Article pubs.acs.org/IECR
Measurement and Correlation of Liquid−Liquid Equilibria for Ternary and Quaternary Systems of Heptane, Cyclohexane, Toluene, and [EMim][OAc] at 298.15 K Sandra Corderí,† Elena Gómez,*,† Noelia Calvar,‡ and Á ngeles Domínguez† †
Advanced Separation Processes Group, Department of Chemical Engineering, University of Vigo, Campus Lagoas-Marcosende, 36310 Vigo, Spain ‡ LSRE-Laboratory of Separation and Reaction Engineering, Associated Laboratory LSRE/LCM, Department of Chemical Engineering, Faculty of Engineering, University of Porto, Rua Dr. Roberto Frias, s/n, Porto 4200-465, Portugal S Supporting Information *
ABSTRACT: In this work, the feasibility of using 1-ethyl-3-methylimidazolium acetate ionic liquid, [EMim][OAc], as a solvent for the extraction of toluene from a mixture of heptane and cyclohexane was analyzed. Liquid−liquid equilibrium (LLE) data for the {heptane (1) + cyclohexane (2) + toluene (3) + [EMim][OAc] (4)} quaternary system and for the corresponding ternary systems were measured at 298.15 K and atmospheric pressure. Selectivities and solute distribution ratios were calculated from the experimental LLE data, and the obtained values were compared with those obtained for the {heptane or cyclohexane (1) + toluene (2) + [EMim][OAc] (3)} ternary systems. Finally, the experimental results for the ternary and quaternary systems were correlated with the nonrandom two-liquid thermodynamic model.
1. INTRODUCTION The separation of aromatic and aliphatic hydrocarbons from naphtha is complicated because of the proximity of their boiling points as well as the formation of azeotropes. Liquid extractions are the industrially common processes for this kind of separation, in which organic solvents such as sulfolane, dimethyl sulfoxide, and ethylene glycols are used.1 The drawback of these solvents is that they are harmful to the environment because of their volatility and toxicity. Moreover, with these solvents, additional distillation steps are required; as a result, additional investments and energy consumption are needed. The substitution of these common organic solvents with greener alternatives such as ionic liquids (ILs) has been a topic of interest over the past several years. ILs are liquid salts with a melting point under 100 °C, which consist of a great variety of cations and anions. They present negligible vapor pressure, a wide liquid range, excellent ability to solvate organic and inorganic compounds, and tailoring ability.2,3 Therefore, ILs present attractive properties for their consideration as alternatives to conventional organic solvents in liquid−liquid extraction processes. In the open literature, ILs have been explored for their use in liquid−liquid extractions. Most of the works include liquid− liquid equilibrium (LLE) data for binary and ternary mixtures of aromatic and aliphatic compounds.4−15 However, because of the fact that petrochemical streams consist of a large variety of compounds, it is important to study systems with more than three components and, thus, that the composition of real mixtures be approached. The feasibility of separating the aromatic compound toluene from a mixture of alkanes using the IL 1-ethyl-3-methylimidazolium acetate, [EMim][OAc], has been evaluated in this work. © 2014 American Chemical Society
This IL not only can be easily acquired but also is easy to handle. Furthermore, the small cation together with the acetate anion (free from halogens) confers on this IL a relatively low toxicity.16,17 We have focused our research on the study of the liquid−liquid equilibrium of heptane, cyclohexane, toluene, and [EMim][OAc] by means of ternary and quaternary systems, namely, {heptane or cyclohexane (1) + toluene (2) + [EMim][OAc] (3)}, {heptane (1) + cyclohexane (2) + [EMim][OAc] (3)}, and {heptane (1) + cyclohexane (2) + toluene (3) + [EMim][OAc] (4)} at 298.15 K and atmospheric pressure. LLE data were analyzed in terms of the solute distribution ratio and selectivity, and the experimental LLE data were correlated by means of the nonrandom two-liquid (NRTL) thermodynamic model.18
2. EXPERIMENTAL SECTION 2.1. Chemicals and Apparatus. The IL [EMim][OAc] was supplied by IoLiTec GmbH. Cyclohexane, heptane, and toluene were purchased from VWR Prolabo. The IL was subjected to vacuum (P = 0.2 Pa) and moderate temperature (T = 343 K) for several days to reduce the initial water content and the levels of other volatile compounds to negligible values. The water content of the IL was determined by Karl Fischer titration (Mettler Toledo, C20 Coulometric KF, accuracy in the measurements of 5%). The other chemicals were used without further purification. To prevent water absorption, all chemicals were always kept and manipulated under an inert gas atmosphere. Table 1 lists the mass fraction purities, water Received: Revised: Accepted: Published: 9471
March 6, 2014 May 7, 2014 May 9, 2014 May 9, 2014 dx.doi.org/10.1021/ie500973k | Ind. Eng. Chem. Res. 2014, 53, 9471−9477
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Table 1. Purities, Water Contents, Densities (ρ), and Refractive Indices (nD) of Pure Components at 298.15 K and Atmospheric Pressurea ρ (g cm−3)
nD
component
purity, mass fraction
water content (ppm)
exptl
lit.
