- Email: [email protected]
Kinetic modeling of polyethylene and polypropylene thermal degradation
Journal of Analytical nnd Applied Pyrolysis
ELSEVIER
40-41
(1997) 305-319
Kinetic modeling of polyethylene and polypropylene thermal degradation
Received
I8 October
19%; aeecpted
12 February
1997
Abstmet
The actual interest towards thermal degradation of plastics lies in the possibility not only of recovering energy but also of producing useful chemicals. This paper presents a mcchanistic kinetic model able to describe the radical chain pyrolysis reactions taking pIace in the liquid phase. The elementary reaction steps are analyzed and their kinetic paramctcrs am proposed startingfrom the well known analogous gas phase reactions. On the basii of a very limited number of indcpcndent kinetic parameters it is then possible to properly describe this degradation process. ‘The simplifying hypotheses needed to describe thii decomposition process in a closed form are carefully discussed. Unfortunately, the enhancing effect of the intermediate formation of unsaturated spscies forces the use of a numcriad approach. As a ctinscquence. the simulalion of the pyrolysis process requires the integration of a large system of ordinary differential equations. A few examples of comparisons hetwzcn mod4 prcdictions and experimental data confirm the validity of the proposed mechanisn for polyethylene and polypropylene degradation. These data refer to very low prcssurc dofompositioo experiments as well as to differential thermal analysis at atmospheric pressure. The effkct of the molecular weight of the polymer is also discussed. 8 i337 Elsevier Scienoz B-V. Xey~orris: Kinetic modeling; Polyethylene; Polypropyknc:
*Corresponding author. Tel: [email protected]&tni.it
+39
2
23993250:
Thermal degradation
fax:
+39
0165'~2~70~97~$17.00 8 1997 Ekvier sciena:B.V. All rightsreserved. PIiSOl65.2370(97)00032-6
2
70638173:
email:
306
E. Rurri
PI ul. i J. Awd. Appi. Pyul~sis 40-41 (1997) 305-319
1. Introdwtian
The amount of plastic wastes is growing year after year and the fraction of plastiL3 in municipal solid wastes (MSW) and in refuse derived fuels (RDF) is progressively Increasing. In Western Europe 6-10% of MSW is composed of plastics (9.3 million tons in 1992) which for the largest part (72%) is disposed of by landfill [I]. Pyrolysis and gasification processes have been recognized as promising routes for the upgrading of solid wastes to more usable and energy dense materials such as gas fuel and/or fuel oil, or to high value feed stocks for the chemical indtistry. The characterization of pyrolysis behavior of plastic wastes is then of interest in the optimization of pyrolysis processes for the recovery of energy rich or valuable product fractions. Nevertheless, a pyrolysis step is always present in the initial stages of gasification and combustion. Technologies for optimal large scale combustion and gasification still need basic research directtid towards thermal efficiency of the system, operational problems and pollutants emission control. Literature reports several papers on pyrolysis and gasification of plastics. The goal of the major part of the work reported so far was to retrieve monomers or other valuable products through thermal processes in various types of reactors. They deal with the characterization of the rate of weight loss during the primary thermal degradation [2-51, and on the primary product characterization [6- 101. However, there is a general lack of fundamental infonnation needed to extrapolate the results towards full scale process description. In the attempt of developing a model for plastic pyrolysis in full scale systems, the first step is to describe the thermal degradation of polymers in terms of an ‘intrinsic’ kinetics, in which heat and mass transfer limitations are not included. Generally lumped-parameter kinetic models are proposed ill literature for plastics and biomasses. These models do not take into account the rigorous and exhaustive description of the chemistry of thermal degradation of polymers and describe the pyrolysis process by means of a simplified reaction pathway. Each single reaction step considered is representative. of a complex network of reactions. As far as poly-olefins concerns, thermal degradation is usually described by a single step degradation [ll-141 in the form of Arrhenius nth order rate equations. A less simplified approach was followed by Darivakis et al. [2], who fitted their experimental data by means of a multiple independent parallel reaction kinetics. Their model assumed that volatilization products were originated from a large number of independent parallel first-order reactions. Calculated best-fit vctues of the activation energy probability density function for polyethylene (PE) pyroiysis resulted very low, indicating the contribution of relatively few degradation pathways to apparent kinetics, or that, if many parallel reactions contribute, they have very similar activation energies. Therefore the single step approach was adequate to describe the apparent kinetics of degradation, at least in a narrow range of heating rates and operating conditions. Various Authors [4,5,12.13] found activation energies ranging from 45 to 73 kcal mol - ‘, and pre-exponential factors between 10” and lOI 5-l.
E. Ram’ er al. i 1. Ad.
Appl. Pplyis
#-4
(1997) 30%;I9
307
These broad variations are essentially due to two reasons: differences in polyethylene or polypropylene (PP) characteristics (molecular weight, presence of weak links, additives) and differences in experimental conditions from which kinetic data are calculated. In particular a single step mode) is not able to cover. with the same kinetic parameters, a wide range of heating rates, temperatures and conversion levels. The possible presence of mass and beat transfer limitations, generally not taken into account in kinetic data abstraction, extends the range of variation of kinetic constants_ As a result of the previous analysis, it emerges that a mechanistic model is needed, which would be able to take into auount differences in starting material and describe the phenomenon in a broad range of reaction severity (i.e., heating rates and temperatures). Above all tbe mechanistic model allows us to predict the detail of the gas product distribution wbicb is the mast significant step in the possibiiity of upgrading solid wastes toward chemica1 reactants. The radical chain pyrolysis reactions taking place in the liquid phase has to be described on the basis of a very limited n.unber of independent kinetic parameters. On the other hand. experiments are needed in order to validate the model. The experimental data have to be produced in vety well controlled and known conditions, in order to try to limit heat and mass transfer effiits.
2. Kinetic m6zchkm
and rate parameters
The thermal degradation of poly-olefins is a typical radical chain mechanism where initiation, H-abstraction, ~-s&ton, and radical recombination reactions are the relevant reaction classes. Rules and criteria adopted in the kinetic m&ling of this liquid phase degradation process are similar to the ones already adopted in the gas-phase pyrolysis of hydrocarbons [15,16]. These radical reactions are CompIetely described on the basis of a limited set of independent kinetic parameters, already discussed elsewhere and evaluated on the basis of analogies and structural contributions. It is important to observe that a lack of direct experimental information for a given reaction does not ncceSSariIy restit in meaningless parameter values because the same parameters are used in al) the different reactions of the same class. In practice, because of the sterical hindrance in the liquid state, molecular rotations of large C-C segments are inhibited and internal isomerization reactions of atkyl radicals can be neglected, at least for the evaluation of the overall degradation process. Exceptions regard the reactions at the gas-liquid interface where the radical rotations krjme more important. During the degradation process, the gas produced increases this interface favoring further intramolecular abstractions. These backbiting reactions can then explain the experimental evidence of a larger amount of C6 and Cl0 species in the final products [Il. A further and deeper analysis of this phenomenon will allow us to better characterize the gas distribution. but that is beyond the goal of this paper. As already mentioned, these aspects should only partially affect the overall liquid phase degradation process.
