(516c) Integrating Kinetic Modeling with Sustainability Assessment to Inform Decision-Making and Process Optimization in Plastic Recycling

The accumulation of plastic waste causes pollution, depletes resources, and imposes economic burdens due to inefficient waste management. As recycling technologies are being developed, understanding their economic viability and environmental impacts becomes essential for ensuring their effective implementation. System analysis studies in this field rely on yield data from experimental studies conducted under specific operating conditions. However, this approach has limitations for optimizing operating conditions and presents challenges when comparing technologies, due to the lack of standardization in life cycle assessment (LCA) and techno-economic analysis (TEA) studies. Additionally, the conditions used in laboratory experiments often differ from those required for large-scale production.

To overcome this, we developed an approach that integrates kinetic modeling into process simulation (eg., in Aspen Plus), using polypropylene decomposition via pyrolysis as a case study¹ . This approach enables dynamic determination of product yields and utility consumption under various operating conditions (vapor residence time and reactor temperature), allowing for the assessment of how operating conditions influence the sustainability (both economic and environmental) of the process. Economic calculations were performed on the basis of a plant with 100 kt/year processing capacity. The main product is pyrolysis oil, which replaces crude oil in refineries, while the by-product, combustible pyrolysis gas, replaces natural gas for heating purposes. The minimum selling price of pyrolysis oil varies between $420/ton (at lower temperatures and longer residence time) and $490/ton (higher temperature and shorter residence time) depending on reaction conditions, indicating that optimization could reduce the price by up to 20%. For this analysis, the equipment was designed based on a worst-case scenario for heat and energy duties. Therefore, further reductions in the minimum selling price are possible with optimized equipment design, where equipment is tailored to operate under optimal condition, along with some overdesigns for reliability.

Life cycle assessment shows that the optimal operating conditions for minimizing environmental impact vary by impact category and often differ from those that maximize profitability, indicating trade-offs across different dimensions of sustainability. When compared to conventional methods like landfilling and incineration, this work reveals that recycling polypropylene plastics through pyrolysis outperforms incineration in almost all impact categories. However, its environmental performance compared to landfilling remains uncertain and is dependent on the specific impact assessment category being analyzed.

Future work should focus on employing mechanistic kinetic models that characterize the formation of chemical species and capture the influence of additional operating parameters (e.g., pressure), This would enable more advanced system analyses that focus on both chemical and fuel production offering a deeper exploration of trade-offs between economic and environmental impacts in plastic recycling.

References

(1) Kulas, D. G.; Zolghadr, A.; Shonnard, D. Micropyrolysis of Polyethylene and Polypropylene Prior to Bioconversion: The Effect of Reactor Temperature and Vapor Residence Time on Product Distribution. ACS Sustainable Chem. Eng. 2021, 9 (43), 14443–14450. https://doi.org/10.1021/acssuschemeng.1c04705.