Ind. Eng. Chem. Res. 2002, 41, 4559-4566
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KINETICS, CATALYSIS, AND REACTION ENGINEERING Kinetic Study of Polyolefin Pyrolysis in a Conical Spouted Bed Reactor Roberto Aguado,* Martı´n Olazar, Beatriz Gaisa´ n, Rube´ n Prieto, and Javier Bilbao Departamento de Ingenierı´a Quı´mica, Universidad del Paı´s Vasco, Apartado 644, 48080 Bilbao, Spain
Kinetic results of polyolefin (polyethylene and polypropylene) pyrolysis obtained in a conical spouted bed reactor are compared with those obtained by thermogravimetry and in a highheating-rate microreactor. The results are evidence of the good performance of this original reactor for kinetic studies of flash pyrolysis of plastics at high temperatures, which is due to bed isothermicity and to the fast and uniform coating of sand particles with melted plastic and, consequently, to lower limitations of heat and mass transfer. Agglomeration problems are minimum in this reactor, which is due to the good gas-solid contact in the spouted bed and to the vigorous particle movement attained with the conical geometry of the reactor. Introduction
of n-th order:
The thermal decomposition of plastics has originally been studied in the literature with the aim of ascertaining the structure of the material1,2 and because pyrolysis is the step prior to gasification and combustion of these materials. At present, pyrolysis is considered as the more suitable way of upgrading waste plastics in order to obtain feedstocks and fuel.3 The importance of the technological development of large scale pyrolysis units is evident in view of the growing amount of postconsumer plastic wastes in developed countries (at present 20 million tons in the United States, 15 million tons in Europe, and the same amount in Japan).4-6 The aim of upgrading is to avoid the environmental deterioration produced by landfilling these materials that are not readily degradable. Furthermore, upgrading by combustion is not a proper solution because it may lead to the formation of highly toxic pollutants, such as dioxins and furans. A knowledge of pyrolysis kinetics is of great importance for the design and simulation of the reactor and in order to establish the optimum process conditions. The fluidized bed reactor is a technology developed at larger scale.7-15 Most of the kinetic studies in the literature have been carried out using thermogravimetric analyzers (TGAs), although other techniques, such as screw reactors, screen heaters,16,17 or laminar flow reactors,18 have been used in order to overcome heat and mass transfer limitations of conventional TGA equipment. Although structural kinetic models based on the mechanisms that take place have been proposed for polyolefin pyrolysis,16,18-22 the more useful kinetic results (because of their simplicity) for their use in reactor design are those fitted to an empirical kinetic equation * Corresponding author. Telephone: 34-94-6015363. Fax: 34-94-4648500. E-mail:
[email protected].
-
dW E ) kWn ) k0 exp Wn dt RT
(
)
(1)
Westerhout et al.23 have reviewed the results obtained in the literature for the parameters of eq 1. Most of the authors fit their results for high conversions to an order of n ) 1 and obtain activation energies in the 80-280 kJ mol-1 range and frequency factors in the 1010-1018 s-1 range. Although from the fact that n ) 0 was found for low conversions,24,25 a value in the 0.5-1 range has also been determined for the reaction order in the whole range of conversions.26 The great differences in the kinetic results obtained in the literature are attributable to several experimental factors, such as the material used (particle size and molecular weight of the polymer) or the operating conditions (heating ramp, gas used and its flow rate, heat transfer rate between sample and crucible).22 Another factor that justifies the difference in the results is the methodology followed for treating the experimental results in order to calculate the kinetic parameters.27 Furthermore, most of the results in the literature are obtained by thermogravimetry, and they correspond to temperatures lower than 450 °C, which means that their extrapolation for the design of reactors that operate at higher temperatures is speculative. In this paper a new reactor, which is a conical spouted bed reactor, is proposed for the kinetic study of polyolefin pyrolysis at high temperatures. The conical spouted bed reactor has the following advantages over conventional spouted beds (cylindrical with conical base): simpler design because no distributor plate is needed, lower pressure drop, cyclic circulation of the particles, high heat and mass transfer between gas and solid (whose circulation is mainly counter-current), and short gas residence time. Furthermore, the conical spouted bed has great versatility as far as gas flow is
10.1021/ie0201260 CCC: $22.00 © 2002 American Chemical Society Published on Web 08/03/2002
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Ind. Eng. Chem. Res., Vol. 41, No. 18, 2002
Figure 1. Diagram of the pyrolysis equipment. Figure 2. Scheme and design parameters of the conical spouted bed reactor.
