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Production of Dl-limonene by vacuum pyrolysis of used tires

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Various samples of used car and truck tires were pyrolyzed in a batch mode under vacuum and in a continuous feed reactor. The pyrolysis temperature varied in the range of 440–570°C. dl-limonene is a major product formed during the thermal decomposition of rubber under reduced pressure conditions. The pyrolysis oils were distilled to obtain a dl-limonene-rich fraction. The difficulty of obtaining a pure dl-limonene fraction is discussed. A high pyrolysis temperature decreases the dl-limonene yield due to the cracking of the pyrolysis oil. Several secondary organic compounds produced by cracking were identified by gas chromatography/mass spectrometry (GC/MS) analysis. These compounds had a boiling point similar to dl-limonene. The dl-limonene yield decreases with an increase of the pyrolysis reactor pressure. The mechanism of the thermal degradation of tires leading to the formation of dl-limonene is discussed. A dl-limonene-rich fraction was obtained following a series of distillation. Sulfur-containing compounds in the dl-limonene-rich fractions were analyzed by GC using a sulfur specific detector. Several thiophene-derivatives were identified. Quantitative analysis of the sulfur compounds in the dl-limonene rich fractions was made. An olfactometry test was performed on a standard thiophene sample in d- and dl-limonene solutions to establish an approximate threshold value to detect the thiophene odor.
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Journal of Analytical and Applied Pyrolysis
57 (2001) 91107
Production of dl-limonene by vacuum pyrolysis
of used tires
Hooshang Pakdel
a
, Dana Magdalena Pantea
a
,
Christian Roy
a,b,
*
a
De´partement de ge´nie chimique,Uni6ersite´La6al,Cite Uni6ersitaire,Sainte-Foy,
Quebec G
1
K
7
P
4
,Canada
b
Institut Pyro6ac Inc.,
333
rue Franquet,Sainte-Foy,Quebec G
1
P
4
C
7
,Canada
Received 17 February 2000; accepted 10 August 2000
Abstract
Various samples of used car and truck tires were pyrolyzed in a batch mode under vacuum
and in a continuous feed reactor. The pyrolysis temperature varied in the range of
440570°C. dl-limonene is a major product formed during the thermal decomposition of
rubber under reduced pressure conditions. The pyrolysis oils were distilled to obtain a
dl-limonene-rich fraction. The difficulty of obtaining a pure dl-limonene fraction is discussed.
A high pyrolysis temperature decreases the dl-limonene yield due to the cracking of the
pyrolysis oil. Several secondary organic compounds produced by cracking were identified by
gas chromatography/mass spectrometry (GC/MS) analysis. These compounds had a boiling
point similar to dl-limonene. The dl -limonene yield decreases with an increase of the
pyrolysis reactor pressure. The mechanism of the thermal degradation of tires leading to the
formation of dl-limonene is discussed. A dl -limonene-rich fraction was obtained following a
series of distillation. Sulfur-containing compounds in the dl-limonene-rich fractions were
analyzed by GC using a sulfur specific detector. Several thiophene-derivatives were identified.
Quantitative analysis of the sulfur compounds in the dl-limonene rich fractions was made.
An olfactometry test was performed on a standard thiophene sample in d- and dl-limonene
solutions to establish an approximate threshold value to detect the thiophene odor. © 2001
Elsevier Science B.V. All rights reserved.
Keywords
:
dl-limonene; Pyrolysis; GC/MS
www.elsevier.com/locate/jaap
* Corresponding author. Tel.: +1 418 656 7406; fax: +1 418 656 5993.
E-mail address
:
croy@gch.ulaval.ca (C. Roy).
0165-2370/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0165-2370(00)00136-4
92 H.Pakdel et al.
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J.Anal.Appl.Pyrolysis
57 (2001) 91107
1. Introduction
Although used tires represent less than 1 wt.% of the industrial, commercial and
domestic wastes, they give rise to disposal problems. Disposal problems arise from
the extent to which whole tires float back to the surface and become partially filled
with water, which serves as an ideal breeding habitat for many insects. Another
problem associated with used tires is the fact that they are a major fire hazard when
dumped in large numbers. The number of tire fires is increasing and the generated
toxic compounds contaminate soil, groundwater and air. The mutagenic emission
factor of tires burning in open air has been found to be 3 4 orders of magnitude
greater than the values reported for the combustion of oil, coal or wood in utility
boilers [1]. Polycyclic aromatic hydrocarbons (PAHs) contribute substantially to the
indirect-acting mutagenic activity of the particulate organics emitted from the open
burning of tires while aromatic amines appear to account for much of the
direct-acting mutagenic activity [1]. Composition of PAHs emission is affected by
the conditions under which the combustion occurs [2].
