**2.2 Thermal and catalytic degradation (Scheirs & Kaminsky, 2006 )**

(a) Thermal and (b) catalytic degradation of heavy hydrocarbons can be comparatively described with the following items

(a) Thermal degradation


(b) Catalytic degradation


### **2.3 Mass balance**

To demonstrate the mass balance, it is essential to determine the product yield for gas, liquid and residue, and also the composition of liquid products at different conditions of the various operating parameters such as temperature, residence time and pressure. From this, it is required to mention the economical aspect. Raw materials in a pyrolysis process contain nonproductive constituents, such as moisture, inorganic material, etc. These loss factors have to take into a consideration for the establishment of mass balance. Generally, mass balance is established by input and output amount, based on 100% of feeding amount. In the pyrolysis process, the important operating point is controlled by the maximum of valuable products and minimum of sludge amount. Thus, the operating margin must reach a reasonable level for mass balance in the economic aspect.

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

(a) Thermal and (b) catalytic degradation of heavy hydrocarbons can be comparatively

4. Wide distribution of molecular weight in the liquid product (poor gasoline

To demonstrate the mass balance, it is essential to determine the product yield for gas, liquid and residue, and also the composition of liquid products at different conditions of the various operating parameters such as temperature, residence time and pressure. From this, it is required to mention the economical aspect. Raw materials in a pyrolysis process contain nonproductive constituents, such as moisture, inorganic material, etc. These loss factors have to take into a consideration for the establishment of mass balance. Generally, mass balance is established by input and output amount, based on 100% of feeding amount. In the pyrolysis process, the important operating point is controlled by the maximum of valuable products and minimum of sludge amount. Thus, the operating margin must reach a

product is monomer recovery with a high fraction.

1. High production of C1s and C2s in the gas product.

2. High production of C3s and C4s in the gas product

3. Some diolefins made at high temperature

5. High fraction of gas and coke products

5. Aromatics produced by olefin cyclization 6. More reactive for larger molecules 7. No reaction for pure aromatics 8. Paraffins produced by H2 transfer

described with the following items

(a) Thermal degradation

2. Olefins less branched.

6. Relatively slow reactions.

(b) Catalytic degradation

selectivity)

**2.3 Mass balance** 

**2.2 Thermal and catalytic degradation (Scheirs & Kaminsky, 2006 )** 

1. Short in the reaction time and low in degradation temperature

9. Product distribution controlled by the selection of a catalyst

reasonable level for mass balance in the economic aspect.

3. Olefins as the primary products and more branched by isomerization 4. More C5-C10 products in the liquid product (high gasoline selectivity)

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

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 in the degradation of heavy hydrocarbons.


\* wt% were determined by GC/MS

Table 1. Yields of liquid, gas and coke produced from thermal and catalytic degradation of waste HDPE with various catalysts at 450OC (Seo et al.,2003).

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

Pyrolysis of Waste Polystyrene and High-Density Polyethylene 181

function of reaction lapsed time are shown in Fig. 2. The experiments were performed with a stirred semi-batch reactor at a catalyst amount of 9.1 wt % and at a temperature of 400 OC

The cumulative amount distributions of liquid products clearly increase with an increase in the mixing proportions of PS against HDPE. These results are due to the fact that the increase of PS content in HDPE and PS mixture has much high yield of liquid product and high degradation rate. This means that pyrolysis of PS is predominant over the pyrolysis of HDPE in the mixture. According to the previous result (Lee et al.,2002), waste PS showed higher liquid yield and higher initial degradation rate in the catalytic degradation than waste PP and PE, because PS is mainly converted into stable aromatic components as liquid

> HDPE:PS=100:0 HDPE:PS=80:20 HDPE:PS=60:40 HDPE:PS=40:60 HDPE:PS=20:80 HDPE:PS=0:100 HDPE:PS=100:0 (no-cat.)

Temperature

Lapse time (min) 0 100 200 300 400

Fig. 2. Cumulative amount distributions of liquid products for catalytic degradation of waste HDPE and PS mixture using spent FCC catalyst at 400OC. (A: Initial degradation

Temperature (

0

100

200

300

400

500

OC)

with the same reaction temperature programming.

phase and also the low degradation temperature.

