Vacuum Pyrolysis of Waste Tires by Continuously Feeding into a

Aug 19, 2010 - The continuous pyrolysis of waste tires under vacuum conditions (25 and 50 kPa) has been studied in a pilot plant equipped with a conic...
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Ind. Eng. Chem. Res. 2010, 49, 8990–8997

Vacuum Pyrolysis of Waste Tires by Continuously Feeding into a Conical Spouted Bed Reactor Gartzen Lopez, Martin Olazar,* Roberto Aguado, Gorka Elordi, Maider Amutio, Maite Artetxe, and Javier Bilbao Department of Chemical Engineering UniVersity of the Basque Country, P.O. Box 644 - E48080, Bilbao, Spain

The continuous pyrolysis of waste tires under vacuum conditions (25 and 50 kPa) has been studied in a pilot plant equipped with a conical spouted bed reactor and operating with continuous feed at 425 and 500 °C. The effects of vacuum on product distribution and properties have been studied. The main effect of vacuum is an increase in the diesel fraction yield of the liquid product. A remarkable yield of isoprene has been obtained operating under vacuum, reaching yields of higher than 7 wt %. Moreover, a positive effect on the quality of the residual carbon black has been observed, given that a decrease in pore blockage gives way to higher surface areas of the carbon blacks obtained. The results show that vacuum operation does not limit the good perspectives for waste tire valorization by pyrolysis in a conical spouted bed, and energy requirements for heating the inert gas and the condensation section are significantly reduced. 1. Introduction Waste tires are a serious environmental problem due to their potential landfill hazard. At present, 3.1 million tonnes per year of waste tires are generated in EU, 4.4 in USA, and 1.2 in Japan, and according to estimations, these figures will increase in future decades.1 Among the routes for the large-scale valorization of waste tires, encouraging results have been obtained in the studies concerning their use as crumb in cement-based materials and road asphalts and as powder in the preparation of thermoplastic elastomers.2-4 Waste tire has high volatile and fixed carbon contents with heating values greater than that of coal and biomass, and, consequently, it is an ideal raw material for thermochemical processes (combustion, pyrolysis, and gasification).5,6 Energy valorization by incinerating waste tires is viable from a technological point of view, being in fact carried out in numerous cement kilns, although it involves problems related to the control of harmful emissions of acid gases (SOx, HCl, HF, NOx), volatile organic compounds (VOCs), PAH, PCB, PCDD, and heavy metals.6 The cofiring of tire with volatile coals is especially interesting for reducing NO emissions in power stations.7 The interest of the tire pyrolysis process lies in the fact that the products obtained by this process (gas, liquid, and solid, whose yields are 10, 45, and 33 wt %, respectively, and the remaining 12 wt % is the metallic residue of the tire) may be easily handled, stored, and transported and then valorized separately according to different objectives. The noncondensable gas is made up of light hydrocarbons (olefins and C1-C4 paraffins) together with H2, CO, CO2, and H2S and can be used to provide the energy requirements of the process, contributing to the design of a cost-effective and thermally integrated process. The use in situ of an acid catalyst (HZSM-5, HY and Hβ zeolites, and MCM-41) is efficient for increasing the content of light olefins (ethylene and propylene), whose demand is increasing in the petrochemical industry.8-11 Tire pyrolysis oil (TPO) can be used as a source of refined chemicals (such as benzene, toluene, xylenes, isoprene, and * Corresponding author e-mail: [email protected].

