Energy & Fuels 2007, 21, 2489-2498
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Kinetic Evaluation of the Pyrolysis of Polyethylene Waste Paula A. Costa,*,† Filomena J. Pinto,† Ana. M. Ramos,‡ Ibrahim K. Gulyurtlu,† Isabel A. Cabrita,† and Maria. S. Bernardo‡ INETI-DEECA, Estrada do Pac¸ o do Lumiar 22, 1649-038 Lisboa, Portugal, and REQUIMTE, Departamento de Quı´mica, FCT-UNL, 2829-516 Caparica, Portugal ReceiVed March 6, 2007. ReVised Manuscript ReceiVed July 5, 2007
The main aim of this study was the identification of possible routes for reaction mechanism for the pyrolysis of polyethylene. In this paper, a kinetic study of the pyrolysis of polyethylene is presented. The pyrolysis reactions were carried out in a six autoclave system using different temperatures and reaction times. It was analyzed if the direct conversion of plastic wastes into gaseous, liquid, and solid products was favored or if parallel reactions and/or reversible elementary steps should be considered. A theoretical model has been ajusted to experimental data. The kinetic parameters of polyethylene pyrolysis were estimated.
Introduction Modern societies are overdependent on petroleum for fuels and for raw material in many industries. In the world, about 42% of this fuel is consumed to produce energy, 45% on transportation, 4% for plastic production, 4% as feedstock for the petrochemical industry, and 5% in other applications1. Hence, efforts have to be undertaken to find alternative means to substitute petroleum for energy. Furthermore, the gradual unattended accumulation of enormous amounts of plastic wastes produced all over the world has negative and hazardous impact on the environment. Plastics waste generation increased by 5.9% between 2001 and 20032. The fraction of plastic in municipal solid wastes (MSW) is continuously rising. In Western Europe, 0.7% (w/w) of MSW is composed of plastics (20.6 million tons in 2002)2. In 2003, the major part of this waste (61%) was deposited in landfills, 22.5% was used for energy recovery, 1.7% was for feedstock recycling, and 14.8% was used in other recycling proposes2. Pyrolysis of plastic wastes may have an important role in converting them into economically valuable hydrocarbons, which can be used either as fuels or as feedstock in petrochemical industry. Pyrolysis of plastic wastes occurs under relatively moderate conditions of pressure and temperature. During this process, the polymeric structure is broken down, producing smaller intermediate species (radicals or ions). These fragments can further react and produce a mixture of smaller hydrocarbon molecules, being liquid, gas, or solid in nature. Apart from depolymerisation reactions, many secondary reactions may occur, including side group or substitute reactions, such as the following: chain scission, unsaturation, cross-linking, group substitution or elimination, cyclization, etc. * Corresponding author. Tel.: 351.21.0924600. Fax: 351.21.7166569. E-mail:
[email protected]. † INETI-DEECA. ‡ FCT-UNL. (1) Waste to energy. Presented at the International Management Industry Conference, Global Business Network Ltd, Brussels, Belgium, December 2000. (2) An analysis of plastic consumption and recoVery in Europe; Association of Plastics Manufacturers in Europe: Brussels, Belgium, 2004.
As polymer thermal decomposition is an endothermic process, the minimum energy required for the dissociation of the C-C bond in the chain has to be supplied to break down the polymer. This is the main factor to determine the polymer stability3. Therefore, a direct relationship between the dissociation energy and the decomposition temperature for different polymers has been found3. Taking this requirement into account, the temperatures used in the present study were between 400 and 470 °C. Several authors4-6 studied the polyethylene pyrolysis process using different reactors and experimental conditions. The most used reactors were of the fluidized bed and the fixed bed types. Most of the studies focused on the effect of experimental conditions (reaction temperature and time) on product yield and composition; others7-10 characterized extensively the products obtained in the polyethylene (PE) pyrolysis. Most of the kinetics studies on PE thermal degradation have been based on thermal gravimetric analysis (TGA) measurements, mainly using a power law model to describe the rate of weight loss11-18. The main difference between these studies was (3) Agrado, J.; Serrano, D. Feedstock Recycling of Plastic Wastes; Royal Society of Chemistry: Letchworth, UK, 1999; Chapter 4, p 73. (4) Herna`ndez, M. R.; Garcı´a, A. N.; Go´mez A.; Agullo´, J.; Marcilla, A. Ind. Eng. Chem. Res. 2006, 45, 8770-8778. (5) Herna`ndez, M. R.; Go´mez A.; Garcı´a, A. N.; Agullo´, J.; Marcilla. Appl. Catal. A: Gen. 2007, 317, 183-194. (6) Mastral, J. F.; Berrueco, C.; Ceamanos, J. Energy Fuels 2006, 20, 1365-1371. (7) Herna`ndez, M. R.; Garcı´a, A. N.; Marcilla, A. J. Anal. Appl. Pyrol. 2007, 78, 272-281. (8) Soja´k, L.; Kubinec, R.; Jurda´kova´, H.; Ha´jekova´, E.; Bajus, M. J. Anal. Appl. Pyrol. 2007, 78, 387-399. (9) Demirbas, A. Energy Sources 2005, 27, 1313-1319. (10) Demirbas, A. J. Anal. Appl. Pyrol. 2004, 72, 97-102. (11) Gao, Z; Amasaki, I.; Nakada, M. J. Anal. Appl. Pyrol. 2003, 67, 1. (12) Conesa, J. A.; Marcilla, A.; Font, R.; Caballero, J. J. Anal. Appl. Pyrol. 1996, 36, 1. (13) Westerhout, R.; Waanders, J.; Kuipers, J.; Swaaij, W. Ind. Eng. Chem. Res. 1997, 36, 1995. (14) Ranzi, E.; Dente, M.; Faravelli, T.; Bozzano, G.; Fabini, S.; Nava, R.; Cozzani, V.; Tognotti, L. J. Anal. Appl. Pyrol. 1997, 40-41, 305319. (15) Bockhorn, H.; Hornung, A.; Hornung, U.; Schawaller, D. J. Anal. Appl. Pyrol. 1999, 48, 93-109. (16) Faravelli, T.; Bozzano, G.; Colombo, M.; Ranzi, E.; Dente, M. J. Anal. Appl. Pyrol. 2003, 70, 761. (17) Darivakis, G. S.; Howard, J. B.; Peters, W. A. Combust. Sci. Technol. 1990, 74, 267-281.
