂nE ⎠ ⎝ ∂ntotal ⎠ ⎝ ∂nG ⎠ (7) 2

Figure 1. The isothermal synthetic schematic apparatus diagram: 1, gas chamber; 2, circulation pump; 3, CO2 storage; 4, isothermal water bath; 5, stainless steel buffer; 6, vacuum pump; 7, buret; 8, equilibrium cell; 9, temperature controller; 10, magnetic stirrer; 11, ceramic heater band; 12, resistance thermometer.

⎛ ∂α ⎞2 ⎛ ∂α ⎞2 2 ⎟⎟ (δp )2 (δnG) = ⎜ ⎟ (δVG) + ⎜⎜ G1 ⎝ ∂VG ⎠ ⎝ ∂pG1 ⎠ 2

⎛ ∂α ⎞2 ⎛ ⎞2 ⎟⎟ (δp )2 + ⎜ ∂α ⎟ (δT )2 + ⎜⎜ G2 ⎝ ∂T ⎠ ⎝ ∂pG2 ⎠

synthetic method mainly consisted of a high pressure equilibrium cell, a gas chamber, the water bath temperature control system, and a vacuum air-moved system. Temperatures of the equilibrium cell and gas chamber were maintained by heating water with JULABO Labortechnik GmbH within an accuracy of ± 0.03 K. Pressures of the two chambers were measured by two pressure sensors (GE PTX5072) with a precision of ± 0.2 kPa ranging from 0 to 1500 kPa. Two PT100 resistance thermometers for a range of 273.15 K to 473.15 K were used for temperature measurement with an accuracy of ± 0.1 K.

(8)

⎛ ∂α ⎞2 ⎛ ∂α ⎞2 ⎛ ∂α ⎞2 2 2 ⎟⎟ (δp )2 (δnE) = ⎜ ⎟ (δVE) + ⎜ ⎟ (δVL) + ⎜⎜ E1 ⎝ ∂VE ⎠ ⎝ ∂VL ⎠ ⎝ ∂pE1 ⎠ 2

⎛ ∂α ⎞2 ⎛ ⎞2 ⎟⎟ (δp )2 + ⎜ ∂α ⎟ (δT )2 + ⎜⎜ E2 ⎝ ∂T ⎠ ⎝ ∂pE2 ⎠ 620

(9)

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⎛ ∂α ⎞2 ⎛ ∂α ⎞2 (δntotal)2 = ⎜ ⎟ (δmEmimAc)2 ⎟ (δmPZ)2 + ⎜ ⎝ ∂mPZ ⎠ ⎝ ∂mEmimAc ⎠

were compared with literature values, and the results were plotted in Figure 2. The figure shows that there is good agreement with literature, which means the device applied is reliable.

(10)

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where δVG = δVE = 0.1 mL, δpG1 = δpG2 = δpE1 = δpE2 = 0.2 kPa, δVL = 0.03 mL, δT = 0.1 K, δmPZ = δm[Emim][Ac] = 0.0001 g. To verify the method and reproducibility of the presented solubility data, the solubilities of CO2 in 30 wt % MEA aqueous solution at 313.15 K were determined. The experimental data

RESULTS AND DISCUSSION Density and Viscosity. The density and viscosity data can be regressed as a function of temperature as the following equations.31 ρ = A 0 + A1(T /K)

(11)

log η = B0 + B1(T /K)

(12)

where ρ is density, η is viscosity, A0, A1, B0, and B1 are the fitting parameters, and T is the temperature. The method of least-squares was used to calculate the fitting parameters. Also standard deviations were calculated by eq 13. n

SD = [∑ (Zexp − Zcal)2 /n]0.5

(13)

i

where SD is standard deviations, Zexp is experimental values (density, viscosity), Zcal is calculated values, and n is the total number of data points.31

Figure 2. Comparison of solubility data for CO2 at 313.15 K in 30 wt % MEA between published data29,30 and this work: △, Jou 1995 GC;29 ●, this work; ○, Lee 1976;30 ★, Jou 1995 BaCO3.29

