Energy Fuels 2010, 24, 6239–6247 Published on Web 11/16/2010
: DOI:10.1021/ef101010n
Study of the Pyrolysis Kinetics of a Mixture of Polyethylene, Polypropylene, and Polystyrene Paula Costa,*,† F. Pinto,† A. M. Ramos,‡ I. Gulyurtlu,† I. Cabrita,† and M. S. Bernardo§ LNEG-UEZ, Estrada do Pac-o do Lumiar 22, 1649-038 Lisboa, Portugal, ‡REQUIMTE, Departamento de Quı´mica, and §UBiA, FCT-Universidade Nova de Lisboa, 2829-516 Caparica, Portugal
†
Received August 3, 2010. Revised Manuscript Received October 15, 2010
The principal aim of this study was the identification of possible routes for the reaction mechanism of a plastic waste mixture pyrolysis. Kinetic studies of a mixture of equal percentages of polyethylene (PE), polypropylene (PP), and polystyrene (PS) were performed in order to analyze if the direct conversion of plastic wastes into gaseous, liquid, and solid products was favored, or if parallel reactions and/or reversible elementary steps should be considered. The experiments were carried out in a six microautoclave system. On the basis of the experimental results obtained at different temperatures and reaction times, a reaction pathway was proposed. The fitting of a kinetic model to the experimental data was performed. The models reasonably fit the results and gave a satisfactory explanation of several experimental data of thermal degradation of the plastic waste mixture tested. The kinetic parameters were estimated, and a dependence of the activation energy and of the pre-exponential factor on temperature was observed, so it was verified that the rate constant of some reactions exhibited nonlinear temperature dependence on the logarithmic form. This fact probably indicates that temperature affects the reaction mechanism. The product composition was also analyzed, and on the basis of the data obtained, reaction mechanisms were proposed.
composition based on plastic waste composition of the mixtures used and on pyrolysis conditions. The studies found in the literature showed that the information on this subject is limited and sometimes contradictory; enough experimental results do not exist to enable an objective and accurate analysis of the pyrolysis of plastic waste mixtures. This process has been reported by some research groups; however, different technologies and experimental parameters have been used. Nevertheless, only a few studies with the aim of finding the possible presence of interactions and synergetic effects during the degradation of mixed plastic wastes were found. In some cases, the results reported are contradictory. For instance, Wu et al.1 did not observe interactions between the different components during the pyrolysis of a mixture of HDPE (high density polyethylene), LDPE (low density polyethylene), PP (polypropylene), PS (polystyrene), ABS (acrylonitrile butadiene styrene), and PVC (polyvinylchloride). Also Westerhout et al.2,3 tested mixtures of PE and PP with different compositions and did not observe significant interactions when they were processed together. However, other authors obtained significantly different results during the thermal degradation of plastic waste mixtures compared with the individual pyrolysis of the same plastics. Some authors observed that the interaction between the compounds produced during the plastic waste pyrolysis was highly dependent on the kind of plastic used. For example, the presence of PS in the waste mixture seemed to affect
Introduction Present societies are over dependent on petroleum for fuels and for raw materials, essentials for many industries. In the world, this fuel is the most consumed to produce energy. Hence, efforts have to be undertaken to implement better management solutions of the petroleum resources, both for optimization of the energy efficiency technologies and for finding alternative routes for obtaining fuels. This aim may be achieved by recycling processes and reutilization of the resources from petroleum, which reduces its increasing utilization. Furthermore, the fraction of plastic in municipal solid wastes (MSW) is continuously rising, being therefore one of the main concerns of the developed societies. The need of finding urgent solutions for the mentioned problems led to the idea of applying pyrolysis technology to plastic waste, to allow its energy recovery whenever mechanical or physical recycling cannot be applied. In the pyrolysis process, not only can industrial plastic waste mixtures be used, that cannot be incorporated in the industrial process, but also the plastic mixtures present in municipal solid wastes can be used. Pyrolysis is a thermochemical process, which has the inherent advantage of high flexibility with respect to feedstock characteristics, so it can be applied to plastic mixtures with contaminants. This is the main advantage of this process. It is fundamental to study the kinetic of the pyrolysis of plastic waste mixtures in order to verify if the reaction pathways proposed for the individual pyrolysis of each kind of plastics can be applied to the pyrolysis of plastic waste mixtures. The present study is very important to test the possibility of foreseeing the product distribution and its
(1) Wu, C.-H.; Chang, C.-Y.; Hor, J.-L.; Shih, S.-M.; Chen, L.-W.; Chang, F.-W. Waste Manage. 1993, 13, 221–235. (2) Westerhout, R. W. J.; Waanders, J.; Kuipers, J. A. M.; van Swaaij, W. P. M. Ind. Eng. Chem. Res. 1998, 37, 2316–2322. (3) Westerhout, R. W. J.; Waanders, J.; Kuipers, J. A. M.; van Swaaij, W. P. M. Ind. Eng. Chem. Res. 1998, 37, 2293–2300. (4) Williams, E. A.; Williams, P. T. J. Chem. Technol. Biotechnol. 1997, 70, 9–20.
