the molecular weight range of pyrolytic oils derived from tyre waste

Journal of Analytical and Applied Pyrolysis 29 (1994) 111-128

JOURNALor ANALYTlCALd APPUED PYROLYSIS

The molecular weight range of pyrolytic oils derived from tyre waste

Paul T. Williams *, David T. Taylor

Department of Fuel and Energy, The University of Leeds, Leeds LS2 9JT, UK

(Received September 13, 1993; accepted December 14, 1993)

Abstract

Pyrolysis oils obtained from the batch pyrolysis of tyre waste were analysed for their molecular weight (MW) distribution using a small molecule, size exclusion chromatography (s.e.c.) system with a dual ultraviolet and refractive index detection system. The tyres were pyrolysed in a nitrogen purged static batch stainless steel reactor heated at 20°C

-’ to a final temperature of 450°C. The evolved pyrolysis vapours were passed di- zyly to a second reactor heated to higher temperatures of 500, 560, 600, 640, 700 and 712°C. The derived oil after secondary cracking was collected in a condensation trap. The analytical system was character&d in terms of the influence of mobile phase flow rate and column temperature on column efficiency, and the behaviour of model hydrocarbons compared to the ideal behaviour of the standard polystyrene calibration curve. The optimum practical operating conditions were 0.26 ml min- ’ flow rate and 0°C column temperature. The pyrolytic oil had a MW range from a nominal 50 to 1200. The oils showed a decrease in MW as the secondary reactor temperature was increased. The oils were fractionated in terms of their chemical classes using liquid chromatography, and each fraction was also analysed using the s.e.c. system. The ‘oils showed an increase in aromatic content and decrease in aliphatic content as the secondary reactor temperature was increased. The aromatisation of the oils was due to a Diels-Alder type reaction involving pyrolysis of alkanes to alkenes, and subsequent cyclisation to aromatic and polycyclic aromatic compounds. The millivolt output of each detector was compared and gave a measure of the aromaticity of the oils when compared to the chemical class fractionation.

Keywords: Analysis; Molecular weight; Pyrolysis; Size exclusion chromatography; Tyre pyrolysis oils

* Corresponding author.

0165-2370/94/$07.00 0 1994 - Elsevier Science B.V. All rights reserved

SSDZ 0165-2370(94)00792-Y

112 P.T. Williams, D.T. Taylor / J. Anal. Appl. Pyrolysis 29 (1994 11 I-128

1. Introduction

Large quantities of scrap automotive tyres are generated each year throughout the World and their disposal represents an increasing environmental problem. Estimates for the generation of scrap tyres are 2.5 x lo6 tonnes in North America, 1.5 x lo6 tonnes in the European Community and 0.5 x lo6 tonnes in Japan [ 11. The majority of tyres are disposed of in open or landfill sites; however, tyres do not degrade in landfill sites and accidental fires can cause problems of pollution to the atmosphere and water course. Thermal processing techniques have received renewed investigation, since such disposal routes maximise the energy potential of the scrap tyres [2,3]. Inciner- ation with energy recovery via electricity generation or district heating has been devel- oped both in the USA and UK [ 31. Pyrolysis of tyres is also currently receiving renewed interest, since the derived oils have a high calorific value and may be used directly as fuels or added to petroleum refinery feedstocks; they may also be an important source of refined chemicals, since they have been shown to contain high-value chemicals [3-51.

The pyrolytic oils have been shown to be chemically complex, containing a large number of chemical compounds with a wide molecular weight range [l-8]. Conse- quently, to aid the characterisation of the oils, researchers have examined analytical techniques which give rapid, broad classifications of the oils; such a technique is size exclusion chromatography (s.e.c.). S.e.c analyses samples in terms of their molecular weight range and has been used to analyse oils from a variety of sources, including oils derived from coal [9], heavy crude petroleum oils [lo] and from the pyrolysis of biomass [ 111. However, there are few data on the application of s.e.c. to pyrolysis oils derived from scrap automotive tyres.

