Ionic Liquids for Aromatics Extraction. Present Status and Future

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Ind. Eng. Chem. Res. 2010, 49, 7530–7540

Ionic Liquids for Aromatics Extraction. Present Status and Future Outlook G. Wytze Meindersma,* Antje R. Hansmeier, and Andre´ B. de Haan Department of Chemical Engineering & Chemistry/SPS, EindhoVen UniVersity of Technology, P.O. Box 513, 5600 MB EindhoVen, The Netherlands

Ionic liquids (ILs) can be used to replace conventional solvents in liquid-liquid extractions of aromatic hydrocarbons. An IL-based extraction process requires fewer process steps and less energy consumption, provided that the mass-based aromatic distribution coefficient and/or the aromatic/aliphatic selectivity are higher than those of the current state-of-the-art solvents such as sulfolane. Only a small number of ionic liquids are able to combine higher mass-based distribution coefficients with selectivities comparable to or higher than those of sulfolane. The most suitable ILs from our analysis are [bmim]C(CN)3, [3-mebupy]N(CN)2, [3-mebupy]C(CN)3, and [3-mebupy]B(CN)4. The mass-based distribution coefficients with these four ILs for benzene, toluene, and p-xylene are factors of 1.2-2.5 higher than those with sulfolane, and the aromatic/ aliphatic selectivities are up to a factor of 1.9 higher than with sulfolane. Based on the performed analysis, it can be concluded that industrial application of ILs for aromatics extraction has not yet materialized because only four of the total of 121 investigated ILs are considered suitable for aromatic/aliphatic separation. Most of the reported ILs do not provide higher mass-based aromatic distribution coefficients and/or higher aromatic/ aliphatic selectivities than those achieved by conventional solvents such as sulfolane. 1. Introduction The separation of aromatic hydrocarbons (benzene, toluene, ethylbenzene, and xylenes) from C4-C10 aliphatic hydrocarbon mixtures is challenging because these hydrocarbons have boiling points in a close range and several combinations form azeotropes. The conventional processes for the separation of these aromatic and aliphatic hydrocarbon mixtures are liquid-liquid extraction, suitable for the range of 20-65 wt % aromatic content; extractive distillation for the range of 65-90 wt % aromatics; and azeotropic distillation for high aromatic content, >90 wt %.1 Typical solvents used are polar components such as sulfolane,2-10 N-methylpyrrolidone (NMP),5,11 N-formylmorpholine (NFM),10,12,13 ethylene glycols,6,14,15 and propylene carbonate.16 This implies additional distillation steps to separate the extraction solvent from both the extract and raffinate phases and to purify the solvent, resulting in additional investments and energy consumption. The costs of regeneration of sulfolane are high, because in the currently used process, sulfolane, which has a boiling point of 287.3 °C, is taken as overhead from the regenerator and returned to the bottom of the aromatics stripper as a vapor.17 Overviews of the use of liquid-liquid extraction and extractive distillation for the separation of aromatic hydrocarbons from aliphatic hydrocarbons can be found elsewhere.18-21 Ionic liquids have been identified as promising solvents to replace conventional solvents in liquid-liquid extraction of aromatic hydrocarbons requiring fewer process steps and less energy consumption, provided that the mass-based aromatic distribution coefficients and/or the aromatic/aliphatic selectivities are higher than those of the current state-of-the-art solvents such as sulfolane.22,23 The data for sulfolane are as follows: Dbenz ) 0.43 kg/kg (0.58 mol/mol), Sbenz/hex ) 28.5;10 Dtol ) 0.26 kg/ kg (0.31 mol/mol), Stol/hept ) 30.9;24 and Dp-xyl ) 0.26 kg/kg (0.27 mol/mol), Sp-xyl/oct ) 24.9.8 The application of ionic liquids for extraction processes is promising because of the negligible vapor pressures of ILs.25,26 * To whom correspondence should be addressed. Tel.:+31 40 247 5808. Fax: +31 40 246 3966. E-mail: [email protected].

However, it has been shown that many ionic liquids can be distilled at low pressure without decomposition.27 A range of pure, aprotic, ionic liquids can be vaporized under vacuum at 200-300 °C and then recondensed at lower temperatures. The very low vapor pressures of ILs facilitate solvent recovery using techniques as simple as flash distillation or stripping. The criteria that a suitable ionic liquid for the separation of aromatic and aliphatic hydrocarbons must meet are the same as for conventional solvents.24,28,29 In general, this means high solubility for the aromatic hydrocarbon in the solvent combined with a high selectivity through a low solubility of the aliphatic components. This application potential of ionic liquids in aromatics extraction has resulted in the publication of a very large number of articles on the separation of aromatic hydrocarbons (benzene, toluene, ethylbenzene, and xylenes) from aliphatic hydrocarbons (hexane, heptane, octane, and cyclohexane) over the past 10 years (121 ILs in Table A in the Supporting Information). Unfortunately, however, most studies lack a comparison of the IL process with a conventional process and report distribution coefficients on a mole basis, whereas their industrial performance is characterized by mass-based distribution coefficients. Furthermore, by far, most authors report only on the distribution coefficients and selectivities based on activity coefficients at infinite dilution and not on the distribution coefficients and selectivities at finite concentrations.30 For these reasons, we have prepared a critical analysis comparing literature and our own data for aromatic/aliphatic separations using ILs with data on their extraction using the benchmark solvent sulfolane. In the open literature, mostly imidazolium-based ionic liquids combined with anions such as [Tf2N]-, CH3SO4-, and BF4- are described as solvents for the extraction of aromatic hydrocarbons from aliphatic components.31-45 Moreover, few data are available for ternary systems containing ionic liquids with pyridinium cations.24,26,39,43,46-48 In this work, we have measured liquid-liquid equilibria with several ionic liquids for the separation of toluene/n-heptane, toluene/n-hexane, benzene/n-hexane, and p-xylene/n-octane. Furthermore, we have compiled literature data concerning

