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Solvent extraction of aromatic sulfur compounds from n-heptane using the 1-ethyl-3-methylimidazolium tricyanomethanide ionic liquid
Accepted Manuscript Solvent extraction of aromatic sulfur compounds from n-heptane using the 1ethyl-3-methylimidazolium tricyanomethanide ionic liquid Marek Królikowski, Klaudia Walczak, Urszula Domańska PII: DOI: Reference:
S0021-9614(13)00209-7 http://dx.doi.org/10.1016/j.jct.2013.05.048 YJCHT 3567
To appear in:
J. Chem. Thermodynamics
Received Date: Revised Date: Accepted Date:
28 March 2013 17 May 2013 28 May 2013
Please cite this article as: M. Królikowski, K. Walczak, U. Domańska, Solvent extraction of aromatic sulfur compounds from n-heptane using the 1-ethyl-3-methylimidazolium tricyanomethanide ionic liquid, J. Chem. Thermodynamics (2013), doi: http://dx.doi.org/10.1016/j.jct.2013.05.048
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J. Chem. Thermodynamics
___________________________________________________________ M. Królikowski, K.Walczak and U. Domańska
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Solvent extraction of aromatic sulfur compounds from n-heptane using the 1-ethyl-3-methylimidazolium tricyanomethanide ionic liquid Marek Królikowskia,*, Klaudia Walczaka, Urszula Domańskaa,b a
Department of Physical Chemistry, Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland b Thermodynamic Research Unit, School of Engineering, University of KwaZulu-Natal, Howard College Campus, King George V Avenue, Durban 4041, South Africa
Keywords: Desulfurization; Extraction; (Liquid + liquid) phase equilibria; Ternary system; [EMIM][TCM]; Ionic liquid;
Received: 26 march 2013;
* To whom the correspondence should be addressed. E-mail: [email protected]. ___________________________________________________________ M. Królikowski, K.Walczak and U. Domańska
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Abstract
The ionic liquid 1-ethyl-3-methylimidazolium tricyanomethanide ([EMIM][TCM]) has been tested as a solvent for the separation of sulfur compounds from aliphatic hydrocarbon. Liquid–liquid phase equilibrium data have been determined for ternary systems containing the ionic liquid, thiophene or benzothiophene and n-heptane. The influence of temperature on the separation of thiophene from n-heptane was determined. High solubility of sulfur compounds and practical immiscibility of aliphatic hydrocarbon in ionic liquid have been found. The values of selectivity and solute distribution ratios have been calculated for all systems and compared with literature data for other 1-ethyl-3-methylimidazolium-based ionic liquids. High values of selectivity were obtained. The experimental data were correlated using the NRTL equation, and the binary interaction parameters have been reported. The phase equilibria diagrams for the ternary mixtures including the experimental and calculated tielines have been presented.
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1. Introduction
Fuel combustion with a high content of aliphatic and aromatic sulfur compounds i.e. mercaptanes, sulfides, disulfides, thiophenes, benzothiophenes, dibenzothiophenes and their alkyl derivatives generates an excessive emission of pollutants, sulfur oxides (SOx) to the atmosphere. SOx is responsible for the formation of acid rain,corrosion of metal structures and is harmful to people and other living organisms. Acid rain causes limited damage to any cities or industrial complexes, but the damage can spread across wide geographic areas if a large amount of acid rain is released. Acidification of lakes, surface waters, groundwater can do damage to aquatic creatures, forests and farming lands. Furthermore high contents of SOx in exhaust fumes lowers the efficiency of catalytic converters in cars. Sulfuric oxides also poison catalysts in catalytic converters used for reducing CO and NOx emissions and this severely affects environment [1]. In order to minimize SOx emission, limits on the sulfur content of fuels: in USA to a maximum of 15 ppm since 2010 [2] and to 10 ppm in Europe since 2009 [3] were introduced. For the past several years the hydrodesulfurization (HDS) process is used on industrial scale to eliminate sulfur compounds [4]. The HDS process uses catalysts for the conversion of organic sulfur to H2S and hydrocarbons. This process is efficient for the removal of thiols, sulfides and thiophenes, but less effective for removing refractory sulfur compounds such as benzothiophene, dibenzothiophene and their alkyl derivatives [5]. The extraction, bio-desulfurization,
reactive or nondestructive adsorption, N-adsorption,
oxidation, alkylation and complexation process can be an alternative for desulphurization of fuel. Among them, the extractive desulfurization (EDS) is considered to be the most attractive [1, 6]. EDS process can be carried out under mild conditions at low energy consumption and without hydrogen consumption.
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Ionic liquids (ILs) with their specific properties such as negligible vapour pressure and the high selectivity in the separation of aromatic sulfur compound from aliphatic hydrocarbons, are considered to be very interesting for new technologies including the EDS process. ILs can successfully replace the conventional volatile and often toxic organic solvents. Therefore, systematic investigations of ternary systems containing ionic liquid, sulfur compound, and aliphatic hydrocarbon as a model of fuel are of great importance. Over the past few years, considerable amount of experimental work has been done using imidazolium-based ionic liquids with different alkyl groups substituted on the ring and different anions: ([EMIM][NTf2] + thiophene + n-heptane) at T = 298.15 K [7], ([EMIM][NTf2] + thiophene + hexane) at T = 298.15 K [8], ([EMIM][SCN] or [DMIM][MP] + thiophene + n-heptane) at T = 298.15 K and 303.15 K [9], ([EMIM][EtSO4] + thiophene + hexane, n-heptane, dodecane, or hexadecane) at T = 298.15 K [10], ([BMIM][OTf] + thiophene + n-heptane) at T = 298.15 K [11], ([OMIM][NTf2] + thiophene + hexane, n-heptane or hexadecane) at T = 298.15 K [12], ([OMIM][NTf2] + thiophene + dodecane) at T = 298.15 K [13], ([OMIM][BF4] + thiophene + n-heptane, dodecane or hexadecane) at T = 298.15 K [14]. The selectivity (S) determine the ability of IL to effectively extraction. The highest selectivity for thiophene / n-heptane extraction process at T = 298.15 K were obtained for 1-ethyl-3methylimidazolium thiocyanate [EMIM][SCN] S = 1598 and 1,3-dimethylimidazolium methylphosphonate [DMIM][MP] S = 1756. In our laboratory, the liquid-liquid phase equilibria for ternary systems (different ILs + thiophene + n-heptane) such as : ([BMPYR][FAP], [BMPYR][TCB] or [BMPYR][TCM] + thiophene + n-heptane) at T = 298.15 K [15], ([COC2mMOR][FAP], [COC2mPIP][FAP] or [COC2mPYR][FAP] + thiophene + n-heptane) at T = 298.15 K [16], [COC2mMOR][NTf2],
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[COC2mPIP][ NTf2] or [COC2mPYR] [NTf2] + thiophene + n-heptane) at T = 298.15 K [17] have been measured. Recently, 1-ethyl-3-methylimidazolium tricyanomethanide [EMIM][TCM], studied in this work, was tested in the separation of toluene from n-heptane [18]. The values of selectivity for the separation were in the range of (18.9 – 47.0) at T = 313.2 K. It is favorable and suggests good results for the extraction of aromatic sulfur compounds from n-heptane. Furthermore Verevkin and Heintz determined the enthalpy of vaporization and molar enthalpy of formation of [EMIM][TCM] using quartz crystal microbalance – QCM technique and combustion calorimetry, respectively [19]. Studies of vaporization enthalpies and vapor pressures of ILs are scarce because of obvious problems with measuring the extremely low vapor pressures and their temperature dependencies. The combination of vaporization enthalpies obtained with the results from combustion calorimetry has allowed to determine gaseous molar enthalpies of formation, which enables additionally characterize the thermodynamic properties of [EMIM][TCM]. In this work, the liquid-liquid phase equilibrium (LLE) data for the ternary systems of {[EMIM][TCM] (1) + thiophene or benzothiophene (2) + n-heptane (3)} were determined at T = 298.15 K (system with thiophene)
and T = 308.15 K (system with thiophene or
benzothiophene) and pressure p = 0.1 MPa. From the experimental results, the values of selectivity S and the solute distribution ratio β were calculated and compared with the values of S and β of other imidazolium-based ILs. The NRTL thermodynamic model [20] was used to correlate the experimental data for the studied ternary systems.
