Kinetic study of the thermal decomposition of polypropylene, oil shale,
and their mixture
J. Gersten, V. Fainberg*, G. Hetsroni, Y. Shindler
Center for Research in Energy Engineering and Environmental Conservation, Faculty of Mechanical Engineering, Technion, Israel Institute of Technology, Haifa 32000, Israel
Accepted 22 December 1999
Abstract
The thermal decomposition behavior of polypropylene, oil shale and a 1:3 mixture of the two was investigated in a thermogravimetric analyzer (TGA) reaction system in an argon atmosphere, with a view to comparing the process of the mixture with those of the individual components. Experiments were conducted at three heating rates of 5, 10, and 15 K min21, in the temperature range of 30±9008C. The obtained activation energies were 250 kJ kg21for polypropylene, 63 kJ kg21for the oil shale organic matter, and 242 kJ kg21for the mixture. The results indicate that the characteristics of the process depend on the heating rate, and that polypropylene acts as a catalyst in the degradation of oil shale in the mixture.q2000 Elsevier Science Ltd. All rights reserved.
Keywords: Polypropylene; Oil shale; Dynamic thermogravimetry; Pyrolysis
1. Introduction
Pyrolysis of polymeric materials has long been a subject of investigation, and is currently attracting increased inter-est in the context of recovery of valuable products from polymeric waste. Plastics at present make up 7±9% by weight (or 20±30% by volume) of the total waste stream, and their proportion is expected to reach about 10% in 2000 [1]. Of this amount, 89% are polyole®nsÐpolyethylene, polypropylene, polystyrene, polyethylene terephthalate, and PVC. Plastics are not biologically degradable, which creates problems of disposal. One of the possible ways to treat this waste is thermal decomposition, a process, which yields both energy and gaseous and liquid products, which can be used in various ways. The kinetics of thermal degra-dation of individual polymers has been described in the literature, but to date there has been no reference to the degradation of a mixed system of polymers and oil shale.
Oil shale is one of the most promising sources of energy in the world, with large deposits situated in almost all the continents. Its energy capacity is 2.5 times than that of coal and 30 times that of petroleum. In Israel, it is the only organic natural resource for producing energy. The deposits of the northern Negev desert accounting for nearly half of
the total reserve (12 billion tons) and at the present level of liquid fuel consumption (ca 8 million tons/year) will suf®ce for the country's requirements for more than 80 years. Unfortu-nately, this plentiful resource is of poor quality: a low organic matter content and oil yield, and a high content of sulfur.
Combined pyrolysis of a mixture of polymers and oil shale can on the one hand to improve the effectiveness of oil shale processing and on the other side provide a solution to the waste problem.
The polymer selected for this purpose in our work is polypropylene.
2. Background
Thermogravimetric analysis (TGA) has been widely used in studying pyrolytic phenomena. Methods for evaluation of mass-loss curves obtained by this means have been studied and reviewed [2±8].
The mathematical model used to describe the kinetics of a system undergoing chemical change is usually expressed in the form:
dX
dt f Xk T 1
As polymer degradations are often chain reactions, f(X) represents the net result of a series of elementary steps, 0016-2361/00/$ - see front matterq2000 Elsevier Science Ltd. All rights reserved.
PII: S0016-2361(00)00002-8
* Corresponding author. Tel.: 1972-4-835-6702; fax: 1 972-4-832-4533.
each of which has its own activation energy, with the corre-sponding response to temperature change.
The rate constant k changes with absolute temperature according to the Arrhenius equation. This model is used almost universally to express the temperature dependence of the rate constant of the reaction and is de®ned as:
k T Aexp 2RET
2
Among the various methods proposed for estimation of overall kinetic parameters from non-isothermal TG data, Flynn's isoconversional method is the most widely used and was adopted in the present study [9]. This method was applied in this research.
The reaction order can be evaluated by means of the Coats±Red±Fern or Piloyan±Novikova methods, assuming a rate law of the type:
dX
dt Aexp 2 E RT
12Xn 3
The plot of ln dX=dt=exp 2E=RT versus ln 12X is a straight line with slope equal tonand an intercept equal to lnA.
