Extraction of Thiophene or Pyridine from n-Heptane Using Ionic

ARTICLE pubs.acs.org/IECR

Extraction of Thiophene or Pyridine from n-Heptane Using Ionic Liquids. Gasoline and Diesel Desulfurization Karolina Ke- dra-Krolik, Mutelet Fabrice,* and Jean-No€el Jaubert Laboratoire Reactions et Genie des Procedes, Nancy-Universite, 1 rue Grandville, BP 20451 54001 Nancy, France ABSTRACT: The aim of this study is to investigate the possible use of ILs as solvents for two separation problems frequently encountered in petroleum industry: {aromatic sulfur compound þ aliphatic hydrocarbon} or {nitrogen compound þ aliphatic hydrocarbon}. This work is focused on three ILs: 1-ethyl-3-methylimidazolium thiocyanate, 1,3-dimethylimidazolium methylphosphonate, and tris-(2-hydroxyethyl)-methylammonium-methylsulfate. In the first part of this article, a study of new three ternary systems is studied in view of defining the capacity of proposed ILs as solvents for extraction of sulfur and nitrogen containing organic compounds from aliphatic hydrocarbons. Therefore, LLE measurements of ternary mixtures for five systems were measured at 298.15 K and at atmospheric pressure: {thiophene þ n-heptane þ1-ethyl-3-methylimidazolium thiocyanate}, {thiophene þ n-heptane þ 1,3-dimethylimidazolium methylphosphonate}, {thiophene þ n-heptane þ tris-(2-hydroxyethyl)-methylammoniummethylsulfate}, {pyridine þ n-heptane þ1-ethyl-3-methylimidazolium thiocyanate}, {pyridine þ n-heptane þ1,3-dimethylimidazolium methylphosphonate}. The second section of this article presents results of extraction of synthetic fuels - model gasoline and model diesel by the use of selected ILs. The influence of extraction time or temperature as well as three stepped procedure using each time a fresh portion of ILs on the final fuel contamination was investigated.

’ INTRODUCTION The separation of mixtures by liquid-liquid extraction is often applied by many industrial procedures due to simplistic operation option, mild processes conditions, and economical advantages of this method. High efficiency of this technique depends largely on precisely selected the most suitable solvent for specific separation process. However, conventional solvents commonly used for extraction are highly volatile, flammable, and often toxic. The environmental regulations all over the world are more stringent so applications of green solvents in classical methods or new economically and environmentally friendly technologies are in great demand. Some of industrially important extraction processes are still a challenge 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. This makes them also impracticable to be separated by distillation. In recent years considerable attention has been given to deep desulfurization of gasoline and diesel due to the higher restrictions concerning sulfur-compounds content level in fuels. Sulfur oxides that form in gasoil combustion process contribute to acid rains, global warming effect, and air pollution. Those affect urban as well as industrial areas and are harmful for human health due to secondary inorganic aerosol gases formation. The European Union gradually reduced the maximum sulfur concentration in fuels to avoid high emission of gases resulting from combustion of heavy fuels. Since 2010 the total sulfur content cannot be higher than 10 ppm. Deep desulfurization of gasoline and diesel is a very complex problem for petroleum industry and has to be solved to minimize the pollutions from fuel oils exhaust gas. Indeed, many research groups carry out deep investigations of environmentally benign, energy saving, and effective methods.1 r 2011 American Chemical Society

The conventional method applied by refineries is hydrodesulfurization (HDS) and consists of catalytic hydrotreating with Co-Mo/Al2O3, Ni-Mo/Al2O3, or Ni-W/Al2O3 catalysts.2 HDS is highly efficient for removal of aliphatic hydrocarbons containing sulfur. However, the challenging task arises with elimination of aromatic sulfur compounds such as thiophene or dibenzothiophene and their alkyl derivatives such as 4-methyldibenzothiophene by use of the HDS method.3 Moreover, HDS processes have to be conducted under elevated temperatures (300-400 C) as well as elevated pressures (20-100 atm of H2) which makes the method cost and energy consumptive. To face these problems with the purpose of regard sulfur contamination within the required levels, researchers have been making several efforts to elaborate new catalysts or to enhance existing catalyst activity and change HDS operation conditions. On the other hand, many investigations have been directed toward expanding on new original approaches to deep desulfurization of fuels, such as processes based on distillation, adsorption, extraction, reactive alkylation, complexation, precipitation, photochemical desulfurization, selective oxidation, biodesulfurization, ultrasounds accompanied extraction, and combinations of mentioned methods.4 The use of ionic liquids (ILs) for liquid-liquid extraction shows a great potential in comparison with conventional organic solvents, due to their unique physicochemical properties. Negligible vapor pressure, high chemical and thermal stability, nonflammability, or recyclability makes them environmentally friendly solvents. Moreover they are able to dissolve a wide range of organic or inorganic substances, and it is possible to adjust Received: September 1, 2010 Accepted: December 6, 2010 Published: January 20, 2011 2296