10.1021/ie100703p  2010 American Chemical Society Published on Web 07/15/2010

Ind. Eng. Chem. Res., Vol. 49, No. 16, 2010

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Table 1. Suppliers and Purities of the Chemicals Used chemical

abbreviation

supplier, purity

1-ethyl-3-methylimidazolium methylsulfonate 1-ethyl-3-methylimidazolium dicyanamide 1-ethyl-3-methylimidazolium thiocyanate 1-butyl-3-methylimidazolium dicyanamide 1-butyl-3-methylimidazolium tricyanomethane 1-butyl-3-methylimidazolium thiocyanate 1-hexyl-3-methylimidazolium thiocyanate 3-methyl-N-butylpyridinium tetrafluoroborate 3-methyl-N-butylpyridinium dicyanamide 3-methyl-N-butylpyridinium tricyanomethane 3-methyl-N-butylpyridinium tetracyanoborate 4-methyl-N-butylpyridinium tetrafluoroborate 4-methyl-N-butylpyridinium dicyanamide 4-methyl-N-butylpyridinium thiocyanate 3,4-dimethyl-N-butylpyridinium dicyanamide 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imidebis(trifluoromethylsulfonyl)imide 1-butyl-1-methylpyrrolidinium dicyanamide 1-butyl-1-methylpyrrolidinium thiocyanate methyltrioctylammonium bis(trifluoromethylsulfonyl)imide triethylsulfonium bis(trifluoromethylsulfonyl)imide benzene toluene ethylbenzene p-xylene n-hexane n-heptane n-octane acetone

[emim]CH3SO3 [emim]N(CN)2 [emim]SCN [bmim] N(CN)2 [bmim]C(CN)3 [bmim]SCN [hmim]SCN [3-mebupy]BF4 [3-mebupy]N(CN)2 [3-mebupy]C(CN)3 [3-mebupy]B(CN)4 [4-mebupy]BF4 [4-mebupy]N(CN)2 [4-mebupy]SCN [3,4-dimebupy] N(CN)2 [mebupyrr][Tf2N]

Basionic, >95% Iolitec, >98% Basionic, >95% Iolitec, >98% Merck, >98% Iolitec, >98% Merck, >98% Merck, >98% Merck, >97% custom-made custom-made Merck, >98% Iolitec, >98% Iolitec, >98% custom-made Iolitec, >98%

[mebupyrr]N(CN)2 [mebupyrr]SCN [N1888][Tf2N] [S222][Tf2N] C6H6 C6H5CH3 C6H5C2H5 C6H4(CH3)2 CH3(CH2)4CH3 CH3(CH2)5CH3 CH3(CH2)6CH3 CH3COCH3

Iolitec, >98% Iolitec, >98% Iolitec, 99% Iolitec, 99% Fluka, >99% Merck, >99% Merck, >99% Merck, >99% Fluka, >99% Merck, >99% Merck, >99% Fluka, >99.9%

aromatic/aliphatic separations from activity coefficients at infinite dilution and/or liquid-liquid equilibria and compared all of these IL data to those for the aromatic/aliphatic separation with the conventional solvent sulfolane. For this comparison, we have calculated the aromatic distribution coefficients of the most promising ILs on a mass basis, because this is the approach used in industry. MacFarlane et al.49 showed that ionic liquids containing the anion dicyanamide have a significantly lower viscosity than ionic liquids combined with other anions. Because low viscosity is a favorable property of a solvent, we also provide some new liquid-liquid equilibrium (LLE) data for pyridinium- and imidazolium-based ionic liquids combined with the anions dicyanamide, N(CN)2-; tricyanomethane, C(CN)3-; tetracyanoborate, B(CN)4-; and thiocyanate, SCN-. The ionic liquids [3-methyl-N-butylpyridinium][dicyanamide], [1-butyl-3-methylimidazolium] [dicyanamide], and [1-butyl-3-methylimidazolium][thiocyanate] are low-viscosity ionic liquids with considerably high capacities and selectivities for aromatic hydrocarbons.50 The objective of this work was to select suitable ionic liquids for the separation of aromatic hydrocarbons from aliphatic hydrocarbons by means of experiments and comparison with literature results. 2. Experimental Section 2.1. Materials and Methods. The chemicals used are listed in Table 1. Prior to the experiments, the ionic liquids were dried in a rotary evaporator (Bu¨chi Rotavapor R-200) at 100 °C and under reduced pressure. Subsequently, the water content of the ionic liquids was determined by means of Karl Fischer titration and was found to be 900 ppm for [3-mebupy]N(CN)2, 500 ppm for [bmim]N(CN)2, and 1400 ppm for [bmim]SCN before the experiments. 2.2. Equipment and Experimental Procedure. Liquid-liquid extraction experiments were carried out in jacketed glass vessels

with a volume of about 70 mL. The vessels were closed with a poly(vinyl chloride) (PVC) cover through which a stirrer shaft was passed. For each experiment, 10 mL of the feed (toluene + n-heptane, toluene + n-hexane, benzene + n-hexane, or p-xylene + n-octane) and 20 mL of ionic liquid were added to a vessel, and the mixture was stirred (1200 rpm) for 15 min to reach equilibrium. In previous work, we reported that a mixing time of 5 min is sufficient to reach equilibrium.24 Nevertheless, to ensure that phase equilibrium was reached in every case, the extraction experiment was continued for 15 min. After stirring was ceased, the two phases were allowed to settle for about 1 h. This was carried out according to the procedure described earlier.24 Two stainless steel propellers, one in the bottom phase and one at the phase interface, with an electronic stirrer (Ika Eurostar) were used for phase mixing. Constant temperature ((0.1 °C) was maintained by means of a water bath (Julabo F32-MW). 2.3. Analysis. After equilibrium was reached, a sample of 0.5 mL of each phase was taken and analyzed by gas chromatography (Varian CP-3800). Acetone was added to the samples to avoid phase splitting and to maintain a homogeneous mixture. Ethylbenzene (0.2 mL for the raffinate samples and 0.1 mL for the extract phase samples) was used as an internal standard for gas chromatography (GC) analysis. The compositions of benzene, toluene, p-xylene, n-hexane, n-heptane, and n-octane in the samples were analyzed by a Varian CP-3800 gas chromatograph with a wall-coated open-tubular (WCOT) fused silica CP-SIL 5CB column (50 m × 0.32 mm; film thickness (df) ) 1.2) and a Varian 8200 AutoSampler. Because ionic liquids have no or a very low vapor pressure, they cannot be analyzed by GC; therefore, only the hydrocarbons of the feed/raffinate and extract phases were analyzed, and the amount of ionic liquid was calculated by means of a mass balance. For a ternary mixture, only two components need to be analyzed; the third one, the ionic liquid, was determined by a mass balance of the measured mass fractions of toluene and n-heptane, toluene and n-hexane, benzene and n-hexane, or p-xylene and n-octane.

