Comparison of Ionic Liquids to Conventional ... - ACS Publications

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Comparison of Ionic Liquids to Conventional Organic Solvents for Extraction of Aromatics from Aliphatics Roberto I. Canales† and Joan F. Brennecke* Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, Indiana 46556, United States ABSTRACT: Commonly used solvents for separation of aromatics from aliphatics using liquid−liquid extraction are reviewed in terms of their physical properties and separation performance. The experimental liquid− liquid measurements for conventional solvents are contrasted with the phase behavior when using an ionic liquid as the extracting component. For comparison purposes, ternary systems of heptane + toluene + extraction solvent are used as representative mixtures, where sulfolane is the main organic extraction solvent discussed. Since ionic liquid properties are drastically changed by changing the anion, comparisons are performed for 1ethyl-3-methylimidazolium based ionic liquids using different anions. Ionic liquids containing cyano-substituted and bis(trifluoromethylsulfonyl)imide anions provide good selectivities and distribution ratios. On the basis of the phase behavior and capacities, as well as their relatively low viscosities, they appear to be competitive alternatives to organic solvents.

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INTRODUCTION The main source of aromatics in industry, for example, benzene, toluene, xylenes, and ethylbenzene, are catalytic reforming and steam cracking processes. During catalytic reforming, low-octane heavy naphtha from paraffinic or naphthenic oil is processed at 723−823 K and 1.7−7.0 MPa to obtain a high-octane reformate gasoline by isomerizing aliphatics, ring-formation from these alkanes, and finally dehydrogenating the cyclic aliphatics to form aromatics. A typical reformate gasoline stream can have an aromatic content ranging from 20 to 65 wt %,1,2 in which the main fraction of aromatics in this product is composed of xylenes and a smaller amount of toluene.3 In the steam cracking process any of a diverse range of oil streams, from light hydrocarbons to heavy naphtha, are fed to the reactor with the objective of producing olefins, such as ethylene, propylene, and other higher olefins. This process is highly endothermic, so near ambient pressure and temperatures over 950 K are required, depending on the feed.4 The product from the steam cracking is called pyrolysis gasoline, which is rich in benzene, and contains between 50 and over 90 wt % of aromatics.1,3 Aromatics from reformate and pyrolysis gasolines are generally not separated by simple distillation because the boiling points of the components in the mixture span a very narrow range and a variety of azeotropes exist. Different processes are used to perform the separation of the aromatics, depending on their concentration in the effluent. When the concentration of aromatics is high, i.e. over 90 wt %, it is economically viable for the aromatics to be separated through azeotropic distillation by adding a strong polar compound. Streams with a medium aromatic concentration, that is, from 65 to 90 wt %, can be processed by extractive distillation using, for example, N-methylpyrrolidone, phenols, glycols, etc. Streams © XXXX American Chemical Society

with lower concentrations of aromatics, that is, from 20 to 65 wt %, are separated by liquid−liquid extraction, which is reviewed in this work.1 For aromatic concentrations below 20 wt %, other techniques have to be pursued to economically remove the aromatics.5 The recovery of the aromatics from the extraction solvents used in the various processes is done by direct distillation or stripping with a secondary solvent, followed by further distillation.1,3 The main solvents reported in the literature for aliphatic− aromatic liquid−liquid extraction, which have also been used in industrial separations,1 are sulfolane6−41 and glycols,35,37,42−59 but many compounds have been tested for application in aliphatic−aromatic separations, including dimethyl sulfoxide,30,39 ethylene and propylene carbonate,30,57,60−64 Nmethylpyrrolidone,65−68 N-formylmorpholine,27,56,69−74 and dimethylformamide,64 among others. The ternary aliphatic + aromatic + solvent systems, with compositional analysis, distribution factors, and selectivities, are detailed in each study, which is useful for performance comparisons among different compound combinations. Moreover, multicomponent systems with more than three constituents have been published. Those publications focus on the addition of a modifier to the extraction solvent or the use of two typical solvents29,30,38,56,64,68,75 for the extraction of mixed aromatics from a single or multiple aliphatics.8,12,13,76−81 Ionic liquids have been introduced as an alternative to conventional solvents for liquid−liquid separations due to their very low vapor pressure, wide liquid range, and high density compared with common aliphatics and aromatics, such as Received: January 26, 2016 Accepted: April 4, 2016

