(727b) Continuous Back to Monomer Recycling of PET from Composite Materials Using a Twin-Screw Extruder

The current Packaging Act requires a 63% quota for the mechanical recycling of plastics from 2022 in Germany [1]. In order to meet this requirement, new efficient recycling technologies are necessary. Many plastics, such as polyethylene terephthalate (PET), are used in combination with other polymers. Apart from energy recovery, no recycling process for such multilayer material is available and it is therefore largely lost to the resource cycle.

The research project revolPET^® specifically addresses this issue through the development of a continuous process for recovering the monomers terephthalic acid (TA) and ethylene glycol (EG) in primary raw material quality from mixed PET-containing waste [2]. While literature mainly reports batchwise depolymerization of PET in stirred tank reactors [3, 4], the revolPET^® process uses an extruder as a continuous chemical reactor.

This contribution presents experimental results on the successful depolymerization of PET in composite materials into a TA salt and EG. The high flexibility of the extruder offers a wide range of options for process design in terms of thermal, mechanical and chemical conditions, so that suitable process windows are identified depending on the feed material. Key to this technology, the twin-screw extruder allows the two reagents to be mixed as solids. This innovative process intensification enables high degrees of depolymerization at average residence times below 2 minutes. In the experiments, a PET depolymerization degree of ≥ 97 % was achieved. Systematic parameter studies reveal the influence of temperature, filling degree of the screw chamber and residence time on the successful PET depolymerization. Figure 1 presents typical results for degree of depolymerization and residence time at varied screw revolutions.

In addition to the technical aspects, the contribution provides insight into the ecological assessment of this new recycling process. In particular, the challenge of the exothermic reaction to identify an optimal process window is shown. This topic is examined in more detail against the background of scaling. The presented experimental results will be discussed in detail and placed in the context of the current political debate.

Literature

[1] § 16 Absatz 2 VerpackG

[2] www.revolpet.eu

[3] A.M. Al-Sabagh, F.Z. Yehia, Gh. Eshaq, A.M. Rabie, A.E. ElMetwally, Egyptian Journal of Petroleum (2016) 25, 53-64, doi: 10.1016/j.ejpe.2015.03.001

[4] A. Oku, L.-C. Hu, E. Yamada, J. Appl. Polym. Sci. (1997) 63, 595-601.