**5. Pyrolysis of municipal plastic waste**

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

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

Pyrolysis of Waste Polystyrene and High-Density Polyethylene 185

Fraction of liquid product (wt%)

0

Paraffins Olefins Naphthenes Aromatics Phenols Methylesters

(a) (b)

Carbon number 4 6 8 10 12 14 16 18 20 22 24

(c)

In the case of low MPW sample, the fractions of liquid paraffin, olefin, naphthene and aromatic products are about 5%, about 76%, about 6% and about 12%, respectively. Primary liquid product is olefin components and secondary is aromatic components. In the liquid product, naphthene and paraffin components are produced with a little amount. This means that low MPW mainly consists of PP polymer, among polyolefinic polymers, with relatively low degradation temperatures and high olefin fraction of liquid product in pyrolysis process (Lee et al., 2002). Also, aromatic products show 10% or more, because of including a little PS in low MPW. These results were demonstrated by the

Fig. 5. Carbon number distribution of liquid paraffin, olefin, naphthene and aromatic products obtained from pyrolysis of MPW at 400OC (specific gravity : <1.0(a), 1.0-1.1(b),

10

20

30

40

50

60

Carbon number 6 8 10 12 14 16 18 20 22 24 26

Paraffins Olefins Naphthenes Aromatics

Paraffins Olefins Naphthenes Aromatics

Carbon number 6 8 10 12 14 16 18 20 22 24 26

Fraction of liquid product (wt%)

0

10

20

30

40

50

60

Fraction of liquid product (wt%)

0

1.1-1.2(c)) (Lee, 2007).

10

20

30

40

50

60

70

over various samples of MPW. Basically, the yield of liquid products in all samples is 75% or over. Note the corresponding liquid yields is in the following order; medium MPW >> low MPW > high MPW. Especially, the medium MPW shows highest liquid yield with about 90%. On the contrary, the order of gas and residue yield shows reverse relationship. It can be explained by the result that the plastic type contained in each MPW separated by a difference of specific gravity is an important key on the product distribution obtained. Lee et al. (Lee et al., 2002) have reported the influence of plastic type on liquid, gas and residue yield for pyrolysis of plastic wastes. The pyrolysis of polystyrene, due to the structure of stable benzene-ring, shows higher liquid yield and lower gas yield than that of polyolefinic polymer (PE, PP) with a straight hydrocarbon structure. Polystyrene is less cracked to gas product of 5 carbon numbers or less. Hence, the product distribution is strongly dependent on the plastic type including in municipal plastic wastes.


Table 3. Product yields obtained from pyrolysis of various MPWs at 400OC (Lee, 2007).

As the characteristics of liquid product, the paraffin, olefin, naphthene and aromatic (PONA) components, etc are compared over three different MPWs, as shown in Table 4. Also, their carbon number distributions are plotted in Fig. 5, respectively. These showed a peculiar product distribution, due to the chemical nature and structure of plastic type in MPW.


Table 4. Liquid product composition obtained from pyrolysis of various MPWs at 400OC (Lee, 2007).

over various samples of MPW. Basically, the yield of liquid products in all samples is 75% or over. Note the corresponding liquid yields is in the following order; medium MPW >> low MPW > high MPW. Especially, the medium MPW shows highest liquid yield with about 90%. On the contrary, the order of gas and residue yield shows reverse relationship. It can be explained by the result that the plastic type contained in each MPW separated by a difference of specific gravity is an important key on the product distribution obtained. Lee et al. (Lee et al., 2002) have reported the influence of plastic type on liquid, gas and residue yield for pyrolysis of plastic wastes. The pyrolysis of polystyrene, due to the structure of stable benzene-ring, shows higher liquid yield and lower gas yield than that of polyolefinic polymer (PE, PP) with a straight hydrocarbon structure. Polystyrene is less cracked to gas product of 5 carbon numbers or less. Hence, the product distribution is strongly dependent

Sample (S.G..) Liquid yield (wt%) Gas yield(wt%) Residue (wt%)

Low MPW (<1.0) 80.9 11.1 8.0 Medium MPW(1.0-1.1) 89.8 2.9 7.3 High MPW (1.1-1.2) 76.0 9.7 14.3

Table 3. Product yields obtained from pyrolysis of various MPWs at 400OC (Lee, 2007).

product distribution, due to the chemical nature and structure of plastic type in MPW.

