Article history:Received 2 October 2013Received in revised form 10 January 2014Accepted 11 January 2014Available online 3 February 2014
Keywords:ExtractionOxidationThiophene compoundsModel fuelParametric study
Thiophenic sulfur removalwas attempted in three differentmodes of operation, such as extraction, oxidation andsimultaneous oxidation–extraction for a model fuel. Extraction was done using different solvents, viz.,acetonitrile, dimethyl sulfoxide and 1-ethyl 3-methyl imidazolium ethyl sulfate at near ambient condition. Thesulfur compounds tested were thiophene, benzothiophene and dibenzothiophene in iso-octane. Titaniumsilicate-1 and titanium-beta catalysts were used for oxidation of the compounds. A detailed parametric studywas performed varying stirring time, extraction temperature and amount of solvent, to investigate their effectson the extraction of the three sulfur compounds. Simultaneous oxidation–extraction process showed a significantamount of sulfur removal. Dimethyl sulfoxide was found to be the best solvent for extraction.
Ultra-low sulfur fuel is of great demand in today's world andmost ofthe countries impose a strict regulation in lowering the sulfur content inpetroleum oils. According to the U.S. guidelines the sulfur level in dieselfuel should not be more than 15 ppmw [1]. In India, the EURO IVstandard, 50 ppmw is the highest limit of sulfur for both gasoline anddiesel since 1st April, 2010 [2].
Themost common practice to remove sulfur from fuel oils in refiner-ies is hydrodesulfurization (HDS), where high temperature and hydro-gen pressure are used to remove sulfur compounds. The ranges oftemperature and hydrogen pressures that are usually used in HDS are290–455 °C and 150–3000 psi respectively with dual transition metal(Co–Mo, Ni–Mo, or Ni–W) catalysts [3,4]. This process is more effectivefor aliphatic and acyclic sulfur compounds but it is less effective for re-fractory thiophenic derivatives [5]. It is necessary to improve the currentHDS technology particularly to achieve the production of light oil withlow level of sulfur. To avoid the need of hydrogen, high pressure and/or high temperature, a low severity alternative desulfurization tech-nique is required to efficiently remove the aromatic sulfur compoundsfrom oils. Hence, a number of non-HDS processes are now investigated,such as, adsorption [6,7], bio-desulfurization [8,9], extraction [10–12],and oxidation [13,14]. Extractive desulfurization is one of the attractivetechniques to remove sulfur, because it can be operated at ambienttemperature and pressure without changing the chemical structure ofthe fuel oil components. The solvents can be regenerated and the sulfurcompounds can be recovered.
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Most of the organic solvents used in extraction are highly volatileand this may cause solvent loss and toxicity in the environment. Ionicliquid (IL) is such a solvent that produces negligible vapor and thus rep-resents a new class of solvents as well as an extracting agent. Because oftheir non-volatility, non-flammability and high thermal stability, ILs areeasy to handle and they produce less environmental pollution [15]. Ithas been observed that, extraction can remove sulfur compoundsfrom fuel oil up to a certain degree whereas, when extraction followsoxidation, the extent of removal is higher [16, 17].
Caero et al. [18] investigated the effects of temperature and amount ofsolvent on oxidative desulfurization using TiO2 supported V2O5 catalystcoupled with extraction with acetonitrile for removing benzothiopheneand dibenzothiophene in synthetic fuel. Zhu et al. [19] reported theoxidative desulfurization of fuels by peroxotungsten and peroxo-molybdenum complexes in the presence of ionic liquids. The removal ofdibenzothiophene from model fuel by extraction using six halogen-freeionic liquids was studied by Mochuzuki and Sugawara [20]. Recently,Wilfred et al. [21]used imidazoliumbased ionic liquids for desulfurizationof dibenzothiophene in dodecane by extraction process.
The objective of the present work is to investigate the performanceof oxidation and extraction, either separately or coupled together, inthe removal of thiophenic compounds from a model fuel containingthiophene (TH), benzothiophene (BT) or dibenzothiophene (DBT) iniso-octane. It has been planned to perform the work in two segments,a) physical extraction of sulfur compounds using organic solvents,acetonitrile and dimethyl sulfoxide (DMSO) and ionic liquid (IL) 1-ethyl-3-methyl imidazolium ethylsulfate (EMIES); and b) single potoxidation–extraction using similar sets of solvents and catalysts. It alsoincludes the study of regeneration and reusability of ionic liquid as anextracting solvent.