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To avoid inaccuracy of the analysis caused by fouling of the GC by the ionic liquid, a linear column and a precolumn were used. Furthermore, measurements were carried out in duplicate to increase accuracy. The deviation in the calibration curves of 1% and a possible contamination of the gas chromatograph can cause a variance in the mole fractions (estimated at 1%). The averages of the two measurements were used in our results. The relative average deviation in the compositions is about 2.5%. 3. Results and Discussion Liquid-liquid extraction is based on the partial miscibility of liquids and is used to separate a dissolved component from its solvent by its transfer into a second solvent. The equilibrium in extraction can be characterized by the distribution coefficient, or partition coefficient, Di, which is defined as the ratio of the concentrations of solute i in the ionic liquid or solvent phase (extract E) and in the organic phase (raffinate R), according to D1)CE1 /CR1 and D2)CE2 /CR2

(1)

The selectivity, S1/2, of species 1 over species 2 is defined as the ratio of the distribution coefficients of species 1 and species 2 S1/2)D1 /D2)(CE1 /CR1 )/(CE2 /CR2 )

(2)

The definition of the distribution coefficient at infinite dilution, Di∞, is given by D∞1 ) 1/γ∞1 and D∞2 ) 1/γ∞2

(3)

where γi∞ is the activity coefficient at infinite dilution of species i. The selectivity at infinite dilution, S∞ 1/2, of species 1 over species 2 is defined as the ratio of the distribution coefficients at infinite dilution of species 1 and species 2 S∞1/2)D∞1 /D∞2 ) γ∞2 /γ∞1

(4)

The results of the extraction experiments with several ionic liquids for the separations of benzene/n-hexane, toluene/nhexane, toluene/n-heptane, and p-xylene/n-octane are reported in Table 2. All ILs, except [emim]SCN, have a higher molebased benzene distribution coefficient than sulfolane, and most ILs, except [emim]CH3SO3, [emim]SCN, and [4-mebupy]BF4, also have a higher mass-based benzene distribution coefficient than sulfolane (Dbenz ) 0.35 mol/mol and Dbenz ) 0.25 kg/kg). Most ILs also have a higher benzene/n-hexane selectivity than sulfolane (Sbenz/n-hex ) 29.1). All tested ILs, except [emim]SCN, show a higher mole-based toluene distribution coefficient and also a higher mass-based toluene distribution coefficient, except for [emim]CH3SO3, [emim]N(CN)2, [emim]SCN, [3-mebupy]BF4, and [4-mebupy]BF4, than sulfolane (Dtol ) 0.31 mol/ mol and Dtol ) 0.26 kg/kg). Most ILs show also a higher toluene/n-heptane selectivity than sulfolane (Stol/n-hept ) 33). The four tested ILs for the p-xylene/n-octane separation all show higher mole-based and mass-based p-xylene distribution coefficients than sulfolane (Dp-xyl ) 0.27 mol/mol and Dp-xyl ) 0.26 kg/kg). The p-xylene/n-octane selectivities of these ILs are comparable to those of sulfolane (Sp-xyl/n-oct ) 24.9). Only the IL [N1888][Tf2N] shows a very low aromatic/aliphatic selectivity. 4. Extraction with Ionic Liquids in Industrial Processes Extraction of aromatics from mixed aromatic/aliphatic streams with ionic liquids is expected to require fewer process steps

and less energy consumption than extraction with conventional solvents, because ionic liquids have a negligible vapor pressure. The recovery of ILs is relatively easy because of this very low vapor pressure. The solvent sulfolane is used as a benchmark for the separation of aromatic and aliphatic hydrocarbons, because it is one of the most commonly used solvents for this separation in industry. 4.1. Economic Evaluation. An economic evaluation was made for the separation of aromatic compounds from the feed of a naphtha cracker with the IL [4-mebupy]BF4, extended to other ILs {[emim]C2H5SO4, [4-mebupy]CH3SO4, [bmim]Cl1.0AlCl3, [emim]Cl-1.0AlCl3, and [3-mebupy]N(CN)2} and compared to that obtained with sulfolane.51,52 The separation of toluene from a mixed toluene/n-heptane stream was used to model the aromatic/aliphatic separation. Most ethylene cracker feeds contain 10-25% aromatic components, depending on the source of the feed (naphtha or gas condensate). The aromatic compounds are not converted into olefins, and even small amounts are formed during the cracking process in the cracker furnaces.53 Therefore, these compounds occupy a part of the capacity of the furnaces, and they put an extra load on the separation section of the stream containing C5-C10 aliphatic compounds. If a major part of the aromatic compounds present in the feed to the crackers could be separated upstream of the furnaces, it would offer several advantages: higher capacity, higher thermal efficiency, and less fouling. The improved margin will be around € 20/ton of feed or M€ 48 per year for a naphtha cracker with a feed capacity of 300 ton/h, due to lower operating costs. For a naphtha feed of 300 ton/h containing about 10% aromatic hydrocarbons, the costs in the sulfolane extraction were estimated by UOP, the supplier of the sulfolane process, to be about M€ 86 for the investment costs and M€ 24 for the energy costs. Sulfolane has to be regenerated with high-pressure steam,17 whereas the regeneration of ILs requires low-pressure steam. The liquid-liquid extraction process with the IL [4-mebupy]BF4 was modeled in Aspen Plus using our own experimental data for this IL, including pilot-plant extraction experiments.51,52 The size of the equipment and the energy requirements for the process steps were determined by means of this simulation. The investment costs for this process were estimated to be about M€ 56, including an IL inventory of M€ 20. An ionic liquid price of € 20/kg was used in the calculations, and BASF, a major producer of imidazole, one of the primary products for ionic liquids, has indicated that it is indeed possible to reach a level of € 10-25/kg with production on a large scale.54-56 The investment and annual costs for the separation of 10% aromatics from a cracker feed with sulfolane and several ionic liquids are shown in Figure 1.51,52 Even with a lower distribution coefficient than for sulfolane, a process with an IL can still be profitable, as can be seen in Figure 1 for the IL 1-ethyl-3methylimidazolium ethylsulfate ([emim]C2H5SO4, IL 1). The investment costs are in the same range as those for the sulfolane process, but the annual costs are much lower, because of the lower regeneration costs of the IL. A higher aromatic/aliphatic selectivity means a purer aromatic product, less extraction of aliphatics, and a lower number of extraction stages. A large aromatic distribution coefficient means a lower solvent-to-feed (S/F) ratio with, consequently, smaller extraction and regeneration units with lower investment costs, including a smaller IL inventory, and lower energy costs. Massbased aromatic distribution coefficients higher than 0.7 kg/kg hardly lead to lower costs, as can be seen in Figure 1. At a


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a

Table 2. Results of Extraction of Aromatic Hydrocarbons with Ionic Liquids solvent sulfolane [emim]CH3SO3 [emim]N(CN)2 [emim]SCN [bmim]N(CN)2 [bmim]C(CN)3

