Valorization of Waste Agricultural Polyethylene Film by Sequential

Ind. Eng. Chem. Res. 2009, 48, 8697–8703

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Valorization of Waste Agricultural Polyethylene Film by Sequential Pyrolysis and Catalytic Reforming Guillermo San Miguel,*,† David P. Serrano,‡,§ and Jose´ Aguado§ Department of Energy Engineering, School of Industrial Engineering (ETSII), UniVersidad Politećnica de Madrid, c/ Jose´ Gutie´rrez Abascal, 2, Madrid, 28006, Spain, Madrid Institute for AdVance Studies on Energy (IMDEA Energıá), Mo´stoles, 28933, Spain, and Department of Chemical and EnVironmental Engineering, ESCET, UniVersidad Rey Juan Carlos, c/ Tulipań s/n, Mo´stoles, 28933, Spain

This paper deals with the potential of using sequential pyrolysis and catalytic reforming for the conversion of agriculture film waste into useful hydrocarbon products. The experiments were conducted in a two-step reaction system consisting of a pyrolytic batch reactor (450 °C) connected in series to a secondary fixed bed reactor where the organic vapors were reformed at temperatures between 425 and 475 °C. Two conventional zeolites (HZSM-5 and Beta) and a mesostructured aluminosilicate Al-MCM-41 were used as catalysts in the reforming stage. Conversion values were not affected by the temperature in the reforming stage and remained fairly constant in all the experiments (89-92 wt %). In the absence of catalyst, the process generated a high proportion of hydrocarbons in the gasoline (C5-C12) and diesel (>C13) range (between 51 and 56 wt % and between 18 and 19 wt %, respectively) and a consequently lower amount of light hydrocarbon products (between 17 and 23 wt %), all of which consisted essentially of a mixture of n-paraffins and olefins. Catalytic reforming over HZSM-5 favored the formation of light hydrocarbons (up to 53 wt %) consisting primarily of C3 and C4 olefins. Catalytic reforming over HZSM-5 also favored the formation of aromatics (up to 12.7 wt %), iso-parafins (8.9 wt %), and naphthenes (4.0 wt %) in the gasoline (C5-C12) fraction. Owing to their weaker acid properties, zeolite Beta and Al-MCM-41 exhibited inferior reforming activities to zeolite HZSM5, as evidenced by the lower proportion of light hydrocarbons products and the reduced concentration of nonparaffinic products in the heavier fractions. The influence of reforming temperature on product distribution was not significant in the range 425-475 °C. Introduction Despite a significant improvement over the past few years and the enactment of increasingly stringent legislation in the field of waste management, 53% of all the waste plastics generated in Europe1 is still disposed in landfills. Feedstock recycling covers a wide range of chemical and thermal processes aimed at transforming the waste polymers into hydrocarbon products for use as petrochemical feedstock. Despite their potential, the commercial application of these technologies still remains very marginal (2%) owing primarily to technical and economic constraints.1,2 Agricultural activities absorb about 2% of all the plastics consumed in Europe.1 In this sector, plastic film is widely used for greenhouse covering and crop protection. This practice enables both water savings and a better temperature control of the crops for increased agricultural output. However, owing to the action of light and oxygen, the plastic loses its mechanical properties and needs to be replaced every 3-4 years.3 Most agriculture film is made of low density polyethylene (LDPE) although it usually contains smaller proportions of other polymers like ethylene vinyl acetate (EVA) copolymer as well as additives for improved performance and resistance to degradation. The characteristics of this plastic residue, relatively homogeneous and scarcely contaminated by other components, make it suitable for valorization. Thermal cracking of polyethylene (PE) has been described to occur according to a random scission mechanism that * To whom correspondence should be addressed. E-mail: g.sanmiguel@ upm.es. † Universidad Politećnica de Madrid. ‡ Madrid Institute for Advance Studies on Energy (IMDEA Energıá). § Universidad Rey Juan Carlos.