A

DOI: 10.1021/acs.jced.6b00077 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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Table 1. Properties of Common Solvents Used in Aliphatic−Aromatic Liquid−Liquid Extraction at 298.15 K and 0.1013 MPa solvents ethylene glycol dimethylformamide dimethyl sulfoxide ethylene carbonate N-methylpyrrolidone propylene carbonate diethylene glycol N-formylmorpholine sulfolane toluene heptane a

MW (g·mol−1) 62.07 73.09 78.13 88.06 99.13 102.09 106.12 115.13 120.17 92.14 100.20

Tb (K) 470.65 426.25 462.25 519.15 475.05 514.75 518.65 513.15 560.45 383.75 371.53

Tm (K)

ρ (g·cm−3)

260.15 212.75 291.75 309.48 248.75 224.35 262.85 296.15 301.55 178.15 182.60

1.10 0.94 1.10 1.32b 1.03 1.20 1.11 1.15 1.26a 0.86 0.68

σ (mN·m−1)

ref

13 370 60 1.3 50 6 17c

48.60 35.83 41.70 50.60 40.25 34.60 27.93

9.1a 3787c 6066c

47.95a 27.76 19.63

188−190 172, 191 172, 192 188, 193−195 172, 191 188, 194−196 188, 197 198, 199 172, 200 188, 191 188,191

μ (mPa·s)

Pvap (Pa)

16.06 0.90 2.00 1.85b 1.67 2.47 30.2 7.67 10.35a 0.56 0.39

c

At 303.15 K. bAt 313.15 K. cEstimated with a correlation.201

heptane and toluene. One of the challenges is trying to find an ionic liquid with low viscosity, high selectivity, high capacity, and low toxicity, in order to improve the existing methods practiced in industry for aliphatic/aromatic separation. Liquid−liquid phase equilibrium, capacity, and selectivity have been reported for several combinations of aliphatics and aromatics using ionic liquids as the extraction solvent. Among the ionic liquids studied, the cations used are mostly imidazolium41,82−128 and pyridinium.41,93,116,128−149 A few studies use pyrrolidinium,150 ammonium,95,131,151 morpholinium,152 and other cations,86,153 while the most investigated ionic liquid anion is bis(trifluoromethylsulfonyl)imide ([Tf2N]−). In addition, aromatic extractions have been performed with ionic liquid mixtures in an attempt to improve both selectivity and capacity.154−159 In some cases, infinite dilution activity coefficients of aromatic and aliphatic compounds in ionic liquids have been used as a first pass evaluation of the suitability of a particular ionic liquid for aliphatic−aromatic separation.160,161 Here we focus on liquid− liquid equilibrium studies of ternary mixtures of heptane + toluene + imidazolium-based ionic liquids, paying particular attention to those containing the 1-ethy-3-methylimidazolium ([emim]+) cation. The method usually proposed for recovery of the aromatic from the ionic liquid extraction solvent is simple distillation or stripping, taking advantage of the thermal stability and low volatility of the ionic liquid.162 Jongmans et al.163 simulated the ethylbenzene/styrene extractive distillation process using 4methyl-N-butylpyridinium tetrafluoroborate ([4-mebupy][BF4]), proposing several regeneration techniques: simple evaporation, two stage evaporation with the second unit at very low pressure, nitrogen stripping, distillation, and supercritical CO2 extraction. The most promising recovery technique according to this author is the low pressure regeneration. Scurto et al.,164,165 Aki et al.,166 Mellein and Brennecke,167 Canales and Brennecke168 and Canales et al.169 have studied the salting-out effect of sub- and supercritical CO2 on mixtures of ionic liquids with different organic solvents and water. The addition of CO2 results in a liquid−liquid phase split, where the organic-rich or aqueous-rich phase can be quite pure. This technique is another promising method for recovering the aromatic and regenerating the ionic liquid for reuse if one can find an ionic liquid that releases the aromatic at moderate pressures and low CO2 compositions in the liquid phase. Lower pressures and use of less CO2 makes the liquid−liquid phase split process more attractive than supercritical CO2 extraction.

