Catalytic flash pyrolysis of HDPE in a fluidized bed reactor for recovery of fuel-like hydrocarbons

J. Anal. Appl. Pyrolysis 78 (2007) 272–281 www.elsevier.com/locate/jaap

Catalytic flash pyrolysis of HDPE in a fluidized bed reactor for recovery of fuel-like hydrocarbons ´ ngela N. Garcı´a, Antonio Marcilla * Ma del Remedio Herna´ndez, A Department of Chemical Engineering, University of Alicante, 03080, P.O. Box 99, Alicante, Spain Received 3 August 2005; accepted 16 March 2006 Available online 12 September 2006

Abstract Products obtained in the flash pyrolysis of HDPE in a fluidized bed reactor, in thermal and catalytic conditions (HZSM-5 or HUSY 20% by weight) at four different temperatures (in the range 500–800 8C) have been analyzed in this work focusing on the liquid fraction. The results obtained showed significant differences between condensable compounds generated in presence and absence of catalysts. The liquid fraction obtained without catalyst was composed principally by linear paraffins (C10–C40) and almost no generation of aromatic compounds was observed. The presence of low amounts of zeolite (HZSM-5 or HUSY) led to a significant reduction of the saturated and unsaturated condensable hydrocarbons, while it favored the formation of aromatics and branched paraffins. Compared with the results reached with HZSM-5 zeolite, HUSY produces higher amount of aromatics and branched alkanes and a narrower distribution of products, independently of the pyrolysis temperature. The reactor employed in this work was a fluidized bed reactor, very similar to that used in generation of gasoline-range hydrocarbons at large scale, which allows to illustrate a very useful method for the recovery of these hydrocarbons. # 2006 Elsevier B.V. All rights reserved. Keywords: Zeolite; High density polyethylene; Fluidized bed reactor; Liquid fraction; Catalytic pyrolysis

1. Introduction Nowadays, plastics provide a fundamental contribution to our society. This kind of material is very frequent in daily life as it is found in food packaging, electricity industry, toy industry, containers, etc. With the improvements in living standards in the second half of the twentieth century the amounts of plastics in the wastes increased, rising 98.1 kg per capita consumption of virgin plastics in 2003 [1]. Because of this it is necessary to offer alternatives to remove the plastics. To send the plastic wastes to landfills has been a very habitual practice for many years. The huge volume of plastics generated requires a wide extension of land, moreover plastics do not degrade and remain in municipal refuse tips for decades [2]. Nowadays, incineration is the more widely used way to eliminate plastic wastes. With this method, electric and calorific energy are generated. Plastics are generally high calorific value products ranging approximately from 18,000 to 38,000 kcal/kg [3]. However an uncontrolled incineration can produce serious health and

* Corresponding author. Tel.: +34 96 590 3789; fax: +34 96 590 3826. E-mail address: [email protected] (A. Marcilla). 0165-2370/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jaap.2006.03.009

environment problems due to the possible emission of dioxins, furans, acid gases and heavy metals [1]. For this reason this kind of elimination is being rejected. An interesting alternative for the reduction of plastic wastes is that known as tertiary recycling. Tertiary recycling is the degradation of a polymer to its monomer components by chemical or thermal means. The primary goal of this kind of recycling is the recovery of the monomers, but the conversion of waste polymers to liquid fuel or other valuable products is also possible [4,5] and in many times, practically unavoidable. The pyrolysis process belongs to this type of recycling. Some polymers, e.g. polystyrene, can be decomposed thermally in high yield to the monomers; but other polymeric materials such as polyethylene or polypropylene have a more complicated pyrolysis pattern and it is difficult to obtain a high yield of ethylene or propylene [4,6]. Domestic plastic wastes typically contain 57 wt% of polyolefins, 14% of poly(vinyl chloride), 19 wt% of polystyrene, 5 wt% of other plastics and paper, and 5 wt% of inorganic materials (additives) [7]. The main polyolefin present in these wastes is polyethylene [8–10] and the development and implementation of a tertiary recycling method for this material would be of great importance.