generates a mixture of linear n-paraffins and R-olefins over a wide range of molecular weights.4,5 However, the commercial viability of converting waste polyethylene into useful hydrocarbons by direct thermal treatment has not been proven, owing primarily to the low market price of the resulting products.2 The use of catalysts opens a whole new range of opportunities in the field of feedstock recycling that may be the key to commercial success. A large number of porous materials exhibiting acid properties have been tested for their catalytic activities in the thermal cracking of PE. The most frequently used include conventional6-10 and modified10-12 zeolites, amorphous silica/alumina,7 clay,13 and mesostructured solids.10,12,14,15 Zeolites are crystalline aluminosilicate minerals characterized by a well-developed micropore structure.16 Owing to their acid properties, synthetic zeolites are commercially used as catalysts in numerous industrial processes, such as the cracking of petroleum fractions (fluid catalytic cracking (FCC)). Al-MCM41 is a noncrystalline aluminosilicate characterized by an extended surface area and mesoporous structure.17 Most of these papers describe the catalytic cracking of PE in batch reactors where the polymer is placed in contact with different proportions of catalyst. An evident benefit of this approach relates to a reduction in the cracking temperature in comparison with the noncatalytic process, which in commercial terms may be translated into energy savings. However, an even more important aspect relates to the product selectivity exhibited by some catalysts, which allow the process to be directed toward a narrower distribution of hydrocarbon products with higher market values. However, direct catalytic cracking of plastics suffers from a number of drawbacks which have prevented its commercial success. The first one relates to the difficulty to recover the

10.1021/ie900776w CCC: $40.75  2009 American Chemical Society Published on Web 08/17/2009

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Ind. Eng. Chem. Res., Vol. 48, No. 18, 2009

catalyst after use, which increases the operating costs and discourages the development and use of more expensive materials with improve catalytic properties. Furthermore, as catalysts are in intimate contact with the plastic, they are prone to be rapidly deactivated due to the formation of char and also to the poisoning effect of impurities. In addition, owing to molecular size restrictions, the diffusion of bulky polymeric molecules through the porosity of microporous catalysts (zeolites) is strongly impeded, thus reducing their catalytic activity and potential use.15 Some of these problems may be overcome separating the pyrolytic stage from the catalytic reforming reaction. This approach has been tested by Bagri and Williams18 and Aguado et al.,19 who investigated the catalytic reforming of hydrocarbon vapors generated from the pyrolysis of pure LDPE and reported an increase in the proportion of aromatic and small molecular weight hydrocarbons. Catalytic conversion of real waste plastics poses greater difficulties than pure polymers owing to differences in their chemical structure derived from prolonged exposure to ambient conditions and also the presence of both organic and inorganic impurities. Hence, we considered that it was essential to investigate the performance of the two stage approach on a real plastic residue. In this paper, we describe the conversion into hydrocarbon products of a waste agriculture film by sequential pyrolysis and catalytic reforming. The pyrolysis reaction was always conducted at 450 °C in order to achieve a comparable evolution of hydrocarbon vapors, while the reforming reaction was carried out at 425, 450, and 475 °C. The catalysts investigated included two types of zeolites (HZSM-5 and Beta) and a mesostructured Al-MCM-41 solid. Conversion values and product yields were determined for each trial, and the hydrocarbon products were characterized for their carbon atom number and hydrocarbon type. Experimental Section Plastic Polymers. Waste plastic film employed as agriculture greenhouse cover in Almeria (Spain) was used in this work. As described elsewhere,9 this material consists primarily of LDPE but also contains 4 wt % of EVA copolymer and 1.6 wt % ash. The waste plastic was supplied in the form of small fragments of size 1-3 mm. Catalysts Synthesis and Characterization. The catalysts employed in this work include conventional HZSM-5 and Beta zeolites and mesostructured Al-MCM-41. These materials were synthesized in our laboratories according to procedures published elsewhere.15 The calcined materials were characterized for their Si/Al atomic ratios by inductively coupled plasma (ICP) using a Varian VISTA AX CCD spectrophotometer. A Micrometrics ASAP 2010 nitrogen gas adsorption analyzer was used to characterize their textural properties. Total surface areas were calculated by application of the BET equation, surface areas not associated to micropores (SEXT+MESOPOR) and total micropore volumes were determined using the t-plot method, and total pore volumes were calculated from the volume of nitrogen adsorbed at p/p0 ) 0.90. Acid properties were determined by ammonia temperature programmed desorption (TPD) in a Micrometrics 2910 TPD/TPR apparatus. Zeolite crystal sizes were estimated by transmission electron microscopy (TEM), using a Philips TECNAI 20 microscope. Thermogravimetric Analyses. Thermogravimetric analyses (TGAs) were conducted using a TA Instruments SDT 2960 simultaneous TGA-DSC (differential scanning calorimeter) analyzer. The plastic samples (10.0 ( 0.2 mg) were loaded into

