Ind. Eng. Chem. Res. 2008, 47, 8389–8395
8389
Binary and Ternary Phase Diagrams of Benzene, Hexafluorobenzene, and Ionic Liquid [emim][Tf2N] Using Equations of State A. Yokozeki*,† and Mark B. Shiflett‡ DuPont Fluoroproducts Laboratory, Chestnut Run Plaza 711, Wilmington, Delaware 19880 and DuPont Central Research and DeVelopment, Experimental Station, Wilmington, Delaware 19880
A binary system of benzene and hexafluorobenzene is known as a system with famous double azeotropes (minimum-and-maximum pressure azeotropes at the isothermal VLE (vapor-liquid equilibrium)). In order to understand how these azeotropic behaviors will be affected by interactions with an ionic liquid, solubility experiments for benzene + ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [emim][Tf2N], and hexafluorobenzene + [emim][Tf2N] systems have been carried out at temperatures of about 283, 298, and 318 K. Both binary systems show immisciblity gaps with concentration ranges from about 77 to 100 and from about 66 to 100 mol % for benzene and hexafluorobenzene systems, respectively. The observed solubility data have been used to develop equation-of-state (EOS) models (with a generic Redlich-Kwong cubic equation) for these binary systems. As for the binary system of benzene and hexafluorobenzene, VLE data in the literature have been employed to develop the EOS model. Thus, binary and ternary phase diagrams for the present three components have been constructed using the present binary interaction parameters. Then, extractive separations of the azeotropes are discussed based on the present EOS model. 1. Introduction During the past several years, intensive studies on roomtemperature ionic liquids (RTILs) and their mixtures have been reported, and the number of studies on the thermophysical and thermodynamic properties as well as their applications is still increasing.1-4 We have also studied the thermodynamic properties of RTIL mixtures with various compounds, particularly with hydrofluorocarbons (HFCs).5-14 The present phase equilibrium study of benzene and fluorinated benzene with RTIL is a part of our continuing investigations to understand the hydrofluorocarbon (HFC) + RTIL interactions (or phase behaviors), and is our first report on fluorinated benzene mixtures with RTILs. Phase behaviors of conjugated hydrocarbons and their fluorinated derivatives would provide further insights on the intermolecular interactions. There are several solubility studies of RTIL mixtures with aromatic hydrocarbons and/or alkanes, with a special interest in the (aromatic/alkane) separation application.15-17 However, to the best of our knowledge, no solubility study of fluorinated benzene + RTIL systems has been reported. Our first report on this kind of mixtures is a binary system of hexafluorobenzene (C6F6) and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([emim][Tf2N]), as well as a benzene (C6H6) + [emim][Tf2N] binary system. Around the period from the mid 1960s to the mid 1970s, strong interests in the intermolecular interactions arose among fluorinated benzene derivatives.18-25 This is largely due to the discoveries of special interactions between C6H6 and C6F6 molecules: 1-1 intermolecular compound in the solid state18 and homogeneous double azeotropes in the vapor-liquid equilibrium (VLE).22 The initially popular view that the interaction was charge-transfer in origin has been abandoned in favor of a model involving electrostatic forces; refer to a review article25 for the intensive investigation of benzene + hexafluorobenzene and related systems. * To whom correspondence should be addressed. E-mail:
[email protected]. † DuPont Fluoroproducts Laboratory. ‡ DuPont Central Research and Development, Experimental Station.
