Ind. Eng. Chem. Res. 1998, 37, 841-847
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Experimental Determination of the Yield of Pyrolysis Products of Polyethene and Polypropene. Influence of Reaction Conditions R. W. J. Westerhout, J. A. M. Kuipers,* and W. P. M. van Swaaij Faculty of Chemical Engineering, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands
The influence of temperature, residence time, concentration level of reactants and products, polymer type, and composition of the polymer mixture on the product spectra obtained from pyrolysis of polyethene (PE) and polypopene (PP) was determined. In our study the temperature was varied between 650 and 850 °C, whereas residence times between 0.1 and 1 s were used. Thermodynamic calculations revealed that aromatics and methane are formed as the main products under conditions pertaining to chemical equilibrium. The ratio between these compounds is governed by the C/H ratio in the original polymer. Experiments were performed using a tubular reactor. The main products of the pyrolysis of PE and PP at 750 °C and a residence time of 1 s are ethene (respectively 45 and 19 wt %), propene (24 and 45 wt %), and butene (23 and 27 wt %). At higher reactor temperatures the yields of ethene and methane increase, while the yields of both propene and butene decrease. The influences of the residence time, product concentration, polymer type, and composition of the polymer mixture on the product distribution are negligible compared to the influence of temperature. 1. Introduction
Table 1. Product Spectrum for PE and PP Pyrolysis Reported by Sawaguchi et al. (1980, 1981)
1.1. General Introduction. In the world large amounts of mixed plastic waste (MPW) are produced every year. The APME (1996) estimated that some 17.505 kton/yr of MPW was produced in Western Europe in 1994. One of the most promising alternatives to dumping and incineration of this waste, mainly consisting of polyethene (PE), polypropene (PP), polystyrene (PS), and poly(vinyl chloride) (PVC), is hightemperature pyrolysis (>600 °C) by which valuable products such as ethene, propene, styrene, etc., can be produced. Reliable information on the yield of the hightemperature pyrolysis products of PE and PP and the influence of important parameters, such as temperature and residence time, is unfortunately very scarce in the literature. Some research on this subject was done by Sawaguchi et al. (1980, 1981), and their reported product yields are summarized in Table 1. In addition to the products that have been reported by Sawaguchi et al., other products (i.e., aromatic) must have been formed also to satisfy the mass balance as the C/H ratio of the gas-phase products per definition exceeds 2, while the C/H ratios of both PP and PE are exactly 2. It is very likely that during the experiments aromatic compounds (benzene, naphthalene) with C/H ratios close to 1 were formed. It is possible that most of the aromatic compounds formed coke, which was deposited in the equipment and was therefore not detected. Conesa et al. (1994) performed some experiments with HDPE in a Pyroprobe 1000. In the pyroprobe it can be assumed that the secondary cracking of products is limited as the residence time in the “hot” zone of the reactor is small. The results of their experiments are summarized in Table 2. * To whom correspondence should be addressed.
product
LDPE (wt %)
i-PP (wt %)
a-PP (wt %)
hydrogen methane ethene ethane propene propane butene
0.9 10.7 45.6 4.4 26.4 1.6 10.4
0.1 5.7 14.6 5.8 48.4 2.4 23.0
0.4 6.0 13.7 6.1 48.6 2.5 22.7
total
100
100
100
Table 2. Yields of Products as a Function of Temperature (Conesa et al., 1994) T (°C)
ethene (wt %)
propene (wt %)
total gas (wt %)
total waxes and tars (wt %)
500 600 700 800 900
0.64 6.08 15.11 22.65 13.76
0.37 2.65 3.75 2.59 2.80
2.42 14.62 33.84 37.23 35.82
97.58 85.38 66.16 62.77 64.18
Some experiments with a microreactor were reported by Sodero et al. (1996). They performed experiments at temperatures ranging from 800 to 900 °C and reaction times between 350 and 2500 ms. At 800 °C the conversion of the plastic material and therefore the yields of products were very low for the forementioned reaction times. At 900 °C a total gas yield of 95 wt % was found at 1000 ms. The results of Sodero et al. (1996) are plotted in Figure 1. The yields of other alkanes (especially ethane) are remarkably high in their experiments. Others investigators determined the product spectrum resulting from PE and PP pyrolysis (for instance, Seeger and Ritter, 1977; Kiang et al., 1980; Madorsky and Straus, 1954), but unfortunately these authors did not report the applied reaction conditions (residence time, concentration, and temperature) as most of these
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842 Ind. Eng. Chem. Res., Vol. 37, No. 3, 1998
Figure 1. Product yield as a function of the reaction time (Sodero et al., 1996).
