Influence of Process Variables on Oils from Tire Pyrolysis and

VOLUME 14, NUMBER 4

JULY/AUGUST 2000

© Copyright 2000 American Chemical Society

Articles Influence of Process Variables on Oils from Tire Pyrolysis and Hydropyrolysis in a Swept Fixed Bed Reactor A. M. Mastral,*,† R. Murillo,† M. S. Calle´n,† T. Garcı´a,† and C. E. Snape‡ Department of Energy and Environment, Instituto de Carboquı´mica, CSIC, P.O. Box 589, Zaragoza 50080, Spain, and Department of Pure and Applied Chemistry, University of Strathclyde, 295 Cathedral Street, Glasgow G1 1XL, Scotland, U.K. Received July 16, 1999

Scrap tires are a growing environmental problem because they are not biodegradable and the components used to manufacture tires cannot readily be recovered. In this investigation, the thermochemical recycling of rubber from old tires by pyrolysis and hydropyrolysis has been studied using a swept fixed bed reactor. This is the first time that such a reactor has been used to carry out tire hydrogenation. The effect of the main process variables (temperature, heating rate, gas flow, reaction time, hydrogen pressure) on yields of oils, gases and solid residue has been determined. The oils, have been characterized using a combination of spectroscopic and chromatographic analytical techniques (TLC-FID, GC-MS, simulated distillation, and FTIR). While the main variable affecting tire conversion is temperature, oil composition is influenced mainly by hydrogen pressure, with the oils becoming lighter as the pressure is raised. No relationship between functional group composition of the oils determined by TLC-FID and FTIR and process variables was found. GC-MS showed that the oils are mainly comprised of single ring alkyl-aromatic species together with a large amount of limonene. Based on this finding, a possible reaction pathway for rubber conversion through polyisoprene depolymerization and further cyclization is discussed.

Introduction The disposal of scrap automotive tires is an increasing environmental problem. For example, in the European Union, a quantity of 2 × 106 Tm is generated every year.1 There are several options available to avoid dumping scrap tires in landfill or open air sites, includ* Author to whom correspondence should be addressed. Fax: 34 976 733977. E-mail: [email protected]. † Instituto de Carboquı´mica, CSIC. ‡ University of Strathclyde. (1) Davidson, R. M. Coprocessing waste with coal; EA Coal Research: London, 1997.

ing combustion,2 pyrolysis,3,4 and hydrogenation.5 Although combustion (incineration) can easily be used to recover energy, the emissions produced (dioxins, PAC, particulate matter, etc.)2 make this possibility unfavorable from an environmental point of view. Tires contain vulcanized rubber in addition to the (2) Mastral, A. M.; Calle´n, M. S.; Murillo, R.; Mayoral, M. C. Coal Science; Ziegler, A., van Heek, K., Eds.; DGMK: Germany, 1997; Vol. 2, p 1155. ISBN 3 931850-22-6. (3) Williams, P. T.; Taylor, D. T. Pyrolysis and Gasification; Elsevier Applied Science: London, U.K., 1989. (4) Cypres, R.; Bettens, B. Pyrolysis and Gasification; Elsevier Applied Science: London, U.K., 1989. (5) Mastral, A. M.; Mayoral, M. C.; Murillo, R.; Calle´n, M. S.; Garcı´a, T.; Tejero, M. P.; Torres, N. Ind. Eng. Chem. Res. 1998, 37, 35453550.