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2. Experimental
2.1. Materials
The
studied
ionic
liquid,
1-ethyl-3-methylimidazolium
tricyanomethanide,
[EMIM][TCM] (CAS No. 666823-18-3) was supplied by IoLiTec - Ionic Liquids Technologies GmbH, Germany and was reported to have a mass fraction purity of > 98%. Immediately before measuring the ionic liquid was degassed using the ultrasonic bath and dried under very low pressure of about 5⋅10-3 Pa at a temperature T = 343 K for about 48 hours. This procedure was used to remove any volatile chemicals and water from the ionic liquid. The structure of the investigated IL is presented in table 1. Thiophene (Aldrich, CAS No. 110-02-1, grade >0.998 mass fraction), benzothiophene (Aldrich, CAS No. 95-15-8, grade >0.995 mass fraction), n-heptane (Sigma-Aldrich, CAS No. 142-82-5, grade >0.998 mass fraction), 1-propanol (Sigma-Aldrich, CAS No. 71-23-8, grade >0.999 mass fraction) and acetone (POCH, CAS No. 67-64-1 pure p. a., >0.997 mass fraction) were used as received and without further purification. Purity was tested by gas chromatography (GC) analysis which did not detect any appreciable peaks of impurities.
2.2. Apparatus and procedure
All weighing involved in the experimental work was carried out using a Mettler Toledo AB 204-S balance, with a precision of ±1·10−4 g. Water content was analyzed by Karl-Fisher titration method (model SCHOTT Instruments TitroLine KF). Samples of [EMIM][TCM] and the solvents were dissolved in
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methanol (POCH, pure p.a.) and titrated using CombiTitrant 2 no. 18802 (Merck) with steps of 0.0025 cm3. Measurement for all substances was performed three times and the analysis have shown that the water content was less than 680 ppm for [EMIM][TCM] and 70 ppm for solvents. Density of [EMIM][TCM] was measured using an Anton Paar GmbH 4500 M vibrating-tube densimeter (Graz, Austria) with precise to within ±1·10−5 g · cm−3, and the uncertainty of the measurements was estimated to be better than ±1·10−4 g · cm−3. Two integrated Pt 100 platinum thermometers provided good precision in temperature control internally (T ± 0.01 K). Densimeter includes an automatic correction for the viscosity of the sample. The experimental data compared with literature [18] are presented in table 2. For the determination of the experimental LLE tie-lines, mixtures with compositions inside the immiscible region of the systems were introduced into a jacketed glass cell with a volume of 10 mL, together with a coated magnetic stirring bar, and properly closed to avoid losses by evaporation or pickup of moisture from the atmosphere. The jackets were connected to a thermostatic water bath (LAUDA Alpha) to maintain a constant temperature of T = (298.15 or 308.15) K in the vessels. The uncertainty of temperature measurements was ±0.05 K. The mixtures were stirred for 6 h to reach the thermodynamic equilibrium. The time needed for the separation of two phases in the thermodynamic equilibrium was visually very short, but allowed to settle for overnight to guarantee that the equilibrium state was
completely reached. After phase separation, approximately (0.1-0.3) cm3 samples from both phases in equilibrium were taken using glass syringes with coupled stainless steel needles, without disturbance of the interface. Sample of the phase was placed in an ampoule with a capacity of 2 cm3. The ampoule closed with septum cap. Acetone (1.0 cm3) was added to the samples to avoid phase splitting and to maintain a homogeneous mixture. 1-Propanol was used as internal standard for the GC-analysis. Ionic liquid cannot be analyzed by GC, ___________________________________________________________ M. Królikowski, K.Walczak and U. Domańska
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therefore, only thiophene, benzothiophene and n-heptane were analyzed. For a ternary mixture, only two components need to be analyzed; the mass fraction of the third component (ionic liquid) is then determined by subtracting the mol fractions of the two other components from one. The composition was analyzed by gas chromatography (PerkinElmer Clarus 580 GC equipped with auto sampler and FID and TCD detectors). The capillary column of the chromatograph was protected with an pre-column to avoid the non-volatile ionic liquid reaching the column in case of leak from the glass wool in the liner. The TotalChrom Workstation software was used to obtain the chromatographic areas for the thiophene, benzothiophene, n-heptane and internal standard, 1-propanol. All samples were injected three times, and the average value was calculated. Details of the operational conditions of the apparatus are reported in table 3. The estimated uncertainty in the determination of mole fraction compositions is ±0.003 for compositions of the hydrocarbon-rich phase and ±0.005 for compositions of IL-rich phase.
3. Results and discussion
The composition of the experimental tie-lines for the ternary systems {[EMIM][TCM] (1) + thiophene (2) + n-heptane (3)} at T = 298.15 K and T = 308.15 K and {[EMIM][TCM] (1) + benzothiophene (2) + n-heptane (3)} at T = 308.15 K are reported in tables 4-5, respectively. The corresponding triangular diagrams with the experimental tie-lines for each system are shown in figures 1-3. All systems presented in this work are type II since [EMIM][TCM] is partially miscible with other components and there is only one immiscibility region [21].
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In the {[EMIM][TCM] (1) + thiophene (2)} binary system, the complete solubility was observed from mole fraction x1 = 0.187 at T = 298.15 K and x1 = 0.195 at T = 308.15 K; For binary systems {imidazolium-based IL with thiocyanate anion + thiophene} it was observed that the solubility decreases with an increasing temperature [22]. The solubility of n-heptane in the [EMIM][TCM] was very low. The high immiscibility gap in the range of (0 to 0.991) IL mole fraction was detected. The solubility dependence on temperature is very small. The diagrams show that the zone of immiscibility is wide and decreases when changing systems from thiophene to benzothiophene. This is due to the stronger π-π interaction between IL and benzothiophene than with thiophene. Through analysis of the composition of the upper (hydrocarbon-rich) phase, the ionic liquid was not observed. Nevertheless, a very small amount may be present in this phase, but the content of the ionic liquid is too small for this analysis. In our previous work [23], [BM4Py][TOS] in the binary system with hexane at the mole fraction xIL = 10-5 was observed. The feasibility of using the ionic liquid as a solvent to perform the extraction of sulphur compound such as thiophene, benzothiophene from an aliphatic hydrocarbons was evaluated by two parameters: selectivity S and solute distribution ratio β. These parameters reported in tables 4 and 5, together with the LLE data are calculated from experimental data according to equations:
S=
x2II x3I x2I x3II
(1)
β=
x2II x2I
(2)
where x is the mole fraction; superscripts I and II refer to the hydrocarbon – rich phase (raffinate) and ionic liquid – rich phase (extract), respectively. Subscripts 2 and 3 refer to sulphur compound (thiophene or benzothiophene) and n-heptane, respectively. Figure 4 shows the selectivity as a function of the mole fraction of solute in the raffinate. The values ___________________________________________________________ M. Królikowski, K.Walczak and U. Domańska
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for analogous systems {[EMIM][NTf2] (1) + thiophene (2) + n-heptane (3)} at T = 298.15 K, {[EMIM][SCN] (1) + thiophene (2) + n-heptane (3)} at T = 298.15 K and T = 303.15 K, {[EMIM][EtSO4] (1) + thiophene (2) + n-heptane (3)} at T = 298.15 K, are also plotted, for comparison. In each case the values of selectivity decrease when the mole fraction of sulphur compound in the raffinate increases. For the ternary system with thiophene the temperature effect is modest, the highest values of selectivity that are: 233.7 for T = 298.15 K and 164.7 for T = 308.15 K were obtained for the first tie-line. It follows from the better solubility of thiophene in the ionic liquid – rich phase at a lower temperature. The values of selectivity for the ternary system {[EMIM][TCM] (1) + benzothiophene (2) + n-heptane (3)} are much higher than that for the ternary system {[EMIM][TCM] (1) + thiophene (2) + n-heptane (3)}. This is due to the fact that much better solubility of benzothiophene in the ionic liquid – rich phase was observed. The high selectivity is mainly due to the practically immiscibility of n-heptane in [EMIM][TCM] ionic liquid. The values of selectivity obtained in this work for ternary system {[EMIM][TCM] (1) + thiophene (2) + n-heptane (3)} at T = 298.15 K are two times higher than for the system with [EMIM][NTf2] [7] and very similar to that for the system with [EMIM][EtSO4] [10] at the same temperature. The highest values of selectivity were obtained for the mixture of {[EMIM][SCN] (1) + thiophene (2) + n-heptane (3)} [9]. For this system the lower values of solute distribution ratio, on an average level 0.7 were determined. Figure 5 shows the solute distribution ratio as a function of the mole fraction of the solute in the raffinate for systems with 1-ethyl-3-methylimidazolium-based ionic liquid. In the case of ILs with [TCM], [NTf2] and [EtSO4] anions the solute distribution ratio decrease with an increase of the mole fraction of the solute in the raffinate. The best values of β were obtained for systems with [EMIM][TCM]. For the {[EMIM][TCM] (1) + thiophene (2) + n-heptane (3)} the values of β were in the range of (0.8 to 2.4) at T = 298.15 K and from (0.8 to 1.9) at T = 308.15 K. Furthermore, the values of β for {[EMIM][TCM] (1) +
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benzothiophene (2) + n-heptane (3)} at T = 308.15 K were observed in the range (0.9 to 3.3). An increase of temperature resulted a slight decrease in β, especially at low mole fraction of the solute. In addition, the knowledge of the physicochemical properties such as: density and viscosity, are very important to design an extraction processes with IL. It should be noted that the density of the investigated IL at T = 298.15 K is equal to 1.08146 g·cm-3 but viscosity is very low and equal to 15.02 mPa·s [18]. The results obtained in this work suggest that [EMIM][TCM] can efficiently perform the separation of thiophene or benzothiophene from n-heptane.