Once we have solubility information about the present binary systems with the ionic liquid, the ternary phase behavior of the C6H6 + C6F6 + [emim][Tf2N] system can be obtained with help of the known VLE information about the C6H6 and C6F6 system.22-24 Then, it is quite interesting to see how the famous double azeotropic system will be influenced by the addition of the ionic liquid. In this study, we apply an equation of state (EOS) method (with a generic Redlich-Kwong cubic equation26) to understand the phase behavior of each binary system and the ternary system. An advantage using the EOS model over solution (activity) models is that the EOS method is more reliable for extrapolating the correlation outside the measured range of experimental data used; refer to our previous works.5,6,10-12,27,28 Any activity model such as the NRTL (nonrandom two liquid) model26,29 or “Gamma/Phi” solution model26 is easy to use and excellent to fit low pressure solubility data without much difficulty in the numerical calculations, but the application range of the correlated model is limited; the model prediction outside the correlation range is often unreliable. A drawback in the use of the EOS method is the numerical difficulty, particularly in the case of very low pressures, and thus, the present work may be regarded as a challenge for modeling a (very) low pressure EOS. This work is organized as follows. First, solubility (or immiscibility) experiments for the binary systems of C6H6 + [emim][Tf2N] and C6F6 + [emim][Tf2N] are reported in Section 2. Then, thermodynamic models and data analyses are described in Section 3. Subsection 3.1 analyzes the present experimental LLE (liquid-liquid equilibrium) data of Section 2, by use of the NRTL activity model.29 In the following subsection, 3.2, several PTx (pressure-temperature-composition) data at vapor-liquid equilibrium (VLE) conditions are generated, based on the “Gamma/Phi” method26 with the NRTL activity coefficients in subsection 3.2. This is because the VLE-PTx data are needed in order to develop the present EOS model for these binary systems. After describing the present EOS model structure in subsection 3.3, the binary interaction parameters for the EOS model are determined in the following subsection
10.1021/ie800754u CCC: $40.75 2008 American Chemical Society Published on Web 09/27/2008
8390 Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008 Table 1. Experimental LLE data for Benzene (1) + [emim][Tf2N] (2) systema T (K)
x1L1 (mol %)
x1L2 (mol %)
j L1b (cm3 · mol-1) V
j L2b (cm3 · mol-1) V
282.5 ( 0.2 298.0 ( 0.2 317.1 ( 0.2
77.5 ( 0.4 77.2 ( 0.4 77.0 ( 0.4
100.0-0.2 100.0-0.2 100.0-0.2
123.4 ( 1.0 125.6 ( 1.0 128.3 ( 1.0
87.5 ( 0.7 89.2 ( 0.7 91.4 ( 0.7
j V
ex c
L1
(cm3 · mol-1)
j V
ex c
L2
-2.1 ( 1.0 -2.1 ( 1.0 -2.4 ( 1.0
a Super- and subscript, L1 and L2, denote one liquid phase and coexisting another liquid phase, respectively. molar volume.
b
(cm3 · mol-1)
-0.1 ( 0.7 -0.1 ( 0.7 -0.1 ( 0.7
Observed molar volume.
c
Excess
Table 2. Experimental LLE data for Hexafluorobenzene (1) + [emim][Tf2N] (2) systema T (K)
x1L1 (mol %)
x1L2 (mol %)
j L1b (cm3 · mol-1) V
j L2b (cm3 · mol-1) V
282.5 ( 0.2 297.9 ( 0.2 317.2 ( 0.2
100.0-0.3 100.0-0.3 100.0-0.5
66.6 ( 1.4 66.2 ( 1.8 64.4 ( 1.5
113.4 ( 1.5 115.5 ( 1.4 119.9 ( 1.5
158.4 ( 2.5 160.4 ( 2.4 165.8 ( 2.6
j V
ex c
L1
(cm3 · mol-1)
2. Experimental Section 2.1. Materials. The ionic liquid [emim][Tf2N] (electrochemical grade, assay g99.5%, C8H11F6N3O4S2, Lot and Catalog no. 259095 IL-201-20-E, CAS registry no. 174899-82-2) was purchased from Covalent Associates Inc. (Woburn, MA). Benzene (HPLC grade, assay g99.9%, C6H6, Product and Batch no. 270709-100 ML 00440TH, CAS registry no. 71-43-2) and hexafluorobenzene (NMR grade, assay g99.5%, C6F6, Product and Batch no. 326720-256 14320CE, CAS registry no. 39256-3) were obtained from Sigma-Aldrich, Inc. (St. Louis, MO). Elemental analysis was used to check the purity of the [emim][Tf2N] sample. The purity was g99.4%, and a detailed description of the analysis can be found in our previous work.6 The [emim][Tf2N] was dried and degassed by filling a borosilicate glass tube with about 10 g of the ionic liquid and pulling a coarse vacuum with a diaphragm pump (Pfeiffer, model MVP055-3, Nashua, NH) followed by further evacuation using a turbopump (Pfeiffer, model TSH-071) to a pressure of about 4 × 10-7 kPa, while simultaneously heating and stirring the ionic liquid at a temperature of about 348 K for 5 days. The final mass fraction of water was measured by Karl Fischer titration and the dried sample contained 188 × 10-6 H2O. 2.2. Experimental Method. A detailed description of the liquid-liquid-equilibria (LLE) experimental equipment and procedures are available in our previous reports.10,11 Therefore, only the basic experimental techniques and measurement uncertainties are given here. LLE experiments were made with samples containing (85.3 and 95.2) mole percent benzene + [emim][Tf2N] and samples containing (85.0 and 95.0) mole percent hexafluorobenzene + [emim][Tf2N]) at constant temperatures from about (283 to 317) K using the volumetric method.10,11 The uncertainty in temperature was ( 0.2 K and was determined using a standard platinum resistance thermometer (SPRT model 5699, Hart Scientific, American Fork, UT, range from 73 to 933 K) and readout (Blackstack model 1560 with SPRT module 2560) with a NIST certified traceable accuracy to (0.005 K. The vapor phase density for benzene which contains a negligible contribution of [emim][Tf2N] was calculated using the REFPROP computer program30 and prop-
ex c
L2
0.0 ( 1.5 -0.1 ( 1.4 0.0 ( 1.5
a Super- and subscript, L1 and L2, denote one liquid phase and coexisting another liquid phase, respectively. molar volume.