limonene),12-14 and as a substitute for diesel fuel it helps to reduce the demand for natural resources.15,16 Tire-derived residual carbon blacks have two different applications: on the one hand, for reuse as carbon black, although a reduction in the sulfur content (approximately 50 wt % of the S in the tire) is needed for that purpose, and, on the other hand, active carbons can be obtained from residual carbon black by carrying out an activation process using steam or carbon dioxide as activation agents. The active carbons derived from waste tire are of high quality, mainly mesoporous, and have been successfully applied to the adsorption of different pollutants and as a support for metallic catalysts.17-20 Different technologies have been applied in the tire pyrolysis process under atmospheric pressure, such as fixed bed reactors,12,13,21,22 rotatory kilns,23,24 moving beds,25,26 and fluidized beds.27,28 The conical spouted bed reactor (CSBR) is an alternative technology to conventional (bubbling) fluid beds due to its excellent performance in handling sticky and irregular materials, such as scrap tires.29 The CSBR is characterized by its versatility for operation with high gas velocities, and, consequently, a vigorous gas-solid contact is generated, which enhances heat and mass transfer between phases, increases the heating rate of the solid, allows attaining an isothermal bed, and avoids bed defluidization by agglomeration of particles, even under severe conditions involving very sticky particles, as happens in the pyrolysis of waste plastics.30-32 The low segregation of the CSBR is an interesting feature for handling solids whose size decreases throughout the process as well as for the joint treatment of solids of different density and granulometry, which allows using catalysts in the bed.9,10,33 A conical spouted bed reactor (CSBR) has been used in this study for tire pyrolysis under vacuum and with continuous feed, in order to compare product yields and compositions with those obtained in a previous paper under ambient pressure.34 The main objective, which is a key factor for process viability, is to reduce the mass flow rate of inert gas (N2). Consequently, the condensation section is simpler, and less energy is required to cool the outlet stream. The literature related to the vacuum pyrolysis of waste tires is limited to the papers by Zhang et al.,35 using a fixed bed reactor, and by Roy’s research group using a moving bed reactor

10.1021/ie1000604  2010 American Chemical Society Published on Web 08/19/2010

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Table 1. Composition (wt%) of the Tire Material components

composition (wt%)

natural rubber styrene-butadiene rubber carbon black zinc oxide sulfur aromatic oil phenolic resin estearic acid IPPD (antioxidant) CBS (vulcanization accelerator) other additives

29.59 29.59 29.59 2.96 0.89 2.37 2.37 0.59 0.89 0.89 0.27

Elemental Analysis

Figure 1. Scheme of the pilot plant.

with cutting-edge technology.36-39 In addition to the aforementioned advantages associated with the decrease in the inert gas mass flow rate, the pyrolysis of waste tires under vacuum gives way to a higher liquid yield and better control of its composition, either by increasing the yield of high value added components, such as dl-limonene,39 or by improving its fuel quality.35 Another advantage of vacuum operation is the improvement in the kinetics of the pyrolysis process, as we reported in a previous paper,40 in which the same tire material used in this work was pyrolyzed in a thermogravimetric analyzer, under both atmospheric pressure and vacuum. This vacuum effect is attributed to the enhancement of the volatilization of primary products and their diffusion within the particle, which reduces the residence time of these products inside the particle. Consequently, the quality of the residual carbon black is improved, given that the undesirable processes of volatile carbonization by secondary reactions are minimized and the surface properties of the carbon black are similar to commercial ones.35,37 The improvement in the kinetics is consistent with the proposal of a limiting step in the pyrolysis of tires, as is the formation of a reactive intermediate, which is accelerated by enhancing volatilization.41 2. Experimental Section 2.1. Experimental Equipment. A scheme of the pyrolysis pilot plant is shown in Figure 1. The design has been based on the extensive knowledge acquired in previous studies on the pyrolysis of other waste materials such as plastics42,43 and biomass.44,45 The lack of knowledge in the literature about the performance of conical spouted bed reactors under vacuum required us to conduct a prior hydrodynamic study in the pyrolysis plant by using different materials and up to a temperature of 600 °C.46 We observed that the minimum spouting velocity increases when the operating pressure is reduced. Nevertheless, this effect due to the lower density of the gas under vacuum conditions attenuates as temperature is increased, which partially offsets the increase in the N2 mass flow rate required for the spouting regime. In the 400-600 °C range (suitable for pyrolysis), the mass flow rate under ambient pressure is twice and 3.5 times those corresponding to 50 and 25 kPa, respectively. Given that the unit described in Figure 1 needs to be fully airtight for operating under vacuum, a leak test has been carried out before each pyrolysis run to ensure no air is entering the reaction medium. The plant is equipped with a system for continuous solid feeding, which allows operating in continuous mode. The system consists of a hopper connected to a pneumatically actuated valve, which measures out the solid and

carbon (wt%) hydrogen (wt%) nitrogen (wt%) sulfur (wt%) ash (wt%)

