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A two-stage process has been developed for the pyrolysis of high-density polyethylene to obtain light olefins, where probable mechanisms for each stage pyrolysis were proposed in our previous study. This work aims to establish molecular-level kinetic models for the first-stage pyrolysis at a lower temperature and for the second-stage pyrolysis at a higher temperature, respectively, to predict the overall reaction performances in accordance to the role of detailed mechanisms. A structure-oriented lumping (SOL) method based molecular-level kinetic model can effectively describe the first-stage pyrolysis, while a detailed molecular-level kinetic model is developed for the second-stage pyrolysis to track the evolutions of all alkanes, olefins, and diolefins, with special focus on the formation of light olefins. The established separate models can achieve the automatic generation of complex reaction networks and present good agreement with experimental data obtained from the first- and second-stage. In addition, the first-stage model indicates that the random scission plays an important role in the pyrolysis process at lower temperatures, while the chain-end scission mechanism dominates the second-stage reaction to produce light olefins at higher temperatures. As a whole, the molecular-level modelling scheme behaves excellently at a wide range of temperature, which provides guidance for upgrading the pyrolysis process. • A molecular level kinetic model is established for the two-stage HDPE pyrolysis. • The reaction network is generated automatically and visualized. • This model achieves excellent agreement with experimental data. • The competition of two scissions and important role of temperature are quantified. [ABSTRACT FROM AUTHOR]

Waste plastics have emerged as a global environmental issue. Polyethylene, accounting for about 25% of global plastic production, has been acknowledged as the major cause of "white pollution". In this paper, a generalized molecular-level kinetic model was proposed to understand the pyrolysis of polyethylene applying structure-oriented lumping, Poisson distribution, and genetic algorithm. The number of ordinary differential equations derived from the reaction kinetics were dramatically reduced by this simplified network with 14-rule design. The generalizability was confirmed by case studies without any change in the rule design. Under most conditions, the mean relative error between experimental and model data could be controlled within 5%, while abnormal deviations indicate the obstacle of heat transfer. The simulation curve could provide guidance for the experiments by locating the peak value of products. From 400–470 °C in Case 2, the increase in the reaction proportion of producing light oil explains the experimental results from the perspective of molecular-level reaction network. The comparison between PE and PS pyrolysis elaborates different pyrolysis mechanisms based on the different molecular structures. • Established a generalized molecular-level model for polyethylene pyrolysis. • Offered an efficient tool avoiding repeated work in pyrolysis simulation. • Confirmed the generalizability by case studies without changing the rule design. • Provided guidance for the experiments by locating the peak value of products. [ABSTRACT FROM AUTHOR]

The accumulation of waste plastics has been one of the most urgent and difficult environmental challenges. In this work, the pyrolysis of polyethylene (PE) has been studied by thermogravimetric analysis (TGA), pyrolysis/gas chromatography/mass spectrometry (PyGC-MS), and a two-stage tube reactor. The pyrolysis behavior of high-density polyethylene (HDPE) and low-density polyethylene (LDPE) is compared, and there are almost no differences in the thermogravimetric curves obtained by TGA and the yield of gaseous products obtained in a two-stage process. The evolution of light olefins (C 2 –C 4) during the pyrolysis of PE is investigated in a two-stage process with a constant condition in the first stage (500 °C) and various conditions in the second stage (700–1000 °C, 0.2–0.4 s). The temperature in the second stage has a more significant effect on the yield and selectivity of light olefins than the residence time, and the ethylene, propylene, and butene yields do not increase monotonously as the temperature rises. The highest yield of light olefins of 76.1 wt% is obtained at 800 °C with a selectivity of 91.7%. The maximum yields of ethylene, propylene, and butene are 45.1 wt%, 17.3 wt%, and 24.3 wt%, respectively. The scission mechanisms are proposed to provide insight into the process during which PE molecules transform into light olefins. • There are almost no differences between the pyrolysis characteristics of high-density and low-density polyethylene. • The evolution of light olefins during the polyethylene pyrolysis is investigated by PyGC-MS and a two-stage tube reactor. • A high yield of light olefins of 76.1 wt% with a high selectivity of 91.7% is obtained at 800 °C. • Probable mechanisms for two-stage pyrolysis of PE are proposed to reveal the interplay of reactions. [ABSTRACT FROM AUTHOR]

The environmental pollution caused by waste plastics and their potential of resource recovery are receiving increasing attention. In this study, a molecular level kinetic model was established to simulate the polypropylene pyrolysis based on the SOL method, Poisson distribution model and genetic algorithm, which was simplified and effective without the introduction of extensive chemical databases and stiff differential equations. The automatic generation of complex reaction network was based on the representation of species and reactions at molecular level and the design of 14 reaction rules consistent with the mechanism. The model prediction achieved good agreement with experimental data affected by the primary and secondary reactions, verifying the generalizability of the models. The model prediction according to the kinetic parameters provided guidance for the pyrolysis experiments. In general, the molecular-level model behaves exquisite in the evaluation, validation, and prediction, and can gain an insight into the pyrolysis of polymers. • A molecular level kinetic model with automatic reaction network generation is established for polypropylene pyrolysis. • The automatic network generation is based on the Poisson distribution model, the SOL method and the design of reaction rules. • This model achieves excellent agreement with experimental data and can predict pyrolysis performance in more details. • The important role of temperature on the pyrolysis and the competition of two scissions are quantified by this model. [ABSTRACT FROM AUTHOR]