Thermal Degradation of Polystyrene - T. Farawelli - M. Pinicroli - F. Pisano - G. Bozzano - M. Dente...
-
Upload
victor-castrejon -
Category
Documents
-
view
218 -
download
1
Transcript of Thermal Degradation of Polystyrene - T. Farawelli - M. Pinicroli - F. Pisano - G. Bozzano - M. Dente...
7/27/2019 Thermal Degradation of Polystyrene - T. Farawelli - M. Pinicroli - F. Pisano - G. Bozzano - M. Dente - E. Ranzi
http://slidepdf.com/reader/full/thermal-degradation-of-polystyrene-t-farawelli-m-pinicroli-f-pisano 1/19
Journal of Analytical and Applied Pyrolysis
60 (2001) 103–121
Thermal degradation of polystyreneT. Faravelli *, M. Pinciroli, F. Pisano, G. Bozzano,
M. Dente, E. Ranzi
CIIC -Dipartimento di chimica Industriale e Ingegneria Chimica, Politecnico di Milano,
Piazza L. da Vinci 32 , 20133 Milano, Italy
Received 14 June 2000; accepted 18 September 2000
Abstract
Thermal degradation of plastic wastes offers the possibility of recovering energy and useful
chemicals. Polyethylene and polypropylene pyrolysis have been discussed already in previous
works (E. Ranzi, M. Dente, T. Faravelli, G. Bozzano, S. Fabini, R. Nava, V. Cozzani, L.
Tognotti, J. Anal. Appl. Pyrol., 40–41 (1997) 305–319 and T. Faravelli, G. Bozzano, C.
Scassa, M. Perego, S. Fabini, E. Ranzi, M. Dente, J. Anal. Appl. Pyrol., 52 (1999) 87–103).
This paper aims to develop a detailed kinetic model of polystyrene thermal degradation. The
predictions of overall rates of degradation and volatile product distribution are compared
with experimental results obtained by different authors at different pressure and temperature
conditions. In order to reduce the computing times required by the numerical integration of
the kinetic model, a flexible lumping procedure has also been introduced. © 2001 Elsevier
Science B.V. All rights reserved.
Keywords: Lumping procedure; Pyrolysis; Gasification
www.elsevier.com/locate/ jaap
1. Introduction
The fraction of plastics in municipal solid wastes (MSW) and in refuse derived
fuels (RDF) is increasing continuously. In Western Europe, 6– 10% of MSW is
composed of plastics (9.3 million tons in 1992). The largest part (72%) is disposed
of by landfill, whereas the remaining part is incinerated or recycled in different ways
[3].
* Corresponding author. Tel.: +39-2-23993282; fax: +39-2-70638173.
E -mail address: [email protected] (T. Faravelli).
0165-2370/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
P I I : S 0 1 6 5 - 2 3 7 0 ( 0 0 ) 0 0 1 5 9 - 5
7/27/2019 Thermal Degradation of Polystyrene - T. Farawelli - M. Pinicroli - F. Pisano - G. Bozzano - M. Dente - E. Ranzi
http://slidepdf.com/reader/full/thermal-degradation-of-polystyrene-t-farawelli-m-pinicroli-f-pisano 2/19
104 T . Fara6elli et al . / J . Anal . Appl . Pyrolysis 60 (2001) 103 – 121
Pyrolysis and gasification are now 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 industry. The
characterization of pyrolysis behavior of plastic wastes is then significant in the
optimization of pyrolysis processes for the recovery of valuable product fractions.
Moreover, a pyrolysis step is always present in the initial stages of gasi fication and
combustion processes.
Literature reports several papers on pyrolysis and gasification of plastics. The
goal of the major part of the works reported so far was to retrieve monomers orother 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 [4 – 7] as well as on the primary product characterization
[8 – 11].
In the attempt of developing a model of 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,
kinetic models with apparent kinetic parameters are proposed in literature for
plastics and biomasses. These models do not take into account the complete and
more rigorous description of the chemistry of polymer thermal degradation and
describe the pyrolysis process by means of a simplified reaction pathway. Eachsingle reaction step is representative of a complex network of reactions. This
approach proved adequate to describe the apparent kinetics, only in a narrow range
of heating rates and operating conditions. In particular, a single step model 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 heat transfer
limitations, generally not taken into account in the identification of kinetic data,
spreads the range of variation of these kinetic constants. The broad variations
between the activation energies and pre-exponential factors found by various
authors [4 – 6] are essentially due to two reasons — differences in properties and
characteristics (molecular weight, presence of weak links, additives) of polystyrene
(PS), and differences in experimental conditions from which kinetic data are
calculated; for example, anionic PS is thermally more stable than thermal PS,
because of the greater number of weak links in the latter.
As a result of the previous considerations, there comes out the interest in a
mechanistic model able to account for the differences in starting material and also
to describe the phenomenon in a wide range of reaction conditions (i.e. heating
rates and temperatures). Furthermore, the mechanistic model would allow to
predict the detail of gas product distribution and this is the most significant step in
the possibility of an upgrading of solid wastes toward chemical reactants.
