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Predictive Thermodynamic Models for Ionic Liquid−SO2 Systems Chengna Dai, Zhigang Lei,* and Biaohua Chen State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Box 266, Beijing 100029, China S Supporting Information *

ABSTRACT: Two types of predictive thermodynamic models, i.e., the UNIFAC model and GCLF EOS (group contribution lattice fluid equation of state), were applied to predict the solubility of SO2 in ionic liquids (ILs). The group interaction parameters were estimated by correlating the solubility data of SO2 in ILs exhaustively collected from the articles published until July 2015. It was verified that both the UNIFAC model and GCLF EOS could be used for predicting the solubility of SO2 in ILs, but the UNIFAC model gives results that are better than those of GCLF EOS. The UNIFAC model then was used to identify the structure−property relation for SO2 solubility and selectivity of CO2 to SO2, whereas GCLF EOS was used to investigate the volume expansivity of ILs upon the addition of SO2. The results showed that volume expansivity is independent of the different combinations of cations and anions and exhibits a linear relationship with SO2 solubility in ILs.

1. INTRODUCTION Sulfur dioxide (SO2) is the main contributor to the formation of acid rain, and is detrimental to human beings and the environment. The removal and capture of SO2 from flue gas is essential. The present approaches for SO2 removal include limestone-based, activated carbon adsorption, pressure swing adsorption flue-gas desulfurizations, and others.1−3 However, there are some obvious disadvantages, e.g., low operation efficiency and high emission of waste. Thus, a desirable capture method is absorption into a liquid. However, at present the most commonly used are amine-based solvents. Unfortunately, SO2 reacts with amine-based component through an irreversible chemical process causing solvent degradation, which results in high cost for the recycle of the solvent.4 In recent years, ionic liquids (ILs) have been proposed for acid gas separation and absorption because of their unique characteristics (e.g., extremely low vapor pressure, high thermal and chemical stability, designer and green solvents). Various studies5−9 have reported on designing functional ILs (including [TMG]- and [MEA]-based ILs) to increase the solubility of SO2 by introducing functional groups (−NH or −NH2) into the framework of ILs. However, the high cost and complex preparation of the functional ILs limit their application at a large scale. Other literature10−18 reported that SO2 is also highly soluble in common ILs, e.g., [BMIM][BF4], [HMPY][Tf2N], and [BMIM][PF6]. However, the studies on solubility of SO2 in ILs are much fewer than those on CO2 in ILs. A series of special precautions should be taken carefully in laboratories during the solubility measurement of SO2, which introduces difficulty into the experimental study. Therefore, it is highly desirable to develop some reliable and applicable predictive models to reduce the amount of experimental work and to provide a better and thorough understanding of the removal of SO2 by ILs. The most commonly used predictive models describing the phase equilibria of the systems with ILs are activity coefficient models and equations of state.19−25 Among others, the UNIFAC model is widely used for the prediction of thermodynamic properties of systems with ILs because of its © 2015 American Chemical Society

simple formulation and fast calculation speed. It is a functional group-contribution model assuming that the interaction energy between particular groups would be constant regardless of the overall structures of the components. Thus, they can be easily extended to other similar systems and can be directly incorporated into some commercial simulation software packages such as ASPEN PLUS and PROII. However, activity coefficient models do not consider the volume effect. Thus, they can not be used to predict the volumetric properties with regard to absorption processes. To remedy this insufficiency, groupcontribution lattice-fluid equation of state (GCLF EOS) was considered in this work. In our previous works,26,27 it was reported that both the UNIFAC model and GCLF EOS exhibit very good predictions for IL−CO2 systems over a wide temperature and pressure range. Recently, these predictive thermodynamic models developed by us have been adopted by many authors. Thus, it is straightforward for us to further extend these models to IL−SO2 systems. The focus of this work is to develop two types of predictive thermodynamic models (i.e., the UNIFAC model and GCLF EOS) for IL−SO2 systems, and this work is organized into the following three parts: (1) extending the group parameters of the UNIFAC model and GCLF EOS by correlating the experimental data collected from the literature; (2) identifying the structure−property relation using the UNIFAC model, considering both SO2 solubility as well as CO2/SO2 selectivity in ILs for simultaneous removal of SO2 and CO2; and (3) predicting the volume expansivity of ILs with the addition of SO2 using GCLF EOS, which is especially important when obtaining the volumetric properties (e.g., density, volume flow rate) of the fluids flowing in the absorption column. The meanings of abbreviations for all anions and cations of ILs throughout this papaer are listed in Table S1 in Supporting Information. Received: Revised: Accepted: Published: 10910