86.57 7.66 0.44 2.14 3.19

can feed up to 300 g h-1 of waste tire. In a first step, the ground tire fills the hollow ball valve assisted by a vibrator inside the hopper. Subsequently, it rotates, and the tire is fed into the reactor forced by a small nitrogen flow. Nitrogen has been used as fluidizing agent, and its flow is controlled by a mass flow controller that allows feeding up to 30 L min-1. Prior to entering the reactor, it is heated to the reaction temperature by means of a preheater. The reactor is the unit’s main component, being a spouted bed of conical geometry with a cylindrical upper section. The total height of the reactor is 34 cm, with the conical section being 20.5 cm, and with an angle of 28°. The diameter of the cylindrical section is 12.3 cm, the bottom diameter is 2 cm, and the gas inlet diameter is 1 cm. These geometric factors provide great bed stability,47,48 and they are especially suitable for handling mixtures of waste tires and sand.49 The reactor’s operation ranges from the regime of spouted bed to a vigorous regime of jet spouted bed (or dilute spouted bed). The volatile products leave the reactor together with the inert gas and the finest carbon black particles. These particles are retained in a high-efficiency cyclone followed by a 25 µm sintered steel filter, both placed at the outlet of the reactor inside a forced convection oven maintained at 270 °C to avoid the condensation of pyrolysis oil in either component. The gases leaving this filter circulate through a volatile condensation system consisting of a condenser and a coalescence filter. The condenser is a double-shell tube cooled by tap water. The coalescence filter ensures the total condensation of volatile hydrocarbons. Vacuum is attained in the plant by means of a Vacuubrand mz2d vacuum pump placed downstream of the coalescence filters. The pressure within the reactor is controlled by means of a needle valve placed between the reactor and the pump. Once an initial period of one minute (unsteady state devolatilization) has elapsed, pressure is constant throughout continuous operation, with pressure and the remaining operating variables being monitored by a software application. 2.2. Tire Material. The tire material (Table 1) is supplied laminated and completely free of steel, carcass, and textiles by JENECAN S.L. (Bilbao, Spain). The material has been frozen in liquid nitrogen and then ground to a particle size below 1 mm using a Retsch ZM 100 grinder. The low heating value (determined in an isoperibolic Parr 1356 bomb calorimeter) is 38.8 MJ kg-1. The thermal behavior of this material was studied in a previous paper by thermogravimetric analysis.40 2.3. Experimental Procedure. The pyrolysis runs have been carried out in continuous regime by feeding 3 g min-1 of ground