3. Experimental procedure
Commercial grade E-50-E polypropylene MW 400;000 and oil shale (Northern Negev deposit, Israel) were used in this study. The polypropylene samples were cryogenically ground, using liquid nitrogen as coolant. The oil shale samples were also ground. The (3:1) oil shale/ polypropylene mixture was blended by tumbling for 30 min in order to achieve homogeneity. In all the
experi-ments, samples of around 23±24 mg with particle sizes ranging approximately from 50 to 400mm, were placed in the platinum crucible of a thermobalance. Weight-loss experiments were carried out on a Setran 92-16.18 thermo-gravimetric analyzer controlled by a PC AT compatible system. The setup is capable of simultaneous mass evolu-tion, and heat ¯ow measurements as function of temperature and time. An argon atmosphere, maintained at a constant ¯ow rate of 25 cm3min21was used as an inert environment.
Experiments were conducted at three heating rates 5, 10 and 15 K min21, with the sample heated from room temperature to
9008C after a 30 min hold at room temperature to ensure that all traces of oxygen were removed from the apparatus. It was reported earlier that different heating rates cause variations in the Arrhenius parameters [10], and that different purge-gas ¯ow rates can also effect the thermal decomposition [11]. This point was not investigated in this work.
On completion of the experiments, the weight-loss curves were normalized to 100%. The percentage of the residual mass and heat ¯ow as function of temperature and time were used to determine the kinetics of the weight-loss process and the apparent activation energies. Their ®t to the experimen-tal data was checked by the Simplex-Flexible optimization method [12]. The function to be minimized (OF) was: OFX Mexp2Mcal2 4
The validity of each model was tested with the aid of the variation coef®cient (VC), calculated as:
VC %
OF= D2P p
Mexp £100 5
Mexpbeing the average of the experimental weight. n Reaction order of pyrolysis reaction (dimensionless)
P Number of parameters for the corresponding model (dimensionless) R Universal gas constant8.3144 J K21mol21)
T Absolute temperature (K) t Pyrolysis time (min)
Tm The temperature at which the reaction rate is maximum (K)
VC Variation coef®cient (dimensionless)
W Weight of the residue at a given temperatureT(mg) Wf Final weight of the residue (mg)
Wi Initial weight of the material (mg)
The relative error, in percentage of the calculated weight-loss data versus the experimental value was calculated as:
Relative error MexpM2Mcal
exp £100 6
4. Results
The thermogravimetric weight-loss kinetics of the three materials: polypropylene, oil shale and their mixture will be presented in this work. The weight-loss curves obtained for the pure materials form the basis, from which the thermal degradation of the mixture can be evaluated. The 3:1 proportion between the mixture components is equivalent to an organic matter proportion of approximately 1:2.4.
4.1. Degradation of polypropylene
Weight loss, heat ¯ow and derivative information curves of polypropylene degradation are presented in Fig. 1.
Only the results obtained at a heating rate of 5 K min21
are presented here, but the curves obtained at all heating rates were similar, except for a shift atTmand the
tempera-ture at which the decomposition began and ended. Exami-nation of this data clearly indicates that polypropylene degrades in a single-stage process. Two peaks correspond-ing to endothermic reactions are observed in the pattern of the heat ¯ow as function of the temperature. The ®rst peak corresponds to melting of the polypropylene, which begins at 430 K and continues up to 481 K, without accompanying weight loss. The second peak corresponds to the decompo-sition reaction. Mass change may also be evaluated with the aid of a plot of the weight derivative versus temperature.
The derivative information clearly shows the temperatures at which the reaction began and ended, and that at which the reaction rate is maximal. The weight of the sample remained almost stable below 613±643 K (the exact temperature depends on the heating rate). Above this temperature weight loss started, and increased abruptly above 673 K up to 753± 768 K (this interval is also a function of the heating rate). The residual weight around 773 K was about 0.2%. As the heating rate increased, the thermograms shifted to the right, with the most rapid decomposition temperatureTmand the
peak rate increasing. This data is listed in Table 1.
The maximum relative error of the ®tted parametersE,A, andnin the isoconversion method is 2.6% at 720 K. At the end of the reaction, when the residual weight is very small (about 2±3%), the relative error is around 9%.
4.2. Degradation of oil shale
Weight loss, heat ¯ow and derivative curves of oil shale degradation are presented in Fig. 2.
There are three stages of weight loss, best illustrated in the derivative curves. The ®rst stage corresponds to drying of the oil shale, which continues until 455 K with a 10% decrease in total weight (corresponding to the moisture content in the sample). Up to 548±563 K, the weight was almost stable. Above this interval the second stageÐ decomposition of the organic matter began with 11% weight loss, ending at 803±828 K. The heat requirement for this reaction is low, as shown by the low peak in Fig. 2. The data for the pyrolysis temperatures is listed in Table 2. As in the case of polypropylene, the characteristic temperatures of the process depend on the heating rate. The third stage, which begins at 938±958 K (again depending on the heating rate), Fig. 1. Weight loss, heat ¯ow and weight loss derivative curves of
poly-propylene degradation (5 K min21).
