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Production of dl-limonene by vacuum pyrolysis of used tires
Journal of Analytical and Applied Pyrolysis 57 (2001) 91 – 107 www.elsevier.com/locate/jaap
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 G1K 7P4, Canada b Institut Pyro6ac Inc., 333 rue Franquet, Sainte-Foy, Quebec G1P 4C7, 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 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. © 2001 Elsevier Science B.V. All rights reserved. Keywords: dl-limonene; Pyrolysis; GC/MS
* Corresponding author. Tel.: + 1 418 656 7406; fax: +1 418 656 5993. E-mail address: [email protected] (C. Roy). 0165-2370/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 5 - 2 3 7 0 ( 0 0 ) 0 0 1 3 6 - 4
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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 conditions 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 B 200°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
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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-containing 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 unsaturated 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 llimonene. 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 concentrations. 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 manufacture. 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 intramolecular 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-
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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 compounds. 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 c H018, 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 countercurrently 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
534c510–570d
500a570b 546 42
500 153 21
14.4 0.8
Product Yields (wt.% on feedstock basis) Naphtha 11.9 13.5 dl-Limonene 2.6 1.6
250 25
10
13
40.9 11.7 38.4
b
Bed temperature. Reactor inside wall temperature. c Molten salt temperature. d Registered from different locations of the reactor inside wall. e Not available.
a
H45 Truck
A120 Truck Batch, 1 l
A121 Truck
Batch, 1 l
A122 Truck
Batch, 15 l
G45 Polyisoprene
23.7 3.6
53.7 7.0 39.3
230 33
480c431–471d
12.0
n.a. 3.3
60 3.6 36.4
0.2 –
480
1.3
n.a. 3.3
43.4 3.2 53.4
0.2 –
440
1.3
n.a. 2.8
n.a.e n.a. n.a.
0.2 –
480
1.3
30.7 9.8
90.3 5.9 3.8
1 –
500
28
Granules B3.8 Granules B3.8 Granules B3.8 Granules B3.8 Granules B3.8 Granules B3.8 Granules 2
(wt.% on feedstock basis) 57.5 56.5 11.9 10.1 30.6 33.4
Product Yields Oil Gas Solid residue
H036 Truck
Horizontal Horizontal Horizontal Batch, 1 l semi-continuous semi-continuous semi-continuous pilot pilot pilot
H018 Truck
12
Multiple hearth semi-continuous pilot Cylindrical form 2.7
Reactor type
Average particle volume (cm3) Total pressure (kPa) Temperature (°C) Total feed (kg) Feed rate (kg h−1)
D014 Car
Exp. c Type
Table 1 Product yields under various pyrolysis conditions
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Fig. 1. Vacuum pyrolysis pilot plant schematic.
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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 B 12 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, CO2, N2, H2O, HCl, HF, NH3 and H2S 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 (GodloeTR). The naphtha fractions were further distilled in a batch mode in a 5 l 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
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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 B 210°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 sulfurcontaining 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
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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 predominate 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 decompose 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 temperatures 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 formation 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 corresponding 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 experiment 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 obtained 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
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Table 2 Principal compounds in the dl-limonene concentrated fraction (run D014) Tentative Structure 2,7,7-trimethylbicyclo(2.2.1)heptane 4-methyl-1-isopropylcyclohexene 1-methyl-3-isopropylbenzene (m-cymeme) dl-limonene Butylbenzene 4-ethyl-1,2-dimethylbenzene 3-tert-butylthiophene 2,5-diethylthiophene a b
Boiling Point (°C)
176 175–176 183 169a,b 181a,b
Concentration (wt.%) 2 3 2 92 0.5 0.5 Trace Trace
Identified by GC/FPD and confirmed by GC/MS. 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) Tentative Structure
Boiling Point (°C)
Concentration (wt.%)
1-methyl-3-isopropylbenzene Trimethylbenzene dl-limonene Indane 1-propynylbenzene Butylbenzene 2-tert-butylthiophene (2 isomers) 2,5-diethylthiophene 2-methyl-5-propylthiophene Others
175 175 175–176 176
13 19 50 8 1 3 0.4 0.2 Trace 5.4
a b
183 169a,b 181a,b \181
Identified by GC/FPD and confirmed by GC/MS. Ref. [36].