Operating parameters for s.e.c., such as flow rate and column temperature, are often neglected in terms of improving chromatographic efficiency. Solvent flow rates tend to be fixed at between 1 and 2 ml min- ‘. Column temperature is usually maintained at ambient [9] or elevated temperature [lo]. However, there are a few data on the operation of s.e.c. at lowered temperatures. In addition, the use of two detectors, for example, ultraviolet (W) and refractive index (RI) detection, has the advantages of enabling differentiation of compounds on the basis of chemical class in addition to molecular weight, and allows more information on the sample to be obtained.

In this paper a small molecule s.e.c. system with dual W and RI detection has been evaluated to determine the optimum solvent flow rate and column temperature in terms of chromatographic efficiency. The system has been used to analyse the molecular weight range of oils derived from the pyrolysis of scrap tyres and chemical class fractions of those oils. In addition, a measure of the aromaticity of the oils using the W/RI single ratio has been obtained.

2. Experimental

2.1. Scrap tyre pyro Iysis

The tyres were pyrolysed in a nitrogen purged, static batch, stainless steel reactor with a nominal capacity of 200 cm3, externally heated by an electrical ring furnace.

P.T. Williams, D.T. Taylor 1 J. Anal. Appl. Pyrolysis 29 (1994 II 1- 128 113

The tyre pieces including the metal core, sized to 2-5 cm2, were placed on a support in the reactor and heated at 20°C min- ’ to a final temperature of 450°C; they were held at that temperature for a minimum of 2 h or until there was no further significant yield of gas. The evolved pyrolysis vapours were passed directly to a second reactor heated to higher temperatures of 500, 560, 600, 640, 700 and 712°C. The derived oil after secondary cracking was collected in a condensation trap maintained at 0°C. The residence time of the gases in the secondary reactor was in the order of 30 s and varied with the rate of evolution of volatiles from the tyres during pyrolysis. Details of the experimental system and detailed analysis of the oils is reported elsewhere [7]. A similar two-stage pyrolysis reactor and secondary reactor have been used by Cypres and Bettens [8] for the pyrolysis of scrap tyres.

2.2. Size exclusion chromatography

Figure 1 shows a schematic diagram of the s.e.c. system used to determine molecular weight (MW) of the tyre oils. The system incorporated two 150 mm x 4.6 mm i.d. columns with Polymer Laboratories 5 pm RPSEC 100 A type packing. A third column of the same material was placed in line between the pump and the injection valve, to ensure pre-saturation of the solvent with the column packing material and also to avoid analytical column dissolution and hence loss of perfor- mance. The solvent used for the mobile phase was tetrahydrofuran (THF). The calibration system used was based on polystyrene samples of low polydispersity in the MW range of 800-860 000; also included was benzene for low MW calibration. Samples were introduced through a 2 ~1 loop injection valve. A W detector from Merck-Hitachi and an RI detector from Varian were used. W scanning of the polystyrene MW fractions in THF indicated that the maximum absorbance was at 262 nm, and all calibration and efficiency measurements were taken at this wavelength. The UV detector would be sensitive to the detection of aromatic compounds and the RI detector would measure elution of all compounds. The output from the detectors was both recorded on a chart recorder to give the millivolt output from each detector and also input to a micro-computer which analysed the data. The absolute recorder response in millivolts from the W detector was compared with that from the RI detector to indicate the.aromaticity

injection valve

det%or RI

detector

t- guard COlUNl temperature controlled

s.e.c. calms

Fig. 1. Schematic diagram of the size exclusion system

114 P.T. Williams, D.T. Taylor / .I. Anal. Appl. Pyrolysis 29 (1994) 11 l-128

of the oils in relation to the MW range. The MW distribution was determined as number and weight average MW; in addition, the polydispersity of the oils was also calculated. Single compounds have a polydispersity in the range 1.0-1.1, measured on the system described in this work.