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tire. The bed was made up of 35 g of sand (particle diameter 0.63-1 mm), which is enough to ensure good heat transfer and bed isothermicity due to the vigorous solid movement that characterizes the conical spouted bed reactor. The gas flow rate was 1.2 times that corresponding to the minimum spouting velocity. The nitrogen flow rates (measured at room conditions atmospheric pressure) were 9.5, 5.3, and 3.0 L min-1 for the experiments carried out under atmospheric pressure and under vacuums of 50 and 25 kPa, respectively. The decrease is not linear with pressure because the minimum spouting velocity increases slightly as pressure is increased.43 The reactor outlet stream of volatiles has been analyzed online by means of a GC Agilent 6890 (HP-Pona column). The sample was injected into the GC by means of a thermostatted line maintained at 250 °C. The purpose of this system is to avoid the condensation of the heavier products in the transfer line and so quantify all the volatile products in the chromatographic device. Moreover, and in order to complete the analysis of the volatile fraction, the noncondensable gases have been injected into a micro GC connected to a mass spectrometer (Agilent 5975B), which allows quantifying and identifying the gaseous products. Furthermore, the liquid obtained by condensation has been analyzed in a GC/MS (Shimadzu UP-2010S provided with a HP-Pona column) for identifying the pyrolysis products. Once a continuous run has been completed (100 g of tire fed), the char collected by the lateral outlet and retained in the cyclone and filter was weighed and collected for subsequent analysis. The mass balance has been carried out using the yields of carbon black and volatiles, and the balance closure was above 95% in all runs. The CSBR allows for continuous operation by selectively removing the carbon black from the bed and thus avoiding its accumulation throughout the pyrolysis process. The fountain region of the CSBR is characterized by the segregation of materials of different densities. In this region, the solids of lower density (carbon black particles) describe higher trajectories, whereas the heaviest particles (sand and unreacted tire) reach lower heights. Char removal has been carried out by means of a pipe placed above the bed surface. The surface area and pore volume of the carbon blacks have been determined from nitrogen adsorption-desorption isotherms provided by a Micromeritics ASAP 2000. The technique based on Hg porosimetry (Micromeritics Autopore II 9220) has been used to characterize macropores. The composition of the carbon black samples obtained in the activation process has been determined in an LECO CHNS932 elemental analyzer. Sulfur content is a parameter of great relevance, given that the carbon black reuse as such requires this content to be lower than 1 wt %.50 3. Results 3.1. Product Yields. The pyrolysis of tire particles has been studied at 425 and 500 °C, and the effect of pressure has also been studied by operating under both vacuum (25 and 50 kPa) and atmospheric pressure. The products have been grouped into five lumps: char (or carbon black), gas (C1-C4 hydrocarbons), nonaromatic liquid fraction (nonaromatic C5-C10 hydrocarbons), aromatic liquid fraction (single ring C10- aromatic hydrocarbons), and tar (including C11+, independently of their aromatic or nonaromatic nature). Figure 2 shows the evolution of the lumps obtained under the four different conditions studied, Figure 2a shows the results obtained at 425 °C, and Figure 2b shows the results obtained at 500 °C.

Figure 2. Effect of pressure on the yields of the different product lumps. Graph a, 425 °C. Graph b, 500 °C.

The more important effect of reducing the operating pressure is the increase in the yield of tar or C11+. Moreover, as tar yield increases, that of the C5-C10 fraction (both aromatics and nonaromatics) decreases. Vacuum operation causes a slight increase in the gas yield, and this effect is more pronounced at 425 °C. The decrease in the yield of single-ring C10- aromatic hydrocarbons is attributed to the negative effect of vacuum on the cyclization and aromatization reactions of pyrolysis primary products.5 This interpretation is in line with the increase in the gas yield, which is explained by the attenuation of olefin condensation by Diels-Alder reactions to give aromatic compounds. The effect of vacuum is explained by the fact that vacuum enhances diffusion toward the outside of the volatiles formed within the porous structure of the tire particle, which is due to the positive pressure gradient generated by vacuum for that flow. The faster diffusion of the volatiles inside the particle reduces their residence time and, consequently, limits the secondary and cracking reactions, increasing the yield of the heavier fraction. This point is reinforced in a previous paper on tire pyrolysis kinetics under vacuum,40 in which a positive effect of vacuum on the thermal degradation process of the tire material was observed and, consequently, the pyrolysis reaction begins at lower temperatures and is faster when performed under vacuum (at the same temperature). This effect of vacuum is attributed

