Recycling of used lubricating oil by solvent extraction: experimental results, Aspen Plus simulation and feasibility study

This study investigates the recycling of used lubricating oil (ULO) by solvent extraction using a ternary solvent consisting of 2-propanol, 1-butanol and methyl ethyl ketone (MEK) followed by a vacuum distillation unit. In order to select the best combination of solvents for the liquid–liquid extraction process, many parameters were first evaluated on a laboratory scale such as the effect of the solvent composition, the solvent-to-oil ratio and the temperature. The oil recovery efficiency and the physicochemical properties of the oil obtained from the extraction process were measured, and the results showed that a ternary system consisting of 50% 1-butanol/25% isopropanol/25% MEK is the best mixture for effectively reducing the contaminants in ULO with a good sludge separation prior to the fractionation. Also, a solvent-to-oil ratio of 3:1 and a temperature of 25 °C are the optimal parameters for the extraction when the aforementioned solvent mixture is used. The whole re-refining process including the vacuum distillation unit was then simulated for a plant capacity of 2125 kg/h using Aspen Plus™. Four pseudo-components (saturate, monoaromatic, diaromatic and polyaromatic) together with the nonrandom two-liquid (NRTL) and universal quasichemical activity coefficient (UNIQUAC) were used to describe the liquid–liquid equilibrium (LLE) in the system solvent/lubricating oils during the extraction phase. The calculated yields on the extraction unit performed for different solvent composition, solvent-to-oil ratio and temperature showed a good agreement with the experimental data performed at the laboratory scale which verifies the right choice of the solvent mixture. The investigation of the amount and composition of oil loss inside the sludge obtained from the simulation showed that the contaminant reduction and the oil recovery depend on the percentage of 1-butanol in the solvent mixture. Also, the results confirmed that the amount of MEK and 2-propanol in the solvent mixture should not exceed 25% and the best results were obtained when 1-butanol was used as a base solvent with an amount of 50%. Finally, to check the feasibility of the process, the investment payback time and the return on investment of the re-refining process were evaluated. Based on the economic study results, the production process was found to be highly profitable with a return on investment equal to 55% and a payout period of 1.9 years in the worst-case scenario where the price of lubricating oil is 2.5 $/L. Graphic abstract