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Assessment of tire devolatilization kinetics
J. Anal. Appl. Pyrolysis 74 (2005) 259–264 www.elsevier.com/locate/jaap
Assessment of tire devolatilization kinetics E Aylo´n a, M.S. Calle´n a, J.M. Lo´pez a, Ana Marı´a Mastral a,*, R. Murillo a, M.V. Navarro a, Slawomir Stelmach b a
Instituto de Carboquı´mica, CSIC, M Luesma Castan 4, 50018 Zaragoza, Spain b Institute for Chemical Processing of Coal, ICHPW, 41-803 Zabrze, Poland Received 7 June 2004; accepted 20 September 2004 Available online 1 April 2005
Abstract In this paper, a kinetic study of the devolatilization of tire rubber has been performed in a thermobalance using nitrogen as carrier gas. The main operation variables have been studied including flow rate, particle size, final temperature and heating rate, observing that only temperature has a remarkable effect on tire rubber conversion. Kinetic parameters have been calculated and used to simulate all experimental curves including the ones obtained at different heating rates. In addition, an approximate tire rubber composition has been obtained by integrating the reaction rate versus temperature curves. The suitability of the kinetic parameters has been demonstrated in real conditions by performing experiments in a fixed bed reactor. Finally, the conclusions achieved from this study can be used not only for pyrolysis process design but also for the interpretation and modeling of the tire rubber combustion process. # 2005 Elsevier B.V. All rights reserved. Keywords: Tire rubber; Devolatilization; Kinetics; Thermobalance
1. Introduction The disposal of used automotive tires is an increasing economical and environmental problem for most of the developed countries. It is estimated that 2.5 million tones per year are generated in the European Union, 2.5 million tones in North America and around 1 million in Japan [1]. Unfortunately, most of the scrap tires generated are dumped in open or landfill sites [2]. It is well known that tire is made of rubber materials (polybutadiene, styrene–butadiene rubber and polyisoprene or natural rubber), carbon black and some fibrous materials [3,4]. It has high volatile and fixed carbon contents with heating value greater than that of coal. This makes rubber from old tire a good raw material for thermochemical processes [5]. On the other hand, scrap tire is bulky and it is not a biodegradable residue and therefore it is not possible to achieve its natural degradation in landfills. As a consequence, open dumping of scrap tire not only occupies a large space, presents an eyesore and could cause
* Corresponding author. Tel.: +34 976 733977; fax: +34 976 733318. E-mail address: [email protected] (A.M. Mastral). 0165-2370/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jaap.2004.09.006
potential health and environmental hazards, but also illustrates wastage of valuable energy resource. Waste tire pyrolysis has been widely studied for years [1–7]. This process seems to be an alternative to combustion processes because no hazardous emissions are produced and the recovery of solid and liquid material is achieved [8,9]. Different variables that could affect the devolatilization process have been studied. Gonza´lez et al. [10] did not find significant influence of temperature on the amount and characteristics of pyrolysis products working over 500 8C and proved that tire-pyrolysis liquids are a complex mixture of C5–C20 organic compounds, with a great proportion of aromatics. After tire pyrolysis, three phases are obtained: solid, liquid and gas and their composition is related to the temperature of the thermal treatment. The solid phase, approximately 40% weight of the initial sample, is mostly constituted of carbon black but also contains the mineral matter initially present in the used tire. The gas phase contains a mixture of light hydrocarbons and carbon dioxide. The pyrolysis gases can be used to provide the energy requirements of the pyrolysis process. Finally, the liquid phase is a complex hydrocarbon mixture in which significant concentrations of some valuable components like
E. Aylo´ n et al. / J. Anal. Appl. Pyrolysis 74 (2005) 259–264
260
Notation ci Ea ko koi n R SDTG t T wfinal wi w0 X Xexp Xi
contribution of each single reaction to the mass global loss activation energy (kJ mol1) pre-exponential factor in Ahrrenius equation (s1) frequency factor for component i reaction order universal gas constant (8.314 J mol1 K1) sorum objective function reaction time (s) temperature (K) sample ash weight (mg) sample weight at any time (mg) sample initial weight (mg) solid fractional conversion solid experimental conversion solid fractional conversion for component i
DL-limonene, benzene or toluene have been identified [7,11,12]. Mastral et al. [7] studied the thermochemical recycling of rubber from old tires by pyrolysis and hydropyrolysis using a swept fixed bed reactor. They analyzed the effect of the main process variables (temperature, heating rate, gas flow, reaction time and hydrogen pressure) on oils, gases and solid residue produced. They found that 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. Conesa et al. [14] proposed a model for the primary devolatilization of tires, considering different heating rates. They proposed a model that includes three organic fractions that do not form char, and a fourth fraction that does not decompose. The authors confirmed this hypothesis by mass spectrometry. The three organic fractions were assigned to different compounds: the first one probably corresponds to the decomposition of an oil fraction, the second one to that of natural rubber and the third to styrene–butadiene rubber. Thermal degradation kinetics of natural rubber, styrene– butadiene rubber and polybutadiene were investigated by Lin et al. [15,16]. They proposed that two or three reactions were involved for the mixed rubbers. The kinetic studies by Chen et al. [17,18] show that one reaction is involved when SBR or epoxy resin is decomposed in an inert gas, but two reactions are involved when oxygen is present in the carrier gas, since thermal decomposition is not only an independent process, but also a first step in the gasification or combustion process. In addition, the rate of heat supply, total energy and operation time are important design parameters not only for a pyrolysis system to recover the gaseous and carbon products but also for combustion processes because devolatilization is the first step in the whole combustion
reaction. Consequently, the devolatilization kinetics of scrap tires deserves more investigation. In this paper, not only kinetic parameters are calculated but also its the model validated in a fixed bed reactor.
