Liquid Extraction of Toluene from Heptane, Octane, or Nonane Using

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Liquid Extraction of Toluene from Heptane, Octane, or Nonane Using Mixed Ionic Solvents of 1‑Ethyl-3-methylimidazolium Methylsulfate and 1‑Hexyl-3-methylimidazolium Hexafluorophosphate Khaled H. A. E. Alkhaldi, Adel S. Al-Jimaz,* and Mohammad S. AlTuwaim

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Chemical Engineering Department, College of Technological Studies, PAAET, P.O. Box 42325, Shuwaikh 70654, State of Kuwait

ABSTRACT: Liquid−liquid extractions of toluene from paraffin compounds, using binary ionic solvent mixtures of 1-hexyl-3methylimidazolium hexafluorophosphate [hmim][PF6] and 1-ethyl-3-methylimidazolium methylsulfate [emim][CH3SO4], have been examined at 313.15 K and an atmospheric pressure of 101 kPa. Based on the experimental liquid−liquid equilibrium data for the pseudoternary systems consisting of heptane, octane, or nonane + toluene + [hmim][PF6] + [emim][CH3SO4], the distribution ratios and selectivity values were calculated and compared to systems using pure ionic liquids or sulfolane. The results showed that the use of a binary mixture of [hmim][PF6] + [emim][CH3SO4] at a solvent mole fraction of 0.9 for [hmim][PF6] improves both the toluene distribution ratio and the selectivity, with respect to those of sulfolane and other pure ionic solvents. Consequently, these mixed ionic liquids could be considered an environmentally benign alternative solvent for the aromatic extraction process. The experimental data were correlated by the nonrandom two-liquid (NRTL) model.

1. INTRODUCTION The separation of aromatic compounds from paraffinic compounds has an important role in industry, resulting in a continuous effort to improve current industrial processes.1,2 Liquid extraction methods are appropriate for extracting aromatic compounds from aliphatic compounds for aromatic contents of 20−65% by weight.3 The selection of suitable solvents for separation processes is key for targeted process magnification.4 Ionic liquids (ILs) were introduced as an alternative to conventional solvents for the extraction of aromatics from aliphatic compounds due to their low volatility. This significant property leads to a greener operation due to easier recovery of the solvent by simple separation processes, such as flash distillation.5−9 This research investigates the extraction of aromatics from petroleum refinery products, including gasoline, kerosene, diesel fuels, and lubricating oil.10−16 The aliphatic compounds of interest are in the C7−C17 range, and the aromatics include toluene, ethylbenzene, xylene, propylbenzene, and butylbenzene. The objective is to enhance the properties of the © XXXX American Chemical Society

products by extracting these aromatics. This work covers aliphatics from C7−C9 and toluene in the gasoline product range. The solvent extraction efficiency is linked to the aromatic distribution coefficients, while the solvent regeneration cost is linked to solvent selectivity. A preferred solvent is one of high distribution coefficient (>0.5) and the highest possible selectivity. Many industrial processes, as well as laboratory procedures, employ mixed solvents due to their many practical applications. There are several reasons why the use of mixed solvents is preferred, including the improvement of certain physical properties and their solvency.17 Solvents with higher distribution ratios and selectivity values than sulfolane may be achieved in an intermediate system by mixing two ILs.18−25 In previous works,10−16 some ILs were evaluated for the separation of aromatics from paraffinic compounds at 313.15 K Received: July 30, 2018 Accepted: November 23, 2018

A

DOI: 10.1021/acs.jced.8b00669 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

and an atmospheric pressure of 101 kPa. Stability in the presence of water (to avoid hydrolysis) and low viscosities were important properties that favored solvents based on alkyl sulfate anions.16 Among these solvents, [emim][CH3SO4] showed high selectivity values.15,16 The second solvent, [hmim][PF6], was selected because of its high aromatic distribution ratio.14 Unlike the tetrafluoroborate-based ionic liquids, the [hmim][PF6] solvent is stable at low to moderate temperatures and does not hydrolyze, even in the presence of water at moderate temperatures.26 First, we have examined the influence of [hmim][PF6] + [emim][CH3SO4] binary mixtures with different compositions on the toluene distribution ratio and the ILs selectivity for the separation of toluene from n-heptane/toluene mixtures containing 25 wt % of toluene at 313.15 K and an atmospheric pressure of 101 kPa. On the basis of these results, liquid−liquid equilibria (LLE) were determined for the pseudoternary systems heptane, octane, or nonane + toluene + [hmim][PF6] + [emim][CH3SO4] at 313.15 K and an atmospheric pressure of 101 kPa, with a fixed [hmim][PF6] mole fraction in the mixed IL solvent of 0.9. The distribution ratio of toluene and the selectivity of the ILs were calculated from the LLE data. The liquid phase diagram was plotted, and the nonrandom two-liquid (NRTL) model was used to correlate the experimental data.27

