JND Reproducible Thermodynamic Modelling and Energy-Normalized Assessment of a Small-Scale Plastic Pyrolysis Reactor for PE, PP, and HDPE Waste

Abstract

Pyrolysis offers a promising route for recovering liquid hydrocarbons from difficult-to-recycle polyolefin waste; however, small-scale reactor studies can provide incomplete assessment when reported oil production is not considered together with the thermal energy required to heat the feedstock, reactor hardware, and compensate for heat losses. This study develops a transparent and reproducible thermodynamic framework for a small-scale pyrolysis reactor processing polyethylene (PE), polypropylene (PP), and high-density polyethylene (HDPE). The model was formulated using direct mass- and energy-balance equations based on the reported fabricated reactor geometry, a 3 kg batch basis, literature-derived thermophysical properties, polymer fusion and pyrolysis enthalpies, reactor-wall heating, and heat-loss calculations. A deterministic MATLAB implementation was used to evaluate thermal input over 400–550 °C and to assess the sensitivity to insulation thickness from 10 to 50 mm. Model outputs were assessed against the reported final plastic-waste-oil (PWO) volumes, while experimental observations, literature-derived properties, modelling assumptions, and legacy numerical results were kept explicitly separated. At 500 °C, the calculated thermal inputs were 10.67 MJ for PE, 10.88 MJ for PP, and 14.00 MJ for HDPE. Increasing the assumed insulation thickness from 10 to 50 mm reduced the calculated thermal input by approximately 9.3–12.0%. The reported PWO volumes were 1.50 L for PE, 1.40 L for HDPE, and 0.90 L for PP. Under a common 500 °C reference condition, the corresponding reference energy-normalized oil-recovery indices (REORI) were 0.141, 0.100, and 0.083 L MJ⁻¹, respectively. REORI provides a common-basis comparative screening indicator by relating reported PWO volume to modelled thermal input, but it should not be interpreted as measured process efficiency. The framework demonstrates the importance of considering reactor thermal mass and heat-loss control alongside feedstock properties when assessing small-scale pyrolysis systems. However, the analysis is limited by the unavailability of direct heater-energy measurements, calibrated reaction-zone temperature data, cooling-water flow records, replicate measurements, and complete primary experimental records. Future work should therefore incorporate direct heater-power or energy-consumption measurements, calibrated reaction-zone temperature monitoring, oil mass-yield determination, condenser heat-removal measurements, and replicated experiments to enable rigorous energy-efficiency evaluation and validation of the thermodynamic framework.