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Study of the gases obtained in thermal and catalytic flash pyrolysis of HDPE in a fluidized bed reactor
J. Anal. Appl. Pyrolysis 73 (2005) 314–322 www.elsevier.com/locate/jaap
Study of the gases obtained in thermal and catalytic flash pyrolysis of HDPE in a fluidized bed reactor ´ ngela N. Garcı´a, Antonio Marcilla * Maria del Remedio Herna´ndez, A Department of Chemical Engineering, University of Alicante, P.O. Box 99, Alicante 03690, Spain Received 27 September 2004; accepted 8 March 2005 Available online 12 April 2005
Abstract A fluidized bed reactor was used to study the production of gases from the thermal and catalytic pyrolysis of high-density polyethylene (HDPE). The effect of the bed temperature and type of catalyst was evaluated. The range of operating temperature was from 400 to 800 8C. The catalysts used in this study were HZSM-5 and HUSY in a proportion of 20% by weight. The analysis of the volatile compounds showed that the main products obtained with both catalysts were similar, but the amount of gases produced with HZSM-5 was higher than the amount obtained with HUSY. The major compounds obtained with both catalysts were C3 and C4 hydrocarbons (propane, propene, trans-butene, 1-butene, isobutene and cis-2-butene) and C5 olefins (1-pentene). # 2005 Elsevier B.V. All rights reserved. Keywords: Zeolite; High-density polyethylene; Fluidized bed reactor; Catalytic pyrolysis
1. Introduction In today’s modern world society, plastics provide a fundamental contribution to all main daily activities. In the middle of the seventies, the world-wide consumption of thermoplastics was around 13 million tonnes year 1, but with the increase of the application of these materials in the following years, the present demand exceeds 70 million tonnes year 1 [1]. Domestic plastic wastes contain around 57 wt% polyolefins, 14 wt% poly(vinyl chloride), 19 wt% polystyrene, 5 wt% other plastics and paper, and 5 wt% inorganic materials (additives). The main polyolefin in the domestic plastic wastes is polyethylene [2–6]. This polymer has a lot of usages such as in the electricity industry, in food packaging, in the toy industry, containers, etc. This versatility of polyethylene is due to its high impact resistance, high chemical resistance and low hardness and stiffness. It is the standard plastic with maximum production levels [7]. Because of this great increase in the usage of polyethylene it is necessary to find alternatives to remove the plastic wastes. The large majority of plastic wastes are landfilled. Polyethylene does not degrade, thus remaining in * Corresponding author. Tel.: +34 96 590 3789; fax: +34 96 590 3826. E-mail address: [email protected] (A. Marcilla). 0165-2370/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jaap.2005.03.001
municipal refuse tips for decades. Due to this situation, the recycling of plastics has been recognized as a necessity. The recycling methods can be grouped as follows: 1. Mechanical processing of the used plastics to form new products. This method has a very limited application, since the plastic properties are lost in the recycling process and the plastics obtained are of low quality. 2. Incineration of the plastics to recover energy [8]. The energetic use is an interesting alternative since this kind of material has a high calorific value (ranging from 18,000 to 38,000 kcal/kg approximately) [9]. The disadvantage is that this method can produce toxic gaseous compounds. In many countries, incineration of plastic waste is forbidden or politically unacceptable because of possible atmospheric contamination [8,10–12]. 3. An interesting alternative is tertiary recycling. This method allows the generation of products with higher value as fuel or chemicals. In this method, the plastic waste is processed to produce basic petrochemicals that can be used as feedstock to make virgin plastic. This process has the advantage of working with mixed plastics since all plastics are reduced to petrochemicals [13]. Thermal and catalytic pyrolysis are included in this kind of recycling [10].
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There are important differences between thermal and catalytic degradation. Thermal cracking of polymers has been widely described in the literature. In general, thermal decomposition of polyolefins consists in a radical mechanism including several phases: initiation, propagation and ending stages. In these phases, the polymer takes part in different types of reaction, such as hydrogen atom abstraction or b-scission reactions. As a result of this cracking mechanism, the products obtained using thermal degradation show a wide product distribution [10]. Furthermore, polymer decomposition is an endothermic process and plastic materials have poor thermal conductivity; thus, thermal pyrolysis, which is a potential recycling method, has the important drawback of the low selectivity and the high energetic consumption [5]. In order to decrease the process temperature and the energy requirements, catalysts have been used [5,14]. In addition, in catalytic degradation, product distribution can be controlled to a certain extent. With the right selection of the catalyst, it is possible to favor the obtention of certain types of compounds that can be more valuable [15]. Moreover, this kind of pyrolysis has other advantages over thermal degradation such as shortening the cracking time, reducing the proportion of solid residue in the final products and reducing the cracking temperature [16]. Normally, the catalysts used in catalytic pyrolysis are acid solids such as zeolites [8,14,15,17,18]. Catalytic cracking evolves through carbenium ion-type intermediate generated by hydrogen transfer reactions [18]. Zeolites have been used because they favor these types of reactions due to the presence of acid centres and help the rupture process of the polymer macromolecules. This rupture process begins in the zeolite surfaces since the polymer needs to be cracked before entering in the internal pores of the zeolites due to their reduced size pores. Zeolites present specific molecular size pores and the access of the plastic molecules to catalyst reactive sites as well as the growth of end products inside the pore is limited to the pore size [3]. This fact allows a narrow margin of product distribution to be obtained. In the present work, high-density polyethylene (HDPE) has been pyrolysed in a fluidized bed reactor. Experiments of catalytic pyrolysis were carried out using HZSM-5 and HUSY zeolites as catalysts. The volatile compounds up to C8 hydrocarbon obtained in thermal and catalytic degradation of polyethylene were compared. The influence of the type of catalyst as well as the pyrolysis temperature on the tendency of the volatiles evolved was evaluated.
2. Experimental 2.1. Materials The HDPE used for this study was a powdered BP Chemicals grade with a density of 935 kg/m3, melt flow
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index (MFI) obtained at 190 8C of 6 g 10 min 1 and 210– 500 mm particle size. The zeolitic catalysts used were HZSM-5 and HUSY supplied by Grace GMBH & Co. KG and their characteristics are shown in Table 1. To measure these properties, the following equipments were used: the SiO2/Al2O3 ratio was measured by fluorescence in a Philips equipment, model PW1480; structural characteristics of zeolite (pore size, BET surface area, pore volume, external surface area and micropore volume) were measured using nitrogen isothermal adsorption at 77 K in an automatic equipment AUTOSORB-6 supplied by Quantachrome; thermal programmed desorption (TPD) of NH3 was carried out in a thermogravimetric balance Netzsch TG 209. Two peaks were detected for HZSM-5 while only one peak was observed with HUSY. Zeolites were analysed by electronic microscopy (SEM and TEM techniques). Many agglomerates formed by different crystals were detected. Zeolites were sieved and agglomerates in the range 37–70 mm were selected for the runs. Both catalysts were dry mixed with the HDPE in a proportion of 20 wt%. Standard gases were used for the identification and quantification of volatile compounds obtained. The standards employed were mixtures of paraffins and olefins of hydrocarbons between C1 and C8, manufactured for Supelco by Scott Specialty Gases. 2.2. Reactor The pyrolysis was carried out in a stainless steel AISI 310 fluidized bed reactor (see Fig. 1), heated by a cylindrical refractory oven. The body of the reactor was a cylinder 71 cm high with 5.8 cm internal diameter. At 46 cm from the bottom of the reactor, a lateral exit for the volatile compounds is located. A porous plate at the bottom of the reactor supports the bed and uniforms the fluidizing gas flow at the entrance. The inert fluidized bed was sand, 70–210 mm particle size, with the following composition: SiO2, 98–99%; CaO, Table 1 Catalyst characteristics Characteristics
HZSM-5
HUSY
SiO2/Al2O3 (wt%) Pore size (nm) BET surface area (m2/g) External surface area (m2/g)a Pore volume (cm3/g)b Particle size (mm) Micropore volume (cm3/g)a Acidity (mmol NH3/g)c
22.2 0.55 341 37.6 0.18 37–70 0.16 1.15
4.8 0.74 614 28.1 0.35 37–70 0.29 2.15
c
Tmax (8C)
166
0.88 416
154
Two peaks at two different temperatures were detected for HZSM-5 corresponding to two types of acid centres. a Obtained by application of the t-plot method. b Measured at p/p0: 0.995. c From ammonia TPD.