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to the enhancement of the volatilization of primary products and their diffusion within the particle, reducing their residence time. Consequently, the secondary reactions of repolymerization and carbonization and char pore blockage are minimized. The concentration of volatile pyrolysis products in the reaction medium is similar in both the vacuum and atmospheric processes, and no effect is expected on the secondary reactions. The effect of vacuum reported by Zhang et al.35 involved a reduction in the residence time in the reactor (fixed bed), which prevented the secondary cracking of volatile vapors. Thus, they obtained a condensed pyrolysis oil mainly made up of compounds with a high boiling point and low naphtha content. The residence time of the volatiles in the CSBR decreases slightly when the operation is carried out under vacuum. Pakdel et al.39 studied the thermal degradation of tires at 500 °C at 10 kPa. The yield of liquid obtained in this case was 57 wt %, which is a slightly lower value than that recorded here. The liquid yield is the sum of the nonaromatic liquid fraction, aromatic liquid fraction and tar, which accounts for 60 wt % at 500 °C. This difference is due to the higher gas yield obtained by Pakdel et al.,39 around 11 wt %. The liquid yields obtained by Zhang et al.35 operating at a pressure of 3.5-4 kPa were even lower, 33-42 wt % in the 450-500 °C temperature range. These lower liquid yields are explained by the higher gas yields and the incomplete degradation of the sample at 450 °C. Nevertheless, the above-mentioned authors did not study the pyrolysis process under both atmospheric pressure and vacuum, so a comparison cannot be readily made of the vacuum effect on product distribution. A slight reduction in char yield is obtained operating under vacuum due to the reduction in the polymerization reactions on the surface of the char particle. This effect is related to the reduction of volatile residence time in the reactor, given that this variable has a significant effect on the degradation of tar components.21 The main effect of temperature is an increase in the gaseous and aromatic fractions and the resulting decrease in the yield of nonaromatic liquid fraction, as observed in previous studies carried out operating under atmospheric pressure in a wider temperature range.14,34,41 The increase in the gaseous fraction with temperature is attributed to the more severe cracking at high temperatures, which gives way to the formation of gases, mainly olefinic compounds. A clear increase in gas yield with temperature has been observed by several authors using different technologies.21,23,24,28,51-54 The yield of aromatics in the pyrolysis of tires is enhanced by high temperatures (due to secondary reactions) and by the presence of aromatics in the original formulation of the tire, as is our case (styrene-butadiene rubber). The yields of tar, or C11+ fraction, and char do not change significantly in the temperature range studied. 3.2. Product Composition. Table 2 shows the yields of the major components in the gaseous fraction obtained in tire pyrolysis under the conditions studied in this paper. The gases are mainly made up of methane and C2-C4 olefins when pyrolysis is performed under atmospheric pressure. However, a significant increase in the yield of alkanes is obtained operating under vacuum. The content of CO2 is very low and negligible in the case of CO, H2, and H2S. The major component in the gaseous phase is 1,3-butadiene, and its yield increases with temperature and vacuum, with the maximum yield being 2.84 wt % at 500 °C under vacuum (25 kPa). The formation of this component is related to the thermal decomposition of styrenebutadiene rubber.28 Although it is higher in pyrolysis under

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Table 2. Comparison of the Yield of Gaseous Products (in wt%) Obtained under Atmospheric Pressure and Vacuum at 425 and 500 °C 425 °C

500 °C

100 kPa 50 kPa 25 kPa 100 kPa 50 kPa 25 kPa CO2 CH4 C2 ethane ethene C3 propane propene C4 isobutene 1,3-butadiene 2-butene unidentified total

