**1. Introduction**

174 Material Recycling – Trends and Perspectives

WRAP (Waste and Resource Action Plan) (2006). Environmental Benefits of Recycling. Final

Yang, X. K., Chen, L. F., Wang, J. A., Noreña, L. E. & Novaro, O. (2009). Study of the Keggin

41 hybrid catalysts. *Catalysis Today*, Vol. 148, pp 160-168.

structure and catalytic properties of Pt-promoted heteropolycompound/Al-MCM-

report.

As the rate of consumption of plastic materials in the world is greatly expanded, more waste plastics are generated. In recent years, their generation amount in Korea becomes about four million tons per year, according to data from the National Institute of Environmental Research. The disposal of waste plastic is mostly achieved by conventional ways such as landfill or incineration. However, these methods have a problem of a social resistance due to the air pollution, soil contamination, and the economical resistance caused by an increase of a space and a disposal cost. Thus, the recycling of plastic wastes as a cheap source of raw materials has become a predominant subject over all countries. The development of technologies acceptable from the environmental and economical fields is one of the most important key factors.

Generally, the recycling methods are classified as the material recycling and chemical recycling. The former is one of the most conventional methods but is limited by difficulties in maintaining the high quality and adequate price of final products, in particular, for the mixture of plastic waste. Thus, application of other procedures such as chemical recycling and energy recovery is required (Al-Salem et al., 2009).

The chemical recycling, referred to as an advanced recycling technology, is included in a tertiary recycling. The process is converted from plastic wastes into smaller molecules corresponding to chemical intermediates through the use of heat and chemical treatment, such as liquids, gases and waxes. These chemical intermediates can be used as the fuel oil and feed stocks of petrochemicals processes, etc. The chemical recycling is described by the routes as follows (Kumar et al., 2011).

The chemical recycling can be mainly explained by the chemical recovery systems, which are classified as a heterogeneous and a homogeneous process. The chemolysis methods as homogeneous process utilize chemical agents as catalysts for depolymerization of polymers to obtain the products with low molecular wieghts. Chemolysis includes the processes such as glycolysis, hydrolysis, methanolysis and alcoholysis. On the other hand, heterogeneous processes are greatly described by gasification and pyrolysis. Gasification as partial oxidation (using oxygen or steam) can generate a mixture of hydrocarbons and synthesis gas (CO and H2), which are dependent on the type of polymer, biomass, coal and comixture, and on quantity of and quality of resulting product.

Pyrolysis of Waste Polystyrene and High-Density Polyethylene 177

degradation process (Miskolczi et al., 2004). The most commonly used catalysts are (1) solid acid catalysts such as zeolite, silica-alumina, FCC catalyst and MCM-41, etc [Miskolczi et al., 2004; Lee et al., 2002; Garcia et al., 2005; Seddegi et al., 2002; Achilias et al., 2007; Miskolczi et al., 2006; Marcilla et al., 2005; Lin & Yang, 2007] and (2) bifunctional catalysts (Buekens & Huang, 1998). In the degradation of the polymer chain using acidic catalyst, the molecular weight of polymer chain could be rapidly reduced through cracking reaction and then carbonium ion intermediates would be rearranged by hydrogen and carbon atoms shifts with producing the isomers of high quality. In the case of bifunctional catalyst consisting of both acidic and metal material as reforming catalyst, the metallic sites catalyze hydrogenation/ dehydrogenation, while the acidic sites on the support catalyze the isomerization reaction. These reactions would improve the octane numbers of light hydrocarbons. Also, hydro-cracking involves the reaction with hydrogen over a bimetallic catalyst at moderate temperatures and pressures, which is focused on obtaining a high quality hydrocarbon product. These catalysts used in refinery hydro-cracking reaction for

With regards to the reactant used in this chapter, high-density polyethylene has a linear structure with no or little branching, while polystyrene is cyclic structure with relatively low degradation temperature. Polyolefinic and polystyrene polymers that have above 70% fraction in plastic waste are the major polymeric materials in a municipal plastic waste stream. In case of western Europe, polyethylene plastics make up over 40% of the total plastic content of municipal solid waste (Onwudili et al., 2009). These polymers consisting of mainly hydrogen and carbon atoms are so close to crude oil that the plastic waste would be processed by the reaction methods such as the thermal and catalytic cracking. In the pyrolysis process, polystyrene can be thermally degraded to the corresponding monomer or aromatics with its high selectivity at lower temperatures, whilst thermal degradation of polyolefinic polymers occurs at higher temperatures and lead to a complex mixture of

This chapter presents the pyrolysis of both polystyrene and high-density polyethylene with different physiochemical properties and also the upgrading of low-grade oil product obtained by thermal degradation. Moreover, the effect of mixing of two plastics and catalyst addition for the pyrolysis would be explained by the yield for gas, liquid, solid products

The pyrolysis is basically degraded for large hydrocarbons into smaller ones. From this process, the polymer is converted into paraffins and olefins, etc., with low molecular weights. Thermal degradation is accompanied with a free radical chain reaction. When free radicals react with hydrocarbons, new hydrocarbons and new free radicals are produced. Also, free radicals can decompose into olefins and new radicals. In the reaction mechanism by polymer type (Scheirs & Kaminsky, 2006), high-density polyethylene consisting of straight long carbon chains is pyrolyzed through the random-chain scission, which is broken up randomly into smaller molecules with various chain lengths. This product is obtained

**2.1 Reaction mechanism of high-density polyethylene and polystyrene** 

heavy hydrocarbons incorporate both cracking and hydrogenation.

aliphatic hydrocarbons.

**2. Basic pyrolysis** 

and the composition of liquid components, etc.


Fig. 1. Schemes of chemical recycling.