A mechanistic model for the polyethylene and polypropylene (PE and PP)
degradation process was developed [1,2]. This work was prepared on very similar
basis. In order to describe properly the phenomenon, particular attention has been
paid to the reaction steps and to the physical aspects of the degradation processsince both play an important role in the final product distribution. As seen by many
authors [4 – 15], the propagation step is the result of a competition between three
7/27/2019 Thermal Degradation of Polystyrene - T. Farawelli - M. Pinicroli - F. Pisano - G. Bozzano - M. Dente - E. Ranzi
http://slidepdf.com/reader/full/thermal-degradation-of-polystyrene-t-farawelli-m-pinicroli-f-pisano 3/19
105T . Fara6elli et al . / J . Anal . Appl . Pyrolysis 60 (2001) 103 – 121
different reaction mechanisms — unzipping, intramolecular and intermolecular H
transfer. As a consequence, these three pathways are introduced in the reaction
scheme through the definition of two coef ficients i and k, which indicate the fractions
of the radicals involved, respectively, in intermolecular and intramolecular abstrac-
tion. The remaining radicals give unzipping reactions with a high production of the
monomer. It is worth observing that this unzipping reaction constitutes a very relevant
propagation mechanism in the usual conditions but it was not accounted, due to its
lower importance, in PE and PP thermal degradation.
The radical chain pyrolysis reactions here considered take place only in the liquidphase and are described on the basis of a very limited number of independent kinetic
parameters.
2. Kinetic mechanism
The thermal degradation of most of the polymers is a typical radical chain
mechanism, where initiation, propagation and termination reactions are the relevant
reaction classes. These radical reactions are described completely by a limited set of
independent kinetic parameters, evaluated on the basis of structural contributions as
well as similarity and analogy rules.
2 .1. Initiation reactions
Initiation reactions determine a C C bond cleavage of polymer chains to form
radicals; the following two types of different initiation reactions can be identified.
a1. Random scission — to form one primary radical (Rp) and one secondary
benzyl radical (Rsb) with a strong benzylic resonance.
a2. Chain-end scission — to form again one secondary benzyl (Rsb) and the
resonantly stabilized allyl benzene radical (Ra).
This second type of initiation reactions has an increasing importance during
degradation process, because of both the growth chain end positions with decreas-ing of the molecular weight and the formation of several unsaturated species, due
to the propagating b-scission reactions (see b3).
7/27/2019 Thermal Degradation of Polystyrene - T. Farawelli - M. Pinicroli - F. Pisano - G. Bozzano - M. Dente - E. Ranzi
http://slidepdf.com/reader/full/thermal-degradation-of-polystyrene-t-farawelli-m-pinicroli-f-pisano 4/19
106 T . Fara6elli et al . / J . Anal . Appl . Pyrolysis 60 (2001) 103 – 121
We do not account for the presence of weak links, which can give surely an
important contribution especially for radical polymerized polystyrene. Depending
on the nature of the polymer, proper adaptive and corrective factors can force these
initiation steps, but this aspect is beyond the main scope of this work.
2 .2 . Propagation reactions
Propagation step consists of the sequence of H-abstraction and b-decomposition
or unzipping reactions. There are following two types of H-abstraction reactions.
b1. Intermolecular abstractions — the radicals abstract the hydrogen from adifferent molecule:
Due to the higher stability of the long lived resonantly stabilized benzylic radicals
formed, it is only considered the intermolecular abstraction on the tertiary
carbons atoms with the formation of Rt.
b2. Intramolecular abstractions — the radicals Rsb and Rp can easily form five-,
six- or seven-membered ring intermediates, with the final result of a 1 – 4, 1 – 5 or1 – 6 isomerization reaction:
These reactions are also called back biting reactions. The six- or seven-membered
ring reaction is favored by the energetic point of view (lower strain energy), while
the 1 – 4 isomerization is favored from the entropic point of view, being lower the
number of degrees of freedom (rotors) to be blocked. As a result of the stericalhindrance in the liquid phase, back biting 1 – 5 reaction is the favored one and the
only one considered here.
b3. The tertiary benzylic radical Rt undergoes a scission of the C C bond in b
position to form a secondary benzylic radical and a polymer species with an
unsaturated end:
As already mentioned, unzipping reactions are b-decomposition reactions of Rsb
radicals with the formation of a monomer and another Rsb radical with amonomeric unit less. These reactions can be considered as the reverse of
poly-addition reactions:
7/27/2019 Thermal Degradation of Polystyrene - T. Farawelli - M. Pinicroli - F. Pisano - G. Bozzano - M. Dente - E. Ranzi
http://slidepdf.com/reader/full/thermal-degradation-of-polystyrene-t-farawelli-m-pinicroli-f-pisano 5/19
107T . Fara6elli et al . / J . Anal . Appl . Pyrolysis 60 (2001) 103 – 121
2 .3 . Termination reactions
Two different second-order termination reactions are considered.c1. Recombination reactions:
c2. Disproportionation reactions of radicals, like:
The main difference between these reactions is the formation of species with an
unsaturated end in the disproportionation reaction.
3. Kinetic parameters
As discussed already in the case of polyethylene and polypropylene pyrolysis, rate
constants determined in gas-phase pyrolysis of hydrocarbons constitute the starting
point in the evaluation of kinetic parameters valid for liquid-phase degradation
process [1]. Significant corrections need to be applied to gas-phase kinetic parame-ters in order to account for the condensed state, because of the inhibition of
molecular rotations of large C C segments [16]. Typically, reactions with low heat
of reactions have a marginal correction when transported from the gas to the liquid
phase, for this reason the propagation reactions are assumed with the same kinetic
parameters in both the phases. On the contrary, chain initiation reactions require
significant corrections. This approach has been already tested and validated in the
case of visbreaking process [17,18], as well as in PE and PP pyrolysis [1,2].
