ARTICLE pubs.acs.org/EF
Deep Fuels Desulfurization and Denitrogenation Using 1-Butyl-3-methylimidazolium Trifluoromethanesulfonate Karolina Ke- dra-Krolik,†,‡ Fabrice Mutelet,*,† Jean-Charles Moïse,† and Jean-No€el Jaubert† † ‡
Laboratoire Reactions et Genie des Procedes, Nancy-Universite, 1 rue Grandville, BP 20451 54001 Nancy, France Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warszawa, Poland ABSTRACT: The aim of this study is to investigate the possible use of 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([BMIM][OTf]) as solvent for three separation tasks which appear in deep desulfurization of fuels: {aromatic sulfur compound þ aliphatic hydrocarbon}, {nitrogen compound þ aliphatic hydrocarbon}, or {sulfur compound þ aliphatic hydrocarbon}. New three ternary systems are studied to investigate the capacity and selectivity of [BMIM][OTf] for extraction of sulfur and nitrogen containing aromatic organic compounds from aliphatic hydrocarbons. Aromatic hydrocarbon capture, which often appears causing unwanted reduction of fuel octane number, is also investigated. Therefore, LLE measurements of ternary mixtures for three systems are presented: {thiophene þ n-heptane þ [BMIM][OTf]}, {pyridine þ n-heptane þ [BMIM][OTf]}, {benzene þ n-heptane þ [BMIM][OTf]}, and {thiophene þ n-heptane þ [BMIM][OTf]} at 298.15 K and atmospheric pressure. The liquidliquid equilibrium (LLE) data are correlated by the use of the NRTL model. Moreover, the extraction experiments of synthetic fuels— model gasoline and model diesel using [BMIM][OTf]—have been performed. The influence of three stepped extraction procedure using each time a fresh portion of [BMIM][OTf] on the final gasoline and diesel compositions is presented.
’ INTRODUCTION The environmental protection strategies in recent years have led to drastic restrictions by European Community concerning sulfur and nitrogen compound content levels in produced fuels.1 Such compounds after gasoil combustion form sulfur and nitrogen oxides which are responsible for acid rain, global warming effects, or air pollution, which are harmful for humans. However, deep desulfurization of gasoline and diesel is a very complex problem for the petroleum industry. Hydrodesulfurization (HDS), which is applied in the fuel industry nowadays, consists of the catalytic hydrotreating process.2 HDS is highly efficient for the removal of aliphatic hydrocarbons containing sulfur but the challenging task is the elimination of aromatic sulfur compounds such as thiophene or dibenzothiophene and their alkyl derivatives. Moreover, this conventional method is highly cost and energy consumptive due to high operating temperatures (300400 °C) as well as elevated pressures (20100 atm of H2). To face these problems, many researchers carry out investigations on environmentally benign, energy saving, and effective method. Several investigations are focused on elaboration of new catalysts to change HDS operation conditions. On the other hand many studies have been directed toward expanding new original approaches to deep desulfurization of fuels, such as processes based on distillation, adsorption, extraction, reactive alkylation, and many others.3 Liquidliquid extraction is an often applied technique in industry for mixtures separation. The advantages of this method are simplistic operation option, mild process conditions, and low energy consumption. However, the extraction efficiency depends largely on a precisely selected solvent for specific separation processes. The conventional solvents used for extraction are highly volatile, flammable, and often toxic. Moreover, some of industrially important extraction processes are still a challenge, r 2011 American Chemical Society