Table 3. Density and Viscosity of PZ (1) + [Emim][Ac] (2) + H2O (3) at Various Temperatures and PZ or [Emim][Ac] Concentrations under p = 0.1 MPaa w1/w2/w3

ρexp/g·cm−3

0.3/0.3/0.4 0.3/0.2/0.5 0.3/0.1/0.6 0.3/0/0.7 0.2/0.3/0.5 0.1/0.3/0.6 0/0.3/0.7

1.0514 1.0405 1.0314 1.0191 1.0458 1.0406 1.0352

0.3/0.3/0.4 0.3/0.2/0.5 0.3/0.1/0.6 0.3/0/0.7 0.2/0.3/0.5 0.1/0.3/0.6 0/0.3/0.7 w1/w2/w3

1.0352 1.0249 1.0155 1.0034 1.0289 1.0252 1.0198 ηexp/g·cm−3 T/K = 313.15 8.633 6.104 4.174 2.909 4.687 2.552 1.432 333.15 4.189 3.230 2.328 1.593 2.634 1.537 0.939

ρcor/g·cm−3

ρexp/g·cm−3

1.0518 1.0406 1.0319 1.0192 1.0461 1.0412 1.0356

1.0433 1.0326 1.0235 1.0112 1.0379 1.0334 1.0278

T/K = 313.15

T/K = 323.15

T/K = 333.15

0.3/0.3/0.4 0.3/0.2/0.5 0.3/0.1/0.6 0.3/0/0.7 0.2/0.3/0.5 0.1/0.3/0.6 0/0.3/0.7 0.3/0.3/0.4 0.3/0.2/0.5 0.3/0.1/0.6 0.3/0/0.7 0.2/0.3/0.5 0.1/0.3/0.6 0/0.3/0.7 a

ρcor/g·cm−3 1.0434 1.0327 1.0236 1.0110 1.0378 1.0329 1.0275 T/K = 343.15

1.0351 1.0247 1.0152 1.0029 1.0294 1.0247 1.0195 ηcor/g·cm−3 8.655 6.096 4.188 2.882 4.694 2.542 1.421 4.181 3.217 2.354 1.609 2.661 1.565 0.949

1.0271 1.0167 1.0075 0.9953 1.0216 1.0163 1.0111 ηexp/g·cm−3 T/K = 323.15 6.059 4.407 3.133 2.140 3.558 1.987 1.143 343.15 2.871 2.340 1.768 1.221 2.017 1.258 0.792

1.0268 1.0168 1.0068 0.9948 1.0211 1.0165 1.0115 ηcor/g·cm−3 6.016 4.429 3.140 2.153 3.534 1.995 1.161 2.906 2.337 1.765 1.202 2.004 1.228 0.776

Standard uncertainties u are u(w) = 0.0005, u(T) = 0.05 K, u(ρ) = 0.0001 g·cm−3, and u(η) = 0.005 mPa·s. 621

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Table 4. Fitting Parameters (A0, A1, B0, B1) and SD for PZ (1) + [Emim][Ac] (2) + H2O (3)a ρ

a

η

w1/w2/w3

A0

104A1

SD

B0

B1

SD

0.3/0/0.7 0.3/0.1/0.6 0.3/0.2/0.5 0.3/0.3/0.4 0.2/0.3/0.5 0.1/0.3/0.6 0/0.3/0.7

1.2732 1.2937 1.2899 1.3128 1.3072 1.2982 1.2873

−8.1127 −8.3598 −7.9600 −8.3357 −8.3382 −8.2084 −8.0383

0.0004 0.0005 0.0001 0.0003 0.0004 0.0005 0.0004

10.1836 10.4500 11.8153 13.5480 10.4313 8.5284 6.6709

−0.029140 −0.028797 −0.031958 −0.036372 −0.028373 −0.024255 −0.020181

0.019 0.015 0.013 0.030 0.020 0.021 0.014

Standard uncertainty u is u(w) = 0.0005.

