TECHNOLOGY
In theory, at least, dissolution exposes the polymer to less thermal and physical stress during the recovery process than conventional mechanical recycling. However, the recovered polymer is likely to require compounding or pelletising to make it suitable for further use, which may mitigate that benefit to some extent. In addition, the cost of the numerous processing steps — pre-treatment, dissolution, filtration, precipitation, solvent removal and reformulation — is likely to make dissolution most attractive for processing of mono-material waste streams with a relatively high level of contaminants that would be difficult to remove mechanically otherwise. Depolymerisation is certainly a chemical
recycling process, typically using heat (and often a catalyst) to convert a polymer back to its building block monomers — for this reason it is sometimes referred to as monomer recovery. It is most suitable for use with step-growth polymers such as PET, which are polymerised by polycondensation. A number of companies are developing various
processes to depolymerise PET, with pilot projects underway at Carbios in France, CuRe Technology and Ioniqa in the Netherlands, Rittec in Germany, and BP Infinia, Eastman and Loop Industries in North America. Depolymerisation of polycondensation poly- mers typically involves reintroducing the molecular component that was eliminated during the original polymerisation process. Several solvolytic process- es are being investigated to do this, including hydrolysis, glycolysis, methanolysis and transesteri- fication. These are all multi-step processes that include 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 thermal depolymerisation technologies that are capable of converting PS polymer back to styrene monomer. By converting polymers back to the original monomers, depolymerisation can lead to new poly- mers of virgin quality. However, it uses highly specific chemical processes so the incoming waste stream has to be consistent in terms of polymer composition, meaning considerable cost may be incurred in pre-sorting. Energy requirements can also be quite high. Thermal cracking converts waste plastic – and
many of the contaminants the waste may carry – back to basic feedstock components such as hydrocarbons and syngas (a gaseous mixture of CO, CO2 CH4
, H2 and ). Two processes are used to thermally crack – or
feedstock recycle – polymers: pyrolysis cracks the polymer chains at high temperature in the absence of oxygen; gasification heats the polymer with a controlled but limited amount of oxygen. Both yield a different mix of end products with targeted applica- tions ranging from fuels to chemical feedstocks. Conventional pyrolysis thermal cracking is a
relatively simple technology. Waste goes through a pre-sorting and shredding process and is then pyrolysed at high temperature — typically 400-600° — to create vapour and gas, which is then purified to create a range of hydrocarbons. These hydrocar- bons can include gas, wax, oils and char. Yields of each can be controlled to some extent by adjusting temperature, pressure, and residence times, as well as through the use of particular catalysts and thermal profiles. As pyrolysis occurs in the absence of oxygen,
Four types of plastics recycling Source: AMI Consulting
8
the process is only really suitable for polymers with a limited oxygen content, such as PE, PP and PS. Polymers containing high levels of oxygen or halogens — particularly PVC and compounds containing brominated flame retardants — must be sorted and removed from the waste input stream. Oxygen and halogen concerns aside, pyrolysis can handle waste streams with a mixed polymer composition that would be highly challenging for either mechanical or dissolution and depolymerisa- tion chemical recycling methods. That said, it is an energy intensive process and the quality and mix of the output materials is still dependent to some extent on the input materials. In addition, much of the gas and oil output from pyrolysis plant is likely to be burnt as fuel, either to provide energy for the process itself or because of the need for additional purification steps to upgrade it to be used as a cracker or chemical plant feedstock. Under most regulatory and accreditation regimes, the use of
Chemical Recycling – Global Insight 2022
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