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An interesting alternative in the search of the solution to this problem is the usage of catalysts. With the right selection of the catalyst it is possible to narrow the distribution of products obtained in the pyrolysis process and produce more valuable compounds [11–15]. Zeolites are normally used for this purpose [12–13]. They favor hydrogen transfer reactions due to the presence of acid centers and help the cracking of the polymer. On the other hand, zeolites have specific molecular size pores and the access of the plastic molecules to the catalyst reactive sites, as well as the growth of end products inside the pore, are limited to the pore size [10]. This fact allows compounds with restricted size to be obtained. In addition to the selection of the products obtained in the degradation process, the catalytic pyrolysis of polyolefins has other advantages such as reducing the energy and time for the cracking of the macromolecules. Polymer decomposition is an endothermic process and plastic materials have a poor thermal conductivity. This is why thermal pyrolysis requires a high temperature for the rupture process. If the catalyst is present in the reaction vessel, the degradation process takes place at a lower temperature than the thermal pyrolysis [8,13,15]. Another aspect that must be improved for the development of a successful tertiary recycling method for polymers is the heating transfer during the degradation. One possibility is to use a fluidized bed reactor. The bed is composed of an inert particulate material such as sand, which is used to convey the required heat for the highly endothermic pyrolysis reactions [4]. The fluidized bed has a number of special advantages for the pyrolysis of high density polyethylene. It is characterized by excellent heat and mass transfer that can reduce the time necessary for the reaction, has a low tendency for clogging with molten polymer during the degradation process and has the ability to maintain a nearly constant temperature throughout the reactor [6,15]. This system of heating based on a bed that covers the polymer for the transfer of heat has been widely used in many studies [4,5,9,16,17]. With regard to the products that can be obtained in the pyrolysis of plastics, three fractions can be collected: solid residue, gaseous products and condensable volatiles. Hydrocarbon oils have a good combustibility and a good calorific value [18,19]. The oils may be used as a feed for a petroleum refinery catalytic cracker in the manufacture of gasoline [20]. Moreover, if the liquids obtained in the degradation process are in the gasoline range or heavier fraction, they can be used directly as fuel for transportation [21]. For these reasons, the study of liquid compounds obtained in the pyrolysis of plastics is an interesting subject. One of the drawbacks of this type of processes was their doubtful economy. The natural dispersion of a residue of this type, the lack of large scale collection and aconditioning facilities, the need of transportation to treatment plants, the wide range of products that they originate and the need of refining or post conditioning, etc., are powerful arguments to be considered. But the knowledge of the effect of different catalysts facilitating the process, narrowing the range of products derived, reducing the temperature and time required and specially the last and deep oil crisis, that has dramatically

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increased the prices of crudes makes the recovery of plastics by catalytic pyrolysis increasingly interesting. The objective of the present work is to study the thermal and catalytic pyrolysis of HDPE in a fluidized bed reactor focusing on the liquid fraction that has been extensively analyzed. This type of reactor provides results complementary to those obtained in other analytical equipments such as TG, and closer to the industrial operation. In the literature, several papers studying the thermal liquid fraction obtained from the polymer pyrolysis [8,11,13,22–26] as well as that obtained from catalytic polymer decomposition [18,20,27–32] can be found. In general, the information shown in these papers is limited to the global yield of non-volatile compounds, total alkane or total alkene yields. Only some of them present the composition of the liquid fraction classified following a molecular weight or boiling point distribution. Most of these works have performed the study at one unique temperature and only few of them evaluate the non-condensable compounds at a temperature range of 400–600 8C in the catalytic cases or up to 700 8C in thermal decomposition. In the present paper, the composition of liquid fraction is analyzed in detail, showing its variation with the carbon number and the process temperature, using two different types of catalysts. Therefore, the present paper shows a different focusing on the study of the pyrolytic condensable compounds which can contribute to the knowledge of this fraction in order to evaluate its specific applicability. 2. Experimental 2.1. Material Powdered BP Chemicals high density polyethylene with a density of 935 kg/m3 and 210–500 mm particle size is used in this study. The zeolites employed as catalyst are HZSM-5 and HUSY supplied by Grace GMBH & Co. KG and their characteristics are shown in Table 1. To measure these properties, the following apparatus are used: the SiO2/Al2O3 ratio is measured by fluorescence in a Philips apparatus, model PW1480; structural characteristics of zeolite (pore size, BET surface area, pore volume, external surface area and micropore Table 1 Catalyst characteristics Characteristics 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 Tmax (8C)c a