dx.doi.org/10.1021/ie101834m | Ind. Eng. Chem. Res. 2011, 50, 2296–2306

Industrial & Engineering Chemistry Research some of their properties such as polarity or miscibility for certain system requirements. They also have high ionic conductivity and wide electrochemical window. These properties make ILs very good solvents for liquid-liquid extractions as well as for a number of applications e.g. catalysis, synthesis, gas separations which was proved by some thermodynamic methods concerning determination of activity coefficients at infinite dilution.5-7 The use of ILs usually causes higher values of selectivities at infinite dilution for a specific separation process comparing to classical solvents. This is generally caused by the high solubility of polar compounds in ionic liquids and the low miscibility of aliphatic compounds. The interactions between aromatic sulfur compounds and ionic liquids were conducted by many research groups.8-10 Revelli and co-workers demonstrated using Nuclear Magnetic Resonance (NMR) that thiophene molecules are accommodated into the ionic pair structure of the ionic liquids enabling the selective and extractive removal of aromatic sulfur compounds from fuels.8 A unique feature of ionic liquids as salts is their mobility and flexibility, which allow for a facile restructuring of the ionic liquids in the process of thiophene dissolution. 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.8 The first investigations of fuels extractive desulfurization using ionic liquids has been reported by B€ossman et al. in 2001.11 These authors studied the dialkylimidazolium chlorides mixed with AlCl3 as an extracting solvent and showed that the steric factors of specific ionic liquids are responsible for S-containing compounds extraction rather than chemical interactions involving the acid proton, as they supposed before. Among many types of studied ionic liquids, those based on imidazolium cations and BF4- or PF6- anions showed high efficiency for organic sulfur as well as organic nitrogen compounds extraction.12,13 It was proved that these ionic liquids interact by coulombian forces with π electrons of aromatic ring. However, several liquid-liquid equilibrium data showed that unwanted effect of aromatic hydrocarbons extraction proceeds parallel to desulfurization and decreases of fuel octane number.14 Due to the stability and corrosion problems as well as the total costs of deep fuel desulfurization the choice of the most suitable ionic liquid is still a challenging task for researchers. Among many investigated ionic liquids, the imidazolium thiocyanate ones, e.g. [BMIM][SCN], [EMIM][SCN] were proved to show very small interactions between alkanes and cycloalkanes and high selectivity with respect to thiophene separation from such solvents, and they were highly useful for the fuel desulfurization process.8,14,15 A less expensive alternative is the use of imidazolium-based phosphoric ionic liquid which showed encouraging results.16-18 The reported desulfurization facility of this type of ILs may be ordered as follows: [BMIM][DBP] > [EMIM][DEP] . [MMIM][DMP]. Additionally the selectivity for sulfur removal decreases from dibenzothiophene through benzothiophene up to 3-methylthiophene.17 On the other hand the fuel solubility increases in the same way, so the compromise is necessary to choose the most effective one which was proved by Nie et al.18 The authors showed that partition coefficients values increases for ionic liquids containing longer alkyl substitute of the anion or cation. Holbrey and co-workers19