Used tire vacuum pyrolysis is an attractive and clean recycling process solution
which has been the subject of several patents [3]. Vacuum pyrolysis produces useful
liquid hydrocarbons and pyrolytic carbon black. Due to the mild pyrolysis condi-
tions used (e.g. low pyrolysis temperature and absence of a carrier gas), vacuum
pyrolysis produces no hazardous emissions. Vacuum pyrolysis, which operates at a
temperature of about 75 100°C lower than atmospheric pyrolysis, produces an oil
with a different chemical composition. PAHs with potential health hazards are
formed from aliphatic hydrocarbons via Diels Alder type aromatization reactions
at high pyrolysis temperature and long residence time in the reactor. Williams and
Taylor [4] reported the formation of individual hazardous PAHs when tire oils were
subjected to secondary cracking reactions in a post-pyrolysis reactor heated to
720°C. Furthermore, Cunliffe and Williams [5] reported that the PAHs content of
the pyrolysis oils obtained under a nitrogen purged static-bed batch reactor
condition increases with an increase of the pyrolysis temperature. They also
reported that the total PAHs concentration in the oils increased from 1.5 to 3.5
wt.% as the pyrolysis temperature was increased from 450 to 600°C. Due to a lower
pyrolysis temperature, the PAHs content of the vacuum pyrolysis oils is expected to
be lower than atmospheric and high temperature pyrolysis.
Except for the Onahama plant in Japan [6], to our knowledge there is no other
proven large scale, continuous feed industrial tire pyrolysis system operating at
present. Common problems include feeding and handling the tire shreds inside the
reactor and finding end-use applications to the pyrolysis products [7]. Pyrolysis
process economics is greatly influenced by the quality and yield of the pyrolysis
products, especially carbon black.
Vacuum pyrolysis of used tires produces approximately 55 wt.% pyrolysis oil.
This oil typically contains 20 25 wt.% of a naphtha fraction with a boiling point
B200°C. The naphtha fraction typically contains 20 25 wt.% dl -limonene. The
pyrolysis oil is also composed of unsaturated branched chain hydrocarbons and
volatile sulfur and nitrogen-containing compounds [8]. The presence in the oil
93H.Pakdel et al.
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J.Anal.Appl.Pyrolysis
57 (2001) 91107
derived from the vacuum pyrolysis of used tires of single-ring nitrogen compounds
(PANH) such as aniline, pyridine and alkylated pyridine and alkylated quinolins
and sulfur-containing compounds (PANSH) such as benzothiasol has been reported
[8,9]. Tire pyrolysis oil has a high calorific value, typically 40 MJ kg
1
, and a
sulfur content of 0.8 1.6 wt.% depending on the tire source and pyrolysis
process conditions used. Unlike crude oil, tire-derived pyrolysis oil sulfur-contain-
ing compounds are generally volatile thiophenic derivatives.
A high proportion of the volatile aromatic hydrocarbons found in pyrolysis oils,
BTX in particular, can be used as an octane booster if the pyrolysis naphtha
fraction is separated and blended with petroleum naphtha. However, the unsatu-
rated nature of the pyrolysis oil is the main obstacle to refining and handling [10].
dl-Limonene (dipentene) is a major component of the pyrolysis oil and is derived
from the thermal decomposition of polyisoprene [11,12]. Limonene is the chief
constituent of citrus oil and is mainly obtained by expression from the fresh peel of
grapefruit, lemon, and orange. Limonene exists in three forms: d-limonene, the
most naturally abundant, l-limonene and dl-limonene, a racemic isomer. Except for
its optical activity, dl-limonene has the same physical properties as d- and l-
limonene. Limonene has extremely fast-growing and wide industrial applications
[11]. Furthermore, the biological activity of limonene, such as its chemopreventive
activity against rat mammary cancer, has been recently investigated [13,14]. The
market demand for limonene fluctuates considerably. Its price was about 1 US$
kg
1
during the period 1986 1988 and increased up to 9 US$ kg
1
in 1995 1996.
Its sale price was 10 US$ kg
1
as of November 1999.
Polyisoprene or natural rubber compose approximately 50 60% of a typical
truck tire formulation [15]. Both represent an ideal source of limonene [16]. Tire
elastomers other than polyisoprene are not the main source of dl-limonene.
However, Madorsky et al. [17] examined the pyrolysis of polybutadiene rubber, and
found that butadiene, vinylcyclohexene and dipentene were formed in high concen-
trations. Pure polyisoprene yields oil with a wide range of hydrocarbon compounds
upon pyrolysis. Under similar conditions, regular tires yield more solid residue,
which is partially due to the presence of carbon black added during tire manufac-
ture. It has been shown that SBR (styrene and butadiene rubber) and BR
(butadiene rubber) are non-charring rubbers and that extender oil has no effect on
the carbon residue [16]. However, extensive charring and condensation reactions
may occur during pyrolysis owing to poor heat transfer throughout the sample,
slow heating rates, and long residence times of the products in the pyrolysis reactor
[18]. The thermal decomposition of different rubbers has been studied earlier by TG
and DTG to predict the behavior of rubber mixtures and their compositions under
atmospheric nitrogen [19,20] and oxygen [21]. Conesa and co-workers indicated a
weight loss of about 65% at 500°C temperature under nitrogen atmosphere [22]
while a stronger heat effect was observed under oxygen atmosphere with a weight
loss over 80% [21]. Since pyrolysis degradation mechanism largely involves in-
tramolecular free radical reactions which take place in the rubber section of the
product, the polymeric structure and sulfur crosslinking in particular tend to
change the pyrolysis product distribution and oil yield. Pyrolysis gas chromatogra-
94 H.Pakdel et al.
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J.Anal.Appl.Pyrolysis
57 (2001) 91107
phy/mass spectrometry (GC/MS) analysis of many polymers showed significant
amounts of monomers, sometimes almost exclusively, sometimes with higher
oligomers. The effect of filler materials like carbon black produces little interference
in general on the pyrolysis products [23]. However, Cypres and Bettens [24] have
shown that pyrolyzing different brands of tires results in significant differences, of
the order of 10%, in the yields of solid, liquid and gaseous products.