4. Modeling
The correlation of the experimental data was done with the non-random liquid equation, (NRTL) developed by Renon and Prausnitz [20]. The equations and algorithms used in the calculation of the compositions of liquid phases follow the method used by Wales [24]. The equations were described by us earlier [15,25]. In this work, the value of NRTL parameter,
αij, was fixed at a value of 0.3, which has given the best results of the correlation. The correlated parameters obtained using the NTRL model along with the root mean square deviations (RMSD) are given in table 6. The RMSD values, which can be taken as a measure of the precision of the correlation, were calculated according the equation:
where x is the mole fraction; the subscript i, l and m provide a designation for the component, phase and the tie-line, respectively. The value k designates the number of tie-lines. The compositions calculated from the correlations are included in figures 1 to 3. Good correlation of
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the experimental values with the NTRL model was obtained. The RMSD values are below 0.004 for {[EMIM][TCM] (1) + thiophene (2) + n-heptane (3)} and 0.006 for {[EMIM][TCM] (1) + benzothiophene (2) + n-heptane (3)} ternary systems.
4. Conclusions
The aim of this study was to measured the experimental liquid-liquid phase equilibria for the ternary systems containing {[EMIM][TCM] (1) + thiophene (2) + n-heptane (3)} at different temperature and {[EMIM][TCM] (1) + benzothiophene (2) + n-heptane (3)} at T = 308.15 K and at ambient pressure. The values of selectivity S and solute distribution ratio β were calculated directly from the experimental data. These parameters for the studied systems are higher than for analogous systems with the same [EMIM] cation and more popular [NTf2] and [EtSO4] anions. This indicates that the ionic liquid [EMIM][TCM] is a perspective solvent for the extraction of studied aromatic sulfur compounds from n-heptane. Apart from that, the low
impact of the temperature upon the phase diagram was observed. Therefore the separation processes can be carried out at ambient temperatures, reducing the energy requirements. The NRTL model was used to correlate the experimental LLE results. In general, the LLE data of the ternary systems studied are well correlated with the NRTL model, as can be seen by comparing the experimental and the calculated data in the ternary diagrams. The RMSD values are below 0.004 for {[EMIM][TCM] (1) + thiophene (2) + n-heptane (3)} and 0.006 for {[EMIM][TCM] (1) + benzothiophene (2) + n-heptane (3)}.
Acknowledgement This work has been supported by the National Science Center project 2011/01/B/ST5/00800. M. Królikowski wishes to thank the START Program of the Foundation for Polish Science.
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TABLE 1 Structure, name, abbreviation of the investigated IL. Structure
Name, abbreviation 1-Ethyl-3-methylimidazolium tricyanomethanide,
N
+
N
N
[EMIM][TCM]
C N
C
-
C
C N
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TABLE 2 The experimental and literature [18] density (ρ) as a function of temperature at p = 0.1 MPa for 1-ethyl-3-methylimidazolium tricyanomethanide.a T/K ρexp/ (g · cm-3) ρlit/ (g · cm-3) 298.15 1.08146 1.08156 308.15 1.07441 1.07464 318.15 1.06742 1.06771 328.15 1.06050 1.06079 338.15 1.05364 1.05386 348.15 1.04685 1.04694 a Standard uncertainties u are u(T) = 0.01K, u(ρ) = 10-4 g · cm-3, u(p) = 0.5 kPa.
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TABLE 3 Operational conditions in the gas chromatograph for the compositional analysis of the phases in equilibrium. Element Columns
Oven
Injector
Detector
Characteristic Type
Flow Carrier gas Temperature program Injection volume Split ratio Temperature Type Temperature
Description Elit-5 PerkinElmer DB-5 (5% diphenyl/95% dimethyl polysiloxane), length 30 m, inner diameter 0.53 mm, film thickness: 1.5 μm Elite-Wax PerkinElmer, length 30 m, inner diameter 0.53 mm, film thickness: 1.0 μm 5 cm3 · min-1 Helium 343.15 K (for [EMIM][TCM] + thiophene + n-heptane ) 343.15 K, 8 min. → (40 K/min) 423.15 K,12min. (for [EMIM][TCM] + benzothiophene + n-heptane ) 0.0001 cm3 10:1 423 K Flame ionization detector (FID) 493 K
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TABLE 4 Compositions of experimental tie-lines, selectivity, (S) and solute distribution ratios, (β) for ternary systems {[EMIM][TCM] (1) + thiophene (2) + n-heptane (3)} at T = 298.15 K and T = 308.15 K, p = 0.1 MPa.a IL – rich phase S β II II x1 x2 T = 298.15 K 0.000 0.000 0.991 0.000 0.000 0.016 0.951 0.038 233.7 2.38 0.000 0.085 0.816 0.174 187.3 2.05 0.000 0.183 0.672 0.317 128.7 1.73 0.000 0.217 0.620 0.369 121.0 1.70 0.000 0.284 0.553 0.436 99.9 1.54 0.000 0.407 0.447 0.541 71.7 1.33 0.000 0.511 0.383 0.605 48.2 1.18 0.000 0.592 0.338 0.650 37.3 1.10 0.000 0.682 0.310 0.679 28.8 1.00 0.000 0.775 0.274 0.716 18.9 0.92 0.000 0.879 0.239 0.752 11.5 0.86 0.000 0.975 0.205 0.792 6.8 0.81 0.000 1.000 0.187 0.813 0.81 T = 308.15 K 0.000 0.000 0.990 0.000 0.000 0.066 0.861 0.128 164.7 1.94 0.000 0.150 0.730 0.258 121.8 1.72 0.000 0.220 0.639 0.349 103.1 1.59 0.000 0.291 0.570 0.418 84.9 1.44 0.000 0.344 0.513 0.475 75.5 1.38 0.000 0.418 0.460 0.527 56.4 1.26 0.000 0.523 0.391 0.595 41.7 1.14 0.000 0.602 0.351 0.637 32.4 1.06 0.000 0.611 0.345 0.642 31.4 1.05 0.000 0.681 0.316 0.672 26.2 0.99 0.000 0.699 0.313 0.674 24.2 0.96 0.000 0.790 0.279 0.710 17.2 0.90 0.000 0.920 0.233 0.760 9.4 0.83 0.000 1.000 0.195 0.805 0.81 a Standard uncertainties u are: u(x) = 0.003 for compositions of the hydrocarbon-rich phase, u(x) = 0.005 for compositions of IL-rich phase, u(T) = 0.02 K, u(p) = 0.5 kPa. Hydrocarbon – rich phase x1I x2I
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TABLE 5 Compositions of experimental tie-lines, selectivity, (S) and solute distribution ratios, (β) for ternary systems {[EMIM][TCM] (1) + benzothiophene (2) + n-heptane (3)} at T = 308.15 K,
p = 0.1 MPa.a Hydrocarbon – rich phase IL – rich phase S β I I II II x1 x2 x1 x2 0.000 0.000 0.990 0.000 0.000 0.012 0.951 0.039 321.1 3.25 0.000 0.041 0.864 0.124 241.7 3.02 0.000 0.064 0.805 0.181 189.1 2.83 0.000 0.131 0.649 0.336 148.6 2.56 0.000 0.154 0.605 0.378 122.1 2.45 0.000 0.244 0.484 0.495 76.7 2.03 0.000 0.297 0.408 0.570 64.2 1.92 0.000 0.427 0.352 0.625 36.5 1.46 0.000 0.480 0.294 0.680 28.3 1.42 0.000 0.590 0.265 0.709 18.9 1.20 0.000 0.735 0.254 0.719 9.6 0.98 0.000 0.817 0.216 0.759 6.8 0.93 0.000 0.900 0.196 0.781 3.9 0.87 0.000 1.000 0.135 0.865 0.87 a Standard uncertainties u are: u(x) = 0.003 for compositions of the hydrocarbon-rich phase, u(x) = 0.005 for compositions of IL-rich phase, u(T) = 0.02 K, u(p) = 0.5 kPa.
