hydrocarbon gases and oils from the recycling of polystyrene waste by catalytic pyrolysis

INTERNATIONAL JOURNAL OF ENERGY RESEARCHInt. J. Energy Res. 2004; 28:31–44 (DOI: 10.1002/er.949)

Hydrocarbon gases and oils from the recycling ofpolystyrene waste by catalytic pyrolysis

Paul T. Williamsn,y and Ranbir Bagri

Department of Fuel and Energy, The University of Leeds, Leeds, LS2 9JT, U.K.

SUMMARY

The yield and composition of oils and gases derived from the pyrolysis and catalytic pyrolysis ofpolystyrene has been investigated. The pyrolysis and catalytic pyrolysis was carried out in a fixed bedreactor. Two catalysts were used, zeolite ZSM-5 and Y-zeolite and the influence of the temperature of thecatalyst, the amount of catalyst loading and the use of a mixture of the two catalysts was investigated. Themain product from the uncatalysed pyrolysis of polystyrene was an oil consisting mostly of styrene andother aromatic hydrocarbons. The gases were found to consist of methane, ethane, ethene, propane,propene, butane and butene. In the presence of either catalyst an increase in the yield of gas and decrease inthe amount of oil produced was found, but there was significant formation of carbonaceous coke on thecatalyst. Increasing the temperature of the Y-zeolite catalyst and also the amount of catalyst in the catalystbed resulted in a decrease in the yield of oil and increase in the yield of gas. Copyright # 2004 John Wiley& Sons, Ltd.

KEY WORDS: pyrolysis; catalysis; waste; plastics

1. INTRODUCTION

It has been estimated that 100 million tonnes of plastics are produced world-wide each year, andthe waste arising from this is largely landfilled or incinerated, with only a small percentage beingrecycled (Williams, 1998). Such recycling mainly involves the production of low grade products,such as, sewer pipes, plastic fencing and garden furniture. Higher technology recycling routeswhich produce higher value end products with an increased potential of economic success havebeen proposed Such processes include, chemical and thermal methods of recycling (Schiers,2001). The recycling of plastic waste by such higher technological routes has the potential torecover the hydrocarbons of the plastic polymer in the form of gas and oil. The hydrocarbonsmay then be recycled back into the petrochemicals industry to produce either virgin plastics,premium grade fuels or chemicals.

Pyrolysis of plastic waste has been proposed as a thermochemical recycling route where theplastic waste materials are processed in an inert atmosphere. The plastic polymer is thermally

Received 30 January 2003Accepted 20 March 2003Copyright # 2004 John Wiley & Sons, Ltd.

yE-mail: [email protected]

Contract/grant sponsor: EPSRC.

nCorrespondence to: P. T. Williams, Department of Fuel and Energy, University of Leeds, Leeds LS2 9JT, U.K.

degraded to produce an oil, gas and char, the proportions of which depend on the type of plasticand the process conditions used (Williams, 1998). For example, pyrolysis of low densitypolyethylene under slow heating rates up to 7008C produced an 80wt% yield oil/wax product,18wt% gas yield and negligible char (Williams and Williams, 1999). Polyethylene terephthalatepyrolysed under the same conditions produced 41wt% oil/wax, 39wt% gas and 16wt% char.Fast heating rates such as those produced in a fluidized bed pyrolysis unit and highertemperatures result in the thermal cracking of the pyrolysis gases and a higher yield of gas andlower yield of oil (Williams and Williams, 1998).

Further developments in the process of feedstock recycling of plastics have included the use ofcatalysts. For example, the use of a catalyst has been proposed as a way to, improve the yield ofvaluable products in the oil, to reduce the temperature of reaction, increase the yield of gas oralter the composition of the oil product (Bagri and Williams, 2002; Isoda et al., 1998; Lin et al.,1998; Zhao et al., 1996; Mordi et al., 1994).

Polystyrene is a major type of thermoplastic used throughout the world in such applicationsas electrical appliances, thermal insulation, tape cassettes, cups and plates (Williams, 1998). InWestern Europe alone, approximately 2.5 million tonnes of polystyrene are produced each year(APME, 1996). Polystyrene comprises 12.3% of the plastic content of municipal solid waste(APME, 1996). In addition, polystyrene represents a plastic which may be collected as a singlesource, for example from food retail outlets and from commercial and industrial premises.