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0.19%; MgO, 0.016%; Na2O, 0.008%; Al2O3, 0.25%; Fe2O3, 0.05%; K2O, 0.30%; TiO2, 0.05%. In all experiments, the static bed depth was maintained around 14.8 cm (approximately 460 g of sand). The fluidization agent was nitrogen. Fluidization velocity changes with temperature: at a low temperature, the fluidization velocity is larger than at a higher one [19]. This velocity was calculated theoretically using the Ergun equation at the different operating temperatures (400, 500, 600, 700 and 800 8C). For the purpose of maintaining the residence time of the volatile generated in all experiments, the velocity of nitrogen was maintained constant inside the reactor independently of the pyrolysis temperature. The value of the flow, maintained constant, was 3500 ml/min measured at the process temperature, which represents 2.6 times the minimum fluidization flow of the sand at 400 8C (the lowest operating temperature) and 3.7 times the minimum fluidization flow of the sand at 800 8C (the highest one) in the reactor used and well below the entrainment velocity of the sand. Thus, the fluidization state of the sand at every operating temperature as well as the heat transfer between hot sand particles and cold polymer was guaranteed. The values of nitrogen flow, measured at room temperature, selected to keep constant the nitrogen flow inside the reactor, are shown in Table 2.
Table 2 Nitrogen flow (measured at room temperature) used for each pyrolysis temperature Temperature (8C)
Nitrogen flow (ml/min)
400 500 600 700 800
1550 1349 1195 1072 972
Two grams of HDPE (or 2 g of HDPE + 0.5 g catalyst in the catalytic runs) was placed in the feed hopper. Prior to the experiment, the feed hopper was purged with nitrogen to guarantee inert atmosphere inside the reactor during the pyrolysis. When the reactor reached the selected temperature, the nitrogen flow was adjusted according to the operating temperature (see Table 2). The experiment began by switching the valve on to allow the nitrogen flow to enter the sampling bag and connecting the cronometer. The sample was simultaneously dropped onto the hot fluidized sand bed. Condensable products generated were trapped in the glass traps while gases were collected in the sampling bag. The experiment lasts between 19 and 25 min approximately, according to the operating temperature (i.e., the nitrogen flow).
2.3. Experimental procedure
2.4. Gas analysis
Experiments were carried out as follows. The reactor was programmed to the selected temperature and the top reactor heating system was switched on. Reactor, glass traps, gasometer and gas sampling bag were connected on line (Fig. 1). A valve allowed the flow direction to change (to the gas sampling bag or to the exit). During the heating time, a nitrogen flow circulated through the system to the exit, in order to purge it.
The gas fraction was collected in a Tedlar bag. The volatile compounds were identified and quantified using standard gases by a Shimadzu GC-17A gas chromatograph with an FID detector and an alumina/KCl column 2-4249 (30 m 0.32 mm i.d.). The column program was: Tin, 35 8C; Tf, 180 8C; heating rate, 5 8C/min; timetotal, 51.5 min. Those peaks that could not be identified with the standards employed were quantified by the response
Fig. 1. Experimental system: (A) manometers; (B) oven; (C) reactor; (D) feed hopper; (E) top reactor heating system; (F) ice–salt bath; (G) gasometer; (H) stainless steel Dixon rings; (I) glass traps; (J) gas sampling bag of 25 l.
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factor of the compound that was closer to them in the chromatogram.
3. Results and discussion 3.1. Effect of temperature and catalyst on the gas yields Three sequences of experiments (thermal degradation, catalytic pyrolysis using HZSM-5 as catalyst and catalytic pyrolysis using HUSY) at five different temperatures (400, 500, 600, 700 and 800 8C) were carried out in order to study the effect of the temperature as well as the catalyst type on the yields of the pyrolytic products obtained. In order to find out the temperature from which the presence of catalyst began to be significant, catalytic experiments using HZSM-5 at temperatures lower than 400 8C were carried out. The values of the gas fraction obtained in each run are shown in Fig. 2. As was expected, in the thermal pyrolysis, the percentage of gases increased with temperature as a consequence of a more effective cracking of polyethylene. Comparing thermal decomposition with HUSY and HZSM-5 catalytic pyrolysis, the latter produced the highest amount of gases. This was indicative that HZSM-5 promoted a more complete degradation of polyethylene than HUSY at any operating temperature. As can be seen in Fig. 2, in the catalytic pyrolysis using the HUSY catalyst, the highest value of a gas fraction was obtained at 700 8C. A slight increase in the percentage of gases is detected from 400 to 700 8C while this percentage diminished above 700 8C. The highest percentages using HZSM-5 catalyst were obtained at 500–600 8C, showing a slight decrease at higher temperatures. Experiments carried out at 200–300 8C with HZSM-5 completed the study of the catalytic effect on the HDPE pyrolysis. This zeolite (HZSM5) was selected for these experiments at lower temperatures since it was the most active catalyst from the two zeolites used in the present study. The results showed the low activity
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of the catalyst at 200 8C (gas yield around 0.3%) as well as a significant increase at 300 8C, by obtaining a gas yield of 2.7%, very close to the gas yield obtained at 400 8C in thermal degradation. According to the results shown in Fig. 2, it seems that the catalytic effect is reduced at the highest temperatures. This fact can be explained considering the deactivation of the catalyst. Studies of the deactivation of the zeolites [20,21] show that the activity of HZSM-5 is clearly affected by temperature, decreasing the activity of this zeolite by increasing the temperature due to structural changes. Apparently, high temperature affects the strong acid centres, principally due to a dealumination process. 27 Al RMN and 29Si RMN spectras of HZSM-5 regenerated at temperatures around 600 and 900 8C show that high temperatures lead to a higher disorder degree and distortion of the bond angles between silicon and aluminium, modifying the acidity of the zeolites [20]. On the other hand, temperature has a lower influence on the HUSY activity, being more affected by the deposition of coke on its pores. These studies show that the yield of coke increases at high temperature, decreasing the activity of this catalyst. As observed, HZSM-5 catalyst showed the maximum gas yield around 500–600 8C decreasing at higher temperatures as a consequence of the zeolite deactivation. In the case of HUSY, the maximum of the yield of volatile compounds was around 700 8C (at a higher temperature than with HZSM-5) while the gas fraction decreased at higher temperatures probably due to the coke deposited on its pores. Sakata et al. [22] pyrolysed HDPE at 430 8C in a glass reactor under semi-batch conditions. The percentage of gases obtained in thermal degradation was 9.6%, meanwhile using HZSM-5, the yield of gases was 44.3%. Park et al. [23] degraded HDPE in thermal and catalytic conditions, at 450– 500 8C, in a fixed-bed semi-batch reactor, using HZSM-5 and Y zeolite among other catalysts. They reported an increase in the yield of gases obtained in the catalytic pyrolysis, from 48.3% (thermal pyrolysis) up to 88.4% (HZSM-5), confirming HZSM-5 as the catalyst, which leads to a higher gas yield. 3.2. Effect of temperature and catalyst in the gas composition Tables 3–5 show the composition of the gas produced in the pyrolysis of HDPE as a function of the fluidized bed temperature under thermal and catalytic conditions. From the results obtained, some interesting aspects can be remarked.
Fig. 2. Variation of the yield of gases obtained in thermal and catalytic pyrolysis.
3.2.1. Effect of temperature in the thermal degradation Within the operating temperature range studied, the gas fraction obtained in the thermal pyrolysis was formed by products distributed in a wide range of molecular weights. This distribution results from the random scission of the long polymeric chain, which is the mechanism followed by the thermal degradation of polyolefinic olygomers [6,15,24].