2. Experimental The raw material used for the kinetic studies was shredded tire rubber supplied by AMSA, a Spanish waste tire recycling company. This residue was sifted to three different average particle sizes of 2, 1 and 0.5 mm and had the following ultimate and proximate analyses: C (daf): 88.3%; H (daf): 7.69%; S (mf): 1.85%, moisture (ar) 0%, ash content 5.30%, volatile matter (ar) 64.7% and fixed carbon (ar) 30%, where daf means dry and ash free, mf means moisture free and ar means as received. Pyrolysis tests were conducted in a thermobalance (SETARAM TG DTA-92). Two different gas flows are introduced into the thermobalance. The first one consisted of pure N2 and passed through the head of the thermobalance in order to protect the microbalance from the possible corrosive gases introduced or generated. The second one was introduced directly at the top of the thermobalance reactor and consisted of nitrogen during heating and reaction times. Both gas streams were controlled by mass flow controllers (BRONKHORST HI-TEC EL-FLOW) and mixed before they reached the sample. The solid sample was deposited in a small platinum basket with a circular base (5 mm diameter and 2 mm height). The thermobalance was provided with an electric oven that could operate up to 1750 8C and it was controlled with a PID temperature controller. Different heating and cooling rates could be programmed with a maximum program time of 89 h. A thermocouple was located close to the platinum basket for temperature monitoring and closing the oven control loop. The solid weight loss, together with other process variables like temperature, was continuously transmitted to a computer through a data acquisition card. The temperature evolution and the time when the different mass flow controllers had to be opened were also controlled through the computer. All the pyrolysis experiments were performed by working at atmospheric pressure. The tested heating rates were 5, 10, 20, 30 and 40 8C/min. The sample was forming a small fixed bed in the platinum basket and an initial mass of 10 mg was used for all the runs. The influence of the particle size was also studied ranging from 0.5 to 2 mm. Different flow rates were tested (100–250 ml/min) to study the influence of this variable on the rubber tire conversion. The experimental conversion (Xexp) was calculated according to Equation (1), where w0 is the initial sample weight, wi is the sample weight at any time and wfinal is the final weight (the weight stable after reaction). Xexp ¼
w0 wi w0 wfinal
(1)
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261
Fig. 1. Swept fixed bed reactor pilot plant.
2.1. Experiments in a fixed bed reactor The swept fixed bed reactor (Fig. 1) was 30 cm long and had an internal diameter of 2 cm. It was provided with a mass flow controller, a PID temperature controller and a furnace. The design of the temperature controller allowed fixing a certain heating rate for the experiment. These experiments were performed at 5 8C/min heating rate, 0.2– 0.4 mm particle size and 2 l/min flow rate.
Fig. 3. Rubber tire conversion vs. time at different particle sizes (550 8C final temperature, 150 ml/min flow rate and 20 8C/min heating rate).
150 ml/min was used for the rest of experiments in order to protect the thermobalance head of possible corrosive gases liberated during the reaction. 3.2. Particle size influence
3. Results and discussion 3.1. Flow rate influence With the aim of assessing the influence of external mass transfer on tire rubber pyrolysis, experiments at different flow rates (250, 200, 150 and 100 ml/min) were performed. All these runs were carried out with a particle size lower than 1 mm, a heating rate of 20 8C/min and a final reaction temperature of 550 8C. Fig. 2 shows the experimental results obtained and it is observed no influence of this variable on the final tire rubber conversion. In all the experiments, the final conversion was 100%. This conversion has been calculated considering that only the polymeric fraction reacts at these experimental conditions because carbon black is not reactive under standard pyrolysis conditions [5]. A flow rate of 100 ml/min seems to be high enough to carry away all the conversion products of the rubber pyrolysis avoiding possible retrogressive or condensation reactions. Since the flow rate does not affect to tire pyrolysis, a value of
The influence of particle size on tire pyrolysis was studied using three different particle sizes: 2, 1 and <1 mm. This a very important variable in the pyrolysis process because it is an endothermic reaction where a temperature profile along the particle diameter occurs. In this way, it is expected that, depending on the solid thermal conductivity, a remarkable temperature gradient inside the particles could be observed. This temperature gradient could affect to the obtained products in the pyrolysis process [19]. In addition, the rubber tire grinding process is quite complicated and energetically demanding. Therefore, it would be beneficial for the whole pyrolysis process that large particle sizes could be used. Fig. 3 shows the pyrolysis results for the studied fractions and the following experimental conditions: 550 8C, 150 ml/min flow rate and 20 8C/min heating rate. It is observed that the maximum conversion was obtained in all the experiments, regardless of the particle diameter and all the curves are overlapping each other. Therefore, no influence of this variable was observed for the pyrolysis of tire rubber, at least working in the 2 mm dust range. Anyway, the particle size fraction smaller than 1 mm was selected for the rest of the runs and the kinetics deduction because the initial sample weight in the thermobalance is quite small, around 9–10 mg. Therefore, a higher sample surface that, at the same time, implies a more representative sample will be achieved when small particles are used. 3.3. Influence of the final temperature
Fig. 2. Rubber tire conversion vs. time at different flow rates (550 8C final temperature, 0.4–0.6 mm particle size and 20 8C/min heating rate).
