Kinetic Model & Analysis for Pyrolysis of Waste Polystyrene Over Laumontite

Kinetic Model & Analysis for Pyrolysis of Waste Polystyrene Over Laumontite

Krishna Kant Kumar Singh, Dr. S.P.Singh

Department of Chemical Engineering, BIT, Sindri, Jharkhand, 828123

Abstract

Laumontite(NZ) has been studied as a catalyst in the degradation (Catalytic Thermolysis) of polystyrene (PS) at 400oC in a semi batch reactor. Its performance on the degradation was compared with HZSM-5, silica-alumina and also with thermal degradation (thermolysis). HNZ was as effective as HZSM-5 for the production of liquid oils with carbon numbers of C5-C12, without any severe deactivation. The main product of PS degradation was styrene for both thermolysis and catalytic thermolysis. Among the catalysts tested, silica-alumina showed the highest yield of ethylbenzene and the lowest one of styrene due to its mesopores. Increase in degradation temperature for HNZ resulted in a decrease of selectivity towards ethylbenzene and propylbenzene, but an increase of styrene selectivity with lower yield of styrene dimers. A kinetic analysis by thermo gravimetric analysis was also carried out using a dynamic model. It offeredreliable kinetic parameters for the catalytic degradation of PS over HNZ. The apparent activation energy was found to be 360kJ/mol, and the catalytic degradation of PS followed rst-order kinetics.

Keywords:Polystyrene;Catalyticthermolysis;Laumontite; TGA

1. Introduction

   Though various kinds of techniques have been proposed for the conversion of waste plastics, it is generally accepted that material recovery is not a longterm solution to the present problem, and that energy or chemical recovery is a more attractive one. Consequently, new technologies are being investigated for the chemical recycling of plastic wastes. One approach is to employ inert gas thermolysis to produce gasoline-like materials. In this method, the waste plastics are thermally or catalytically degraded into gases and oils, which can be used as sources in fuels or chemicals [1].

   Thermal degradation (thermolysis) of waste plastics into fuel oils was studied extensively, but the oils obtained showed a wide-ranged distribution of carbon atom numbers and contained a signicant fraction of olens [2 4]. The olens are not favorable for fuel oils because these are easily polymerized into unusable compounds during storage and transportation. In contrast, the oils produced by catalytic degradation (catalytic thermolysis) are known to contain a relatively narrow distribution of hydrocarbons, lower amount of olens and higher amount of aromatics compared to the oils from the thermal degradation. An excellent summary of the catalytic recycling of polymers was reported by Uemichi [2].

   Tomar S.S. et al., reported maximum yield of liquid when polymer catalyst ratio was 4:1 and after this ratio the liquid yield decreases. The degradation of waste plastic was over two commercial grade cracking catalysts, HZY

   (20) & HZY (40) in a semi-batch reactor [3]. The most commonly used catalysts in the catalytic degradation of polymers are solid acids and bases [49].

   Plastic wastes for the catalytic degradation processes are mainly limited to polyolenic wastes (polyethylene and polypropylene) and polystyrene (PS). In contrast to polyethylene and polypropylene, PS can be thermally depolymerized to obtain the monomer styrene with a high selectivity. Zhang et al., [10] Obtaineda styrene yield of 70 wt. % by PS degradation at 350oC using a semi-batch

   reactor with a continuous ow of nitrogen. On the contrary, Audisio et al. [11] reported very low selectivity (< 5 wt. %) to the styrene in PS degradation with solid acids such as silica-alumina and HY or REY zeolites at 350oC. The main products in their study were benzene, ethylbenzene and cumene. Ukei et al. [7] reported that

   solid bases, especially BaO, were more effective catalysts than solid acids (HSM-5 and silica-alumina) for the degradation of PS to styrene monomer and dimer at 350oC.

   During the last two decades, thermogravimetric analysis (TGA) has been continuously used to understand the reaction mechanism of thermal degradation of polymers [1214]. The degradation studies using TGA can offer valuable informations on the activation energy, the overall reaction order and the pre-exponential factor. Park et al., developed new mathematical methods for the kinetic analysis and reported a kinetic analysis of thermal degradation of polymer using a dynamic model, and compared this model to other methods. However, most of the previous works are related to thermal degradation of

   polyethylene and polypropylene. There are only very few works are reported on the degradation of PS. The dynamic method has not get been applied for the catalytic degradation of PS.