2.3. Chemical class fractionation of the oils

The oils were analysed by liquid chromatography to separate them into chemical class fractions. The liquid chromatography system consisted of 100 mm x 10 mm glass columns packed with silica, Bondesil (sepralyte) sorbent, pre-treated at 105°C for 2 h prior to use. To prevent the formation of a solid phase with the addition of the pentane mobile phase, and to improve solvent contact with the oil, the oil was thoroughly mixed with Chromosorb G/AW/DMCS 60-80 support and packed in the column above the analytical phase. The column was then sequentially eluted with n-pentane, benzene, ethyl acetate and methanol to produce, aliphatic, aromatic, heteroatomic and polar fractions respectively. The n-pentane fraction was further subdivided into the first eluted n-pentane fraction (pentane-1) which contained the aliphatic compounds and the second eluted n-pentane fraction (pentane- 2) which contained lower MW aromatic compounds. Previous work [7] has shown that the pentane-2 fraction contains mainly single- and 2-ring PAH, and the benzene fraction contains mainly 3-, 4- and 5-ring PAH, although there is some overlap of 2- and 3-ring PAH in each fraction. The methanol elution produced no significant percentage mass fraction and was not analysed further. The collected fractions were carefully prepared for s.e.c. analysis by removal of the solvent mobile phase using nitrogen blow-down. Each fraction was analysed by FT-IR and gas chromatography/mass spectrometry to confirm that the chemical class fractionation was complete and efficient.

3. Results and Discussion

3.1. Evaluation of the s.e.c. system

Figure 2 shows the retention volume in relation to 1ogMW for the polystyrene standards used to calibrate the s.e.c. system. Also shown is benzene, which was included in the calibration. Figure 2 shows that the retention volume of the MW fractions of polystyrene and benzene were linear over a very wide range, from molecular mass 78 to 860 000 (Fig. 2 shows a slight curvature to the calibration as should be expected).

Figure 3 shows the deviation of n-alkanes, n-alkenes and polycyclic aromatic hydrocarbons (PAH), and Fig. 4 shows the deviation of alkylated cyclic hydrocarbons from measured MW and known MW using the UV detector. There are systematic variations in the measured MW of n-alkanes, n-alkenes, PAH and alkylated cyclic hydrocarbons. Increasing chain length for n-alkanes, n-alkenes, and size for PAH, showed an increasing deviation of the measured MW from the known

P.T. Williams, D.T. Taylor 1 J. Anal. Appl. Pyrolysis 29 (1994) 11 I - 128 115

2 3 4 5 6

Log blw

Fig. 2. Calibration graph for the size exclusion system; retention volume versus 1ogMW (MW range 78-860 000).

MW. Similarly, as the alkyl group chain length associated with the cyclic hydrocarbons was increased, there was an increase in the measured MW compared to the known MW. Deviations from polystyrene calibration curves have also been shown previously for other model compounds [ 9,12,13]. For example, Bartle et al. [9] working with coal derivatives showed that alkanes and polycyclic aromatic compounds (PAC) deviated significantly from the calibration curve of standard polystyrene MW fractions. It has been suggested [ 141 that PAH may exhibit adsorption effects with the solid phase material of the column, causing enhanced retention times and lower apparent MW results, as found in this work. Johnson and Chum [ 1 l] in their work on biomass pyrolysis oils used polystyrene MW fractions, and showed that aromatic acids and naphthalenes deviated significantly from the calibration curve. The tyre pyrolysis oils have been previously shown to contain a

116 P.T. Williams, D.T. Taylor 1 J. Anal. Appl. Pyrolysis 29 (1994) 111- 128

n n-alkenes

X n-alkanes

l PAH a X

0 700 200 300

Known M

Fig. 3. Comparative MW for n-alkanes, n-alkenes and PAH; known versus measured MW by the size exclusion system.

wide variety of compounds, including n-alkanes, n-alkenes, alkylated cyclic hydrocarbons and PAH [l--S], and consequently the MW shown in this work represent an apparent MW.

