Purification and characterisation of post-consumer plastic pyrolysis oil fractionated by vacuum distillation

Abstract

Valorisation of post-consumer plastic pyrolysis oil in steam crackers and their application as precursors to chemicals is currently hindered by the complexity of these oils in terms of their boiling range, the risk of fouling by unsaturated compounds, and the presence of contaminations (heteroatoms, metals, and halogens). To gain insights into which of these challenges is actually a potential showstopper, we have studied the vacuum distillation of pyrolysis oil derived from two sorted post-consumer waste plastics, i.e. polypropylene (PP) rigids and mixed polyolefins (MPO) rigids, in a batch fractionating column. The crude pyrolysis oil samples were distilled in three fractions i.e., the light fraction (final boiling point <175 °C), middle fraction (175–360 °C), and heavy fraction (initial boiling point >360 °C). Subsequently, comprehensive characterisation, such as physicochemical properties, the level of contaminations, and chemical composition, was performed. The suitability of the light and middle fractions for steam cracking and precursors to chemical feedstock was assessed. It was found that the density and viscosity values of light and middle fractions from the two pyrolysis oil samples were comparable to fossil naphtha and diesel, whereas total acid number (TAN) values were notably higher compared to petroleum-based oil fractions (>0.5 mgKOH/g). Analyses via GC-MS and ATR-FTIR revealed predominant concentrations of olefins. ICP-OES analysis showed that distillation removes more than 97.5% of metals in all fractions. The chlorine content (>3 ppm), nitrogen (≥0.1%), and oxygen (≥0.1%) exceed thresholds. Hence, this study shows that vacuum distillation effectively improved most, but not all, of the properties of the distilled fractions and can thus be an essential downstream process step for the application of post-consumer plastic pyrolysis oils as feedstock in steam crackers and precursors to chemicals.