Pyrolysis probe GC/MS analysis of polyisoprene indicates that isoprene is one of
the main degradation products. Vulcanized polyisoprene with various cross-link
densities was also detected by pyrolysis GC/MS and the structure and composition
of the degradation products were determined [25]. The authors reported a decrease
of monomer and dimer content of the pyrolysis product with an increase in
cross-link density. The same authors reported a maximal dl-limonene yield at
434°C. Optimum conditions can be designed to selectively produce a narrow range
of hydrocarbon types and possibly dl-limonene, which is the main objective of this
work.
Insufficient or non-uniform heating process tends to generate heavy aliphatic
hydrocarbons. High temperatures favor volatile aromatics such as benzene. Tamura
et al. [26] suggested that benzene might be formed as a direct result of the thermal
degradation of the rubber polymer via the formation of conjugated double bonds
in the polymer chain. Diels Alder cyclization reaction of alkenes, formed under
extensive secondary reactions of the pyrolysis vapor at either high temperature
and/or long vapor residence times, has been reported to produce benzene and
polycyclic aromatic hydrocarbons [27]. Dehydrogenation of cyclohexene and
derivatives under severe degradation conditions also produces aromatic com-
pounds. Any restrictions to the removal of the vapor products will accelerate the
recondensation and cokefaction reactions.
This paper discusses the optimum operating conditions for the production of
dl-limonene in a large scale vacuum pyrolysis reactor. The limonene formation and
separation methods from the pyrolysis oil as well as the major impurities found in
the limonene fraction are also discussed.
2. Experimental
2
.
1
.Pyrolysis
A schematic diagram of the large scale pyrolysis experimental unit used in this
study (runs cH018, H036 and H045, Table 1) is illustrated in Fig. 1. The
pyrolysis unit is a semi-continuous pilot plant reactor 3-m long with a diameter of
600 mm. The reactor is equipped with two horizontal heating plates, one on top of
the other, each 350-mm wide. Commercial eutectic molten salts circulate counter-
currently with the feedstock through tubes below the heating plates supporting the
bed of tire particles. The salt leaving the reactor is collected in a tank, which is
equipped with a vertical pump to circulate the molten salts through the system and
to the salt reheating unit. During the pyrolysis experiments, the temperature was
95H.Pakdel et al.
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J.Anal.Appl.Pyrolysis
57 (2001) 91107
Table 1
Product yields under various pyrolysis conditions
A121Exp. cA122D014 G45H018 H036 H45 A120
Truck Truck PolyisopreneTruckType Car TruckTruckTruck
Batch, 1 lHorizontal Batch, 1 l Batch, 15 lHorizontalHorizontalMultipleReactor type Batch, 1 l
semi-continuoussemi-continuous semi-continuoushearth
pilotpilotsemi-continuous pilot
pilot
Granules B3.8Average Granules B3.8Cylindrical Granules B3.8 Granules B3.8 Granules B3.8 Granules B3.8 Granules 2
particle form 2.7
volume
(cm
3
)
12 12.0 1.3 1.3 1.3 2813Total pressure 10
(kPa)
480 440 480 500534
c
510–570
d
480
c
431–471
d
500
a
570
b
500Temperature
(°C)
230 0.2 0.2 0.2 1153Total feed (kg) 546 250
––––3321 42Feed rate (kg 25
h
1
)
Product Yields (wt.% on feedstock basis)
43.4 n.a.
e
Oil 90.357.5 56.5 40.9 53.7 60
3.2 n.a. 5.93.67.0Gas 11.710.111.9
53.4 n.a.Solid residue 3.830.6 33.4 38.4 39.3 36.4
Product Yields (wt.% on feedstock basis)
n.a. n.a.13.5 30.711.9 14.4 23.7 n.a.Naphtha
3.3dl-Limonene 2.82.6 9.81.6 0.8 3.6 3.3
a
Bed temperature.
b
Reactor inside wall temperature.
c
Molten salt temperature.
d
Registered from different locations of the reactor inside wall.
e
Not available.
96 H.Pakdel et al.
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J.Anal.Appl.Pyrolysis
57 (2001) 91107
Fig. 1. Vacuum pyrolysis pilot plant schematic.
97H.Pakdel et al.
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J.Anal.Appl.Pyrolysis
57 (2001) 91107
monitored within the different regions of the reactor by means of thermocouples,
which were installed at various locations (Table 1). The process is designated as
Pyrocycling™, and is developed by Pyrovac (Sainte-Foy, P. Quebec, Canada).
Shredded tires were fed at a flow rate comprised between 21 and 42 kg h
1
into
the Pyrocycler™ reactor (see Table 1). The feedstock is conveyed over both heating
plates while being agitated using a novel patented device [28]. As a result, the heat
transfer between the reactor and the pyrolyzed material is significantly increased.
