*3.3.3 Plastic-to-monomer*

The production of monomers that would be firther used for polymerisation could be the only solution for closed-loop recycling of PE and PP waste. Depolymerisation of polyolefins is not easy as bonds between carbon atoms in the chain are relatively strong. As described before, thermal or thermocatalytic cracking leads to a mixture of hydrocarbons with different chain lengths. Gas fraction is produced, but alkenes content in it is usually low. A research was conducted to maximise the gas fraction and olefin content, but the highest ethylene concentration in the gas stream achieved was 25%. Still, a minimum of 40% of liquid product was obtained [7]. In other research, steam was used in a fluidised bed reactor for pyrolysis of plastic waste to maximise olefin yields. 20–31% ethene and 14–18% propene were obtained. Additionally, 19–23% gasoline was produced. These yields are similar to standard naphtha steam cracker's product, but the research was run at lab-scale, and no information about scale-up is available [25].

The most extensively investigated process currently is the production of feedstock for commercial steam crackers. Plastic waste is cracked thermally or catalytically into a liquid fraction with proper boiling range and further mixed with fossil feedstock. Some research presented that it would be possible to use liquid from plastic pyrolysis only, receiving results similar to use naphtha [26]. But to avoid coking of the colder section of the steam cracking reactor and contamination of created streams with heteroatoms present in pyrolysis oils, co-cracking with standard feeds is preferred. Purification of the pyrolysis oil by hydrotreatment upfront steam cracking could be a solution that increases the total cost of the process. Considering the capacities of chemical recycling plants and current steam crackers (millions of tons of ethylene), significant dissolution of pyrolysis oil in fossil-based feedstock might be a solution for this issue, as pyrolysis oil can be only a small part of the inlet stream to the steam cracker.

Although making monomers and then polymers from polyolefin waste look like a very promising route for closed-loop recycling of polyethylene and polypropylene, several concerns should be considered. First, the basic question mark is about yields of the process. If it is assumed that pyrolysis oil has similar properties to commercial naphtha, then yields of ethylene and propylene that can be used for further polymerisation is limited, as presented in **Table 2** [27]. In the case of higher boiling fractions (like gasoil or diesel fraction) or oils reach in branched or aromatic


**Table 2.** *Products of steam cracking of naphtha [27].*

#### *Chemical Recycling of Polyolefins (PE, PP): Modern Technologies and Products DOI: http://dx.doi.org/10.5772/intechopen.99084*

hydrocarbons, yields of ethylene and propylene are lower, and the yield of liquid products rises [28]. These products can be used as feedstock for chemical processes but are currently used primarily as fuels, which is not considered as recycling according to European regulations.

As the plastic-based liquid has to be blended with naphtha or other steam cracking fossil-based feeds, it is challenging to trace the flow of materials along the supply chain. For that purpose, a mass balance accounting system is required. It is a set of rules for allocating the recycled content to different products in order to be able to claim the recycled content. Products can be accredited by the independent scheme, for example, the International Sustainability and Carbon Certification Plus (ISCC) scheme. NGOs challenge the currently used calculation method as requiring more clarification and a more strict approach as it can be misused, claiming incorrect recycled contents [29]. It is understandable as long as detailed and correct data is not shared. For example, in one of the published Life Cycle Assessments (LCA) for the process, the considered amounts of naphtha from chemical recycling that is needed to produce 1 kg of LDPE were 1,2–2,0 kg with the baseline of 1,5 kg of naphtha per 1 kg of LDPE produced. These numbers are not in line with ethylene yields from fossil naphtha (as presented in **Table 2**) and require a broader explanation [30].

The final consideration is about the overall environmental impact of the process. For evaluating the influence of the process or product, a systematic analysis of the environmental impacts, called Life Cycle Assessment (LCA), is used. Currently, only two executive summaries of LCA were published, which were also criticised by NGOs [31]. As not enough data is publicly available, it is difficult to evaluate these concerns. What is sure, the process chain is very long and complex, as presented in **Figure 3**, and requires the use of fossil-based feedstocks and only part of the plastic pyrolysis oil is converted back to a polymer. The yield of the fraction that can be processed in a steam cracker in the plastic pyrolysis process is unknown. In this case, LCA analysis should consider yields and processing of other products from plastic pyrolysis and steam cracking to present the whole impact. Lastly, the feasibility of these processes are a matter of concern, especially if hydrotreatment is used for pyrolysis oil purification.

