Diffusion Coefficients of CO2 in Ionic Liquids Estimated by Gravimetry

Article pubs.acs.org/IECR

Diffusion Coefficients of CO2 in Ionic Liquids Estimated by Gravimetry Cristian Moya,† Jose Palomar,*,† Maria Gonzalez-Miquel,‡ Jorge Bedia,† and Francisco Rodriguez‡ †

Departamento de Química Física Aplicada (Sección de Ingeniería Química), Universidad Autónoma de Madrid, Cantoblanco, 28049 Madrid, Spain ‡ Departamento de Ingeniería Química, Universidad Complutense de Madrid, 28040 Madrid, Spain S Supporting Information *

ABSTRACT: The estimation of diffusion coefficients of CO2 in ionic liquids by gravimetry is analyzed with the aim of establishing a measurement method that provides consistent values of diffusivity. Absorption kinetic curves of CO2 in three common ILs were measured at different temperatures (293−323 K) and pressures (1−20 atm) by using a high pressure sorption analyzer with magnetic suspension balance operating in dynamic mode. A mass diffusion model widely used in the literature was applied to estimate effective diffusion coefficients for CO2−IL systems from time-dependent absorption data. The measuring conditions (IL mass, dimension of sample container, gas flow) in the dynamic absorption experiments were modified to verify the assumptions of the diffusion model. Obtained results were compared to available data. In addition, the suitability of theoretical methods commonly used for estimating diffusion coefficients of CO2 in ILs was analyzed, in order to select a computational approach for preliminary selection of ILs with favorable transport properties for CO2 capture. Chen et al.14 defined an absorption rate parameter to characterize the kinetics of CO2 in ILs through gravimetric analysis. Diffusion coefficients of CO2 in ILs have also estimated by the semi-infinitive volume approach,27 the lagtime technique,28 the transient thin-liquid method,29,30 FTIR measurements,31 and the high-pressure quartz spring method.33 Recently, our group carried out gravimetric measurements by a magnetic suspension balance to analyze the influence of both the anion and the cation on the values of diffusion coefficients of CO2 in ILs.16,17 It was concluded that the structural features of ILs, as well as the operating temperature, have significant influence on the mass transfer rate of physical absorption of CO2 in ILs, whereas pressure (or solute concentration) seems to play a minor rule. In addition, it was evident by the kinetics and the thermodynamics of CO2 absorption may follows opposite trends depending on the anion or cation structure for, respectively, common-cation or common-anion IL series,16,17 which must be considered to select the most favorable solvent for CO2 capture. In general, reported experimental equilibrium data for CO2− IL systems from gravimetric measurements at different temperatures and pressures show adequate level of consistency and reproducibility. In contrast, the analysis of the available diffusion coefficient values of CO2 in IL solvents reveals higher data dispersion depending on the measurement method. The main aim of this work is setting up a procedure to obtain reproducible coefficient diffusion values of CO2 in ILs from a gravimetric balance method. For this purpose, time-dependent absorption experiments were carried by using a high pressure

1. INTRODUCTION The potential application of ionic liquids (ILs) in gas separation processes is receiving increasing attention, mainly due to the particular advantages of ILs as absorbents: high and tailorable absorption capacity, very low vapor pressure, wide liquid window, and adequate thermal stability. In particular, ILs are being intensively investigated as absorbents for CO2 capture in order to develop novel technologies of capture, which overcome the drawbacks associated with the amine-based systems.1−9 The design of the separation processincluding the equipment sizingfor CO2 capture by ILs implies a deep knowledge of both thermodynamics and kinetics of the absorption phenomena. Various experimental techniques (gravimetric balance method, quartz crystal microbalance method, isochoric saturation method, bubble point method, and others) have been applied to measure the solubility of CO2 in ILs.10 Among them, the gravimetry has been proved to be an adequate technique to accurately determine the equilibrium isotherms of CO2 absorption in ILs.11−17 Indeed, systematic experimental information about the thermodynamics of CO2 physical absorption in ILs has been reported, taking into account both the operating conditions and the IL nature.13,18−26 Experimental evidence indicate that the anion structure mainly determine the CO2 absorption capacity of the IL solvent, observing that highly fluorinated anions increase the physical solubility of CO2 in ILs. As expected, significant effects of the absorption temperature and pressure on the CO2 solubility in ILs were reported. Comparatively, however, the available studies focused on characterizing the absorption rate of CO2 in IL-based systems are scarce.11,12,16,17,27−33 Shifflet and Yokozeki estimated the diffusion coefficients of CO2 in [bmim][PF6] and [bmim][BF4] from thermogravimetric timedependent absorption data by using a simple diffusion model.11 © 2014 American Chemical Society

