(587m) A Mechanistic Model for the Product Distribution of Fast Pyrolysis of High Density Polyethylene Waste

Fast pyrolysis emerges as an interesting alternative for the recycling of plastic waste on an industrial scale, despite incomplete information about the fundamental principles governing this process. Among the most important research areas is the development of an accurate kinetic model to predict the formation of different hydrocarbon classes when plastic is subjected to high temperatures and different residence times in an inert atmosphere. Several researchers have developed kinetic models that attempt to estimate the pyrolysis products distribution at different temperatures and residence times. One of the first models was proposed by Bockhorn et al.¹, it predicted the thermal degradation of polyethylene and polypropylene by using mass spectrometry measurements of pyrolysis products under isothermal conditions. Costa et al.² established a simplified model by grouping the products of polyethylene pyrolysis into four classes. This study idealized a series of chemical reactions that gave a kinetic model with only nine elemental first order reactions. Other models attempted to predict the product distribution of over 100 intermediate and hydrocarbon products using over 10,000 reactions.^3,4 Nevertheless, these models require complex methods for solving the differential equations that are not easily accessible. Mastral et al.⁵ developed the sole kinetic model for fast pyrolysis of polyethylene under short residence times (0.52-2.07 s) and high temperatures (500-1000 °C). This model predicted the production of 8 groups of linear hydrocarbon and aromatic products. Random chain scission has been proposed as the mechanism responsible for the degradation of polyolefins into shorter hydrocarbon molecules through a set of free radical-based chain reactions. In this mechanism, polymers are broken down through a series of initiation, propagation, and termination steps. The complexity of these sequential reactions suggests that new insights may be gained by isolating the primary reactions from the secondary reactions. Therefore, this project attempts to develop a kinetic model to estimate the product distribution of primary and secondary products of the fast pyrolysis of waste HDPE. Primary reactions are carried out in a micro-pyrolysis CDS Analytical 5200 HP unit while secondary reactions occur in reactor extension attached to the micro-pyrolyzer. Products of waste HDPE degradation will be tested at high temperatures (650 – 700 °C) and short residence times (~0.0 to 5.3 s) where no char is produced and be analyzed using a gas chromatography-mass spectrometry equipment. The developed model will be based on the random chain scission mechanism described previously and will describe the product distribution of four classes of linear hydrocarbon compounds and aromatics. Possible reactions that describe the formation of upcoming hydrocarbon species will be discussed.

References

1. Bockhorn H, Hornung A, Hornung U, Schawaller D. 1999. Kinetic study on the thermal degradation of polypropylene and polyethylene. Journal of Analytical and Applied Pyrolysis 48(2):93-109.
2. Costa PA, Pinto FJ, Ramos AM, Gulyurtlu IK, Cabrita IA, Bernardo MS. 2007. Kinetic Evaluation of the Pyrolysis of Polyethylene Waste. Energy & Fuels 21(5):2489-2498.
3. Németh A, Blazsó M, Baranyai P, Vidóczy T. 2008. Thermal degradation of polyethylene modeled on tetracontane. Journal of Analytical and Applied Pyrolysis 81(2):237-242.
4. Levine SE, Broadbelt LJ. 2009. Detailed mechanistic modeling of high-density polyethylene pyrolysis: Low molecular weight product evolution. Polymer Degradation and Stability 94(5):810-822.
5. Mastral JF, Berrueco C, Ceamanos J. 2007. Theoretical prediction of product distribution of the pyrolysis of high density polyethylene. Journal of Analytical and Applied Pyrolysis 80(2):427-438.