Conference Agenda

Circularity in the chemical industry is crucial for achieving sustainability, particularly as it transitions from traditional, linear, fossil-based production models to more sustainable, resource-efficient processes. Circular processes aim to minimize waste, optimize resource use, and close the loop by utilizing renewable or waste resources, which is becoming a key strategy to address environmental and economic challenges [1]. Repurposing existing infrastructure to integrate circular processes offers a cost-effective and resource-efficient solution, particularly in the petrochemical sector, where capital outlay and long-term economic viability pose challenges to adopting new technologies [2].

This work presents a systematic approach to process repurposing, focusing on matching circular processes with existing petrochemical infrastructure through superstructure-based optimization.

First, we evaluate the viability of incorporating ethylene derived from waste PE pyrolysis into existing naphtha-based steam cracking operations. To do so, we developed process simulation models using Aspen Plus software, followed by a comprehensive techno-economic assessment (TEA) and life cycle assessment (LCA). The findings indicate that the integrated approach offers competitive ethylene production costs of 0.745 €/kg of ethylene, in comparison to the business as usual cost of 0.747€/kg [3] and the 0.839 €/kg cost of standalone pyrolysis. A similar trend is observed in the environmental analysis, with a small reduction of the CO₂ eq. emissions of 1.4396 kg CO kg CO₂ eq./kg of ethylene with the best integration scenario, in comparison to the business as usual cost of 1.4408 kg CO kg CO₂ eq./kg; while the standalone pyrolysis is more favourable in this metric, with 1.2433 kg CO kg CO₂ eq./kg.

In conclusion, this study demonstrates the potential for combining chemical recycling with traditional ethylene production methods, highlighting the importance of balancing economic and environmental factors for sustainable chemical processing. Further research should focus on scaling this approach for industrial use and validating the results with real-world data.

References:

[1] Slootweg, J.; One Earth, 2024. Sustainable chemistry: Green, circular, and safe-by-design.

[2] Télessy, K.; Barner, L.; Holz, F.; International Journal of Hydrogen Energy, 2024. Repurposing natural gas pipelines for hydrogen: Limits and options from a case study in Germany.

[3] Spallina, V., et al. Energy Conversion and Management, 2017. Techno-economic assessment of different routes for olefins production through the oxidative coupling of methane (OCM): Advances in benchmark technologies.

The transition to a circular economy (CE) requires agents in circular supply chain (SC) networks to take a variety of different initiatives, many of which are dynamic in nature. However, there is a lack of generic mathematical models for circular initiatives that incorporate the time dimension, and their combined effects on different agents and overall SC circularity is not well understood. We use a system dynamics (SD)-based approach to develop a generic framework for dynamic modeling of CE networks and use it to model the supply chain for Polyethylene Terephthalate (PET) plastic packaging, a significant contributor to pollution in landfills and waterways. Novel contributions include generic quantitative models for material quality loss and a model for a consumer that includes both continuous and discrete product reuse.

We propose a prototypical circular SC network by combining dynamic models for five agents: a manufacturer, consumer, material recovery facility (MRF), recycling facility, and the Earth. We use the planetary boundaries framework to quantify the absolute environmental sustainability of the network while accounting for feedback effects between different Earth-system processes. We apply this framework to the case study of the PET SC by considering different scenarios over a 65-year time horizon in the US, including both “slow-down-the-loop” initiatives (i.e., those that extend product use time through demand reduction or reuse) and “close-the-loop” initiatives (i.e., those that reintroduce product to the supply chain through recycling) by the consumer, as well as capacity expansion of the MRF and recycling facilities.

We find that given the current recycling infrastructure in the U.S., “slow-down-the-loop” initiatives are more effective than “close-the-loop” initiatives, which require capacity expansion to accommodate the increased recycle rate and an associated time delay. However, combining the two eliminates the need for capacity expansion and leads to the highest circularity. Sensitivity analyses are performed to analyze the effect of consumer behavior on network circularity. As the consumer recycle rate increases, circularity increases until reaching a plateau. This plateau may be due to recycling capacity limitations or quality loss due to the mechanical recycling process; above this plateau, there may be a trade-off between circularity and sustainability. Overall, we conclude that although chemical recycling technologies have the potential to eliminate quality loss and may be promising long-term solutions, such technologies have a significantly higher cost and environmental impact than mechanical recycling and are not currently widespread in the U.S. Thus, in the short term, “slow-down-the-loop” initiatives are more promising solutions for a CE transition.

This material is based upon work supported by the National Science Foundation under Award No. 2339068 (NSF CAREER Award PI AI Torres). Disclaimer: Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

The European Green Deal's objectives for circularity and climate neutrality aims for Europe to achieve net-zero emissions by 2050, proving that economic growth can occur without increasing resource consumption. In the context of plastics, this is potentially achievable by coupling mechanical and chemical recycling to ensure circular economy principles– the former allows for reshaping of the plastic waste to form plastic pellets, while the latter can restore plastic to its original components, maintaining both the quality and mass. Additionally, industrial symbiosis enables sharing resources within industries to reduce waste and increase resource efficiency. This promotes sustainability and circularity by turning one company’s waste into another’s raw material.

This study explores several scenarios for the end-of-life of two common plastic types, polyethylene and polypropylene. To this end, we model the two supply chains, starting from raw material extraction all the way to end-of-life, using input-output data gathered from the literature. Most of the scenarios generated assume the two supply chains work independently, i.e., by exploiting their own resources and showing no interactions with the neighboring system. However, we also explore one scenario implementing industrial symbiosis principles, where the focus is to exchange by-products between the two supply chains so as to decrease resource material extraction and plastic waste. This is particularly appealing when combined with chemical recycling, that can produce a myriad of products, as not all the products generated with this process can be used to produce the original plastic. With these scenarios, we then perform a detailed techno-economic assessment of the different options, while also calculating the degree of circularity they achieve.

Results reveal that the scenario combining mechanical and chemical recycling, within each individual supply chain, already leads to a decrease in the extraction of fossil-based resources, while increasing circularity compared to the combination of recycling with incineration or landfill. Some of these benefits are further improved in the industrial symbiosis scenario, achieving a 14% decrease in raw material extraction at an expense of a moderate decrease in revenues due to the exchange of the byproducts within the systems. The other scenarios show an inferior performance in all the metrics assessed and should be considered inferior options.

This holistic approach to plastic production demonstrates that combining recycling strategies and industrial symbiosis can reduce resource consumption. A combination of the recycling technologies while sharing raw materials within industries can be a key drive for addressing the growing concern of accumulation of plastic waste, thereby improving resource use, reducing waste formation and creating closed-loop systems that can ultimately reach the EU Green Deal targets.