platinum microcrucibles and heated from 50 to 700 °C, at 10 °C min-1 in flowing nitrogen (100 mL min-1). Catalytic experiments involved placing 1.0 ( 0.1 mg of catalyst into the microcrucible prior to the addition of the polymer for a total mass of 10.0 ( 0.2 mg. Temperatures marking maximum degradation rates (Tmax) were determined from the derivative thermogravimetric (DTG) plots. Duplicate analyses conducted on selected samples produced deviations lower than (1 °C. Catalytic Conversion in the Two-Stage Reactor. The reaction system employed in this work, described in more detail elsewhere,19 consists of two independent electrically heated quartz vessels connected in series one on top of the other. The bottom reactor is cylindrical in shape (140 mm high and 20 mm i.d.) and is loaded with the plastic material for its thermal cracking. The vapors generated in the pyrolytic reactor are carried over to the fixed bed catalytic reactor (160 mm long and 13 mm i.d.) where the catalytic reforming takes place. In a typical experiment, 5.0 g of plastic are loaded into the pyrolytic reactor and 0.5 g of catalyst (HZSM-5, Beta, or AlMCM41) are placed inside the catalytic reactor. The system is purged with 40.0 mL/min of nitrogen, and the catalytic reactor is heated to a final set temperature of either 425, 450, or 475 °C. When the catalytic reactor reaches its final temperature, the pyrolysis reactor is heated at a constant 10 °C/min to 450 °C and kept at this temperature for 120 min, which allowed a steady evolution of pyrolytic vapors. Experiments using similar reaction conditions but in the absence of catalyst were also conducted for comparative purposes. Volatile products coming off the catalytic reactor were directed into an ice trap where heavier hydrocarbons were condensed and separated from the gas fraction, which was collected in a Tedlar bag for characterization. Conversion values describe the proportion of polymer not remaining inside the reaction vessel after the completion of each trial. Liquid fraction yields were determined by weight from the amount of products condensed in the ice trap. Gas fraction yields were determined by weight taking into consideration the volume and the composition of the noncondensable fraction. Transformation of gas volumes into mass values was calculated under ideal conditions (25 °C and 1 atm). Analysis of Hydrocarbon Products. Liquid and gas products were independently characterized by PIONA analysis (nparaffins, iso-paraffins, olefins, naphthenes, and aromatics). The analyses were conducted using a VARIAN CP-3800 chromatograph with a Chrompack CP SIL PIONA (100 m length, 0.25 mm id) column and flame ionization detector (FID). Liquid samples were injected after dilution in carbon disulfide (CS2), and gas samples were injected from the Tedlar bag. The resulting chromatograms were analyzed using VARIAN Detailed Hydrocarbon Analysis (DHA) Star software. Results and Discussion Catalyst Characterization. The results in Table 1 show similar aluminum contents in both zeolites HZSM-5 (Si/Al ratio 30) and Beta (Si/Al ratio 33), and a comparatively lower aluminum content in Al-MCM-41 (Si/Al ratio 40). The presence of aluminum in the silicate structure generates an electron imbalance that is considered to be the origin of the acid properties exhibited by these solids. Of the two zeolites, HZSM-5 exhibited a larger number of stronger acid sites than Beta, as evidenced by its higher ammonia adsorption capacity (0.538 mequiv g-1 compared to 0.481 mequiv g-1) and desorption temperature (375 °C compared to 322 °C). This observation may been attributed partly to the higher aluminum content in


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Table 1. Physicochemical and Textural Properties of the Acid Catalysts Employed in This Work aciditya

surface areac

Catalysts

TMAX (°C)

NH3 desorp. (mequiv g-1)

Si/Alb

SBET (m2 g-1)

SEXT+MESOP (m2 g-1)