Fig. 4 shows the results obtained in the pyrolysis of rubber tire at different final temperatures. All the runs were carried out at 150 ml/min, 20 8C/min heating rate and the lower particle size (lower than 1 mm). By plotting conversion versus temperature (Fig. 5), it can be observed that pyrolysis reaction starts at 200 8C when the heating rate is 20 8C/min. It is also
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for reactor design because in pyrolysis processes both, the product distribution and the final conversion achieved may depend on this variable, especially when fossil fuels are used. Experiments using different heating rates (5, 10, 20, 30 and 40 8C/min) were performed with a final temperature and a flow rate of 550 8C and 150 ml/min, respectively, for all the runs. Fig. 5 shows the conversion results obtained at the different heating rates. It is observed that the influence of this variable is only slightly appreciable at the slowest heating rates. Higher conversions are achieved for the same temperature when the slowest heating rates are used because the sample is under reaction conditions for a longer time when slow heating rates are used. Fig. 4. Rubber tire conversion vs. time at different final temperatures (0.4– 0.6 mm particle size, 150 ml/min flow rate and 20 8C/min heating rate).
Fig. 5. Rubber tire conversion vs. temperature at different heating rates (550 8C final temperature, 150 ml/min flow rate and 0.4–0.6 mm particle size).
observed in the curves obtained for 500, 550 and 600 8C that the reaction finishes between 480–490 8C and therefore, no influence of the final temperature is observed. Total conversion has been achieved before the final temperature has been reached and no weight loss is registered. Conversion does not improve by increasing temperature over 500 8C because the polymer has completely disappeared from the solid sample and carried away by the nitrogen stream and the remaining carbon black is not reactive under these experimental conditions. With respect to the experiment performed at 450 8C, it is observed that the conversion evolution is identical to the rest of experiments until the final temperature is reached. At this moment, the tire rubber total conversion has not been achieved. Although the observed trend is different, finally, the total conversion is obtained. 3.4. Heating rate influence Another important variable that has been studied is the influence of heating rate. This is a very important variable
3.5. Kinetic analysis Usually, mathematical models for describing overall decomposition of complex wastes and biomass consider independent parallel reactions [20]. Rubber tire DTG curves are characterized by the presence of shoulders and/or double peaks because more than one reaction is occurring simultaneously. In this case, the process has been modeled considering that rubber tire is comprised of four components that can be separately but simultaneously reacting. These components are: additives (non polymeric material added during the manufacturing process including oils, plasticizers, etc.), a first polymeric material (probably polybutadiene or styrene–butadiene rubber), a second polymer (probably natural rubber) and the solid residue (the carbon black and mineral matter added as reinforcing material and filler). In the DTG curves, only three peaks are detected and it is assumed that the last tire rubber component does not show weight loss under the experimental conditions tested. The general kinetic law for each single reaction is: dXi ¼ koi eEai =RT ð1 Xi Þni (2) dt where Xi is the solid fractional conversion for component i calculated according to Equation (1), t is the reaction time, R is the gas universal constant, T is the absolute temperature, koi is the frequency factor for component i, Eai is the activation energy and ni is the reaction order for component i. As the sample components decompose independently, the overall pyrolysis rate for N reactions occurring in parallel can be described by the following equation: X N N dX X dXi ¼ ci ðci koi eEai =RT ð1 Xi Þni Þ (3) ¼ dt dt i¼1 i¼1 where ci is a coefficient that expresses the contribution of each single reaction to the mass global loss. In this way, Equation (3) allows us to calculate coefficient ci after the determination of the frequency factor, activation energy and reaction order. In the kinetics evaluation, a nonlinear least squares (NLS) algorithm (specifically Nelder and Mead algorithm) was used. With this procedure, it is possible to identify
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Table 1 Correlation parameters for the rubber tire devolatilization reaction
Eai (kJ/mol) koi (s1) ni
Fig. 6. Experimental and calculated reaction rates vs. temperature (550 8C final temperature, 20 8C/min heating rate, 0.2–0.4 mm particle size and 150 ml/min flow rate).
