the pyrolysis of individual plastics and a plastic mixture in a fixed bed reactor

J. Chem. T ech. Biotechnol. 1997, 70, 9È20

The Pyrolysis of Individual Plastics and a PlasticMixture in a Fixed Bed Reactor*

Elizabeth A. Williams & Paul T. Williams”Department of Fuel and Energy, University of Leeds, Leeds LS2 9JT, UK(Received 21 June 1996 ; revised version received 18 November 1996 ; accepted 5 December 1996)

Abstract : Six thermoplastics, which represent more than two-thirds of allpolymer production in western Europe, were pyrolysed in a static batch reactorin a nitrogen atmosphere. These were high density polyethylene (HDPE), lowdensity polyethylene (LDPE), polystyrene (PS), polypropylene (PP), polyethyleneterephthalate (PET) and polyvinyl chloride (PVC). The heating rate used was25¡C min~1 to a Ðnal temperature of 700¡C. These six plastics were then mixedtogether to simulate the plastic fraction of municipal solid waste found inEurope. The e†ect of mixing on the product yield and composition was exam-ined. The results showed that the polymers studied did not react independently,but some interaction between samples was observed. The product yield for themixture of plastics at 700¡C was 9É63% gas, 75É11% oil, 2É87% char and 2É31%HCl. The gases identiÐed were H2 , CH4 , C2H4 , C2H6 , C3H6 , C3H8 , C4H8 ,

and CO. The composition of oils were determined using FourierC4H10 , CO2Transform infra-red spectrometry and size exclusion chromatography. Analysisshowed the presence mainly of aliphatic compounds with small amounts of aro-matic compounds.

J. Chem. T ech. Biotechnol. 70, 9È20 (1997)No. of Figures : 11. No. of Tables : 7. No. of Refs : 26

Key words : plastics, pyrolysis, Fourier Transform infra-red spectrometry, chemi-cal production

INTRODUCTION

In western Europe total plastic consumption in 1991was over 24 million tonnes1 with packaging represent-ing 39% of this. The amount wasted was estimated atalmost 17 million tonnes.1 Less than 4% of petroleumresources are currently being used for polymer pro-duction,2 though this Ðgure looks set to increase withthe development of new applications for both polymersand copolymers multiplying. The car industry, forexample, is tending towards signiÐcantly increasingpolymer usage in new vehicles.3

In 1990, less than 2% of eight major commoditypolymers were recycled, the rest being landÐlled orincinerated.4 Recent Ðgures show that 80% of wastegoes to landÐll, 10% is recycled and 10% is inciner-ated.5 Many governments have identiÐed the plasticwaste problem as an area to be carefully controlled. The

* Presented in part at Clean Tech Ï96, London, 19È21 June1996.” To whom correspondence should be addressed.

government in the UK has set a challenge of recycling25% of all household waste by the year 20006 and torecover value from 40% of municipal waste by the year2005.7

In the UK 7É4% of polymers were recycled via sec-ondary recycling in 1991.18 The polymers used arethose separated from other waste. Uses of such recycledmaterials are similar to those for the original virginpolymer. However, in many cases the physical proper-ties are inferior. In addition, in the food and drinksindustries for example, government legislation makesrecycled plastics unusable in food applications. Takinginto account the markets available for recycled plasticmaterials it has been estimated that material recyclingrates can increase no higher than between 15 and 20%of total waste generated.4,8,9 Therefore if governmenttargets are to be met, more mixed, and possibly con-taminated, plastic will have to be dealt with. Currenttechnology for secondary recycling would Ðnd it diffi-cult to produce a product with enough added value forthe materials market.

91997 SCI. J. Chem. T ech. Biotechnol. 0268-2575/97/$17.50. Printed in Great Britain(

10 E. A. W illiams, P. T . W illiams

TABLE 1CaloriÐc Value of Some Major Plastics Compared with Fuels

Sample CaloriÐc value (MJ kg~1)

Polyethylene 46É50Polypropylene 46É50Polystyrene 41É90

Kerosene 46É50Gas oil 45É20Heavy oil 42É50

Source : Ref. 23.

Table 1 lists the caloriÐc values of some polymerscompared with some conventional fuels. The caloriÐcvalue of those polymers are comparable to the fuelsthemselves, making their utilisation conceivable. There-fore, alternatives such as tertiary recycling are beingexamined. This is deÐned as the recovery of monomersor oil from waste plastic by a depolymerisationprocess.4

Pyrolysis is such a tertiary recycling process. Thepolymer sample is heated in an inert atmospherecausing the carbonÈcarbon bonds to break along thepolymer backbone. This depolymerisation processresults in monomers or short chained compounds beingformed. Generally three product types are formed froma pyrolysis reaction, that is a gas, oil and char. All havethe potential for use as a fuel or chemical feedstock andthus bring an intrinsic value to the whole process.