HZSM-5 Beta Al-MCM-41

375 322 271

0.538 0.481 0.331

30 33 40

404 539 1168

2 33 1168

a

b

porosityc DMESOP (nm)

VMIC (cm3 g-1)

VTOTAL (cm3 g-1)

2.26

0.179 0.271 0

0.188 0.271 0.927

c

Ammonia TPD. ICP-AES measurements. Nitrogen adsorption at 77 K.

the HZMS-5 but also to the stronger acidity associated with the MFI structure, characteristic of HZSM-5 zeolites.16 As expected, the Al-MCM-41 material exhibited inferior acid properties than the two zeolites, both in terms of acid strength (271 °C) and the number of acid sites (0.331 mequiv g-1). This observation has been associated with both its lower aluminum content and also to its amorphous structure, which makes the aluminum sites less accessible and less evenly distributed than in crystalline materials. With respect to their textural properties, the results in Table 1 show that Al-MCM-41 exhibited a much larger BET surface area (1168 m2 g-1) and total pore volume (0.927 cm3 g-1) than the two zeolites. As expected, the contribution of micropores in this sample was negligible, exhibiting an average mesopore diameter of 2.26 nm, as determined by the BJH method. In contrast, the results confirm the essentially microporous character of both zeolites. Comparatively, zeolite Beta exhibited a higher BET surface area (539 m2 g-1) than zeolite HZSM-5 (404 m2 g-1). Zeolite Beta also exhibited a more extensive external surface area (33 m2 g-1) than HZSM-5 (2 m2 g-1), which was attributed primarily to the comparatively smaller crystal size of the former (200 nm compared to 3000 nm). Catalytic Activities Determined by TG Analysis. The catalytic activities of zeolites HZSM-5 and Beta and of AlMCM-41 in the cracking of the waste agriculture film were investigated using thermogravimetric analysis. Figure 1 shows the mass loss (TG) and the derivative of mass loss with time (dw/dt) represented against temperature (DTG) for each one of the plastic/catalyst mixtures. The catalytic activity of each solid may be related to its capacity to shift the degradation reaction to lower temperatures. In the absence of catalysts, the thermal analysis of the agriculture film showed a single area of weight loss starting at around 425 °C and finishing at around 500 °C, with a peak maximum located at 479 °C. This thermal behavior is characteristic of low density polyethylene. A 1.4 wt % ash residue remained at temperatures above 600 °C, which was attributed to inorganic impurities in the waste plastic. The agriculture film degraded at lower temperatures when the experiments were conducted in the presence of 10% catalyst. The lowest catalytic activity was exhibited by Al-MCM-41, which reduced the degradation temperature of the plastic by only 10 °C. This behavior was attributed to the weaker acid properties of this solid compared to the two zeolites. Despite its strong acid properties, zeolites HZSM-5 and Beta exhibited comparable catalytic activities to each other (peak maxima at 463 and 454 °C, respectively). It is believed that the larger crystal size and narrower pore dimensions of HZSM-5 may pose diffusional impediments for bulky polymer molecules to access the internal actives sites of the catalyst. Hence, the weaker acid properties of zeolite Beta are partly compensated by its comparatively larger pore dimensions and subsequent lower diffusional hindrances. Sequential Pyrolysis and Catalytic Reforming of Agriculture Film Waste. Conversion Values and Product Yields. In the two-stage reaction system, the agriculture film was converted

Figure 1. Thermogravimetric analyses (TG) and derivative thermogravimetric (DTG) plots showing the effect of 10 wt % catalysts on the cracking of waste agriculture film.

