A multi-modal and process intensification study of a sequential acid-solvent protocol for valorizing waste lubricant oil into high-quality base stock

Abstract

The transition to a circular economy for hydrocarbons demands advanced recycling paradigms for waste lubricant oil (WLO). This study presents a holistic, multi-modal investigation of an intensified sequential process integrating acid pre-treatment with methanol extraction for WLO valorization. To complement conventional analysis, High-Resolution Mass Spectrometry was employed to deconvolute the molecular-level transformation, identifying and quantifying the removal of more than 500 distinct contaminant species. The mechanism of acid treatment was elucidated through in-situ FTIR spectroscopy, which quantified second-order reaction kinetics (k = 2.3 × 10⁻³ L mol⁻¹ s⁻¹) for metal-carbonate dissociation. Density Functional Theory calculations provided a quantum-mechanical rationale for solvent efficacy, revealing strong, spontaneous binding between methanol and key oxidation products (e.g., ΔGbind= -5.2 kcal/mol with stearic acid). Microwave-assisted extraction enhanced process efficiency, reducing processing time by 95.8 % and increasing yield by 8 %. The reclaimed oil exceeded key API Group I specifications, with a kinematic viscosity of 16.49 cSt at 100°C, a flash point of 240°C and a total acid number of 0.05 mg KOH/g. A comparative Life Cycle Assessment demonstrated a 65% reduction in global warming potential compared to virgin base oil production. This work provides a validated framework for sustainable WLO valorization by coupling atomistic-level mechanistic insights with a validated, intensified process.