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Thermo-oxidative decomposition of polyvinyl chloride
J. Anal. Appl. Pyrolysis 74 (2005) 215–223 www.elsevier.com/locate/jaap
Thermo-oxidative decomposition of polyvinyl chloride Ignacio Aracil *, Rafael Font, Juan A. Conesa Departamento de Ingenierı´a Quı´mica, Universidad de Alicante, Ap. 99, 03080 Alicante, Spain Received 29 July 2004; accepted 5 August 2004 Available online 17 March 2005
Abstract Pyrolysis and combustion of polyvinyl chloride resin have been studied by thermogravimetric analysis. Nine different runs with about 5 mg sample mass have been carried out in three different atmospheres (He; He:O2 9:1; and He:O2 4:1) and heating rates (5, 10 and 20 8C/ min). Adequate kinetic simplified models have been proposed and all heating rates have been simultaneously correlated with the same set of kinetic constants, obtaining good results. The pyrolysis model consists of one first reaction producing gases and solid residue followed by two parallel reactions of previous solid, whereas the combustion model adds a third parallel reaction with oxygen and three new combustion reactions to burn char formed in the three previous reactions. Different considerations have been taken into account during the optimization to achieve the best results. Parameters obtained have been discussed and also compared with others from literature. # 2005 Elsevier B.V. All rights reserved. Keywords: TG; PVC; Kinetic model; Air; Combustion
1. Introduction Over the last few decades, many efforts have been made looking for alternative treatments to landfilling of plastic waste. Both recycling and energy recovery can be effective processes to consider, and their application will depend on different technical, economic, legal and ecological considerations. Polyvinyl chloride (PVC) is one of the most consumed plastics and has received great attention because of chlorinated combustion byproducts, which have been shown to be toxic. There are some primary applications for this thermoplastic, such as window frames, pipes, flooring, wallpaper, bottles, cable insulation, credit cards, toys or medical products [1]. According to annual studies conducted by the Association of Plastic Manufacturers in Europe (APME), total consumption of PVC in the European Community has risen from 6.9 million tonnes in 1999 to 7.9 million tonnes in 2003 [1]. In 1999 total annual PVC waste quantity was about 4.1 million tonnes in the European Community, and is predicted to increase 30% in 2010 and 80% up to 7.3 million tonnes in 2020 [2]. * Corresponding author. Tel.: +34 965 903 867; fax: +34 965 903 826. E-mail address: [email protected] (I. Aracil). 0165-2370/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jaap.2004.08.005
Thermal decomposition of PVC has been widely studied by many researchers. Some works are found concerning analysis of PVC by thermogravimetric techniques, since they provide very useful information about kinetic reaction parameters like activation energies, pre-exponential factors, reaction orders, number of processes involved, etc. Papers usually focus on mechanistic aspects and some of them have revealed the main structural changes occurring during thermal decomposition regarding initiation and propagation of both dehydrochlorination and aromatic formation [3–13]; several works face the great amount of reactions which appear to take place during the process [14–15], whereas others propose non-complicated mechanisms consisting of simplified reactions which gather a complex number of reactions [16–20]. This latter procedure has the advantage of being able to simulate rather accurately the thermal degradation behavior by using a relatively small number of equations. However, it is important to cover a wide range of operating conditions to assure the model can be applicable at different conditions. The main aim of this work has been to propose a simplified model to describe both pyrolysis and combustion of pure PVC at different atmospheres and heating rates. On the other hand, it is interesting to point out the fact that, whereas a lot of papers deal with pyrolytic decomposition of
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PVC, fewer papers have been found including the effect of oxygen in the atmosphere of the reaction when studying thermal decomposition of PVC [3,7,21–24], so the results presented here might be useful for a better knowledge of the thermal behavior of PVC inside a pyrolyzer or a combustor. Some interesting considerations have been taken into account when the optimization of selected parameters has been conducted. Finally, the results obtained have been discussed and compared with other previous works.
2. Experimental Experiments were performed on white powdery PVC resin Etinox-450 obtained by emulsion polymerization. This resin is basically used in low-viscosity plastisols. Molecular weight was measured in a previous work [18] by GPC in tetrahydrofuran, resulting 104,000 and 40,000 for Mw and Mn, respectively. The particle size distribution was determined with a Coulter LS 230 particle size meter suspending PVC solid particles in water, obtaining a distribution between 1.4 and 24.4 mm and a medium value of 6.0 mm (Fig. 1); this equipment allows particles to be measured in a range between 0.4 and 2000 mm. Besides, the chemical composition of PVC, shown in Table 1, was analysed by two complementary techniques: Elemental analysis, for C, H, S and N with a Perkin-Elmer 2400 CHN.
X-ray fluorescence, for elements with an atomic weight higher than that of magnesium. The equipment used was an automatic sequential spectrometer of X-ray fluorescence model PW1480. The TG runs were carried out in a Setaram TG-DTA 9216.18 thermobalance controlled by a PC compatible system, which records the temperature measured by a thermocouple situated under the holder. The total gas flow was 60 mL/min, consisting of helium in the pyrolysis runs and a mixture of helium and oxygen in the combustion runs. The reproducibility of the runs was tested and also the buoyancy effect was subtracted for each run by comparing with the corresponding blank runs. Some metals with known melting points (In, Sn, Pb, Al, Zn, Ag) are periodically used to calibrate the equipment temperature in the range 150–960 8C. As well as this, one experiment with Avicel cellulose was carried out at 5 8C/min in helium to test the good working of the equipment by calculating the kinetic parameters of this decomposition and checking that the results were in good agreement with those obtained in a round-robin study of Avicel cellulose pyrolysis [25]. The PVC was heated from 25 to 700 8C in every experiment. Nine runs with about 5 mg sample mass were carried out changing both the heating rate (5, 10 and 20 8C/ min) and the atmosphere (He; He:O2 9:1; and He:O2 4:1) in order to study both pyrolytic and thermo-oxidative decomposition.
3. Results and discussion Figs. 2–7 show the experimental weight loss variation with temperature obtained at different heating rates and atmospheres. Runs at different heating rates can be compared in Figs. 2–4, while the effect of the atmosphere is highlighted in Figs. 5–7. In the figures, w is defined as the residual mass fraction of the solid (including residue formed and non-reacted initial solid), i.e. the ratio between solid mass at any time (m) and initial solid mass (m0). Fig. 1. Particle size distribution of PVC Etinox-450.