decrease of PZ concentration when the water composition is fixed. The viscosity of a hybrid absorbent is much lower than that of pure [Emim][Ac], and it has much potential for further research.31 The data here are very essential for the design and operation of the gas treatment. Solubility of CO2 in Hybrid Absorbents. Solubilities of CO2 in different concentrations of PZ or [Emim][Ac] at 313.15 K and pressures up to 1 MPa were listed in Table 5. Solubilities of CO2 in 0.1 PZ + 0.3 [Emim][Ac] + 0.6 H2O at temperatures ranging from 313.15 K tp 343.15 K and pressures up to 1 MPa were listed in Table 6. Solublities decrease as the absorption temperature rises and increase as pressure ascends, which is consistent with the solubility law in other solvents. As solubilities of CO2 are so sensitive to temperature and pressure, elevation of temperature or reduction of pressure can be used for regeneration of hybrid absorbents. As shown in Table 5, solubilities of 0.3 [Emim][Ac] aqueous solution are much less than those of 0.3 PZ aqueous solution. CO2 loading of the latter under about 30 kPa is 100 times as that of the former. As the pressure increases to 880 kPa, CO2 loading of the latter is 6 times as that of the former. It shows that the difference of CO2 loading between the two solutions decreases with an increase of the pressure. The result can be explained as follows. CO2 loading of 0.3 [Emim][Ac] aqueous solution under the whole pressure range consists of a relatively homogeneous linear rise, indicating that the chemical absorption characteristic is not obvious and physical absorption behavior may be dominant. However, PZ aqueous solution at the same mass fraction under low pressure has shown strong dissolving ability. In addition, the CO2 loading of aqueous PZ solution did not change much with an increase of pressure, which shows evident chemical absorption characteristics. To study the effect of different PZ concentrations on the absorption ability of hybrid absorbents, nCO2/nILs values of 0.3 [Emim][Ac] + 0.7 H2O, 0.1 PZ + 0.3 [Emim][Ac] + 0.6 H2O, 0.2 PZ + 0.3 [Emim][Ac] + 0.5 H2O, and 0.3 PZ + 0.3 [Emim][Ac] + 0.4 H2O were calculated and plotted in Figure 5. As can be seen in Figure 5, nCO2/nILs of 0.1 PZ + 0.3 [Emim][Ac] + 0.6 H2O at about 30 kPa is 0.62, while nCO2/nILs of 0.3 [Emim][Ac] + 0.7 H2O is only 0.01. Meanwhile, nCO2/nILs of 0.1 PZ + 0.3 [Emim][Ac] + 0.6 H2O at about 880 kPa is 0.89 and nCO2/nILs of 0.3 [Emim][Ac] + 0.7 H2O is 0.20. It can be concluded that the CO2 absorption ability of the hybrid absorbent improves highly in low pressure and ascends slightly in high pressure as PZ concentration increases, which agrees with the rule of chemical absorption. Besides, the solubilities of CO2 in hybrid absorbents with the addition of PZ are much larger than those of [Emim][Ac] aqueous solution. Absorption behavior of hybrid absorbent contained PZ are extremely excellent under low

Experimental and correlated data of thermophysical properties (density and viscosity) about hybrid absorbents were listed in Table 3. The fitting parameters along with standard deviations are presented in Table 4. As we can see in Figures 3 and 4, the density declines slightly while the viscosity decreases greatly with the increase of

Figure 3. Densities of PZ (1) + [Emim][Ac] (2) + H2O (3) hybrid absorbents as a function of temperature: w1/w2/w3 = ■, 0.3/0.3/0.4; ●, 0.3/0.2/0.5; ▲, 0.3/0.1/0.6; ▼, 0.3/0/0.7; ○, 0.2/0.3/0.5; △, 0.1/0.3/ 0.6; ▽, 0/0.3/0.7.