Table 1
Different temperature values for the three heating rates (polypropylene decomposition)
Heating rate (K min21) Beginning pyrolysis
temperature (K) Maximum peakrate (% min21) Tdecomposition rate (K))m(temperature of maximum End pyrolysistemperature (K)
5 613 18.9 729 754
10 627 32.9 742 762
15 644 42.9 750 767
the high peak in the heat ¯ow. Decomposition of the carbo-nates in the mineral matter of the shale at these high temperatures is accompanied by considerable heat losses, which necessitate additional heat input for the process and reduces the heating value of the gas. This is the main reason why oil shale is usually retorted at 763±803 K, apart from the fact that a maximal oil yield is obtained under these conditions. The total loss of this stage is 25%. At the end of the reaction the residual weight is 54.5%.
The maximal relative error of the weight loss for the organic decomposition stage, calculated by means of the set of kinetic parameters is 4.9% at 703 K. At the end of this reaction at 778 K, the correlation is poor. The maximal relative error obtained for the mineral decomposition stage is 3.7% at 1030 K.
4.3. Degradation of mixture
Weight loss, heat ¯ow curves, and the derivative are presented in Fig. 3. The phenomena characterizing the degradation of the separate components appear also in the mixture. Removal of the water from the oil shale occurs in the same range of temperatures as melting of the polypro-pylene. For this reason, there is one peak in the derivative and two peaks in the heat ¯ow. Removal of the water is re¯ected in the DTG peak and in the heat ¯ow peak, while melting of the polypropylene is re¯ected in the second heat ¯ow peak. The organic decomposition stage begins at 556±569 K (depending on the heating rate). The different temperatures are listed in Table 3.
In order to study the catalytic effect of the oil shale on the degradation process of the polymer we had to compare the thermograms of the components and the mixture. At the beginning of the reaction, when the small pieces of poly-propylene degrade the oil shale particles are already degraded, therefore the temperature shown by the mixture thermogram is not indicative that oil shale impact the
degra-dation of the polypropylene. In these circumstances, we looked for temperature above 550 K (the onset temperature of oil shale degradation at 5 K min21) at which the weight
loss of the mixture would exceed that of the unmixed oil shale. Such temperature below 613 K (the onset temperature of polypropylene degradation at 5 K min21) if it exists would
indicate that the polypropylene began to decompose in the presence of oil shale earlier than in its unmixed state. Such a situation, with a 3% difference in favor of the mixture, was actually observed at 599 K, indicating a 14 K advance of the decomposition onset for the polypropylene. The same phenomenon was observed at the other heating rates. The onset temperatures at 10 and at 15 K min21 were 614 and
627 K, respectively, i.e. advanced of 13 and 17 K. The maxi-mal relative error of the weight loss for the organic decom-position stage at 5 K min21, calculated with the aid of the
kinetic parameters is 5.7% at 733 K. At the mineral decom-position stage the maximal relative error is 1.9% at 1113 K. Close results were obtained for the other heating rates.
5. Discussion
5.1. Polypropylene degradation kinetics
The degradation rate as function of the temperature for Fig. 3. Weight loss, heat ¯ow and weight loss derivative curves of the mixture oil shale±polypropylene (3:1) degradation (5 K min21).
Table 3
Different temperature values for the three heating rates (oil shale/polypropylene 3:1)
Heating rate (K min21) Beginning pyrolysis
temperature (K) Maximum peakrate (% min21) Tdecomposition rate (K))m(temperature of maximum End pyrolysistemperature (K)
5 556 9.4 732 783
10 564 7.6 745 799
the three heating rates, is presented in Fig. 4. The decom-position rate is 0.8 mg min21at 700 K and reaches a
maxi-mum of around 4±8 mg min21 (depending on the heating
rate) at 730±750 K. The Arrhenius parameters are presented in Fig. 5, which indicates apparent activation energies around 250 kJ mol21. The slope and the intercept of the
straight line in Fig. 6 yield the reaction order of the decom-position reaction as 0.99. Close values were obtained for the other heating rates.
