Mixtures of rubber tyre and plastic wastes pyrolysis: A kinetic study

Energy 58 (2013) 270e282

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Mixtures of rubber tyre and plastic wastes pyrolysis: A kinetic study Miguel Miranda*, I. Cabrita, Filomena Pinto, I. Gulyurtlu LNEG, Estrada Paço do Lumiar, 22, 1649-038 Lisboa, Portugal

a r t i c l e i n f o

a b s t r a c t

Article history: Received 19 November 2012 Received in revised form 12 June 2013 Accepted 14 June 2013 Available online 16 July 2013

The study performed aimed at analysing possible routes for pyrolysis reaction mechanisms of polymeric materials namely RT (rubber tyre) and plastic wastes (PE (polyethylene), PP (polypropylene) and PS (polystyrene)). Consequently, and seeking sustainable transformation of waste streams into valuable chemicals and renewable liquid fuels, mixture of 30% RT, 20% PE, 30% PP and 20% PS was subjected to pyrolysis. Different kinetic models were studied using experimental data. None of the mechanisms found in literature led to a numerical adjustment and different pathways were investigated. Kinetic studies were performed aiming to evaluate direct conversions into new solid, liquid and gaseous products and if parallel reactions and/or reversible elementary steps should be included. Experiments were performed in batch system at different temperatures and reaction times. Kinetic models were evaluated and reaction pathways were proposed. Models reasonably fit experimental data, allow explaining wastes thermal degradation. Kinetic parameters were estimated for all temperatures and dependence of Ea and pre-exponential factor on temperature was evaluated. The rate constant of some reactions exhibited nonlinear temperature dependence on the logarithmic form of Arrhenius law. This fact strongly suggests that temperature has a significant effect on reaction mechanism of pyrolysis of mixtures of rubber tyre and plastic wastes. Published by Elsevier Ltd.

Keywords: Kinetic model Rubber tyre Plastic Wastes Pyrolysis

1. Introduction At the advent of the new century energy underpins human, technologic and economic development contributing significantly to improve quality of life, which usually results in an increase of both consumption of goods and production of wastes. The dependence on fuel and derived petroleum products will continuously increase in the next generations. This makes issues like environmental protection, waste management and efficient use of resources most important to tackle. The continuous increase of end-use polymeric materials, the dependence on petroleum for fuels and energy, and feedstocks for the industrial sector could result in a negative impact on environment [1e3]. Significant efforts have been undertaken to find more economical and environmentally-friendly solutions aiming to reduce the dependence on fossil fuels through more efficient energy technologies. Such actions have been taken to ensure alternative routes for fuels production, implementation of waste management policies and integration of smarter products and services to ensure a sustainable development [4e7].

* Corresponding author. I.P. (LNEG), National Laboratory of Engineering and Geology, Estrada Paço Lumiar, 22, 1649-038 Lisbon, Portugal. Tel.: þ351 210924417. E-mail address: [email protected] (M. Miranda). 0360-5442/$ e see front matter Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.energy.2013.06.033

Pyrolysis technology could be a solution for a better environment when applied to polymeric-base waste streams (e.g. tyres and plastics) allowing the recovery of both energy and organic content, whenever physical and/or mechanical recycle could not be implemented. Furthermore, industrial polymeric wastes that cannot be incorporated in industrial streams may be economically valorized through pyrolysis. This thermochemical process offers the advantage of having high flexibility with respect to feedstock characteristics, as it allows converting different polymeric-base wastes (with certain degree of contaminants) into added valuable products. Rubber tyre and most plastics are non-biodegradable materials and present high volatile matter and carbon contents. Moreover, these wastes present heating values higher than coal and biomass, offering great potential for recovery [8]. Many research groups have focused their research in this area, reporting valuable information on technology and reactor types and experimental parameters. Detailed reaction mechanisms and simulation models, including the elimination of redundant species and reactions, rate-ofproducts and reaction flux analysis and uncertainty analysis have been reported [9e15]. This information has been used as valuablebase data to ensure the reliability of kinetic mechanisms of pyrolysis [16]. A few kinetic studies were found in the literature review applied to rubber tyres and plastic waste mixtures, considering the formation of solids, liquids and gaseous products. Most of the

M. Miranda et al. / Energy 58 (2013) 270e282

information found refers to TGA (Thermogravimetric analysis) studies, where only transformations related to weight variation are considered. However, the information on this subject is still limited and in many cases contradictory. Moreover, enough experimental data do not exist to enable an objective and accurate analysis as the majority of the kinetics studies was carried out by thermobalance, DTA (Differential thermal analysis) and TGA measurements [17e 22]. These studies allow identifying different types of rubber and plastic approaching exclusively kinetic mechanisms of devolatilization without taking into account the formation of secondary products and interactions between different materials. As all reactions involve changes in enthalpy, those equipments present a significant disadvantage. They are unable to provide information about the nature of the reactions. As a result, these techniques solely describe the kinetic mechanisms involving loss of mass (devolatilization) [23]. Thus, chemical transformations that occur in the formation of products without loss of mass are not evaluated [2]. In addition, hydrocarbon pyrolysis is based on multi-step chemical reactions. These include a set of reversible free-radical reactions such as: radical recombination; radical and molecular disproportionation and its reverse; radical addition to unsaturated carbon; isomerization and hydrogen abstraction; b-scission and its reverse; and homolytic dissociation and its reverse [24,25]. Yumiko et al. [26] reported in PP catalytic decomposition into liquid fraction that C9 fraction was obtained by b-scission of rearranged ions while C4 and C5 fractions were obtained by previously C9 fraction formed. Radical mechanisms often appear more complicated due to the number of elementary steps involved. Pyrolysis of moderate complex molecules usually has a significant number of elementary steps. Furthermore, pyrolysis reactions occur not only in the gas phase, in which is possible to find a large number of thermochemical and kinetic data, but also in liquid and solid phases where additional diffusional mechanisms interfere with chemical kinetics [27]. The kinetic mechanisms of solid state reactions generally cannot be assumed to follow simple rate laws that are applicable to gas-phase state reactions. In solid phase, molecular movement is highly restricted and reactions are dependent on local structure and activity where the concept of concentration dependence is not of major importance. Faravelli et al. [28] reported that each single reaction step can be representative of a complex network of reactions and referred the need to apply significant corrections in gas-phase kinetic parameters to account for the condensate state (inhibition of molecular rotations of large CeC segments). Other research groups [29e32] estimated different kinetic parameters and reported heat and mass transfer limitations in different types of reactions as well as interactions of different polymers in mixtures. Katsuhide et al. [27] reported the influence of pressure in CeC links and proposed a mechanism of chain-end scission for polymers. Similar approaches to the present work were undertaken by some authors [33e35], regarding different types of plastics and their mixtures. Costa et al. [36] reported a number of rate constants of some steps considered in the models did not present linear temperature dependence. This could be explained by the fact that some reaction mechanisms altered with the change of temperature or the order of the reactions were different than those assumed. For rubber tyre wastes, Leung et al. [8] considered a three-component simulation model based on independent and irreversible firstorder degradation reactions, while Su et al. [37] reported the first stage is associated to the decomposition of oils, moisture, plasticizers and additives and the second stage to the decomposition of different rubber materials. Lin et al. [38] reported three stages of decomposition for SBR (styreneebutadiene rubber). Other group of investigators [23] proposed a kinetic model for tyre pyrolysis taking into account the formation of an intermediate fraction and parallel reactions while Aguado et al. [22] referred the need of taking into

