Ind. Eng. Chem. Res. 2001, 40, 4749-4756
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Microwave-Induced Pyrolysis of Plastic Wastes Carlos Ludlow-Palafox* and Howard A. Chase Department of Chemical Engineering, University of Cambridge, New Museums Site, Pembroke Street, Cambridge CB2 3RA, United Kingdom
The performance of a novel microwave-induced pyrolysis process was evaluated by studying the degradation of high-density polyethylene and aluminum/polymer laminates in a semibatch bench-scale apparatus. The results showed that the new process has the same general features as other, more traditional, pyrolytic processes but with the advantage that it is able to deal with problematic wastes such as laminates. Degradation experiments were performed between 500 and 700 °C and the relationship between temperature, residence time of the pyrolytic products in the reactor, and the chemical composition of the hydrocarbon fraction produced was investigated. Toothpaste tubing was used as an example of a laminated material to be treated with the novel process. Clean aluminum was recovered together with hydrocarbons and the trial proved that the process has excellent potential for the treatment of plastic wastes on a commercial scale. 1. Introduction The consumption of plastics in Europe in 1998 was about 30.4 million tonnes. The main components of this were linear low and low-density polyethylene (L/LDPE 23%), high-density polyethylene (HDPE 15%), and polypropylene (PP 20%).1 Around 58% of this consumption became waste, resulting in nearly 17.6 million tonnes of plastic disposal over the same period. Almost 60% of the total production was destined for the household market, mainly for use in packaging, and nearly 70% of these plastics ended up in the municipal solid waste (MSW). Because of increasing political concern in the governments and society as a whole toward environmental issues, a great deal of research has been done over the last 2 decades into recycling methods to deal with this amount of waste in an economical and ecological way. Numerous processes have been developed from this research, some of them focused on the recovery of energy from the plastics (incineration) and others focused on the combined recovery of the energetic and chemical value of the plastic. Pyrolysis, thermolysis, or gasification are the names associated with this latter kind of process and in this report they will be referred to as feedstock recycling or pyrolytic processes. Pyrolytic processes have been studied previously using several different types of equipment such as fluidized beds,2 rotary kilns,3 and rotating reactors.4 The research has been carried out mainly with homogeneous waste or pure plastics rather than with more realistic materials that contain other contaminants. However, some investigations have been carried out using real wastes or have tried to imitate the composition found in the MSW or other sources of real wastes.5-8 Microwave-induced pyrolysis is a new process developed initially by Tech-En Ltd.9 (Hainault, U.K.). The process involves mixing plastics, which are known to have a very high transparency to microwaves, with a highly microwave-absorbent material, namely, carbon. When carbon is exposed to a microwave field, it can * To whom correspondence should be addressed. Tel.: +44 (0) 1223 330132. Fax: +44 (0) 1223 334 796. E-mail: cl232@cheng. cam.ac.uk.
reach temperatures up to 1000 °C in a few minutes. If shredded plastics are mixed with the carbon, prior to or during heating, the energy absorbed from the microwaves is transfered to the plastics by conduction, providing a very efficient energy transfer and a highly reducing chemical environment. The latter avoids formation of undesired hydrocarbon products containing oxygen. Microwave heating has many advantages over conventional heating including more even distribution of heat and better control over the heating process. Sources of microwave radiation allow high temperatures and high rates of heating to be obtained and show excellent efficiencies both for conversion of electrical energy into heat (80-85%) and for heat transfer to the load. Modern equipment has very high reliability and is competitive with other heating methods. As mentioned above, some pyrolytic processes cannot operate when the plastic is mixed with other contaminants. Materials such as aluminum/polymer laminates, used mainly as packaging for food and beverage and other products such as toothpaste, are often found in MSW and do not currently have a suitable feedstock recycling method; most waste of this kind ends up as landfill. Our initial tests showed the potential of the microwave-induced pyrolysis process for the treatment of this kind of waste. These materials consist of a variety of products, all containing a thin foil of aluminum (typically with a thickness of ≈6-30 µm essentially to give long-life protection from gas intrusion and mechanical support) laminated in conjunction with paper and plastic layers. In addition to the aforementioned advantages of microwave-induced pyrolysis, the process can be mechanically gentle, and therefore very fragile materials, like the aluminum within laminate wastes, can be recovered clean and ready for reuse following smelting. This new process provides not only the way to recover the chemical and energetic value of the plastic waste but also the possibility of recovering another material with commercial value. In the current investigation the recovery of a material such as aluminum from these wastes is an important goal because of the volume of such waste generated annually and the value
10.1021/ie010202j CCC: $20.00 © 2001 American Chemical Society Published on Web 10/05/2001
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Ind. Eng. Chem. Res., Vol. 40, No. 22, 2001 Table 1. Aluminum-Polymer Toothpaste Packaging Laminate Composition layer material paint and inks for decoration polyethylene polyethylene copolymer aluminum polyethylene copolymer polyethylene
Figure 1. Schematic drawing of the microwave-induced pyrolysis apparatus. (Numbers refer to components described in main text.)