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have investigated {dibenzothiophene þ dodecane þ IL} ternary systems for a range of ionic liquids with different cation type (imidazolium, pyridinium, and pyrrolidinium) and a range of anion types. The partition coefficient of dibenzothiophene to the ionic liquids varies following the order with cation class (dimethylpyridinium > methylpyridinium > pyridinium ≈ imidazolium ≈ pyrrolidinium). The anion type influence is much less important. The aim of this study is to investigate the possible use of ILs as solvents for two separation problems frequently encountered in petroleum industry: {aromatic sulfur compound þ aliphatic hydrocarbon} or {nitrogen compound þ aliphatic hydrocarbon}. This work is focused on three ILs: 1-ethyl-3-methylimidazolium thiocyanate, 1,3-dimethylimidazolium methylphosphonate, and tris-(2-hydroxyethyl)-methylammonium-methylsulfate. The first part of presented work consists on a study of new ternary systems to acquire information about application of proposed ILs as solvents for extraction of sulfur and nitrogen containing organic compounds from aliphatic hydrocarbons. Therefore, LLE measurements of ternary mixtures for five systems were measured at 298.15 K and atmospheric pressure: {thiophene þ n-heptane þ1-ethyl-3-methylimidazolium thiocyanate}, {thiophene þ n-heptane þ1,3-dimethylimidazolium methylphosphonate}, {thiophene þ n-heptane þ tris-(2-hydroxyethyl)-methylammonium-methylsulfate}, {pyridine þ n-heptane þ1-ethyl-3-methylimidazolium thiocyanate}, {pyridine þ n-heptane þ1,3-dimethylimidazolium methylphosphonate}. Moreover the temperature influence for the best one of used ILs and LLE measurement at 303.15 K and atmospheric pressure for {thiophene þ n-heptane þ1-ethyl-3-methylimidazolium thiocyanate} system is shown. Experimental data were correlated using the NRTL and UNIQUAC activity coefficient models. The second section of this article presents results of extraction of synthetic fuels - model gasoline and model diesel by the use of selected ILs. The influence of extraction time or temperature and three-step extraction using each time a fresh portion of ILs on the final fuel contamination were investigated.

’ EXPERIMENTAL SECTION Thiophene and pyridine were supplied by Sigma-Aldrich with purity of 99% and 99.8%, respectively. N-Heptane was purchased from Fisher Scientific, with purity of 99%. 2,2,4-Trimethylpentane was 99% pure from Merck. Toluene, dibenzothiophene, and n-hexane were supplied by Across Organics with a quoted purity of 99% and 99% and 95%, respectively. N-Dodecane and n-hexadecane were taken from Fluka and were over 98% pure. All those chemicals were used for experiments without any further purification. The investigated ionic liquids: 1-ethyl-3-methylimidazolium thiocyanate, [EMIM][SCN], and tris-(2-hydroxyethyl)-methylammoniummethylsulfate, [TEMA][MeSO4], were purchased from Fluka and were over 95% pure. 1,3-Dimethylimidazolium methylphosphonate, [DMIM][MP], was obtained from Solvionic with reported purity of 98%. Before measurements, the ionic liquids were purified under vacuum for approximately 12 h to remove possible traces of solvents and moisture. The water content of the ionic liquids determined using the Karl Fischer technique was from 300 to 700 ppm. ’ DETERMINATION OF TIE LINES OF TERNARY MIXTURES The LLE measurements of ternary mixtures have been performed in jacketed glass cells. The experimental set up 2297

dx.doi.org/10.1021/ie101834m |Ind. Eng. Chem. Res. 2011, 50, 2296–2306

Industrial & Engineering Chemistry Research Table 1. GC Operating Conditions for Composition Analysis injector temperature carrier gas

250 C helium

capillary column

WCOT Ulti-Metal coated with HT-SIMDIST-CB (10 m 0.53 mm 0.53 μm) with

flow rate

an empty precolumn 2 mL.min-1

column oven

70 Cf125 C (5 C/min), 5 min

detector type

FID

detector temperature

250 C

consists of a cell with an internal volume of about 30 cm3 kept at constant temperature of 298.15 K using a thermostated bath. The temperature inside the cell is measured by a platinum resistance thermometer PT-100 with an accuracy of (0.1 K. The ternary mixtures, with compositions inside the immiscible region of the system, are weighed using a METTLER analytical balance with a precision of (0.0001 g. All the mixtures were vigorously mixed using a Teflon coated magnetic stirrer for 2 h to reach a good contact between both phases and 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. For determination of the ionic liquid composition, density measurements of both phases were performed at 298.15 or 302.15 K with a vibrating tube densimeter (Anton Paar, model DMA601). The uncertainty in the measurements is (10-5 g 3 cm-3. The estimated uncertainty in the determination of mole fraction compositions is (10-4.

’ MODELED FUELS DESULFURIZATION EXPERIMENTS The commercial gasoline was modeled by the 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 26 wt % heptane, 26 wt % dodecane, 26 wt % hexadecane, 10 wt % toluene, 3 wt % thiophene, 3 wt % dibenzothiophene, and 6 wt % pyridine. The influence of time of extraction on the model-fuel composition was investigated by introduction of equal volumes of model gasoline and IL into thermostated, 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 gasoline phase samples were taken using syringe with needle after 0.5 h, 1 h, 2 h, and 6 h after stopping the stirring and their compositions were analyzed by GC. The extraction in steps was conducted three times by using new portion of IL and gasoline or diesel as a feed in following extraction stages. The fuel composition after each step was determined by GC. ’ RESULTS AND DISCUSSION Thermodynamic Study of the Ternary Systems {Thiophene or Pyridine/n-Heptane/IL}. The investigations of pos-

sible use of selected ionic liquids for sulfur or nitrogen containing organic compounds extraction in fuel industry have been performed. For this purpose, LLE of five ternary systems were determined: {thiophene þ n-heptane þ [EMIM][SCN]}, {pyridine þ n-heptane þ [EMIM][SCN]}, {thiophene þ n-heptane þ [DMIM][MP]}, {pyridine þ n-heptane þ [DMIM][MP]},

