Phase Behavior of Nitrate Based Ionic Liquids with Thiophene and

Article pubs.acs.org/IECR

Phase Behavior of Nitrate Based Ionic Liquids with Thiophene and Alkanes Babak Mokhtarani,* Hossein Mansourzareh, and Hamid Reza Mortaheb Chemistry and Chemical Engineering Research Center of Iran, P. O. Box 14335-186, Tehran 1496813151, Iran S Supporting Information *

ABSTRACT: In this study, the extraction of thiophene from alkane compounds (hexane, heptane, and octane) was investigated by using 1-butyl 3- methylimidazolium nitrate ([Bmim][NO3]) and 1-methyl 3-octylimidazolium nitrate ([Omim][NO3]). Liquid−liquid equilibrium (LLE) data for ternary systems of ionic liquid + thiophene + alkane were reported at atmospheric pressure and 298.15 K. The calculated selectivity and solute distribution coefficient values from the experimental data demonstrate that the selectivities increase with increasing alkane chain length while the solute distribution coefficients decrease. Moreover, the selectivity values for the systems containing [Bmim][NO3] are higher than those containing [Omim][NO3]. This implies that [Bmim][NO3] is a more suitable solvent than [Omim][NO3] for desulfurization of alkane. The nonrandom twoliquid (NRTL) and universal quasi-chemical (UNIQUAC) models were used to correlate the experimental data. The results show both models can correlate the experimental data with good accuracies.

1. INTRODUCTION Sulfur oxides (SOx) originating from combustion of fuels increasingly threaten human life with environmental problems such as global warming and water pollution.1 Moreover, the presence of sulfur in fuels decreases their export values and causes corrosion in pipelines and equipment. The environmental limitations are restricted severely as the United States Environmental Protection Agency (EPA) mandated a maximum sulfur content in diesel to 15 ppm in 2006 and below 10 ppm in the near future.2 The conventional hydrodesulfurization (HDS) process for desulfurization is based on the reaction of hydrogen with sulfur compounds where the sulfur compounds are converted to elemental sulfur via the Claus process.2 The process has some disadvantages, such as high consumption of hydrogen and catalysts, and severe operating conditions (temperature of 300 °C and pressure of 3−10 MPa). Besides, the HDS process can hardly remove aromatic sulfurs such as thiophenes, benzothiophenes, and dibenzothiophenes.3 Among the alternative processes including extraction desulfurization (EDS), oxidation desulfurization (ODS), absorption desulfurization (ADS), and biological desulfurization (BDS), which are used individually or in combinations,4 extractive desulfurization is a common process. This is because the process is carried out at ambient temperature and atmospheric pressure without any hydrogen consumption. The appropriate solvent for desulfurization must remove sulfur compounds selectively without affecting the octane number of the fuel.5 Many organic solvents are used for removal of sulfur compounds, but none of them can yield satisfactory results. Ionic liquids (ILs) may be applied as new solvents for extractive desulfurization. ILs are organic salts that are liquid below 100 °C6 and have advantages such as low vapor pressure, acceptable solubility, nonflammability, and high stability in most chemical processes.7 In addition, some type of ILs can be distilled at low pressures without any decomposition.8 © 2013 American Chemical Society

The suitability of an IL for a separation process can be determined by measurement of the activity coefficient at infinite dilution. From these data, the selectivities and capacities at infinite dilution can be directly calculated.9 The ratio of activity coefficients of solutes at infinite dilution in the solvent (IL) is the selectivity of a solute at infinite dilution, and the inverse of the activity coefficient of each solute at infinite dilution in IL is defined as the capacity of that solute at infinite dilution. In recent years, several research studies on desulfurization using ionic liquids have been carried out. Alonso et al.10 investigated the effect of an alkane’s chain length on the separation of thiophene from aliphatic compounds using [Omim][NTf2] (Omim = 1-methyl 3-octylimidazolium). They found that the selectivity increases with an increase in the alkane’s chain length while the distribution coefficient decreases. Jiang et al.11 used imidazolium cation based and alkyl phosphate anion based ILs for desulfurization. The pyridinium based ILs were tested by Francisco et al.12 They determined the liquid− liquid equilibrium data for [hmmpy][NTf2], thiophene, and three aliphatic compounds (hexane, dodecane, and hexadecane). The obtained selectivities and distribution coefficients showed that although this IL can be used as a promising solvent for extractive desulfurization of the fuels with low thiophene concentrations, large consumption of solvent would be needed ́ for desulfurization. Rodriguez-Cabo et al.13 studied the effect of the position of the alkyl side chains of IL using hexyl dimethyl pyridinium. They showed that the asymmetry induced by changing the position of alkyl side chains on the ring of the IL’s cation increases the efficiency of desulfurization. In another work ́ by Rodriguez-Cabo et al,14 desulfurization of fuel oils (aliphatic compounds) and other aromatic compounds of the fuel Received: Revised: Accepted: Published: 1256