Figure 4. Viscosities of PZ (1) + [Emim][Ac] (2) + H2O (3) hybrid absorbents as a function of temperature: w1/w2/w3 = ■, 0.3/0.3/0.4; ●, 0.3/0.2/0.5; ▲, 0.3/0.1/0.6; ▼, 0.3/0/0.7; ○, 0.2/0.3/0.5; △, 0.1/0.3/ 0.6; ▽, 0/0.3/0.7.

temperature for the same absorbent.20 When PZ concentration is constant, the density of hybrid absorbent goes down lightly and the viscosity extremely descends with the decline of the [Emim][Ac] concentration. Similarly, both densities and viscosities of the hybrid absorbent go up with a rise of the PZ concentration when [Emim][Ac] concentration is invariable. In a word, it is effective to lessen the viscosity of the hybrid absorbents with an increase of [Emim][Ac] concentration and the decrease of PZ concentration with constant water content. On the contrary, it is beneficial to lower the density of hybrid absorbents with an increase of [Emim][Ac] concentration and a 622

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Table 5. Solubility of CO2a in the Solvent PZ (1) + [Emim][Ac] (2) + H2O (3) of Different Compositions at T = 313.15 Kb p/kPa

α/mol·mol−1

27.2 92.2 169.1 338.7 578.9 860.3

0.9086 0.9744 1.0100 1.0481 1.0897 1.1318

20.2 84.4 185.2 376.1 569.9 857.6

0.8444 0.9148 0.9570 0.9995 1.0305 1.0667

31.4 86.7 202.3 426.4 585.3 870.3

0.6856 0.7247 0.7608 0.7977 0.8214 0.8474

27.5 70.3 174.8 369.9 595.7 872.0

0.6053 0.6404 0.6723 0.7043 0.7290 0.7524

29.5 61.5 159.5 354.5 558.4 838.4

0.5137 0.5420 0.5839 0.6210 0.6506 0.6871

28.6 71.1 161.9 355.9 571.7 880.4

0.3762 0.4047 0.4260 0.4645 0.4994 0.5389

28.9 57.8 158.6 373.8 598.1 874.2

0.0096 0.0162 0.0527 0.1022 0.1530 0.1995

u(α)c/mol·mol−1

mCO2/mol·kg−1

u(mCO2)d/mol·kg−1

3.1644 3.3935 3.5175 3.6502 3.7951 3.9417

0.0060 0.0061 0.0061 0.0062 0.0064 0.0067

3.4369 3.7234 3.8952 4.0682 4.1944 4.3417

0.0061 0.0064 0.0067 0.0069 0.0071 0.0075

3.1933 3.3755 3.5436 3.7155 3.8259 3.9470

0.0060 0.0060 0.0061 0.0062 0.0064 0.0068

3.1749 3.3590 3.5264 3.6942 3.8238 3.9465

0.0061 0.0062 0.0062 0.0064 0.0066 0.0069

2.0981 2.2137 2.3848 2.5364 2.6573 2.8063

0.0052 0.0053 0.0054 0.0056 0.0057 0.0059

1.0998 1.1831 1.2454 1.3579 1.4600 1.5754

0.0041 0.0042 0.0044 0.0046 0.0048 0.0051

0.0169 0.0286 0.0929 0.1801 0.2697 0.3516

0.0004 0.0011 0.0013 0.0014 0.0022 0.0024

w1/w2/w3 = 0.3/0/0.7 0.0016 0.0016 0.0016 0.0016 0.0017 0.0018 w1/w2/w3=0.3/0.1/0.6 0.0014 0.0015 0.0015 0.0015 0.0016 0.0017 w1/w2/w3=0.3/0.2/0.5 0.0012 0.0012 0.0012 0.0012 0.0013 0.0014 w1/w2/w3=0.3/0.3/0.4 0.0010 0.0011 0.0011 0.0011 0.0011 0.0012 w1/w2/w3=0.2/0.3/0.5 0.0013 0.0013 0.0013 0.0014 0.0014 0.0015 w1/w2/w3=0.1/0.3/0.6 0.0018 0.0018 0.0018 0.0018 0.0019 0.0020 w1/w2/w3=0/0.3/0.7 0.0003 0.0003 0.0003 0.0004 0.0005 0.0006

Solubility of CO2 in the solvent is expressed as α and mCO2: α = nCO2/(namine + nIL) = nCO2/(nPZ + nEmimAc); mCO2 = nCO2 per kilogram of solvent, where the solvent refers to the hybrid absorbent that contains PZ, EmimAc, and H2O. bStandard uncertainties u are u(w) = 0.0005, u(T) = 0.1 K and u(p) = 0.2 kPa. cu(α) refers to standard uncertainty of α. du(mCO2) refers to standard uncertainty of mCO2. a