Uncatalysed pyrolysis of polystyrene has been shown to produce 83wt% conversion to a lowviscosity oil which consisted mainly of styrene and a gas yield and char yield of less than 5wt%each (Williams and Williams, 1999). Scott et al. (1990) found a similar conversion ofpolystyrene, under fast pyrolysis conditions, to a liquid condensate of 88.5wt% at 5328C whichwas shown to be largely composed of styrene with a yield of 76.2wt%. Buekens and Schoeters(1980) and Bouster et al. (1989) also reported high yields of styrene from pyrolysis ofpolystyrene with yields of 76 and 78.7wt%, respectively. Catalytic pyrolysis of polystyrene hasshown that the yield of styrene is reduced and a range of aromatic hydrocarbons, includingsingle ring aromatic compounds and polycyclic aromatic hydrocarbons are produced (Williamset al., 1993; Puente and Sedran, 1998). For example, Puente and Sedran (1998) found thatcatalytic pyrolysis of polystyrene using ZSM-5 zeolite produces oil, which contained mostlysingle ring aromatics, such as toluene and ethylbenzene.

Whilst there has been some research into the catalytic pyrolysis of polystyrene as a potentialroute for recycling polystyrene waste there are few data detailing the yield and composition ofthe products in relation to process parameters. In this paper polystyrene has been pyrolysedunder slow pyrolysis conditions in a fixed bed reactor with and without the presence of acatalyst. Two catalysts were examined, Y-zeolite and zeolite ZSM-5 catalysts and thetemperature of the catalyst and the amount of catalyst in relation to the mass of polystyrenewere investigated. The yield and composition of the derived gas and oil products in terms oftheir hydrocarbon composition were determined.

2. MATERIALS AND METHODS

2.1. Plastic and catalyst characteristics

The polystyrene was obtained in the form of 2-3mm sized pellets from BP Chemicals Ltd.,Grangemouth, U.K. Two catalysts were used to investigate the catalytic pyrolysis of

Copyright # 2004 John Wiley & Sons, Ltd. Int. J. Energy Res. 2004; 28:31–44

P. T. WILLIAMS AND R. BAGRI32

polystyrene, zeolite ZSM-5 and Y-zeolite catalysts obtained from BDH, Ltd. (U.K.) and theircharacteristics are shown in Table I. The cation present in the zeolite ZSM-5 structure washydrogen (H-ZSM-5). The Y-zeolite had a larger pore size and higher surface area compared tothe ZSM-5 catalyst. The pore size is significant in determining the size selectivity of the reactantsand products, which can enter and leave the active sites of the catalyst.

2.2. Catalytic pyrolysis reactor

Figure 1 shows a schematic diagram of the fixed bed catalytic reactor. The reactor consisted of apyrolysis section and a catalyst section, both constructed of stainless steel and separately heatedexternally by electric ring furnaces. Temperatures throughout the reactor were controlled andmonitored by thermocouples. The reactor had an internal diameter of 37mm and a height of400mm.The reactor was continually purged with nitrogen. A stainless steel crucible supportedby metal gauze was used to hold the polystyrene in the pyrolysis section. Pyrolysis was carriedout at a heating rate of 108Cmin�1 to a final temperature of 5008C and held at that temperaturefor 20min to ensure complete pyrolysis. Pyrolysis conditions of temperature and heating rateand mass of polystyrene were the same throughout the experimental programme. The relativelylong residence times of gases in each reactor and the separate heating regimes ensured thatdistinctly separate temperatures in each reactor were maintained. The zeolite catalyst was placedin the catalyst section and held in place with metal gauze. The catalyst section was attacheddirectly to the pyrolysis section so that the pyrolysis gases generated were passed directly overthe fixed catalyst bed. After each experiment, the catalyst was removed and the formation ofcarbonaceous coke on the catalyst was determined by the difference in mass before and aftercatalytic regeneration. The regeneration took the form of heating the used catalyst in a furnaceat a temperature of 5508C in the presence of air for a period of eight hours. The gases exiting thereactor were condensed in the condenser system which consisted of a water cooled condenserfollowed by a series of glass condensers filled with glass wool cooled by CO2/acetone to trap thevolatile hydrocarbons. The condenser system was weighed before and after the experiment todetermine the total oil yield. A 100 l Tedlar gas bag was used to collect all the gaseous product.

