Nov-2024

Upgrading pyrolysis oil for greater plastic circularity (ERTC 2024)

Our current use of plastics is ahead of our ability to handle waste plastic volumes. Only around 15% of plastics produced each year are recycled, with predictions that the global recycling rate of commodity plastics will reach just 16.5% by 2030.

Dan Miskin and Nirav Shah
Evonik

Article Summary

A circular solution is needed; however, numerous barriers must be overcome to achieve such success. The varied international management of the problem poses one such challenge; the EU is looking to ensure all plastic packaging is reused or recycled by 2030, whereas the US is in an early stage of development with its plastics waste management system.

Other difficulties include the make-up of individual plastic products, which cannot all be recycled using the same process, and accessing plastic waste feedstock at a scale with consistent quality and cost-effectiveness. Advanced methods of recycling, such as pyrolysis, present a viable solution to some of these issues, helping drive forward the circular economy.

Current picture
Mechanical recycling has been around for decades and is now established on an international scale. Keeping the polymer chain intact, this method has limited pools of appropriate feedstock; the resulting material properties limit end-market applications.

For plastic waste streams that are unsuitable for mechanical recycling – whether lower quality or mixed waste streams – chemical recycling, particularly pyrolysis, can be used as a solution. These streams can be converted into feedstock to be utilised by the petrochemical industry.

Pyrolytic oil can help reduce dependence on fossil fuels, replacing fossil-based naphtha in steam crackers and refineries as a more sustainable feedstock. It also represents a more sustainable fuel option for hard-to-abate industries, such as marine transport and air travel.

However, this technology is not yet at scale. Although large chemical companies are starting to invest in pyrolysis oil production, these initial commercial plants vary between 10 and 50 kta compared to the typical steam cracker size of 500-1,000 kta. Overall, further development of key technologies, waste collection, and sorting are required. For the greater adoption of pyrolytic oil as a raw feedstock, issues with purity and compositional diversity need to be overcome. Mixed plastics like PVC, PET, fillers, and additives can introduce contaminants (such as chlorides, nitrogen, oxygen, and silicones), while oil instability and poor flow characteristics also pose issues.

Cleaning out contaminants
Chlorides, a significant contaminant in plastic waste streams, can cause corrosion issues in steam cracking furnaces in petrochemical plants, resulting in plant and equipment breakdown. Additionally, reduction of contaminants is required to meet ethylene plant specifications.

However, the complexity of organic chlorides pose a challenge. They tend to exist in roughly equal concentrations throughout the boiling point range of the oil, attach to varying carbon number hydrocarbons, and have differing levels of steric hindrance.

or years, Chlorocel® – Evonik’s chloride adsorbent line – has been used to great effect in varied refining processes. The company’s researchers are also taking industrially proven processes and adsorbents used elsewhere in the industry, applying and adapting them to plastic waste recycling processes to introduce new brands of products for purification and upgrading.

Rocket technology has proven effective in pretreating feedstock. The skid-mounted modular purification unit utilises regenerable adsorbents, ensures optimum fluid dynamics for better performance, and is fully automated for efficient monitoring of adsorption and regeneration. It can treat used motor, industrial, and transformer oils in addition to pyrolysis oil. Work is currently underway to see how regenerable this technology could be in PyOil² service.

Zeolite-based sorbents are an established technology for removing organic chlorides in fossil feedstocks but have proven less effective with plastic pyrolysis oil. Bauxite-based adsorbents have been shown to be effective for organic chloride removal – and for other impurities including metals, colour, and odour – when applied to the development of plastic pyrolysis oil. If developed to make regenerable, these adsorbents could contribute to better sustainability and circularity.

Another option includes hydroprocessing catalysts, allowing for the separation of impurities and contamination during production. Boasting regenerability and reusability, these catalysts not only remove metals and organic chlorides, but also reduce olefins and aromatics for decreased heater fouling in steam crackers and better yields.
With the challenge of oil instability – which can cause fouling and corrosion within process equipment and sludge accumulation on the bottom of storage tanks – Evonik’s SiYPro® additives assist with stabilising finished pyrolysis oil and preventing fouling.

Polyolefin-rich plastic waste feedstocks, which tend to produce highly aliphatic pyrolysis oils, lead to oil with a high wax content that can solidify at relatively high temperatures. Viscoplex® additives minimise wax crystallisation, significantly reducing the pour point and viscosity of plastic pyrolysis oil, and can eliminate the need for heat tracing.

Advancing to the next step
Organic chloride removal from plastic pyrolysis oils is a challenging new problem for the circular plastic industry. Constant innovation is vital to achieving a shared solution.
Evonik has recently developed a new adsorbent specifically for removing organic chlorides from plastic pyrolysis oil. It demonstrates significant improvements over existing sorbents (Figure 1) and significantly reduces chloride in highly contaminated light and heavy pyrolysis oil (Figure 2).

Industrially produced pyrolysis oil was distilled into a naphtha and residual fraction with a cut-off point of 400°F (200°C). Both fractions saw significant chloride removal of 300 parts per million (ppm), with removal efficiency higher for the naphtha fraction.

Conclusion
Expanding the amounts, types, and qualities of plastic waste that can be recycled, pyrolysis is a viable and complementary method to grow the recycling landscape and contribute to a circular solution. Wider adoption will lower capital cost per metric ton and reduce CO₂ footprint per metric ton. Its maturation will improve yield, efficiency, and reliability for higher throughput and lower Opex.

Contaminants pose a barrier, affecting output quality, quantity, and process efficiency. Evonik’s new generation of pyrolysis oil-specific catalysts and adsorbents makes it possible to meet chloride and other purity specifications for even the most stringent petrochemical complexes. Ongoing research will only continue to strengthen this portfolio for improved circularity.

This short article originally appeared in the 2024 ERTC Newspaper, which you can VIEW HERE