Octanoic acid pyrolysis in a stainless-steel tube: What is the role of the coke formed on the wall?

A way of upgrading waste cooking oils into usable products or energy is pyrolysis. Used cooking oils contain free fatty acids. Therefore, octanoic acid was selected as a model molecule to investigate the behavior of carboxylic acid pyrolysis. The experiments were first performed in a stainless-steel reactor that obviously led to coke deposits on the wall. To analyze the effects of reaction temperature, residence time of the reactant, and composition of the wall material, octanoic acid pyrolysis was conducted in reactors made of different metals and with different internal diameters. The obtained results showed that, in addition to the significant effects of reactor wall itself, octanoic acid pyrolysis was catalyzed by Fe, Ni, and Cr metal content of the coke formed at the initial stage of the reaction. Continuous analysis of CO, CO2 and H2 production throughout octanoic acid pyrolysis confirmed this result and showed that the coke formed in three stages during which it firstly acted as a pyrolysis accelerator with an activity decreasing with time, and then as a passivator-like agent. Hence, two types of coke were formed: first coke containing a certain amount of metal, then a non-metallic coke. Also, when using reactors of different construction materials, different types of coke deposits were observed demonstrating that coke deposition and activity depends on the surface metal content of the reactor. The conclusion of this work clearly confirms that reactors built of materials as chemically inert as possible (i.e. quartz) are a prerequisite to generate kinetic data aimed at validating or extending homogeneous gas phase kinetic models.