Temperature selective condensation of tyre pyrolysis oils to

maximise the recovery of single ring aromatic compoundsq

Paul T. Williams*, Alexander J. Brindle

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

Received 22 November 2002; accepted 23 November 2002; available online 30 January 2003

Abstract

Scrap tyres were pyrolysed in a fixed bed reactor and the evolved pyrolysis gases were passed through a condenser system maintained at

separately controlled temperature to determine the yield and composition of the condensed oil. The main objective was to maximise the

selective condensation of single ring aromatic compounds which are of known higher commercial value. In addition, the molecular weight

range of the condensed oils was also determined. The influence of condenser packing material was also examined. The results showed that

the type of packing material within each condenser was also examined and found to be important in determining the yield and composition of

the condensed oil. Similarly, condenser temperatures determined the yield and also composition of the oils. Maximum concentrations of

single ring aromatic compounds in relation to the selective condensation process showed significant increases in the concentration of certain

compounds could be achieved in the condensed oils collected in each condenser.

q 2003 Elsevier Science Ltd. All rights reserved.

Keywords: Pyrolysis; Tyres; Recycling; Chemicals

1. Introduction

The annual production of scrap tyres throughout the

world is estimated at 1000 million, representing a significant

treatment and disposal problem. The tyre also represents a

potentially valuable source of energy, raw materials and

chemicals. However, in most countries, the preferred option

for disposal is landfill or stockpiling. Other treatment

options for tyres have included the retreading of scrap tyres

and the use of crumbed tyres in sports fields, playgrounds,

pavements and roads. Energy recovery schemes have

included for example, tyre incinerators with energy

recovery and the use of tyres in cement kilns. With

increasing emphasis on recycling of waste, alternative

process routes for tyres have been examined with the aim of

recovering some of the value in the tyre [1]. One process

which has received considerable recent attention is

pyrolysis of the tyre to produce an oil, gas and char in

addition to the steel cord, all of which have the potential to

be recycled [2,3]. Process conditions can be altered to

maximise the yield of char, oil or gas, for example, higher

pyrolysis temperatures cause the higher molecular weight

oil species to thermally degrade to gas. Rapid removal of the

pyrolysis gases from the hot zone of the reactor such as with

fluidised bed reactors, gas purged fixed bed reactors or

vacuum pyrolysis systems tend to maximise the production

of oil [2–5].

There is much interest in the production of hydrocarbon

liquids from waste materials, particularly tyres, since the

production of a liquid product increases the ease of

handling, storage and transport and hence the oil product

does not have to be used at or near the pyrolysis plant. The

yield of oil from tyres can be up to 58 wt% of the tyre rubber

and the oil has broad fuel properties similar to commercial

grade light fuel oil/diesel fuel. For example, the energy

value of the oil is 42 MJ kg21 and sulphur content between

0.5 and 1.5 wt% depending on process conditions, and

therefore the pyrolysis oils may be combusted directly [6].

The oils are chemically very complex, consisting of a range

of alkanes, alkenes and aromatic species with a molecular

weight range from ,50 to .1000 Da [2]. However, of most

0016-2361/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved.

doi:10.1016/S0016-2361(03)00016-4

Fuel 82 (2003) 1023–1031

www.fuelfirst.com

q Published first on the web via Fuelfirst.com—http://www.fuelfirst.com

* Corresponding author. Tel.: þ44-1132-33-2504; fax: þ44-1132-46-

7310.

E-mail address: p.t.williams@leeds.ac.uk (P.T. Williams).

interest, the oils have been shown to contain valuable

chemicals such as, benzene, toluene, xylenes and limonene

in significant concentrations which are used in the chemical

industry [3,6–8]. The economic importance of such single

ring aromatic chemicals suggests that identification of

processes which maximise the concentrations of such

chemicals would generate a product oil which has potential

as a chemical feedstock rather than a liquid fuel.

In this paper, scrap tyres have been pyrolysed in a fixed

bed reactor and the pyrolysis gases have been passed

directly to a condenser system designed to selectively

condense the pyrolysis gases at different temperatures. The

aim of the work was to obtain condensed fractions of oil

containing increased concentrations of single ring aromatic

compounds. The selective condensation process is mainly

dependant on the dew point of the components of the oil,

which is closely linked to the boiling point of that

compound so it becomes apparent that selective conden-

sation would prove useful for the fractionation of pyrolysis

oil into different temperature indicative cuts. The tempera-

ture selective condensation is analogous to the separation of

the components of oils via distillation. However, tempera-

ture selective condensation differs from distillation in that

fractionation is achieved by cooling gaseous products to

liquids rather than in the case of distillation where oil is

transformed from its liquid state to a vaporous state to afford

separation. The parameters examined were the type of

packing material used in the condenser and the condenser

temperature.

