Chemical recycling methods generally aim for breaking down the polymers into their building blocks, purifying them separately and combining them again as needed to create new polymers. The carbon bound in the polymer is held in a closed loop, and emissions are minimised. In pyrolytic processes at elevated temperatures and under vacuum, polymers are broken down into a mixture of low molecular substances. In case the low molecular substances are predominantly monomers, this process is called depolymerisation. Out of the standard polymers, only polystyrene exhibits this outstanding property. Thermal depolymerisation allows for almost all post-consumer plastic feedstock, as it has only moderate requirements to feedstock purity.
As a matter of principle, different reactor types can be used to depolymerise polystyrene: simple rotary kilns and autoclaves, continuous fluidised bed processes or reactors where the material is moved with screws. All reactor types operate in a temperature range between 350 and 600 °C, where polystyrene is subjected to high thermal stress and thus breaking down. The styrene monomer is liquid at room temperature and evaporates at these high temperatures, to be subsequently condensed.
In the ResolVe project, a large, lab-scale steel batch reactor is being used to study the decomposition process and to evaluate different feedstock sources. To study the behaviour in a small pilot scale, a reconfigured extruder is being used in a continuous operation mode. Extruders are standard tools to process and granulate polymers, and they are found in nearly every polymer processing company. In contrast to established procedures, the extruder is optimised to generate a maximum output of gaseous decomposition products.
The flow diagram shows how the technology works: PS enriched polymer waste is thermically depolymerised. Styrene is formed as the primary product, aside from oligomers and other pyrolysis products that are condensated and collected as styrene oil. Pigments, fillers or adherent dirt from the waste input form a solid residue. The styrene is then purified via distillation or other separating technologies to the extent that a utilization in the polymer production is enabled. Further by-products may be isolated and used as raw materials as well, or they may be used in conventional steam cracking processes as a replacement for fossil fuel. Only residues that cannot be used in further processes are thermally recovered and contribute to the generation of the necessary process energy.