The calculated apparent activation energies reported in literature varied over a wide range. Day and Cooney [13] found energies between 130 and 195 kJ mol21as function of
the fractional weight loss with a ®rst-order reaction rate, while Chart and Balke [14] obtained 324^12 kJ mol21 also with a ®rst-order reaction rate.
5.2. Oil shale degradation kinetics
The degradation rate as function of the temperature for
the three heating rates is presented in Fig. 7. At 5 K min21,
decomposition begins at 550 K and reaches a maximum of 0.2 mg min21 at 700 K, at 15 K min21 a maximum of
0.8 mg min21at 730 K. The decomposition rates of the oil
shale are 10 times lower than those of polypropylene. The Arrhenius parameters for the three heating rates are presented in Fig. 8, whereby the average apparent activation energy of the organic matter decomposition stage is 63 kJ mol21. The reaction order likewise obtained from
the slope and intercept of the straight line in Fig. 9 is 1.03. Comparison with literature data shows that the kinetic parameters are unique to each individual case of oil shale. For example, Skala and Kopsch [15] investigated three deposits from oil shales Knjazevac (Yugoslavia), Northern Korea and Estonia and obtained for the organic stage, assuming a ®rst-order reaction at 5 K min21activation
ener-gies of 55, 133 and 150 kJ mol21, respectively. This
diver-sity is due to differences in behavior of the oil shale during pyrolysis, attribution to structural differences between the corresponding kerogens. Israeli oil-shale kerogen contains many heteroatoms like sulfur and oxygen, which have weaker bonds than carbon±carbon or carbon±hydrogen systems, which is why its activation energy is lower than, for example, the Estonian case.
5.3. Mixture degradation kinetics
The derivative information for the three heating rates is presented in Fig. 10. The decomposition rate of the organic material depends on temperature and the heating rate, increasing with the latter: 2, 4 and 5 mg min21at 5, 10 and
15 K min21, respectively. The maximum decomposition rate 0
2 4 6 8 10
620 670 720 770 Temperature (K)
Rate, mg/min
5 *K/min 10 *K/min 15 *K/min
Fig. 4. Polypropylene decomposition rate versus temperature.
Fig. 5. Ln(k) versus 1=Tfor polypropylene decomposition.
Fig. 6. Variation of ln dX=dt=exp 2E=RTwith ln 12Xfor determina-tion of reacdetermina-tion order and frequency factor for polypropylene degradadetermina-tion.
Fig. 7. Decomposition rate of oil shale±organic matter versus temperature (mg min21).
temperature of the mixture is higher than that of oil shale by 20±30 K and close to the maximum of polypropylene. The decomposition rate of the mixture is close to that of poly-propylene and much higher than that of oil shale.
The behavior of the mixture decomposition rate constant is of importance in understanding its degradation process. In the 560±620 K, interval the rate constant depends only on the heating rate but not on temperature (Fig. 11), the reason being that the decomposition process is related to the ther-mal conductivity.
We saw already that the characteristic temperatures of the pyrolysis depend on the heating rate. If we take the initial and ®nal temperatures (Ti and Tf, respectively) of the
decomposition stages for the studied materials, one can see that a linear relation exists between them (Fig. 12).
The following relation can characterize this dependence:
Ti;f YB1G 7
whereYhas the dimensions of time and refers to the time needed to heat the substance andGcharacterizes its thermal stability, their values, calculated from the initial tempera-ture, are listed in Table 4, which shows that oil shale with moisture has the maximum thermal conductivity. Dry oil shale has lower thermal conductivity, but still high because the organic volatiles transfer heat very well between the
mineral species. After complete decomposition of the kero-gen and removal of the volatiles, the thermal conductivity of the residue (spent shale) is extremely low. A similar tendency was indicated by the values ofY andGobtained from the ®nal temperature.
Polypropylene, which contains neither moisture nor mineral components, also has a very low thermal con-ductivity 20.0012 W m21K21 (STM C177), therefore its
Y value is high (3.1) compared to the other substances. The thermal conductivity of oil shale is around 0.02 W m21K21; that of the mixture during the drying process,
and at the beginning of the organic decomposition is close to the values of oil shale when it is reacted alone, and lower at the beginning of the mineral decomposition. Hence, for accelerating the decomposition of waste polypropylene addition of oil shale is preferable than addition of spent shale. By this means, the thermal conductivity of the mixture will increase and the ef®ciency of the retorting process will be improved.
