Jul-2023

Purifying and upgrading of waste plastics pyrolysis oils

Closing the plastics production disposal loop requires the ability to recycle at scale by combining chemical recycling with pyrolysis technology.

Artem Vityuk and Sanaz Norouzi
BASF Corporation

Article Summary

Plastics as materials feature unique benefits that have proven indispensable for modern society. As their use areas span building materials and mobility to consumer goods and machinery, low costs and wide availability make plastics a universal solution. However, they are also extremely difficult to replace.

Production volumes have also been impressive. Global production of thermoplastic resins (PE, PP, PS, PET, PVC) reached 300 MMt/yr in 20221 and continues posting growth rates above global GDP. While industry has been able to supply much-needed plastics volumes to support economic growth, the issue of plastics waste has never been truly resolved.

Plastics are among the least recycled materials, with an average global recycling rate of only 9%, compared to steel at 85%, aluminium at 75%, paper at 60%, and glass at 50%.2 Fundamentally, the problem of waste plastics is not only an environmental or pollution problem but also a resource efficiency issue. As industries look to transition from linear to circular economy models to reduce associated GHG emissions, the reuse of materials and specifically plastics is crucial.

Looking at recycling methods that are used commercially, mechanical recycling plays an important role. However, they only apply to specific polymer grades and are not utilised for hard-to-recycle plastics, limiting their impact on a global scale.1 While there are alternative methods in development and commercialisation, the only feasible option for recycling waste plastics at scale is chemical recycling, where pyrolysis technology has been proven instrumental.³

There are certain unique benefits associated with pyrolysis. However, the key features include scalability (pyrolysis and its modifications have been known to industry for decades) and ease of integration into existing olefin production assets (steam crackers). Waste plastics pyrolysis facilities produce pyrolysis oil (pyoil), which can be used as a feedstock for naphtha crackers towards ethylene and propylene production, the core monomer building blocks of most plastics.^4,⁵

Ideally, it is possible to divert hard-to-recycle waste plastics from landfill and dumps (unsuitable for mechanical recycling) to pyrolysis sites supplying polyolefin complexes producing brand-new plastics and in essence close the plastics production disposal loop. That exciting recycling solution is gaining momentum in chemical and petrochemical companies engaged in olefins value chains.

There are challenges; pyrolysis of waste plastics at a commercial scale is not mature. The most pronounced technical challenge limiting the wider use of waste plastics-derived pyoils in downstream petrochemical assets includes purity and compositional diversity. For example, steam cracker feedstocks feature very tight specifications that need to be satisfied if pyoil is added as a feedstock.

High contamination of pyoils is not surprising as mixed waste plastics are normally a very complex combination of polymers that often contain electronics waste, multilayer films, plastics composites, and other side components contributing to contamination of the resulting pyoils. Regional and country-specific factors also influence the composition of mixed waste plastics. 

Thus, the purification of waste plastics pyoils is an absolute must if industry aims to reach large industrial use of this technology and reduce plastics waste by chemical recycling. Alternatives are to apply extensive presorting or modify the pyrolysis process by reducing useful liquids yields – cost-prohibitive measures that would limit wide industrial usage.

Solving the pyoil purification challenge
Solving these bespoke challenges and building a more circular economy for plastics requires innovation and joint efforts across the value chain. To enable more efficient plastics recycling, the proprietary PuriCycle line of high-performance products has been developed. They use novel catalysts and adsorbents to purify most of the investigated complex waste plastics pyrolysis feeds (pyoils). A wide range of the pyoil’s impurities are selectively converted or removed, enabling downstream processing of circular plastics streams.

Purification of pyoils obtained from waste plastics is among the most demanding technical tasks in chemical plastics recycling. Impurities, such as halogen, nitrogen, and oxygen compounds, plus higher levels of reactive components, such as dienes, complicate the downstream use and impose strict limitations on further processing of such streams in new polymers production.

PuriCycle is not a purification process, but a set of catalysts and adsorbents designed to purify plastic pyrolysis feeds. It covers products in four application segments: decontamination, prehydrogenation, dehalogenation, and hydrotreating. These process technologies are generally known to industry as, for example, prehydrogenation and hydrotreating (widely used in refining). However, the compositional complexity of waste plastics pyoils imposes new requirements on adsorbents and catalysts formulations designed in accordance with the ‘fingerprints’ of impurities seen in pyoils.

Decontamination
A wide range of pyrolysis process configurations and plant arrangements exist, with some plants producing lower-grade pyoils requiring some basic purification (removal of microns range-sized ash particulates and heavy deposits). Activated alumina-based adsorbents are well positioned to perform this duty. A wide range of high pore volume, high surface area alumina sphere and granule adsorbents can be provided as a drop-in solution. Depending on customer needs, they can be used at the production site to improve pyoil colour properties, making them suitable for further purification.

Prehydrogenation
Pyoils often contain certain amounts of highly reactive compounds, mostly dienes and styrenes, although there is a wider spectrum of polyunsaturates. The exact level of these components is highly dependent on the composition of mixed plastics waste and pyrolysis process design. Values typically range from about 1-10 wt%. Due to high reactivity, these species tend to oligomerise when subject to elevated temperatures, substantially impacting the properties of pyoil and making it less stable.

Pyoils often contain certain amounts of highly reactive compounds, mostly dienes and styrenes, although there is a wider spectrum of polyunsaturates. The exact level of these components is highly dependent on the composition of mixed plastics waste and pyrolysis process design. Values typically range from about 1-10 wt%. Due to high reactivity, these species tend to oligomerise when subject to elevated temperatures, substantially impacting the properties of pyoil and making it less stable.

As a result, such pyoils are more challenging to process and transport. For these conditions, PuriCycle SH chemistries are designed to selectively hydrogenate reactive compounds frequently seen in waste plastics pyrolysis streams, improving feed stability. Prehydrogenation process conditions are comparatively mild, with temperatures ranging from 80-150°C and hydrogen pressures from 15-25 barg, allowing for easy integration with available on-site plant infrastructure.