Co-solvent effects for aromatic extraction with ionic liquids

Co-solvent effects for aromatic extraction with ionic liquids

Journal of Molecular Liquids 180 (2013) 145–153 Contents lists available at SciVerse ScienceDirect Journal of Molecular Liquids journal homepage: ww...

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Journal of Molecular Liquids 180 (2013) 145–153

Contents lists available at SciVerse ScienceDirect

Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq

Co-solvent effects for aromatic extraction with ionic liquids C.V. Manohar, Tamal Banerjee ⁎, Kaustubha Mohanty Department of Chemical Engineering, Indian Institute of Technology Guwahati, Guwahati – 781039, Assam, India

a r t i c l e

i n f o

Article history: Received 26 November 2012 Received in revised form 24 January 2013 Accepted 25 January 2013 Available online 8 February 2013 Keywords: Aromatic extraction Ionic liquid Co-solvent Selectivity

a b s t r a c t This work tries to solve the current problems associated with ionic liquid (IL) mediated liquid–liquid extraction such as large viscosity and high cost. The performance of IL-solvent mixtures instead of pure ILs as extractant was systematically studied here. In this work we have separated benzene (aromatic component) from hexane (aliphatic component) by using the extractant IL-Acetonitrile. The quaternary LLE experiments i.e., IL(1)-Acetonitrile(2)-Benzene(3)-Hexane(4) was conducted at T = 298.15 K and 1 bar pressure. 1-ethyl3-methylimidazolium based ionic liquid coupled with the anions ethylsulphate [EtSO4] and Acetate [Ac] was used to separate benzene using the IL and acetonitrile as the solvent. It was found that for both the systems, the hexane composition in extract phase and the solvent composition in raffinate phase are nearly zero. This implies that a mixture of IL-Acetonitrile can be beneficial since it avoids cross-contamination which is usually a problem with conventional solvents. Although the selectivities are very high (~150), the capacities are low. With respect to selectivity values [EMIM][Ac] seems to be a more economical choice of solvent in terms of cross contamination. Both the NRTL and UNIQUAC models gave RMSD's less than unity. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Aromatic compounds are the key chemicals in petrochemical and chemical industry. The aromatic compounds are basic raw materials for many intermediate and fine chemicals. Aromatics are extracted from hydrocarbon feed stocks where it remains as a mixture of aromatic and aliphatic compounds. The aromatic compounds extracted from hydrocarbon feed stocks is useful in the catalytic cracking feed stocks with its commercial importance in the oil industry [1]. The separation of aromatics from aliphatic mixtures is a very challenging job in oil industry. These compounds have close boiling points which lead to the formation of azeotropic mixtures [2]. The conventional separation process for aromatics and aliphatic hydrocarbon mixtures are liquid-liquid extraction [3]. The aromatics are separated from aliphatic by using extractive distillation. This process is usually applied when the aromatic content ranges from 20 wt.% to 60 wt.%. For higher aromatic content (~90%) separation, azeotropic distillation is preferred. Liquid– liquid separation process is generally favorable as cost and energy requirement is very less. Further the LLE data provide important technical information in developing processes for separation of desired products (extract) from mixtures of hydrocarbons. The commercial solvents which are used for aromatic separation from naphtha or gasoline are acetonitrile, sulfolane, N-methylepyrolidine (NMP), N-formylmorpholine, glycols and poly propylene carbons [4–8]. Organic solvents are basically toxic, flammable, very volatile compounds and easily evaporate to

⁎ Corresponding author. Tel.: +91 361 2582266; fax: +91 361 2582291. E-mail address: [email protected] (T. Banerjee). 0167-7322/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molliq.2013.01.019