More research concerning plastic pyrolysis has beencarried out in the last decade than ever before. Japanhas developed a method for decomposing polyoleÐns.The Fuji process uses catalysts to form a gasoline-richoil from polyethylene, polypropylene and polystyrene.10Work in Europe has dominated the area andresearchers at Hamburg University have played animportant role. Polymers such as polyethylene, poly-propylene and tyres under di†erent reaction conditionshave been pyrolysed.11 Also in Germany, the energyand chemical company, Veba, is operating a plasticrecycling process by which used polymers are hydro-cracked into a synthetic oil.8 BASF has recently built apilot plant for the production of naphthas, oleÐns andaromatics from mixed plastic wastes.8 In the UK, BPChemicals is leading a consortium in developing a plas-tics recycling plant. Their process is based on Ñuid bedtechnology and the resultant waxes have been shown tobe suitable for feeding straight into a steam cracker forethylene production.8 Pinto et al. pyrolysed plasticwaste at 430¡C in an autoclave to yield more than 80%liquid and less than 5% gas. Roy13 in Canada hasfocused on plastic pyrolysis using vacuum technology.

Much of the previous research has mainly concen-trated on the pyrolysis of single polymers. This paperexamines the issue of pyrolysing both single and mixedplastic wastes for further utilisation

MATERIALS AND METHODS

Pyrolysis reactor

The pyrolysis unit used was a Ðxed bed, batch reactor.A schematic diagram of it is presented in Fig. 1. Thiswas stainless steel, having an internal diameter of37 mm and being 250 mm in length. The top was sealedwith a stainless steel Ñange and reinforced gasket. Thecarrier gas was nitrogen and this was passed into thetop of the reactor at a rate of 200 cm3 min~1. Thispurge gas removed the reaction products from the reac-tion site and reduced the extent of secondary reactions.These can occur if volatiles are subjected to longresidence times in the hot reactor.14 The nitrogen Ñowrate used gave a residence time of 25 s through the hotreaction zone.

High density polyethylene (HDPE), low density poly-ethylene (LDPE), polystyrene (PS), polypropylene (PP),polyethylene terephthalate (PET), and polyvinyl chlo-ride (PVC) were the thermoplastics used. These are thesix polymers most commonly found in domestic wasteand represent more than two-thirds of all polymer pro-duction in western Europe.15 A mixture of these whichrepresented 95% of the plastic fraction of municipalwaste found in Europe1 was also used. The compositionof this fraction is shown together with the normalisedpercentages for the polymers actually pyrolysed in thisstudy in Table 2. The polymer sample was placed in astainless steel crucible inside the reactor. This avoidedloss of sample during melting and before the pyrolysistemperature was reached. The temperature of thesample was measured using a type K thermocouple.This was in direct contact with the sample. The wholereactor was externally heated by an electrically heatedring furnace. The temperature programme used was25¡C min~1 to a Ðnal temperature of 700¡C. This tem-perature was held until no more gas was produced.

The pyrolysis vapours were swept down through thebottom of the reactor and rapidly condensed in a seriesof four glass condensers. Dry ice and acetone was usedas the cooling agent giving an approximate temperatureof [78¡C. A glass wool trap was put in series after the

TABLE 2Percentage of Each Polymer Used in the Simulated Plastic

Fraction of Municipal Solid Waste Used in this Study

Polymer Percentage (%)

LDPE 31É25HDPE 31É25PP 7É29PS 13É5PVC 11É46PET 5É21

Source : Ref. 1.

Pyrolysis of plastics in a Ðxed bed reactor 11

Fig. 1. Schematic diagram of Ðxed bed reactor.

condensers to catch any oil particles which were nottrapped in the condensers. The remaining gases werethen collected in a gas bag.

HCl analysis

During the pyrolysis of PVC and the plastic mixture, adreschel bottle of de-ionised water was placed in-line.All the gas was bubbled through with the aim of dis-solving any HCl produced by PVC degradation. TheHCl in samples were analysed using a CorningH2OpH/ion meter 135 with a chloride electrode calibratedbetween 100 and 1000 ppm. A double junction referenceelectrode Ðlled with 1 molar with a Ðxed poten-KNO3tial was used in conjunction with the chloride electrode.The chloride ions migrating to the chloride electroderesulted in a di†erence in potential compared with thereference and a direct ppm reading could be taken.