Accumulative amount (g)

0

region, B: Final degradation region) (Lee et al., 2004).

40

80

120

160

A

B

200


\*Others mean hydrocarbons containing oxygen or unidentified organic compounds.

Table 2. Weight fraction of each PONA Group in oil products from thermal and catalytic degradation of HDPE with various catalysts at 450OC (Seo et al.,2003).

Catalytic degradation using ZSM-5 with small size increases aromatic hydrocarbons up to 59wt%, as a shape selectivity of catalyst, which is mainly consisting of the alkyl-aromatics with one-benzene ring structure. ZSM-5 is superior to zeolite Y in terms of aromatic formation. Also, the hydrogen atoms in ZSM-5 catalytic degradation contribute to the formation of naphthenes with largely C6-C8 hydrocarbons. Paraffins and olefins contain mostly lighter hydrocarbons.

It has been demonstrated that rare earth exchanged zeolite Y is more active than silicaalumina as cracking catalyst (Lin&Yang, 2007; Onwudili et al., 2009), because zeolite can provide a greater acidic site density. Since zeolite Y has more favorable shape selectivity for aromatic formation than non-zeolite catalyst, some intermediate carbenium ion formed by acidic zeolite will choose a pathway to aromatic formation, and some will be left over as olefin. Thus, the oil product from zeolite Y was mostly consisted of C6-C9 molecules which would be produced as largely light olefins and some cyclics. Zeolite Y improved the formation of branched isomers by the isomerization of light olefins and in cyclic products, naphthenes and aromatics by cyclization were mostly consisted of C6 and C7-C10 molecules, respectively.

The oil product over mordenite, among zeolites, appeared differently from other zeolites. This product distribution was similarly shown with that of thermal degradation, rather than other zeolites. This contrasting result of both mordenite and other zeolites seems to be correlated with the crystalline pore structure. Since this physical property is adopted for greater diffusion, mordenite with large pore size of one-dimensional pore structure can provide a greater initial activity than zeolite Y, but it would tend to lose activity more rapidly with time on stream. Coke formation in mordenite is known to be significant in a literature(Chen et al., 1989). As the result, the lighter molecules were less formed in mordenite.

### **4. Pyrolysis of mixture of waste HDPE and PS**

When the pyrolysis is conducted to obtain the oil product, the effects of the mixing of HDPE and PS are described in this section. For the catalytic degradation of two polymers with a different mixing proportions, the cumulative amount distributions of liquid products as a

(Total-Paraffin) Total-

Thermal cracking 40.75 40.47 0.28 39.93 18.50 0.68 0.14 ZSM-5(powder) 1.63 1.51 0.12 16.08 23.55 58.75 0.01 ZeoliteY(powder) 5.39 0.00 5.39 79.92 7.68 7.01 0.00 Zeolite Y(pellet) 25.10 20.68 4.42 49.28 12.05 8.43 5.14 Mordenite(pellet) 31.07 30.89 0.18 57.07 11.51 0.13 0.22

Table 2. Weight fraction of each PONA Group in oil products from thermal and catalytic

Catalytic degradation using ZSM-5 with small size increases aromatic hydrocarbons up to 59wt%, as a shape selectivity of catalyst, which is mainly consisting of the alkyl-aromatics with one-benzene ring structure. ZSM-5 is superior to zeolite Y in terms of aromatic formation. Also, the hydrogen atoms in ZSM-5 catalytic degradation contribute to the formation of naphthenes with largely C6-C8 hydrocarbons. Paraffins and olefins contain

It has been demonstrated that rare earth exchanged zeolite Y is more active than silicaalumina as cracking catalyst (Lin&Yang, 2007; Onwudili et al., 2009), because zeolite can provide a greater acidic site density. Since zeolite Y has more favorable shape selectivity for aromatic formation than non-zeolite catalyst, some intermediate carbenium ion formed by acidic zeolite will choose a pathway to aromatic formation, and some will be left over as olefin. Thus, the oil product from zeolite Y was mostly consisted of C6-C9 molecules which would be produced as largely light olefins and some cyclics. Zeolite Y improved the formation of branched isomers by the isomerization of light olefins and in cyclic products, naphthenes and aromatics by cyclization were mostly consisted of C6 and C7-C10 molecules,