atmosphere. The solvent's costs thus become very high without the absence of any process for the recovery of the evaporated solvent [9]. Further the capacity and selectivity with these solvents necessitates high investments and exhibit large energy consumption. The high energy consumption is mainly due to the need of additional steps for the solvent recovery and purification of product streams. The improvement of such a technology depends on using new solvents such as ionic liquids which has the potential to fulfill these requirements. Studies have indicated higher aromatic distribution coefficient for aromatic/aliphatic selectivity than conventional solvents [10–15]. Ionic liquids forms two immiscible liquid layers i.e., ionic liquid rich layer containing the aromatics and the raffinate phase containing the aliphatic. Another important aspect is that they are eco-friendly in nature. Although it has high selectivity; a higher viscosity limits it application to practical problems. Usually the high electrostatic and hydrogen-bonding interaction between the cation and anion leads to high viscosity. This also affects the mass-transfer rates which thus limits its performance. Thus it is difficult to design an IL which are less dense and also possess low viscosity. For benzene-hexane separation, most of the work reported earlier deals with ternary liquid–liquid equilibrium involving single ionic liquid as solvent [16]. Further in our earlier work [17], extraction based on mixture of ionic liquid as solvent also did not give satisfactory results in terms of capacity. One way of choosing the Ionic Liquid is by preparing a pseudo solvent mixture with ionic liquid as one of the solvent. Some solvent properties which include polarity, hydrogen-bond acidity/basicity and hydrophobicity can be adjusted by a wise selection of molecular solvents. From previous work [18], it was noticed that by adding molecular solvents as co-solvents the viscosity of ionic liquid can be reduced. The molecular solvent

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used will mainly break the hydrogen bonds which appear between cation and anion [19–21]. This will lead to higher mixing rates and also retain the inherent property of ionic liquids. Yang et al. [22], in their work separated alcohol from aliphatic mixtures by using mixed tocopherol and ionic liquids. They reported that the separation efficiency improved by using mixed tocopherol. Based on their finding a similar approach has been adopted in this work. It was shown from our previous work that a smaller cation like [EMIM] gave higher selectivity [23]. Also via COSMO-RS the anions such as [Ac], [EtSO4] and [MeSO3] gave high values of selectivities. Thus in this work we have chosen the following ILs namely: 1-ethyl-3-imidazolium acetate and 1-ethyl-3-imidazolium ethyl sulfate (Fig. 1). We have separated benzene from hexane by using acetonitrile as co-solvent via liquid–liquid extraction at T = 298.15 K.

Table 1 Densities of pure components at atmosphere pressure. S.NO

Components

Density(ρ),(gm./cm3) Experimental data (this work)

Density(ρ),(gm./cm3) Literature data

1 2 3 4 5

[EMIM][EtSO4] [EMIM][Ac] Acetonitrile Benzene Hexane

1.2435 1.1005 0.7814 0.8776 0.6597

1.2423[25] NA 0.7860[41] 0.8761[41] 0.6548[41]

time of 13 h was given after which we separated the two layers with the help of 2 mL syringes. The layers were then transferred to 5 mL cuvettes.

2. Experimental details 2.3. Compositional analysis

Benzene (SRL chemicals, India), hexane (Sigma Aldrich, Germany) and acetonitrile (Merck Specialties Private Limited, India) were supplied at 99% purity. In order to test the purity of benzene, hexane and acetonitrile, densities of the pure components were measured at atmospheric pressure with Anton Par DSA-4500MA digital vibrating U-tube densitometer. It is reported in Table 1. The solvent extraction has been carried out by using the imidazolium based ionic liquids such as 1-ethyl-3-methyl imidazolium ethyl sulphate of purity> = 99%, and 1-ethyl-3-methylimidazolium acetate of purity 99%. Both the ILs were supplied by Sigma-Aldrich and produced by BASF. For the characterization of both the phases, DMSO-D6 of purity>= 99.8% was used for the NMR analysis and was supplied by Merck, Germany. 2.2. Procedure Feed mixture was prepared such that concentrations of ionic liquid, acetonitrile, benzene and hexane always form two layers. Samples were prepared by referring the data of ternary IL-liquid–liquid systems [24]. In this process the average concentration of the component(s) in both the phases were taken as the feed composition. Thereafter individual volumes were found from mole fraction, molecular weight and density values. For obtaining the IL/acetonitrile mixture ratios, quantum chemical COSMO-RS model [25] was applied to predict the benzene/hexane selectivities and capacities at different compositions of acetonitrile namely 20, 40, 60 and 80%. It was found that the selectivity and capacity was highest at 80% composition of acetonitrile. Fig. 2 shows the variation of capacity versus mole fraction of benzene in raffinate phase. The initial feed fractions were obtained from an IL ternary system reported earlier [24].Here the average of both phases were taken as feed concentrations. Thereafter the molar concentration of 20% Ionic Liquid and 80% aceonitrile was taken for predictions. In the experimatal studies, the total volume was kept at 8 mL and then inserted into 15 mL cuvette after covering it with para-film tape as to prevent the loss of compounds to atmosphere. The cuvettes were kept in the shaker bath (Dailhan Lab, China) for 6 h at100 rpm with temperature set to 298.15 K. The temperature was kept within ±0.01 K. An equilibrium