10.1021/ef070115p CCC: $37.00 © 2007 American Chemical Society Published on Web 08/31/2007
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that some used isothermal methods, while others employed nonisothermal ones. One of the most important limitations of using models based on TGA measurements is that they are only able to describe the steps which are normally associated with weight loss3. For instance, the significant degradation that takes place in the polymer between 350 and 400 °C leading to a large decrease in the average molecular weight is not detected in the TGA measurements. Such transformations take place mainly through a random scission mechanism, producing intermediate species (heavy waxes and tars), which are not volatilized at these relatively low temperatures3. By using TGA in kinetic studies, the evaporation rate of products is determined, but not the intrinsic chemical reaction rate, because not all the bonds that are broken lead to the evaporation of a product, only product fragments which are small enough will evaporate and lead to a decrease of the polymer mass13. Thermal degradation of polyolefins is usually described by a single step degradation following the Arrhenius nth-order rate equations14. The single step approach has been used to describe the apparent kinetics of degradation in a narrow range of heating rates and operational conditions. However, this approach is not able to cover a wide range of heating rates, temperatures, and conversion levels, with the same kinetic parameters16. Only a few detailed kinetic models describing the polymer degradation are reported in the literature. A less simplified approach was followed by Darivakis et al.17, who fitted the experimental data obtained by means of an infinite number of independent parallel reactions. This model assumed that volatilization products were formed from a large number of independent parallel first-order reactions with activation energies following a Gaussian distribution. Activation energy of about 208 kJ mol-1 was obtained for PE with this model17. The main differences between the studies reported in the literature were the kinetic models used to describe the pyrolysis kinetics and the values of Arrehenius parameters obtained with the different models tested11. The kinetic parameters presented for the different models used differ significantly, namely activation energies varying between 160 and 320 kJ mol-1 and pre-exponential factors ranging from 1011 to 1021 s-1 12. One of the few studies that did not use TGA measurements was performed by Ramdoss et al.19 These authors studied the mechanism of liquefaction of a mixture of polypropylene (PP) and PE in a tubing bomb microreactor, with an operating temperature between 475 and 525 °C, a hydrogen initial pressure of 790 KPa and 6 g of plastic material. A kinetic model was fitted to experimental data. From the results of this study, it was observed that a short residence time (5-10 min) was required at a relatively high temperature (500 °C) for maximizing the liquid yield19. Regarding to the kinetic parameters, presented in the literature, it was observed that, depending on the conversion degree, different reaction orders were obtained, and consequently, the kinetic constants are significantly different within the conversion ranges. However, in many studies, the simple power law model is used over a large conversion range; this is probably the most important reason for the observed differences between the values obtained for the kinetic parameters6. Other reasons can, probably, be due to the use of different types of PE, the presence of impurities in the polymer, and the influence of the different experimental conditions used. Most of the studies done in PE pyrolysis kinetics used TGA measurements and the power law model to describe the rate of (18) Marongiu, A.; Faravelli, T.; Ranzi, E. J. Anal. Appl. Pyrol. 2007, 78, 343-362.
Costa et al.
weight loss. However, as described, this method has some limitations. Mainly, there are only reported in the literature the global kinetic parameters for the overall degradation of PE. In this work, the pyrolysis of PE was investigated in order to identify possible routes for reaction pathways using experimental data obtained in pyrolysis. It was studied if the direct conversion of plastic wastes into gaseous, liquid, and solid products was favored or if parallel reactions should be considered. On the basis of the experimental results obtained, a theoretical kinetic model was developed and a reaction pathway for the pyrolysis of PE was proposed. The experimental results were compared with those predicted by the model. The kinetic parameters of the conversion of PE into different products based on the pyrolysis results were calculated. Previous work done by the authors20 to study the effect of experimental parameters on plastic waste pyrolysis considering product yield and composition, seem to show that chemical reactions should be very fast and that reaction times should be relatively short20. To test reaction times lower than 10 min, a system with six 160 mL autoclaves was used. The results obtained could also be used for kinetic studies. This paper presents and discusses the results obtained. Experimental Section The plastic used was a mixture of low-density polyethylene (LDPE) and high-density polyethylene (HDPE) collected from households and then mechanically recycled and extruded in small pellets with a diameter of 0.5 cm. These wastes were used as received. The plastic samples used were also characterized by elemental analysis to carbon, hydrogen, and nitrogen (CHN) were determined and always presented a C:H ratio of 6.15 (being 85.5% of C, 13.9% of H,
