Process Design and Optimization for Desulfurization of Diesel Using Ionic Liquids

Abstract

Sulfur dioxide emissions to the atmosphere have been known to cause detrimental health and environmental effects. The currently used hydrodesulfurization (HDS) method employed by refineries has several drawbacks, such as excessive hydrogen consumption, high energy demand and inability to remove complex organosulfur compounds. These drawbacks have limited its ability to produce ultra-low sulfur diesel (ULSD) at reasonable operating and capital costs. Ionic liquids (ILs) have been widely researched in efforts to develop industrial processes that can complement or replace the conventional HDS. However, their success has only been proven on an experimental level with limited research conducted with regards to their industrial scale feasibility and their integration into process simulators such as ASPEN Plus. In this work, quantum and statistical thermodynamic calculations and property estimations methods have been successfully combined to generate an IL database that contains all properties necessary for simulating IL processes in ASPEN Plus using COSMO-SAC property package for a total of 26 commercially available ILs. Upon the integration of all components, several possible process configurations have been conceptualized and the performance of each IL was assessed. In particular, the challenge of ionic liquid regeneration, which has largely been ignored in literature, has also been addressed and several potential regeneration methods have been proposed including extractive regeneration (E-RE) and stripping regeneration using nitrogen/air as stripping media (S-RE). The results indicated that 1-butyl-3-methylimidazolium thiocyanate is the most promising IL among all 26 ILs under study in terms of EDS, E-RE and S-RE. It was found that E-RE was effective in the removal of dibenzothiophene (DBT) while S-RE was more effective in the removal of thiophene and benzothiophene (BT). As a result, an optimized diesel desulfurization process that is a combination of all configurations understudy has been proposed. This was necessary to obtain ULSD, maximum removal of thiophene, BT and DBT from spent IL stream without imposing contaminants such n-hexane, and with minimum losses of n-hexadecane (diesel). The proposed process achieved ULSD with 6.53 PPM total sulfur, 0.06% loss of n-hexadecane, and 100% IL recycling with the recycled stream containing 0% thiophene, 10% BT and 12% DBT.