271

account heat and mass transfer limitations in the kinetic studies. Different values were reported by Yang et al. [39] for NR (natural rubber), styreneebutadiene rubber (SBR) and BR (butadiene rubber) elastomers ranging from 152 to 215 kJ mol1 for Ea and 4.15  10þ10 to 2.36  10þ16 min1 for frequency factor. The differences found in kinetic parameters reported by different groups of investigators may be attributed to different blending and materials constituents, types of reactors, particle size, experimental conditions and heat and mass transfer limitations. The kinetic study of mixtures of rubber tyre and plastic wastes (PE, PP and PS) is fundamental to respond to the above mentioned problems. The present study used a theoretical model which fits successfully the experimental data allow foreseeing the product composition and relative distribution based on initial waste composition. 2. Experimental section Polymeric wastes used were collected from two different suppliers. Rubber tyre waste was collected from Portuguese automobile tyre households after mechanically shredded into small strips with an average length of 2e3 cm. Plastic wastes were constituted by polyethylene (PE), polypropylene (PP) and polystyrene (PS) obtained from plastic households after mechanically processed (small pellets with average diameter of 0.5 cm). Both wastes were characterized by elemental analysis equipment, LECO CHN 2000, to determine carbon, hydrogen and nitrogen contents. C/H mass ratio of rubber tyre was 11.3 and sulphur content was 1.47% (0.07), as determined by LECO SC-144DR equipment. Proximate analysis showed fixed carbon content of 33.5% (w/w), moisture content of 2.0% (w/w), ash of 2.9% (w/w), volatile matter of 61.6% (w/w). The high heating value determined was around 38.5 MJ kg1. Rubber tyre wastes main components were natural rubber (NR), styreneebutadiene rubber (SBR) and butadiene rubber (BR). X-ray fluorescence analysis indicated the presence of Zn, Na and S in high concentrations; Si, Cl, Al, K and Ca were detected in lower concentrations and some traces of Br, P, Pb and Fe were also found. Plastic wastes characterization showed a C/H mass ratio of 5.9 for PE, 6.0 for PP and 11.7 for PS. PE proximate analysis presented fixed carbon content of 0.1% (w/w), moisture content of 0.0% (w/w), ash of 0.1% (w/w), volatile matter of 99.8% (w/ w). The high heating value determined was around 46.4 MJ kg1. The proximate analysis performed on PP revealed fixed carbon content of 0.1% (w/w), moisture content of 0.1% (w/w), ash content of 17.2% (w/w) and volatile matter of 82.6% (w/w). The high heating value determined was around 37.6 MJ kg1. In the case of PS proximate analysis the values determined are as follows: fixed carbon content e 0.2% (w/w); moisture content e 0.3% (w/w); ash e 0.0% (w/w); volatile matter e 99.5% (w/w). The high heating value determined was around 39.0 MJ kg1. XRF (X-ray fluorescence) analysis indicates the presence of Ti in high concentrations for PE and Cl and Ca for PP; regarding lower concentrations it was found Pb, S, Ca, Si, Al and Mg for PE; Pb, Zn, Ti, Ca, S, K, Fe, Si and Al, for PP; and, S, Fe, Si, Al, Mg and Na for PS. Traces of Zn, Cl, Mg, Na and K were found for PE, whilst for PP it was found Sr, Cr, P and Mg, and for PS it was found Ti, Cl and Zn. RSM (response surface methodology) was employed [40] to optimize experimental conditions aiming at maximizing the dependent variable e liquid fraction. The experimental standard deviation was 3.4%. Although the results obtained are specific to the analysed domain (type of reactor, independent variables chosen, domain of experimental conditions and dependent variable), this information can be used as a first approach in an industrial facility. This base-information was used in the kinetic studies that were carried out in a series of six small batch reactors (Hastelloy C276 microautoclaves) of 0.16 L capacity built by Parr Instruments [40]. For each run the reactors were loaded with a total polymeric waste blend of 20 g (30% rubber tyre, 20% PE, 30% PP and 20% PS). The

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reactors were then closed, purged and pressurized with nitrogen gas to a previously preset value and then placed in a pre-heated oven at 900  C. After the desired reaction temperature was reached, the reactor was maintained in the oven during the reaction time previously established. Time zero was considered the time in which the pre-set reaction temperature was reached. Experiments were carried out at a heating rate of 30  C min1, with a 2  C min1 deviation. Reaction time varied between 12 and 908 s for temperatures of 350, 370, 390, 410, 430 and 450  C. After reaching the pre-set reaction time in each experiment, the reactors were cooled in an ice bath, to quench all reactions. When the temperature inside the reactor reached near room temperature, the reactor was depressurized, the volume of the gaseous fraction was measured and analysed by a HewlettePackard 6890 gas chromatography using Porapak Q and Molecular Sieves A packed columns. The reactors were then opened and both liquid and solid products were collected, weighed and analysed. Results showed an experimental standard deviation lower than 5%. The solid fraction was submitted to a solideliquid extraction process to recover the remaining liquid, first with CH2Cl2 (dichloromethane) and then with THF (tetrahydrofuran). These two solvents were selected due to their capability for hydrocarbons extraction and for not interfering with the subsequent gas chromatography analysis. As a result of the extraction process, three fractions were obtained: a light liquid fraction soluble in CH2Cl2, a heavy liquid fraction soluble in THF and a solid fraction insoluble in these two solvents. Pyrolysis liquid fractions were distillated and three other fractions were obtained, a light liquid fraction with a distillation point lower than 150  C, a heavy liquid fraction with a distillation range between 150 and 270  C and a more viscous liquid with a distillation point higher than 270  C. The first two fractions of the distillation process and both the extracted light and heavy liquid fractions were analysed using a HewlettePackard 6890 gas chromatograph. More than 70% (v/v) of liquids obtained from pyrolysis of rubber tyre and plastic wastes mixture present a distillation point lower than 270  C. Distillation curves of liquid products are within the range of standard gasoline and gas oils curves, although around 20% (v/v) did not distilled below 270  C (liquid residue). More than 60% of all compounds analysed by chromatography were clearly identified. Chemical and physical properties of pyrolysis products were determined by ASTM standards [41]. 3. Results and discussion