The pyrolysis vapors are evacuated from the reactor by means of a vacuum pump,
which maintained a total pressure B12 kPa in the reactor. The vapors are
condensed in two packed towers indirectly cooled with tap water. Non-condensable
gases are driven out of the condensing towers and burned in a gas burner. An
on-line FTIR spectrometer from BOMEM monitored CO, CO
2
,N
2
,H
2
O, HCl,
HF, NH
3
and H
2
S gases on a real time basis. A summary of all pyrolysis
experimental conditions and the pyrolysis product yields obtained are shown in
Table 1.
Experiment D010 was performed several years before in a semi-continuous
multiple hearth reactor. The reactor description is available elsewhere [27]. Tests
A120 to A122 and G45 were performed in two small batch reactors of 1 l and 15
l capacities, respectively. These two pyrolysis reactor configurations have also been
described in detail elsewhere [29,30].
2
.
2
.Distillation
The pyrolysis oil fractions from experiments D014, H18, H036 and H045 were
distilled batchwise in a 300 l capacity pilot column to recover the naphtha fractions.
The distillation was performed in a 750 mm long and a 45 mm i.d. glass column
with 25 theoretical plates at a 1:30 reflux ratio and packed with a metallic material
(Godloe
TR
). The naphtha fractions were further distilled in a batch mode ina5l
capacity column to recover a concentrated dl-limonene fraction.
2
.
3
.Solid liquid chromatography on dual silica gel and alumina column
The limonene rich fractions were fractionated to simple sub-fractions to analyze
their compositions. Details of the fractionation technique can be found elsewhere
[8].
2
.
4
.Analysis
2
.
4
.
1
.GC analysis for the sulfur containing compounds
A Hewlett Packard model 5970 GC equipped with a flame photometric detector
(FPD) was used to scan the oil fractions to detect the sulfur-containing compounds.
The analytical conditions were as follows: detector temperature 230°C, air flow rate
100 ml min
1
, hydrogen 75 ml min
1
, nitrogen 30 ml min
1
, capillary column
HP-5MS with dimensions 30 m ×0.25 mm i.d. and 0.25 mm of film thickness. The
column temperature was maintained at 30°C for 3 min, then the temperature was
98 H.Pakdel et al.
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J.Anal.Appl.Pyrolysis
57 (2001) 91107
raised to 90°C at 5°C min
1
, then finally to 250°C at 30°C min
1
. The column
temperature was maintained at the final temperature for 10 min.
2
.
4
.
2
.GC/MS analysis
The oil fractions were analyzed by a HP-5890 GC with split injection at 290°C.
The column was a 30 m ×0.25 mm i.d. HP5-5MS fused silica capillary with 0.25
mm film thickness from Hewlett Packard. Helium was the carrier gas with a flow
rate of about 1 ml min
1
. The GC initial oven temperature was 35°C for 3 min,
then programmed to increase to 110°C at 4°C min
1
and then to 250°C at 30°C
min
1
. The oven temperature was held at 250°C for 5 min. The end of the column
was introduced directly into the ion source of a HP-5970 series quadrupole mass
selective detector. The transfer line was set at 270°C and the mass spectrometer ion
source was at 250°C with a 70 eV ionization potential. Data acquisition was carried
out with a PC base G1034C Chemstation software and a NBS library data base.
The mass range of m/z=30 350 Da was scanned every second.
3. Results and discussion
All the pyrolysis oil samples were recovered and subjected to dl-limonene analysis
using naphthalene as an internal standard. In addition, the pyrolysis oils were
distilled to recover the naphtha fractions (bp B210°C). The naphtha fractions
were further subjected to an additional distillation step in a high efficiency
distillation column to recover the dl-limonene-rich fractions. The recovered
limonene fractions were analyzed for impurities and trace concentration of sulfur-
containing compounds. Table 1 shows the limonene yields obtained under various
experimental conditions.
3
.
1
.Formation of limonene
Limonene formation is dependent on the pyrolysis pressure, temperature and
vapor residence time inside the reactor, as well as the sample size and nature. Low
pyrolysis pressure and temperature and short vapor residence time increase the
limonene yield. Implementation of all these conditions in a large-scale pyrolysis
reactor is challenging. The results indicate that two energy dependent reaction
mechanisms exist. One mechanism involves a high energy radical reaction while the
other mechanism involves a low energy dimerization reaction. The high energy
reactions produce hydrocarbons with a high C:H ratio while the low energy
dimerization reactions form hydrocarbons, such as limonene, with a low C:H ratio.
During pyrolysis, both mechanisms occur but at different rates and different
temperatures depending on the location and residence time of the gases in the
reactor. The polyisoprene part of the rubber thermally decomposes through a
b-scission mechanism to an isoprene intermediate radical. It is then transformed to
isoprene (depropagation). The possibility of intramolecular cyclization to form
dl-limonene cannot be ruled out. Isoprene molecules in the gas phase dimerize to
99H.Pakdel et al.
