Ind. Eng. Chem. Res. 1998, 37, 2293-2300
2293
Recycling of Polyethene and Polypropene in a Novel Bench-Scale Rotating Cone Reactor by High-Temperature Pyrolysis R. W. J. Westerhout, J. Waanders, J. A. M. Kuipers,* and W. P. M. van Swaaij Reaction Engineering Group, Faculty of Chemical Engineering, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands
The high-temperature pyrolysis of polyethene (PE), polypropene (PP), and mixtures of these polymers was studied in a novel bench-scale rotating cone reactor (RCR). Experiments showed that the effect of the sand or reactor temperature on the product spectrum obtained is large compared to the effect of other parameters (for instance, residence time). In general, it can be concluded that the amount of polymer converted into propene and butene decreases with higher cracking severity (higher temperatures or longer residence times), while the fraction methane increases. About 80 wt % of the polymer is converted into gas at a reactor temperature of 898 K, while 20 wt % is converted into intermediate waxlike compounds or aromatics in the case of PE. The gas yield increases slightly with the reactor and/or sand temperature to 88 wt % at higher temperatures. The total amount of alkenes decreases with increasing cracking intensity, which suggests that the reactor should be operated at the lowest possible temperature. Our results indicate that the reactor offers a few significant advantages compared to other reactors (no fluidization gas necessary, good solid-polymer mixing, no cyclones necessary) and a competitive product spectrum. However, significant improvements are still possible to make the reactor concept technically and economically more attractive. 1. Introduction 1.1. General Introduction. Every year large amounts of mixed plastic waste (MPW), mainly consisting of polyethene (PE), polypropene (PP), polystyrene (PS), and poly(vinyl chloride), (PVC) are produced. At the moment this waste is usually dumped or incinerated together with household waste, but due to environmental concerns, governments, companies, and universities are looking at alternatives for the disposal of this waste. One very promising alternative to dumping or incineration is high-temperature pyrolysis of the MPW to recover valuable chemicals, like ethene, propene, and styrene. Several high-temperature pyrolysis processes were developed in the past using bubbling fluidized beds (BFB’s; Sinn, 1974; Sinn et al., 1976) or circulating fluidized beds (CFB’s; Batelle Memorial Institute, 1992). At University of Twente a novel reactor, a rotating cone reactor (RCR), has been developed for the pyrolysis of biomass (Wagenaar, 1994). A schematic drawing of this reactor, termed the bench-scale RCR ([B]RCR), is presented in Figure 1. Polymer or biomass is fed into the reactor together with preheated sand, which is used to supply heat to the reactor, to enhance the heat-transfer characteristics and to prevent sticking of particles to the reactor wall. The sand and polymer particles are thoroughly mixed on the bottom plate of the reactor and are subsequently transported upward by the rotating action of the cone. In the reactor the polymer particles are heated and pyrolyzed. The sand leaves the reactor at the top, while the gaseous products are removed at the bottom of the reactor. This type of reactor has some advantages compared to conventional reactors for the pyrolysis of biomass and polymers: * To whom correspondence should be addressed.
Figure 1. Schematic representation of the bench-scale RCR.
(a) No (external) fluidization gas is required, which reduces the required reactor volume and also enables downsizing or elimination of auxiliary equipment. (b) Short gas- and solid-phase residence times. (c) Good polymer-sand mixing. (d) High-intensity reactor (high throughput-volume ratio). (e) No cyclone necessary for gas-solid separation at the exit of the reactor.
S0888-5885(97)00704-5 CCC: $15.00 © 1998 American Chemical Society Published on Web 05/08/1998
2294 Ind. Eng. Chem. Res., Vol. 37, No. 6, 1998
Figure 2. Reaction scheme for the pyrolysis of polymers.
a
b
Figure 3. (a) Dimensions of inner cones. (b) Dimensions of rotating cones.
