(365c) Process Systems Engineering: Limits of Performance of the “Cyclic” Waste Plastic Economy
Much of the attention has been focused on the idea of a âcyclic plastic economyâ; the principle of this idea is that plastics are first mechanically recycled (washed, repaired and reused) as much as possible until their condition has deteriorated to a point where the plastic needs to be chemically broken into their chemical intermediates. This transformation is primarily accomplished using pyrolysis, which breaks the plastics down to alkene molecules. These alkenes can then be re-polymerized into new plastic products.
In principle, it should be possible to maintain an entirely closed and independent cycle, with negligible environmental impact. Old plastics enter the cycle while new plastics leave the cycle. How realistic is this, though? Just how âcyclicâ is the cycle? This work seeks to investigate this question by applying high-level systems engineering to identify the limits of performance for such a cycle.
The proposed system to investigate this question is comprised of two major sections:
- Pyrolysis, which is an endothermic reaction that involves using heat to split the polymer chains into predominantly alkene components that is typically carried out at temperatures in the region of 823K.
- Polymerization, which is an exothermic process that is carried out at temperatures around 373K. Normally, coordination polymerization of ethylene monomers is used. However, pyrolysis does not produce ethylene alone but rather a range of alkene products. The re-polymerization of pyrolysis products for this system would require reversing the pyrolysis reaction using something analogous to radical polymerization. However, for this work, the mechanism of polymerization is not important but only that it is the reverse of pyrolysis.
Since pyrolysis and polymerization are simple reverses of each other, it seems reasonable to think that the energy generated by the exothermic polymerization of new plastics could be used to facilitate the endothermic pyrolysis of old plastics. This is the entire premise of the cyclic economy. However, the difference in temperatures makes this impractical. The exothermic polymerization does not operate at a high enough temperature for the endothermic pyrolysis. The quality of the energy from polymerization needs to be upgraded before it can be used for pyrolysis. The second law of thermodynamics already allows for heat pumps that accomplish this task but they require an input of power to function.
Previous efforts have shown that waste polyethylene can be used to generate power. If some amount of the waste polyethylene feed is diverted to generate power, a heat pump can use this power to upgrade the energy from polymerization to a temperature appropriate for pyrolysis.
Analysis found this system to have a carbon efficiency of 97.5%. While this efficiency is quite high, it is important to note that, for the cyclic economy to be completely âcyclicâ, the efficiency has to be 100%. The loss of 2.5% efficiency is due to a portion of waste plastic being used to generate power for a heat pump and not to generate new plastic.
Conventional power production invariably requires some form of combustion but the use of renewable energy avoids this issue and presents the possibility of achieving the desired 100% carbon efficiency needed for a truly cyclic system. It was found that a 1.5 MW wind turbine, operating at 30% efficiency could provide the energy requirements for a cyclic system processing 45 tons/day of waste polyethylene. A 1000 kW concentrated solar power installation, operating at 40% efficiency, could process 40 tons/day. These renewable energy sources carry substantial capital costs of between 4 and 6 million US-dollars, for the renewable energy units alone. However, they do achieve the required 100% carbon efficiency needed for the cyclic system.
All of these values represent an idealized limit of performance, the very highest efficiency any process could hope to achieve. Real processes would always perform worse than this limit, general energy loses and fluctuations in renewable energy power supply are just some of inefficiencies present in real systems.
The implication posed by this analysis is that polymers contain a significant amount of chemical potential locked away in the polymer structure. There could be great potential in viewing waste plastics not as a âwasteâ but rather as an opportunity, a potential new fuel source waiting to be unlocked.
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