Feedstock recycling of polyethylene in a two-step thermo-catalytic reaction system

Abstract

The conversion of low density polyethylene (LDPE) into high value hydrocarbons has been investigated using a two-step reaction system consisting of an initial pyrolytic furnace followed by an independent reactor containing nanosized n-HZSM-5 zeolite or Al-MCM-41 mesostructured material where the catalytic reforming of the pyrolytic vapours took place. The system was run at temperatures between 425 and 475 °C and the results compared with those obtained in the absence of catalyst. Temperatures of 450 °C and above were required to reach conversion values in excess of 90 wt%. At that temperature and in the absence of catalysts, thermal cracking of LDPE generated almost exclusively α-olefins and n-paraffins over a wide range of molecular weights, most of which (74.7 wt%) collected as liquid products. Catalytic reforming over n-HZSM-5 was effective at 425 °C and caused a significant increase in the proportion of gaseous hydrocarbons (73.5 wt% selectivity at 450 °C) that consisted primarily of olefins. The remaining liquid products contained a high proportion of valuable aromatic and branched species in the gasoline size range (C 5–C 12) making it suitable for blending with commercial gasolines. Owing to its weaker acid properties, catalytic reforming over Al-MCM-41 required the use of higher temperatures (450 °C and above) to produce a lower proportion of gas products (54–58 wt%) but a consequently higher amount of liquid (34–42 wt%) hydrocarbons. Besides, formation of aromatics was less significant than with n-HZSM5, but the resulting oils contained a good combination of olefins and iso-paraffins. This product composition is comparable to commercial transportation fuel.