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{thiophene þ n-heptane þ [TEMA][MeSO4]}. The influence of water contents of 2 and 5 wt % on the desulfurization selectivity and capacity of [EMIM][SCN] at 298.15 and 303.15 K have been studied. Moreover, synthetic gasoline and diesel were prepared, and their desulfurizations by the use of selected ILs have been conducted. Table 2 shows the compositions of the experimental tie lines for the five investigated ternary systems at 298.15 K. The equilateral triangular diagrams with LLE representation of these systems are presented in Figures 1 - 5. All equilateral triangular diagrams show a behavior that corresponds to the Type 2 with two of their binary systems exhibiting partial immiscibility and with only one immiscibility region. Values of solute distribution ratio β and selectivity S reported in Table 2 are calculated from experimental data according to equations xIL 1 ð1Þ β ¼ HC x1 S ¼

HC xIL 1 3 x2 IL xHC 1 3 x2

ð2Þ

where x is the mole fraction, subscripts 1 and 2 refer to solute (pyridine, thiophene) and to hydrocarbon (n-heptane), respectively, and superscripts HC and IL indicate the hydrocarbon-rich phase and the IL-rich phase, respectively. In these ternary phase diagrams, the low thiophene or pyridine concentration part should be considered in order to investigate deep desulfurization of fuels using ionic liquids. Among investigated ionic liquids, [EMIM][SCN] shows the highest selectivity for thiophene and pyridine (see Table 2). [DMIM][MP] is also highly selective for thiophene, but the solute distribution ratio is twice smaller than with [EMIM][SCN]. At low molar fraction of thiophene, the selectivity with [EMIM][SCN] is about 1600 at 298.15 K. This high selectivity is mainly due to the immiscibility of n-heptane in this ionic liquid. The capacity of [EMIM][SCN] for thiophene extraction from hydrocarbon solvent decreases with increment of temperature. Selectivity of numerous ternary systems {thiophene þ heptane þ ionic liquid} has been reported in the literature. Our previous investigation on [BMIM][SCN] showed lower values of selectivity ranging from 19 up to 284 than with [EMIM][SCN].8 Nevertheless, the solute distribution ratio increases when the alkyl chain length grafted on the cation increases. Arce and co-workers have also investigated some other ionic liquids for the extraction of thiophene from n-heptane: [HMMpy][Ntf2], [OMIM][Ntf2], [OMIM][BF4], and [EMIM][EtSO4].20-24 For these ternary systems, the highest selectivity was found for [EMIM][EtSO4], and the capacity was ranging from 0.7 to 1.68. Compared to [EMIM][SCN], [EMIM][EtSO4] has a lower selectivity but a solute distribution ratio twice higher. Figures 6 and 7 show the evolution of the solute distribution ratio as a function of thiophene or pyridine mole fraction. The thiophene distribution coefficients are relatively low in all ternary systems containing thiophene. Values of β are below the unit for all three investigated ionic liquids. In case of ternary systems containing pyridine, β values are over the unit for [DMIM][MP] and [EMIM][SCN] in the whole range of concentration. The highest β values are obtained for [EMIM][SCN] indicating that this ionic liquid would be the most advantageous as a solvent for hydrocarbon fuels denitrification. Figure 8 shows the influence of water content of 2 or 5 wt % in ionic liquid for the ternary system 2298


Industrial & Engineering Chemistry Research

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Table 2. Compositions of Experimental Tie Lines, Solute Distribution Ratios β, and Selectivity S for All Ternary Systems hydrocarbon-rich phase