parameters (koi, Eai, ni and ci) that minimize values of the objective function SDTG given below (see Equation (4)) and proposed by Sorum et al. [20]. " calc #2 N X dw exp dw SDTG ¼ (4) dt dt j j j¼1 In Equation (4), ðdw=dtÞexp is the experimentally observed DTG curve and ðdw=dtÞcalc is the calculated DTG curve obtained by numerical solution by Runge–Kutta method of the kinetic differential equation with the given set of parameters. Subscript j denotes discrete values of ðdw=dtÞ. Sørum’s modified objective function would be similar to the regression variance of the fit. In this way, it shows the dispersion of the experimental values versus the calculated ones. Fig. 6 shows the experimental and calculated reaction rates versus temperature working at a final temperature of 550 8C, at a constant heating rate of 20 8C/min, with particle size between 0.2–0.4 mm and a flow rate of 150 ml/min. Three different zones can be distinguished in this plot: Temperature between 150 and 310 8C: in this zone the drying of the particles has occurred. However, as the moisture content of tire rubber is very low, it is not appreciable the corresponding weight-loss. Following the particles drying, it is observed that the tire rubber decomposition begins. Low reaction rates are observed specially between 150 and 200 8C. In this stage of the pyrolysis reaction, probably the tire rubber additives are slowly decomposed and incorporated to the flow rate. During their decomposition, it is not observed a sharp peak compared to the other tire rubber components because they are not constituted by only one product but they are a mixture of different products (extender oils, plasticizers and other additives) with different properties and therefore different behavior under pyrolysis conditions. Temperature between 310 and 430 8C: in this zone, it is observed a sharper peak than the previous one probably
Additives
Polymer 1
Polymer 2
70 10000 2.8
212 8.20E + 14 1.4
265 3.20E + 17 1.9
because it corresponds to the decomposition of only one polymer and not a mixture of different compounds. In addition, this polymer will be present in rubber tire with a higher percentage than the additives because the area under the curve is much higher. Conesa et al. [14] and Williams and Besler [13] have suggested that this peak could correspond to the decomposition of natural rubber. Temperature between 350 and 490 8C: this last peak is also sharper than the first one but not as sharp as the second one. In addition, its area is lower than the one of the second peak and it has been reported [13] that could be assigned to the decomposition of styrene–butadiene rubber. At higher temperatures than 490 8C, a reaction rate close to zero is observed. All the polymeric material and the one that is able to be transformed into gases in the studied temperature range has been liberated and swept along the thermobalance by the nitrogen flow. Table 1 compiles the kinetic parameters obtained from the experimental results by fitting the reaction rate versus temperature curves. The obtained parameters are of the same range than the obtained by other authors like Conesa et al. [14] and Sorum et al. [20]. In addition, it is observed that the higher the temperature of the maximum in the different peaks, the higher the activation energy. All the experimental results obtained in this research were fitted using the deduced kinetic parameters and obtaining correlation coefficients higher than 0.95 in all the cases confirming the suitability of these parameters. With this methodology, it was also possible to find an approximate tire rubber composition. It was found that the sample studied had around 11.6% of additives, 32.6% of the first polymer, 18.0% of the second polymer and 37.8% of carbon black and mineral matter. This composition is only applicable to the tire rubber used: a mixture of rubbers from different brands, and it does not have to fit with the results reported by other authors who used a specific rubber tire. 3.6. Validation of kinetic results in a fixed bed reactor Finally, simulated experimental curves have been compared with real experimental results obtained in a fixed bed reactor (see Fig. 1) in order to validate calculated kinetic parameters. Experiments were stopped at different final temperatures in order to obtain the evolution of conversion with temperature. Fig. 7 shows the experimental and calculated conversion versus temperature. A good correlation between experimental results obtained in the fixed bed reactor and simulated values
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Acknowledgements Authors would like to thank Spanish Science and Technology Ministry (Project PPQ-4145), the Ramo´ n y Cajal Program (R. Murillo and M.S. Calle´ n) contracts and the General Council of Arago´ n, D.G.A., Spain, (J.M. Lo´ pez Pre-Doc grants).
References
Fig. 7. Experimental and calculated conversion vs. temperature in the SFBR (5 8C/min heating rate, 0.2–0.4 mm particle size and 2 l/min flow rate).
calculated from the thermobalance kinetics have been found. Therefore, it has been confirmed the suitability of these kinetic parameters in real conditions.
4. Conclusions Different operation variables have been studied in the tire rubber devolatilization process observing that only temperature has a remarkable effect on tire rubber conversion when this variable is lower than 500 8C. Total tire rubber conversion is achieved in these conditions for every component but the carbon black remains unconverted. Kinetic parameters have been deduced for every tire rubber component and have been used to simulate all the experimental results demonstrating their suitability. Finally, an approximate composition of the rubber tire sample has been calculated. The kinetic parameters obtained in the thermobalance were confirmed in a fixed bed reactor obtaining experimental conversion values close to the calculated ones.