*To whom correspondence should be addressed. Fax: 351.21.7166569. E-mail:
[email protected]. r 2010 American Chemical Society
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product composition. Williams et al. investigated the pyrolysis of a plastic waste mixture (31.25% LDPE, 31.25% HDPE, 7.29% PP, 13.5% PS, 11.46% PVC, and 5.21% PET) in a fixed bed reactor at 700 °C and observed the presence of higher percentages of aromatic compounds and oxygenated groups than expected by simple accumulation of the liquid fraction formed by the degradation of each plastic. This result seems to indicate that the primary products formed by the degradation of each plastic can react with others formed during the decomposition of other plastics present in the mixture, thus leading to significant changes in the product distribution. Onwudili et al.5 studied product composition of a mixture of PE and PS in a closed batch reactor and verified that the oil produced was a blend of the compounds obtained by pyrolysis of separated plastic. Wong and Broadbelt6 compared the results obtained in the pyrolysis of a mixture with equal percentage of PP and PS and suggested that the presence of PP did not influence the degradation of PS, but the degradation of PP seemed to have been retarded. Similar results were observed by Bockhorn et al.7 that studied the influence of the presence of PS on PE degradation. These authors detected a significant reduction on the activation energy of the PE when pyrolyzed in the presence of PS. These authors proposed that the initiation reaction was controlled by the formation of radicals from the decomposition of PS. Williams and Williams8 also studied the influence of the presence of PS on HDPE, LDPE, PP, PVC, and PET pyrolysis and verified a clear interaction of the different kind of plastic used with PS, leading to the production of higher gas yields in the presence of this polymer. In the liquid fraction, obtained from pyrolysis of PEAD/PS, PEBD/PS, and PP/PS mixtures, more aromatic compounds were detected than expected. Recently, Walendziewski9 studied the degradation of two mixtures, one with 70% (w/w) PP and 30% (w/w) PS and other with 90% (w/w) PE and 10% (w/w) PS, using temperatures of 420 and 500 °C. An increase in the liquid yield was verified, mainly in the lighter compound fraction when more PS was used. Also, Ciliz et al.10 noticed an increase in the liquid yield in the presence of PS when mixtures of PP and PS were used. They also observed an increase in the solid residue when higher percentages of PP were used. On the other hand, Bockhorn et al.7 noticed that the influence of the presence of PE in the PS degradation was very low, since the kinetic parameters of the PS degradation did not change with the presence of PE. Regarding the kinetic studies of plastic waste mixtures, most of them used thermogravimetric analysis (TGA) measurements and the power law model to describe the rate of weight loss. In the literature, only the global kinetic parameters for the overall degradation of plastic waste mixtures were found, since the steps, which are not accompanied by a weight loss, as it happens in the initial stages of random chain cleavage, cannot be described by this method. Faravelli11 investigated the effect of the mixing scale of polymers and their interactions
in the melt. Several thermogravimetric analyses were performed on small samples of PE and PS mixtures. Marcilla et al.12 presented a thermogravimetric study of the influence of different catalysts on the decomposition of HDPE and PP mixtures and compared those results with the thermal decomposition behavior. This authors tested three different zeolite catalysts (ZSM5, fluid catalytic cracking, and USY) and observed that, when a catalyst was employed, the decomposition of these two polymers was strongly affected, specifically the temperature of maximum degradation rate was much lower than the one observed without catalyst. They also noticed that the shift to lower temperatures observed was different depending on the catalyst used. Regarding kinetic parameters, Bockhorn et al.7 calculated the activation energy and a reaction order for the thermal degradation of a PE and PS mixture. These authors obtained values of 166 ( 12 kJ mol-1 for the activation energy and a reaction order between 1 and 1.6. Other values for the activation energy were obtained by Ramdoss et al.13 These authors proposed a kinetic model based on the results obtained in runs performed to study the liquefaction (with hydrogen atmosphere) mechanism of a mixture of PP and PE. They compared the theoretical model with the experimental results. A range of temperatures from 475 to 525 °C and an initial hydrogen pressure of 790 kPa were used. From the results obtained, the authors concluded that higher temperatures and lower reaction times (5-10 min) were necessary to achieve the maximization of liquid yield. The activation energy and the pre-exponential factor for the different steps of the liquefaction model proposed were calculated. In this model, it was considered that the products of the liquefaction of a mixture of PE and PP were light and heavy liquid fractions, gases, and solids.13 In this work, the pyrolysis of a mixture of PE, PP, and PS was studied. Using the experimental data obtained, kinetic parameters for different steps of the plastic waste mixture pyrolysis were determined in order to identify possible routes for the reaction mechanism. On the basis of the results obtained, a theoretical kinetic model was developed and a reaction pathway was proposed. The experimental results were compared with those predicted by the model. The kinetic parameters of the conversion of a mixture of PE, PP, and PS into different products based on the pyrolysis results were calculated. Previous work carried out by the authors in order to study the effect of experimental parameters on plastic waste pyrolysis considering product yield and composition seem to show that chemical reactions should be very fast and that reaction times should be relatively short. 14 To test reaction times lower than 10 min, a system with six 160 mL microautoclaves was used. The results obtained were also used for kinetic studies. This paper presents and discusses the results obtained. Experimental Section
(5) Onwudili, J. A.; Insura, N.; Williams, P. T. J. Anal. Appl. Pyrolysis 2009, 86, 293–303. (6) Wong, H.-W.; Broadbelt, L. J. Eng. Chem. Res. 2001, 40, 4716– 4723. (7) Bockhorn, H.; Hentschel, J.; Bockhorn, A.; Hornung, U. Chem. Eng. Sci. 1999, 54, 3043–3051. (8) Williams, P. T.; Williams, E. A. Energy Fuels 1999, 13, 188–196. (9) Walendziewski, J. Fuel Process. Technol. 2005, 86, 1265–1278. (10) Ciliz, N. K.; Ekinci, E.; Snape, C. E. Waste Manage. 2004, 24, 173–181. (11) Faravelli, T.; Bozzano, G.; Colombo, M.; Ranzi, E.; Dente, M. J. Anal. Appl Pyrolysis 2003, 70, 761–777.