N

O-

O

S

20% Acetonitrile 40% Acetonitrile

0.70

60% Acetonitrile

0.65

80% Acetonitrile

0.60 0.55 0.50 0.45

0.35 O-

O

1-ethyl-3-methylimidazolium acetate

Pure Ionic Liquid

0.75

N

O

O

0.80

0.40

N+

N+

In recent times 1H NMR spectra is used for the determination of composition of phases at equilibrium [26–31]. The equilibrium phase samples were separated via 2 mL syringes. Thereafter we have taken 0.1 mL sample from both the phases by using micropipette. The samples were then transferred to the NMR tubes (thrift Grade). In NMR, DMSO-d6 was used as a solvent for both the phases. In the NMR tubes, 0.1 mL of sample were inserted along with 0.3 mL of NMR solvent i.e. DMSO-d6. These NMR tubes were then put into the NMR spectrometer of 11.74 Tesla (20 MHz response of 1H). The 1H NMR shift peaks for IL(1)+acetonitrile(2)+Benzene(3)+Hexane(4) system was thus obtained. The mole fractions were obtained by the peak areas of the hydrogen atom of each component. In the NMR spectra, aromatics peaks are represented between 7 to 8.5 and aliphatic in 0.5 to 1.5 ranges. For acetonitrile, peaks were observed at 2. For hexane, the peak of the methylene groups is not required as it overlaps with the ethyl group of the ionic liquid thereby losing quantitative accuracy. Thus the CH3 peaks were taken ~1.33 even though it overlaps with the peak of the terminal methyl group of the alkyl-substituent chain. The peaks corresponding to 3 hydrogen atoms of the ionic liquid terminal methyl group were subtracted from the total peak integration to obtain the contribution of hexane to the peak integration. It should be noted that the peaks for benzene and imidazolium ring are near to each other, so equations are formed by adding the respective areas of hydrogen and then solved simultaneously. Composition calculation for benzene, hexane,

Distribution Coefficient

2.1. Chemicals and materials

1-ethyl-3-methylimidazolium ethylsulphate

Fig. 1. Structure of ionic liquids used in this work.

0.30 0.00

0.20

0.40

0.60

0.80

1.00

Mole Fraction of Benzene in Raffinate Phase(x) Fig. 2. COSMO-RS predicted capacities with mole fraction of benzene in Raffinate Phase.

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S, is the selectivity, XB is the mole fraction of benzene, XH is the mole fraction of hexane. E and R refers to the extract and raffinate phase respectively. The distribution coefficient is used to understand quantitatively of how the particular solute is distributed between the phases. Thus, the distribution coefficient is defined as the ratio of mole fraction of the solute in the extract phase (ionic liquid rich phase) to the mole fraction of the solute in the raffinate phase(hydrocarbon rich phase).

acetonitrile and ionic liquids via NMR spectrum is done using this formula: xi ¼

Hi 3 X Hi

ð1Þ

i¼1

where xi is the mole fraction of samples,Hi is the single hydrogen peak area for sample(s). Fig. 3 shows the extract (IL rich) phase NMR spectra while Fig. 4 depicts the raffinate (hexane rich) phase spectra. As can be seen from the Fig. 4, the raffinate phase is free from any ionic liquid because of the absence of peaks between 2 and 7. The peaks within this region mainly consist of the anions of the ionic liquids i.e. acetate and ethylsulphate. For checking the accuracy we prepared some known mixtures in the homogenous region close to the binodal curve and then obtained the 1H NMR. The measured results were found to be in good agreement (+/−0.001 mole fraction) with the actual compositions.