The column efficiency was determined by calculation of the peak separation efficiency (TZ) and number of theoretical plates using a polystyrene MW fraction of 13 000 and a benzene one of MW 78. Evaluation of the flow rate data in relation to the s.e.c. efficiency parameters, TZ (78, 13 000), and number of theoretical plates (N) showed (Table 1) that the maximum efficiency was obtained at a flow rate of 0.26 ml min - l. Higher flow rates in particular drastically reduced efficiency, for example, a flow rate of 1 .O ml min - ’ reduced the TZ (78, 13 000) efficiency by 20%. Consequently, all the tyre pyrolysis oils were analysed at a mobile phase flow rate of 0.26 ml min - ‘.

The influence of column temperature on the efficiency of the s.e.c. system was also examined. Temperature control was maintained using a conventional column heater or freezing mixtures to obtain a column temperature range between - 67 and + 36°C. Varying the temperature should produce an influence of two compet- ing effects as the temperature of operation is increased; the solubility of the sample should increase and the solvent viscosity decrease, leading to improved efficiency at

P.T. WiIliams, D.T. Taylor 1 J. Anal. Appl. Pyrolysis 29 (1994) 11 I - 128 117

700 200

Known MV

Fig. 4. Comparative MW for cyclic hydrocarbons; known versus measured MW by the size exclusion system.

Table 1 Column efficiency in relation to flow rate and temperature

Flow Rate (ml min-‘)

TZ (78, 13 000) N78 N 13ooo

0.05 3.35 1600 182 0.10 3.40 1780 176 0.20 3.45 1970 182 0.40 3.49 1770 182 0.60 3.20 1610 171 0.80 2.90 1500 137 1 .oo 2.85 1410 107

Temp. (“C) TZ (78, 13 000)

-67 2.81 1320 129 -18 3.21 1720 162 -4 3.33 1720 162

2 3.45 1755 176 14 3.57 1670 190 28 3.46 1600 181 36 3.46 1560 172

118 P.T. Williams, D.T. Taylor / J. Anal. Appl. Pyrolysis 29 (1994) ill- 128

elevated temperatures; acting counter to this would be the increased reactivity of solvent/solute interactions leading to significantly decreased efficiency. Table 1 shows the TZ (78, 13 000) and number of theoretical plates (N) for benzene and polystyrene MW fractions of 13 000. The results showed that the maximum efficiency occurred between 2 and 14°C. For ease of practical set-up, the system was operated at 0°C and an ice/water mixture was used to maintain the column temperature at 0°C throughout the work. The s.e.c. system was used intermittently over a period of three years and a standard of benzene (MW 78) was applied before each period of use. The number average MW for the UV detector was 79 (maximum 86, minimum 70) and for the RI detector was 75 (maximum 84, minimum 68) over the period of use, from over 80 determinations. The results indicate the stability of the system and consistency of operating parameters. Column temperature is usually maintained at ambient [9] or elevated temperature [lo]. Determann [ 151 suggests that elution volume is largely independent of the flow rate and temperature; however, the work quoted by Determann was at elevated temperatures above ambient column temperature, i.e. 55, 90 and 125°C. Yau et al. [ 161 also examined the effect of elevated temperature on column efficiency for s.e.c. and found that there was a slight shift towards smaller retention volumes with increasing temperature. However, the work reported here shows that optimum efficiency was obtained at lower than ambient temperatures.