308
E. Ran:i
rr al. ,‘J. AIIUI. Appl. Pp~~\:~i.s 40-41
(1997) 305. .119
As stated by Benson [IX], ‘experience shows that reactions in condensed phases between non polar or slightly polar molecules do not differ greatly from gas phase reactions’. As a consequence, most of rhe kinetic parameters reported in the following directly comes from the analogous gas phase reactions. H-abstraction and /?-decomposition reactions do not require significant corrections. When a few corrections are needed they will be specifically discussed. This approach has been already tested and validated in the case of visbreaking process [19,20]. As clearly indicated in the previously referred papers, chain initiation reactions do require some corrections. The C-C bond cleavages of the polymer structure can be conveniently classified on the basis of the different type of the formed radicals. Chain initiation to form two primary radicals (Rpm): k,
F
=
- 4 I 300 + F(rr,)
IO’*.”exp [
T
1W’I
(1)
Chain initiation to form one primary (RP-) and one secondary k,,.
s
=
IO"."exp
radical
[R,m):
- 40500 + F( tJc)
T
I
(2)
(s-7
Chain initiation to form one primary (R,*) and one ally1 type radical (R,*); kp
u
=
lo”,s exp
- 36lXKI+ F{n,) T
1
b- ‘1
&I,) is an additive correction contribution to the activation energy for the transposition to the liquid phase and rr, is the number of carbon atoms characterizing, in the free volume theory. the flow unit for the polymer diGsion. The polymer molecules move through the liquid phase by the coordinated migration of segments of the polymer chain. The critical volume [length) for this migration of a polymer chain
is the volume of this jumping unit which is only a small segment of the complete chain. This LengthOr,)can be estimated from the energy (E,) required for
;:E mobility of the polymer segment and it is experimentally measured as a temperature function [2t]. In orciz: to account for the transposition between the gas and the condensed phase it is convenient
to refer to the Trouton-Meissner
rule for the estimation
of the
heat of evaporation: AH_ t 21 T&J
I 3200 ~zf” (kcal kmnl - ‘1
By accounting for an average difference between products and reactants. it is possible to derive:
&AH&,))
3200 “c [j ’’ -t (tt, - j )I” - rr: ‘1 dj 2 lO70& z n, s 0
(kcal kmol- ‘)
On these basis, the proper correclion contribution has been written as:
E. Ranti et d /J. And. Appl. Fyro~sis 40-41 (1997) 305-319
309
This expression, expcrimcntally tuned also in the casz of visbr&.ing reactions [ZO] presents the advantage to be limited for high values of n,. At high temperatures, the asymptotic values of n, 24 for poIyethykne and 36 for polypropylene are evaluated. The reductions in the activation energy of chain initiation reaction (F(:(n,)) then results in about 5.3 kcal mol- ’for PE and 8.0 kcal mol - I for PP. It seems also relevant to oiiscrve that these corrections are in a quite good agreement with the activation energies proposed by Van Krevelen [2l] for the temperature-viscosity correlation of PE and PP. Similar corrections are required by radical recombination reactions whose kinetic parameters, in the condensed phase, become [22]:
q_ is the molar volume of the fiow unit:
&-
q-n, P
[
m3
hOI
1
6)
unit (14 kg kmol-‘) and p is the density of the initial polymer. Finally, W is the corrective factor for the recombination rate constant and takes into account Ihe symmetry, resonance steric and surface effects. In the case of PE and always referring to the flow unit of 24 carbon atoms, ti can be estimated to be 0.016 and then the kinetic constant for chain termination reactions of primary-primary radicals simply kcomes:
nr, is the molecular weight of the -CH,-
k, = lOlo-f-&j)-exp(-
y) [j--&-l
(7)
On the basis of the steady state hypothesis, deepIy discussed in the case of polystyrene by Lehrle et al. 1231, it is possible lo evaluate the overall radical concentration inside the system: qJ = rinil- r,,, = 0 dt where rinil = 2kp. p[ - cHZ
is
- I
the initiation reaction race and ri,, =
is the
WI2
termination
[RI =
reaction rate. As a result, the radical conumtration “2
2. k,, ,a[-CH,-] k,
]
=
r>;;‘3”’
becomes:
@I
Let’s define P, the polymer, R,* the radical and On the alkeoe with R carbon atoms. of the H-abstraction reactions on P,, and
Chain propagation steps, as a result
/Ldecompositions, are the following:
R,* + -(CH, CH2 CH2)- 1: f, + -(CH> CH*CH,)-
R,* *1: 4 + R,,
_
j)*
(91
Rate constants for these reactions can be directly taken from the corresponding ones already determined in the gas phase [24]: k,= IlP
k,=
cxp( - y) [&-I
(10)
IDX.lrrp( $0) [&I
I$,= NYexp(
-F)
(11)
(12)
[&-I
The small difference between the activation energies of kr and k, accounts for the primary nature of Ra in respect of the secondary one of R,* radicals. Assuming the steady state conditions. the mass balance of radical R,= becomes: F
= C,m[R]P, - k;[CH,][RJ
-k,,
-(R,] =0
(13)
and it is possible to evaluate the radical concentration as: (14) The production of the atkene 4 can be simply expressed as:
2 =
k,’ . [RJ
=
k,
kr*[R1Pn= Kf
kp+ kr-p/w,
. [RJP,
(151
where KP is the rate constant of the apparent propagation reaction. H-abstraction reactions on the H atoms in the liquid phase are extremely faster than a-decomposition reactions (k, sk, (pjnr,)). As a consequence, B-decomposition is the rate determining step in the usual conditions (600-800 K) and the formed smaller radicals generate the parent molecule before undergoing a new p-s&ion. On these bases, KP simply becomes:
h&r
” =k,j t k,pIni,
m. s-.-z
p
k& k,
10’24.exp
[-y?] [&I
(16)
Smaller alkane and alkene species are produced through /?-decomposition reactions of the radicals formed by H-abstraction reactions on the alkanes and alkenes. The
alkenes can also come from the dialkenes. Ultimately, a,w-dialkene species (D,) are produced in the same way from heavier alkenes and dialkenes. Conjugate dialkenes are only the result of allyl-type radical decomposition reactions. Schematically, the overall degradation process can be reduced to the following steps:
E. RunA er al./J.Anal. Appl. Pyrolysis 40-41 (1997)305-319
PndOi+PnT.i
0” + 0*5(Di + f* _j) +
O+S(Oj
+
0” -$
(17)
On-j
Dfi+Dj+
311
where j can vary along the carbon chain and stoichiometry is justified by the random s&ion mechanism. This implies that the ratios among alkanes. alkenes and diaikenes should exactly be I ;2:1. Literature data [17,25] indicate a larger amount of alkenes and suggest the importance of the effect of the already discussed inharnolecdar H-transfer reactions (back-biting).
On the basis of the previously referred kinetic parameters, it is then possibk to describe the degradation process by considering the overall system of mass balance equations of the different alkane, alkene and dialkcne species. As already mentioned, the net result of the elementary reaction sequence Eq. (9) is: P,,+R* 5 RH+O,_,+tS_
WI
and the disappearance term of the P,, species (involving all the n carbon atoms in the polymer chain) can be expressed as: rdu = KP-[RI-n -P,
(19)
At the same time P. can be formed by larger alkanes: P,+R-y
RH+Om_a+R,*
m
with the successive H-abstraction reaction of R,* to give P,. Therefore, a first productim term of P,, rises from all the alkanes with at least n + 2 carbon atoms: r pmd.1
=j_$+2
(21)
KP’[Rl’Pj
In a similar way, also alkene species with m 2 n + 4 can form the alkane P,: O,+ R- “,’RH+D,,,_m+
R,m
WI
This reaction competes with the alternative path which leads to a couple of alkenes. On the basis of the random s&ion mechanism, this second contribution to the formation of P, is:
’2
r pmct.~=~j_n+4~P.[Rl~~j
(23)
As a result, the mass balance equation of alkane P,, is: dP,_ - dt
rdia +
rpd.l
+
r,rad.Z
(24)
312
E. Runzi er al. /J
And. Appl. Pyrolysis 40-41 (1997) 305-319
These general formation and disappearance terms, as well as the lower limit of the sums, are modified according to two different reasons. First, terminal methyl and vinyl groups have been neglected, both. for initial decomposition and for the H-abstraction rzactions, due to their lower\ reacti4ty. Moreover, the characteristics of ally1 radicals and ally1 H atoms*‘ilavc 3een singled out. C,,, and Cz, are the enhancing factors respectively for the FLabsl:;;actiw reactions in the ally1 position and for the B-s&ion reactions to form ally1 radicals. They can be estimated, always with reference to the similar gas phase reactions, on the basis of the relative stability of the ally1 type radicals. For instance, the reactivity of ally1 H atoms is aI least three limes higher than that of the alkyl ones. With these hypotheses, the molar balances can be applied to all the alkane, alkene and dialkene species in the liquid phase and the following overall system is obtained:
+ j i+
4KP[RJDj
The initial unimoiecuIar decomposirion reactions, essential steps for the definition of the averall radical concenrration Eq. (8), can be neglected in comparison with the chain propagations, so they do not appear in the system Eq. (25). The following initial conditions [moles] arc assumed: P&=0)=$(1
-p)2.$-’
O,(f = 0) = 0
D,(r = 0) = 0
(26)
where p is the parameter of the SchuItFIory distribution [21]. With the hypothesis of a fix& nverall degradation constant (K* = KP[R]) for alkanes, alkenes and dialkenes, and so not considering separately the enhancing factors C,,, and C$,, the previous system can be simplified and becomes:
E. Ran5 CI 01. /J. Awl.
Appt. Pyrolysis 40-41
(f997j 305-319
313
It is then possible, via Laplace transforms, to obtain the analytical solution of this system 1221;
Unfortunately, the enhancing effect of the formation of intermediate alkenes and dialkenes species cannot be disregarded. Moreover, also the need to analyze temperature variations inside the system forces the use of a numerical approach to integrate the original system. As a consequcna, the simulation of the degradation process requires the numerical integration of the large system nf ordinary difl&ntial Eq. (25). As it will be discussed in the next section, the upper limit of the sums can lo reasonably reduced to a few thousand carbon atoms (N) and the overall dimension of the system (3 N) is easily handled by modem computers. These balance equations refer to all the components in the liquid phase. ClausiusClapeyron and Trouton-Meissner equations allow to d&e, as a function of system pressure [atm] and temperature [K], the lower limit of the carbon number (L) corresponding to species in the liquid phase: L=5_46x
10-T
(
I --
1nP 2 10.5>
Formed species with C atom number lower than L are considered instantaneously evaporating. When temperature increases, the non-continuous transition toward heavier species in the gas phase is responsible of small &continuities of the predicted TG curves, as can be observed in the figures reported in the next section.
Kniunann and Bockhom [i2] report data on thermal degradation of different plastics in a well controlled thermogravimetric (TG) apparatus. Sample weight and heating rates are such that these data can be considered unaffected by heat and mass transfer limitations,
314
E. Ran-i et al. /J.
Anal. AppL Pyrolysis 40-41
(1997) 305-319
Mucha [3] studied five different PE samples of 4 mg, with molecular weight ranging from 28 to 53.5 x 103and a number of CHJ groups per 1000 carbon atoms, from 2 to 35. The different behavior in the weight loss curves of the five samples shows the influence cf the molecular weight. Apparent activation energies of single-step kinetics: decrease with ‘1.The efIect of molcc~lz.r weight Gu the thermal degradation of PE has been also analyzed by Jellinek [26] and Oakes acd Richards 151.The activation energj of PE in the range of molecular weight 9- I30 x IO”was 40-70 kcal mol - ‘, w’$>ut significant effect of the molecular weight. However, Jellinek ~~~~~C.! l/3 decrease of apparent activation energy by doubling molecular weight, in the range 23-44 x 103. Madorsky [4] reported a slight variaiion of activation energy between low molecular weight polyethylene and branched polyethylene. Peculiarity of the data performed by Anderson and Freeman [27] is that they were obtained in vacuum conditions (I mmHg), using a sample of 100 mg and a constant heating rate of YC min- ‘. This allows to validate the model in different operating conditions in respect with the usual ones, The validation of the kinetic model has been carried out through the comparison not only with literature but also with new experimental data. The need of producing new data on poly-olefin thermal degradation is due to the lack of a complete detail of information on the amount of oolymer used and heating rates for evaluating the possible presence of mass and heat transfer limitntions. As a matter of fact, only few papers completely describe all the details of the experiments. Mainly for these reasons it has ken useful to produce new experimental data. PE and PP pyrolysis was studied by TG analysis, measuring weight loss historys of semples subjected to well defined heating conditions in an inert atmosphere of pure nitrogen. A Mettler TA-3000 system was used, which allows heating rates (1 -.lOO”C min - ‘) typical of conventional pyrolysis reactors. Pyrolysis curves can he obtained both in isothermal and constant heating rate conditions. Experimental data here presented were carried out at different constant heating rates. A constant N, flow rate of 100 cm3 min - ’was fed in the apparalus as a purge gas, both to prevent the presence of air in the pyrolysis zone, and to remove gaseous and condensable products evolved during pyrolysis, thus minimizing secondary interactions between volatile and condensate “I’tactions.Then 2- 10 mg of Plastic were put in the sample holder of 6 mm diameter, 4.5 mm height and a thickness of I mm. In order to verify the importance of heat transfer limitations and temperature distribution within the sample, the Biot number (Bi = hL/k) has been evaluated as a function of sample size and composition. The sample is assumed as a composite, formed by the holder and the plastic material). k is the PE thermal conductivity (0.33 WK - ’ m- ‘) and h, heat transfer coefficient, was calculated assuming a Nu equal to 2 (since the nitrogen flux is small) and the nitrogen conductivity of 4.4 x IO-* WE=-’ m-l. The results in 29 WK- ’m* and Biot Number ranges between lO- * and IO- I. It means that the sample can be considered isothermal, i.e., no internal temperature distribution is present during the runs.
6. Rani cl al. /J. Ana/. Appt. Pyrolpi_~ 40-41 (1997) MS-339
315
The characteristic heating time of the system tH, calculated considering only conductive and convective heat transfer external resistance, is less than 20 s. In other words, if the external pyrolysis number PY=is defined as the ratio between the charactcljstic times of thermal degradation (the time to get SO?% conversion) and sample heating I~, vah~~ of 5-20 are obtained. Sample heating does not influence the weight loss curve since pyrolysis reactions are the controlling step. PE and PP are melting before decomposition. At sample size of about 10 mg the decomposition occurs in thin films of liquefied polymer and there is no effect of sample size on the degradation rate, As already mentioned, the kinetic model has been compared with both the experiments here presented and the literature data. The molecular weight of the sam$es is quite difficult to bt dctermincd especially for PE. The predicted effect of the molecular weight on TG curves has been then calculated and the result is shown in Fig. 1. As can be expected, increasing the molecular weight the degradation rate decreases, reaching asymptotic values at about 10000 g mol - I, In all the success& examples, this average molecular weight, corresponding to about 700 -C& units, will be assumed in the model. The average carbon numhr is about 700 and, in order to account for the initial polymer distribution, it is convenient to assume 2MNlas the upper limit of the sums (NJ In this way, the truncation error in the nom~atized distribution is lower than IO- ‘. This leads to a total number of 6000 equations in the overall ditTercntial system. Further experimental work should be done to co&m this assumption or better to verify the predicted trends.
1 0.95 0.9
0 85 0.8 Residue Wdght FlIKliOl? 0.75 0.7 0.65 0.6 0.55
400
410
420
430 TV
440
450
460
I”cl
Fig. I. Calculated poIyerhyknc degradation CUIVCS for different mokcular weights. (Ekatiag rate 1WC min -I, P = I aim). Average Mokculat ueighu are, respccrivdy, IOW, 2U00. H100, IDO and tOWI g mo1- ‘.