*Scheme of polyethylene and polypropylene from the plastic waste production process.*

Currently, big polyolefin producers are involved in projects for the chemical recycling of plastics into monomers, like BASF, SABIC, Borealis or Chevron Philips Chemical, cooperating with companies experienced in pyrolysis, like Plastic Energy, Quantafuel or Nexus Fuels. In Geleen, the Netherlands, a plant for cracking of polyolefins is constructed and a hydrotreating system for purification of pyrolysis oil, which will later be fed to a steam cracker. This project is a joint investment of Plastic Energy and SABIC [32]. The plant is expected to have a capacity of 15–20 000 t/a and to become operational in 2022.

#### *3.3.4 Plastic to chemicals (upcycling)*

The production of valuable chemicals from waste, called upcycling, is an interesting alternative. The mixture of hydrocarbons obtained from polyolefins' pyrolysis can be upgraded or separated into different hydrocarbon types. What is more, the flexibility of the cracking process enables the maximisation of target fractions.

The major advantage of plastic-to-chemicals processes is that most of the proposed solutions offer final market products that do not require further processing in petrochemical plants. In this case, a mass balance approach is not required as products are based entirely on plastic waste. As products are not dedicated to be burned for energy production, these technologies can be classified as open-loop recycling also under European regulations. What is more, special, niche applications enable higher margin than compared to naphtha or fuel. On the other hand, these applications are limited when products are produced from waste, require high purity (virgin) polymers or complex pre- or post-treatment and purification, which may significantly influence the feasibility.

Benzene, toluene and xylenes (BTX) are important aromatics used by petrochemical industry to produce valuable chemicals like polystyrene, nylons, methacrylates, polyurethanes, plasticisers and many more. The pyrolysis process of polyethylene and polypropylene can be controlled to maximise aromatic hydrocarbons. A presence of polystyrene in the raw material could increase yields of BTX fraction. Nevertheless, it is possible to obtain 53% and 32% BTX from PP and PE, respectively [33–35]. These aromatics have to be further separated from the pyrolysis oil.

Encina from the US is an example of a company that provides a technology of catalytic cracking for BTX and propylene production but is currently not at a commercial scale. The planned unit will produce about 90 000 t/a of chemicals [36].

Polyolefin waxes can also be produced by the cracking of polymers. These kinds of waxes are widely used in PVC production, surface modifiers, additives to other waxes etc., and can be produced as a by-product of polyolefins production. Some companies, like Mitsui Chemicals America, Hana Corp., EPChem or Merlob, crack virgin polymers for the purpose of wax production. In this case, an issue of contamination by additives does not exist. If polyolefin wastes are considered, then a proper purification process should be implemented, or the application range would be significantly limited to those where colour and presence of inorganics is not an issue.

GreenMantra Technologies from Canada produces waxes by catalytic pyrolysis of plastic wastes at elevated pressure (4,5–25 bar). Wax products under the name of Ceranovus can be used for bitumen and asphalt modification, polymer processing or adhesives production. As an addition, fuel oil is produced. The current plant (TRL 9) has a capacity of 5 000 t/a [37]. Another company producing waxes ("EnviroWax") from plastic waste through pyrolysis is Trifol from Ireland. The by-products are fuels: diesel/kerosene and naphtha. The company has a pilot plant (TRL 6–7) with plans for scale-up to 37 000 t/a [38].

*Chemical Recycling of Polyolefins (PE, PP): Modern Technologies and Products DOI: http://dx.doi.org/10.5772/intechopen.99084*

**Figure 4.** *Clariter products.*

Clariter carries out the most complex process for plastic waste conversion into chemicals. Aliphatic solvents ("Solventra"), white oils ("Oilter") and paraffin waxes ("Clariwax") of high purity are produced from polyolefin waste via thermal cracking, hydrotreatment, and distillation and are alternatives to similar fossil-based products available in the market (**Figure 4**). To maximise on profit, target products are deeply purified from heteroatoms and hydrogenated so they can potentially be used in the cosmetic industry. Other applications are: paints, inks, degreasers, wax emulsions, paper and wood impregnation, lubrication, car or furniture polishes, silicone sealants and others. The company owns a pilot plant in Poland (TRL 5) and an Industrial-scale Plant in South Africa (TRL 7) with scale-up plans for 60 000 t/a facilities. Most interestingly, the company claims to achieve a net negative carbon footprint which is unique compared to other LCA's published [39].
