(131c) Design and Optimization of Circular Economy Networks: Recycling of Polyethylene Terephthalate As a Case Study

Circular network design is an emergent approach amongst various sectors in achieving
sustainability. Its applications range from food waste mitigation in the food sector to
prolonging material life and reducing reliance on virgin resource utilization in the textiles
industry. Studies in the literature highlight the positive environmental and economic impacts
of implementing a circular design. In this work, we design circular networks for recycling
polyethylene terephthalate (PET). PET is the primary plastic product used in single-use
plastics and textiles, thus constituting a large share of created plastic and generated waste.
The circular PET supply chain includes recycling by mechanical, chemical, and other physical
processes; the vast amount of options makes it difficult to assess the best pathway and the
extent of material circularity.

This work proposes a superstructure optimization approach to assess circular PET value
chains. The superstructure considers fossil and biomass-based feedstocks for producing
virgin PET via the two most common linear pathways. Additionally, we implement circularity
with mechanical recycling and several chemical recycling options to recover monomers.
The superstructure optimization problem is formulated as a mixed integer linear program
(MILP). We obtain the Pareto optimal solutions that minimize the cost and consumption
of new material. Preliminary results show that a combination of mechanical recycling and
alcoholysis provides the circular network with the lowest operational cost. As expected,
material circularity is improved by adding chemical recycling processes at the expense of
higher costs.

The presentation will discuss the PET linear and circular superstructures, the proposed
MILP model, and the Life cycle analysis of the economically-optimal PET circular value
chain.

This work is funded by Carnegie Mellon University, the GEM fellowship, and the Thomas
and Adrienne Klopack Graduate Fellowship in Engineering.