2.3. Gas analysis

The gas collected in the sample gas bag was analysed for hydrocarbon and non-hydrocarbongases using packed column gas chromatography. Hydrocarbons from C1 to C4 were analysedusing a Pye-Unicam gas chromatograph with a 2.2m� 6mm column packed with n-octane

Table I. Characteristics of the ZSM-5 and Y zeolite catalysts.

Catalyst property ZSM-5 Y-zeolite

Particle size (mm) 2 2Pore size (in�10) 5.5 7.4Pore volume (m�3 kg�1) 0.48 0.64Surface area (m2 g�1) 300 440Bulk density (kgm�3) 0.72 0.61Silica:alumina ratio 50 11Support Clay binder Clay binder


RECYCLING OF POLYSTYRENE WASTE BY CATALYTIC PYROLYSIS 33

Porasil C of 80–100 mesh size, nitrogen carrier gas and with a flame ionisation detector.Nitrogen, hydrogen, oxygen and carbon monoxide were analysed with a Pye Unicam series 204gas chromatograph, with a stainless steel 1.8m� 6mm column packed with 5 (AA silica molecularsieve with argon as the carrier gas and with a thermal conductivity detector. Carbon dioxide wasanalysed using a Gow–Mac Spectra gas chromatograph with a 1.8m� 6mm stainless steelcolumn packed with 100–120 mesh silica gel, helium carrier gas and thermal conductivitydetector. The total mass of gas evolved was calculated from the measured concentrations andmolecular masses of the gases.

2.4. Oil analysis

Functional group chemical analysis of the derived oils and waxes was carried out using Fouriertransform infra-red spectrometry (FTIR). A Perkin-Elmer 1750 spectrometer was used whichhad data processing and spectral library search facilities. A small amount of the oil or wax wasmounted on a potassium bromide disc which had been previously scanned as a background. Theinfra-red spectra of the sample was then taken. The resulting spectra was normalised to the C–Hpeak around 3000 cm�1, direct comparisons of relative peak intensities could then be taken.

The composition of oils derived from the pyrolysis and catalytic pyrolysis of polystyrene wereanalysed by capillary column gas chromatography with flame ionisation detection and alsocompound identifications for aromatic species were confirmed by gas chromatography/massspectrometry. Identification was also by the extensive use of relative retention indices and the

Figure 1. Schematic diagram of the reactor system.



use of internal standards. The gas chromatograph/flame ionisation system was a Carlo Erba3500 chromatograph with cold on-column injection. The capillary column was a HT-5 type,25m long� 0.32mm internal diameter, high temperature, fused silica, aluminium coatedcolumn with a 5% phenyl polycarborane-siloxane stationary phase.

3. RESULTS AND DISCUSSION

3.1. Product yield

Table II shows the product yields obtained from the pyrolysis of polystyrene for the absence of acatalyst, and for the zeolite ZSM-5 and Y-zeolite catalysts in relation to catalyst temperature,respectively. Pyrolysis at 5008C in the absence of any catalyst produced a yield of 96.6wt% oil,negligible char and low gas yield Similar high yields of oil and low char and gas yields have beenreported by other workers for the uncatalysed pyrolysis of polystyrene (Williams and Williams,1999; Scott et al., 1990; Kaminsky, 1992). In the presence of both catalysts, there was a decreasein the oil yield and increase in gas yield and a marked formation of carbonaceous coke on thecatalyst. As the catalyst temperature was increased, the gas yield increased with a consequentdecrease in oil yield. For both catalysts, the formation of coke increased with increasing catalysttemperature.

The formation of carbon on zeolite catalysts has been attributed to a wide variety of chemicalgroups, but certain groups such as aromatic compounds, naphthenes, alkenes and alkanes haveall been cited as particularly leading to increased formation of carbon on the catalyst (Venutoand Habib, 1979). In particular, aromatic species have a greater predisposition to being involvedin pathways to coke formation because of their ability to easily involve themselves in hydrogentransfer and cyclisation reactions. The analysis of pyrolysis oils derived from polystyrene havebeen shown to be very high in aromatic compounds and the composition is dominated by thepresence of styrene (Williams and Williams, 1999; Williams et al., 1993), consequently, leadingto high carbon formation on the catalysts.

