To overcome this, we developed an approach that integrates kinetic modeling into the Aspen Plus process simulator using polypropylene decomposition via pyrolysis as a case study. This will enable us to dynamically monitor product yields and utility consumption under various operating conditions, allowing us to optimize the conditions to achieve predefined goals, such as maximizing process profitability and/or minimizing environmental impacts. The lumped-type kinetic model1 integrated in this work is comprised of 10 reactions and 6 lumped species, and by solving the system of ordinary differential equations (ODEs), it is possible to predict the product distribution at different temperature levels and vapor residence times. The system of ODEs is solved numerically using the 4th order Runge-Kutta (RK4) method, and is applied to a calculator block which determines the yields of species at reactor’s specified temperature and vapor residence time. The data from the simulator is used for an excel based life cycle assessment.
Additionally, a code for calculating the minimum selling price (MSP) is developed and applied to a flowsheet calculator in Aspen Plus. The MSP is calculated numerically using the iterative "modified secant" root-finding method. This approach enabled us to determine the MSP of the primary product of the process (pyrolysis oil) under varying operating conditions influencing utility consumption (heat duties and power) and product yields. Additional case studies and analyses are performed in Excel.
Our analysis reveals 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. The uncertainty analysis showed that achieving long-term profitability for the process is still a significant challenge. When compared to traditional waste management methods such as landfilling and incineration, recycling polypropylene through pyrolysis performs better than incineration in almost all environmental impact categories. However, its advantage over landfilling is less clear and depends on which specific impact category is being evaluated.
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.