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TABLE 6 Binary interaction parameters and root mean square deviation (σx) for the NRTL equation for the ternary systems {[EMIM][TCM] (1) + thiophene, or benzothiophene (2) + n-heptane (3)}.a
gij/(J · mol-1) gji/(J · mol-1) RMSD σx {[EMIM][TCM] (1) + thiophene (2) + n-heptane (3)} at T = 298.15 K 12 -2229.4 29435 13 11373 12411 0.004 23 5388.1 -1772.6 {[EMIM][TCM] (1) + thiophene (2) + n-heptane (3)} at T = 308.15 K 12 -2351.4 29611 13 11658 12121 0.004 23 5648.0 -2066.5 {[EMIM][TCM] (1) + benzothiophene (2) + n-heptane (3)} at T = 308.15 K 12 478.84 37573 13 9925.5 11505 0.006 23 4064.1 1084.7 Parameter αij = 0.3 ij
a
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Captions to the figures
FIGURE 1. Plot of the experimental (●, solid lines) versus calculated with NRTL equation (○, dotted lines) for the composition tie lines of the ternary system {[EMIM][TCM] (1) + thiophene (2) + n-heptane (3)} at T = 298.15 K. FIGURE 2. Plot of the experimental (●, solid lines) versus calculated with NRTL equation (○, dotted lines) for the composition tie lines of the ternary system {[EMIM][TCM] (1) + thiophene (2) + n-heptane (3)} at T = 308.15 K. FIGURE 3. Plot of the experimental (●, solid lines) versus calculated with NRTL equation (○, dotted lines) for the composition tie lines of the ternary system {[EMIM][TCM] (1) + benzothiophene (2) + n-heptane (3)} at T = 308.15 K. FIGURE 4. Plot of the selectivity, (S) as a function of the mole fraction of solute in the hydrocarbon – rich phase for the ternary systems: ●, {[EMIM][TCM] (1) + thiophene (2) + n-heptane (3)} at T = 298.15 K; ■, {[EMIM][TCM] (1) + thiophene (2) + n-heptane (3)} at
T = 308.15 K; ▲,{[EMIM][TCM] (1) + benzothiophene (2) + n-heptane (3)} at T = 308.15 K; ○,
{[EMIM][NTf2] (1) + thiophene (2) + n-heptane (3)} at T = 298.15 K [7]; ♦,
[EMIM][SCN] (1) + thiophene (2) + n-heptane (3)} at T = 298.15 K [9]; ◊, [EMIM][SCN] (1) + thiophene (2) + n-heptane (3)} at T = 303.15 K [9]; ∆, {[EMIM][EtSO4] (1) + thiophene (2) + n-heptane (3)} at T = 298.15 K [10]. FIGURE 5. Plot of the solute distribution ratio, (β) as a function of the mole fraction of solute in the hydrocarbon – rich phase for the ternary systems: ●, {[EMIM][TCM] (1) + thiophene (2) + n-heptane (3)} at T = 298.15 K; ■, {[EMIM][TCM] (1) + thiophene (2) + n-heptane (3)} at T = 308.15 K; ▲,{[EMIM][TCM] (1) + benzothiophene (2) + n-heptane (3)} at T = 308.15 K; ○,
{[EMIM][NTf2] (1) + thiophene (2) + n-heptane (3)} at T = 298.15 K [7]; ♦,
[EMIM][SCN] (1) + thiophene (2) + n-heptane (3)} at T = 298.15 K [9]; ◊, [EMIM][SCN] (1) ___________________________________________________________ M. Królikowski, K.Walczak and U. Domańska
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+ thiophene (2) + n-heptane (3)} at T = 303.15 K [9]; ∆, {[EMIM][EtSO4] (1) + thiophene (2) + n-heptane (3)}at T = 298.15 K [10].
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FIGURE 1.
FIGURE 2.
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FIGURE 3.
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600 500
S
400 300 200 100 0 0.0
0.2
0.4
0.6
x2
0.8
1.0
I
FIGURE 4.
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3.0 2.5 2.0
β
1.5 1.0 0.5 0.0 0.0
0.2
0.4
0.6
x2
0.8
1.0
I
FIGURE 5.
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HIGHLIGHTS > LLE data for (EMIMTCM + thiophene/benzothiophene + n-heptane) were determined. > High S and β for the extraction of thiophene/benzothiophene from n-heptane was found. > Results of S and β were compared with available literature. >The NRTL model satisfactorily correlates the LLE data.
___________________________________________________________ M. Królikowski, K.Walczak and U. Domańska
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References
[1] P. S. Kulkarni, C. A. M. Afonso, Green Chem. 12 (2010) 1139-1149. [2] http://www.epa.gov/otaq/fuels/gasolinefuels/index.htm accessed May 2013. [3] http://www.dieselnet.com/standards/eu/fuel.php accessed May 2013. [4] M. Francisco, A. Arce, A. Soto, Fluid Phase Equilib. 294 (2010) 39–48. [5] I.V. Babich, J.A. Moulijn, Fuel 82 (2003) 607–631. [6] H. Zhao, S. Xia, P. Ma, J. Chem. Technol. Biotechnol. 80 (2005)1089–1096. [7] H. Rodríguez, M. Francísko, A. Soto, A. Arce, Fluid Phase Equilib. 298 (2010) 240– 245. [8] B. Rodríguez-Cabo, A. Soto, A. Arce, J. Chem. Thermodyn. 57 (2013) 248–255. [9] K. Kędra-Królik, F. Mutelet, J.N. Jaubert, Ind. Eng. Chem.. Res. 50 (2011) 2296–2306. [10] L. Alonso, A. Arce, M. Francisco, A. Soto, Fluid Phase Equilib. 270 (2008) 97–102. [11] K. Kędra-Królik, F. Mutelet, J. Ch. Moïse, J.N. Jaubert, Energy Fuels 25 (2011) 1559– 1565. [12] L. Alonso, A. Arce, M. Francisco, A. Soto, Fluid Phase Equilib. 263 (2008) 176–181. [13] L. Alonso, A. Arce, M. Francisco, A. Soto, J. Chem. Thermodyn. 40 (2008) 265–270. [14] L. Alonso, A. Arce, M. Francisco, A. Soto, J. Chem. Thermodyn. 40 (2008) 966–972. [15] U. Domańska, E.V. Lukoshko, M. Królikowski, J. Chem. Thermodyn. 61 (2013) 126– 131. [16] A. Marciniak, M. Królikowski, J. Chem. Thermodyn. 49 (2012) 154–158. [17] A. Marciniak, M. Królikowski, Fluid Phase Equilib. 321 (2012) 59– 63.
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[18] M. Larriba, P. Navarro, J. García, F. Rodríguez Ind. Eng. Chem. Res. 52 (2013) 2714– 2720. [19] V. N. Emel’yanenko, D. H. Zaitsau, S. P. Verevkin, A. Heintz, J. Phys. Chem. B 115 (2011) 11712–11717. [20] H. Renon, J.M. Prausnitz, AIChE J. 14 (1968) 135–144. [21] J. M. Sørensen, W. Arlt, Liquid–Liquid Equilibrium Data Collection, DECHEMA Chemistry Data Series, Frankfurt, 1980. [22] U. Domańska, M. Królikowski, K. Ślesińska, J. Chem. Thermodyn. 41 (2009) 1303– 1311. [23] U. Domańska, M. Królikowski, A. Pobudkowska, T. M. Letcher, J. Chem. Eng. Data 54 (2009) 1435–1441 [24] S.W. Walas, Phase Equilibria in Chemical Engineering, Butterworth Publishers: Boston, (1985). [25] U. Domańska, A. Pobudkowska, M. Królikowski, Fluid Phase Equilib. 259 (2007) 173– 179.