3.2. Molecular weight range of tyre pyrolysis oils

Figure 5 shows the MW range measured using the RI and UV detectors of the tyre pyrolysis oils pyrolysed at 450°C and subjected to secondary cracking at either 500, 550, 600, 640, 700 and 712°C. The MW range of the oils was from a nominal 50 to over 1200. As the secondary reactor temperature was increased there was a shift to a lower MW range. This shift is reflected in the average MW data for the oils, shown in Table 2, for the number and weight average MW, and for both the RI and UV detectors. Also shown in Table 2 is the polydispersity of the oils. The polydispersity reflects the deviation of the MW distribution from the gaussian distribution of an ideal single compound. The increase in polydispersity for both the RI and UV detectors indicates a broader range of MW distribution reflecting a wider range of compounds present in the sample. The decrease in MW range with increasing secondary reactor temperature reflects the changing composition of the tyre pyrolysis oils which will be discussed later.

3.3. Chemical class fractionation of tyre pyrolysis oils

Figure 6 shows the aliphatic, low and higher MW aromatic and heteroatomic fractions resulting from liquid column chromatography fractionation of the tyre oil samples in relation to secondary reactor temperature. There was a marked decrease in the percentage mass of the aliphatic fraction and an increase in the aromatic fractions as the secondary reactor temperature was increased from 500 to 712°C. Detailed analysis of these tyre pyrolysis oils has been reported previously by the

P.T. Williams, D.T. Taylor / J. Anal. Appl. Pyrolysis 29 (1994) 11 l-128 119

- 640 “C (a) RI . . . . . . 15 700 OC -

100 1000 70000

t”w

- 640 OC fbl uv . . . . . .

75 7& DC

* 1 9 --- 712 OC

co L 0 70 - 5 3 *

Fig. 5. MW range of tyre pyrolysis oils in relation to secondary reactor temperature measured by RI and

W detectors.

authors [7], showing that as the secondary reactor temperature was increased, formation of mono- and polycyclic aromatic compounds occurred via pyrolysis of alkanes. The alkanes are pyrolysed to alkenes which are subsequently aromatised by a Diels-Alder type reaction to form PAH [ 7,8,17,18]. Cypres [ 171 pyrolysed model aliphatic compounds and confirmed the Diels-Alder route to aromatic compounds. For example, pyrolysis of n-decane produced alkenes by thermal degradation; post-pyrolysis reactions of the alkenes between 600 and 9OO”C, showed’ a decrease in light alkenes, and the formation of single-ring aromatic compounds such as benzene, toluene and alkyl-aromatic compounds by Diels-Alder reactions.

Cypres and Bettens [ 81 and Williams and Taylor [7] have shown for tyre pyrolysis, with secondary cracking of the derived pyrolysis vapours at higher temperatures, as in this work, that Diels-Alder aromatisation reactions do occur. Their work also shows that an increase in single-ring compounds, such as benzene, xylene, toluene and styrene, occurs as the temperature of the secondary cracking

120 P. T. Williams, D.T. Tayior / .I. Anal. Appl. Pyrolysis 29 (1994) If I - 128

Table 2 Molecular weight of tyre pyrolysis oil in relation to secondary reactor temperature

W detection

Temperature (“C)

No. average (Weight)

Weight average (Weight)

Polydispersity

500 168 244 1.46 560 163 250 1.53 600 164 236 1.43 640 151 227 1.50 700 130 194 1.49 712 129 201 1.55

RI detection

Temperature “C

No. average

(MW)

Weight average (MW)

Polydispersity

500 278 323 1.16 560 265 309 1.17 600 249 290 1.16 640 228 268 1.18 700 169 207 1.23 712 167 209 1.25

reactor is increased. PAH are also formed by further associative reactions. Conse- quently, as secondary cracking occurs, higher concentrations of single-ring aromatic compounds are formed by thermal degradation of alkanes, some of which will be of high MW; The increased production of single-ring aromatic compounds and decrease of higher MW alkenes will shift the measured MW to a lower range. Indeed, for the 712°C tyre pyrolysis oil shown in Fig. 5 the lower MW compounds can clearly be seen as a separate peak.