Teflon-coated magnetic stirrer before it was allowed to settle for 4 h to attain equilibrium. All experiments were measured at a temperature of 313.15 K and atmospheric pressure of 101 kPa. 2.3. Measurements of Phase Compositions. To evaluate the influence of the composition of the IL solvent mixture on its extractive properties, binary mixtures of [hmim][PF6] + [emim][CH3SO4] were used as solvents in LLE screening experiments over the whole range of compositions. The binary IL mixtures were mixed in the cells with the same volume of a hydrocarbon mixture comprised of n-heptane and toluene, with a 25% toluene fraction based on mass. Evaluation of the mixed solvents’ extractive properties during the screening allowed for selection of the most suitable IL binary mixture composition. Then, LLE data for the n-heptane (1) + toluene (2) + [hmim][PF6] (3) + [emim][CH3SO4] (4) pseudoternary system, with a [hmim][PF6] mole fraction of 0.9 in the mixed solvent at 313.15 K and atmospheric pressure of 101 kPa, were determined. The aliphatic rich and ionic layers were separated and weighed. Then, samples from both layers were removed using a syringe, and the compositions of the organic compounds (nheptane, n-octane, n-nonane and toluene) were determined using an Agilent gas chromatograph 7890B (5977A GC/ MSD). The GC details are shown in Table 2. The ionic liquids

2. EXPERIMENTAL METHODS 2.1. Chemicals. Heptane, octane, nonane, and toluene were purchased from Sigma-Aldrich, and [hmim][PF6] and [emim][CH3SO4] were purchased from Iolitec GmbH. The purity of all chemicals was confirmed using GC. A newgeneration Mettler Toledo C20 Compact Coulometric Karl Fischer Titrator was used to measure the water content in all substances. All chemicals were utilized without any additional purification. Table 1 shows the mass fractions purities and water content of the pure chemicals that were used in this study.

Table 2. GC/MSD Operating Conditions for Composition Analysis Agilent 7890B GC injector 523.15 K temperature carrier gas helium capillary Agilent HP-5 ms Ultra Inert (30 m × 0.250 mm × 0.25 μm) column with an empty precolumn flow rate 2 cm3 min−1 column oven 313.15 K → 523 K (20 K/min), 15.5 min detector type FID detector 573.15 K temperature Agilent 5977A MS

Table 1. Details of the Chemicals: Purities and Water Contents compound

supplier

[hmim] [PF6] [emim] [CH3SO4] heptane

Iolitec GmbH Iolitec GmbH SigmaAldrich SigmaAldrich SigmaAldrich SigmaAldrich

octane nonane toluene

CAS number

mass fraction purity

water content (mass fraction)

30468035-1 51647401-4 142-82-5

0.99

≤0.0005

0.98

≤0.0005

0.997

≤0.00004

111-65-9

0.99

≤0.00004

111-84-2

0.99

≤0.00003

108-88-3

0.995

≤0.00005

ion source source temperature quad temperature electron energy scan

EI 493.15 K 453.15 K 70.0 eV 10−510 m/z

cannot be analyzed by the GC because they have insignificant vapor pressures. The ionic liquids were collected by a precolumn to protect the primary column and to avoid any inaccuracy that can disrupt analysis, which could be caused by tainting the GC with ionic liquid. A “three point” calibration method was used to reduce systematic errors. Mole fractions of three validation samples were gravimetrically prepared and repeatedly analyzed 10 times using the gas chromatograph. All GC analyses were repeated 10 times, and the average value was recorded to reduce random errors. The compositions of the ILs [hmim][PF6] and [emim][CH3SO4] were determined by material balance calculations. The standard deviations of the mole fraction measurements were calculated and corresponded to an experimental uncertainty of 0.001.