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HIGHLIGHTS > LLE data for (EMIMTCM + thiophene/benzothiophene + n-heptane) were determined. > High S and β for the extraction of thiophene/benzothiophene from n-heptane was found. > Results of S and β were compared with available literature. > The NRTL model satisfactorily correlates the LLE data.
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S0021-9614(13)00209-7 http://dx.doi.org/10.1016/j.jct.2013.05.048 YJCHT 3567
To appear in:
J. Chem. Thermodynamics
Received Date: Revised Date: Accepted Date:
28 March 2013 17 May 2013 28 May 2013
Please cite this article as: M. Królikowski, K. Walczak, U. Domańska, Solvent extraction of aromatic sulfur compounds from n-heptane using the 1-ethyl-3-methylimidazolium tricyanomethanide ionic liquid, J. Chem. Thermodynamics (2013), doi: http://dx.doi.org/10.1016/j.jct.2013.05.048
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J. Chem. Thermodynamics
___________________________________________________________ M. Królikowski, K.Walczak and U. Domańska
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Solvent extraction of aromatic sulfur compounds from n-heptane using the 1-ethyl-3-methylimidazolium tricyanomethanide ionic liquid Marek Królikowskia,*, Klaudia Walczaka, Urszula Domańskaa,b a
Department of Physical Chemistry, Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland b Thermodynamic Research Unit, School of Engineering, University of KwaZulu-Natal, Howard College Campus, King George V Avenue, Durban 4041, South Africa
Keywords: Desulfurization; Extraction; (Liquid + liquid) phase equilibria; Ternary system; [EMIM][TCM]; Ionic liquid;
Received: 26 march 2013;
* To whom the correspondence should be addressed. E-mail: [email protected]. ___________________________________________________________ M. Królikowski, K.Walczak and U. Domańska
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Abstract
The ionic liquid 1-ethyl-3-methylimidazolium tricyanomethanide ([EMIM][TCM]) has been tested as a solvent for the separation of sulfur compounds from aliphatic hydrocarbon. Liquid–liquid phase equilibrium data have been determined for ternary systems containing the ionic liquid, thiophene or benzothiophene and n-heptane. The influence of temperature on the separation of thiophene from n-heptane was determined. High solubility of sulfur compounds and practical immiscibility of aliphatic hydrocarbon in ionic liquid have been found. The values of selectivity and solute distribution ratios have been calculated for all systems and compared with literature data for other 1-ethyl-3-methylimidazolium-based ionic liquids. High values of selectivity were obtained. The experimental data were correlated using the NRTL equation, and the binary interaction parameters have been reported. The phase equilibria diagrams for the ternary mixtures including the experimental and calculated tielines have been presented.
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1. Introduction
Fuel combustion with a high content of aliphatic and aromatic sulfur compounds i.e. mercaptanes, sulfides, disulfides, thiophenes, benzothiophenes, dibenzothiophenes and their alkyl derivatives generates an excessive emission of pollutants, sulfur oxides (SOx) to the atmosphere. SOx is responsible for the formation of acid rain,corrosion of metal structures and is harmful to people and other living organisms. Acid rain causes limited damage to any cities or industrial complexes, but the damage can spread across wide geographic areas if a large amount of acid rain is released. Acidification of lakes, surface waters, groundwater can do damage to aquatic creatures, forests and farming lands. Furthermore high contents of SOx in exhaust fumes lowers the efficiency of catalytic converters in cars. Sulfuric oxides also poison catalysts in catalytic converters used for reducing CO and NOx emissions and this severely affects environment [1]. In order to minimize SOx emission, limits on the sulfur content of fuels: in USA to a maximum of 15 ppm since 2010 [2] and to 10 ppm in Europe since 2009 [3] were introduced. For the past several years the hydrodesulfurization (HDS) process is used on industrial scale to eliminate sulfur compounds [4]. The HDS process uses catalysts for the conversion of organic sulfur to H2S and hydrocarbons. This process is efficient for the removal of thiols, sulfides and thiophenes, but less effective for removing refractory sulfur compounds such as benzothiophene, dibenzothiophene and their alkyl derivatives [5]. The extraction, bio-desulfurization,
reactive or nondestructive adsorption, N-adsorption,
oxidation, alkylation and complexation process can be an alternative for desulphurization of fuel. Among them, the extractive desulfurization (EDS) is considered to be the most attractive [1, 6]. EDS process can be carried out under mild conditions at low energy consumption and without hydrogen consumption.
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Ionic liquids (ILs) with their specific properties such as negligible vapour pressure and the high selectivity in the separation of aromatic sulfur compound from aliphatic hydrocarbons, are considered to be very interesting for new technologies including the EDS process. ILs can successfully replace the conventional volatile and often toxic organic solvents. Therefore, systematic investigations of ternary systems containing ionic liquid, sulfur compound, and aliphatic hydrocarbon as a model of fuel are of great importance. Over the past few years, considerable amount of experimental work has been done using imidazolium-based ionic liquids with different alkyl groups substituted on the ring and different anions: ([EMIM][NTf2] + thiophene + n-heptane) at T = 298.15 K [7], ([EMIM][NTf2] + thiophene + hexane) at T = 298.15 K [8], ([EMIM][SCN] or [DMIM][MP] + thiophene + n-heptane) at T = 298.15 K and 303.15 K [9], ([EMIM][EtSO4] + thiophene + hexane, n-heptane, dodecane, or hexadecane) at T = 298.15 K [10], ([BMIM][OTf] + thiophene + n-heptane) at T = 298.15 K [11], ([OMIM][NTf2] + thiophene + hexane, n-heptane or hexadecane) at T = 298.15 K [12], ([OMIM][NTf2] + thiophene + dodecane) at T = 298.15 K [13], ([OMIM][BF4] + thiophene + n-heptane, dodecane or hexadecane) at T = 298.15 K [14]. The selectivity (S) determine the ability of IL to effectively extraction. The highest selectivity for thiophene / n-heptane extraction process at T = 298.15 K were obtained for 1-ethyl-3methylimidazolium thiocyanate [EMIM][SCN] S = 1598 and 1,3-dimethylimidazolium methylphosphonate [DMIM][MP] S = 1756. In our laboratory, the liquid-liquid phase equilibria for ternary systems (different ILs + thiophene + n-heptane) such as : ([BMPYR][FAP], [BMPYR][TCB] or [BMPYR][TCM] + thiophene + n-heptane) at T = 298.15 K [15], ([COC2mMOR][FAP], [COC2mPIP][FAP] or [COC2mPYR][FAP] + thiophene + n-heptane) at T = 298.15 K [16], [COC2mMOR][NTf2],
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[COC2mPIP][ NTf2] or [COC2mPYR] [NTf2] + thiophene + n-heptane) at T = 298.15 K [17] have been measured. Recently, 1-ethyl-3-methylimidazolium tricyanomethanide [EMIM][TCM], studied in this work, was tested in the separation of toluene from n-heptane [18]. The values of selectivity for the separation were in the range of (18.9 – 47.0) at T = 313.2 K. It is favorable and suggests good results for the extraction of aromatic sulfur compounds from n-heptane. Furthermore Verevkin and Heintz determined the enthalpy of vaporization and molar enthalpy of formation of [EMIM][TCM] using quartz crystal microbalance – QCM technique and combustion calorimetry, respectively [19]. Studies of vaporization enthalpies and vapor pressures of ILs are scarce because of obvious problems with measuring the extremely low vapor pressures and their temperature dependencies. The combination of vaporization enthalpies obtained with the results from combustion calorimetry has allowed to determine gaseous molar enthalpies of formation, which enables additionally characterize the thermodynamic properties of [EMIM][TCM]. In this work, the liquid-liquid phase equilibrium (LLE) data for the ternary systems of {[EMIM][TCM] (1) + thiophene or benzothiophene (2) + n-heptane (3)} were determined at T = 298.15 K (system with thiophene)
and T = 308.15 K (system with thiophene or
benzothiophene) and pressure p = 0.1 MPa. From the experimental results, the values of selectivity S and the solute distribution ratio β were calculated and compared with the values of S and β of other imidazolium-based ILs. The NRTL thermodynamic model [20] was used to correlate the experimental data for the studied ternary systems.