Table 1 Chemical composition of PVC resin (wt.%) Element
Elemental analysis
H C O Na Al Si S Cl K Ca Zn
4.80 38.4 – – – – 1.40 – – – –
X-ray fluorescence (semiquantitative) – – 0.076 0.12 0.0057 0.0095 55.2 0.018 0.017 0.034
Fig. 2. Experimental data for pyrolysis runs in He.
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Fig. 3. Experimental data for combustion runs in He:O2 9:1.
Fig. 6. Experimental data for TG runs at 10 8C/min.
Fig. 4. Experimental data for combustion runs in He:O2 4:1.
Fig. 7. Experimental data for TG runs at 20 8C/min.
On the one hand, when the heating rate increases, the weight loss curves move to higher temperature regardless of the atmosphere, as expected. Pyrolytic decomposition is generally accepted to consist of two main steps. The first one (at about 500–650 K) is dehydrochlorination, and it is believed to be initiated primarily at unsaturated sites (allylic chlorines) or at other defect sites in the polymer chain [4,6]. It takes place to form HCl and a linear polyene structure that has also been shown to produce unsubstituted aromatics (benzene, naphthalene) through intramolecular cyclization from polyene radicals formed by chain scission [5,8,11].
Then, polyene chains can react through intermolecular reactions and these crosslinked chains undergo further reactions to form alkyl aromatic hydrocarbons and char residue. Alkyl aromatic formation appears to take place again by intramolecular cyclization but, in this case, there is H–H exchange between polyene chains [9–11]. The main interest is focused on comparing runs at different atmospheres. When pyrolytic runs are compared with those in helium:oxygen atmospheres, an interesting behavior is noted: the first step is slightly sped up in the presence of oxygen, then the second step is delayed and finally no char residue remains. This, in a simplified explanation, could indicate that oxygen enhances dehydrochlorination, then it could partially oxidize polyene chain and, in the end, char would be completely oxidized and the resulting products volatilized. According to the behavior observed, and taking into account some previous models proposed in literature which will be briefly described later, two models have been developed, one for pyrolysis and one for combustion, the latter including the effect of the presence of oxygen. 3.1. Pyrolysis model
Fig. 5. Experimental data for TG runs at 5 8C/min.
A two-stage model has been proposed (Scheme 1). It consists of a first reaction in which dehydrochlorination and
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w R1 w R1 r1 formed r1 reacted v1 v12 v22 ¼ ¼ a1 a12 a22 1 r1 r1 r12 r1 r22 The kinetic laws can be expressed as
wR1 ¼ r1 Scheme 1.
the formation of some unsubstituted aromatics take place together with the remaining polyene chain, and a second stage involving two parallel reactions which can produce both volatiles and final solid residue. Since experimental curves give different char yield at the different heating rates, these two reactions have been proposed to be competitive. In the previous scheme, S1 is the initial solid, V1 and Vi2 are the possible gases + volatiles evolved in the corresponding reactions and R1 and Ri2 are the solid residues formed in the decomposition. The procedure followed here is similar to that described in a previous paper [26]. From now on, we will work on a normalized basis, and hence we define ws1 , v1 , wR1 , vi2 and wRi2 (these latter two for both reactions (12) and (22)) which represent the corresponding kg of compounds with respect to kg of initial reactive solid S1 (mass fractions). On the other hand, each fraction has a yield coefficient that represents the maximum mass fractions obtainable by means of each reaction. It can be easily deduced that each yield coefficient for each product fraction ranges between 0 and the corresponding reactive yield coefficient (1 in the case of reaction (1) and r1 for reactions (12) and (22)). It is very important to point out that, in the definition of yield coefficient for competitive reactions, the term ‘maximum’ for one reaction refers to the highest possible yield obtained in the case in which the parallel competitive reaction was not taken into account, so r12 and (r1 r12) are the maximum yields obtained should reaction (22) not take place, and the same can be said for (r1 r22). For the above-mentioned reason, it is very useful to introduce the concept of the conversion factor for each reaction, which is defined as the ratio between the mass fraction of volatiles obtained at any time during which the reaction is taking place and the corresponding yield coefficient, so: v1 v1 ðwR1 Þformed ¼ ¼ ¼ 1 ws1 (1) v11 1 r1 r1 vi2 vi2 ai2 ¼ ¼ ; i ¼ 1; 2 (2) vi21 r1 ri2 Thus, according to the definitions previously commented, a1 ranges between 0 and 1 during the experiment, whereas both a12 and a22 cannot reach the value 1 (unless only one of the two reactions were active), but together they do add up to 1 at the end of the process at infinite time. The definitions are valid for solid residue taking into account that in Eq. (1), wR1 is not the solid residue existing at any time, but the residue formed from reaction (1) as if it did not subsequently react, as indicated. From a mass balance it can be deduced that a1 ¼
(3)
dðwR1 =r1 Þformed dðv1 =ð1 r1 ÞÞ ¼ k1 ðws1 Þn1 ¼ dt dt da1 (4) ¼ k1 ð1 a1 Þn1 ¼ dt dðwR1 =r1 Þreacted dðvi2 =ðr1 ri2 ÞÞ ¼ dt dtni2 wR1 ¼ ki2 ¼ ki2 ða1 a12 a22 Þni2 r1 dai2 ; i ¼ 1; 2 (5) ¼ dt since R1 is formed from reaction (1) and is consumed in reactions (12) and (22). In the previous equation n1, n12 and n22 are the reaction orders for reactions (1), (12) and (22), respectively, and k1, k12 and k22 are the kinetic constants for the corresponding reactions. These kinetic constants follow the Arrhenius law, so: E1 k1 ¼ k1 exp (6) RT Ei2 ki2 ¼ ki20 exp ; i ¼ 1; 2 (7) RT where k1, k120 and k220 are the pre-exponential factors for reactions (1), (12) and (22), respectively, and E1, E12 and E22 the apparent activation energies for the corresponding reactions. When setting out the equations, it has been considered more convenient to work in terms of conversion factors. The procedure performed consisted of optimizing 12 parameters (r, E, n and kinetic constants for each one of the three reactions) best fitting calculated and experimental weight loss (wcalc and wexp , respectively). Starting with initial values of conversion factors equal to 0 at initial time, these parameters have been numerically calculated along the time, and then: vtot ¼ v1 þ v12 þ v22 ¼ a1 ð1 r1 Þ þ