Paraffins 4.61 1.82 0.06 Olefins 75.93 0.02 0.10 Naphthenes 6.08 0 0.46 Aromatics 11.97 97.19 22.24 Phenols 0.25 - 17.05 Nitro-aromatics 0.15 0.97 1.88 Aldehydes 1.01 - - Methylesters 0 - 58.21 <C13 92.89 92.05 99.13 C13-C24 7.01 7.95 0.87 >C24 0.10 0 0

Table 4. Liquid product composition obtained from pyrolysis of various MPWs at 400OC

Low MPW (<1.0)

As the characteristics of liquid product, the paraffin, olefin, naphthene and aromatic (PONA) components, etc are compared over three different MPWs, as shown in Table 4. Also, their carbon number distributions are plotted in Fig. 5, respectively. These showed a peculiar

Sample (S.G.)

Medium MPW (1.0-1.1)

High MPW (1.1-1.2)

on the plastic type including in municipal plastic wastes.

Product Composition

(Lee, 2007).

Fig. 5. Carbon number distribution of liquid paraffin, olefin, naphthene and aromatic products obtained from pyrolysis of MPW at 400OC (specific gravity : <1.0(a), 1.0-1.1(b), 1.1-1.2(c)) (Lee, 2007).

In the case of low MPW sample, the fractions of liquid paraffin, olefin, naphthene and aromatic products are about 5%, about 76%, about 6% and about 12%, respectively. Primary liquid product is olefin components and secondary is aromatic components. In the liquid product, naphthene and paraffin components are produced with a little amount. This means that low MPW mainly consists of PP polymer, among polyolefinic polymers, with relatively low degradation temperatures and high olefin fraction of liquid product in pyrolysis process (Lee et al., 2002). Also, aromatic products show 10% or more, because of including a little PS in low MPW. These results were demonstrated by the

Pyrolysis of Waste Polystyrene and High-Density Polyethylene 187

pA

process.

pA

pA

pA

pA

pA

pA

5 10 15 20 25 30 35 40 min

5 10 15 20 25 30 35 40 min

5 10 15 20 25 30 35 40 min

5 10 15 20 25 30 35 40 min

5 10 15 20 25 30 35 40 min

5 10 15 20 25 30 35 40 min

5 10 15 20 25 30 35 40 min

Fig. 6. GC peaks of product oils for thermal degradation of raw pyrolytic oil under

Also, the catalytic degradation of pyrolytic oil using powder type FCC catalyst as a commercial cracking catalyst is investigated by a stirred tank reactor. The purpose of the catalytic degradation is to identify the possibility for utility of spent FCC catalyst as a waste catalyst, as well as the application of fresh FCC catalyst. A simple pyrolysis and catalytic degradation using spent or fresh FCC catalyst are compared by cumulative amount distribution of liquid product as a function of lapse time of reaction, as shown in Fig. 7. When a little catalyst (10%) is quickly loaded in the reactor at 420OC, the cumulative yield of liquid product is improved by the effects of catalyst, due to more cracking of heavy hydrocarbons into liquid product. Also, the cumulative yield distribution from catalytic degradation using both spent and fresh FCC catalysts is slightly deviated. This shows that spent FCC catalyst, compared to fresh FCC catalyst, has an effective result on the pyrolysis

degradation temperature programming (Lee, 2009).

carbon number distribution of liquid PONA products over the case of low MPW, as shown in Fig. 4(a). Main liquid product was light olefin component with 9 of carbon number. This result was consistent with that of Sakata et al.(Sakata et al., 1999), who produced much more light hydrocarbon with 9 of carbon number from thermal degradation of PP at relatively low degradation temperature.

On the other hand, medium MPW showed highest fraction of liquid product with about 90% and lowest fraction of gas product, among three samples. In liquid product, aromatic components showed about 97% fraction and the rest was less than 2% fraction, respectively, as shown in Table 4. Moreover, phenol, aldehyde and methylester components in liquid products were not appeared and only nitro-aromatic products showed less than 2% fraction. It can be explained by the results that plastic type contained in medium MPW is mostly consisting of polymers with benzene-ring structures, especially PS among these polymers. This result can be reflected by carbon number distribution of liquid product, as shown in Fig. 4(b). Here, carbon number distribution was very short, mainly ranged from 8 to 9, as aromatic components. This result show a similar tendency in a comparison with that of Demirbas study (Demirbas, 2004), which is mainly consisting of 50-60% fraction of styrene and 10-20% fraction of C5-C8 hydrocarbons.

For pyrolysis of high MPW sample, the distribution of liquid products shows about 58% fraction in methylmethacrylate component, about 22% fraction in aromatic components and about 17% fraction in phenol components, as a main liquid product. However, straight hydrocarbon and naphthene components mainly obtained from pyrolysis of polyolefinic polymers are produced with very little amount (Lee et al., 2004). This result indicates that high MPW sample did not almost contain polyolefinic polymer type, and was mostly consisting of PMMA and then a little PS. This is demonstrated by the carbon number distribution of liquid products, as shown in Fig. 4 (c). Note the main product is methylmethacrylate monomer, producing from the pyrolysis of PMMA (Smolders & Baeyens, 2004).