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Fig. 2. Gas chromatograms of the four naphtha fractions obtained under different pyrolysis conditions.
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Table 4 Principal compounds in the dl-limonene concentrated fraction (run H036) Tentative Structure 1-methyl-4-isopropenylcyclohexene Trimethylbenzene 1-methyl-3-isopropylbenzene dl-limonene Indane 1-ethyl-3,5-dimethylbenzene 1-propenylbenzene 1-methyl-3-propylbenzene 4-ethyl-1,2-dimethylbenzene 2-tert-butylthiophene 2,5-diethylthiophene Others a b
Boiling Point (°C)
175 175 175–176 176
169a,b 181a,b
Concentration (wt.%) 3 5 22 62 1 2 – 2 – 0.38 0.31 3
Identified by GC/FPD and confirmed by GC/MS. 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 2 or 3-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
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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 (C6H7S)+ , m/z = 125 for (C7H9S)+ and m/z = 139 for (C8H11S)+ were identified as the principal fragment ions of dimethylthiophene, tert-buthylthiophene and diethylthiophene, 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
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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: Csulfur =Csulfur compound ×N × 32.06/M where N is the number of sulfur atoms in the sulfur compound and M is the molecular weight of the compound. ln S values of three standards, namely thiophene, dimethylthiophene and dodecanethiol, were plotted in various concentrations versus ln C by 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 S2 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 decomposition, 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-enriched 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 indicated 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 dllimonene 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.
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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. Preliminary 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 concentration 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.
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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.
References [1] D.M. DeMarini, P.M. Lemieux, J.V. Ryan, L.R. Brooks, R.W. Williams, Environ. Sci. Technol. 28 (1994) 136–141. [2] M. Mastral, M. Calle´n, R. Murillo, Fuel 75 (1996) 1533 – 1536. [3] C. Roy, European Patent Application Number 0 669 388 A2. 1995; Canadian Patent Number 1,334,433. 1995; Australia Patent Number 651,029. 1994; U.S. Patent Number 5,229,099. 1993; U.S. Patent Number 5,208,401. 1993; Australia Patent Number 636,350. 1993; U.S. Patent Number 5,099,086. 1992; U.S. Patent Number 5,087,436. 1992; Canadian Patent Number 1,271,151. 1990; Canadian Patent No 600,208. 1989; U.S. Patent Number 4,740,270 (1988). [4] P.T. Williams, D.T. Taylor, Fuel 72 (1993) 1469 – 1474. [5] M. Cunliffe, P.T. Williams, J. Anal. Appl. Pyrolysis 44 (1998) 131 – 152. [6] H. Kohno, H. Asao, T. Amano, Use of Alternative Fuel at Onahama. Third AIME Annual Meeting. Dallas, Texas. February 14–18, 1982. [7] C. Roy, A. Chaala, H. Darmstadt, J. Anal. Appl. Pyrolysis 51 (1999) 201 – 221. [8] S. Mirmiran, H. Pakdel, C. Roy, J. Anal. Appl. Pyrolysis 22 (1992) 205 – 215. [9] P.T. Williams, R.P. Bottrill, Fuel 74 (1995) 736. [10] Benallal, C. Roy, H. Pakdel, S. Chabot, M.A. Poirier, Fuel 74 (1995) 1589 – 1594. [11] R. Gelling, M.J. Loadman, B.D. Sidek, J. Polym. Sci. Polym. Chem. 17 (1979) 1383. [12] H. Pakdel, C. Roy, H. Aubin, G. Jean, S. Coulombe, Environ. Sci. Technol. 25 (1991) 1646 – 1649. [13] P.L. Crowell, Breast Cancer Res. Treat. 46 (1997) 191 – 197. [14] Nakaizumi, M. Baba, H. Uehara, H. Iishi, M. Tatsuta, Cancer Lett. 117 (1997) 99 – 103. [15] R.F. Ohm (Ed.), Rubber Handbook, Norwalk, CA: R.T. Vanderbilt Company, 1990, pp. 600 – 612. [16] P.T. Williams, S. Besler, Fuel 74 (1995) 1277 – 1283. [17] S.L. Madorsky, S. Straus, D. Thompson, L. Williamson, J. Res. Natl. Bur. Stand. 