Conversion of a Polyolefin Mixture into Light Olefins by Fast Pyrolysis and In-Line Catalytic Cracking on HZSM‑5

A model polyolefin mixturecomposed of 50% HDPE, 40% LDPE, and 10% PPhas been converted into valuable light olefins on an HZSM-5-based catalyst in a two-step continuous process: fast pyrolysis in a conical spouted bed reactor followed by upgrading in a catalytic fixed-bed reactor. The effect of the operating conditions in the catalytic step (space-time of 10 and 15 g_catalyst min g_plastic ^-1 and temperature of 450 and 500 °C) on product yields and catalyst stability has been investigated up to a time on stream (TOS) of 4 h. Light olefins represented the most abundant product, with a maximum yield of 77% obtained at 500 °C with a space-time of 10 g_catalyst min g_plastic ^-1 under zero-time conditions. Further increase in space-time led to a reduction in light olefin yield and to an increase of benzene, toluene, and xylene isomers (BTX) as well as light alkanes. Characterization of spent catalyst samples (coke deposition, physical properties) revealed the presence of two different types of coke and a higher coke deposition in the first section of the catalytic bed (in contact with incoming waxes). The catalyst proved to be stable at 500 °C, while higher deactivation was observed after 4 h of operation at the lower temperature (450 °C).