**3.2 Product composition**

As mentioned before, the cracking of polyolefin chains produces different hydrocarbons. In general, three types of streams are created during the pyrolysis of polyethylene and polypropylene: gas fraction that consists of hydrocarbons with the lowest molecular weights, a liquid or semisolid fraction (pyrolysis oil) that consists

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

**Figure 2.** *Cracking of polyolefins by random scission reaction, redrawn from [7].*

of hydrocarbons that were created in the form of vapours which after cooling were condensed and the process residue (char), that consists of plastic's additives and coke, which was created during the process. The composition and yields of each of these products depend on the feedstock's composition and process conditions such as temperature, pressure, residence time, and catalyst used.

For example, during the process in a rotary kiln reactor with quartz sand used as a heat carrier, pyrolysis oil consisting of 43,1% of aliphatic hydrocarbons and 55,5% of aromatic hydrocarbons were obtained from polyethylene. Pyrolysis oil from polypropylene consisted of 44,7% aliphatic and 52,9% aromatic hydrocarbons [9]. On the other hand, during thermal cracking of PE and PP in a microreactor at different temperatures gave products consisting of 59,7% alkanes, 31,90% alkenes, 8,40% cycloalkanes and 66,55% alkanes, 25,7% alkenes and 7,58% cycloalkanes, respectively. No aromatics were identified [10]. These two examples already indicate how different products can be obtained, depending on the process conditions.

In general, it can be observed that raising the temperature and residence time can increase arenes creation and can also raise the alkane to alkene ratio in the product, for example, in [11]. Aromatics content can also be significantly raised by the use of certain catalysts, like zeolites. The type of catalyst also influences the alkane/alkene ratio [12]. It should be noted that catalysts can be rapidly deactivated, limiting their use in continuous processes [13]. An increase in the temperature can increase the yield of long-chain hydrocarbons due to reduced residence time. However, it can also favour increased gas and lower molecular weight product formation by increasing the number of secondary reactions if the residence time is long enough. The majority of cracking processes are conducted at atmospheric pressures. However, some investigations present that higher pressure can increase the gas formation at lower temperatures, but with the increase of the temperature, the effect was diminished. A decrease of double bonds formation with the pressure increase was also observed [14]. What is more, different polymers can have a synergistic effect on co-pyrolysis [15]. Polystyrene (PS) is also a valuable feedstock for pyrolysis. The product of PS cracking is almost fully aromatic, with styrene monomer as a major product [16]. It can also be co-pyrolysed with polyolefins. Poly(methyl methacrylate) (PMMA) is another polymer that can be pyrolysed [17]. Poly(vinyl chloride) (PVC) produces large quantities of corrosive hydrogen chloride and can contaminate all – gas, liquid, and residue. PET gives low yields of

oil, and the thermal cracking of polyurethanes provides products reach in organic nitrogen components [18]. Pyrolysis of biomass converts waste into oil with high oxygen content and increase coke formation [19]. That is why most of the research and developed technologies are based on polyolefins, optionally with the addition of polystyrene, while other plastics and biomass are treated as impurities.

Pyrolysis of plastics is a complex process with many variables that produce hydrocarbons from polyolefin feedstock. It makes the process difficult but flexible at the same time. That is why many different solutions are used (other types of reactors), but also different product types for different applications are obtained.

### **3.3 Product applications**

As described in the previous section, cracking of polyethylene and polypropylene can lead to many different products. The composition of the products – hydrocarbon type and chain length – will determine their application.

#### *3.3.1 Plastic-to-intermediate*

Pyrolysis oil obtained during thermal or thermocatalytic cracking of polyolefins is a complex mixture of hydrocarbons with different chain lengths (5 to 30 and more carbon atoms). Linear and branched paraffins and olefins, together with aromatics: mono and polyaromatics – with and without aliphatic side chains, are obtained. Such a complex mixture does not have a direct application without additional treatment. However, as a hydrocarbon product, it can be mixed with refinery and petrochemical streams and processed together with crude. The process is simple, consisting of only a cracking reactor, product cooling system, residue discharge system and gaseous product burning unit (for energy production).