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2. Experimental
2.1. Materials
The
studied
ionic
liquid,
1-ethyl-3-methylimidazolium
tricyanomethanide,
[EMIM][TCM] (CAS No. 666823-18-3) was supplied by IoLiTec - Ionic Liquids Technologies GmbH, Germany and was reported to have a mass fraction purity of > 98%. Immediately before measuring the ionic liquid was degassed using the ultrasonic bath and dried under very low pressure of about 5⋅10-3 Pa at a temperature T = 343 K for about 48 hours. This procedure was used to remove any volatile chemicals and water from the ionic liquid. The structure of the investigated IL is presented in table 1. Thiophene (Aldrich, CAS No. 110-02-1, grade >0.998 mass fraction), benzothiophene (Aldrich, CAS No. 95-15-8, grade >0.995 mass fraction), n-heptane (Sigma-Aldrich, CAS No. 142-82-5, grade >0.998 mass fraction), 1-propanol (Sigma-Aldrich, CAS No. 71-23-8, grade >0.999 mass fraction) and acetone (POCH, CAS No. 67-64-1 pure p. a., >0.997 mass fraction) were used as received and without further purification. Purity was tested by gas chromatography (GC) analysis which did not detect any appreciable peaks of impurities.
2.2. Apparatus and procedure
All weighing involved in the experimental work was carried out using a Mettler Toledo AB 204-S balance, with a precision of ±1·10−4 g. Water content was analyzed by Karl-Fisher titration method (model SCHOTT Instruments TitroLine KF). Samples of [EMIM][TCM] and the solvents were dissolved in
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methanol (POCH, pure p.a.) and titrated using CombiTitrant 2 no. 18802 (Merck) with steps of 0.0025 cm3. Measurement for all substances was performed three times and the analysis have shown that the water content was less than 680 ppm for [EMIM][TCM] and 70 ppm for solvents. Density of [EMIM][TCM] was measured using an Anton Paar GmbH 4500 M vibrating-tube densimeter (Graz, Austria) with precise to within ±1·10−5 g · cm−3, and the uncertainty of the measurements was estimated to be better than ±1·10−4 g · cm−3. Two integrated Pt 100 platinum thermometers provided good precision in temperature control internally (T ± 0.01 K). Densimeter includes an automatic correction for the viscosity of the sample. The experimental data compared with literature [18] are presented in table 2. For the determination of the experimental LLE tie-lines, mixtures with compositions inside the immiscible region of the systems were introduced into a jacketed glass cell with a volume of 10 mL, together with a coated magnetic stirring bar, and properly closed to avoid losses by evaporation or pickup of moisture from the atmosphere. The jackets were connected to a thermostatic water bath (LAUDA Alpha) to maintain a constant temperature of T = (298.15 or 308.15) K in the vessels. The uncertainty of temperature measurements was ±0.05 K. The mixtures were stirred for 6 h to reach the thermodynamic equilibrium. The time needed for the separation of two phases in the thermodynamic equilibrium was visually very short, but allowed to settle for overnight to guarantee that the equilibrium state was
completely reached. After phase separation, approximately (0.1-0.3) cm3 samples from both phases in equilibrium were taken using glass syringes with coupled stainless steel needles, without disturbance of the interface. Sample of the phase was placed in an ampoule with a capacity of 2 cm3. The ampoule closed with septum cap. Acetone (1.0 cm3) was added to the samples to avoid phase splitting and to maintain a homogeneous mixture. 1-Propanol was used as internal standard for the GC-analysis. Ionic liquid cannot be analyzed by GC, ___________________________________________________________ M. Królikowski, K.Walczak and U. Domańska
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therefore, only thiophene, benzothiophene and n-heptane were analyzed. For a ternary mixture, only two components need to be analyzed; the mass fraction of the third component (ionic liquid) is then determined by subtracting the mol fractions of the two other components from one. The composition was analyzed by gas chromatography (PerkinElmer Clarus 580 GC equipped with auto sampler and FID and TCD detectors). The capillary column of the chromatograph was protected with an pre-column to avoid the non-volatile ionic liquid reaching the column in case of leak from the glass wool in the liner. The TotalChrom Workstation software was used to obtain the chromatographic areas for the thiophene, benzothiophene, n-heptane and internal standard, 1-propanol. All samples were injected three times, and the average value was calculated. Details of the operational conditions of the apparatus are reported in table 3. The estimated uncertainty in the determination of mole fraction compositions is ±0.003 for compositions of the hydrocarbon-rich phase and ±0.005 for compositions of IL-rich phase.
3. Results and discussion
The composition of the experimental tie-lines for the ternary systems {[EMIM][TCM] (1) + thiophene (2) + n-heptane (3)} at T = 298.15 K and T = 308.15 K and {[EMIM][TCM] (1) + benzothiophene (2) + n-heptane (3)} at T = 308.15 K are reported in tables 4-5, respectively. The corresponding triangular diagrams with the experimental tie-lines for each system are shown in figures 1-3. All systems presented in this work are type II since [EMIM][TCM] is partially miscible with other components and there is only one immiscibility region [21].
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In the {[EMIM][TCM] (1) + thiophene (2)} binary system, the complete solubility was observed from mole fraction x1 = 0.187 at T = 298.15 K and x1 = 0.195 at T = 308.15 K; For binary systems {imidazolium-based IL with thiocyanate anion + thiophene} it was observed that the solubility decreases with an increasing temperature [22]. The solubility of n-heptane in the [EMIM][TCM] was very low. The high immiscibility gap in the range of (0 to 0.991) IL mole fraction was detected. The solubility dependence on temperature is very small. The diagrams show that the zone of immiscibility is wide and decreases when changing systems from thiophene to benzothiophene. This is due to the stronger π-π interaction between IL and benzothiophene than with thiophene. Through analysis of the composition of the upper (hydrocarbon-rich) phase, the ionic liquid was not observed. Nevertheless, a very small amount may be present in this phase, but the content of the ionic liquid is too small for this analysis. In our previous work [23], [BM4Py][TOS] in the binary system with hexane at the mole fraction xIL = 10-5 was observed. The feasibility of using the ionic liquid as a solvent to perform the extraction of sulphur compound such as thiophene, benzothiophene from an aliphatic hydrocarbons was evaluated by two parameters: selectivity S and solute distribution ratio β. These parameters reported in tables 4 and 5, together with the LLE data are calculated from experimental data according to equations:
S=
x2II x3I x2I x3II
(1)
β=
x2II x2I
(2)
where x is the mole fraction; superscripts I and II refer to the hydrocarbon – rich phase (raffinate) and ionic liquid – rich phase (extract), respectively. Subscripts 2 and 3 refer to sulphur compound (thiophene or benzothiophene) and n-heptane, respectively. Figure 4 shows the selectivity as a function of the mole fraction of solute in the raffinate. The values ___________________________________________________________ M. Królikowski, K.Walczak and U. Domańska
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for analogous systems {[EMIM][NTf2] (1) + thiophene (2) + n-heptane (3)} at T = 298.15 K, {[EMIM][SCN] (1) + thiophene (2) + n-heptane (3)} at T = 298.15 K and T = 303.15 K, {[EMIM][EtSO4] (1) + thiophene (2) + n-heptane (3)} at T = 298.15 K, are also plotted, for comparison. In each case the values of selectivity decrease when the mole fraction of sulphur compound in the raffinate increases. For the ternary system with thiophene the temperature effect is modest, the highest values of selectivity that are: 233.7 for T = 298.15 K and 164.7 for T = 308.15 K were obtained for the first tie-line. It follows from the better solubility of thiophene in the ionic liquid – rich phase at a lower temperature. The values of selectivity for the ternary system {[EMIM][TCM] (1) + benzothiophene (2) + n-heptane (3)} are much higher than that for the ternary system {[EMIM][TCM] (1) + thiophene (2) + n-heptane (3)}. This is due to the fact that much better solubility of benzothiophene in the ionic liquid – rich phase was observed. The high selectivity is mainly due to the practically immiscibility of n-heptane in [EMIM][TCM] ionic liquid. The values of selectivity obtained in this work for ternary system {[EMIM][TCM] (1) + thiophene (2) + n-heptane (3)} at T = 298.15 K are two times higher than for the system with [EMIM][NTf2] [7] and very similar to that for the system with [EMIM][EtSO4] [10] at the same temperature. The highest values of selectivity were obtained for the mixture of {[EMIM][SCN] (1) + thiophene (2) + n-heptane (3)} [9]. For this system the lower values of solute distribution ratio, on an average level 0.7 were determined. Figure 5 shows the solute distribution ratio as a function of the mole fraction of the solute in the raffinate for systems with 1-ethyl-3-methylimidazolium-based ionic liquid. In the case of ILs with [TCM], [NTf2] and [EtSO4] anions the solute distribution ratio decrease with an increase of the mole fraction of the solute in the raffinate. The best values of β were obtained for systems with [EMIM][TCM]. For the {[EMIM][TCM] (1) + thiophene (2) + n-heptane (3)} the values of β were in the range of (0.8 to 2.4) at T = 298.15 K and from (0.8 to 1.9) at T = 308.15 K. Furthermore, the values of β for {[EMIM][TCM] (1) +
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benzothiophene (2) + n-heptane (3)} at T = 308.15 K were observed in the range (0.9 to 3.3). An increase of temperature resulted a slight decrease in β, especially at low mole fraction of the solute. In addition, the knowledge of the physicochemical properties such as: density and viscosity, are very important to design an extraction processes with IL. It should be noted that the density of the investigated IL at T = 298.15 K is equal to 1.08146 g·cm-3 but viscosity is very low and equal to 15.02 mPa·s [18]. The results obtained in this work suggest that [EMIM][TCM] can efficiently perform the separation of thiophene or benzothiophene from n-heptane.