10.1021/ef990183e CCC: $19.00 © 2000 American Chemical Society Published on Web 06/17/2000

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rubberized fabric with reinforcing textile cords, steel or fabric belts, and steel-wire bead.6 The most commonly used tire rubber is styrene-butadiene copolymer (SBR) containing about 25 wt % styrene. Other rubbers used in tire manufacture include natural rubber (cis-polyisoprene), synthetic cis-polyisoprene, and cis-polybutadiene. Another important component in tire manufacturing is carbon black, which is used to strengthen the rubber and aid abrasion resistance. In addition, extender oil, a mixture of aromatic hydrocarbons, is added to soften the rubber and to improve workability. In the rubber vulcanization process, sulfur is used to cross link the polymer chains within the rubber, and it also hardens and prevents excessive deformation at elevated temperatures. In general, the sulfur content of rubber from tire is up to 1.5%. This sulfur percentage performs a positive role in hydrogenating processes due to the SH radical behavior in hydrogenation reactions.7 An accelerator, an organic-sulfur compound, is added together with ZnO and stearic acid to control the vulcanization process and also to enhance the physical properties of the rubber. Currently, more than one hundred different compounds can be added depending on the specific trademark and on the specific use to be given to the tire. The pioneering work on tire pyrolysis carried out by Williams et al.6,8 used a swept fixed bed reactor and nitrogen as carrier gas. They obtained around 55% of liquid product mainly comprised of substituted alkyl benzene. In addition some experiments in a 2-Tm-perday batch reactor were performed obtaining results similar to the laboratory scale ones.9 Other authors have been working with other types of reactors, including fluidized beds10 and rotary kilns.11 However, higher temperatures have been used than in the laboratory investigation by Williams et al.,6,8 giving rise to lower liquid product and higher gas yields. Other studies have been concerned with the production active carbon from scrap tire pyrolysis.12-16 It has been found that the residual solids have surface areas between 50 and 120 m2/g, depending on the pyrolysis temperature, but this surface area can be easily increased to 1000 m2/g through a CO2 or steam activation process. The solids, mainly derived from carbon black, could also be recycled for the manufacture of new tires after suitable treatments to reduce mineral matter. Co-processing of scrap tires with coal to enhance (6) Williams, P. T.; Besler, S.; Taylor, D. T. Fuel 1990, 69 (12), 14781482. (7) Mastral, A. M.; Mayoral, C.; Izquierdo, M. T. ACS Div. Fuel Chem. 1993, 38 (1), 121. (8) Williams, P. T.; Besler, S. Fuel 1995, 74 (9), 1277-1283. (9) Williams, P. T.; Besler, S.; Taylor, D. T. Proc. Inst. Mech. Eng.1993, 207, 55-63. (10) Kaminsky, W.; Sinn H. Thermal conversion of solid wastes and biomass; American Chemical Society Symposium Series 130; ACS Publishers: Washington, DC, 1980. (11) Kawakami, S.; Inone, K.; Tanaka, H.; Sakai, T. American Chemical Society Symposium Series; ACS Publishers: Washington, DC, 1980; p 130. (12) Cunliffe, A. M.; Williams, P. T. Environ. Technol1998, 19 (12), 1177-1190. (13) SanMiguel, G.; Fowler, G. D.; Sollars, C. J. Ind. Eng. Chem. Res. 1998, 37 (6), 2430-2435. (14) Darmstadt, H.; Summchen, L.; Roland, U.; Roy, C.; Kaliaguine, S.; Adnot, A. Surf.Interface Anal.1997, 25 (4), 245-253. (15) Teng, H.; Serio, M.; Wo´jtowicz, M. A.; Bassilakis, R.; Solomon, P. R. Ind. Eng. Chem. Res. 1995, 34, 3102-3111. (16) Merchant, A. A.; Petrich, M. A. AIChE J. 1993, 39 (8), 13701376.