42 (1949) 499. [18] P.T. Williams, S. Besler, D.T. Taylor, Fuel 69 (1990) 1474 – 1482. [19] J.A. Conesa, A. Marcilla, J. Anal. Apply. Pyrolysis 37 (1996) 95 – 110. [20] J. Yang, S. Kaliaguine, C. Roy, Rubber Chem. Tech. 66 (1993) 213 – 229. [21] J.A. Conesa, R. Font, A. Fullana, J.A. Caballero, Fuel 77 (1998) 1469 – 1475. [22] J.A. Conesa, R. Font, A. Marcilla, J. Anal. Appl. Pyrolysis 43 (1997) 83 – 96. [23] M.J. Matheson, T.P. Wampler, W.J. Simonsick, J. Anal. Appl. Pyrolysis 29 (1994) 129. [24] R. Cypres, B. Bettens, in: G.L. Ferrero, K. Maniatis, A. Buekens, A.V. Bridgwater (Eds.), Pyrolysis and Gasification, Elsevier, London, 1989. [25] J. Xigao, L. Huiming, J. Anal. Appl. Pyrolysis 3 (1981) 49 – 57. [26] S. Tamura, K. Mukarami, H. Kuwazoe, J. Appl. Polym. Sci. 28 (1983) 3467 – 3484. [27] S. Mirmiran, Ph.D Thesis, Universite´ Laval, Quebec 1994. [28] C. Roy, D. Blanchette, B. de Caumia, Horizontal Moving and Stirred Bed Reactor. US Patent Claim Number 08/811 172. March 4, 1997. Canadian Patent Claim Number 2,196,841. February 5, 1997. [29] Roy, A. Rastegar, S. Kaliaguin, H. Darmstadt, V. Tochev, Rubber Compos. Proces. Appl. 23 (1995) 21. [30] Roy, H. Darmstadt, B. Benallal, C. Amen-Chen, Fuel Proc. Tech. 50 (1997) 87 – 103. [31] S.A. Groves, L.S. Roy, J. Anal. Appl. Pyrolysis 19 (1991) 301 – 309. [32] Napoli, Y. Soudais, D. Lecomte, S. Castillo, J. Anal. Appl. Pyrolysis 40 – 41 (1997) 373 – 382. [33] K. Bhowmick, S. Rampalli, K. Gallagher, R. Seeger, D.J. McIntyre, J. Anal. Appl. Polym. Sci. 33 (1987) 1125.
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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 G1K 7P4, Canada b Institut Pyro6ac Inc., 333 rue Franquet, Sainte-Foy, Quebec G1P 4C7, 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 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. © 2001 Elsevier Science B.V. All rights reserved. Keywords: dl-limonene; Pyrolysis; GC/MS
* Corresponding author. Tel.: + 1 418 656 7406; fax: +1 418 656 5993. E-mail address: [email protected] (C. Roy). 0165-2370/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 5 - 2 3 7 0 ( 0 0 ) 0 0 1 3 6 - 4
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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 conditions 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 B 200°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
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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-containing 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 unsaturated 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 llimonene. 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 concentrations. 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 manufacture. 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 intramolecular 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-
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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 compounds. 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 c H018, 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 countercurrently 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
534c510–570d
500a570b 546 42
500 153 21
14.4 0.8
Product Yields (wt.% on feedstock basis) Naphtha 11.9 13.5 dl-Limonene 2.6 1.6
250 25
10
13
40.9 11.7 38.4
b
Bed temperature. Reactor inside wall temperature. c Molten salt temperature. d Registered from different locations of the reactor inside wall. e Not available.
a
H45 Truck
A120 Truck Batch, 1 l
A121 Truck
Batch, 1 l
A122 Truck
Batch, 15 l
G45 Polyisoprene
23.7 3.6
53.7 7.0 39.3
230 33
480c431–471d
12.0
n.a. 3.3
60 3.6 36.4
0.2 –
480
1.3
n.a. 3.3
43.4 3.2 53.4
0.2 –
440
1.3
n.a. 2.8
n.a.e n.a. n.a.
