Optimization of a novel series-condenser pyrolysis reactor for densified polypropylene waste: Maximizing oil yield and reducing greenhouse gas emissions via gas recycling

Post-consumer polypropylene (PP) waste was converted into fuel-grade pyrolysis oil using a novel vacuum pyrolysis system with a non-water-cooled six-series condenser. Response Surface Methodology-Box-Behnken Design (RSM-BBD) optimized gas flow rate, reaction time, and feedstock density. The quadratic model showed an excellent fit (R² > 99%), with feedstock density and its interactions with gas flow rate and reaction time significantly affecting oil yield (P < 0.05). The optimal conditions (15.98 L/min, 51.36 min, and 0.60 g/cm³) achieved a maximum yield of 96.89 g, a 7% improvement over baseline and a total pyrolysis oil yield of 82 wt%. The pyrolysis oil exhibited fuel-grade properties, and in engine tests, the 50:50 v/v pyrolysis oil-diesel blend reduced carbon monoxide (CO), nitrogen oxides (NOₓ), hydrocarbons (HC), and smoke emissions, while delivering brake thermal efficiency (BTE) and brake specific fuel consumption (BSFC) closer to conventional diesel. Life cycle assessment (LCA) indicated a carbon footprint of 1,385.1 kgCO 2 e/y, or 240.4 kgCO 2 e per tonne of oil, partially offset by recycling non-condensable gases. Gas recirculation reduced the carbon footprint by 25%. The system demonstrated promising economic viability, with a short payback period, from a techno-economic perspective. Optimizing feedstock density was identified as a key factor, enhancing liquid yield, increasing reactor capacity, and reducing greenhouse gas (GHG) emissions. This optimization not only improved environmental performance but also facilitated the transition toward net-zero GHG emissions, which are perceived to have lower investment risks while enhancing environmental sustainability. • Response surface methodology with Box-Behnken design efficiently optimizes pyrolysis oil yield, reveals critical variable interactions, and generates accurate predictive equations. • Feedstock density emerges as a novel control parameter, enhancing heat transfer and enabling higher loading capacity despite marginal linear effects. • Integrated optimization of density, gas flow, and reaction time establishes a key principle for maximizing pyrolysis oil recovery from plastic waste. • Numerical optimization validates process improvement hypotheses while minimizing experimental requirements. • Non-water-cooled multi-stage condensers without catalyst efficiently separate high-viscosity liquids from diesel-range hydrocarbons, enabling effective pyrolysis oil production. • Blending pyrolysis oil with conventional diesel upgrades combustion performance and reduces emissions. • Non-condensable gas recycling decreases fossil fuel use while substantially reducing greenhouse gas emissions. • Densified plastic waste pyrolysis delivers positive economic returns while reducing environmental impact.