Coking mechanism and abatement strategy of used lubricating oil resource regenerative utilization

Coking has a major impact on equipment fouling, catalyst deactivation and pipe clogging, in biomass utilization, petroleum refinery processes and other related fields. Therefore, coking is one of important aspects of research in these fields and generates mechanism as intricate as the phenomenon itself. Complexity is essentially coupled to the mechanisms and promote the utilization of renewable resources can be better guided if fully understanding this complexity. In this study, the evolution of gas-liquid phase coke structures of used lubricating oil demetallization pretreatment at temperatures below 400 °C was first investigated. Then, the main influencing factors and mechanism of used lubricating oil pyrolysis pretreatment gas-phase coking are further presented. Finally, a method for preventing coking is proposed and verified. Our results show that metal catalysts promote the formation of liquid-phase coke, where the coke formed was mainly filamentous. And gas-phase reactions at the early stage of coke formation were mainly attributed to HCl-catalyzed Diels-Alder type reaction of olefins produced by used lubricating oil thermal cracking. The planar expansion of the polycyclic compound generated by the Diels–Alder reaction in the sheet was accompanied by partial dehydrogenation, and finally, three-dimensional particles are formed by the vertical connection between these planes. Starting with simple aromatics, dehydrogenation and condensation reactions occurred to produce soot particles or tar droplets that could be liquid or even solid in a thermal cracking reactor. They adhered to the surface and were incorporated in the coke layer because the outer surface of the droplets was not completely dehydrogenated. Next, dehydrogenation reactions were possible, and the coke layer could grow further. Meanwhile, coke contains Cl, which further catalyzed the cracking leading to the formation of autocatalytic coke. On the other hand, the condensation of high boiling products on the gas deposition sheet and eventual dehydrogenation to form coke were the secondary causes. Based on this research, we established a pilot plant with an annual output of 100 t/ year, which has achieved continuous and stable operation for 624 h. And the pretreatment process has been verified by the hydrogenation process. It also has great theoretical guiding significance for gas-phase coking of biomass, bio-oil pyrolysis and pyrolysis gas-phase coking of other substances. Significantly, our results provide reliable and effective models relating to ULO regenerative pretreatment, which should be helpful in future ULO hydrorefining and resource utilization.