of the reclaimed product. Data from the Aluminium Foil Recycling Campaign indicates that the UK market for aluminum laminated plastic and paper packaging is 15300 ton/year. The vast majority of this material ends up in landfill, even though it contains ≈1500 tonnes of aluminum. Across Europe, the proportion of collected cartons containing aluminum foil varies between countries from 10% to over 80%, giving a potential recovery of 4000 tonnes of aluminum from beverage cartons alone. This study presents the development of a bench-scale semibatch microwave-induced pyrolysis apparatus. This is tested with pure plastics to compare the characteristics of the process with the results of previous investigations. The equipment is then used to treat more realistic aluminum/polymer laminate waste. 2. Experimental Equipment and Procedures 2.1. Apparatus. The experimental apparatus developed and used during this investigation is shown in Figure 1. It consists of a modified catering microwave oven (1) with a maximum power output of 5 kW. The oven has 4 magnetrons, each of which is controlled by a separated switch in such a way that the power output can be controlled to either 25, 50, or 75% of the maximum, with a continuous generation of microwaves rather than with on/off cycles. The reactor (2) is a quartz vessel of 180-cm diameter. This vessel is placed in a specially moulded base made of a microwave-transparent/insulating material (VF1500AK prefired, M. H. Detrick, Mokena, IL). The reactor has an agitation system that consists of an impeller with two 45° pitched blades, 11-mm-diameter (pipe) stainless steel shaft, and a motor (3) with a maximum speed of 6 rpm. The temperature of the carbon load in the system is monitored using two thermocouples; one is ducted into the carbon through the center of the shaft and the other one enters the reaction chamber through a port on the top of the reactor. Both thermocouples are in direct contact with the carbon inside the reactor. The thermocouples are connected via a data acquisition card (DT302, Data Translation, Marlboro, MA) to a computer. The computer runs a control program developed for the VEE package (Agilent Technologies, Palo Alto, CA) to read the temperature at a rate of 100 Hz, average the reading, and send on/off commands back to the magnetrons to maintain the desired temperature. The reactor is gravity fed using the top feeder (4). Valves permit an inert gas to purge the incoming material to avoid the presence of oxygen in the reactor. The flow
layer thickness (µm) insignificant 110 45 30 30 60
rate of the purging/carrier gas is monitored using a rotameter (5). The products of pyrolysis leave the reactor and pass through a system of condensers that has a main collection flask (6), two water-cooled Liebig condensers (7), and two cold traps (8,9). After the two cold traps the uncondensable gases flow through a cotton wool filter (10) to collect any aerosols present, before leaving the system. 2.2. Materials and Methods. The particulate carbon used was provided by Tech-En Ltd. and is a waste material consisting of coke that forms the bottoms residue of distillation towers. When received, the carbon had a sulfur content of up to 3.5% and therefore had to be heated to ≈1000 °C over 45 min in an inert atmosphere to remove the sulfur. Different sizes of carbon particles were separated by sieving and only particles 106 µm; 5.5% > 75 µm). Most experiments were carried out using HDPE pellets (Rigidex 5502XA from BP Chemicals), average diameter 3 mm and average height 1 mm, with a density of 954 kg/m3 and a weight-average molecular weight of 141600 amu. The experiments with aluminum/polymer laminates were carried out with toothpaste tube laminate with an aluminum content of ≈30% (w/w); the composition and layer thickness of the laminate is shown in Table 1. Throughout all experiments a mixing speed of 6 rpm was used. The experiments were carried out using water flowing through condensers at 60 °C, the first cold trap submerged in a cold water bath set at 1 °C (using a refrigeration system) and the second cold trap at -78 °C (using dry ice). The experiments were started by heating the carbon and purging the reactor. The carbon, typically 1000 g, was heated to the reaction temperature while a small flow (0.5 L/min) of purging gas (N2) flowed through the reactor. During this heating-up period the carbon lost water that had been previously adsorbed from the environment; therefore, during this time the reactor was detached from the condensation