Polypropylene (PP) and low-density polyethylene (LDPE) are the main polymers used in the production of plastic packaging, which makes up about forty percent of all waste plastics. These polymers cannot be mechanically recycled, as such it has been proposed that chemical recycling is applied to transform these waste plastics into feedstock. A two-stage process has been proposed which consists of the pyrolysis of polyolefins and subsequent catalytical upgrading via hydroprocessing. Extensive knowledge on the pyrolysis of LDPE and PP is available in literature. The hydroprocessing of these feeds is however largely undocumented and, as far as we know, there is no research comparing the hydroprocessing of PP and LDPE. The main scope of this thesis is to identify the main characteristics for the hydroprocessing of pyrolysed PP and LDPE. By performing comprehensive analysis of the pyrolysed PP and LDPE, it was shown that the pyrolysis oils have a distinctly different composition. Pyrolysed PP consists of light, extensively branched hydrocarbons, while pyrolysed LDPE has a broader carbon distribution and a significantly large fraction of linear hydrocarbons. Furthermore, it was shown that hydrogenation and hydrocracking were the main reactions that had occurred during hydroprocessing. It was found that hydrogenation occurred to the largest extent. Despite the different amounts of unsaturated compounds in pyrolysed PP (69 wt%) and LDPE (26 wt%), the amount of unsaturated compounds could be reduced in both cases to approximately 10 wt% by hydrogenation. Additionally, it was shown that the temperature had a minimal effect on the composition of the hydroprocessed plastics. In summary, it is concluded that there are significant differences in composition between pyrolysed PP and LDPE. The differences in structure are relatively unaffected by hydroprocessing, while the differences in the amount of unsaturated compounds were nullified by hydrogenation. The properties and ease of processing of the pyrolysis oils are affected by their structure and therefore, it is suggested that PP and LDPE are processed differently. The hydroprocessed products are not directly applicable in fuel applications as the remaining unsaturated fraction would impact fuel stability. More severe hydroprocessing conditions, or an additional hydrogenation step are required to reduce this fraction to acceptable levels. It is suggested that processed PP would serve best as gasoline and Fluidized Catalytic Cracking (FCC) feedstock due to their comparable structure. Furthermore, it is suggested that processed LDPE could be used in various applications ranging from FCC feedstock to waxes due to its broad composition and significant unbranched fraction. |
