Recovering Valuable Hydrocarbon Molecules from Used Lubricating Oils via Supercritical CO2 Extraction

With the escalating global energy crisis and burgeoning environmental concerns as a result of the accumulation of used lubricating oil, the urgency and economic potential of recycling this waste is undeniably critical. Conventional methods, such as distillation and hydrogenation, often stumble upon operational difficulties as a result of thermochemical decomposition, while advanced procedures, like molecular distillation and membrane separation, face industrial scale-up, owing to prohibitive costs and limited separation capabilities. Herein, this study heralds the emergence of supercritical CO2 (SC-CO2) extraction, which can exemplify superior efficacy in addressing these concerns. Leveraging the high solvency under extreme pressure and low-temperature conditions, SC-CO2 can selectively separate undesirable components, reducing coke formation and simplifying subsequent refining stages. The comprehensive analysis, conducted using gas chromatography–mass spectrometry and high-resolution mass spectrometry, revealed the selectivity of SC-CO2 toward saturates with a smaller molecular weight. On the basis of this analysis, the optimal extraction condition was determined. Implementing a two-stage process at 70 °C and 10–20 MPa, we effectively eradicated residual small molecules and large polar compounds, yielding an intermediate fraction rich in isoparaffins and 1–4 ring naphthenes with a carbon distribution of C25–40. Notably, the recovery rate of saturates was 60%. Critically, the removal rate of undesirable components in recycled lubricating oil is remarkably high, reinforcing the practical viability and exceptional advantages of SC-CO2 extraction for lubricating oil recycling as a sustainable and economically rewarding solution to pressing global energy and environmental predicaments.