Received: Revised: Accepted: Published: 13782

May 12, 2014 July 18, 2014 August 8, 2014 August 8, 2014 dx.doi.org/10.1021/ie501925d | Ind. Eng. Chem. Res. 2014, 53, 13782−13789

Industrial & Engineering Chemistry Research

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sorption analyzer with magnetic suspension balance operating in dynamic mode. The kinetic curves of CO2 absorption in IL were measured at different temperatures (298−323 K) and pressures (1−20 bar), during enough time to achieve the gas− liquid equilibrium. Consequently, the corresponding absorption isotherms for CO2−IL system were also obtained at the different operating conditions. Three ILs were included in the study, all of them largely evaluated in bibliography as absorbents for CO2 capture: 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, [emim][NTf2]; 1-octyl-3methylimidazolium bis(trifluoromethanesulfonyl)imide, [omim][NTf2]; and 1-butyl-3-methylimidazolium hexafluorophosphate, [bmim][PF6]. In order to assess a gravimetric measuring method for reproducible absorption kinetic data, several aspects of the experimental procedure were checked: (i) the IL absorbent sample mass; (ii) the dimensions of balance container; (iii) the CO2 gas flow; and (iv) the pressure gaps used in the absorption experiments. Then, the diffusion coefficients of CO2 in IL were estimated from time-dependent absorption data by applying a mass diffusion model previously used in gas−IL systems. The reliability of the assumptions established by the mass diffusion model is checked by experiments specifically designed with this purpose. Obtained values of diffusion coefficient by different procedures were compared each other and, also, with the available reported data. To complete the analysis, gas−liquid equilibrium data obtained at saturation were systematically compared to previous results in literature, in order to ensure the reliability of absorption measurements. The consistent values of diffusion coefficients estimated in current work extend the still scarce database of available kinetic parameters for CO2−IL systems. In this context, the availability of a theoretical model which reasonably estimates the diffusion coefficient of CO2 in ILs is of a great interest: it would minimize the cost of experimentation by doing a preliminary selection of ILs with adequate transport properties for CO2 capture. In this work, the application of diffusion models was revised in view of the new available experimental data.

In this kind of measurements, it is necessary to take into account the buoyancy effect that affect the mass reading. To evaluate this effect, it is needed to precisely know the mass and volume of the system. For this reason a blank and a buoyancy measurements were made before the absorption measurements. Two sample containers with diameters of 1 and 1.5 cm were used in absorptions experiments. A complete procedure for blank measurements, reactivation of the sample and buoyancy measurements can be found in previous works.16,17 The CO2 absorption isotherms for each ILs were determined at different temperatures (298, 308, and 323 K), and pressures up to 20 bar. Once the sample is loaded (mass from 50 to 650 mg) the sample is degassed in vacuum until mass remains constant (weight change rate < 0.02 mg/h). Then, the pressure was increased stepwise using CO2 (gas flow rate = 50, 100, or 200 mL/min), and the increment in the sample weight measured. For each pressure step the equilibrium is reached when weight change rate < 0.02 mg/h. Afterward, the buoyancy effect was corrected and the mass of CO2 absorbed can be obtained. CO2 solubility in IL is represented in absorption isotherms in terms of the molar fraction of gas absorbed: XCO2 =

mCO2 /MCO2 ms /MIL + mCO2 /MCO2

(1)