P k1

k2

L

k3

k6

k4

HL k7

G

S

k5 Fig. 1. Reaction pathway for rubber tyre and plastic wastes mixtures (P ¼ rubber tyre and plastic wastes mixtures; G ¼ non-condensable fraction; L ¼ light liquid fraction; HL ¼ heavy liquid fraction; S ¼ solid lower molecular weight polymer).

that solids could be: (i) initial polymer-base material; (ii) polymer of lower molecular weight and/or (iii) other long-chain solid molecules initially formed. On the basis of the above mention assumptions and taking into account the initial reaction mechanism proposed in Fig. 1, a theoretical kinetic model was developed. This model was calculated by solving the system of differential equations (1)e(5). For the present kinetic model seven rate constants were estimated for temperatures of 350, 370, 390, 410, 430 and 450  C. The rate constants of elementary first-order reactions were estimated through the resolution of differential equations by using MicromathÒ ScientistÒ program for WindowsÔ and the RungeeKutta method based on the Taylor theorem. Stineman method with least-squares analysis was used to calculate the numerical adjustments of interactions. As initial program procedure, authors used the information found in literature for the development of the proposed models as well as for the initial values of all constant rates. This information was introduced in the program to initiate the least-squares minimization in order to estimate the final constants rates to be adjusted to the experimental results.

dP=dt ¼ k1 P  k2 P  k3 P  k4 P

(1)

dG=dt ¼ k1 P

(2)

dL=dt ¼ k2 P þ k5 S þ k6 HL

(3)

dHL=dt ¼ k3 P  k6 HL þ k7 S

(4)

dS=dt ¼ k4 P  k5 S  k7 S

(5)

3.1. Reaction mechanism and kinetic model Given the high complexity of both reactants and products the proposed reaction mechanisms were develop taking into account: (i) the temperature dependence of the rate constants is described by Arrhenius equation; (ii) the physical state of the products obtained at room temperature; (iii) all elementary reactions are firstorder and irreversible and, (iv) there are no mass transfer resistances/limitations in the reaction medium. The initial proposed reaction pathway (Fig. 1) involves a combination of series and parallel reactions. Pyrolysis of rubber tyre and plastic wastes mixture (P) resulted in the formation of four different products namely: G (non-condensable fraction), L (light liquid fraction), HL (heavy liquid fraction) and S (solid polymer fraction) composed of lower molecular weight than the initial materials. The adopted methodology was based on liquid products solubility in the solvents used. As a result, light liquid fraction was soluble in CH2Cl2 whilst THF was a more suitable solvent for the heavy liquid fraction. Similar approaches were also adopted by other groups of researchers [35,36,42e46]. Due to the formation of solid compounds in the early stage of the process (low reaction times) it was assumed

Table 1 presents the values of seven rate constants determined by the theoretical model in the formation of non-condensable products (G), light liquid (L) and heavy liquid (HL) and solid lower molecular weight polymer (S). These rate constants were obtained from the theoretical model after suitable fitting between Table 1 Rate constants of pyrolysis products: non-condensable compounds (G), light liquid (L), heavy liquid (HL) and solid lower molecular weight polymer (S) (104 s1). Rate constants (104 s1)

Reaction temperature 350 ( C)

370 ( C)

390 ( C)

410 ( C)

430 ( C)

450 ( C)

k1 k2 k3 k4 k5 k6 k7

39.0 549.1 101.9 338.1 2.6 63.2 91.8

37.9 545.5 242.2 312.6 1.4 29.3 0.0

33.2 536.5 223.3 341.9 2.5 155.9 0.0

58.5 1452.1 120.6 300.3 1.6 87.6 78.5

50.0 812.2 204.5 292.1 2.9 393.0 0.1

153.3 2119.2 93.6 180.0 0.7 0.0 0.0

M. Miranda et al. / Energy 58 (2013) 270e282 P [theoretical] L [theoretical] L [experimental]

S [theoretical] G [theoretical] HL [experimental]

P [theoretical] L [theoretical] L [experimental]

HL [theoretical] G [experimental] S [experimental]

100

HL [theoretical] G [experimental] S [experimental]

90

80

80

70

Productyield (%w/w)

Product yield (%w/w)

S [theoretical] G [theoretical] HL [experimental]

100

T = 350ºC; P = 0,48MPa Rubber tyre, PE, PP and PS

90

273

60 50 40 30

70

T = 390ºC; P = 0,48MPa Rubber tyre, PE, PP and PS

60 50 40 30

20

20

10

10 0

0 0

100

200

300

400

500

600

700

800

0

900

100

200

300

400

500

600

700

800

900

Time (s)

Time (s)

Fig. 2. Model validation. Comparison between model output and experimental data for reaction temperature of 350  C.

Fig. 4. Model validation. Comparison between model output and experimental data for reaction temperature of 390  C.

theoretical and experimental data. Weight percentages are referred as the ratio between product weight fraction and initial mass used (mixture of rubber tyre and plastics wastes). The adjustments of experimental results with those predicted by the kinetic model are presented in Figs. 2e7. The points correspond to the experimental results and the thick lines to the output data obtained from the kinetic model. The results predicted by the model fits well with the experimental data for all temperatures and reaction times studied, allowing a reasonable understanding of the mechanism prevailing during pyrolysis of these wastes. No tests were conducted for reaction temperatures higher than 450  C due to experimental constrains (maximum operating oven temperature). In addition, higher reaction temperatures did not lead to higher liquid yields as previously reported [41]. As regards to reaction time, values do not exceed 905 s due to stabilization of pyrolysis products (Figs. 2e7). Information presented in Figs. 2e7 suggests a good fitting between experimental results and the data obtain by the theoretical model (Fig. 1). This information supports the experimental results regarding the formation of light liquid compounds, ranging between 60% and 85% (w/w), followed by heavy liquids and solids both presenting yields lower than 25% (w/w). Non-condensable compounds present yields lower than 10% (w/w) for all reaction temperatures studied. The formation of heavy liquid tends to occur mainly for reaction times lower than 300 s while solid products occur at earlier moments of the reaction. Both fractions seem to be converted into smaller molecules resulting in the formation of light

liquid for higher reaction times. For reaction times higher than 300 s, the decrease occurs through the conversion of both solids and heavy liquids into lighter compounds (light liquid). Other possible explanation may occur due to the increase of reaction time, as more energy was provided to reaction medium promoting the breaking of CeC bonds. For reaction temperatures higher than 430  C and reaction times higher than 550 s heavy liquid seems to be totally converted into light liquid as both solid and noncondensable products are stable. Some steps initially proposed in the reaction pathway for mixtures of rubber tyre and plastic wastes pyrolysis (Fig. 1) probably do not take place as some estimated constant rates were zero, suggesting a reduced probability of these reactions to occur. These steps may vary strongly with temperature, which could result in changes in the initially proposed reaction pathway. Consequently, in Fig. 8 is presented the modify reaction mechanism for reaction temperatures of 350, 370, 390, 410, 430 and 450  C in which were removed all initially steps with zero and near zero constant rate values. These steps are presented in dashed lines. The reaction mechanisms presented in Fig. 8 suggest that for all temperatures studied the formation of non-condensable products (G) were not significantly affected by reaction temperature resulting only from the direct conversion of the reactant P (reaction k1). These results are in agreement with the information presented in Table 1 (low values of rate constants) and Figs. 2e7 where the formation of non-condensable compounds does not exceed 10% (w/w). However,