3.4, using the VLE-TPx data mentioned above for the C6H6 + [emim][Tf2N] and C6F6 + [emim][Tf2N] systems. As for the binary C6H6 + C6F6 system, VLE-TPx data in the literature22 are used for the EOS parameters. The validity of the present EOS correlation is also discussed here. Section 4 predicts isothermal ternary phase diagrams for the present system, and discusses the C6H6/C6F6 azeotrope separation with the ionic liquid addition. Then, concluding remarks follow.
j V
b
(cm3 · mol-1)
-2.3 ( 2.5 -3.3 ( 2.4 -3.4 ( 2.6
Observed molar volume.
c
Excess
Table 3. NRTL Binary Interaction Parameters in eq 2 system (1)/(2)
τ(0) 12
τ(1) 12 ⁄ K
τ(0) 21
τ(1) 21 ⁄ K
C6H6/[emim][Tf2N] C6F6/[emim][Tf2N]
9.3428 7.5042
136.38 394.38
-2.6571 -0.98914
-113.78 -358.91
erly accounted for in the mass balance equations.10,11 The vapor density for hexafluorobenzene was approximated using the ideal gas law and the vapor pressure data.29 Observed liquid phase compositions and molar volumes are shown in Tables 1 and 2, where excess molar volumes of each liquid phase are also listed, being calculated with known molar volumes of the pure compounds in the literature.19,30 3. Results and Analysis 3.1. Activity Model for LLE. Liquid-liquid equilibria (LLE) for an N-component system at low pressures can be described by the following set of nonlinear equations with liquid mole fractions, xi, and activity coefficients, γi26: L1 L2 L2 γL1 i xi ) γi xi (i ) 1,..,N),
(1)
where N ) 2 for a binary system, superscripts, L1 and L2, indicate two liquid phases of LLE, and i ) 1 designates species 1 (here, benzene or hexafluorobenzene), while i ) 2 corresponds to species 2, the ionic liquid [emim][Tf2N]. For the activity coefficient, we use the NRTL (nonrandom two liquids) solution model29 in the present study. The binary activity coefficients of the NRTL model are explicitly given in the electronic supporting material (SM) of this article. The temperaturedependent binary interaction parameter (τij) of the NRTL is modeled in this work as follows: (1) (0) (1) τ12 ) τ(0) 12 + τ12 ⁄ T, and τ21 ) τ21 + τ21 ⁄ T
(2)
The third NRTL adjustable parameter R (nonrandomness parameter) was assumed to be a constant of 0.2 in the present analysis. Then, the two unknown adjustable parameters τ12 and τ21 at a given T can be determined from one set of LLE experimental (the present volumetric method) data, using eq. 1 (two nonlinearly coupled equations). We have three sets of such LLE data for each binary system, as shown in Tables 1 and 2, and can obtain a set of τ12 and τ21 at a respective temperature. Thus, the empirical parameters in eq. 2 have been determined from the obtained τij data and are given in Table 3. Once the binary interaction parameters of the NRTL model have been determined as a function of temperature, we can calculate the Tx (temperature-composition) phase diagram at any temperature by solving eq 1. Numerical solutions of eq 1
Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008 8391 26
have been obtained by use of a robust flash algorithm. Calculated Tx diagrams for the present LLE systems were successfully constructed and compared well with the experimental solubility data in Figures S1(a) and S1(b) of the Supporting Information. 3.2. VLE Data Generation from LLE Correlation. In order to have the information about the mutual solubility in the entire composition range, we must obtain the solubility data in the VLE region, besides the LLE data obtained and correlated above. Instead of collecting low pressure VLE data experimentally, we decided to generate such VLE data in this study, based on the present LLE correlation and the low pressure solution model, often called “Gamma/Phi” method,26 as described below. The PTx data at a VLE condition can be calculated for the present binary systems using the following equation with the NRTL binary interaction parameters obtained above (τ12, τ21). P ) γ1P01x1 ⁄ Φ1 + γ2P02x2 ⁄ Φ2
(3)
where the second term (for the ionic liquid) can be neglected, since the vapor pressure (P20) is safely assumed to be zero at temperatures of interest.