November 14, 2013 December 17, 2013 December 17, 2013 December 17, 2013 dx.doi.org/10.1021/ie4038592 | Ind. Eng. Chem. Res. 2014, 53, 1256−1261

Industrial & Engineering Chemistry Research

Article

(toluene) by [Emim][NTf2] (Emim = 1-ethyl-3-methylimidazolium) was investigated. The extractive/oxidative desulfurization of dibenzotiophene (DBT) and diesel oil using a C5H9NO· SnCl2 coordinated ionic liquid was investigated by Li et al.15 Removal efficiencies of DBT from diesel in the absence and presence of H2O2 and CH3COOH as oxidants were 87.6% and 94.8% in 30 min, respectively. Domańska et al.16 studied the desulfurization of thiophene from alkane by three types of pyrrolidinium based ILs. Their results showed that 1-butyl-1methylpyrrolidinium tricyanomethanide, [BMPYR][TCM], might be an appropriate solvent for desulfurization. Recently, Królikowski et al.17 investigated the separation of the aromatic sulfur compounds such as benzothiophene and thiophene from n-heptane using 1-ethyl-3-methylimidazolium tricyanomethanide

([Emim][TCM]) at different temperatures. The results indicated that temperature changes do not affect the efficiency of desulfurization. In this study, removal of thiophene from different alkanes (hexane, heptane, and octane) are studied using 1-butyl 3- methylimidazolium nitrate ([Bmim][NO3]) and 1-methyl 3-octylimidazolium nitrate ([Omim][NO3]) ILs. The nitrate based ILs were selected in the present study because they are halogen-free and thus more environmentally friendly than other types of ILs. The liquid−liquid equilibrium (LLE) data for the ternary systems of [Bmim][NO3] + thiophene + alkane and [Omim][NO3] + thiophene + alkane are measured. The effects of alkane type and the cation’s chain length on the extraction of thiophene from alkane are also investigated. Finally, the LLE data are correlated using nonrandom two-liquid (NRTL) and universal quasi-chemical (UNIQUAC) models.

Table 1. Purities and Suppliers of the Chemicals chemical name

supplier

mass fraction purity

hexane heptane octane thiophene [Bmim][NO3] [Omim][NO3]

Merck Merck Merck Merck synthesized in laboratory synthesized in laboratory

0.99 0.99 0.99 0.99 0.99 0.99

2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. Hexane, heptane, octane, and thiophene were purchased from Merck Co., and their purities are reported in Table 1. The studied ILs ([Bmim][NO3] and [Omim][NO3]) were synthesized in our laboratory. These ILs were prepared from [Bmim][Cl] and [Omim][Cl] according to the procedure in the

Figure 1. Experimental LLE of the ternary systems of [Omim][NO3] (1) + alkane (2) + thiophene (3) at 298.15 K. Solid lines (●) indicate the experimental data, and dashed lines (Δ) indicate the NRTL correlation. 1257

dx.doi.org/10.1021/ie4038592 | Ind. Eng. Chem. Res. 2014, 53, 1256−1261


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Figure 2. Experimental LLE of the ternary systems of [Bmim][NO3] (1) + alkane (2) + thiophene (3) at 298.15 K. Solid lines (●) indicate the experimental data, and dashed lines (Δ) indicate the NRTL correlation.

literature.18 [Bmim][Cl] and [Omim][Cl] were synthesized according to the procedure in the literature19,20 as described in our previous work.21,22 The structures of synthesized ILs were verified with nuclear magnetic resonance (NMR) spectroscopy. The ILs were kept under vacuum for 24 h at 343.15 K. The measured mass fractions of the ILs by a 684 Karl Fischer coulometer were less than 1 × 10−3. 2.2. Apparatus and Procedure. Ternary systems of IL + thiophene + alkane with total mass of 5 g were prepared in 20 mL glass vials. Each component was weighted by a laboratory balance with the precision of ±10−4 g. The mixture in the glass vial was closed firmly with a cap and septum and stirred by a shaker (IKA HS-260) for 180 min at 300g. The glass vial was placed for 12 h into a water bath with a precision of ±0.1 K (Julabo, FP50) set at 298.15 K. The two phases in the glass vial were separated and analyzed after the equilibrium condition was attained. The composition of each phase was determined using gas chromatography (GC; Varian, cp 3800) with a flame ionization detector (FID) and capillary column (Chrompack, 30 m × 0.25 mm × 1.2 μm). The experimental uncertainties in the determination of top and bottom phase compositions were less than ±2.10−3 and ±1.10−3 in mole fraction, respectively. Because the NMR spectroscopy detected no IL in the top phase, the composition of the top phase was calculated using the calibration