July 23, 2015 October 7, 2015 October 7, 2015 October 7, 2015 DOI: 10.1021/acs.iecr.5b02710 Ind. Eng. Chem. Res. 2015, 54, 10910−10917

Article

Industrial & Engineering Chemistry Research Table 1. New Obtained Group Binary Interaction Parameters (αmn and αnm) in UNIFAC Model

Table 2. New Obtained Group Binary Interaction Parameters αmn in GCLF EOS

main groups

main groups

m

n

αmn

αnm

m

n

αmn (αmn = αnm)

SO2 SO2 SO2 SO2 SO2 SO2 CH2 CHCH ACH CCOO CH2O SO2

CH2 [MIM][BF4] [MIM][Tf2N] [MIM][PF6] [MIM][MeSO4] [MPY][Tf2N] [MIM][Ac] [MIM][Ac] [MIM][Ac] [MIM][Ac] [MIM][Ac] [MIM][Ac]

191.9017 −41.7884 2.5440 −197.2417 −123.0372 69.1635 371.5589 500.1731 464.6787 409.6365 412.5778 −413.4426

145.0950 −95.7849 −106.3500 181.7151 −326.5951 −125.1685 667.4794 286.9818 186.2460 −2.5585 4.5897 89.0608

SO2 SO2 SO2 SO2 SO2 SO2 CH2 SO2

CH2 [MIM][BF4] [MIM][Tf2N] [MIM][PF6] [MIM][MeSO4] [MPY][Tf2N] [MIM][Ac] [MIM][Ac]

−1.1546 −0.7370 −0.3876 −0.6910 −0.8184 −0.4726 −0.5854 −0.5575

July 2015. The average relative deviation (ARD) minimized was adopted as the objective function (OF): OF =

2. THERMODYNAMIC MODELS Both the UNIFAC model and GCLF EOS are based on the functional group concept. As usual, ILs are divided into one whole IL group including the main skeleton of cation and anion and several CH2 or CH3 groups for the remaining alkyl chain on the cation. 2.1. UNIFAC Model. The gas−liquid equilibrium for SO2 (1) and IL (2) system at low and medium pressures is y1Pϕ1 = x1γ1P1s

N

∑ 1

xcal − xexp xexp

(3)

where xexp and xcal are the experimental SO2 solubility data and calculated values by the UNIFAC model, respectively, and N is the number of data points. The detailed solubility data, experimental methods, and the corresponding cited literature are given in Table S2 in Supporting Information. As for [MIM][Ac]-based ILs, the group interaction parameters between [MIM][Ac] and other groups (i.e., CH2, CHCH, CCOO, and CH2O) were obtained by correlating the experimental activity coefficient at infinite dilution reported in the literature using the following objective function (OF):

(1)

where x1 and y1 are the mole fractions of SO2 in liquid and gas phases, respectively; ϕ1 is the gas-phase fugacity coefficient determined using the Peng−Robinson equation;28 P is the system pressure; Ps1 is the saturated vapor pressure determined by Antoine equation;29 and γ1 is the activity coefficient determined by the UNIFAC model. The gas phase is assumed to be pure SO2 (y1 = 1) because of the negligible vapor pressure of ILs.30 In the UNIFAC model, the activity coefficient is expressed as

ln γi = ln γiC + ln γi R

1 N

OF =

γ∞ exp

1 N

N

∑ 1

∞ ∞ − γexp γcal ∞ γexp

(4)

γ∞ cal

where and are the activity coefficient of organic solvent in ILs at infinite dilution obtained by experiment and calculation by the UNIFAC model, respectively. The detailed activity coefficients, experimental methods, and the corresponding cited literature35−40 are given in Table S3 in Supporting Information. The SOLVER function with the optimization algorithm of Newton’s central difference in Microsoft Excel 2007 was used to correlate the group binary interaction parameters (αmn and αnm), and the procedure is similar that detailed in our previous publications.26,27 Accordingly, the group binary interaction parameters (αmn and αnm) are derived and listed in Table 1. The current UNIFAC model parameter matrix is illustrated in Figure 1. 2.2. GCLF EOS. GCLF EOS is based on statistical thermodynamics, and each component or functional group is assumed to occupy several lattice sites. The equation form is written as