2. Materials and methods

2.1. Tyres

The tyre rubber used for pyrolysis was obtained from

shredded and crumbed passenger car tyres. The crumbed tyre

rubber was sieved to produce a size range of 1.0–1.4 mm. A

typical analysis of the tyre, on a steel- and fabric-free basis

was, carbon 86.4 wt%, hydrogen 8.0 wt%, nitrogen 0.5 wt%,

and sulphur 1.7 wt%. The proximate analysis of the tyre

crumb was, volatiles, 62.2 wt%, fixed carbon 29.4 wt%, ash

content 7.1 wt% and moisture content 1.3 wt%. The tyre

rubber had a calorific value of 40.0 MJ kg21.

2.2. Pyrolysis reactor and temperature selective

condensation system

Fig. 1 shows a schematic diagram of the fixed bed

pyrolysis reactor and temperature selective condensation

system. The reactor was constructed of stainless steel and

was 10 cm diameter £ 15 cm high. Each experiment con-

sisted of the pyrolysis of 150 g of tyre crumb. Heating was

via an external electrical heater and temperatures were

Fig. 1. Schematic diagram of the pyrolysis reactor and temperature selective condensation system.

P.T. Williams, A.J. Brindle / Fuel 82 (2003) 1023–10311024

controlled to give the sample a ramped heating rate of

5 8C min21 to the final tyre pyrolysis temperature of 570 8C

and held at that temperature for 1 h or until pyrolysis was

complete. The pyrolysis temperature of 570 8C was selected

to ensure complete pyrolysis of the tyre. Temperatures were

monitored throughout the reactor. A continuous purge of

inert nitrogen was introduced to the reactor via a perforated

ring located at the base of the reactor. The nitrogen flow rate

gave a maximum gas residence time of approximately 30 s

within the reactor. The tyre was contained in a sample cage

which fitted into the reactor, flush with the reactor walls. The

sample cage was removable allowing the sample to be

introduced and post-reaction solid product to be removed

easily. Gases exited the reactor downwards via an exit tube

located above the sample cage.

The pyrolysis gases were passed directly to the temperature

controlled condensation system. The first three condensers

were constructed of stainless steel followed by a set of four

glass condensers (impingers). The stainless steel condensers

were located inside separate furnaces and could be tempera-

ture controlled at selected temperatures, monitored by a

thermocouple placed inside the condenser. Condensers were

tested with no packing, a mesh packing (rather like packed

wire wool) and pall rings (similar to perforated steel tubes).

The steel gauze mesh and pall ring packings were stainless

steel and were designed to provide a high surface area and

impingement surface for the pyrolysis gases to condense onto.

These packing materials were compared to experiments where

the condensers had no packing material. Condenser tempera-

ture was tested for temperatures of 100, 150, 200 and 250 8C to

ascertain its effect on separating the oil into separate

condensed oil fractions. For each set of experiments, all

three stainless steel condensers were held constant at one

particular temperature, being either 100, 150, 200 or 250 8C.

The glass condensers were all maintained at about 270 8C

using solid carbon dioxide/acetone. The oil was removable

from each condenser at the end of the experiment. Throughout

each experiment gas samples were taken using PTFE gas

sample syringes and the gases were analysed off-line using

packed column gas chromatography. The gas samples were

taken after the oil condensation system.

Oil yield data for inclusion in the determination of mass

closure was obtained by weighing each condenser, includ-

ing packing material, connecting pipes and condensed oil

before and after pyrolysis. Additionally the mass of tyre

before pyrolysis and mass of char after reaction were

determined by weighing. The mass of each gas produced

was obtained from the gas composition analysis using gas

chromatography, the gas yield profile of each gas through-

out the duration of the experiment together with the

molecular mass data for each gas.