As already described, we consider two types of initiation reactions:
a1. random scission {PSRsb+Rp}
k sr=5×1013
exp−63700
RT
7/27/2019 Thermal Degradation of Polystyrene - T. Farawelli - M. Pinicroli - F. Pisano - G. Bozzano - M. Dente - E. Ranzi
http://slidepdf.com/reader/full/thermal-degradation-of-polystyrene-t-farawelli-m-pinicroli-f-pisano 6/19
108 T . Fara6elli et al . / J . Anal . Appl . Pyrolysis 60 (2001) 103 – 121
a2. allyl scission {PSRsb+Ra}
k sa=5×1012 exp−58700
RT
These activation energies are taken directly for the equivalent C C bond cleavage
in the gas phase [18] and contain already a correction of about 3800 cal mol−1 for
the transposition to the liquid phase [1]. Further corrections are also required in the
case of radical recombination reactions. In fact, the kinetic parameters of thetermination reactions in the condensed phase become,
k t=1012.8T
400V s exp
−E v
RT
b2
V s is the molar volume of the flux unit
V S=PMS
z=
num0
z
where m0 is the molecular mass of monomer and z is the liquid density, which can
be considered constant reasonably during the process and equal to 900 kg m−3,
that is the estimated value (400°C) starting from the polymer density of 1050 kgm−3 (25°C) [19]. nu is the monomeric units of polystyrene characterizing the flux
unit for the molecular momentum transfer and a value of nu=7 is assumed, on the
basis of Eyring’s free volume theory [19].
E v is the energy required for the mobility of the molecular flux unit, and b2, the
corrective factor that takes into account the symmetry, resonance, steric and surface
effects [16]. In the case of polystyrene, the kinetic constant for chain termination
reactions simply becomes,
k t=5×106T exp−14 000
RT
On the basis of the kinetic constant calculated for the initiation and termination
reactions, it is then possible to evaluate the global concentration of radicals. We
assume that all the different radicals (primary, secondary benzylic and tertiary, with
different chain length) are equivalent to a unique lumped radical R. Initiation and
termination reactions can be written as follows,
initiation PSk s
2R
termination 2R
k tPS
Assuming the steady state hypothesis, the concentration of this pseudo radical is
evaluated from its mass balance,
d[R]
dt=2k
s[PS]−2k
t[R]2=0
7/27/2019 Thermal Degradation of Polystyrene - T. Farawelli - M. Pinicroli - F. Pisano - G. Bozzano - M. Dente - E. Ranzi
http://slidepdf.com/reader/full/thermal-degradation-of-polystyrene-t-farawelli-m-pinicroli-f-pisano 7/19
109T . Fara6elli et al . / J . Anal . Appl . Pyrolysis 60 (2001) 103 – 121
[R]=' k s[PS]
k t
Taking into account both the random and the allyl initiation steps, the previous
expression becomes,
[R]=' k sr[PSsr]+k sa[PSsa]
k t
where [PSsr] and [PSsa] are, respectively, the concentration of the C C bonds, which
could undergo random scission and allyl scission.The concentration of allyl and primary radicals is negligible, because they can be
obtained only by initiation steps, whereas secondary benzyl radicals, which can also
be formed by b-scission reactions, are the predominant ones.
Radical chain mechanism is the result of propagation reactions of R t and Rsb
radicals.
As far as the tertiary benzyl radicals are concerned, there are two possible paths,
b-decomposition reactions{RtPS+Rsb};
k i=1013 exp−27 000
RT
H abstraction reactions {Rt+PSPS+R%t};
k er=5×107 exp−16 500
RT
As far as the secondary benzyl radicals are concerned, there is the competition
among three different reaction classes,
H abstraction reactions of a tertiary benzyl hydrogen {Rsb+PSPS+Rt};
k ef =5×107 exp−13 500
RT
unzipping reactions {RsbStyrene+Rsb};
k u=1013 exp−26 000
RT
back biting reactions {RsbRt}.
k bb(1,5)=109 exp−16 000
RT
As mentioned already, the kinetic parameters above reported are taken directly
from the analogous well defined gas phase reactions (inter and intra molecular
H-abstractions, b-scissions).
7/27/2019 Thermal Degradation of Polystyrene - T. Farawelli - M. Pinicroli - F. Pisano - G. Bozzano - M. Dente - E. Ranzi
http://slidepdf.com/reader/full/thermal-degradation-of-polystyrene-t-farawelli-m-pinicroli-f-pisano 8/19
110 T . Fara6elli et al . / J . Anal . Appl . Pyrolysis 60 (2001) 103 – 121
On this basis, a unique kinetic expression for the propagation step involving
tertiary radicals can be derived,
H-abstraction PSn+R
k ef R
tn+PS;
b-decomposition Rtn
k iR
s(n−k )+PSk ;
re-abstraction PS+Rtn
k erR
t+PSn.
where Rtn is the tertiary radical of length n, whilst R
t is a generic tertiary radical,
with the same chain length as PS.The production of polymer species of length k (PSk ) can be expressed as,
d[PSk ]
dt=k i[R
tn] where n]k +1
The steady-state assumption for the radicals Rtn becomes,
d[Rtn]
dt=k ef [PSn][R]−k i[R
tn]−k er[Rtn][PS]=0
[Rtn]=
k ef
k i+k er[PS][R][PSn]
Thus,
d[PSk ]
dt=
k ef k i
k i+k er[PS][R][PSn]=k p[PSn]
where
k p=k ef k i
k i+k er[PS][R]
is the equivalent rate constant of the apparent propagation reactions involving the
tertiary radicals.