Gas analysis

The evolved gas was collected in a 10 dm3 TeÑon bagand then analysed o†-line using three packed columngas chromatographs. Hydrocarbons were analysedusing a GCD chromatograph with a 2É2 m ] 6 mm

glass column packed with 80È100 mesh n-octanePorasil C. Permanent gases were identiÐed on a Pye 204chromatograph with a 1É8 m ] 6 mm stainless steelcolumn packed with molecular sieve Carbon5 Ó.dioxide analysis was carried out on a Gow Mac chro-matograph with a 1É8 m ] 6 mm stainless steel columnpacked with 100È120 mesh silica gel. A Ñame ionisationdetector identiÐed the hydrocarbons and thermal con-ductivity detectors were used to determine the othergases. Data manipulation was carried out using aHarley Peakmaster data analysis system. Three sampleswere injected into each gas chromatograph to increasethe conÐdence of the results. The gases identiÐed werepermanent gases, CO and and hydrocarbons up toCO2

The total weight of gas produced was calculated byC4 .comparison to the known nitrogen Ñow rate.

Elemental analysis (CHN)

During pyrolysis a char was formed in the metal cruci-ble where, after cooling, it was retrieved and analysed.Polyvinyl chloride and polyethylene terephthalate werethe only polymers to produce signiÐcant amounts ofchar. Chars were analysed on a Perkin-Elmer model240C elemental analyser. Each sample was analysedtwice.


Fourier Transform infra-red spectrometry (FTIR)

Functional group analysis of each pyrolytic oil and waxwas carried out using Fourier Transform infra-red spec-trometry. A Perkin-Elmer 1750 spectrometer was usedwhich had data processing and spectral library searchfacilities. A small amount of the liquid fraction derivedfrom the polymer pyrolysis was mounted on a hand-made potassium bromide (KBr) disc which had beenpreviously scanned as a background. The infra-redspectra of the sample was then taken. The resultingspectra were normalised to the CwH peak around3000 cm~1. Direct comparisons of peak intensity couldthen be taken.

Size exclusion chromatography (SEC)

Size exclusion chromatography, also known as gel-permeation chromatography, is a non-destructivemethod which gives information on the molecularweight distribution of the sample. The equipment usedwas similar to that of HPLC. The columns were two4É6 ] 150 mm 5 km RPSEC type in series. This100 Ómethod di†ers from HPLC in that the elution order isin reverse molecular size, i.e. the largest molecule iseluted Ðrst. Two detectors were used in series, thesebeing a refractive index detector and an ultra-violetdetector, enabling the extent of sample aromaticity tobe determined. The analytical equipment was linked toa chart recorder in addition to a computer with dataprocessing facilities. A small amount of the oils andwaxes were dissolved in 10 cm3 of tetrahydrofuran(THF). This solvent was also the eluent through thesystem.

RESULTS

Mass balances

The product yields for each individual plastic and theplastic mixture are shown in Table 3. All polymers pro-duced a hydrocarbon gas and an oil or waxy fraction.Some polymers produced a small amount of char. Poly-vinyl chloride also produced HCl gas upon heating.Polypropylene gave the largest amount of pyrolytic oil

and PVC the smallest with the respective yields being85% and 31É69%. Oil was the major product formedfrom the pyrolysis of all polymers with the exception ofPVC where HCl gas was the dominant product.

The appearance of the liquid products was varied.HDPE, LDPE and PP all gave a light coloured waxyproduct. PS produced a red/brown liquid, whereas bothPVC and PET gave a dark almost black heavy liquid.

The gases produced were a comparatively minorcomponent when compared with the oil yield. However,in the main, all gases were deemed potentially usefuldue to their hydrocarbon nature. HDPE, LDPE andPP yielded similar quantities of gas. PS and PVC pro-duced only very small amounts. PET made the largestvolume of gas. However, due to the oxygen present inits structure (see Fig. 2), upon depolymerisation themajor part was carbon dioxide, with this representingmore than 66% of the gas yield.

PET and PVC gave a large amount of char, the yieldsbeing 15É5% and 16É92% respectively. Wenning16 notedthat PVC showed a coke formation of approximately20% by weight.

A theoretical product yield for the plastic mixturewas calculated from the experimental yields obtainedfor each individual polymer. The theoretical and experi-mental results were comparable. The liquid fraction col-lected was a dark waxy substance, though less viscousthan the waxes from HDPE, LDPE or PP pyrolysis.The gas yield of 9É64% was lower than the theoreticalvalue of 13%. The reason for this may simply be due toa smaller number of secondary reactions. The HCl col-lected from the mixture was almost one-third of thatexpected. This may be due to the occurrence of organo-chloride compounds in the oil fraction. Comparisons ofthe product yields between plastics and products aremore clearly presented in Figs 3a and 3b.

The mass balances obtained by other researchersdi†er widely depending on the reaction conditions used.Kaminsky11 found that using polyethylene as a feed, upto 50% of the input material can be retrieved in theliquid form. Here the wax yield for polyethylene wasmore than 80% at 700¡C. Using mixed plastic waste ina rotary kiln pyrolysis unit Wenning16 obtained aproduct composition of gas 30È50%, oil 40È50% andcoke 5È15% at pyrolysis temperatures between 650 and

TABLE 3Comparison of Product Yields from the Pyrolysis of Polymers in a Fixed Bed

Reactor (%)

HDPE L DPE PS PP PET PV C Mixture

Gases 16É77 15É02 3É41 13É63 33É99 2É47 9É63Oil 79É72 84É25 83É77 84É44 41É30 31É69 75É12Char 0É00 0É00 3É50 0É15 15É55 13É78 2É87HCl 0É00 0É00 0É00 0É00 0É00 52É93 2É31Total 96É48 99É27 90É67 98É23 90É84 100É87 89É93


Fig. 2. Structures of repeating units of the thermoplasticsused.

