Extraction of sulphur compounds from model fuels with use of ionic liquids.

Abstract

One of the most promising applications of ionic liquids in the modern chemical industry is its use as extractants in separation methods, primarily extraction and extractive distillation. Sulphur emissions are regulated widely by governments and international organisations, to ensure the lowest possible emission of sulphur oxides to the atmosphere, with many stipulations that the amount of sulphur compounds in the fuel must not exceed 10 ppm. This study focused on the screening of the potential extractants for the desulphurisation of the fuels by performing experimental measurements for five different ionic liquids via two-phase separation techniques, namely, gas-liquid chromatography (GLC) and liquid-liquid extraction (LLE) measurements. The 1-butyl-1-methypyrrolidinium dicyanamide [BMPYR][DCA] was used as a solvent in a packed column during gas-liquid chromatography measurements of the retention times which allowed for the calculation of activity coefficients at infinite dilution. This technique was used to ensure coverage of the whole spectrum of the experimental methods and thermodynamic principles for assessing the suitability of ILs for the intended purpose. Four ionic liquids: dihydroxyimidazolium bis{(trifluoromethyl)sulphonyl}imide [OHOHIM][NTf2], 1-butyl-3-methylimidazolium trifluoromethanesulphonate [BMIM][OTf], 1-butyl-1-methylpiperidinium dicyanamide [BMPIP][DCA] and tri-iso-butylmethylphosphonium tosylate [P-i4,i4,i4,1][TOS], were were assessed via liquid-liquid equilibria measurements for suitability as extractants. Ternary LLE measurements for the systems {IL+thiophene+hydrocarbon} were performed, and the selectivity (S), distribution ratio (β) and performance index (PI) of the ILs investigated in this work was compared to ILs reported in the literature. The LLE tielines were modelled using the Non-Random Two Liquid (NRTL) model, which showed a satisfactory correlation of the experimental data, with a maximum absolute average deviation of 0.01 in the mole fractions. The most promising IL determined in this study is [BMIM][OTf] with PI equal to 54.8 and 193 for octane and hexadecane systems, respectively. This conforms with the findings in literature as imidazolium ILs with relatively short alkyl substituents as reported to be the most promising extractants in terms of performance index (PI). The least promising results were obtained for [BMPIP][DCA]. Dicyanamide anion was not strongly represented in LLE literature, yet PIs calculated using activity coefficients data for ILs with this anion provided justification for further investigations. The recommendation from this work is that laboratory-scale extraction measurements be performed to assess the viability and reuse of this IL.