2.2. Apparatus and Procedure. The LLE experimental systems were mixed in six 60 cm3 glass cells and heated by a water bath utilizing a Haake DC1 thermostat. The cells’ temperatures were measured using a PT100 platinum resistance thermometer with an accuracy of ±0.1 K. Initially, 20 g of each paraffin compound was mixed with 20 g of the mixed solvent with different amounts of toluene. All quantities were weighed by a METTLER analytical balance accurate to ±0.0001 g. Every mixture was stirred vigorously for 1 h using a B



Article

3. RESULTS AND DISCUSSION 3.1. Screening LLE Experiments with [hmim][PF6] + [emim][CH3SO4] IL Binary Mixtures. Pure [hmim][PF6] and [emim][CH3SO4] ILs and their binary mixtures were investigated for the liquid−liquid extraction of toluene from heptane/toluene mixtures, with a 25% toluene content based on mass. This screening was performed to select the most suitable composition for the IL mixture. The LLE experimental data as a function of [hmim][PF6] mole fraction (ϕ3) in the mixed IL solvents are shown in Table 3, together with the estimated uncertainties for the compositions. Table 3. Experimental Equilibrium Data on Mole Fraction (xi), Distribution Coefficient (K) and Selectivity (S) of the Pseudoternary System Heptane (1) + Toluene (2) + [hmim][PF6] (3) + [emim][CH3SO4] (4) As a Function of [hmim][PF6] Mole Fraction in the Mixed IL Solvent (ϕ3) at T = 313.15 K and P = 101 kPa and 25% of Toluene in Mass Basis in Hydrocarbon Feeda heptane rich phase x2

Figure 1. (■) Selectivity (S) and (●) Distribution coefficient (K), against mole fraction of [hmim][PF6] in [emim][CH3SO4] + [hmim][ PF6] mixed solvent (ϕ3) at 313.15 K for the system: heptane (1) + toluene (2) + [emim][CH3SO4] (3) + [hmim][PF6] (4).

IL-rich phase x2

ϕ3

x1

x2

x1

x2

K

S

0.00 0.07 0.15 0.23 0.32 0.42 0.52 0.62 0.74 0.86 1.00

0.766 0.766 0.766 0.767 0.767 0.768 0.769 0.770 0.770 0.771 0.771

0.234 0.234 0.234 0.233 0.233 0.232 0.231 0.230 0.230 0.229 0.229

0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0.010 0.011 0.012

0.088 0.090 0.093 0.097 0.100 0.106 0.112 0.119 0.124 0.129 0.133

0.37 0.39 0.40 0.41 0.43 0.46 0.49 0.52 0.54 0.56 0.58

159.31 105.86 80.47 66.11 56.72 51.70 47.98 45.22 42.36 40.32 37.80

a

Standard uncertainties (u) are u(T) = 0.2 K, u(P) = 1 kPa, u(x) = 1.0 × 10−3, u(ϕ) = 1.0 × 10−2. Figure 2. Percentage extraction of toluene (●), against mole fraction of [hmim][PF6] in [emim][CH3SO4] + [hmim][ PF6] mixed solvent (ϕ3) at 313.15 K for the system: heptane (1) + toluene (2) + [emim][CH3SO4] (3) + [hmim][PF6] (4).

The influence of mixed [hmim][PF6] + [emim][CH3SO4] ILs on the solute distribution ratio (K) and selectivity (S) values was examined. The K and S values are also present in Table 3, and they were calculated from the LLE experimental data as follows: K = x 2II/x 2I

(1)

S = x 2IIx1I/x 2Ix1II

(2)

phase (raffinate) and the IL-rich phase (extract) of the three pseudoternary mixtures at 313.15 K and 101 kPa. Figure 3 shows the experimental tie lines of the three pseudoternary mixtures at 313.15 K as ternary diagrams. A small amount of paraffin was present in the IL-rich phase, as shown in Figure 3. 3.3. Distribution Ratio and Selectivity. Tables 4−6, in addition to the LLE data, include the corresponding toluene distribution ratio and selectivity values for the three pseudoternary systems. These parameters are widely used for evaluating the feasibility of using a solvent in a liquid extraction process. The distribution ratio for toluene, a measure of solvent capacity for extraction, was calculated for the three pseudoternary systems and included in Tables 4−6. Figure 4 presents the relationship between the calculated distribution ratios and the toluene mole fractions in the solvent-rich phase (x2) for the three pseudoternary systems. The distribution ratio values decreased with the aromatic mole fraction in the solvent-rich phase. As shown in this figure, the distribution ratio values increase in the following order: heptane < octane
0.4) to achieve the feasible separation of aromatics from paraffin compounds. The selectivity of the solvent was used to measure the effectiveness of the extraction of toluene from paraffin compounds using [hmim][PF6] + [emim][CH3SO4]. The selectivity of the mixed IL, which is a measure of the ability of the solvent to separate toluene from paraffin compounds, was calculated. The selectivity values decreased as the toluene mole fraction in the solvent-rich phase (x2) increased, as shown by the selectivity data presented in Figure 6. Further, this figure shows that the selectivity values increase in the following order: heptane < octane < nonane. A comparison of the data for the pseudoternary system heptane + toluene + [hmim][PF6] + [emim][CH3SO4] at 298.15 and 313.15 K with data from previous studies using sulfolane27 and other ILs is presented in Figure 7. In the present study, we can see that the [hmim][PF6]+[emim][CH3SO4] mixed solvent gives better selectivity than sulfolane and other ILs, which were used by other workers.29 As expected, the selectivity values of this work are less than those reported with pure [emim][CH3SO4] in the literature30,31 due to the higher solubility of aliphatics in [hmim][PF6]. The selectivity values in the systems studied are higher than unity, which ensures that the separation of toluene from paraffin compounds is feasible.

