Depolymerization - Mixing Technology in Plastics Recycling

There are many advantages to using plastic, as it is a very versatile material and can be adapted to specific technical requirements. It is also low-cost and affordable, lightweight, easy to form into different shapes and resistant to corrosion. Consequently, plastic has changed our consumption habits and displaced other materials previously used for the applications where polymers now dominate, such as wood, metal and glass.

Consequently, there is still a strong and sustained demand for polymers. Global polymer production was estimated at 390.7 million tonnes in 2021 and could grow to 460 million tonnes by 2030(1).

However, polymers are typically derived from non-renewable, fossil resources and our use-and-throw-away approach has led to a significant problem of plastic pollution. It is no surprise that plastic production and plastic pollution have become a growing environmental problem in many societies around the world. In 2017, the total amount of plastic produced since the 1950s was estimated at 8.3 billion tonnes, of which 6.3 billion became plastic waste. Only 9 percent of this waste was recycled, 12 percent was incinerated and 79 percent ended up in landfills or the environment (2). In the face of the so-called plastic waste crisis and a change in attitudes that leads to considering all waste as a valuable material for new uses, plastic recycling is one of the key elements in the transition to a circular approach that transforms plastic waste into a valuable new resource.

Polymer Recycling Methods

Polymer recycling comprises various mechanical and chemical recycling technologies which complement each other.

Mechanical recycling which processes plastic waste into secondary raw material without significantly changing the chemical structure of the material is a well-established technology. It is very effective for products made of a single polymer, such as a PET-bottles, or for those whose various components can be properly separated. However, when polymers are repeatedly shredded, melted down and processed by mechanical recycling, they degrade in quality and can only be recycled mechanically a few times before they are too degraded to be reused.

Chemical recycling, also called advanced recycling, can turn plastic back into the monomers, i.e. the molecular building blocks that it was built from. Therefore, chemical recycling processes deliver in the best-case high-quality monomers or building blocks and can be a substitute for monomers from virgin fossil feedstock. Chemical recycling can also help to increase recycling rates, as it can also utilize plastic waste that is colored, multi-layered or contaminated and therefore difficult to be recycled mechanically. The main chemical recycling technologies cover pyrolysis, gasification, hydro-cracking and solvolysis which break down the long hydrocarbon chains of waste polymers into shorter hydrocarbon fractions or monomers.

Stirred Reactors in Plastic Recycling

Pyrolysis and solvolysis are preferably conducted in stirred reactors, which offer great flexibility to handle a wide range of material properties. Postconsumer plastic is intrinsically heterogeneous and of undefined quality. This variation in chemical composition will impact on selectivity, yield, and performance unless the reactor system is designed to allow for sufficient process flexibility and robustness to adapt to changing plastic waste qualities.

This article summarizes the practical approaches to address these challenges and shows EKATO’s capabilities for various development and optimization strategies from lab, through piloting and demonstration scale, before building a commercial reactor or plant system. EKATO supports in the creation of customer-specific system solutions for recycling quantities of up to 100’000 tons per year.

Stirred Reactor Technology for Pyrolysis Processes

One option for a chemical recycling of polymers is pyrolysis, where very high temperatures (450-750?°C) are applied to decompose polymer chains of plastic waste in the absence of oxygen.

The pyrolysis process is suitable for polyolefins (PO), such as polyethylene (PE), polypropylene (PP) and polymers like polystyrene (PS) or polymethyl methacrylate (PMMA). A major benefit of the pyrolysis process is that it can be operated with mixed plastic waste. This is especially true with regard to the complex processing or sorting of the plastic, which is required for other processes.

In addition to the pyrolysis reactor itself, typical pyrolysis plants also comprise several other mixed tanks. In Figure 2, the main process steps of a pyrolysis process are illustrated.

Special know-how is needed how to achieve the high temperatures of 450-750°C required for pyrolysis processes in large-scale plants. Here classical heating media such as steam or thermal oil cannot be used anymore. Therefore, heating methods such as electric heating systems or direct gas firing are applied.