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Table 3 Yield (g compound/100 g polyethylene) of volatile compounds in thermal pyrolysis of HDPE
Table 5 Yield (g compound/100 g polyethylene) of volatile compounds in catalytic pyrolysis of HDPE with HUSY
Compounds
Compounds
Methane Ethane Ethene Propane Propene Isobutane n-Butane trans-Butene + 1-butene Isobutene cis-2-Butene n-Pentane 1,3-Butadiene 1-Pentene 2-Butyne 1-Butyne n-Hexane 1-Hexene Benzene Toluene Xylene Isopentane cis-2-Hexene Heptane 1-Heptene Non-identified HDPE (g)
Yield (g compound/100 g HDPE) 405 8C
509 8C
604 8C
710 8C
800 8C
0.01 0.03 0.02 0.06 0.36 0.05 0.04 0.30 0.25 0.10 0.03 0.01 0.07 0.01 0.03 0.03 0.07 0.07 0.02 0.01 0.04 0.05 0.02 0.03 1.3 1.9
0.26 0.50 0.56 0.34 1.6 0.02 0.19 1.3 0.64 0.34 0.15 0.12 0.50 0.04 0.01 0.26 0.76 0.35 0.14 0.07 0.02 0.29 0.13 0.66 6.0 1.7
0.51 0.65 1.5 0.48 5.6 0.03 0.26 3.4 1.8 1.1 0.19 0.52 0.86 0.11 0.01 0.56 1.0 0.50 0.23 0.09 0.03 0.53 0.38 0.79 11.2 1.9
4.2 2.9 12.5 0.70 7.6 0.01 0.26 3.9 0.30 0.55 0.13 4.2 1.9 0.31 0.01 1.8 2.5 1.8 1.0 0.12 0.02 0.26 0.65 1.5 8.6 1.9
4.8 3.0 16.5 0.74 10.5 0.01 0.27 5.1 0.24 0.53 0.10 5.0 2.3 0.47 0.02 2.0 2.6 2.2 0.80 0.08 0.02 0.16 0.76 1.5 6.7 2.1
Table 4 Yield (g compound/100 g polyethylene) of volatile compounds in catalytic pyrolysis of HDPE with HZSM-5 Compounds
Methane Ethane Ethene Propane Propene Isobutane n-Butane trans-Butene + 1-butene Isobutene cis-2-Butene n-Pentane 1,3-Butadiene 1-Pentene 2-Butyne 1-Butyne n-Hexane 1-Hexene Benzene Toluene Xylene Isopentane cis-2-Hexene Heptane Non-identified HDPE (g) Catalyst (%)
Yield (g compound/100 g HDPE) 400 8C
509 8C
608 8C
705 8C
804 8C
0.02 0.06 1.3 3.1 16.0 2.5 1.6 7.9 9.0 3.6 0.69 0.02 9.7 0.37 0.42 0.35 0.12 0.34 0.54 0.08 1.3 0.21 0.12 10.2 2.0 20.8
0.10 0.14 2.8 3.8 19.7 4.4 2.0 8.5 7.9 3.8 0.85 0.05 8.3 0.64 1.2 0.50 0.18 0.62 1.9 1.1 3.1 0.24 0.21 14.2 1.6 22.1
0.17 0.26 4.3 5.6 20.4 3.1 2.6 8.2 7.4 3.6 0.94 0.06 6.5 0.50 0.40 0.49 0.11 0.59 1.6 0.22 1.6 0.16 3.1 15.6 2.2 19.4
0.33 0.35 3.9 4.1 21.3 2.0 1.9 9.6 7.0 4.0 0.74 0.18 7.1 0.30 0.31 0.56 0.24 0.69 0.91 0.10 1.1 0.28 0.15 11.3 2.3 18.6
1.4 0.80 5.8 2.9 20.5 1.4 1.4 8.6 6.1 3.6 0.53 0.51 5.8 0.21 0.23 0.86 0.18 0.89 1.3 0.09 0.81 0.26 0.14 9.3 1.7 20.6
Methane Ethane Ethene Propane Propene Isobutane n-Butane trans-Butene + 1-butene Isobutene cis-2-Butene n-Pentane 1,3-Butadiene 1-Pentene 2-Butyne 1-Butyne n-Hexane 1-Hexene Benzene Toluene Xylene Isopentane cis-2-Hexene heptane 1-Heptene 2,2-Dimetilhexane Non-identified HDPE (g) Catalyst (%)
Yield (g compound/100 g HDPE) 402 8C
509 8C
608 8C
705 8C
807 8C
0.03 0.03 0.10 0.26 3.7 2.6 0.30 2.9 0.69 1.3 0.25 0.01 4.3 0.22 1.9 0.25 0.24 0.56 0.04 0.01 3.3 0.35 0.18 0.07 0.43 18.0 2.2 18.6
0.08 0.06 0.88 0.79 10.1 3.7 0.63 5.2 4.4 2.2 0.39 0.03 5.3 0.38 1.7 0.37 0.17 0.27 0.52 0.35 3.6 0.26 0.25 0.07 0.62 18.6 2.0 20.9
0.19 0.11 0.57 0.68 8.3 4.0 0.71 5.6 3.7 2.4 0.53 0.04 5.7 0.37 2.8 0.55 0.27 0.68 0.56 0.40 4.7 0.41 0.36 0.12 0.57 25.7 1.9 20.9
0.41 0.22 0.67 0.67 8.5 3.2 0.72 6.5 4.2 2.7 0.57 0.14 6.7 0.33 2.3 0.69 0.48 1.0 0.78 0.45 3.9 0.63 0.45 0.18 0.68 29.0 1.8 19.9
0.94 0.37 1.1 0.69 7.8 3.2 0.69 5.7 3.7 2.3 0.50 0.29 5.4 0.26 1.7 0.69 0.41 0.61 0.22 0.03 3.6 0.44 0.31 0.08 0.29 18.8 1.9 19.3
In general, most of the products increased their yield by increasing pyrolysis temperature. Obviously, at the lowest operating temperature (400 8C) the yield of the compounds obtained was very low and no compounds whose composition reaches a yield equal or higher than 1% were detected, although considering volumetric fraction, some compounds, such as propene, reach a percentage higher than 17% (v/v). At this reactor temperature, around 26% of the HDPE fed melts without decomposing. At 500 8C, only two compounds (propene and the sum of trans-butene and 1-butene) exceed 1%, although all compounds increased their yield significantly. In the range 400–600 8C, the major compound obtained is propene. At 700 8C the yield of methane, ethene and 1,3-butadiene increased significantly. Some products such as isobutene, npentane, cis-2-butene or cis-2-hexene showed a maximum value at 600 8C while others such as toluene and xylene showed this maximum at 700 8C. The presence of a maximum clearly shows the role of the cracking secondary reactions at high temperatures. It seems that as a result of these cracking reactions, at temperatures higher than 600 8C, the major compound is ethene instead of propene (35%, v/v, of ethene at 800 8C). It is difficult to avoid secondary reactions using a fluidized bed reactor. The presence of secondary reactions in the cracking of polymers using this type of equipment has
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been reported previously. Liu et al. [25] reported a similar behaviour in the pyrolysis of PS in a fluidized bed reactor, showing that primary products were stable until 600 8C and secondary reactions take place when the pyrolysis temperature exceeds this value, and can modify the tendency of the products obtained. From this point on, the cracking of some heavy compounds began, decreasing their yield while the percentage of lighter products increased, due to the fragments of cracking products in secondary reactions. The results reported in this paper about the thermal pyrolysis of HDPE are in agreement with others found in literature. Williams and Williams [6] reported that the main gases produced with LDPE in a fluidized bed reactor from 500 to 700 8C were ethene (from 2.19% at 500 8C to 26.86% at 800 8C), propene (from 1.82% to 18.59%) and butene (from 2.91% to 7.63%). Sakata et al. [22,26] showed that the major gaseous compounds obtained from the pyrolysis of HDPE at 430 8C were propane, propene, ethane and ethene. Mastral et al. [27] reported that C3 compounds were the main gaseous compounds at 645 8C, while ethane was the major product at 780–850 8C. Serrano et al. [15] reported a preferencial formation of C3–C4 hydrocarbons when LDPE was pyrolysed at low temperature, indicating that this fact could be caused by radical b-scission at the polymer backbone and at the LDPE branchings, respectively. This mechanism would explain the results found in this study where propene is the main gas product obtained at low temperatures, before the secondary cracking began to be significant. 3.2.2. Effect of temperature and type of catalyst in catalytic pyrolysis As commented previously, the catalytic pyrolysis produced a higher yield of gas products than the thermal pyrolysis, especially when using the HZSM-5 catalyst. Furthermore, as can be seen in Fig. 2, the temperature in the catalytic decomposition played a less significant role than in thermal degradation. Thus, the yield of gaseous compounds generated under catalytic conditions does not show so important variations as a function of temperature as those detected in the absence of a catalyst. This fact is more evident in HZSM-5 runs than in HUSY experiments (see Tables 4 and 5). The results obtained indicate that the catalysts studied, especially HZSM-5, produce a narrower distribution of products unlike the thermal degradation, probably due to the role of the zeolitic pore specific size on the catalytic pyrolysis process. This distribution is centred in C3 and C4 hydrocarbons, specifically propene, trans-butene, 1-butene, isobutane, isobutene and cis-2-butene; C5 hydrocarbons also show a significant increase, such as 1-pentene and isopentane. The highest differences between thermal and catalytic pyrolysis results are found in the C3–C5 range. Under catalytic conditions, the major compound obtained is propene independently of the pyrolysis temperature (about 30%, v/v, of propene with HZSM-5 and around 20%, v/v, with HUSY).
