General Process Description

Pyrolysis (or Cracking) can be defined as the chemical and thermal Degradation of Polymeric Materials by heating in the absence of Oxygen in an inert atmosphere e.g. Nitrogen. The Pyrolysis temperature is the most influential Pyrolysis parameter and values between 400 and 800°C are generally employed depending on the Feedstock being processed, whether or not a Catalyst is used, and on the target products. Heating systems for Reactors can be dynamic (e.g. some Batch Reactor configurations) or isothermal (e.g. Fluidised Bed), with isothermal systems being most frequently applied.^[1] A typical Pyrolysis Process is presented in Fig. 1.

Figure 1 - Schematic of a small-scale Pyrolysis Pilot Plant^[1]

Legend: 1-transportation, 2-selective collection, 3-shredding, 4-washing, 5-drying, 6-waste storage, 7-catalyst storage, 8-reactor, 9-heating gas storage, 10--separation unit, 11-catalyst filter

Temperature-dependent Process Yields

The process yields carbonised Char and volatiles that may be separated into Hydrocarbon Oil/Wax and non-condensable Gas. As the Pyrolysis temperature of Polyolefins (PO) decreases, increasing Wax and partially converted Feedstock (Residue) Fractions are observed in the yield structure. The Pyrolysis Products can be applied as Fuels and Petrochemicals. Thermal cracking of POs are usually carried out either at high temperatures (>700°C), to produce an Olefin mixture of C₁–C₄ Gases and Aromatic Compounds (Benzene, Toluene and Xylene) or at low temperatures (400–500°C) where the yield structure comprises a high-calorific value Gas, condensable Hydrocarbon Oils and Waxes.^[1]

The Formation and yields of the various products formed during the Pyrolysis of Mixed Polyolefin Waste depend largely on operating conditions as shown in Fig. 2. It is noteworthy that commercial Processes are mostly geared towards Liquid production under high-temperature conditions (Fast Pyrolysis), rather than towards the production of Waxes under low-temperature conditions (Slow Pyrolysis), probably due to economic imperatives.    

Figure 2 - Pyrolysis Process Trade-offs^[2]

Waste Plastic Feedstock suitable for Pyrolytic Treatment 

From a practical perspective, few Polymers lend themselves to selective Depolymerization: PMMA typically yields about 90% Methyl Methacrylate monomer^[3], and Polystyrene depolymerizes into 65% Styrene and 15% Ethylbenzene under optimized conditions^[4].  On the contrary, most other Polymers produce a complex mixture of gazeous, liquid and solid Products when submitted to thermolytic treatment conditions, such as Polyester (PET) yielding mostly gaseous Products as a major fraction and liquids not exceeding 39% by weight of the total Products, and PVC being mostly gasified - HCl being formed as a main Component - and also producing a high solid Fraction (Char) depending upon the Reaction conditions.^[5]

For that reason, Mixed Plastic Waste is not a suitable feedstock for the Pyrolysis Process as both selectivity and yields of useful Products are too low for commercial purposes as reported in case of the VEBA OIL Technology already 30 years ago^[5], the article outlining that a feedstock made of 60% Polyethylene, 15% Polystyrene, 10% PVC and 5% each of Polypropylene, PET and Polyamide yields a Product Mixture composed of 30 to 50% gaseous Products, 40 to 55% Liquids and 5 to 15% Solids by weight (Fig. 3).

Figure 3 - Pyrolysis of Mixed Plastic Waste via the VEBA OIL Process^[6]

Claims about the Pyrolysis Process being suitable for Mixed Plastic Waste abound, although similarly low selectivity and yields are consistently reported. Such is the case of the FRAUNHOFER iCycle^® Technology^[7], for which the yields of gaseous, liquid and solid Products from the Pyrolysis of Mixed Plastic Waste are reported to be respectively 39.4%, 45.3% and 15.3% by weight (Fig. 4).     

Figure 4 - Pyrolysis of Mixed Plastic Waste via the FRAUNHOFER iCycle^® Process^[7]

It is publicly acknowledged that commercial-stage Pyrolysis plants operate with Mixed Polyolefin Waste as a Feedstock, obtained via either dedicated collection schemes or selective sorting and cleaning of Mixed Plastic Waste. From the Conversion of this Mixed Polyolefin Waste, the following yields have been reported (Tab. 1): 

Table 1 - Published Yields of Mixed Polyolefin Waste Pyrolysis

Post-Processing Requirements

The Liquid Fraction produced in the Pyrolysis Process is Pyrolysis Fuel Oil, commonly denominated PyOil, a form of Gasoil (also known as Diesel) as it covers a broad range of Hydrocarbon Molecules as presented in in Tab. 1, from C₆ to C₂₈. This PyOil is largely made of olefinic Compounds^[5] and requires both Hydrotreatment to saturate the Olefin Groups and Fractionation of the Hydrotreated PyOil to isolate the lighter Components (Naphtha, Diesel) before they can be returned to the Steam Cracker for conversion into Monomers such as Ethylene, Propylene and Butadiene. 

References

1. Butler, E., Devlin, G. & McDonnell, K. Waste Polyolefins to Liquid Fuels via Pyrolysis: Review of Commercial State-of-the-Art and Recent Laboratory Research. Waste Biomass Valor 2, 227–255 (2011). https://doi.org/10.1007/s12649-011-9067-5

2. Adapted from Andrew N. Rollinson and Jumoke Oladejo, Gaia, June 2020: Chemical Recycling: Status, Sustainability, and Environmental Impact

3. W. Kaminsky * and J. Franck: Monomer recovery by pyrolysis of poly( methyl methacrylate) (PMMA), Journal of Analytical and Applied Pyrolysis, 19 (1991) 311-318. https://doi.org/10.1016/0165-2370(91)80052-A

4. Ali Karaduman: Pyrolysis of Polystyrene Plastic Wastes with Some Organic Compounds for Enhancing Styrene Yield, Energy Sources, 24:7, 667-674, (2002). https://www.tandfonline.com/doi/abs/10.1080/00908310290086590

5. Shafferina Dayana Anuar Sharuddin et al.: A review on pyrolysis of plastic wastes, Energy Conversion and Management, 115 (2016) 308–326, 12 Feb 2016. https://doi.org/10.1016/j.enconman.2016.02.037

6. H.P. Wenning: The VEBA OEL Technologie pyrolysis process, Journal of Analytical and Applied Pyrolysis, 25 (1993) 301-310. https://doi.org/10.1016/0165-2370(93)80049-6

7. ONLINE Plastic Waste 2 Plastic Conference, Fraunhofer UMSICHT, Dr. Alexander Hofmann, 9 Nov 2020: Chemical Recycling of Plastic Waste –Pyrolysis and downstream processing of pyrolysis oils

8. Plastic Waste 2 Plastic Conference, Plastic Energy, Carlos Monreal, Founder & CEO, 9 Nov 2020: Chemical Recycling for the Circular Economy: Transforming Plastic Waste into Virgin-Quality Plastics

9. QUANTAFUEL, 4Q 2019 report (no longer available online)

10. Georgia Recycling Coalition Inc., 2015, Nexus Fuel, LLC: Plastic Waste to Fuel