850¡C. The aim of the work presented here is oil pro-duction. More than 75% of the yield from the pyrolysisof a mixture was a liquid fraction.

Gas analysis

The gases identiÐed were hydrocarbons up to C4including methane, ethane, ethene, propane, propene,butane, butene, plus carbon dioxide, carbon monoxideand hydrogen.

Fig. 3a. Comparison of product yields with the pyrolysis ofsix thermoplastics and a plastic mixture in a Ðxed bed reactor.

Fig. 3b. Comparison of products from the pyrolysis of sixthermoplastics and a plastic mixture in a Ðxed bed reactor.

The evolution of gases from HDPE, LDPE, PP andPVC were similar. This is due to their comparablestructures (Fig. 2). Each polymer contains the mono-valent group of atoms with their di†er-CH2CHwXences being introduced with a change in the X-group.All thermally degrade through random scission17,18 togive a wide product size distribution. The temperatureat which degradation begins is between 300¡C and450¡C. The thermal behaviour of polyvinyl chloridediverges slightly from the other similarly structuredpolymers. The CwCl bond in PVC has a lower bondenergy than other bonds in its structure,19 as Table 4shows, and upon heating it is this bond which is pri-marily broken. Therefore, PVC degradation beginsaround 150¡C which is a much lower temperaturethan for the other polymers. The intermolecular chaintransfer reaction which follows leads todehydrochlorination20 and HCl gas is evolved. Thiscontains until 90% of the Cl present in the structure hasbeen given o†. The empirical formula of PVC is suchthat, upon heating, the chlorine yield is 56% of the orig-inal sample. Once dehydrochlorination is completePVC yields hydrocarbon gases consistent with a vinylpolymer.

The results of the gas evolution from the pyrolysis ofall polymers are given in Table 5. The Ðgures are rep-resented in terms of weight/weight percentage, i.e. as apercentage of the original sample. It is evident that thepercentage of alkenes, less than or equal to is muchC4 ,greater than alkanes. In some cases the ratio is 3 : 1.Hydrogen is the smallest contributor to the gas stream.

TABLE 4Bond Energies Found in the Polymers Used

Bond Dissociation energy(kcal mol~1)

CwC 83CxC 146CwH 99CwO 86CwCl 81


TABLE 5Total Gases Evolved from the Pyrolysis of Polymers and a Polymer Mixture (Wt/Wt%)

Hydrogen Methane Ethane Ethene Propane Propene Butane Butene CO2 HCl

HDPE 0É12 1É90 2É21 6É08 1É31 4É56 0É22 0É36 1É26 0É00LDPE 0É05 1É14 1É67 4É00 1É33 4É00 0É32 2É00 0É59 0É00PS 0É04 0É53 0É08 0É26 0É02 0É05 0É00 0É06 1É16 0É00PP 0É05 0É93 1É45 3É52 1É00 3É53 0É23 1É29 3É31 0É00PET 0É31 0É71 0É03 1É41 0É13 0É09 0É00 0É00 22É71 0É00PVC 0É12 0É77 0É47 0É15 0É24 0É19 0É11 0É15 0É26 52É93Mixture 0É08 0É97 1É01 1É67 0É70 0É83 0É14 2É20 2É06 2É31

Kaminsky and Sinn,21 in their research of PE and PPpyrolysis, found much higher percentages of gaseoushydrocarbons at temperatures of 740¡C and 830¡C.21However, the trend of more alkenes than alkanes withnegligible amounts of hydrogen was the same as foundhere. Thereafter, there is a tendency for the smalleralkenes to dominate the mixture, with the trend beingethene[ propene[ butene. This is particularly appar-ent with HDPE, LDPE and PP pyrolysis. Taylor22found this trend with both alkanes and alkenes forpolyethylene, polypropylene and tyre pyrolysis. PVCevolved a larger proportion of small compounds thanlarge compounds. However, methane evolution domi-nated the hydrocarbon product mixture. Polystyreneand polyethylene terephthalate both yielded negligibleamounts of hydrocarbon gases although methane andethene were the major contributors in each case. Thegas trends are more clearly seen in Fig. 4.