e.g. separations of aliphatic and aromatic hydrocarbons or alcohols and aliphatic hydrocarbons. The components of such systems have very narrow range of boiling points and several combinations form azeotropes. Ionic liquids can be used as ideal substitutes of organic solvents to improve the environmental friendliness of conventional extraction techniques because of their negligible vapor pressure, high chemical and thermal stability, nonflammability, nontoxicity, or recyclability.4 Moreover, they are known as “designer solvents” due to possibility of tuning their properties by combining different cations and anions and their respective structure tailoring. In this way one can obtain for given system required properties of IL such as polarity, miscibility, or viscosity. The use of proper IL for a given system causes higher values of selectivities at infinite dilution for a specific separation process compared to classical solvents. This is usually caused by the high solubility of polar compounds in ionic liquids and the low miscibility of aliphatic compounds. These properties make ILs ideal solvents for liquidliquid extractions as well as for a number of applications, which was proved by determination of activity coefficients at infinite dilution using experimental techniques.57 The interactions between aromatic sulfur compounds and ionic liquids were studied by many research groups.810 Among many types of studied ionic liquids, those based on imidazolium cations and SCN, MP, BF4, PF6, or NTf2 anions showed high efficiency for organic sulfur as well as organic nitrogen compounds extraction.1014 Revelli and co-workers have shown using nuclear magnetic resonance (NMR) that thiophene molecules accommodate the ionic pair structure of the ionic liquids. This proves the Received: January 31, 2011 Revised: March 11, 2011 Published: March 14, 2011 1559
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Table 1. GC Operating Conditions for Composition Analysis injector temperature
250 °C
carrier gas
helium
capillary column
WCOT Ulti-Metal coated with HT-SIMDIST-CB (10 m 0.53 mm 0.53 μm) with an empty precolumn
flow rate
2 mL 3 min1
column oven detector type
70 f 125 °C (5 °C 3 min1), 5 min FID
detector temperature
250 °C
ability of [BMIM][SCN] for selective extraction of aromatic sulfur compounds from fuels.10 An unique feature of ionic liquids and salts is their mobility and flexibility, which allows for a facile restructuring of the ionic liquids while thiophene is dissolved. The restructuring process is primarily driven by the interaction of thiophene with the imidazolium cations of the ionic liquids, and the maximum absorption capacity of thiophene by ionic liquids is primarily determined by the size and structure of both cations and anions.10 This work is focused on 1-butyl-3-methylimidazolium trifluoromethanesulfonate as an extracting solvent for two separation problems frequently encountered in the petroleum industry: aromatic sulfur and nitrogen compound extraction from aliphatic hydrocarbons. The first part of the presented work consists of a study of three new ternary systems to acquire information about the application of a proposed IL as an extracting solvent. The compounds selected for the presented work are the components of model gasoil which represents the composition of real fuel. Thiophene represents sulfur-containing aromatic hydrocarbons, pyridine, nitrogen-containing aromatic hydrocarbons, benzene, aromatic hydrocarbons, and n-heptane, aliphatic hydrocarbons. Therefore, liquidliquid equilibrium (LLE) measurements of ternary mixtures for three systems were measured at 298.15 K and atmospheric pressure: {thiophene þ n-heptane þ [BMIM][OTf]}, {pyridine þ n-heptane þ [BMIM][OTf]}, {benzene þ n-heptane þ [BMIM][OTf]}. Classical parameters which characterize extraction processes, such as solute distribution ratio and selectivity, were calculated for all investigated systems and fully discussed. Moreover, experimental data have been correlated of by means of NRTL model. The second part of this work concerns extraction of model gasoline and model diesel by the use of [BMIM][OTf]. The three-stepped extraction using each time a fresh portion of IL on the final fuel contamination were investigated.
