(322a) Microkinetic Modelling of Polyolefin Binary Mixture Pyrolysis

With about 400 million tonnes of plastic waste generated per year, plastic pollution is a major crisis currently facing humanity [1]. A significant portion of the plastic waste is made up of polyolefins such as polyethylenes and polypropylene. Pyrolysis is an effective chemical recycling strategy that is reasonably agnostic to the type of plastic feedstock and can therefore be applied to mixed streams. Pyrolysis of polyolefins results in hydrocarbons with low carbon numbers that can be used as fuels.

Since pyrolysis results in the formation of a wide range of products, detailed mechanistic models are needed to predict the product distribution. A microkinetic model is a multiscale model as it includes kinetic information of the mechanistic steps involved and predicts reactor level measurables such as conversion and product selectivity. The Broadbelt group has developed microkinetic models for pyrolysis of several polyolefins based on the population balance approach [2], [3], [4]. In this approach, the reaction mixture is divided into classes based on their characteristics that are relevant for the reactions (species types, end types, etc.), and the chain length distributions of these species classes are tracked using their moments. These models also have the capability to track explicit species up to specified carbon numbers for identification of the product distribution.

Polyolefin pyrolysis proceeds by a radical mechanism. When a mixture of plastics is pyrolyzed, they behave as a kinetically coupled system where the chain propagation steps are facilitated by the radicals from all the polymers. However, if the polymers in the mixture are immiscible, this crossover of radicals is not feasible for longer chains. In such situations, smaller radical species can still diffuse into other phases and facilitate chain propagation. This behavior affects the reaction time of each individual polymer in a mixture compared to what is observed in the pyrolysis of pure samples [5]. The rates of cross-propagation and self-propagation of radicals impact the depolymerization rates [6]. In this work, we present a microkinetic modelling study of binary mixture pyrolysis involving polypropylene (PP), high density polyethylene (HDPE) and linear low density polyethylene (LLDPE). PP/HDPE form an immiscible mixture whereas HDPE/LLDPE form a miscible mixture when LLDPE has a low branch content. Our preliminary results suggest that, in a PP/HDPE mixture, the degradation of HDPE is enhanced while PP is retarded in the mixture. A detailed analysis of the reaction mechanisms will be presented.

Figure 1: Comparison of decay trend of number average molecular weights of polyolefins during mixture and pure component pyrolysis of PP/HDPE mixture at 653.15 K.

References

[1] UNEP (United Nations Environment Programme), From pollution to solution: a global assessment of marine litter and plastic pollution, vol. 237, no. 3169. 2021. [Online]. Available: https://www.unep.org/resources/pollution-solution-global-assessment-mari...

[2] T. M. Kruse, O. Sang Woo, and L. J. Broadbelt, “Detailed mechanistic modeling of polymer degradation: application to polystyrene,” Chem Eng Sci, vol. 56, no. 3, pp. 971–979, Feb. 2001, doi: 10.1016/S0009-2509(00)00312-2.

[3] T. M. Kruse, H. W. Wong, and L. J. Broadbelt, “Mechanistic modeling of polymer pyrolysis: Polypropylene,” Macromolecules, vol. 36, no. 25, pp. 9594–9607, 2003, doi: 10.1021/ma030322y.

[4] S. E. Levine and L. J. Broadbelt, “Detailed mechanistic modeling of high-density polyethylene pyrolysis: Low molecular weight product evolution,” Polym Degrad Stab, vol. 94, no. 5, pp. 810–822, 2009, doi:10.1016/j.polymdegradstab.2009.01.031.

[5] T. M. Kruse et al., “Binary mixture pyrolysis of polypropylene and polystyrene: A modeling and experimental study,” J Anal Appl Pyrolysis, vol. 73, no. 2, pp. 342–354, 2005, doi: 10.1016/j.jaap.2005.03.006.

[6] C. Lamarca, C. Libanati, and M. T. Klein, “Design of Kinetically Coupled Complex Reaction Systems,” Chem Eng Sci, vol. 45, no. 8, pp. 2059–2065, 1990.