   Hwang EY et al.,reported that the natural clinoptilolite zeolite was good catalyst for the degradation of poly- propylene [5] and polyethylene [16]. The purpose of this study is to evaluate the performance of the natural zeolite (Laumontite) in the catalytic degradation of PS. The emphasis of this study is also placed on the kinetic analysis of catalytic degradation of PS. A dynamic model

   [15] was applied to predict the catalytic degradation of PS over NZ by TGA method.

2. Experimental(Materials and catalysts) PS, in granular form, was collected from local market (Melt flow Index=7.5 g/10 min, density=1.03 g/cm3). The PS samples of 60-150 mesh size were used for this study. Several types of solid acid catalysts such as silica-alumina (Loba Chemie, India), ZSM-5 (Himedia Lab.) and Laumontite(NZ)was collected from local market. ZSM-5 and NZ were ion-exchanged three consecutive times with 1 M NH4Cl solution for 20 h. The zeolites exchanged with NH+ were dried at 110oC for 6 h, and then calcined in air at 400oCto obtain the proton (H+)exchanged zeolites like HZSM-5 and HNZ.

3. Apparatus and procedure

   The catalytic thermolysis of PS was carried out in a semi- batch reactor.The thermolysis setup used in this experiment is shown in Figure 1. It consists of a semi batch reactor (steel made) of volume 2 liters, vacuum- packed with two outlet tubes towards vacuum pump and condenser. Vacuum pump with a gage is attached to the reactor so as carryout the reaction in vacuum. The condenser is attached to collect condensed Liquid hydrocarbons and gases separately. The reactor is heated externally by an electric furnace, with the temperature being measured by a Cr-Al: K type thermocouple fixed inside the reactor, and temperature is controlled by an external PID controller. 100 gm of waste plastics sample were loaded in each thermolysis reaction. The condensable liquid products was collected through the condenser and weighed. After thermolysis, the solid residue left inside the reactor was weighed. Then the weight of gaseous/volatile product was calculated from the material balance. The uncondensed gases were separated out in a bladder from condenser. Apparatus setup and samples for testing are shown in Fig. 1(a, b).

   Figure 1(a) Apparatus setup

   The kinetics of PS degradation at non-isothermal conditions has been investigated using TGA at various heating rates between 10 and 50oC/min. The initial weight of PS sample was 100 gm and that of HNZ catalyst was 10 gm. The experiments were carried out in vacuum.

   1. Analysis

      The bulk structure of the NZ was conrmed by XRD analysis with Cu-Ka radiation. The composition of the NZs was determined by atomic absorption spectroscopy (Perkin-Elmer AAS 5100). The acidic properties of the catalysts were determined by a conventional temperature programmed desorpion experiment of ammonia in the temperature range of 100 to 700oC at a constant heating rate of 5 oC/min. The specic surface area and pore size of the catalysts were measured by a BET apparatus.

      The degradation of the PS gave o gases, liquids and residues. The residue means the carbonaceous compounds remaining in the reactor and deposited on the wall of the reactor. The amount of coke deposit on the catalyst was calculated by measuring the desorbed amount of carbon dioxide during temperature programmed oxidation of used catalysts. The gases were analyzed by GC. The condensed liquid samples were analyzed by GC-MS. The physical properties and the composition of the liquids were also measured.

      Figure 1(b) Samples for testing

   2. Kinetic analysis

      In the kinetics of polymer degradation using TGA, it is very common to use the basic rate equation of conversion for thermal degradation in inert atmosphere [15] as

      = (1 ) exp (1)

      whereA is pre-exponential factor (l/ min), E is the activation energy (J/mol), Tis the temperature of reaction

      (K) and Ris the gas constant (8.314 J/mol K), and n

      denotes the overall reaction order. However, A is not

      2 11 1

      =

      =

      2 2 2

      2 2 2

      (8)

      strictly constant but depends, according to collision theory [17], on T0.5. Therefore, if the basic Eq. (1) is taken and a heating rate = dT/dt(K/min) is employed, it can be

      = 1

      0 2(1)

      (9)

      shown that

      = 0 = 1/2 exp(/)(1 ) (2)

      If the temperature rises at a constant heating rate , and the kinetic parameters at any weight loss fraction are

      If the factor A0 is determined, the n and E values at any weight loss fraction can be obtained from Eqs. (8) and (9) by numerical methods. The average reaction order and activation energy can be calculated from Eqs.(10) and

      (11) as the followings:

      approximately equal to those of its neighboring weight loss fraction, then by dierentiation of Eq. (2), Eq. (3) can be obtained.

