Chemical recycling through pyrolysis

This article examines chemical recycling via pyrolysis, which converts waste plastics into valuable petrochemicals. The benefits and challenges of integrating pyrolysis into the circular economy will be discussed, as well as the process principles, factors affecting product distribution, and potential applications of the petrochemical products. Potential strategies to optimise the technology for commercial-scale implementation will also be outlined.

Pyrolysis process

In an oxygen-free environment, pyrolysis involves the thermal breakdown of carbonaceous materials (municipal solid waste [MSW], biomass, waste plastics). It is possible to produce gaseous, liquid, and solid products through a variety of reaction pathways. Depending on the desired product distribution, the process usually takes place between 300°C and 900°C. Condensable output can be created by pyrolysis-based equipment, which is very useful when processing plastics or tyres. In such cases, the operating temperature should not exceed 600°C. A higher processing temperature is required if syngas is to be the primary output – nominally above 800°C.

Product distribution is influenced by temperature, residence time, feedstock composition, and catalysts. Generally, gaseous products prefer higher temperatures, but careful temperature management and residence time can have a significant impact on petrochemicals and refined fuels.

In pyrolysis, temperature plays a crucial role in determining reaction rate and product distribution. Due to the faster rate of reaction at higher temperatures, gaseous products such as hydrogen and methane are produced. Alternatively, lower temperatures promote the formation of liquid and solid products, such as pyrolysis oil and char.

Reactor residence time refers to the amount of time a feedstock remains inside the reactor. A longer residence time can result in more secondary reactions, resulting in more liquids and solids. In contrast, shorter residence times may favour gaseous formation.

Product distribution is heavily influenced by feedstock composition. Generally, polyethylene (PE) and polypropylene (PP) produce more liquid products, while polyvinyl chloride (PVC) and polyethylene terephthalate (PET) produce more solid products. Considering that PET already has a significant recycling presence, liquid fuels can easily avoid this in the feedstock. In general, PVC should be avoided, except when naturally occurring in waste streams (e.g., hospital waste), where the system should isolate chlorine compounds and transform waste streams accordingly.

The use of catalysts improves the yield and selectivity of products produced by pyrolysis. The use of zeolite catalysts can enhance hydrocarbon production, while metal-organic frameworks (MOFs) can be used to selectively target chemical intermediates. Catalysts should be optional in a system and used only when necessary.

Pyrolysis can be performed in batch reactors, continuous reactors, and fluidised bed reactors. There are advantages and disadvantages to each reactor type, depending on the scale and distribution of the product. Chemical recycling will most likely succeed commercially with continuous reactors, which offer significant economic and efficiency advantages. As an example, if one is favouring syngas as the main output, continuous phase reactors should be used (with up to 1000 kg/hr swallowing capacity, and multiples of 1000 kg/hr units up to 5 tph), then flash pyrolysis is optimal and takes up less real estate in a vertical solution. Phase pyrolysis is frequently the best solution for producing liquid fuels.

This article was originally published in the November 2023 issue of Hydrocarbon Engineering magazine. To read the full article, sign in or register for a free subscription.

Written by Dan Nienhauser, Stellar3, USA,.

Read the article online at: https://www.hydrocarbonengineering.com/petrochemicals/18032024/chemical-recycling-through-pyrolysis/