(2)

ln γCi

where stands for the combinatorial contribution, which is due to only the differences in the size and shape of functional groups, and contains two group parameters Rk and Q k. The group parameters for functional groups involved in this work were obtained from the published UNIFAC group-interaction parameter matrix except for [MIM][Ac] and SO2.31−33 As for [MIM][Ac] and SO2 groups, the group parameters were calculated by Rk = (V × NA)/VVW and Q k = (A × NA)/AVW, where NA is Avogadro’s number (6.023 × 1023 mol−1); V and A are group volume and surface area calculated by the COSMO-RS (conductor-like screening model for real solvents) model; and VVW and AVW are 15.17 cm3·mol−1 and 2.5 × 109 cm2·mol−1, respectively, as suggested by Bondi.34 The results for [MIM][Ac] are Rk = 6.8459 and Q k = 4.3335, while for SO2 group they are 0.9011 and 0.8480, respectively. ln γiR stands for the residual contribution to activity coefficient due to the energetic interaction between different groups, which is a function of the group interaction parameters anm and amn. The group interaction parameters (anm and amn) between the IL group and CH2 can be found from our previous publication.32 The unknown group interaction parameters (anm and amn) between the IL group and SO2 were obtained by correlating the experimental SO2 solubility data exhaustively collected from the literature contributions published prior to

⎛ ṽ ⎞ θ2 P̃ z ⎛ v ̃ + q/r − 1 ⎞ ⎟ + ⎟− = ln⎜ ln⎜ ⎝ ṽ − 1 ⎠ ⎠ T̃ 2 ⎝ ṽ T̃ (5) where P̃, T̃ , and ṽ are the reduced pressure, temperature, and molar volume, respectively; q is the interaction surface area parameter; r is the number of lattice sites occupied by a molecule or a functional group; and z is a fixed coordination number (z = 10). 2Pv h P T 2RT v , T̃ = , ṽ = P̃ = = = = v hr P* zε* T* zε * v* (6)

θ= 10911

q/r , v ̃ + q/r − 1

zq = (z − 2)r + 2

(7)

DOI: 10.1021/acs.iecr.5b02710 Ind. Eng. Chem. Res. 2015, 54, 10910−10917

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Industrial & Engineering Chemistry Research

Figure 1. Current UNIFAC parameter matrix for ILs. Solid black squares, previously published parameters;31,41−44 striped squares, previously published parameters;26,32,33,45,46 pink squares, new parameters (this work); white squares, no parameters available.

Figure 2. Current GCLF EOS parameter matrix for ILs. Solid black squares, previously published parameters;24,48 striped squares, previously published parameters by our group;27,49,50 pink squares, new parameters (this work); white squares, no parameters available.

where vh is the volume of one lattice site (vh = 9.75 × 10−3 m3·kmol−1) and R is the universal gas constant (8.314 J·mol−1). For a given component, there are two adjustable parameters in eqs 5−7, i.e., the molecular interaction energy, ε*, and the molecular reference volume, v*. For a pure component, the two adjustable parameters (ε* and v*) can be obtained by the following mixing rule: εi* =

∑∑ k

Θ(ki)Θ(mi)(ekkemm)1/2

m

⎛T ⎞ ⎛ T ⎞2 ekk = e0, k + e1, k ⎜ ⎟ + e 2, k ⎜ ⎟ ⎝ T0 ⎠ ⎝ T0 ⎠

Θ(ki) =

(8)

R kk =

vi* =

∑ nk(i)R kk k

(9)

(10)

nk(i)Q k ∑n nn(i)Q n

(11)

⎡ ⎛T ⎞ ⎛ T ⎞2 ⎤ 1 ⎢ R + R + R ⎜ ⎟ ⎟⎥ 0, k 1, k 2, k ⎜ 103 ⎢⎣ ⎝ T0 ⎠ ⎝ T0 ⎠ ⎥⎦

(12)

where ei,k and Ri,k are constants for a certain group k; T is the system temperature; T0 is 273.15 K; and Q k is the dimensionless surface area parameter of group k, as used in the UNIFAC model.

where ekk is the group interaction energy between like groups (i) k, Θ(i) k the surface area fraction of group k, nk the number of group k, and Rkk the group reference volume of group k. 10912


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Industrial & Engineering Chemistry Research For a binary mixture, one binary interaction parameter k12 was introduced to calculate the molecular interaction energy and the molecular reference volume: ̇ Δε , εi* = θ1̅ ε11 + θ2̅ ε22 − θ1̅ θ2̅ Γ12 Δε = ε11 + ε22 − 2ε12 ε12 = (ε11ε22)1/2 (1 − k12),

(13)