0.001 0.05 0.16 0.07 0.09 0.25 0.17 0.08 1.10 0.03 1.08 0.25 1.81

0.001 0.23 0.68 0.11 0.57 0.70 0.59 0.11 2.20 0.08 1.63 0.28 3.82

0.001 0.26 0.81 0.22 0.59 0.80 0.66 0.14 2.86 0.14 2.19 0.31 4.73

0.002 0.19 0.65 0.16 0.49 0.66 0.09 0.58 2.48 0.04 2.04 0.40 0.26 4.24

0.002 0.16 0.60 0.32 0.28 0.66 0.35 0.31 2.72 0.07 2.01 0.38 4.14

0.002 0.24 1.02 0.50 0.52 0.91 0.43 0.49 3.68 0.07 2.84 0.44 5.85

vacuum than under atmospheric pressure, the yield of the gas fraction is low and best burnt to produce energy for the pyrolysis process. Table 3 shows the yields of the main components in the C5-C10 fraction obtained under different pressures and temperatures. The liquid fraction is a complex mixture of hydrocarbons whose individual composition is generally low, but it also contains interesting hydrocarbons in a high proportion, such as isoprene, limonene, and styrene. Vacuum operation has a positive effect on the yield of isoprene at both temperatures studied, with a maximum yield of 7.52 wt % at 500 °C and 25 kPa. The opposite occurs in the case of the other main compound, limonene, whose yield is lower under vacuum conditions. The reduction in limonene yield is approximately 60% at both temperatures studied operating at 25 kPa. The yields of isoprene and limonene are closely related, i.e. isoprene is formed by β-scission of polyisoprene in the degradation of the polymer and it undergoes dimerization in the reaction medium to yield limonene.55 It seems that vacuum inhibits the dimerization reaction of isoprene to yield limonene, resulting in an increase in the yield of isoprene. A similar trend as that of limonene is observed in the case of styrene, whose yield is considerably reduced operating under vacuum (around 35% reduction at both temperatures studied when pyrolysis is carried out under 25 kPa). The yield of certain aromatic compounds, such as toluene, ethylbenzene, and xylenes, increases with temperature, which has also been reported by other authors.21,24,28 That increase in aromatic products is related to Diels-Alder reactions that promote the formation of aromatic compounds from olefins.55 However, the yield of these compounds is not greatly affected by the operating pressure in the range studied in this paper. Table 4 shows the yields of the more significant components in the tar fraction. The tar is made up of hydrocarbons heavier than C10, whose characterization is complex due to the low concentration of its components and to the limitations of GC/ MS techniques. As observed in Table 4, vacuum operation favors the formation of most of the compounds in the C11+ range. Most of the components in the C11-C13 range are of aromatic nature, but those heavier than C14 are mainly longchain paraffins. 3.3. Pyrolysis Product Properties. Tire pyrolysis oil (TPO) has a rather high content of olefinic and aromatic compounds, which limits its direct applications as automotive fuel. Murugan et al.15,16 studied the performance of the tire pyrolysis oil in a diesel engine. The results obtained were promising in terms of efficiency, emissions, and lack of operational problems, but they

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Table 3. Comparison of the Yields of the Main Individual Components in the C5-C10 Fraction (in wt%) Obtained under Atmospheric Pressure and Vacuum at 425 and 500 °C 425 °C

C5 isoprene 2-pentene methylbutenes C6 cyclohexene hexadienes 1,3,5-hexatriene 1-methyl-1,3-cyclopentadiene benzene C7 methylhexatrienes 1-methylcyclohexene toluene C8 4-ethenilcyclohexene 1-ethyl-3-methylencyclopentene styrene ethylbenzene xylenes C9 indene methylstyrene trimethylbenzenes ethylmethylbenzenes C10 l-limonene d-limonene dimethyloctadienes methyl(methylethyl)benzenes methyl-1H-indenes other compounds unidentified total

500 °C

100 kPa

50 kPa

25 kPa

100 kPa

50 kPa

25 kPa

5.94 5.01 0.54 0.22 1.03 0.13 0.13 0.15 0.07 0.14 1.66 0.14 0.09 0.74 8.91 1.33 0.41 4.49 0.56 1.27 4.18 0.65 1.14 0.08 0.89 27.21 1.10 19.30 1.36 0.52 12.47 5.94 54.96

7.39 5.26 0.83 0.39 1.56 0.19 0.25 0.09 0.19 0.25 2.29 0.09 0.16 1.36 6.27 0.60 0.13 3.12 0.64 0.87 3.57 0.64 0.24 0.33 18.36 0.73 10.15 1.00 0.89 0.40 10.64 3.55 42.98

9.65 7.20 1.30 0.39 1.72 0.19 0.23 0.09 0.18 0.50 2.64 0.10 0.17 1.77 6.54 0.98 2.96 0.61 1.47 2.57 0.26 0.19 0.30 0.29 16.78 0.80 9.00 0.74 0.75 0.22 9.21 2.65 42.56

