Application of 1-pentyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide for desulfurization, denitrification and dearomatization of FCC gasoline
Introduction
Production of fuels of ultra-low sulfur, nitrogen and aromatic content has been widely investigated due to the ever increasing environmental requirements [1]. Diesel and gasoline that are rich in aromatics and sulfur and nitrogen compounds will produce exhaust gases containing significant amounts of SOx, NOx, CO, CO2, hydrocarbons and solid particles which will contribute to the environmental pollution.
Motor gasoline is a blend of several types of gasoline, mostly produced with secondary conversion processes, such as catalytic cracking, reforming and alkylation. FCC (fluid catalytic cracking) gasoline is one of the most important ingredients of commercial gasoline, with overall contribution of sulfur up to 98%. This is why FCC gasoline desulfurization is highly necessary [2]. Today, commonly used industrial process for desulfurization of gasoline is hydrodesulfurization (HDS), in which organic sulfur compounds are converted to H2S and the corresponding hydrocarbons. The process requires high temperatures and pressures, and large amounts of hydrogen, making HDS expensive and relatively environmentally unfriendly. The HDS process is highly efficient in removing thiols, sulfides and disulfides; however it is less effective for thiophene and its derivatives [3], [4], [5]. The sulfur compounds that remain in motor fuels are mainly thiophene and its derivatives, which make deep-desulfurization by HDS extremely difficult.
Nitrogen compounds inhibit the conventional HDS process [6] because of their role in catalytic deactivation [7]. In addition, they are responsible for the formation of deposits [8]. Denitrification of feeds before HDS would enhance the depth of desulfurization [9]. Nitrogen compounds can be removed from the mixture of hydrocarbons by hydrotreating with the selection of the appropriate catalyst [1] or by means of liquid–liquid extraction.
Apart from the mentioned environmental demands, high aromatic content also lowers the fuel quality. Therefore, the separation of aromatic from aliphatic hydrocarbons is a very important process in petrochemical industry as well [10].
Different separation processes can be used for the separation of aromatic hydrocarbons from C4 to C10 aliphatic hydrocarbon mixtures depending on the aromatic content. Liquid–liquid extraction is one possibility [11]. Any commercial organic solvent would usually require an additional separation step, since solvent must be removed from both phases. Since liquid–liquid extraction has many advantages, such as mild operation conditions, it is a reasonable choice to investigate application of new environmentally acceptable solvents that could replace volatile organic solvents. In principle, extraction is based on better solubility of sulfur and nitrogen compounds as well as aromatic hydrocarbons in an appropriate solvent with respect to non-aromatics. The process efficiency, for the most part, depends on the proper solvent selection, which means that a compromise must be reached between solvent selectivity and capacity, and other properties including environmental and toxicological restrictions. For the mentioned reasons many researchers investigated the effectiveness of different types of ionic liquids for desulfurization [12], [13], [14], [15], [16], [17], [18], [19], [20], denitrification [17], [18], [19], [20], [21] and dearomatization [21], [22], [23] of model solutions and real refinery streams. Ionic liquids are very good solvents for a wide range of organic, inorganic and polymeric compounds. Owing to their negligible vapor pressure, ionic liquids are considered as green solvents. Ionic liquids can be regenerated by evaporation of volatile components under high vacuum [24]. This is very important because commercially used solvents in liquid–liquid extraction usually generate organic waste which at the end requires disposal.
In this article liquid–liquid equilibria have been experimentally determined for eight systems, at T = 298.15 K and 101325 Pa, with thiophene or pyridine as the key component to be distributed between a hydrocarbon (toluene, n-hexane, n-heptane or i-octane) and ionic liquid component 1-pentyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. The applicability of [C5mim][Tf2N] for desulfurization, denitrification and dearomatization of model solutions and real FCC gasoline by means of liquid–liquid extraction was tested experimentally. This ionic liquid was selected according to the previously published results obtained with several ionic liquids [20]. Among six tested ionic liquids, [C5mim][Tf2N] exerted the best potential as a selective solvent for desulfurization and denitrification of model solution that represents FCC gasoline. The highest extraction efficiencies and the most appropriate properties [20] (low viscosity, high density difference, low interfacial tension) for selective solvent were observed with [C5mim][Tf2N].
Section snippets
Experimental
In this work, the equilibrium compositions for the eight systems comprising a common ionic liquid component [C5mim][Tf2N] (IL, 1-pentyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide) as component (3) were experimentally determined. Component (1) was selected among four hydrocarbons (n-hexane, n-heptane, i-octane, toluene) and component (2) was either thiophene or pyridine. The measurements were performed at 101325 Pa and at temperature of 25 °C. All the composition determinations were
Binodal curves
The investigation of liquid–liquid equilibria in systems comprising ionic liquids has gained much attention recently. However, to the best of our knowledge, the systems with [C5mim][Tf2N] as IL component have not been investigated so far. Binodal curve data are presented in table 2. The data are indicated by the corresponding interpolation lines in FIGURE 1, FIGURE 10, FIGURE 11, FIGURE 2, FIGURE 3, FIGURE 4, FIGURE 5, FIGURE 6, FIGURE 7, FIGURE 8, as well.
Solubility of ionic liquid in
Conclusions
Liquid–liquid equilibria for the three-component systems with ionic liquid [C5mim][Tf2N] – (pyridine or thiophene) – hydrocarbon (toluene or n-hexane or n-heptane or i-octane) were determined at T = 298.15 K and atmospheric pressure using a variant of the turbidimetric titration method and refractometry. Based on the calculated values of solute distribution ratios and ionic liquid selectivity, it can be concluded that the partial dearomatization will follow processes of desulfurization and
Acknowledgments
The authors would like to thank INA-industrija nafte d.d. and Croatian Academy of Sciences and Arts for the financial support.
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2021, FuelCitation Excerpt :In order to address this scientific question, a comprehensive literature survey is necessary. According to the literature considering the reported LLE data for about 84 ternary systems [7,9,17–45], some paradoxical points were found. For example, the mole fractions of thiophene in the IL-rich phase (i.e., Y2) and the hydrocarbon-rich phase (i.e., X2) have a positive linear correlation in each constant system in the expanded range of X2.
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