Pyrolysis of high-density polyethylene in free-fall reactors in series

Polyethylene pyrolysis has been studied analyzing the influence of temperature and residence time on the product distribution. The study was performed in an installation comprising two free-fall reactors in series, enabling the influence for both primary and secondary reactions to be studied separately. This also allows for the temperature to be increased in the second reaction zone, which is a parameter of great influence in the formation of aromatics. The use of reactors of different volume in the second zone also allows for the study of a broader range of residence times. The results obtained in the present work show qualitative trends similar to those obtained in other works of polyethylene pyrolysis in fluidized bed. At low temperatures, the main products obtained are solid waxes, while almost no aromatic compounds appear. The cracking rate increases with the rise in temperature, favoring the reaction of the heaviest fractions. Nevertheless, there are some differences. In general, ethylene, the direct product of the ‚-scission of hydrocarbon chains, is the main compound in the gaseous fraction. The yields to methane and hydrogen increase as the temperature rises, with these being the final products of the degradation process and in the formation of aromatics. The production of aromatics and polycyclic aromatic hydrocarbons increases with the temperature and residence time.