Two parameters i and k are useful to define the fractions of secondary benzyl
radicals which, respectively, follow H-abstraction and back biting reactions,
i=k ef [PS]
k u+k ef [PS]+k b
k=k b
k u+k ef [PS]+k b
The k b rate constant has been obtained in analogy with the previous k p but
considering the back-biting as abstraction mechanism,
k b=k bb(15)
k i
k i+k er[PS]
The remaining fraction (1-i-k) of Rsb undergoes unzipping reactions [20]. In theusual conditions, with the proposed rate constant, i ranges between 0.1 and 0.2 and
k is about 0.1.
7/27/2019 Thermal Degradation of Polystyrene - T. Farawelli - M. Pinicroli - F. Pisano - G. Bozzano - M. Dente - E. Ranzi
http://slidepdf.com/reader/full/thermal-degradation-of-polystyrene-t-farawelli-m-pinicroli-f-pisano 9/19
111T . Fara6elli et al . / J . Anal . Appl . Pyrolysis 60 (2001) 103 – 121
4. Analytical and numerical solution of mass balance equations
In the analysis of the kinetic mechanism of polystyrene thermal degradation,
there is a progressive formation of unsaturations in the end positions of the
molecules. Each molecule in the system is identified by the number of phenyl
groups contained and on the basis of the different end unsaturations. Thus, it is
possible to have in the system components with alkane backbone (P), without
double bonds, alkene backbone (O) and a – v dialkene backbone (D), respectively,
with one and both ends unsaturated.
However, these simple assumptions would not allow to distinguish molecules
with similar structures. For example, O1 or the alkene backbone with only one
phenyl group would include both styrene and h-metyl-styrene. It is then convenient
to consider three types of chain end for each one of the previously considered
species. This classification is shown schematically in Table 1.
The total nine families of different species are reduced to five, with the hypothesis
that the initial polymer is constituted only by type I alkane backbone (P I). On the
basis of the proposed mechanism the system is only composed by the following
species — PI and PIII alkane backbone, OI and OII alkene backbone, and DII
dialkene backbone. As briefly sketched in Fig. 1, both the alkene backbone families
come from b-scission of tertiary radicals. DII is formed by the b-scission of both OI
and OII. Finally, the H abstraction of secondary radicals produce either PI or PIII
according to their structure. All the main degradation products observed experi-
mentally are easily taken into account by these five families.
Table 1
Families of species formed during polystyrene degradation
7/27/2019 Thermal Degradation of Polystyrene - T. Farawelli - M. Pinicroli - F. Pisano - G. Bozzano - M. Dente - E. Ranzi
http://slidepdf.com/reader/full/thermal-degradation-of-polystyrene-t-farawelli-m-pinicroli-f-pisano 10/19
112 T . Fara6elli et al . / J . Anal . Appl . Pyrolysis 60 (2001) 103 – 121
Fig. 1. Sample of formation of the different backbone families, starting from the assumed original
polymer structure.
Thermal degradation is a cracking process taking place in a liquid phase and
reaction products go away as volatiles. Cracking reactions in gas phase areneglected and it is necessary to distinguish the molecules in the liquid phase from
the gaseous ones.
Clausius – Clapeyron and Trouton – Meissner equations [21] allow to define, as a
first approximation, the lower limit of the number of monomeric units (L)
corresponding to species in the liquid phase as a function of system pressure (atm)
and temperature (K),
L=1
8
T 2
(136)2
1−ln P
10, 5
2The good agreement found by this relationship in comparison with the experi-
mental data is shown in Fig. 2, where the estimated boiling temperatures of hydrocarbons with different numbers of carbon atoms are compared with the
experimental values. It has to be noted that at very high molecular weight, this
7/27/2019 Thermal Degradation of Polystyrene - T. Farawelli - M. Pinicroli - F. Pisano - G. Bozzano - M. Dente - E. Ranzi
http://slidepdf.com/reader/full/thermal-degradation-of-polystyrene-t-farawelli-m-pinicroli-f-pisano 11/19
113T . Fara6elli et al . / J . Anal . Appl . Pyrolysis 60 (2001) 103 – 121
approach does not predict correctly the boiling temperature, but these temperature
conditions are quite far from those of interest. Formed species with a number of
monomeric units lower than L are considered directly pertaining to the gas phase.
On the basis of the previous assumptions and hypotheses, it is then possible to
obtain the following mass balance equations for the five families in the system,
dPIn
dt=− k p(n−1, 5) PIn+iRPIn
dPIIIn
dt=−k p(n−2)PIIIn+iRPIIIn
dOIn
dt=−k p(n−2, 5)OIn+
1
2k p %
n+1
PI j +k p %
n+1
PIII j +1
2k p %
n+2
OI j +iROIn
dOIIn
dt=−k p(n−2)OIIn+
1
2k p %
n+1
PI j +1
2k p %
n+2
OII j
dDIIn
dt=−k p(n−3)DIIn+
1
2k p %
n+1
OI j +1
2k p %
n+1
OII j +k p %
n+2
DII j
where
Fig. 2. Comparison between predicted (line) and experimental (dots) boiling temperatures of aliphatic
hydrocarbons. Boiling temperatures of styrene and dimer are also reported.