1.15 166

HZSM-5

HUSY

22.2 0.55 341 37.6 0.18 <37 0.16

4.8 0.74 614 28.1 0.35 <37 0.29 2.15 154

2.15 154

Obtained by application of the t-plot method. Measured at p/p0: 0.995. c From ammonia TPD. Two peaks at two different temperatures were detected for HZSM-5 corresponding to two types of acid centres. b

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volume) are measured using nitrogen isothermal adsorption at 77 K in an automatic apparatus AUTOSORB-6 supplied by Quantachrome; thermal programmed desorption (TPD) of NH3 is carried out in a thermogravimetric balance Netzsch TG 209, and two peaks are detected for HZSM-5. Zeolites are analyzed by electronic microscopy (SEM and TEM techniques) detecting many agglomerates formed by different crystals. Zeolites are sieved and agglomerates with a size lower than 37 mm are selected for the runs. In the catalytic experiments, the zeolite used is mixed with HDPE in a proportion of 20% by weight, in a Brabender plasticorder mixer at 140–160 8C. Rods obtained are then cut, ground and sieved. A particle size in the range 210–420 mm is selected for the experiments. Previous experiments in a TG equipment show that this ratio is enough to guarantee a large extent of catalytic decomposition with polyethylene in the presence of HZSM-5, reaching their maximum effect at a ratio equals to 10%, keeping almost constant this effect above this proportion. Manos et al. [33] show similar curves for the case HDPE-HUSY catalyst observing that proportion above 33% of catalyst slightly increase the overall degradation rate, 20% proportion being a proportion close to the maximum effect reached. Different standards in the range C10–C23 compounds are used to identify the hydrocarbons (saturated and unsaturated) and aromatics obtained. Solutions in hexane of these standards in the range 1–500 ppm are prepared in order to obtain three average response factors to quantify the compounds obtained: paraffins, olefins and aromatics. 2.2. Experimental system The equipment used for the flash pyrolysis of polyethylene is a fluidized bed reactor. A diagram of the reactor employed is shown in Fig. 1. As can be seen, the reactor, glass traps, gasometer and sampling bag are connected on line. The body of the reactor is a 71 cm high cylinder with 5.8 cm of 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 at the entrance. The reactor is heated by a cylindrical refractory oven. The range of temperatures studied is 500–800 8C.

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 and (J) gas sampling bag of 25 l.