The objective of this brief review is to show the performance of ionic liquids in liquid−liquid separation of aromatic from aliphatics compared with common organic solvents. We will focus on physical and transport properties (e.g., density and viscosity) and the performance of the solvents in terms of selectivity and distribution ratio.

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LIQUID−LIQUID SEPARATION OF AROMATICS FROM ALIPHATICS USING COMMON SOLVENTS Liquid−liquid separation is the most popular process in industry to extract aromatics from aliphatics. Selection of a suitable solvent is the crucial first step in performing successful liquid−liquid extractions. The best solvents have good selectivity and capacity, high thermal stability, good availability, low cost, high surface tension and low to moderate viscosity.170 More environmentally friendly solvents, which have a smaller impact on human health and the environment, is another feature to consider. These compounds, frequently called “green” solvents, must have a very low toxicity, persistence, and volatility.171 Table 1 presents some properties, such as boiling temperature at 0.1013 MPa, melting point, density, viscosity, vapor pressure and surface tension at 298.15 K of the typical solvents used for liquid−liquid extraction of aromatics from aliphatics. These properties are compared with the representative aliphatic and aromatic used in this work: heptane and toluene. The main characteristics observed for the conventional extraction solvents are their relatively low vapor pressure, low viscosity, and high surface tension. The low vapor pressure is an important factor for avoiding significant loss of the solvent, which would require replacement and add cost. Low viscosity is essential to decrease mass transfer problems in the mixing and separation of the liquid phases. A high surface tension is necessary for avoiding the presence of emulsions.170 All of the solvents have higher density than either heptane or toluene. This is virtually a requirement for selective extraction of the aromatic, since the aromatics are more dense than the aliphatics. If the solvent is to have good capacity and selectivity for the aromatic compound, the extract will inevitably be the more dense of the two liquid phases, facilitating their gravimetric separation. The aliphatics, with a small portion of solvent and aromatics, will be the raffinate, which is the low density upper phase in the liquid−liquid equilibrium. All the solvents show a liquid range of approximately 200 K and some of them melt close to the room temperature, such as sulfolane, dimethyl sulfoxide, ethylene carbonate, and N-formylmorpholine. The boiling point of the solvent is also important because B



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it has to be significantly higher than that of the aromatic if the aromatic is to be recovered by simple distillation. It is also noted that the molecular weight of the extraction solvents are comparatively close to the values for heptane and toluene. The molecular structures of the different solvents shown in Table 1 are presented in Figure 1.

Figure 1. Molecular structure of some typical solvents used in aromatic/aliphatic liquid−liquid extraction. Figure 2. Comparison of ternary liquid−liquid equilibrium data for the heptane + toluene + sulfolane system at 298.15 K13,14,26,34,36 in (a) mole fraction and (b) mass fraction.

Sulfolane is the most studied separation solvent in the literature and is widely used in industry in extractive distillation and liquid−liquid aromatic extraction.1 In Figure 2, a comparison of the literature data for liquid−liquid equilibrium of the heptane + toluene + sulfolane system at 298.15 K is presented in mole and mass fraction bases, showing high consistency in the tie lines and coexistence curves among results from different researchers. Therefore, sulfolane is a good choice as a “base-case” solvent with which to compare experimental measurements of aliphatic + aromatic + solvent liquid−liquid equilibrium. However, note that sulfolane is more highly toxic than some other extraction solvents.172 It is not surprising that the mole and weight fraction diagrams are similar since the molecular weights of the three components are similar. There is usually some temperature effect on liquid−liquid equilibrium, with larger miscibility gaps generally observed at lower temperatures. However, the pressure effect on the phase behavior is negligible close to atmospheric conditions.170 Figure 3 presents a comparison of the tie lines in the system heptane + toluene + sulfolane at three different temperatures: 298.15 K, 313.15 K, and 323.15 K. In this mixture, the immiscible area is slightly larger at the lower temperature, as expected, but the tie lines follow the same slope and the compositions in both phases are not considerably different. Thus, there is not a strong temperature effect for this mixture over the relatively small temperature range shown in Figure 3. Figure 4 shows a comparison of the immiscibility gaps when different solvents are used for separating toluene from heptane at 313.15 K, where the largest immiscible region is with ethylene carbonate and the smallest is with N-methylpyrroli-

