Pyrolysis process and mechanism for calcium elimination of used lubricating oil

Abstract

The present work is mainly focused on the mechanism for used lubricating oils (ULOs) elimination of calcium using calcium dodecyl benzenesulfonate (Ca-DDBS) as a model compound for the first time. Intermediate and final products formed during thermal degradation in air and N2 atmosphere were determined by combining TG-DSC, XRD, TG-MS, and TG-IR techniques. Bond dissociation energies (BDEs) of Ca-DDBS were calculated systematically using the Gaussian 16 program, and primary, secondary decomposition intermediates and transition states of decomposition reactions have been optimized using the B3LYP method. More importantly, the thermolysis mechanism of Ca-DDBS was determined by combining quantum-chemical calculations and experimental results. Results disclosed that the thermolysis process of Ca-DDBS involves a cascade process with multiple steps, each having its individual characteristics. The model compound first initiates thermolysis through the homolytic cleavage of the C-S bond. Through the pyrolysis mechanism, it is speculated that the solid product of thermal decomposition is a mixture of CaSO4, CaSO3, and CaO, owing to the interconversion of decomposition products, which is also confirmed by XRD analysis. Finally, the decomposition process of the model compound in base oil was similar to that of monomers in air. Significantly, this study provides important theoretical support for the resource utilization of ULOs.