Journal of Analpical and Applied F’yrolysis

ELSEWER 40-41 (1997) 347-363

Analysis of products derived from the fast pyrolysis of plastic waste

Elizabeth A. Williams *, Paul T. Williams

Plastic Waste has become an increased problem in many industrialiscd countries. Concern over the vol~~mes king created has lead to a numlzcr of govermnen ts introducing new legislation for its recovery as a rcsoutrr. Traditional recovq methods have gem concentrated on recycling of materials or Mncration for energy. Tk majority of poIynws in waste are mainly hydrocarbon in nature and so can bc used as a feedstock for the chemical industries or as a fuel. Pyrolysis is a tertiary rccycIi.ng process and has an ability to provide three end products: a gas, an oil and a char which all havt the potential to lx further utilised. It is anticipated that if waste recovery targets currently being set are to h met then methods. such as pyrolysis, will have to be cioseIy examined. The py~111ysis of a plastic mixture in a fluid&d bed reactor was studied. The mixture was a simulated fraction of that found in municipal solid waste in Europe. The reactor was constructed of stainkss steel and was 70 cm high x 10 cm diameter, with a 17 cm high x 20 cm diameter expander s&on. The sample was introduced batchwist into the bed. The iniluence of temperature on product yield and composition was studied. Pyrolysis was carried out at temperatures bctwat 500 and 7OO*C. This gave widely diRering product yields of bctwcen 9.79 and 88.76% gas and between 18.44 and 57.1 f % oil. The oils were analysed for their functional groups using Fourier transform infrared spcctrosuopy. The molccuIar weight distribution was also dctcr- mined using size exclusion chromatography. It was found that as temperature was &reascd the amount of aromatic compounds in the oil increased. The molecular weight range was also affected. Potential applicatioas for the oils was also investigated. Q 1997 EIscvier Science B.V.

Keywords: Pyrolysis: Plastic waste; Analysis

??Compnding author. Tel: + 44 I13 2332504, fax: + 44 I13 2440572.

Oh%-2370/97/$17.00 0 1997 Ekvier Sckna B.V. All rights rmrvcd. PII SOl65-2370(97)00048-X

348 E. A. Williun~s. P. T. Wiiliunu 1 J. Anal. Appl. Pp!rsix 40-41 (I 997) 347. 363

1. Introduction

Polymers are becoming a necessity in modern life in many Western countries. Their discovery and subsequent utilisation meant that by the mid 1960s total world consumption of thermoplastics alone was I3 million tonnes [I]. With an increasing number of applications being found for these materials, today the demand for the main commodity thermoplastics is more than 70 million tonnes [I]. Although a significant amount of the thermoplastics are utilised in products with a long life span, the majority are used in short term applications such as packaging. Thus, the quantity of thermoplastics found in waste is increasing correspondingly.

Plastic waste can originate from a multitude of sources. The major areas of waste creation are from the distribution industries representing 21.7% of all plastic waste 121 and municipal solid waste (MSW) which accounts for 60.4O/o [2]. Municipal solid waste is being targeted as an issue for improvement. However, the nature of MSW is very complex, making its treatment difficult. Other forms of waste are easier to handle, because of their uniformity and can be treated as they are created by, for example. feeding back into the production process.

Currently 80% of MSW goes to landfifl, 10% is recycled and 105’0 is incinerated in the USA [3]. However, landfill is becoming an iucreasingly expensive option. Not only are suitable sites less available close to the point of waste generation. leading to a rise in transportation costs, but costs of disposal are set to become even more inflated. A landfill tax of E7 per tonne has been introduced by the UK Government which becomes effective this year [4]. It is hoped that this will reduce the amount of waste going to landfill and in conjunction with this aim, the Government in the UK has set targets for waste recovery. The aim is to recycle 25% of all household waste by the year 2ooO [5] and to reclaim for energy recovery 40% of waste by 2005 [41.