The inert bed is sand of 70–210 mm particle size supplied by Resacril, s.l. The fluidization agent is nitrogen. The velocity of nitrogen is maintained constant inside the reactor independently of the temperature. The flow used inside the reactor is 3500 ml/min measured at the process temperature, which is assumed to be 2.8 times the minimum fluidization velocity of the sand at 500 8C (the lowest operating temperatures) and 3.7 times the minimum fluidization velocity of the sand at 800 8C (the highest operating temperature). The reactor is not isothermal (temperature ranging from that of the sand bed to approximately 300 8C at the reactor head). A rigorous correlation model developed previously which includes primary and secondary reactions, heat transfer processes and expansion of overall volatiles [34,35] allows us to estimate an average residence time for the volatiles in the reactor. This value is in the range 1.3–1.7 s for the experiments evaluated in this paper. 2.3. Experimental procedure Experiments are carried out as follows: the reactor is programmed to the selected temperature and the top reactor heating system is switched on to keep the temperature of this zone at 300 8C. A valve allowed the flow direction to change (to the gas sampling bag or to the exit). During the heating time, the system is purged with nitrogen. Two grams of HDPE (or 2 g of mixture 80% HDPE + 20% catalyst in the catalytic runs) are placed in the feed hopper and dropped onto the hot fluidized sand bed when the experiment began. At that moment, the nitrogen flow is shifted to the sampling bag, and the time is recorded. Condensable products generated are trapped in the stainless steel Dixon rings of the glass traps while gases are collected in a sampling bag (25 l). 2.4. Product analysis In each experiment four fractions are collected: the gas fraction, the condensable compounds, the wax fraction and the solid residue. The gas fraction is collected in a Tedlar bag and its composition is analyzed by gas chromatography and reported elsewhere [36]. The stainless steel Dixon rings of the glass traps are washed with hexane to collect the liquid and wax fractions. Both fractions are separated by filtering and the liquid one is concentrate with a rotavapor. These compounds are identified and quantified by an Agilent GC–MS (GC 6890N-MD 5973N) with a HP-5MS column (30 m  0.25 mm i.d.). The column program is: injector temperature, 250 8C; initial column temperature, 40 8C; initial time, 5 min; heating rate, 12 8C/min; final temperature, 320 8C; final time, 25 min; run time, 53.33 min; carrier gas, He, 1 ml/ min; average velocity, 38 cm/s; solvent delay, 6 min. Library Wiley 275 is used for the identification of the saturation degree of the liquid compounds (paraffins, olefins or

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Table 2 Yield of fraction distribution in thermal and catalytic pyrolysis (%, by weight) Thermal pyrolysis T (8C) Gases Liquids Waxes Solid residue a

a

500 15.2 40.9 7.4 36.5

600 32.4 37.6 5.4 24.6

HZSM-5 pyrolysis 700 57.8 24.0 6.7 11.5

800 66.3 13.6 5.8 14.3

500 88.3 4.3 5.0 5.6

600 82.6 4.4 6.7 8.5

HUSY pyrolysis 700 90.9 8.8 2.5 3.1

800 87.2 6.8 1.8 9.4

500 83.4 9.7 1.0 10.9

600 79.3 12.6 0.1 7.9

700 86.9 11.6 0.6 4.3

800 90.2 9.8 0.4 4.8

In the thermal case, solid residue was calculated by difference.

aromatics). On the other hand, the carbon number of the hydrocarbons detected is determined by their coincidence with the standards injected (1-decene, 1-hexedecene, n-hexadecane, 1-eicosene, n-eicosane, n-docosane, n-tricosane, naphthalene, 5-tert-butyl-m-xylene, isobutylbenzene and 1,3-diisopropenylbenzene) as well as the compounds identified in the thermal liquid (n-paraffins, 1-alkenes and diolefins) previously studied, where, due to the compound distribution, the identification of peaks is much easier. Paraffins and olefins different from those present in thermal pyrolysis (branched paraffins, other olefins different from 1-alkenes) are quantified by groups. Thus, paraffins or olefins located between Cn and Cn+1 thermal paraffins are quantified together and assigned to Cn carbon number. The solid fraction is calculated by weight difference after calcination of the sand together with the solid residue. 3. Results and discussion 3.1. Effect of temperature and catalyst on the product yields Three sequences of experiments (thermal degradation and catalytic pyrolysis using HZSM-5 or HUSY as catalyst) at four different temperatures (500, 600, 700 and 800 8C) are carried out in order to study the effect of the temperature and the