IL-rich phase x3HC

x1IL

x2IL

x3IL

β

x1HC

x2HC

0.0824

0.9176

0

0.0523

0.0004

0.9473

0.64

1598.5

0.1561

0.8439

0

0.1082

0

0.8918

0.69

-

0.2045

0.7955

0

0.1287

0.0020

0.8694

0.63

252.5

0.3088

0.6596

0.0317

0.2311

0

0.7689

0.75

-

0.3701

0.6159

0.0140

0.2660

0

0.7340

0.72

-

0.4486

0.5514

0

0.3396

0

0.6604

0.76

-

0.5702

0.4298

0

0.4030

0

0.5970

0.71

-

0.7361

0.2639

0

0.5153

0.0017

0.4830

0.70

106.5

0.1133

0.8867

0

0.0683

0.0015

0.9302

0.60

346.8

0.2451

0.7549

0

0.1695

0.0010

0.8295

0.69

497.3

0.3482

0.6061

0.0457

0.2370

0

0.7630

0.68

0.3945

0.5750

0.0305

0.2575

0

0.7425

0.65

0.4386

0.5036

0.0578

0.3237

0.0019

0.6744

0.74

193.4

0.6035

0.3846

0.0119

0.4363

0.0017

0.5620

0.72

165.2

0.6884 0.7874

0.3116 0.2126

0 0

0.4790 0.5390

0.0018 0

0.5192 0.4610

0.70 0.68

119.0

0.1133

0.8867

0

0.0683

0.0015

0.9302

0.60

346.8

0.2200

0.7555

0.0245

0.0923

0.0002

0.9075

0.42

1756.2

0.2700

0.7300

0

0.1155

0.0002

0.8842

0.43

1261.2

0.3589

0.6411

0

0.1574

0.0013

0.8413

0.44

217.1

0.4777

0.5223

0

0.1927

0.0007

0.8067

0.40

303.7

0.5618

0.4305

0.0078

0.2039

0

0.7961

0.36

0.6154 0.6917

0.3846 0.3083

0 0

0.2003 0.2254

0.0026 0.0008

0.7971 0.7739

0.33 0.33

48.8 133.8

0.9273

0.0727

0

0.2402

0.0010

0.7588

0.26

18.9

0.1079

0.8873

0.0047

0.0087

0.0026

0.9887

0.08

27.5

0.2942

0.7058

0

0.0175

0.0024

0.9801

0.06

17.6

0.4443

0.5557

0

0.0227

0.0031

0.9742

0.05

9.3

0.4952

0.5048

0

0.0400

0.0007

0.9593

0.08

56.3

0.6279

0.3721

0

0.0453

0.0014

0.9534

0.07

19.7

0.6667 0.7905

0.3310 0.2095

0.0022 0

0.0525 0.0641

0.0011 0.0002

0.9465 0.9358

0.08 0.08

24.5 102.0

0.8321

0.1679

0

0.0733

0.0013

0.9254

0.09

11.1

0.0719

0.9155

0.0126

0.2769

0.0064

0.7167

3.85

549.3

0.1940

0.8060

0.0000

0.5485

0.0133

0.4382

2.83

170.7

0.0346

0.9654

0.0000

0.1670

0.0040

0.8290

4.82

1156.4

0.1104

0.8896

0.0000

0.4363

0.0133

0.5504

3.95

264.0

0.1542 0.5110

0.8458 0.4890

0.0000 0.0000

0.5078 0.7201

0.0187 0.0202

0.4734 0.2597

3.29 1.41

148.7 34.2

0.7164

0.2836

0.0000

0.8783

0.0507

0.1616

1.12

6.8

0.0263

0.9604

0.0133

0.0992

0.0030

0.8978

3.77

1208.9

0.0731

0.9269

0

0.0450

0.0115

0.9436

0.61

49.6

0.1408

0.8592

0

0.0924

0.0322

0.8754

0.66

17.5

0.1936

0.8064

0

0.2298

0.0393

0.7310

1.19

24.4

S

Thiophene (1) þ Heptane (2) þ [EMIM][SCN] (3) at T = 298.15 K

Thiophene (1) þ Heptane (2) þ [EMIM][SCN] (3) at T = 303.15 K

Thiophene (1) þ Heptane (2) þ [DMIM][MP] (3) at T = 298.15 K

Thiophene (1) þ Heptane (2) þ [TEMA][MeSO4] (3) at T = 298.15 K

Pyridine (1) þ Heptane (2) þ [EMIM][SCN] (3) at T = 298.15 K

Pyridine (1) þ Heptane (2) þ [DMIM][MP] (3) at T = 298.15 K

2299



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Table 2. Continued hydrocarbon-rich phase

IL-rich phase x1IL

x2IL

x3IL

β

0

0.2976

0.0417

0.6607

1.02

17.3

0 0

0.3499 0.3729

0.0355 0.0292

0.6146 0.5979

0.99 0.88

18.1 17.2

0.5445

0

0.4104

0.0469

0.5428

0.90

10.5

0.4957

0

0.4404

0.0382

0.5214

0.87

11.3

0.3260

0

0.4947

0.0380

0.4674

0.73

6.3

x1HC

x2HC

0.2924

0.7076

0.3529 0.4254

0.6471 0.5746

0.4555 0.5043 0.6740

x3HC

Figure 1. Tie lines for ternary mixture {Thiophene þ Heptane þ [EMIM][SCN]} at 298.15 K. Experimental data: þ and ;. NRTL Correlation (red) and ---- (red). UNIQUAC Correlation 0 (green) and -- (green).

