(642a) Effect of Thermal Pre-Treatment on Pyrolytic Conversion of Polyolefins

As of 2018, only 8.7% of all the plastics produced are recycled; the rest are landfilled, incinerated, or leaked into the environment. Chemical upgrading provides a potentially attractive pathway to valorize waste plastics and promote a carbon-circular economy. Polyolefins are the most abundantly used plastics. However, the conversion of polyolefins is hindered by their high thermal stability and the hydrocarbon products with a widely distributed molecular range. The pyrolysis oil of polyolefins often contains significant amounts of waxes (>C20 hydrocarbons, solids at room temperature). Due to their high boiling points, the waxes in pyrolysis oil can cause reactor clogging and increase the reactor temperatures during catalytic upgrading. In our work, we overcome this challenge by preheating polyolefins before pyrolysis. The preheating temperatures were determined for different polyolefins based on their respective physiochemical behavior at the molten phases. At the optimal preheating temperatures, the chain fragmentation overruled the crosslinking reactions, and the viscosities of the molten plastics also reached the lowest. It shows that pyrolysis of preheated polyolefins can produce hydrocarbons with significantly narrower molecular weight distribution, decreasing wax contents without affecting gas products. As a result, more gasoline and diesel range hydrocarbons could be produced. When raw HDPE and the HDPE preheated at 295 ℃ were pyrolyzed, the wax yields reduced from 49 to 34 C% and 41 to 25 C%, respectively, for the pyrolysis temperatures of 550 and 600 °C. Notably, the respective gas yields only increased from 3.9 to 4.5 C% and 5.7 to 7.3 C%. Similar results were also obtained with a preheated LDPE. Catalytic pyrolysis of the raw and preheated HDPE was also compared. It shows that the selectivity of alkylated single-ring aromatics increased with the preheated HDPE.