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2. Materials and methods
2.1. Materials
Tetra n-butyl orthotitanate (98%) from Merck, India, tetra ethylammoniumhydroxide (25% aqueous solution) and thiophene fromSpectrochem, India, anhydrous sodium aluminate, fumed silica, hydro-gen peroxide (30%) and iso-octane (99.5%) from Sigma Aldrich, India,benzothiophene from Himedia, India and dibenzothiophene from AlfaAesar were procured.
Acetonitrile and DMSO from Merck, India, and EMIES IL from SternChemicals, USA were purchased.
2.2. Experimental methods
The model fuel was prepared by dissolving thiophene (TH),benzothiophene (BT) or dibenzothiophene (DBT) in iso-octane at aconcentration of 1000 ppmw sulfur.
The experiments of desulfurization, either by simultaneous oxida-tion and extraction or by sole extraction were performed in a 100 mlglass vessel fitted with a glass stirrer and condenser. The vessel waskept in a water bath whose temperature wasmaintained by a tempera-ture controller cum indicator. In the extraction experiment, extractingsolvent andmodel fuel at desired ratio weremixed together and heatedat 303 K under stirring for 30 min. After that, the whole content of thevessel was kept undisturbed at that temperature until the solvent and
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Fig. 2. Sulfur removal using liquid-liquid extraction for TH, BT and DBT in model fuelby different solvents Temperature: 333K, S-concentration: 1000 ppm, solvent: modelfuel :: 0.5:1 (v/v).
Fig. 3. Sulfur partition between extracting agent and model fuel of (a) TH, (b) BT and(c) DBT.Temperature: 303 K, extraction time: 30 min, solvent: model fuel :: 5:1 (v/v).
iso-octane phases were completely separated. Samples from the iso-octane phase were collected to be analyzed in HPLC.
The thiophenic sulfur compounds were oxidized by hydrogenperoxide in the presence of nano-titanium silicate-1 (TS-1) andmesoporous titanium beta (Ti-beta) catalysts. TS-1 was used for THand Ti-beta was used for BT and DBT oxidation. Preparation andcharacterization of the catalysts and oxidation of TH, BT and DBTwere done by the authors in their previous works [22. 23].
Fig. 4. Effect of extractant to fuel ratio on desulfurization efficiency (a) TH, (b) BT and (c)DBT.Temperature: 333K, S-concentration: 1000 ppm, model fuel : 5 ml, extraction time:30 min.
Single pot oxidation–extraction experiments were carried out in theglass vessel with a desired amount of extracting solvent and 20 ml ofmodel fuel with required catalyst at a particular temperature andstirring speed. Hydrogen peroxide was mixed as an oxidizing agent tothis mixture. Samples at different intervals of time were withdrawn tobe analyzed in HPLC.
The analysis of samples was done in HPLC (Perkin Elmer, USA) usingreverse phaseMerck SP-18 column. Thewavelength of UV detector wasset at 254 nm.
3. Results and discussion
The effects of different process parameters on the solvent extractionprocess with different solvents and the oxidation in the presence ofextracting solvents are presented in this section.
3.1. Extraction of sulfur compounds
The removal of TH, BT andDBT frommodel fuel by solvent extractionwith acetonitrile, DMSO and EMIES IL was investigated. The concentra-tion of sulfur compounds in model fuel was 1000 ppmw of S.
3.1.1. Selection of optimum extraction timeThe effect of time on desulfurization efficiency of three different
solvents is presented in Fig. 1, where BT at the concentration of1000 ppmw of sulfur was used in model fuel. The ratio of model fuelto extracting agent was set to 4:1 (v/v) for this experiment. The timeof extraction was varied performing different extraction experimentswith 5 min, 10 min, 20 min, 30 min and 40 min. The liquid–liquidequilibrium was found to be achieved within 30 min confirming arapid extraction process as observed from the figure. The maximumdesulfurization efficiency for acetonitrile, DMSO and EMIES IL wasfound to be 32, 53.5 and 46% respectively. After 30 min of continuousstirring, the final sulfur concentration in extracting solvent is leveledoff for all the solvents and increasing extraction time does not help inimproving the efficiency. Similar results (10–25 min) were reportedfor extraction equilibrium at room temperature [24, 25].