octane rich phase

IL-rich phase

x1

x2

x1

x2

K

S

1.000 0.962 0.944 0.927 0.909 0.869 0.832 0.797 0.764 0.706 0.655 0.610 0.570 0.534 0.503 0.475 0.450 0.427

0.000 0.038 0.056 0.073 0.091 0.131 0.168 0.203 0.236 0.294 0.345 0.390 0.430 0.466 0.497 0.525 0.550 0.573

0.012 0.012 0.012 0.012 0.012 0.011 0.011 0.010 0.010 0.009 0.008 0.008 0.007 0.007 0.006 0.006 0.006 0.006

0.000 0.027 0.039 0.050 0.061 0.086 0.107 0.126 0.143 0.173 0.197 0.216 0.230 0.240 0.253 0.264 0.273 0.280

0.71 0.70 0.68 0.67 0.65 0.64 0.62 0.61 0.59 0.57 0.55 0.53 0.52 0.51 0.50 0.50 0.49

55.83 54.60 53.52 52.50 51.25 50.13 49.04 47.96 46.75 45.57 44.38 43.18 41.98 40.76 39.53 38.26 36.95

a

Standard uncertainties (u) are u(T) = 0.2 K, u(P) = 1 kPa, u(x) = 1.0 × 10−3, u(ϕ) = 1.0 × 10−2.

nonane. A comparison of the solvent efficiency between this work on a mixed solvent of {[hmim][PF6] + [emim][CH3SO4]}, other IL solvents,29−31 and sulfolane,27 to extract D



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Figure 3. Experimental and predicted LLE data at T = 313.15 K for the system: (a) n-heptane (1) + toluene (2) + [emim][CH3SO4] (3) + [hmim][PF6] (4), (b) n-octane (1) + toluene (2) + [emim][CH3SO4] (3) + [hmim][PF6] (4), (c) n-nonane (1) + toluene (2) + [emim][CH3SO4] (3) + [hmim][PF6] (4). (●) experimental; () NRTL.

Figure 4. Measured distribution coefficient (K) against aromatic mole fraction in the solvent rich phase (x2) at 313.15 K for the system: paraffin (1) + toluene (2) + [emim][CH3SO4] (3) + [hmim][PF6] (4) at ϕ3 = 0.9: (●) heptane, (○) octane, and (▼) nonane.

Figure 6. Measured selectivity (S) against aromatic mole fraction in the solvent rich phase (x2) at 313 K for the system: paraffin (1) + toluene (2) + [emim][CH3SO4] (3) + [hmim][PF6] (4) at ϕ3 = 0.9: (●) heptane, (○) octane, and (▼) nonane.

Figure 5. Measured distribution coefficient (K) against toluene mole fraction in the solvent rich phase (x2) at 313.15 K for the system: (●) heptane (1) + toluene (2) + [emim][CH3SO4] (3) + [hmim][PF6] (4) at ϕ3 = 0.9; (□) heptane (1) + toluene (2) + sulfolane (3);29 (▼) heptane (1) + toluene (2) + [emim][C2H5SO4] (3);29 (■) heptane (1) + toluene (2) + [bmim][CH3SO4] (3);29 (△) heptane (1) + toluene (2) + [mmim][CH3SO4] (3);29 (○) heptane (1) + toluene (2) + [mebupy][BF4] (3);29 (◆) heptane (1) + toluene (2) + [emim][CH3SO4] (3);30 (◇) and heptane (1) + toluene (2) + [emim][CH3SO4] (3).31