presence of catalyst on the yield of the pyrolytic products obtained. Table 2 shows the evolution of the fractions analyzed with temperature in catalytic and thermal degradation. In the catalytic pyrolysis, the mass balance calculated is always in the range 99–105%. In the thermal case, solid residue was calculated by difference. By analyzing the results shown in Table 2, it can be observed that the influence of the temperature on the gas fraction yield is more significant in thermal than in catalytic pyrolysis. According to the results presented in Table 3, the presence of zeolites in the pyrolysis favors the yield of C3–C5 olefins. Paraffins in this carbon range also show a significant increase. In general, it seems that HZSM-5 tends to increase the yield of C3–C4 compounds more than HUSY, but HUSY produces higher yields of C5 hydrocarbons. Gas composition is studied extensively elsewhere [36]. The amount of liquid compounds generated is much lower in catalytic than in thermal pyrolysis. As can be seen in Table 2, thermal liquids decreased monotonally by increasing temperature. A less marked reduction is also observed in the wax fraction. According to the values shown in Table 2, it seems that thermal cracking follows a different behavior than the catalytic one. Thus, by increasing temperature in thermal pyrolysis, the wax and liquid fraction decompose increasing gas yields. In catalytic pyrolysis, gas yield is almost independent of temperature and yields as high as 88.3% with HZSM-5 or

Table 3 Yields of C1–C7 volatile compounds obtained in thermal and catalytic HDPE pyrolysis Compounds

Thermal

HZSM-5

500 8C

600 8C

Paraffins C1 C2 C3 C4 C5 C6 C7

0.26 0.50 0.34 0.21 0.17 0.26 0.13

Olefins C2 C3 C4 C5 C6 C7

0.56 1.62 2.36 0.50 1.05 0.66

HUSY

700 8C

800 8C

500 8C

600 8C

700 8C

800 8C

500 8C

600 8C

700 8C

800 8C

0.51 0.65 0.48 0.28 0.22 0.56 0.38

4.18 2.91 0.70 0.26 0.15 1.80 0.65

4.76 2.99 0.74 0.28 0.12 2.05 0.76

0.17 0.21 5.86 6.58 4.06 0.19 0.14

0.53 0.39 5.51 6.18 3.66 0.15 0.16

4.44 1.57 5.12 5.10 3.00 0.05 0.23

11.01 2.27 2.19 1.49 0.66 1.78 0.16

0.72 0.38 0.85 5.94 7.16 0.99 0.00

0.83 0.44 0.85 5.70 6.69 0.99 0.43

1.83 0.93 1.19 6.37 7.04 1.28 0.33

4.88 1.73 1.19 4.88 4.85 0.71 0.20

1.50 5.65 6.88 0.86 1.54 0.79

12.48 7.62 9.04 1.86 2.79 1.45

16.46 10.49 10.86 2.28 2.81 1.49

2.99 19.46 25.67 10.30 0.49 0.00

4.64 21.66 23.02 6.39 0.30 0.00

9.52 25.99 22.61 4.67 0.25 0.00

15.14 20.50 15.05 2.91 0.38 0.00

0.87 7.60 13.06 6.31 1.49 0.19

0.97 7.89 13.41 6.23 1.45 0.16

2.03 11.04 16.31 6.78 1.25 0.14

5.63 13.98 17.77 5.65 1.03 0.11

C4 paraffins: isobutane and n-butane; C5 paraffins: n-pentane and isopentane; C4 olefins: trans-butene, 1-butene, isobutene, cis-2-butene and 1,3-butadiene; C6 olefins: 1-hexene and cis-2-hexene.

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Fig. 2. Evolution of n-alkane compounds (C10–C40) in the HDPE thermal pyrolysis.