system to avoid condensation and capture of water in the collection flasks (cold traps). When the temperature stabilized and the reactor and the condensation systems were attached, the nitrogen flow was set to a value between 0 and 1.4 L/min and the plastic or laminate load, usually 50 g, was dropped into the carbon bed using the top feeder. Just after loading the temperature reading showed a slight drop but it recovered promptly. The temperature was maintained at the set point (1% by switching on or off different magnetrons. The volume of condensed products collected in the main collection vessel was recorded throughout the experiment. When the accumulation of condensed products stopped and the visual inspection showed no plastic left in the reactor and that its quartz walls were clean, a small amount of purging gas was allowed to flow through the system before switching the oven off. The carbon was allowed to cool to 200 °C with the whole apparatus
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sealed before dismantling the condensation system. The cold traps, along with all the glassware connectors, and the cotton-wool filter were weighed to determine the amount of polymer that had been converted into liquid or waxy products. The carbon in the reactor was weighed. In the case of the experiments with laminate, the clean aluminum foil was separated from the carbon particles by sieving and both materials were weighed separately. After the experiment the mass balance was calculated and the yield of oils and waxes was established. The loss of carbon in all experiments was no more than 0.5% of the initial amount and there was no evidence of coke formation within the reactor. Therefore, it is believed that all the material to be degraded was transformed into products and therefore the amount of gases produced was calculated by the difference. 2.3. Product Analysis. To preserve the nature of the oils/waxes collected in the condensing system, these were transferred to 50-mL vials without the use of solvent. Aliquots of 1 mL were taken to be analyzed by gas chromatography-mass spectrometry using a 5973/ 6890 GC-MS (Agilent Technologies, Palo Alto, CA) apparatus. Before injection, the 1-mL samples were placed in an oven to melt them; 2-µL injections were performed using a warm 100µL syringe. The GC-MS was operated in nonisothermal mode, ramping from 30 to 325 °C using a 15-m fused silica capillary column HP5MS, cross-linked 5% PH ME Siloxane, i.d. 0.25 mm, film thickness 0.25 µm. The carrier gas used was helium, analytical grade, with a constant flow rate of 1.1 mL/min. During the experiments, gas samples were taken after the filter and collected in 10-L Teflon bags. These samples were analyzed by GC-MS in isothermal mode at 40 °C using the same fused silica capillary column as before but now increased to 60-m in length with the same carrier gas flow rate. The total ion chromatogram obtained from the GCMS analysis was integrated using the ChemStation integrator provided with the machine and equipped with the Wiley library of chemical compounds (6th ed.). The chromatograph integrator was programmed in two different modes, allowing the quantification of compounds by both species and size. In this way, a single analysis by GC-MS permitted the identification of the products and the classification of the sample by chain length. The GC-MS was not calibrated for the individual compounds found; hence, the quantifications presented as a result of the integration show the total ion content percentage (TIC%) that, although must not be confused with a weight percentage (wt %), gives a good approximation of the composition by mass of the sample. 3. Results and Discussion 3.1. Pyrolysis of Polyethylene Pellets. The degradation of polyethylene pellets was carried out at different temperatures between 450 and 700 °C and with different flows of carrier gas through the reactor to study the rate of generation of volatiles, the yields of the process, and the product composition. The results obtained are discussed below and are compared with some results reported in the literature. 3.1.1. Rate of Degradation. During this investigation, the reactor temperature had the greatest effect on the rate of reaction/decomposition, observed by the rate of condensation in the main collection flask (vessel #6
Figure 2. Cumulative yield of products in the main collection vessel from HDPE pellets pyrolysis at 500 and 600 °C, following the addition of 50 g of HDPE to a preheated bed of carbon at time 0.