The plastic wastes used were collected from households and were mechanically recycled. These plastic wastes were presented in small pellets with a diameter of 0.5 cm and were used as received. (12) Marcilla, A.; Garcı´ a-Quesada, J. C.; Sanchez, S.; Ruiz, R. J. Anal. Appl Pyrolysis 2005, 74, 387–392. (13) Ramdoss, P. K.; Tarrer, A. R. Fuel 1998, 77, 293–299. (14) Pinto, F.; Gulyurtlu, I.; Costa, P.; Cabrita, I. J. Anal. Appl Pyrolysis 1999, 51, 39–55.
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The plastic samples, PE, PP, and PS, used were characterized by elemental analysis to carbon, hydrogen and nitrogen (CHN) determination and presented a C/H mass ratio of 6.2 for PE, 6.3 for PP, and 12.3 for PS. In the present study, a system with six 160 mL microautoclaves built by Hastelloy C276, by Parr Instruments, was used. The autoclaves were first loaded with 20 g of a mixture with equal percentages of PE, PP, and PS, then closed, purged, and pressurized to 0.41 MPa with nitrogen. The autoclaves were placed inside a heated oven at 900 °C, one by one. When the desired temperature inside the autoclave was reached (run temperature), the reactor was left in the oven during the reaction time previously preset. The heating rate was about 37 °C/min, so it takes about 1 h to reach the desired temperature, depending on the chosen temperature and plastic waste mixture used. The deviation in the heating rate during all the runs carried out was lower than 3 °C/min. The effect of the heating rate on the results obtained was studied, but this parameter did not seem to influence the product yields or its distribution. The time zero was considered the time at which the chosen temperature was reached. The reaction times used varied between 34 and 724 s, and the reaction temperatures changed between 380 and 420 °C. The temperature range and initial pressure used were chosen on the basis of previous work performed by the authors,14 which showed that the maximization of the liquid product was attained at lower temperatures, with the significant decrease in the solid yield in the PS pyrolysis and in the gas yields for all the three kinds of plastic tested. At temperatures higher than 420 °C, no significant changes in product yield and composition were observed. After the reaction time previously established by the authors for each experiment, the autoclaves were cooled in an ice bath to try to quench the reactions as fast as possible. At least three tests with determinations of product yield were done, for each run. The results presented are the average of the values obtained in the three runs. However, a run was only considered if the deviation of the values obtained from the average was lower than 5%. The variation between the experimental conditions used was always less than 7 °C and 4 s. The variation of the temperature during the run was not higher than 6 °C. When the temperature inside the reactor was near room temperature, the autoclave was depressurized and the volume of the gaseous fraction was measured using an analogical gas meter. The gaseous compounds were collected in a sampling bag for direct analysis using a Hewlett-Packard 6890 gas chromatograph. Porapak Q and Molecular Sieves A˚ packed columns were used. The autoclaves were, then, opened, and the liquid and solid products were collected, weighed, and analyzed. The solid fraction was extracted with two solvents, first with dichloromethane and then with tetrahydrofuran (THF). These two solvents were selected, because a large set of hydrocarbons was extracted and the two fractions could be analyzed by gas chromatography. Three fractions were obtained: the extracted light liquid fraction (soluble in dichloromethane), the extracted heavy liquid fraction (soluble in THF), and the solid fraction (insoluble in the two solvents). Each liquid fraction was distilled, and also, three fractions were collected: the light liquid fraction, with a boiling point lower than 150 °C, the heavy liquid fraction that distilled between 150 and 270 °C, and the liquid residue. The solubility of these three distilled fractions in dichloromethane was confirmed. The first two fractions of distillation, the extracted light liquid fraction and the extracted heavy liquid fraction, were analyzed using a Hewlett-Packard 6890 gas chromatograph and a TOF/ GC/MS mass spectrometer from Leco Instruments.