β¼

LLE experimental data for the systems: [EMIM][Ac](1)-acetonitrile (2)–benzene(3)–hexane(4) and [EMIM][EtSO4](1)-acetonitrile(2)– benzene(3)–hexane(4) at 298.15 K are reported in Tables 2 and 3 respectively. Quaternary tie lines are plotted in ternary plots by assuming ionic liquid and acetonitrile as pseudo solvent. Hence for visualization purpose, their mole fractions are added. In Table 2, it is observed that the benzene concentration ranges from 0.066 to 0.150 in extract phase for [EMIM][EtSO4](1)-acetonitrile(2)–benzene(3)–hexane(4) system. This is very low as compared to extraction carried out using a single IL [32] where benzene concentrations as high as 0.9 has been reported in extract phase (hexane rich). However on close inspection it can be seen that the hexane composition in extract phase and the solvent composition in raffinate phase is nearly zero. This implies that a mixture

ð2Þ

N

O O

N+

S

O-

N O

O O

N+

ð3Þ

3.1. Liquid–liquid equilibrium of IL-acetonitrile–benzene–hexane

Selectivity of ionic liquid is measured by the affinity of aromatic compounds in feed mixtures towards the ionic liquid. By its value, the amount of solute absorbed in ionic liquid is obtained. It is defined as the ratio of solute in extract phase (ionic liquid rich phase) to solute in raffinate phase (hexane rich phase) [31] xEB xRH xRB xEH

xEB xRB

3. Results and discussions

2.4. Selectivity and distribution coefficient



147

S

O-

N O

Fig. 3. Extract phase 1H NMR spectrum for IL(1)-acetonitrile(2)–benzene(3)–hexane(4).

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Fig. 4. Raffinate phase1NMR spectrum for IL(1)-acetonitrile(2)–benzene(3)–hexane(4). Note that there is no evidence of IL peak in the raffinate phase.

of IL-Acetonitrile can be beneficial since it avoids cross-contamination which is usually seen with conventional solvents. Although the selectivities are very high, however the capacities are low. Figs. 5 and 6 shows the ternary plot of [EMIM][EtSO4](1)-acetonitrile(2)–benzene(3)– hexane(4) which depicts negative slope at higher concentrations of benzene. This is also visible in the distribution values (Table 2) which are above one at low concentration (positive slope at lower end of triangular diagrams) and below one at high concentrations. In Table 3 for the [EMIM][Ac](1)-acetonitrile(2)–benzene(3)– hexane(4) system, it is observed that the benzene concentration ranges from 0.005 to 0.347 in extract phase which is higher than that observed for [EMIM][EtSO4](1)-acetonitrile(2)–benzene(3)–hexane(4) (Table 1). This again is very less as compared to the reported data [32]. The selectivity (Table 2) values are lower as compared to [EMIM] [EtSO4](1)-acetonitrile(2)–benzene(3)–hexane(4) (Table 1). However as with the earlier system, the hexane composition in extract phase and the solvent composition in raffinate phase are nearly zero. So as compared to [EMIM][EtSO4], [EMIM][Ac] seems to be more economical choice of solvent in terms of cross contamination. In Figs. 7 and 8, the ternary plot for [EMIM][Ac](1)–acetonitrile(2)–benzene(3)–hexane(4) are given. It again depicts a negative slope at higher concentrations of benzene.

The distribution coefficient is found to increase with benzene concentration in feed mixture for [EMIM][Ac](1)-acetonitrile(2)– benzene(3)–hexane(4) system (Table 3), while it decreases for [EMIM] [EtSO4](1)-acetonitrile(2)–benzene(3)–hexane(4) system (Table 2). However the distribution ratios for both the ILs are less than one which implies less amount of IL is required for aromatic extraction. The distribution coefficient values increases from 0.035 to 0.44 for [EMIM] [Ac](1)–acetonitrile(2)–benzene(3)–hexane(4)system while it decreases from 1.2 to 0.27 in [EMIM][EtSO4](1)-acetonitrile(2)–benzene(3)– hexane(4) in the respective feed mixtures. Thus from both the systems it is seen that benzene is distributed mainly in the alkane-rich phase. Increasing the number of stages and recycling the raffinate back to the extraction unit can increase the separation of aromatic compounds from aliphatic compounds [32]. However the variation of both selectivity (Fig. 9) and distribution values (Fig. 10) with benzene concentration does not point out to specific trend for both the IL + Acetonitrile mixtures. It can be seen that slopes in Figs. 5–8 are very similar to the ones reported for the separation of benzene and heptane using a pyridinium based Ionic Liquid [33].Thus the values of selectivity and capacity obtained in Tables 2 and 3 are in the same order and range as observed for the separation of heptane and toluene using the same ionic liquid

