(672g) Analysis of Polyethylene Pyrolysis Kinetics Using a Cost-Effective Rapid-Heating System: A 5-Lump Isothermal Study.

The accumulation of plastics in sea and land urges governments, corporations, and communities to look for sustainable solutions. The current mechanical recycling efforts are lacking, with low contribution (<10%). Chemical recycling methods, especially pyrolysis, can potentially have a higher impact as they can deal with mixed and contaminated feed.

The design of pyrolysis reactors requires high-quality kinetic data. The literature has ∼200 kinetic data points for polyethylene pyrolysis with a wide distribution of activation energies of 42-533 kJ/mol. Most of these studies use non-isothermal data collected via thermal gravimetric techniques. Other literature uses batch systems to conduct isothermal kinetic studies, but most come with a long heating time of 20-120 min. The reactor choice regarding temperature gradient and thermal dwell time has the highest impact on these values. Therefore, studying true isothermal kinetics is challenging, as a significant polymer conversion would be reached before the isothermal temperature.

This paper presents a novel and affordable semi-batch reactor system heated by a thick copper-block designed to deliver energy at ∼125 °C/min. We used this system to measure the isothermal kinetic parameters of polyethylene decomposition using discrete lump methodology at temperatures of 411, 420, 429, 437, and 449 °C. We have identified five lumps that fit the kinetics data well: three representative polymer chains, liquid, and gas. The rate constants were fitted via an optimized solution of coupled ordinary differential equations. The activation energy and pre-exponential factor were found via the linearized form of the Arrhenius equation. The initial two decomposition steps have an apparent activation energy of ∼360 kJ/mol, matching the computational activation energy reported for the C-C homolysis. The net apparent activation energy of the four reactions amounts to 234 kJ/mol, very close to the non-isothermal thermal gravimetric result obtained in this study of 247 kJ/mol.