done. All the solvents appear to have a plait point, so high toluene concentration mixtures cannot be separated. Comparison of the ternary liquid−liquid phase equilibrium diagrams is useful in determining the compositions of the initial heptane + toluene mixtures that can be separated with a particular solvent. However, the best analysis is provided by calculating the capacity and selectivity of the solvent at different compositions of the two phases. The capacity of the solvent to dissolve heptane or toluene is defined as the distribution ratio (or distribution factor) Di. It can be defined in mole fraction (Dx,i in eq 1) or mass fraction (Dw,i in eq 2) basis where xαi or β is the mole fraction of compound i in the α or β phase and wαi or β is the mass fraction of compound i in the α or β phase, calling α as the lower extract phase and β as the upper raffinate phase, Dx , i = Dw , i =

xiα xiβ

(1)

wiα wiβ

(2)

A solvent with good capacity for separating the aromatic has to show a lower distribution ratio for the aliphatic than the aromatic. A high distribution ratio of the aromatic is required to decrease the solvent usage in the liquid−liquid extraction. The selectivity (S in eq 3) is calculated as the distribution ratio of C



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Figure 3. Temperature effect on the liquid−liquid equilibrium for heptane + toluene + sulfolane mixtures36 in (a) mole fraction and (b) mass fraction: (black square) 298.15 K, (red circle) 313.15 K, and (blue triangle) 323.15 K.

Figure 4. Liquid−liquid equilibrium of the heptane + toluene + solvent system at 313.15 K in (a) mole fraction and (b) mass fraction: (black square) sulfolane,36,41 (red circle) ethylene carbonate,30 (blue triangle) N-formylmorpholine,56,69 (green diamond) N-methylpyrrolidone,68 (cross) ethylene glycol59

the aromatic divided by the distribution ratio of the aliphatic in mole or mass fraction. S=

β α β Daromatic xα /xaromatic waromatic /waromatic = aromatic = α β α β Daliphatic xaliphatic /xaliphatic waliphatic /waliphatic

As shown in Figure 2, there is good agreement among literature data for the heptane + toluene + sulfolane system at 298.15 K. In Figure 6, the selectivity and distribution ratio are shown as a function of xβtoluene (and wβtoluene for mass distribution ratio) for the same data shown in Figure 2 at 298.15 K. Despite the apparent agreement in the liquid−liquid equilibrium data, there are significant differences in the selectivities at low toluene concentrations. This is because the selectivity is very sensitive to small variations in the mole fractions of the two liquid phases at low toluene concentrations. For example, the data of Tripathi et al.36 gives a selectivity of 44.9 at xβtoluene = 0.0898 but for Chen et al.13 S = 28.6 at xβtoluene = 0.083. These large variations in selectivity are decreased when xαtoluene > 0.2, where the compositions of the two liquid phases are closer to the plait point. All of the reports show increasing distribution ratio with increasing toluene composition, but as seen with the selectivity, there is more disparity at low toluene compositions. To evaluate data from different research groups, experimental compositions and selectivities should be analyzed for thermodynamic consistency, as is done with the Gibbs− Duhem equation for vapor−liquid equilibrium. Consistency of liquid−liquid equilibrium data is commonly checked by applying the correlations proposed by Hand173 and Othmer and Tobias,174 but it has been demonstrated that these “tests” are not sensitive enough to weight fraction and selectivity

(3)

As a consequence, a good liquid−liquid separation solvent or a mixture of extracting solvents has to have a high distribution ratio for the aromatic, and a low distribution ratio for the aliphatic that results in a high selectivity for the target compound, in order to obtain low amounts of aromatic in the aliphatic-rich phase. Selectivity is highly dependent on composition, so it also depends on the temperature. Figure 5 shows the selectivity and distribution ratio for the heptane + toluene + sulfolane system at different temperatures obtained from the work of Tripathi et al.,36 where selectivity decreases and the distribution ratio tends to increase by enhancing the concentration of toluene in the upper phase (xβtoluene). The most significant temperature effect on selectivity is observed at the lowest xβtoluene, where the selectivity is about 45 at 298.15 K but closer to 30 at 323.15 K. There is no clear temperature effect on the distribution ratio in the heptane + toluene + sulfolane system. Distribution ratio values decrease slightly when based on mass fraction because the molecular weight of sulfolane is bit higher than the molecular weights of toluene and heptane. D