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Isopentane yield showed a significant increase under catalytic pyrolysis. This product showed a low production in thermal degradation, the maximum yield obtained was 0.04% at 400 8C, while it reached 3.1% at 500 8C using HZSM-5 and 4.7% with HUSY at 600 8C. As commented previously, a significant decrease of the volatile yield can be distinguished at high temperature under catalytic conditions, probably due to the catalyst deactivation. This decrease is also observed in the product yields. By comparing the yield of compounds present in the gaseous fraction, in thermal and catalytic pyrolysis, shown in Tables 3–5, it is observed that the yield of gaseous compounds was higher in the pyrolysis with HZSM-5 than in pyrolysis with HUSY or in thermal degradation, at all the temperatures evaluated. At 400 8C, the yield of propene obtained using HZSM-5 was 44 times higher than in thermal degradation, whereas using HUSY the yield of propene was around 10 times higher than the yield obtained in thermal pyrolysis. At higher temperatures, the yield of propene obtained in thermal degradation increased and the difference between the yields obtained in thermal and catalytic pyrolysis decreased. Only at 700 and 800 8C, some compounds reached yields higher in thermal degradation than in pyrolysis with HZSM-5 or HUSY. Compounds that showed more differences at higher temperatures between thermal and catalytic pyrolysis were methane, ethane and ethene. This fact can be the result of secondary reactions. Data related to catalytic pyrolysis of HDPE were found in literature. Sakata et al. [22,26], by pyrolysing HDPE at 430 8C, indicated that the HZSM-5 catalyst decreased the content of C2–C3 components and increased the content of C4–C5 hydrocarbons significantly. Manos et al. [28] pyrolysed HDPE with a heating rate of 5 K/min and 633 K as final reactor temperature and showed that HZSM-5 produced, fundamentally, alkenes in the range C3–C5, while HUSY produced, especially isobutane and isopentane. These results agree with those reported in this paper. According to some researchers [10,24], the preference of these catalysts towards this range of products can be explained considering the mechanism of degradation over the zeolites. The large macromolecules have to react on the external surface of the zeolite catalyst first, which could be the limiting reaction step. Smaller cracked fragments diffuse into the zeolite pores and undergo further reactions [10]. The primary compounds produced seem to be C1–C5 hydrocarbons, being formed by end-chain scission reactions. The other compounds obtained could be a result of the oligomerization reactions of the olefinic gaseous fractions and random cracking at any position of the polymer chain [12]. The pore size of the zeolite is an important characteristic in the diffusion sep of reactives and products, since it can limit the size product obtained in catalytic degradation favoring some products over other ones, as has been shown previously. This preference is one of the reasons
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Fig. 3. Evolution of major volatile products (C1–C3) vs. temperature in thermal pyrolysis.
Fig. 5. Evolution of major volatile products (C1–C3) vs. temperature in catalytic pyrolysis with HZSM-5.
why zeolites are the preferred catalysts for conversion of plastic wastes [29]. Figs. 3 and 4 show the general tendency of the gaseous compounds obtained as a function of temperature in thermal pyrolysis: on one hand, temperature favors the decomposition of polyethylene and the amount of noncondensable volatiles increases. On the other hand, temperature can also favor the secondary cracking of primary hydrocarbons producing decreases in the yields of some hydrocarbons (such as isobutene or cis-2-butene) at temperatures above 700 8C. Other compounds (such as propene and butene) modify their tendency and at high temperatures do not increase their yield as much as was expected. The yield of compounds such as methane, ethane and ethene increases significantly at temperatures above 600–700 8C, probably due to the contribution of the secondary reactions. As can be seen in Figs. 5–8, the general tendency of products versus temperature in catalytic degradation is different from thermal pyrolysis. Most of them (C3–C5 hydrocarbons mainly) are hardly affected by pyrolysis temperature, and the most significant changes can be found in the range 400–500 8C. This behaviour is more clearly observed using HZSM-5 instead of HUSY. From an applied
point of view, this fact has important consequences since catalytic degradation allows the reduction of pyrolysis temperature without significant modifications in product yield and distribution. This would suggest an important reduction in the amount of necessary energy to obtain a specific product. Lighter hydrocarbons seem to be more dependent on temperature and more affected by secondary reactions. Thus, independently of the catalyst used, propene shows a maximum at 500–600 8C, ethene changes its tendency at the same temperature and methane and ethane increase their yields continuously. In Fig. 9, the percentage of volatiles obtained in the pyrolysis with HZSM-5 at 200 and 300 8C as well as the yield obtained in the thermal degradation at 400 8C are shown. In this figure, it can be seen that at 200 8C, HZSM-5 is hardly active and the yields of gaseous compounds obtained were still low. However, at 300 8C, using the HZSM-5 as a catalyst, some compounds reached a yield higher than those obtained at 400 8C in the thermal degradation. These compounds were olefins with 3–5 carbon number, range of products favored in presence of HZSM-5. Thus, at 300 8C the activity of HZSM-5 began to be significant.
Fig. 4. Evolution of major volatile products (C4–C5) vs. temperature in thermal pyrolysis.
Fig. 6. Evolution of major volatile products (C3–C5) vs. temperature in catalytic pyrolysis with HZSM-5.
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Fig. 7. Evolution of major volatile products (C1–C3) vs. temperature in catalytic pyrolysis with HUSY.
Fig. 8. Evolution of major volatile products (C3–C5) vs. temperature in catalytic pyrolysis with HUSY.
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1. The influence of temperature on volatile yield is more significant in thermal than in catalytic pyrolysis. 2. In thermal degradation, the yield of gases increases with temperature in the range 400–800 8C. 3. At 300 8C the activity of HZSM-5 begins to be significant whereas the thermal degradation begins at 400 8C. 4. HZSM-5 favors gas production in a greater extension (87.4% at 600 8C) as compared to HUSY (76.0% at 700 8C) or thermal degradation (66.3% at 800 8C). 5. In catalytic pyrolysis the distribution of the products obtained is narrower than in thermal degradation. The range of products C3–C5 is favored under catalytic conditions, increasing their yield significantly in relation to thermal degradation. 6. HZSM-5 favors mainly olefins, while HUSY favors olefins and some paraffins in the same range of carbon number. The main products obtained with HZSM-5 and HUSY are propane, propene, trans-butene, 1-butene, isobutene, cis-2-butene, 1-pentene. Isopentane is formed in catalytic pyrolysis, fundamentally, being favored with HUSY versus HZSM-5. 7. The highest yields obtained in thermal pyrolysis are reached for ethene (16.5%) and propene (10.5%) at 800 8C. In catalytic pyrolysis using HZSM-5, the main volatile compounds are propene (21.3% at 700 8C) followed by trans-butene + 1-butene (9.6% at 700 8C) and 1-pentene and isobutene (9.7% and 9.0%, respectively at 400 8C). Using HUSY, propene reaches the highest yield at 500 8C (10.1%), followed by transbutene + 1-butene and 1-pentene (6.5% and 6.7%, respectively, at 700 8C); isopentane shows the highest yield at 600 8C (4.7%). 8. In relation to thermal degradation, the catalytic pyrolysis allows the reduction of the degradation temperature in 300–400 8C in order to obtain a significant yield of propene. A yield of propene similar to that obtained under thermal pyrolysis at 800 8C is reached at 400 8C using HZSM-5 and at 500 8C using HUSY.
Acknowledgements The authors wish to thank CICYT PPQ2001-0733, FEDER, GV (project GRUPOSO3/159) and Ministry of Education, Culture and Sport for financial support.
Fig. 9. Yield of volatile compounds in thermal degradation at 400 8C and in pyrolysis with HZSM-5 at 200 and 300 8C.