Since the gaseous fraction is a signiÐcant portion ofthe products it is important to determine whether theresultant gas has any value. Many researchers have sug-

gested that the gas generated from the pyrolysis reac-tion could be reburnt so as to increase the energybalance of the process.12 The caloriÐc value of the gas istherefore considered to be important. North Sea gas hasa gross caloriÐc value of 38É6 MJ kg~1.23 The hydro-carbon gases produced here all have a gross caloriÐcvalue greater than 45 MJ kg~1 23 and so it is clear thatthe gases produced could provide energy to the process.

Elemental analysis (CHN)

Table 6 shows the elemental analysis of the chars pro-duced. The majority element is carbon in all cases. Thisis to be expected, since during depolymerisation theweakest points of the structure break Ðrst resulting inthe production of a more thermally stable product.Carbon to carbon triple and double bonds are amongthe hardest to break (see Table 4) and so it follows thatmainly carbon will remain in the char. Little hydrogenwas present and there was a negligible amount of nitro-

Fig. 4. Comparison of hydrocarbon gas yields from the pyrolysis of each individual plastic and a plastic mixture in a Ðxed bedreactor.


TABLE 6Elemental Analysis of Chars from the Pyrolysis of Polymers

and Polymer Mixture in a Fixed Bed (%)

Pet PV C Mixture

Carbon 84É93 90É15 87É73Hydrogen 2É48 2É55 1É99Nitrogen 0É00 0É15 0É13Ash 5É86 2É91 4É23

gen, indicating no interaction between the sample andthe carrier gas.

If the percentage of oxygen in the chars is taken asbeing the di†erence between the whole char and theresults presented in Table 6 then it is evident that thechar from PET pyrolysis has a signiÐcant amount ofoxygen, that is 6É73%. This may be due to the presenceof oxygen within the original polymer structure. Theamount of ash in the PET char is also quite high, espe-cially when compared with that of PVC.

A theoretical elemental composition for the charobtained from the mixture can be made from the datagained from the individual polymer chars. It wasobserved that the theoretical and experimental resultswere similar. However, the experimental results showless carbon and hydrogen and a notable increase in theamount of char.

Oil analysisÈFourier Transform infra-red spectrometry

The FTIR spectra obtained for the pyrolysis oils fromHDPE, LDPE, PP, PS and PVC are similar due to

their similar structure. Figure 5a shows the spectra ofHDPE, LDPE and PP pyrolysis oils, Fig. 5b shows thespectra of PS, PVC and PET pyrolysis oils and Fig. 5cshows the spectrum obtained for the polymer mixturepyrolysis oil.

HDPE, L DPE and PP pyrolysis oilsThe area of the spectrum between 3000 cm~1 and2800 cm~1 shows the presence of andwCH3 , wCH2CwH groups. The peak preceding these at 3050 cm~1is an indication of the presence of alkenes. HDPE,LDPE and PP all have sharp peaks in these regionswhich dominate their spectra. This illustrates the highlyaliphatic nature of the oils produced. More evidence ofsaturated alkane groups and substituted alkenes appearbetween 1500 and 1400 cm~1. Peaks found at approx-imately 1380 cm~1 and 1320 cm~1 reinforce the identi-Ðcation of molecules with and CwH bondsCwCH3respectively.

PV C pyrolysis oilPolyvinyl chloride, as expected, has strong absorptionpeaks in aliphatic regions of the spectra. It also haspeaks which are between 3500 and 3000 cm~1 showingthe presence of xCwH stretches and, consequently,aromatic compounds, though the intensity of theabsorption is only small. As mentioned earlier almost90% of the chlorine content of PVC is evolved as HCl.So, the presence of absorption bands representative ofCl bonded groups was negligible within the oil. Thepyrolytic oil from PVC is therefore very similar to that

Fig. 5a. FTIR spectra of HDPE, LDPE and PP pyrolysis oils.


Fig. 5b. FTIR spectra of PVC, PS and PET pyrolysis oils.

produced from the pyrolysis of polyethylene and poly-propylene. There does, however, seem to be an increasein the amount of aromatic compounds here.

PS pyrolysis oilPolystyrene di†ers slightly from the other vinyl poly-mers. Again and groups are identiÐedCH3 CH2

between 2950 and 2850 cm~1 showing the presence ofshort and long chains of both alkanes and alkenes. Thepeaks between 1420 cm~1 and 1400 cm~1 show thepresence of alkenes and those at 1460 cm~1 and1380 cm~1 again relate to an alkane structure. In addi-tion three peaks just above 3000 cm~1 are very intense,illustrating the highly aromatic character of the oil orig-

Fig. 5c. FTIR spectra of the pyrolysis oil derived from the plastic mixture.


inating from the benzene ring present in the polymerstructure. Benzene ring substitution patterns also occurin the region between 1950 and 1650 cm~1. Mono-, di-and tri-substituted benzene rings could attribute for theabsorption peaks in the region 780È750 cm~1.

