Tatu Köli from School of Chemical Engineering at Aalto University recently completed his Master’s thesis touching upon the circular plastics theme in the SPIRIT programme. The thesis, titled “Pyrolysis of multilayer plastics: Influence of temperature and feedstock composition” is part of the Urban Mill co-innovation project led by VTT.
With his freshly approved thesis soon making him a M.Sc (Chem.), Tatu Köli is now working as Research Scientist at VTT. In his role, he will continue to work with the Urban Mill co-innovation project, which develops an industrial chemical recycling concept for plastic waste. Tatu’s thesis focused on the pyrolysis of multilayer plastics, studying particularly the influence of temperature and feedstock composition. This is supporting one of the objectives of the SPIRIT programme, namely, to develop the concept for chemical recycling technology.
Pyrolysis is an emerging technology for treating plastic waste streams enabling recycling of streams unsuitable for mechanical recycling, such as multilayer plastics (MLP). Tatu explains: “The objective of my thesis was to gain an overall understanding of the pyrolysis of polymers commonly used in the MLPs, to study the influence of process conditions and to assess the suitability of pyrolysis products for steam cracking. It aimed to generate data to support the scale-up of the pyrolysis process and planning of further processing and purification methods for the pyrolysis concept.” According to Tatu, there were some very good findings and ideas for further studies for the Urban Mill project and its scale-up. Tatu is looking forward to developing and monitoring test runs to continue the studies of the pyrolysis process for plastics.
In the study, a series of lab-scale pyrolysis experiments on mixtures containing polyolefins (PO) and varying amounts of polyamide-6 (PA-6) and polyethylene terephthalate (PET) was carried out in a batch fixed bed reactor. The aim was to investigate the effect of temperature and feedstock composition of the product characteristics. The influence of montmorillonite clay catalyst was also studied.
Some of the findings:
- The liquid product derived from mixed plastics clearly exceeded the typical naphtha steam cracker feedstock specifications for nitrogen and oxygen. Most of the nitrogen and oxygen present in PA-6 was retained in the liquid and solid products, whereas almost half of the oxygen in PET evolved as CO and CO2 at the highest studied pyrolysis temperature of 600 °C.
- Caprolactam and benzoic acid were the main liquid products of PA-6 and PET with yields up to 41 and 23 wt-%, respectively. Caprolactam is the PA-6 monomer and accounted for a major share of nitrogen and oxygen present in the liquid product. This emphasizes the importance of efficient removal of caprolactam in the product post-treatment. As the value-added PA-6 monomer is recovered, the nitrogen and oxygen content declines increasing the product quality and suitability for steam cracker feedstock.
- Compared to PET and PA-6, the pyrolysis of PO gave significantly higher naphtha yields in the simulated distillation. The naphtha fraction with boiling point up to 200 °C is often desirable steam cracker feedstock, and therefore fractioning by distillation could be considered a viable step for upgrading the product. Further distillation experiments on the liquid product should be carried out to study the feasibility of distillation for product upgrading.
- The introduction of montmorillonite catalyst increased the gas yield, reduced the liquid yield and shifted the boiling point distribution towards lighter products. Caprolactam and benzoic acid content in the liquid product was reduced to an undetectable level in the presence of catalyst. As the catalytic pyrolysis has shown to affect the product distribution significantly, the performance of other low-cost catalysts in terms of optimizing the product distribution should be investigated in future studies.
“Chemical recycling, and especially the pyrolysis route will complement the mechanical plastic recycling in the coming years,” says Jaakko Tuomainen from Borealis. “The pyrolysis process is less sensitive to the feedstock composition, but it is not at all insensitive to that! This thesis adds understanding, what is a feasible level of PA and PET in the multilayer plastics, and how are these materials best recovered in the chemical recycling process via pyrolysis. That will improve the plastic recycling rate in Finland, which is one of the SPIRIT targets.”
Joona Lahtinen from VTT adds: ”This master’s thesis provides invaluable data that directly supports the mission of the UrbanMill project to create the whole value chain of chemical plastic recycling. The results derived from this work hold significance for our future tasks, guiding us in our future research steps and helping to establish clear boundaries and operational parameters for pyrolysis within the project.”