exptl
lit.
[EMim][OAc] heptane cyclohexane toluene
>0.950 >0.990 >0.995 >0.998
1754 − − −
1.09966 0.67957 0.77388 0.86223
1.09968b 0.67946d 0.77389d 0.86219d
1.50097 1.38529 1.42356 1.49398
1.49992c 1.38511d 1.42354d 1.49390e
Standard uncertainties are as follows: u(ρ) = ±0.00003 g cm−3, u(nD) = ±0.00004, and u(T) = ±0.01 K. bFrom ref 19. cFrom ref 20. dFrom ref 21. From ref 22.
a e
temperature controller, with a precision of ±0.01 K). Samples were vigorously mixed for 6 h to ensure a good contact between both phases. Then, the cells were left to settle overnight to achieve a clear phase separation. Afterward, a sample from each equilibrium phase was taken with a syringe and analyzed by means of gas chromatography. The internal standard method was applied to obtain quantitative results, and octane was the standard compound used for this purpose. Samples of upper and lower phases were diluted with methanol and introduced into chromatographic vials, where a known amount of the internal standard is present. The compositions of cyclohexane, heptane, and toluene in the samples were analyzed with a Hewlett-Packard 5890 Series II gas chromatograph with a Hewlett-Packard 5971 mass selective detector and a Hewlett-Packard-5MS capillary column (60 m × 0.250 mm × 0.25 μm). Because the ILs have negligible vapor pressure, they cannot be analyzed by gas chromatography, and therefore, their composition is calculated through a mass balance. An empty precolumn was also used to protect the column and collect the IL that could not be retained by the liner. The following temperature program was used: initial temperature of 343.15 K for 10.30 min, ramp of 15 K min−1, and final temperature of 368.15 K for 4.30 min. The injector and detector were maintained at 553.15 K, and the helium carrier gas flow rate was kept constant at 1 mL min−1 in the column. The injection was done by splitless with an injection volume of 1 μL. Two analyses were performed for each sample to obtain a mean value. To obtain the error in the equilibrium molar fraction compositions, ternary and quaternary mixtures with a wellknown composition were prepared by mass. These mixtures were analyzed with the chromatographic method, and their compositions were compared with those obtained by mass. The maximal error in the compositions was estimated to be ±0.004 in mole fraction.