[bmim]SCN [hmim]SCN [3-mebupy]BF4 [3-mebupy]N(CN)2

[3-mebupy]C(CN)3

[3-mebupy]B(CN)4

[4-mebupy]BF4 [4-mebupy]N(CN)2 [4-mebupy]SCN [3,4-dimebupy]N(CN)2 [mebupyrr][Tf2N] [mebupyrr]N(CN)2 [mebupyrr]SCN [N1888][Tf2N] [S222][Tf2N] a

separation

T (°C)

aromatics (mol %)

Darom (mol/mol)

Darom (mass/mass)

Sarom/aliph

benzene/n-hexane toluene/n-hexane toluene/n-heptane toluene/n-heptane benzene/n-hexane toluene/n-hexane benzene/n-hexane toluene/n-hexane toluene/n-heptane benzene/n-hexane toluene/n-hexane toluene/n-heptane benzene/n-hexane toluene/n-hexane toluene/n-heptane p-xylene/n-octane benzene/n-hexane toluene/n-hexane toluene/n-heptane benzene/n-hexane toluene/n-hexane toluene/n-heptane benzene/n-hexane toluene/n-hexane toluene/n-heptane benzene/n-hexane toluene/n-hexane toluene/n-heptane p-xylene/n-octane benzene/n-hexane toluene/n-hexane toluene/n-heptane p-xylene/n-octane benzene/n-hexane toluene/n-hexane toluene/n-heptane p-xylene/n-octane toluene/n-heptane benzene/n-hexane toluene/n-hexane toluene/n-heptane benzene/n-hexane toluene/n-hexane toluene/n-heptane toluene/n-heptane benzene/n-hexane toluene/n-hexane toluene/n-heptane benzene/n-hexane toluene/n-hexane toluene/n-heptane benzene/n-hexane toluene/n-hexane toluene/n-heptane benzene/n-hexane toluene/n-hexane benzene/n-hexane toluene/n-hexane

30 30 40 40 30 30 30 30 40 30 30 30 30 30 40 40 30 30 40 30 30 40 30 30 40 30 30 30 40 30 30 30 40 30 30 30 40 40 30 30 40 30 30 40 40 30 30 40 30 30 40 30 30 40 30 30 30 30

2.0 4.0 10.0 10.0 2.0 4.0 2.0 4.0 10.0 2.0 4.0 10.0 2.0 4.0 10.0 10.0 2.0 4.0 10.0 2.0 4.0 10.0 2.0 4.0 10.0 2.0 4.0 10.0 10.0 2.0 4.0 10.0 10.0 2.0 4.0 10.0 10.0 10.0 2.0 4.0 10.0 2.0 4.0 10.0 10.0 2.0 4.0 10.0 2.0 4.0 10.0 2.0 4.0 10.0 2.0 4.0 2.0 4.0

0.35 0.29 0.31 0.44 0.57 0.33 0.31 0.41 0.27 0.75 0.58 0.63 1.54 1.08 0.85 0.60 0.70 0.43 0.50 0.74 0.63 0.75 0.67 0.57 0.54 1.44 0.93 0.86 0.54 1.84 1.50 1.12 0.83 1.92 1.54 1.48 1.09 0.51 0.83 0.75 0.63 0.67 0.57 0.51 0.90 1.73 1.19 1.22 0.71 0.52 0.55 0.73 0.61 0.53 1.64 1.51 1.23 0.87

0.25 0.21 0.26 0.22 0.28 0.17 0.16 0.11 0.16 0.32 0.25 0.28 0.61 0.43 0.39 0.31 0.31 0.19 0.26 0.29 0.25 0.35 0.25 0.21 0.23 0.60 0.39 0.42 0.30 0.70 0.56 0.51 0.42 0.74 0.60 0.64 0.51 0.22 0.34 0.31 0.31 0.28 0.24 0.25 0.42 0.39 0.27 0.34 0.30 0.22 0.27 0.32 0.27 0.27 1.11 1.03 0.28 0.20

29.1 24.4 33.0 25.4 30.1 18.3 114.3 78.6 147.3 30.5 23.8 59.0 32.3 22.8 49.3 23.1 55.4 33.9 65.8 20.9 18.0 16.8 36.2 30.4 51.5 35.3 22.9 44.7 25.2 34.8 27.9 34.8 25.8 27.0 21.9 38.5 22.0 45.5 34.3 31.3 35.1 45.2 38.7 45.0 25.6 20.7 14.4 15.6 15.4 11.3 42.0 17.3 14.6 47.7 1.8 1.7 32.2 23.0

A mixture of benzene, toluene, n-hexane, and n-hexene was used for the benzene/n-hexane and toluene/n-hexane separations.

toluene distribution coefficient of about 0.6-0.7 kg/kg, the investment costs can be reduced to about M€ 25-30, in which case the annual costs will be in the range of M€ 16-18/year. Therefore, ILs suitable for the extraction of aromatics from mixed aromatic/aliphatic streams must have a high aromatic distribution coefficient, higher than 0.3 kg/kg, and an aromatic/ aliphatic selectivity at least comparable to that of sulfolane. 4.2. Activity Coefficients at Infinite Dilution versus LLE Experiments. Table A, in the Supporting Information, lists a large number of known distribution coefficients on a mole basis and aromatic/aliphatic selectivities for several aromatic/

aliphatic separations from the literature. Most authors have reported the aromatic distribution coefficients on a mole basis only. Furthermore, it is clear from Table A in the Supporting Information that, by far, most authors have determined activity coefficients at infinite dilution and calculated the distribution coefficients and the aromatic/aliphatic selectivity from those coefficients. Only a few authors have carried out liquid-liquid equilibrium studies to determine the distribution coefficients and the aromatic/aliphatic selectivity at finite concentrations. It is also apparent from this table that there are considerable differences in values of the distribution coefficients and

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Figure 1. Investment and variable costs for extraction with ionic liquids.51,52 1, [emim]C2H5SO4; 2, [mebupy]BF4; 3, [mebupy]CH3SO4; 4, [3-mebupy]N(CN)2; 5, [bmim]Cl-1.0AlCl3; 6, [emim]Cl-1.0AlCl3. Horizontal line: margin for the naphtha cracker, M€ 48/year.

Figure 2. Mole-based distribution coefficients calculated from activity coefficients at infinite dilution and from LLE experiments.