εii =

∑ ∑ Θ(ki)Θ(mi)(ekkemm)1/2 k

m

(14)

v* =

∑ xivi*

(15)

i

where Γ̇12 is the nonrandomness parameter between components 1 and 2. ̇ Γ22 ̇ ⎛ Δε ⎞ Γ11 ⎟ = exp⎜θ 2 ⎝ RT ⎠ Γ̇12

(16)

The interaction parameter k12 in eq 14 can be calculated by k12 =

∑ ∑ Θ(mM)Θ(nM)αmn m

Θ(M) k

n

(Θ(M) k

(17)

(i) ∑in(i) k Q k/∑p∑ink Q k)

where = is the surface area fraction of group k in the mixture containing all groups and αmn is the group binary interaction parameter. In GCLF EOS, the mole fraction activity coefficient of component i is calculated by ln γi = ln

φi =

⎛ v ̃ vĩ − 1 ⎞ ai ṽ = ln φi − ln xi + ln i + qi ln⎜ ⎟ xi ṽ ⎝ v ̃ − 1 vĩ ⎠

⎛ 2θi , p − θ zq θ⎞ + qi⎜ − ⎟ + i ln Γi̇ 2 T̃ ⎠ ⎝ Tĩ

(18)

xivi* xiri = * ∑j xjrj ∑j xjvj

(19)

where xi is the mole fraction of component i in the liquid phase, Γ̇i the nonrandomness parameter, φi the volume fraction of component i in the mixture, and θi,p the surface area fraction of pure component i at the same temperature and pressure as in mixture. For SO2 and [MIM][Ac]-based ILs, the group interaction energy parameters (e0,k, e1,k, and e2,k) and reference volume parameters (R0,k, R1,k, and R2,k) were estimated beforehand by correlating the molar volumes of respective components. For SO2, the density data at different temperatures (280−350 K) and pressures (0.01−0.5 MPa) were obtained from NIST Chemistry WebBook (http://webbook.nist.gov/chemistry/). As for [MIM][Ac]-based ILs, the densities at different temperatures (293.15−358.15 K) were derived from linear generalized model as proposed by Valderrama and Rojas.47 The group interaction energy parameters and reference volume parameters for SO2 and [MIM][Ac] groups were obtained in the way similar to that reported in our previous work.48 The density of ILs decreases with increasing temperature; however, the variation is not obvious. For [EMIM][Ac], the density changes slightly from 1.113 to 1.108 g·cm−3 when the temperature increases from 293.15 to 358.15 K. Thus, it is assumed that for the [MIM][Ac] group e1,k = e2,k = 0 and R1,k = R2,k = 0. The results for SO2 are as follows: e0,k = 646.4545, e1,k = 238.3813, e2,k = 54.2257; R0,k = −7.4229, R1,k = 52.5179, and R2,k = −1.1827.

Figure 3. Solubility of SO2 in three common ILs at various temperatures and pressures. (a) [EMIM][BF4]: 298.15 K, ref 12.; (b) [BMIM][PF6]: 298.15 K; ref 12.; (3) [HMIM][Tf2N]: black squares, 298.15 K; red circles, 313.15 K; blue triangles, 333.15 K; ref 15. Scattered points, experimental data; solid lines, results predicted by the UNIFAC model; dashed lines, results predicted by GCLF EOS.

The parameters for [MIM][Ac] are e0,k = 1440.99 and R0,k = 104.01. The SOLVER function then was adopted to correlate the group binary interaction parameters (αmn) between SO2 and IL groups using the objective function as given in eq 2. In this way, the group binary interaction parameters were derived and are listed in Table 2. The current GCLF EOS parameter matrix is illustrated in Figure 2.

3. RESULTS AND DISCUSSION 3.1. Prediction of Solubility of SO2 in ILs. The predicted SO2 solubility values by the UNIFAC model and GCLF EOS in 10913


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Table 3. Comparison of SO2 Solubility in ILs between Experimental and Predicted Results by UNIFAC Model and GCLF EOS ARD (%) ILs

T range (K)

P range (bar)

UNIFAC

GCLF EOS

no. of data points

ref

[BMIM][Ac] [BMIM][BF4] [BMIM][MeSO4]

282.80−348.20 298.15 282.90−348.20 308.15−328.15 298.15 298.15 293.15−413.15 298.15 298.15 282.40−348.20 298.15−333.15 298.15

0.05−3.02 0.16−2.01 0.05−3.00 0.25−1.00 0.19−2.66 0.17−2.48 1.00 0.17−2.51 0.16−3.08 0.11−3.00 0.24−3.19 0.29−2.96