3.4. A4olecular weight range of chemical class fractions

The chemical class fractions, pentane-1, pentane-2 and benzene, fractionated from the tyre pyrolysis oils representing the aliphatic, low and higher MW aromatic fractions were analysed separately for their MW range. The ethyl acetate fraction was not analysed, since it showed no significant change in percentage mass fraction of the oil in relation to secondary reactor temperature. Tables 3-5 show the s.e.c. analysis of the aliphatic, low and higher MW aromatic fractions respectively. The Tables show the number and weight average MW as calculated using the RI and UV signals; polydispersity is also shown. The UV detector showed little signal response to aliphatic compounds; however, the data are included for completion. The aliphatic fraction (Table 3) shows a decrease in both number and weight

P.T. Williams, D.T. Taylor 1 J. Anal. Appl. Pyrolysis 29 (1994) ill-128 121

High MV

Aliphatics

u b . A A Heteroatomicz

70 b

Ob * 500 550 600 650 700

Secondary Reactor Temperature IT}

Fig. 6. Chemical class fractionation of tyre pyrolysis oils in relation to secondary reactor temperature.

average MW as the secondary reactor temperature is increased from 500 to 712°C. The data in Fig. 6 showed that the aliphatic fraction decreased in concentration as the secondary reactor temperature was increased. The MW data also indicate that the compounds within the aliphatic fraction decrease in MW as secondary cracking reactions occur with increased secondary reactor temperature. The higher MW compounds formed on pyrolysis of the tyres undergo cracking to lower MW compounds. The polydispersity data also indicate that the oils become more complex as secondary reactions occur, producing a wider range of compounds, indicated by the wider range of MW. Table 4 shows the s.e.c. analysis of the low MW aromatic compounds of the pentane-2 fraction, which mainly contains single- ring aromatic and 2-ring PAH. As was the case for the aliphatic compounds, the results showed a decrease in number and weight average MW for both the RI and UV detectors as the secondary reactor temperature was raised from 500 to 712°C.

122 P.T. Williams, D.T. Taylor / J. Anal. Appl. Pyrolysis 29 (1994) Ill- 128

Table 3 Molecular weight of tyre pyrolysis oil aliphatic fraction in relation to secondary reactor temperature

UV detection

Temperature No. average Weight average

(“Cl (MW) (MWl

500 280 462 560 283 456 600 291 450 640 242 419 700 174 319 712 161 343

Polydispersity

1.65 1.60 1.55 1.73 1.83 2.13

RI detection


(“Cl (MW) (MW)

Polydispersity

500 350 487 1.39

560 360 497 1.38

600 354 464 1.31 640 334 469 1.40 700 308 430 1.40 712 216 456 1.65

Table 4 Molecular weight of tyre pyrolysis oil low molecular aromatic fraction in relation to secondary reactor temperature

UV detection


(“Cl (MW) (MW)

Polydispersity

500 208 333 1.60 560 204 334 1.63 600 197 328 1.67 640 180 306 1 .I0 700 158 285 1.80 712 141 251 1.78

RI detection


(“Cl (MW) (MW)

Polydispersity

500 219 385 1.38 560 276 393 1.42 600 284 394 1.39 640 242 374 1.54 700 223 343 1.54 712 170 290 1.71

Table 5

P.T. Williams, D.T. Taylor 1 J. Anal. Appl. Pyrolysis 29 (1994) 1 II- 128 123

Molecular weight to tyre pyrolysis oil heavier aromatic fraction in relation to secondary reactor temperature

Temperature

(“C)

500 560

600 640

700 712

Temperature

(“Cl _

500 560 600 640

700 712

UV detection

No. average

(MW)

218 224 193

198

177 178

RI detection

Weight average

(MW)

320 327 285

291

272 299

Polydispersity

1.47 1.46 1.48

1.47

1.54 1.70

No. average Weight average

(MW) (MW)

293 372 302 383 255 324 270 337

228 281 224 306

Polydispersity

1.27 1.27 1.27 1.25

1.23 1.37

The detailed analysis of the tyre pyrolysis oils reported previously by the authors [ 71 has shown that there was an increase in single-ring aromatic compounds, such as benzene, toluene, xylene and styrene, present in the oil as the secondary reactor temperature was increased. These aromatic compounds would elute in this pentane- 2 fraction, resulting in the shift to lower MW compounds (see Table 4). The Diels-Alder formation of aromatic compounds increases the concentration of lower MW aromatic species in the pyrolysis oils. The polydispersity of the low MW aromatic fraction also showed an increase as the secondary reactor temperature was increased. The polydispersity reflects the increasing chemical complexity of the oils, producing a wider range of MW.