where MCO2 and MIL are, respectively, the molar masses of CO2 and the ionic liquid. More detailed description of the absorption experiment procedure is given elsewhere.16 2.3. Mass Diffusion Model. The diffusion coefficients were estimated by applying a mass diffusion model reported by Shifflet and Yokoezi11 for CO2−IL systems, which was previously applied for gas absorption in lubricant oils34 and later has been also applied for other solute (NH3, toluene)−IL systems.35−37 This model makes the following assumptions:11 (1) Gas dissolves through a one-dimensional (vertical) diffusion process, in which there is no convective flow in the liquid; (2) A thin boundary layer between the gas and liquid phases exists, where the thermodynamic equilibrium is instantly established with the saturation concentration and where the concentration is constant all the time at a given temperature and pressure; (3) Temperature and pressure are kept constant; (4) The gas-dissolved liquid is a highly dilute solution, and so the relevant thermophysical properties of the solution do not change. Then, the process may be described by one-dimensional mass diffusion due to the local concentration difference.

2. EXPERIMENTAL PROCEDURE 2.1. Materials. Carbon dioxide, CO2, and nitrogen, N2, were obtained from Praxair, Inc., with a minimum purity of 99.999%. The ionic liquids 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([emim][NTf2]), 1-octyl-3methylimidazolium bis(trifluoromethanesulfonyl)imide ([omim][NTf2]), and 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) were obtained from Io-Li-Tec (Ionic Liquid Technologies), all of them with a minimum purity of 99%. Before absorption experiments, ILs were dried and degassed under vacuum (10−3 mbar) at 298 K over 24 h. Water content in ILs was analyzed by Karl Fischer titration (Mettler Toledo NMR DL31) and NMR technique (Bruker DRX-500 spectrometer), obtaining estimations lower than 200 ppm in all cases. 2.2. Absorption Measurements. The measurements of gas solubility were performed with a Gravimetric High Pressure Sorption Analyzer (ISOSORP GAS LP-flow, Rubotherm) with magnetic suspension balance (MSB). The instrument can determine masses in the range of 0−10 g with a precision of 0.01 mg and work in a wide range of temperatures (ambient up to 150 °C) and pressures (from 10 to 6 to 30 bar). Detailed descriptions of the experimental setup are given elsewhere.16

Shiflett and Yokozeki

In the model, the CO2 mass balance is expressed as

∂C ∂ 2C = D· 2 ∂t ∂z

(2)

with an initial conditionC = C0 when t = 0 and z < 0 < L and boundary conditions(i) C = Cs when t > 0 and z = 0 and (ii) ∂C/∂z = 0 at z = L. C is the concentration of gas dissolving in the IL as a function of time, t, and vertical location, z. L is the depth of IL in the container [obtained using the sample container diameter (1 or 1.5 cm), the IL mass sample, and the experimentally determined density by the buoyancy measurements], and C0 is the CO2 initial concentration at each temperature and pressure. D is the diffusion coefficient, which is assumed to be constant. However, since the obtained CO2 solutions in IL cannot be considered highly dilute, the diffusion 13783

dx.doi.org/10.1021/ie501925d | Ind. Eng. Chem. Res. 2014, 53, 13782−13789


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coefficients must be taken as “effective” diffusion constants. A detailed description of the procedure followed to apply the diffusion model to estimate D values in ILs is given elsewhere.16 The MSB needs time to pressurize (∼10 min); during this time, the partial pressure of CO2 varies over the time and, in consequence, the concentration at the interface also varies. Since the diffusion model takes into account that this concentration is constant, only the points obtained at the fixed pressure and temperature were used in the model fit. In addition, to describe more precisely the main trend of the kinetic curve, the absorption data obtained after 90% of Cs were discarded for obtaining D values by this diffusion model.

3. RESULTS One main first decision for measuring absorption data by a gravimetric balance is the mass of the solvent used in the

Figure 2. CO2 absorption time-dependent data and pressure profiles for [emim][NTf2] at 298 K using a IL mass of (A) 50 and (B) 500 mg.

