Maximising naphtha-range hydrocarbons from thermal pyrolysis of polyolefin-rich mixed plastic waste by split-plot response surface methodology

The pyrolysis of polyolefin (PO)-rich mixed plastic waste represents a promising pathway for recycling plastic waste into liquid hydrocarbons, particularly in the naphtha range, for use as a refinery input. However, assessments of naphtha production from complex plastic waste remain limited. This work systematically investigates the batch pyrolysis of a PO-rich mixed plastic waste derived from a sorted household waste stream (DKR-350) using Design of Experiments (DoE) and response surface methodology (RSM). Oil yield, naphtha-range hydrocarbon yield, and other responses were modelled as functions of relevant processing parameters, such as batch time, temperature, and plastic pre-treatment method. Experiments were conducted in a batch autoclave under various process conditions (380-450 °C, 0-4 h, with varying feedstock pre-treatments, gas types, and pressures). Time and temperature were the most critical factors for achieving the highest oil yield (73% at 420 °C and a batch time of 2.6 h). The highest naphtha yield in the experimental design range was 42% (448 °C, 3.1 h), achieved using a feedstock that had been pre-treated by dry-washing. Additionally, this work gives key insights into the pyrolysis mechanism of plastic waste. For instance, the formation of CO and CO₂ was linked to the presence of specific biogenic and non-polyolefinic impurities at various stages during pyrolysis. The composition of the naphtha fraction also becomes increasingly richer in C₅-C₉ as pyrolysis severity increases. This work explores the potential of pyrolysis as part of the solution to global plastic waste challenges within a more circular economy.