83.4% with HUSY are obtained at 500 8C as compared to 15.2% in thermal conditions. The wax fraction obtained in presence of HZSM-5 is higher than that obtained with HUSY at all temperatures evaluated, opposite to the liquid fraction that present higher yields using HUSY as catalyst. Results found in literature are in agreement with this tendency. Several researchers [32,37,38] have pyrolyzed different polymers such as HDPE, LLDPE and PP using different catalysts including HZSM-5 and HUSY. Their results showed that HUSY tends to produce higher yields of liquids than HZSM-5. 3.2. Effect of temperature on the liquid composition in thermal pyrolysis In the analysis of the liquid compounds obtained in the thermal degradation, triplets formed by alkadiene, alkane and 1-alkene with the same carbon number, from C10 to C40 can be distinguished. Other minor ‘‘non-terminal’’ olefins are also detected which showed a better resolution and a significant intensity at low temperature and molecular weight. Some researchers have shown similar groups of hydrocarbons in the pyrolysis of different polymers. Williams and Williams [9] reported series of triplets of alkadienes, alkenes and alkanes with the same carbon number by pyrolyzing LDPE in a fluidized bed reactor from 500 to 700 8C. Bagri and Williams [20] reported similar triplets from the degradation of LDPE in a fixed bed reactor at 500 8C. Predel and Kaminsky [39] pyrolyzed HDPE, PP and PS at 510 8C in a fluidized bed reactor and showed that the liquid fraction obtained was formed

by alkadienes, alkenes and alkanes with the same carbon number, from C15 to C40. McGrattan [40] also showed that triplets of -diene, -ene and -ane compounds were present in EVA degradation products. Figs. 2–4 show the variation of yields of alkanes, 1-alkenes and alkadienes with carbon number in thermal pyrolysis at each temperature. By observing these figures, it can be affirmed that, at 500 8C, alkanes are the major compounds of thermal liquids while, at higher temperatures (600–800 8C), 1-alkenes reach the highest yields. As can be observed, the paraffins yield decreased by increasing temperature (from 500 to 800 8C), the alkane behavior being very similar at any temperature evaluated: paraffins distribution is located in the range C10–C33 without observing a clear maximum in the distribution. Only a slight increase of the paraffin yields in the range C14–C24 could be detected. The maximum yields of 1-alkenes and alkadienes are observed at 600 8C. As in the case of paraffins, the olefin distribution is spread up to C30 hydrocarbons, with a slight maximum centered in this case around C13–C16. In the alkadiene distribution, heavier compounds than C28 are not detected. The maximum of the distribution is centered around C15–C18. As was commented on previously, at low temperatures (500– 600 8C) and low molecular weighs (up to C20, approximately), other olefins different from 1-alkenes are detected. Their yields

Fig. 3. Evolution of 1-alkene compounds (C10–C40) in the HDPE thermal pyrolysis.

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Fig. 4. Evolution of alkadiene compounds (C10–C30) in the HDPE thermal pyrolysis.

decrease by increasing the temperature and the molecular weight of the alkene. A brief summary of the thermal liquid composition can be seen in Table 4. 3.3. Effect of temperature on the liquid composition in catalytic pyrolysis Significant differences can be observed between thermal and catalytic results, since with catalyst, the triplets formed by alkanes, alkenes and alkadienes, typical in thermal pyrolysis, are not distinguished. Contrarily to the thermal pyrolysis, where only linear hydrocarbons are detected, in the catalytic degradation carried out using both zeolites, HZSM-5 or HUSY, the presence of two different types of alkanes is observed: Table 4 Yields of n-alkanes, branched alkanes, total olefins and aromatics obtained using HUSY as catalyst (g compound/100 g HDPE pyrolyzed) Compounds

Thermal N-Alkanes Branched alkanes 1-Olefins Non-terminal olefins Alkadienes Aromatics

Temperatures (8C) 500 8C

600 8C

700 8C

800 8C

18 – 15 6.7 1.7 –

6.0 – 23 3.6 5.6 –

3.9 – 14 1.6 4.9 –

1.6 – 7.5 0.52 3.9 –

HZSM-5 N-Alkanes Branched alkanes 1-Olefins Non-terminal olefins Alkadienes Aromatics