Figure 3. Aromatic/aliphatic selectivities calculated from activity coefficients at infinite dilution and from LLE experiments.

selectivities reported for same separation obtained through activity coefficients at infinite dilution and actual LLE experiments. The largest difference in aromatic distribution coefficients is found with the IL [bmim]PF6, a factor of 4.3 in the distribution coefficient of benzene (Figure 2), and in the aromatic/aliphatic selectivity with the IL [mmim]CH3SO4, a factor of 8.1 in the selectivity for the separation of toluene/n-heptane (Figure 3). Figure 2 shows the values of the mole-based aromatic distribution coefficients obtained from activity coefficients at infinite dilution and from LLE experiments for the same separation. It is apparent from Figure 2 that the mole-based aromatic distribution coefficients measured from LLE experiments are generally higher than those obtained from activity coefficients at infinite dilution. The average difference is a factor of 1.5 with a standard deviation of 0.6. Figure 3 shows the aromatic/

aliphatic selectivities obtained from activity coefficients at infinite dilution and from LLE experiments. Also here, the selectivities measured from LLE experiments are generally higher than those calculated from activity coefficients at infinite dilution. The average difference between the two values is a factor of 1.6, but the standard deviation (SD) is higher than for the difference in distribution coefficient, SD ) 1.6, which is to be expected as the selectivity is the ratio of the aromatic and aliphatic distribution coefficients. 4.3. Comparison of Ionic Liquids for Toluene/ n-Heptane Separation. Because toluene/n-heptane proved to be a good representative for aromatic/aliphatic separation, the criteria for the selection of the ILs were a comparable or higher toluene distribution coefficient and a comparable or higher toluene/n-heptane selectivity than sulfolane (Dtol ) 0.31 mol/ mol and Stol/n-hept ) 30.9): Dtol > 0.2 mol/mol and Stol/hept > 30.9 or Stol/hept > 20 and Dtol > 0.31 mol/mol at T ) 40 °C and at a toluene concentration of about 10 wt %. The ILs that show a relatively low aromatic/aliphatic selectivity but a high aromatic distribution coefficient and the ILs that show a relatively low aromatic distribution coefficient but a high aromatic/aliphatic selectivity can still be suitable for the separation of aromatic and aliphatic hydrocarbons, depending on other process conditions. Table 3 lists the ILs suitable for toluene/n-heptane separation, and in Figure 4, the toluene/nheptane selectivity is shown as a function of the distribution coefficient (mol/mol) of toluene. In Table 3, the ILs that meet both criteria, Dtol > 0.31 mol/mol and Stol/hept >30.9, are in boldface. The IL [bmim]I3 (17) shows the highest toluene distribution coefficient (2.32) with a toluene/n-heptane selectivity of 30.1, but this IL and also the IL [emim]I3 (8) are extremely corrosive and, therefore, not suitable for industrial use.57 Although the AlCl3-containing ILs show high aromatic distribution coefficients and high aromatic/aliphatic selectivities (ILs 11 and 21 in Figure 4), these ionic liquids are not suitable because of their reaction with water. In addition, ILs with BF4- as the anion (ILs 27-30 and 34 in Figure 4) are unsuitable for this separation, because the price of these ILs will be too high, as almost all ILs with F-containing anions are expensive. The most suitable ILs from Table 3 and Figure 4 are [bmim]N(CN)2 (18) with Dtol ) 0.63 mol/mol and Stol/hept ) 59.0, [bmim]C(CN)3 (19) with Dtol ) 0.85 mol/mol and Stol/hept ) 49.3, [3-mebupy]N(CN)2 (31) with Dtol ) 0.86 mol/mol and Stol/hept ) 44.7, [3-mebupy]C(CN)3 (32) with Dtol ) 1.12 mol/ mol and Stol/hept ) 34.8, and [3-mebupy]B(CN)4 (33) with Dtol ) 1.48 mol/mol and Stol/hept ) 38.5; all values considerably higher than those of sulfolane. 4.4. Comparison of Ionic Liquids for Benzene/ n-Hexane and p-Xylene/n-Octane Separation. Next to toluene/ n-heptane, benzene/n-hexane is used to model aromatic/aliphatic separations. The benzene distribution coefficient for sulfolane is 0.58 mol/mol, and the benzene/n-hexane selectivity is 28.5. The criteria for selecting ionic liquids were thus Dbenz > 0.45 mol/mol and Sbenz/hex > 28.5 or Sbenz/hex > 20 and Dbenz > 0.58 mol/mol at T ) 40 °C and at a benzene concentration of about 10 wt %. Table 4 lists the ILs suitable for benzene/n-hexane separation, and in Figure 5, the benzene/n-hexane selectivity is shown as a function of the distribution coefficient (mol/mol) of benzene. The ILs that meet both criteria, Dbenz > 0.58 mol/mol and Sbenz/hex >28.5, are in boldface in Table 4. The IL [bmim]Cl-2.0AlCl3 (16) shows the highest benzene distribution coefficient and the highest benzene/n-hexane selectivity, but this IL is not suitable for industrial practice because


7535

Table 3. Overview of Toluene Distribution Coefficients in Mole and Mass Fractions and Toluene/n-Heptane Selectivities, Obtained from LLE or 1/γ∞a

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 a

solvent

Dtoluene (mol/mol)

Dtoluene (mass/mass)

Stol/n-hept

remarks

ref(s)

sulfolane [mmim][Tf2N] [emim]BF4 [emim]CH3SO3 [emim]C2H5SO4 [emim]CH3C6H4SO3 [emim][Tf2N] [emim]I3 [emim]N(CN)2 [emim]SCN [emim]Cl-1.0AlCl3 [emmim][Tf2N] [bmim]BF4 [bmim]PF6 [bmim]CH3SO4 [bmim]CF3SO3 [bmim]I3 [bmim]N(CN)2 [bmim]C(CN)3 [bmim]SCN [bmim]Cl-1.0AlCl3 [C10H17O2mim]Br [CN(CH2)3mim][Tf2N] [CN(CH2)3mim]N(CN)2 [CN(CH2)3mmim][Tf2N] [N-epy][Tf2N] [N-bupy]BF4 [N-hepy]BF4 [2-mebupy]BF4 [3-mebupy]BF4 [3-mebupy]N(CN)2 [3-mebupy]C(CN)3 [3-mebupy]B(CN)4 [4-mebupy]BF4 [4-mebupy]CH3SO4 [4-mebupy]N(CN)2 [4-mebupy]SCN [3,4-dimebupy]N(CN)2 [N-C3OHpy](C2F5)3PF3 [mebupyrr]CF3SO3 [mebupyrr]N(CN)2 [mebupyrr]SCN [pmpip][Tf2N] [Et3NH]Cl-2.0AlCl3 [S222][Tf2N]