4. Conclusions From the study of thermal and catalytic gases obtained in the flash pyrolysis of HDPE in a fluidized bed reactor, the following conclusions can be deduced:
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Study of the gases obtained in thermal and catalytic flash pyrolysis of HDPE in a fluidized bed reactor ´ ngela N. Garcı´a, Antonio Marcilla * Maria del Remedio Herna´ndez, A Department of Chemical Engineering, University of Alicante, P.O. Box 99, Alicante 03690, Spain Received 27 September 2004; accepted 8 March 2005 Available online 12 April 2005
Abstract A fluidized bed reactor was used to study the production of gases from the thermal and catalytic pyrolysis of high-density polyethylene (HDPE). The effect of the bed temperature and type of catalyst was evaluated. The range of operating temperature was from 400 to 800 8C. The catalysts used in this study were HZSM-5 and HUSY in a proportion of 20% by weight. The analysis of the volatile compounds showed that the main products obtained with both catalysts were similar, but the amount of gases produced with HZSM-5 was higher than the amount obtained with HUSY. The major compounds obtained with both catalysts were C3 and C4 hydrocarbons (propane, propene, trans-butene, 1-butene, isobutene and cis-2-butene) and C5 olefins (1-pentene). # 2005 Elsevier B.V. All rights reserved. Keywords: Zeolite; High-density polyethylene; Fluidized bed reactor; Catalytic pyrolysis
1. Introduction In today’s modern world society, plastics provide a fundamental contribution to all main daily activities. In the middle of the seventies, the world-wide consumption of thermoplastics was around 13 million tonnes year 1, but with the increase of the application of these materials in the following years, the present demand exceeds 70 million tonnes year 1 [1]. Domestic plastic wastes contain around 57 wt% polyolefins, 14 wt% poly(vinyl chloride), 19 wt% polystyrene, 5 wt% other plastics and paper, and 5 wt% inorganic materials (additives). The main polyolefin in the domestic plastic wastes is polyethylene [2–6]. This polymer has a lot of usages such as in the electricity industry, in food packaging, in the toy industry, containers, etc. This versatility of polyethylene is due to its high impact resistance, high chemical resistance and low hardness and stiffness. It is the standard plastic with maximum production levels [7]. Because of this great increase in the usage of polyethylene it is necessary to find alternatives to remove the plastic wastes. The large majority of plastic wastes are landfilled. Polyethylene does not degrade, thus remaining in * Corresponding author. Tel.: +34 96 590 3789; fax: +34 96 590 3826. E-mail address: [email protected] (A. Marcilla). 0165-2370/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jaap.2005.03.001
municipal refuse tips for decades. Due to this situation, the recycling of plastics has been recognized as a necessity. The recycling methods can be grouped as follows: 1. Mechanical processing of the used plastics to form new products. This method has a very limited application, since the plastic properties are lost in the recycling process and the plastics obtained are of low quality. 2. Incineration of the plastics to recover energy [8]. The energetic use is an interesting alternative since this kind of material has a high calorific value (ranging from 18,000 to 38,000 kcal/kg approximately) [9]. The disadvantage is that this method can produce toxic gaseous compounds. In many countries, incineration of plastic waste is forbidden or politically unacceptable because of possible atmospheric contamination [8,10–12]. 3. An interesting alternative is tertiary recycling. This method allows the generation of products with higher value as fuel or chemicals. In this method, the plastic waste is processed to produce basic petrochemicals that can be used as feedstock to make virgin plastic. This process has the advantage of working with mixed plastics since all plastics are reduced to petrochemicals [13]. Thermal and catalytic pyrolysis are included in this kind of recycling [10].
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There are important differences between thermal and catalytic degradation. Thermal cracking of polymers has been widely described in the literature. In general, thermal decomposition of polyolefins consists in a radical mechanism including several phases: initiation, propagation and ending stages. In these phases, the polymer takes part in different types of reaction, such as hydrogen atom abstraction or b-scission reactions. As a result of this cracking mechanism, the products obtained using thermal degradation show a wide product distribution [10]. Furthermore, polymer decomposition is an endothermic process and plastic materials have poor thermal conductivity; thus, thermal pyrolysis, which is a potential recycling method, has the important drawback of the low selectivity and the high energetic consumption [5]. In order to decrease the process temperature and the energy requirements, catalysts have been used [5,14]. In addition, in catalytic degradation, product distribution can be controlled to a certain extent. With the right selection of the catalyst, it is possible to favor the obtention of certain types of compounds that can be more valuable [15]. Moreover, this kind of pyrolysis has other advantages over thermal degradation such as shortening the cracking time, reducing the proportion of solid residue in the final products and reducing the cracking temperature [16]. Normally, the catalysts used in catalytic pyrolysis are acid solids such as zeolites [8,14,15,17,18]. Catalytic cracking evolves through carbenium ion-type intermediate generated by hydrogen transfer reactions [18]. Zeolites have been used because they favor these types of reactions due to the presence of acid centres and help the rupture process of the polymer macromolecules. This rupture process begins in the zeolite surfaces since the polymer needs to be cracked before entering in the internal pores of the zeolites due to their reduced size pores. Zeolites present specific molecular size pores and the access of the plastic molecules to catalyst reactive sites as well as the growth of end products inside the pore is limited to the pore size [3]. This fact allows a narrow margin of product distribution to be obtained. In the present work, high-density polyethylene (HDPE) has been pyrolysed in a fluidized bed reactor. Experiments of catalytic pyrolysis were carried out using HZSM-5 and HUSY zeolites as catalysts. The volatile compounds up to C8 hydrocarbon obtained in thermal and catalytic degradation of polyethylene were compared. The influence of the type of catalyst as well as the pyrolysis temperature on the tendency of the volatiles evolved was evaluated.
2. Experimental 2.1. Materials The HDPE used for this study was a powdered BP Chemicals grade with a density of 935 kg/m3, melt flow
315
index (MFI) obtained at 190 8C of 6 g 10 min 1 and 210– 500 mm particle size. The zeolitic catalysts used were HZSM-5 and HUSY supplied by Grace GMBH & Co. KG and their characteristics are shown in Table 1. To measure these properties, the following equipments were used: the SiO2/Al2O3 ratio was measured by fluorescence in a Philips equipment, model PW1480; structural characteristics of zeolite (pore size, BET surface area, pore volume, external surface area and micropore volume) were measured using nitrogen isothermal adsorption at 77 K in an automatic equipment AUTOSORB-6 supplied by Quantachrome; thermal programmed desorption (TPD) of NH3 was carried out in a thermogravimetric balance Netzsch TG 209. Two peaks were detected for HZSM-5 while only one peak was observed with HUSY. Zeolites were analysed by electronic microscopy (SEM and TEM techniques). Many agglomerates formed by different crystals were detected. Zeolites were sieved and agglomerates in the range 37–70 mm were selected for the runs. Both catalysts were dry mixed with the HDPE in a proportion of 20 wt%. Standard gases were used for the identification and quantification of volatile compounds obtained. The standards employed were mixtures of paraffins and olefins of hydrocarbons between C1 and C8, manufactured for Supelco by Scott Specialty Gases. 2.2. Reactor The pyrolysis was carried out in a stainless steel AISI 310 fluidized bed reactor (see Fig. 1), heated by a cylindrical refractory oven. The body of the reactor was a cylinder 71 cm high with 5.8 cm internal diameter. At 46 cm from the bottom of the reactor, a lateral exit for the volatile compounds is located. A porous plate at the bottom of the reactor supports the bed and uniforms the fluidizing gas flow at the entrance. The inert fluidized bed was sand, 70–210 mm particle size, with the following composition: SiO2, 98–99%; CaO, Table 1 Catalyst characteristics Characteristics
HZSM-5
HUSY
SiO2/Al2O3 (wt%) Pore size (nm) BET surface area (m2/g) External surface area (m2/g)a Pore volume (cm3/g)b Particle size (mm) Micropore volume (cm3/g)a Acidity (mmol NH3/g)c
22.2 0.55 341 37.6 0.18 37–70 0.16 1.15
4.8 0.74 614 28.1 0.35 37–70 0.29 2.15
c
Tmax (8C)
166
0.88 416
154
Two peaks at two different temperatures were detected for HZSM-5 corresponding to two types of acid centres. a Obtained by application of the t-plot method. b Measured at p/p0: 0.995. c From ammonia TPD.