0.54 0.13 0.10 0.57 – 2.9

0.49 0.12 0.10 0.57 – 3.2

2.3 2.7 0.09 0.22 – 3.9

0.26 0 0.10 0.07 – 6.4

HUSY N-Alkanes Branched alkanes 1-Olefins Non-terminal olefins Alkadienes Aromatics

0.45 3.1 – 0.36 – 5.8

0.64 4.5 – 0.63 – 6.9

0.51 3.3 – 0.41 – 7.3

0.48 2.0 – 0.47 – 6.9

n-alkanes, similar to those obtained in thermal degradation, and branched alkanes which are not detected under thermal conditions. Olefins similar to those observed in thermal degradation (1-olefins) are not detected in catalytic pyrolysis with HUSY, while they are obtained using HZSM-5 in low proportion. On the other hand, ‘‘non-terminal’’ alkenes are found in the liquid fraction generated with both catalysts. Contrarily to thermal liquids, aromatic compounds have also been identified in the condensables obtained under catalytic conditions. In Figs. 5–11 the yields reached for the compounds detected at every temperature evaluated are shown. It can be seen that, in general, the influence of temperature on the liquid composition in the presence of HUSY is not significant, contrarily to the influence of this parameter on the HZSM-5 liquids. The results obtained seem to indicate that the behavior of HZSM-5 is more thermally affected than HUSY, the compounds generated being dependent on the temperature used during the pyrolysis. In all the cases evaluated, paraffins are the main linear compounds in the liquid fraction, although different types of paraffins could be distinguished depending on the catalyst used: with HZSM-5 the main compounds obtained are n-paraffins, meanwhile branched paraffins show the highest yields using HUSY. In all the cases evaluated, components in the catalytic liquid reach lower yields than those in thermal liquid except for aromatics. In the catalytic degradation using HZSM-5 (Figs. 5–8) it can be seen how the highest yields of n-alkanes shift to higher carbon numbers by increasing temperature in the range 500– 700 8C. Thus, at 500 8C, the highest yield is observed at C9; at 600 8C the maximum of the distribution is centred around C12– C15 while at 700 8C, the maximum is found at higher carbon numbers, around C29 (Fig. 5). At 700 8C n-alkanes show higher yields than at 500–600 8C. The yield of n-alkanes at 800 8C is very low and the presence of a maximum with the carbon number is not distinguished. In the case of branched alkanes the tendency observed is similar to that shown by n-alkanes. At low temperatures, these compounds are centered in the range C10– C15 and no branched paraffins higher than C15 are detected. On the contrary, at 700 8C, heavier branched alkanes appear and

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Fig. 5. Evolution of n-alkane compounds (C8–C40) in the HDPE catalytic pyrolysis using HZSM-5 as catalyst.

Fig. 6. Evolution of branched alkane compounds (C8–C34) in the HDPE catalytic pyrolysis using HZSM-5 as catalyst.

those with a carbon number lower than 20 showed negligible yields. At 800 8C, branched paraffins are not found at any carbon number (Fig. 6). In the case of HUSY (Fig. 9), n-paraffins are present in the liquid fraction, up to 22–24 atoms of carbon, while branched alkanes are not detected above C19 (Fig. 10). The narrower distribution of branched alkanes is also observed by analyzing the interval of paraffins with the highest yields. Thus, the highest yields of linear alkanes are located around C10–C16 at all temperatures evaluated, while the yield distribution of branched alkanes is centred in the range C10–C13.

Alkenes detected in the catalytic liquids with both zeolites show different behaviors, HZSM-5 producing higher yields and heavier olefins than HUSY. Using HZSM-5 as catalyst, 1-alkenes are detected in the range C11–C26, and the distribution ‘‘yield versus carbon number’’ shows also a maximum shift: it is centered at C11–C13 at 500 8C, at C13 at 600 8C and at C16 at 700–800 8C (Fig. 7). As was commented on previously, this type of olefins is not obtained using HUSY at any temperature evaluated. Other alkenes different from 1-olefins (‘‘non-terminal’’ alkenes), which were hardly found in thermal degradation, are

Fig. 7. Evolution of 1-alkene compounds (C8–C26) in the HDPE catalytic pyrolysis using HZSM-5 as catalyst.

