Continued demand for polyolefins can be met by recycling plastic materials back to their constituent monomers, ethylene and propylene, via thermal cracking in a pyrolysis reactor. During pyrolysis, saturated polyolefin chains break carbon-carbon and carbon-hydrogen bonds, yielding a distribution of alkanes, alkenes, aromatic chemicals, light gases, and solid char residue at temperatures varying from 400-800 °C. To design a pyrolysis reactor that optimizes the chemistry for maximum yield of light olefins, a detailed description of the chemical mechanisms and associated kinetics is required. To that end, the reaction kinetics of isothermal films of low-density polyethylene (LDPE) have been measured by the method of ‘Pulse-Heated Analysis of Solid Reactions,’ or PHASR, which allows for quantification of intrinsic kinetics via isothermal reaction-controlled experimental conditions. The evolution of LDPE films from 20 milliseconds to 2.0 seconds for five temperatures (550, 575, 600, 625, and 650 °C) was characterized by measurement of the yield of chromatography-detectable compounds (<C20) in addition to the total yield of volatile products. The kinetics of volatile product evolution was interpreted via a lumped kinetic model with activation energy 225 ± 16 kJ/mol, compared with existing kinetic models of polyethylene pyrolysis, and validated from first principles.

The growing global plastic waste challenge requires the development of new plastic waste management strategies such as pyrolysis that will enable a circular plastic economy. Pyrolyzed plastics thermally convert into a complex mixture of intermediates and products that includes their constituent monomers. Developing optimized, scalable pyrolysis reactors capable of maximizing the yield of desired olefinic products requires a fundamental understanding of plastic pyrolysis mechanisms and reaction kinetics. Accordingly, the intrinsic reaction kinetics of polypropylene (PP) pyrolysis have been evaluated by the method of Pulse-Heated Analysis of Solid Reactions (PHASR), which enables the time-resolved measurement of pyrolysis kinetics at high temperature absent heat and mass transfer limitations. The yield of gas chromatography-detectable light species (<C20) and the total yield of volatile products were quantified at five temperatures (525, 550, 575, 600, and 625 °C) for reaction times of 20 ms to 2.0 s, generating polypropylene pyrolysis product evolution curves that were compared to literature data. The overall reaction kinetics were described by a lumped first-order consumption model with an activation energy of 242.0 ± 2.9 kJ/mol and a pre-exponential factor of 35.5 ± 0.6 ln(1/s). Additionally, the production of the solid residues formed during polypropylene pyrolysis was investigated, revealing a secondary kinetic regime.