Fast Pyrolysis Product Analysis of Commercial Polyethylenes

The pyrolysis of eight commercial polyethylene grades in a fluidized bed reactor (FBR, Hamburger Verfahren) operating at 750 °C (residence time of ∼3 s) is carried out to show that the gaseous and liquid product spectra are very similar. The reaction temperature is the sole parameter of major impact for the outcome of the pyrolysis in the FBR between 700 and 800 °C. The temperature of 750 °C gives a good yield of ethylene (31 wt %) and propylene (15 wt %) and moderate amounts of methane (11 wt %). Solid products (soots) are hardly formed. A stochastic analysis shows that the molar amounts of the linear alkane and alkene products with less than 4-5 carbon atoms (making up 75 wt % of the products) are confidently described by geometric distributions Geom(1 - α). The remaining 25 wt % of the products are mainly aromatics with 6 or more carbon atoms. The formation of alkanes at a 750 °C bed temperature is interpreted to result from random chain scission with a probability α of 0.82 in n-alkyls, followed by a hydrogen abstraction: 1-alkenes result from a competition between β-scission and 1,2-hydrogen shifts with an α of 0.64 for β-scission in linear alkyl radicals. The intermediate formation of n-alkyls and primary radicals is key; weak bond scissions (at branching points) are not relevant for polyethylene decomposition at 750 °C. The α geometric parameters are temperature dependent, giving access to activation energies of 47(4) kJ/mol for alkane formation and 54(2) kJ/mol for the difference in β-scission and 1,2-hydrogen shift on the interval between 600 and 900 °C. The geometric distributions suggest that the 75 wt % gaseous products are directly formed and transported away from the hot zone (surface ablation). The same analysis applied to published data on polyethylene pyrolysis shows that different geometric distributions for alkanes and alkenes are present too.