in Figure 1). Figure 2 shows the cumulative volume of oils collected for two temperatures as a function of reaction time. Zero time refers to the point at which the plastic load was dropped into the reactor. The volumes shown account for ≈70 wt % of the total oil/waxes accumulated in the whole collection system. As can be seen, the reaction proceeds quickly, and at 600 °C, after less than 120 s, no further gas condensation was observed in the main collection vessel. Because of limitations in the present reactor configuration, it was not possible to achieve successful degradation below 500 °C. Evolution of gaseous products at 450 °C was very slow, and the formation of a sticky mass of carbon and melted plastic interfered with proper agitation of the carbon by the impeller and prevented accurate control over the temperature. Experiments carried out at 700 °C were extremely fast and resulted in an abrupt “flash” of the plastic load; hence, it was not possible to take readings of the volume collected over time. Data found in the literature regarding the rate of degradation of PE come principally from kinetic and mechanistic studies of its degradation. These studies have been performed using samples of a few milligrams and particle sizes ranging from ≈60 to ≈400 µm10-13 and demonstrate degradation times much shorter than the ones presented in Figure 2. It is clear that the difference between this and other investigations are limitations on heat and mass transfer due to the size of the sample and polymer particles used. Some authors have tried to establish relationships that can predict the relative importance of transport mechanisms and kinetics in plastic degradation reactions and therefore account for said differences. Among these, Pyle and Zaror14 proposed the use of the “pyrolysis number” to represent the relation between the apparent reaction rate constant for a pseudo-first-order pyrolysis reaction and the time constant for the internal heat conduction in spheres in the transient state. The pyrolysis number is defined by the following equation:
Py )
K kFCpr2
(1)
Clearly, when Py . 1, the reaction proceeds quickly compared to the heat penetration, and when Py , 1, the physical characteristics of the particle limit the heat penetration and therefore the rate of the degradation process. With the physical characteristics of the polymers used in this investigation, the pyrolysis number was found to be on the order of 0.05-0.3, confirming
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Table 2. Product Yield (wt %) from the Pyrolysis of PE Reported in the Literature Together with the Results of the Present Study (N.R. ) Not reported) 500 °C Conesa et al.20 batch fluidized bed Conesa et al.20 Pyroprobe 1000 Cozzani et al.21 fixed bed Williams and Williams22 fluidized bed this work; microwave-induced pyrolysis
600 °C
gas
oil/wax
solid
gas
oil/wax
solid
≈7-16 2.42 ≈8-12 10.8 19.0
N.R. 97.5 ≈83-90 89.2 81.0
N.R. N.R. ≈2-5 0 0
∼18-60 14.6 ≈20-35 24.2 20.9
N.R. 85.3 ≈55-74 75.8 79.1
N.R. N.R. ≈6-10 0 0
that the experiments in the current investigation do not allow an accurate assessment of the chemical kinetic rate of the degradation reaction. It has also been suggested that the presence of a large number of free electrons in a pyrolytic system may alter the reaction mechanism.15 In this process the carbon heats up because of electric currents induced on the particles by the microwaves; therefore, free electrons are also present and a possible change in reaction mechanism could occur. New equipment would be necessary to study these effects with small particles and samples. Unfortunately, there appear to have been few investigations that report the “rate” of degradation of polymers at a scale in which the size of the sample and/or the particles are close to those found in real wastes and at the temperatures reported in this investigation. McCaffrey et al.16-18 and Uddin et al.19 presented charts of liquid yield vs time for the pyrolysis of PE at lower temperatures (430-440 °C), with degradation times in the range 90-300 min. Although it is clear that the longer times are due mainly to the lower reaction temperature, they can also be linked to the process characteristics. In the microwave-induced pyrolysis the plastic is dropped into a hot chamber and thereby the carbon at the degradation temperature “surrounds” the particles, reducing the problems associated with heat transfer and increasing the process rate. In most of the investigations mentioned above the load was placed inside a reactor and heated by external means such as heating mantles, causing a much slower rise in the polymer temperature. 