316
E. Ran:i tv al. .iJ. Anal.
Appl. Pp/pis
40-41 (1997) 3OS-319
Fraction
300
400 TanpaatlJ.re [“CI
35il
450
Fig. 2. TG curves for polyethylene (healing r&e 5°C min- I, P = 1 mmHg): -
500
calculuted, - ? ?-
experimentaldata 1271. Fig. 2 shows the comparison between predicted results and TG experiments [27] carried out under a vacuum of 1 mm Hg pressure and a constant heating rate of 5°C min-l. The agreement is good both in terms of decomposition beginning (at about 4UO*C)and in the sharp reactivity increase between 420 and 470°C. Two different TG analysis are reported in Fig. 3 and compared with the model computations. These new conditions refer to atmospheric pressure and allow to highlight the effect of heating rate. Data coming from Mucha [3] are carried out at 8°C min- ’ while this work experiments refer to 2WC min- ‘. The model agrees
ResidueWeight Fraction
400
420
440
460 480 Tmlpr&nlrc [“Cl
500
520
Fig. 3. TG curves for polyethylene (P = I atm): calculated, - ? ?- experimental data [3]. Healing rate S°C min- I. - A - bperimenlal data [this work]. Heating mte 20°C min- I.
317
E. Ransi et al. ,‘J. Anal. Appl. Pyro&sir 40-41 {I9971 HIS-319
400
420
440
460 rcmpcranrrr
480
500
520
[“Cl
Fig. 4 TG cuwss for polyethyknc (heating rate 10°C min- 1, P= 1 atm): experimental data 112J. - A - Experimental data (this work).
cak&tcd.
- a -
quite we11 with both the measurements and correctly predicts the shift toward highertemperatures when the heating rate is increased. Reactivity is underpredictLti in comparison with Mucha’s experimental data. Deviations are in the order of IO’C especially in the 48O-490°C temperature range. The agreement is more satisfactory in comparison with our data where the model shows a small overprediction of the overall reactivity. In Fig. 4 the predicted results are compared with new experimental data and also with literature data 1121. Both these data refer to atmospheric pressure and a constant heating rate of 10°C min -‘. The model agrees quite well with both the experiments, being in the middle of the measured TG CWWS. The observed deviations in the experimental data can he partially explained with the possible differences in the analyzed samples, like the weight, the molectdarweight distribution and/or the purity of the polymer. Finally the same approach, with only minor modifications, has been applied to polypropylene degradation. As aheady mentioned, in this case chain initiation reactions producing one primary and one secondary radical Eq. (2) requires a lower activation energy. Also the termination constant k, presents the same expression discussed for PE Eq. (5). The carbon number in the flow unit (I?~)is now 36 and the collision factor (02 is 0.006. Always assuming the same hypotheses proposed for PE degradation, the overall apparent propagation rate constant becomes: KP= IO”.‘enp[ - ‘;m]
[&I
130)
Referring now to the monomeric unit (-C,H,-), it is easy to obtain an overall differcntiaf system of mass balance equations with the same structure of system Eq. (25). Fig. 5 shows the comparison between model and experimental data for two different constant heating t-ales. The agreement is very good also in this case and allows to further extend the proposed approach.
318
E. Pmri
rt al. 11. Anal. Appt. PyolysiJ
40-41
(1997) 31X-319
ResidueWcYc:$t Fraction
Fig. 5. TG curves for polypropylene (P = I atm):calcularcd. - 0 - experimental data (this work). Heating rate 10°C min - I, - A - experimentaldata (this work). Heating rate 20’T min - ‘.
5. Conclusions General rules, already successfully tested in liquid hydrocarbon pyrolysis, are proposed in this paper, for the quantitative characterization of the kinetic elementary steps typical of pctlyolefins thermal degradation. The modei reproduces quite *?ge of operating pressure and heating rate. The well experimen:al data in a wide ro. adopted mechanistic approach allows extrapolation rules and the complete deterrnination of the gas product distribution and it offers particular advantages when applied to the simulation of possible real process, where mass and heat transfer interferes with kinetics in determining the effective reaction rates. M~eover the distribution of the gas phase is the starting point fo. successive pyrolysis processes. Further work should support the development of this model and spec9ic activity has to be addressed mainly in the direction of a complete comparison with detailed experimental data on the distribution of gas phase pyrolyzate.
Acknowledgements
This work has been carried out under the financial support of MURST (40%). Useful discussions with Dr Marinella Levi and Prof. Giuseppe Tieghi are acknowledged.
References [I] R. Zevcnhoven. M. Karlsson. M. Frankenhaeuserand M. Hupa, Laboratory scale characterization of plasticderived fuels. Borealis Polymer Oy, Report 9513, Borga, Finland, 1995.
E. Roth [Z] [3] [4] [S] [6] [7] [S] [9] [IO] [II]
ef al. / 1. Anal. ApgI. Pyralysir
40-41
(1997)
305-319
319
G.S. Darivakis, J.B. Howard, W.A. Peters. Combust. 52. Tech. 74 (19!Xl) 267. M. Mucha. J. Polym. Sci. Polym. Symp. 57 (1976) 25. &L. Madorski, I. Polym. Sci. 9 (1952) 133. W.G. Oakes. R.B. Richards, 1. Cbern. Sot. 619 (1949) 2929. J.H. Hodgkin, M.N. Galbraith. Y.K. Choog. J.Macromol. Sci. Chem. Al7 (I) (1982) 35. N.A. Khahurinskii. J. Therm. Anal. 32 (1987) 1675. S.L. Madorsky. Thermal Degradation of Organic Polymers, Inters&ace. NY, 1965. Y. Sugimura, S. Tsuge, Maccomolecuks 12 (1979) 512. W. Kaminsky. Makromol. Cbem.. Macromol. Symp. 57 (1992) 145. H. Bockhom, A. Hornung, U. Homung. S. Teepe and J_ Weichnunn. Combustion Meeting. Napoli, Italy, July 25-28. 1995.
[I21 R. Kniimann. H. Bockhorn. Cornbust. Sci. Tech. I01 (1994) 285. 1131 V. Conani. L. Petarca, L. Tognotri, Fuel 746 (1995) 903-912. [14] V. Cozani. C. Nicoklla, M. Rovatti, L. Tognot& Ind. Eng. Chcm. Res. 35 (1996) 90. [I 5] M. Dente and E. Rati in ‘Pyrolysis Theory and industrial Pm&x’. Albrigbl, Boyceand Comxan eds.. Academic Press San Diego, Chapter7, 1983. [I61 E. Ran& M. Dente and G. Bopano in G. CodniIlo, S. CrisciteUi and P.G. Lignola (Eds.), Analysis, Simulation, Dynti of Chemical Reactors. CUEN. Napoli, 1995. [17] Y. Tsuchiya. K. Sumi, J. Polyp. Sci. 6 (1968) 415. [lB] S. Benson. Thcm~&cmical Kin&s. 2nd e&t.. Wiley. New York, 1976. [I91 M. Dcntc. G. Boeano and M. Rossi, Prdings First Conference on Chemical and Prouss Engineering. Firenze. ItaIy. lw3, 163-172. [20] M. Dcnte, G. Bozzano and G. Bwsani. Comput. Ckm. Eng, 19%. in press. 1211 D,L. Van Knvelen. Properties of Pofymers. Elsevicr+ Amsterdam, 2nd *jfl.. 1966. 1221G. Cianflone Mottola and P. Nannini. Valutazioni preliminari per un possibik shema cinetico di degradazionc di poliolefint commerciali, Thesis Politccoico di MiIano. 1993. (231 R.S. Lehrk. R.E. Pcakman, J.C. Robb. Eur. Polym. J. 18 (1982) 517. 1241 M. Dente, E. Ranzi, A. Goossens. Comput. Chem. Eng. 3 (1979) 61. [2S] R.P. Lattimer. J. Anal. Appl. Pyrol@ 31 (1995) 203. [26] H.H.G. Jellinek, J. Polym. Sci. 4 (1949) 13. 1271 D.A. Anderson, ES. Freeman, J. Polym. Sci. 54 (l%l) 253.