3.1.2. Desulfurization pattern of different sulfur compounds with respect toextractants
A comparison of physical extraction using solvents, acetonitrile,DMSO and EMIES IL, was done for three thiophenic compounds, TH,BT and DBT at 333 K under atmospheric pressure and depicted inFig. 2. It is observed that for all the solvents, the removal of thiophene
Fig. 5. Desulfurisation efficiency of different solvents for DBT removal at differentextraction temperatures Solvent : model fuel :: 0.4:1 (v/v), amount of model fuel : 10ml, S-concentration: 1000 ppm, extraction time: 30 min.
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is the lowest among sulfur compounds tested. DMSO is observed to bethe best solvent for all the three sulfur compounds with a highestremoval for DBT, 70.89%.
3.1.3. Partition coefficient of solvents for different sulfur compoundsThe values of the Nernst partition coefficient (KN) for extraction using
the aforementioned three solvents for three different thiophenic com-pounds are shown in Table 1. The coefficient is defined as the ratio ofthe concentration of sulfur in extract phase (mg [S]·kg [extractant]−1)to that in oil (mg [S]·kg [oil]−1). Higher KN value represents better
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Fig. 6.Multistage desulfurization of (a) TH, (b) BT and (c) DBT compounds by acetonitrile,DMSO and EMIES IL.Temperature: 303 K, S-concentration: 1000 ppm,model fuel : solvent:: 4:1 (v/v).
desulfurization efficiency. Equilibrium sulfur contents (TH, BT, DBT)between model fuel and different extracting agents presented inFig. 3(a)–(c) explain that the sulfur content in the extract phase is a linearfunction of sulfur content in the model fuel at equilibrium in the studiedrange of sulfur concentration. This is the behavior of an ideal equilibriumfor dilute solution. Hence, the partition coefficient is dependent on sulfurconcentration. The values of the partition coefficients obtained by usingacetonitrile and DMSO are larger than those of EMIES IL. The descendingorder of partition coefficient for three sulfur compounds using IL is DBTN BTN THwhereas, for acetonitrile the order is reverse. ForDMSO, thepar-tition coefficient decreases following the order BT N DBT N TH.
3.1.4. Effect of solvent to oil ratioFig. 4(a)–(c) shows the influence of the extractant to model fuel
ratio (v/v) on desulfurization efficiency for the removal of the threesulfur compounds at 303K. It has been observed that thedesulfurizationefficiency increases for all three extractants with an increase in volumeratio. However, the rate of increase of desulfurization efficiency isslower at high extractant to model fuel ratio. Therefore, an increase inextractant amount for getting high desulfurization does not fulfill thepurpose and hence makes the process less efficient. Similar resultswere reported in the earlier works [25,12].
3.1.5. Effect of extraction temperatureThe effect of temperature on extractive desulfurization of model fuel
containing DBT with extractant to model fuel ratio 0.4:1 (v/v) by vary-ing temperature from 298 to 333 K, is shown in Fig. 5. This temperaturerange was chosen as the extracting solvents are quite stable at thatrange. The mild decreasing trend of the desulfurization efficiency withan increase in temperature was observed for all the extractants. Thesimilar trend was reported elsewhere [26].This decreasing trend maybe due to the change in the distribution of S-compounds in iso-octaneand extracting solvent with temperature, which in turn affects thepartition coefficient. The suitable extracting temperature was chosenfor subsequent runs to be 303 K for all the extracting solvents, whichis close to room temperature and convenient for operation.
3.1.6. Desulfurization by multistep extractionThe results of multistage desulfurization of model fuel containing
thiophene (1000 ppmw S) with acetonitrile, DMSO and EMIES ILextracting solvents are shown in Fig. 6(a)–(c). The extractionprocess was carried out at 303 K using model fuel to extractantratio of 4:1 (v/v). It has been observed that the sulfur content inthe model fuel decreased from 1000 ppmw to below 10 ppmwafter eight extraction steps with acetonitrile and DMSO, whereas, IL
Fig. 7. Sulfur partition coefficients between EMIES and model fuel at different water con-tents. Temperature: 303 K, S-concentration: 1000 ppm.
Fig. 9. Sulfur removal by simultaneous oxidation and extraction process.Catalyst: TS-1forTH and Ti-beta for BT and DBT, Reaction temperature: 333K, time: 2 h, H2O2 to S moleratio :: 10:1, amount of catalyst :7.5 g/L-1, S-concentration: 1000 ppm.
Table 2The desulfurization of model fuel by regenerated [EMIES].
required fourteen steps to get the same result. Similar types ofresults were obtained for BT and DBT.