Figure 1. Reaction pathway for the pyrolysis of an equivalent mixture of PS, PP, and PE. (P, plastic mixture; P1, solid lower molecular weight polymer; G, gas; L , light liquid fraction; H, heavy liquid fraction).
in Figure 1. In the development of this pathway, it was assumed that, in the pyrolysis of a mixture of PE, PP, and PS, four fractions were produced: solid lower molecular weight polymer (P1), gas (G), light liquid fraction (L), and heavy liquid fraction (H). The solid fraction was detected at very low reaction times and at lower temperatures, so it was considered to be a polymer of lower molecular weight than that of initial plastics, or other molecules of long chain initially formed. The approach adopted was to group the compounds produced on the basis of their solubility in the solvents used, so the light liquid fraction included the compounds soluble in dichloromethane and the heavy liquid fraction included the compounds soluble in tetrahydrofuran. It was confirmed that the distilled fractions were soluble in dichloromethane. The compounds not soluble in these two solvents were identified as solid fraction and were included in the lower molecular weight polymer (P1). Others reaction pathways13 were tested, but they did not fit the experimental results obtained in this study. The reaction pathway proposed involves a combination of series and parallel reactions. The following assumptions were made in the development of the reaction pathway: (1) all the reactions are first order. (2) all the reactions are irreversible. (3) there are no mass transfer resistances. (4) temperature dependence of the rate constants is described by the Arrhenius equation. These assumptions are usually made by the authors that studied plastic pyrolysis kinetics.13 On the basis of the reaction pathway proposed, a kinetic model was developed. Ten rate constants were estimated, using the results obtained at three different temperatures: 380, 400, and 420 °C. These parameters were calculated by solving the system of differential equations shown below, eqs 1-5, concerning the reactant P and all the products considered. The system of differential equations was solved using the program Micromath Scientist for Windows and the Runge-Kutta method based on the Taylor theorem. The numerical adjustment interactions were undertaken using the Stineman method with the least-squares analysis. The models defined by the authors were introduced in this software. The program required initial values for the different constant rates to start the least-squares minimization. These initial values used were found in the literature. The program used these values to generate the final estimated constant rates adjusted to the experimental results obtained.
Results and Discussion Kinetic Study of Product Yield. The reaction pathway proposed for the pyrolysis of plastic waste mixtures is presented 6241
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Table 1. Rate Constants for the Formation of Liquid Fraction, Gases and Polymer with Lower Molecular Weight temperature (°C) rate constants (s-1) k1 k2 k3 k4 k5 k6 k7 k8 k9 k10
380
400 -2
2.95 10 3.63 10-2 2.10 10-3 8.11 10-2 2.1 10-3 0.0000 4.00 10-5 0.0000 9.90 10-3 0.0000
420 -1
1.02 10 6.04 10-2 3.90 10-3 6.30 10-2 1.03 10-2 0.0000 2.00 10-5 0.0000 2.17 10-2 9.80 10-3
1.90 10-2 9.94 10-2 6.00 10-8 2.51 10-2 1.97 10-2 0.0000 5.00 10-8 4.00 10-3 1.81 10-2 0.0000
Figure 3. Model validation. Comparison between the model and experimental data at 400 °C.
Figure 2. Model validation. Comparison between the model and experimental data at 380 °C.
Figure 4. Model validation. Comparison between the model and experimental data at 420 °C.
Table 1 presents the values of the rate constants determined by the model. The deviation determined for these parameters was lower than 7%.