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yields from liquefaction processes has received considerable attention.17-29 In most of the studies carried out, it was observed that tire does not affect the total coal conversion (by comparison of experimental and theoretical values), but it increases the liquids yield, mainly that of asphaltenes. This fact has been attributed to the recombination of coal and tire-derived radicals. Some work on the reaction pathways involved in the pyrolytic degradation of tires has been reported. Groves et al.30 analyzed the oil derived from the pyrolysis of natural rubber in pyrolysis-gas chromatography at 500 °C. They showed that the major products were the monomer (isoprene) and the dimer (dipentene), together with other oligomers up to the hexamer present in significant concentrations. Tamura et al.31 have shown that isoprene and dipentene are formed in high concentration in natural rubber pyrolysis and have suggested that both these products are produced by depolymerization from polymer radicals occurring by β-scission at double bonds. The polymer radicals are liable to form six-member rings, especially under mild pyrolysis conditions, so the dipentene is formed predominantly at lower temperatures. Chien et al.32 pyrolyzed natural rubber in helium at 384 °C and identified isoprene and dipentene as the main products. In this paper, the influence of some important process variables (temperature, heating rate, time, and hydrogen pressure) in tire conversion in hydropyrolysis has been studied using a swept fixed bed reactor. In addition, the oils have been characterized and a mechanism for rubber degradation is postulated. Experimental Section Discarded tires supplied by AMSA (a rubber recycling enterprise), ground and sifted to a particle size of 0.9 mm, were used. The steel thread and the textile netting had previously been removed. The proximate analysis (moisture: 0.94%; ash: 3.83%; volatile matter: 67.30%; fixed carbon: 31.14%) indicates the low ash and high volatile matter of the sample used. The fixed carbon comprises the carbon black used, whereas the volatile matter mainly consists of the polymeric materials present in tire. The ultimate analysis (% C: 88.64; (17) Farcasiu, M.; Smith, C. M. Prepr. Pap.s Am. Chem. Soc., Div. Fuel Chem. 1992, 37 (1), 472-479. (18) Farcasiu, M. CHEMTECH 1993, 23 (1), 22-24. (19) Liu, Z.; Zondlo, J. W.; Dadyburjor, D. B. Energy Fuels 1994, 8 (3), 607-612. (20) Mastral, A. M.; Murillo, R.; Pe´rez-Surio, M. J.; Calle´n, M. S. Energy Fuels 1996, 10 (4), 941-947. (21) Tang, Y.; Curtis, C. W. Fuel Process. Technol. 1996, 46 (3), 195215. (22) Tang, Y.; Curtis, C. W. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1996, 41 (3), 1057-1061. (23) Orr, E. C.; Shi, Y.; Shao, L.; Liang, J.; Ding, W.; Anderson, L. L.; Eyring, E. M. Fuel Process. Technol. 1996, 49 (1/3), 233-246. (24) Orr, E. C.; Shi, Y.; Ji, Q.; Shao, L.; Villanueva, M.; Eyring, E. M. Energy Fuels 1996, 10 (3), 573-578. (25) Mastral, A. M.; Murillo, R.; Calle´n, M. S.; Pe´rez-Surio, M. J.; Mayoral, M. C. Energy Fuels 1997, 11 (3), 676-680. (26) Anderson, L. L.; Calle´n, M. S.; Ding, W.; Liang, J.; Mastral, A. M.; Mayoral, M. C.; Murillo, R. Ind. Eng. Chem. Res. 1997, 36, 47634767. (27) Liu, Z.; Dadyburjor, D. B. Energy Fuels 1995, 9, 673. (28) Mastral, A. M.; Murillo, R.; Mayoral, M. C.; Calle´n, M. S. Energy Fuels 1997, 11, 813-818. (29) Mastral, A. M.; Murillo, R. U.S. Patent 5,936,134, August 1999. (30) Groves, S. A.; Lehrle, R. S.; Blazso, M.; Szekely, T. J. Anal. Appl. Pyrolysis 1991, 19, 301. (31) Tamura, S.; Murakami, K.; Kowazoe, H. J. Appl. Polym. Sci. 1987, 33, 1122. (32) Chien, J. C.; Kiang, J. K. Eur. Polym. J. 1979, 15, 1059.