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0.19%; MgO, 0.016%; Na2O, 0.008%; Al2O3, 0.25%; Fe2O3, 0.05%; K2O, 0.30%; TiO2, 0.05%. In all experiments, the static bed depth was maintained around 14.8 cm (approximately 460 g of sand). The fluidization agent was nitrogen. Fluidization velocity changes with temperature: at a low temperature, the fluidization velocity is larger than at a higher one [19]. This velocity was calculated theoretically using the Ergun equation at the different operating temperatures (400, 500, 600, 700 and 800 8C). For the purpose of maintaining the residence time of the volatile generated in all experiments, the velocity of nitrogen was maintained constant inside the reactor independently of the pyrolysis temperature. The value of the flow, maintained constant, was 3500 ml/min measured at the process temperature, which represents 2.6 times the minimum fluidization flow of the sand at 400 8C (the lowest operating temperature) and 3.7 times the minimum fluidization flow of the sand at 800 8C (the highest one) in the reactor used and well below the entrainment velocity of the sand. Thus, the fluidization state of the sand at every operating temperature as well as the heat transfer between hot sand particles and cold polymer was guaranteed. The values of nitrogen flow, measured at room temperature, selected to keep constant the nitrogen flow inside the reactor, are shown in Table 2.
Table 2 Nitrogen flow (measured at room temperature) used for each pyrolysis temperature Temperature (8C)
Nitrogen flow (ml/min)
400 500 600 700 800
1550 1349 1195 1072 972
Two grams of HDPE (or 2 g of HDPE + 0.5 g catalyst in the catalytic runs) was placed in the feed hopper. Prior to the experiment, the feed hopper was purged with nitrogen to guarantee inert atmosphere inside the reactor during the pyrolysis. When the reactor reached the selected temperature, the nitrogen flow was adjusted according to the operating temperature (see Table 2). The experiment began by switching the valve on to allow the nitrogen flow to enter the sampling bag and connecting the cronometer. The sample was simultaneously dropped onto the hot fluidized sand bed. Condensable products generated were trapped in the glass traps while gases were collected in the sampling bag. The experiment lasts between 19 and 25 min approximately, according to the operating temperature (i.e., the nitrogen flow).
2.3. Experimental procedure
2.4. Gas analysis
Experiments were carried out as follows. The reactor was programmed to the selected temperature and the top reactor heating system was switched on. Reactor, glass traps, gasometer and gas sampling bag were connected on line (Fig. 1). A valve allowed the flow direction to change (to the gas sampling bag or to the exit). During the heating time, a nitrogen flow circulated through the system to the exit, in order to purge it.
The gas fraction was collected in a Tedlar bag. The volatile compounds were identified and quantified using standard gases by a Shimadzu GC-17A gas chromatograph with an FID detector and an alumina/KCl column 2-4249 (30 m 0.32 mm i.d.). The column program was: Tin, 35 8C; Tf, 180 8C; heating rate, 5 8C/min; timetotal, 51.5 min. Those peaks that could not be identified with the standards employed were quantified by the response
Fig. 1. Experimental system: (A) manometers; (B) oven; (C) reactor; (D) feed hopper; (E) top reactor heating system; (F) ice–salt bath; (G) gasometer; (H) stainless steel Dixon rings; (I) glass traps; (J) gas sampling bag of 25 l.
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factor of the compound that was closer to them in the chromatogram.
3. Results and discussion 3.1. Effect of temperature and catalyst on the gas yields Three sequences of experiments (thermal degradation, catalytic pyrolysis using HZSM-5 as catalyst and catalytic pyrolysis using HUSY) at five different temperatures (400, 500, 600, 700 and 800 8C) were carried out in order to study the effect of the temperature as well as the catalyst type on the yields of the pyrolytic products obtained. In order to find out the temperature from which the presence of catalyst began to be significant, catalytic experiments using HZSM-5 at temperatures lower than 400 8C were carried out. The values of the gas fraction obtained in each run are shown in Fig. 2. As was expected, in the thermal pyrolysis, the percentage of gases increased with temperature as a consequence of a more effective cracking of polyethylene. Comparing thermal decomposition with HUSY and HZSM-5 catalytic pyrolysis, the latter produced the highest amount of gases. This was indicative that HZSM-5 promoted a more complete degradation of polyethylene than HUSY at any operating temperature. As can be seen in Fig. 2, in the catalytic pyrolysis using the HUSY catalyst, the highest value of a gas fraction was obtained at 700 8C. A slight increase in the percentage of gases is detected from 400 to 700 8C while this percentage diminished above 700 8C. The highest percentages using HZSM-5 catalyst were obtained at 500–600 8C, showing a slight decrease at higher temperatures. Experiments carried out at 200–300 8C with HZSM-5 completed the study of the catalytic effect on the HDPE pyrolysis. This zeolite (HZSM5) was selected for these experiments at lower temperatures since it was the most active catalyst from the two zeolites used in the present study. The results showed the low activity
317
of the catalyst at 200 8C (gas yield around 0.3%) as well as a significant increase at 300 8C, by obtaining a gas yield of 2.7%, very close to the gas yield obtained at 400 8C in thermal degradation. According to the results shown in Fig. 2, it seems that the catalytic effect is reduced at the highest temperatures. This fact can be explained considering the deactivation of the catalyst. Studies of the deactivation of the zeolites [20,21] show that the activity of HZSM-5 is clearly affected by temperature, decreasing the activity of this zeolite by increasing the temperature due to structural changes. Apparently, high temperature affects the strong acid centres, principally due to a dealumination process. 27 Al RMN and 29Si RMN spectras of HZSM-5 regenerated at temperatures around 600 and 900 8C show that high temperatures lead to a higher disorder degree and distortion of the bond angles between silicon and aluminium, modifying the acidity of the zeolites [20]. On the other hand, temperature has a lower influence on the HUSY activity, being more affected by the deposition of coke on its pores. These studies show that the yield of coke increases at high temperature, decreasing the activity of this catalyst. As observed, HZSM-5 catalyst showed the maximum gas yield around 500–600 8C decreasing at higher temperatures as a consequence of the zeolite deactivation. In the case of HUSY, the maximum of the yield of volatile compounds was around 700 8C (at a higher temperature than with HZSM-5) while the gas fraction decreased at higher temperatures probably due to the coke deposited on its pores. Sakata et al. [22] pyrolysed HDPE at 430 8C in a glass reactor under semi-batch conditions. The percentage of gases obtained in thermal degradation was 9.6%, meanwhile using HZSM-5, the yield of gases was 44.3%. Park et al. [23] degraded HDPE in thermal and catalytic conditions, at 450– 500 8C, in a fixed-bed semi-batch reactor, using HZSM-5 and Y zeolite among other catalysts. They reported an increase in the yield of gases obtained in the catalytic pyrolysis, from 48.3% (thermal pyrolysis) up to 88.4% (HZSM-5), confirming HZSM-5 as the catalyst, which leads to a higher gas yield. 3.2. Effect of temperature and catalyst in the gas composition Tables 3–5 show the composition of the gas produced in the pyrolysis of HDPE as a function of the fluidized bed temperature under thermal and catalytic conditions. From the results obtained, some interesting aspects can be remarked.
Fig. 2. Variation of the yield of gases obtained in thermal and catalytic pyrolysis.
3.2.1. Effect of temperature in the thermal degradation Within the operating temperature range studied, the gas fraction obtained in the thermal pyrolysis was formed by products distributed in a wide range of molecular weights. This distribution results from the random scission of the long polymeric chain, which is the mechanism followed by the thermal degradation of polyolefinic olygomers [6,15,24].