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279

Fig. 8. Evolution of ‘‘non-terminal’’ alkenes compounds in the HDPE catalytic pyrolysis using HZSM-5 as catalyst.

Fig. 9. Evolution of n-alkane compounds (C8–C29) using HUSY as catalyst.

obtained using both zeolites. With HZSM-5, this type of unsaturated compounds are detected in the range C11–C26 (Fig. 8). The maximum yields of these hydrocarbons are reached by olefins with 12–13 carbon atoms at 500–600 8C, decreasing the yield at higher carbon numbers. At 700 8C, a clear maximum is not observed and at 800 8C, significant yields of these olefins are only found around C12–C14. In the case of HUSY (Fig. 11) the distribution range is narrower and no alkenes heavier than C18 are found. The highest yields are centred around C16–C17 at any temperature evaluated.

Alkadienes are not detected in the catalytic liquids neither using HZSM-5 nor HUSY. Aromatic compounds (including benzene derivates, naphthalene, naphthalene derivates and up to three ring aromatic hydrocarbons) present the highest yields of the condensable products at all the temperatures studied using both catalysts, although they are not observed under thermal conditions. However, benzene, toluene and xylene are detected in gas fraction in catalytic as well as in thermal pyrolysis in a significant proportion [36]. The aromatic values shown in Table 4 show that, in presence of HUSY, the yields

Fig. 10. Evolution of branched alkane compounds using HUSY as catalyst.

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280

Fig. 11. Evolution of alkenes obtained using HUSY as catalyst.

are very similar (around 6–7%), practically independent of the process temperature. This tendency is different from that observed in the presence of HZSM-5, where the aromatic yields seem to increase at high temperature (700–800 8C). Some results found in the literature agree with those present in this paper. For example, Bagri and Williams [20] reported that oils obtained in the degradation of LDPE between 400 and 600 8C, in a fixed bed reactor, contained single aromatic rings and polycyclic aromatic hydrocarbons with HZSM-5 and Yzeolite. Sakata et al. [41] pyrolyzed HDPE at 430 8C under semi-batch conditions, using different catalysts including HZSM-5. They reported that liquids obtained from the thermal degradation contained paraffins and olefins but no aromatics, while aromatic compounds were detected under catalytic conditions. Different authors [20,37] found that catalytic pyrolysis of HDPE with HUSY or Y-zeolite produces higher yields of aromatic liquid compounds than catalytic pyrolysis with HZSM-5. This fact is different from that observed in the gas fraction, where the percentage of aromatics reached is higher with HZSM-5 than with HUSY in most of the cases analyzed [40]. A brief summary of the catalytic liquid composition can also be seen in Table 4. It can be observed that the amount of branched alkanes and aromatics is significantly higher in the presence of HUSY than in the other two cases (HZSM-5 and thermal pyrolysis), at all the temperatures evaluated. Linear paraffins and olefins reach much higher yields in thermal than in catalytic conditions. Differences between HUSY and HZSM5 are not significant for yields of these two types of compounds. 4. Conclusions In this work, the yields of liquid compounds obtained in the flash pyrolysis of HDPE are shown. The effect of bed temperature (500–800 8C) and the presence of HZSM-5 and HUSY as catalysts have been evaluated. From the experiments carried out, the following conclusions can be obtained: 1. In the thermal degradation, liquids contain a wide distribution of products. Groups of diolefins, olefins and paraffins with the same carbon number, from C10 to C40 are present in thermal liquids. 2. In thermal pyrolysis, n-paraffins are the main hydrocarbons obtained at 500 8C decreasing by increasing the temperature.

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