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J.Anal.Appl.Pyrolysis
57 (2001) 91107
dipentene. This depends on the reactor configuration and heat transfer limitation
inside the reactor. If the reactor temperature is too high and the vapor residence
time is too long, then the dl-limonene molecules will decompose to isoprene along
with many other compounds. The high energy mechanism then starts to predomi-
nate following a free radical pathway which will lead to the formation of pyrolysis
oil. It is very unlikely that ideal conditions will be achieved in a large scale pyrolysis
unit. It is believed that all mechanisms occur during the course of pyrolysis to some
extent. Analytical pyrolysis-GC is a suitable technique to perform a controlled
pyrolysis reaction. Groves and Roy [31] used the pyrolysis-GC/MS technique to
study the natural rubber pyrolysis mechanism. The authors indicated that if the
monomer residence time increases in the melt, the relative yield of dimer would be
greater for the thicker samples. They postulated a Diels Alder mechanism for the
formation of dl-limonene. In contrast, it is believed that dl-limonene will decom-
pose above 450°C if it is not quickly removed from the reaction zone.
The mechanism of thermal degradation of tires and tire rubber components
suggests that the major initial products of pyrolysis are isoprene and dl-limonene
and other dimers. Further reaction results in the formation of a wide variety of
compounds, such as aromatic compounds, directly by polymer chain scission or via
degradation products of isoprene and dl-limonene or partially at higher tempera-
tures or long residence times via secondary reactions. Due to their structural
differences, natural rubber decomposes at lower temperatures than styrene and
butadiene rubbers.
Earlier results from the authors’ laboratories indicated that dl-limonene forma-
tion ended below 450°C [12]. Napoli et al. [32] reported that dl-limonene is the
main component of sidewall rubber pyrolysis under a flow of nitrogen at 450°C but
the authors provided no quantitative results. dl-limonene was formed in trace
quantities as the pyrolysis temperature was increased to 550°C. Furthermore, Roy
et al. [30] reported an increase of about 30 40% in the dl -limonene yield as the
pyrolysis pressure of polyisoprene was reduced from 28 kPa to 0.8 kPa. Bhowmick
et al. [33] reported that dl-limonene starts to form at a temperature of about 300°C.
Earlier results of the authors indicated that the six hearth vacuum pyrolysis vertical
reactor operating at 250, 300, 350, 400, 450 and 510°C yielded six fractions of oils
[12]; dl-limonene was mainly found in the oil fractions of hearths 3 5 correspond-
ing to the temperature range of 350 450°C. At any temperature higher than
450 480°C, dl -limonene is believed to decompose to trimethylbenzene, m-cymene
and indane (see Tables 2 and 4). A reaction mechanism for the formation of
trimethylbenzene from the degradation of limonene has been proposed earlier
[34,35]. The A121 experiment was performed at a lower temperature than experi-
ment A120, resulting in a decrease in oil yield. However, the overall dl-limonene
yield in the oil was not affected. As the pyrolysis pressure increased in experiment
A122, the total dl-limonene yield immediately dropped. Similar results were ob-
tained earlier by the authors during the pyrolysis of polyisoprene [30].
Tables 2 4 show the chemical composition of three dl -limonene rich fractions.
Trimethylbenzene, m-cymene and indane are believed to arise from the thermal
decomposition of dl-limonene and other compounds and inhibit the purification of
100 H.Pakdel et al.
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J.Anal.Appl.Pyrolysis
57 (2001) 91107
Table 2
Principal compounds in the dl-limonene concentrated fraction (run D014)
Boiling Point (°C)Tentative Structure Concentration (wt.%)
2,7,7-trimethylbicyclo(2.2.1)heptane 2
34-methyl-1-isopropylcyclohexene
21761-methyl-3-isopropylbenzene (m-cymeme)
92175–176dl-limonene
0.5183Butylbenzene
0.54-ethyl-1,2-dimethylbenzene
Trace169
a,b
3-tert-butylthiophene
2,5-diethylthiophene 181
a,b
Trace
a
Identified by GC/FPD and confirmed by GC/MS.
b
Ref. [36].
dl-limonene. Fig. 2 illustrates the gas chromatograms of the four naphtha fractions
obtained under different conditions (see Table 1). Peaks 1 4 in Fig. 2 were
identified as trimethylbenzene, m-cymene, dl-limonene and indane respectively. As
shown in Tables 2 4, those three compounds have a boiling point similar to that
of dl-limonene and are difficult to separate from dl-limonene by distillation without
substantial additional operating costs. The ratio of dl-limonene in D014 (Fig. 2a)
and H045 (Fig. 2b) to trimethylbenzene, m-cymene and indane is lower than that
of H018 and H036 oils (see Fig. 2c and d). Oils D014 and H045 were obtained at
lower pyrolysis temperatures than oils H018 and H035 (see Table 1). The total
dl-limonene yield also decreased in H018 and H036 pyrolysis oils compared with
D014 and H045. Thus, it can be concluded that dl-limonene undoubtedly has
degraded to form trimethylbenzene, m-cymene and indane when subjected to a
pyrolysis temperature higher than 500°C.
Table 3
Principal compounds in the dl-limonene concentrated fraction (run H018)
Boiling Point (°C) Concentration (wt.%)Tentative Structure
1751-methyl-3-isopropylbenzene 13
19175Trimethylbenzene
175–176dl-limonene 50
176Indane 8
1-propynylbenzene 1
Butylbenzene 3183
169
a,b
0.42-tert-butylthiophene (2 isomers)
181
a,b
2,5-diethylthiophene 0.2
Trace2-methyl-5-propylthiophene \181
Others 5.4
a
Identified by GC/FPD and confirmed by GC/MS.
b
Ref. [36].
101H.Pakdel et al.