The results were shown in Figure 6. When the concentration of PZ is constant, hybrid absorbents, which consist of a 0.3 PZ solution with different concentrations of [Emim][Ac], absorbed basically the same amount of CO2 per mol PZ. It can be concluded that [Emim][Ac] has little effect on absorption ability of hybrid absorbents with high concentration of PZ, which is in accord with literature.21 The absorption ability of hybrid

pressure, and they are suitable for CO2 capture under low CO2 partial pressure. The nCO2/nILs data of 0.3 PZ + 0.7 H2O, 0.3 PZ + 0.1 [Emim][Ac] + 0.6 H2O, 0.3 PZ + 0.2 [Emim][Ac] + 0.5 H2O, and 0.3 PZ + 0.3 [Emim][Ac] + 0.4 H2O were calculated and compared to investigate the effect of different [Emim][Ac] concentrations on the absorption ability of the hybrid absorbents. 623

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Table 6. Solubility of CO2a in the solvent 0.1 PZ + 0.3 [Emim][Ac] + 0.6 H2O at temperatures ranged from 313.15 to 343.15 Kb p

α

u(α)c

mCO2

u(mCO2)d

kPa

mol·mol−1

mol·mol−1

mol·kg−1

mol·kg−1

28.6 71.1 161.9 355.9 571.7 880.4

0.3762 0.4047 0.4260 0.4645 0.4994 0.5389

1.0998 1.1831 1.2454 1.3579 1.4600 1.5754

0.0041 0.0042 0.0044 0.0046 0.0048 0.0051

25.5 86.4 199.1 384.5 591.0 869.9

0.3577 0.3872 0.4172 0.4481 0.4764 0.5103

1.0457 1.1320 1.2197 1.3100 1.3927 1.4918

0.0040 0.0041 0.0042 0.0045 0.0047 0.0050

34.4 95.5 216.3 408.4 607.9 881.0

0.3453 0.3782 0.4056 0.4364 0.4603 0.4885

1.0095 1.1056 1.1857 1.2758 1.3457 1.4281

0.0034 0.0041 0.0042 0.0044 0.0045 0.0047

36.7 103.6 195.2 407.7 589.7 888.7

0.3360 0.3660 0.3894 0.4225 0.4430 0.4744

0.9823 1.0700 1.1384 1.2351 1.2951 1.3869

0.0033 0.0036 0.0042 0.0043 0.0044 0.0047

T = 313.15 K 0.0018 0.0018 0.0018 0.0018 0.0019 0.0020 T = 323.15 K 0.0017 0.0017 0.0017 0.0017 0.0018 0.0019 T = 333.15 K 0.0018 0.0018 0.0019 0.0019 0.0019 0.0020 T = 343.15 K 0.0018 0.0018 0.0018 0.0018 0.0019 0.0020

Figure 6. Influence of [Emim][Ac] on the load of CO2 of 0.3 PZ solution: ■, 0 [Emim][Ac]; ●, 0.1 [Emim][Ac]; ▲, 0.2 [Emim][Ac]; ▼, 0.3 [Emim][Ac].

Table 7. Solubility Comparison with Literature Data15,17,32 T

p

CO2 loadinga

hybrid absorbents

K

kPa

mol·mol−1

0.2 MEA + 0.2 [Bmim][BF4]18 0.2 MEA + 0.2 bheaab,18 0.15 [N1111][Gly]c + 0.15 MDEA33 0.15 [N2222][Gly]d + 0.15 MDEA33 0.15 [N1111][lys]e + 0.15 MDEA33 0.15 [N2222][lys]f + 0.15 MDEA33 4 M MDEA + 1 M [gua][OTf]g,16 0.1 PZ + 0.3 [Emim][Ac] 0.2 PZ + 0.3 [Emim][Ac] 0.3 PZ + 0.3 [Emim][Ac] 0.3 PZ + 0.2 [Emim][Ac] 0.3 PZ + 0.1 [Emim][Ac]