PET pyrolysis oilThe presence of oxygen within the PET structure leadsto the identiÐcation of many di†erent functional groups.There is a very broad stretching around 3400 and3200 cm~1. This could be due to intramolecular OH orhydrogen-bonded OH groups. Thus, there is a likeli-hood that this pyrolytic oil contains some alcohols andpossibly some carboxylic acids. Some aldehyde struc-tures have been noted primarily around the 950È900 cm~1 region. There is also an indication ofaromatic CwH around 3040 cm~1 and 3020 cm~1 inthe polyethylene terephthalate spectra. Again, peaks areindicative of some alkenes being present. The pyrolyticoil from polyethylene terephthalate pyrolysis has thefunctional groups which correspond to the presence ofaldehydes, ketones, carboxylic acids, alcohols and aro-matic compounds possibly substituted with one of thesefunctional groups.

Polymer mixture pyrolysis oilThe polymer dominating this simulated plastic fractionof municipal solid waste is polyethylene. It follows thatthe spectrum of the mixture is extremely similar in itsoutline to that of polyethylene. However, on closerexamination the spectrum is much more complex.

The peak between 3100 and 3000 cm~1 could be dueto the presence of xCwH stretch associated with aro-matic homocyclic compounds. Aromatic compoundscould be expected with the inclusion of polystyrene inthe mixture. Aromatic absorption bands also featureelsewhere in the spectrum. A small peak at 1500 cm~1shows in-plane deformation of CxC. The much largerpeak at between 1460 and 1440 cm~1, which is possiblyan overlap of two peaks, could also infer CxC in-planedeformation. The peaks between approximately 950 and680 cm~1 show aromatic groups. Those, particularly at900, 890 and 780 cm~1 represent CwH out-of-planedeformations which are characteristic of aromaticgroups.

Groups with an oxygen atom are also reasonably dis-tinct. There is broad stretching around 3400 and3200 cm~1 due to intramolecular OH or hydrogen-bonded OH groups. A strong absorption peak at1380 cm~1 can be attributed to CwO stretching andOwH in-plane deformations in tertiary alcohols andphenols. A smaller peak around 1320 cm~1 could bedue to primary and secondary alcohols. CxO stretch-ing vibrations observed at between 1680 cm~1 and1650 cm~1 suggest, therefore, the presence of carboxylicacids in addition to alcohols. Aldehydes and ketonesalso contribute to the oil matrix. Peaks at around

975 cm~1 and 1280 cm~1 show the presence of thesegroups.

In summary, the FTIR spectrum of the polymermixture shows the presence of many of the groups men-tioned in the matrix of each individual polymer pyrol-ysis oil. However, the aromatic groups and someoxygenated groups in particular seem to account for amuch larger amount than can be attributed simply tothe accumulation of the single polymer degradationreactions. It is possible, therefore, that the primary pro-ducts formed initially from each polymer react with dif-ferent products than if only a single polymer were beingpyrolysed. This results in di†erent secondary productsbeing formed. This is not necessarily true for all colli-sions but there is some evidence that interactionbetween primary degradation products do occur.

Oil analysisÈsize exclusion chromatographyIt has been suggested that one possible use of the liquidproducts from plastic pyrolysis is as a chemical feed-stock.12,24,25 If this is to become a reality then the oilmust have sufficient properties to be substituted intoexisting petrochemical processes.

BP Chemicals found that the wax produced from thepyrolysis of plastics can be mixed with naphtha at up to20%.8 This can then be fed to a steam cracker withoutsigniÐcantly altering the process. Naphtha is a cut ofcrude oil and has a boiling range between 120¡C and180¡C. It consists of a complex mixture of hydrocarbonsand could have a similar composition to the waxes pro-duced in this research. One important characteristic offeedstocks used in petrochemical processes, such assteam cracking and catalytic cracking, is the molecularweight distribution and the boiling point range. It wastherefore thought important to look at the molecularweight characteristics of the pyrolysis oils to seewhether they are comparable with existing petrochemi-cal feedstocks.

Size exclusion chromatography was used to deter-mine the molecular weight distribution of each pyrolysisoil and wax sample. The results of SEC are presentedboth as RI and UV. n-Alkanes are not absorbed in UVlight and so the use of the two detectors can yield valuesfor both aliphatics and aromatics in the oil. Analysis ofthe oils and waxes produced from both the individualpolymers and the polymer mixture gave a weight dis-tribution between a molecular weight value of 64 and2000 daltons (Da). However the vinyl polymers had thelargest range due to the random nature of the depoly-merisation reaction.