P [theoretical] L [theoretical] L [experimental]

S [theoretical] G [theoretical] HL [experimental]

P [theoretical] L [theoretical] L [experimental]

HL [theoretical] G [experimental] S [experimental]

100

90

80

80 Product yield (%w/w)

Product yield (%w/w)

HL [theoretical] G [experimental] S [experimental]

100

T = 370ºC; P = 0,48MPa Rubber tyre, PE, PP and PS

90

S [theoretical] G [theoretical] HL [experimental]

70 60 50 40 30

T = 410ºC; P = 0,48MPa Rubber tyre, PE, PP and PS

70 60 50 40 30

20

20

10

10 0

0 0

100

200

300

400

500

600

700

800

900

Time (s)

Fig. 3. Model validation. Comparison between model output and experimental data for reaction temperature of 370  C.

0

100

200

300

400

500

600

700

800

900

Time (s)

Fig. 5. Model validation. Comparison between model output and experimental data for reaction temperature of 410  C.

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M. Miranda et al. / Energy 58 (2013) 270e282 P [theoretical]

S [theoretical]

HL [theoretical]

L [theoretical]

G [theoretical]

G [experimental]

L [experimental]

HL [experimental]

S [experimental]

TL

L

HL

G

S

2

100 90

1

T = 430ºC; P = 0,48MPa Rubber tyre, PE, PP and PS

70

Ln k (s-1)

Product yield (%w/w)

80

60 50 40

0 1.35

1.40

1.45

1.50

1.55

1.60

1.65

-1

-2

30 20

-3

10

1/T*103 (1/K)

-4

0 0

100

200

300

400

500

600

700

800

900

Fig. 9. Arrhenius representation for pyrolysis of rubber tyre and plastic wastes mixtures.

Time (s)

Fig. 6. Model validation. Comparison between model output and experimental data for reaction temperature of 430  C.

P [theoretical]

S [theoretical]

HL [theoretical]

L [theoretical]

G [theoretical]

G [experimental]

L [experimental]

HL [experimental]

S [experimental]

the formation of light liquid compounds is strongly affected by temperature resulting not only from the direct conversion of reactant P (reaction k2) especially for temperatures higher than 410  C, but also through the cracking of chemical bonds of heavier compounds (HL) (reaction k6) mostly at 390 and 430  C. The formation of HL results mainly from the direct conversion of reactant P (reaction k3) and in small amounts through reaction k7 at temperatures of 350 and 410  C. The formation of solid compounds (S) only results from the direct conversion of reactant P through reaction k4. For all range of experimental conditions, non-condensable compounds present no significant changes being almost 10% (w/w). For the same temperature and reaction time range, light liquid fraction (L) presents the highest yields ranging between 40 and 90% (w/w) followed by heavy liquid (HL) and solid compounds (S) being both lower than 30% (w/w). Nevertheless, reaction temperatures lower than 390  C and short reaction times favoured the formation of liquids with lower molecular weight ranging from 42% to 71% (w/w), while for reaction temperatures higher than 390  C, the formation of this liquid fraction increased between 71% and 88% (w/w). This increase could be related with the direct

100 90

Productyield(%w/w)

80 70

T = 450ºC; P = 0,48MPa Rubber tyre, PE, PP and PS

60 50 40 30 20 10 0 0

100

200

300

400

500

600

700

800

900

Time (s)

Fig. 7. Model validation. Comparison between model output and experimental data for reaction temperature of 450  C.

P

k1

k2

P

k3

k4

k1

k2

k3

HL G

L

k6

S

G

L

L

k2

S

G

L

k4

k1

k2

k6

T = 410ºC

k4

k6

S

T = 390ºC

P

k3

k3

HL

k6

P

k3

HL G

k1

T = 370ºC

P

k2

k4

HL k7

T = 350ºC

k1

P

k4

k1

k2

k3

k4

HL k7

S

G

L

k6

T = 430ºC

S

G

L

HL

S

T = 450ºC

Fig. 8. Modify reaction pathway of mixtures of rubber tyre and plastic wastes for reaction temperatures of 350, 370, 390, 410, 430 and 450  C (P ¼ reactant composed rubber tyre, PE, PP and PS; G ¼ non-condensable products; L ¼ light liquid; HL ¼ heavy liquid; S ¼ solid lower molecular weight polymer).

M. Miranda et al. / Energy 58 (2013) 270e282 Table 2 Kinetic parameters for pyrolysis of rubber tyre and plastic wastes mixtures. Reaction

Kinetic parameters A (s1)

Ea (kJ mol1) þ13

4.74  10 6.71  10þ14 n.l.d. 9.81  10þ5 n.l.d.

TL L HL G S

73.3 80.6 n.l.d. 46.0 n.l.d.

n.l.d. e non-linear temperature dependence.

conversion of the polymer mixture initially pyrolysed and with the conversion of heavier liquid compounds into lighter ones since, with the increase of reaction time, more energy was provided to the reaction medium favouring the breaking of CeC bonds. In addition, experimental results suggested that light liquid and heavy liquid compounds are related presenting opposite trends during the cracking process. For reaction times higher than 150 s, heavy liquid yields decrease earlier with the increase of reaction temperature suggesting the cracking of molecules with higher molecule weight

Aromatics

Alkanes

into smaller molecules (light liquid). Solid lower molecular weight polymer presents similar behaviour, decreasing from 30% to 10% (w/w) with the rise of both reaction temperature and time. The results obtained are in agreement with those estimated for the kinetic constants and presented in Table 1. The dependence of kinetic constants on temperature is presented in Fig. 9 with the aim of estimate the Ea (activation energy) and pre-exponential factor, according to the Arrhenius law. The kinetic parameters presented in Table 2 are referred to total liquid (TL), light liquid (L), heavy liquid (HL), non-condensable compounds (G) and solid compounds (S). The results suggest that a number of rate constants present a non-linear dependence with the temperature. This fact may be due to changes in the kinetic mechanism with the variation of temperature, to reactions which are not first order as initially assumed and also due to partial conversion before run temperature is reached. In addition, besides chemical effects, diffusional effects can also be found in the pyrolysis of rubber tyre and plastic wastes mixtures, although they were not considered as rate-controlling steps of the reactions. Ea and pre-exponential factors were estimated for the reaction steps with linear dependence with temperature ranging, respectively,