into gas and liquid hydrocarbons by sequential pyrolysis and catalytic reforming. In all the trials, the conditions in the pyrolytic reactor were identical (heating to 450 at 10 °C/min) in order to ensure a comparable evolution of vapors into the reforming stage. Catalytic reforming of the pyrolytic vapors was conducted at three different temperatures (425, 450, and 475 °C) and using three different catalysts (HZSM-5, Beta, AlMCM-41). The same experiments were also conducted in the absence of catalyst for comparative purposes. Figure 2 illustrates the conversion values in terms of weight percentage (wt %) of the original plastic mass and also the yields (wt %) of light (C1-C4), gasoline (C5-C12), and diesel (>C13) range hydrocarbons. Overall conversion values may be calculated by addition of the different (light, gasoline, and diesel) product yields. As expected, the experimental results show that conversion values were primarily controlled by the conditions in the pyrolytic reactor and remained relatively constant (89-92 wt %) regardless of the temperature in the reforming stage. With respect to product yields, Figure 2 describes the formation of a high proportion of hydrocarbons in the gasoline and diesel range (between 51 and 56 wt % and between 18 and 19 wt %, respectively) and a consequently lower amount of light products (between 17 and 23 wt %) when the reforming

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Figure 2. Conversion values and product yields resulting from the sequential pyrolysis and catalytic reforming of waste agriculture film at different reforming temperatures (425, 450, and 475 °C).

reactor was run on empty (no catalyst). The slightly higher proportion of light hydrocarbons observed when the reforming reactor was run at higher temperatures (17 wt % gas yield at 425 °C compared to 23 wt % at 475 °C), was attributed to the thermal cracking of the hydrocarbons evolving from the pyrolytic stage. The presence of acid catalysts in the reforming reactor favored the formation of light hydrocarbons and, consequently, reduced the proportion of gasoline and diesel range products. Of the three catalysts, zeolite HZSM-5 was the most active in the production of C1-C4 hydrocarbons, generating comparable product yields (between 51-53 wt %) when using reforming temperatures between 425 and 475 °C. The high catalytic activity of zeolite HZSM-5 has been attributed primarily to its strong acid properties. Owing to their weaker acid properties, catalytic reforming over zeolite Beta and Al-MCM-41 generated a lower proportion of light C1-C4 hydrocarbon products (between 44 and 45 wt % for zeolite Beta and between 27 and 32 wt % for Al-MCM-41). Consequently, the yields of heavier hydrocarbon species was notably higher (47-51 wt % combined gasoline and diesel for Beta reforming; and 36-42 wt % combined gasoline and diesel for Al-MCM-41). Unlike in TG experiments, these results show a good correlation between acid properties and catalytic activity. This suggests that the inhibition observed when the catalysts are in intimate contact with the plastic polymer does not appear to occur when the reforming and the pyrolytic stages are separate. It is believed that the latter approach may reduce the formation of char deposits and/or the poisoning of acid centers, thus favoring the performance of the catalysts. Furthermore, the smaller molecular size of the hydrocarbons reaching the

reforming stage, compared to the original polyethylene molecules, is believed to pose fewer diffusional hindrances. Characterization of Hydrocarbon Products. The hydrocarbon products derived from the thermocatalytic conversion of waste agriculture film were analyzed by gas chromatography (PIONA/GC). This technique was precise in the characterization of hydrocarbon products in the C1-C12 range. This section describes the characteristics of these products by carbon atom number and hydrocarbon type. Characterization by Carbon Atom Number. Figure 3 shows the distribution by carbon atom number (between C1 and C12) of the hydrocarbons derived from the sequential pyrolysis and catalytic reforming of agriculture film waste at the three reforming temperatures (425, 450, and 475 °C). In general terms, the results show that the reforming temperature did not have a significant effect on the hydrocarbon profiles. Hence, for ease of discussion and unless otherwise indicated, the results in this section refer only to the results obtained at the intermediate reforming temperature of 450 °C. When the reforming reactor was run on empty (no catalyst), the results illustrate the formation of hydrocarbon products over a wide molecular size range, with no preferences being observed in the formation of molecules of specific dimensions. These results are in agreement with the random scission mechanism that has been described for the thermal degradation of polyethylene.4 The results show a significant increase in the formation of light hydrocarbon products, primarily in the range C1-C4, when the pyrolytic vapors were subjected to catalytic reforming. However, significant differences were observed depending on the characteristics of the catalyst employed. Zeolite HZSM-5


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Figure 3. Characterization by carbon atom number of the hydrocarbon products derived from the sequential pyrolysis and catalytic reforming (at 450 °C) of waste agriculture film.