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Figure 1. Yields obtained in tire pyrolysis as a function of temperature (15 min, 0.56 m/s N2, 300 °C/min). % H: 8.26; % N: 0.43; % S: 1.43) indicates a high relatively high atomic H/C ratio (1.1) and the level of sulfur present from the vulcanization process. The experimental installation used consists of a swept fixed bed reactor coupled to a quadrupole mass spectrometer detector. The internal reactor diameter of the incoloy reactor used was 1 cm and the length of the hot zone was 5 cm. The resistively heated reactor is connected to the gas supply (hydrogen or nitrogen), an ice-cooled condenser where oils are collected, and finally to a quadrupole mass spectrometer which monitors the evolved gases. Further details of the experimental setup can be found elsewhere.33,34 The tire sample (1.2 g) was loaded into the reactor, and after pressurizing and testing for leaks, the required gas flow rate was established using a rotameter. The sample was heated to the desired temperature at rates of 25, 100, and 300 °C min-1, and the on-line mass spectrometer recorded the evolution of selected components in the flue gas. After cooling and disconnecting the system, the condenser is weighed to calculate the oils yield. The oils were recovered in dichloromethane (DCM) and stored in small vials under an inert atmosphere. The remaining solid residue, which is insoluble in DCM, was also weighed and stored. Total conversion, oils yield, and gas yield were calculated according to eqs 1-3. The calculated experimental error was carried out by repetition of the same experiment six times. This way, the standard deviation was found to be 1% for conversion, 1.5% for oil yield, and 1% for gas yield.

% conversion ) 100 ×

(tire)mf - DCMinsolubles

% oils ) 100 ×

(tire)daf oils (tire)daf

% gases ) % conversion - % oils

(1) (2) (3)

The oils have been analyzed by a number of techniques. To identify and quantify the different functional groups, thin-layer chromatography coupled to a flame ionization detector (TLCFID) has been used. In this technique, a fast fractionation on silica-alumina chromrods into saturate, aromatic, and polar compounds is achieved. Saturate compounds are eluted with n-hexane for 15 min, aromatic compounds with toluene for 10 min, and finally polar compounds with a mixture of DCM/ methanol (95:5) for 2 min. The apparatus used was an Iatroscan Mk-5 model. Individual compounds in each of the fractions were identified by GC-MS using a Hewlett-Packard 5890 gas chromatograph with a 30-m capillary DB-5 coupled (33) Ismail, K.; Brown, S. D.; Mitchell, S. C.; Snape, C. E.; Buchanan, A. C., III; Britt, P. F.; Franco, D.; Maes, II; Yperman, J. Energy Fuels 1995, 9, 707-716. (34) Brown, S. D.; Sirkecioglu, O.; Snape, C. E.; Eglinton, T. I. Energy Fuels 1997, 11, 532.

to a Hewlett-Packard 5971 mass spectrometer. To determine the boiling point distributions of the oils, simulated distillation in capillary GC column was performed. A Varian GC Star 3400 gas chromatograph equipped with a 60-m capillary DB-5 column and a FID detector was used. The fractions determined were gasoline (Tb < 200 °C), kerosene (200 °C < Tb < 275 °C), gas-oil (275 °C < Tb < 325 °C), heavy gas-oil (325 °C < Tb < 400 °C), vacuum gas-oil (400 °C < Tb < 540 °C), and vacuum residue (Tb > 540 °C). In addition, some oils were characterized by FTIR using a Nicolet Magna 550 instrument in transmission mode with thin films of oil dispersed on NaCl plates. Regarding the solids, their surface area is analyzed by CO2 and N2 adsorption in a Micromeritics ASAP 2000 apparatus. When CO2 was used, the adsorption was carried out at 273 K and the Dubinin-Ashtakov isotherm applied to calculate the surface area. In the case of N2, the adsorption temperature was 77 K and the classical BET equation was used.