Heptane Ethylcyclohexane Nonane Tetradecane Pentacosane Octacosane

Alkenes

100 Temperature = 350 ºC Initial pressure = 0.48

80 70 60 50 40 30 20 10 0 0

100 200 300 400 500 600 700 800 900

Alkanes concentration (% v/v)

Relative distribution (%v/v)

90

95 85 75 65 55 45 35 25 15 5 0

100 200 300 400 500 600 700 800 900 Time (s)

3-Methyl-2-Cyclopentene

Dipentene

Undecene

Dodecene

Tridecene

Octadecene

Docosene

95 85 75 65 55 45 35 25 15 5

Temperature = 350 ºC Initial pressure = 0.48 MPa

0

100 200 300 400 500 600 700 800 900 Time (s)

c) Alkenes concentration - effect of reaction temperature and reaction time

b) Alkanes concentration - effect of reaction temperature and reaction time Ethylbenzene 3-Methylpiridine n-Butylbenzene Octylbenzene

Aromatic concentration (% v/v)

Alkenes concentration (% v/v)

Nonene

Decene

Octane 1,1-Dimethylcyclohexane Dodecane Pentadecane Hexacosane

Temperature = 350 ºC Initial pressure = 0.48 MPa

Time(s)

a) Distribution of liquid pyrolysis – effect of reaction temperature and reaction time Isoprene

275

95

o-Xylene Cumene 4-Ethyl-2-Methoxyphenol Decylbenzene

Temperature = 350 ºC Initial pressure = 0.48 MPa

85 75 65 55 45 35 25 15 5 0

100 200 300 400 500 600 700 800 900 Time (s)

d) Aromatic concentration - effect of reaction temperature and reaction time

Fig. 10. Liquid distribution and relative concentration of mixtures of rubber tyre and plastic wastes pyrolysis e reaction temperature 350  C.

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M. Miranda et al. / Energy 58 (2013) 270e282

from 46.0 to 80.6 kJ mol1 and 9.81  10þ5 to 6.71  10þ14. Correlation coefficients obtained for TL, L and G curves where respectively 0.994, 0.968 and 0.852. The lowest value of Ea was obtained for non-condensable compounds (G) which result from the direct conversion of initially polymer mixture (reaction k1) while the highest value was obtained in the formation of light liquid (L), which result both from the direct conversion of reactant (P) (reaction k2) and from heavy liquid (HL) through reaction k6. The reported values are consistent with experimental data. Nevertheless, the results obtained for solid compounds (S) and heavy liquid (HL) suggest the application of the Arrhenius equation in the estimation of Ea and pre-exponential factor may not be the best approach, due to possible changes in the reaction mechanism with the temperature and/or the order of the reactions might be different than those initially assumed. Unlike to what was observed in the present work, Costa et al. [35,36] in a similar batch reactor reported that temperatures below 400  C did not favoured the formation of liquid products. These authors [36] in pyrolysis of mixtures of PE, PP and PS wastes, estimated ten rate constants ranging from 4.0  105 to 8.1  102, 9.8  103 to 1.0  101 and 5.0  108 to 9.9  102 respectively for

Aromatics

Alkanes

380, 400 and 420  C. This group also reported that rate constants of a number of steps were zero. For the pyrolysis of PE [22] Ea and preexponential factor range from 185 to 301 kJ mol1 and 5.8  109 to 1.1  1021 respectively. Fei Ding et al. [46] also used a batch reactor and reported activation energies of HDPE (High-density polyethylene) conversion and plastic mixtures, values of 217.6 kJ mol1 and 178.5 kJ mol1 respectively. Leung and Wang [8] in a threeelastomer simulation model of scrap tyres reported values of Ea and pre-exponential factor for NR (43 kJ mol1 and 4.5  10þ3 for oil and 207 kJ mol1 and 3.0  10þ14 for elastomer), for BR (43 kJ mol1 and 2.3  10þ3 for oil and 215 kJ mol1 and 7.1  10þ14 for elastomer) and for SBR (48 kJ mol1 and 6.9  10þ3 for oil and 152 kJ mol1 and 3.1  10þ10 for elastomer). Aguado et al. [22] in scrap tyre pyrolysis (microreactor) reported similar kinetic parameters. Olazar et al. [23] in scrap tyres pyrolysis estimated kinetic constants for seven lumps: gas (C4), non-aromatic liquids (C5eC10), aromatics (C10), tar (C11þ), intermediate, char of low grade carbon black and original tyre. Different kinetic parameters were reported by other groups of investigators in pyrolysis of tyres wastes [1,38,39]. The differences found in a number of reported works could result from differences in kinetic models, waste-base materials and mixtures, experimental

Heptane Ethylcyclohexane Nonane Dodecane Pentadecane Pentacosane

Alkenes

90

Temperature = 370 ºC Initial pressure = 0.48

80 70 60 50 40 30 20 10 0 0

100 200 300 400 500 600 700 800 900

Alkanes concentration (% v/v)

Relative distribution (%v/v)

100

95 85 75 65 55 45 35 25 15 5

Temperature = 370 ºC Initial pressure = 0.48 MPa

0

100 200 300 400 500 600 700 800 900

Time(s)

Time (s)

Nonene

Decene

3-Methyl-2-Cyclopentene

Dipentene

Undecene

Dodecene

Tridecene

Octadecene

Docosene

95 85 75 65 55 45 35 25 15 5

Temperature = 370 ºC Initial pressure = 0.48 MPa

0

100 200 300 400 500 600 700 800 900 Time (s)

c) Alkenes concentration - effect of reaction temperature and reaction time

b) Alkanes concentration - effect of reaction temperature and reaction time Ethylbenzene m/p-Xylene n-Butylbenzene

95 Aromatic concentration (% v/v)

Alkenes concentration (% v/v)

a) Distribution of liquid pyrolysis – effect of reaction temperature and reaction time Isoprene

Octane 1,1-Dimethylcyclohexane Decane Tetradecane Tetracosane

o-Xylene Cumene 4-Ethyl-2-Methoxyphenol

Temperature = 370 ºC Initial pressure = 0.48 MPa

85 75 65 55 45 35 25 15 5 0

100 200 300 400 500 600 700 800 900 Time (s)

d) Aromatic concentration - effect of reaction temperature and reaction time

Fig. 11. Liquid distribution and relative concentration of mixtures of rubber tyre and plastic wastes pyrolysis e reaction temperature 370  C.