4. Modeling
The correlation of the experimental data was done with the non-random liquid equation, (NRTL) developed by Renon and Prausnitz [20]. The equations and algorithms used in the calculation of the compositions of liquid phases follow the method used by Wales [24]. The equations were described by us earlier [15,25]. In this work, the value of NRTL parameter,
αij, was fixed at a value of 0.3, which has given the best results of the correlation. The correlated parameters obtained using the NTRL model along with the root mean square deviations (RMSD) are given in table 6. The RMSD values, which can be taken as a measure of the precision of the correlation, were calculated according the equation:
where x is the mole fraction; the subscript i, l and m provide a designation for the component, phase and the tie-line, respectively. The value k designates the number of tie-lines. The compositions calculated from the correlations are included in figures 1 to 3. Good correlation of
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the experimental values with the NTRL model was obtained. The RMSD values are below 0.004 for {[EMIM][TCM] (1) + thiophene (2) + n-heptane (3)} and 0.006 for {[EMIM][TCM] (1) + benzothiophene (2) + n-heptane (3)} ternary systems.
4. Conclusions
The aim of this study was to measured the experimental liquid-liquid phase equilibria for the ternary systems containing {[EMIM][TCM] (1) + thiophene (2) + n-heptane (3)} at different temperature and {[EMIM][TCM] (1) + benzothiophene (2) + n-heptane (3)} at T = 308.15 K and at ambient pressure. The values of selectivity S and solute distribution ratio β were calculated directly from the experimental data. These parameters for the studied systems are higher than for analogous systems with the same [EMIM] cation and more popular [NTf2] and [EtSO4] anions. This indicates that the ionic liquid [EMIM][TCM] is a perspective solvent for the extraction of studied aromatic sulfur compounds from n-heptane. Apart from that, the low
impact of the temperature upon the phase diagram was observed. Therefore the separation processes can be carried out at ambient temperatures, reducing the energy requirements. The NRTL model was used to correlate the experimental LLE results. In general, the LLE data of the ternary systems studied are well correlated with the NRTL model, as can be seen by comparing the experimental and the calculated data in the ternary diagrams. The RMSD values are below 0.004 for {[EMIM][TCM] (1) + thiophene (2) + n-heptane (3)} and 0.006 for {[EMIM][TCM] (1) + benzothiophene (2) + n-heptane (3)}.
Acknowledgement This work has been supported by the National Science Center project 2011/01/B/ST5/00800. M. Królikowski wishes to thank the START Program of the Foundation for Polish Science.
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TABLE 1 Structure, name, abbreviation of the investigated IL. Structure
Name, abbreviation 1-Ethyl-3-methylimidazolium tricyanomethanide,
N
+
N
N
[EMIM][TCM]
C N
C
-
C
C N
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TABLE 2 The experimental and literature [18] density (ρ) as a function of temperature at p = 0.1 MPa for 1-ethyl-3-methylimidazolium tricyanomethanide.a T/K ρexp/ (g · cm-3) ρlit/ (g · cm-3) 298.15 1.08146 1.08156 308.15 1.07441 1.07464 318.15 1.06742 1.06771 328.15 1.06050 1.06079 338.15 1.05364 1.05386 348.15 1.04685 1.04694 a Standard uncertainties u are u(T) = 0.01K, u(ρ) = 10-4 g · cm-3, u(p) = 0.5 kPa.
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TABLE 3 Operational conditions in the gas chromatograph for the compositional analysis of the phases in equilibrium. Element Columns
Oven
Injector
Detector
Characteristic Type
Flow Carrier gas Temperature program Injection volume Split ratio Temperature Type Temperature
Description Elit-5 PerkinElmer DB-5 (5% diphenyl/95% dimethyl polysiloxane), length 30 m, inner diameter 0.53 mm, film thickness: 1.5 μm Elite-Wax PerkinElmer, length 30 m, inner diameter 0.53 mm, film thickness: 1.0 μm 5 cm3 · min-1 Helium 343.15 K (for [EMIM][TCM] + thiophene + n-heptane ) 343.15 K, 8 min. → (40 K/min) 423.15 K,12min. (for [EMIM][TCM] + benzothiophene + n-heptane ) 0.0001 cm3 10:1 423 K Flame ionization detector (FID) 493 K
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TABLE 4 Compositions of experimental tie-lines, selectivity, (S) and solute distribution ratios, (β) for ternary systems {[EMIM][TCM] (1) + thiophene (2) + n-heptane (3)} at T = 298.15 K and T = 308.15 K, p = 0.1 MPa.a IL – rich phase S β II II x1 x2 T = 298.15 K 0.000 0.000 0.991 0.000 0.000 0.016 0.951 0.038 233.7 2.38 0.000 0.085 0.816 0.174 187.3 2.05 0.000 0.183 0.672 0.317 128.7 1.73 0.000 0.217 0.620 0.369 121.0 1.70 0.000 0.284 0.553 0.436 99.9 1.54 0.000 0.407 0.447 0.541 71.7 1.33 0.000 0.511 0.383 0.605 48.2 1.18 0.000 0.592 0.338 0.650 37.3 1.10 0.000 0.682 0.310 0.679 28.8 1.00 0.000 0.775 0.274 0.716 18.9 0.92 0.000 0.879 0.239 0.752 11.5 0.86 0.000 0.975 0.205 0.792 6.8 0.81 0.000 1.000 0.187 0.813 0.81 T = 308.15 K 0.000 0.000 0.990 0.000 0.000 0.066 0.861 0.128 164.7 1.94 0.000 0.150 0.730 0.258 121.8 1.72 0.000 0.220 0.639 0.349 103.1 1.59 0.000 0.291 0.570 0.418 84.9 1.44 0.000 0.344 0.513 0.475 75.5 1.38 0.000 0.418 0.460 0.527 56.4 1.26 0.000 0.523 0.391 0.595 41.7 1.14 0.000 0.602 0.351 0.637 32.4 1.06 0.000 0.611 0.345 0.642 31.4 1.05 0.000 0.681 0.316 0.672 26.2 0.99 0.000 0.699 0.313 0.674 24.2 0.96 0.000 0.790 0.279 0.710 17.2 0.90 0.000 0.920 0.233 0.760 9.4 0.83 0.000 1.000 0.195 0.805 0.81 a Standard uncertainties u are: u(x) = 0.003 for compositions of the hydrocarbon-rich phase, u(x) = 0.005 for compositions of IL-rich phase, u(T) = 0.02 K, u(p) = 0.5 kPa. Hydrocarbon – rich phase x1I x2I
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TABLE 5 Compositions of experimental tie-lines, selectivity, (S) and solute distribution ratios, (β) for ternary systems {[EMIM][TCM] (1) + benzothiophene (2) + n-heptane (3)} at T = 308.15 K,
p = 0.1 MPa.a Hydrocarbon – rich phase IL – rich phase S β I I II II x1 x2 x1 x2 0.000 0.000 0.990 0.000 0.000 0.012 0.951 0.039 321.1 3.25 0.000 0.041 0.864 0.124 241.7 3.02 0.000 0.064 0.805 0.181 189.1 2.83 0.000 0.131 0.649 0.336 148.6 2.56 0.000 0.154 0.605 0.378 122.1 2.45 0.000 0.244 0.484 0.495 76.7 2.03 0.000 0.297 0.408 0.570 64.2 1.92 0.000 0.427 0.352 0.625 36.5 1.46 0.000 0.480 0.294 0.680 28.3 1.42 0.000 0.590 0.265 0.709 18.9 1.20 0.000 0.735 0.254 0.719 9.6 0.98 0.000 0.817 0.216 0.759 6.8 0.93 0.000 0.900 0.196 0.781 3.9 0.87 0.000 1.000 0.135 0.865 0.87 a Standard uncertainties u are: u(x) = 0.003 for compositions of the hydrocarbon-rich phase, u(x) = 0.005 for compositions of IL-rich phase, u(T) = 0.02 K, u(p) = 0.5 kPa.