Results and Discussion Effect of Temperature. To check the influence of temperature, experiments at 400, 500, and 600 °C were carried out in which the carrier gas (N2), the heating rate (300 °C/min), the reaction time (15 min), and the gas velocity inside the reactor (0.56 m/s) were kept constant. Figure 1 shows that temperature is critical in that below 500 °C both the total conversion and oil yield decrease around 10%. Indeed, 500 °C appears to be the optimum temperature, since at higher temperatures (600 °C) neither the total conversion nor oil yield increases further. This plateau suggests that all the polymeric materials have been converted into oils and gases. This is further substantiated by the close agreement between the unconverted material (37%) and the amount of fixed carbon (35%) obtained in proximate analysis. The surface areas of the residues occurred in the range 50-70 m2/g, and no relationship to processing conditions was found. The surface areas for N2 and CO2 were identical which demonstrates that no narrow microporosity exists in these solids. The calculated micropore volume applying the BJH model was around 0.25 cm3/g and the average pore diameter was 16 nm. The inorganic content measured as ash content of the residues was close to 12%. Simulated distillation of the oils (Table 1) indicates that higher temperatures are promoting the formation of lighter products since both the contents of gasoline and kerosene increase with temperature. This fact is probably due to hydrocracking of the primary volatile released. It means that internal hydrogen redistribution (in inert atmosphere) takes place giving as consequence lighter and heavier products, simultaneous cracking and retrogressive reactions, keeping constant the oils percentage. Figure 2 shows the CH4 evolution profile as a function of reaction temperature. At high temperature hydrocracking and retrogressive reactions of the previously released products seem to reach equilibrium and a plateau in oils yields is observed. Clearly, the amount of CH4 produced is higher working at 600 °C than at lower temperatures. Figure 2 also reveals that the conversion of tire into oils and gases is a relatively fast process, since after 6 min of reaction time practically no more CH4 is produced. Temperature has no discernible effect on the group composition of the oils determined by TLC-FID (Table

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Mastral et al. Table 2. TLC-FID Analyses of Oils Obtained from Tire Hydropyrolysis as a Function of Different Process Variables process variable

Figure 2. CH4 evolution profile as a function of reaction temperature in tire pyrolysis (15 min, 300 °C/min, 0.056 m/s N2). Table 1. Simulated Distillation of Oils as a Function of Reaction Temperature (300 °C/min, 15 min, 0.56 m/s) % gasoline % kerosene % gas-oil % heavy gas-oil % vacuum gas-oil % vacuum residue

400 °C

500 °C

600 °C

10.8 6.9 10.8 19.6 40.5 11.4

12.5 9.5 13.0 19.0 35.9 10.1

19.2 10.6 11.6 17.2 35.0 6.4

temperature (°C) 400 500 600 heating rate (°C/min) 25 100 300 reaction time (min) 15 30 60 gas velocity (m/s) 0.056 0.560 1.120 H2 pressure (bar) 100 50 10

% saturate

% aromatic

% polar

(300 °C/min, 15 min, 1 bar, 0.56 m/s N2) 6.8 73.8 19.5 5.6 72.9 21.6 7.0 73.3 19.7 (500 °C/min, 15 min, 1 bar, 0.56 m/s N2) 5.3 74.5 20.3 6.6 71.0 22.5 6.3 71.9 21.8 (500 °C, 300 °C/min, 0.056 m/s H2) 3.5 73.4 23.2 6.6 72.6 20.8 6.7 71.8 21.5 (500 °C, 300 °C/min, 15 min, 10 bar H2) 3.5 73.4 23.2 5.9 76.9 17.3 7.1 78.7 14.3 (500 °C, 300 °C/min, 0.011 m/s, 15 min) 3.5 73.4 23.2 9.9 65.6 24.6 10.7 68.5 20.8

Figure 4. Yields obtained as a function of reaction time in tire hydropyrolysis (10 bar, 500 °C, 300 °C/min, 0.056 m/s H2).

Figure 3. Yields obtained as a function of heating rate in tire pyrolysis (15 min, 0.56 m/s N2).

