A catalytic reactive distillation approach to high density polyethylene pyrolysis – Part 1 – Light olefin production

Abstract

An integrated reactor/separation semi-batch catalytic pyrolysis reactor system, based on a distillation approach, was developed to produce gaseous hydrocarbons, to be used as feedstock or fuel in the perspective of a Circular Economy approach. This work intends to be a proof of concept and to test the operating conditions and performance of the reactor to control the molecular weight of the products. For that purpose, thermal and catalytic pyrolysis experiments were run using high density polyethylene (HDPE) under several experimental conditions. Different temperatures were used (400, 450 and 500 °C) for both thermal and catalytic pyrolysis, with 1% (w/w) of HZSM-5. The influence of the temperature of the coolant at the outlet of the reactor was also investigated for the catalytic degradation of the HDPE at 450 °C, providing a way to control the average molecular weight of the recovered products, which increased with increasing coolant temperature. The focus of the study was to develop an integrated reactor/separation system where only the lighter compounds produced could leave the reactor and thus direct the production to chemical feedstock to be reused in petrochemistry, including in the production of new plastics. The analysis of the volatile products indicated that, for both thermal and catalytic pyrolysis, the products obtained were within the range of C2 to C8. Higher amounts of C3 to C5 were formed, with a particular emphasis on olefins, with very high yields in the case of catalytic pyrolysis. The activity of the catalysts after use was also checked through the ARDT index (Ability to Reduce the Decomposition Temperature), with values for this index above 50% which indicate a good possibility for reuse of the catalyst.