of binary systems of thiophene and the aliphatic components. The bottom phase’s compositions were calculated by diluting the bottom phase’s sample with acetone and direct injection of the diluted samples to the GC. Because the GC cannot detect the IL composition, the IL content of the sample was filtered by a filter in the injection line.

3. RESULTS AND DISCUSSION LLE data for the ternary systems of [Bmim][NO3] (1) + alkane (2) + thiophene (3) and [Omim][NO3] (1) + alkane (2) + thiophene (3) determined at 298.15 K and atmospheric pressure are reported in Tables S1 and S2 of the Supporting Information. The selectivity (S) and the solute distribution coefficient (β) are defined as follows: β=

S=

x3II x3I

(1)

x3IIx 2I x3Ix 2II

(2)

where x is the mole fraction, superscripts II and I refer to the alkane- and IL-rich phases, and subscripts 2 and 3 refer to alkane and thiophene, respectively. The higher selectivity reduces the 1258



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number of stages, and the higher distribution coefficient decreases the amount of solvent that causes reduction in the capital and operating costs.23 The ternary phase diagrams are shown in Figures 1 and 2. As seen in the figures, there is no IL in the alkane-rich phase and the studied systems show a wide immiscibility area. Since the desulfurization process corresponds to the regions with low thiophene concentrations, the lowest part of the diagram should be investigated. In the low thiophene concentration, the slopes of the tie lines are low and positive. As the thiophene concentration increases, the slopes of tie lines are increased and with a further increase in the thiophene concentration, the slopes decrease again. The variation of tie lines’ slopes represents the solutropic behavior for the studied systems. Francisco et al.12 and Alonso et al.10,24 reported this behavior for desulfurization using other types of ILs. The selectivity and solute distribution coefficient versus solute mole fraction (thiophene) in the alkane-rich phase are shown in Figures 3−6. By increasing the thiophene concentration, the

Figure 5. Selectivity vs thiophene mole fraction in the alkane-rich phase for the ternary system of [Bmim][NO3] (1) + alkane (2) + thiophene (3) at 298.15 K: ●, hexane; ■, heptane; ▲, octane.

Figure 6. Distribution coefficient vs thiophene mole fraction in the alkane-rich phase for the ternary system [Bmim][NO3] (1) + alkane (2) + thiophene (3) at 298.15 K: ●, hexane; ■, heptane; ▲, octane.

Figure 3. Selectivity vs thiophene mole fraction in the alkane-rich phase for the ternary system of [Omim][NO3] (1) + alkane (2) + thiophene (3) at 298.15 K: ●, hexane; ■, heptane; ▲, octane.

Figure 7. Selectivity vs thiophene mole fraction in the alkane-rich phase for the ternary system IL (1) + heptane (2) + thiophene (3) at 298.15 K: ●, [C8mim][BF4];25 ■, [COC2mMOR][NTF2];26 ▲, [Bmim][NO3]; ◆, [Omim][NO3].

Figure 4. Distribution coefficient vs thiophene mole fraction in the alkane-rich phase for the ternary system of [Omim][NO3] (1) + alkane (2) + thiophene (3) at 298.15 K: ●, hexane; ■, heptane; ▲, octane.

selectivity and distribution coefficient values decrease. These figures show that as the length of the alkane chain increases, the selectivity value increases while the solute distribution coefficient decreases. In addition, the selectivity of different ILs against thiophene mole fraction in the alkane-rich phase is compared with other types of ILs25,26 in Figure 7. As seen in the figure, the selectivity values of the system containing [Bmim][NO3] is greater than

those of the system containing [Omim][NO3]. However, the solute distribution coefficient of the system containing [Bmim][NO3] is less than that of the system containing [Omim][NO3]. This implies that although [Bmim][NO3] is a more suitable solvent than [Omim][NO3] and that it requires fewer stages for removal of thiophene from the alkane systems, the required amount of [Bmim][NO3] is more than that of [Omim][NO3]. 1259



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data and the obtained values from the UNIQUAC model is made in Figures S1 and S2 of the Supporting Information. As the figures show, there is good agreement between the experimental data and the correlated results by the UNIQUAC model. The comparison of rmsd calculated for the NRTL and UNIQUAC models shows that the accuracy of prediction by the NRTL model is better than that of the UNIQUAC model.