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Figure 5. (a) Selectivity, (b) distribution ratio in mole fraction basis and (c) distribution ratio in mass fraction basis versus composition of toluene in the upper raffinate phase for the heptane + toluene + sulfolane36 system at (black square) 298.15 K, (red circle) 313.15 K, and (blue triangle) 323.15 K.

Figure 6. (a) Selectivity, (b) distribution factor in mole fraction basis and (c) distribution factor in mass fraction basis versus composition of toluene in the upper raffinate phase for the heptane + toluene + sulfolane system at 298.15 K. (black square) Tripathi et al.;36 (red circle) Lin et al.; (blue triangle) Chen et al.;13 (green diamond) De Fré and Verhoeye;14 (cross) Rawat and Gulati.34

variations and do not have a strong theoretical basis.175 Unfortunately, to our knowledge, there are no procedures to check thermodynamic consistency of liquid−liquid equilibrium data based on a robust theoretical technique.

to evaluate the economic feasibility of using ionic liquids for aliphatic−aromatic separations. Typical cations and anions used for aliphatic/aromatic liquid−liquid separation are shown in Tables 2 and 3, respectively. Ionic liquids using the 1-ethyl-3-methylimidazolium ([emim]+) cation are commonly studied since the short alkyl chains result in good selectivity and distribution ratios (as discussed below). Table 4 presents densities, viscosities, and surface tensions of several [emim]+ based ionic liquids with different anions, in comparison to sulfolane, heptane, and toluene. The density and surface tension of ionic liquids are very similar to common liquid−liquid extraction solvents. The

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LIQUID−LIQUID SEPARATION OF AROMATIC FROM ALIPHATICS USING IONIC LIQUIDS Ionic liquids are usually immiscible with aliphatics, with very small mutual solubilities. On the other hand, the solubility of aromatics in many ionic liquids is quite high, but generally they are not completely miscible. This behavior allows the ionic liquid to separate aromatic/aliphatic mixture at any feed composition but a selectivity and capacity analysis is necessary E



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Table 2. Ionic Liquid Cations Used for Liquid−Liquid Extraction of Aromatics from Aliphatics

Table 3. Ionic Liquid Anions Used for Liquid−Liquid Extraction of Aromatics from Aliphatics

a A, B, C, D, n = number of carbons in the alkyl chain; X, Y = initial of alkyl chain.

main difference is the higher viscosities of the ionic liquids, ranging from 15 to 320 mPa·s. On the basis of viscosity, the most competitive ionic liquid alternatives are ones with the [emim]+ cation and anions containing cyano groups, such as [DCA]−, [TCM]−, and [SCN]−, as well as [Tf2N]−, all of which have viscosities similar to sulfolane. The vapor pressures of the ionic liquids are generally very small compared to organic solvents and this represents an advantage for the reduction of the gas emissions to the atmosphere and the reduction of the regeneration cost of the solvent. The molecular weights of ionic liquids are usually much higher than those of common extraction solvents. A typical ternary diagram for heptane + toluene +1-ethyl-3methyl imidazolium bis(trifluoromethylsulfonyl)imide ([emim][Tf2N]) is shown in Figure 7 at two different temperatures in mole and mass fractions. There is very little difference in the data at 298.15 K and 313.15 K, from the groups of Arce et al.84 and Garcia et al.,176 respectively, and tie lines have similar slope. When compared with the results of Corderi ́ et al. at 298.15 K, there is a small variation in the compositions of the lower phase and the slopes of their tie lines are different than the other groups when the toluene composition is over 0.2 mole fraction. These differences could lead to significant variation in the selectivities. When compositions are shown in mass fractions, the difference between the different experimental values is more difficult to distinguish because the values are dominated by the high molecular weight of [emim][Tf2N]. In Figure 8, the effect of the length of the alkyl chains of the imidazolium cation (1-alkyl-3-methylimidazolium) on the selectivity and the distribution ratio of toluene at two different temperatures (298.15 and 313.15 K) are explored. The effect of temperature on the selectivity and distribution ratio is very small, as seen in the heptane + toluene + [emim][Tf2N] and