The oil product over mordenite, among zeolites, appeared differently from other zeolites. This product distribution was similarly shown with that of thermal degradation, rather than other zeolites. This contrasting result of both mordenite and other zeolites seems to be correlated with the crystalline pore structure. Since this physical property is adopted for greater diffusion, mordenite with large pore size of one-dimensional pore structure can provide a greater initial activity than zeolite Y, but it would tend to lose activity more rapidly with time on stream. Coke formation in mordenite is known to be significant in a literature(Chen et al., 1989). As the result, the lighter molecules were less formed in

When the pyrolysis is conducted to obtain the oil product, the effects of the mixing of HDPE and PS are described in this section. For the catalytic degradation of two polymers with a different mixing proportions, the cumulative amount distributions of liquid products as a

n-paraffin i-paraffin

\*Others mean hydrocarbons containing oxygen or unidentified organic compounds.

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

Olefin Naphthene Aromatics Others\*

Catalyst Total-

mostly lighter hydrocarbons.

respectively.

mordenite.

**4. Pyrolysis of mixture of waste HDPE and PS** 

Paraffin

function of reaction lapsed time are shown in Fig. 2. The experiments were performed with a stirred semi-batch reactor at a catalyst amount of 9.1 wt % and at a temperature of 400 OC with the same reaction temperature programming.

The cumulative amount distributions of liquid products clearly increase with an increase in the mixing proportions of PS against HDPE. These results are due to the fact that the increase of PS content in HDPE and PS mixture has much high yield of liquid product and high degradation rate. This means that pyrolysis of PS is predominant over the pyrolysis of HDPE in the mixture. According to the previous result (Lee et al.,2002), waste PS showed higher liquid yield and higher initial degradation rate in the catalytic degradation than waste PP and PE, because PS is mainly converted into stable aromatic components as liquid phase and also the low degradation temperature.

Fig. 2. Cumulative amount distributions of liquid products for catalytic degradation of waste HDPE and PS mixture using spent FCC catalyst at 400OC. (A: Initial degradation region, B: Final degradation region) (Lee et al., 2004).

Pyrolysis of Waste Polystyrene and High-Density Polyethylene 183

catalyst containing zeolite. Both the degradation of plastic mixture and the characteristics of oil product obtained are significantly influenced by plastic type in the mixture, as well as

> PS content (wt.%) 0 20 40 60 80 100

Fig. 4. The distribution of liquid paraffin, olefin, naphthene and aromatic (PONA) products for catalytic degradation of waste HDPE and PS mixture using spent FCC catalyst in the initial degradation time is presented with a function of PS content (Lee et al., 2004).

Pyrolysis is a suitable process for thermoplastics like polyethylene and polystyrene. For a small mixture of polyvinyl chloride (PVC) and polyethylene terephthalate (PET) included in municipal plastic waste (MPW), an issue of environment and operation problems occurs in pyrolysis process. Thus, the removal of PVC and PET in MPW may be conducted by separation methods such as water separation, because of relatively high density of PVC and PET in a comparison for polyethylene and polystyrene with specific gravity 1.2 or less. Also, after the pretreatment of MPW, the inorganic materials contained with very low content are deposited in solid carbon residue during the pyrolysis. The MPWs are classified as low MPW(<1.0), medium MPW(1.0-1.1) and high MPW(1.1-1.2), based on the specific gravity 1.2

Table 3 shows the yields of liquid, gas and residue products obtained by the pyrolysis of three different MPWs at a stirred batch reactor of 1.1 liter volume size, under the same experimental conditions. From these results, the product distribution is clearly different

or less. The pyrolysis corresponding to three type MPWs is conducted.