2). The analysis confirms that tire-derived oils are mainly aromatic despite only one aromatic ring present in the SBR monomer, one of the main tire components. However, the composition of the aromatic fraction could be affected by cyclization and aromatization reactions of the primary volatiles. The origin of the polar compounds is not clear, but they could be formed from the recombination of sulfur added to rubber during the vulcanization process. Effect of Heating Rate. Heating rate has been studied because previous work6 has shown that it could have some influence on conversion and product distribution. In addition, heating rate would be an important parameter in possible commercial applications because it would depend on the reactor used. Figure 3 shows the total conversion and oil yield obtained as a function of the heating rate (25, 100, and 300 °C/min). The experiments were carried out at 500 °C with a reaction time of 15 min and a gas velocity (N2) of 0.56 m/s. It can be observed that for the two fastest rates (100 and 300 °C/min) very close results are obtained. Further, the TLC-FID analyses of the oils (Table 2) showed that no significant differences arise as a function of heating rate, with the aromatic fraction prevailing. While these

heating rates are relevant to operation in a rotary kiln, they are still considerably lower than those achieved in fluidized bed reactors. Reaction Time. To check the influence of reaction time, hydropyrolysis was performed at three different reaction times, namely 15, 30, and 60 min, keeping the temperature, heating rate, and flow rate constant (500 °C, 300 °C/min, 0.056 m/s H2, respectively). Only a slight increase in the total conversion is observed at 60 min (see Figure 4), but the difference observed is probably within experimental error. In fact, Figure 5 shows that, for the evolution of the methane, no difference is observed for the three reaction times studied. In all cases the reaction is practically complete after 14 min. As expected, no significant differences were evident in the group composition of the oils (Table 2). Carrier Gas Velocity. From an economical point of view, low gas velocities imply lower gas consumption and smaller pumping equipment, which is particularly important when H2 is used. However, from a scientific point of view, the carrier gas velocity is directly related to vapor residence time inside the reactor and that could influence both product distribution and composition. Three different H2 velocities were used here, namely 0.056, 0.56, and 1.12 m/s. The other conditions used were a temperature of 500 °C, a reaction time of 15 min, a heating rate of 300 °C/min, and a pressure of 10 bar. From Figure 6, it can be appreciated that the differences

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Figure 5. Methane evolution as a function of reaction time in tire hydropyrolysis (10 bar, 500 °C, 300 °C/min, 0.056 m/s H2).

Figure 6. Yields obtained as a function of gas velocity in the reactor (500 °C, 300 °C/min, 10 bar, 15 min).

are small, but small increases in conversion and oil yield with increasing gas velocities are evident. However, from a processing standpoint, the increases are so low that they do not justify such a considerable increase (up to 20 times) in H2 circulation. Something similar could be said in relation to the oils composition (Table 2) where increases in saturate and aromatic concentrations are evident with increasing gas velocity. Hydrogen Pressure. The influence of the inert or hydrogenating atmosphere was checked, respectively, in N2 and H2 atmospheres. Independently of the atmosphere, the hydrogen involved in tire conversion is significative due to the inherent tire hydrogen content. In nitrogen atmosphere there is an internal redistribution into the conversion products. The influence of inert

Figure 7. Yields obtained as a function of H2 pressure (500 °C, 300 °C/min, 0.011 m/s, 15 min).