ELSEVIER
40-41
(1997) 305-319
Kinetic modeling of polyethylene and polypropylene thermal degradation
Received
I8 October
19%; aeecpted
12 February
1997
Abstmet
The actual interest towards thermal degradation of plastics lies in the possibility not only of recovering energy but also of producing useful chemicals. This paper presents a mcchanistic kinetic model able to describe the radical chain pyrolysis reactions taking pIace in the liquid phase. The elementary reaction steps are analyzed and their kinetic paramctcrs am proposed startingfrom the well known analogous gas phase reactions. On the basii of a very limited number of indcpcndent kinetic parameters it is then possible to properly describe this degradation process. ‘The simplifying hypotheses needed to describe thii decomposition process in a closed form are carefully discussed. Unfortunately, the enhancing effect of the intermediate formation of unsaturated spscies forces the use of a numcriad approach. As a ctinscquence. the simulalion of the pyrolysis process requires the integration of a large system of ordinary differential equations. A few examples of comparisons hetwzcn mod4 prcdictions and experimental data confirm the validity of the proposed mechanisn for polyethylene and polypropylene degradation. These data refer to very low prcssurc dofompositioo experiments as well as to differential thermal analysis at atmospheric pressure. The effkct of the molecular weight of the polymer is also discussed. 8 i337 Elsevier Scienoz B-V. Xey~orris: Kinetic modeling; Polyethylene; Polypropyknc:
*Corresponding author. Tel: [email protected]&tni.it
+39
2
23993250:
Thermal degradation
fax:
+39
0165'~2~70~97~$17.00 8 1997 Ekvier sciena:B.V. All rightsreserved. PIiSOl65.2370(97)00032-6
2
70638173:
email:
306
E. Rurri
PI ul. i J. Awd. Appi. Pyul~sis 40-41 (1997) 305-319
1. Introdwtian
The amount of plastic wastes is growing year after year and the fraction of plastiL3 in municipal solid wastes (MSW) and in refuse derived fuels (RDF) is progressively Increasing. In Western Europe 6-10% of MSW is composed of plastics (9.3 million tons in 1992) which for the largest part (72%) is disposed of by landfill [I]. Pyrolysis and gasification processes have been recognized as promising routes for the upgrading of solid wastes to more usable and energy dense materials such as gas fuel and/or fuel oil, or to high value feed stocks for the chemical indtistry. The characterization of pyrolysis behavior of plastic wastes is then of interest in the optimization of pyrolysis processes for the recovery of energy rich or valuable product fractions. Nevertheless, a pyrolysis step is always present in the initial stages of gasification and combustion. Technologies for optimal large scale combustion and gasification still need basic research directtid towards thermal efficiency of the system, operational problems and pollutants emission control. Literature reports several papers on pyrolysis and gasification of plastics. The goal of the major part of the work reported so far was to retrieve monomers or other valuable products through thermal processes in various types of reactors. They deal with the characterization of the rate of weight loss during the primary thermal degradation [2-51, and on the primary product characterization [6- 101. However, there is a general lack of fundamental infonnation needed to extrapolate the results towards full scale process description. In the attempt of developing a model for plastic pyrolysis in full scale systems, the first step is to describe the thermal degradation of polymers in terms of an ‘intrinsic’ kinetics, in which heat and mass transfer limitations are not included. Generally lumped-parameter kinetic models are proposed ill literature for plastics and biomasses. These models do not take into account the rigorous and exhaustive description of the chemistry of thermal degradation of polymers and describe the pyrolysis process by means of a simplified reaction pathway. Each single reaction step considered is representative. of a complex network of reactions. As far as poly-olefins concerns, thermal degradation is usually described by a single step degradation [ll-141 in the form of Arrhenius nth order rate equations. A less simplified approach was followed by Darivakis et al. [2], who fitted their experimental data by means of a multiple independent parallel reaction kinetics. Their model assumed that volatilization products were originated from a large number of independent parallel first-order reactions. Calculated best-fit vctues of the activation energy probability density function for polyethylene (PE) pyroiysis resulted very low, indicating the contribution of relatively few degradation pathways to apparent kinetics, or that, if many parallel reactions contribute, they have very similar activation energies. Therefore the single step approach was adequate to describe the apparent kinetics of degradation, at least in a narrow range of heating rates and operating conditions. Various Authors [4,5,12.13] found activation energies ranging from 45 to 73 kcal mol - ‘, and pre-exponential factors between 10” and lOI 5-l.
E. Ram’ er al. i 1. Ad.
Appl. Pplyis
#-4
(1997) 30%;I9
307
These broad variations are essentially due to two reasons: differences in polyethylene or polypropylene (PP) characteristics (molecular weight, presence of weak links, additives) and differences in experimental conditions from which kinetic data are calculated. In particular a single step mode) is not able to cover. with the same kinetic parameters, a wide range of heating rates, temperatures and conversion levels. The possible presence of mass and beat transfer limitations, generally not taken into account in kinetic data abstraction, extends the range of variation of kinetic constants_ As a result of the previous analysis, it emerges that a mechanistic model is needed, which would be able to take into auount differences in starting material and describe the phenomenon in a broad range of reaction severity (i.e., heating rates and temperatures). Above all tbe mechanistic model allows us to predict the detail of the gas product distribution wbicb is the mast significant step in the possibiiity of upgrading solid wastes toward chemica1 reactants. The radical chain pyrolysis reactions taking place in the liquid phase has to be described on the basis of a very limited n.unber of independent kinetic parameters. On the other hand. experiments are needed in order to validate the model. The experimental data have to be produced in vety well controlled and known conditions, in order to try to limit heat and mass transfer effiits.
2. Kinetic m6zchkm
and rate parameters
The thermal degradation of poly-olefins is a typical radical chain mechanism where initiation, H-abstraction, ~-s&ton, and radical recombination reactions are the relevant reaction classes. Rules and criteria adopted in the kinetic m&ling of this liquid phase degradation process are similar to the ones already adopted in the gas-phase pyrolysis of hydrocarbons [15,16]. These radical reactions are CompIetely described on the basis of a limited set of independent kinetic parameters, already discussed elsewhere and evaluated on the basis of analogies and structural contributions. It is important to observe that a lack of direct experimental information for a given reaction does not ncceSSariIy restit in meaningless parameter values because the same parameters are used in al) the different reactions of the same class. In practice, because of the sterical hindrance in the liquid state, molecular rotations of large C-C segments are inhibited and internal isomerization reactions of atkyl radicals can be neglected, at least for the evaluation of the overall degradation process. Exceptions regard the reactions at the gas-liquid interface where the radical rotations krjme more important. During the degradation process, the gas produced increases this interface favoring further intramolecular abstractions. These backbiting reactions can then explain the experimental evidence of a larger amount of C6 and Cl0 species in the final products [Il. A further and deeper analysis of this phenomenon will allow us to better characterize the gas distribution. but that is beyond the goal of this paper. As already mentioned, these aspects should only partially affect the overall liquid phase degradation process.