Treatment of homogeneous materials available in large quantities such as paper, glass and metal has been shown to be viable via materials recycling. These can also be separated from MSW for material recycling. The plastic fraction of MSW found in Europe represents approximately 7% by weight. In Germany 80% of all packag- ing waste, including plastics, must be separated from other waste [I ] and a% is to be recycled as material. However, viable markets for certain recycled plastics are only available at rates of IO-15% plastic recycling [6] due to its inferior quality when compared to virgin polymers.

So, if enough industrial and domestic waste is to be utilised so as to meet new recycling targets then not only will contaminated and mixed plastics have to be dealt with but also alternatives to landfill and materials recycling will have to be found.

Incineration currently handles approximately 10% of waste produced in Europe [3] to give energy via electricity generation. district heating or combined heat and power schemes. However, incineration units emit dioxins, furans, acids gases and heavy metals which can cause damage to the environment and to health. Limits of emissions have been set so as to reduce any hazardous compounds. However this means new clean-up systems will have to be incorporated into many existing plants. dramatically increasing their operating costs,

&.A. lVit/ianu. P.T. Williums jJ. And. Appl. Fydyk 40-41 (1997) 347-363 w9

Pyrolysis is an alternative process to incineration and materials recycling. This approach of hydrocarbon processing has been investigated previously [7j but only recently has this interest increased [S]. If applications for the products formed are found, then perhaps it coutd IX a potential route for chemical or fuel production.

To study this potential a mixture of plastics was pyrolysed using a sand Auidising bed to produce gases, oils and chars which were collected and subsequently analysed.

2. ExperimeIltal

The plastic mixture used in this study was a simulated fraction of that found in MSW throughout Europe. The actual composition of the polymers used can be seen in Table 1. The polymers were virgin pellets of 2-5 mm in diameter, which were obtained from BP Chemicals in Grangemouth, UK. A small particle size was chosen to reduce the effect of heat transfer.

The pyrolysis was carried out using a Buidised bed reactor. A schematic diagram of the reactor can be seen in Fig. I. The sample was fed in batches via a gate valve. In total three batches each of 3 g in weight were dropped in at each temperature. The reactor was stainless steel with a 10 cm diameter and approximately 70 cm total height. An expander section at the top of the reactor enabled the velocity of the carrier gas and pyrolysis vapours to be reduced before they entered into the cyclone. Silica sand was used as the bed material and this was 10 cm high at rest. Nitrogen was used as the fluidising gas which was set at 12 l min- ’ from the cylinder at room temperature. This provided enough fluidising gas to em ble the bed to be operated at 3 tunes the minimum fluidising velocity (Umf), which had an average value of 2.4 cm s--I at the temperatures studied. This gave good mixing of

Table 1 Camposition of polymer sample used in the study

Polymw Pcrccntage (Yu)

LDPE 31.25 HDPE 31.25 PP 7.29 Ps 13.50 PVC Il.44 PET 5.21

,

I: _ _ :f’ Fig. I. khcmalic diagram of !luidised bed rcxtor. (I ) Control Pund. (2) Rotmeicr, (3) Fccdcr. (4) Main Rci~cior, (5) Ring Furnace. (6) Sund Bed, (7) Cyclone, (8) Cutchyol, (9) Knirtmesh, (10) Mclut Curchpol. 11 I) Glass Condcnxrs. (12) ColdTrups. (13) Gas Sampling Bny, (141 Ciluss Wool

the sand. The residence time of gases in the reactor was 53 s at room temperature and approximately I5 s at the range of operating temperatures of the reactor.

Heat was supplied to the unit via nitrogen preheated to a temperature of 450°C. The reactor was also externally heated using an electric ring furnace. The tempera- ture in the reactor was varied between 500 and 700°C, Five thermocouples were used to give a temperature profile of the entire height of the reactor. These were positioned evenly along the reactor section with the tip being exposed to the sand or gas stream. After the pyrolysis vapours and fluidising gas had passed through the cyclone to remove any particulate matter, all vapours were passed through a heated metal line filled with an icdustrial de-mister. This had the effect of knocking heavy particles out of the gas stream where they were then collected in a metal catchpot. The remaining lighter components were condensed using four glass condensers in dry ice and acetone traps. A vessel containing glass wool was put in-line after the condensers to catch any oil particles which had failed to be retrieved. In addition a dreschel bottle of de-iocised water was placed in-line, so all the gas could be bubbled through with &he aim of dissolving any HCI produced by PVC degrada- tion. The HCI in water samples were analysed using a Coming pH/ion meter !35 with a chloride electrode calibrated between 100 and 1000 ppm.

