Use of Ultrasound and Ionic Liquids to Enhance the Extractive Desulfurization of Oils

Abstract

Desulfurization of diesel oil has been a widely investigated process, due to the detrimental impact on health and the environment caused by sulfur dioxide emissions resulting from the combustion of diesel oil. This has recently impelled many governments to reduce the maximum sulfur content of diesel to 10 ppm. The current desulfurization technology, hydrodesulfurization (HDS), is energy intensive, uses large quantities of hydrogen, and is ineffective at removing some sulfur compounds. The need for cost effective desulfurization methods to achieve this tighter specification gave rise to the study of replacement methods. In laboratory scale equilibrium studies, the use of ionic liquids (ILs) within the context of liquid-liquid extraction (LLE) has been demonstrated to be a viable, low energy, substitute to HDS. However, mass transfer limitations and scale-up issues have not previously been investigated. Since ILs suffer from high viscosities, which reduces the rate of interphase mass transfer, this would require more separation stages and larger processing equipment. Recently, the effect of ultrasound (US) on mass transfer has been investigated to establish whether it can be used to enhance mass transfer in industrial processes. This work aims to study the process development issues associated with extractive desulfurization of diesel using ILs: interphase mass transfer, phase dispersion and settling behavior, and IL regeneration. The impact of US irradiation on each of these process steps is investigated. The study involved the use of model diesel, in which hexadecane was used along with dibenzothiophene (DBT) as the model sulfur compound. The extraction solvent used was 1-ethyl-3-methylimidazolium acetate ([C2mim][CH3COO]) IL. This study has shown that sonification can greatly enhance the rate of mass transfer of DBT from diesel to the IL, resulting in a 57% sulfur removal for a 75% decrease in extraction time, and leading to a reduction in the number of extraction stages required. Furthermore, the phase settling rate was found to be increased by both sonification and the addition of a surfactant. The IL was easily regenerated by extraction using hexane, ultimately obtaining a regeneration of 93% through multiple extractions. However, sonification was found to have no significant benefit in this step of the process.