0.31 0.50 0.30 0.44 0.22 0.36 0.83 0.84 0.24 0.27 1.80 0.61 0.34 0.34 0.33 0.43 2.32 0.63 0.85 0.50 1.57 0.30 0.30 0.23 0.30 0.52 0.43 0.67 0.52 0.54 0.86 1.12 1.48 0.43 0.61 0.63 0.51 0.90 0.75 0.45 0.55 0.53 0.79 1.06 0.63

0.26 0.14 0.16 0.22 0.10 0.12 0.22 0.17 0.13 0.16 0.69 0.16 0.15 0.12 0.14 0.15 0.71 0.32 0.39 0.26 0.57 0.09 0.07 0.11 0.07 0.14 0.20 0.28 0.23 0.24 0.41 0.51 0.64 0.21 0.24 0.31 0.25 0.42 0.14 0.16 0.27 0.27 0.20 0.29 0.17

30.9 29.8 34.1 25.4 50.5 28.0 27.7 48.6 43.4 147.3 69.6 22.7 46.1 21.3 31.2 20.2 30.1 59.0 49.3 65.8 35.7 43.1 40.6 53.8 26.8 24.7 74.4 25.6 60.0 51.5 44.7 34.8 38.5 50.0 42.3 35.1 45.0 25.6 37.5 26.7 42.0 47.7 20.2 23.1 21.2

extraction activity coefficients extraction extraction extraction extraction, 75 °C extraction, 25 °C extraction, 45 °C, corrosive extraction, 50 °C extraction extraction activity coefficients extraction activity coefficients extraction extraction, 45 °C extraction, 35 °C, corrosive extraction, 30 °C extraction extraction, 30 °C extraction activity coefficients activity coefficients, 30 °C activity coefficients, 30 °C activity coefficients, 30 °C activity coefficients extraction extraction extraction extraction extraction, 30 °C extraction, 30 °C extraction, 30 °C extraction extraction extraction extraction extraction activity coefficients, 45 °C activity coefficients, 45 °C extraction extraction activity coefficients, 45 °C extraction, 20 °C activity coefficients, 45 °C

24, 58 34 24 this work 24, 58 24 59 60 61 this work 62 63 24 64 58 65 60 66 this work 66 62 67 68 68 68 69 70 70 43 this work 66 this work this work 58, 71 24 this work this work this work 72 73 this work this work 74 75 76

T ) 40 °C, ∼10 wt % toluene.

Figure 4. Toluene/n-heptane separation with ionic liquids, mole-based, ∼10 wt % toluene, T ) 40 °C, component numbers in Table 3.

of its reaction with water. The ILs [Me3-NH]Cl-2.0AlCl3 (36) and [Et3-NH]Cl-AlCl3 (37) are not suitable for the same reason. The most suitable ILs for the separation of benzene and n-hexane are [bmim]C(CN)3 (14) with Dbenz ) 1.54 mol/mol

and Sbenz/hex ) 32.3, [3-mebupy]N(CN)2 (25) with Dbenz ) 1.45 mol/mol and Sbenz/hex ) 54.3, [3-mebupy]C(CN)3 (26) with Dbenz ) 1.84 mol/mol and Sbenz/hex ) 34.8, and [3-mebupy]B(CN)4 (27) with Dbenz ) 1.92 mol/mol and Sbenz/hex ) 27.0. These ILs are the same ILs that are most suitable for the toluene/n-heptane separation. Sulfolane shows a p-xylene distribution coefficient of 0.27 mol/mol and a p-xylene/n-octane selectivity of 24.9 (See Table 5). The criteria for selecting ionic liquids for the p-xylene/noctane separation were Dxyl > 0.20 mol/mol and Sxyl/oct > 24.9 or Sxyl/oct > 20 and Dxyl > 0.27 mol/mol at T ) 40 °C and at a p-xylene concentration of about 10 wt %. Table 5 lists the ILs suitable for the p-xylene/n-octane separation, and in Figure 6, the p-xylene/n-octane selectivity is shown as a function of the distribution coefficient (mol/mol) of p-xylene. The ILs that meet both criteria, Dxyl > 0.27 mol/mol and Sxyl/oct >24.9, are in boldface in Table 5. The most suitable ILs for the separation of p-xylene/n-octane are [bmim]C(CN)3 (5) with Dxyl ) 0.60 mol/mol and Sxyl/oct ) 23.1, [3-mebupy]N(CN)2 (7) with Dxyl ) 0.54 mol/mol and Sxyl/ oct ) 25.2, [3-mebupy]C(CN)3 (8) with Dxyl ) 0.83 mol/mol

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Table 4. Overview of Benzene Distribution Coefficients in Mole and Mass Fractions and Benzene/n-Hexane Selectivities, Obtained from LLE or from 1/γ∞a

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 a

solvent

Dbenzene (mol/mol)

Dbenzene (mass/mass)

Sbenz/n-hex

remarks

ref(s)

sulfolane [mmim][Tf2N] [emim]BF4 [emim]C2H5SO4 [emim]CF3SO3 [emim][Tf2N] [emim]N(CN)2 [emmim][Tf2N] [bmim]BF4 [bmim]PF6 [bmim]CH3SO4 [bmim][MDEGSO4]a [bmim]N(CN)2 [bmim]C(CN)3 [bmim]SCN [bmim]Cl-2.0AlCl3 [hmim]BF4 [hmim]SCN [CN(CH2)3mim][Tf2N] [CN(CH2)3mmim][Tf2N] [N-epy]C2H5SO4 [N-epy][Tf2N] [1-et-3-mepy]C2H5SO4 [3-mebupy]BF4 [3-mebupy]N(CN)2 [3-mebupy]C(CN)3 [3-mebupy]B(CN)4 [4-mebupy]BF4 [4-mebupy]N(CN)2 [4-mebupy]SCN [N-C3OHpy](C2F5)3PF3 [mebupyrr]CF3SO3 [mebupyrr][Tf2N] [pmpip][Tf2N] [C2][Tf2N] b [Me3NH]Cl-2.0AlCl3 [Et3NH]Cl-2.0AlCl3 [N111(C2OH)][Tf2N] [P1444]CH3C6H4SO3 [S222][Tf2N]

0.58 0.74 0.47 0.59 0.45 1.20 0.57 0.91 0.57 0.55 0.81 0.49 0.75 1.54 0.70 1.96 0.61 0.74 0.56 0.53 0.49 1.24 0.90 0.67 1.45 1.84 1.92 0.95 0.83 0.67 1.09 0.68 1.73 1.07 1.24 1.54 1.63 0.67 0.70 1.23