Table II. Product yield from the catalytic pyrolysis of polystyrene in relation to catalyst temperature.

Catalyst Pyrolysistemperature (8C)

Catalysttemperature (8C)

Oil(wt%)

Gas(wt%)

Carbon(wt%)

No catalyst 500 500 96.6 0.1 0.0

Zeolite ZSM-5 500 400 92.4 0.7 9.7500 450 91.6 1.6 9.2500 500 91.2 2.5 10.2500 550 87.0 3.2 10.2500 600 86.2 3.6 11.4

Y-zeolite 500 400 75.0 3.2 22.0500 450 70.6 4.1 23.9500 500 71.0 5.8 22.0500 550 69.0 7.0 24.0500 600 69.6 6.8 24.0



There were significant differences between the two catalysts. The zeolite ZSM-5 catalystproducing lower gas and carbon yields and consequently, higher oil yields. The formation ofcarbon on the Y-zeolite catalyst was high compared to the ZSM-5 catalyst. Lin et al. (1998)examined the catalytic pyrolysis of polypropylene and polyethylene over ZSM-5 and Y-zeolitecatalysts. They also reported a lower carbonaceous coke formation on the catalyst for the ZSM-5 catalyst, which they attributed to the more restrictive pore size of the ZSM-5 catalystcompared to the Y-zeolite catalyst The yields of carbonaceous coke on the catalysts found byLin et al. (1998) for polyethylene and polypropylene were much lower than the coke valuesfound in this work for polystyrene. They reported coke value of 1.7wt% for polyethylene and1.1wt% for polypropylene for the ZSM-5 zeolite catalyst, but were higher for the Y-zeolitecatalyst at 3.9wt% and 3.7wt% for polyethylene and polypropylene, respectively. The pyrolysisproducts from polyethylene and polypropylene have been shown to largely aliphatic alkanesand alkenes (Kaminsky, 1992). However, the products of pyrolysis for polystyrene are highlyaromatic, resulting in higher coke formation. Puente and Sadran (1998) have also shown thatfor the fluidized bed catalytic pyrolysis of polystyrene with zeolite ZSM-5 catalyst producedcoke levels of between 15 and 28wt% depending on catalyst contact time. Zhang et al. (1995)however, found somewhat lower yields of coke on zeolite ZSM-5 catalyst of 5.9wt% for thecatalytic pyrolysis of polystyrene. However, their work was carried out at a reactiontemperature of 3508C. The results shown in Table II, clearly show the influence of catalysttemperature, suggesting that at lower catalyst temperatures such as those used by Zhang et al.(1995), similar coke formation would be produced.

Table III shows the influence of the polystyrene:catalyst ratio for the Y-zeolite catalyst. Thedata represent the pyrolysis of 5 g of polystyrene in relation to the mass of the Y-zeolite. As themass of catalyst was increased there was a marked reduction in the yield of oil from 94.2wt% ata polystyrene-.catalyst ratio of 2:1 to 71wt% at a polystyrene:catalyst ratio of 1:3. There was acorresponding increase in gas yield The carbon formation on the catalyst was markedly affectedby the increasing polystyrene:catalyst ratio, increasing from 4 to 22wt% as the polystyrene:catalyst ratio was increased.

Table IV shows the influence of mixing the zeolite ZSM-5 catalyst and the Y-zeolite catalysts.The two catalysts have different pore sizes and surface acidities. The Y-zeolite had a pore size7.8 (AA and a Si:Al ratio of 5.4 representing a higher acidity catalyst, and the zeolite ZSM-5 had apore size 5.6 (AA and Si:Al ratio of 40 representing a lower acidity catalyst. The lowersilica:alumina ratio results in an increase in the surface acidity of the catalyst by increasingthe relative surface concentration of aluminium (Venuto and Habib, 1979, Campbell, 1988). Thedifference in the pore size between the two catalysts provides the reactant selectivity ofthe catalyst. The smaller pore size restricts the size of hydrocarbons entering the pore system

Table III. Product yield from the catalytic pyrolysis of polystyrene in relation to plastic:catalyst ratio.