Table 7 shows the size exclusion data for the pyrol-ysis oils. The polydispersity gives an indication of thenumber of di†ering sized components in the sample. So,if the sample was a pure compound then its polydisper-sity is equal to one. The polydispersity values for PSand PET are 1É21 and 1É09 respectively, whereas for the


TABLE 7SEC Data for Pyrolysis Oils from Polymers and a Polymer Mixture in the Fixed Bed Reactor

Polymer

HDPE L DPE PS PP PET PV C Mixture

UV detectorNumber average MW 178 208 116 280 210 230 136Weight average MW 281 250 147 439 276 384 209Polydispersity 1É58 1É21 1É27 1É57 1É32 1É67 1É53

RI detectorNumber average MW 240 209 145 241 195 251 125Weight average MW 360 306 176 403 212 334 165Polydispersity 1É50 1É46 1É21 1É67 1É09 1É28 1É32

other pyrolysis oils the Ðgure is much larger, rangingbetween 1É67 and 1É28.

The size exclusion chromatography graphs have beenplotted with weight percentage against a log molecularweight. A log scale enables examination of the data forlow molecular weight compounds which would other-wise be restricted on a linear scale.22 Figures 6a, 6b and6c show the molecular weight of pyrolysis oils frompolystyrene, polyethylene and the plastic mixturerespectively. For most of the polymers there were tworegions of di†ering molecular weights within eachsample. The Ðgures from the refractive index detector,illustrating the oil/wax as a whole, show a peak betweena log molecular weight value of 1É8 and 2É7 and then asecond peak in the range 2É7È3É4 for all polymers exceptPVC and PET. Here only one molecular weight rangewas observed.

For LDPE, HDPE, PP and PS the Ðrst peak rep-resenting smaller molecular weight compounds is larger

Fig. 6a. Molecular weight and boiling point distribution ofLDPE pyrolysis oils.

than that showing heavier molecules. The amount of theLDPE wax with a molecular weight less than log 2É7 or500 Da is 87%. For HDPE, PP and PS this Ðgure is75%, 70% and 62% respectively. Although only 62% ofthe polystyrene pyrolysis oil had a molecular weight of500 Da or less the maximum molecular weight of acomponent was found to be only 907 Da. Polystyrene isknown to degrade with an unzipping reaction to formmainly its monomer.26 So a small Ðgure for the molecu-lar weight range is expected. When presented as boilingpoints, (Figs 6a, 6b and 6c) the results indicate that atleast 70% of the waxes and 62% of the polystyrene oilhave a boiling range between approximately 46¡C and379¡C. The modal boiling point for LDPE, HDPE andPP is 270¡C. For PS this is greatly reduced to only126¡C.

The results for the pyrolysis oils from PVC and PETare very similar to the others in that their log molecularweight is between 1É8 and approximately 3É4. However,

Fig. 6b. Molecular weight and boiling point distribution ofPS pyrolysis oils.


Fig. 6c. Molecular weight and boiling point distribution ofpyrolysis oils derived from a plastic mixture.

only one range is observed. Again the modal boilingpoint for the oils is 270¡C.

The pyrolysis oil from the plastic mixture has amolecular weight range similar to the wax from poly-ethylene pyrolysis. There are two molecular weightranges, with 73% of the oil having a molecular weightless than log 2É4 or 250¡C. The modal boiling point (seeFig. 6c) is much less than that found for either high orlow density polyethylene, but is more in agreement withthat found for a polystyrene oil. Therefore, even thoughpolyethylene was the major component of the originalsample mixture, the characteristics of the oil do notmatch so that the presence of several polymers togetherresults in smaller products being formed upon heating.

The resultant oil from the mixture seems at Ðrstglance to be better placed than either HDPE, LDPE orPP to be used as a feed for a steam cracker. The boilingrange is quite low, around that of naphtha itself.Analysis of the oil has shown that a signiÐcant propor-tion of the oil product has an aromatic nature. If it is tobe used alongside naphtha, then more detailed investi-gations of its performance in such processes will have tobe carried out. Work done so far shows promisingresults.

CONCLUSIONS

The quantity of the oil fraction produced from four ofthe six thermoplastics and the polymer mixture isapproximately equal to, or greater than, 80%. This highyield is clearly beneÐcial to any process utilising the oilfraction. In addition, the gaseous fraction consists ofhigh heating value hydrocarbons which have the poten-tial to be used as a fuel.

Analysis of the pyrolysis oils has shown that the com-position of those from HDPE, LDPE, PP, PVC and thepolymer mixture is mainly aliphatic. PS has a morecomplex composition with mainly aromatic com-ponents and PET yields mainly oxygenated and aro-matic compounds. All have the potential for chemicalfeedstock production.

The molecular weight of each pyrolysis oil is between64 and 2000 Da. For the majority of the samples eachhas a modal boiling point of 270¡C. When mixedtogether the boiling point range was reduced giving apeak at only 126¡C. This is within the boiling pointrange of the naphtha feed used in a steam cracker forethylene production and so pyrolysis oil from mixedplastic could possibly be substituted into this chemicalprocess.

The pyrolysis of individual polymers and theirmixture give three products which all have the potentialto be used as fuels or chemical feedstocks. Moreresearch into optimum reaction conditions for the for-mation of the most useful products will prove invalu-able for future waste utilisation e†orts.

