Kinetic modelling of the cracking of HDPE pyrolysis volatiles on a HZSM-5 zeolite based catalyst

Continuous catalytic pyrolysis of high density polyethylene has been carried out for obtaining light olefins in a two step reaction system: (i) thermal pyrolysis in a conical spouted bed at 500 °C; (ii) cracking of the volatiles (mainly waxes, C21+) formed in the first step, in a fixed bed reactor using a catalyst based on HZSM-5 zeolite. A kinetic model based on lumps including catalyst deactivation by coke deposition has been established to quantify the product distribution obtained in the second step and its evolution with time on stream. Experimental data have been obtained at 400, 450 and 500 °C, with space-times of 0, 1, 2, 4, 6 and 8 gcat min gHDPE−1 and for 5 h time on stream. The discrimination of kinetic models has been carried out based on statistics calculated for Fischer distribution (stepwise procedure), and the best fits have been obtained for a model considering six steps: cracking of waxes to produce non-aromatic C5–C11 compounds, single-ring aromatics, light alkanes and light olefins; cracking of non-aromatic C5–C11 compounds to yield light olefins and light olefin condensation reactions to produce single-ring aromatic compounds. The deactivation kinetic equation considers waxes as main coke precursors, whose deposition rate decreases with time on stream.