formed, not only by the initial conversion of polymers but also by the cracking of the chemical bonds of solid compounds initially formed. At this temperature, the gaseous compounds do not appear to have been formed by breaking the bonds of the molecules of heavy liquid. The reaction scheme proposed for the plastic waste mixture pyrolysis was compared with the ones obtained for the individual plastic waste pyrolysis. 15,16 At 400 °C, the reaction scheme proposed is similar to the one obtained for the pyrolysis of PP at 380 and 400 °C.15 Figure 5 shows that, at the highest temperature tested (420 °C), the reaction pathway proposed for the mixture pyrolysis was similar to the scheme proposed for the pyrolysis of PP at the same reaction temperature.16 The experimental results obtained in the pyrolysis of the mixture containing the same percentage of PE, PP, and PS runs were analyzed. Regarding the experimental values obtained for the product yields, some of these values were slightly above 100% (∼103; w/w) due to the experimental deviation in the measurements of the product fractions. The weight percentages presented refer to the ratio between the weight of the product fraction obtained and the initial mass of the plastic waste used. From the analysis of the experimental results, it was observed that the production of light liquid fraction increased from 27% to 71% (w/w), while the heavy liquid decreased from 30% to 10% with the increase of the reaction temperature. The solid fraction yield also decreased from 40% to 16% (w/w) with the rise of temperature. For all temperatures tested, the yield of light liquid fraction and gas
dP=dt ¼ - k1 P - k2 P - k3 P - k4 P
ð1Þ
dP1=dt ¼ k4 P - k8 P1 - k9 P1 - k10 P1
ð2Þ
dH=dt ¼ k1 P - k6 H - k5 H þ k10 P1
ð3Þ
dL=dt ¼ k2 P þ k5 H þ k9 P1 - k7 L
ð4Þ
dG=dt ¼ k3 P þ k6 H þ k7 L þ k8 P1
ð5Þ
where P is plastic mixture, P1 is polymer of lower molecular weight, G is gas, L is light liquid fraction, and H is heavy liquid fraction. Figures 2, 3, and 4 show the adjustments of the values calculated by the kinetic model, to the experimental results. The data predicted by the model are quite close to the experimental values, which can give a possible explanation for the results obtained from the pyrolysis of a mixture containing the same percentage of three types of plastics. The value of the rate constants of some of the steps proposed in the reaction scheme shown in Figure 1 was zero, so it was assumed that if the rate constant was zero or near zero it was not probable that the reaction would have taken place. These steps vary with the temperature used, changing the reaction scheme originally proposed. Thus, Figure 5 presents the modified schemes for the different temperatures used. It can be seen in Figure 5 that at 380 °C the solid and heavy liquids appear to have only been converted into light liquid fraction and the gas seems to have been formed by the conversion of the initial polymers and of the light liquid fraction. At 400 °C, the heavy liquid seems to have been
(15) Costa, P.; Pinto, F.; Ramos, A. M.; Gulyurtlu, I.; Cabrita, I.; Bernardo, M. Chem. Eng. Trans. 2007, 11, 485–490. (16) Costa, P.; Pinto, F.; Ramos, A. M.; Cabrita, I.; Gulyurtlu, I.; Bernardo, M. S. Energy Fuels 2007, 21, 2489–2498.
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Figure 5. Reaction pathway for the pyrolysis of an equivalent mixture of PS, PP, and PE for the different temperatures tested (380, 400, and 420 °C). (P, plastic mixture; P1, solid lower molecular weight polymer; G, Gas; L, light liquid fraction; H, heavy liquid fraction).
increased with the reaction time, while the heavy liquid fraction and solid yield decreased. This decrease was, probably, due to the conversion of the polymer with lower molecular weight initially formed and to the heavy liquid fraction conversion into light liquid fraction, since, with the increase of reaction time, more energy was provided to the system, promoting the breaking of C-C bonds of these molecules with higher molecular weight, resulting in lighter molecules. The reaction time, for which the maximum conversion to light liquid fraction was obtained, decreased with the raise of temperature. At 380 °C, a constant value of about 89% (w/w) for light liquid fraction yield was achieved after 544 s; at 400 °C, the yield of these compounds was 99% (w/w) for a reaction time of 360 s, and at the temperature of 420 °C, the same value (99% (w/w)) was attained just after 190 s. The other products were detected in very low percentages (gas) or even not detected (solid and heavy liquid fraction). These results can, probably, be explained by the fact that, as the reaction temperature increased and more energy was supplied to the system, the reaction time required for the stabilization of the light liquid fraction concentration decreased. The yield of gaseous compounds increased almost linearly with reaction time for all temperatures tested. The products yields obtained by pyrolysis of mixed waste plastics (Figures 2, 3, and 4) showed a similar trend to the ones obtained in the pyrolysis of PP for the same temperatures tested (380, 400, and 420 °C).15 Therefore, as for PP, there was an increase in the light liquid fraction and gaseous compounds yields and a decrease in the yields of solid fraction and heavy liquid fraction with the raise of temperature and reaction time. However, in the pyrolysis of the PP, PE, and PS mixture, using similar experimental conditions, at 380 and 400 °C, higher yields of heavy liquid fraction and
lower of lighter oil than those observed in PP pyrolysis were found. For a temperature of 400 °C and reaction times lower than about 60 s, a heavy liquid fraction yield much higher than the expected value was verified. The expected value for each product yield was calculated as the average of the results for each type of plastic used individually. This higher yield of heavy liquid fraction might be due to a decrease in the thermal degradation rate of PP and PS when processed together with PE, which would lead to a decrease in the yield of lighter compounds by comparison to those obtained for the pyrolysis of PP and PS separately. Since the rate of degradation of PP and PS is much higher than that of PE, it is possible that the degradation of these two polymers has been delayed by the presence of PE. This decrease would be immediately detected in a very short reaction time, as it was experimentally verified. Thus, it was observed that for very low reaction times, the yield of light liquid fraction was lower, by approximately 32%, compared to the expected theoretical value. The solid fraction yield, at 400 °C, was always much lower than expected. This decrease can suggest that the formation of solid compounds in the degradation of PS, in the presence of other types of plastic, might have not been favored. The value of the yield of gaseous compounds, obtained at this temperature, was similar to the one expected. For the highest temperature tested, 420 °C (Figure 4), the yield of all products obtained for the pyrolysis of the mixture and for the pyrolysis of PP were similar.15 The yield of solid and heavy liquid fraction was higher at lower temperatures (380 and 400 °C), which was consistent with the highest rate constants of formation of these compounds from plastic wastes. The rate constants of the reactions of formation of P1, H, and L from the initial polymers 6243