Figure 2. Tie lines for ternary mixture {Thiophene þ Heptane þ [DMIM][MP]} at 298.15 K. Experimental data: þ and ;. NRTL Correlation (red) and ---- (red). UNIQUAC Correlation 0 (green) and -- (green).

{thiophene þ heptane þ [EMIM][SCN]}. The solute distribution ratio is about three times higher for low thiophene concentration in the case of presence of water. An increase of temperature decreases the solute distribution ratio.

S

Figure 3. Tie lines for ternary mixture {Thiophene þ Heptane þ [TEMA][MeSO4] } at 298.15 K. Experimental data: þ and ;. NRTL Correlation (red) and ---- (red).

Figure 4. Tie lines for ternary mixture {Pyridine þ Heptane þ [EMIM][SCN]} at 298.15 K. Experimental data: þ and ;. NRTL Correlation and (red) and ---- (red). UNIQUAC Correlation 0 (blue) and -- (blue).

The LLE data of the investigated ternary systems were correlated using the Non-Random Two-Liquid equation (NTRL) proposed by Renon and Prausnitz25 and the UNIversal QUAsi-Chemical (UNIQUAC) theory developed by Abrams and Prausnitz.26 2300



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Figure 5. Tie lines for ternary mixture {Pyridine þ Heptane þ [DMIM][MP]} at 298.15 K. Experimental data: þ and ;. NRTL Correlation (red) and ---- (red). UNIQUAC Correlation and 0 (green) and -- (green). Figure 7. Solute distribution ratio β as function of mole fraction of pyridine in the hydrocarbon-rich phase for two systems at 298.15 K.

Figure 6. Solute distribution ratio β as function of mole fraction of thiophene in the hydrocarbon-rich phase for the three systems at 298.15 K.

NRTL Model. For the NRTL Model, the activity coefficient γi, for any component i of the ternary system, is given by m P

ln γi ¼

0

τji Gji xj

j¼1 m P

l¼1

þ Gli xl

m P

1

xr τrj Grj C m X xj Gij B C B r¼1 C ð3Þ B τ ij m m A @ P P j¼1 Glj xl Glj xl l¼1

Figure 8. Water influence on solute distribution ratio β as function of mole fraction of thiophene in the hydrocarbon-rich phase for [EMIM][SCN] at 298.15 K.

UNIQUAC Model. For the UNIQUAC Model, the activity coefficient γi, for any component i of the ternary system, is given by m Φi z θi Φi X xj lj - qi lnðθj τji Þ ln γi ¼ ln þ qi ln þ li xi 2 Φi xi j ¼ 1

l¼1

with Gji = exp(-Rjiτji), τji = (gji-gii)/(RT) = (Δgji)/(RT), and Rji = Rij = R where g is an energy parameter characterizing the interaction of species i and j, xi is the mole fraction of component i, R is the nonrandomness parameter. Although R can be treated as an adjustable parameter, in this study it was set equal to 0.3.27

þ qi - qi

m X j¼1

Pm

θj τji m P θk τkj

ð4Þ

k¼1

P where Φi = (rixi)/( j=1rjxj), θi = (qixi)/( m j=1qjxj), lj = z/ 2(rj-qj)-(rj-1), and τji = exp((-Δuij)/(RT)). Here, the lattice 2301



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Table 3. Values of Binary Parameters for the NRTL Equation for the Ternary Mixtures system

ij

Δgij (J.mol-1)

Δgji (J.mol-1)

R

rmsd

Thiophene (1) þ Heptane (2) þ [DMIM][MP] (3)

12

5622.87

1771.01

0.3

0.0321

13

24275.07

6054.1

23

9001.6

20975.87

12

19100.2

5400.98

0.3

0.0056

13

18505.7

5836.4

23

9466.4

20557.2

12

12351.06

9629.64

0.3

0.0005

13 23

7913.05 15198.1

9420.3 5861.5

12

4521.7

1465.4

0.3

0.0338

13

12560.4

-3810

23

12602.3

19510.5

12

4605.5

-2679.5

0.3

0.0024

13

18003

5526.6

23

12058

12393

Thiophene (1) þ Heptane (2) þ [BMIM][SCN] (3)