2 X
ai2 ðr1 ri2 Þ (8)
i¼1
wcalc ¼ 1 vtot
(9)
The objective function (OF) to minimize was the sum of the square differences between experimental and calculated weight loss values (Eq. (10)). All the thermograms obtained at different heating rates have been simultaneously correlated with the same set of kinetic parameters. OF ¼
3 X N X
exp
2 ðwm j wcalc mj Þ ;
m¼1 j¼1
m heating rates; j points
(10)
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The model validity has been tested calculating the variation coefficient (VC): pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi OF=ðN PÞ VC ¼ 100 (11) wexp where N and P are the number of data and parameters fitted, respectively, and wexp is the average of the experimental weights. According to the procedure suggested by Martı´nGullo´ n et al. [27], the great interrelation existing among the pre-exponential factor, the apparent activation energy and the reactor order can be decreased when optimization is performed in terms of a ‘comparable kinetic constant’ Ki defined in Eq. (12), instead of optimizing k0i. This constant is calculated in a temperature around the maximum decomposition rate (Tmax). Since Ki , Ei and ni are optimized, the pre-exponential factor k0i is readily calculated from Eq. (12): Ei Ki ¼ ki ð0:64Þni ¼ k0i exp ð0:64Þni RTmax
(12)
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The results obtained are shown in Table 2. There are some interesting results to comment. On the one hand, r12 equals r1, whereas r22 equals 0. This means that each competitive reaction only gives one kind of compounds (char residue in reaction (12) and volatiles in reaction (22)), so Scheme 1 can be simplified as Scheme 1a. On the other hand, according to the previous results, the activation energy E12 corresponding to char formation reaction is lower than the activation energy E22 of volatile formation reaction, what agrees with the fact that the lower the heating rate the more char is formed (see Fig. 5). Experimental and calculated values are given in Fig. 8, showing a good agreement. There are some other papers that have studied other kinetic models for pure PVC with some similarities, and below some of them are schematized as appearing in the original papers. Calculated activation energies for each reaction (in kJ/mol) are shown above the corresponding arrows:
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Marcilla and Beltra´ n [18] proposed two different kinetic models with an intermediate polyene chain in the second model according to that suggested by Montaudo and Puglisi [11]. The last evolution of volatiles and char was put into one single reaction instead of considering two competitive reactions because no differences in char yield were observed at different heating rates (5, 10, 25 and 50 8C/min). In the first model, which assumed a sole reaction for the first step, the results were 146.4 kJ/mol, 3.52 1012 min1 and 1.32 for E1, k10 and n1, respectively. Anthony [19] suggested a similar model to Marcilla and Beltra´ n’s second model but proposing that the intermediate stage consisted of two competitive reactions to produce benzene and volatiles, based on the work of Montaudo and Puglisi [11]. Despite the model is general, it was only validated for one heating rate in the paper (20 8C/min), so it is not possible to compare at different heating rates. On the other hand, Wu et al. [17] introduced two initial parallel reactions concerning formation of volatiles and intermediate from two different H–T and H–H polyene chain configurations and two final competitive reactions to give char and more volatiles. These latter reactions are equivalent to reactions (12) and (22) of the present work, and the results Table 2 Kinetic parameters obtained for pyrolysis model (kij0 in s1, Eij in kJ/mol) 0.372 0.372 0 1.51 1.71 2.59 140.2 188.6 289.7 3.71 1010 5.97 1010 4.03 1018 0.0916 1.545
r1 r12 r22 n1 n12 n22 E1 E12 E22 k10 k120 k220 OF VC (%)
Scheme 1a.
Fig. 8. Experimental (-) and calculated (—) data for pyrolysis runs in He.
fitted for three heating rates (1, 2 and 5.5 8C/min) were 218 kJ/mol, 5.6 1012 s1 and 1.5 for E12, k120 and n12, respectively, and 267 kJ/mol, 4.8 1016 s1 and 1.5 for E22, k220 and n22, respectively. Activation energies are similar to those obtained in the present work, but pre-exponential factors are two magnitude orders higher and lower for reactions (12) and (22), respectively. Differences could be due to the previously commented great interrelation between the three parameters E, k0 and n. In order to prove this, we tried to fit the model keeping reaction orders equal to 1. The results showed good agreement between calculated and experimental values and, however, differences of some magnitude orders in pre-exponential factors with respect to the values given in Table 2, but no great differences in activation energies, what means that it is difficult to assure that the calculated values obtained by one method are likely to be the best ones. Anyway, it must not be forgotten that all kind of models are correlation models. Of course, another source of discrepancy may be the differences of PVC resin properties. It could be questioned that the model proposed in this work is much too simplified, and this is probably true, but the main aim of the pyrolytic model was to be taken as a starting point to develop a kinetic model for combustion. 3.2. Oxidative pyrolysis and combustion model Scheme 2 presents the three-stage model proposed to explain the behavior obtained at two different helium:oxygen atmospheres and three heating rates.
Scheme 2.
