**2. Basic pyrolysis**

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

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

Pyrolysis of Waste Polystyrene and High-Density Polyethylene 179

Although the catalytic degradation of polyethylene over a wide variety of catalysts have been tested, zeolites have proven effective by many researchers [[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; Buekens & Hunang, 1998]. Seo et al (Seo et al.,2003) reports that the product characteristics for both thermal and catalytic degradation of waste HDPE using various zeolites are relatively compared as the yields of gas, liquid and residue, and carbon number distribution of liquid products, as shown in Table 1. Yields of liquid were over 70% using all zeolites, with the exception of ZSM-5, as well as thermal degradation. However, the catalytic degradation was produced much more light hydrocarbons (C6-C12) than that of thermal degradation, and moreover ZSM-5 and zeolite Y were more effective than mordenite. ZSM-5 and zeolite Y have a unique threedimensional micropore structure as well as a strongly acidic property, whereas mordenite has only a parallel one-dimensional pore structure with a restricted diffusion of reactant. Especially, ZSM-5 with a smaller pore size, rather than that of zeolite Y was more cracked into light hydrocarbons such as C6-C12 components and gas products. Since the initially degraded materials on the external surface of catalyst can be dispersed into the smaller internal cavities of catalyst, they can be further degraded to the smaller size of gaseous hydrocarbons. These findings mean that the pore properties of catalyst are important factor

> Yield Liquid fraction\* Liquid Gas Coke C6-C12 C13-C23 ≥C24

Thermal cracking 84.00% 13.00% 3.00% 56.55% 37.79% 5.66% ZSM-5 (powder) 35.00% 63.50% 1.50% 99.92% 0.08% 0% Zeolite Y (powder) 71.50% 27.00% 1.50% 96.99% 3.01% 0% Zeolite Y (pellet) 81.00% 17.50% 1.50% 86.07% 11.59% 2.34% Mordenite (pellet) 78.50% 18.50% 3.00% 71.06% 28.67% 0.27%

Table 1. Yields of liquid, gas and coke produced from thermal and catalytic degradation of

In the characteristics of oil product, paraffin, olefin, naphthene and aromatic (PONA) distribution is one of the important factors which can determine the quality of oil product, as shown in Table 2. Oil product from thermal degradation of HDPE consists of 40.47wt% paraffins, 39.93wt% olefin, 18.50wt% naphthenes and a trace amount of aromatics. Relative to thermal degradation of HDPE, catalytic degradation is known to occur at a faster reaction rate and lead to subsequent reactions including isomerization and aromatization, as well as cracking reaction (Vento & Habib, 1979). Subsequent reactions proceeding through carbenium ion-type intermediate generated by acidic catalysts contribute to the greater

waste HDPE with various catalysts at 450OC (Seo et al.,2003).

formation of olefins and aromatics, as shown in Table 2.

**3. Pyrolysis of pure waste high-density polyethylene and polystyrene** 

in the degradation of heavy hydrocarbons.

\* wt% were determined by GC/MS

Catalysts

with a wide distribution of molecular weight, including hydrocarbons with high boiling point and/or low valuable products like wax. Thus, this means that the addition of catalyst in the pyrolysis can be a more efficient method to produce high valuable products with mainly gasoline range components. On the other hand, pyrolysis of polystyrene with cyclic structure is occurred by both end-chain and random-chain scissions. This polymer is broken up from the end groups successively yielding the corresponding monomers, as well as its breakage randomly into smaller molecules of one or more benzene-ring structures. This product is monomer recovery with a high fraction.