The fluid&d bed used m this study, and described above. was similar in design and operation to reactors used by other researchers. Kaminsky [9] used a quartz sand bed heated !o between 600 and 9tWC to pyrolyse samples of plastic waste and tyres. Conesa et al. [IO] used a similar reactor, this being 6.9 cm in diameter and 43.2 cm in height. Scott et al. [7j also had a comparable fluid&d bed to the one used here, though the residence time was only 0.5 s.

The entire product gas was collected in a 100 I Teflon bag. The samphng therefore continued for approximately g min to ensure all gases had been collected. This was then analysed off-line using three packed column gas chromatographs. Hydrocarbons were analysed using a GCD chromatograph with a 2.2 m x 6 mm glass column packed with 80-100 mesh n-octane Porasil C. Permanent gases wcrc identified on a Pye 204 chromatograph with a 1.8 m x 6 mm stainless steel cohunn packed with molecular sieve 5 A. Carbon dioxide analysis was carried out on a Gow Mac chromatagraph with a 1.8 m x 6 mm stainless steel column packed with 100-120 mesh silica gel. A flame ionisaiion detector identified the hydrocarbons and thermal conductivity detectors were uscd to determine the other gases. Data manipulation was carried out using a Harley Peakmaster data analysis system. Three samples were injected into each gas chromatograph, The gases identified were permanent gases, CO and CO,, and hydrocarbons up to C,. The total weight of gas produced was calculated by comkariwn to the known nitrogen flow rate and a mass balance was obtained.

2.4. Elemental umlpis

During pyrolysis a char was formed which remained in tht and btd. It was thepefore only retrieved after the reactor had cooled to room temperature and could be dismantled. Chars were analysed on a Perkin-Elmer model 240C elemental analyzer. The chars tended to form a fused mass with a portion of the sand bed. These were then ground and sieved so that a uniform sample was achieved. Each sample was analysed twice.

2.5. Fourier trunsform ittfrtweti specrroscopy (FTIR)

Functional group analysis of each pyrolytic oil and wax was carried out using Fourier Transform infra-red spectroscopy. A Perkin-Elmer 1750 spectrometer was used which had data processing and spectral library search facilities. A small amount of the liquid fraction derived from the polymer pyrolysis was mountec’ on a potassium bromide (KBr) disc which had been previously scanned as a back- ground, The infra-red spectra of the sample was then taken. The multing spectra were normalised to the C-H peak around 3000 cm - I_ Direct comparisons of peak intensity were then taken,

2.6. Size exclusion chronaatogruphy {SEC]

Size exclusion chromatography, which is aIso known as gel-permeation chro- matography, is a non-destructive method which gives information on the molecular weight distribution of the sample. The equipment used was similar t? that of KPLC. The cohunas used were two 4.6 K 150 mm 5 pm RPSEC IO0 A type in series. This method differs from HPLC in that here the elution order is in reverse

352 E.A. Willims, P, T. Wihms /J, Arral, Appl. qvmlysis 4U-4J (1997) 347-363

molecular size. That is, the targest molecule is eluted firs:. Two detectors were used, these being a refractive index (RI) detector and an ultra-violet (UV) detector. The RI detector gave information on the molecular mass of the whole oil and the UV detector gave an indication of the sample’s aromaticity. The analytical set-up was linked to a chart recorder in addition to a computer with data processing facilities. A small amount of the oils and waxes were dissolved in 10 ml of tetrahydrofuran (THF). This solvent was also the eluent through the system.