Pyridine (1) þ Heptane (2) þ [DMIM][MP] (3)

Pyridine (1) þ Heptane (2) þ [BMIM][SCN] (3)

Thiophene (1) þ Heptane (2) þ [TEMA][MeSO4]

Table 4. Values of the Binary Parameters for the UNIQUAC Equation system

ij

Δuij (J.mol-1)

Thiophene (1) þ Heptane (2) þ [DMIM][MP] (3)

12

1386

64

13

2720

524

Thiophene (1) þ Heptane (2) þ [BMIM][SCN] (3)

Pyridine (1) þ Heptane (2) þ [DMIM][MP] (3)

Pyridine (1) þ Heptane (2) þ [BMIM][SCN] (3)

Δuji (J.mol-1)

23

1072

3300

12

6400

-810

13

2600

450

23 12

-150 -340

3960 2298

13

-212

2498

23

6929

-172

12

510

2330

13

4910

-860

23

4900

150

rmsd 0.0055

0.0112

0.0058

0.0117

coordination number z is assumed to be equal to 10, and ri and qi are respectively a relative volume and surface area of the pure component i. Parameters ri and qi are respectively relative to molecular van der Waals volumes and molecular surface areas. They are calculated as the sum of the group volume and group area parameters Rk and Qk X X ri ¼ υik Rk and qi ¼ υik Qk ð5Þ k

k

where υik is the number

of groups of type k in molecule i. The group parameters Rk and Qk are obtained from van der Waals group volumes and surface areas, and Vk and Ak are taken from the UNIFAC group contributions28 Vk Ak and Qk ¼ Rk ¼ ð6Þ 15:17 2:5 109 The values of 15.17 and 2.5 109 are respectively the standard segment volume and standard segment area of a methylene group.28 This means that a total of two adjustable parameters per binary Δgji or Δuji have to be fitted for both models. Interaction parameters for binary systems {thiophene þ n-heptane} and {pyridine þ n-heptane} are not available in the literature at 298 K. These parameters are then fitted using the three ternary mixture data.

Figure 9. Evolution of model gasoline components weight % changes during extractive desulfurization using [EMIM][SCN] as a solvent at 298.15 and 303.15 K.

Binary interaction parameters for both NRTL and UNIQUAC equations are those which minimize the difference between the 2302



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Figure 10. Evolution of model gasoline components weight % changes during extractive desulfurization using [DMIM][MP] as solvent, 298.15 K and 303.15.

Figure 11. Evolution of model gasoline components weight % changes in three-steps extraction process using [EMIM][SCN] as solvent at 298.15 K.

experimental and calculated mole fractions Fobj: ¼

N X 3 X k¼1 i¼1

I , exp fðxi, k

I , calc - xi, k Þ2

II , exp þ ðxi, k

II , calc - xi, k Þ2 g

ð7Þ

I,calc where N is the number of tie lines in the data set, xI,exp i,k and xi,k are the experimental and calculated mole fractions of one phase, and and xII,calc are the experimental and calculated mole fractions of xII,exp i,k i,k the second phase. The binary parameters and root mean-square deviation (rmsd), calculated using the procedure above, of the NRTL and UNIQUAC equations are given in Tables 3 and 4, respectively. The rmsd values, which provide a measure of the accuracy of the correlations, were calculated according to the following equation:

0

11=2 N P 3 P I , exp II , exp I , calc II , calc fðxi, k - xi, k Þ2 þ ðxi, k - xi, k Þ2 gC B Bk ¼ 1 i ¼ 1 C rmsd ¼ B ð8Þ C @ A 6N

Figure 12. Evolution of model gasoline components weight % changes in three-steps extraction process using [DMIM][MP] as solvent at 298.15 K.

Figure 13. Evolution of model diesel components weight % changes in three-steps extraction process using [EMIM][SCN] as solvent at 298.15 K.

For the ternary system {Thiophene þ Heptane þ [TEMA][MeSO4] }, the UNIQUAC model was not applied due to the lack of volume, Rk, and area, Qk, parameters of van der Waals groups for this ionic liquid. As can be inferred from the rmsd values, fairly good correlation of the experimental values with NTRL and UNIQUAC models was obtained. In the case of ternary systems {Thiophene þ n-heptane þ [DMIM][MP]} and {Pyridine þ n-heptane þ [BMIM][SCN]}, the UNIQUAC model fits much better than the NRTL equation.