7/27/2019 Thermal Degradation of Polystyrene - T. Farawelli - M. Pinicroli - F. Pisano - G. Bozzano - M. Dente - E. Ranzi
http://slidepdf.com/reader/full/thermal-degradation-of-polystyrene-t-farawelli-m-pinicroli-f-pisano 12/19
114 T . Fara6elli et al . / J . Anal . Appl . Pyrolysis 60 (2001) 103 – 121
RPIn=1
2k p %
n+2
PI j +1
2k p %
n+2
OII j +(1-i-k)RPIn+1+kRPIn+3
RPIIIn=1
2k p %
n+1
PI j +k p %
n+2
PIII j +1
2k p %
n+2
OI j +(1-i-k)RPIIIn+1+kRPIn+3
ROIn=1
2k p %
n+2
OI j +1
2k p %
n+1
OII j +k p %
n+2
DII j +(1-i-k)ROIn+1+kROIn+3
are the mass balance equations for secondary benzyl radicals of different types
present in the system.n is the chain length and terms like (n−2) means that not all the positions in the
molecule are equivalent and can be involved in the reaction, may be due to a
different or lower reactivity (see for instance the end groups).
The contributions of initial decomposition and termination reactions are negligi-
ble in the overall balance when compared with the chain propagation ones and,
therefore, they have been neglected.
The balance equations of the species in the liquid phase are characterized by a
first term of disappearance, whereas the ones of gaseous species contain only
formation terms.
Mass balance equations of styrene and of the trimer show the contributions of
unzipping and back biting reactions,
dOI1
dt=(1-i-k)
%2
(RPI j +RPIII j )+%
3
ROI j
ndOI3
dt=dOI3
dt
0
+k %
L+1
(RPI j +RPIII j +ROI j )
Initial conditions are needed to integrate the system of ordinary differential
equations. It has been assumed that only PI species are present initially. Initial
molecular weight distribution curve is assumed on the basis of Schultz ‘most
probable distribution’ [22],
xi = 1 n1− 1
ni −1
where xi is the mole fraction of molecules with degree of polymerization i , and nis the average degree of polymerization. The maximum length N is assumed on the
basis of a total loss lower than 0.1% [20]. N becomes the upper limit of sums of
mass balance equations. The resulting dimension of the overall differential system
is 5×N . The solution is obtained after a numerical integration through an implicit
multi-step Adams – Moulton method [23].
Because of the heavy computing times (about 10-min on a PC) owing to the
initial high molecular weights, a lumping procedure has been introduced [24 – 26]. It
is based on the grouping of the longest species into lumps. It is possible to de fine
the critical length beyond which species are grouped and the number of species of each group. This approach strongly reduces the calculation time without a signifi-
cant effect on the predicted results.
7/27/2019 Thermal Degradation of Polystyrene - T. Farawelli - M. Pinicroli - F. Pisano - G. Bozzano - M. Dente - E. Ranzi
http://slidepdf.com/reader/full/thermal-degradation-of-polystyrene-t-farawelli-m-pinicroli-f-pisano 13/19
115T . Fara6elli et al . / J . Anal . Appl . Pyrolysis 60 (2001) 103 – 121
Fig. 3. Isothermal degradation curves for PS. Comparison between experimental data found by
Bockhorn et al. [4] and model results.
5. Experimental data and validation of the model
In this work, three types of experimental data are compared with the model
predictions — isothermal data, TGA curves, and gas product distributions.
Recent isothermal data at atmospheric pressure are reported by Bockhorn et al.
[4]. Their comparison with model results are shown in Fig. 3. The agreement is very
good at 360, 400 and 410°C. The intermediate isothermal curves at 370, 380 and
390°C are slightly underpredicted.
Experimental data reported by Bouster et al. [5] are obtained in experimental
conditions (temperatures and pressure) similar to those already discussed. Theydiffer mainly in the polymer molecular weight (100 000 instead of 186 000 g mol−1).
The comparisons with these data are shown in Fig. 4. Also in this case, the
agreement is satisfactory and even better than in the previous example.
Fig. 5 shows the comparison between predicted results and isothermal experimen-
tal data presented by Madorsky [6]. At 348°C, the agreement is very good, but at
lower temperatures, the model seems to forecast a faster decomposition. Due to the
very low experimental pressure (about 10−5 mmHg), these discrepancies can be
justified with only 2 and 4°C, respectively, at 338 and 328°C (i.e. within experimen-
tal uncertainty).
Even if the comparison with the experimental data is generally good, the partial
observed disagreement is confirmed by the differences between the overall kineticparameters proposed in literature. The activation energies respectively proposed by
Bockhorn, Bouster and Madorsky are 41, 49 and 55 kcal mol−1. None of these
7/27/2019 Thermal Degradation of Polystyrene - T. Farawelli - M. Pinicroli - F. Pisano - G. Bozzano - M. Dente - E. Ranzi
http://slidepdf.com/reader/full/thermal-degradation-of-polystyrene-t-farawelli-m-pinicroli-f-pisano 14/19
116 T . Fara6elli et al . / J . Anal . Appl . Pyrolysis 60 (2001) 103 – 121
experimental activation energies is able to cover all the temperature ranges investi-gated here and only a phenomenological approach can span over all the experi-
ments with the same level of accuracy. The apparent activation energy calculatedfrom the detailed kinetic model presented here is about 47 kcal mol−1.