However, since the ILs can be recycled easily, this does not cause a serious problem.

4. THERMODYNAMIC MODELING The experimental data can be modeled for further design and simulation studies of the desulfurization process. The NRTL and UNIQUAC models were used for correlating the experimental data. In the NRTL model,27 the activity coefficients are expressed as n

ln γi =

∑ j = 1 τjixjGji n

∑k = 1 xkGki

n ⎞⎛ ∑m = 1 τmixmGmi ⎞ ⎟ ⎟ ⎜ τ − n ij n ∑ x G ⎟⎜ ∑k = 1 xkGkj ⎟⎠ j = 1 ⎝ k = 1 k kj ⎠⎝ (3) n

+

⎛

5. CONCLUSION Experimental data for extraction of thiophene from alkane (hexane, heptane, and octane) were obtained using [Bmim][NO3] and [Omim][NO3] as the solvent at atmospheric pressure and 298.15 K. The selectivity and the solute distribution coefficient values were calculated from the experimental data. The selectivity values increase with an increase in the length of the alkane’s chain while the thiophene distribution coefficients decrease. The length of IL’s cationic chain has a great effect on the selectivity values; that is, the selectivity values decrease with an increase in the length of IL’s cationic chain while the thiophene distribution coefficients increase. This implies that [Bmim][NO3] is a more suitable solvent than [Omim][NO3] for desulfurization of the studied systems although it requires a higher amount of the solvent. The experimental data were correlated using NRTL and UNIQUAC models. Although both models correlate the experimental data with reasonable accuracies, the predicted values by the NRTL model have higher accuracies than those by the UNIQUAC model.

∑ ⎜⎜

xjGij

where x is the mole fraction, R is the gas constant, T is absolute temperature, and α is the nonrandomness parameter, set to 0.3. The energy parameters are calculated by minimization of differences between the experimental and calculated mole fractions of each component. The following equation is used as the objective function. ⎛ x exp − x cal ⎞2 ijk ijk ⎟ F = ∑ ∑ ∑ ⎜⎜ exp ⎟ x ijk ⎠ i j k ⎝ m n−1 2

(4)

where m is the number of tie lines, n is the number of components, k is the number of phases, and superscripts of “exp” and “cal” refer to experimental and calculated mole fraction values, respectively. The minimization was performed with Solver in Excel software, and the calculated energy parameters are given in Table S3 of the Supporting Information, in which the root-mean-square deviation is defined as follows: 1/2 ⎡ ⎛ x exp − x cal ⎞2 ⎤ ⎢ ∑im ∑nj − 1 ∑k2 ⎜ ijk exp ijk ⎟ ⎥ ⎝ xijk ⎠ ⎥ ⎢ rmsd = ⎢ ⎥ × 100 2mn ⎢ ⎥ ⎣ ⎦

■

S Supporting Information *

Tables S1−S5 listing the experimental data and calculated distribution coefficients (β) and selectivities (S), the binary interaction parameters for NRTL and UNIQUAC models, and the UNIQUAC structural parameters and Figures S1 and S2 showing the comparisons between experimental data with the UNIQUAC model. This material is available free of charge via the Internet at http://pubs.acs.org.

(5)

where m is the number of tie lines and n is the number of the components in the mixtures. The comparisons of experimental data with those obtained from the NRTL model are plotted in Figures 1 and 2. As the figures show, there is good agreement between the experimental and correlated results. The experimental data were also correlated using the UNIQUAC model:28

■

c ⎛ ∑ xjlj + qi⎜⎜1 − ln ∑ θτj ji − j=1 j=1 ⎝ c

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +98 21 44580751. Fax: +98 21 44580781. Notes

⎛Φ ⎞ ⎛θ ⎞ z ln γi = ln⎜ i ⎟ + qi ln⎜ i ⎟ + li 2 ⎝ xi ⎠ ⎝ Φi ⎠ Φ − i xi

ASSOCIATED CONTENT

The authors declare no competing financial interest. c

∑ j=1

■

⎞ ⎟ c ∑k = 1 θkτkj ⎟⎠ θτ j ij

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(6)

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Phase Behavior of Nitrate Based Ionic Liquids with Thiophene and

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