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were the highest at 380 and 400 °C, as shown in Table 1. At the highest temperature tested (420 °C), the yield of heavy liquid fraction formed was much lower, even for smaller reaction times. These low values may indicate that, at this temperature, the direct conversion of the polymers into light liquid fraction was favored over the formation of an intermediate (heavy liquid fraction) that from the break of their bonds would lead to lighter compounds. The experimental results were consistent with the values obtained for the rate constants, since, at 380 °C, k4 was higher than k1 and k2, because a higher yield of solid compounds was obtained. At 400 °C, the value of k2 was lower than k1 and quite close to k4, but at 420 °C, this order was reversed, with the value of k2 being the highest. At 420 °C, the rate constant for the conversion of heavy liquid fraction into a light liquid fraction (k5) was higher than that for the conversion of the solid product into a light liquid fraction (k9), but at lower temperatures became smaller. The formation of a heavy liquid fraction at 420 °C seems to have been due only to the cracking of the C-C bonds of the initial polymers, since the rate constant for the conversion of solid into heavy liquid fraction was zero at this temperature (k10 = 0). The source of the gaseous fraction also varied with temperature. That is, at 380 and 400 °C, the gaseous compounds appeared to have been, mostly, formed from the initial polymers, since the values of k3 were much higher than k7, and k6 and k8 were zero. However, at 420 °C, the value of k8 became higher than k3 and k7, which seems to indicate that, at this temperature, the formation of gaseous compounds from the solid waste has not been favored. As in the pyrolysis of PE and PP,15,16 these results appeared to indicate that the cracking of the C-C bonds of the new polymer formed may have occurred at the end of the polymer chain. The yield of gaseous compounds obtained was very low, even lower than the one obtained in the pyrolysis of each of the three types of plastics individually pyrolyzed. At 380 °C, its value did not exceed 3.2% (w/w), and at 400 and 420 °C, it was less than 6.6% (w/w). The dependence of kinetic constants of the reactions on temperature was represented in order to calculate the activation energy and pre-exponential factor of the various steps shown in the reaction pathway proposed. However, the rate constants did not exhibit a linear dependence with temperature, as shown in Figure 6. This can, probably, be explained by the change of reaction mechanism with the temperature, or by the fact that the multiple reactions that take place during the plastic waste pyrolysis are not first order, as assumed. The activation energy and pre-exponential factor were calculated for the steps in which these two parameters were not temperature dependent (reactions 2 and 5). Estimated values of 94.7 ( 7 and 211.4 ( 17 kJ mol-1 for Ea and of 1.60E5 s-1 and 1.96E14 s-1 for the pre-exponential factor were obtained. The value of Ea obtained for the formation of light liquid fraction by direct conversion of mixed waste plastics (reaction 2) was much lower than the one observed for the conversion reaction of heavy liquid fraction to light liquid fraction (reaction 5). These values are consistent with the experimental results. The values of the kinetic parameters for the reactions in which these values varied with the temperature could be calculated for different temperature ranges, but more experimental results (a wider temperature range) were needed to calculate these parameters with accuracy. It was not possible to test more reaction temperatures, because previous work done by the authors14 showed that below 380 °C the conversion to liquid products was very low,
Figure 6. Arrhenius plot of rate parameters.
Figure 7. Effect of reaction time and temperature on gaseous alkane relative distribution.
and at temperatures higher than 420 °C, no significant variations in product yields and composition were detected. With the results obtained, it was observed that the application of the Arrhenius equation for the calculation of Ea and of pre-exponential factor is not, probably, the best approach, not only because of the possible change in the reaction mechanism with temperature, but also because the order, of all reactions, might not be one, as assumed. Unlike what was observed in this work, Ramdoss et al.13 did not find any dependence of the Arrhenius parameters with the temperature, so the values of Ea and of the pre-exponential factor for different steps in the model proposed were calculated. These authors obtained a value of Ea for the formation of light liquid fraction from the heavy liquid fraction (11.8 kJ mol-1) lower than the one for the direct conversion of mixed plastics into the light liquid fraction (34.7 kJ mol-1). This difference in the results obtained might be due, to the fact that this study relates to the liquefaction (using hydrogen) and not pyrolysis of a mixture of only PE and PP, and also, because different experimental conditions (temperatures, pressures, and times reaction) and different reactor types were used. Ramdoss et al.13 also proposed a kinetic model for the liquefaction of the mixture of PP and PE. The reaction scheme proposed by these authors13 was tested in this study but did not adjust to the experimental results obtained. Kinetic Study of Products Composition. Figures 7 and 8 present the evolution of the distribution of gaseous alkanes and alkenes in the function of residence time and temperature. It was found that at the lowest temperature tested (380 °C) the formation of alkanes increased at reaction times higher than 100s. At 420 °C and at lower reaction times, more gaseous alkanes were produced. Similar results were obtained at 400 and 420 °C, except for the two higher reaction times, in which, at 420 °C, a greater increase in the concentration of alkanes 6244
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Figure 8. Effect of reaction time and temperature on gaseous alkene relative distribution. Figure 10. Effect of reaction time and temperature on liquid alkene production.