12.10 19.40 10.42 21.91 2.78 5.74 14.70 1.58 3.86 7.15 19.46 6.34

12.62 11.16 18.30 16.64 7.10 30.79 29.98 11.90 12.45 22.19 22.56 17.41

50 10 62 12 6 10 7 12 13 38 30 11

10 12 10 13 12 12 11 12 12 14 15 15

[BMIM][PF6] [BMIM][Tf2N] [EMIM][BF4] [HMIM][BF4] [HMIM][Tf2N] [HMPY][Tf2N]

three common types of ILs, i.e., [EMIM][BF4], [BMIM][PF6], and [HMIM][Tf2N], along with the experimental data obtained from the literature, are illustrated in Figure 3. It is evident that predicted values and experimental data exhibit the same trend, but the UNIFAC model has better prediction than GCLF EOS. The ARDs (average relative deviations, (1/N)∑i N= 1|(xi,pre − xi,exp)/xi,exp|) for [EMIM][BF4], [BMIM][PF6], and [HMIM][Tf2N] using the UNIFAC model are 1.58, 2.78, and 7.15%, respectively, while they are 11.90, 7.10, and 22.19%, respectively, using GCLF EOS. As a whole, both the UNIFAC model and GCLF EOS can be extended to IL−SO2 systems effectively. The comparison between experimental and predicted solubility for the UNIFAC model and GCLF EOS is summarized in Table 3. It can be seen that several ARDs are a little large mainly because of the limited experimental data in a narrow range of temperature or pressure. In further work, more experimental data of SO2 solubility in a wide range of temperature and pressure are needed for further improvement of the predictions. 3.2. Structure−Property Relation for SO2 Solubility and Selectivity of CO2 to SO2 in ILs. The UNIFAC model was used to identify the relationship between molecular structures of ILs and separation performances concerning SO2 solubility and selectivity of CO2 to SO2. In this work, Henry’s constant (Hi) was calculated by Hi(T ) = lim (γiPis) = γi∞Pis xi → 0

(20)

γ∞ i

where γi and are the activity coefficients of gas i in ILs at finite and infinite dilution, respectively, which are predicted by the UNIFAC model; Psi is the saturated vapor pressure of gas i obtained using the Antoine equation.29 The comparison between the calculated and experimental Henry’s constants of SO2 in ILs obtained from the literature12,15,51,52 is given in Table S4 in Supporting Information, and both agree in most cases. Moreover, the selectivity of CO2 to SO2 (SCO2/SO2) was used to evaluate the separation performance of various ILs, which is defined as SCO2 /SO2 =

Figure 4. Henry’s constants of SO2 (a) and selectivity of CO2 to SO2 (b) in [RnMIM][X] families predicted by the UNIFAC model at 298.15 K. Black squares, [RnMIM][BF4]; red circles, [RnMIM][PF6]; blue triangles, [RnMIM][Tf2N].

SO2 was investigated using the UNIFAC model, and the results are shown in Figure 4. It can be seen that with the increase of the carbon number, the Henry’s constants decrease for all ILs. For ILs with shorter carbon number in the alkyl chain on the cation (when n < 4), the Henry’s constants are in the order of [Tf2N] > [PF6] > [BF4], while for ILs with longer carbon number the Henry’s constants are in the order of [Tf2N] > [BF4] > [PF6]. Therefore, both cations and anions have significant influence on SO2 solubility. From Figure 4b, it is seen that the influence of carbon number on the selectivity of CO2

HCO2 HSO2

(21)

The influence of carbon number in the alkyl chain on the imidazolium cation-based ILs with different anions (i.e., [BF4], [PF6], and [Tf2N]) on SO2 solubility and selectivity of CO2 to 10914


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4. CONCLUSIONS In this work, two types of predictive thermodynamic models, i.e., the UNIFAC model and GCLF EOS, which are commonly used by general chemical engineers, were extended to IL−SO2 systems. The group binary interaction parameters between SO2 and IL groups were obtained by correlating the solubility data exhaustively collected recently. The strong predictive power of the UNIFAC model and GCLF EOS for predicting the solubility of SO2 in ILs was confirmed. Moreover, the selectivity of CO2 to SO2 in various ILs was also investigated using the UNIFAC model. It was found that SO2 solubility and selectivity exhibits a reverse trend. Thus, the balance between solubility and selectivity should be addressed during the selection of suitable ILs. Finally, volume expansivity of ILs with the addition of SO2 was studied using GCLF EOS. The results demonstrate that volume expansivity is independent of the different combinations of cations and anions, showing a linear relationship with SO2 solubility. Moreover, volume expansivity of IL−SO2 systems is a litter larger than that of IL−CO2 systems. To the best of our knowledge, this is the first work to disclose the volume expansivity of IL−SO2 systems, which is especially important in both experimental solubility measurement and absorption process design.

