Mechanical and chemical plastics recycling offer potential for the environment and process technology
3/4/2024 Circular economy & recycling Article

Mechanical and chemical plastics recycling offer potential for the environment and process technology

The circular economy is gaining momentum: Plastics in particular are in the spotlight in view of growing mountains of waste. But how does recycling actually work? And what processes and machines will be needed in the future?

The letters of the word

Eleven billion tonnes is no mean feat. That's how much plastic was produced in the world between 1950 and the summer of 2023. Because eleven billion tonnes is hard to imagine, it helps to make a comparison: if you add up all the buildings, Berlin, for example, comes to a weight of two billion tonnes. The problem is that not even 10 per cent of the plastic produced so far was recycled - almost 80 per cent ended up in landfills or in nature - a mountain that is equivalent to four times Berlin stacked on top of each other, or even six times in terms of volume.

Plastics are probably the easiest way to explain the principle and also the problem of the hitherto linear economy: "take-produce-dispose". Yet plastic waste is a valuable resource: it not only contains a lot of energy from crude oil and natural gas, but also a lot of valuable hydrocarbons that are needed in the (chemical) industry. Industry has now recognised this potential - and the public and politicians are increasingly demanding its use.

Circular economy can reduce the waste mountain

One key to this is the circular economy - a model of production and consumption in which existing materials and products are used, reused, repaired, refurbished and recycled for as long as possible. And this requires a lot of process engineering. The starting point is mechanical recycling: this is suitable for clean, unmixed plastic waste and makes it possible to reuse polymers for new products with comparatively little effort. This is particularly the case when it comes to thermoplastics such as PET. Sorting is particularly important here. Here the trend is towards highly automated sorting systems in which analytical equipment such as near-infrared spectroscopy (NIR), X-ray fluorescence, terahertz technology or multispectral imaging are used to identify plastics. Artificial intelligence and machine learning in combination with robotic pickers make it possible to significantly increase grade purity in the recycling of plastics. This is an El Dorado for creative specialists in mechanical process engineering - because in addition to classic machine technology, as used for sorting, washing, drying, melting, etc., the combination with high tech is increasingly in demand here!

However, if plastic waste cannot be sorted cleanly enough, chemical recycling is called for: in this process, the waste is broken down into its molecular components, from which new plastics or other chemical products are finally produced again. Common processes are pyrolysis, in which the polymers are decomposed into their components by heating in the absence of oxygen, or gasification, in which a synthesis gas consisting of carbon monoxide and hydrogen is produced using steam and oxygen. Other processes include solvent-based recycling and depolymerisation. Compared to mechanical recycling, however, chemical recycling is relatively expensive and energy-intensive. Nevertheless, many plastics and chemical producers are currently looking at this option: the Plastics Europe association expects companies to invest €7.2 billion in chemical recycling by 2030.

Chemical recycling on an industrial scale

Whether BASF, Covestro or Evonik - the big chemical companies are currently researching options for plastics recycling and are already testing on a pilot scale. The motto of the chemical giant DOW is not to spill the beans, but to make a big splash. Together with the British recycling specialist Mura Technology, the company is planning and building a plant at the chemical site in Böhlen, Saxony, in which up to 120,000 tonnes of plastic waste will be chemically recycled annually from 2025. As a producer of plastics and a purchaser of the raw materials resulting from recycling, Dow is thus very close to the vision of a closed cycle. And because the two project partners are also looking at further projects in Europe and the USA with a total capacity of 600,000 tonnes per year, we want to take a closer look at the process.

The first special feature of the technology called HydroPRS is the undemanding nature of the input material: even multi-layer plastics, which were previously considered non-recyclable, can be cracked with the "Hydrothermal Plastics Recycling Solution" and converted into usable chemicals. The process begins with the preparation of the plastic. The raw material is shredded and impurities such as glass or metals are removed. Then the shredded plastic mixture is melted and supercritical water is added. This is because above the critical point of 374 °C and 221 bar there is no longer any difference in density between liquid water and water vapour - the single-phase fluid can thus penetrate the plastic waste like a gas, but at the same time has the density of a liquid - a property that is also used to generate electricity in steam power plants.

The thermal energy splits the plastics in the reactor into liquid hydrocarbons and gas. The pressure is then reduced and the resulting products are separated into individual fractions. The energy for this separation process comes from the expansion of the reactor, and the resulting high-energy process gas is used to generate supercritical steam. The products of the process are then used to produce new plastics such as polyethylene, polypropylene or others (PET, PS, ABS and polyamides), but in some cases they are also used as a substitute for fossil fuels (naphtha, gas oil).

Partnerships for closed value-added cycles

In contrast to traditional licensors, the British process developer Mura relies entirely on the methodology of the circular economy - and this is based on close partnerships between companies along a value chain. For example, the company cooperates not only with Dow but also with the plastics giants Chevron Phillips Chemical and LG Chem. In Germany, the plain bearing and energy chain specialist Igus has also recognised the potential of the technology. For a rapid roll-out of the technology, the large-scale plant manufacturer KBR is also on board. The best prerequisites, therefore, for putting a stop to the further growth of the global plastic waste mountain.

Numerous developments and technologies for the mechanical and chemical recycling of plastics will also be on show at the upcoming POWTECH TECHNOPHARM.


Armin Scheuermann

Armin Scheuermann

Chemical engineer and freelance specialised journalist