3.1.2. Process Yields. Many studies available in the literature show the yields of the different type of compounds formed during the degradation of polyethylene and other thermoplastics. Although it is clear from previous research that the yield of gases, liquid/waxes, and solid residue depends significantly on the process characteristics and variables, there is a general consensus that the higher the process temperature, the higher the yield of gases. Table 2 shows the results of other researchers for the degradation of PE at 500 and 600 °C along with the results obtained using the microwave-induced pyrolysis apparatus. As can be seen during the present work, the increase from 500 to 600 °C caused little difference in the yields of the products. These results, contrary to most previous findings, may be explained by the configuration and the characteristics of the degradation process. McCaffrey et al.16 noted a decrease in the gas yield with an increase in temperature for a stirred reactor. They attributed this to the fact that their reactor configuration allowed for some of the long hydrocarbon chains in the gaseous phase to condense and be recycled into the reactor if they were not “volatile” enough to escape to the condensation system. Other researchers20,21 have found that higher yields of gases are achieved at higher reaction temperatures, especially if those gases remain at the reaction temperature for a
certain amount of time. When the primary reaction products leave the reaction conditions quickly, as in the case of Pyroprobe tests,20 the results give more accurate information about the initial chemistry of the pyrolysis but provide less information about the behavior of larger systems useful for scale-up work. The examples mentioned above illustrate that the final yield and hence product composition of the pyrolysis products are determined mainly by the secondary and tertiary reactions that occur after the polymer molecules have first been cracked, especially in systems using “large” equipment and/or samples. This fact, combined with the evidence of an increase in the reaction rate with temperature, could explain the yield results obtained in this investigation: as temperature increased from 500 to 600 °C, the reaction rate increased and so did the production rate of primary gaseous products. Because of the constant volume of the reactor, the pressure increased and resulted in more rapid flow of gas out of the reaction hot zone; the residence time therefore became dependent on the reaction temperature. Given the configuration and dimensions of the equipment, the increase in the level of cleavage attained due to an increase in temperature in the range 500 to 600 °C is counterbalanced by a decrease in the residence time in the reactor due to an increase in the gas production rate. 3.1.3. Product Analysis: Oils/Waxes. The relationship between reactor temperature and the residence time of gaseous products outlined above was confirmed by the GC-MS analysis of the degradation products of the process. A typical total ion chromatogram for the oils/waxes obtained during the experiments is shown in Figure 3. The chromatogram obtained is very similar to those obtained by other researchers.7,16-18,22,23 On the basis of the TIC% of the different compounds identified, it was possible to directly obtain the estimates for the molecular weight distribution of the products from the GC-MS results. Figure 4 shows the carbon number distribution of the oil/waxes obtained from the pyrolysis of HDPE pellets at 500, 600, and 700 °C. Table 3 shows the number-average and weightaverage molecular weights (Mn and Mw), degrees of polymerization (µn and µw), and polydispersity for the same products. Previous researchers22 found that the average molecular weight and chain length showed a continuous reduction with an increase in process temperature contrary to what can be observed in Figure 4 and Table 3. In these results, it is noticeable that the oils/waxes produced at 500 and 700 °C have similar molecular weight distributions and these are smaller than those produced at 600 °C. This confirms the suggestion that at 600 °C the effect of higher temperature compared to that at 500 °C is balanced by the reduction of the residence time. However, at 700 °C the temperature effect dominates, causing the same level of cleavage as that at 500 °C but in a much shorter reaction time.
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Figure 3. Total ion chromatogram of the oils/waxes produced during the pyrolysis of HDPE pellets in the microwave-induced pyrolysis apparatus at 700 °C.