[hmim][Tf2N] systems, from the work of Arce et al.84 at 298.15 K and Garcia et al.104 at 313.15 K. As noted previously for the data in the ternary diagram of Figure 7, the selectivities calculated from Corderi ́ et al.93 will underestimate the values for [emim][Tf2N] compared to the other researchers, but show similar distribution ratios at low toluene compositions. When the distribution ratios are recalculated in terms of the mass fractions, the values are decreased dramatically, as expected, due to the high molecular weights of the ionic liquids. The selectivities for the different [Cxmim][Tf2N] ionic liquids is [hmim] < [bmim] < [emim] < [mmim], which means that a longer alkyl chain decreases the selectivity. The F



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Table 4. Properties of [emim]+ Ionic Liquids Used in Liquid−liquid Extraction of Aromatics from Aliphatics at 298.15 K solvents sulfolane [SCN]− [OAc]− [TCM]− [DCA]− [CH3SO3]− [ESO4]− [DEP]− [Tf2N]− toluene heptane a

MW (g·mol−1) 120.17 169.25 170.21 201.23 177.21 206.26 236.29 264.26 391.31 92.14 100.20

ρ (g·cm−3)

μ (mPa·s)

a

a

1.26 1.11684 1.09778 1.08162 1.10198 1.2398 1.24018 1.146 1.51900 0.86 0.68

10.35 22.15 132.91 15.02 19.90 155 101.42 320.89 32.99 0.56 0.39

σ (mN·m−1)

ref

47.95a 57.76 42.9 50.94 40.3 45.1 46.9 34.46 35.2 27.76 19.63

172, 200 202 203 118, 204 203 205, 206 203 207 208, 209 188,191 188,191

At 303.15 K.

0.799), Dx,heptane = 0.294 (Dw,heptane = 0.144) and Dx,toluene = 0.823 (Dw,toluene = 0.404), obtaining a S = 2.80. At these conditions, the change achieved by increasing the alkyl chain length from butyl to hexyl is ΔDx,heptane = 0.114 (ΔDw,heptane = 0.059), ΔDx,toluene = 0.037 (ΔDw,toluene = 0.034) and ΔS = −1.56, from which one can conclude that the effect on the heptane mole distribution ratio is three times greater (twice in the case of mass fraction) than the effect on the toluene distribution ratio. Figure 9 shows the selectivity and toluene distribution ratio for separating toluene from heptane for ionic liquids with the [emim]− cation and a variety of different anions. The results are compared with sulfolane. The sulfolane data selected for comparison was the data that showed the higher selectivity and distribution ratio of toluene compared with other values from literature.36 In terms of selectivity, all the ionic liquids presented in Figure 9 are competitive with sulfolane, since the lowest value of selectivity for ionic liquids is for [emim][Tf2N], which is very close to the sulfolane curve. The highest selectivity is when [emim][SCN] is used, with selectivities over 90 at the lowest compositions. High selectivities are also observed with [emim][CH3SO3] and [emim][DCA]. In the case of the toluene mole distribution ratio comparison, only three ionic liquids are competitive with sulfolane: [emim][DEP], [emim][TCM], and [emim][Tf2N]. Ionic liquids with the highest selectivities fall far below the sulfolane curve in terms of distribution ratio, which means that a large amount of solvent would be required for separating the aliphatic/aromatic mixture. The problem with [emim][Tf2N] is the higher concentration of aliphatic dissolved in the ionic liquid. In general, the ionic liquids presented in Figure 9 show better or comparable selectivities to sulfolane, but just [emim][TCM], [emim][DEP], and [emim][Tf2N] have a higher or similar mole distribution ratio of toluene. The main disadvantage of using [emim][DEP] is its high viscosity. However, [emim][TCM] and [emim][Tf2N] may be viable alternatives to replace sulfolane. For mass fraction distribution ratios, the values for all the ionic liquids fall below the sulfolane line. Once again, this is due to the high molecular weight of the ionic liquids. Consequently, a larger mass of ionic liquid than sulfolane would be necessary to separate heptane from toluene, which could increase operating and capital costs. To overcome the general trend for ionic liquids of high selectivity but low distribution ratio, Larriba et al.158,159 proposed using a mixture of ionic liquids: one with high selectivity, such as [emim][DCA], and another with high toluene distribution ratio, such as [4empy][Tf2N]. There is still