      =

      =1

      1

      1

      (10)

      2

      1

      1

      1 1

      =

      =1

      (11)

      2

      2

      2 = (1 )

      + +

      2

      (3)

      The maximum degradation rate occurs when d2/dT2is zero. Therefore, Eq. (3) at maximum rate gives

      wheref is the nal weight loss fraction and N denotes the total number of TG data.

      0 1

      1

4. Results and discussion

   =

   2 1

   Table 1
   Catalyst	Structure	Pore size (Ã… )	Si/Al ratio	SBET (m 2/g)
   HNZa	Laumontite	7.6 X 30 3.3 X 4.6	4	271
   SA	Amorphous	60-100	13% Al2O3	504
   HZSM-5	MFI	5.3X5.6 5.1X5.5	22	417
   a Relative weight ratio of other metal components based on Al is as follows; Si=4.0, Na=0.21, K=0.08, Ca=0.02, Ti=0.01, Fe=0.03.

   Table 1
   Catalyst	Structure	Pore size (Ã… )	Si/Al ratio	SBET (m 2/g)
   HNZa	Laumontite	7.6 X 30 3.3 X 4.6	4	271
   SA	Amorphous	60-100	13% Al2O3	504
   HZSM-5	MFI	5.3X5.6 5.1X5.5	22	417
   a Relative weight ratio of other metal components based on Al is as follows; Si=4.0, Na=0.21, K=0.08, Ca=0.02, Ti=0.01, Fe=0.03.

   + 1 1 (4)

   4.1 Characterization of catalysts

   2

   2

   whereTmax(maximum peak temperature) and max(weight loss fraction at the peak temperature). Using Murray and Whites expression [18],integration of Eq. (2) results in,

   1 1

   1 1 1 1

   = 0 5/2 1 5 × (5)

   The main physicochemical properties of the catalysts used

   2

   in the PS degradation are shown in Table 1. The HNZ is a

   If Eq. (4) is combined with Eq. (5), the following result is obtained:

   silicarich member of the Laumontite family [20]. Its porestructure is characterized by two main channels parallel to the c-direction, one formed by a 10-member

   (1

   )1 = 1 1 + 1(6)

   2

   ring (7.6 X 3.0 Ã…) and the other by an eight-member ring(3.3~4.6 Ã…). BET analysis on pore volume distributionin Fig. 2 shows that most of pores in HNZ are

   Eq. (6) does not contain the heating rate except as Tmaxvaries with heating rate. The product n(1-max)n-1 is not only independent of , but is very nearly equal to unity. By substituting this value in Eq. (4) and taking the natural logarithm, one obtains:

   micro- pores of about 5 Ã…, and it has practically no mesopores. The total pore volume of pores between 4.5 to

   9.1 Ã… is0.11 cm3/g. Silica-alumina has a wide-range mesopores with surface area of 504 m2/g. HZSM-5 has Si/Al ratio of 22 and surface area of 417 m2/g.

   = 0 + 2 + 1

   (7)

   3 2

   The plot of lnversus 1/Tmax should give a straight line with the slope-E/R giving the activation energy E atmaximum rate, and A0 can be calculated from the activation energy E and the intercept on the Y axis [19].

   Eqs. (2) and (3) give the following expressions for nand E.

   Figure 2.Cumulative pore volume distribution of HNZ.

   4.2 Degradation of PS

   PS, alone or together with catalysts (HNZ, HZSM-5, SA) in the reactor, was degraded at 400oC for 2 h. Table 2 lists the gases, liquids, residues and cokes on the catalyst from the degradation experiment. The amount of gases products was calculated by subtracting the sum of weights for liquids, residues and catalyst with coke, from the total weight of PS sample and fresh catalyst initially loaded to the reactor. Carbonaceous compounds adhering to the reactor wall were dissolved inn-hexane and were measured as degradation residues. In all cases, the liquid oils were main products. The HNZ showed a comparable performance with SA inproducing liquid oils for the degradation of PS without producing much residues and cokes.