Table 5 shows the number and weight average MW data for both the RI and UV detectors for the higher MW aromatic fraction of tyre pyrolysis oils after secondary cracking at 500-712°C. As was the case for the aliphatic fraction and the low MW aromatic fraction, there was a decrease in the average MW as the secondary reactor temperature was increased. At the higher secondary reactor temperatures, higher molecular mass 4- and 5-ring PAH are formed, accompanied by increased concentrations of the lower MW 3-ring PAH. The lower MW compounds have a disproportionate effect on the average MW data, producing a marked shift to lower MW. The polydispersity of the higher MW aromatic fraction also showed an increase as the secondary reactor temperature was increased. The polydispersity reflects the increasing chemical complexity of the oils, producing a wider range of MW.

124 P.T. Williams, D.T. Taylor / J. Anal. Appl. Pyrolysis 29 (1994) 11 l-128

-640 OC (al Aliphatics

Y I5

.*..*. 700 OC

9 --- 712 OC

E

k 10.

-640 OC (b) Low W Aromatics

15 . ..‘.. 700 DC

- ---772 OC

3

700 7000 70000

I -640 “C (cl High MW Aromatics

75 - . . . ...700 OC

---772 “C

Fig. 7. MW range of tyre pyrolysis oil chemical class fractions in relation to secondary reactor temperature measured by RI.

P. T. Williams, D. T. Taylor / J. Anal. Appl. Pyrolysis 29 (1994) 111- 128 123

Figure 7 shows the aliphatic, low and high MW fraction MW ranges measured by RI detection. Only the 640, 700 and 712°C data are included for clarity. The shift to low MW as the temperature of the secondary reactor was increased, illustrated by the average MW data in Tables 3-5, can be clearly seen. The appearance of low MW species as the temperature was increased can also be seen, particularly for the 712°C data for all the fractions. In addition, the high MW aromatic fraction data for 712°C shows that higher MW species are produced; this is further evidence for the Diels-Alder aromatisation reaction producing high MW PAH.

The average MW of the aliphatic fraction appears to be higher than the low or high MW aromatic fractions. However, the behaviour of aliphatic compared to aromatic compounds in s.e.c. produces different apparent MW, as illustrated by Fig. 3. Figure 3 shows that the alkanes and alkenes have an apparent MW consistently higher than the known MW, whereas PAH are consistently lower. Therefore, interpretation of s.e.c. data in terms of the MW of specific classes of chemical compounds should be used with caution. However, the use of this type of analysis on a comparative basis as in this work is valid. The determination of chemical mechanisms such as the influence of secondary reactions on pyrolysis vapours can be aided by s.e.c., as in this work, particularly if the work is supplemented with detailed analysis of chemical class fractions.

3.5. Estimation of the aromaticity of the tyre pyrolysis oils

The application of RI and UV detectors, with the RI detector used to measure all eluting compounds and the UV detector specific for aromatic compounds, can be extended to estimate the aromaticity of the tyre pyrolysis oils. The estimation was determined by the ratio of the millivolt output signals from the W and RI detectors, which were compared with the aromatic chemical class fraction (pentane- 2 plus benzene fractions) shown in Fig. 6. The results are shown in Fig. 8. The W/RI ratio of the tyre pyrolysis oils shows an increase as the secondary reactor temperature increases from 500 to 712°C. The chemical class fractionation involving liquid column chromatography also shows an increase in aromatic content of the oils with increasing secondary reactor temperature. The results, although not absolute, indicate the increase in aromatic compounds compared to the rest of the chemical matrix of the sample. Consequently, this simple, quick and non-destructive analysis of the samples by s.e.c. with dual RI and W detectors allows an estimatation of the relative aromatic content of the oils.