Catalyst Plastic: catalyst ratio Catalyst mass (g) Oil (wt%) Gas (wt%) Carbon (wt%)

No catalyst – – 96.6 0.1 0.0Y-zeolite 2:1 2.5 94.2 1.4 4.0

1:1 5.0 80.6 2.5 16.41:2 10.0 76.4 4.4 18.01:3 15.0 71.0 5.8 22.0



of the catalyst and which may then undergo catalytic cracking and reformation reactions.Mixing the catalysts was investigated since the combined properties of the two catalysts mayprovide for a wider range of species which can react with the catalyst and a consequent morealtered product stream. The data in Table IV suggest that the Y-zeolite catalyst produces thelarger influence on the product stream, as shown previously. Mixing the catalysts will have agreater influence on the composition of the products and will be discussed later.

3.2. Gas composition

Figures 2 and 3 show the composition of the gases produced from the catalytic pyrolysis ofpolystyrene in the presence of the zeolite ZSM-5 catalyst and Y-zeolite catalyst, respectively. Asdiscussed previously, Table II showed that the total gas yield in the presence of the Y-zeolitecatalyst was significantly higher compared to the zeolite ZSM-5 catalyst and total gas yieldincreased with increasing catalyst temperature for both catalysts. The uncatalysed pyrolysisof polystyrene produced a negligible gas yield of 0.1wt%. Figures 2 and 3 show that the maingases produced from the catalytic pyrolysis of polystyrene were alkane and alknene gases fromC1 to C4. Ethene and propene and lower concentrations of methane, ethane and propane werethe main gases produced with the zeolite ZSM-5 catalyst, whereas equal concentrations ofethane, propene, methane, ethane and propane were produced with the Y-zeolite catalyst.Isobutane was produced in significant concentrations for the Y-zeolite. Puente and Sedran

Table IV. Product yield from the catalytic pyrolysis of polystyrene in relation to ZSM-5:Y-zeolite catalystratio.

Catalyst ZSM-5 (wt%) Y-Zeolite (wt%) Oil (wt%) Gas (wt%) Carbon (wt%)

No catalyst – – 96.6 0.1 0.0

Mixed catalyst 100.0 0.0 91.2 2.5 10.283.3 16.7 83.6 3.9 14.866.7 33.3 77.4 4.2 18.00.0 100.0 71.0 5.8 22.0

Figure 2. Gas composition in relation to catalyst temperature for the catalytic pyrolysis of polystyrenewith the zeolite ZSM-5 catalyst



(1998) also reported a marked increase in C2 and C3 gases for the catalysed pyrolysis ofpolystyrene compared to uncatalysed pyrolysis.

Figure 4 shows the composition of the gas derived from the catalytic pyrolysis of polystyrenein relation to the amount of catalyst present in the catalytic section of the reactor for theY-zeolite. There was a marked increase in the total gas yield as shown in Table III. Increasingthe catalyst bed loading resulted in an increase in the concentration of all of the gases.

The influence of mixing the zeolite ZSM-5 and Y-zeolite catalysts on the yield andcomposition of the gaseous products is shown in Figure 5. The temperature of the catalyst bedwas 5008C throughout. As shown in Figs. 2 and 3, the 100% use of the zeolite ZSM-5 catalystproduced a lower gas yield compared to 100% of the Y-zeolite. Figure 6 shows a calculated gascomposition for the relative mixture proportions of 83:17, 66:33 and 50%:50%, based on theexperimental gas composition data for 100% zeolite ZSM-5 and 100% Y-zeolite. Comparedwith Figure 5, for the experimental data on the 83:17 and 66%:33% catalyst mixtures, the totalgas yield was higher than would have been predicted by merely adding the gas proportionally

Figure 3. Gas composition in relation to catalyst temperature for the catalytic pyrolysis of polystyrenewith the Y-zeolite catalyst.