Table 2. Compositions of expErimental Tie Lines, Solute Distribution Ratios β, and Selectivity S for Investigated Ternary Systems at 298.15 K hydrocarbon-rich phase xHC 1
xHC 3
IL-rich phase xIL 1
xIL 2
xIL 3
β
S
thiophene (1) þ heptane (2) þ [BMIM][OTf] (3)
’ EXPERIMENTAL SECTION Thiophene, benzene, hexane, 2,2,4-trimetylpentane, dodecane, and hexadecane were supplied by Sigma-Aldrich with purity of 99%. N-heptane was purchased from Fluka and was of 99% pure. Pyridine and toluene were from Sigma-Aldrich with purity 99.8%. Dibenzothiophene was purchased from Across Organics with a quoted purity of 99%. All chemicals were used for experiments without any further purification. The investigated ionic liquid 1-butyl-3-methylimidazolium trifluoromethanesulfonate was purchased from Solvionic with a purity of 99.5%. Before measurements, the ionic liquids were dried in oven under high vacuum at a temperature of 70 °C for approximately 12 h to remove possible traces of solvents and moisture. Determination of Tie Lines of Ternary Mixtures. The LLE measurements of ternary mixtures have been performed in jacketed glass cells. The experimental set up consists of a cell with an internal volume of about 30 cm3 which were kept at constant temperature of 298.15 K using a thermostatted bath. The temperature inside the cell was measured by a
xHC 2
0.0402
0.9277
0.0321
0.0577
0.0292
0.9132
1.31
33
0.1285
0.8715
0.0000
0.1760
0.0231
0.8010
0.99
10
0.2142
0.7858
0.0000
0.3512
0.0378
0.6109
1.17
26
0.3070 0.3325
0.6930 0.6675
0.0000 0.0000
0.3837 0.4363
0.0390 0.0263
0.5773 0.5374
1.19 0.96
20 7
0.3733
0.6267
0.0000
0.4497
0.0265
0.5238
1.20
28
0.4053
0.5947
0.0000
0.4745
0.0271
0.4983
0.93
6
0.4378
0.5622
0.0000
0.5216
0.0335
0.4449
1.25
22
0.6277
0.3723
0.0000
0.6211
0.0357
0.3432
0.88
5
0.7065
0.2935
0.0000
0.6764
0.0386
0.2850
1.64
34
0.7652
0.2348
0.0000
0.7097
0.0341
0.2562
0.85
4
0.8288 0.9226
0.1712 0.0774
0.0000 0.0000
0.7305 0.7820
0.0298 0.0159
0.2397 0.2021
1.37 0.82
52 3
0.9842
0.0147
0.0012
0.8108
0.0040
0.1852
1.43
46
0.0000
pyridine (1) þ heptane (2) þ [BMIM][OTf] (3) 0.9811 0.0189 0.0000 0.0204 0.9796
0.0381
0.9619
0.0000
0.2256
0.0941
0.6804
5.92
61
0.0612
0.9388
0.0000
0.4519
0.0971
0.4510
7.38
71
0.1553
0.8447
0.0000
0.6018
0.1072
0.2910
3.88
31
0.2166
0.7834
0.0000
0.6956
0.1222
0.1822
3.21
21
0.3200 0.2972
0.6800 0.7028
0.0000 0.0000
0.7287 0.7396
0.1372 0.1464
0.1341 0.1140
2.28 2.49
11 12
0.3689
0.6311
0.0000
0.7623
0.1522
0.0856
2.07
9
0.3824
0.6176
0.0000
0.7649
0.1711
0.0640
2.00
7
0.4914
0.5086
0.0000
0.7489
0.2137
0.0374
1.52
4
0.0166
benzene (1) þ heptane (2) þ [BMIM][OTf] (3) 0.9834 0.0000 0.1042 0.0329 0.8628 6.28
188
0.2417
0.7583
0.0000
0.4173
0.0435
0.5392
1.73
30
0.3094
0.6906
0.0000
0.4958
0.0385
0.4658
1.60
29
0.3581 0.5313
0.6419 0.4687
0.0000 0.0000
0.5482 0.6205
0.0377 0.0515
0.4141 0.3279
1.53 1.17
26 11
0.6481
0.3519
0.0000
0.6931
0.0285
0.2785
1.07
13
0.7186
0.2814
0.0000
0.7953
0.0286
0.1761
1.11
11
platinum resistance thermometer PT-100 with an accuracy of (0.1 K. The ternary mixtures, with compositions inside the immiscible region of the system, were weighted using a Mettler analytical balance with a precision of (0.0001 g. All the mixtures were vigorously mixed using Teflon coated 1560