0.2 –
480
1.3
30.7 9.8
90.3 5.9 3.8
1 –
500
28
Granules B3.8 Granules B3.8 Granules B3.8 Granules B3.8 Granules B3.8 Granules B3.8 Granules 2
(wt.% on feedstock basis) 57.5 56.5 11.9 10.1 30.6 33.4
Product Yields Oil Gas Solid residue
H036 Truck
Horizontal Horizontal Horizontal Batch, 1 l semi-continuous semi-continuous semi-continuous pilot pilot pilot
H018 Truck
12
Multiple hearth semi-continuous pilot Cylindrical form 2.7
Reactor type
Average particle volume (cm3) Total pressure (kPa) Temperature (°C) Total feed (kg) Feed rate (kg h−1)
D014 Car
Exp. c Type
Table 1 Product yields under various pyrolysis conditions
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Fig. 1. Vacuum pyrolysis pilot plant schematic.
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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 B 12 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, CO2, N2, H2O, HCl, HF, NH3 and H2S 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 (GodloeTR). The naphtha fractions were further distilled in a batch mode in a 5 l 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
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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 B 210°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 sulfurcontaining 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
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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 predominate 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 decompose 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 temperatures 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 formation 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 corresponding 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 experiment 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 obtained 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
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Table 2 Principal compounds in the dl-limonene concentrated fraction (run D014) Tentative Structure 2,7,7-trimethylbicyclo(2.2.1)heptane 4-methyl-1-isopropylcyclohexene 1-methyl-3-isopropylbenzene (m-cymeme) dl-limonene Butylbenzene 4-ethyl-1,2-dimethylbenzene 3-tert-butylthiophene 2,5-diethylthiophene a b
Boiling Point (°C)
176 175–176 183 169a,b 181a,b
Concentration (wt.%) 2 3 2 92 0.5 0.5 Trace Trace
Identified by GC/FPD and confirmed by GC/MS. 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) Tentative Structure
Boiling Point (°C)
Concentration (wt.%)
1-methyl-3-isopropylbenzene Trimethylbenzene dl-limonene Indane 1-propynylbenzene Butylbenzene 2-tert-butylthiophene (2 isomers) 2,5-diethylthiophene 2-methyl-5-propylthiophene Others
175 175 175–176 176
13 19 50 8 1 3 0.4 0.2 Trace 5.4
a b
183 169a,b 181a,b \181
Identified by GC/FPD and confirmed by GC/MS. Ref. [36].
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Fig. 2. Gas chromatograms of the four naphtha fractions obtained under different pyrolysis conditions.
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Table 4 Principal compounds in the dl-limonene concentrated fraction (run H036) Tentative Structure 1-methyl-4-isopropenylcyclohexene Trimethylbenzene 1-methyl-3-isopropylbenzene dl-limonene Indane 1-ethyl-3,5-dimethylbenzene 1-propenylbenzene 1-methyl-3-propylbenzene 4-ethyl-1,2-dimethylbenzene 2-tert-butylthiophene 2,5-diethylthiophene Others a b
Boiling Point (°C)
175 175 175–176 176
169a,b 181a,b
Concentration (wt.%) 3 5 22 62 1 2 – 2 – 0.38 0.31 3
Identified by GC/FPD and confirmed by GC/MS. 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 2 or 3-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
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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 (C6H7S)+ , m/z = 125 for (C7H9S)+ and m/z = 139 for (C8H11S)+ were identified as the principal fragment ions of dimethylthiophene, tert-buthylthiophene and diethylthiophene, 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
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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: Csulfur =Csulfur compound ×N × 32.06/M where N is the number of sulfur atoms in the sulfur compound and M is the molecular weight of the compound. ln S values of three standards, namely thiophene, dimethylthiophene and dodecanethiol, were plotted in various concentrations versus ln C by 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 S2 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 decomposition, 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-enriched 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 indicated 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 dllimonene 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.
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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. Preliminary 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 concentration 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.
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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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