Figure 1. CO2 solubility (molar fraction) in [emim][NTf2] measured in current work with different IL mass (50−700 mg) (circles) and previous works by Kim et al.38 (gray boxes) and Yokozeki et al.39 (open boxes) at 298 K.

Table 1. Estimated Diffusion Coefficients of CO2 in [emim][NTf2] and [omim][NTf2] at 298 K with Different Amounts of IL (50−650 mg) and at Different Pressures (1− 20 bar)

sample container. The ISOSORP GAS LP-flow sorption measuring instrument used has a resolution of 0.01 mg and allows working with solvent loads up to 10 g. In previous works, we showed that this equipment, using 50 mg of absorbent in balance container, provides fast (approximately 2 h for each T, P setpoint) and accurate (uncertainty < 0.1%) determination of equilibrium absorption data of CO2 in ILs.16,17 Other authors used IL mass sample ranging from 60 mg to 1 g in gravimetric analyzer with similar precisions.11−15,38,39 Figure 1 collects the solubility data of CO2 in [emim][NTf2] obtained varying the IL sample mass from 50 to 700 mg at 298 K and pressures between 1 and 20 bar. Very close values of gas−liquid equilibrium data for CO2 + [emim][NTf2] system were obtained independently of IL mass used in measurements (for more detail, see Supporting Information Table S1). In addition, good agreement was found when comparing to the available values of CO2 solubility in this IL at similar pressure and temperature ranges, reported by other authors.38,39 In contrast, the mass of IL used in balance measurements affects remarkably the shape and quality of the kinetic curves obtained for CO2 absorption in ILs. Figure 2 compares the timedependent data obtained for 50 and 500 mg of IL [emim][NTf2] at 298 K. Using higher amount of absorbent allows obtaining more definite kinetic curves, that have higher

D (10−11 m2/s) P (bar) IL [emim] [NTf2]

[omim] [NTf2]

mass (mg)

1

5

10

15

20

average

50 150 250 350 500 650 50 150 250 350 500 650

4.3 14.5 31.1 44.2 45.4 57.1 1.7 10.7 24.8 37.6 42.9 50.6

10.8 22.4 36.8 52.1 58.5 60.4 3.8 23.3 36.5 39.5 44.9 52.7

7.5 18.2 39.2 51.7 64.3 66.4 5.1 24.2 38.1 52.4 51.8 55.1

5.2 21.0 42.6 66.5 75.3 73.6 3.8 18.3 37.9 63.2 60.0 56.0

5.3 22.1 50.6 65.1 76.3 66.5 4.2 21.8 41.8 67.5 59.5 60.2

7 20 40 56 64 65 4 20 36 52 52 55

reproducibility and a greater number of points for model fit (see Supporting Information Figure S2). The diffusion model proposed by Shifflet and Yokozeki11 were used to fit the experimental data at different temperatures and pressure, obtaining R2 correlation coefficients >0.98 when using 500 13784



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Figure 3. Dependence of estimated diffusivity of CO2 in IL on solvent mass loaded into sample container for [emim][NTf2] (●) and [omim][NTf2] (▲) at 298 K.

Figure 5. Kinetic curves obtained for [emim][NTf2] at 1 bar and 298 K using different gas flows: 50 mL/min (▲), 100 mL/min (●) and 200 mL/min (■).

effective diffusion coefficient (D) of CO2 in IL was calculated for each temperature (Table 1). For an IL sample mass of ∼50 mg, the D coefficient had a value of 7 × 10−11 m2/s, whereas a D value of 64 × 10−11 m2/s was estimated when ∼500 mg of IL was used in measurements at 298 K. These values are too different from each other. For this reason, the effect of the IL mass loaded to balance container on the CO2 absorption kinetic curves was analyzed with more detail by further gravimetric experiments. Time-dependent CO2 absorption data were measured using increasing mass (from 50 to 650 mg) of IL absorbent ([emim][NTf2] and [omim][NTf2]) at 298 K and a pressure range from 1 to 20 bar (Table S1). Average D values in Table 1 were estimated by using the kinetic curves at 1−20 atm of three independent experiments, obtaining standard deviations below 15%. Figure 3 reveals that the estimation of the effective diffusion coefficients (D) of CO2 in both ILs remarkably depends on the mass sample loaded until certain value, after which the D value remains constant. This effect was related to the depth (L) value of IL in the container used in eq 2 of the mass diffusion model. At amounts of solvent higher than 500 mg, the liquid film of IL filled the complete bottom section of sample container, whereas using mass sample lower of 500 mg implied that the IL is not distributed uniformly over the surface of the sample holder, observing IL drops at the bottom surface. Therefore, it should be ensured the adequate estimation of L values by using enough