or hydrogenating atmosphere is not as important in the conversion products distribution as in the oils nature. The influence of the hydrogen atmosphere is reflected on oil quality. The most important variable affecting tire conversion into oils is H2 pressure (see Figure 7). It is observed that while total conversion is not affected, oil yield is improved from 44% at 10 bar to 56% at 100 bar. Since depolymerization is a mainly thermal process, and in most of the experiments carried out similar conversions have been obtained, it could be thought that the initial H2 pressure does not influence product distribution. However, the increase in oil yield at high hydrogen pressures could be due to the tire-derived radicals being stabilized and so preventing retrogressive reactions that could lead to some heavier oils and light hydrocarbon gas formation. Increasing the H2 pressure has improved oil quality with increases in saturates and corresponding decreases in aromatics and polars being observed (Table 2). The simulated distillation of the oils also shows a considerable improvement in quality when working at high pressure with higher concentrations of gasoline and kerosene being obtained (Table 3). It is observed that lighter oils are obtained working at higher H2 pressures. Therefore, it seems that the high H2 pressure helps to stabilize radical species, preventing polymerization reactions occurring that could give rise to the heavier compounds in the oils25,28 as well as increasing hydrocarbon gas yields. Oil Analysis by GC-MS and FTIR. The thermal degradation of scrap tire produces highly complex oils.

Scheme 1

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Table 3. Simulated Distillation of Oils as a Function of H2 Pressure (500 °C, 300 °C/min, 0.011 m/s, 15 min) % gasoline % kerosene % gas-oil % heavy gas-oil % vacuum gas-oil % vacuum residue

100 bar

50 bar

10 bar

28.5 16.7 17.8 22.4 14.6 0.0

19.0 11.0 12.9 19.7 33.6 3.7

17.8 6.9 10.8 16.6 40.7 7.2

GC-MS analysis showed a great abundance of different products and it was not possible to establish a clear relationship between the experimental conditions and specific compounds. Some sulfur-containing products such as dibenzothiophene, naphthothiophene, and benzonaphthothiophene were identified by GC, but they were not quantified because sulfur in tire is 1.43%, lower percentage than that of most of the coals shown and the studied processes were those generally used at coal conversion. The products comprise mainly substituted benzene species, although some alkylated naphthalenes were also detected as well as some long chain hydrocarbons. However, the most abundant product (around 12%) under all the reaction conditions was limonene (C10H12):

The high yields of limonene found can be explained taking into account that natural rubber (polyisoprene), used in tire manufacturing, is an isoprene polymer, together with the thermal decomposition mechanism now described. At reaction conditions the polyisoprene is depolymerized forming dimeric species. As this activation giving the diradical species takes place in the absence of oxygen, a cyclization could be produced. This dimer species, a short-life radical, could be stabilized through a two-step process driving to limonene by pyrolytic isomerization, as it happens from propylene (35) Sykers, P.; Mechanism reactions in organic chemistry; Roca, M., Ed.; 1971.

Figure 8. FTIR spectra of the oils obtained in the pyrolysis (15 min, 1 bar, 0.56 m/s N2, 500 °C, 300 °C/min) and hydropyrolysis (15 min, 100 bar, 0.011 m/s H2, 500 °C, 300 °C/min) of the tire.

to cyclopropane35 according to the following mechanism of Scheme 1: The oils gave very similar FTIR spectra, independent of the experimental conditions employed as demonstrated in Figure 8 which shows that for the spectra of oils obtained at 100 bar of H2 pressure and atmospheric N2 pressure. The spectra corroborate the presence of -CH2, -CH3 groups and aromatic groups in similar proportions in the two oils. Conclusions The recycling of waste tires by pyrolysis seems to be a feasible solution to the serious environmental problem originated. The conversion products are comprised of oils and gases together with a solid that corresponds to the unconverted carbon black. The conversion obtained for the scrap tire is always close to the maximum possible at 500 °C and the oil yield is always higher than 40%. No remarkable differences using hydrogen or nitrogen were found in total conversion and oil yield. However, hydrogen pressure was the most important variable regarding oil composition, with much lighter products being obtained at high pressure. High yields of limonene were obtained in all the conditions investigated and this has been attributed to a cyclization reaction of the isoprene liberated. Acknowledgment. The authors thank the European Commission, DG XVII (Project No. 7220/EC/043), and Spanish CICYT (Project Amb-0059) for their financial support. EF990183E