7.40 5.73 0.91 0.40 1.97 0.17 0.15 0.25 0.16 0.27 3.01 0.23 0.17 1.51 11.24 1.06 0.31 6.08 1.11 1.53 4.27 0.46 0.97 0.10 1.17 16.94 1.16 9.13 1.47 0.75 0.30 8.08 7.64 52.41

4.69 3.29 0.65 0.28 1.07 0.12 0.13 0.08 0.14 0.17 1.64 0.08 1.12 7.71 0.86 4.05 1.09 1.33 3.90 0.75 0.11 0.63 0.11 20.64 0.96 8.44 0.86 0.97 0.74 10.71 5.80 43.46

10.31 7.52 1.38 0.58 2.04 0.14 0.24 0.18 0.29 0.42 2.25 0.11 1.55 6.86 0.81 3.47 0.69 1.51 2.40 0.44 0.12 0.38 0.09 10.28 0.64 4.43 0.54 0.63 0.32 9.19 4.54 38.67

Table 4. Comparison of the Yields of the Main Individual Components in the Tar Fraction (C11+) (in wt%) Obtained under Atmospheric Pressure and Vacuum at 425 and 500 °C 425 °C C11-C13

C14-C16

C17+

benzothiazole pentylbenzene dihydromethylnaphtalenes methylnaphtalenes biphenyl methylbiphenyl trimethylnaphtalenes trimethyldodecatretraenes pentadecane 1-phenil-3,4-divinil cyclohexane hexadecane heptadecane octadecane nonadecane eicosane methyleicosane other compounds unidentified total

500 °C

100 kPa

50 kPa

25 kPa

100 kPa

50 kPa

25 kPa

3.17 0.30 0.06 0.13 0.05 0.05 0.05 1.15 0.11 0.04 0.14 0.09 1.01 0.31 0.06 0.07 0.05 0.05 3.63 4.08 9.33

4.99 0.51 0.14 0.16 0.22 0.34 0.34 0.06 6.15 0.26 0.42 0.09 1.10 0.33 0.06 0.06 0.26 0.20 8.78 7.09 19.33

9.01 0.47 0.33 0.30 0.42 1.18 0.54 0.07 5.25 0.34 0.22 0.37 0.16 0.12 10.47 4.24 18.87

3.94 0.33 0.06 0.15 0.05 0.09 1.16 0.14 0.09 0.09 0.62 0.27 0.02 0.04 0.04 0.05 4.01 3.56 9.24

8.86 1.15 0.12 0.17 0.82 0.21 0.10 0.04 4.20 0.10 0.24 0.28 0.07 1.44 0.51 0.07 0.05 0.10 0.07 10.39 3.67 18.17

8.93 0.79 0.10 0.15 0.60 1.01 0.15 0.15 5.89 0.04 0.05 0.43 0.09 0.67 0.15 0.08 0.03 11.67 6.02 21.53

concluded that the main challenge is to reduce the aromatic content and viscosity for its use as a fuel in diesel engines. Consequently, a mild hydrocracking treatment would be sufficient for upgrading the liquid,56 or it may also be fed with other feedstocks from wastes into refinery units like those for FCC, coking, or thermal cracking.38 Furthermore, given that it has a high heating value (40 MJ kg-1) and a relatively low sulfur

content, another option is the use of this liquid in industrial furnaces.24,57,58 Figure 3 shows the distillation curves for the pyrolysis oils obtained operating at 425 °C (Figure 3a) and 500 °C (Figure 3b). The distillation curves have been obtained with a HYSYS commercial simulator, using over 50 compounds heavier than C5 (prevailing ones in the GC analysis) to define the different

