Tertiary recycling of polyethylene waste by fluidised-bed reactions in the presence of various cracking catalysts

A mixture of post-consumer polyethylene waste (HDPE/LDPE) was pyrolyzed over various catalysts using a fluidised-bed reactor operating in the 290–430 °C range under atmospheric pressure. The yield of volatile hydrocarbons for zeolitic catalysts (HZSM-5 > HUSY > HMOR) gave higher yield than non-zeolitic catalysts (SAHA ≈ MCM-41). Observed differences in product yields and product distributions can be attributed to the nature of catalyst and the reaction temperature. Product distributions with HZSM-5 contained more olefinic materials with about 60 wt% in the range of C3–C5. However, both HMOR and HUSY produced more paraffinic streams with large amounts of isobutane (i-C4) and both catalysts deactivated during the course of the degradation. MCM-41 and SAHA showed the lowest conversion and generated an olefin-rich product with a rise to the broadest carbon range of C3–C7. The selectivity could be influenced by the change in reaction condition with the olefins and iso-olefins produced by low temperatures. A kinetic model based on a lumping reaction scheme for the observed products and catalyst deactivation has been investigated. The model gave a good representation of experiment results. It is demonstrated that under appropriate reaction temperatures and suitable catalysts can have the ability to control both the product yield and product distribution from polymer degradation, potentially leading to a cheaper process with more valuable products.