These high temperatures imply new challenges for the equipment:

• The materials used have to be permanently suitable for such temperatures.
• Mechanical equipment such as agitators, mechanical seals, motors, and gearboxes have to be equipped with reliable cooling devices.
• The operating and ambient conditions have to be safe for the employees.

EKATO can assist in the choice of appropriate materials and mechanical equipment based on many years of experience in the field of mixing in high temperature units.

Stirred Reactor Technology for Solvolysis Processes

Solvolysis of plastic waste involves using a reagent to decompose the polymer matrix. The name of the exact solvolysis process often depends on the reagent used: hydrolysis when the reagent is water, alcoholysis when it is an alcohol, and glycolysis when glycol is used. All these processes do not affect carbon-carbon-bonds, but attack bonds between carbon and heteroatoms in the skeleton of the polymers. Therefore, these methods can only be applied for polymers with heteroatoms in their backbone such as PET, PA, PLA, and PU. In figure 3, the typical process steps of a solvolysis are illustrated:

These mixed reactors are typically applied to run the various solvolysis reactions in a batch-operated reactor or in a continuously operated cascade of reactors. The main mixing tasks are fully homogenization of shredded and often irregular shaped particles as well as heat transfer for heating and cooling during the reaction. The actual depolymerization often takes place at elevated pressure and temperature at the presence of a catalyst.

Due to the process contains different process steps like first diffusion of the solvent into the polymer than swelling it and afterwards depolymerize, viscosity changes over time cannot be neglected. It will significantly impact the reaction time and heat transfer capability unless the agitation system is designed flexible enough to cover the full viscosity range.

A high concentration of suspended plastic matter is desired to increase the productivity and to decrease the amount of needed solvent.

Behind the depolymerization reactor purification of the monomer often is achieved by an agitated crystallization step. The crystallization strategy depends on the starting product and on the corresponding monomer quality to be achieved for re-polymerization.

Many of these processes have in common that they already work well in the laboratory or pilot scale but have not been transferred to the production scale yet. And this is precisely where EKATO provides support with its experience in the field of scale up. In EKATO’s R&D center processes such as solvolysis can be reproduced at different scale. The process parameters required for scale-up, such as power input, and temperatures, can thus be determined directly on the original product. In addition, these tests offer the possibility to identify and implement various optimization potentials. In this way various processes for recycling PET or polyester clothing have already been mapped and optimized. The subsequent scale-up from our 50-liter stainless steel plant (Figure 4) to pilot plant sizes of up to 10 m3 could already be accomplished successfully.

Summary

Solvolysis and pyrolysis are two established technologies for chemical recycling and the demand for these processes continues to grow. In addition to the existing processes and licenses to be acquired, there are currently many companies that are developing and improving these processes. Both of the technologies described pose demanding tasks for mixing technology. Requirements include, for example, the handling of highly viscous non-Newtonian media, very high process temperatures that the equipment must cope with, or strongly fluctuating properties of the stirred material during the process. In addition to the sophisticated stirring technology, test equipment is often also required to bring the development of such a process from the laboratory scale to the demonstration or production scale. EKATO can provide support as a development partner for all of these requirements, be it during scale-up, measurements and tests in test plants or the delivery of the equipment. Right from the process development stage, EKATO can give a development project new impetus and support the design and delivery of reliable equipment with cooperation partners who are active in the field of plastics recycling.

Free webinar:

If you are interested in more details, register for our free Depolymerization webinar on July 18: https://www.ekato.com/ekato-group/workshops-and-seminars/depolymerization/

(1) Plastics Europe in ?Plastics – The Facts 2022”, October 2022

(2) “Production, use and fate of all plastic ever made “ by Roland Geyer, Jenna R. Jambeck , and Kara Lavender Law in Science Advances, 19 Jul 2017, Vol 3, Issue 7

(3) “Chemical Recycling – Status, trend and Challenges” by Nova Institute (2020)