Circular Economy Twin Screw Extruder for Chemical Plastic Recycling

Editor: Ahlam Rais

Coperion is supplying an extrusion system to Ghent University in Belgium for comprehensive research and development tasks in chemical recycling of mixed plastic waste. The company designed this laboratory system, built upon a ZSK 18 Megalab twin screw extruder, especially for chemical recycling of post-consumer waste within a throughput range of 1-10 kg per hour.

Due to its intensive dispersion and devolatilization output, Coperion’s ZSK twin screw extruder is extremely well suited for energy-efficient chemical recycling of mixed plastic waste.
Due to its intensive dispersion and devolatilization output, Coperion’s ZSK twin screw extruder is extremely well suited for energy-efficient chemical recycling of mixed plastic waste.
(Source: Coperion)

Along with the extruder, it includes a feeder from Coperion K-Tron as well as a vacuum unit, states the firm.

The company’s twin screw extruder technology is particularly well suited for chemical recycling of plastics. After post-consumer waste, either shredded or compacted, is reliably added to the extruder’s process section by the feeder, a great deal of mechanical energy is introduced into the material in the shortest time thanks to the continuous surface renewal as well as intensive dispersion and shearing along the twin screws, shares the firm.

Within about 30 seconds, a homogeneous, highly devolatilized melt with a temperature of up to 350°C is produced, into which the energy has been introduced very efficiently.

Further materials, such as catalysts, can be added and mixed in as needed. In some cases, residual water or chlorides from PVC are introduced into the extruder in minute quantities along with the plastic waste. According to the company, both are reliably extracted via vacuum devolatilization on the extruder's process section.

Twin screw extruders possess numerous advantages that are especially beneficial in chemical recycling. The technology covers a broad range of throughputs. On larger ZSK extrusion machines, throughputs of up to 20 t/h can be realized using this process. Polymers of various viscosities are reliably plastified thanks to the highly effective mode of operation of the twin screws. Plastic energy dissipation takes place in no time. When needed, corrosion and wear protection of all product-contact parts within the process section can assure long extruder lifetime even when processing very aggressive materials, mentions the firm.

Reclamation of Raw Materials

Within the reactor, the melt, which was previously heated to up to 350°C in the twin screw extruder, is further heated. At up to 500°C pyrolysis of the polymers takes place, the splitting of polymer chains into shorter units in an oxygen-free environment.

The pyrolysis of polymers utilizes the random scission mechanism, where free radicals are generated. At the same time, chain reactions are initiated which lead to cracking polymers into a broad mixture of hydrocarbons in liquid and gaseous phase. The most important factors for driving this process are residence time, temperature, and the type of pyrolysis agent, opines the firm.

Most of the inorganic components of the post-consumer waste remain in the reactor's sump and are removed. The polymers’ organic hydrocarbons evaporate. They are transformed into monomers, petrochemical raw materials, or synthesis gases and then processed further in a destillator into marketable products such as oil, heavy fuel, or waxes.

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