A number of experiments were performed to understand the influence of the flow rate of carrier gas through the reactor on the final product spectra at constant temperature. The motivation behind these experiments was that it was anticipated that at higher carrier gas flow rates the average molecular weights would be larger than those obtained with lower carrier gas flow rates or with no carrier gas at all. Table 4 shows the average molecular weight found for the pyrolysis of HDPE pellets with different rates of nitrogen flowing though the reaction zone. The values correspond to the expected results and help to confirm the explanations outlined in previous paragraphs. However, increasing the gas flow rate at 500 °C by 40% does not result in the reduced level of chain cleavage that was observed, with the 100 °C increase in temperature in the experiments at 600 °C. Unfortunately, it was not possible to carry out experiments with even higher flow rates of carrier gas because of reductions in the performance of the condensation system. The main chemical components of the oil/wax collected were found to be R-alkenes followed by alkanes and dialkenes of different chain lengths along with a number of other aliphatic and aromatic compounds ranging from C3 to approximately C56. Each set of two higher peaks in Figure 3 represents the pair of monounsaturated and saturated compounds, respectively. It is noticeable that the mono-unsaturated compound is always the more abundant compound for certain chain lengths. As Conesa et al.20 noted, there is little available information describing the products found in the degradation of polyethylene. While it is generally accepted that the main products are aliphatic linear hydrocarbons, it is clear that the yields of these along with cyclic and aromatic compounds depend to a large extent on the pyrolytic conditions. There are very few studies that report the analysis of oils/waxes produced at the operating temperatures in this investigation. There are a number of contradictions in other work conducted below and above the temperatures considered in this work. While some investigations12,23-21 found less alkenes than alkanes below 500 °C, other researchers16 reported values for the overall degree of unsaturation between 58 and 70%. At the other extreme of the temperature
range studied, Kaminsky2,25 claimed to have obtained up to 19.1% of benzene and 3.9% of toluene at 740 °C in fluidized-bed equipment. These values differ from the ones presented by Conesa et al.20 of 1.88% of benzene and 0.33% of toluene also measured in a fluidized-bed reactor at 700 °C. Table 5 presents the yields of some groups of compounds or specific compounds obtained in the microwaveinduced pyrolysis of HDPE pellets. Integration of the total ion chromatograms confirmed that the highest peaks (linear hydrocarbons) observed in Figure 3 represent a very high percentage of the total amount of oils/ waxes and accounts for almost 93% of the condensable compounds produced at 600 °C. The rest are a large number of cyclic, branched, and aromatic compounds. Table 5 also shows that the oils/waxes obtained at 500 and 700 °C, although possessing similar levels of cleavage (as evidenced by similar molecular weight distributions), show important differences in the identities of the compounds present. It is known that the first mechanism involved in the pyrolysis of polyolefins is a random scission of the polymer backbone (see, for example, ref 12). This cleavage causes the appearance of two (highly reactive) primary radicals that have to be stabilized. The two main mechanisms that follow are either an intermolecular or an intramolecular transfer. In the former case, the free radical is stabilized by withdrawal of a hydrogen atom from a different molecule close to the radical, leading to the formation of an alkane. In the latter, the hydrogen used for the stabilization is obtained from the same molecule that contains the radical by transferring it from one place to the other and leads to the creation of an alkene. It is generally accepted that, in the degradation of polyethylene, the most probable mechanism is intramolecular transfer because of the abundance of hydrogen in the polymer backbone,22 resulting in the observed high concentration of alkenes. The purpose of this work was not to elucidate the reaction mechanism, but the results summarized in Table 5 interpreted in light of the above discussion of mechanisms suggest that the increase in temperature caused a restriction of intermolecular transfer processes, thus diminishing the yield of alkanes. This “blocking” of the intermolecular reactions is associated with an excess of
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Ind. Eng. Chem. Res., Vol. 40, No. 22, 2001 Table 5. Yields of Products (TIC%) Obtained in the Oils/Waxes from HDPE Pyrolysis in the Microwave-Induced Pyrolysis Apparatus temperature (°C) linear hydrocarbons of which:
alkanes alkenes dialkenes
methylcyclopentene benzene cyclohexene toluene ethylbenzene xylene propylbenzene methylethylbenzene
500
600
700
81.1 37.0 52.2 10.9
92.8 29.5 60.1 10.4
88.0 11.8 60.0 28.2
0.09 0.10 0.07 0.32 0.27 0.33 0.11 0.31
0.01 0.02 0.01 0.11 0.11 0.14 0.06 0.15
0.08 0.11 0.12 0.25 0.18 0.14 0.12 0.19
Table 6. Yields of products with carbon number g3 (TIC%) in the gases obtained from HDPE pyrolysis in the Microwave Induced Pyrolysis apparatus.