Figure 4. Diffusivity values vs temperature for different ILs, [emim][NTf2] (circles), [bmim][PF6] (squares), and [omim][NTf2] (triangles), obtained from different measurements: from this work, different mass loaded 500 mg (black symbols) and 50 mg (gray symbols) and other authors11,27,28,30 (white symbols).

mg of IL, whereas lower values (0.96 > R2 > 0.80) where generally obtained when using 50 mg of [emim][NTf2] in absorption experiments. After adjustment to this model the kinetic curve points for each pressure, an average value of

Table 2. Average Values of Diffusion Coefficients of CO2 in Different ILs Obtained in This Work

a

IL

P (bar)

mass (mg)

diametera (cm)

gas flow (mL/min)

T (K)

D (10−11 m2/s)

standard deviation

[emim][NTf2]

1−20

500

1.5 1.0 1.5

100

298

64 61 61 60 78 104 52 61 83 29 37 49

13 10 5 8 13 5 8 3 6 4 6 8

50 200 100

[omim][NTf2]

1−20

500

1.5

100

[bmim][PF6]

1−20

500

1.5

100

308 323 298 308 323 298 308 323

Diameter of the sample container used in experiments. 13785



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Figure 7. Values of effective diffusivity (D) vs viscosity (μ) of C0 2 in ILs: (black symbols) values from this work; (white symbols) other authors.29,30 Bottom figure in logarithm scale.

authors27,28,30 (Figure 3); whereas a systematic underestimation of effective diffusion coefficient is observed when using relatively low mass (respect to sample container capacity) of solvent in gravimetric measurements.11,16,17 In order to develop a gravimetric method to obtain reproducible diffusion parameter, further experiments have been carried out to meet other assumptions of the model (see complete information in Table S1 of the Supporting Information). In first place, CO2 absorption experiments with [emim][NTf2] at 298 K were performed using different diameter sizes (1.5 and 1.0 cm) of the sample container but the same IL mass (500 mg), in order to ensure the simplification of one-dimensional diffusion. The results collected in Table 2 showed close D values obtained for the different height/with ratio of IL volume in sample container. To ensure the absence of gas diffusion problems and the assumption of constant saturation concentration in the gas− liquid boundary layer, various absorption tests with [emim][NTf2] at 298 K have been conducted using different CO2 continuous gas flow (50, 100, and 200 mL/min). Virtually identical kinetic curves were obtained for the different gas flows (Figure 5), which implies very close values of D estimations (Table 2). On the other hand, the gravimetric sorption analyzer

Figure 6. Diffusivity values obtained for (A) [emim][NTf2], (B) [bmim][PF6], and (C) [omim][NTf2] at 298 (white symbols), 308 (gray symbols), and 318 K (black symbols).

IL mass in the balance container (as example for [emim][NTf2] case, L > 1.3 mm for a sample container diameter of 1.5 cm and considering a IL density of 1.52 g/cm3) in order to obtain consistent estimations of the effective diffusion coefficients (D) of CO2 + IL systems by eq 2. Figure 4 compares the reported D values for CO2 in [emim][NTf2], [omim][NTf2], and [bmim][PF6] at different temperatures, estimated by different experimental methods.11,27,28,30 It is observed that current D estimationsobtained using higher IL depth (L)match the experimental trend of CO2−IL kinetic data reported for several 13786



