28. November 2023 | Magazine:

On the way to the (almost) perfect material cycle TU Braunschweig tests new PET recycling process

Disposable drinks bottles, clothing, trays and other everyday items – polyethylene terephthalate, better known as PET, has become an integral part of our lives. However, a major drawback of the material is that its recyclability is currently very limited. To change this, scientists at the Institute of Chemical and Thermal Process Engineering at Technische Universität Braunschweig are researching a new recycling process. The aim of this process is to return PET to its original materials. This could close the material cycle almost completely and give plastic waste a new value as a raw material. Prof. Stephan Scholl, head of the Institute for Chemical and Thermal Process Engineering, explains how the process works and why it offers completely new possibilities.

When plastics such as PET are recycled, they are usually shredded into flakes and then incorporated into new plastic polymers. For example, recycled PET drinks bottles or textiles with recycled content are produced in this way. However, this process has two major drawbacks: First, it only works with single-origin plastics. Other substances that contaminate the plastic must be separated out. For example, if the plastic contains dyes, it cannot be recycled because the process cannot filter out the dyes. Secondly, shredding the flakes shortens the polymer chains, which means they can only be recycled a limited number of times.

“A PET bottle can be recycled up to eight times using this process. It is then used for energy recovery. In other words, it is incinerated and lost to the material cycle,” explains Prof. Stephan Scholl.

A newly developed process from the Institute of Chemical and Thermal Process Engineering at the Technical University of Braunschweig could now provide a remedy for these limitations. The approach is to return plastics such as PET to their original materials and thus close the material cycle. Here, too, the plastic is first shredded into flakes. These are then washed and depolymerised in an extruder to produce a doughy mass of monoethylene glycol, disodium terephthalate as valuable components and impurities such as colour particles. The doughy mass is then dissolved in water. The valuable components are completely soluble in water and the water-insoluble impurities can be filtered out. In the next step, water-soluble impurities are adsorbed, leaving only water and the valuable components in solution. Finally, sulphuric acid is added. This produces terephthalic acid, which precipitates as a solid from the water solution and can also be filtered out. The remaining monoethylene glycol and sodium sulphate can then be easily separated from the water. The resulting materials can then be used to make new PET.

Focus on sustainability and economics

The new process takes three to four hours to convert PET back into its raw materials. This means that the method developed by the Institute of Chemical and Thermal Process Engineering is just as fast as the conventional recycling process. The process was designed by the team at the Institute of Chemical and Thermal Process Engineering with a short process time in mind, says Professor Scholl. “The process only has a short throughput time because large quantities of PET residue have to be processed. The longer the material remains in the system, the larger the system has to be. If economic efficiency is important, the retention time must be kept as short as possible. The new process is also very climate-friendly.

“Compared to the virgin route, i.e. the production of plastics from crude oil, the production of PET from recycled raw materials can save around 50% in terms of environmental impact.

Development in dialogue with practical partners

The new process is already being tested on a small scale in a pilot plant with customers and partners in collaboration with RITTEC 8.0 Umwelttechnik GmbH.This plant has a material throughput of 15 to 20 kilos of plastic per hour and is currently operated on an hourly basis.”The next milestone would be to run the plant around the clock,” says Prof Scholl.”To achieve this, the system needs to be further automated.That’s what we’re working on at the moment.

Photo Credit: Scholl/TU Braunschweig

The scientists are currently focusing on what other materials can be recycled using the process.The project is working with packaging manufacturers to test the recyclability of packaging.The research team then gives feedback to the manufacturers on which materials cause difficulties and how they could be replaced.

“Recyclability should also be considered in product development,” says Prof Scholl.

It is already clear that all polycondensates can be recycled using the new process.This could also put an end to the incineration of fast-fashion clothing.The garments, which are often made of synthetic fibres, are incinerated during overproduction and returns because they have not been recyclable so far due to dyes and other foreign substances.”The good selectivity of the depolymerisation for polyester and the filterability of the process would also make it unnecessary to sort the cotton and polyester parts of the clothing beforehand, as we can reliably filter out foreign substances, including undecomposed cotton, with the new process,” says Prof. Scholl.

New recycling process opens up new possibilities

“Thanks to the recycling process, 95 to 97 percent of PET can be recycled.We are therefore on the way to an almost perfect material cycle.Three to five percent material loss in the process is an immensely good value.This is very close to zero waste,” explains Professor Stephan Scholl.

In contrast to recycling with shredded PET flakes, the new process can be repeated as often as required.The time required is also similar. In addition, the investment and operating costs are not significantly higher than for building and operating a conventional mechanical PET recycling plant.These advantages open up new opportunities for plastic waste recycling worldwide, especially in the Global South.”Often there are already informal structures that collect plastic waste and sell it to recycling companies,” explains Prof. Scholl.The wider range of applications for the new recycling process opens up much greater opportunities for value creation.”A larger proportion of plastic waste can be returned to the material cycle.This means that even the individual PET bottle has a value at the end of the recycling chain, as materials can be recovered from it for the production of new PET,” says Scholl.The project is working with partners in Brazil, Thailand and Ghana to evaluate its applicability in countries in the global south.The aim of these collaborations is to develop structures that enable local plastic waste collectors to add value by providing a steady supply of plastic waste to a local recycling plant.

Next steps towards industrial use

With a throughput of 15 to 20 kilos per hour, the pilot plant is still too small to serve as a model for a large commercial scale. “The next step towards commercialisation would be a larger plant with a throughput of around 500 kilos per hour, running continuously. However, this would require investors and suppliers to provide the recycling material. The target size for such plants would be 20,000 to 30,000 tonnes per year (about 100 tonnes per day) if they are to be used in industry,” predicts Prof Scholl. The next step, however, will be the continuous operation of the existing pilot plant.