How bubbles formation and growth affect plastic pyrolysis process: A phenomenological investigation through dimensionless numbers

Polyethylene (PE) pyrolysis is an essential process for converting plastic waste into valuable products, but the interplay between chemical kinetics and physical phenomena, such as bubble formation, heat transfer, and devolatilization, remains a challenge for reactor-scale design. While the influence of transport limitations under fast heating is recognized, their quantitative integration into predictive models is still lacking. This study investigates PE pyrolysis using Thermogravimetric Analysis (TGA) and Pyroprobe experiments over a wide range of heating rates (5 °C/min to 20,000 °C/s). TGA results show that heat transfer resistance delays degradation onset, with apparent activation energies ranging from 338 to 385 kJ/mol (non-isothermal) and 65–155 kJ/mol (isothermal). Pyroprobe data reveal diffusion-controlled behavior, bubble entrapment, and incomplete devolatilization. To evaluate the timing between melting and pyrolysis, we introduce the dimensionless “H number,” which compares melting time to reaction time and identifies whether pyrolysis begins before the polymer is fully molten. Together with the Biot and internal pyrolysis numbers, it helps distinguish kinetic from transport-limited regimes and predict the onset of physical constraints. Our findings show that only sub-milligram samples reach true kinetic control. This integrated approach offers a practical framework to assess heat and mass transfer effects, supporting the design of more efficient and selective pyrolysis reactors for industrial plastic waste valorization.