concerned, which allows for a vigorous solid movement. This is interesting when the solid is of irregular texture, its size is not uniform, or there is a size distribution or it is sticky. In view of these properties, the conical spouted bed reactor performs well in the pyrolysis of vegetable biomass28,29 and in catalytic polymerization.30,31
sand particles and coats them. The vigorous cyclic movement of the particles allows for obtaining a uniform coating of the particles without agglomeration problems, as is shown in the scheme of Figure 2b. Figure 2a shows a diagram of the reactor where the dimensions are shown: HT ) 0.340 m; Hc ) 0.205 m, γ ) 28°; Dc ) 0.123 m; Di ) 0.02 m; D0 ) 0.01 m. These dimensions have been established from previous hydrodynamic studies carried out in the spouted bed regime in conical contactors32,33 and bearing in mind the versatility of the equipment. When the inert gas flow rate is increased, the gas-solid contact regime evolves from the conventional spouted bed to a dilute spouted bed (which characterizes conical contactors) and the gas residence time decreases from a few seconds to approximately 20 ms. Temperature is measured by means of three thermocouples placed at different radial positions in the reactor and provided with free vertical movement. Bed isothermicity is noteworthy, and it is attained because of vigorous particle circulation. The reaction temperature is reached by means of two electric resistances covered with ceramic material. One resistance is located within the tube for heating the inert gas before it enters the reactor, and the second one surrounds the conical section of the reactor. The temperature in these two positions is measured by two thermocouples. The two heating zones are thermally insulated. The pyrolysis product stream is carried by the inert gas toward a condensing system made up of a cooler, an ice trap, and a 25 µm sintered steel filter, where wax is collected. The stream of noncondensable gases (all the products except the wax, at reaction conditions) is analyzed by gas chromatography (Hewlett-Packard 6890) using a BPX5 capillary column of 50 m. The runs for the kinetic study have been carried out by feeding 1 g of plastic (with a particle size of approximately 1 mm) and using 30 g of sand (with a size
Experimental Section The materials studied are low-density polyethylene (LDPE), high-density polyethylene (HDPE), and polypropylene (PP). These materials are supplied by Dow Chemical (Tarragona, Spain), and their properties are set out in Table 1. The thermogravimetric equipment used is a symmetrical Setaram TAG 24 thermobalance connected to a vacuum system (10-3 mmHg). Two sets of experiments have been carried out; one set at constant temperature in the 390-440 °C range and the second one following temperature-time ramps at the rates 2, 5, and 10 °C min-1. This second set allows for obtaining kinetic parameters corresponding to higher temperatures. The microreactor used is a commercial piece of equipment, Pyroprobe 1000, which is connected on-line to a Hewlett-Packard 6890 gas chromatograph. The sample of plastic is placed in a quartz tube, which is surrounded by a platinum filament. The runs have been carried out at 500, 550, and 600 °C for times between 15 and 2000 s, depending on the material. The run temperature is reached at the heating rate 10 °C ms-1. As the maximum pyrolysis time allowed for the equipment is 100 s, the runs of longer duration have been carried out by repeating reaction cycles of 100 s. At the end of the time established for each run, the conversion is determined by weighing the tube containing the sample. Figure 1 shows a diagram of the laboratory setup furnished with a conical spouted bed reactor. As the plastic material enters the reactor, it melts onto the Table 1. Properties of the Materials Studied material
