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

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 degrees C and the results compared with those obtained in the absence of catalyst. Temperatures of 450 degrees 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 cx-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 degrees C and caused a significant increase in the proportion of gaseous hydrocarbons (73.5 wt% selectivity at 450 degrees 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 degrees 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. (C) 2006 Elsevier B.V. All rights reserved.