   HZSM-5 catalyst has a MFI structure with intersecting

   Table 2
   Catalyst	Contents of products (wt.%)
   	Gases	Liquids	Residues	Cokes
   Thermal	8.5	81.7	9.8
   HNZ	9.7	81.5	4.9	3.9
   HZSM-5	14.7	75.1	6.3	3.9
   SA	7.7	83.5	4.8	4.0
   Product distribution in the degradation of PS at 400oC for 2 h

   Table 2
   Catalyst	Contents of products (wt.%)
   	Gases	Liquids	Resides	Cokes
   Thermal	8.5	81.7	9.8
   HNZ	9.7	81.5	4.9	3.9
   HZSM-5	14.7	75.1	6.3	3.9
   SA	7.7	83.5	4.8	4.0
   Product distribution in the degradation of PS at 400oC for 2 h

   5.3 X 5.6 and 5.1 X 5.5 Ã… channels [21]. Therefore, the initially cracked fragments can diuse through the pores of HZSM-5 and react further in the cavities created at the intersection of two channels, yielding more gaseous products.

   The distributions of liquid products by carbon atom number obtained in both thermolysis and catalytic thermolysis are shown in Fig. 3. The main products are C8 hydrocarbons, with C9 and C7. The hydrocarbons of C14 or higher, mainly dimers of styrene and small amount of trimers, are 14-15 wt.% for thermal degradation and for HNZ. However, those for HZSM-5 and SA are 27-28 wt.%. These results are dierent from the ones obtained in the catalytic thermolisis of polyethylene or polypropylene, where the use of HNZ, HZSM-5 and SA catalysts shows no formation of hydrocarbons for C14 or higher [5, 16].

   Figure3. Product distribution of liquid formed in the degradation of PS over H-form catalysts and SA at 400oC.

   Table 3 shows the distribution of liquid products byhydrocarbon type. Both thermolysis and catalytic thermolysis showed only aromatic compounds, since they arestable enough not to be further cracked or hydrogenated to parans or olens. Only silica-alumina shows very small amount of isoparans. The main products, aromatic hydrocarbons, are identied in detail as shown in Table 4. Thermal degradation shows the highest amount of styrene (70.1 wt.%) and the lowest amount of ethylbenzene (8.8 wt.%). Thermolysis degradation of PS starts with a random initiation to form polymer radicals [22], the main products being styrene and its corresponding dimers and trimers.

   Table 3
   Catalyst	Distribution of liquid (wt.%)
   	n- Paraffins	iso- Paraffins	Olefins	Naphthenes	Aromatics
   Thermal	0.01	0.27	0.24	0.27	99.21
   HNZ	0.04	0.52	0.09	0.22	99.14
   ZSM-5	0.02	0.2	0.13	0.22	99.42
   SA	0.15	1.63	0.6	0.79	96.83
   Distribution of liquid product in the degradation of PS at 400oC for 2 h

   The catalytic degradation over amorphous SA exhibits the lowest amount of styrene (48.2 wt.%) with quite a big increase in the selectivity towards ethylbenzene(30.2 wt.%) and benzene (2.5 wt.%) compared to the degraded aromatic products for HNZ (styrene=60.1 wt.%, ethylbenzene=16.0 wt.%) and HZSM-5 (styrene=62.6 wt.%, ethylbenzene=13.0 wt.%). The same phenomena on the distribution of styrene and ethyl-benzene are also reported by Serrano et al. [23] for the catalytic degradation of PS over SA and HZSM-5. They obtained 16 wt.% of styrene for SA and 50 wt.% of styrene for HZSM-5 after the PS degradation at 375oC for 30 min.

