Distribution of sulfur-containing aromatics between [hmim][Tf2N] and supercritical CO2: a case study for deep desulfurization of oil refinery streams by extraction with ionic liquids

Abstract

Stringent regulations of the maximum content of sulfur in transportation fuels have prompted an intense search for new processes of deep desulfurization to complement common catalytic hydrodesulfurization. One of the alternative processes under development involves extraction of sulfur-containing aromatic compounds (SAs) from diesel fuel and gasoline with ionic liquids (ILs). In these applications, recycling of ILs will be highly important, and reextraction of SAs from ILs with supercritical carbon dioxide (scCO₂) appears to be a possible method to regenerate ILs. The design and feasibility assessment of supercritical reextraction requires the partition coefficients of SAs between ILs and scCO₂. We used open tubular capillary-column chromatography to measure the partition coefficients of several SAs between 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([hmim][Tf₂N]) and scCO₂, with [hmim][Tf₂N] serving as the stationary liquid and scCO₂ as the mobile phase (carrier fluid). The results confirm that the partition coefficients of SAs can be tuned within wide limits (over a decadic order of magnitude) by relatively modest changes in temperature (40–80 °C) and pressure (8.7–17.6 MPa). However, even the most favourable values of the partition coefficients of SAs between [hmim][Tf₂N] and scCO₂ suggest that feasibility of a large-scale supercritical reextraction process is questionable at best.