Table 2 Experimental tie-lines, selectivity (S) and distribution ratio (β) for [EMIM][EtSO4](1)-acetonitrile(2)–benzene(3)–hexane(4) at 298.15 K. S.No

1 2 3 4 5 6 7 8

IL-Rich phase

Hexane-rich phase

Xionic

xacetonitrille

Xbenzene

Xhexane

Xionic

xacetonitrile

Xbenzene

Xhexane

0.365 0.342 0.303 0.410 0.429 0.431 0.452 0.487

0.567 0.580 0.614 0.497 0.464 0.437 0.401 0.361

0.066 0.074 0.081 0.091 0.105 0.125 0.139 0.150

0.002 0.004 0.002 0.002 0.002 0.007 0.008 0.002

0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000

0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000

0.059 0.143 0.212 0.330 0.365 0.375 0.521 0.553

0.940 0.856 0.787 0.669 0.634 0.624 0.478 0.446

S

β

525.7 110.7 150.3 92.2 91.1 29.7 15.9 60.4

1.12 0.52 0.38 0.27 0.28 0.33 0.26 0.27

C.V. Manohar et al. / Journal of Molecular Liquids 180 (2013) 145–153

149

Table 3 Experimental tie-lines, selectivity (S) and distribution ratio (β) for [EMIM][Ac](1)–acetonitrile(2)–benzene(3)–hexane(4) at 298.15 K. S.No

IL-rich phase

1 2 3 4 5 6 7 8

Hexane-rich phase

Selection parameter

Xionic

xacetonitrile

Xbenzene

Xhexane

Xionic

xacetonitrile

Xbenzene

Xhexane

S

β

0.913 0.896 0.831 0.440 0.725 0.150 0.254 0.592

0.081 0.046 0.097 0.463 0.157 0.707 0.497 0.059

0.005 0.056 0.071 0.094 0.116 0.142 0.247 0.347

0.001 0.002 0.001 0.003 0.002 0.001 0.002 0.002

0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000

0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000

0.148 0.262 0.303 0.436 0.552 0.687 0.739 0.783

0.851 0.737 0.696 0.563 0.447 0.312 0.260 0.216

28.7 78.7 163.1 40.4 46.9 64.4 43.4 47.8

0.03 0.21 0.23 0.21 0.21 0.21 0.33 0.44

[32] and for the separation of benzene and heptane using a pyridinium based cation [33].The greater scatter of the selectivity for the quaternary mixtures may be due to the higher volatility of hexane [34,35]. These trends do agree with the reported work of Meindersma et al. [32], where toluene and heptane was separated using the IL:[EMIM] [EtSO4]. A similar observation was also reported by Deenadayalu et al. [36], where they used [EMIM][OcSO4] (Oc = Octyl) as a solvent for separating benzene and hexane.

lnγ i ¼ ln

    c Φi z θ Φ X þ qi ln i þ li − i xl xi Φi xi j¼1 j j 2 1 0

C B c c X X B θj τij C C þ qi B θj τ ji − C B1− ln c X A @ j¼1 j¼1 θk τkj

ð5Þ

k¼1

where,   Gji ¼ exp −α ji τji

3.2. NRTL and UNIQUAC correlations The reliability of experimental data was validated using the NRTL and UNIQUAC model and the results were analyzed in terms of RMSD values between experimental and predicted tie lines. According to the NRTL [37] and UNIQUAC model, the non-ideal liquid phase activity coefficient (γ of componenti) are given by Eqs. (4) and (5) as given below: c X