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Table 3 Yield (g compound/100 g polyethylene) of volatile compounds in thermal pyrolysis of HDPE
Table 5 Yield (g compound/100 g polyethylene) of volatile compounds in catalytic pyrolysis of HDPE with HUSY
Compounds
Compounds
Methane Ethane Ethene Propane Propene Isobutane n-Butane trans-Butene + 1-butene Isobutene cis-2-Butene n-Pentane 1,3-Butadiene 1-Pentene 2-Butyne 1-Butyne n-Hexane 1-Hexene Benzene Toluene Xylene Isopentane cis-2-Hexene Heptane 1-Heptene Non-identified HDPE (g)
Yield (g compound/100 g HDPE) 405 8C
509 8C
604 8C
710 8C
800 8C
0.01 0.03 0.02 0.06 0.36 0.05 0.04 0.30 0.25 0.10 0.03 0.01 0.07 0.01 0.03 0.03 0.07 0.07 0.02 0.01 0.04 0.05 0.02 0.03 1.3 1.9
0.26 0.50 0.56 0.34 1.6 0.02 0.19 1.3 0.64 0.34 0.15 0.12 0.50 0.04 0.01 0.26 0.76 0.35 0.14 0.07 0.02 0.29 0.13 0.66 6.0 1.7
0.51 0.65 1.5 0.48 5.6 0.03 0.26 3.4 1.8 1.1 0.19 0.52 0.86 0.11 0.01 0.56 1.0 0.50 0.23 0.09 0.03 0.53 0.38 0.79 11.2 1.9
4.2 2.9 12.5 0.70 7.6 0.01 0.26 3.9 0.30 0.55 0.13 4.2 1.9 0.31 0.01 1.8 2.5 1.8 1.0 0.12 0.02 0.26 0.65 1.5 8.6 1.9
4.8 3.0 16.5 0.74 10.5 0.01 0.27 5.1 0.24 0.53 0.10 5.0 2.3 0.47 0.02 2.0 2.6 2.2 0.80 0.08 0.02 0.16 0.76 1.5 6.7 2.1
Table 4 Yield (g compound/100 g polyethylene) of volatile compounds in catalytic pyrolysis of HDPE with HZSM-5 Compounds
Methane Ethane Ethene Propane Propene Isobutane n-Butane trans-Butene + 1-butene Isobutene cis-2-Butene n-Pentane 1,3-Butadiene 1-Pentene 2-Butyne 1-Butyne n-Hexane 1-Hexene Benzene Toluene Xylene Isopentane cis-2-Hexene Heptane Non-identified HDPE (g) Catalyst (%)
Yield (g compound/100 g HDPE) 400 8C
509 8C
608 8C
705 8C
804 8C
0.02 0.06 1.3 3.1 16.0 2.5 1.6 7.9 9.0 3.6 0.69 0.02 9.7 0.37 0.42 0.35 0.12 0.34 0.54 0.08 1.3 0.21 0.12 10.2 2.0 20.8
0.10 0.14 2.8 3.8 19.7 4.4 2.0 8.5 7.9 3.8 0.85 0.05 8.3 0.64 1.2 0.50 0.18 0.62 1.9 1.1 3.1 0.24 0.21 14.2 1.6 22.1
0.17 0.26 4.3 5.6 20.4 3.1 2.6 8.2 7.4 3.6 0.94 0.06 6.5 0.50 0.40 0.49 0.11 0.59 1.6 0.22 1.6 0.16 3.1 15.6 2.2 19.4
0.33 0.35 3.9 4.1 21.3 2.0 1.9 9.6 7.0 4.0 0.74 0.18 7.1 0.30 0.31 0.56 0.24 0.69 0.91 0.10 1.1 0.28 0.15 11.3 2.3 18.6
1.4 0.80 5.8 2.9 20.5 1.4 1.4 8.6 6.1 3.6 0.53 0.51 5.8 0.21 0.23 0.86 0.18 0.89 1.3 0.09 0.81 0.26 0.14 9.3 1.7 20.6
Methane Ethane Ethene Propane Propene Isobutane n-Butane trans-Butene + 1-butene Isobutene cis-2-Butene n-Pentane 1,3-Butadiene 1-Pentene 2-Butyne 1-Butyne n-Hexane 1-Hexene Benzene Toluene Xylene Isopentane cis-2-Hexene heptane 1-Heptene 2,2-Dimetilhexane Non-identified HDPE (g) Catalyst (%)
Yield (g compound/100 g HDPE) 402 8C
509 8C
608 8C
705 8C
807 8C
0.03 0.03 0.10 0.26 3.7 2.6 0.30 2.9 0.69 1.3 0.25 0.01 4.3 0.22 1.9 0.25 0.24 0.56 0.04 0.01 3.3 0.35 0.18 0.07 0.43 18.0 2.2 18.6
0.08 0.06 0.88 0.79 10.1 3.7 0.63 5.2 4.4 2.2 0.39 0.03 5.3 0.38 1.7 0.37 0.17 0.27 0.52 0.35 3.6 0.26 0.25 0.07 0.62 18.6 2.0 20.9
0.19 0.11 0.57 0.68 8.3 4.0 0.71 5.6 3.7 2.4 0.53 0.04 5.7 0.37 2.8 0.55 0.27 0.68 0.56 0.40 4.7 0.41 0.36 0.12 0.57 25.7 1.9 20.9
0.41 0.22 0.67 0.67 8.5 3.2 0.72 6.5 4.2 2.7 0.57 0.14 6.7 0.33 2.3 0.69 0.48 1.0 0.78 0.45 3.9 0.63 0.45 0.18 0.68 29.0 1.8 19.9
0.94 0.37 1.1 0.69 7.8 3.2 0.69 5.7 3.7 2.3 0.50 0.29 5.4 0.26 1.7 0.69 0.41 0.61 0.22 0.03 3.6 0.44 0.31 0.08 0.29 18.8 1.9 19.3
In general, most of the products increased their yield by increasing pyrolysis temperature. Obviously, at the lowest operating temperature (400 8C) the yield of the compounds obtained was very low and no compounds whose composition reaches a yield equal or higher than 1% were detected, although considering volumetric fraction, some compounds, such as propene, reach a percentage higher than 17% (v/v). At this reactor temperature, around 26% of the HDPE fed melts without decomposing. At 500 8C, only two compounds (propene and the sum of trans-butene and 1-butene) exceed 1%, although all compounds increased their yield significantly. In the range 400–600 8C, the major compound obtained is propene. At 700 8C the yield of methane, ethene and 1,3-butadiene increased significantly. Some products such as isobutene, npentane, cis-2-butene or cis-2-hexene showed a maximum value at 600 8C while others such as toluene and xylene showed this maximum at 700 8C. The presence of a maximum clearly shows the role of the cracking secondary reactions at high temperatures. It seems that as a result of these cracking reactions, at temperatures higher than 600 8C, the major compound is ethene instead of propene (35%, v/v, of ethene at 800 8C). It is difficult to avoid secondary reactions using a fluidized bed reactor. The presence of secondary reactions in the cracking of polymers using this type of equipment has
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been reported previously. Liu et al. [25] reported a similar behaviour in the pyrolysis of PS in a fluidized bed reactor, showing that primary products were stable until 600 8C and secondary reactions take place when the pyrolysis temperature exceeds this value, and can modify the tendency of the products obtained. From this point on, the cracking of some heavy compounds began, decreasing their yield while the percentage of lighter products increased, due to the fragments of cracking products in secondary reactions. The results reported in this paper about the thermal pyrolysis of HDPE are in agreement with others found in literature. Williams and Williams [6] reported that the main gases produced with LDPE in a fluidized bed reactor from 500 to 700 8C were ethene (from 2.19% at 500 8C to 26.86% at 800 8C), propene (from 1.82% to 18.59%) and butene (from 2.91% to 7.63%). Sakata et al. [22,26] showed that the major gaseous compounds obtained from the pyrolysis of HDPE at 430 8C were propane, propene, ethane and ethene. Mastral et al. [27] reported that C3 compounds were the main gaseous compounds at 645 8C, while ethane was the major product at 780–850 8C. Serrano et al. [15] reported a preferencial formation of C3–C4 hydrocarbons when LDPE was pyrolysed at low temperature, indicating that this fact could be caused by radical b-scission at the polymer backbone and at the LDPE branchings, respectively. This mechanism would explain the results found in this study where propene is the main gas product obtained at low temperatures, before the secondary cracking began to be significant. 3.2.2. Effect of temperature and type of catalyst in catalytic pyrolysis As commented previously, the catalytic pyrolysis produced a higher yield of gas products than the thermal pyrolysis, especially when using the HZSM-5 catalyst. Furthermore, as can be seen in Fig. 2, the temperature in the catalytic decomposition played a less significant role than in thermal degradation. Thus, the yield of gaseous compounds generated under catalytic conditions does not show so important variations as a function of temperature as those detected in the absence of a catalyst. This fact is more evident in HZSM-5 runs than in HUSY experiments (see Tables 4 and 5). The results obtained indicate that the catalysts studied, especially HZSM-5, produce a narrower distribution of products unlike the thermal degradation, probably due to the role of the zeolitic pore specific size on the catalytic pyrolysis process. This distribution is centred in C3 and C4 hydrocarbons, specifically propene, trans-butene, 1-butene, isobutane, isobutene and cis-2-butene; C5 hydrocarbons also show a significant increase, such as 1-pentene and isopentane. The highest differences between thermal and catalytic pyrolysis results are found in the C3–C5 range. Under catalytic conditions, the major compound obtained is propene independently of the pyrolysis temperature (about 30%, v/v, of propene with HZSM-5 and around 20%, v/v, with HUSY).