2.3. Gas analysis

Gas composition throughout each experiment was

determined from the analysis of the gases sampled using

the gas syringes. The gases were analysed for hydro-

carbon and non-hydrocarbon gases. Hydrocarbons from C1

to C4 were analysed using a Pye-Unicam gas chromato-

graph with flame ionisation detector, glass column

2.2 m £ 6 mm packed with n-octane Porasil C of 80–

100 mesh size and nitrogen carrier gas. Nitrogen,

hydrogen, oxygen and carbon monoxide were analysed

with a Pye Unicam series 204 gas chromatograph with a

thermal conductivity detector. The column used was made

of stainless steel being 1.8 m £ 6 mm dimensions with

argon as the carrier gas. The silica packing was 5 A sized

molecular sieve. Carbon dioxide was analysed using a

Gow – Mac Spectra gas chromatograph with a

1.8 m £ 6 mm stainless steel column packed with 100–

120 mesh silica gel, helium carrier gas and thermal

conductivity detector.

2.4. Oil analysis

The oils collected in each of the three stainless steel

condensers was analysed separately. The oils from the four

glass condensers were bulked and analysed together. The

weight average molecular weight of the oils was determined

using size exclusion chromatography. The analytical system

consisted of a 30 £ 0.78 cm column packed with silica

beads of an average pore size of 50 A, giving the column a

molecular weight exclusion limit of 3000 Da. Samples were

injected with a Rheodyne injection valve with a 20 ml

sample loop. The eluant used was tetrahydrofuran with dual

ultra-violet and refractive index detection. The ultra-violet

data only is presented here since it gave better detection

levels, superior signal to noise ratios and inherent stability.

Calibration of the size exclusion chromatography system

was with a range of standards, including polystyrene

standards with molecular weights of 480, 1050, 1350,

1770 and 2550. Additionally, aromatic and polycyclic

aromatic hydrocarbon standards of lower molecular weights

were used for calibration including benzene (MW of 78),

naphthalene (MW of 128), anthracene (MW of 178),

chrysene (MW of 228) and benzanthracene (MW of 228).

The oils were analysed for their composition of single

ring aromatic compounds, in particular, toluene, xylenes

and limonene, using capillary column gas chromatography.

The chemical complexity of tyre pyrolysis oils usually

involves complex analytical methodologies [3], however,

for this work, as only certain single ring aromatic

compounds were to be analysed, a simple analytical scheme

was adopted. The pyrolysis oils collected from the

condensers were diluted with a suitable solvent to a

known concentration and analysed directly using gas

chromatography with flame ionisation detection. The gas

chromatograph used was a Fisons 8000 with on-column

injection. The column used was a non-polar polydimethyl-

siloxane HT-5 capillary column 30 m in length by 0.32 mm

diameter giving 70,000 theoretical plates/meter and the

carrier gas was helium at a flow rate of 2 ml min21 at 40 8C.

P.T. Williams, A.J. Brindle / Fuel 82 (2003) 1023–1031 1025

The temperature programme of the oven was 40 8C for

2 min followed by a ramp rate of 5 8C min21 to 380 8C, at

which point the oven was held isothermally for 20 min to

ensure elution of all low volatility material.

Toluene, m-, p-, and o-xylene and limonene standard

compounds were used for identification and quantification

via calibration curves in the range 50–1000 ppm. Identifi-

cation of the single ring aromatic species was also aided by

coupled gas chromatography/mass spectrometry.

3. Results and discussion

3.1. Product yield and gas composition

The pyrolysis of scrap tyres was undertaken at the same

conditions of pyrolysis, but with alterations to the

condensation system, in terms of the type of packing

material used in the condenser and the condenser tempera-

ture. Therefore, as expected, all of the pyrolysis experiments

yielded the same mass balance, since pyrolysis conditions

were identical. The pyrolysis conditions were, a heating rate

of 5 8C min21 to a final pyrolysis temperature of 570 8C,

with a nitrogen gas flow rate of 1.5 l min21. The mean

product yield was a total gas yield of 4.3 wt%, oil yield of

57.1 wt% and char yield of 38.7 wt%. The analysis of the

repeatability of the experiments showed excellent precision

giving standard deviations of 0.19, 0.17 and 0.05 wt% for

the gas, oil and char yields, respectively. Similarly, the gas

composition was the same for each experiment.

