Chemical Recycling Mixing Technology for Plastic Recycling

Solvolysis and pyrolysis are two established technologies for chemical recycling. Both of these technologies involve demanding tasks for the mixing technology. This article summarizes the practical approaches to address these challenges — from lab, through piloting and demonstration scale, before building a commercial reactor or plant system.

The use of plastic offers many advantages as it is a very versatile material and can be tailored to meet specific technical needs. It is also cost-effective and affordable, lightweight, easy to mold into various shapes and resistant to corrosion. Consequently, plastic has transformed the way we consume and has displaced other materials that were previously used for the applications, such as wood, metal, and glass.

Consequently, there is still a strong and continuing demand for polymers. Global polymer production was estimated to be 390.7 million metric tons in 2021 with a potential to grow to 460 million metric tons in 2030 [Source: Plastics Europe in “Plastics — The Facts 2022”, October 2022].

However, polymers typically come from non-renewable, fossil resources and our use-once-and-discard approach has led to a significant plastic pollution problem. No wonder that plastic production and plastic pollution has become a growing environmental concern in many societies around the world. In 2017, the total amount of plastic produced since the 1950s was estimated to be 8.3 billion tons. Of that, 6.3 billion tons had turned into plastic waste. Only 9 percent of that waste was recycled, 12 percent was incinerated, and 79 percent ended up in landfills or the environment [Sources: “Production, use and fate of all plastic ever made” by Roland Geyer, in Science Advances, 19 Jul 2017, Vol 3, Issue 7]. In the face of the so-called plastic waste crisis and an attitude that is moving towards considering all waste as a valuable material for new uses, plastic recycling is one of the key elements to move to a circular approach by transforming plastic waste into a valuable new resource.

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

Polymer Recycling Methods

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, so they 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 components that it was built from. In the best case, chemical recycling processes therefore deliver high-quality monomers — chemical building blocks — that can 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 recycle mechanically. The main chemical recycling technologies — pyrolysis, gasification, hydro-cracking and solvolysis — all break down the long hydrocarbon chains of waste polymers into shorter hydrocarbon fractions or monomers.

Pyrolysis and solvolysis are preferably conducted in stirred reactors, which offer great flexibility to handle a wide range of material properties. Post-consumer plastic waste 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.

Stirred Reactor Technology for Pyrolysis Processes

One option for the 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) and polypropylene (PP), and other polymers like polystyrene (PS) and 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 that 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.

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Special know-how is needed on how to achieve the high temperatures of 450–750 °C required for pyrolysis processes in large-scale plants. These temperatures are too high for traditional heating media such as steam or thermal oil. Instead, 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 must be permanently suitable for such temperatures.

• Mechanical equipment such as agitators, mechanical seals, motors, and gearboxes must be equipped with reliable cooling devices.

• The operating and ambient conditions must be safe for the employees.

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

In addition to the demanding mechanical conditions, mixing quality is a major success criterion for the pyrolysis process. The plastic waste added to the reactor at low temperatures has to be heated to a high temperature. Here it is important to homogenize the reactor content efficiently to avoid high temperature gradients — otherwise, long, and uneconomic processing times occur. It should also be considered that pyrolysis reactors can be designed for batch and continuous operation, with each method requiring its own heating strategy.

In cooperation with various customers, Ekato has accompanied and successfully carried out the scale-up of laboratory pyrolysis plants, via pilot scale, up to operating scales of 100,000 tonnes per year. Sound scale-up requires accurate product data such as the viscosity of the melt, which has a significant influence on heat transfer through the vessel wall.

Normally the viscosity of the molten plastic mass behaves in a way that is inversely proportional to the heat transfer coefficient. Depending on the ratio of the various plastics and the temperature, the viscosity of the molten mass is not easy to predict or to measure. In fact, the viscosity can change by a factor of ten during the process cycle. An additional challenge is that the mass usually behaves in a non-Newtonian way. This means that the viscosity of the heated mass is not only a function of the temperature but can also be a function of shear rate and time. Besides its influence on heat transfer, the viscosity has a significant effect on the power requirement of the agitator. A wrongly designed reactor agitator can therefore have a substantial negative impact on the overall efficiency of the pyrolysis system. This means it is especially necessary to determine the viscosity of the melt at the elevated process temperature when examining the feasibility of the complete process.

To address this topic Ekato has a rheometer available for particularly high temperatures up to 600 °C. An integrated camera allows the sample to be observed while measuring. The results of the measurements then <make it possible to design the reactor agitator for the correct viscosity and thus to precisely determine the required agitator power demand and heat transfer performance.

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. As a result, these methods can only be applied to polymers with heteroatoms in their backbones, such as PET, PA, PLA, and PU. In Figure 3, the typical process steps of a solvolysis are illustrated.

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Typically agitated reactors are applied to run the various solvolysis reactions in either batch operation or a continuously operated cascade of reactors. The main mixing tasks are the homogenization of shredded and often irregularly shaped particles with the solvent, and heat transfer for heating and cooling during the reaction. The actual depolymerization often takes place at elevated pressure and temperature in the presence of a catalyst.

Since the process comprises several steps — first the diffusion of the solvent into the polymer, then swelling of the polymer, and subsequently depolymerization — viscosity changes over time cannot be neglected. As for the pyrolysis process, the viscosity will significantly affect the reaction time and heat transfer capability. The agitation system therefore has to be designed flexibly enough to cover the full viscosity range occurring over the different process steps. As a high concentration of suspended plastic matter is desired to increase the productivity and decrease the amount of solvent needed, viscosity becomes even more challenging.

Downstream of the depolymerization reactor, purification of the monomer is often 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 the fact that they work well in the laboratory or at pilot scale but have not yet been transferred to production scale. 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 scales. The process parameters required for scale-up, such as power input and temperature, can thus be determined directly with 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 flakes or polyester clothing have already been performed and optimized. The subsequent scale-up from our 50-liter stainless steel vessel (Figure 4) to pilot plant sizes of up to 10 m³ has already been accomplished successfully.

Summary

There are many companies currently developing and improving solvolysis and pyrolysis processes. Challenges include, for example, the handling of highly viscous non-Newtonian media, very high process temperatures which need to be handled by the equipment, or strongly varying properties of the stirred material throughout the process. Due to the demanding agitation technology, test equipment is often required to manage the development of such a process from laboratory scale to demonstration or production scale.

Ekato Rühr- und Mischtechnik can provide support as a development partner addressing all these requirements, be it for scale-up, physical measurements and trials in test facilities or for the delivery of equipment. Starting from initial process development, Ekato can support new development projects and support in designing and supplying reliable equipment as a cooperation partner in the field of plastics recycling.

Meet the Ekato experts at this year’s Interpack in Duesseldorf/Germany (May 4–10): Hall 16, booth 16C37

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