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J.Anal.Appl.Pyrolysis
57 (2001) 91107
Fig. 2. Gas chromatograms of the four naphtha fractions obtained under different pyrolysis conditions.
102 H.Pakdel et al.
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J.Anal.Appl.Pyrolysis
57 (2001) 91107
Table 4
Principal compounds in the dl-limonene concentrated fraction (run H036)
Boiling Point (°C) Concentration (wt.%)Tentative Structure
31-methyl-4-isopropenylcyclohexene
5175Trimethylbenzene
175 221-methyl-3-isopropylbenzene
dl-limonene 62175–176
1Indane 176
21-ethyl-3,5-dimethylbenzene
1-propenylbenzene
21-methyl-3-propylbenzene
4-ethyl-1,2-dimethylbenzene
0.38169
a,b
2-tert-butylthiophene
2,5-diethylthiophene 181
a,b
0.31
3Others
a
Identified by GC/FPD and confirmed by GC/MS.
b
Ref. [36].
3
.
2
.Separation of dl-limonene
Compositional analysis of the dl-limonene fractions is of prime importance for
their application and end-use.
dl-Limonene concentrated fraction of D014 oil had an unpleasant S-containing
compound odor. The limonene-rich oil was fractionated into narrow sub-fractions
for its detailed compositional analysis in order to identify malodorous compounds.
The following compounds were identified. All these compounds have similar boiling
points to that of dl-limonene and are not classified as hazardous compounds.
2,7,7-trimethylbicycloheptane
1,4-dimethyl-1,3-cyclohexadiene
1-methyl-4-ethylbenzene
1-methylpropylbenzene
1-methyl-3-isopropylbenzene (m-cymene)
2-methyl-1-propenylbenzene
2or3-tert-butylthiophene
Diethylthiophene
4-methyl-1-isopropylcyclohexene
dl-limonene
Indane
4-methylene-1-isopropylcyclohexene
Butylbenzene
Butenylbenzene
1-methyl-5-isopropenylcyclohexene
2-ethenyl-1,4-dimethylbenzene
Diethylbenzene
Diethylbenzene
103H.Pakdel et al.
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J.Anal.Appl.Pyrolysis
57 (2001) 91107
Dimethylethylbenzene
Trimethylbenzene
Ethyl-3,5-dimethylbenzene
Benzene and cyclohexadiene derivatives are thermal degradation products of
limonene or isoprene units obtained during the first stage of pyrolysis. m-cymene
and dl-limonene are two compounds with an acceptable odor, tert-butylthiophene
and diethylthiophene on the other hand are malodorous. Most sulfur-containing
compounds, typically thiophene and benzothiazol and their derivatives, were easily
removed during the first and second distillation stages. Adsorption and membrane
techniques have been tested for the deodorization of the dl-limonene rich fractions.
These methods are currently under development in the authors’ laboratories. All
the pyrolysis oil samples and their fractions were analyzed with a GC equipped with
a flame photometric detector (GC/FPD) and the removal of sulfur compounds was
monitored during the dl-limonene enrichment process.
3
.
3
.Sulfur analysis
3
.
3
.
1
.GC/MS analysis
Due to the complexity of the pyrolysis oil, the determination of their sulfur
content is quite difficult. However, sub-fractionation of the dl-limonene rich
fractions as outlined in Section 2.3, enabled a detailed GC/MS analysis and led to
a positive identification of trace amounts of sulfur compounds. Formation of sulfur
compounds in the pyrolytic oils is due to the thermal degradation of additives such
as vulcanization agents and accelerators added during the tire fabrication. Their
presence hampers dl-limonene separation and purification. Approximately 60 80%
of the sulfur-containing compounds were removed during the primary distillation of
the pyrolysis oil. The remaining sulfur compounds were analyzed and identified
following their sub-fractionation. By GC/MS analysis of the separated fractions,
dimethylthiophene, tert-butylthiophene and diethylthophene were found to be the
major sulfur compounds in the dl-limonene rich fractions. m/z=111 for (C
6
H
7
S)
+
,m/z=125 for (C
7
H
9
S)
+
and m/z=139 for (C
8
H
11
S)
+
were identified as the
principal fragment ions of dimethylthiophene, tert-buthylthiophene and diethylthio-
phene, respectively, in the dl-limonene rich fractions [37]. The distribution of these
sulfur compounds was monitored by GC/MS by selecting m/z 111, 125 and 139
ions. The analysis of the dl-limonene rich fractions revealed a similar sulfur
compound distribution to that of GC/MS.
3
.
3
.
2
.GC/FPD analysis
GC/FPD analysis was applied as a fingerprint GC analysis of pyrolysis oils and
the dl-limonene fractions during the course of the enrichment process. FPD is a
fast, robust selective and sensitive method of sulfur analysis but suffers from the
quenching effect of the high concentration of hydrocarbons in fuel samples due to
104 H.Pakdel et al.
/
J.Anal.Appl.Pyrolysis
57 (2001) 91107
the incomplete separation of the sulfur compounds from the hydrocarbons and
non-linearity of the detector response. Preliminary separation of the oil fractions
can significantly reduce the quenching effect for a meaningful quantitative analysis.