Table 1 shows the average gas composition. The gas

composition was mainly composed of alkane and alkene

gases, carbon monoxide, carbon dioxide and hydrogen. The

rubber types used in tyre manufacture include, styrene–

butadiene–rubber, natural rubber (polyisoprene), nitrile

rubber, chloroprene rubber and polybutadiene rubber. The

thermal degradation process will produce highly reactive

free radicals which are often sub-units of the original rubber

molecule [9]. Consequently, the high concentration of

alkenes and dienes and butadiene in particular, are probably

derived as primary degradation products from the thermal

degradation of the tyre rubber. Secondary reactions of

the pyrolysis gases in the hot zone of the reactor will also

lead to the formation of butadiene, methane and hydrogen.

3.2. Influence of condenser packing material

The influence of the packing material placed in the three

stainless steel condensers was examined, using no packing,

a mesh packing, and large and small pall rings. For this

section of the experimental work, all of the steel condensers

were kept at the same temperature of 100 8C. A set of four

glass condensers (impingers), were placed after the three

stainless steel condensers to trap all of the oil. It has

been shown that the presence of a packing material within

the condensers would be beneficial to the efficiency of the

column [10]. The distribution of oil throughout the

condensation system in relation to the packing material

compared to no packing material being present in the

condensers is shown in Fig. 2. It should be noted that the oil

yield was the same for each experiment at 57.1 wt%, only

the distribution of the oil in each condenser changing with

experimental conditions.

For each experiment, all of the three stainless steel

condensers contained either no packing material, mesh,

small pall rings or large pall rings. It was apparent that the

use of pall rings made a significant difference to the

efficiency of the condensation units in condensing oil from

the vapour stream compared to no packing material or mesh.

Both types of pall rings were found to give similar oil

distributions between the three condensation units and the

cooled glass impingers. The improvement in the collection

efficiency of the stainless steel condensation system with the

pall rings was significant. The pall ring packed first column

collected nearly 45% of the pyrolysis oil whereas an

unpacked column condensed only 32% from the vapour

phase. The three packed condensation units removed nearly

70% of the oil from the vapour stream as opposed to the

unpacked condenser train which removed just under 60%.

Pall rings greatly increase the contact surface area within the

condenser. The condenser works by encouraging mass

transfer from the gaseous phase by providing a surface of

condensable temperature. Increasing this surface allows

more of the vapour stream to come into contact with the

surface and allow condensation of condensable vapours at

that given temperature. This reduced the amount of

potentially condensable vapour that bypassed the conden-

sation unit. The improved efficiency of the pall rings was

also attributed to their ability to make the flow of vapour

tortuous and fragmented without reducing the volume of the

condensation unit by a significant amount. Pall rings are

designed to have an extremely high surface area and a high

void volume. Once introduced into the condensation reactor

shell they impart maximum surface area (leading to

Table 1

Gas composition from the pyrolysis of scrap tyres

Gas Yield (wt%)

Hydrogen 0.25

Carbon monoxide 0.22

Carbon dioxide 0.60

Methane 0.96

Ethane 0.51

Ethene 0.40

Propane 0.43

Propene 0.60

Isobutane 0.16

Butane 0.12

But-2-ene 0.02

But-1-ene 0.07

Butadiene 2.13

Total 4.27

P.T. Williams, A.J. Brindle / Fuel 82 (2003) 1023–10311026

increased mass transfer) whilst reducing the volume of the

condenser by only a small amount.

The use of the mesh packing was found to give

efficiencies of condensation which were only slightly better

than those of the unpacked columns. This indicated that the

mesh was not as effective as pall rings in promoting mass

transfer processes. However, this may have been due to flow

channelling since the mesh was rolled up and placed in the

condenser leading to open channelling within the mesh.

The oil condensed in each of the three stainless steel

condensers and a bulked sample of oil collected from all of

the set of glass condensers was analysed for their weight

average molecular weight using size exclusion chromatog-

raphy. This gave four sets of oils for each experiment. This

data was used to examine the effectiveness of the

condensation system in condensing the tyre pyrolysis oil

into condensed fractions of different molecular weights.

Fig. 3 shows the influence of the type of packing material

on the average molecular weight of the oils collected from the

various condensed fractions of the online condensation

system. Using no packing at all gave the smallest range of

molecular weights between condensed fractions. The

difference in average molecular weight between the oil

collected in the first condenser and that collected in the

cooled impingers was only 40 Da. In comparison, the use of

mesh packing produced condensed oils with a significantly

wider range of average molecular weights from 305 Da in the

first condenser to 240 Da in the cooled glass impingers,

a range of 65 Da. The presence of the pall ring type packing

material further increased this range of average molecular

weights to over a 100 Da. Increasing the range of average

molecular weights of the oils collected across the first,

second, third steel condensers and the set of glass impingers

indicated that the condensation system was working more

efficiently, fractionating the condensable oils into more

distinct molecular weight streams. The small range of

molecular weights between the first condenser and the

glass impingers of the condenser system without packing

material, suggested that a significant amount of higher

molecular weight material had been carried through the

condenser system to be collected in the final glass impingers.

