Production of dl-limonene by vacuum pyrolysis of used tires

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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.

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 <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 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 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 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 chromatography/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.

Section snippets

Pyrolysis

A schematic diagram of the large scale pyrolysis experimental unit used in this study (runs # 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

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 < 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 sulfur-containing compounds.

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

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