(376h) Development of Kinetic Model for Depolymerization of Polyethylene to Ethylene in a Microwave Reactor

Polyethylene is the world's largest plastic resin based on production volume. Despite accounting for 34% of plastics production, polyethylene has a notably low recovery rate (about 14% in 2015)¹, with the majority ending up in landfills or the environment. These factors motivate the development of processes that convert waste plastics, notably polyethylene, to light olefins in an economical manner. Chemical upcycling technologies, such as pyrolysis can convert plastic waste to fuels and chemicals that are then integrated into petroleum refineries for further processing. One important consideration in scaling-up this depolymerization process is to develop an accurate kinetic model, which is essential for reactor design. However, there are very few models available in the literature that accurately model the depolymerization process. Levine and Broadbelt² modeled the pyrolysis of polyethylene in the absence of a catalyst at the mechanistic level to predict the formation of low molecular weight products. However, pyrolysis processes are generally energy intensive, with significant greenhouse gas production, and lower light-olefin selectivity, which requires expensive downstream separation³.

Recently, there has been significant interest in microwave-enhanced pyrolysis of plastics in the presence of catalysts, which can improve the yield of olefins at lower temperatures⁴. In this research, a novel microwave process is developed to convert waste plastic into ethylene in the presence of a catalyst. Low-density polyethylene (LDPE, average molecular weight 4000) is mixed with a catalyst and then loaded into a reactor within a Sairem microwave system to generate a mixture of olefins. The fraction of alkanes and alkenes in the gas and liquid phase are measured experimentally. Preliminary experimental results indicate that the light olefin selectivity is about 60% with ethylene selectivity of 30% (by weight). Furthermore, the microwave process requires significantly less energy compared to the conventional thermal process. Low-molecular weight product (LMWP) yields are predicted using a kinetic Monte-Carlo (kMC) model describing the time-dependent depolymerization of polyethylene in the liquid phase, following dominant modes of decomposition of unzipping, random scission and backbiting⁵. These predictions are then validated against experimentally derived LMWP yields. The effects of microwave heating and catalyst presence on elementary kinetic parameters are then determined using machine learning (ML) to conduct parameter optimization. The kinetic model is utilized for designing a microwave-enhanced catalytic reactor that can be used for industrial-scale production of ethylene from LDPE.

References

1. Conk RJ, Hanna S, Shi JX, Yang J, Ciccia NR, Qi L, Bloomer BJ, Heuvel S, Wills T, Su J, Bell AT, Hartwig JF. “Catalytic deconstruction of waste polyethylene with ethylene to form propylene,” Science, 30:1561-1566 (2022)
2. Levine, SE, and Broadbelt, LJ, “Detailed mechanistic modeling of high-density polyethylene pyrolysis: Low molecular weight product evolution,” Polymer Degradation and Stability, 94, 810–822 (2009)
3. Aguado, J, Serrano, D, and Escola, J, “Fuels from waste plastics by thermal and catalytic processes: A review,” Eng. Chem. Res., 47, 7982-7992 (2008)
4. Hu, X, Ma, D, Zhang, G, Ling, M, Hu, Q, Liang, K, Lu, J, Zheng, Y, “Microwave-assisted pyrolysis of waste plastics for their resource reuse: A technical review,” Carbon Resources Conversion, 6, 3, 215-228 (2023)
5. Bui Viet, D, Weihs, GF, Rajarathnam, G, Abbas, A, “Highly accelerated kinetic Monte Carlo models for depolymerisation systems,” Computers & Chemical Engineering, 193, 108945 (2025)