lnγi ¼

2 τji Gji xj

j¼1 c X

þ Gki xk

0

τij Gij xi C7 B c 6 X 6 Gij xj B C7 6 C7 Bτij − i¼1 6X C7 B c c X A5 @ j¼1 4 Gkj xk Gkj xk

k¼1

k¼1

q i xi qT

ð7Þ

Φi ¼

r i xi rT

ð8Þ

τji ¼

g ji −g ii Aji ¼ RT T

ð9Þ

θi ¼

13

c X

ð4Þ

ð6Þ

qT ¼ ∑ qk xk

ð10Þ

r T ¼ ∑ r k xk

ð11Þ

k

k¼1

k

Benzene 0.0 0.1

1.0 Experimental NRTL model prediction

0.9

0.2

0.8

0.3

0.7

0.4

0.6

0.5

0.5

0.6

0.4

0.7

0.3

0.8

0.2

0.9

0.1

1.0

Hexane

0.0

0.0 0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

[EMIM][EtSO4]-Acetonitrile

Fig. 5. Experimental and NRTL predicted tie-lines for quaternary system: [EMIM][EtSO4](1)-acetonitrile(2)–benzene(3)–hexane(4) at T = 298.15 K.

150

C.V. Manohar et al. / Journal of Molecular Liquids 180 (2013) 145–153

Benzene 0.0

1.0

0.1

Experimental UNIQUAC model prediction

0.9

0.2

0.8

0.3

0.7

0.4

0.6

0.5

0.5

0.6

0.4

0.7

0.3

0.8

0.2

0.9

0.1

1.0

Hexane

0.0

0.0

0.1

0.3

0.2

0.4

0.5

0.6

0.7

0.8

0.9

1.0

[EMIM][EtSO4]-Acetonitrile

Fig. 6. Experimental and NRTL predicted tie-lines for quaternary system: [EMIM][Ac](1)-acetonitrile(2)–benzene(3)–hexane(4) at T = 298.15 K.

li ¼

z ðr −qk Þ þ 1−r k 2 k

ð12Þ

Eqs. (7)–(9) describes the area fraction (θ), segment fraction (Ф) and interaction parameter (τ). The pure component surface and volume parameter is represented by r and q respectively. The mixture surface and volume parameter is given by Eqs. (10) and (11) respectively. Mole fraction of liquid phase is represented as x. The coordination number is represented by z (Eq. (12)) which is taken as 10. The non randomness parameter i.e. αij=αji =0.2 was used in the UNIQUAC model

regression. gji represents the average interaction energy for the interaction of molecules of component j with molecules of component i .R and T represents the gas constant and temperature in Eq. (9). The accurate regression and subsequent prediction of NRTL and UNIQUAC interaction parameters are one of the most important outcome of LLE data. This is usually done by fitting the experimental LLE data to NRTL/UNIQUAC model. The interaction parameters obtained can then be used for predicting properties at intermediate conditions. However both the model incorporates a parameter estimation process, which is an optimization without a unique result. This mostly runs into conditions where the optimization problem is nonconvex with several

Benzene 0.0 0.1

1.0

Experimental NRTL model prediction

0.9

0.2

0.8

0.3

0.7

0.4

0.6

0.5

0.5

0.6

0.4

0.7

0.3

0.8

0.2

0.9

0.1

1.0

Hexane

0.0

0.0 0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

[EMIM][Ac]-Acetonitrile

Fig. 7. Experimental and UNIQUAC predicted tie-lines for quaternary system:[EMIM][EtSO4](1)-acetonitrile(2)–benzene(3)–hexane(4) at T = 298.15 K.