Figure 4. Carbon number distribution for the pyrolysis of HDPE pellets with a flow of 1 L/min of carrier gas at (a) 500 °C, (b) 600 °C, and (c) 700 °C. Table 3. Average Molecular Weights and Degrees of Polymerization for the Pyrolysis of HDPE Pellets with a Flow of 1 L/min of Carrier Gas at 500, 600, and 700 °C temperature (°C)
µw
Mw (amu)
µn
500 600 700
10.9 14.5 10.3
305 406 289
8.2 11.8 8.5
Mn (amu) polydispersity 229 330 238
1.3 1.2 1.2
Table 4. Average Molecular Weights and Degrees of Polymerization for the Pyrolysis of HDPE Pellets at 500 °C carrier gas flow (L/min)
µw
Mw (amu)
µn
0 0.4 1 1.4
9.4 10.0 10.9 11.6
264 280 305 323
7.4 7.8 8.2 9.4
Mn (amu) polydispersity 207 220 229 262
1.3 1.3 1.3 1.2
hydrogen becoming available during the stabilization of the free radicals (see, for example, ref 18). The reason that this happens at higher temperatures (>600 °C) was
carbon number
500 °C
600 °C
3 4 5
46.0 27.8 13.7
50.0 26.0 14.1
carbon number
500 °C
600 °C
6 7
8.9 1.1
8.8 1.1
attributed by Cozzani et al.21 based on Blazso26 to the presence of repolymerization reactions, leading to an increase in hydrogen content in the evolved gas and a higher degree of unsaturation in the hydrocarbons in the oils/waxes. When the aromatic compounds are considered, it is reported that there is an increase in the aromaticity of the waxes as the temperature increases.22 Conesa et al.20 noted this same effect in the yields of this class of compounds (with the exception of toluene) over the temperature range discussed in the present work. In this study, however, the yields of the aromatic compounds showed a minimum at 600 °C. In light of the explanations discussed above, this result may show that temperature is not the only factor influencing the secondary reactions leading to the production of aromatic compounds and that an increase in the residence time may also promote secondary reactions that increase the aromaticity of the products. This last statement also seems to apply to the aliphatic nonlinear and branched compounds produced in the pyrolysis. 3.1.4. Product Analysis: Gases. Because of the experimental set-up, the analysis of the noncondensable gas fraction revealed little information about what actually occurred within the reactor. However, this analysis is important for the economic contribution that these gases can make to a commercial process because they could be burnt and could help to make the process more self-sustained on an energy basis. The main compounds found in the gases were linear alkenes and alkanes as were found in the oils/waxes fraction, but with a lower molecular size, ranging from C1 to C7. Table 6 shows the compositions in terms of length of the carbon chain of the gases obtained at 500 and 600 °C with a flow of 1 L/min of carrier gas through the reactor. The composition is calculated on a basis free of C1 and C2 compounds because the procedure employed for the analysis of the gases was not able to achieve adequate chromatographic separation of the evolved methane, C2 compounds, and the nitrogen employed as carrier gas in the reactor. The composition of the gases
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is very similar at each of the temperatures shown; nevertheless, there is a significant difference between these results. At 500 °C the mass fraction of compounds with carbon number g3 represented 56% of the total of noncondensable gases evolved whereas at 600 °C they represented 65% of the gaseous mixture. These figures are consistent with results presented in previous sections as they show an increase in the molecular weight distribution of the products formed at the latter temperature. In terms of the specific compounds in the gaseous mixture, the analysis of the gases exhibited similar results to those presented elsewhere.20-22 Although it was not possible to obtain separation of low molecular weight compounds, the search for ions with a mass of ≈2 generated very low abundance, signifying very low concentrations of hydrogen. It is important to notice that compounds that reduce the potential use of the gases as a fuel or energy source for the process were found in relatively low concentrations. Levels of benzene and other aromatics were below 0.25%. The linear alkenes and alkanes, as present in the oils/waxes, accounted for almost 90% of the mixture of compounds, with the rest of the mixture consisting mainly of cyclic aliphatic compounds. The concentrations of aromatics and compounds with ratios H/C < 2 were not found in great amounts, meaning that the products had a H/C ratio of ≈2, a value expected because of the original polyethylene feedstock; this is another reason that explains the absence of a considerable amount of hydrogen in the gases produced. 3.2. Pyrolysis of Toothpaste Packaging Laminate. One of the most important features of the process under study is its capacity to deal with real wastes that cause other pyrolytic processes to fail. It is this ability that compensates for the necessity of the generation of heat via microwave-heating procedures with some reduction in the overall thermal efficiency of the process. While there has been some research27,28 on the pyrolysis of aluminum/PVC laminates, there is no information in the literature concerning the pyrolysis of aluminum/ polyethylene composites for feedstock recycling. The results outlined below show that microwave-induced pyrolysis could be a commercial alternative for the recycling of this kind of waste. When a load of 50 g of laminate was added to the reactor, 14.7 g (29.4%) of clean aluminum was liberated, in agreement with the expected amount (30% by mass) predicted from the composition of the laminate. The solid aluminum was separated easily from the carbon by sieving and showed a shiny and clean surface. A very thin layer of a fine white powder was found adhered to the side walls of the quartz reactor. It was established that this was titanium dioxide, a compound commonly used as a white pigment in paints and inks. Clearly, this compound was originally present in the painted surface of the toothpaste tube, was separated from the organic content of the laminate during pyrolysis, and was deposited on the walls of the reactor. There was no significant differences in the yields of oils/waxes and noncondensable compounds produced from the laminate and from HDPE pellets in pyrolysis experiments at 500 and 600 °C. The molecular weight distribution of the oils/waxes in both cases were very similar with a slight increase on the average molecular weights in the case of the laminate, especially in the weight-average, causing a slight increase in the prod-
Figure 5. Carbon number distribution for the pyrolysis at 500 °C without carrier gas of (a) HDPE pellets and (b) aluminum/ polymer laminate (toothpaste tube).