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Figure 1. Tie lines for ternary mixture {thiophene þ heptane þ [BMIM][OTf]} at 298.15 K: experimental data (black lines and boxes) and NRTL correlation (red lines and boxes). magnetic stirrer for 2 h to reach a good contact between both phases and were kept overnight in order to settle down. Then, samples of both layers were taken from the cell using a syringe. The compositions of organic compounds such as n-heptane, thiophene, or pyridine were determined by gas chromatography. The GC operating conditions are given in Table 1. All GC analysis were repeated three times to check reproducibility. Density measurements of both phases were performed at 298.15 K to determine the ionic liquid phase composition by the use a vibrating tube densimeter (Anton Paar, model DMA601). The uncertainty in the measurements was (105 g 3 cm3. The estimated uncertainty in the determination of mole fraction compositions was (104. NMR analysis of the hydrocarbon rich phase was also performed to check its possible contamination with ionic liquid. Modeled Fuel Desulfurization Experiments. The model of commercial gasoline was demonstrated by a mixture of 26 wt % hexane, 26 wt % heptane, 26 wt % iso-octane, 10 wt % toluene, 6 wt % thiophene, and 6 wt % pyridine. Commercial diesel was represented by model mixture containing of 26 wt % heptane, 26 wt % dodecane, 26 wt % hexadecane, 10 wt % toluene, 3 wt % thiophene, 3 wt % dibenzothiophene, and 6 wt % pyridine. The stepped extraction of the model fuels was investigated by introduction of equal volumes of model gasoil and [BMIM][OTf] into thermostatted, 30 cm3 cells. The mixture was vigorously stirred to achieve good contact of both phases and stopped after 15 min to separate the phases. The extraction in steps was conducted three times by using new portion of IL and gasoline or diesel as a feed in the following extraction stages. The fuel composition after each step was determined by GC.
’ RESULTS AND DISCUSSION Experimental LLE Data. Sulfur or nitrogen containing organic compound extractions were investigated by the use of
[BMIM][OTf] as a solvent for deep desulfurization of fuels. LLE of three ternary systems were determined: {thiophene þ n-heptane þ [BMIM][OTf]}, {pyridine þ n-heptane þ [BMIM][OTf]}, and {benzene þ n-heptane þ [BMIM][OTf]}, which are shown in Table 2. The equilateral triangular diagrams with LLE representation of these systems are presented in Figures 13. Equilateral triangular diagrams for thiophene and benzene show a behavior corresponding to type II with two of their binary systems exhibiting partial immiscibility and with only one immiscibility region. The ternary system containing pyridine presents a phase behavior of type I with one of the binary systems exhibiting partial immiscibility. Values of solute distribution ratio β and selectivity S reported in Table 2 are calculated from experimental data according to equations: xIL 1 xHC 1
ð1Þ
HC xIL 1 x2 IL xHC 1 x2
ð2Þ
β¼
S¼
where x is the mole fraction, subscripts 1 and 2 refer to solute (pyridine, thiophene, or benzene) and to hydrocarbon (n-heptane or n-dodecane), respectively, and superscripts HC and IL indicate the hydrocarbon-rich phase and the IL-rich phase, respectively. The solute distribution ratios indicates the solute-carrying capacity of the investigated ionic liquid. For effective extraction, it is desired to have β higher than 1. Figure 4 shows solute distributions ratios for three investigated solutes as a function 1561
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Figure 2. Tie lines for ternary mixture {pyridine þ heptane þ [BMIM][OTf]} at 298.15 K: experimental data (black lines and boxes) and NRTL correlation (red lines and boxes).