This was undesirable since the aim of the experiments was to

try and fractionate the heavier weight molecular weight

material separately from the lower molecular weight oils.

The use of pall rings greatly improved this molecular

weight difference between the fractions of condensed oils

Fig. 2. Influence of the presence and type of packing material in the condensation system on the fraction of oil collected in each condenser.

Fig. 3. Molecular weight range of the condensed tyre pyrolysis oils in relation to the presence and type of packing material in the condensation system.

P.T. Williams, A.J. Brindle / Fuel 82 (2003) 1023–1031 1027

indicating better separation of higher molecular weight

material from lower molecular weight material.

The selective condensation system was examined for its

effectiveness at increasing the higher value chemicals,

toluene, m/p-xylene, o-xylene and limonene in the various

oil fractions condensed in each condenser. In relation to the

type of packing material, the yields of these chemicals

derived from tyre pyrolysis under the fixed conditions of

pyrolysis were constant since the pyrolysis parameters

remained the same, only the type of condensation system

examined changed between experiments. The concentration

of these chemicals in the total tyre pyrolysis oil was

determined by collecting the oil in a single oil fraction and

analysing as a whole. The concentrations determined were,

o-xylene, 0.2 wt%, m/p-xylene 1.1 wt%, toluene, 1.1 wt%

and limonene 3.6 wt%.

Fig. 4 shows the influence of the condenser packing

material on the yields of xylenes, toluene and limonene in

the oils collected in each condenser. The effect of the

packing material was to aid in the selectivity of the

condensation system by increasing the yields of high

value chemicals found in the final glass impinger conden-

sation fraction with a consequent reduction in concentration

in high value chemicals in the condenser fractions 1–3. The

presence of the mesh packing in the stainless steel

condensers was found to be marginally beneficial to the

selectivity of the system compared to the presence of no

packing material, increasing the concentrations of the high

value chemicals in the final glass impinger condenser set.

The biggest increase in selectivity was demonstrated by the

use of the pall ring packing material which increased the

concentration of limonene condensed in the final glass

impinger fraction from 4.8 to 6.8 wt%. Similar relative

increases in the final condensation set were seen for toluene

which increased in the oil fraction collected in the glass

impingers from 1.9 to 3.2 wt% and for m/p-xylene which

increased from 1.8 to 2.8 wt%. As a consequence of these

increases in concentration of chemicals in the impinger

stream there was a corresponding decrease in the concen-

tration of these chemicals in the preceding stainless steel

condensers, particularly condenser 1. The improved selec-

tivity towards higher concentrations of single ring aromatic

compounds in the condensed oils collected in the final glass

impinger condenser was reflected in the oil molecular

weight data shown in Fig. 3. That data showed that the final

glass impinger condenser collected oil with a lower

molecular weight distribution when pall rings were added

compared to no packing material being present in the

stainless steel condensers.

3.3. Influence of condenser temperature

Fig. 5 shows distribution of condensed oil in the stainless

steel condensers and glass condensers when the stainless

steel condensers were all maintained at either 100, 150, 200

or 250 8C. For this set of experiments, the three stainless

steel condensers were packed with the small pall ring

packing material, since these produced the highest con-

denser efficiency. It should be noted that for each

experiment, all three stainless steel condensers were held

at the same temperature and the glass condensers were

maintained at 270 8C and collected all of the remaining oil

Fig. 4. Concentration of xylenes, toluene and limonene in the condensed tyre pyrolysis oils in relation to the presence and type of packing material in the

condensation system.

P.T. Williams, A.J. Brindle / Fuel 82 (2003) 1023–10311028

which passed through the first three condensers. Fig. 5

shows that increasing the condenser temperature reduced

the mass balance of oils distributed to the steel condensation

units, a higher mass percentage of oil exited the steel

condensers and was trapped in the final glass impingers.

When the three steel condensers were maintained at 100 8C,

67% of the oil was condensed in the steel condensers, at a

condenser temperature of 250 8C, 50% was collected in the

first three steel condensers. This was reflected in the

collected oil fraction for condenser 1 which collected 42%

at 100 8C and at 250 8C it contained just 25%.