C.V. Manohar et al. / Journal of Molecular Liquids 180 (2013) 145–153

151

Benzene 0.0

1.0

0.1

Experimental UNIQUAC model prediction

0.9

0.2

0.8

0.3

0.7

0.4

0.6

0.5

0.5

0.6

0.4

0.7

0.3

0.8

0.2

0.9

0.1

1.0

Hexane

0.0

0.0 0.1

0.2

0.3

0.4

0.5

0.6

0.8

0.7

0.9

1.0

[EMIM][Ac]-Acetonitrile

Fig. 8. Experimental and UNIQUAC predicted tie-lines for quaternary system:[EMIM][Ac](1)-acetonitrile(2)–benzene(3)–hexane(4) at T = 298.15 K.

local extrema. The focus of this work is also on the application of genetic algorithm (GA) to find global extrema for estimation of interaction parameters. Thus the fitted parameters are not global or unique. It implies that different runs with GA will give different values of parameters. However we report the interaction parameters for the least Objective function value (F) (Eq. (13)). From the objective function F (Eq. (13)) defined below, the quaternary LLE data was regressed to obtain the binary interaction parameters. It can be written as: Maximize :

F0

1

ð13Þ

with respect to Aij or τij @ where i; j ¼ 1; 2; 3 A and j≠i m X II X c   X l 2 l l wik xik −x^ik ; ¼−

l

wik ¼ 1

k¼1 l¼I i¼1 l and x^lik refers to the experimental and predicted mole fracHere xik tions respectively. In recent times Genetic Algorithm (GA) program toolbox in MATLAB has been used to maximize the Objective function F by regressing the obtained experimental data [25,38]. A negative sign for F has been used as the GA toolbox is used for maximization. In our earlier work [25,38] we used a population size of 100 and number

of generations as 200. Modified Rachford–Rice algorithm [39] was used to predict the tie lines of the quaternary systems. The pure components parameters (Table 4) have been calculated by GEnerating POLyhedra (GEPOL) [40], which is based on the concept of solvent excluding surface. Here Aij and τij refers to the interaction parameter between component i and j in UNIQUAC and NRTL model respectively. The root mean square deviation is defined as: 2   3 j 2 1=2 j m X c X 2 ^ik x − x X ik 6 7 RMSDðin % Þ ¼ 100  4 5 2mc k¼1 i¼1 j¼1

ð14Þ

where ‘m’ refers to the number of tie lines, ‘c’ the number of compol and x^lik are the experimental nents and ‘2’ is the number of phases. xik and predicted values of composition (mole fraction) for component i for the k th tie line in phase l, respectively. The values of r and q for the components have been presented in Table 3. The tie lines predicted by NRTL and UNIQUAC models were compared with the experimental tie lines. The ternary tie line plot comparison between experimental and predicted tie lines are shown in

1.2 1000

Distribution coefficient (β)

[EMIM][EtSO4](1)-Acetonitrile(2)-Benzene(3)-Hexane(4)

Selectivity(S)

[EMIM][Ac](1)-Acetonitrile(2)-Benzene(3)-Hexane(4)

100

10

[EMIM][Ac](1)-Acetonitrile(2)-Benzene(3)-Hexane(4) [EMIM][EtSO4](1)-Acetonitrile(2)-Benzene(3)-Hexane(4)

1.0 0.8 0.6 0.4 0.2 0.0

1 0

0.1

0.2

0.3

0.4

0.5

0.6

Benzene concentration in raffinate phase(x) Fig. 9. Selectivity versus Benzene concentration in raffinate phase for [EMIM][EtSO4](1)acetonitrile(2)–benzene(3)–hexane(4) and [EMIM][Ac](1)-acetonitrile(2)–benzene(3)– hexane(4) at T=298.15 K.

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Benzene concentration in raffinate phase(x) Fig. 10. Distribution coefficient versus Benzene concentration in raffinate phase for [EMIM] [EtSO4](1)-acetonitrile(2)–benzene(3)–Hexane(4) and [EMIM][Ac](1)-acetonitrile(2)– benzene(3)–hexane(4) at T=298.15 K.

152

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Table 4 UNIQUAC volume and surface area structural parameters for different components. S.No

Compounds

Volume parameter (r)

Surface area parameter (q)

1 2 3 4 5

[EMIM][Ac]a [EMIM][EtSO4]a Benzene Acetonitrile Hexane

8.75 8.39 3.19 1.87 4.50

5.56 6.62 2.40 1.72 3.86

a

Calculated from [40].