3. Results and discussion

3. I. Praducr yield

Table 2 shows the product yield from the pyrolysis of the plastic mixture. It is evident i.rat as the temperature is increased the percentage of gas increases. Burton et al, [I 1) stated that high temperature-high heating rate environments favour increased gas formation as the molecules breakdown and form a wide range of smaller organic molecules. In addition, with the higher amount of energy available at the higher temperature there is a tendency for an increased number of secondary reactions. The amount of oil and wax decreases with an increase in temperature, Thus, it is reasonable to suggest that the cracking down of the liquid products formed initially is the cause of increased gas evolution at high temperatures. Scott et al. [7] also showed in their work that at temperatures lower than 700°C the major part of thh product was solid. Whereas at higher temperatures the main product was a gas [7]. Research by Conesa [IO] again noted this observation.

Two liquid fractions were collected. The first was trapped by the industrial de-mister in the metal condensers (No. 10. Fig. I) and the second in the glass condensers (No. 12, Fig. I). At pyrolysis temperatures of between SO0 and 600°C a wax was found in both traps. !-Iowever, above 600°C an oil was found only in the glass condensers. The fact that no heavy wax was produced at the higher tcmpera- tures would also suggest that the increase in energy was resulting in these waxes being broken up to form either lighter waxes or gaseous products.

Product yields from the pyrolysis of plastic mixtures with change in temperature

Product ‘Tempcmture of finidised bed reactor IT)

500 550 600 650 700

Gas 9.79 24 52 43.33 88.76 68.81 Wax 17 28 18.5’> 8.72 000 o.lH Oil 37.79 38.55 34 44 20.49 18.44 Char 2.82 5.87 7.59 -a -4

HCI 1.76 I.42 I .43 0.80 0.51

” Unavailable.

E.A. Wiliiams. P.T. Wilhhs/J. Anal. Appl. P_wdys~ IO--U IlW7) 37-363 353

Fig. 2. Cwmpuriron of gaseous mixrum produced from rhe pyrolysis of a pl;lsric mixlurc in a Ruidixd bed reactor with change in !cmperarurr.

3.2. Gas analysis

As has been mentioned the affect of temperature on the quantity of gases evolvPed was quite significant. At the lowest temperature 4.79% of gas was produced with this figure more than quadrupling to 88.76% at 65OT and then decreasing marginally to 68.81% at 700°C. Coma et al. [lo] produced similar results during their pyrolysis of polyethylene in a fluid&d bed. They found that the gas yield represented only 5.7% of the total products collected at XM’C whereas at 800°C the gases represented 96.5% [IO]. The majority of the gases collected here were hydrocarbon in nature with a smaller amount oi carbon dioxide. No carl~n monoxide was detected during pyrolysis. The gas chromatograph was accurate down to approximately 0.1% of the total gas analyzed. It can therefore be assumed that if carbon monoxide was present the quantity was less than 0.1%.

Examination of the actual change in composition with temperature found that a significant proportion of the gaseous mixture remained remarkably similar. Fig. 2 shows the contribution, by weight percentage, of each gas in the gas stream. The gases hydrogen and butane are those with the lowest conc~~~tration and are hardly detectabk on the chart. Methane and ethane fluctuated between 0 and 1% of the original weight of polymer, remaining essentially constant regardless of tempera- ture. Therefore, the pyrolysis temperature has little effect on the concentration of these gases.

The rest of the gases were, however, influenced by temperarun?. At the lower pyrolysis temperature of 5OO’C the major gaseous species was propenc, with smaller, though significant quantities of ethene. As the temperature was increased

354 E.A. Wil1imz.c. P. T. Wihrnr ,‘J. Ad. Appi. Pyrolpiris JO -41 (1997) 347-363

the amount of propene decreased with a corresponding increase in ethene forma- tion. The reduction in the amount of propene has also been highlighted by Kaminsky [I 21 and Conesa et al [I OJ. The overall order, in increasing concentration, was butane, propane, ethane, methane, bu!ene, propene and ethene.

Thus, the trend of gases produced shows that there is a tendency for the smaller gases in any series to be formed in the majority. That is, there is more ethene than propene and in turn more of this than butenc. The same is true of the alkant gas distribution. The reason for this is that smaller gaseous molecules could be more stable than larger ones. Also the probability of a smaller molecule forming during random s&ion is greater than the probability of large gaseous species forming. In addition to this, as the temperature is increased and the energy in the system is greater then this series of formation is emphasised with the smaller hydrocarbons increasing their contribution.