308
E. Ran:i
rr al. ,‘J. AIIUI. Appl. Pp~~\:~i.s 40-41
(1997) 305. .119
As stated by Benson [IX], ‘experience shows that reactions in condensed phases between non polar or slightly polar molecules do not differ greatly from gas phase reactions’. As a consequence, most of rhe kinetic parameters reported in the following directly comes from the analogous gas phase reactions. H-abstraction and /?-decomposition reactions do not require significant corrections. When a few corrections are needed they will be specifically discussed. This approach has been already tested and validated in the case of visbreaking process [19,20]. As clearly indicated in the previously referred papers, chain initiation reactions do require some corrections. The C-C bond cleavages of the polymer structure can be conveniently classified on the basis of the different type of the formed radicals. Chain initiation to form two primary radicals (Rpm): k,
F
=
- 4 I 300 + F(rr,)
IO’*.”exp [
T
1W’I
(1)
Chain initiation to form one primary (RP-) and one secondary k,,.
s
=
IO"."exp
radical
[R,m):
- 40500 + F( tJc)
T
I
(2)
(s-7
Chain initiation to form one primary (R,*) and one ally1 type radical (R,*); kp
u
=
lo”,s exp
- 36lXKI+ F{n,) T
1
b- ‘1
&I,) is an additive correction contribution to the activation energy for the transposition to the liquid phase and rr, is the number of carbon atoms characterizing, in the free volume theory. the flow unit for the polymer diGsion. The polymer molecules move through the liquid phase by the coordinated migration of segments of the polymer chain. The critical volume [length) for this migration of a polymer chain
is the volume of this jumping unit which is only a small segment of the complete chain. This LengthOr,)can be estimated from the energy (E,) required for
;:E mobility of the polymer segment and it is experimentally measured as a temperature function [2t]. In orciz: to account for the transposition between the gas and the condensed phase it is convenient
to refer to the Trouton-Meissner
rule for the estimation
of the
heat of evaporation: AH_ t 21 T&J
I 3200 ~zf” (kcal kmnl - ‘1
By accounting for an average difference between products and reactants. it is possible to derive:
&AH&,))
3200 “c [j ’’ -t (tt, - j )I” - rr: ‘1 dj 2 lO70& z n, s 0
(kcal kmol- ‘)
On these basis, the proper correclion contribution has been written as:
E. Ranti et d /J. And. Appl. Fyro~sis 40-41 (1997) 305-319
309
This expression, expcrimcntally tuned also in the casz of visbr&.ing reactions [ZO] presents the advantage to be limited for high values of n,. At high temperatures, the asymptotic values of n, 24 for poIyethykne and 36 for polypropylene are evaluated. The reductions in the activation energy of chain initiation reaction (F(:(n,)) then results in about 5.3 kcal mol- ’for PE and 8.0 kcal mol - I for PP. It seems also relevant to oiiscrve that these corrections are in a quite good agreement with the activation energies proposed by Van Krevelen [2l] for the temperature-viscosity correlation of PE and PP. Similar corrections are required by radical recombination reactions whose kinetic parameters, in the condensed phase, become [22]:
q_ is the molar volume of the fiow unit:
&-
q-n, P
[
m3
hOI
1
6)
unit (14 kg kmol-‘) and p is the density of the initial polymer. Finally, W is the corrective factor for the recombination rate constant and takes into account Ihe symmetry, resonance steric and surface effects. In the case of PE and always referring to the flow unit of 24 carbon atoms, ti can be estimated to be 0.016 and then the kinetic constant for chain termination reactions of primary-primary radicals simply kcomes:
nr, is the molecular weight of the -CH,-
k, = lOlo-f-&j)-exp(-
y) [j--&-l
(7)
On the basis of the steady state hypothesis, deepIy discussed in the case of polystyrene by Lehrle et al. 1231, it is possible lo evaluate the overall radical concentration inside the system: qJ = rinil- r,,, = 0 dt where rinil = 2kp. p[ - cHZ
is
- I
the initiation reaction race and ri,, =
is the
WI2
termination
[RI =
reaction rate. As a result, the radical conumtration “2
2. k,, ,a[-CH,-] k,
]
=
r>;;‘3”’
becomes:
@I
Let’s define P, the polymer, R,* the radical and On the alkeoe with R carbon atoms. of the H-abstraction reactions on P,, and
Chain propagation steps, as a result
/Ldecompositions, are the following:
R,* + -(CH, CH2 CH2)- 1: f, + -(CH> CH*CH,)-
R,* *1: 4 + R,,
_
j)*
(91
Rate constants for these reactions can be directly taken from the corresponding ones already determined in the gas phase [24]: k,= IlP
k,=
cxp( - y) [&-I
(10)
IDX.lrrp( $0) [&I
I$,= NYexp(
-F)
(11)
(12)
[&-I
The small difference between the activation energies of kr and k, accounts for the primary nature of Ra in respect of the secondary one of R,* radicals. Assuming the steady state conditions. the mass balance of radical R,= becomes: F
= C,m[R]P, - k;[CH,][RJ
-k,,
-(R,] =0
(13)
and it is possible to evaluate the radical concentration as: (14) The production of the atkene 4 can be simply expressed as:
2 =
k,’ . [RJ
=
k,
kr*[R1Pn= Kf
kp+ kr-p/w,
. [RJP,
(151
where KP is the rate constant of the apparent propagation reaction. H-abstraction reactions on the H atoms in the liquid phase are extremely faster than a-decomposition reactions (k, sk, (pjnr,)). As a consequence, B-decomposition is the rate determining step in the usual conditions (600-800 K) and the formed smaller radicals generate the parent molecule before undergoing a new p-s&ion. On these bases, KP simply becomes:
h&r
” =k,j t k,pIni,
m. s-.-z
p
k& k,
10’24.exp
[-y?] [&I
(16)
Smaller alkane and alkene species are produced through /?-decomposition reactions of the radicals formed by H-abstraction reactions on the alkanes and alkenes. The
alkenes can also come from the dialkenes. Ultimately, a,w-dialkene species (D,) are produced in the same way from heavier alkenes and dialkenes. Conjugate dialkenes are only the result of allyl-type radical decomposition reactions. Schematically, the overall degradation process can be reduced to the following steps:
E. RunA er al./J.Anal. Appl. Pyrolysis 40-41 (1997)305-319
PndOi+PnT.i
0” + 0*5(Di + f* _j) +
O+S(Oj
+
0” -$
(17)
On-j
Dfi+Dj+
311
where j can vary along the carbon chain and stoichiometry is justified by the random s&ion mechanism. This implies that the ratios among alkanes. alkenes and diaikenes should exactly be I ;2:1. Literature data [17,25] indicate a larger amount of alkenes and suggest the importance of the effect of the already discussed inharnolecdar H-transfer reactions (back-biting).
On the basis of the previously referred kinetic parameters, it is then possibk to describe the degradation process by considering the overall system of mass balance equations of the different alkane, alkene and dialkcne species. As already mentioned, the net result of the elementary reaction sequence Eq. (9) is: P,,+R* 5 RH+O,_,+tS_
WI
and the disappearance term of the P,, species (involving all the n carbon atoms in the polymer chain) can be expressed as: rdu = KP-[RI-n -P,
(19)
At the same time P. can be formed by larger alkanes: P,+R-y
RH+Om_a+R,*
m
with the successive H-abstraction reaction of R,* to give P,. Therefore, a first productim term of P,, rises from all the alkanes with at least n + 2 carbon atoms: r pmd.1
=j_$+2
(21)
KP’[Rl’Pj
In a similar way, also alkene species with m 2 n + 4 can form the alkane P,: O,+ R- “,’RH+D,,,_m+
R,m
WI
This reaction competes with the alternative path which leads to a couple of alkenes. On the basis of the random s&ion mechanism, this second contribution to the formation of P, is:
’2
r pmct.~=~j_n+4~P.[Rl~~j
(23)
As a result, the mass balance equation of alkane P,, is: dP,_ - dt
rdia +
rpd.l
+
r,rad.Z
(24)
312
E. Runzi er al. /J
And. Appl. Pyrolysis 40-41 (1997) 305-319
These general formation and disappearance terms, as well as the lower limit of the sums, are modified according to two different reasons. First, terminal methyl and vinyl groups have been neglected, both. for initial decomposition and for the H-abstraction rzactions, due to their lower\ reacti4ty. Moreover, the characteristics of ally1 radicals and ally1 H atoms*‘ilavc 3een singled out. C,,, and Cz, are the enhancing factors respectively for the FLabsl:;;actiw reactions in the ally1 position and for the B-s&ion reactions to form ally1 radicals. They can be estimated, always with reference to the similar gas phase reactions, on the basis of the relative stability of the ally1 type radicals. For instance, the reactivity of ally1 H atoms is aI least three limes higher than that of the alkyl ones. With these hypotheses, the molar balances can be applied to all the alkane, alkene and dialkene species in the liquid phase and the following overall system is obtained:
+ j i+
4KP[RJDj
The initial unimoiecuIar decomposirion reactions, essential steps for the definition of the averall radical concenrration Eq. (8), can be neglected in comparison with the chain propagations, so they do not appear in the system Eq. (25). The following initial conditions [moles] arc assumed: P&=0)=$(1
-p)2.$-’
O,(f = 0) = 0
D,(r = 0) = 0
(26)
where p is the parameter of the SchuItFIory distribution [21]. With the hypothesis of a fix& nverall degradation constant (K* = KP[R]) for alkanes, alkenes and dialkenes, and so not considering separately the enhancing factors C,,, and C$,, the previous system can be simplified and becomes:
E. Ran5 CI 01. /J. Awl.
Appt. Pyrolysis 40-41
(f997j 305-319
313
It is then possible, via Laplace transforms, to obtain the analytical solution of this system 1221;
Unfortunately, the enhancing effect of the formation of intermediate alkenes and dialkenes species cannot be disregarded. Moreover, also the need to analyze temperature variations inside the system forces the use of a numerical approach to integrate the original system. As a consequcna, the simulation of the degradation process requires the numerical integration of the large system nf ordinary difl&ntial Eq. (25). As it will be discussed in the next section, the upper limit of the sums can lo reasonably reduced to a few thousand carbon atoms (N) and the overall dimension of the system (3 N) is easily handled by modem computers. These balance equations refer to all the components in the liquid phase. ClausiusClapeyron and Trouton-Meissner equations allow to d&e, as a function of system pressure [atm] and temperature [K], the lower limit of the carbon number (L) corresponding to species in the liquid phase: L=5_46x
10-T
(
I --
1nP 2 10.5>
Formed species with C atom number lower than L are considered instantaneously evaporating. When temperature increases, the non-continuous transition toward heavier species in the gas phase is responsible of small &continuities of the predicted TG curves, as can be observed in the figures reported in the next section.
Kniunann and Bockhom [i2] report data on thermal degradation of different plastics in a well controlled thermogravimetric (TG) apparatus. Sample weight and heating rates are such that these data can be considered unaffected by heat and mass transfer limitations,
314
E. Ran-i et al. /J.
Anal. AppL Pyrolysis 40-41
(1997) 305-319
Mucha [3] studied five different PE samples of 4 mg, with molecular weight ranging from 28 to 53.5 x 103and a number of CHJ groups per 1000 carbon atoms, from 2 to 35. The different behavior in the weight loss curves of the five samples shows the influence cf the molecular weight. Apparent activation energies of single-step kinetics: decrease with ‘1.The efIect of molcc~lz.r weight Gu the thermal degradation of PE has been also analyzed by Jellinek [26] and Oakes acd Richards 151.The activation energj of PE in the range of molecular weight 9- I30 x IO”was 40-70 kcal mol - ‘, w’$>ut significant effect of the molecular weight. However, Jellinek ~~~~~C.! l/3 decrease of apparent activation energy by doubling molecular weight, in the range 23-44 x 103. Madorsky [4] reported a slight variaiion of activation energy between low molecular weight polyethylene and branched polyethylene. Peculiarity of the data performed by Anderson and Freeman [27] is that they were obtained in vacuum conditions (I mmHg), using a sample of 100 mg and a constant heating rate of YC min- ‘. This allows to validate the model in different operating conditions in respect with the usual ones, The validation of the kinetic model has been carried out through the comparison not only with literature but also with new experimental data. The need of producing new data on poly-olefin thermal degradation is due to the lack of a complete detail of information on the amount of oolymer used and heating rates for evaluating the possible presence of mass and heat transfer limitntions. As a matter of fact, only few papers completely describe all the details of the experiments. Mainly for these reasons it has ken useful to produce new experimental data. PE and PP pyrolysis was studied by TG analysis, measuring weight loss historys of semples subjected to well defined heating conditions in an inert atmosphere of pure nitrogen. A Mettler TA-3000 system was used, which allows heating rates (1 -.lOO”C min - ‘) typical of conventional pyrolysis reactors. Pyrolysis curves can he obtained both in isothermal and constant heating rate conditions. Experimental data here presented were carried out at different constant heating rates. A constant N, flow rate of 100 cm3 min - ’was fed in the apparalus as a purge gas, both to prevent the presence of air in the pyrolysis zone, and to remove gaseous and condensable products evolved during pyrolysis, thus minimizing secondary interactions between volatile and condensate “I’tactions.Then 2- 10 mg of Plastic were put in the sample holder of 6 mm diameter, 4.5 mm height and a thickness of I mm. In order to verify the importance of heat transfer limitations and temperature distribution within the sample, the Biot number (Bi = hL/k) has been evaluated as a function of sample size and composition. The sample is assumed as a composite, formed by the holder and the plastic material). k is the PE thermal conductivity (0.33 WK - ’ m- ‘) and h, heat transfer coefficient, was calculated assuming a Nu equal to 2 (since the nitrogen flux is small) and the nitrogen conductivity of 4.4 x IO-* WE=-’ m-l. The results in 29 WK- ’m* and Biot Number ranges between lO- * and IO- I. It means that the sample can be considered isothermal, i.e., no internal temperature distribution is present during the runs.
6. Rani cl al. /J. Ana/. Appt. Pyrolpi_~ 40-41 (1997) MS-339
315
The characteristic heating time of the system tH, calculated considering only conductive and convective heat transfer external resistance, is less than 20 s. In other words, if the external pyrolysis number PY=is defined as the ratio between the charactcljstic times of thermal degradation (the time to get SO?% conversion) and sample heating I~, vah~~ of 5-20 are obtained. Sample heating does not influence the weight loss curve since pyrolysis reactions are the controlling step. PE and PP are melting before decomposition. At sample size of about 10 mg the decomposition occurs in thin films of liquefied polymer and there is no effect of sample size on the degradation rate, As already mentioned, the kinetic model has been compared with both the experiments here presented and the literature data. The molecular weight of the sam$es is quite difficult to bt dctermincd especially for PE. The predicted effect of the molecular weight on TG curves has been then calculated and the result is shown in Fig. 1. As can be expected, increasing the molecular weight the degradation rate decreases, reaching asymptotic values at about 10000 g mol - I, In all the success& examples, this average molecular weight, corresponding to about 700 -C& units, will be assumed in the model. The average carbon numhr is about 700 and, in order to account for the initial polymer distribution, it is convenient to assume 2MNlas the upper limit of the sums (NJ In this way, the truncation error in the nom~atized distribution is lower than IO- ‘. This leads to a total number of 6000 equations in the overall ditTercntial system. Further experimental work should be done to co&m this assumption or better to verify the predicted trends.
1 0.95 0.9
0 85 0.8 Residue Wdght FlIKliOl? 0.75 0.7 0.65 0.6 0.55
400
410
420
430 TV
440
450
460
I”cl
Fig. I. Calculated poIyerhyknc degradation CUIVCS for different mokcular weights. (Ekatiag rate 1WC min -I, P = I aim). Average Mokculat ueighu are, respccrivdy, IOW, 2U00. H100, IDO and tOWI g mo1- ‘.