M. Miranda et al. / Energy 58 (2013) 270e282

conditions, types of reactors and unavoidable heat and mass transfer limitations. The kinetic parameters estimated for rubber tyre and plastic wastes mixtures, which varied with temperature, were calculated for reaction temperature range of 350e450  C. Nevertheless, thermal conversion of these mixtures for temperatures lower than 350  C result in the formation of very low amounts of liquids (being some of them in a semi-state of solid fraction) while for temperatures higher than 450  C the liquid fraction yield decreased. Thus 370  C was suggested as an optimum temperature for maximum liquid production [41]. 3.2. Product composition Pyrolysis of mixtures of rubber tyre and plastic wastes result in the formation of non-condensable, liquid and solid products. Nevertheless, the identification of non-condensable products was impossible to carry out due to the very small amount of gas produced. Thus, gas yields were calculated by difference to the total mass initially pyrolysed after weighting both liquid and solid fractions. Liquid hydrocarbons which are very interesting

Aromatics

Alkanes

from both energetic and chemical point of view are composed by dozens of compounds (identified by GCeMS (Gas chromatography–mass spectrometry)). In the range of temperatures analysed, most organic compounds acquire sufficient vibration energy to cause breaking of bonds with formation of free radicals. Thus, alkanes, alkenes and aromatics compounds are expected to undergo rupture of CeC (carbonecarbon) and CeH (carbonehydrogen). This may result in the formation of radicals which can further react giving rise to lower hydrocarbon molecules and hydrogen. Moreover, higher-molecular-weight compounds can also be formed resulting from their recombination. Thermal breakdown of complex structures leads to very complex mixtures of products arising from concurrent dissociation, elimination, and bond fission. As many compounds have been identified, they were grouped in alkanes, alkenes and aromatic compounds. In this very complex mixture is worth mentioning the formation of alkanes (mainly pentacosane, ethylcyclohexane and decane), the monomer and dimer of natural rubber isoprene and aromatics compounds such as xylene, ethylbenzene and 4ethyl-2-methoxyphenol. Both aromatic compounds and other wide range of hydrocarbons may result from non-selective

Heptane Ethylcyclohexane Nonane Dodecane Pentadecane Tetracosane Triecontane

Alkenes

Temperature = 390 ºC Initial pressure = 0.48

80 70 60 50 40 30 20 10 0 0

100 200 300 400 500 600 700 800 900

Alkane concentration (% v/v)

Relative distribution (% v/v)

100 90

95 85 75 65 55 45 35 25 15 5 0

100 200 300 400 500 600 700 800 900 Time (s)

Dipentene

Undecene

Dodecene

Tridecene

Tetradecene

Octadecene

Docosene

95 85 75 65 55 45 35 25 15 5

Temperature = 390 ºC Initial pressure = 0.48 MPa

0

100 200 300 400 500 600 700 800 900 Time (s)

c) Alkenes concentration - effect of reaction temperature and reaction time

b) Alkanes concentration - effect of reaction temperature and reaction time Ethylbenzene m/p-Xylene Styrene 4-Ethyl-2-Methoxyphenol

Aromatic concentration (% v/v)

Alkene concentration (% v/v)

Decene

3-Methyl-2-Cyclopentene

Octane 1,1-Dimethylcyclohexane Decane Tetradecane Hexadecane Pentacosane

Temperature = 390 ºC Initial pressure = 0.48 MPa

Time (s)

a) Distribution of liquid pyrolysis – effect of reaction temperature and reaction time Isoprene

277

95

o-Xylene Cumene n-Butylbenzene

Temperature = 390 ºC Initial pressure = 0.48 MPa

85 75 65 55 45 35 25 15 5 0

100 200 300 400 500 600 700 800 900 Time (s)

d) Aromatic concentration - effect of reaction temperature and reaction time

Fig. 12. Liquid distribution and relative concentration of mixtures of rubber tyre and plastic wastes pyrolysis e reaction temperature 390  C.

278

M. Miranda et al. / Energy 58 (2013) 270e282

breakage of bonds which result in the formation of a significant number of molecular structures. The evolution of the relative distribution of total liquid compounds (formed by light and heavy liquid fractions) is presented in Figs. 10e15 separated into alkanes, alkenes and aromatic compounds (Fig. a), for all temperatures studied as a function of reaction time. Given the particular complexity of the mixtures and for an easier interpretation, only liquid compounds with concentrations higher than 5% (v/v) are presented, separated in alkanes, alkenes and aromatic compounds (Figs. bed of Figs. 10e15). The relative distribution of alkanes, alkenes and aromatic compounds was not significantly affected by the rise of both reaction temperature and time. The highest concentration of aromatic compounds was around 60%e70% (v/v), alkenes between 15% and 30% (v/v) and alkanes of 8%e18% (v/v). Nevertheless, for all reaction temperatures the reported information suggests a close relationship between the formation of alkenes and aromatic compounds, showing evidences of mutual conversion. Therefore, the conversion of alkenes into aromatic compounds might have occurred in the initial moments of the reaction mainly for temperatures of 350e370  C and 430e 450  C. In general, alkanes ranging from C5 to C30 were identified

Aromatics

Alkanes

being pentacosane, dodecane, tetradecane and hexacosane those ones formed in higher concentrations. Moreover, the formation of decane occurred both for reaction temperatures higher than 430  C and reaction times higher than 251 s, while the formation of nonadecane only occurred at 450  C. Regarding alkenes fraction, compounds in range from C5 to C22 were identified and no significant changes were found with the increase of both reaction temperature and time. Isoprene was the compound formed at higher concentration, followed by docosene and undecane. It is also worth mention the formation of dipentene at 390 and 430  C, undecane for temperatures higher than 410  C, tetradecane for temperatures lower than 430  C and octadecane at 450  C. In what concerns the aromatic fraction, GCeMS analysis revealed the existence of compounds ranging from C6 to C16 being o-xylene, ethylbenzene and 4ethyl-2-methoxyphenol those formed in higher concentrations for all reaction temperatures studied. Nevertheless, benzene and toluene were not observed, probably because the reaction medium was reactive enough to promote the conversion of initial products formed into other ones by different cracking reactions. Furthermore, rubber tyre and plastics additives could promote catalytic reactions of these compounds. Other worth mentioned compounds

Ethylcyclohexane Decane Tetradecane Tetracosane Hexacosane

Alkenes

90

Temperature = 410 ºC

80

Initial pressure = 0.48

70 60 50 40 30 20 10 0 0

Alkanes concentration (% v/v)

Relative distribution (%v/v)

100

95 85 75 65 55 45 35 25 15 5

Temperature = 410 ºC Initial pressure = 0.48 MPa

0

100 200 300 400 500 600 700 800 900

100 200 300 400 500 600 700 800 900

Time(s)

Time (s)