Looking at the hydrogen chloride (HCI) gas evolved it is evident that the volume decreases with increased temperature. This may be because at the elevated temper- atures there are an increased number of radicals available to react with the free HCI from the first step of PVC degradation. At the lower temperatures fewer reactive species are awilable to combine with the HCI and so it is able to escape from the system as HCi gas. The amount of HCI actually determined was smaller than that anticipated. The monomer unit of PVC, which can be seen in Fig. 3. shows that the chlorine molecule contributes more than 56% by weight of the whole polymer, and 6.4% of the samnle pyrolysed. Here, only between 1.76 and 0.51% was trapped using the de-ioni%; v.ater, which represents less than one third of that potentially available. Thus, either the high flow rate of gas was such that insuffi- cient time was given for complete dissolution IO occur and part of the HCI formed was atrowed to escape. Alternatively, chloro-organic compounds could be present in the derived pyrolysis oil.

The char produced during pyrolysis was found in the sand bed amalgamated with sand. This mixture was heated to &KKK in a furnace to quantify the weight of char. At 500°C 2.82% of the original sample produced char, whereas at the higher temperature of 600’C the amount collected was 7.59?/b illustrating an increase in the char yield. This trend was also obsetvecl by Williams et al. [13].

The results of the elemental analysis are shown in Table 3. The ash values were extremely high. However, a large proportion of this was thought to be sand which remained unseparated from the char during analysis. To be suitable as a fuel then chars should be low in ash. The values of ash seemed to decrease with an increase in temperature. However. due to the presence of sand this effect cannot be confirmed. An efficient method for separating the sand from ;he char should be dtveloped enabling any trend to be established.

E.A. Wilhms, P.T. WilCwns /J. Anal. Appl. P~rol+s M-41 (JS7) 347-363 355

3.4. Oil analysis

3.4. I. I;TIR The FTIR spectra taken of all the pyrolytic oils produced between 500 and 700°C

are shown in Fig. 4. It is evident that temperature increase has a significant effect on the resuttant oil. The spectra of the !XWC pyrolytic oil is far less complex than that obtained at 7OO”C, indicating a greater variety of compound types present at higher temperatures.

The initial sample pyrolysed was mainly hydrocarbon in nature, with the exception of PET. Thcrrfore, the compounds expected are also hydrocarbons. The peaks bctwccn 3000 and 2800 cm-’ and again bctwccn 1350 and 1450 cm-’ represent C-H deformations and show the presence of -CH,, CH, and C-H groups which highlight the aliphatic nature of the oils. HDPE, LDPE and PP represent more than 60% of the initial sample. This dtrmir?stion in each of the spectra by these absorbar&s is therefore expected because oi the random depoly-

Pd~~lCaTarpblhrlrrO

+c+; i c-o-c-H*-m-0 3 a

Fig. 3. Structures of rcpcatiag units of the thermoplastics used.

Table 3 Elemental analysis of chars obtained [torn the pyrolysis of a plastic mixture in a fluidised bed reactor with change in temperurure

Temperature of Ouidised bed reactor (“C)

500 550 600 650 700

Nitro,gen Carbon Hydrogen Ash

0.05 (1 0 02 0.32 0.77 1.73 -L _. 2.35 13.49 34.53 KU7 _- *L 0.08 Q.fO O.SI

96.92 77.86 81.20 53.81

U Unavailuble.

merisation process which takes place during HDPE, LDPE and PP pyrolysis resulting in an array of smaller oligomer compounds. Alkenes are also shown to be in the oil. Peaks between 1575 and 1675 cm- ’ as wet1 as those between 875 and 950 cm - ’ indicate C-C stretches. At the higher pyrolysis temperatures the stretches around I650 c,m.-’ appear at a lower wavenumber than those present in the low temperature pyrolysis oils. This suggests a move from monosubstituted groups to dienes [14].