316
E. Ran:i tv al. .iJ. Anal.
Appl. Pp/pis
40-41 (1997) 3OS-319
Fraction
300
400 TanpaatlJ.re [“CI
35il
450
Fig. 2. TG curves for polyethylene (healing r&e 5°C min- I, P = 1 mmHg): -
500
calculuted, - ? ?-
experimentaldata 1271. Fig. 2 shows the comparison between predicted results and TG experiments [27] carried out under a vacuum of 1 mm Hg pressure and a constant heating rate of 5°C min-l. The agreement is good both in terms of decomposition beginning (at about 4UO*C)and in the sharp reactivity increase between 420 and 470°C. Two different TG analysis are reported in Fig. 3 and compared with the model computations. These new conditions refer to atmospheric pressure and allow to highlight the effect of heating rate. Data coming from Mucha [3] are carried out at 8°C min- ’ while this work experiments refer to 2WC min- ‘. The model agrees
ResidueWeight Fraction
400
420
440
460 480 Tmlpr&nlrc [“Cl
500
520
Fig. 3. TG curves for polyethylene (P = I atm): calculated, - ? ?- experimental data [3]. Healing rate S°C min- I. - A - bperimenlal data [this work]. Heating mte 20°C min- I.
317
E. Ransi et al. ,‘J. Anal. Appl. Pyro&sir 40-41 {I9971 HIS-319
400
420
440
460 rcmpcranrrr
480
500
520
[“Cl
Fig. 4 TG cuwss for polyethyknc (heating rate 10°C min- 1, P= 1 atm): experimental data 112J. - A - Experimental data (this work).
cak&tcd.
- a -
quite we11 with both the measurements and correctly predicts the shift toward highertemperatures when the heating rate is increased. Reactivity is underpredictLti in comparison with Mucha’s experimental data. Deviations are in the order of IO’C especially in the 48O-490°C temperature range. The agreement is more satisfactory in comparison with our data where the model shows a small overprediction of the overall reactivity. In Fig. 4 the predicted results are compared with new experimental data and also with literature data 1121. Both these data refer to atmospheric pressure and a constant heating rate of 10°C min -‘. The model agrees quite well with both the experiments, being in the middle of the measured TG CWWS. The observed deviations in the experimental data can he partially explained with the possible differences in the analyzed samples, like the weight, the molectdarweight distribution and/or the purity of the polymer. Finally the same approach, with only minor modifications, has been applied to polypropylene degradation. As aheady mentioned, in this case chain initiation reactions producing one primary and one secondary radical Eq. (2) requires a lower activation energy. Also the termination constant k, presents the same expression discussed for PE Eq. (5). The carbon number in the flow unit (I?~)is now 36 and the collision factor (02 is 0.006. Always assuming the same hypotheses proposed for PE degradation, the overall apparent propagation rate constant becomes: KP= IO”.‘enp[ - ‘;m]
[&I
130)
Referring now to the monomeric unit (-C,H,-), it is easy to obtain an overall differcntiaf system of mass balance equations with the same structure of system Eq. (25). Fig. 5 shows the comparison between model and experimental data for two different constant heating t-ales. The agreement is very good also in this case and allows to further extend the proposed approach.
318
E. Pmri
rt al. 11. Anal. Appt. PyolysiJ
40-41
(1997) 31X-319
ResidueWcYc:$t Fraction
Fig. 5. TG curves for polypropylene (P = I atm):calcularcd. - 0 - experimental data (this work). Heating rate 10°C min - I, - A - experimentaldata (this work). Heating rate 20’T min - ‘.
5. Conclusions General rules, already successfully tested in liquid hydrocarbon pyrolysis, are proposed in this paper, for the quantitative characterization of the kinetic elementary steps typical of pctlyolefins thermal degradation. The modei reproduces quite *?ge of operating pressure and heating rate. The well experimen:al data in a wide ro. adopted mechanistic approach allows extrapolation rules and the complete deterrnination of the gas product distribution and it offers particular advantages when applied to the simulation of possible real process, where mass and heat transfer interferes with kinetics in determining the effective reaction rates. M~eover the distribution of the gas phase is the starting point fo. successive pyrolysis processes. Further work should support the development of this model and spec9ic activity has to be addressed mainly in the direction of a complete comparison with detailed experimental data on the distribution of gas phase pyrolyzate.
Acknowledgements
This work has been carried out under the financial support of MURST (40%). Useful discussions with Dr Marinella Levi and Prof. Giuseppe Tieghi are acknowledged.
References [I] R. Zevcnhoven. M. Karlsson. M. Frankenhaeuserand M. Hupa, Laboratory scale characterization of plasticderived fuels. Borealis Polymer Oy, Report 9513, Borga, Finland, 1995.
E. Roth [Z] [3] [4] [S] [6] [7] [S] [9] [IO] [II]
ef al. / 1. Anal. ApgI. Pyralysir
40-41
(1997)
305-319
319
G.S. Darivakis, J.B. Howard, W.A. Peters. Combust. 52. Tech. 74 (19!Xl) 267. M. Mucha. J. Polym. Sci. Polym. Symp. 57 (1976) 25. &L. Madorski, I. Polym. Sci. 9 (1952) 133. W.G. Oakes. R.B. Richards, 1. Cbern. Sot. 619 (1949) 2929. J.H. Hodgkin, M.N. Galbraith. Y.K. Choog. J.Macromol. Sci. Chem. Al7 (I) (1982) 35. N.A. Khahurinskii. J. Therm. Anal. 32 (1987) 1675. S.L. Madorsky. Thermal Degradation of Organic Polymers, Inters&ace. NY, 1965. Y. Sugimura, S. Tsuge, Maccomolecuks 12 (1979) 512. W. Kaminsky. Makromol. Cbem.. Macromol. Symp. 57 (1992) 145. H. Bockhom, A. Hornung, U. Homung. S. Teepe and J_ Weichnunn. Combustion Meeting. Napoli, Italy, July 25-28. 1995.
[I21 R. Kniimann. H. Bockhorn. Cornbust. Sci. Tech. I01 (1994) 285. 1131 V. Conani. L. Petarca, L. Tognotri, Fuel 746 (1995) 903-912. [14] V. Cozani. C. Nicoklla, M. Rovatti, L. Tognot& Ind. Eng. Chcm. Res. 35 (1996) 90. [I 5] M. Dente and E. Rati in ‘Pyrolysis Theory and industrial Pm&x’. Albrigbl, Boyceand Comxan eds.. Academic Press San Diego, Chapter7, 1983. [I61 E. Ran& M. Dente and G. Bopano in G. CodniIlo, S. CrisciteUi and P.G. Lignola (Eds.), Analysis, Simulation, Dynti of Chemical Reactors. CUEN. Napoli, 1995. [17] Y. Tsuchiya. K. Sumi, J. Polyp. Sci. 6 (1968) 415. [lB] S. Benson. Thcm~&cmical Kin&s. 2nd e&t.. Wiley. New York, 1976. [I91 M. Dcntc. G. Boeano and M. Rossi, Prdings First Conference on Chemical and Prouss Engineering. Firenze. ItaIy. lw3, 163-172. [20] M. Dcnte, G. Bozzano and G. Bwsani. Comput. Ckm. Eng, 19%. in press. 1211 D,L. Van Knvelen. Properties of Pofymers. Elsevicr+ Amsterdam, 2nd *jfl.. 1966. 1221G. Cianflone Mottola and P. Nannini. Valutazioni preliminari per un possibik shema cinetico di degradazionc di poliolefint commerciali, Thesis Politccoico di MiIano. 1993. (231 R.S. Lehrk. R.E. Pcakman, J.C. Robb. Eur. Polym. J. 18 (1982) 517. 1241 M. Dente, E. Ranzi, A. Goossens. Comput. Chem. Eng. 3 (1979) 61. [2S] R.P. Lattimer. J. Anal. Appl. Pyrol@ 31 (1995) 203. [26] H.H.G. Jellinek, J. Polym. Sci. 4 (1949) 13. 1271 D.A. Anderson, ES. Freeman, J. Polym. Sci. 54 (l%l) 253.