Figure 4. Gas composition in relation to catalyst loading for the catalytic pyrolysis of polystyrene with theY-zeolite catalyst.



based on the 100% zeolite ZSM-5 and 100% Y-zeolite experimental data. In fact, each of thealkane and alkene gases were produced in higher concentrations than would have beenpredicted. Consequently, there must have been further interaction of the pyrolysis gases andcatalytic products to increase the yield of alkane and alkene gases.

The gases produced from the catalytic pyrolysis of polystyrene are hydrocarbon with asignificant calorific value, enough, under the majority of process conditions to provide theenergy requirements for the pyrolysis and catalytic pyrolysis process and consequently off-setthe costs of recycling.

3.3. Oil composition

Figure 7 shows the Fourier transform infra-red (FT-IR) spectral analysis of oils obtained for thepyrolysis and the catalytic pyrolysis of polystyrene for the Y-zeolite catalyst in relation to

Figure 5. Gas composition in relation to a mixture of zeolite ZSM-5 catalyst and Y-zeolite catalyst for thecatalytic pyrolysis of polystyrene.

Figure 6. Gas composition in relation to a mixture of zeolite ZSM-5 catalyst and Y-zeolite catalyst for thecatalytic pyrolysis of polystyrene. 100%:0% data are experimental results, 83:17, 66:33 and 50%:50% are

calculated data.



catalyst temperature. The peaks at the 2980,2950 and 2930 cm�1 wavelengths represent C–Hstretching vibrations of the chemical functional groups, -CH3 -CH2 and -CH respectively. Thepresence of C=C stretching vibrations between approximately the wavelengths 1765 and1625 cm�1 suggest the presence of alkenes. The presence of aromatic groups is indicated by thepeaks between 3000 and 3100 cm�1, at 1450 cm�1, and between 675 and 900 cm�1. The peakslocated at 980 and 920 cm�1 represent CH stretching and deformation vibrations of alkenestructures. Figure 7 for the FT-IR spectra for the oil derived from the uncatalysed pyrolysis ofpolystyrene shows that the oil is compositionally complex containing aromatic and aliphaticgroups. The addition of the Y-zeolite catalyst resulted in a significant change in the FT-IRspectra as shown in Figure 7. The aromatic groups indicated by the peaks at 3000 and 3100, at1450 and 675–900 cm�1 show a marked increase in intensity and consequently concentration.Increasing the temperature of the Y-zeolite catalyst resulted in an increase in aromatic peakintensity.

Figure 8 shows the FT-IR spectral analysis of oils obtained for the pyrolysis and the catalyticpyrolysis of polystyrene for the zeolite ZSM-5 catalyst in relation to catalyst temperature. TheFT-IR spectra were similar to that obtained for the Y-zeolite, with increases in peak intensitiesfor the aromatic groups indicated at 3000 and 3100 cm�1, at 1450 and 675 -900 cm�1.

Figure 9 shows the FT-IR spectra of the oil derived from the catalytic pyrolysis of polystyrenein relation to the catalyst bed loading for the Y-zeolite catalyst. The effect of increased aromaticcontent is indicated even at the lower catalyst concentration, but there was also an increase inaromatic peak intensity as the amount of catalyst was increased. Figure 10 shows the influenceof mixing the two catalysts on the FT-IR spectra of the oils derived from the catalytic pyrolysis

Figure 7. FT-IR spectra of the oil derived from the catalytic pyrolysis of polystyrene in relation toY-zeolite catalyst temperature.



Figure 8. FT-IR spectra of the oil derived from the catalytic pyrolysis of polystyrene in relation to zeoliteZSM-5 catalyst temperature.

Figure 9. FT-IR spectra of the oil derived from the catalytic pyrolysis of polystyrene in relation toY-zeolite catalyst loading.



of polystyrene. All the spectra were quite similar since for each experiment there was 15 g ofcatalyst present in the catalytic section of the reactor.

The oils were analysed using capillary column gas chromatography with either flameionisation detection or mass spectrometry detection to determine the composition of the oilsderived from the catalytic pyrolysis of polystyrene. Table V shows the range of aromatic speciesidentified in the oils, ranging from 1 to 4 ring aromatic compounds. The highly aromatic natureof oils derived from the catalytic pyrolysis of polystyrene has been reported before. Forexample, Puente and Sedran (1998) examined the oils derived from the catalytic pyrolysis ofpolystyrene using ZSM-5 zeolite and showed that the oil consisted of mostly single ring

Figure 10. FT-IR spectra of the oil derived from the catalytic pyrolysis of polystyrene in relation tomixtures of Y-zeolite catalyst and zeolite ZSM-5 catalyst loading.