mol wt, g mol-1
polydispersity
density, kg m-3
heating value, MJ kg-1
pyrolysis heat, kJ kg-1
LDPE HDPE PP
92200 46200 50000-90000
5.13 2.89 2.00
923 940 890
43 43 44
534.6 429.8 581.9
Ind. Eng. Chem. Res., Vol. 41, No. 18, 2002 4561 Table 2. Kinetic Parameters Obtained in Thermobalance in Runs Carried out at Constant Temperature with the Three Materials material
T, °C
LDPE
HDPE
0.02-0.27
410
0.04-0.4 0.4-0.52
435
0.18-0.4 0.4-0.85 0.85-0.99 0.01-0.11
410 435 390 415 440
a
0.05-0.4 0.4-0.74 0.74-0.98 0.16-0.4 0.4-0.74 0.74-0.98 d
E, kJ mol-1
k0, s-1
263 ((24)a 287 ((26)b
9.36 ((0.89) × 1015 9.23 ((0.91) × 1017
1.49 × 10-4 2.08 × 10-4 5.22 × 10-4 4.49 × 10-4 5.89 × 10-4 1.07 × 10-3 6.38 × 10-5 8.97 × 10-4
386 ((33)a 162 ((15)b 115 ((12)b
1.65 ((0.14) × 1025 4.95 ((0.46) × 108 3.51 ((0.34) × 105
8.10 × 10-4 1.09 × 10-3 1.81 × 10-3 1.66 × 10-3 2.25 × 10-3 3.53 × 10-3
258 ((26)c 255 ((23)c 109 ((11)d
1.71 ((0.17) × 1016 1.40 ((0.16) × 1016 3.40 ((0.31) × 105
3.9 ×
0 0.65 1 0 0.65 1 0 0.65 1 0 0.65 1 0 0.65 1 0 0.65 1 0 0.65 1 0 0.65 1 0 0.65 1
0.06-0.4 0.4-0.8 0.8-0.87 0.1-0.4 0.4-0.8 0.8-0.989 0.11-0.4 0.4-0.69
400-435 °C. b 410-435 °C. c 390-440 °C.
k, s-1
n
400
400
PP
X
10-5
6.84 × 10-5 9.54 × 10-5 3.86 × 10-4 5.7 × 10-4 1.42 × 10-3 9.93 × 10-6
415-440 °C.
between 0.63 and 1 mm). The temperatures studied are 500, 550, and 600 °C, and the reaction times are between 1 and 1080 s (sufficient time for the reaction to be complete at 500 °C). Results Thermogravimetry. As an example of the thermogravimetric results obtained at constant temperature, Figure 3 shows the evolution of conversion with time for LDPE at three temperatures. The results of conversion versus time have been fitted to eq 1, which, expressed in terms of conversion and linearized, is
ln
dX ) ln k + n ln(1 - X) dt
(2)
where
X)
W0 - W W0
(3)
Figure 4 shows the results of ln(dX/dt) versus ln(1 - X) calculated from Figure 3. The reaction order, n, and the kinetic constant, k, are determined from Figure 4. The value n ) 0 has been determined for low conversions (the entire range corresponding to 400 °C), and n ) 1, for high conversions, whereas, for intermediate conversions, n ) 0.65 has been obtained. The results are qualitatively similar for the other materials studied. The results calculated for the reaction order and for the kinetic constants and their corresponding conversion ranges are set out in Table 2. Likewise, the values of activation energies and frequency factors obtained by fitting the results of k to the Arrhenius equation are also set out. These results are shown with a 95% confidence interval. It is observed that activation energy decreases as conversion levels increase for pyrolysis of HDPE and PP. This result is explained on the basis of
Figure 3. Results of evolution of conversion with time obtained in thermobalance at different temperatures for LDPE pyrolysis.
the pyrolysis mechanisms, as the Random Chain Dissociation Model, which distinguishes between the type of bond and its stability. Thus, at low conversions, there is a preferential cracking of the long main chains, with the subsequent cracking of the resulting branched chains, which are thermally less stable.23 Constant temperature runs may only be carried out at low temperatures,
