Aromatization of hydrocarbons during thermochemical conversion of polyethylene: Investigation on the influence of steam and potassium

Abstract

Within the transition towards a circular economy society, a more resource-efficient way of handling the processed carbon atoms is needed. The valorization of the generated plastic wastes by using them as feedstock to generate new monomers could be an adequate strategy to reach this goal. This valorization could be achieved by thermochemical conversion (or recycling) of these wastes. During the thermochemical conversion of plastics, the aromatization of potentially valuable hydrocarbons occurs. If the reaction mechanism governing the formation of aromatics is known, the process could be better optimized towards olefins production. Olefins such as ethylene and propylene are monomers used as feedstock to produce some of the most conventional plastics on the market, e.g. polyethylene (PE), polypropylene (PP), and polyvinyl chloride (PVC). In this work, the underlying reactions of aromatization during the decomposition of PE have been reviewed, along with the impact of the addition of steam and potassium. The review was conducted by the literature study on (1) the decomposition of PE in a high-temperature environment; (2) the possible reactions leading towards aromatic structures and; (3) the interactions of potassium with hydrocarbons. Based on the literature review, a hypothesis on how interaction from the presence of potassium and steam affect the aromatization of the hydrocarbons is formulated. Then, through the experimental work, the potential effect of steam and potassium in the thermochemical process was studied. The experiments showed that the gas and aromatic yields were affected both by the presence of steam and potassium. The addition of steam increased the overall yields, both of valuable gases and aromatics compared to the results obtained from the experiments conducted with only nitrogen. The addition of potassium increased the production of H2, CO, and CO2, while the aromatic content was reduced. When solid KOH was added to the reactor the olefin yield was reduced. But when K-enriched olivine was used, that phenomenon was not seen, and the yield of olefins was unaffected by the potassium. However, there are insufficient experimental data to confirm or reject the hypothesis. Furthermore, during this work, a new lab reactor and analysis technique capable of detecting/measuring KOH during thermochemical conversion in a fluidized bed was developed. From preliminary experiments, differences are observed between the UV-spectras generated by the decomposition of PE in either nitrogen, steam/nitrogen, and steam/nitrogen environment with potassium present. However, further research is needed to shed light upon the governing mechanism behind the observed differences due to the presence of steam and potassium.