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Table 3 Kinetic parameters obtained for combustion model (kij0 in s1, ki0 j0 in s1 atmbij and Eij in kJ/mol) r1 r12 r22 r32 r13, r33 n1 n12 n22 n32 n13, n33 b32 E1 E12 E22 E32 E13, E33 k10 k120 k220 0 k320 0 , k0 k130 330 b13, b33 OF VC (%)
0.372 0.372 0 0.372 0.372 1.51 1.71 2.59 1.69 0.72 0.76 128.3 188.6 289.7 276.1 90.6 4.00 109 5.97 1010 4.03 1018 2.77 1018 6.51 103 0.52 0.343 2.229
It has been assumed that the structure of reactions (12) and (22) is that obtained in the previous part. Now, the model introduces a third competitive reaction (32) in the second step to account for the delay in weight loss with respect to pyrolysis, allowing the polyene to give both char residue (R32) and volatiles (V32) by oxidation of polyene chain. Besides this, the non-existence of final char involves that some oxidation to give volatile products or gases (CO, CO2, H2O) occurred from char produced in reactions (12) and (32), and this is taken into account in reactions (13) and (33). It has also been considered that the term V in reactions (32), (13) and (33) includes the volatilized mass of solid residue, which is different from the total gas + volatiles produced, since the latter includes oxygen contribution. In order to simplify the model, it has been assumed that reactions (13) and (33) have the same kinetic parameters. Finally, the first step has been considered to be accelerated by the presence of oxygen (only as an enhancer of the decomposition rate). The papers studying the effect of oxygen in thermal degradation also showed that the HCl generation rate increased when compared to inert atmospheres, probably due to the formation of peroxide radicals [3,7,21–23]. Nagy et al. [24] developed a simplified model for the thermo-oxidative decomposition of PVC; the primary initiation step was identical to that of the purely thermal decomposition and then, rapid dehydrochlorination followed resulting in the formation of polyenes which reacted readily with oxygen even at approximately 100 8C to form peroxides and hydroperoxides. The definitions of the new yield coefficients can be easily deduced taking into account the explanations given in the
Fig. 9. Experimental (-) and calculated (—) data for combustion runs in He:O2 9:1.
previous part. Of course, coefficients r13 and r33 are equal to r12 and r32, respectively. New conversion factors corresponding to the new reactions are now defined: v32 v32 a32 ¼ ¼ (13) v321 r1 r32 ai3 ¼
vi3 vi3 ¼ ; vi31 ri3
i ¼ 1; 3
(14)
Following a deduction similar to that shown in Eqs. (3)– (5), the resulting equations for compounds involved in the second stage (reactions (12), (22) and (32)) are: w R1 wR1 wR1 ¼ r1 r1 formed r1 reacted v1 v12 v22 v32 ¼ 1 r1 r1 r12 r1 r22 r1 r32 ¼ a1 a12 a22 a32
(15)
ni2 dðwR1 =r1 Þreacted dðvi2 =ðr1 ri2 ÞÞ wR1 ¼ ki2 ¼ dt dt r1 ¼ k2 ða1 a12 a22 a32 Þni2 dai2 ; i ¼ 1; 2; 3 ¼ dt
(16)
Fig. 10. Experimental (-) and calculated (—) data for combustion runs in He:O2 4:1.
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And the same can be applied for reactions (13) and (33), obtaining: wRi2 wRi2 wRi2 vi2 vi3 ¼ ¼ ri2 ri2 formed ri2 reacted r1 ri2 ri3 ¼ ai2 ai3 ;
i ¼ 1; 3
(17)
ni3
dðwRi2 =ri2 Þreacted dðvi3 =ri3 Þ wRi2 ¼ ki3 ¼ dt dt ri2 dai3 ; ¼ ki3 ðai2 ai3 Þni3 ¼ dt
Scheme 2a.
i ¼ 1; 3 (18)
In order to calculate conversion factors for each stage, Eqs. (4), (16) and (18) have been used (in terms of conversion factors) in this case. The next step is to calculate vtot ¼ a1 ð1 r1 Þ þ
3 X ai2 ðr1 ri2 Þ þ a13 r13 þ a33 r33 i¼1
(19) wcalc ¼ 1 vtot (20) As in the previous part, the kinetic parameters have been optimized in order to minimize the differences between experimental and calculated weight loss data according to Eq. (10). Note that all the data of the two atmospheres and the three heating rates have been simultaneously fitted. Pyrolysis parameters have been kept and only the preexponential factor k10 and the activation energy E1 have been recalculated. As well as this, other nine parameters have been optimized: r32, n32, E32 from reaction (32), n13, E13 from reactions (13) and (33), and different k320 and k130 for runs at the two different helium:oxygen atmospheres (four pre-exponential factors altogether) to take into account the effect of partial pressure of oxygen (which equals 0.10 and 0.20 atm for helium:oxygen 9:1 and 4:1, respectively), since different behavior is observed when comparing corresponding thermograms. The pre-exponential factors for reactions with oxygen have been considered to consist of two terms, one a typical Arrhenius pre-exponential factor ki0 j0 and the other one the partial pressure of oxygen pO2 raised to the power of an order bij, as indicated in Eqs. (21) and (22): 0 k320 ¼ k320 ð pO2 Þb32 0 ki30 ¼ ki30 ð pO2 Þbi3 ;
Regarding activation energy Ei3 and pre-exponential factor 0 , the values obtained appear to be a bit low. It is ki30 interesting to observe that the yield coefficient for reaction (32) (r32) is equal to r1, what means that only char residue is formed from this reaction, as occurred with reaction (12), so Scheme 2 can be simplified to Scheme 2a. On the other hand, activation energy E1 value has decreased, what agrees with the fact that oxygen is like a promoter that enhances dehydrochlorination rate. Finally, Figs. 11 and 12 show experimental and calculated mass fraction of volatiles v and conversion factors a at 5 8C/min in He:O2 4:1. These graphs are useful to observe the evolution of the different fractions throughout the heating process. As can be observed from the results, good agreement has been obtained with a relatively simple model both in inert and oxidative atmosphere, taking into account the effect of
Fig. 11. Calculated mass fraction of volatiles (vi j ) at 5 8C/min in He:O2 4:1.
(21) i ¼ 1; 3
(22)
Thus, once k320 and ki30 have been calculated for the two 0 , b , different atmospheres, Eqs. (21) and (22) allows k320 32 0 and b to be calculated. The results obtained are given in ki30 i3 Table 3, and the experimental and calculated values of weight loss in helium:oxygen 9:1 and 4:1 are shown in Figs. 9 and 10, respectively. It seems that experimental values have been satisfactorily fitted with the calculated parameters according to the proposed model, taking into account that the six experiments have been simultaneously fitted and parameters obtained in pyrolysis of reactions (12) and (22) have been kept.
Fig. 12. Calculated conversion factor (aij) at 5 8C/min in He:O2 4:1.
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the helium:oxygen ratio with the same set of kinetic parameters. Results for the pyrolytic model are comparable to those presented in several previous papers with similar reactions for thermal decomposition of pure PVC. The combustion model has been mainly suggested on the basis of experimental behavior observed.
[7] [8] [9] [10] [11] [12] [13]
Acknowledgements Support for this work was provided by Ministerio de Educacio´ n y Ciencia of Spain and research projects PPQ2002-00567 and PPQ2002-10548-E.