At the end of the organic decomposition stage, the mixture has a low thermal conductivity similar to that of dolomit, calcium carbonate and ash, and its thermal stability is reduced, it thus begins to decompose at lower temperatures. Oil shale has a larger content of volatiles than polypropylene, thus its thermal stability is much lower. The thermal Fig. 9. Variation of ln dX=dt=exp 2E=RTwith ln 12Xfor determination of reaction order and frequency factor. Left: oil shale±mineral matter degrad-ation, right: oil shale±organic matter degradation.
Fig. 10. Decomposition of the organic matter of a mixture of oil shale±
stability of the mixture is between that of polypropylene and oil shale, but closer to that of the latter. This again leads to the conclusion that oil shale, as an admixture is preferable from the kinetic point of view than spent shale to the waste. By contrast, in the 710±760 K interval the rate constant is independent of the heating rate, the reason being conversion of the decomposition process from diffusion to kinetic regime. The obtained activation energy is 242 kJ mol21,
close to that for polypropylene (250 kJ mol21) and much
higher than that for oil shale (63 kJ mol21). Hence, up to
750 K, the mixture decomposition kinetics is determined by the polypropylene degradation. In the 750±770 K interval, the mixture decomposition rate constant is lower than that of polypropylene but higher than that of oil shale (Fig. 13), so that the former accelerates the decomposition of oil shale in the mixture. The reaction order obtained from the slope and intercept of the straight line in Fig. 14 equals 1.06.
The radicals formed during decomposition of the poly-propylene react with the organic matter of the oil shale in the mixture and accelerate its degradation. The activation energy of this process is equal to that of polypropylene degradation.
In a previous paper by us [16], a new method for utiliza-tion of waste polymers through combined pyrolysis with oil shale was proposed. Quantitative analysis of the degradation products of the oil shale±polypropylene mixture in the pilot reactor con®rmed that the polypropylene acts as a catalyst, initiating the thermal degradation of the organic matter in the oil shale, and increasing the yield of hydrogen and organic gas and oil compounds. The observations presented in this paper are helpful in understanding the kinetic char-acteristics of this process, and of practical signi®cance in the creation of an advanced thermal destruction system. 6. Conclusions
The main conclusion, which can be drawn from the kinetic behavior of the oil shale±polypropylene mixture, is that the polypropylene accelerates the decomposition of the organic matter in the oil shale. The degradation of the studied materials can be considered as a ®rst-order reaction, as was indicated by the results of the isoconversion method. The calculated activation energy of organic matter decom-position of the mixture is 242 kJ mol21, substantially higher
than that for the unmixed oil shale (63 kJ mol21) and close
to that of polypropylene (250 kJ mol21).
Acknowledgements
This research was supported by the Center for Absorption Fig. 12. Initial temperatures of organic decomposition stage as a function of
heating rate.
Table 4
YandGvalues for the investigated materials
Substance Yvalues Gvalues
Stage of decomposition
Drying Organic matter Mineral matter Drying Organic matter Mineral matter
Oil shale 0.6 1.0 1.9 356 544.7 928.5
Polypropylene ± 3.1 ± ± 597.5 ±
Mixture 0.9 1.3 2.8 360.5 549.5 919.0
Fig. 13. Ln(k) versus 1=T for the middle decomposition of the organic matter of a mixture oil shale±polypropylene (3:1).
1986.
[4] Petrovic ZS, Zavargo ZZ. Reliability of methods for determination of kinetic parameters from thermogravimetry, and DSC measurements. Journal of Applied Polymer Science 1986;32:4353±67.
[5] Dickens B, Flynn JH. ACS advances in chemistry series, vol. 203. Washington, DC: American Chemical Society, 1983 (p. 209±32). [6] Ozawa T. Bulletin of the Chemical Society, Japan 1965;38:1881±6. [7] Broido A. Journal of Polymer Science, Part A 1969;2(7):1761±73. [8] Kissinger HE. Reaction kinetics in differential thermal analysis.
Analytical Chemistry 1957;29:1702±6.
[13] Day M, Cooney D, Mackinnon M. Degradation of contaminated plas-tics: a kinetic study. Polymer Degradation and Stability 1995;48:341±9. [14] Chart J, Balke S. Thermal degradation kinetics of polypropylene. Part 3. Thermogravimetric analysis. Polymer Degradation and Stability 1997;57:135±49.
[15] Skala D, Kopsch M, Sokic M, et al. Kinetic and modeling of oil shale pyrolysis. Fuel 1990;69:490±6.