Figure 7. Measured selectivity (S) against toluene mole fraction in the solvent rich phase (x2) at 313.15 K for the system: (●) {heptane (1) + toluene (2) + [emim][CH3SO4] (3) + [hmim][PF6] (4) at ϕ3 = 0.9; (□) heptane (1) + toluene (2) + sulfolane (3);29 (▼) heptane (1) + toluene (2) + [emim][C2H5SO4] (3);29 (■) heptane (1) + toluene (2) + [bmim][CH3SO4] (3);29 (△) heptane (1) + toluene (2) + [mmim][CH3SO4] (3);29 (○) heptane (1) + toluene (2) + [mebupy][BF4] (3);29 (◆) heptane (1) + toluene (2) + [emim][CH3SO4] (3);30 and (◇) heptane (1) + toluene (2) + [emim][CH3SO4] (3).31 E



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3.4. LLE Correlation by the NRTL Model. The nonrandom two liquid (NRTL) model proposed by Renon and Prausnitz27,28,32 was used to correlate our experimental data. The NRTL model was fitted to the experimental data using Sørensen’s iterative computer program based on the flash calculation method28 to correlate the LLE results of the pseudoternary systems under study in this paper. The binary interaction parameters aij and aji were optimized, and the third nonrandomness parameter (αij) of the NRTL equation was fixed to 0.2, which is the usual value for this parameter when fitting LLE data for systems with mixed ILs as extraction solvents. Values of aij and aji parameters of the NRTL model for the pseudoternary systems at 313.15 K and atmospheric pressure

investigated. The results showed that mixing [emim][CH3SO4], a solvent of high selectivity, and [hmim][PF6], a solvent of higher aromatic distribution ratio, unveiled a higher toluene extraction percentage than those of pure ILs. Moreover, the toluene distribution ratio and the separation factor values were higher than those of sulfolane for a mixed [hmim][PF6] + [emim][CH3SO4] IL with a [hmim][PF6] mole fraction of 0.90. In conclusion, the toluene distribution ratios and selectivities of the studied systems indicate that separation of toluene from its mixtures with heptane, octane, or nonane is feasible using [hmim][PF6] + [emim][CH3SO4] as a mixed solvent.

■

*E-mail: [email protected].

Table 7. NRTL Interaction Parameters aij and aji and Root Mean Square Deviation (RMSD) for Three Pseudoternary Systems: Paraffins (1) + Toluene (2) + [hmim][PF6] (3) + [emim][CH3SO4] (4) at T = 313.15 K and P = 101 kPaa

ORCID

Adel S. Al-Jimaz: 0000-0002-1205-4968 Mohammad S. AlTuwaim: 0000-0002-9638-8710 Notes

The authors declare no competing financial interest.

NRTL i

j

aij/K

heptane heptane

toluene [hmim][PF6] (3) + [emim] [CH3SO4] (4) [hmim][PF6] (3) + [emim] [CH3SO4] (4) toluene [hmim][PF6] (3) + [emim] [CH3SO4] (4) [hmim][PF6] (3) + [emim] [CH3SO4] (4) toluene [hmim][PF6] (3) + [emim] [CH3SO4] (4) [hmim][PF6] (3) + [emim] [CH3SO4] (4)

−177.69 1710.80

676.13 876.56

1673.20

−170.62

−224.79 1700.50

679.45 932.92

1739.30

−246.50

−252.61 1695.60

720.54 940.99

1781.20

−276.74

toluene octane octane toluene nonane nonane toluene

aji/K

AUTHOR INFORMATION

Corresponding Author

■

RMSD

REFERENCES

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0.1631

0.1731

0.1718

τij = aij/T = (gij − gjj)/RT.

a

are listed in Table 7, together with the root-mean-square deviation (RMSD) of the fit calculated as follows: 1/2 | l o o o o RMSD = 100o m ∑ ∑ ∑ (xijk ,exp − xijk ,cald)2 /6no}oo o o o k j i o n ~

(3)

where x is the mole fraction; n is the number of tie lines; and subscripts exp, cald, i, j, and k represent experimental, calculated, components, phases, and tie lines, respectively. The interaction parameters for the NRTL model were used to calculate the NRTL tie lines for the present systems, as shown in Figure 3. The calculations based on the NRTL model gave a good representation of the tie line data for those systems, as well as analysis of the mean RMSD.33−36

4. CONCLUSIONS Liquid−liquid equilibria experimental data were studied for three pseudoternary mixtures composed of heptane, octane, or nonane + toluene + [hmim][PF6] + [emim][CH3SO4] at 313.15 K and 101 kPa. In addition, to adequately assess the correlation between the experimental LLE data, we utilized the NRTL model. The influence of the mixed IL composition on the toluene distribution ratio and the solvent selectivity was F



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Liquid Extraction of Toluene from Heptane, Octane, or Nonane Using

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