Figs. 5 and 6 for NRTL and UNIQUAC model respectively for the system [EMIM][EtSO4](1)-acetonitrile(2)–benzene(3)–hexane(4).Similarly Figs. 7 and 8 shows the NRTL and UNIQUAC comparison for [EMIM] [AC](1)-acetonitrile(2)–benzene(3)–Hexane(4). As can be seen from Figs. 5 to 8, all the tie lines matches excellently with the experimental tie lines. This is manifested in Tables 5 and 6 which gave RMSD's values less than unity for both the systems. The NRTL and UNIQUAC interaction parameters are also shown in Tables 5 and 6. The average RMSD value for NRTL and UNIQUAC are 0.3% and 0.54% respectively for the two systems.

Nomenclature Latin symbols: [EMIM] Cation: 1-ethyl 3-methylimidazolium [Ac] Anion: acetate [EtSO4] Anion: ethyl sulphate S Selectivity F Objective function RMSD Root mean square deviation R Universal gas constant, J K −1 mol −1 T Temperature, K Normalized volume parameter for the Staverman– ri Guggenheim combinatorial term Normalized surface area parameter for the Staverman– qi Guggenheim combinatorial term m Number of tie lines c Number of components in the LLE system Mole fraction of component i of phase I in the LLE system xiI Peak area under NMR spectra of species i Hi z Coordination number (= 10) l Staverman–Guggenheim combinatorial term parameter Average interaction energy for the interaction of molecules gji of component j with molecules of component i feed concentration of the ith component zi Liquid feed rate of phase 1 LI

4. Conclusion The quaternary LLE experiments were conducted for the selection of potential solvents which may reduce the viscosities for benzene extraction from hexane. In this work, the 1-ethyl-3-methylimidazolium based ionic liquid coupled with the anions ethylsulphate [EtSO4] and Acetate [Ac] was used to separate benzene using the IL and acetonitrile as a solvent. It was found that for both the systems, the hexane composition in extract phase and the solvent composition in raffinate phase were nearly zero. This implies that a mixture of IL-acetonitrile can be beneficial since it avoids cross-contamination which is usually associated with the conventional solvents. The selectivities values were very high (~150), however the capacities were found to be low. With respect to selectivity values [EMIM][Ac] seems to be more economical choice of solvent in terms of cross contamination. Both the NRTL and UNIQUAC models gave satisfactory results.

Greek symbols Activity coefficient of solute i in solution S γi/S Peak area under NMR spectra of species i Hi β Distribution coefficient θ Area fraction in UNIQUAC equation τ NRTL/UNIQUAC interaction parameter Φ Segment fraction in UNIQUAC equation α NRTL non-randomness parameter

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Table 5 NRTL and UNIQUAC interaction parameters for [EMIM][Ac](1)–acetonitrile(2)–Benzene(3)–hexane(4) at T= 298.15 K. i-j

NRTL model parameters τij

τji

UNIQUAC model parameters F*

System: EMIM][Ac](1)-acetonitrile(2)–benzene(3)–hexane(4) 1–2 5.7764 6.4207 −4.35 ×10-04 1–3 0.8973 9.59 1–4 2.6931 8.6898 2–3 12.546 3.9814 2–4 6.4811 9.44 3–4 12.013 3.2128

RMSD**

Aij/K

Aji/K

F*

RMSD**

0.0026

−822.65 317.56 997.85 116.62 154.79 −233.94

273.01 −74.744 376.67 285.6 999.99 371.14

−2.11 ×10-03

0.0057

Table 6 NRTL and UNIQUAC interaction parameters for [EMIM][EtSO4](1)–acetonitrile(2)–benzene(3)–hexane(4) at T = 298.15 K. i-j

NRTL model parameters Τij

τji

UNIQUAC model parameters F*

System: EMIM][EtSO4](1)-acetonitrile(2)–benzene(3)–hexane(4) 1–2 −8.7522 15.984 −7.41 ×10-04 1–3 3.4646 16.8 1–4 18.571 4.9546 2–3 19.306 8.159 2–4 19.986 4.1689 3–4 7.5504 0.80621

RMSD**

Aij/K

Aji/K

F*

RMSD**

0.0034

564.68 625.42 542.09 103.5 999.79 −266.88

−642.63 −278.89 123.32 −130.6 998.54 748.68

−1.62 ×10-03

0.0050

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