0.43 0.18 0.21 0.22 0.15 0.31 0.28 0.21 0.23 0.20 0.30 0.13 0.32 0.61 0.31 0.53 0.22 0.29 0.12 0.11 0.19 0.30 0.34 0.25 0.62 0.70 0.74 0.39 0.34 0.28 0.18 0.21 0.39 0.25 0.33 0.70 0.47 0.16 0.17 0.28

28.5 27.3 45.2 66.4 30.2 27.8 30.1 23.0 32.7 34.8 46.4 35.0 30.5 32.3 55.4 80.0 21.3 20.9 70.0 34.9 62.3 29.7 26.1 36.2 54.3 34.8 27.0 55.9 34.3 45.2 36.5 31.8 20.7 20.5 20.5 35.0 35.0 41.9 22.7 32.2

extraction activity coefficients activity coefficients extraction activity coefficients extraction extraction, 30 °C activity coefficients activity coefficients activity coefficients, benzene/n-hept extraction activity coefficients, 35 °C extraction, 30 °C extraction, 30 °C extraction, 30 °C extraction, benz/n-hept, 20 °C activity coefficients, 30 °C extraction, 30 °C activity coefficients, 30 °C activity coefficients, 30 °C extraction, 25 °C extraction extraction, 25 °C extraction, 30 °C extraction, 30 °C extraction, 30 °C extraction, 30 °C extraction extraction, 30 °C extraction, 30 °C activity coefficients, 45 °C activity coefficients, 45 °C extraction, 30 °C activity coefficients, 45 °C extraction, 25 °C extraction, benz/n-hept, 20 °C extraction, benz/n-hept, 20 °C extraction activity coefficients, 35 °C extraction, 30 °C

10 34 77 41 78 31 this work 63 79 64 42 80 this work this work this work 75 35 this work 68 68 81 39 47 this work 66 this work this work 58, 71 this work this work 72 73 this work 74 82 75 75 61 83 this work

1-butyl-3-methylimidazolium 2-(2-methoxyethoxy) ethylsulphate. b ethyl(2-hydroxyethyl)dimethylammonium. a T ) 40 °C, ∼10 wt % benzene.

Figure 5. Benzene/n-hexane separation with ionic liquids, mole-based, ∼10 wt % benzene, T ) 40 °C, component numbers in Table 4.

and Sxyl/oct ) 25.8, and [3-mebupy]B(CN)4 (9) with Dxyl ) 1.09 mol/mol and Sxyl/oct ) 22.0. These ILs are the same ILs that are most suitable for the toluene/n-heptane and benzene/nhexane separations. 4.5. Comparison of Ionic Liquids for Aromatic/ Aliphatic Separations on Mass Basis. However, in industrial practice, an S/F ratio based on mass, and not on mole basis, is used because the weight amount of solvent determines the costs

of the separation. Abu-Eishah and Dowaidar rightly pointed out that, for comparison of an IL-based extraction process with a conventional process, only mass-based distribution coefficients should be used.23 Therefore, the distribution coefficients on a mole basis of the most suitable ILs in Tables 3-5 were converted into distribution coefficients on a mass basis. The values of the aromatic/aliphatic selectivity are not changed as they are the same on both mole and mass bases. The ILs with a higher mass-based toluene distribution coefficient and a higher toluene/n-heptane selectivity than sulfolane are depicted in Figure 7, those with a higher mass-based benzene distribution coefficient and a higher benzene/n-hexane selectivity than sulfolane are shown in Figure 8, and those with a higher massbased p-xylene distribution coefficient and a higher p-xylene/ n-octane selectivity than sulfolane are shown in Figure 9. ILs with fluorinated anions displayed high mole-based distribution coefficients, but their high molecular weights result in significantly lower mass-based distribution coefficients compared to sulfolane. For instance, the ILs [mmim][Tf2N] (Dbenz ) 0.74 mol/mol, Dtol ) 0.50 mol/mol) and [emim][Tf2N] (Dbenz ) 1.20 mol/mol, Dtol ) 0.83 mol/mol, Dxyl ) 0.37 mol/mol) both have higher mole-based aromatic distribution coefficients than sulfolane (Dbenz ) 0.58 mol/mol, Dtol ) 0.31 mol/mol, Dxyl ) 0.27 mol/mol), but they have lower mass-based distribution coefficients than sulfolane (for [mmim][Tf2N], Dbenz ) 0.18 kg/kg


7537

Table 5. Overview of p-Xylene Distribution Coefficients in Mole and Mass Fractions and p-Xylene/n-Octane Selectivities, Obtained from LLE or from 1/γ∞a

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 a

solvent

Dp-xylene (mol/mol)

Dp-xylene (mass/mass)

Sxyl/n-oct

remarks

ref(s)

sulfolane [emim][Tf2N] [emmim][Tf2N] [bmim]PF6 [bmim]C(CN)3 [N-epy][Tf2N] [3-mebupy]N(CN)2 [3-mebupy]C(CN)3 [3-mebupy]B(CN)4 [4-mebupy]BF4 [4-mebupy]BF4 [4-mebupy]BF4 [N-C3OHpy](C2F5)3PF3 [mebupyrr]CF3SO3 [pmpip][Tf2N] [C2][Tf2N]b [S222][Tf2N]

0.27 0.48 0.39 0.22 0.60 0.38 0.54 0.83 1.09 0.22 0.36 0.25 0.50 0.30 0.58 0.30 0.43

0.26 0.15 0.11 0.09 0.31 0.11 0.30 0.42 0.51 0.11 0.18 0.12 0.10 0.12 0.17 0.09 0.13

24.9 32.3 21.5 22.8 23.1 27.7 25.2 25.8 22.0 29.0 42.7 31.7 36.9 23.7 20.2 27.0 20.8

extraction, 35 °C extraction, 25 °C activity coefficients activity coefficients extraction activity coefficients extraction extraction extraction activity coefficients, m-xylene/n-octane extraction, m-xylene/n-octane activity coefficients activity coefficients, 45 °C activity coefficients, 45 °C activity coefficients, 45 °C extraction, m-xyl/n-oct, 25 °C activity coefficients, 45 °C

8 40 63 64 this work 69 this work this work this work 84 58, 71 84 72 73 74 82 76

T ) 40 °C, ∼10 wt % p-xylene. b Ethyl(2-hydroxyethyl)dimethylammonium.

Figure 6. p-Xylene/n-octane separation with ionic liquids, mole-based, ∼10 wt % p-xylene, T ) 40 °C, component numbers in Table 5.