was the most active of the three in the formation of light products, with a particularly high selectivity toward the formation of C3 (27.6 wt %) and C4 hydrocarbons (28.3 wt %). It was also observed that catalytic reforming over zeolite HZSM-5 generated a relatively high concentration of C8 hydrocarbons (8.7 wt %), which has been associated with the dimerization of C4 olefins. This reaction pathway, already proposed in the work of Serrano et al.,12 is evidenced by the formation of a high proportion of alkyl aromatic gasoline range products, as will be discussed in the Characterization by Hydrocarbon Type section. The comparatively weaker acid properties exhibited by zeolite Beta and Al-MCM-41 were reflected in their reduced capacity to produce light hydrocarbons. For instance, the selectivity of zeolite Beta and Al-MCM-41 toward the formation of four carbon atom hydrocarbons (C4) was 24.0 and 17.7 wt %, respectively. Characterization by Hydrocarbon Type. Hydrocarbon type analysis is essential to ascertain the potential use of a hydrocarbon mixture directly as a fuel or as refinery feedstock. PIONA/GC analysis was used to characterize the hydrocarbons derived from the thermocatalytic conversion of waste agriculture film into five categories: n-paraffins, iso-paraffins, olefins, naphthenes, and aromatics. Results in Figure 4 represent the composition by hydrocarbon type of the light (C1-C4) and gasoline (C5-C12) fractions. The composition of the diesel (>C12) fraction may not be accurately determined by PIONA analysis. Experimental results showed few differences in the composition of hydrocarbons generated at different reforming temperatures (425, 450, and 475 °C). Hence, for ease of discussion, the results in this section only refer to the trials conducted at 450 °C.

Figure 4 shows that, when the experiment was conducted in the absence of catalysts, the resulting products consisted essentially of linear n-paraffins and olefins, with a higher proportion of the latter. These results confirm that, despite the residual nature and the presence of impurities in the waste polymer, its cracking mechanism and the range of products generated is similar to that described for pure LDPE.19 The composition of the gasoline fraction is not appropriate for its direct use as automobile fuel. However, this product may still be employed as refinery feedstock where it would be subjected to further cracking and reforming. The catalytic reforming of the pyrolytic vapors increased the formation of light (C1-C4) olefins and iso-paraffins, as well as heavier (C5-C12) isoparaffins, naphthenes, and aromatic products. This occurred primarily as a consequence of a reduction in the concentration of heavier n-paraffins and olefins. The experimental results show that catalytic reforming over zeolite HZSM-5 generated a higher proportion of light (C1-C4) olefins (47.3 wt %) than zeolite Beta (31.0 wt %) and Al-MCM41 (30.3 wt %), which has been associated with the stronger catalytic activity of the former. The results also show the formation of relatively high concentrations of light iso-paraffins (between 5.2 and 9.5 wt %), particularly when the reforming reaction was conducted over zeolite Beta. Regarding the gasoline fraction (C5-C12), the results illustrate the formation of a mixture of n-paraffins, iso-paraffins, olefins, naphthenes, and aromatic products. Catalytic reforming over zeolite HZSM-5 produced a gasoline fraction with the highest concentration of aromatic hydrocarbons (12.7 wt %) and a low proportion of n-paraffins (5.4 wt %) and olefins (8.9 wt %). Considering only the gasoline range hydrocarbons derived from HZSM-5 reforming, it may be calculated that 33 wt % of this

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Figure 4. Characterization by hydrocarbon type of the products derived from the sequential pyrolysis and catalytic reforming (at 450 °C) of waste agriculture film.