3.1.7. Regeneration of EMIES ILIn the extraction by ionic liquid, regeneration and recycling are
having immense value. The EMIES IL, which is used in this process asextractant is hydrophilic in nature while all sulfur compounds arehydrophobic. Separation of sulfur compounds from IL can be achievedby the addition of water in IL [27,28]. Therefore, extraction experimentswere performed at 303 K by using IL with different dilutions in waterfrom 0 to 70% and the results are presented in Fig. 7. It is observedfrom the figure that the partition coefficient (KN) decreases with anincrease of water dilution. A rapid drop of KN value is noticed within15% and KN shows almost zero value at 70%, indicating the completeremoval of sulfur compounds from IL at this dilution. Hence, it can besaid that at 70% dilution, all the sulfur compounds are transferredfrom IL phase to oil phase.
Used IL from extraction experiments was regenerated from theextractants by the water dilution. An excess amount of water (1.5:1w/w) was added to the used IL and two phases are separated, anaqueous-EMIES phase free of sulfur compounds and an oil phasecontaining sulfur compounds. The oil phase is the small amount of oilwhich was entrained to IL during extraction process. Water from ILwas removed by distilling at 377 K for 2 h. A trace amount of watermay be present in IL after distillation [29]. To remove water from IL asmuch as possible, the concentrated IL after distillation was kept at 253K under vacuum for overnight to remove the rest of the water. Thisregenerated IL is used three times to extract TH, BT and DBT frommodel fuel at 303 K. The results presented in Table 2 indicate that theregenerated IL could be recycled three times without any significantdecrease in desulfurization efficiency.
3.2. Sulfur removal by simultaneous oxidation and extraction
Fig. 8 shows the conversion of TH oxidation with TS-1 and BT andDBT oxidation with Ti-beta using H2O2 as oxidant for 2 h, without
Fig. 8. Oxidation of TH, BT and DBT in model fuel by TS-1 (for TH) and Ti-beta (for BT andDBT) Reaction temperature: 333K, time: 2 h, H2O2 to S mole ratio :: 10:1, amount of cata-lyst :7.5 g/L-1, S-concentration: 1000 ppm.
extraction at 333 K. It has been observed from the figure that theconversion of TH is 88.83% and those of BT and DBT are 68.08 and72.49 respectively.
The experiments were carried out to study the removal of sulfurcompounds from model fuel with simultaneous oxidation and extrac-tion process at 333 K for 2 h and the results in the form of histogramare presented in Fig. 9. Desulfurization efficiency was enhanced whenextraction is coupled with oxidation in a single pot. It has been notedthat the removal of BT increases from 68 (without extraction) to 83%when oxidation is combined with extraction using DMSO as solvent.Whereas, there is a negligible increase in the removal of TH, the changeis only 1.2% by using same combined process. This is because, the oxida-tion reaction is fast enough to convert TH into its products, sulfuric acid,butane dioic acid, and propane dioic acid [30],which are highly solublein aqueous phase. Hence, the use of extracting solvent does not affectthe desulfurization efficiency much. The increase in desulfurizationefficiency by the addition of extracting solvent in the oxidation reactionfor BT and DBT might be due to the fact that the reactant and productsboth are transported to some extent to the solvent, which in turnincrease the overall efficiency. The products of oxidation of BT andDBT are their corresponding sulfone and sulfoxides, which are polarand they have appreciable solubility in DMSO [31].
4. Conclusion
Desulfurization by liquid–liquid extraction and its influence incatalytic oxidation of sulfur-laden model fuel were studied. Theoptimum time for extraction was 30 min, above which no significantdesulfurization was observed. DMSO was the best among the threesolvents, acetonitrile, DMSO and EMIES IL in terms of desulfurizationefficiency, which was observed as 49.87 for TH, 67.54 for BT with ahighest removal for DBT, 70.89% at 30 min with solvent: model fuelratio as 0.5:1 (v/v). For all the sulfur compounds and solvents, partitioncoefficients were found to be in a linear relation with the sulfur contentof the fuel. The partition coefficients of TH, BT and DBT using DMSOwere calculated as 1.39, 2.86 and 1.77 respectively. Ionic liquid EMIESused in extraction could be regenerated and reused. Multistep extrac-tion was found to be more effective than the single step extractionand acetonitrile and DMSO both required eight cycles to draw downthe sulfur content from 1000 ppm to less than 10 ppm in the modelfuel, whereas, EMIES required fourteen steps for the same operation.
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Single pot oxidation–extraction showed that the maximum removalof TH was 88.83, and those of BT and DBT were 68.08 and 72.49%respectively using DMSO as extractant.
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