ACKNOWLEDGEMENTS

The authors would like to thank the Department ofTrade and Industry via the Energy Technology SupportUnit, Harwell, Oxfordshire for support for this work(Grant number B/T1/00397/00/00) and also, BP Chemi-cals for the supply of polymer samples.

REFERENCES

1. APME, Plastics Recovery. In Perspective : Plastics Con-sumption and Recovery in W estern Europe 1989È1991,Association of Plastics Manufacturers in Europe, Brussels,1993.

2. Stein, R. S., Polymer recycling : thermodynamics and eco-nomics. In Plastics, Rubber and Paper RecyclingÈA Prag-matic Approach, ACS Symposium Series 609, eds C. P.Rader et al. 1995.

3. Rader, C. P. & Stockel, R. F., Polymer recycling : an over-view. In Plastics, Rubber and Paper RecyclingÈA Pragma-tic Approach, ACS Symposium Series 609, eds C. P. Raderet al. 1995.

4. Mackey, G., A review of advances recycling technology. InPlastics, Rubber and Paper RecyclingÈA PragmaticApproach, ACS Symposium Series 609, eds C. P. Rader etal. 1995.

5. Rowatt, R. J., The plastic waste problem. Chemtech,January (1993), pp. 23È4.

6. Olgivie, S. M., Achieving the government target of 25%recycling of household waste by 2000 : some technicalchoices. Presented to institute of local government studiesseminar on recycling : opportunities for local authorities,4th March, 1991.

7. Making W aste W ork, A Strategy for Sustainable W asteManagement in England and W ales. HMSO, London,1995.


8. Lee, M., Feedstock recycling : new plastics for old. Chem-istry in Britain, July (1995), pp. 515È6.

9. Majors in polymer recycling link-up. Plastics and RubberW eekly, 20th November, 1993 p. 2.

10. Fuji Recycle Industry K. K., Useful oil reclaimed fromwaste plastics (polyoleÐn). Japan Chemical W eek, May 31,1990, p. 44.

11. Kaminsky, W., Thermal recycling of polymers. Journal ofAnalytical and Applied Pyrolysis, 8 (1995) 439È48.

12. Pinto, F., Gulyurtlu, I., Goncalves, M. & Cabrita, I., Anew perspective for plastics recyclingÈthermolysis ofplastics in an inert atmosphere. Recycling Ï95 Congress,February 1È3, 1995, Geneva.

13. Roy, C., Vacuum pyrolysis of automobile shredderresidue. I.G.T Conference : Energy from Biomass andW aste XV I, March, 1992.

14. Garcia, A., Font, R. & Marcilla, A., Kinetic studies of theprimary pyrolysis of MSW in a pyroprobe 1000. Journalof Analytical and Applied Pyrolysis, 23 (1993) 99È119.

15. Prospects for Plastics, Shell BrieÐng Service, LeaÑet No. 1,1993.

16. Wenning, H. P., The VEBA OEL technologie pyrolysisprocess. Journal of Analytical and Applied Pyrolysis, 25(1993) 301È10.

17. Wampler, T. P., Thermometric behaviour of polyoleÐns,Journal of Analytical and Applied Pyrolysis, 5 (1989)187È95.

18. Shalaby, S. W., Thermoplastic polymers. In T hermalCharacterisation of Polymeric Materials, eds E. A. Turin.Academic Press, New York, 1981, pp. 237È64.

19. Driver, W. E., Plastics, Chemistry and T echnology. VanNostrand Reinhold Co.

20. Lattimer, R. P. & Kroenke, W. J., The formation of vola-tile pyrolzates from PVC. Journal of Applied PolymerScience, 25 (1980) 101È110.

21. Kaminsky, W. & Sinn, H., Pyrolysis of plastic waste andscrap tyres using a Ñuidised bed process. In ACS Sympo-sium 130: T hermal Conversion of Solid W astes andBiomass, eds J. L. Jones & S. B. Radding. AmericanChemical Society, Washington, DC, 1980.

22. Taylor, D., Analysis of products from the pyrolysis ofwastes. PhD thesis, University of Leeds, 1993.

23. Osborn, P. D., Handbook of Energy Data and Calculations.Butterworths, London, 1985.

24. Tesoro, G. C., Recycling of synthetic polymers for energyconservationÈthe state of the art. Polymer News, 12(1987) 265È8.

25. Kaminsky, W., Petrochemicals by thermolysis of plastics.In T hermolysis, A T echnology for Recycling and Depollu-tion, International Meeting ISSep, 24È25 March, 1994,Wallonia, Session III, pp. 139È60.

26. Williams, P. T., Horne, P. A. & Taylor, D. T., Polycyclicaromatic hydrocarbons in polystyrene derived oil. Journalof Analytical and Applied Pyrolysis, 25 (1993) 325È34.