(345e) Extractive Desulfurization of Fuels Using Ionic Liquids

Conventional hydro desulfurization (HDS) process is considered as an efficient sulfur removal process in oil refining industry. However, in the case of aromatic sulfur compounds (thiophenes, dibenzothiophenes and their alkylated derivatives), HDS process has some weak points. High level of hydrogen consumption and operating at high temperature and pressure which make it a hazardous process are the main challenges and impose high level of fixed and commercial costs. In this regard, research for finding alternative deep desulfurization techniques have been continued during last years and still is going on. Among various methods, extractive desulfurization using ionic liquids (ILs) as green solvents attracted more attention during last decades. This process can be run at ambient temperature and pressure. Meanwhile ILs have great capacity because of their special properties compared to common organic solvents. They are usually thermally stable and have negligible vapor pressure which makes them environmental friendly solvent. On the other hand, ILs formed by an anion and cation that can be easily designed for specific tasks by an appropriate combination of their ions. Previous works in this field show that imidazolium based ILs have greater selectivity while pyridinium based ILs are stronger extractive solvents. Some anions are not stable and decompose to harmful and toxic byproducts during extraction. Besides solubility and selectivity concerns, density and viscosity are important in extractive desulfurization as well. Usually, ILs with lower density show higher desulfurization efficiency.

Considering above facts, in this work various ILs have been preliminary screened based on their physical and chemical properties. Finally, extraction of thiophene from normal alkanes using ILs including nitrate [NO3]^- or thiocyanate [SCN]^- as anion and imidazolium or pyridinium as cation have been studied. Several ionic liquids such as 1-Octyl-3-methyl imidazolium thiocyanate [Omim][SCN], 1-hexyl-3-methyl imidazolium thiocyanate [Hmim][SCN], 1-butyl-3-methyl imidazolium thiocyanate [Bmim][SCN], 1-octyl-3-methyl imidazolium nitrate [Omim][NO3], 1-buyl-3-methyl imidazolium nitrate [Bmim][NO3], 1-hexyl-4-pyridinium nitrate [HPy][NO3] and 1-butyl-4-pyridinium nitrate [BPy][NO3] were synthesized. The structure of the synthesized ILs was checked using nuclear magnetic resonance (NMR) spectroscopy. The purified IL was dried and degassed for 24 hr. at 343.15 K under vacuum, then were kept in bottle under argon gas. The mass fractions of water in the dried products, measured by Karl Fischer coulometer, were less than 1×10^-3. These ILs were proved to be stable and inexpensive. Meanwhile they have low viscosity and density while are environmental friendly.

Liquid-Liquid Equilibrium (LLE) data for ternary mixtures containing sulfur compound, fuel model and IL were experimentally measured. Selectivity and distribution coefficient were calculated using experimental data. The experimental LLE were determined by preparation of immiscible mixtures of three components of IL+ n-alkane+ thiophene with total mass of 4gr. Each component was weighed by a laboratory balance with the precision of ±10^â??4 g. The mixture in the glass vial was closed firmly with a cap and septum and stirred by a shaker for 120 min at 300 strokes per minute. The glass vials were placed for at least 12 hr into a water bath with a precision of ±0.1° K. The two phases in the glass vials were separated and analyzed after reaching the equilibrium condition. The composition of each phase was determined using gas chromatography (GC). All analyses were repeated three times and the experimental uncertainties for the top and bottom phase compositions were less than ±1.10^â??3 and ±2.10^â??3in mole fraction, respectively.

The experimental LLE data for ternary mixtures showed that cation chain length of IL has a remarkable effect on selectivity of aromatic sulfur compounds. It was found that the IL with lower molecular weight of cation has higher selectivity and lower solute distribution coefficients. Besides, the experimental data demonstrated that the selectivity increases as alkane chain length of fuel model increases while the solubility of ionic liquids in the fuel models decreases, hence the costs of desulfurization process will be decreased. Moreover, the Results showed that for all systems when thiophene concentration decreases, the selectivity and distribution coefficient values increases. It means that ILs are good solvent at deep extractive desulfurization process. Meanwhile The selectivity remarkably increases as chain length of ILâ??s cation decreases for all systems while the solute distribution coefficient decreases slightly.

In order to check, the temperature on selectivity, extraction of thiophene from n-decane by BMIM[NO₃] and OMIM[NO₃] have been studied three different temperatures (288, 298 and 313 K). Temperature showed no great effect in desulfurization and the study showed that the selectivity increases as the solute mole fractions in the hydrocarbon-rich phase decreases. Also the selectivity and distribution ratio of thiophene was reduced with increasing the temperature.

Our results proved that [Bmim][SCN] has the highest selectivity and lowest alkane solubility, in the system containing thiophene and n-decane. In other words [Bmim][SCN] is a more suitable ionic liquid solvent than others for extractive desulfurization of higher alkane.

Finally measured experimental data were correlated using non- randomness two liquid (NRTL) theory and the model parameters were obtained. Ternary phase equilibrium diagrams for mentioned ternary mixtures including the experimental and calculated tie-lines have been reported. It can be seen that the model can correlate the experimental data efficiently.