If the peak area (S) and sulfur concentration (C) is expressed as ln S/ln C, the
response/concentration curve becomes linear. The sulfur compounds concentration
can then be calculated from the following equation:
C
sulfur
=C
sulfur compound
×N×32.06/M
where Nis the number of sulfur atoms in the sulfur compound and Mis the
molecular weight of the compound. ln Svalues of three standards, namely thio-
phene, dimethylthiophene and dodecanethiol, were plotted in various concentra-
tions versus ln Cby linear regression. These compounds did not give a single curve
as demonstrated by Zoccolillo et al. [38], but the following equations were derived.
Any of the following equations could be used to calculate the sulfur content in the
dl-limonene enriched fraction.
Y=2.0738x+1.5170, R
2
=0.9977 for thiophene
Y=2.0683x+1.4390, R
2
=0.9932 for 2,5-dimethylthiophene
Y=2.0192x+1.5568, R
2
=0.9942 for dodecanethiol
These equations produced a proportionality constant factor, the exponential n
factor, and a linearity factor very close to 2. Due to S
2
emissions, the response of
the FPD in the sulfur mode is generally assumed to be proportional to the square
of the input sulfur concentration if the detector is functioning satisfactorily [39]. It
was found that the FPD response to sulfur only depends on the number of sulfur
atoms in the molecules and is independent of the chemical structure [40]. Farwell
and Barinaga [41] discussed the following reasons for the deviation from the
linearity n-factor of 2: non-optimum flame conditions, compound-dependent de-
composition, competitive flame reactions, experimental imprecision, non-gaussian
chromatographic sample introduction and quenching effects. Quantitative sulfur
analysis of the limonene enriched fractions using the thiophene equation or the
other equations yielded similar sulfur contents. The results of three dl-limonene-en-
riched fractions are shown in Tables 2 4.
The naphtha fractions corresponding to runs H018 and H036 were submitted to
total sulfur analysis using the bomb calorimetric method (ASTM No. D516-86).
The sulfur content was respectively 0.48 and 0.71%. The GC/FPD method indi-
cated sulfur content of 0.44 and 0.51% respectively. The difference is attributed to
the quenching effect of the hydrocarbon content of the naphtha fractions. A similar
sulfur content for tire-derived pyrolysis oil produced by a different pyrolysis process
from vacuum pyrolysis has been reported in the literature [42]. The H018 dl-
limonene enriched fraction was found to have a sulfur content of 0.18% using both
ASTM and GC/FPD methods, confirming the absence of the quenching effect after
dl-limonene enrichment.
105H.Pakdel et al.
/
J.Anal.Appl.Pyrolysis
57 (2001) 91107
3
.
4
.Olfactometric test
Due to their trace sulfur content, the dl-limonene enriched fractions from run
D014 (91% dl-limonene and 280 ppm S) and run H045 (53% dl-limonene and 1130
ppm S) had a relatively unpleasant smell. One of the most objective measurements
of odor intensity is its threshold value which reflects the intensity of only one
specific odorant concentration, i.e., the weakest that can be detected. Pure samples
of commercial d-limonene, dl-limonene and thiophene from Aldrich (Oakfield,
Ontario, Canada) were chosen for a laboratory olfactometry test. Olfactometry
using the human sense of smell is reported to be the most valid means of measuring
odor [43]. Detailed information about odor measurement and factors affecting
olfactometry panel performance can be found elsewhere [44]. Two sets of pure
d-limonene and dl-limonene samples with thiophene contents were prepared over a
range of 5 51 ppm. Panelists were selected from the local department staff and
graduate students. Threshold values of about 15.4 ppm and 20.6 ppm were
measured respectively for dl-limonene and d-limonene samples. An exponential
dependence between the intensity of odor and its concentration was observed. The
sulfur concentration of the dl-limonene-enriched fraction of D014 oil was about 228
ppm, which is above the dl-limonene threshold value. A limonene-rich fraction with
a pleasant odor was later obtained using a membrane purification method. Prelim-
inary results obtained at the laboratory scale by the authors using an asymmetric
polyimide capillary membrane tube led to a limonene fraction virtually free of
unpleasant sulfurous odor.
4. Conclusion
The maximum yield of dl-limonene (3.6 wt.%) was obtained from truck tires in
a pilot plant pyrolysis reactor.
dl-Limonene is formed by the dimerization of isoprene units following a low
energy reaction mechanism. Intramolecular cyclization to form dl-limonene is
also possible.
A pyrolysis temperature higher than 500°C tends to crack the limonene
molecules to trimethylbenzene, m-cymene and indane which have boiling points
similar to dl-limonene.
The dl-limonene yield increases as the pyrolysis pressure decreases.
Any heat and mass transfer limitations during the pyrolysis hamper dl-limonene
formation and favor a secondary degradation of the pyrolysis products.
Dimethylthiophene, diethylthiophene and tert-butylthiophene found at a concen-
tration of about 228 ppm are the major source for the unpleasant odor in the
dl-limonene rich fraction.
The quantitative determination of the total sulfur content of a dl-limonene rich
fraction with GC/FPD can be made using thiophene, dimethylthiophene or
dodecanthiol as the external standards.
106 H.Pakdel et al.
/
J.Anal.Appl.Pyrolysis
57 (2001) 91107
The sub-fractionation of the oil samples eliminated the quenching effect for
reliable GC/FPD analysis.