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Thermo-oxidative decomposition of polyvinyl chloride Ignacio Aracil *, Rafael Font, Juan A. Conesa Departamento de Ingenierı´a Quı´mica, Universidad de Alicante, Ap. 99, 03080 Alicante, Spain Received 29 July 2004; accepted 5 August 2004 Available online 17 March 2005
Abstract Pyrolysis and combustion of polyvinyl chloride resin have been studied by thermogravimetric analysis. Nine different runs with about 5 mg sample mass have been carried out in three different atmospheres (He; He:O2 9:1; and He:O2 4:1) and heating rates (5, 10 and 20 8C/ min). Adequate kinetic simplified models have been proposed and all heating rates have been simultaneously correlated with the same set of kinetic constants, obtaining good results. The pyrolysis model consists of one first reaction producing gases and solid residue followed by two parallel reactions of previous solid, whereas the combustion model adds a third parallel reaction with oxygen and three new combustion reactions to burn char formed in the three previous reactions. Different considerations have been taken into account during the optimization to achieve the best results. Parameters obtained have been discussed and also compared with others from literature. # 2005 Elsevier B.V. All rights reserved. Keywords: TG; PVC; Kinetic model; Air; Combustion
1. Introduction Over the last few decades, many efforts have been made looking for alternative treatments to landfilling of plastic waste. Both recycling and energy recovery can be effective processes to consider, and their application will depend on different technical, economic, legal and ecological considerations. Polyvinyl chloride (PVC) is one of the most consumed plastics and has received great attention because of chlorinated combustion byproducts, which have been shown to be toxic. There are some primary applications for this thermoplastic, such as window frames, pipes, flooring, wallpaper, bottles, cable insulation, credit cards, toys or medical products [1]. According to annual studies conducted by the Association of Plastic Manufacturers in Europe (APME), total consumption of PVC in the European Community has risen from 6.9 million tonnes in 1999 to 7.9 million tonnes in 2003 [1]. In 1999 total annual PVC waste quantity was about 4.1 million tonnes in the European Community, and is predicted to increase 30% in 2010 and 80% up to 7.3 million tonnes in 2020 [2]. * Corresponding author. Tel.: +34 965 903 867; fax: +34 965 903 826. E-mail address: [email protected] (I. Aracil). 0165-2370/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jaap.2004.08.005
Thermal decomposition of PVC has been widely studied by many researchers. Some works are found concerning analysis of PVC by thermogravimetric techniques, since they provide very useful information about kinetic reaction parameters like activation energies, pre-exponential factors, reaction orders, number of processes involved, etc. Papers usually focus on mechanistic aspects and some of them have revealed the main structural changes occurring during thermal decomposition regarding initiation and propagation of both dehydrochlorination and aromatic formation [3–13]; several works face the great amount of reactions which appear to take place during the process [14–15], whereas others propose non-complicated mechanisms consisting of simplified reactions which gather a complex number of reactions [16–20]. This latter procedure has the advantage of being able to simulate rather accurately the thermal degradation behavior by using a relatively small number of equations. However, it is important to cover a wide range of operating conditions to assure the model can be applicable at different conditions. The main aim of this work has been to propose a simplified model to describe both pyrolysis and combustion of pure PVC at different atmospheres and heating rates. On the other hand, it is interesting to point out the fact that, whereas a lot of papers deal with pyrolytic decomposition of
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PVC, fewer papers have been found including the effect of oxygen in the atmosphere of the reaction when studying thermal decomposition of PVC [3,7,21–24], so the results presented here might be useful for a better knowledge of the thermal behavior of PVC inside a pyrolyzer or a combustor. Some interesting considerations have been taken into account when the optimization of selected parameters has been conducted. Finally, the results obtained have been discussed and compared with other previous works.
2. Experimental Experiments were performed on white powdery PVC resin Etinox-450 obtained by emulsion polymerization. This resin is basically used in low-viscosity plastisols. Molecular weight was measured in a previous work [18] by GPC in tetrahydrofuran, resulting 104,000 and 40,000 for Mw and Mn, respectively. The particle size distribution was determined with a Coulter LS 230 particle size meter suspending PVC solid particles in water, obtaining a distribution between 1.4 and 24.4 mm and a medium value of 6.0 mm (Fig. 1); this equipment allows particles to be measured in a range between 0.4 and 2000 mm. Besides, the chemical composition of PVC, shown in Table 1, was analysed by two complementary techniques: Elemental analysis, for C, H, S and N with a Perkin-Elmer 2400 CHN.
X-ray fluorescence, for elements with an atomic weight higher than that of magnesium. The equipment used was an automatic sequential spectrometer of X-ray fluorescence model PW1480. The TG runs were carried out in a Setaram TG-DTA 9216.18 thermobalance controlled by a PC compatible system, which records the temperature measured by a thermocouple situated under the holder. The total gas flow was 60 mL/min, consisting of helium in the pyrolysis runs and a mixture of helium and oxygen in the combustion runs. The reproducibility of the runs was tested and also the buoyancy effect was subtracted for each run by comparing with the corresponding blank runs. Some metals with known melting points (In, Sn, Pb, Al, Zn, Ag) are periodically used to calibrate the equipment temperature in the range 150–960 8C. As well as this, one experiment with Avicel cellulose was carried out at 5 8C/min in helium to test the good working of the equipment by calculating the kinetic parameters of this decomposition and checking that the results were in good agreement with those obtained in a round-robin study of Avicel cellulose pyrolysis [25]. The PVC was heated from 25 to 700 8C in every experiment. Nine runs with about 5 mg sample mass were carried out changing both the heating rate (5, 10 and 20 8C/ min) and the atmosphere (He; He:O2 9:1; and He:O2 4:1) in order to study both pyrolytic and thermo-oxidative decomposition.
3. Results and discussion Figs. 2–7 show the experimental weight loss variation with temperature obtained at different heating rates and atmospheres. Runs at different heating rates can be compared in Figs. 2–4, while the effect of the atmosphere is highlighted in Figs. 5–7. In the figures, w is defined as the residual mass fraction of the solid (including residue formed and non-reacted initial solid), i.e. the ratio between solid mass at any time (m) and initial solid mass (m0). Fig. 1. Particle size distribution of PVC Etinox-450.