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Table 3. Statistical Results of Fitting Diffusion Coefficient Values Estimated Using Experimental and Theoretical Models diffusion model Arnold

0.5

D = [0.01/A1A2]{(1/MIL + 1/MCO2)/[μ (VCO2 VIL0.33)]} D = 3.7 × 10−10μ−0.59VCO2−1ρIL−2

Morgan28 29

D= D=

Hou and Baltus Wilke−Chang30,41 modified Wilke−Chang a

N

expression

30,40

0.33

+

6.7 × 10 μ MIL−0.89ρIL4.8T−3.3 A[(ϕMIL)0.5T/μVCO20.6]

A1A2 = 0.34

5 −0.66

D = 7.4 × 10−8[ϕ0.5MILAT/μBVCO20.6]

R2

SDa

MAPEb −10

67

0.60

3.9 × 10

53%

67

0.57

3.5 × 10−10

36%

−10

A = 7.4 × 10−8, ϕ = 7.5

67 67

0.47 085

5.4 × 10 4.1 × 10−10

82% 46%

A = 0.67, B = 0.58, ϕ = 0.14

67

0.84

3.9 × 10−10

25%

b

Standard deviation. Mean absolute percentage error.

clear relationship between CO2 diffusivity and viscosity values for 14 ILs which present remarkably different cation and anion structures. As can be observed, the rate of CO2 absorption in ILs can be remarkably enhanced by selecting appropriate ILs and operating conditions, what should be considered when designing gas separation processes based on ILs. Because of the expanding number of available ILs, the experimental evaluation of CO2 diffusivities in ILs can be a tedious and lengthy process.16,30 In this context, several models of diffusivity have been applied to calculate the diffusion coefficient of CO2 in ILs at infinite dilution of liquid solution, based on hydrodynamic theory, kinetic theory of liquids and semiempirical equations based on these theories.30 One main interest of such approximations is to obtain preliminary estimations of the absorption rate of CO2 in ILs at different temperatures, in order to provide kinetic criteria to select the appropriate solvents to be studied experimentally. Table 3 reports the statistical results of fitting different general expression of diffusivity to the extended available experimental D values, including 14 different ILs at 283−323 K temperature range (see Table S2 of the Supporting Information). Based on the statistical parameters of fitting, it is concluded that Wilke−Chang model provides reasonable estimations of the CO2 diffusivity behavior for the wide sample of ILs considered with R2 = 0.85 and standard deviation (4.1 × 10−10 m2/s) in the same order than the dispersion of experimental diffusion coefficients (for example, average SD of experimental D values for [emim][NTf2] at each temperature in the 298−343 K range is 1.7 × 10−10 m2/s). As can be seen in Table 3, several authors have proposed diffusion models which attempted to describe the D dependence on the viscosity and molar weigh by using different power for such terms.28−30 Figure 8 and Table 3 reported the results of fitting the 67 experimental D values of CO2 in ILs to a modified Wilke−Chang model where the viscosity and molar weight present adjustable power parameters. The modified Wilke− Chang expression corrected the general underestimation of measured D values from the original model, providing a mean absolute percentage error (MAPE; 25%) significantly lower than that obtained using reported the Wilke−Chang equation (46%) or other diffusion models (36−82%) in Table 3. Therefore, it is proposed as an affordable approach to estimate CO2 diffusion in ILs (with the limitation of not taking into account concentration effects),42 with the aim of screening the suitability of ILs as absorbent also considering kinetic criteria.

Figure 8. Comparison of diffusivity values estimated from experiments and using the modified Wilke−Chang model for CO2 in ILs: (black symbols) experimental values from this work; (white symbols) experimental values from other authors.29,30