The peaks at 2850 (CH,) and 2950 cm- ’ (CH,) change with increasing tempera- ture. At SOO*C they are quite sharp and defined whereas at 650 and 7OO’C they

Fig. 4. FUR speara of plustic mixlure pyrolysis oils from the Auidised bed with change in remperature.

E.A. WiJJJums, P.T. Williams /.I. Awl. Appl. PyroJpir 40-41 (1997) 347-363 357

Fig. 5. Changes in intec;ity of absorbuncy pnks fra*? the FTIR spectra of oils prdud kom Iht: pyrolysis of a plarric mixture in a fluidiscd bed rcaclor.

become less separated. This could be due to the increase in unsaturated compounds in the system which will tcad to broadening of CHJ and CH, absorption bands. This broadening is therefore indicative of au increase in mndary reactions resulting in an increased amount of compounds with alkenc end groups,

Polystyrene (PS) represents 13.5% and polyethykne terephlhalatc (PET) 5.21% of the original sample. Both their structures contain a benzene ring and so the decomposition products are likely to be aromatic in nature. Looking at the XKPC spectra there is some evidence of aromatic functional groups. Two peaks in the region 7F?D-750 cm - ’ could mean the presence of mono, di and tri substituted benzeao rings. The absorbance values of these groups at 780-750 cm - ’ are apprximately 10% at !iWC, whereas at XWC the absorb-attcy incream dramati- cally to more than 1 W/o. As eack spectra is ~ormlkd to the 2850 cm - ’ peak then this increase represents it higher concentration of these aromatic groups at higher temperatures.

The peak at 910 cm -I represents C-H ouMf-plane deformations which are characteristic of monosubstitutcd alkcncs. As the substitution imxeaxs, so the wxenumber can shift higher or lower, Fig. 5 shows that the baud at 910 cm-’ does not follow a constant rise with tcmperatuhe increase. The intensity of the absorbance peaks at 550°C and then falls above this temperature. The reason for this is the change in compound types at these hi&r temperatures. An increase in substitution and the formation of dienes would result in these spectral change Gent formation was noted earlier due to changes in the C=C stretches and so would seem to support this theory.

358 E.A. Wihmr, P. T. Williumr /J. Anal. Appt. P,wuljxir 40 -41 (1997) 347-363

This reduction in the 910 cm-’ band is coupled with an increase in the CH out-of-plane bending between 690 and 840 cm -. ’ which is representative of uro- matic rings. Thus the FTIK spectra illustrates the move from mainly alkanes to alkenes, dienes and ultimately aromatic compounds with an increase in pyrolysis temperature.

Peaks which are between 3500 and 3CHM cm - ’ also show the presence of =C- H stretches and consequently alkene groups in addition to aromatic groups if attached to a ring. possibly mono or poly substituted benzene compounds. The intensity of these is hardly deteclable in the spectra of the 500°C wax. However, al 700°C their contribution to the sample matrix is quite significant. Fig. 5 illustrates this increase.

Polyethylene terephthalate (PET) contains an oxygen in its structure, The monomer unit (Fig. 3) shows that 33% of the molecular weight represents oxygen. This translates to 1.74% of the initial sample being representative of oxygen. The gas analysis showed that at 500°C 1.65% and at 7WC 1.93% was evolved as CO, which leaves a minute amount in the oil fraction. In general, stretches around 1695 cm - ’ show the presence of oxygenated compounds. Absorbances of around O,I were found in atl spectra. though there was no distinct trend with change in temperature.

The chlorine content of PVC is almost 57% (Fig. 3). It has been documented [ 151 that up to 90% of this is evolved as HCI during pyrolysis. Therefore negligible amounts of orgdno-chloride compounds were expected. These generally occur within the absorbance bands 625-675 cm - I. No absorbance bunds associated with these groups were identified.

Both the light and heavy liquid fractions were analysed using size exclusion chromatography to determine the molecular weight of the oils and waxes. All samples were made up to a 1% solution in tetrahydrofuran (THF). Of this, 5 ~1 was injected onto the column.