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coking or degraded tire deposition on the carbon black. The residual carbon black obtained has an overly high sulfur content that may limit its direct reuse. This sulfur content is around 3 wt % and is similar to that reported by other authors.24,54,58,59 Nevertheless, the quality of the char obtained operating under vacuum conditions is better than that under atmospheric pressure. At 425 °C, BET surface area increases from 46 m2 g-1 under atmospheric pressure to 96 m2 g-1 under 25 kPa vacuum. The results at 500 °C follow the same trend - an increase from 65 to 91 m2 g-1 by reducing the pressure from 100 to 25 kPa. The results in terms of BET surface area obtained by other authors operating under atmospheric conditions are below the values obtained in this paper with pyrolysis under vacuum.5,13,22,24,28,54,60 The improvement in the porous structure properties of the residual carbon black obtained operating under vacuum reinforces the possibilities for carbon black recycling.61 The positive effect of vacuum on the porous structure of the char is related to two factors: i) vacuum facilitates the devolatilization and diffusion of the volatiles within the particle; and ii) vacuum minimizes the deposition of carbon material on the porous structure and so reduces the blockage of pores.62 Lua and Yang also pyrolyzed biomass under vacuum, and they observed a positive effect on the solid fraction properties.63 They noted that charcoals prepared by vacuum pyrolysis yielded a higher surface area than those produced by atmospheric carbonization. This difference was attributed to the more open pore structures in the vacuum pyrolysis charcoals. 4. Conclusions

Figure 3. Comparison of the simulated distillation curves of the pyrolysis oils obtained under different pressures. Graph a, 425 °C. Graph b, 500 °C.

liquid fractions. The distillation curve of the liquids obtained operating under atmospheric pressure is characterized by a long plateau at a temperature around 175 °C. This plateau is due to the high concentration of limonene and to other C10 compounds in the liquid. This plateau is shorter for the liquid obtained under vacuum, which is due to the increase in the light and heavy fractions in the liquid. Thus, the fraction of the liquid that distils below 100 °C is 9% higher in the liquids obtained operating under vacuum at both temperatures studied, which is explained by the higher isoprene yield obtained operating under vacuum. The increase in the heavier fraction is more significant when operating under vacuum. Thus, the liquid fraction with a boiling point higher than 200 °C (diesel fraction) increases, especially at 500 °C and 25 kPa, which accounts for 36 wt % of all the liquid, whereas this fraction accounts for only 13 wt % under atmospheric conditions. Consequently, these results show that the good performance of the atmospheric conical spouted bed for fuel production by waste tire pyrolysis is improved by operating under vacuum, which means that pressure is a significant variable for fitting the range of boiling points in the fuels to market requirements. The amount of char obtained is slightly higher than the sum of the original carbon black plus the inorganic components in the tire (Table 1). This implies that there is a certain degree of

The continuous vacuum pyrolysis of waste tires in a conical spouted bed reactor is interesting for increasing process viability. Thus, nitrogen mass flow rate is lower, and, consequently, the condensation of the liquid product is easier and less energy is required. Vacuum has a significant effect on the distribution of products and their composition but does not have any negative consequences of importance, which means that vacuum operation maintains the good performance of the conical spouted bed reactor for waste tire pyrolysis. The main advantages of vacuum operation over atmospheric operation are the increase in the yield of the liquid fraction corresponding to diesel fuel and the improvement in the surface area of the residual carbon black, although the high content of aromatics and sulfur in the former requires a hydroreforming treatment for commercial use. Moreover, an increase in the yield of isoprene has been obtained operating under vacuum, but limonene yields are lower. The increase in the yield of tar fraction (or C11+ fraction) may be tempered by increasing temperature. The lower adulteration of the carbonaceous material deposited on the residual carbon black surface gives way to higher BET surface areas. The surface area values obtained operating under vacuum are higher than 90 m2 g-1. Acknowledgment This work was carried out with the financial support of the Department of Education of the Basque Government (Project GIC07/24-IT-220-07), the Ministry of Science and Education of the Spanish Government (Project CTQ2007-061167), and the Department of Industry of the Basque Government (Project ETORTEK-08/27).

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ReceiVed for reView January 11, 2010 ReVised manuscript receiVed March 10, 2010 Accepted July 27, 2010 IE1000604