As a further comparison, Fig. 6 shows the model prediction and TG experimentaldata presented by Anderson and Freeman [7] under a vacuum of 1 mm Hg and aconstant heating rate of 5°C min−1. The agreement is especially good at the
beginning of the degradation (until 390°C). In the figure, there is also the curve
obtained with the Bockhorn’s model.As mentioned already, the kinetic model was compared also with the experimen-
tal gas product distributions [8]. The experimental results obtained by Audisio andBertini are reported in Table 2. Styrene is the most important product of the
thermal degradation. The model is in quite good agreement with molecular weighteffect, both for the monomer yield and also other secondary products. On thecontrary, the agreement is not so good with the temperature variation.
Moreover, the assumed kinetic mechanism does not explain the formation of benzene and light hydrocarbons, which are observed and measured in some
experiments. Two are the possible explanations. From one side, it is possible tohave secondary cross-linking reactions with additive substitutions of secondary ortertiary radicals on the different rings. A second explanation can refer to successive
gas phase reactions. On the contrary, it is quite dif ficult to invoke an electrophilicattack on the aromatic ring, because the ionic mechanism is significant only in thepresence of acid catalysts, while the pure thermal degradation is governed by a
radical depolymerization [27,28].
Fig. 4. Isothermal degradation curves for PS. Comparison between experimental data found by Bouster
et al. [5] and model results.
7/27/2019 Thermal Degradation of Polystyrene - T. Farawelli - M. Pinicroli - F. Pisano - G. Bozzano - M. Dente - E. Ranzi
http://slidepdf.com/reader/full/thermal-degradation-of-polystyrene-t-farawelli-m-pinicroli-f-pisano 15/19
117T . Fara6elli et al . / J . Anal . Appl . Pyrolysis 60 (2001) 103 – 121
T a b l e 2
P r o d u c t
d i s t r i b u t i o n ( w t . % ) f r o m p o l y s t y r e n e p
y r o l y s i s a
M W =
2 1 0 0
M X =
1 1 0 0 0 0
M W =
3 8 0 0 0 0
M X =
7 5 0 0
P y r o l y s i s
M X =
3 0 0 0 0
p r o d u c t
C a
l c u l a t e d
E x p e r i m e n t a l C a l c u l a t e d
E x p e r i m e n t a l
E x p e r i m e n t a l C a l c u l a t e d
C a l c u l a t e d
E x p e r i m e n t a l C a l c u l a t e d
E x p e r i m e n t a l
T =
6 0 0 °
C
4 . 9
1
–
2 . 9
3
–
2 . 0
4
–
1 . 8
4
–
–
L i g h t h y
d r o -
3 . 6 2
c a r b o n
s
–
1 . 4
1
–
0 . 9
9
–
1 . 5
–
B e n z e n e
2 . 0
2
–
1 . 6 2
1 . 0
9
4 . 3
8
0 . 9
7
3 . 3
1
0 . 9
3
T o l u e n e
5 . 9
3
2 . 6
9
5 . 1 4
1 . 5
5
4 . 6
5
0 . 2
4
0 . 8
8
0 . 0
7
0 . 5
7
0 . 0
2
0 . 8
9
E t h y l b e n
z e n e
0 . 9
1 . 0 5
1 . 8
5
1 . 1
3
8 2 . 8
5
7 9 . 5
3
8 1 . 3
3
8 3 . 4
9
8 2 . 4
8 7 . 1
8 2 . 7
6 4 . 7
3
8 0 . 7 4
7 7 . 1
3
S t y r e n e
0 . 6
1
0 . 4
5
0 . 6
0 . 4
5
0 . 5
1
0 . 4
8
0 . 4
5
0 . 5
a - M e t h y l -
0 . 6 4
0 . 7
6
s t y r e n e
T =
7 5 0 °
C
3 . 1
5
–
2 . 7
5
L i g h t h y
d r o -
–
5 . 9
3
2 . 1
2
–
–
4 . 2 5
–
c a r b o n
s
–
3 . 6
7
–
2 . 5
8
–
–
4 . 1
4 . 9 9
–
5 . 2
7
B e n z e n e
0 . 5
9
4 . 7
4
0 . 4
5
3 . 8
1
T o l u e n e
0 . 4
1
7 . 1
1
2 . 1
5
5 . 7 3
1 . 1
5 . 4
0 . 2
6
1 . 0
1
0 . 0
7
0 . 7
1
0 . 0
2
1 . 1
3
1 . 6
0 . 9
2
1 . 2 2
E t h y l b e n
z e n e
1 . 5
8 8 . 1
9
8 2 . 5
8 9 . 7
4
8 5 . 5
9
9 0 . 1
7
S t y r e n e
7 0 . 1
7
6 5 . 8
8
7 3 . 6 8
8 2 . 3
4
7 7 . 7
9
1 . 5
0 . 2
1 . 1
9
0 . 1
9
1 . 1
1
0 . 2
3
0 . 1
9
a - M e t h y l -
1 . 6 3
0 . 3
8
1 . 9
s t y r e n e
a
C o m
p a r i s o n b e t w e e n m o d e l p r e d i c t i o n s a n d
e x p e r i m e n t a l d a t a o f A u d i s i o a n d B e r t i n i [ 8 ] .
7/27/2019 Thermal Degradation of Polystyrene - T. Farawelli - M. Pinicroli - F. Pisano - G. Bozzano - M. Dente - E. Ranzi
http://slidepdf.com/reader/full/thermal-degradation-of-polystyrene-t-farawelli-m-pinicroli-f-pisano 16/19
118 T . Fara6elli et al . / J . Anal . Appl . Pyrolysis 60 (2001) 103 – 121
Fig. 5. Isothermal degradation curves for PS. Comparison between experimental data found byMadorsky [6] and model results.