fraction corresponds to aromatics, 19 wt % olefins, 14 wt % n-paraffins, 23% iso-paraffins, and 10 wt % naphthenes. Comparatively, the gasoline fraction obtained by catalytic reforming over zeolite Beta and Al-MCM-41 contained fewer aromatics (8.8 and 4.8 wt %, respectively) and exhibited a higher concentration of n-paraffins (12.8 and 13.5 wt %, respectively) and olefins (12.8 and 15.2 wt %, respectively). This is evidence of the lower catalytic activity and reduced selectivity of these two acid solids compared to HZSM-5. Considering only the gasoline (C5-C12) fraction generated by catalytic reforming over Al-MCM-41, it may be calculated that it consists of 31 wt % olefins, 28 wt % n-paraffins, 21 wt % iso-paraffins, 10 wt % naphthenes, and 10 wt % aromatics. The compositions described for the C5-C12 fractions obtained by sequential pyrolysis and catalytic reforming of waste agriculture film would make these products valuable for blending directly with commercial transport fuels. Furthermore, the high concentration of light olefins obtained in the C1-C4 fraction would have a greater market value than the liquid n-paraffin/ olefin mixtures generated from the purely thermal cracking. Catalytic transformation of heavier saturated hydrocarbons into lighter unsaturated and aromatic products involves an economic advantage, which may be the key to the commercial viability of the valorization process. The market value of long chain linear saturated and monounsaturated hydrocarbons, like the ones generated as a result of direct thermal decomposition of waste polyethylene, may be comparable to light sweet crude oil (45 USD/barrel, equivalent to approximately 0.25 USD/kg). Market prices for unsaturated hydrocarbon gases like ethylene and propylene are comparatively higher, between 0.45 and 0.54 USD/kg and 0.41 and 0.44 USD/kg, respectively. Aromatic species like toluene, styrene, or xylene are valued between 0.41 and 0.57 USD/kg, and liquid fuels like regular unleaded gasoline (0.37 and 0.38 USD/kg) or MTBE (0.52 and 0.54 USD/kg) also

compare favorably against the unprocessed product (US market in January 2009).20 Conclusions Agriculture film waste may be converted into valuable hydrocarbon products by sequential pyrolysis and catalytic reforming using a two-stage reaction system. The results have shown that a pyrolysis temperature of 450 °C was sufficient to ensure steadily high conversion values (above 89 wt %). It was also observed that the conditions in the reforming reactor did not have any significant effect on conversion values. When the reforming reaction was conducted in the absence of catalysts, the thermal conversion of waste agriculture film generated a mixture of olefins and n-paraffins over a wide molecular size range. For a reforming temperature of 450 °C, the yield of light (C1-C4), gasoline (C5-C12), and diesel (>C12) range hydrocarbons was 18, 56, and 18 wt %, respectively. These fractions may be used as low quality fuels or refinery feedstock. Owing to its strong acid properties, zeolite HZSM-5 was very active in the catalytic reforming of the products derived from the pyrolysis of the agriculture film. This activity was evidenced by a notable increase in the yield of light hydrocarbon products (52 wt % at a reforming temperature of 450 °C) and a consequent reduction in the proportion of heavier fractions. The catalytic reforming process also favored the formation of light (C1-C4) olefins and iso-paraffins and promoted the formation of heavier (C5-C12) iso-paraffins, naphthenes, and aromatic products. Comparatively, zeolite Beta and Al-MCM-41 exhibited weaker catalytic activities than zeolite HZSM-5 which implied a higher selectivity toward the formation of heavier hydrocarbons containing less aromatic and iso-paraffinic products. Gasoline range (C5-C12) hydrocarbons derived from the


catalytic conversion of waste agriculture film contain a wellbalanced mixture of n-paraffins, olefins, aromatics, iso-paraffins, and naphthenes, making this product suitable for blending with transport fuels, as a low quality transportation fuel or refinery feedstock. Acknowledgment The authors gratefully acknowledge financial support from Ministerio de Ciencia y Tecnologia (project CICYT CTQ200509078). One of the authors, G.S.M., wishes to thank Ministerio de Ciencia y Tecnologıá for his scholarship granted under the Ramon y Cajal programme. Literature Cited (1) APME. The Compelling Facts About Plastics, An analysis of plastics production, demand and recoVery for 2005 in Europe; Plastics Europe, The Association of Plastic Manufacturers: Belgium, 2007. (2) Tukker A. Plastic waste feedstock recycling, chemical recycling and incineration, Rapra Review Reports Vol. 3 (4), Report 148; Rapra Technology Ltd.: Shropshire, United Kingdom, 2002. (3) Dintcheva, N. T.; La Mantia, F. P.; Scaffaro, R.; Paci, M.; Acierno, D.; Camino, G. Reprocessing and restabilization of greenhouse films. Polym. Degrad. Stab. 2002, 75, 459–464. (4) Pielichowski, K.; Njuguna, J. Thermal Degradation of Polymeric Materials; Rapra Technology Limited: Shropshire, United Kingdom, 2005. (5) Williams, P. T.; Williams, E. A. Fluidised bed pyrolysis of low density polyethylene to produce petrochemical feedstock. J. Anal. Appl. Pyrol. 1999, 51, 107–115. (6) Pinto, F.; Costa, P.; Gulyurtlu, I.; Cabrita, I. Pyrolysis of plastic wastes. 1. Effect of plastic waste composition on product yield. J. Anal. Appl. Pyrol. 1999, 51 (1-2), 39–55. (7) Seo, Y.-H.; Lee, K.-H.; Shin, D.-H. Investigation of catalytic degradation of high-density polyethylene by hydrocarbon group type analysis. J. Anal. Appl. Pyrol. 2003, 70 (2), 383–398. (8) Serrano, D. P.; Aguado, J.; Escola, J. M.; Garagorri, E. Performance of a continuous screw kiln reactor for the thermal and catalytic conversion of polyethylene-lubricating oil base mixtures. Appl. Catal. B. EnViron. 2003, 44 (2), 95–105.