Table 1 Chemical composition of PVC resin (wt.%) Element
Elemental analysis
H C O Na Al Si S Cl K Ca Zn
4.80 38.4 – – – – 1.40 – – – –
X-ray fluorescence (semiquantitative) – – 0.076 0.12 0.0057 0.0095 55.2 0.018 0.017 0.034
Fig. 2. Experimental data for pyrolysis runs in He.
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Fig. 3. Experimental data for combustion runs in He:O2 9:1.
Fig. 6. Experimental data for TG runs at 10 8C/min.
Fig. 4. Experimental data for combustion runs in He:O2 4:1.
Fig. 7. Experimental data for TG runs at 20 8C/min.
On the one hand, when the heating rate increases, the weight loss curves move to higher temperature regardless of the atmosphere, as expected. Pyrolytic decomposition is generally accepted to consist of two main steps. The first one (at about 500–650 K) is dehydrochlorination, and it is believed to be initiated primarily at unsaturated sites (allylic chlorines) or at other defect sites in the polymer chain [4,6]. It takes place to form HCl and a linear polyene structure that has also been shown to produce unsubstituted aromatics (benzene, naphthalene) through intramolecular cyclization from polyene radicals formed by chain scission [5,8,11].
Then, polyene chains can react through intermolecular reactions and these crosslinked chains undergo further reactions to form alkyl aromatic hydrocarbons and char residue. Alkyl aromatic formation appears to take place again by intramolecular cyclization but, in this case, there is H–H exchange between polyene chains [9–11]. The main interest is focused on comparing runs at different atmospheres. When pyrolytic runs are compared with those in helium:oxygen atmospheres, an interesting behavior is noted: the first step is slightly sped up in the presence of oxygen, then the second step is delayed and finally no char residue remains. This, in a simplified explanation, could indicate that oxygen enhances dehydrochlorination, then it could partially oxidize polyene chain and, in the end, char would be completely oxidized and the resulting products volatilized. According to the behavior observed, and taking into account some previous models proposed in literature which will be briefly described later, two models have been developed, one for pyrolysis and one for combustion, the latter including the effect of the presence of oxygen. 3.1. Pyrolysis model
Fig. 5. Experimental data for TG runs at 5 8C/min.
A two-stage model has been proposed (Scheme 1). It consists of a first reaction in which dehydrochlorination and
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w R1 w R1 r1 formed r1 reacted v1 v12 v22 ¼ ¼ a1 a12 a22 1 r1 r1 r12 r1 r22 The kinetic laws can be expressed as
wR1 ¼ r1 Scheme 1.
the formation of some unsubstituted aromatics take place together with the remaining polyene chain, and a second stage involving two parallel reactions which can produce both volatiles and final solid residue. Since experimental curves give different char yield at the different heating rates, these two reactions have been proposed to be competitive. In the previous scheme, S1 is the initial solid, V1 and Vi2 are the possible gases + volatiles evolved in the corresponding reactions and R1 and Ri2 are the solid residues formed in the decomposition. The procedure followed here is similar to that described in a previous paper [26]. From now on, we will work on a normalized basis, and hence we define ws1 , v1 , wR1 , vi2 and wRi2 (these latter two for both reactions (12) and (22)) which represent the corresponding kg of compounds with respect to kg of initial reactive solid S1 (mass fractions). On the other hand, each fraction has a yield coefficient that represents the maximum mass fractions obtainable by means of each reaction. It can be easily deduced that each yield coefficient for each product fraction ranges between 0 and the corresponding reactive yield coefficient (1 in the case of reaction (1) and r1 for reactions (12) and (22)). It is very important to point out that, in the definition of yield coefficient for competitive reactions, the term ‘maximum’ for one reaction refers to the highest possible yield obtained in the case in which the parallel competitive reaction was not taken into account, so r12 and (r1 r12) are the maximum yields obtained should reaction (22) not take place, and the same can be said for (r1 r22). For the above-mentioned reason, it is very useful to introduce the concept of the conversion factor for each reaction, which is defined as the ratio between the mass fraction of volatiles obtained at any time during which the reaction is taking place and the corresponding yield coefficient, so: v1 v1 ðwR1 Þformed ¼ ¼ ¼ 1 ws1 (1) v11 1 r1 r1 vi2 vi2 ai2 ¼ ¼ ; i ¼ 1; 2 (2) vi21 r1 ri2 Thus, according to the definitions previously commented, a1 ranges between 0 and 1 during the experiment, whereas both a12 and a22 cannot reach the value 1 (unless only one of the two reactions were active), but together they do add up to 1 at the end of the process at infinite time. The definitions are valid for solid residue taking into account that in Eq. (1), wR1 is not the solid residue existing at any time, but the residue formed from reaction (1) as if it did not subsequently react, as indicated. From a mass balance it can be deduced that a1 ¼
(3)
dðwR1 =r1 Þformed dðv1 =ð1 r1 ÞÞ ¼ k1 ðws1 Þn1 ¼ dt dt da1 (4) ¼ k1 ð1 a1 Þn1 ¼ dt dðwR1 =r1 Þreacted dðvi2 =ðr1 ri2 ÞÞ ¼ dt dtni2 wR1 ¼ ki2 ¼ ki2 ða1 a12 a22 Þni2 r1 dai2 ; i ¼ 1; 2 (5) ¼ dt since R1 is formed from reaction (1) and is consumed in reactions (12) and (22). In the previous equation n1, n12 and n22 are the reaction orders for reactions (1), (12) and (22), respectively, and k1, k12 and k22 are the kinetic constants for the corresponding reactions. These kinetic constants follow the Arrhenius law, so: E1 k1 ¼ k1 exp (6) RT Ei2 ki2 ¼ ki20 exp ; i ¼ 1; 2 (7) RT where k1, k120 and k220 are the pre-exponential factors for reactions (1), (12) and (22), respectively, and E1, E12 and E22 the apparent activation energies for the corresponding reactions. When setting out the equations, it has been considered more convenient to work in terms of conversion factors. The procedure performed consisted of optimizing 12 parameters (r, E, n and kinetic constants for each one of the three reactions) best fitting calculated and experimental weight loss (wcalc and wexp , respectively). Starting with initial values of conversion factors equal to 0 at initial time, these parameters have been numerically calculated along the time, and then: vtot ¼ v1 þ v12 þ v22 ¼ a1 ð1 r1 Þ þ
2 X
ai2 ðr1 ri2 Þ (8)
i¼1
wcalc ¼ 1 vtot
(9)
The objective function (OF) to minimize was the sum of the square differences between experimental and calculated weight loss values (Eq. (10)). All the thermograms obtained at different heating rates have been simultaneously correlated with the same set of kinetic parameters. OF ¼
3 X N X
exp
2 ðwm j wcalc mj Þ ;
m¼1 j¼1
m heating rates; j points
(10)