of their molar fraction in the hydrocarbon-rich phase. The β values for pyridine and benzene for low and medium solutes concentrations are high, up to 7.4 and decreases with increasing mole fraction of solutes but are still over 1. The system {thiophene þ heptane þ [BMIM][OTf]} presents a solutropic behavior. Thiophene distribution ratio is lower, with maximum value 1.4 but does not decrease rapidly for higher mole fraction range and is close to 1. As observed in Figure 5, [BMIM][OTf] is selective for pyridine, thiophene, and benzene, especially for low molar fraction of the solute. The value of selectivity is related to the number of stages in the separation process. Investigated ionic liquid shows the highest selectivity for pyridine with a maximum 71 while thiophene and benzene show maxima over 30. Selectivity decreases for higher solute concentrations up to the minimum value, 4. Obtained results suggest that applied ionic liquid [BMIM][OTf] could efficiently perform the separation of thiophene and pyridine from heptane. However, together with fuel desulfurization and denitrogenation, benzene content will also decrease, due to values of selectivity and solute distribution ratio for {benzene þ heptane þ [BMIM][OTf]} system. Thus, due to the aromatic ring interaction with the cation of the ionic liquids, decrease of octane number of treated fuel would be observed in similar way as it was earlier observed for different types of ILs.1,1014 Our previous study showed that [SCN] anion shows higher efficiency as extracting solvent due to higher values of selectivity and solute distribution ratio in order to extract thiophene or pyridine.10 Arce and co-workers have investigated some other ionic liquids for the extraction of thiophene from aliphatic hydrocarbons: [HMMpy][Ntf2], [OMIM][Ntf2], [EMIM][Ntf2],
[OMIM][BF4], or [EMIM][EtSO4].1417 Among these ILs in ternary systems with thiophene and heptane, the highest selectivity (from 3.6 to 74.6) was found for [EMIM][Ntf2] and the solute distribution ratio for this ionic liquid was ranging from 0.78 up to 1.96.14 Comparing to [BMIM][OTf] investigated in this work, [EMIM] [Ntf2] has similar values of selectivity and solute distribution ratio. Correlation. The LLE data of the investigated ternary systems were correlated using the non-random two-liquid equation (NRTL) proposed by Renon and Prausnitz.18 For the NRTL model, the activity coefficient γi, for any component i of the ternary system is given by 0 1 m m τji Gji xj x τ G r rj rj C m xj Gij B j¼1 B C r¼1 ln γi ¼ m þ Bτij m C ð3Þ m @ A j¼1 Gli xl Glj xl Glj xl
∑
∑
∑
l¼1
∑
∑
l¼1
∑
l¼1
with Gji = exp(Rjiτji), τji = (gji gii)/(RT) = Δgji/(RT), and Rji = Rij = R and where g is an energy parameter characterizing the interaction of species i and j, xi is the mole fraction of component i, and R the nonrandomness parameter.19 Binary interaction parameters for NRTL equations are those which minimize the difference between the experimental and calculated mole fractions: Fobj ¼
N
3
I, exp II, exp I, calc II, calc fðxi, k xi, k Þ2 þ ðxi, k xi, k Þ2 g ∑ ∑ k¼1 i¼1
ð5Þ
I,calc Where N is the number of tie lines in the data set, xI,exp i,k and xi,k
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Figure 3. Tie lines for ternary mixture {benzene þ heptane þ [BMIM][OTf]} at 298.15 K: experimental data (black lines and boxes) and NRTL correlation (red lines and boxes).
Figure 4. Solute distribution ratio β as function of mole fraction of solute (thiophene, pyridine, and benzene) in hydrocarbon-rich phase, at 298.15 K.
Figure 5. Selectivity coefficient as function of mole fraction of solute (thiophene, pyridine, and benzene) in hydrocarbon-rich phase, at 298.15 K.
are the experimental and calculated mole fractions of one phase, and xII,calc are the experimental and calculated mole and xII,exp i,k i,k fractions of the second phase.
