thermal degradation of polystyrene - t. farawelli - m. pinicroli - f. pisano - g. bozzano - m. dente...

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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



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



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).




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:




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




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




[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).




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.




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




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




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.




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.




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




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.




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


C a l c u l a t e d



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 ] .




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-




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


C a l c u l a t e d


C a l c u l a t e d

C a l c u l a

t e d



( 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 ] .




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.

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