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The model validity has been tested calculating the variation coefficient (VC): pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi OF=ðN PÞ VC ¼ 100 (11) wexp where N and P are the number of data and parameters fitted, respectively, and wexp is the average of the experimental weights. According to the procedure suggested by Martı´nGullo´ n et al. [27], the great interrelation existing among the pre-exponential factor, the apparent activation energy and the reactor order can be decreased when optimization is performed in terms of a ‘comparable kinetic constant’ Ki defined in Eq. (12), instead of optimizing k0i. This constant is calculated in a temperature around the maximum decomposition rate (Tmax). Since Ki , Ei and ni are optimized, the pre-exponential factor k0i is readily calculated from Eq. (12): Ei Ki ¼ ki ð0:64Þni ¼ k0i exp ð0:64Þni RTmax
(12)
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The results obtained are shown in Table 2. There are some interesting results to comment. On the one hand, r12 equals r1, whereas r22 equals 0. This means that each competitive reaction only gives one kind of compounds (char residue in reaction (12) and volatiles in reaction (22)), so Scheme 1 can be simplified as Scheme 1a. On the other hand, according to the previous results, the activation energy E12 corresponding to char formation reaction is lower than the activation energy E22 of volatile formation reaction, what agrees with the fact that the lower the heating rate the more char is formed (see Fig. 5). Experimental and calculated values are given in Fig. 8, showing a good agreement. There are some other papers that have studied other kinetic models for pure PVC with some similarities, and below some of them are schematized as appearing in the original papers. Calculated activation energies for each reaction (in kJ/mol) are shown above the corresponding arrows:
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Marcilla and Beltra´ n [18] proposed two different kinetic models with an intermediate polyene chain in the second model according to that suggested by Montaudo and Puglisi [11]. The last evolution of volatiles and char was put into one single reaction instead of considering two competitive reactions because no differences in char yield were observed at different heating rates (5, 10, 25 and 50 8C/min). In the first model, which assumed a sole reaction for the first step, the results were 146.4 kJ/mol, 3.52 1012 min1 and 1.32 for E1, k10 and n1, respectively. Anthony [19] suggested a similar model to Marcilla and Beltra´ n’s second model but proposing that the intermediate stage consisted of two competitive reactions to produce benzene and volatiles, based on the work of Montaudo and Puglisi [11]. Despite the model is general, it was only validated for one heating rate in the paper (20 8C/min), so it is not possible to compare at different heating rates. On the other hand, Wu et al. [17] introduced two initial parallel reactions concerning formation of volatiles and intermediate from two different H–T and H–H polyene chain configurations and two final competitive reactions to give char and more volatiles. These latter reactions are equivalent to reactions (12) and (22) of the present work, and the results Table 2 Kinetic parameters obtained for pyrolysis model (kij0 in s1, Eij in kJ/mol) 0.372 0.372 0 1.51 1.71 2.59 140.2 188.6 289.7 3.71 1010 5.97 1010 4.03 1018 0.0916 1.545
r1 r12 r22 n1 n12 n22 E1 E12 E22 k10 k120 k220 OF VC (%)
Scheme 1a.
Fig. 8. Experimental (-) and calculated (—) data for pyrolysis runs in He.
fitted for three heating rates (1, 2 and 5.5 8C/min) were 218 kJ/mol, 5.6 1012 s1 and 1.5 for E12, k120 and n12, respectively, and 267 kJ/mol, 4.8 1016 s1 and 1.5 for E22, k220 and n22, respectively. Activation energies are similar to those obtained in the present work, but pre-exponential factors are two magnitude orders higher and lower for reactions (12) and (22), respectively. Differences could be due to the previously commented great interrelation between the three parameters E, k0 and n. In order to prove this, we tried to fit the model keeping reaction orders equal to 1. The results showed good agreement between calculated and experimental values and, however, differences of some magnitude orders in pre-exponential factors with respect to the values given in Table 2, but no great differences in activation energies, what means that it is difficult to assure that the calculated values obtained by one method are likely to be the best ones. Anyway, it must not be forgotten that all kind of models are correlation models. Of course, another source of discrepancy may be the differences of PVC resin properties. It could be questioned that the model proposed in this work is much too simplified, and this is probably true, but the main aim of the pyrolytic model was to be taken as a starting point to develop a kinetic model for combustion. 3.2. Oxidative pyrolysis and combustion model Scheme 2 presents the three-stage model proposed to explain the behavior obtained at two different helium:oxygen atmospheres and three heating rates.
Scheme 2.
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Table 3 Kinetic parameters obtained for combustion model (kij0 in s1, ki0 j0 in s1 atmbij and Eij in kJ/mol) r1 r12 r22 r32 r13, r33 n1 n12 n22 n32 n13, n33 b32 E1 E12 E22 E32 E13, E33 k10 k120 k220 0 k320 0 , k0 k130 330 b13, b33 OF VC (%)
0.372 0.372 0 0.372 0.372 1.51 1.71 2.59 1.69 0.72 0.76 128.3 188.6 289.7 276.1 90.6 4.00 109 5.97 1010 4.03 1018 2.77 1018 6.51 103 0.52 0.343 2.229
It has been assumed that the structure of reactions (12) and (22) is that obtained in the previous part. Now, the model introduces a third competitive reaction (32) in the second step to account for the delay in weight loss with respect to pyrolysis, allowing the polyene to give both char residue (R32) and volatiles (V32) by oxidation of polyene chain. Besides this, the non-existence of final char involves that some oxidation to give volatile products or gases (CO, CO2, H2O) occurred from char produced in reactions (12) and (32), and this is taken into account in reactions (13) and (33). It has also been considered that the term V in reactions (32), (13) and (33) includes the volatilized mass of solid residue, which is different from the total gas + volatiles produced, since the latter includes oxygen contribution. In order to simplify the model, it has been assumed that reactions (13) and (33) have the same kinetic parameters. Finally, the first step has been considered to be accelerated by the presence of oxygen (only as an enhancer of the decomposition rate). The papers studying the effect of oxygen in thermal degradation also showed that the HCl generation rate increased when compared to inert atmospheres, probably due to the formation of peroxide radicals [3,7,21–23]. Nagy et al. [24] developed a simplified model for the thermo-oxidative decomposition of PVC; the primary initiation step was identical to that of the purely thermal decomposition and then, rapid dehydrochlorination followed resulting in the formation of polyenes which reacted readily with oxygen even at approximately 100 8C to form peroxides and hydroperoxides. The definitions of the new yield coefficients can be easily deduced taking into account the explanations given in the
Fig. 9. Experimental (-) and calculated (—) data for combustion runs in He:O2 9:1.
