TECHNOLOGIES
Chemical recycling: a simple explainer
Chemical recycling is a simple term encompassing a wide variety of technologies. Chris Smith discusses the key features
Chemical recycling — referred to as advanced recycling by some — is a simple name that brings together a quite broad portfolio of technologies, not all of which are, strictly speaking, chemical in nature. The first three most certainly are: depoly- merisation, pyrolysis and gasification. The fourth — dissolution — does not fall so clearly into the “chemical” classification but is arguably much closer in process terms than it is to established mechanical recycling. Depolymerisation is clearly a chemical recycling
process, typically making use of heat and selected catalysts or enzymes to convert polymer back to building block monomers. It is most suitable for use with step-growth polymers such as PET, which are polymerised by polycondensation. A number of companies are developing processes to depolymer- ise PET, with pilot projects underway from Carbios in France, CuRe Technology and Ioniqa in the Nether- lands, Rittec in Germany, and Eastman in the US. Depolymerisation of polycondensation poly- mers generally involves reintroducing the molecu- lar component eliminated during the original polymerisation process. Several solvolytic process- es are being investigated to do this, including hydrolysis, glycolysis, methanolysis and transesteri- fication. They involve multiple process steps
Chemical Recycling – Global Insight 2024
including pre-treatment of the waste, followed by depolymerisation, monomer recovery, repolymeri- sation, and finally extrusion and pelletising. Solvolytic depolymerisation techniques are not suitable for use with polymers produced by chain- growth or polyaddition reactions, such as PE, PP and PS. However, some companies — including Pyrowave in Canada and Agilyx in the US — are working with alternative depolymerisation technologies that have been shown to be capable of converting PS polymer back to styrene monomer at pilot scale. Depolymerisation technologies produce new polymer of virgin quality. However, the chemistry used is highly specific so the incoming stream has to be consistent in terms of polymer composition, meaning considerable cost may be incurred in pre-sorting. Process energy requirements can also be quite high. Pyrolysis is a thermal cracking technology that
converts waste plastic — and contaminants that waste may carry — back to basic feedstock compo- nents such as hydrocarbons and syngas (a gaseous mixture of CO, CO2
, H2 and CH4
Main image: Chemical recycling technolgies include numerous processes with very different inputs and outputs
). It involves heating
the pre-sorted and shredded waste to temperatures of 400-600°C in an oxygen-free system to produce a range of hydrocarbons. These include gases, waxes, oils and char. As in conventional oil cracking,
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IMAGE: SHUTTERSTOCK
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