Figure 7. Liquid−liquid equilibrium of the ternary heptane + toluene + [emim][Tf2N] system in (a) molar compositions and (b) mass compositions at 298.15 K (black square) Arce et al.,84 (red circle) Corderi ́ et al.; and at 313.15 K (blue triangle) Garcia et al.104

distribution ratio of toluene follows the opposite trend: a longer alkyl chain increases the distribution ratio. This difference can be explained because the ionic liquids with longer alkyl chains increase the aromatic solubility, but also increases the aliphatic solubility. This directly affects the distribution ratios, causing a larger effect for the aliphatic than the aromatic. For example, Garcia et al.104 report Dx,heptane = 0.180 (Dw,heptane = 0.085) and Dx,toluene = 0.786 (Dw,toluene = 0.370), producing S = 4.36, for xβtoluene = 0.816 (wβtoluene = 0.803) when using [bmim][Tf2N]. However, with [hmim][Tf2N] at xβtoluene = 0.812 (wβtoluene = G



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Figure 8. Effect of the cation alkyl chain length on the (a) selectivity, (b) distribution factor in mole fraction basis, and (c) distribution factor in mass fraction basis versus composition of toluene in the upper raffinate phase for the heptane + toluene + [Cxmim][Tf2N] system. Solid symbols, 298.15 K; open symbols, 313.15 K; (open green diamond) [mmim][Tf2N];104 (solid black square) [emim][Tf2N];84 (solid gray square) [emim][Tf2N];93 (open black square) [emim][Tf2N];104 (open red triangle) [bmim][Tf2N];176 (solid blue circle) [hmim][Tf2N];91 (open blue circle) [hmim][Tf2N].104

Figure 9. Effect of the anion on the (a) selectivity and (b) distribution factor in mole fraction basis and (c) distribution factor in mass fraction basis versus composition of toluene in the upper raffinate phase for the heptane + toluene + [emim]+ cation-based ionic liquids systems. Solid symbols, 298.15 K; open symbols, 313.15 K. (Solid black square) [Tf2N]−;84 (open black square) [Tf2N]−;104 (solid blue circle) [DEP]−;87 (solid green diamond) [OAc]−;92 (open orange triangle) [CH 3 SO 3 ] − ; 107 (open purple star) [CF 3 SO 3 ] − ; 107 (cross) [CHF2CF2SO3]−;107 (solid red inverted triangle) [ESO4]−;113 (open red inverted triangle) [ESO4]−;41 (×) [SCN]−;117 (asterisk) [DCA]−;118 (dash) [TCM]−;118 and (black square with line) sulfolane.36

a need to expand the knowledge of ionic liquids for use in aromatic/aliphatic separation processes in order to improve the mass distribution ratios using low cost ionic liquids. A broader overview on the use of different ionic liquids in several types of aliphatic/aromatic mixtures and the effect of diverse modifications in the molecules on the distribution ratio and selectivity is analyzed in the work of Ferreira et al.177

the extraction solvent and the aromatic has to be separated to obtain a pure aromatic and to recover the solvent for reuse in the process. This separation, as mentioned above, is usually done by direct distillation or solvent stripping followed by distillation. For ionic liquids solvents, the separation by distillation should be easier due to the high thermal stability and low volatility of the ionic liquids. Despite the large number

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AROMATIC + IONIC LIQUID BINARY SYSTEMS After the liquid−liquid separation is performed for the aliphatic/aromatic mixture, the extract composed mainly of H