Figure 9. Effect of reaction time and temperature on liquid alkane production. Figure 11. Effect of reaction time and temperature on liquid aromatic production.
was noticed. For the three temperatures tested, there was a decrease in the concentration of gaseous alkanes until a reaction time of about 100 s was reached. After this time, the trend was reversed. A complex mixture of alkanes, alkenes, and aromatic compounds was detected in the liquid products (light and heavy liquid fraction). The values presented for these compounds production refers to the identified compounds by GC/MS; the authors suppose that the unidentified compounds can be heavier compounds, that were not able to be analyzed by gas chromatography or higher volatile compounds that were lost during the sample manipulation. The aromatic compounds were always produced in larger percentages, with the highest value obtained being 20.5% (v/v). The maximum value for the concentration of alkanes was 13.7% (v/v) (Figures 10, 11, and 12). The alkenes were produced in lower concentrations, and its value did not exceed 5% (v/v). Alkanes with 5-30 carbon atoms and alkenes with 6-22 carbon atoms were detected. As also observed for PS pyrolysis, the fraction of aromatic compounds contained mainly: styrene, R-methylstyrene, toluene, ethylbenzene, and cumene. There was an increase in the concentration of alkanes with the increase of the reaction time for all temperatures tested, as shown in Figure 9. However, the equilibrium value seems to have been reached at about 200 s, when the two lower temperatures were used. At 420 °C, the concentration of alkanes was always higher than the values obtained at the other temperatures tested and this difference was more significant for higher reaction times. Liquid alkene concentration also increased with the reaction time up to 100 s, and then, this trend was reversed. The values obtained at different temperatures were very close,
Figure 12. Evolution of styrene, R-methylstyrene, toluene, ethylbenzene, and cumene concentrations in the liquid fraction of the pyrolysis of a mixture containing PE, PP, and PS with reaction time at 380 °C.
and there was no clear tendency in the results with the increase in reaction temperature, as shown in Figure 10. Similar trends were obtained for both aromatic compounds and alkanes (Figure 11). Thus, the production of alkanes and aromatic compounds appears to have been favored by the rise of the reaction temperature. It was not possible to determine the Arrhenius parameters for the direct conversion of mixed plastic waste for gaseous alkanes and alkenes and (17) Costa, P.; Pinto, F.; Ramos, A. M.; Cabrita, I.; Gulyurtlu, I.; Bernardo, M. S. Chem. Eng. Trans. 2007, 12, 359–364.
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initiation reaction, forming a secondary radical (RPS; reaction 1).18 The apparent activation energy of this reaction is relatively low, due to the high stability of these radicals. This reaction should not be affected by the initiation reactions of degradation of PE through reaction 2 and PP by pathway 3, since these processes are much slower. Thus, as the thermal degradation of PS starts at relatively lower temperatures than PE and PP, it is assumed that the cracking of the polymer chain of PS occurs first, providing the primary radicals to PE (reaction 4) and PP (reaction 5) degradation. In these reactions, tertiary radicals are formed in the case of PP (reaction 5) and secondary radicals are formed in the case of PE (reaction 4). This result is consistent with the fact that the tertiary radicals of PS are more stable than those of PP, and then, the initiation of the reaction by the removal of hydrogen by the radical polystyrene becomes predominant.19 In the subsequent reactions of the secondary radicals, it is observed that the mechanism of PE decomposition seems to remain unchanged, which may explain the high presence of alkanes. In the case of PP, instead of forming a primary and a secondary radical in the initiation reaction, only a primary radical seems to have been formed by intramolecular transfer of hydrogen, thus forming a tertiary radical. This radical seems to follow the reactions proposed for the degradation of PP.15 Similar results were observed by Bockhorn et al.7 when they studied the influence of the presence of PS in the degradation of PE. These authors have proposed a mechanism similar to that of Ramdoss et al.13 for the pyrolysis of an equal mixture of PE and PS. The reaction mechanisms of PP and PE only seem to have suffered changes in the initiation reaction in the presence of PS, which may explain the high formation of alkanes and alkenes, as found in the pyrolysis of this kind of plastics. Aromatic compounds must have been formed, mostly during the degradation of PS. Wong and Broadbelt19 compared the results obtained in the pyrolysis of a mixture with the same percentages of PP and PS with those obtained in the pyrolysis of each one individually, using a batch reactor at temperatures from 350 to 420 °C, and also found that the aromatic compounds present in the final products were formed only from the degradation of PS. However, in this work, at higher temperatures and reaction times, higher percentages of aromatics and lower percentages of olefins were found than those expected by simple addition of compounds formed by the degradation of each type of plastic used individually. These results had already been observed in the study of the effect of waste composition in the pyrolysis of mixed plastic.14 This may indicate that the aromatic compounds formed during PS degradation and the olefins formed by PE and PP degradation might have reacted with each other and, eventually, led to the formation of alkylbenzenes. These reactions, designated by condensation of an aromatic hydrocarbon with an alkene,20 are simply addition reactions to the double bond, since the break of the benzene ring is not favored, due to its high stability.21 Figure 16 shows two examples of these reactions. No interactions were found between the degradation of PP and PE, which seems to indicate
Figure 13. Evolution of styrene, R-methylstyrene, toluene, ethylbenzene, and cumene concentrations in the liquid fraction of the pyrolysis of a mixture containing PE, PP, and PS with reaction time at 400 °C.