Isoprene

Nonene

Decene

Dipentene

Undecene

Tetradecene

Octadecene

Docosene

b) Alkanes concentration - effect of reaction temperature and reaction time Ethylbenzene o-Xylene Styrene

95 Temperature = 410 ºC Initial pressure = 0.48 MPa

85 75 65 55 45 35 25 15 5 0

100 200 300 400 500 600 700 800 900 Time (s)

c) Alkenes concentration - effect of reaction temperature and reaction time

Aromatic concentration (% v/v)

Alkenes concentration (% v/v)

a) Distribution of liquid pyrolysis – effect of reaction temperature and reaction time

95

1,1-Dimethylcyclohexane Dodecane Pentadecane Pentacosane

3-Methylpiridine Cumene 4-Ethyl-2-Methoxyphenol

Temperature = 410 ºC Initial pressure = 0.48 MPa

85 75 65 55 45 35 25 15 5 0

100 200 300 400 500 600 700 800 900 Time (s)

d) Aromatic concentration - effect of reaction temperature and reaction time

Fig. 13. Liquid distribution and relative concentration of mixtures of rubber tyre and plastic wastes pyrolysis e reaction temperature 410  C.

M. Miranda et al. / Energy 58 (2013) 270e282

Aromatics

Alkanes

Heptane Ethylcyclohexane Nonane Undecane Tridecane Pentadecane Pentacosane

Alkenes

90

Temperature = 430 ºC Initial pressure = 0.48

80 70 60 50 40 30 20 10 0 0

100 200 300 400 500 600 700 800 900

Alkane concentration (% v/v)

Relative distribution (%v/v)

100

95 85 75 65 55 45 35 25 15 5 0

100 200 300 400 500 600 700 800 900 Time (s)

b) Alkanes concentration - effect of reaction temperature and reaction time Ethylbenzene Cumene 4-Ethyl-2-Methoxyphenol

Decene Tridecene Docosene

95

95

Temperature = 430 ºC Initial pressure = 0.48 MPa

85 75 65 55 45 35 25 15 5 0

100 200 300 400 500 600 700 800 900 Time (s)

c) Alkenes concentration - effect of reaction temperature and reaction time

Aromatic concentration (% v/v)

Alkene concentration (% v/v)

Nonene Undecene Octadecene

Octane 1,1-Dimethylcyclohexane Decane Dodecane Tetradecane Hexadecane

Temperature = 430 ºC Initial pressure = 0.48 MPa

Time(s)

a) Distribution of liquid pyrolysis – effect of reaction temperature and reaction time Isoprene Dipentene Tetradecene

279

o-Xylene Styrene Octylbenzene

Temperature = 430 ºC Initial pressure = 0.48 MPa

85 75 65 55 45 35 25 15 5 0

100 200 300 400 500 600 700 800 900 Time (s)

d) Aromatic concentration - effect of reaction temperature and reaction time

Fig. 14. Liquid distribution and relative concentration of mixtures of rubber tyre and plastic wastes pyrolysis e reaction temperature 430  C.

formed at 390  C were n-butylbenzene (between 41 and 111 s) and styrene (between 547 and 902 s), while the formation of m/p-xylene only occurred at the highest reaction temperature studied (450  C). For the lowest temperature (350  C) no evidences were found of mutual conversion between alkenes and aromatic compounds. Nevertheless, for reaction temperature of 370  C (Fig. 11a) the conversion of alkenes into aromatic compound can be observed for reaction times lower than 187 s, probably due to double bond breakdown of unsaturated molecules. For reaction times higher than 187 s no significant changes were found. According to Fig. 11b, pentacosane was the alkane formed at the highest concentration (around 25% (v/v)) followed by dodecane (around 15% (v/v)). These compounds present a constant trend with the increase of reaction time. Isoprene (Fig. 11c) was the alkene formed at the highest concentration mainly in the first moments of reaction (52 s) as well as reaction times between 187 and 252 s (around 45% (v/v)). For reaction times higher than 252 s isoprene decreased significantly (22% (v/v)), while both undecane and docosene increased to values around 17% and 12% (v/v), respectively. Aromatic compounds (Fig. 11d) formed at the highest concentrations were o-xylene

and ethylbenzene, with concentrations of 36% and 18% (v/v) respectively. In Fig. 12 is presented the liquid distribution and relative concentration for reaction temperature of 390  C. As it was observed for temperature of 370  C, the concentration of aromatic compounds increased in the first moments of the reaction (187 s) at the expense of a decrease in alkenes fraction while alkane fraction remains constant. For higher reaction times it was observed a slight decrease of aromatic compounds. Regarding alkanes concentration (Fig. 12b), reaction times lower than 253 s favoured the formation of dodecane reaching the content of 28% (v/v), while the formation of nonane was around 15% (v/v) for the first moments of the reaction (111 s). For reaction times higher than 374 s pentacosane was the alkane formed in higher concentration being around 22% (v/v). Regarding alkene formation, isoprene was the compound formed in higher concentration, reaching 49% (v/v) for a reaction time of 56 s. In Fig. 12d is presented the evolution found for aromatic compounds. The increase of reaction time did not favoured the formation of o-xylene as it decreased from 39% to 26% (v/v), while the formation of ethylbenzene increased up to 24% (v/v), being constant with the increase of reaction time.

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M. Miranda et al. / Energy 58 (2013) 270e282

Aromatics

Alkanes

Alkenes

90

Temperature = 450 ºC Initial pressure = 0.48

80 70 60 50 40 30 20 10 0 0

Heptane

Octane

Decane

Undecane

Dodecane

Tetradecane

Pentadecane

Heptadecane

Nonadecane

Pentacosane

95 Alkane concentration (% v/v)

Relative distribution (% v/v)

100

Temperature = 450 ºC Initial pressure = 0.48 MPa

85 75 65 55 45 35 25 15 5 0

100 200 300 400 500 600 700 800 900

100 200 300 400 500 600 700 800 900

Time (s)

Time (s)

a) Distribution of liquid pyrolysis – effect of reaction temperature and reaction time

95 85 75 65 55 45 35 25 15 5

Temperature = 450 ºC Initial pressure = 0.48 MPa

0

Ethylbenzene 3-Methylpiridine Cumene 4-Ethyl-2-Methoxyphenol

Nonene 3-Methyl-2-Cyclopentene Undecene Tetradecene Docosene

100 200 300 400 500 600 700 800 900 Time (s)

c) Alkenes concentration - effect of reaction temperature and reaction time

Aromatic concentration (% v/v)

Alkene concentration (% v/v)

Isoprene Decene Dipentene Dodecene Octadecene

b) Alkanes concentration - effect of reaction temperature and reaction time

95

o-Xylene m/p-Xylene Styrene

Temperature = 450 ºC Initial pressure = 0.48 MPa

85 75 65 55 45 35 25 15 5 0

100 200 300 400 500 600 700 800 900 Time (s)

d) Aromatic concentration - effect of reaction temperature and reaction time

Fig. 15. Liquid distribution and relative concentration of mixtures of rubber tyre and plastic wastes pyrolysis e reaction temperature 450  C.