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TABLE 6 Binary interaction parameters and root mean square deviation (σx) for the NRTL equation for the ternary systems {[EMIM][TCM] (1) + thiophene, or benzothiophene (2) + n-heptane (3)}.a
gij/(J · mol-1) gji/(J · mol-1) RMSD σx {[EMIM][TCM] (1) + thiophene (2) + n-heptane (3)} at T = 298.15 K 12 -2229.4 29435 13 11373 12411 0.004 23 5388.1 -1772.6 {[EMIM][TCM] (1) + thiophene (2) + n-heptane (3)} at T = 308.15 K 12 -2351.4 29611 13 11658 12121 0.004 23 5648.0 -2066.5 {[EMIM][TCM] (1) + benzothiophene (2) + n-heptane (3)} at T = 308.15 K 12 478.84 37573 13 9925.5 11505 0.006 23 4064.1 1084.7 Parameter αij = 0.3 ij
a
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Captions to the figures
FIGURE 1. Plot of the experimental (●, solid lines) versus calculated with NRTL equation (○, dotted lines) for the composition tie lines of the ternary system {[EMIM][TCM] (1) + thiophene (2) + n-heptane (3)} at T = 298.15 K. FIGURE 2. Plot of the experimental (●, solid lines) versus calculated with NRTL equation (○, dotted lines) for the composition tie lines of the ternary system {[EMIM][TCM] (1) + thiophene (2) + n-heptane (3)} at T = 308.15 K. FIGURE 3. Plot of the experimental (●, solid lines) versus calculated with NRTL equation (○, dotted lines) for the composition tie lines of the ternary system {[EMIM][TCM] (1) + benzothiophene (2) + n-heptane (3)} at T = 308.15 K. FIGURE 4. Plot of the selectivity, (S) as a function of the mole fraction of solute in the hydrocarbon – rich phase for the ternary systems: ●, {[EMIM][TCM] (1) + thiophene (2) + n-heptane (3)} at T = 298.15 K; ■, {[EMIM][TCM] (1) + thiophene (2) + n-heptane (3)} at
T = 308.15 K; ▲,{[EMIM][TCM] (1) + benzothiophene (2) + n-heptane (3)} at T = 308.15 K; ○,
{[EMIM][NTf2] (1) + thiophene (2) + n-heptane (3)} at T = 298.15 K [7]; ♦,
[EMIM][SCN] (1) + thiophene (2) + n-heptane (3)} at T = 298.15 K [9]; ◊, [EMIM][SCN] (1) + thiophene (2) + n-heptane (3)} at T = 303.15 K [9]; ∆, {[EMIM][EtSO4] (1) + thiophene (2) + n-heptane (3)} at T = 298.15 K [10]. FIGURE 5. Plot of the solute distribution ratio, (β) as a function of the mole fraction of solute in the hydrocarbon – rich phase for the ternary systems: ●, {[EMIM][TCM] (1) + thiophene (2) + n-heptane (3)} at T = 298.15 K; ■, {[EMIM][TCM] (1) + thiophene (2) + n-heptane (3)} at T = 308.15 K; ▲,{[EMIM][TCM] (1) + benzothiophene (2) + n-heptane (3)} at T = 308.15 K; ○,
{[EMIM][NTf2] (1) + thiophene (2) + n-heptane (3)} at T = 298.15 K [7]; ♦,
[EMIM][SCN] (1) + thiophene (2) + n-heptane (3)} at T = 298.15 K [9]; ◊, [EMIM][SCN] (1) ___________________________________________________________ M. Królikowski, K.Walczak and U. Domańska
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+ thiophene (2) + n-heptane (3)} at T = 303.15 K [9]; ∆, {[EMIM][EtSO4] (1) + thiophene (2) + n-heptane (3)}at T = 298.15 K [10].
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FIGURE 1.
FIGURE 2.
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FIGURE 3.
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600 500
S
400 300 200 100 0 0.0
0.2
0.4
0.6
x2
0.8
1.0
I
FIGURE 4.
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3.0 2.5 2.0
β
1.5 1.0 0.5 0.0 0.0
0.2
0.4
0.6
x2
0.8
1.0
I
FIGURE 5.
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HIGHLIGHTS > LLE data for (EMIMTCM + thiophene/benzothiophene + n-heptane) were determined. > High S and β for the extraction of thiophene/benzothiophene from n-heptane was found. > Results of S and β were compared with available literature. >The NRTL model satisfactorily correlates the LLE data.
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References
[1] P. S. Kulkarni, C. A. M. Afonso, Green Chem. 12 (2010) 1139-1149. [2] http://www.epa.gov/otaq/fuels/gasolinefuels/index.htm accessed May 2013. [3] http://www.dieselnet.com/standards/eu/fuel.php accessed May 2013. [4] M. Francisco, A. Arce, A. Soto, Fluid Phase Equilib. 294 (2010) 39–48. [5] I.V. Babich, J.A. Moulijn, Fuel 82 (2003) 607–631. [6] H. Zhao, S. Xia, P. Ma, J. Chem. Technol. Biotechnol. 80 (2005)1089–1096. [7] H. Rodríguez, M. Francísko, A. Soto, A. Arce, Fluid Phase Equilib. 298 (2010) 240– 245. [8] B. Rodríguez-Cabo, A. Soto, A. Arce, J. Chem. Thermodyn. 57 (2013) 248–255. [9] K. Kędra-Królik, F. Mutelet, J.N. Jaubert, Ind. Eng. Chem.. Res. 50 (2011) 2296–2306. [10] L. Alonso, A. Arce, M. Francisco, A. Soto, Fluid Phase Equilib. 270 (2008) 97–102. [11] K. Kędra-Królik, F. Mutelet, J. Ch. Moïse, J.N. Jaubert, Energy Fuels 25 (2011) 1559– 1565. [12] L. Alonso, A. Arce, M. Francisco, A. Soto, Fluid Phase Equilib. 263 (2008) 176–181. [13] L. Alonso, A. Arce, M. Francisco, A. Soto, J. Chem. Thermodyn. 40 (2008) 265–270. [14] L. Alonso, A. Arce, M. Francisco, A. Soto, J. Chem. Thermodyn. 40 (2008) 966–972. [15] U. Domańska, E.V. Lukoshko, M. Królikowski, J. Chem. Thermodyn. 61 (2013) 126– 131. [16] A. Marciniak, M. Królikowski, J. Chem. Thermodyn. 49 (2012) 154–158. [17] A. Marciniak, M. Królikowski, Fluid Phase Equilib. 321 (2012) 59– 63.
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[18] M. Larriba, P. Navarro, J. García, F. Rodríguez Ind. Eng. Chem. Res. 52 (2013) 2714– 2720. [19] V. N. Emel’yanenko, D. H. Zaitsau, S. P. Verevkin, A. Heintz, J. Phys. Chem. B 115 (2011) 11712–11717. [20] H. Renon, J.M. Prausnitz, AIChE J. 14 (1968) 135–144. [21] J. M. Sørensen, W. Arlt, Liquid–Liquid Equilibrium Data Collection, DECHEMA Chemistry Data Series, Frankfurt, 1980. [22] U. Domańska, M. Królikowski, K. Ślesińska, J. Chem. Thermodyn. 41 (2009) 1303– 1311. [23] U. Domańska, M. Królikowski, A. Pobudkowska, T. M. Letcher, J. Chem. Eng. Data 54 (2009) 1435–1441 [24] S.W. Walas, Phase Equilibria in Chemical Engineering, Butterworth Publishers: Boston, (1985). [25] U. Domańska, A. Pobudkowska, M. Królikowski, Fluid Phase Equilib. 259 (2007) 173– 179.
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HIGHLIGHTS > LLE data for (EMIMTCM + thiophene/benzothiophene + n-heptane) were determined. > High S and β for the extraction of thiophene/benzothiophene from n-heptane was found. > Results of S and β were compared with available literature. > The NRTL model satisfactorily correlates the LLE data.
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