{
Φ1 ≡ exp
(B1 - VL1)(P - P01) RT
}
(4)
where B1: the second virial coefficient of benzene or hexafluorobenzene; using a generic RK EOS, B ) 0.08664035RTc/ Pc-0.4274802(RTc)2/PcRT. V1L: saturated liquid molar volume of benzene or hexafluorobenzene19,30 at a given T. P10: vapor pressure of benzene or hexafluorobenzene29 at a given T. Equation 3 is a pressure-implicit equation, because eq 4 is a function of P. Therefore, a proper iterative method must be used to obtain a pressure at given T and composition (x1); we have made such a computer routine. PTx data at VLE have been thus generated at temperatures close to the present experiments (T ) 283.15, 298.15, and 318.15 K) with a composition range from 10 to 60 (interval of 10) mol % for hexafluorobenzene and from 10 to 70 (interval of 10) mol % for benzene, respectively. These generated VLE-PTx data will be used in order to develop the EOS model in the following subsections, and the validity of the present method is discussed in Section 3.4. 3.3. Equation-Of-State (EOS) Model. In order to study the thermodynamic phase behavior, we have developed an equation of state (EOS), which has been successfully applied for refrigerant/lubricant mixtures,31 and various hydrofluorocarbon, and CO2 mixtures with ionic liquids.5,10-12,28 It is based on a generic Redlich-Kwong (RK) type of cubic EOS: P)
a(T) RT V - b V(V + b)
a(T) ) 0.427480
(5)
R2T2c R(T) Pc
(6)
RTc Pc
(7)
b ) 0.08664
The temperature-dependent part of the a parameter in the EOS for pure compounds is modeled by the following empirical form: e3
R(T) )
∑ β (1 ⁄ T - T ) , where T ≡ T ⁄ T k
k
r
r
r
c
(8)
k)0
The a and b parameters for general N-component mixtures are modeled in terms of their respective binary interaction parameters.27,31
N
a)
∑
i,j)1
√aiaj(1 - fijkij)xixj,
ai ) 0.427480
R2T2ci R (T) (9) Pci i
fij ) 1 + cijT, where cij ) cji, and cii ) 0. kij )
lijlji(xi + xj) , where kii ) 0. ljixi + lijxj
N
b)
∑
1 (b + bj)(1 - mij)xixj, 2 i,j)1 i
bi ) 0.08664
(10) (11)
RTci , Pci
where mij ) mji, mii ) 0 (12) Tci: critical temperature of the i-th species. Pci: critical pressure of the i-th species. R: universal gas constant. xi: mole fraction of the i-th species. In the above model, there are a maximum of four binary interaction parameters: lij, lji, mij, and cij for each binary pair. The fugacity coefficient φi of the i-th species for the present EOS model, which is needed for the phase equilibrium calculation, is given by: ln φi ) ln
a 1 RT + b′i + P(V - b) V - b RTb(V + b) a a′i b′i V - + 1 ln (13) RTb a b V+b
(
)
(
)
where a′i≡(∂na/∂ni)nj+iand b′i≡(∂nb/∂ni)nj+i: n ) total mole number and ni) mole number of i-th species (or xi ) ni/n). The explicit forms of ai′ and bi′ may be useful for readers and are given as follows: N
a′i ) 2
∑ √a a x
{
i j j
j)1
1 - fijkij -
fijlijlji(lij - lji)xixj (ljixi + lijxj)2
}
- a (14)
N
b′i )
∑ (b + b )(1 - m )x - b i
j
ij
j
(15)
j)1
Here, it should be mentioned that the mixing rules in the present EOS (eqs 9, 12, 14, and 15) are different from those used in our previous articles5,10-12,28 for ionic liquid mixtures. The simpler mixing rule (eq 15) makes critical locus calculations easier.27 Vapor liquid equilibria (VLE) for an N-component system can be obtained by solving the following equilibrium conditions: xiφLi ) yiφVi (i ) 1,.., N)
(16)
where xi: liquid mole fraction of the i-th species, yi: vapor mole fraction of the i-th species, φLi : liquid-phase fugacity coefficient of the i-th species, φiV: vapor-phase fugacity coefficient of the i-th species. In the case of three phase equilibria (VLLE), equations corresponding to eq 16 become: L1 L2 L2 V V xL1 i φi ) xi φi ) yi φi (i ) 1,.., N)
(17)
where superscripts, L1 and L2, denote one liquid phase (1) and another coexisting liquid phase (2) of VLLE, respectively. Numerical solutions of eqs 16 or 17 (nonlinearly coupled equations) can be obtained by use of the TP-Flash (RachfordRice) method.26 3.4. EOS Model Parameters. Pure component EOS parameters for benzene and hexafluorobenzene were determined based on data from ref 29. The coefficients, βk, were determined so as to reproduce the vapor pressure of these pure compounds. For the ionic liquid, the critical parameters (Tc and Pc) were taken from our previous work,6 and values in βk of eq 8 were determined in the present work, together with the binary
