From plastic waste to chemical building blocks – 20.03.2024

New technologies are needed to enhance the recycling rates of end-of-life plastics and to convert them back into new plastics or other valuable products.

The EU-funded TREASoURcE project aims to boost the market development of plastic waste streams from municipal, agricultural and industrial sources through mechanical and thermochemical recycling. During TREASoURcE, the recyclability of these streams will be demonstrated with a target recycling rate of 90 %.

Mixed plastic streams from municipal and industrial sectors are usually extremely heterogeneous, containing various plastic grades, such as polyethylene (PE), polypropylene (PP), polystyrene (PS) and polyethylene terephthalate (PET). These are all used in packaging of our everyday commodities, such as shampoo bottles (HDPE), grocery shopping bags (LDPE), charcuterie packaging (PP), yogurt containers (PS) and water and soft-drink bottles (PET).

In addition to their heterogeneous composition, impurities and additives are also present in mixed plastic streams, which limit their use in new products through mechanical recycling, where high purity or excellent mechanical properties are needed. Chemical recycling can on the other hand deal with more heterogeneous plastic feedstocks and convert them into chemical building blocks for new plastics.

Chemical recycling through pyrolysis

Pyrolysis is one of the most promising chemical recycling technologies for mixed plastic streams. In pyrolysis, degradation of organic materials takes place under the effect of heat in the absence of oxygen, producing monomers or smaller hydrocarbon molecules. These gaseous and liquid intermediates can be refined into virgin quality plastics or added value products.

For plastic pyrolysis, various reactor technologies including fluidised bed, rotary kiln, screw kiln, vacuum pyrolysis, melting vessels, plasma pyrolysis and microwave have been developed. Rotary kilns and fluidised beds are already in commercial use for plastic pyrolysis, while other technologies, such as plasma and microwave assisted pyrolysis, are promising but still require considerable research and development.

The advantages of the fluidized bed technology are its excellent heat and mass transfer and scalability. In fluidised bed reactors, the residence time is typically very short, which results in fewer secondary reactions and side products. Another advantage of the fluidized bed is the possibility to use catalyst as heat transfer media in the reactor, a process known as catalytic pyrolysis. Using the proper catalyst can enhance the degradation of plastics and promote specific reaction pathways, leading to higher yields of the desired target products.

Circular BTX

Our main goal in the TREASoURcE project is to demonstrate the feasibility and scalability of the catalytic pyrolysis of unrecycled plastic waste into valuable aromatic chemicals, including benzene, toluene and xylene (BTX). These aromatics are key chemical building blocks in the chemical industry and used in plastics, synthetic fibers, detergents, pharmaceutical, paints and more. Extensive testing of different catalysts in our fixed bed laboratory scale reactor has shown that plastic waste can be converted into BTX components in a cost-effective way. With the best catalyst combination, a BTX yield up to 40 % was achieved at 550 °C.

Our next target is to continue the work in larger scale with plastic samples originating from industrial, household and agricultural sources. Before pyrolysis, the plastic samples will be shredded, sorted, washed and compounded into granulates with a modular extruder (MODIX). For larger scale experiments a process development unit (PDU) operating at 2 kg/h and a pilot operated at 20 kg/h will be used. Both units include separate regenerators for the catalyst to obtain steady state operation. After the reactors, vapours are quenched using fractional condensers into aromatic liquids. The liquids will be purified through distillation before their final quality evaluation through detailed analyses.

20.03.2024 | Christian Lindfors, Elmeri Pienihäkkinen, Pouya Sirous-Rezaei, Muhammad Khan (VTT).

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