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(9) Serrano, D. P.; Aguado, J.; Escola, J. M.; Garagorri, E.; Rodrı´guez, J. M.; Morselli, L.; Palazzi, G.; Orsi, R. Feedstock recycling of agriculture plastic film wastes by catalytic cracking. Appl. Catal. B EnViron. 2004, 49 (4), 257–265. (10) Serrano, D. P.; Aguado, J.; Escola, J. M.; Rodrı´guez, J. M.; San Miguel, G. An investigation into the catalytic cracking of LDPE using PyGC/MS. J. Anal. Appl. Pyrol. 2005, 74 (1-2), 370–378. (11) Manos, G.; Yusof, I. Y.; Papayannakos, N.; Gangas, N. H. Catalytic cracking of polyethylene over clay catalysts. Comparison with an ultrastable Y zeolite. Ind. Eng. Chem. Res. 2001, 40 (10), 2220–2225. (12) Serrano, D. P.; Aguado, J.; Escola, J. M. Catalytic cracking of a polyolefin mixture over different acid solid catalysts. Ind. Eng. Chem. Res., 2000, 39 (5), 1177–1184. (13) Gobin, K.; Manos, G. Polymer degradation to fuels over microporous catalysts as a novel tertiary plastic recycling method. Polym. Degrad. Stab. 2004, 83 (2), 267–279. (14) Jalil, P. A. Investigations on polyethylene degradation into fuel oil over tungstophosphoric acid supported on MCM-41 mesoporous silica. J. Anal. Appl. Pyrol. 2002, 65 (2), 185–195. (15) Aguado, J.; Serrano, D. P.; San Miguel, G.; Escola, J. M.; Rodrı´guez, J. M. Catalytic activity of zeolitic and mesostructured catalysts in the cracking of pure and waste polyolefins. J. Anal. Appl. Pyrol. 2007, 78 (1), 153–161. (16) Baerlocher, C.; Meier, W. M.; Olson, D. H. Atlas of Zeolite Framework Types; fifth ed.; Elsevier: New York, 2001. (17) Trong On, D.; Desplantier-Giscard, D.; Danumah, C.; Kaliaguine, S. Perspectives in catalytic applications of mesostructured materials. Appl. Catal. A Gen. 2001, 222 (1-2), 299–357. (18) Bagri, R.; Williams, P. T. Catalytic pyrolysis of polyethylene. J. Anal. Appl. Pyrol. 2002, 63, 29–41. (19) Aguado, J.; Serrano, D. P.; San Miguel, G.; Castro, M. C.; Madrid, S. Feedstock recycling of polyethylene in a two-step thermo-catalytic reaction system. J. Anal. Appl. Pyrol. 2007, 79 (1-2), 415–423. (20) ICIS Pricing. http://www.icispricing.com. Reed Business Information Limited: Quadrant House, The Quadrant, Sutton, Surrey (accessed July 2009).

ReceiVed for reView May 14, 2009 ReVised manuscript receiVed July 24, 2009 Accepted July 28, 2009 IE900776W

Valorization of Waste Agricultural Polyethylene Film by Sequential

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