8392 Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008 Table 4. EOS Constants for Pure Compounds Used in the Present Study benzene -1
hexafluorobenzene
molar mass (g · mol ) 78.11 562.2a Tc (K) 4.890a Pc (MPa) 353.2a Tb (K) 278.7a Tm (K) β0 1.00241 β1 0.41385 -0.051636 β2 β3 0.0072632 a Ref 29. work.
b
a
Ref 6. c Ref 38.
d
a
186.05 516.7a 3.300a 353.4a 278.2a 1.00183 0.57849 -0.083191 0.013733
[emim][Tf2N] 391.31b 808.8b 2.028b 647.1b 258,c 255,d 254e 1.0 5.62317
Ref 37. e Ref 36. βi: determined in this
Table 5. Optimal Binary Interaction Parameters in eqs 6-8 system (1)/(2)
l12
l21
m12 ) m21
c12 ) c21/K-1
0.0 -1.3851 × 10-3 C6H6/ [emim][Tf2N] -0.4949 -0.5966 0.0 -1.2124 × 10-3 C6F6/[emim][Tf2N] -0.6610 -0.6911 C6H6/C6F6 -0.06302 -0.1163 -0.02957 -1.5919 × 10-3
interaction parameters, by a method described in refs 28 and 31. Table 4 shows these parameters and other thermodynamic property data of the present pure compounds. Binary interaction parameters, lij, lji, mij, and cij in eqs 10-12, for benzene + [emim][Tf2N] and hexafluorobenzene + [emim][Tf2N] were determined using nonlinear regression analyses of VLE-PTx data, generated in Section 3.2. Figures S2(a) and S2(b) of the Supporting Information show excellent fits in pressures of these VLE (PTx) data, and the optimal binary interaction parameters of the EOS model are given in Table 5. Then, in order to see whether the present EOS model thus obtained (without using actual experimental VLE data) is valid or reliable, we have calculated binary PTx phase diagrams for the entire composition range. The model predicted correctly the VLLE behaviors (liquid-liquid immiscibities) as observed experimentally; refer to Figure 1. Here, it should be remembered that the information about the VLLE was not used in the EOS model development above; only (generated) VLE data from the LLE correlation were used. Next, we develop the EOS model for the binary system of benzene and hexafluorobenzene using experimental VLE (PTx) data by Gaw and Swinton.22 The experimental data are composed of five isothermal PTx (a range from 303.15 to 343.15 K with 14 < P < 75 kPa), covering the entire composition range (from 0 to 100 mol %); the total data points amount to 60. The binary EOS interaction parameters have been determined by the nonlinear least-squares analysis of relative pressures: ∑i (1-Pobs(T,x)i/Pcalc(T,x)i)2. The optimal parameters thus obtained are listed in Table 5; the relative AAD (absolute average deviation) in pressure was 0.28%. Predicted binary phase (PTxy) diagrams by the present EOS model are shown in Figure 2 for three selected temperatures (283.15, 343.15, and 406.15 K). The double azeotropes exist between about 303 and 406 K, and below 303 K, only one (minimum pressure) azeotrope exists, whereas at high temperatures, the double azeotropes become a saddle-point azeotrope around 406 K and beyond that no azeotrope exists up to the vapor-liquid critical locus. The predictions of these high-pressure behaviors are wellsupported by experiments by Ewing et al.23 A further validity of the present EOS correlation (for the benzene + hexafluorobenzene system) comes from the correct prediction of VLE critical locus calculations. The critical locus line has been calculated using a method applied previously,27,32 and is plotted on a PT projection in Figure 3, compared with several observed data.23 The agreement seems excellent. In
Figure 1. Isothermal PTx phase diagrams for (a) benzene + [emim][Tf2N] and (b) hexafluorobenzene + [emim][Tf2N] binary systems. Solid lines: calculated with the present EOS model. Solid squares and dotted (tie) lines: the present experimental data. Open circles: experimental data in ref 17.