   Table 4
   Aromatics	Thermal	HNZ	HZSM-5	SA
   Benzene	0.06	0.24	1.25	2.45
   Toluene	7.73	7.93	7.85	6.34
   Ethylbenzene	8.79	15.97	12.96	30.19
   Xylene	0.02	0.03	0.17	0.02
   Styrene	70.08	60.08	62.6	48.21
   Isopropylbenzene	2.11	3.45	2.58	0.06
   -Methylstyrene	10.57	11.45	10.48	10.09
   Trimethylbenzene	0.34	0.04	0.42	0.57
   Indane	0.02	0.01	0.15	0.1
   Indene	0.02	0.06	0.23	0.14
   Butylbenzene	0.02	0.36	0.01	0.3
   Methylpropylbenzene	0.09	0.2	0.07	1.09
   Diethylbenzene	0.02	0.04	0.08	0.07
   Dimethylethylbenzene	0.02	0.02	0.26	0.08
   Tetramethylbenzene	0.04	0.02	0.39	0.06
   Methylindane	0.01	0	0.04	0
   Naphthalene	0.04	0.03	0.43	0.05
   Methylnaphthalene	0	0.08	0.03	0.22
   Composition of aromatic compounds (wt.%) formed in the degradation of polystyrene at 400oC for 2 h

   The acid catalyzed cracking of PS is of carbenium nature [24]. The most likely reaction pathway involvesthe attack of proton associated with a Brönsted acid site to the aromatic rings of PS, due to the reactivity of its side phenyl groups towards electrophilic reagents. The resulting carbenium may undergo -scission followed by a hydrogen transfer. The possible production pathways of benzene, styrene,methlystyrene, toluene, ethylbenzene, isopropylbenzene and indane derivaties are reported by Audisio et al. [11].

   Alternatively, the protonated polymer backbone may proceed through cross-linking reactions among adjacent polymeric chains or even inside the same polymer [25]. Serrano et al. [23] reported that the cross-linking reactions were promoted by the strong Brönsted acid site of HZSM- 5, and therefore it showed very low conversion in the cracking of PS at low temperature (below 375oC). They explained the low activity of silica-alumina for the cross- liking reactions and its high activity for the cracking reactions by its medium acid strength distribution with a large amount of Lewis acid site [26] and by its mesopores.

   Benzene and ethylbenzene can also be formed by further cracking and hydrogenation of the product styrene. Therefore, their highest amounts for SA in Table 4 may be due to these reactions in mesopores of SA where styrene molecule can easily enter inside the mesopores. However, HNZ and HZSM-5 having micropores will have less opportunity for these reactions.

   Figure4. Effect of temperature on the distribution of liquid formed over HNZ.

   The eect of temperature on the PS degradation was studied using HNZ catalysts over 350-425oC for 2 h.Table 5 shows the product distribution. At low temperatures the amount of residues for HNZ catalyst was high, 78.2 and

   26.3 wt.% for 350 and 375oC, respectively. But at higher

   temperatures their amounts wee very small. This indicates that the competitive cross-linking reactions take place rst, especially at low temperatures; therefore the cracking of the resulting cross-linked polymer becomes harder. Even though the amount of liquid products varies much with temperature, the distributions of the liquids are compared to understand the degradation mechanism. Fig.

   4 showsthe distributions of liquid products by carbon atom number. The amount of C8 hydrocarbon increases and that of C14 and higher decreases withtemperature by the increase of degradation rate.

   Table 5
   Temp.	Contents of products (wt. %)
   	Gases	Liquid	Residues	Cokes
   350	5.3	9	78.2	7.5
   375	7.2	61.4	26.3	5.1
   400	9.7	81.5	4.9	3.9
   425	9.4	84.4	3	3.2
   Effect of temperature on the product distribution of PS degradation with HNZ for 2 h.

   The distribution of aromatic hydrocarbons is shown

   inTable 6. At 350oC indane is produced with anappreciable amount. It is interesting that the highest selectivity towards styrene is obtained at 425oC, with lowest selectivity to ethylbenzene and propylbenzene. Ethyl-benzene and propylbenzene can be produced by inter-molecular hydride transfer of the corresponding polymer ion formed by -scission of C-C bond in PS [11]. If the hydrogenation of the product styrene, faster at high temperature, occurs mainly to produce ethyl-benzene, the temperature dependence of the amount of ethylbenzene and styrene in Table 6 will be inversed. Therefore, it may be concluded that the further cracking or hydrogenation of styrene are very hard to occur in HNZ having micropores. For the competitive pathways for styrene and ethylbenzene (or prophylbenzene) productions, the latter seems favorable at lower temperature and the former at high temperature.