[1] I. de Marco, M.F. Laresgoiti, M.A. Cabrero, A. Torres, M.J. Chomon, B. Caballero, Fuel Process. Technol. 72 (2001) 9. [2] D. Pantea, H. Darmstadt, S. Kaliaguine, C. Roy, J. Anal. Appl. Pyrolysis 67 (2003) 55. [3] P.T. Williams, S. Besler, D.T. Taylor, Fuel 69 (1990) 1474. [4] P.T. Williams, S. Besler, D.T. Taylor, R.P. Bottrill, J. Inst. Energy 68 (1995) 11. [5] A.M. Mastral, R. Murillo, M.S. Callen, T. Garcia, N. Torres, Resour. Conserv. Recycl. 29 (2000) 263. [6] H. Pakdel, D.M. Pantea, C. Roy, J. Anal. Appl. Pyrolysis 57 (2001) 91. [7] A.M. Mastral, R. Murillo, M.S. Calle´ n, T. Garcı´a, C.E. Snape, Energy Fuels 14 (2000) 739. [8] J.A. Conesa, R. Font, A. Marcilla, J. Anal. Appl. Pyrolysis 43 (1) (1997) 83. [9] P.T. Williams, S. Besler, D.T. Taylor, Proc. Inst. Mech. Eng. 207 (1993) 55. [10] J.F. Gonza´ lez, J.M. Encinar, J.L. Canito, J.J. Rodriguez, J. Anal. Appl. Pyrolysis 58–59 (2001) 667. [11] A. Cunliffe, P.T. Williams, J. Anal. Appl. Pyrolysis 44 (1998) 131. [12] H. Pakdel, C. Roy, H. Aubin, G. Jean, S. Coulombe, Environ. Sci. Technol. 25 (1992) 1646. [13] P.T. Williams, S. Besler, Fuel 74 (1995) 1277–1283. [14] J.A. Conesa, R. Font, A. Fullana, J.A. Caballero, Fuel 77 (1998) 1469. [15] J.P. Lin, C.Y. Chang, C.H. Wu, S.M. Shih, Polym. Degrad. Stab. 53 (1996) 295. [16] J.P. Lin, C.Y. Chang, C.H. Wu, J. Chem. Technol. Biotechnol. 66 (1996) 7. [17] K.S. Chen, R.Z. Yeh, Y.R. Chang, Combust. Flame 6 (1997) 45. [18] K.S. Chen, R.Z. Yeh, Y.M. Chou, Environ. Eng. Sci. 14 (1997) 175. [19] R.C. Bansal, J.B. Connet, F. Stoeckli, Active Carbon, Marcel Dekker, Inc., New York, 1988. [20] L. Sørum, M.J. Grønli, J.E. Hustad, Fuel 80 (2001) 1217.
Assessment of tire devolatilization kinetics E Aylo´n a, M.S. Calle´n a, J.M. Lo´pez a, Ana Marı´a Mastral a,*, R. Murillo a, M.V. Navarro a, Slawomir Stelmach b a
Instituto de Carboquı´mica, CSIC, M Luesma Castan 4, 50018 Zaragoza, Spain b Institute for Chemical Processing of Coal, ICHPW, 41-803 Zabrze, Poland Received 7 June 2004; accepted 20 September 2004 Available online 1 April 2005
Abstract In this paper, a kinetic study of the devolatilization of tire rubber has been performed in a thermobalance using nitrogen as carrier gas. The main operation variables have been studied including flow rate, particle size, final temperature and heating rate, observing that only temperature has a remarkable effect on tire rubber conversion. Kinetic parameters have been calculated and used to simulate all experimental curves including the ones obtained at different heating rates. In addition, an approximate tire rubber composition has been obtained by integrating the reaction rate versus temperature curves. The suitability of the kinetic parameters has been demonstrated in real conditions by performing experiments in a fixed bed reactor. Finally, the conclusions achieved from this study can be used not only for pyrolysis process design but also for the interpretation and modeling of the tire rubber combustion process. # 2005 Elsevier B.V. All rights reserved. Keywords: Tire rubber; Devolatilization; Kinetics; Thermobalance
1. Introduction The disposal of used automotive tires is an increasing economical and environmental problem for most of the developed countries. It is estimated that 2.5 million tones per year are generated in the European Union, 2.5 million tones in North America and around 1 million in Japan [1]. Unfortunately, most of the scrap tires generated are dumped in open or landfill sites [2]. It is well known that tire is made of rubber materials (polybutadiene, styrene–butadiene rubber and polyisoprene or natural rubber), carbon black and some fibrous materials [3,4]. It has high volatile and fixed carbon contents with heating value greater than that of coal. This makes rubber from old tire a good raw material for thermochemical processes [5]. On the other hand, scrap tire is bulky and it is not a biodegradable residue and therefore it is not possible to achieve its natural degradation in landfills. As a consequence, open dumping of scrap tire not only occupies a large space, presents an eyesore and could cause
* Corresponding author. Tel.: +34 976 733977; fax: +34 976 733318. E-mail address: [email protected] (A.M. Mastral). 0165-2370/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jaap.2004.09.006
potential health and environmental hazards, but also illustrates wastage of valuable energy resource. Waste tire pyrolysis has been widely studied for years [1–7]. This process seems to be an alternative to combustion processes because no hazardous emissions are produced and the recovery of solid and liquid material is achieved [8,9]. Different variables that could affect the devolatilization process have been studied. Gonza´lez et al. [10] did not find significant influence of temperature on the amount and characteristics of pyrolysis products working over 500 8C and proved that tire-pyrolysis liquids are a complex mixture of C5–C20 organic compounds, with a great proportion of aromatics. After tire pyrolysis, three phases are obtained: solid, liquid and gas and their composition is related to the temperature of the thermal treatment. The solid phase, approximately 40% weight of the initial sample, is mostly constituted of carbon black but also contains the mineral matter initially present in the used tire. The gas phase contains a mixture of light hydrocarbons and carbon dioxide. The pyrolysis gases can be used to provide the energy requirements of the pyrolysis process. Finally, the liquid phase is a complex hydrocarbon mixture in which significant concentrations of some valuable components like
E. Aylo´ n et al. / J. Anal. Appl. Pyrolysis 74 (2005) 259–264
260
Notation ci Ea ko koi n R SDTG t T wfinal wi w0 X Xexp Xi