Increasing the condenser temperature above the tem-

perature at which a compound condenses will keep it in the

vapour phase. Hence, compounds that condensed in the

condenser system at 100 8C were eluted to the final glass

impinger condenser system at 250 8C. In addition, by

increasing the temperature of the condensers, the gas flow

velocity would also increase and therefore a reduction in the

gas residence time would occur. The gas residence time at a

condenser temperature of 250 8C was calculated to be 30%

less than at a 100 8C condenser temperature. This would

have an effect on the efficiency of the condensation units

since residence time is an important factor in obtaining good

equilibration and better mass transfer.

Fig. 6 shows the influence of the stainless steel condenser

temperature on the weight average molecular weight

throughout condensation system, where the packing

material was small pall rings. Increasing the temperature

of the heated stainless steel condensation system was, as a

whole, found to give increased molecular weights for oils

collected in the steel condensers. A higher condenser

temperature resulting in fewer compounds of lower

molecular weight condensing until they reach the glass

condenser system which was cooled to 270 8C. This effect

was particularly noticeable in steel condenser 1 where the

weight average molecular weight increased from 306 to

368 Da. The two further condensers also showed similar,

but smaller, increases in the molecular weight of the

collected oil. Reference to Fig. 5 for the oil distribution data,

showed that although less oil was collected at the highest

condenser temperature for the three steel condensers and

condenser 1 in particular, it was of a higher molecular

Fig. 5. Influence of the temperature of the three stainless steel condensers on the fraction of oil collected in each condenser.

Fig. 6. Molecular weight range of the condensed tyre pyrolysis oils in relation to the temperature of the three stainless steel condensers.

P.T. Williams, A.J. Brindle / Fuel 82 (2003) 1023–1031 1029

weight. One of the effects of increasing the condenser

temperature could have been the exit of more higher

molecular weight material to the glass impinger, condenser

system because of their inability to condense out of the

vapour phase. However, this was not the case, and

increasing stainless steel condenser temperature appeared

to have improved the selectivity of the system. Fig. 6 shows

that increasing condenser temperature actually decreased

the molecular weight of the oil found in the glass impingers.

Hence, the effect of increasing condenser temperature was

to improve the efficiency and selectivity in relation to the

average molecular weight of the oils collected in each

condenser.

The influence of the temperature of the stainless steel

condensers on the concentration of xylenes, toluene and

limonene in the condensed oils was examined and the

results are shown in Fig. 7. Fig. 7 shows that as the

temperature of the steel condensation system was

increased, so too did the difference in concentrations of

the toluene, xylenes and limonene in the glass impingers

compared to the steel condensers. For example, the

concentration of limonene in the oil fraction collected in

condenser 1 at a condensation system operating tem-

perature of 100 8C was 1.5 wt%, whilst in the glass

impingers it was 6.8 wt%. At a steel condenser

temperature of 250 8C the concentration of limonene in

the oil in condenser 1 was reduced to 0.5 wt% a factor of

three reduction. In addition, the concentration of

limonene in the glass impingers was also reduced, but

to a lesser extent, at 5.6%. Similarly, at a condensation

system operating temperature of 100 8C both toluene and

m/p-xylene were present throughout the condensation

system at low levels (<0.3%) whilst at 250 8C these

chemicals were only found in the glass impinger

condensed oil. It should be noted that the overall yields

of high value chemicals remained similar because

pyrolysis conditions had not changed; the differing

concentrations were the effect of condensation par-

ameters. Totalling the concentration of high value

chemicals of a particular stream with the amount of oil

collected in that stream gave yields similar to those

where all the oil was condensed and bulked to give one

oil sample at o-xylene, 0.2 wt%, m/p-xylene 1.1 wt%,

toluene, 1.1 wt% and limonene 3.6 wt%. It has been

suggested that limonene degrades readily to benzene,

xylene, toluene, trimethylbenzene, styrene and methyl-

styrene [7]. However, this degradation occurs above

temperatures of about 450 8C rather than the maximum

condenser temperature used in this work of 250 8C.

At the higher steel condensation temperatures in

condensers 1–3, reduced concentrations of toluene, xylene

and limonene were found. This was due to in part, the

Fig. 7. Influence of the temperature of the three stainless steel condensers on the concentration of xylenes, toluene and limonene in the condensed tyre pyrolysis

oils.