addition, a critical locus line as a Tx projection is also shown in Figure S3 of the Supporting Information, compared well with reported values.23 Present success on our EOS correlation may seem fortuitous or surprising. However, we had confidence on the present EOS method, based on our previous experiences. For example, a highly nonideal binary system (ammonia + n-butane) was successfully modeled by the similar EOS,27 using only a limited range of VLE-PTx data, and behaviors in very high pressure regions (including critical loci) and very low pressure VLLE regions were very well predicted. 4. Ternary-System Predictions and Discussion Now that all of the binary EOS models have been verified, we may be able to predict the solubility behavior of the present ternary system with some confidence. First, we examined the ternary VLLE (or immiscible LLE gap), since the immiscibility gaps existed for both binary C6H6 + [emim][Tf2N] and C6F6 + [emim][Tf2N] systems, as shown in Figure 1. Then, the immiscibility region does exist for the present ternary system as well and is shown in Figure 4 as a shaded region, where several LLE tie lines are also shown at an isothermal condition of 343.15 K; the corresponding VLLE pressures are from about 71 to 74 kPa. A similar plot is given in Figure S4 of the Supporting Information at an isothermal condition of 298.15 K. The temperature dependence of the immiscible LLE gap is weak, as seen from Figures 4 and S4 of the Supporting Information. Here, it may be worth to mention that the immiscibility borderline at the ionic liquid-rich side solution
Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008 8393
RA ⁄ B )
( )⁄( ) yA xA
yB xB
(18)
where xA (or xB) and yA (or yB) are the mole fractions of A (or B) in a liquid phase and a vapor (or, another coexisting liquid) phase, respectively. Here, we denote C6F6 (hexafluorobenzene) as A (or 2) and C6H6 (benzene) as B (or 1), and for the selectivity we simply write RA/B (or R2/1 with A ) 2 and B ) 1). Vapor-phase selectivity predictions with two different feed compositions (with the two nearly azeotropic compositions) are shown in Figure 5, parts (a) and (b), as a function of temperature at an isobaric condition (P ) 101.325 kPa). The selectivity R2/1 (or RA/B), is significantly improved with having values of about 1.5-2.7, compared to near 1.0 without help of the ionic liquid addition. Similar plots at isothermal conditions (T ) 373.15 and 393.15 K) as a function of the system pressure are shown in Figure S5, parts (a) and (b), of the Supporting Information. Next, using the LLE property, the liquid-liquid extractive separation for the benzene-hexafluorobenzene azeotropic mix-
Figure 3. Vapor-liquid critical locus of benzene + hexafluorobenzene binary systems. Solid line: predicted by the present EOS model. Solid squares: experimental data.23 A plot of Tc vs. composition is also shown in Figure S3 of the Supporting Information which provides the composition for the experimental data.