In Fig. 13 is presented the information related to 410  C. For this reaction temperature and for reaction times lower than 547 s (Fig. 13a), it is only worth mentioning the increase of alkene fraction from 23% to 32% (v/v) at the expenses of aromatic compounds, which decreased from 61% to 52% (v/v). No significant changes were found for alkane relative distribution (16% (v/v)). For alkanes relative concentration (Fig. 13b), results suggested the formation of pentacosane, tetracosane and decane of around 17% (v/v) and isoprene (Fig. 13c) reaching values around 44% (v/v) at 199 s, decreasing to 28% (v/v) with the increase of reaction time. According to the information of Fig. 13d, aromatic compounds formed in higher concentrations were ethylbenzene (28% (v/v)) and 4-ethyl-2-methoxyphenol (19% (v/v)). All information obtained at 430  C is presented in Fig. 14. Once again, it was observed mutual conversion between alkene and aromatic compounds. According to experimental results, in the first instants of reaction (45 s) aromatic compounds increased at expenses of alkene fraction, reaching the highest value of 67% (v/v). After this period and for reaction times lower than 367 s, opposite behaviour was observed whilst no significant changes were found in alkane fraction (12% (v/v)). In accordance to Fig. 14, experimental

results suggested the formation of decane (Fig. 14b) in the first moments of reaction (62 s) which increased substantially for higher reaction times (725 s) to values of 47% (v/v). Moreover, this temperature seems to favour the formation of both dodecane (24% (v/v)) for reaction times until 251 s and tetradecane (13% (v/v)) for reaction times lower than 185 s. Once again, isoprene was the alkene formed in higher concentration increasing up to 46% (v/v) for reaction times lower than 185 s (Fig. 14c). Experimental results also suggested the formation of dipentene in the first moments of the reaction (60 s) and the formation of undecene, tetradecene and docosene for all reaction time study. Regarding the formation of aromatic compounds (Fig. 14d) it is only worth mentioning the formation of ethylbenzene, 4-ethyl-2-methoxyphenol and o-xylene. The results obtained for the highest reaction temperature (450  C) are presented in Fig. 15. The first moments of reaction (91 s) favoured the formation of aromatic compounds at the expenses of a decrease in alkene fraction, as alkane fraction presented no significant changes (Fig. 15a). After this period, the stabilization of products seems to be achieved being around 64% (v/v) for aromatic compounds and 24% (v/v) for alkene fraction. Taking into

M. Miranda et al. / Energy 58 (2013) 270e282

account Fig. (a) of Figs. 13e15 the results obtained suggested that the increase of reaction temperature from 410 to 450  C favoured the stabilization of products for shorted reaction times (earlier stage). For alkanes relative concentration (Fig. 15b) the increase of reaction time up to 185 s favoured the formation of decane (13%e46% (v/v)) while short reaction times (49 s and 91 s) seems to favour both the formation of dodecane (23% (v/v)) and pentacosane (21% (v/v)). According to Fig. 15c isoprene was the alkene formed at the highest concentration followed by undecene in the first moments of reaction until 241 s and for reaction times higher than 721 s. With regard to the formation of aromatic compounds (Fig. 15d) ethylbenzene and 4-ethyl-2-methoxyphenol present similar behaviour to the one reported for reaction temperature of 430  C. Furthermore, the formation of o-xylene increases from 5% to 27% (v/v) in the first moments of reaction (124 s) and small amounts of m/p-xylene, cumene and styrene are also worth mentioned. According to the experimental results presented in Figs. 10e15, pyrolysis of polymeric-based materials (rubber tyre and different plastic) favoured the formation of a complex mixture of liquid hydrocarbons, which resulted from the direct thermochemical conversion of these wastes as well as from the formation and recombination of other wide range of compounds. 4. Conclusions A mixture of rubber tyre and different plastic wastes (PE, PP and PS) was pyrolysed. Reaction pathways were proposed and experimental results were compared with those obtained through the kinetic model. The proposed kinetic model and experimental data fit rather well and kinetic constants were estimated. Rate constants of certain reaction steps considered in the models did not exhibit linear temperature dependence in a logarithmic form of simplified Arrhenius law. Ea and pre-exponential factor were estimated for those steps that present linear dependence with temperature being 73.3 kJ mol1 and 4.74  10þ13 for total liquid (TL), 80.6 kJ mol1 and 6.71  10þ14 for light liquid (L) and 46.0 kJ mol1 and 9.81  10þ5 for non-condensable compounds (G). Pyrolysis of these polymeric wastes resulted in the formation of a wide range of liquid compounds, followed by solid fraction and in very small amounts of non-condensable compounds. The effect of reaction temperature and residence time on liquid fraction composition was analysed. These compounds were grouped in alkanes, alkenes and aromatic compounds. Liquid fraction analysis revealed concentrations ranging from 8% to 18% (v/v) of alkanes, 15% to 30% (v/v) of alkenes being aromatics compounds the major group formed with contents around 60%e75% (v/v). In addition, experimental results suggest mutual conversion of aromatic and alkenes compounds. References [1] Aylon E, Callen MS, Lopez JM, Mastral Ana Marıa, Murillo R, Navarro MV, et al. Assessment of tire devolatilization kinetics. J Anal Appl Pyrolysis 2005;74: 259e64. [2] Miranda M, Pinto Filomena, Gulyurtlu I, Cabrita I. Pyrolysis of rubber tyres wastes: a kinetic study. Fuel 2012;103:542e52. [3] Pinto Filomena, Miranda Miguel, Gulyurtlu I, Cabrita I. Study of pyrolysis process to recycle tyres and plastic wastes. Int Symp Recycl Reuse Tyres 2001:27e38. [4] Miranda Miguel, Pinto Filomena, Gulyurtlu I. Polymer wastes pyrolysis. Recycling: processes, costs and benefits. Nova Science Publishers, ISBN 978-161209-507-3; 2011. [5] Miranda Miguel, Pinto Filomena, Gulyurtlu I. Polymer wastes pyrolysis for liquid fuel production. Liquid fuels: types, properties and production. Nova Science Publishers, ISBN 978-1-61470-513-0; 2011. [6] Miranda Miguel, Pinto Filomena, Gulyurtlu I. Organic potential of rubber tyre wastes. In Advances in materials science research, vol. 13. Nova Science Publishers, ISBN 978-1-62100-804-0; 2012. [7] Tabasová Andrea, Kropác Jirí, Kermes Vít, Nemet Andreja, Stehlík Petr. Wasteto-energy technologies: impact on environment. Energy 2012;44:146e55.

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