’ MODEL FUELS EXTRACTION The weight percentage of model gasoline components changes as a function of extraction time using [EMIM][SCN] and [DMIM][MP] as an extractive solvent are shown at Figures 9 and 10, respectively. In both experiments, the concentration of sulfur and nitrogen -containing compounds reached the minimum after 2 h. Thus, the extraction period applied in all fuel desulfurization stages was 2 h. The higher temperature of extraction process causes the higher 2303



Figure 14. Evolution of model diesel components weight % changes in three-steps extraction process using [DMIM][MP] as solvent at 298.15 K.

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Figure 16. Evolution of thiophene content (wt%) in simulated gasoline for investigated ionic liquids in three-step desulfurization process.

Figure 17. Evolution of thiophene content (wt%) in simulated diesel for investigated ionic liquids in three-step desulfurization process. Figure 15. Evolution of model diesel components weight % changes in three-steps extraction process using [TEMA][MeSO4] as solvent at 298.15 K.

equilibrium content of thiophene as well as pyridine in refined gasoline (Figures 9 and 10). Thus desulfurization conducted at 298.15 K for both examined [EMIM][SCN] and [DMIM][MP] is more efficient, and this temperature was applied in all further experiments presented in this work. Model gasoline has been desulfurized in three steps at 298.15 K using [EMIM][SCN] and [DMIM][MP] as solvent. Figures 11 and 12 show the composition of model gasoline mixture and changes of weight percent of each component after one, two, and three extraction steps. The same experiments have been performed for simulated diesel oil. Desulfurization were conducted with [EMIM][SCN], [DMIM][MP], and [TEMA][MeSO4] as an extractive solvents. Results are presented at Figures 13-15. Sulfur and nitrogen containing compounds are extracted from hydrocarbon fuels by all free investigated ionic

liquids. The best results have been obtained for [EMIM][SCN] where after the third stage most of the thiophene and all of the pyridine have been extracted. These results are in agreement with obtained LLE data of ternary systems. However, deep desulfurization is also accompanied with unwanted reduction of aromatic hydrocarbon concentration, which causes lower octane number of the fuels. Comparison of investigated ionic liquids for gasoline and diesel desulfurization is presented in Figures 16-18. [EMIM][SCN] ensures 94% of thiophene content reduction in model gasoline and 88% in model diesel oil, while dibenzothiophene is completely extracted from diesel after three process stages. This ionic liquid [TEMA][MeSO4] chosen due to its relatively low price presents the lowest capacity for fuels desulfurization. The use of [TEMA][MeSO4] allows for extraction of about 10% of thiophene and 20% of dibenzothiophene from simulated diesel. It shows that this IL is useless for fuel desulfurization purpose. The third investigated ionic liquid [DMIM][MP] allows for more than 50% of sulfur containing compounds 2304



Figure 18. Evolution of dibenzothiophene content (wt%) in simulated diesel for investigated ionic liquids in three-step desulfurization process.

extraction, effective denitrogenation without fuel dearomatization.

’ CONCLUSIONS (Liquid þ liquid) equilibrium data for the five ternary systems {thiophene þ n-heptane þ [EMIM][SCN]}, {pyridine þ n-heptane þ [EMIM][SCN]}, {thiophene þ n-heptane þ [DMIM][MP]}, {pyridine þ n-heptane þ [DMIM][MP]}, and {thiophene þ n-heptane þ [TEMA][MeSO4]} at T = 298.15 K were determined. All ternary systems correspond to the Type 2 category, with partial or total immiscibility of the IL with the thiophene and the hydrocarbons. The NRTL and UNIQUAC models were used to correlate the experimental LLE results. In general, the LLE data of the ternary systems studied are better correlated with the UNIQUAC model than with the NRTL model. Then, it was found that [EMIM][SCN] has a good capacity for fuels desulfurization. Indeed, [EMIM][SCN] ensures 94% of thiophene content reduction in model gasoline and 88% in model diesel oil, while dibenzothiophene is completely extracted from diesel in the third process stage. This ionic liquid [TEMA][MeSO4] chosen due to its relatively low price presents the lowest capacity for fuels desulfurization.

’ AUTHOR INFORMATION Corresponding Author

*Phone: þ33 3 83 17 51 31. Fax: þ33 3 83 17 53 95. E-mail: [email protected].

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Extraction of Thiophene or Pyridine from n-Heptane Using Ionic

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