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Isopentane yield showed a significant increase under catalytic pyrolysis. This product showed a low production in thermal degradation, the maximum yield obtained was 0.04% at 400 8C, while it reached 3.1% at 500 8C using HZSM-5 and 4.7% with HUSY at 600 8C. As commented previously, a significant decrease of the volatile yield can be distinguished at high temperature under catalytic conditions, probably due to the catalyst deactivation. This decrease is also observed in the product yields. By comparing the yield of compounds present in the gaseous fraction, in thermal and catalytic pyrolysis, shown in Tables 3–5, it is observed that the yield of gaseous compounds was higher in the pyrolysis with HZSM-5 than in pyrolysis with HUSY or in thermal degradation, at all the temperatures evaluated. At 400 8C, the yield of propene obtained using HZSM-5 was 44 times higher than in thermal degradation, whereas using HUSY the yield of propene was around 10 times higher than the yield obtained in thermal pyrolysis. At higher temperatures, the yield of propene obtained in thermal degradation increased and the difference between the yields obtained in thermal and catalytic pyrolysis decreased. Only at 700 and 800 8C, some compounds reached yields higher in thermal degradation than in pyrolysis with HZSM-5 or HUSY. Compounds that showed more differences at higher temperatures between thermal and catalytic pyrolysis were methane, ethane and ethene. This fact can be the result of secondary reactions. Data related to catalytic pyrolysis of HDPE were found in literature. Sakata et al. [22,26], by pyrolysing HDPE at 430 8C, indicated that the HZSM-5 catalyst decreased the content of C2–C3 components and increased the content of C4–C5 hydrocarbons significantly. Manos et al. [28] pyrolysed HDPE with a heating rate of 5 K/min and 633 K as final reactor temperature and showed that HZSM-5 produced, fundamentally, alkenes in the range C3–C5, while HUSY produced, especially isobutane and isopentane. These results agree with those reported in this paper. According to some researchers [10,24], the preference of these catalysts towards this range of products can be explained considering the mechanism of degradation over the zeolites. The large macromolecules have to react on the external surface of the zeolite catalyst first, which could be the limiting reaction step. Smaller cracked fragments diffuse into the zeolite pores and undergo further reactions [10]. The primary compounds produced seem to be C1–C5 hydrocarbons, being formed by end-chain scission reactions. The other compounds obtained could be a result of the oligomerization reactions of the olefinic gaseous fractions and random cracking at any position of the polymer chain [12]. The pore size of the zeolite is an important characteristic in the diffusion sep of reactives and products, since it can limit the size product obtained in catalytic degradation favoring some products over other ones, as has been shown previously. This preference is one of the reasons
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Fig. 3. Evolution of major volatile products (C1–C3) vs. temperature in thermal pyrolysis.
Fig. 5. Evolution of major volatile products (C1–C3) vs. temperature in catalytic pyrolysis with HZSM-5.
why zeolites are the preferred catalysts for conversion of plastic wastes [29]. Figs. 3 and 4 show the general tendency of the gaseous compounds obtained as a function of temperature in thermal pyrolysis: on one hand, temperature favors the decomposition of polyethylene and the amount of noncondensable volatiles increases. On the other hand, temperature can also favor the secondary cracking of primary hydrocarbons producing decreases in the yields of some hydrocarbons (such as isobutene or cis-2-butene) at temperatures above 700 8C. Other compounds (such as propene and butene) modify their tendency and at high temperatures do not increase their yield as much as was expected. The yield of compounds such as methane, ethane and ethene increases significantly at temperatures above 600–700 8C, probably due to the contribution of the secondary reactions. As can be seen in Figs. 5–8, the general tendency of products versus temperature in catalytic degradation is different from thermal pyrolysis. Most of them (C3–C5 hydrocarbons mainly) are hardly affected by pyrolysis temperature, and the most significant changes can be found in the range 400–500 8C. This behaviour is more clearly observed using HZSM-5 instead of HUSY. From an applied
point of view, this fact has important consequences since catalytic degradation allows the reduction of pyrolysis temperature without significant modifications in product yield and distribution. This would suggest an important reduction in the amount of necessary energy to obtain a specific product. Lighter hydrocarbons seem to be more dependent on temperature and more affected by secondary reactions. Thus, independently of the catalyst used, propene shows a maximum at 500–600 8C, ethene changes its tendency at the same temperature and methane and ethane increase their yields continuously. In Fig. 9, the percentage of volatiles obtained in the pyrolysis with HZSM-5 at 200 and 300 8C as well as the yield obtained in the thermal degradation at 400 8C are shown. In this figure, it can be seen that at 200 8C, HZSM-5 is hardly active and the yields of gaseous compounds obtained were still low. However, at 300 8C, using the HZSM-5 as a catalyst, some compounds reached a yield higher than those obtained at 400 8C in the thermal degradation. These compounds were olefins with 3–5 carbon number, range of products favored in presence of HZSM-5. Thus, at 300 8C the activity of HZSM-5 began to be significant.
Fig. 4. Evolution of major volatile products (C4–C5) vs. temperature in thermal pyrolysis.
Fig. 6. Evolution of major volatile products (C3–C5) vs. temperature in catalytic pyrolysis with HZSM-5.
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Fig. 7. Evolution of major volatile products (C1–C3) vs. temperature in catalytic pyrolysis with HUSY.
Fig. 8. Evolution of major volatile products (C3–C5) vs. temperature in catalytic pyrolysis with HUSY.
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1. The influence of temperature on volatile yield is more significant in thermal than in catalytic pyrolysis. 2. In thermal degradation, the yield of gases increases with temperature in the range 400–800 8C. 3. At 300 8C the activity of HZSM-5 begins to be significant whereas the thermal degradation begins at 400 8C. 4. HZSM-5 favors gas production in a greater extension (87.4% at 600 8C) as compared to HUSY (76.0% at 700 8C) or thermal degradation (66.3% at 800 8C). 5. In catalytic pyrolysis the distribution of the products obtained is narrower than in thermal degradation. The range of products C3–C5 is favored under catalytic conditions, increasing their yield significantly in relation to thermal degradation. 6. HZSM-5 favors mainly olefins, while HUSY favors olefins and some paraffins in the same range of carbon number. The main products obtained with HZSM-5 and HUSY are propane, propene, trans-butene, 1-butene, isobutene, cis-2-butene, 1-pentene. Isopentane is formed in catalytic pyrolysis, fundamentally, being favored with HUSY versus HZSM-5. 7. The highest yields obtained in thermal pyrolysis are reached for ethene (16.5%) and propene (10.5%) at 800 8C. In catalytic pyrolysis using HZSM-5, the main volatile compounds are propene (21.3% at 700 8C) followed by trans-butene + 1-butene (9.6% at 700 8C) and 1-pentene and isobutene (9.7% and 9.0%, respectively at 400 8C). Using HUSY, propene reaches the highest yield at 500 8C (10.1%), followed by transbutene + 1-butene and 1-pentene (6.5% and 6.7%, respectively, at 700 8C); isopentane shows the highest yield at 600 8C (4.7%). 8. In relation to thermal degradation, the catalytic pyrolysis allows the reduction of the degradation temperature in 300–400 8C in order to obtain a significant yield of propene. A yield of propene similar to that obtained under thermal pyrolysis at 800 8C is reached at 400 8C using HZSM-5 and at 500 8C using HUSY.
Acknowledgements The authors wish to thank CICYT PPQ2001-0733, FEDER, GV (project GRUPOSO3/159) and Ministry of Education, Culture and Sport for financial support.
Fig. 9. Yield of volatile compounds in thermal degradation at 400 8C and in pyrolysis with HZSM-5 at 200 and 300 8C.
4. Conclusions From the study of thermal and catalytic gases obtained in the flash pyrolysis of HDPE in a fluidized bed reactor, the following conclusions can be deduced:
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