The binary parameters and root mean-square deviation (rmsd), calculated using the procedure above, of the NRTL equation are given in Table 3. The rmsd values, which provide a 1563
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Table 3. Values of Binary Parameters for the NRTL Equation for the Ternary Mixtures system thiophene (1) þ heptane (2) þ [BMIM][OTf] (3)
pyridine (1) þ heptane (2) þ [BMIM][OTf] (3)
benzene (1) þ heptane (2) þ [BMIM][OTf] (3)
Δgij (J 3 mol1)
ij
Δgji (J 3 mol1)
12
6972.1
664.1
13
9748.35
1923.74
23
10850.9
12
3770.1
1031.9
13
71.0
5066.7
23
10850.9
6624.3
6161.4
1333.8
13 23
9619.8 10850.9
1325.3 6624.3
measure of the accuracy of the correlations, were calculated according to the following equation: 0
11=2 N 3 I, exp II, exp I, calc 2 II, calc 2 ∑ ∑ fðx x Þ þ ðx x Þ g i, k i, k i, k Bk ¼ 1 i ¼ 1 i, k C C rmsd ¼ B @ A 6N ð6Þ As can be inferred from the rmsd values for the three ternary systems studied in this work, fairly good correlation of the experimental values with NRTL model was obtained. Desulfurization and Denitrogenation of Model Fuels. Figure 6 shows the composition of a model gasoline mixture and the weight percent of each component changes of after one, two, and three extraction steps using [BMIM][OTf]. The same experiments have been performed for simulated diesel oil, and the results are presented in Figure 7. The extraction time applied in all fuel desulfurization stages was 1 h, and the temperature was 298.15 K. Sulfur as well as nitrogen containing compounds are extracted well from hydrocarbon mixtures by [BMIM][OTf].
rmsd
0.3
0.005
0.3
0.009
0.3
0.007
6624.3
12
Figure 6. Evolution of model gasoline component weight percent changes in the three-step extraction process using [BMIM][OTf] as a solvent at 298.15 K.
R
Figure 7. Evolution of model diesel component weight percent changes in the three-step extraction process using [BMIM][OTf] as a solvent at 298.15 K.
The best results have been obtained for pyridine elimination from fuels. Two steps of extraction are needed to denitrogenate gasoline as well as diesel in 98%. Desulfurization of model gasoline in three stages ensures 96% of thiophene content reduction. Sulfur containing compound extraction from model diesel by [BMIM][OTf] ensures elimination of 97.5% of dibenzothiophene and thiophene removal in more than 93%. These results are slightly better than for [EMIM][SCN] presented in our earlier work.20 However, [BMIM][OTf] has very high solute distribution ratio compared to benzene which is reflected in relatively high extraction of aromatic hydrocarbons from model fuels. Toluene contamination is lower in about 45% in both modeled gasoils so their octane number is remarkably decreased.
’ CONCLUSIONS Liquidliquid equilibrium data for new three ternary systems {thiophene þ n-heptane þ [BMIM][OTf]}, {pyridine þ n-heptane þ [BMIM][OTf]}, {benzene þ n-heptane þ [BMIM][OTf]}, at T = 298.15 K were determined. 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. Then, it was found that [BMIM][OTf] has a good capacity for deep fuel desulfurization as well as for 1564
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Energy & Fuels denitrogenation. It enables full denitrogenation of both modeled fuels in three extraction steps. Desulfurization of model gasoline in three stages ensures 96% thiophene content reduction. Sulfur containing compound extraction from model diesel by [BMIM][OTf] ensures elimination of 97.5% of dibenzothiophene and thiophene removal in more than 93%. However, lowering of the treated gasoil octane number takes place due to aromatic hydrocarbon extraction.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Telephone number: þ33 3 83 17 51 31. Fax number: þ33 3 83 17 53 95.
ARTICLE
desulfurization: Phase equilibria. J. Chem. Thermodyn. 2010, 42 (6), 712–718. (17) Alonso, L.; Arce, A.; Francisco, M.; Soto, A. Phase behaviour of 1-methyl-3-octylimidazolium bis[trifluoromethylsulfonyl]imide with thiophene and aliphatic hydrocarbons: The influence of n-alkane chain length. Fluid Phase Equilib. 2008, 263, 176–181. (18) Renon, H.; Prausnitz, J. M. Local compositions in thermodynamic excess functions for liquid mixtures. AIChE J. 1968, 14, 135–144. (19) Simoni, L. D.; Chapeaux, A.; Brennecke, J. F.; Stadtherr, M. A. Asymmetric framework for predicting liquid - Liquid equilibrium of ionic liquid - Mixed-solvent systems. 2. Prediction of ternary systems. Ind. Eng. Chem. Res. 2009, 48, 7257–7265. (20) Ke-dra-Krolik, K.; Mutelet, F.; Jaubert, J.-N. Ind. Eng. Chem. Res., in press.
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