(ISOSORP GAS LP-flow Rubotherm) controls the set values of temperature and the pressure with uncertainties of ±0.01 K and ±0.01 bar, respectively. Figure S1 in the Supporting Information shows the profiles of temperature and pressure related the time-dependent absorption data collected in Figure 2B. It is concluded that the diffusion model assumption of constant T and P during the kinetic curve measurement can be considered fulfilled. Finally, it has been mentioned that the estimated D coefficients must be considered as “effective” diffusion constants, since obtained CO2 solution into the IL in the 1−20 bar pressure range cannot defined as highly diluted. Figure 6 presents the diffusion coefficients of CO2 estimated at different pressures for the ILs [emim][NTf2], [omim][NTf2,] and [bmim][PF6], determined from the isothermal data at 298, 308, and 323 K (also collected in Table 2). Diffusivity value generally increases with the pressure (i.e., the concentration of CO2 absorbed in the medium), as it was reported previously and was related to the lower viscosity of these IL−CO2 solutions.16,17 The standard deviation of D values in the 1− 20 atm range for the studied ILs is below 14 × 10−11 m2/s. The current procedure provides effective diffusion coefficients of CO2 in ILs with reproducibility error of 5−20%. As it is expected, the temperature has main effects of D values, increasing the absorption rate of CO2 in the three ILs studied at higher temperatures. Previous works have demonstrated that the diffusivity of CO2 in ILs increases when decreasing the viscosity of the solvent, due to their structural features or the higher temperature.16,17,30 Figure 7 revealed a

4. CONCLUSIONS This work describes a procedure to obtain consistent diffusion coefficients of CO2 in ionic liquids by using gravimetry. Several operating variables to obtain the kinetic curves of CO 2 absorption in IL solvent have been analyzed (solvent mass, sample container dimension, gas flow, pressure, and temper13787