A better agreement was found in comparison with the experimental data pro-
posed by Bouster et al. [9] and presented in Table 3. Toluene prediction agrees in
this case with the experimental observations. In these data, the yields of dimer and
trimer are also reported. The trends with the temperature and molecular weight are
quite good. The relative yields of dimer and trimer are not reproduced correctly,
even if their sum matches quite correctly the experimental results.
It has to be noted that more experimental information is needed to characterize
the model better. For instance, it is quite evident that the increase of the amount of
1,3 diphenylpropane with the temperature cannot be explained with the proposed
model. Higher temperatures make easier the transformation of alkane chains in
alkenes.
6. Conclusion
In this paper, a detailed model of polystyrene thermal degradation has been
presented. The model is able not only to describe the weight loss during the process,
but overall to predict the gas phase composition. The kinetic parameters are derived
from the well-known values proposed already for the gas phase pyrolysis, with the
proper modifications to be applied in the liquid phase. The results are encouragingeven though not as accurate as in the detailed kinetic models of PE and PP.
Nevertheless, the proposed model already allows reliable predictions. Some uncer-
7/27/2019 Thermal Degradation of Polystyrene - T. Farawelli - M. Pinicroli - F. Pisano - G. Bozzano - M. Dente - E. Ranzi
http://slidepdf.com/reader/full/thermal-degradation-of-polystyrene-t-farawelli-m-pinicroli-f-pisano 17/19
119T . Fara6elli et al . / J . Anal . Appl . Pyrolysis 60 (2001) 103 – 121
T a b l e 3
P r o d u c t
d i s t r i b u t i o n ( w t . % ) f r o m p o l y s t y r e n e p
y r o l y s i s a
M W =
5 0 0 0 0
M W =
8 0 0 0
P y r o l y s i s p r o d u c t s
M W =
5 0 0 0 0
M W =
2 2 0 0
C a l c u l a t e d
E x p e r i m e n t a l
C a l c u l a t e d
E x p e r i m e n t a l
C a l c u l a t e d
C a l c u l a
t e d
E x p e r i m e n t a l
E x p e r i m e n t a l
( a ) E f f e c t o f t h e m o l e c u l a r w e i g h t a t T =
6 0 0 ° C
7 8 . 2
7 6 . 5
6 7 . 0
1
7 8 . 2
6 9 . 2
4 8 . 2
S t y r e n e
7 6 . 3
7 2 . 7 3 . 2
1 . 7
0 . 8
3 . 5
0 . 8
1 . 3
3 . 1
1 . 0
D i m e r
8 . 6
1 0 . 5
8 . 2
1 0 . 0
6 . 1
9 . 0
T r i m e r
5 . 5
4 . 4
0 . 7
1 . 4
1 . 7
0 . 8
1 . 7
3 . 0
1 . 0
2 . 1
T o l u e n e
0 . 2
0 . 6
0 . 6
0 . 2
0 . 5
0 . 2
0 . 3
0 . 7
1 , 3 - D i p h
e n y l p r o p a n e
T r a c e
T r a c e
0 . 1
0 . 8
T r a c e
T r a c e
2 . 4
0 . 1
E t h y l b e n
z e n e
0 . 1
0 . 3
T r a c e
0 . 2
0 . 0
0 1
0 . 2
a - M e t h y l s t y r e n e
0 . 3
0 . 2
( b ) E f f e c t o f p y r o l y s i s t e m p e r a t u r e ( M W =
1 0 0 0 0 0 g m o l −
1 )
T = 8
0 0 ° C
T =
7 0 0 ° C
T =
6 0 0 ° C
T =
5 0 0 ° C
P y r o l y s i s p r o d u c t s
7 6 . 4
6 6 . 8
8 5 . 4
7 8 . 4
8 9 . 6
6 5 . 6
7 2 . 7
7 8 . 2
S t y r e n e
3 . 4
4 . 6
0 . 3
1 . 9
0 . 1
3 . 1
3 . 2
0 . 8
D i m e r
6 . 2
1 . 3
4 . 5
5 . 0
8 . 6
T r i m e r
1 0 . 0
1 2 . 7
1 5 . 0
1 . 0
T o l u e n e
0 . 9
0 . 7
0 . 6
3 . 0
0 . 7
1 . 7
0 . 7 1 . 6
0 . 1
2 . 0
0 . 1
0 . 2
0 . 6
0 . 6
1 , 3 - D i p h
e n y l p r o p a n e
0 . 3
T r a c e
0 . 1
0 . 0
1
0 . 3
0 . 0
1
0 . 1
T r a c e
E t h y l b e n
z e n e
T r a c e
T r a c e
0 . 5
T r a c e
0 . 9
T r a c e
0 . 2
a - M e t h y l s t y r e n e
0 . 0
1
0 . 3
a
C o m
p a r i s o n b e t w e e n m o d e l p r e d i c t i o n s a n d
e x p e r i m e n t a l d a t a [ 9 ] .