The molecular weight distribution of the samples collected using the incldstrial demister are shown in Fig. 6(z) and (b), whereas Fig. 7(a) and (b) show the lighter fractions. The range shown by the refractive index detector is indicative of the whole oil, since RI is capable of detectin, ,. n 11: hydrocarbons. Aliphatic compounds are not detectable in the UV raugc and so only aromatic compounds were able to be detected using the ultra-violet detector.

It is clear that in Fig. 6(a) the heavy fractions do indeed reveal a high molecular weight distribution. No sample is present with a molecular weight in the region between log I.5 and 2, that is, with a weight less than 100 Do. This confirms that the &mister used in the pyrolysis system is capable of collecting high molecular weight compounds which may normally pass through the system. The maximum weight in lhis range had a log vaIue of more than 3.5 at all temperatures. It is also evident that two molecular weight ranges are present in the sample matrix. That is, more than and less than log 2.6 (400 Da), with the peaks at a molecular weight of 2.3 and 2.7. SEC data can also yield information concerning the boiling point of these waxes. Here, the two peaks in the distribution gave boiling points of 300 and

E.A. Wllium. P.T. Willimm/J. And. Appt. Pyrolysis 40-41 (1#7) 347-M 359

7WC The atmospheric residue cut of crude oil has a boiling range greater than 350°C and the wax produced here appears comparable in this characteristic.

The effect which temprature had on this distribution of products is quite distinct. At pyrolysis temperatures of 500 and SWC the largest percentage of the wax had a molecular weight higher than 400 Da. Whereas at 600°C the opposite is

IS 2 2s 3 5.5 I

~_L%+!F!E~_... -- _ ..- --___-._--__-. ._.

+5oaT -=-rwc +mFc --..... - _..... --._.-_.. _ .._ ___-_-_L

111

I? 2 2.5 3 3.5 .

Lq-Wig -- -._-..-- ------ ---__

;b) tswc +5m +6lax

Fig. 6. (a) Comparison of molecular wei@ distribution of heavy waxes pro&toed from Ux pyrol@s of a plastic mixture in II fluidiscd LUG reactor with cbatqc in tcmpra~urc-Rl d~cctor. (b) Ccmparkm of mol-xular weight distribution of heavy waxes produced fmm the pyrolysis of a plastic rli..ture in a fluid&d bed nactor with change in temperature-LJV dctcctor.

360 E.A. Williar,ts, P.T, Wifliun~s /J. Anal. Appl. Pwo~vsis 40-41 (1997) 347-363

. . . . _ - ., - - _ , I5

“+-5cwC 4-55GT -4-6aPC *650oc +mT _ ,, . _ __ _ _ .._.

14

r-r

I I5 2 25 3 35 4

Log Vddsulu Wci$r ,,. ._.. - .._.. __ _-- _....

+5cwc -s-P.oT &6woc -)- 65WC -=-?wc __~~~~_. .__ ~ -.-- ._... .---.-- (b)

FIN. 7. (a) Comparison ur molecular weight distribution of oils producd from ihe pyrolysis of a plastic mixtur.7 in a fluid&d bed reactor with change in temperature-LV detector. (b] Comparison of moleculcr weight distribution of oils produced from the pyrolysis of a plastic mixture in a fluid&d bed reactor with change in temperature-RI detector.

true. Here 55% of the sample has a moiecular weight less than 400 Da. Thus, at higher temperatures, the peak boiling point was shifted to a value around 3WC. The explanation for this may be that due to an increase in temperature some of the heavy molecules break up to Form much lighter compounds. It is thought that the major component of these heavy waxes is long chained aliphatic compounds.

E.A. Williams, P.T. Willian~/J. Ad. Appt. Pyrolysis 40-41 (1997) 347-363 351

During their time in the hot zone of the reactor random s&ion [I61 is occurring. At higher temperatures, breaking of long chains may halve the chain length and thus the mokcular weight leading to a shift in peak values from 1000 to apprctxi- malely 500 Da.

Sample detection with the W detector illustrates the presence of aromatic compounds within the waxes. Fig. 6(b), again illustrates a shift to lower molecular weight compounds with an increase in temperature.