P.T. Williams, A.J. Brindle / Fuel 82 (2003) 1023–10311030

reduced gas residence time at the higher temperature of

about 30% as the condensation system was increased from

100 to 250 8C. This would allow less time for compounds to

equilibrate with the surroundings, especially important

if their dew point was just below the condensation

temperature. In addition, as the condensation temperature

was increased, compounds which condense at 100 8C are

less likely to do so at 250 8C therefore greatly reducing their

condensation. It should be noted that the condensation

system design was not optimal with respect to equilibrium

and kinetic conditions.

Fig. 7 also shows that there was a steady decrease in

toluene, xylenes and limonene condensed in the glass

impinger condenser system as the temperature of the steel

condensers 1–3, was increased. For example, the concen-

tration of limonene in the oil collected in the glass impinger

condensation fraction was reduced from 6.8 to 5.6 wt% and

the concentration of m/p-xylene was reduced from 2.8 to

2 wt%. This data should be compared to Fig. 5 which shows

that the lower concentration of toluene, xylenes and

limonene in relation to increased condenser temperature

was linked to the higher oil yield carried through to the final

glass impinger. That is, the concentration of these chemicals

in the oil had decreased, but the amount had increased.

Since the same amount of toluene, xylenes and limonene

were obtained overall, irrespective of the condensation

system temperature, it is apparent that the enhanced overall

amount of these chemicals in the oil collected in the final

glass impingers aids the selective concentration by selective

temperature condensation of these chemicals.

4. Conclusions

The pyrolysis of scrap tyres was investigated with a

condensation system which could be controlled to different

temperatures. The influence of the condenser temperature

on the distribution of oil and the composition of the oil in

each fraction was examined. However, initial experiments

were carried out to determine the most suitable type of

condenser packing material which would aid the conden-

sation process. The distribution of oil in the condenser

system was found to be optimal in terms of the amount of oil

condensed, the range of molecular weights of the oil and

the concentrations of toluene, xylene and limonene when

pall rings were used as the condenser packing material.

When the temperature of the first three steel condensers was

increased, it was found that more oil was carried through to

the final glass impinger condensers. It was also found that

the condensed oils in the initial condensers contained higher

molecular weight material. The oils condensed in the final

glass impingers at the higher condensation temperatures had

lower concentrations of toluene, xylenes and limonene

compared to the lower condenser temperatures. However,

whilst the concentration of each species was lower, the total

mass in each oil fraction increased, since the yield of oil in

the final glass condenser had increased.

Acknowledgements

The authors gratefully acknowledge the support of the

UK Engineering and Physical Science for support for this

work (Grant No. GR/L35331/01).

References

[1] Dhir RK, Limbachiya MC, Paine KA, editors. Recycling and reuse of

used tyres. London: Thomas Telford Publishing; 2001.

[2] Williams PT, Besler S, Taylor DT, Bottrill RP. J Inst Energy 1995;68:

11–21.

[3] Cunliffe AM, Williams PT. Environ Technol 1998;19:1177–90.

[4] Roy C, Unsworth J. In: Ferrero GL, Maniatis K, Buekens A,

Bridgwater AV, editors. Pyrolysis and gasification. London: Elsevier

Applied Science; 1989.

[5] Williams PT, Brindle AJ. Fuel 2002;81:2425–34.

[6] Williams PT, Bottrill RP, Cunliffe AM. Trans Inst Chem Engrs, Part

B, Process, Safety Environ Protect 1998;76:291–301.

[7] Pakdel H, Roy C, Aubin H, Jean G, Coulombe S. Environ Sci Technol

1992;25:1646–9.

[8] Kaminsky W, Sinn H. In: Jones JL, Radding SB, editors. Thermal

conversion of solid wastes and biomass. ACS Symposium Series 130.

Washington, DC: American Chemical Society; 1980.

[9] Dodds J, Domenico WF, Evans DR, Fish LW, Lassahn PL, Toth WJ.

Scrap tyres: a resource and technology evaluation of tyre pyrolysis

and other selected alternative technologies. US Department of Energy

Report EGG-2241; 1983.

[10] Henley EJ, Seader JD. Equilibrium stage separation operations in

chemical engineering. New York: JWS Publishers; 1981.

P.T. Williams, A.J. Brindle / Fuel 82 (2003) 1023–1031 1031