313.15 298.15 298.15 298.15 298.15 298.15 303.15 313.15 313.15 313.15 313.15 313.15

895 871 400 400 400 400 800 880.4 838.4 872.0 870.3 857.6

0.5075 0.8893 0.562 0.643 0.694 0.740 0.667 0.5389 0.6871 0.7524 0.8474 1.0667

a CO2 loading: α = nCO2/(namine + nIL). bBis(2-hydroxyethyl)ammonium acetate. cTetramethylammonium glycinate. dTetraethylammonium glycinate. eTetramethylammonium lysinate. fTetraethylammonium lysinate. gGuanidinium trifluoromethanesulfonate.

Solubility of CO2 in the solvent is expressed as α and mCO2: α = nCO2/(namine + nIL) = nCO2/(nPZ + nEmimAc); mCO2 = nCO2 per kilogram of solvent, where the solvent refers to the hybrid absorbent that contains PZ, EmimAc, and H2O. bStandard uncertainties u are u(w) = 0.0005, u(T) = 0.1 K and u(p) = 0.2 kPa. cu(α) refers to standard uncertainty of α. du(mCO2) refers to standard uncertainty of mCO2. a

Solubilities of CO2 in hybrid absorbents studied in this paper were compared with those in the literature, and the results are shown in Table 7. It can be observed that PZ + [Emim][Ac] + H2O are at par with the absorbents in literature. Also among these absorbents, 0.3 PZ + 0.1 [Emim][Ac] particularly shows excellent absorption behavior, and warrants further research.

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CONCLUSION Conventional IL is viscous and has a low solvency for CO2, which are serious issues for application in industrial gas separation. A good solution to these problems is to add water and amine into the absorbent. In this paper, hybrid absorbents piperazine (1) + [Emim][Ac] (2) + H2O (3) with different mass compositions (w1/w2/w3 = 0.3/0/0.7, 0.3/0.1/0.6, 0.3/0.2/0.5, 0.3/0.3/0.4, 0.2/0.3/0.5, 0.1/0.3/0.6, 0/0.3/0.7) were selected to study thermophysical properties (density and viscosity) at T = 313.15 K to 343.15 K. The results showed that it was advantageous to decrease the viscosities of the hybrid absorbents by increasing [Emim][Ac] concentration with constant water content, and densities were vice versa. The viscosities of the hybrid absorbents were low, which improved practical significance of the hybrid absorbents for CO2 capture. The solubility data of CO2 in the hybrid absorbents were determined under T = 313.15 K to 343.15 K and p = 0 MPa to 1.0 MPa. The addition of PZ highly improved the absorption ability of the hybrid absorbent, especially at low pressures with invariable concentration of [Emim][Ac]. However, [Emim][Ac] has little influence on the CO2 absorption

Figure 5. Influence of PZ on the load of CO2 of 0.3 [Emim][Ac] solution: ■, 0 PZ; ●, 0.1 PZ; ▲, 0.2 PZ; ▼, 0.3 PZ.

absorbents mainly depends on the mass fraction of PZ. As discussed in the Introduction, substitution of a part of the water with IL decreases the heat capacity and vaporization heat of the solvent. As a result, energy consumption for thermal regeneration of the absorbent is reduced. It certainly is great news for CO2 capture that the addition of IL in the absorbent can save energy on the premise that the CO2 absorption capacity remains constant. 624

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ability of highly concentrated PZ aqueous solutions. The [Emim][Ac] is helpful for CO2 capture as it is added into hybrid absorbents, because energy consumption due to solvent regeneration is saved, and the absorption capacity of the hybrid absorbent is constant. Also 0.3 PZ + 0.1 [Emim][Ac] + 0.6 H2O is found as a potential CO2 absorbent for further research.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel./Fax: +86-10-6441-6406. Funding

This work is supported by the National Natural Science Foundation of China (51276010, 21306007), and the National Basic Research Program of China (2011CB201306). Notes

The authors declare no competing financial interest.

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