7/27/2019 Thermal Degradation of Polystyrene - T. Farawelli - M. Pinicroli - F. Pisano - G. Bozzano - M. Dente - E. Ranzi
http://slidepdf.com/reader/full/thermal-degradation-of-polystyrene-t-farawelli-m-pinicroli-f-pisano 18/19
120 T . Fara6elli et al . / J . Anal . Appl . Pyrolysis 60 (2001) 103 – 121
Fig. 6. TG curves for PS. Comparison between experimental data of Anderson and Freeman [7] and
model results.
tainties are still present like the dif ficulty in predicting benzene formation as
observed by some authors. At the same time, the dif ficulties related with the
experimental measures, the mass and heat transfer limitations, the possible presence
of successive reactions in the gas products and the small amount of reliable data
and experiments ask for further investigations.
This work on the thermal degradation of poly-styrene adds a further step to the
overall characterization of pyrolysis of plastics. Nowadays, polyethylene and
polypropylene and polystyrene models are available. The major interest in this
research activity is to found an alternative route for the upgrading of solid wastes
to more usable and energy-dense materials.
Acknowledgements
This work was supported by EU under the ‘HALOCLEANCONVERSION’
project, contract n. G1RD-CT 1999-00082.
References
[1] E. Ranzi, M. Dente, T. Faravelli, G. Bozzano, S. Fabini, R. Nava, V. Cozzani, L. Tognotti, J.
Anal. Appl. Pyrol. 40 – 41 (1997) 305 – 319.
[2] T. Faravelli, G. Bozzano, C. Scassa, M. Perego, S. Fabini, E. Ranzi, M. Dente, J. Anal. Appl.
Pyrol. 52 (1999) 87 – 103.
[3] R. Zevenhoven, M. Karlsson, M. Frankenhaeuser, M. Hupa, Laboratory scale characterization of
plastic-derived fuels, Borealis Polymer Oy, Report 95/3, Borga, Finland, 1995.
7/27/2019 Thermal Degradation of Polystyrene - T. Farawelli - M. Pinicroli - F. Pisano - G. Bozzano - M. Dente - E. Ranzi
http://slidepdf.com/reader/full/thermal-degradation-of-polystyrene-t-farawelli-m-pinicroli-f-pisano 19/19
121T . Fara6elli et al . / J . Anal . Appl . Pyrolysis 60 (2001) 103 – 121
[4] H. Bockhorn, A. Hornung, U. Hornung, in: The Twenty-seventh International Symposium on
Combustion, University of Colorado, Boulder, 1998.
[5] C. Bouster, P. Vermande, J. Veron, J. Anal. Appl. Pyrol. 1 (1980) 297 – 313.
[6] S.L. Madorsky, Thermal Degradation of Organic Polymers, Interscience, New York, 1964.
[7] D.A. Anderson, E.S. Freeman, J. Polym. Sci. 54 (1961) 253 – 260.
[8] G. Audisio, F. Bertini, J. Anal. Appl. Pyrol. 24 (1992) 61 – 74.
[9] C. Bouster, P. Vermande, J. Veron, J. Anal. Appl. Pyrol. 15 (1989) 249 – 259.
[10] M.M. Shapi, A. Hesso, J. Anal. Appl. Pyrol. 18 (1990) 143 – 161.
[11] M.T. Sousa Pessoa De Amorim, C. Bouster, P. Vermande, J. Veron, J. Anal. Appl. Pyrol. 3 (1981)
19 – 34.
[12] A. Guyot, Poly. Deg. Stab. 15 (1986) 219 – 235.
[13] L. Costa, G. Camino, A. Guyot, M. Bert, A. Chiotis, Poly. Deg. Stab. 4 (1982) 245 – 260.
[14] L. Costa, G. Camino, A. Guyot, M. Bert, G. Clouet, J. Brossas, Poly. Deg. Stab. 14 (1986) 85 – 93.
[15] M. Swistek, G. Nguyen, D. Nicole, J. Anal. Appl. Pyrol. 37 (1996) 15 – 26.
[16] S. Benson, The Foundations of Chemical Kinetics, McGraw-Hill, New York, 1960.
[17] M. Dente, G. Bozzano, M. Rossi, in: Proceedings of the First Conference on Chemical and Process
Engineering, Florence, Italy, 1993, pp. 163 – 172.
[18] M. Dente, G. Bozzano, G. Bussani, Comp. Chem. Eng. 21 (1997) 1125 – 1234.
[19] D.L. Van Krevelen, Properties of Polymers, Elsevier, Amsterdam, 1990.
[20] M. Pinciroli, F. Pisano, Degradazione Termica del Polistirene, Thesis, Politecnico di Milano, 1999.
[21] O.A. Hougen, K.M. Watson, R.A. Ragatz, Chemical Process Principles, Wiley, New York, 1954.
[22] U.W. Gedde, Polymer Physics, Chapman & Hall, London, 1995.
[23] A. Hindmarsh, in: R.S. Stepleman, et al. (Eds.), ODEPACK, A Systematized Collection of Ode
Solvers in Scientific Computing, North-Holland, Amsterdam, 1983, pp. 55 – 64.[24] P.G. Coxson, K.B. Bischoff, Ind. Eng. Chem. Res. 26 (1987) 1239 – 1248.
[25] P.G. Coxson, K.B. Bischoff, Ind. Eng. Chem. Res. 26 (1987) 1251 – 1257.
[26] J. Wei, J.C.W. Kuo, Ind. Eng. Chem. Fundam. 8 (1969) 114 – 123.
[27] P. Carniti, A. Gervasini, P.L. Beltrame, G. Audisio, F. Bertini, Appl. Catal. A 127 (1995) 139 – 155.
[28] R. Lin, R.L. White, J. Appl. Polym. Sci. 63 (1997) 1287 – 1298.
.