Research guides future of plastic waste chemical recycling

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Cornell College of Engineering has released new research that aims to simplify the process of chemical recycle. This emerging industry could transform waste products into natural resources. Plastic is physically broken down into smaller molecules.

A new paper, "Consequential life cycle assessment and optimization of high-density polyethylene plastic waste chemical recycling," has been published by Fengqi You, Roxanne E. and Michael J. Zak Professors in Energy Systems Engineering and doctoral student Xiang Zhao. It describes a framework that incorporates several mathematical models and methodologies. These models factor in everything, from the market for end products to the environment and environmental effects.

This framework is the first to quantify the life-cycle environmental effects of plastic waste chemical recycling such as climate change, human toxicity, and other factors.

Plastic has been manufactured in billions of tons since 1950s. However, 91% of this plastic, according to a frequently cited study, has not been recycled. Some see the failure to reduce or reuse plastic as an economic opportunity.

This is why chemical recycling, an emerging industry, is attracting the attention of waste industry professionals and researchers like You. You is helping to identify the best technologies for chemical recycling and is providing a roadmap for future growth.

Chemical recycling not only creates a circular economy in which waste products can be turned into natural resources, but it also opens up the possibility for plastics like high-density polyester to be recycled more often.

Your framework can quantify market dynamics' environmental effects in a way that traditional life-cycle sustainability assessments might not be able to. It is also the first to combine superstructure Optimizationa computational technique that searches over a large combinatorial area of technology paths for minimizing cost, life-cycle analysis and market information.

This paper compares consequential life-cycle optimization with other analytical tools. Life-cycle optimization was used to optimize economic outcomes and minimize environmental impacts. It produced an increase of more than 14% in greenhouse gas emissions, and more than 60% in photochemical air pollution, compared with the traditional attributional approach commonly used in environmental assessments.

The analysis provides industry professionals and policy makers with a broad pathway to promote chemical recycling and a circular economics for plastics. However, there are many options and variables that must be considered along the technological path. The framework suggests that a particular type of chemical separation technology be used if there is sufficient demand for basic chemicals such as ethylene or propylene. However, it is recommended that another type of technology be used if isobutene and butane are required.

You stated, "It is a chemical process and there's so many possibilities." What technology would you use if we wanted to invest in chemical recycling? It all depends on the type of waste and the polyethylene plastic variants. Also, it is dependent on current market prices of end products such as fuels and hydrocarbons.

Chemical recycling has environmental consequences. This depends on variables like the supplier process for chemical feedstocks. The framework showed that butene can be produced locally rather than being supplied, which reduces photochemical air pollution by almost 20%. However, onsite natural gas use increases the risk of ionizing radiation by more than 37%.

You stated that there is always something that can be twisted or adjusted in technology and processes, which is the tricky part. He also said that, as new recycling techniques and markets evolve, life-cycle optimization will continue to be a powerful tool for guiding emerging industries.

More information: Xiang Zhao et. al. Consequential life cycle assessment and optimization of high-density polyethylene plastic waste chemical recycling, ACS Sustainable Chemistry & Engineering (2021). Xiang Zhao et. al. Consequential life cycle assessment and optimization of high-density polyethylene plastic waste chemical recycling, (2021). DOI: 10.1021/acssuschemeng.1c03587