contribution of each single reaction to the mass global loss activation energy (kJ mol1) pre-exponential factor in Ahrrenius equation (s1) frequency factor for component i reaction order universal gas constant (8.314 J mol1 K1) sorum objective function reaction time (s) temperature (K) sample ash weight (mg) sample weight at any time (mg) sample initial weight (mg) solid fractional conversion solid experimental conversion solid fractional conversion for component i
DL-limonene, benzene or toluene have been identified [7,11,12]. Mastral et al. [7] studied the thermochemical recycling of rubber from old tires by pyrolysis and hydropyrolysis using a swept fixed bed reactor. They analyzed the effect of the main process variables (temperature, heating rate, gas flow, reaction time and hydrogen pressure) on oils, gases and solid residue produced. They found that 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. Conesa et al. [14] proposed a model for the primary devolatilization of tires, considering different heating rates. They proposed a model that includes three organic fractions that do not form char, and a fourth fraction that does not decompose. The authors confirmed this hypothesis by mass spectrometry. The three organic fractions were assigned to different compounds: the first one probably corresponds to the decomposition of an oil fraction, the second one to that of natural rubber and the third to styrene–butadiene rubber. Thermal degradation kinetics of natural rubber, styrene– butadiene rubber and polybutadiene were investigated by Lin et al. [15,16]. They proposed that two or three reactions were involved for the mixed rubbers. The kinetic studies by Chen et al. [17,18] show that one reaction is involved when SBR or epoxy resin is decomposed in an inert gas, but two reactions are involved when oxygen is present in the carrier gas, since thermal decomposition is not only an independent process, but also a first step in the gasification or combustion process. In addition, the rate of heat supply, total energy and operation time are important design parameters not only for a pyrolysis system to recover the gaseous and carbon products but also for combustion processes because devolatilization is the first step in the whole combustion
reaction. Consequently, the devolatilization kinetics of scrap tires deserves more investigation. In this paper, not only kinetic parameters are calculated but also its the model validated in a fixed bed reactor.
2. Experimental The raw material used for the kinetic studies was shredded tire rubber supplied by AMSA, a Spanish waste tire recycling company. This residue was sifted to three different average particle sizes of 2, 1 and 0.5 mm and had the following ultimate and proximate analyses: C (daf): 88.3%; H (daf): 7.69%; S (mf): 1.85%, moisture (ar) 0%, ash content 5.30%, volatile matter (ar) 64.7% and fixed carbon (ar) 30%, where daf means dry and ash free, mf means moisture free and ar means as received. Pyrolysis tests were conducted in a thermobalance (SETARAM TG DTA-92). Two different gas flows are introduced into the thermobalance. The first one consisted of pure N2 and passed through the head of the thermobalance in order to protect the microbalance from the possible corrosive gases introduced or generated. The second one was introduced directly at the top of the thermobalance reactor and consisted of nitrogen during heating and reaction times. Both gas streams were controlled by mass flow controllers (BRONKHORST HI-TEC EL-FLOW) and mixed before they reached the sample. The solid sample was deposited in a small platinum basket with a circular base (5 mm diameter and 2 mm height). The thermobalance was provided with an electric oven that could operate up to 1750 8C and it was controlled with a PID temperature controller. Different heating and cooling rates could be programmed with a maximum program time of 89 h. A thermocouple was located close to the platinum basket for temperature monitoring and closing the oven control loop. The solid weight loss, together with other process variables like temperature, was continuously transmitted to a computer through a data acquisition card. The temperature evolution and the time when the different mass flow controllers had to be opened were also controlled through the computer. All the pyrolysis experiments were performed by working at atmospheric pressure. The tested heating rates were 5, 10, 20, 30 and 40 8C/min. The sample was forming a small fixed bed in the platinum basket and an initial mass of 10 mg was used for all the runs. The influence of the particle size was also studied ranging from 0.5 to 2 mm. Different flow rates were tested (100–250 ml/min) to study the influence of this variable on the rubber tire conversion. The experimental conversion (Xexp) was calculated according to Equation (1), where w0 is the initial sample weight, wi is the sample weight at any time and wfinal is the final weight (the weight stable after reaction). Xexp ¼
w0 wi w0 wfinal
(1)
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261
Fig. 1. Swept fixed bed reactor pilot plant.
2.1. Experiments in a fixed bed reactor The swept fixed bed reactor (Fig. 1) was 30 cm long and had an internal diameter of 2 cm. It was provided with a mass flow controller, a PID temperature controller and a furnace. The design of the temperature controller allowed fixing a certain heating rate for the experiment. These experiments were performed at 5 8C/min heating rate, 0.2– 0.4 mm particle size and 2 l/min flow rate.
Fig. 3. Rubber tire conversion vs. time at different particle sizes (550 8C final temperature, 150 ml/min flow rate and 20 8C/min heating rate).