4. Conclusions

(1) Scrap tyres were pyrolysed in a static batch reactor at 450°C with secondary cracking of the derived pyrolysis vapours at temperatures of 500 to 712°C. The oils produced were analysed by small molecule size exclusion chromatography (s.e.c.) to determine the MW range of the oils.

P.T. Williams, D.T. Taylor / J. Anal. Appl. Pydysis 29 (1994) II f - 128

uv/RI

Aromatics

3.0

w/RI

Ratio

2.5

2.0

500 550 600 650 700

Secondary Reactor Tenperature (“Cl

Fig. 8. Aromaticity of tyre pyrolysis oils in relation to secokary reactor temperature measured by

UV/RI detection compared to chemical class fractionation.

(2) The s.e.c. system was optimised at a flow rate of 0.26 ml min - ’ and a column temperature of between 2 and 14”C, under which conditions the maximum column efficiency was found. For practical ease, a column temperature of 0°C was used. There was a marked decrease in efficiency at lower temperatures and significant decrease at higher temperatures.

(3) The s.e.c. system was calibrated with polystyrene MW fractions and produced a calibration graph of retention volume versus log MW between MW 78 and MW 860 000. Model chemical classes of compounds, including n-alkanes, n-alkenes, PAH and cyclic hydrocarbons, were analysed, and the results showed that deviations of the measured MW from the known MW of the model compounds occurred. There was an increasing deviation from ideality with increasing n-alkane

P. T. Williams, D.T. Taylor / J. Anal. Appl. Pyrolysis 29 (1994 Ill - 128 127

and n-alkene chain length, increasing size of PAH and increasing number of alkyl side chains on a cyclic hydrocarbon. There was also a wide spread in the deviations of the various PAH.

(4) Chemical class fractionation of the pyrolysis oils showed that increasing the secondary reactor temperature produced oils with a decreased concentration of aliphatic compounds and increased concentration of low and high MW aromatic compounds. The heteroatomic chemical class fraction remained essentially unchanged during secondary cracking. The chemical changes were due to a Diels-Alder type cyclisation giving the following sequence of reactions: alkanes-, alkenes + single-ring aromatic -+ PAH.

(5) The pyrolysis oils showed a decrease in average MW with increasing secondary reactor temperature. S.e.c. analysis of the chemical class fractions showed that for the aliphatic, low and high MW aromatic fractions there was a decrease in MW. This was due to the formation of lower MW species, attributed to the Diels-Alder type of reaction, which increased with increasing secondary reactor temperature.

(6) The ratio of the RI and UV detectors was used to estimate the aromaticity of the tyre pyrolysis oils. The estimation was determined by the absolute output from the detectors in millivolts. The ratio of the signals from the two detectors was compared with the aromatic chemical class fraction of the oils and showed a good correlation. The method provides a simple, quick and non-destructive estimate of the relative aromatic content of the oils.

5. Acknowledgements

The authors would like to thank the UK Science and Engineering Research Council for their support for this work via three research grants, GR/F/06074, GR/F/87837 and GR/H/83355.

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[2] P.T. Williams, S. Besler and D.T. Taylor, Fuel, 69 (1990) 1474-1482. [3] P.T. Williams, S. Besler and D.T. Taylor, Proc. Inst. Mech. Engrs., Part A. J. Power Energy, 207

(1993) 55-63. [4] W. Kaminsky and H. Sinn, in J.L. Jones and S.B. Radding (Eds.), Thermal conversion of solid

wastes and biomass, ACS Symposium Series 130, American Chemical Society, Washington, DC, 1980.

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the molecular weight range of pyrolytic oils derived from tyre waste

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