(672c) Chemical Kinetics and Mass Transfer Interplay during High-Density Polyethylene Pyrolysis

Plastics play a crucial role in various applications in life due to their durability and versatility. However, the recycling rate of plastic waste remains low, leading to significant environmental challenges. One promising approach to address the plastic waste problem is pyrolysis. High-density polyethylene (HDPE), a widely used thermoplastic, can be pyrolyzed to yield wax, liquid oil, gas, and char. Despite vast amount of experimental data on HDPE pyrolysis using different reactor configurations are available, variable product distributions exist in the literature even under the same reaction time and temperature (which are supposed to give the same chemical kinetics), particularly when the system pressure is changing. This variability underscores the potential existence of additional heat and mass transfer effects and the interplay between transport and chemical kinetics. For pyrolysis of a macromolecule, such as HDPE, complex primary and secondary reactions exist, creating a diverse mixture of intermediates and primary and secondary products in the melt phase. On the other hand, the most critical effect reaction pressure induces in a reactive system is to alter the rates of species vaporization.

In this work, we hypothesize that there exists a competition between continued decomposition (i.e. chemical kinetics) and evaporation (i.e. mass transfer) of a HDPE pyrolysis product, where the overall product distribution from HDPE pyrolysis is determined on the relative importance of these two processes of each individual species. To test this hypothesis, microreactor experiments of batch HDPE pyrolysis were conducted. Our experiments showed that as pressure increased, product distributions from HDPE pyrolysis shifted towards lighter hydrocarbons. Our work also shows that the dimensionless Damköhler number, which compares a species’ decomposition rate and its evaporation rate, could be used to describe the interplay of the two processes and predict the product distributions from HDPE pyrolysis (Figure 1).