**1. Introduction**

A real circular economy may be realized if products at the end of their life cycle are reutilized or transformed into raw materials giving them the possibility for another life. The energy balance among the content of the new products and that one required by the transformation must be positive; that is, the process must require less energy of those present in the products; otherwise, the process will have a deleterious effect on the environment. More positive is the balance less will be the contamination. This goal is one of the main challenges in which scientists are involved to solve the problems concerning both global warming and the end of mineral resources. These recycled sources also represent a strong driving force for developing sustainable

industrial processes. Many industrial firms are now involved in environmental issues, and they are, even in a different way, engaged in solving this problem [1].

The world plastic production in 2019 was 368.0 x 106 tons with an increase of 2.5% concerning for 2018 [2] while in the same year, the European production was 57.9 x 106 tons between thermoplastic and thermosetting polymers. Among them, polyolefins are the most produced and employed material in everyday life for industrial, domestic, and technological applications [3]. They are thermoplastic polymers and are mainly used for packaging; their life cycle is very short, which means they must be disposed of in a short time after their production. A less important amount of them is employed to realize furniture, insulating materials, automotive parts, and so on, and their life cycle is considerably longer (10 years or more).

A very recent study by the Organization for Economic Co-operation and Development (OCSE) was published in the "Global Plastic Outlook" [4] where it is reported that less than 10% of plastics are recycled around the world. Considering that the world production is estimated to be 460.0 x 106 tons, higher than the total waste (353.0 x 106 tons) produced in the same year, many actions are required to dispose of these plastics.

Mechanical recycle of polymeric materials is easily through environmental-friendly and economic processes, as reported in **Figure 1**. In this way, renewed objects are realized by recycled plastic materials avoiding disposal of these plastics through landfilling or combustion and reducing the use of mineral oil for their production. This process, however, may be applied only when a single plastic material is available, and it is not contaminated or strongly deteriorated. In the other cases, different environmentally friendly routes may be followed, such as thermochemical processes, thus contributing to the realization of a circular economy and reducing the emission of greenhouse gases

**Figure 1.** *Pathway for mechanical recycling of thermoplastic materials.*

#### *Microwave-Assisted Pyrolysis Process: From a Laboratory Scale to an Industrial Plant DOI: http://dx.doi.org/10.5772/intechopen.104925*

(GHG). Polymer thermochemical processes may supply fuels and chemicals using end life plastic materials as a feedstock, and it may be an alternative to oil-based raw materials [5]. These research studies led to the development of new technologies able to convert waste into resources minimizing the environmental impact of their treatment, avoiding the production of by-products, or limiting the amount of secondary waste.

Chemical recycling technologies [1, 6] are likely to play a crucial role in the transition toward a circular economy and close the recycling of materials giving compounds such as hydrocarbons, available for the production of new compounds. These technologies are able to remove hazardous substances, eventually present in the waste, thus giving new recycled feedstocks. Agreements with materials and chemical producers the use of raw materials from secondary sources are necessary for developing sustainable, feasible, and cost-efficient chemical recycling.

Among chemical recycling technologies, pyrolysis is one of the most important, also referred to as thermolysis or chemical recycling. It represents the transformation of organic materials, under the effect of heat, in the absence of oxygen. Depending on the process conditions, pyrolysis typically yields a mixture of molecules in the form of liquid or wax as the main products. This liquid or wax can be refined to obtain chemicals or fuels, as well as solid and gas may be used for the production of energy.

In this way, waste or contaminated plastic materials may be transformed into oil for industrial purposes, reducing the request and the environmental impact of mineral oil. The potential for recycling is enormous. In 2018, in Europe, plastic production reached almost 62.0 x 106 tons, and all this plastic should be utilized at the end of its life cycle, in one way or the others. For the oil industry, the use of waste plastic as an alternative feedstock represents a new business.

From a chemical point of view, pyrolyzed plastics are a good raw base material for the oil industry so long the remaining impurities are removed from the plastics, and the oil can be used as feedstock for oil refineries. Pyrolysis involves the use of heat and anoxic conditions to break down plastic waste into compounds containing smaller molecules, yielding valuable hydrocarbons in the form of liquid, waxes, and gases. The end products of pyrolysis can be monomers, heating oil, refinery feedstock, transportation fuels, and chemicals.

Chemical recycling can be mainly used to recycle mixed post-consumer plastic waste when sorting single components is not economical. That is, pyrolysis is a very flexible method that allows the use of various feedstocks. Another method of chemical recycling is gasification. This process converts carbonaceous materials into gases. The main product of gasification is synthesis gas (syngas: CO, and H2), which can be further processed into various final products such as gasoline, diesel, methanol, and synthetic methane. Among these processes, the microwave technologies have taken large attention due to their high efficiency in supplying the energy required for a plethora of industrial processes. The main performances of the use of microwaves as correlated to a classical heating system are resumed in **Table 1**. Some examples of products obtained from pyrolysis of different polymers using Microwave-Assisted Pyrolysis (MAP) are reported in **Table 2**.

This chapter aims to offer a comprehensive description of the path from laboratory research to the realization of an industrial plant able to transform up to 2,000 Kg/d of waste plastic into valuable products. These products are available to synthesize new plastic materials, and they may be certified as renewed plastics. However, for plastic waste-based pyrolysis products to become a reality on an industrial scale, ardent development in technologies, value chains, and supporting legislation is needed. Despite these hurdles, the missing links in the plastic recycling loop can be addressed and eventually fixed to establish an actual circular economy for plastics.


#### **Table 1.**

*Correlation between microwave and conventional heating.*


*PE: Polyethylene; PP: Polypropylene; PS: Polystyrene, PVC: Poly(vynil chloride); PET: Poly(ethylene therephtalate); PLA: Poly(lactic acid); WEEE: Waste electric and electronic equipment; MPB: Multilayer packaging beverage; CDP: Corn-derived plastic bag.\* Pyrolysis using classic thermal heating.*

#### **Table 2.**

*Products obtained from microwave-assisted pyrolysys of plastics.*

A circular economy is one of the drivers boosting chemical recycling because it may realize, more and more, so reusing all waste materials. Chemical recycling, such as pyrolysis, is therefore needed, but legislative issues are required for this big challenge of improving the development of this technology.