Figure 3. Schematic representation of the reaction scheme for ternary gas-phase reactions.
Figure 2. Schematic representation of the reaction equation.
studies were aimed at the elucidation of the reaction mechanism. None of the studies distinguished between the primary devolatilization reaction and the secondary and ternary gas-phase reactions. The aim of this study was to assess the influence of the temperature, gas-phase residence time, concentration of products, polymer type, and composition of the polymer mixture on the product yield of the pyrolysis of PE and PP. For this purpose an isothermal tubular reactor with approximate plug flow characteristics was designed and constructed. In this reactor the gas-phase residence time and the temperature are well defined, which offers the possibility of obtaining the maximum achievable or “ultimate” yield of valuable products during the pyrolysis reaction. In large-scale reactors the residence time and temperature control are much more difficult and far from ideal, which may cause excessive cracking of the desired intermediate products to undesired side products. Knowledge about the ultimate yield and the influence of different parameters can be used to evaluate and optimize the product spectrum obtained in these largescale reactors. 1.2. Reaction Scheme. The thermal decomposition or pyrolysis of both PE and PP proceeds via a random degradation mechanism, yielding a broad product spectrum (C1 to C50) (see, for instance, Seeger and Cantow, 1975; Seeger and Ritter, 1977). The primary devolatilization reaction mainly yields waxlike materials with a typical carbon number in the range of 20-50. In the gas phase these products are cracked further (secondary gas-phase reactions) to smaller hydrocarbons, like ethene and propene. These products are thermodynamically unstable at these high temperatures and react to form aromatic compounds like benzene and toluene (ternary gas-phase reactions), while at even longer residence times coke is formed. Other products formed at long residence times are methane and hydrogen. A schematic representation of the reaction scheme is given in Figure 2, while the reaction scheme for the ternary gasphase reactions is given in Figure 3.
Figure 4. Product spectrum of PE/PP in the equilibrium situation as a function of temperature (main products).
2. Thermodynamics At very long residence time and/or at very high temperatures an equilibrium product spectrum will be obtained. In the equilibrium state the free Gibbs enthalpy of the system will be minimal, which implies that the following relation applies: n
µj dnj ) 0 f dG ) 0 ∑ i)1
(1)
The equilibrium composition resulting from the pyrolysis of PE, PP, PS, and PVC was computed with the aid of the ASPEN simulation package. The physical properties of the polymers (formation enthalpy, heat capacity), required for the simulations, were not available in the data banks of ASPEN and were calculated using the group contribution method of Benson and Jacob (ASPEN documentation), while the Lee-KeslerPlocker equation of state was used. In these calculations a very long hydrocarbon (C35H70) was taken as a model compound for the polymers. From the ASPEN computations it was found that in the equilibrium state the pyrolysis of PE or PP mainly yields coke (aromatic compounds with two or more benzene rings) and methane as the main reaction products (see Figure 4). The C/H ratio of coke is close to 1, while the C/H ratio in the original polymers is 2. Therefore, compounds have to be formed with a C/H ratio exceeding 2 to satisfy the overall mass balance, which is indeed the case since a large fraction of the polymer is converted into methane. The ratio at which coke and methane are formed is therefore governed by the C/H ratio in the polymer. PE or PP pyrolysis (C/H ratio 2) yields a relatively large amount of methane, while almost no methane is formed if the polymer is PS (C/H ratio 1). In typical high-temperature pyrolysis
Ind. Eng. Chem. Res., Vol. 37, No. 3, 1998 843
Figure 5. Product spectrum of PE/PP in the equilibrium situation as a function of temperature (minor products).
Figure 6. Product spectrum of PVC in the equilibrium situation as a function of temperature (main products).