previous part. Of course, coefficients r13 and r33 are equal to r12 and r32, respectively. New conversion factors corresponding to the new reactions are now defined: v32 v32 a32 ¼ ¼ (13) v321 r1 r32 ai3 ¼
vi3 vi3 ¼ ; vi31 ri3
i ¼ 1; 3
(14)
Following a deduction similar to that shown in Eqs. (3)– (5), the resulting equations for compounds involved in the second stage (reactions (12), (22) and (32)) are: w R1 wR1 wR1 ¼ r1 r1 formed r1 reacted v1 v12 v22 v32 ¼ 1 r1 r1 r12 r1 r22 r1 r32 ¼ a1 a12 a22 a32
(15)
ni2 dðwR1 =r1 Þreacted dðvi2 =ðr1 ri2 ÞÞ wR1 ¼ ki2 ¼ dt dt r1 ¼ k2 ða1 a12 a22 a32 Þni2 dai2 ; i ¼ 1; 2; 3 ¼ dt
(16)
Fig. 10. Experimental (-) and calculated (—) data for combustion runs in He:O2 4:1.
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And the same can be applied for reactions (13) and (33), obtaining: wRi2 wRi2 wRi2 vi2 vi3 ¼ ¼ ri2 ri2 formed ri2 reacted r1 ri2 ri3 ¼ ai2 ai3 ;
i ¼ 1; 3
(17)
ni3
dðwRi2 =ri2 Þreacted dðvi3 =ri3 Þ wRi2 ¼ ki3 ¼ dt dt ri2 dai3 ; ¼ ki3 ðai2 ai3 Þni3 ¼ dt
Scheme 2a.
i ¼ 1; 3 (18)
In order to calculate conversion factors for each stage, Eqs. (4), (16) and (18) have been used (in terms of conversion factors) in this case. The next step is to calculate vtot ¼ a1 ð1 r1 Þ þ
3 X ai2 ðr1 ri2 Þ þ a13 r13 þ a33 r33 i¼1
(19) wcalc ¼ 1 vtot (20) As in the previous part, the kinetic parameters have been optimized in order to minimize the differences between experimental and calculated weight loss data according to Eq. (10). Note that all the data of the two atmospheres and the three heating rates have been simultaneously fitted. Pyrolysis parameters have been kept and only the preexponential factor k10 and the activation energy E1 have been recalculated. As well as this, other nine parameters have been optimized: r32, n32, E32 from reaction (32), n13, E13 from reactions (13) and (33), and different k320 and k130 for runs at the two different helium:oxygen atmospheres (four pre-exponential factors altogether) to take into account the effect of partial pressure of oxygen (which equals 0.10 and 0.20 atm for helium:oxygen 9:1 and 4:1, respectively), since different behavior is observed when comparing corresponding thermograms. The pre-exponential factors for reactions with oxygen have been considered to consist of two terms, one a typical Arrhenius pre-exponential factor ki0 j0 and the other one the partial pressure of oxygen pO2 raised to the power of an order bij, as indicated in Eqs. (21) and (22): 0 k320 ¼ k320 ð pO2 Þb32 0 ki30 ¼ ki30 ð pO2 Þbi3 ;
Regarding activation energy Ei3 and pre-exponential factor 0 , the values obtained appear to be a bit low. It is ki30 interesting to observe that the yield coefficient for reaction (32) (r32) is equal to r1, what means that only char residue is formed from this reaction, as occurred with reaction (12), so Scheme 2 can be simplified to Scheme 2a. On the other hand, activation energy E1 value has decreased, what agrees with the fact that oxygen is like a promoter that enhances dehydrochlorination rate. Finally, Figs. 11 and 12 show experimental and calculated mass fraction of volatiles v and conversion factors a at 5 8C/min in He:O2 4:1. These graphs are useful to observe the evolution of the different fractions throughout the heating process. As can be observed from the results, good agreement has been obtained with a relatively simple model both in inert and oxidative atmosphere, taking into account the effect of
Fig. 11. Calculated mass fraction of volatiles (vi j ) at 5 8C/min in He:O2 4:1.
(21) i ¼ 1; 3
(22)
Thus, once k320 and ki30 have been calculated for the two 0 , b , different atmospheres, Eqs. (21) and (22) allows k320 32 0 and b to be calculated. The results obtained are given in ki30 i3 Table 3, and the experimental and calculated values of weight loss in helium:oxygen 9:1 and 4:1 are shown in Figs. 9 and 10, respectively. It seems that experimental values have been satisfactorily fitted with the calculated parameters according to the proposed model, taking into account that the six experiments have been simultaneously fitted and parameters obtained in pyrolysis of reactions (12) and (22) have been kept.
Fig. 12. Calculated conversion factor (aij) at 5 8C/min in He:O2 4:1.
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the helium:oxygen ratio with the same set of kinetic parameters. Results for the pyrolytic model are comparable to those presented in several previous papers with similar reactions for thermal decomposition of pure PVC. The combustion model has been mainly suggested on the basis of experimental behavior observed.
[7] [8] [9] [10] [11] [12] [13]
Acknowledgements Support for this work was provided by Ministerio de Educacio´ n y Ciencia of Spain and research projects PPQ2002-00567 and PPQ2002-10548-E.
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