A proven and economical method to deodorize dl-limonene-enriched fractions is
needed.
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This paper demonstrates the inherent error that can be introduced when a FPD response linearizer with a fixed exponential n-factor of 2.0 is used for the analysis of sulfur-containing compounds. The magnitude of the potential resultant error with such a device is shown to be in the range of 10% to approximately 400% when n varies from 2.0 to 1.2. The experimental n-factors for H2S, CH3SCH3, CH3SSCH3, and SO2were found to be less than the theoretical value of n = 2. Thus, the error due to a linearization circuit that assumes that the FPD response is proportional to the square of the sulfur concentration can be considerable and may severely limit the usefulness of the commercial linearizers. Specific recommendations are discussed to minimize this type of error.
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This article assesses flame-photometric (FPD) sulfur-selective detectors with respect to: known sulfur flame chemistry, response classifications, non-linear response models, response quenching, background sulfur doping effects, capillary column compatibility, suggestions for improvements in current FPDs, and potential alternatives to the FPD available now or in the future. Also described is a phenomenological sulfur-response model that incorporates contributions from sulfur analyte, background sulfur, analyte-derived non-sulfur emissions, background flame species, and quenching effects. Certain common FPD idiosyncrasies are also examined and explained in view of present information.
Article
Emission responses of twenty one compounds containing sulfur were examined with a flame photometric detector (FPD) using the 394 nm filter. It was found that the response to sulfur compounds (containing C1-C12) depended only on the number of sulfur atoms in a molecule and not on its chemical structure. The corrected detector response value (√Hw), which was found proportional to the sulfur concentration, is proposed for comparing the response of sulfur compounds. The effect of experimental conditions such as hydrogen, air, carrier gas flow rates, and detector temperature and the presence of hydrocarbon simultaneously eluting with the sulfur compounds on sulfur response were discussed. For comparison of the sulfur response, experimental conditions mentioned above should be kept constant. It was concluded that the optimum hydrogen and air flow rate varied with the geometry of the detector. It is always necessary to monitor hydrocarbons by the FID for accurate measurement of sulfur compounds. It was shown that quantitative determination of all types of sulfur compounds could be done easily by using one standard sulfur compound for intensity calibration.
Article
A method of identifying and quantifying elastomers and processing oil in tire rubber, called DTG curve simulation method, is proposed. This method is based on DTG (derivative thermogravimetry) analysis of the straight elastomers NR, BR and SBR, which produces a series of decomposition rate data and pairs of kinetic constants. DTG curves of binary elastomer systems NR/SBR, NR/BR and BR/SBR are measured. The samples are qualitatively identified following the shape of their DTG curves. Quantitative determination is achieved by DTG curve simulation using a least squares method. When correct elastomers and proper compound ratio are chosen, minimum simulation error is achieved leading to effective identification. Satisfactory results were obtained when applying the method to elastomer mixtures of known composition and to tire rubbers of unknown composition. The study advanced the capability of the TG-DTG technique for quantitative determination of elastomers and blends.
Article
The decomposition of tyre wastes at different heating rates in an oxidizing atmosphere is explained by means of a kinetic model including a first step of pyrolysis, which assumes three organic fractions not forming residues, and a step of combustion. The atmosphere used varied from 10 of oxygen to 20% (v/v). The activation energy for the step of combustion is in the range 221–235 kJ/mol, and there exists a dependence of the rate of decomposition on the partial pressure of oxygen.
Article
The decomposition of tyre wastes at three different heating rates (1, 5 and 25°C min−1) is explained by means of kinetic model including three organic fractions that do not form residues and an inorganic fraction of gases that takes place in three different stages, confirming that three different groups of compounds are pyrolyzed.
Article
The technique of pyrolysis—gas chromatography (Py—GC) has been used to study the thermal degradation of natural rubber. The monomer/dimer (M/D) ratio has been measured over the temperature range 300 to 500°C, using sample sizes of the order of 0.3 mg. The M/D ratio has also been measured as a function of sample thickness, using samples in the range 30 nm to 3 μm (0.1 μg to 10 μg).The dependence of M/D on temperature may be interpreted in terms of (a) different activation energies for depropagation versus intramolecular cyclisation, (b) dissociation of dimer to monomer at higher temperatures, or (c) reduced residence time of monomer in the melt at higher temperatures allowing less opportunity for its recombination.The results for the dependence of M/D on thickness (at constant temperature) indicate that interpretation (c) is the most probable.The present work therefore suggests that monomer recombination, possibly by a Diels-Alder mechanism is an important contributor to dimer formation in rubber pyrolysis.
Article
The determination of the cross-link density of vulcanized polyisoprene by means of pyrolysis—gas chromatography—mass spectrometry was investigated. The main degradation products were isoprene, 1,5-dimethyl-5-vinylcyclohexene, 1-methyl-4-isopropyl-cyclohexene (dipentene) and 3-methyl-1,3-pentadiene. The quantitative results indicated that the amounts of monomer and dimer decreased with increasing cross-link density, but the yield of 3-methyl-1,3-pentadiene was independent of the cross-link density. A linear relationship exists between dipentene to 3-methyl-1,3-pentadiene (ratio of the peak areas or peak heights) and the cross-link density. The reproducibility of the results was generally good (the relative standard deviation was less than 2.0%).