Table V. Aromatic compounds identified in the oils derived from the catalytic pyrolysis of polystyrene.

Single ring compoundsBenzene, toluene, styrene, m-xylene, o-xylene, p-xylene, ethylmethylbenzene, propenylbenzene,

methylstyrene

Two ring compoundsIndene, methylindene, naphthalene, 2-methylnaphthalene, 1-methylnaphthalene, biphenyl, methylbi-

phenyl, dimethylnaphthalene, trimethylnaphthalene, tetramethylnaphthalene, ethylbiphenyl

Three ring compoundsPhenanthrene, methylphenanthrene, dimethylphenanthrene, trimethylphenanthrene

Four ring compoundsPyrene, methypyrene, dimethylpyrene, chrysene, methylchrysene



aromatic compounds, such as benzene, toluene, xylenes, styrene and ethylbenzene and lowerconcentrations of other higher molecular weight aromatic compounds. Zhang (1995)investigated the ZSM-5 catalytic pyrolysis of polystyrene and also identified, benzene, toluene,ethylbenzene and styrene in the product oils. Similarly, Mertinkat (1999) investigated thecatalytic degradation of polystyrene and also found that there was a high yield of single ringaromatic compounds present in the product oil, of which styrene was dominant However, thereare few data identifying the higher molecular weight 2–4 ring aromatic species in oils derivedfrom the catalytic pyrolysis of polystyrene. Table V shows that the oils also contained polycyclicaromatic compounds of 2–4 rings.

The presence of polycyclic aromatic compounds in the product oils is significant in that thisgroup of chemical compounds comprises the largest group of potential carcinogens (Lee et al.,1981). Some of the PAH shown in Table V have been shown to be carcinogenic and/ormutagenic. Lee et al. (1981) list the relative carcinogenicities of certain polycyclic aromaticcompounds and show that chrysene, methylchrysene and trimethylphenanthrene have beenshown to give positive results in carcinogenicity tests. Longwell (1983) has also shown thatphenanthrene and the methylphenanthrenes are mutagenic in both human and bacterial celltests. Therefore, the presence of such compounds in the oils derived from the catalytic pyrolysisof polystyrene have consequences for the safe handling and use of such an oil.

The presence of PAH in the derived pyrolysis and catalysis oils also has further significance ifthe oil is to be used as a liquid fuel. Since increased concentrations of PAH in the fuel may leadto increased emissions of PAH when the fuel is combusted. It has been shown that PAH canoccur in petroleum derived liquid fuels such as furnace gas oil and diesel fuel (Guerin, 1978;Herlan, 1978; Williams et al., 1989). The subsequent formation of PAH on the derived sootfrom the combustion of such fuels has been linked to the PAH content of the original fuel(Herlan, 1978; Williams, 1989). The fuel PAH survive the combustion process and are depositedon the soot as an unburned fuel fraction. The presence of PAH adsorbed on the soot emissionfrom combustion of the polystyrene derived pyrolysis and catalytic pyrolysis oil may thereforerepresent a health hazard.

4. CONCLUSIONS

The investigation into the pyrolysis and catalytic pyrolysis of polystyrene reported in this workhas shown that catalysis results in an increase in the yield of gas and reduction in the oil yieldand significant formation of carbonaceous coke on the catalyst. The derived oil has also beenshown to contain significant concentrations of potentially harmful polycyclic aromaticcompounds. The use of catalysts to produce an oil with a more valuable potential end usehas proved successful for other plastic wastes. However for polystyrene, pyrolysis in the absenceof any catalyst can produce very high (580wt%) concentrations of styrene, which represents apotentially valuable end product, without the further processing costs of catalysis.

ACKNOWLEDGEMENTS

The authors would like to acknowledge the support of an EPSRC Research Scholarship toR. Bagri. We would also like to thank David Wilson, BP Chemical, Grangemouth, for thesupply of polymer samples and technical discussions.



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