contents, densities, and refractive indices of pure chemicals at 298.15 K and atmospheric pressure. These experimental densities and refractive indices are in good agreement with values previously reported in the literature,19−22 thus confirming the purity of the chemicals. Densities of pure components were measured using an Anton Paar DSA-5000M digital vibrating-tube densimeter with an uncertainty in the density measurement of ±3 × 10−5 g cm−3. Refractive indices were determined using an automatic refractometer Abbemat-HP Dr. Kernchen with an uncertainty in the experimental measurements of ±4 × 10−5. Furthermore, a Mettler AX-205 Delta Range balance with an uncertainty of ±3 × 10−4 g was used for the preparation of samples by weight just before their use. 2.2. Liquid−Liquid Equilibrium Compositions. The liquid−liquid equilibrium experiments for the investigated ternary and quaternary systems were conducted in the glass equilibrium cells at 298.15 K and atmospheric pressure. The same experimental methodology previously reported was used for the ternary mixtures.13,14 In the case of the quaternary system, a sectional plane (M1), in which the mole fraction of the IL is constant (x4 = 0.211), was chosen in the tetrahedron phase diagram as illustrated Figure 1. This plane is
3. RESULTS AND DISCUSSION 3.1. Experimental LLE Data. The LLE data of the {heptane or cyclohexane (1) + toluene (2) + [EMim][OAc] (3)} and {heptane (1) + cyclohexane (2) + [EMim][OAc] (3)} ternary systems at 298.15 K and atmospheric pressure are listed in Table S1 of the Supporting Information, whereas panels a and b of Figure 2 show their corresponding triangular diagrams. The LLE data of the {heptane (1) + cyclohexane (2) + toluene (3) + [EMim][OAc] (4)} quaternary system at 298.15 K and atmospheric pressure are listed in Table S2 of the Supporting Information, and Figure 3 shows the experimental and calculated tie-lines. In this figure, only the tie-lines for a fixed initial feed of cyclohexane (x2 = 0.083) were included to avoid the overlapping of the tie-lines. As shown in panels a and
Figure 1. Schematic representation of the {heptane (1) + cyclohexane (2) + toluene (3) + [EMim][OAc] (4)} quaternary system and the quaternary section plane, M1.
perpendicular to the slopes of the tie-lines. The compositions of the initial quaternary mixtures were systematically selected in the sectional plane, completely covering its surface. Hence, tielines within the whole heterogeneous region of the quaternary system were obtained. Ternary and quaternary liquid mixtures inside the immiscible region together with a magnetic stirrer were introduced into the cells. These were sealed with rubber caps and subsequently placed in a thermostatic circulating bath (PoliScience digital 9472
dx.doi.org/10.1021/ie500973k | Ind. Eng. Chem. Res. 2014, 53, 9471−9477
Industrial & Engineering Chemistry Research
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Figure 2. Experimental and calculated LLE data for the (a) {heptane (1) + toluene (2) + [EMim][OAc] (3)} and (b) {cyclohexane (1) + toluene (2) + [EMim][OAc] (3)} ternary systems at 298.15 K and atmospheric pressure. Filled circles and solid lines represent experimental tie-lines, and empty squares and dashed lines represent calculated data using the NRTL model.
b of Figure 2 and in Figure 3, the tie-line slope is negative, manifesting the higher solubility of toluene in the studied alkanes than in the IL. However, toluene is partially soluble in the studied IL, and heptane and cyclohexane show reduced solubility in [EMim][OAc] (cyclohexane > heptane). Therefore, a high ratio of toluene in the extract phase (lower phase) could be obtained.
Together with the LLE experimental data, Tables S1 and S2 of the Supporting Information include the corresponding values for the solute distribution ratio, β, and the selectivity, S, as defined below:
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β = x 2II/x 2I
(1)
S = (x2IIx1I)/(x2Ix1II)
(2)
dx.doi.org/10.1021/ie500973k | Ind. Eng. Chem. Res. 2014, 53, 9471−9477
Industrial & Engineering Chemistry Research
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Figure 3. Experimental and calculated LLE data for the {heptane (1) + cyclohexane (2) + toluene (3) + [EMim][OAc] (4)} quaternary system at 298.15 K and atmospheric pressure. Tie-lines for an initial feed of cyclohexane and [EMim][OAc] (x2 = 0.083 and x4 = 0.211, respectively). Filled circles and solid lines represent experimental tie-lines, and empty squares and dashed lines represent calculated data using the NRTL model.
decrease in β was observed when the concentration of the aromatic was increased. Additionally, these values were always