   Table 6
   Aromatics	350 OC	375 OC	425OC	475 OC
   Benzene	0.56	0.41	0.24	0.44
   Toluene	2.74	5.48	7.93	8.34
   Ethylbenzene	17.19	16.68	15.97	12.68
   Xylene	0.05	0.03	0.03	0.01
   Styrene	48.48	59.82	60.08	67.81
   Isopropylbenzene	9.73	6.41	3.45	0.02
   a-Methylstyrene	6.78	8.3	11.45	9.6
   Trimethylbenzene	0.43	0.21	0.04	0.05
   Indane	3.7	1.23	0.01	0.01
   Indene	0.2	0.12	0.06	0.08
   Butylbenzene	0.02	0.23	0.36	0.42
   Methylpropylbenzene	0.23	0.22	0.2	0.05
   Diethylbenzene	0.34	0.26	0.04	0
   Dimethylethylbenzene	7.66	1.34	0.02	0
   Tetramethylbenzene	0.12	0.04	0.02	0
   Methylindane	0.4	0.12	0	0.03
   Naphthalene	0.86	0.1	0.03	0.03
   Methylnaphthalene	0.14	0	0	0.07
   Composition of aromatics (wt.%) formed in the degradation PS at different temperatures

   4.3 Kinetic studies on the PS degradation

   The kinetic studies on the catalytic PS degradation over HNZ were carried out by TGA. Fig. 6 shows a typical TG curves at various heating rates. The symbols in this gures represent some of calculated values. It was found that the curves were displaced to higher temperatures due to the heat transfer lag and shorter contact time with increasing heating rate. Fig. 6 shows the plot of ln vs. 1/Tmax. Form the straight line the activation energy E at maximum degradation rate, and thus A0 can be obtained at each heating rate from the intercept in Eq. (7). Then these A0 values are substituted to Eq. (9) for the calculation of E and reaction order n by numerical method to solve Eq. (8)

   and (9). The calculated average values of nave and Eave are summarized in Table 7. However, average values of reaction order and activation energy are almost the same for all the dierent heating rates. The calculated values of weight loss fraction by using 4thorder Runge-Kutta numerical integration are also presented in Fig. 6. It can be seen that the computed values agree very well with the TG data. From this kinetic study, the degradationof PS follows rst-order reaction and apparent activation energy for the catalytic degradation of PS with HNZ is about 360kJ/mol. In a separate experiment of thermal degradation of PS with TGA, we obtained the apparent activation energy of about380kJ/mol, higher than thatof catalytic degradation. Audisiol et al. [11] also reported the decrease of activation energy in the catalytic degradation of PS using Y zeolite and silica-alumina under vacuum. They used Freeman and Carrols method [28] for the calculation of the apparent activation energy.

   Figure5. Comparison of TG curves of experimental and calculated data (symbols) for PS degradation with HNZ.

   Figure 6.Plot of ln versus 1/Tmax for the determination of pre-exponential factor A0 in the PS degradation with HNZ.

   Table 7
   Heating rate, (K/min)	Reaction order,	Activation energy,
   10	1.01	359.0
   20	1.01	360.8
   30	1.00	361.3
   50	1.02	361.0
   Reaction order (n) and activation energy (E) for the degradation of polystyrene with HNZ

5. Conclusion

The natural Laumontite zeolite (HNZ) showed good catalytic performance for the catalytic degradation of PS, without much formation of residues or cokes at 400oC for

2 h. All the catalysts tested (HNZ, HZSM-5, SA) produced aromatic liquid oils with over 99% of selectivity. While styrene is the major product in both thermolysis and catalytic thermolysis over the solid acid catalysts, signicant dierences are observed in the aromatic products distribution. HNZ and HZSM-5 showed a decrease of styrene and an increased selectivity towards ethylbenzene and propylbenzene compared to thermal degradation.Amorphous Silica-alumina with ranges of mesopores showed the lowest amount of styrene and a big increase of ethylbenzene. Temperature increase shifted the carbon member istribution to lighter hydrocarbons. An increase of styrene monomer and a decrease of ethylbenzene and propylbenzene are observed for thePS degradation with HNZ. The kinetic analysis by TGAusing a dynamic model explained well the catalytic thermolysis of PS over HNZ. The apparent activationfor the reaction was 360kJ/mol, and the apparent reaction order was close to one.

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