Figure 8. Schematic representation of the tubular reactor.
or temperature the formation of larger amounts of methane and aromatics can be expected. Figure 7. Product spectrum of PVC in the equilibrium situation as a function of temperature (minor products).
reactors the main product of PE pyrolysis is ethene (Sinn, 1974). However, it can be seen from Figure 5 that ethene is thermodynamically not stable and will therefore react with other compounds to produce aromatic compounds provided that the residence time and/ or temperature in the reactor are high enough. The main equilibrium products resulting from the pyrolysis of PVC are HCl and coke (see Figure 6). It can also be seen from Figure 7 that some chlorinated aromatic compounds will be formed during the pyrolysis reactions, but our thermodynamic calculations have revealed that the amount of chlorinated alkanes or alkenes formed during the reaction will be very low. This implies that organic chlorine in the gaseous products will most likely not pose a serious problem but that the liquid product will be contaminated with chlorinated compounds. This is important in connection with any further use of the products as chlorine in the product may poison catalysts or cause corrosion problems in downstream processes. The calculations show that the desired products (ethene, propene) are thermodynamically unstable and that any high-temperature pyrolysis reactor should therefore be operated at short residence times to avoid thermodynamic equilibrium and the formation of undesired aromatic compounds. For this reason very short residence times (0.1-1 s) were chosen for this study. This means that the product spectrum at the reactor conditions used in this study is determined by kinetic phenomena, which allows for the recovery of the thermodynamically unstable intermediate compounds (i.e., ethene and propene). The thermodynamic calculations clearly show that with increasing residence time and/
3. Equipment and Experimental Procedures To study the influence of temperature, gas-phase residence time, concentration level of reactants and products, polymer type, and composition of polymer mixtures, a tubular reactor setup was designed and constructed, which is schematically shown in Figure 8. Two tubular reactors made of stainless steel with diameters of respectively 17 mm for short residence times (0.1 s) and 50 mm for long residence times (0.5-2 s) were used in our experimental study. Both reactors had a length of approximately 1 m. In the top section of the tubular reactor, steel wool supported by a ciperm plate was used to collect the polymer fed to the reactor, which was converted in situ to an intermediate waxlike material by the primary devolatilization reaction (see Figure 2). The intermediate waxlike primary product was rapidly removed from the steel wool in the top of the reactor by a preheated nitrogen gas stream and fed to the reactor, where the products were converted further. By using this experimental setup, it was possible to separate the effects caused by the primary devolatilization reaction and the effects due to the occurrence of the secondary and ternary gas-phase reactions. By varying the tube diameter and the gas flow rate, the gas-phase residence time could be varied. No special care was taken to operate the reactor in the turbulent regime, although this was done if possible to prevent the development of a velocity profile in the reactor. However, even in the laminar regime no significant velocity profiles will exist as the reactor is too short for these profiles to develop. The tubular reactor was kept at the desired temperature with the aid of four electrical ovens. The temperatures of the tube wall and the gas stream were controlled and checked using 4 K type thermocouples. The temperatures of the gas stream and
844 Ind. Eng. Chem. Res., Vol. 37, No. 3, 1998 Table 3. Standard Experimental Conditions parameter
value
parameter
value
polymer type T (°C)
LDPE 750
τ (s) c [(w)ppm]
1 30.000
the tube wall were chosen to be the same during all experiments. The reactor temperature was defined as the temperature of the reactor wall. At the reactor exit the gas-phase products were rapidly quenched by injecting cold nitrogen with a large flow rate. The unconverted primary reaction products and liquids were collected in six washing bottles made of glass (diameter and length approximately 50 and 300 mm) and an aerosol filter. The fractions of the reaction products in the gas phase were determined by analyzing gas-phase samples using a Varian 3400 GC (Haysep Q column, FID detector, column temperature 200 °C). The aerosol filter and washing bottles were weighed before and after each experiment to determine the amount of unconverted primary reaction products and liquids. On the basis of these results and the data obtained from the GC, the mass balance could be evaluated. Coke formation was not accounted for in the mass balance, since this process is relatively unimportant (750 °C) was butadiene found in the product spectrum for PP pyrolysis experiments (