The molecular weight range of the oils from the condensers is simiIar to the heavier fractions. Though, as expected there is a much greater proportion of the sample with a low molecular weight. Again the peak molecular weight vaIue of the sample changes with temperature. Fig. 7(a) highlights this eBct. It rev& three major regions of molecular weight. These are around log v&es 2,2.3 and 2.7 that is, actual molecular weights of 100, 200 and 500 Da. At pyrolysis temperatures of between 500 and 6CWC two of these peaks are or=!, that is at 2.3 and 2.7. This illustrates that a large proportion of the sample has a heavy molecular weight. The distribution between these two peaks is approximately 5050.

At the higher temperatures the distribution changes. The oils produced at 6M and 700°C have a mokcuitrr weight peak of approximately bg 2.2, &at is ahnost I60 Da. This is very similar to the boiling range of naptha [ 17]_ The region around log 3 contributes much less to the composition of the oil, with this being around 25% of the sample, much less than with the other lower temperature oils. Again, it seems that at higher temperatures much smaller molecules are formed than at the lower temperatures. This is probably due to the heavier mohxuks breaking up to form the smaller ones. Fig. 7(b), that of the W detector, shows that aromatic compounds are displayed in the sample. However, no clear trends in molecular weight with change in pyrolysis temperature are observed.

3.5 Product applications

The commercial application of the gases could be as a fwl. The calor& value (CV) of all hydrocarbon gases 1181 produced is more than or qd to 40 MJ kg-‘. Their accumulative CV is therefore sufficient to fuel the pyrolysis process itself. if high temperatuB were conmntrated on then more than 80% of the original feedstr;ck can be converted into gaseous products, Ethcne and propene are valuabk gases within the chemical industry. Ethene is a key intermediate in the macufiacture of polyethylene, vinyl chloride, styrene, ethanol, and other important products [19]. Propene is used to make polypropylene and aetone, and is an intermediate in the production of polyurethane foam [17].

In addition to the direct use of gaseous ole6ns in the production of chemicals, heavier fractions of crude oil are cracked down to form these gaseous intermediates before reprocessing. For example, napthas, a complex mixture of allanes, cy- cloalkanes and aromatics [t7], are cracked to produce ethene as the main product and propene as a by-product. Gasoline, a widely demanded fuel, GUI be produced from the heaviest fraction of crude oil, atmospheric residue, by catalytic cracking 1171.

362 EA. William, P. TV Williumr / 1. Anal. Appl. Pyrolysis 40-41 Cl9971 347-363

These and other products are of great importance in the chemical industries. So, if the products of plastic pyrolysis can be utilised in these processes then economic benefit will he forthcoming.

4. conclusions

Two valuable products are formed from the pyrolysis of mixed plastic waste. These are a gas and oil. Depending on the reaction conditions used the products have varying compositions and properties.

At lower pyrolysis temperatures a heavy wax is formed. This is comparable in its molecular weight range, boiling point and functional group composition to an atmospheric residue cut from crude oil, Thus, catalytic cracking could be carried out on the wax forming a gasoline type mixture of hydrocarbons. A lighter, mainly aliphatic wax is also a product of pyrolysis at temperatures below 550°C which again could be commerciaIly cracked to form gasoline.

At higher temperatures, the oil formed is much more aromatic and has a lower modal boiling point of between 120 and 220°C. It is similar to naptha and could be cracked to form ethene. A hydrocarbon gas is produced at all temperatures with its concentration increasing at higher temperatures.

The results therefore indicate that depending on the product type required the operating conditions can be changed accordingly. It can also be inferred that if the temperature was increased to higher than 700°C then due to increased secondary reactions, it is possible that the resultant oil would directly resemble gasoline without any further processing.

It is therefore evident, that pyrolysis of mixed plastic waste shows a huge potential not only in dealing with waste reduction but also in producing products which show a real ability to be adapted for chemical feedstock production.

Acknowledgements

The authors would like to thank the Department of Trade and Industry via the Energy Technology Support Unit, Harwelf, Oxfordshire for support for this work (Grant number B/TI/O0397/00/00)+ Also, David Wilson of BP Chemicals for the supply of polymer samples.

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