Figure 14. Evolution of styrene, R-methylstyrene, toluene, ethylbenzene, and cumene concentrations in the liquid fraction of the pyrolysis of a mixture containing PE, PP, and PS with reaction time at 420 °C.
liquid alkanes, alkenes, and aromatics, since a change of mechanism with temperature seems to have occurred. The compounds formed in higher percentages were styrene, R-methylstyrene, toluene, ethylbenzene, and cumene, as it was also observed during pyrolysis of PS.17 Figures 12, 13, and 14 show the evolution of the concentration of these compounds with the increase of reaction time, for the three temperatures tested. This pattern was similar to that observed for the pyrolysis of PS,17 since there was also a decrease in styrene and R-methylstyrene and an increase in the formation of toluene, ethylbenzene, and cumene with the rise of reaction time. At the highest temperature tested (420 °C) and at reaction times higher than 360 s, there was a slight decrease in the concentration of these compounds, but it was much lower than the ones observed in the pyrolysis of PS.17 As it happened for the pyrolysis of PS, this decrease may be due to the formation of compounds with higher molecular weight. However, in the pyrolysis of mixed plastic waste, this drop was not accompanied by the formation of solid compounds, which may indicate that the products produced by the reactions between the lighter compounds initially formed do not have a molecular weight as high as those founded in individual pyrolysis of PS. Figure 15 shows a possible reaction mechanism for the degradation of the mixture with equal percentages of the three types of plastic studied. The thermal degradation of PS, PE, and PP occurs through a radical mechanism. The decisive step of thermal degradation of PS seems to be the
(18) Guyot, A. Polym. Degrad. Stab. 1986, 15, 219–235. (19) Wong, H.-W.; Broadbelt, L. J. Ind. Eng. Chem. Res. 2001, 40, 4716–4723. (20) Arnaud P. Cours de Chimie Organique, Librairie l’ etourdi, 1979; Bordas: Paris, Chapter 7, 8, and 11 (21) Morrison, R.; Boyd, R. Organic Chemistry, 4th ed.; Eds.; Allyn and Bacon Inc.: Boston, MA, 1983; Chapters 1-3, 8, 14, and 16.
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Figure 15. Initiation reactions of thermal degradation of a mixture of PS, PP, and PE.
plastic, which may be explained by the influence of the presence of PS in the initiation reactions. Conclusions Kinetic studies for the pyrolysis of a mixture containing equal percentages of PE, PP, and PS were performed, and a reaction pathway was proposed. The experimental results were compared with those obtained by a proposed kinetic model, which showed that the experimental results fit rather well those predicted by the model. The kinetic constants were calculated. The rate constants of some reaction steps considered in the proposed models did not exhibit linear temperature dependence, in the logarithmic form of the Arrhenius law, so the application of the Arrhenius equation for the calculation of Ea and pre-exponential factor, probably, is not the best approach, not only because of the possible change in the reaction mechanism with the temperature, but also because the order of all reactions might not be one, as assumed. It was observed that the effect of reaction time and temperature on product yields obtained by PE, PP, and PS pyrolysis was similar to that observed in PP pyrolysis. The degradation mechanism of PP and PE seems to have suffered some changes only in initiation step, due to the presence of PS. PS seems to have provided primary radicals to PE and PP. The following reactions of the decomposition mechanism of PE and PP appear to be unchanged. No interactions were found between the degradation of PP and PE, which seems to indicate that they have occurred independently.
Figure 16. Condensation reaction of an aromatic hydrocarbon with an alkene.
that they occurred independently. Westerhout et al.2,3 also did not find significant interactions during pyrolysis of PE and PP mixtures with different compositions. Thus, these authors considered that the results obtained with PE and PP mixtures were equivalent to the sum of the results obtained with the plastics pyrolyzed separately. After the analysis of the results, it was found that the compounds formed during the pyrolysis of the mixture were similar to those obtained from the decomposition of each plastic separately. However, the presence of higher percentages of aromatics and lower percentages of olefins was detected, compared with those expected by pyrolysis of each type of 6247