Paraffins Olefins Naphthenes Aromatics

zeolite type in the catalyst.

Weight fraction(%)

0

**5. Pyrolysis of municipal plastic waste** 

10

20

30

40

50

60

70

80

90

100

The slope of the cumulative amount of liquid product versus reaction lapsed time represents as the degradation rate of HDPE and PS mixtures which is needed to obtain liquid products. The initial liquid product was obtained at around 400 OC of reaction temperature. These can be classified as two region of initial (A; initial degradation region) and final (B; final degradation region) lapse time in the reaction time and were appeared as initial and final degradation rate with a function of PS content, as shown in Fig. 3. The initial degradation rates are exponentially increased with increasing PS content in the mixture, while the final degradation rates were also suddenly decreased with increasing PS content, due to the influence of HDPE in the mixture. These results show that the polymers studied do not react independently, but some interaction between samples was observed.

Fig. 3. Initial and final degradation rate as a function of PS content for catalytic degradation of waste HDPE and PS mixture using spent FCC catalyst at 400OC (Lee et al., 2004).

The commercial pyrolysis process yields the pyrolytic oil from the reactor at short contact time of 1-2 hours. It is necessary to know the characteristics of product oil in initial degradation region of Fig. 2. For these results, the distribution of liquid paraffin, olefin, naphthene and aromatic (PONA) products is presented in Fig. 4. Hydrocarbon group compositions of degraded products are strongly dependent on chemical properties of plastic type in plastic waste. As PS is included in the mixture, even though it is low or high, the pyrolysis of this mixture greatly improves the formation of aromatics, whereas the olefins produced by pyrolysis of polyolefins mainly has a much low fraction. This can be explained by the fact that the acceleration of aromatic products stems from the aromatic fragments of PS degradation as well as the cyclization of paraffinic and olefinic intermediates in FCC

The slope of the cumulative amount of liquid product versus reaction lapsed time represents as the degradation rate of HDPE and PS mixtures which is needed to obtain liquid products. The initial liquid product was obtained at around 400 OC of reaction temperature. These can be classified as two region of initial (A; initial degradation region) and final (B; final degradation region) lapse time in the reaction time and were appeared as initial and final degradation rate with a function of PS content, as shown in Fig. 3. The initial degradation rates are exponentially increased with increasing PS content in the mixture, while the final degradation rates were also suddenly decreased with increasing PS content, due to the influence of HDPE in the mixture. These results show that the polymers studied do not react

> PS content (wt%) 0 20 40 60 80 100

Fig. 3. Initial and final degradation rate as a function of PS content for catalytic degradation

The commercial pyrolysis process yields the pyrolytic oil from the reactor at short contact time of 1-2 hours. It is necessary to know the characteristics of product oil in initial degradation region of Fig. 2. For these results, the distribution of liquid paraffin, olefin, naphthene and aromatic (PONA) products is presented in Fig. 4. Hydrocarbon group compositions of degraded products are strongly dependent on chemical properties of plastic type in plastic waste. As PS is included in the mixture, even though it is low or high, the pyrolysis of this mixture greatly improves the formation of aromatics, whereas the olefins produced by pyrolysis of polyolefins mainly has a much low fraction. This can be explained by the fact that the acceleration of aromatic products stems from the aromatic fragments of PS degradation as well as the cyclization of paraffinic and olefinic intermediates in FCC

of waste HDPE and PS mixture using spent FCC catalyst at 400OC (Lee et al., 2004).

Final degradation rate (g/min)

0.0

0.1

0.2

0.3

0.4

0.5

independently, but some interaction between samples was observed.

Initial degradation rate (g/min)

0

1

2

3

4

5

6

7

catalyst containing zeolite. Both the degradation of plastic mixture and the characteristics of oil product obtained are significantly influenced by plastic type in the mixture, as well as zeolite type in the catalyst.

Fig. 4. The distribution of liquid paraffin, olefin, naphthene and aromatic (PONA) products for catalytic degradation of waste HDPE and PS mixture using spent FCC catalyst in the initial degradation time is presented with a function of PS content (Lee et al., 2004).