However, the capacity of commercial chemical recycling plants is limited due to plastic waste availability and the process itself – polymers have a low thermal conductivity which makes the scale-up of the pyrolysis reactor challenging. The biggest pyrolysis plants have a capacity of about 100 000 tons per year which is very small compared to the standard refinery size of about 4–10 million tons per year. This means that the recycled stream is highly dissolved in the refinery. As a result, the product can be contaminated, so there is no need for expensive detailed sorting and washing of the plastic waste or purification of the pyrolysis oil. Even though this makes the process much cheaper, the solution is not economically feasible.

Cracking is an endothermic process that requires a lot of energy to melt the plastic and break polymer bonds, as plastics are excellent insulators. The residue obtained from cracking is usually a high-calorific by-product and contains a high level of contamination, limiting its use in incinerators, especially if the raw material used for pyrolysis was not properly separated and cleaned. For example, the presence of PVC significantly raises the chlorine content, which is limited in incinerators' feedstock specifications. Special treatment of this residue increases the overall cost of operations.

Pyrolysis of plastic waste into feedstock for refineries was very popular a couple of years ago. For example, in Poland until 2007, many commercial (Technology Readiness Level, TRL = 9) plants were operated, but their profitability was based on relief in excise tax. When regulation changed, they were all bankrupted. Low prices of crude oil caused the closure of other companies worldwide, or they changed their profile. For example, Agilyx from the US had to shut down its flagship plant in Tigard in 2016, later changing its activity profile to polystyrene recycling [20].

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

### *3.3.2 Plastic-to-fuel*

The most popular application for products from the chemical recycling of polyethylene and polypropylene are fuels and fuel components. Hydrocarbon product can be separated into more narrow fractions like gasoline, diesel, light and heavy fuel oil. Hydrocarbons with the highest molecular weights (waxes) can be circulated back to the cracking reactor or cracked in an additional catalytic process. Proper process parameters can also limit the quantity of waxy hydrocarbons, but it usually causes high gas yields. Pyrolysis reactors are followed by distillation units. The ratio between different products depends on technology. An example of two companies' products slates are presented in **Table 1** [21].

From one side, fuels obtained from the pyrolysis of polyolefins are characterised by low concentrations of sulphur (less than 0,1%) and are easily burned as hydrocarbon fractions. On the other hand, high olefin content reduces oxidation stability. Furthermore, reactive alkenes relatively easily undergo polymerisation reaction creating gums with high molecular weights. That is why products from PE and PP cracking should not be stored for a longer time. This tendency to polymerisation can also cause issues in distillation units where resins deposit at surfaces of elevated temperatures, reducing heat transfer coefficient in heaters and heat exchangers, also plugging the distillation columns and reducing mas flow in these units. Foaming during distillation is also observed [22, 23].

Hydrotreatment (catalytic reactions with hydrogen) of the products from pyrolysis could be a solution – olefins can be saturated into paraffins, stabilising the product. But it would raise the total cost of the process as it usually carries out at elevated pressures and requires special, separate units. What is more, products reach in linear paraffins may have high pour point of diesel and light fuel oil fractions. Unsuitable gasoline fraction octane number and cold-temperature behaviour of heavier fractions limit their use in combustion. To keep proper fuel parameters, blends of hydrotreated fractions from chemical recycling of plastics and commercial fuels can be prepared. But to keep proper parameters, a maximum of 1% of gasoline fraction, 10% of diesel fraction and 20% of light fuel fraction from polyolefins' cracking can be used [24]. If the process is controlled to produce a highly aromatic product, then higher octane gasoline and lower cetane diesel could be obtained.

Fuels and fuel components obtained from plastic waste compete in price with fossil fuels, making the profitability of the process challenging. Also, this type of application in European regulation is seen as energy recovery, so it is not considered chemical recycling. Nevertheless, there are several companies that are focused on the production of fuels. For example, Bightmark Energy is building a 100 000 t/a commercial facility (TRL 9) in Ashley (US), which is planned to be commissioned in 2021. Braven Environmental is planning a 65 000 t/a plant in Virginia. Nexus Fuels commercial-scale plant's capacity is 50 t/d, similar to one module of Integrated Green Energy Solutions' plant constructed in Amsterdam.


**Table 1.** *Product slate examples [21].* Polish company Handerek Technologies develops technology for fuels production by pyrolysis of plastics and further hydrotreatment at atmospheric pressure, using syngas (mixture of hydrogen and carbon oxide). The process at an early stage of commercialisation, presenting only a small scale pilot plant (TRL 4–5) with plans to build commercial plants with a capacity of 10 000 t/a.