150 ml/min was used for the rest of experiments in order to protect the thermobalance head of possible corrosive gases liberated during the reaction. 3.2. Particle size influence
3. Results and discussion 3.1. Flow rate influence With the aim of assessing the influence of external mass transfer on tire rubber pyrolysis, experiments at different flow rates (250, 200, 150 and 100 ml/min) were performed. All these runs were carried out with a particle size lower than 1 mm, a heating rate of 20 8C/min and a final reaction temperature of 550 8C. Fig. 2 shows the experimental results obtained and it is observed no influence of this variable on the final tire rubber conversion. In all the experiments, the final conversion was 100%. This conversion has been calculated considering that only the polymeric fraction reacts at these experimental conditions because carbon black is not reactive under standard pyrolysis conditions [5]. A flow rate of 100 ml/min seems to be high enough to carry away all the conversion products of the rubber pyrolysis avoiding possible retrogressive or condensation reactions. Since the flow rate does not affect to tire pyrolysis, a value of
The influence of particle size on tire pyrolysis was studied using three different particle sizes: 2, 1 and <1 mm. This a very important variable in the pyrolysis process because it is an endothermic reaction where a temperature profile along the particle diameter occurs. In this way, it is expected that, depending on the solid thermal conductivity, a remarkable temperature gradient inside the particles could be observed. This temperature gradient could affect to the obtained products in the pyrolysis process [19]. In addition, the rubber tire grinding process is quite complicated and energetically demanding. Therefore, it would be beneficial for the whole pyrolysis process that large particle sizes could be used. Fig. 3 shows the pyrolysis results for the studied fractions and the following experimental conditions: 550 8C, 150 ml/min flow rate and 20 8C/min heating rate. It is observed that the maximum conversion was obtained in all the experiments, regardless of the particle diameter and all the curves are overlapping each other. Therefore, no influence of this variable was observed for the pyrolysis of tire rubber, at least working in the 2 mm dust range. Anyway, the particle size fraction smaller than 1 mm was selected for the rest of the runs and the kinetics deduction because the initial sample weight in the thermobalance is quite small, around 9–10 mg. Therefore, a higher sample surface that, at the same time, implies a more representative sample will be achieved when small particles are used. 3.3. Influence of the final temperature
Fig. 2. Rubber tire conversion vs. time at different flow rates (550 8C final temperature, 0.4–0.6 mm particle size and 20 8C/min heating rate).
Fig. 4 shows the results obtained in the pyrolysis of rubber tire at different final temperatures. All the runs were carried out at 150 ml/min, 20 8C/min heating rate and the lower particle size (lower than 1 mm). By plotting conversion versus temperature (Fig. 5), it can be observed that pyrolysis reaction starts at 200 8C when the heating rate is 20 8C/min. It is also
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for reactor design because in pyrolysis processes both, the product distribution and the final conversion achieved may depend on this variable, especially when fossil fuels are used. Experiments using different heating rates (5, 10, 20, 30 and 40 8C/min) were performed with a final temperature and a flow rate of 550 8C and 150 ml/min, respectively, for all the runs. Fig. 5 shows the conversion results obtained at the different heating rates. It is observed that the influence of this variable is only slightly appreciable at the slowest heating rates. Higher conversions are achieved for the same temperature when the slowest heating rates are used because the sample is under reaction conditions for a longer time when slow heating rates are used. Fig. 4. Rubber tire conversion vs. time at different final temperatures (0.4– 0.6 mm particle size, 150 ml/min flow rate and 20 8C/min heating rate).
Fig. 5. Rubber tire conversion vs. temperature at different heating rates (550 8C final temperature, 150 ml/min flow rate and 0.4–0.6 mm particle size).
observed in the curves obtained for 500, 550 and 600 8C that the reaction finishes between 480–490 8C and therefore, no influence of the final temperature is observed. Total conversion has been achieved before the final temperature has been reached and no weight loss is registered. Conversion does not improve by increasing temperature over 500 8C because the polymer has completely disappeared from the solid sample and carried away by the nitrogen stream and the remaining carbon black is not reactive under these experimental conditions. With respect to the experiment performed at 450 8C, it is observed that the conversion evolution is identical to the rest of experiments until the final temperature is reached. At this moment, the tire rubber total conversion has not been achieved. Although the observed trend is different, finally, the total conversion is obtained. 3.4. Heating rate influence Another important variable that has been studied is the influence of heating rate. This is a very important variable
3.5. Kinetic analysis Usually, mathematical models for describing overall decomposition of complex wastes and biomass consider independent parallel reactions [20]. Rubber tire DTG curves are characterized by the presence of shoulders and/or double peaks because more than one reaction is occurring simultaneously. In this case, the process has been modeled considering that rubber tire is comprised of four components that can be separately but simultaneously reacting. These components are: additives (non polymeric material added during the manufacturing process including oils, plasticizers, etc.), a first polymeric material (probably polybutadiene or styrene–butadiene rubber), a second polymer (probably natural rubber) and the solid residue (the carbon black and mineral matter added as reinforcing material and filler). In the DTG curves, only three peaks are detected and it is assumed that the last tire rubber component does not show weight loss under the experimental conditions tested. The general kinetic law for each single reaction is: dXi ¼ koi eEai =RT ð1 Xi Þni (2) dt where Xi is the solid fractional conversion for component i calculated according to Equation (1), t is the reaction time, R is the gas universal constant, T is the absolute temperature, koi is the frequency factor for component i, Eai is the activation energy and ni is the reaction order for component i. As the sample components decompose independently, the overall pyrolysis rate for N reactions occurring in parallel can be described by the following equation: X N N dX X dXi ¼ ci ðci koi eEai =RT ð1 Xi Þni Þ (3) ¼ dt dt i¼1 i¼1 where ci is a coefficient that expresses the contribution of each single reaction to the mass global loss. In this way, Equation (3) allows us to calculate coefficient ci after the determination of the frequency factor, activation energy and reaction order. In the kinetics evaluation, a nonlinear least squares (NLS) algorithm (specifically Nelder and Mead algorithm) was used. With this procedure, it is possible to identify
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Table 1 Correlation parameters for the rubber tire devolatilization reaction
Eai (kJ/mol) koi (s1) ni
Fig. 6. Experimental and calculated reaction rates vs. temperature (550 8C final temperature, 20 8C/min heating rate, 0.2–0.4 mm particle size and 150 ml/min flow rate).