Figure 8. Benzene/n-hexane separation with ionic liquids, mass-based, ∼10 wt % benzene, T ) 40 °C. Note: For [bmim]Cl-2.0AlCl3, [Me3NH]Cl-2.0AlCl3, and [Et3NH]Cl-2.0AlCl3, data are for benzene/nheptane separation, T ) 20 °C.

Figure 7. Toluene/n-heptane separation with ionic liquids, mass-based, ∼10 wt % toluene, T ) 40 °C.

and Dtol ) 0.14 kg/kg; for [emim][Tf2N], Dbenz ) 0.31 kg/kg, Dtol ) 0.22 kg/kg, and Dxyl ) 0.26 kg/kg; for sulfolane, Dbenz ) 0.43 kg/kg, Dtol ) 0.26 kg/kg, and Dxyl ) 0.26 kg/kg); see Tables 3-5. Overall, pyridinium ILs were found to provide higher aromatic distribution coefficients and higher selectivities than other ILs because of their aromatic character. An effective IL should have a low molar volume and a high polarizability. A strongly localized negative charge on the anion (e.g., halides) competes with the aromatics to share the positive charge of the cation. Therefore, anions with a delocalized charge [N(CN)2-, RSO4-, AlCl4-, etc.) perform better.

Figure 9. p-Xylene/n-octane separation with ionic liquids, mass-based, ∼10 wt % p-xylene, T ) 40 °C.

It is apparent from Figures 7 and 8 that ILs with a halogencontaining anion show both a high aromatic distribution coefficient and a high aromatic/aliphatic selectivity. However, because of their corrosiveness and their instability in water, these ILs will not be used in industry. The only suitable ILs in Figures 7-9 are thus [bmim]C(CN)3, [3-mebupy] N(CN)2, [3-mebupy]C(CN)3, and [3-mebupy]B(CN)4. Of these ILs, [3-mebupy]B(CN)4 has the best combination of high mass-based aromatic distribution coefficients (Dbenz ) 0.74 kg/kg, Dtol ) 0.64 kg/kg, and Dxyl ) 0.51 kg/kg) and

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reasonable to high aromatic/aliphatic selectivities (Sbenz/hex ) 27.0, Stol/hept ) 38.5, and Sxyl/oct ) 22.0) (comparable values for sulfolane are as follows: Dbenz ) 0.43 kg/kg, Dtol ) 0.26 kg/kg, and Dxyl ) 0.26 kg/kg, and Sbenz/hex ) 28.5, Stol/hept ) 30.9, and Sxyl/oct ) 24.9). 5. Conclusions From our analysis, it is clear that there can be considerable differences in both distribution coefficients and selectivities determined from activity coefficients at infinite dilution and actually measured values from LLE data. Ionic liquids can replace conventional solvents in liquid-liquid extraction of aromatic hydrocarbons, provided the mass-based aromatic distribution coefficient and/or the aromatic/aliphatic selectivity are higher than those with sulfolane. The main conclusion of the process evaluation is that ILs that show a high aromatic distribution coefficient with a reasonable aromatic/ aliphatic selectivity could reduce the investment costs of aromatic/aliphatic separations by a factor of 2. The best ILs for the separation of aromatic and aliphatic hydrocarbons are [bmim]C(CN)3, [3-mebupy]N(CN)2, [3-mebupy]C(CN)3, and [3-mebupy]B(CN)4. The aromatic distribution coefficients of these ILs are factors of 1.2-2.5 higher and the aromatic/aliphatic selectivities are up to a factor of 1.9 higher than those of sulfolane. Activity coefficients at infinite dilution of ionic liquids are useful for screening purposes, but for industrial applications of separations with ionic liquids, real distribution coefficients and selectivities at finite dilutions have to be obtained, as these are concentration-dependent. Based on the performed analysis, it can be concluded that industrial application of ILs for aromatic extractions has not yet materialized, because most of the currently reported ILs do not provide higher mass-based aromatic distribution coefficients and/or higher aromatic/aliphatic selectivities than those achieved by conventional solvents such as sulfolane. Supporting Information Available: Table A: Overview of aromatic distribution coefficients (mol/mol) and aromatic/ aliphatic selectivities. This information is available free of charge via the Internet at http://pubs.acs.org/. Literature Cited (1) Weissermel, K.; Arpe, H.-J. AromaticssProduction and Conversion. In Industrial Organic Chemistry, 4th ed.; Wiley-VCH: Weinheim, Germany, 2003; pp 313-336. (2) Chen, J.; Duan, L.-P.; Mi, J.-G.; Fei, W.-Y.; Li, Z.-C. Liquid-liquid equilibria of multi-component systems including n-hexane, n-octane, benzene, toluene, xylene and sulfolane at 298.15 K and atmospheric pressure. Fluid Phase Equilib. 2000, 173 (1), 109–119. (3) Chen, J.; Li, Z.; Duan, L. Liquid-Liquid Equilibria of Ternary and Quaternary Systems Including Cyclohexane, 1-Heptene, Benzene, Toluene, and Sulfolane at 298.15 K. J. Chem. Eng. Data 2000, 45 (4), 689–692. (4) Choi, Y. J.; Cho, K. W.; Cho, B. W.; Yeo, Y. K. Optimization of the Sulfolane Extraction Plant Based on Modeling and Simulation. Ind. Eng. Chem. Res. 2002, 41 (22), 5504–5509. (5) Krishna, R.; Goswami, A. N.; Nanoti, S. M.; Rawat, B. S.; Khanna, M. K.; Dobhal, J. Extraction of aromatics from 63-69 °C naphtha fraction for food-grade hexane production using sulfolane and NMP as solvents. Indian J. Technol. 1987, 25 (12), 602–6. (6) Yorulmaz, Y.; Karpuzcu, F. Sulfolane versus diethylene glycol in recovery of aromatics. Chem. Eng. Res. Des. 1985, 63 (3), 184–90. (7) De Fre´, R. M.; Verhoeye, L. A. Phase equilibria in systems composed of an aliphatic and an aromatic hydrocarbon and sulfolane. J. Appl. Chem. Biotechnol. 1976, 26 (9), 469–87. (8) Lee, S.; Kim, H. Liquid-Liquid Equilibria for the Ternary Systems Sulfolane + Octane + Benzene, Sulfolane + Octane + Toluene and Sulfolane + Octane + p-Xylene. J. Chem. Eng. Data 1995, 40 (2), 499– 503.

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ReceiVed for reView March 22, 2010 ReVised manuscript receiVed June 23, 2010 Accepted June 28, 2010 IE100703P

Ionic Liquids for Aromatics Extraction. Present Status and Future

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