(131b) Process Systems Engineering Tools for Assessing Upcycling Technologies of Plastic Waste

Global plastic production is estimated at >350 million tons per year and it is projected to increase to over 1200 million tons by 2050.^1,2 Most plastics are discarded after their first use. Only 9% of the United States (US) plastics are recycled and 16% are incinerated.³ The remaining 75% is disposed into landfills representing an average estimated annual loss of $7.2 billion of market value and 3.4 EJ as embodied energy⁴ (equivalent to 3% of the total energy consumption in the US⁵). The most widespread recycling technology is mechanical since it has low processing costs. However, it degrades the polymer properties, and the plastic can only be recycled up to 6 times. Thermochemical recycling techniques are one alternative that can produce virgin monomers (recycling) and added-value products (upcycling).⁶ The most widespread thermochemical technologies involve pyrolysis, gasification, and hydrothermal liquefaction. Recently, the intensification with catalysts and new heat transfer technologies have increased the yield of valuable petrol fractions (olefins and aromatics). Furthermore, other technologies like direct hydrocracking and hydrogenolysis have also been developed.⁷ These technologies reduce the direct energy consumption operating at mild temperatures but utilize H₂ as a depolymerization agent. Understanding the economic and environmental potential of these and conventional ones is necessary to guide technology selection.

In this presentation, we demonstrate the potential of process systems engineering tools to provide this guidance. We structure the work in three parts:

1. Simulation-based process design of the thermochemical depolymerization technologies with Techno-Eeconomic Analysis (TEA) and Life Cycle Assessment (LCA) for determining the costs and environmental impacts of each of the technologies.
2. A Supply Chain (SC) framework for determining the optimal technology, location, and means of transport. This supply chain integrates the economics and environmental impact analyses of previous depolymerization technologies in parametric and piecewise linear models with the costs and environmental impacts of transportation. The model is formulated as a Mixed Integer Linear Programming problem (MILP) with 1.5 million variables and ~2 million equations.
3. A superstructure optimization determining the optimal use of the pyrolysis products. This tool is formulated as a MILP problem with ~2,000 variables and 1,700 equations.

In the first stage, we compare the recent technologies with conventional technologies. The analysis determines pyrolysis as the most profitable technology and hydrogenolysis supplied with blue hydrogen as the most sustainable. Both are the only two profitable technologies (positive internal rate of return (IRR)), but pyrolysis has a higher IRR if microwave heating is used than hydrogenolysis. At the second stage of the analysis considering the entire SC, pyrolysis is also recommended an economic objective is used. In the design of the SC to minimize the global warming potential, a combination of mechanical recycling, hydrogenolysis, and hydrocracking technologies is recommended. Apart from using the two objectives separately, multi-objective optimization is performed using the epsilon constraint method for determining the Pareto set of solutions. These intermediate points show how pyrolysis is preferred as we are closer to the economic objective, substituting first hydrogenolysis, then mechanical recycling and finally hydrocracking. It should be noted that for these cases, pyrolysis is considered to produce olefins and subsequently lubricants as the final product. However, olefins can be transformed into multiple products. Thus, at the last stage of our analysis, we optimize the superstructure of alternative routes for pyrolysis products. The economic optimization propose a multiproduct refinery with the most valuable products, aldehydes and lubes. Among them, aldehydes are very energy and emission intensive since they need H₂ and they are substituted in the environmental optimization that minimizes the GWP. The production of low-density polyethylene and acetone is preferred instead.

References

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2. Ellen-Macarthur-Foundation (2016). The New Plastics Economy: Rethinking the future of plastics. Available in: https://ellenmacarthurfoundation.org/the-new-plastics-economy-rethinking-the-future-of-plastics

3. United-States-Environmental-Protection-Agency (2020). Advancing Sustainable Material Management: 2018 Tables and Figures. Assessing Trends in Materials Generation and Management in the United States. Available in: https://www.epa.gov/sites/default/files/2021-01/documents/2018_tables_and_figures_dec_2020_fnl_508.pdf

4. Milbrandt, A., Coney, K., Badgett, A., and Beckham, G.T. (2022). Quantification and evaluation of plastic waste in the United States. Resour Conserv Recy 183. ARTN 106363. 10.1016/j.resconrec.2022.106363.

5. EIA (2022). Energy Consumption by Sector, Mothly. . Available in: https://www.eia.gov/totalenergy/data/monthly/pdf/sec2_3.pdf Last access: 31st December 2022. .

6. Dogu, O., Pelucchi, M., Van de Vijver, R., Van Steenberge, P.H.M., D'hooge, D.R., Cuoci, A., Mehl, M., Frassoldati, A., Faravelli, T., and Van Geem, K.M. (2021). The chemistry of chemical recycling of solid plastic waste via pyrolysis and gasification: State-of-the-art, challenges, and future directions. Prog Energ Combust 84. ARTN 100901. 10.1016/j.pecs.2020.100901.

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