ucts. Figure 5 compares the distributions obtained for both materials under the same process conditions at 500 °C. It is not possible to perform the pyrolysis of laminates at 700 °C due to the fact that the aluminum content melts at 660 °C. In terms of individual compounds, again the GC-MS analysis of the samples provided results very similar to those found for pure polyethylene and the presence of oxygenated compounds, as a result of the degradation of other organic compounds such as the ink used in the label, was not detected. The performance of the microwave-induced pyrolysis process appeared to be efficient in extracting the oxygen from the organic content, perhaps converting it to CO2. The analysis of the gases showed essentially the same results as the experiments performed with the pure polyethylene. 4. Conclusions The product yields, MWD, and chemical compositions were established for the pyrolysis of HDPE pellets operating a new semibatch bench-scale apparatus using the microwave-induced pyrolysis process. The yields of liquid and gases between 500 and 700 °C did not vary with respect to temperature, in the manner reported by previous investigations, mainly because of the change in the reaction rate that affected the residence time of the primary pyrolysis products in the hot reaction zone and hence the extent of secondary and ternary reactions. The molecular weight distribution of the oils/waxes produced helped to confirm the proposed relationship between temperature and residence time. Higher average molecular weights were found for the oils/waxes produced at 600 °C than for those produced at 500 °C.
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However, at 700 °C the average molecular weight presented values similar to those obtained at 500 °C, meaning that a similar extent of cracking was accomplished but in a much shorter time. The main products from the degradation were linear hydrocarbons, primarily alkenes, alkanes, and dialkenes, which accounted for ≈81-93% of the oils/waxes, the rest being a very complex mixture of cyclic and branched aliphatic and aromatic compounds. Similarly, the analysis of the gases showed that linear alkenes and alkanes were the main products with only very small amounts of potentially problematic compounds. No evidence of the production of significant amounts of hydrogen was found. The microwave process was successfully used to treat aluminum/polymer laminates (toothpaste tube) as an example of a real waste that presents problems to other, more conventional, pyrolytic processes. The degradation of this waste generated a similar hydrocarbon fraction to that from the degradation of pure HDPE pellets in both the condensable and the noncondensable products. 100% recovery of the aluminum present in the laminate was achieved and the metal showed a clean and shiny surface. Acknowledgment The authors are grateful to the Engineering and Physical Sciences Research Council for the financial support of this project. They are also grateful to TechEn Ltd. for the invaluable advice, materials, and equipment. Carlos Ludlow-Palafox acknowledges the National Science and Technology Council of Mexico and the ORS award for the financial assistance provided. Nomenclature Cp ) heat capacity (J kg-1 K-1) EVOH ) ethylene-vinyl-alcohol-copolymer GC-MS ) gas chromatography-mass spectrometry HDPE ) high-density polyethylene K ) thermal conductivity (W m-1 K-1) k ) first-order reaction rate constant (s-1) LDPE ) low-density polyethylene LLDPE ) linear low-density polyethylene Mw ) weight-average molecular weight (amu) Mn ) number-average molecular weight (amu) MSW ) municipal solid waste PE ) polyethylenes PP ) polypropylenes Py ) pyrolysis number as defined in eq 1 r ) radius of particle (m) F ) density (kg m-3) µn ) number-average degree of polymerization µw ) weight-average degree of polymerization
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Received for review March 1, 2001 Revised manuscript received July 10, 2001 Accepted July 10, 2001 IE010202J