The pyrolysis involves the degradation of the polymeric materials by heating in the absence of oxygen. The method has the routes as the thermal cracking, catalytic cracking and hydro cracking. The recycling of waste plastics by thermal and catalytic degradation processes can be an important source producing alternative fuel oil from the view point of an economical aspect and contributing to the environmental protection from the view point of an environmental aspect (Demirbas, 2004). The method of pyrolysis takes advantage over the incineration and landfill methods because it is based on relatively simplicity into the oil for all thermoplastic mixtures without using the separation treatment for plastic type in the mixture and to lower the environment resistance for air pollutant and soil contamination.

In the pyrolysis, thermal degradation is a simple method for upgrading plastic waste into liquid product at medium temperature (400-600 OC) in the absence of oxygen. However, this process requires relatively high energy consumption, due to a low thermal conductivity of waste plastic and to an endothermic reaction by degradation of waste plastic. Moreover, the oil obtained by pyrolysis of plastic wastes has a wide molecular weight distribution with poor economical value, which does not have a sufficient quality to use as alternative fuel oils (Marcilla et al., 2009). The pyrolysis of polyethylene with high proportion in mixed plastic produces much more unstable heavy compounds with high viscosity as low grade product (Marcilla et al., 2009; Lee & Shin, 2007). The characteristics of these products depend on the nature of plastic waste and process conditions.

The catalytic degradation process, based on the addition of catalyst, can be conducted at low temperatures and high quality products are obtained in a comparison with thermal

Cracking

Gasification

Thermal Cracking

Catalytic Cracking

Hydro Cracking

Methanolysis

Glycolysis

Alcoholysis

Heterogeneous

Homogeneous

Systems Incineration Technology

Process Chemolysis

The pyrolysis involves the degradation of the polymeric materials by heating in the absence of oxygen. The method has the routes as the thermal cracking, catalytic cracking and hydro cracking. The recycling of waste plastics by thermal and catalytic degradation processes can be an important source producing alternative fuel oil from the view point of an economical aspect and contributing to the environmental protection from the view point of an environmental aspect (Demirbas, 2004). The method of pyrolysis takes advantage over the incineration and landfill methods because it is based on relatively simplicity into the oil for all thermoplastic mixtures without using the separation treatment for plastic type in the mixture and to lower the environment resistance for air pollutant and soil contamination.

In the pyrolysis, thermal degradation is a simple method for upgrading plastic waste into liquid product at medium temperature (400-600 OC) in the absence of oxygen. However, this process requires relatively high energy consumption, due to a low thermal conductivity of waste plastic and to an endothermic reaction by degradation of waste plastic. Moreover, the oil obtained by pyrolysis of plastic wastes has a wide molecular weight distribution with poor economical value, which does not have a sufficient quality to use as alternative fuel oils (Marcilla et al., 2009). The pyrolysis of polyethylene with high proportion in mixed plastic produces much more unstable heavy compounds with high viscosity as low grade product (Marcilla et al., 2009; Lee & Shin, 2007). The characteristics of these products depend on the

The catalytic degradation process, based on the addition of catalyst, can be conducted at low temperatures and high quality products are obtained in a comparison with thermal

Process

Chemical Recovery

Energy Recovery

Fig. 1. Schemes of chemical recycling.

nature of plastic waste and process conditions.

Systems

degradation process (Miskolczi et al., 2004). The most commonly used catalysts are (1) solid acid catalysts such as zeolite, silica-alumina, FCC catalyst and MCM-41, etc [Miskolczi et al., 2004; Lee et al., 2002; Garcia et al., 2005; Seddegi et al., 2002; Achilias et al., 2007; Miskolczi et al., 2006; Marcilla et al., 2005; Lin & Yang, 2007] and (2) bifunctional catalysts (Buekens & Huang, 1998). In the degradation of the polymer chain using acidic catalyst, the molecular weight of polymer chain could be rapidly reduced through cracking reaction and then carbonium ion intermediates would be rearranged by hydrogen and carbon atoms shifts with producing the isomers of high quality. In the case of bifunctional catalyst consisting of both acidic and metal material as reforming catalyst, the metallic sites catalyze hydrogenation/ dehydrogenation, while the acidic sites on the support catalyze the isomerization reaction. These reactions would improve the octane numbers of light hydrocarbons. Also, hydro-cracking involves the reaction with hydrogen over a bimetallic catalyst at moderate temperatures and pressures, which is focused on obtaining a high quality hydrocarbon product. These catalysts used in refinery hydro-cracking reaction for heavy hydrocarbons incorporate both cracking and hydrogenation.

With regards to the reactant used in this chapter, high-density polyethylene has a linear structure with no or little branching, while polystyrene is cyclic structure with relatively low degradation temperature. Polyolefinic and polystyrene polymers that have above 70% fraction in plastic waste are the major polymeric materials in a municipal plastic waste stream. In case of western Europe, polyethylene plastics make up over 40% of the total plastic content of municipal solid waste (Onwudili et al., 2009). These polymers consisting of mainly hydrogen and carbon atoms are so close to crude oil that the plastic waste would be processed by the reaction methods such as the thermal and catalytic cracking. In the pyrolysis process, polystyrene can be thermally degraded to the corresponding monomer or aromatics with its high selectivity at lower temperatures, whilst thermal degradation of polyolefinic polymers occurs at higher temperatures and lead to a complex mixture of aliphatic hydrocarbons.

This chapter presents the pyrolysis of both polystyrene and high-density polyethylene with different physiochemical properties and also the upgrading of low-grade oil product obtained by thermal degradation. Moreover, the effect of mixing of two plastics and catalyst addition for the pyrolysis would be explained by the yield for gas, liquid, solid products and the composition of liquid components, etc.