Review

calculating the selectivity of a system using data from different authors, even when no significant variations are detected in the ternary phase equilibrium diagrams. Selectivity remains the key parameter for choosing a better extraction solvent. As suggested by the results shown here, the mole fraction basis is the best way to present the ternary liquid−liquid equilibrium diagrams because it is more sensitive to changes when the extraction solvent has a high molecular weight, but the mass fraction basis is recommended for presentation of distribution ratios, due to its importance in defining the mass of solvent required to perform the separation. Further studies are needed to evaluate if the ionic liquid solvent can be recovered and reused in the process. Finally, the economic feasibility of the liquid−liquid extraction using an ionic liquid with the recovery of the ionic liquid solvent using different techniques needs to be explored in more depth in order to determine the real applicability of ionic liquids for aliphatic−aromatic separations.

of studies of liquid−liquid equilibrium of ionic liquids and aromatics, heats of mixing and excess properties for these binary systems are extremely scarce.178−180 Aromatics have a large solubility in ionic liquids but a miscibility gap is commonly observed. The high solubility of aromatics in, for example, imidazolium ionic liquids with alkyl chains is attributed to interactions of the strong electrostatic fields of the aromatic rings in both the ionic liquid and the toluene, where the van der Waals interactions of the toluene with nonpolar chains on the cation of the ionic liquid are also an important component of the wide mutual miscibility.181,182 According to Holbrey et al.183 this behavior supports the formation of clathrates, because a small interaction produces miscibility or immiscibility of the binary system and strong interactions produce the crystallization of the salt, but in this case there is a partial miscibility. Further understanding in this topic is needed because, for example, some ionic liquids, such as [P66614][Tf2N], are completely miscibility with benzene,86 so the clathrate theory is not applicable for this case. The solubility of the ionic liquid in the aromatic-rich phase is very low or even not detectable, depending on the nature of the ionic liquid and the aromatic in the binary mixture. Several works suggest the appearance of an upper critical solution temperature (UCST) in different combinations of [Tf2N]− ionic liquids with aromatics.184−187 Lachwa et al.186 reported liquid−liquid equilibrium for binary systems, including [Cnmim][Tf2N] + benzene, toluene, and α-methylstyrene where the UCST is also observed and a detectable concentration of ionic liquid is detected over 300 K for benzene and over 390 K for α-methylstyrene in [hmim][Tf2N]. An increase in the alkyl chain length for the imidazolium cation produces a decrease in the immiscibility area due to the increasing aromatic solubility in the ionic liquid.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +1 (574) 631-5847. Fax: +1 (574) 631-8366. Present Address †

Laboratory of Thermodynamics, Departament of Biochemical and Chemical Engineering, Technische Universität Dortmund, Emil-Figge-Str. 70, D-44227 Dortmund, Germany Funding

We acknowledge support from the University of Notre Dame Incropera-Remick Fund for Excellence. Notes

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The authors declare no competing financial interest.

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SUMMARY Ionic liquids are potentially competitive alternatives to currently used organic solvents for successfully separating aromatics from aliphatics. Many ionic liquids have densities and surface tensions that would be acceptable. For the [emim]+ cation, there are several choices of anions (e.g., [SCN]−, [DCA]−, [TCM]− and [Tf2N]−), which yield ionic liquids with sufficiently low viscosities (only 2 or 3 times that of sulfolane). Some anion−cation combinations show comparable or better selectivity than sulfolane, but the challenge of decreasing the amount of solvent used due to their low mass distribution ratio is still an issue to consider. The [emim]+ based ionic liquids with better selectivities than sulfolane involve [SCN]−, [CH3SO3]−, and [DCA]− anions. Selectivities of [emim][SCN] and [emim][DCA] are over four times that of sulfolane but their mass distribution ratios are only about 30% of the value for sulfolane. The [emim]+ ionic liquids with the best mass distribution ratios have the following anions: [TCM]−, [Tf2N]−, and [DEP]−. [emim][TCM] and [emim][Tf2N] have double the selectivity of sulfolane, but their mass distribution ratios are still a bit lower (0.64 and 0.52 times the value of the sulfolane, respectively). Nonetheless, [emim][Tf2N] and [emim][TCM] appear to be the better choices for heptane/toluene separation from among the ionic liquids considered in this work, due to their high selectivity and reasonably high mass distribution ratios. Selectivity is very sensitive to small variations in the experimental liquid−liquid equilibrium compositions, so large differences can be observed under certain conditions when

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