Figure 2. Isothermal PTxy phase diagrams for benzene + hexafluorobenzene binary systems. (a) T ) 283.15 K (b) T ) 343.15 K and (c) T ) 406.15 K. Solid lines (bubble points) and dotted lines (dew points) are calculated with the present EOS model. Symbols in (b): experimental data22 for bubble (solid circles) and dew (open circles) points.
depends rather sensitively on the magnitudes of the binary interaction parameters of the C6H6 and C6F6 pair. For example, we could fit the experimental PTx data “perfectly”, instead of shown in Figure 2(b). However, the interaction parameters for oVerfittings (or “perfect” fittings) of the VLE-PTx data of this binary system provided unrealistically large immiscibility regions in the ternary phase diagram. Now, let us investigate the possible separation of azeotropic mixtures with the ionic liquid addition. In order to assess the feasibility of the gas separation by the extractive distillation or the liquid-liquid extraction, a selectivity coefficient RA/B, ability to separate species A and B in the gaseous or liquid phase is commonly defined as follows:17,33-35
Figure 4. Isothermal ternary phase diagram, constructed by the present EOS model, for the benzene + hexafluorobenzene + [emim][Tf2N] system at T ) 343.15 K. A shaded region is in the liquid-liquid equilibrium (LLE) of the VLLE; selected several LLE tie lines are shown. A similar plot for T ) 298.15 K is shown in Figure S4 of the Supporting Information.
8394 Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008
K. Again, the selectivity coefficient is sufficiently high (from about 1.5 to 3.0), relative to nearly 1.0 in the homogeneous azeotropic mixtures without the ionic liquid. 5. Conclusions Using the mass-volumetric method, binary liquid-liquid equilibria (LLE) of benzene + [emim][Tf2N] and hexafluorobenzene + [emim][Tf2N] systems have been measured at about 283, 298, and 348 K. The former system has a smaller immiscibility gap by about 10 mol% than the latter in the soluterich side mixtures. The LLE data have been correlated well with the NRTL activity model. It is found that a low pressure solution model (“Gamma/Phi” method) with the present NRTL activity coefficients can generate reliable VLE-PTx data for the present binary systems, in order to develop the present (a generic Redlich-Kwong) EOS model. For the benzene-hexafluorobenzene binary system, low-pressure VLE data in the literature have been used to develop the EOS model, which shows reliable predictions for the high pressure properties as well. With all the determined EOS binary interaction parameters, phase diagrams and solubility properties of the benzene + hexafluorobenzene + [emim][Tf2N] ternary system have been constructed. The vapor-phase or liquid-liquid-phase separation of the azeotropic C6H6-C6F6 mixtures seems quite practical with addition of the present ionic liquid. Acknowledgment
Figure 5. Vapor-phase selectivity, R2/1, by the present EOS model for hexafluorobenzene (2) over benzene (1) as a function of temperature at P ) 101.325 kPa. (a) 50 mol % of [emim][Tf2N] with a near azeotropic composition (benzene/hexafluorobenzene mole ratio ) 19/81; see Figure 2b). (b) 50 mol % of [emim][Tf2N] with another near azeotropic composition (benzene/hexafluorobenzene mole ratio ) 82/18; see Figure 2b). Solid circles: in the case of no ionic liquid.
The authors thank Mr. Joe Nestlerode at the DuPont Experimental Station for his assistance with LLE measurements. The present work was supported by DuPont Central Research and Development. Supporting Information Available: Binary NRTL activity model, LLE-Tx diagrams (Figure S1), VLE-PTx diagrams (Figure S2), VLE T-x critical locus (Figure S3), Ternary phase diagram at 298.15 K (Figure S4), Plots similar to Figure 5 at isothermal conditions (Figure S5). This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited
Figure 6. Liquid-phase selectivity, R2/1, by the present EOS model for hexafluorobenzene (2) over benzene (1) as a function of benzene mole % in the ionic liquid rich-side solution at the VLLE conditions; see Figure 4 and Figure S4 of the Supporting Information. Solid line: T ) 298.15 K (VLLE pressures are about 11.4 to 12.6 kPa). Broken line: T ) 343.15 K (VLLE pressures are about 71.8 to 73.3 kPa).
tures may also be practical. To demonstrate this method, we can use the calculated VLLE results in Figures 4 and S4 of the Supporting Information. The liquid phase selectivity is plotted in Figure 6 as a function of a benzene mol% in the ionic liquidrich solution, at the isothermal conditions of 298.15 and 343.15
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ReceiVed for reView May 9, 2008 ReVised manuscript receiVed August 5, 2008 Accepted August 7, 2008 IE800754U