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(11) Shiflett, M. B.; Yokozeki, A. Solubilities and diffusivities of carbon dioxide in ionic liquids: bmim PF6 and bmim BF4. Ind. Eng. Chem. Res. 2005, 44 (12), 4453−4464. (12) Shiflett, M. B.; Yokozeki, A. Solubility of CO2 in room temperature ionic liquid hmim Tf2N. J. Phys. Chem. B 2007, 111 (8), 2070−2074. (13) Soriano, A. N.; Doma, B. T., Jr.; Li, M.-H. Carbon dioxide solubility in some ionic liquids at moderate pressures. Journal of the Taiwan Institute of Chemical Engineers 2009, 40 (4), 387−393. (14) Chen, Y.; Han, J.; Wang, T.; Mu, T. Determination of Absorption Rate and Capacity of CO2 in Ionic Liquids at Atmospheric Pressure by Thermogravimetric Analysis. Energy Fuels 2011, 25 (12), 5810−5815. (15) Pinto, A. M.; Rodriguez, H.; Colon, Y. J.; Arce, A., Jr.; Arce, A.; Soto, A. Absorption of Carbon Dioxide in Two Binary Mixtures of Ionic Liquids. Ind. Eng. Chem. Res. 2013, 52 (17), 5975−5984. (16) Gonzalez-Miquel, M.; Bedia, J.; Abrusci, C.; Palomar, J.; Rodriguez, F. Anion Effects on Kinetics and Thermodynamics of CO2 Absorption in Ionic Liquids. J. Phys. Chem. B 2013, 117 (12), 3398− 3406. (17) Gonzalez-Miquel, M.; Bedia, J.; Palomar, J.; Rodriguez, F. Solubility and Diffusivity of CO2 in hxmim NTf2, omim NTf2, and dcmim NTf2 at T = (298.15, 308.15, and 323.15) K and Pressures up to 20 bar. J. Chem. Eng. Data 2014, 59 (2), 212−217. (18) Anthony, J. L.; Maginn, E. J.; Brennecke, J. F. Solubilities and thermodynamic properties of gases in the ionic liquid 1-n-butyl-3methylimidazolium hexafluorophosphate. J. Phys. Chem. B 2002, 106 (29), 7315−7320. (19) Anthony, J. L.; Anderson, J. L.; Maginn, E. J.; Brennecke, J. F. Anion effects on gas solubility in ionic liquids. J. Phys. Chem. B 2005, 109 (13), 6366−6374. (20) Muldoon, M. J.; Aki, S. N. V. K.; Anderson, J. L.; Dixon, J. K.; Brennecke, J. F. Improving carbon dioxide solubility in ionic liquids. J. Phys. Chem. B 2007, 111 (30), 9001−9009. (21) Yokozeki, A.; Shiflett, M. B. Separation of Carbon Dioxide and Sulfur Dioxide Gases Using Room-Temperature Ionic Liquid hmim Tf2N. Energy Fuels 2009, 23, 4701−4708. (22) Finotello, A.; Bara, J. E.; Camper, D.; Noble, R. D. Roomtemperature ionic liquids: Temperature dependence of gas solubility selectivity. Ind. Eng. Chem. Res. 2008, 47 (10), 3453−3459. (23) Kerle, D.; Ludwig, R.; Geiger, A.; Paschek, D. Temperature Dependence of the Solubility of Carbon Dioxide in Imidazolium-Based Ionic Liquids. J. Phys. Chem. B 2009, 113 (38), 12727−12735. (24) Lim, B.-H.; Choe, W.-H.; Shim, J.-J.; Ra, C. S.; Tuma, D.; Lee, H.; Lee, C. S. High-pressure solubility of carbon dioxide in imidazolium-based ionic liquids with anions PF6 and BF4. Korean Journal of Chemical Engineering 2009, 26 (4), 1130−1136. (25) Carvalho, P. J.; Alvarez, V. H.; Marrucho, I. M.; Aznar, M.; Coutinho, J. A. P. High pressure phase behavior of carbon dioxide in 1butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide and 1butyl-3-methylimidazolium dicyanamide ionic liquids. J. Supercrit. Fluids 2009, 50 (2), 105−111. (26) Mattedi, S.; Carvalho, P. J.; Coutinho, J. A. P.; Alvarez, V. H.; Iglesias, M. High pressure CO2 solubility in N-methyl-2-hydroxyethylammonium protic ionic liquids. J. Supercrit. Fluids 2011, 56 (3), 224− 230. (27) Camper, D.; Becker, C.; Koval, C.; Noble, R. Diffusion and solubility measurements in room temperature ionic liquids. Ind. Eng. Chem. Res. 2006, 45 (1), 445−450. (28) Morgan, D.; Ferguson, L.; Scovazzo, P. Diffusivities of gases in room-temperature ionic liquids: Data and correlations obtained using a lag-time technique. Ind. Eng. Chem. Res. 2005, 44 (13), 4815−4823. (29) Hou, Y.; Baltus, R. E. Experimental measurement of the solubility and diffusivity of CO2 in room-temperature ionic liquids using a transient thin-liquid-film method. Ind. Eng. Chem. Res. 2007, 46 (24), 8166−8175. (30) Moganty, S. S.; Baltus, R. E. Diffusivity of Carbon Dioxide in Room-Temperature Ionic Liquids. Ind. Eng. Chem. Res. 2010, 49 (19), 9370−9376.

ature) by using a high pressure sorption analyzer with magnetic suspension balance operating in dynamic model. As a result, the effective diffusion coefficients of [emim][NTf2], [omim][NTf2] and [bmim][PF6] at 298, 308, and 323 K were provided. In view of the new available diffusivity data, the capability of reported theoretical models to estimate the diffusion coefficient of CO2 in ionic liquids at different temperatures has been revisited. A modified Wilke−Chang model is proposed as general expression to preliminary screen ionic liquids with favorable CO2 absorption rates.

■

ASSOCIATED CONTENT

S Supporting Information *

Table S1: Solubility and diffusivity data of CO2 in various ILs measured in this work. Table S2: Experimental and calculated D data used in the statistical fitting of Table 3. Figure S1: Temperature and pressure profiles during absorption of CO2 in [emim][NTf2] at 298 K. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 34 91 4976938. Fax: 34 91 4973516. E-mail: pepe. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the “Ministerio de Economiá y Competitividad” and “Comunidad de Madrid” for financial support (Projects CTQ2011-26758 and P2013/MAE-2800, respectively).

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REFERENCES

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Diffusion Coefficients of CO2 in Ionic Liquids Estimated by Gravimetry

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