parameters (koi, Eai, ni and ci) that minimize values of the objective function SDTG given below (see Equation (4)) and proposed by Sorum et al. [20]. " calc #2 N X dw exp dw SDTG ¼ (4) dt dt j j j¼1 In Equation (4), ðdw=dtÞexp is the experimentally observed DTG curve and ðdw=dtÞcalc is the calculated DTG curve obtained by numerical solution by Runge–Kutta method of the kinetic differential equation with the given set of parameters. Subscript j denotes discrete values of ðdw=dtÞ. Sørum’s modified objective function would be similar to the regression variance of the fit. In this way, it shows the dispersion of the experimental values versus the calculated ones. Fig. 6 shows the experimental and calculated reaction rates versus temperature working at a final temperature of 550 8C, at a constant heating rate of 20 8C/min, with particle size between 0.2–0.4 mm and a flow rate of 150 ml/min. Three different zones can be distinguished in this plot: Temperature between 150 and 310 8C: in this zone the drying of the particles has occurred. However, as the moisture content of tire rubber is very low, it is not appreciable the corresponding weight-loss. Following the particles drying, it is observed that the tire rubber decomposition begins. Low reaction rates are observed specially between 150 and 200 8C. In this stage of the pyrolysis reaction, probably the tire rubber additives are slowly decomposed and incorporated to the flow rate. During their decomposition, it is not observed a sharp peak compared to the other tire rubber components because they are not constituted by only one product but they are a mixture of different products (extender oils, plasticizers and other additives) with different properties and therefore different behavior under pyrolysis conditions. Temperature between 310 and 430 8C: in this zone, it is observed a sharper peak than the previous one probably
Additives
Polymer 1
Polymer 2
70 10000 2.8
212 8.20E + 14 1.4
265 3.20E + 17 1.9
because it corresponds to the decomposition of only one polymer and not a mixture of different compounds. In addition, this polymer will be present in rubber tire with a higher percentage than the additives because the area under the curve is much higher. Conesa et al. [14] and Williams and Besler [13] have suggested that this peak could correspond to the decomposition of natural rubber. Temperature between 350 and 490 8C: this last peak is also sharper than the first one but not as sharp as the second one. In addition, its area is lower than the one of the second peak and it has been reported [13] that could be assigned to the decomposition of styrene–butadiene rubber. At higher temperatures than 490 8C, a reaction rate close to zero is observed. All the polymeric material and the one that is able to be transformed into gases in the studied temperature range has been liberated and swept along the thermobalance by the nitrogen flow. Table 1 compiles the kinetic parameters obtained from the experimental results by fitting the reaction rate versus temperature curves. The obtained parameters are of the same range than the obtained by other authors like Conesa et al. [14] and Sorum et al. [20]. In addition, it is observed that the higher the temperature of the maximum in the different peaks, the higher the activation energy. All the experimental results obtained in this research were fitted using the deduced kinetic parameters and obtaining correlation coefficients higher than 0.95 in all the cases confirming the suitability of these parameters. With this methodology, it was also possible to find an approximate tire rubber composition. It was found that the sample studied had around 11.6% of additives, 32.6% of the first polymer, 18.0% of the second polymer and 37.8% of carbon black and mineral matter. This composition is only applicable to the tire rubber used: a mixture of rubbers from different brands, and it does not have to fit with the results reported by other authors who used a specific rubber tire. 3.6. Validation of kinetic results in a fixed bed reactor Finally, simulated experimental curves have been compared with real experimental results obtained in a fixed bed reactor (see Fig. 1) in order to validate calculated kinetic parameters. Experiments were stopped at different final temperatures in order to obtain the evolution of conversion with temperature. Fig. 7 shows the experimental and calculated conversion versus temperature. A good correlation between experimental results obtained in the fixed bed reactor and simulated values
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Acknowledgements Authors would like to thank Spanish Science and Technology Ministry (Project PPQ-4145), the Ramo´ n y Cajal Program (R. Murillo and M.S. Calle´ n) contracts and the General Council of Arago´ n, D.G.A., Spain, (J.M. Lo´ pez Pre-Doc grants).
References
Fig. 7. Experimental and calculated conversion vs. temperature in the SFBR (5 8C/min heating rate, 0.2–0.4 mm particle size and 2 l/min flow rate).
calculated from the thermobalance kinetics have been found. Therefore, it has been confirmed the suitability of these kinetic parameters in real conditions.
4. Conclusions Different operation variables have been studied in the tire rubber devolatilization process observing that only temperature has a remarkable effect on tire rubber conversion when this variable is lower than 500 8C. Total tire rubber conversion is achieved in these conditions for every component but the carbon black remains unconverted. Kinetic parameters have been deduced for every tire rubber component and have been used to simulate all the experimental results demonstrating their suitability. Finally, an approximate composition of the rubber tire sample has been calculated. The kinetic parameters obtained in the thermobalance were confirmed in a fixed bed reactor obtaining experimental conversion values close to the calculated ones.
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