to SO2 exhibits a trend similar to that of the Henry’s constant of SO2. But at a given temperature, SO2 solubility and selectivity in these ILs exhibit a roughly inverse trend. Thus, the balance between solubility and selectivity should be considered during the selection of suitable ILs. If both high solubility and high selectivity are needed, the absorption process may be recommended to operate at low temperature like CO2 capture with ILs.45 However, in this case, the high viscosity at low temperature is a critical problem to be solved. 3.3. Prediction of Volume Expansivity of ILs Using GCLF EOS. The dissolution of gas into conventional solvents or ILs would lead to an increase of liquid volume. Volume expansivity (ΔV/V), which is an important physical parameter in the design of an absorption column, was used to investigate the increase of IL volume with the addition of SO2. The volume expansivity based on molar volume can be calculated by53,54 Ṽ (T , P , x) − VIL̃ (T , P0) ΔV %= M × 100 V VIL̃ (T , P0)

(22)

where Ṽ M(T,P,x) is the molar volume of solution with SO2 mole fraction x at a given temperature (T) and pressure (P); and Ṽ IL(T,P0) is the molar volume of pure IL at temperature (T) and ambient pressure (P0 = 1 bar). Because there are no experimental data of the volume expansivity of ILs upon the dissolution of SO2 available from references, the comparison between the experimental and predicted results is not given in the current work. The comparison of volume expansivity of ILs upon the addition of CO2 between experimental and predicted data can be seen in our previous work,27 indicating the validation of the prediction. Herein, the influence of various ILs on volume expansivity was investigated using GCLF EOS, and the results are shown in Figure 5. It can be seen that volume expansivity shows a liner

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b02710. Detailed definitions of abbreviations for all IL anions and cations, experimental data, and predicted results by both the UNIFAC model and GCLF EOS (XLS)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-1064433695. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China under Grants 21476009, 21406007, and U1462104.

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REFERENCES

(1) Kiil, S.; Michelsen, M. L.; Dam-Johansen, K. Experimental investigation and modeling of a wet flue gas desulfurization pilot plant. Ind. Eng. Chem. Res. 1998, 37, 2792−2806. (2) Al-Enezi, E.; Ettouney, H.; El-Dessouky, H.; Fawzi, N. Solubility of sulfur dioxide in seawater. Ind. Eng. Chem. Res. 2001, 40, 1434− 1441. (3) Hansen, B. B.; Kiil, S.; Johnsson, J. E.; Sonder, K. B. Foaming in wet flue gas desulfurization plants: the influence of particles, electrolytes, and buffers. Ind. Eng. Chem. Res. 2008, 47, 3239−3246. (4) Rao, A. B.; Rubin, E. S. Energy efficient production of hydrogen and syngas from biomass: development of low-temperature catalytic process for cellulose gasification. Environ. Sci. Technol. 2002, 36, 4467− 4475. (5) Wu, W.; Han, B.; Gao, H.; Liu, Z.; Jiang, T.; Huang, J. Desulfurization of flue gas: SO2 absorption by an ionic liquid. Angew. Chem., Int. Ed. 2004, 43, 2415−2417. (6) Jin, M.; Hou, Y.; Wu, W.; Ren, S.; Tian, S.; Xiao, L.; Lei, Z. Solubilities and thermodynamic properties of SO2 in ionic liquids. J. Phys. Chem. B 2011, 115, 6585−6591.

Figure 5. Volume expansivity upon the addition of SO2 for various ILs predicted by GCLF EOS.

relationship with the solubility of SO2 (x) (i.e., (ΔV/V)% = −85.5068x with the correlation factor of R2 = 0.9922) and is almost independent of the type of ILs. Moreover, when compared with the volume expansivity of CO2 in ILs ((ΔV/V)% = −89.1950x), the dissolution of SO2 gas leads to a little larger volume expansivity, indicating that there are more free volumes in IL−SO2 solution than in IL−CO2 solution. This phenomenon may explain why the solubility of SO2 is generally higher than that of CO2 in the same common IL under the same operating conditions. 10915


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