The aim of this study is to explore the suitability of the rotating cone reactor concept for the high-temperature pyrolysis of PE, PP, and mixtures of these polymers and to identify the optimal operating conditions for this novel reactor. 1.2. Reaction Scheme for the Pyrolysis of PE and PP. Both PE and PP degrade thermally via a random degradation mechanism, yielding a broad product spectrum (C1-C50) (see, for instance, Seeger et al., 1975, 1977). The primary devolatilization reaction yields mainly intermediate waxlike products. In the gas phase these products are cracked further (secondary gas phase reactions) to smaller hydrocarbons (for instance, ethene and propene). However, the low alkenes and alkanes are thermodynamically unstable at these high temperatures and are converted into aromatic compounds like benzene and toluene (ternary gas-phase reactions). At sufficiently high residence times, significant amounts of coke are formed. Other products obtained at long residence times are methane and hydrogen. A schematic reaction scheme for the overall conversion process is given in Figure 2. 2. Experimental Equipment and Procedures 2.1. Pyrolysis Experiments. The bench-scale rotating cone reactor ([B]RCR) used in this study is identical with the one used by Wagenaar (1994) to study the pyrolysis of biomass. The main dimensions of the [B]RCR are shown in parts a and b of Figure 3. Prior to an experiment the reactor was preheated to the desired temperature using an electrical oven. During the heat-up period the reactor volume was purged
using a small nitrogen stream to remove all oxygen from the reactor. Most experiments were conducted using low-density polyethene (LDPE1) with a density of 917 kg/m3 and an average initial molar weight of 350 000 g/mol and polypropene (PP) with an unknown initial molar weight. In addition, some experiments were performed using high-density polyethene (HDPE) with an average initial molar weight of 125 000 g/mol and linear low-density polyethene (LLDPE) of unknown initial molar weight. The diameter of the polymer particles was typically less than 300 µm. The polymer particles were fed to the reactor using a vibratory feeder at a mass flow rate of approximately 1 g/s through a water-cooled feeding pipe positioned in the center of the reactor. A small cold nitrogen purge stream was fed through the feeding pipe to prevent hot pyrolysis gases from entering the feeding pipe. Prior to the actual pyrolysis experiment, 30 kg of sand was heated to approximately 150 K above the reactor temperature using a small sand bunker surrounded by three electrical ovens. The preheated sand (diameter 0.5-0.8 mm) was introduced in the cone reactor in the center of the bottom plate at an angle of π/4 with respect to the polymer feed stream to ensure good mixing between polymer and sand. The sand mass flow rate in most experiments was approximately 4-5 g/s. The disadvantage of this configuration is that the sand exhibits preferential flow paths through the reactor, which effectively means that only part of the reactor is used. It was not possible to modify the existing reactor to prevent this phenomenon. In the [B]RCR part of the interior volume was blocked using an inner cone to limit the gas-phase residence time to prevent conversion of the desired intermediate products (ethene, propene, and butene) to undesired, less valuable products (methane and aromatic compounds). During all experiments a cone rotating frequency of 600 rpm was used. By the rotating action of the cone, the sand-polymer mixture was transported to the top of the reactor. The sand particles exited the reactor at the top of the reactor and were subsequently collected in an annular space surrounding the cone (see Figure 1). There is no sand outlet present in the reactor. The gaseous products exited the reactor at the bottom and were separated in the product collection section. No cyclone was needed, because no sand particles were entrained in the gaseous product stream. The gaseous products were cooled using six ice-cooled bottles in which the largest fraction of the liquid and solid products could be collected. Aerosols present in the gas flow were collected using a cotton-wool filter. The cotton-wool filter and the ice-cooled bottles were weighed before and after each experiment to determine the fraction of polymer converted to liquid products. The gas flow rate was measured using a volumetric gas flowmeter. During the experiment gas samples were taken after the aerosol filter, which were subsequently analyzed using a Varian 3400 GC with a Haysep Q column (temperature 473 K) and a flame ionization detector (FID), which unfortunately meant that hydrogen could not be detected. An experiment was started by opening the valve in the sand pipe connecting the sand bunker and the reactor. After ensuring that a proper sand flow was established, the actual pyrolysis experiments were
Ind. Eng. Chem. Res., Vol. 37, No. 6, 1998 2295 Table 2. Standard Cases for the Pyrolysis of PE and PP in the Bench-Scale RCR parameter
standard case I
standard case II
polymer Tr [K] Ts [K] Qp [g/s] Qs [g/s] QN2 [g/s] cone angle [rad] inner cone
PE 998 1123 1 5 0.4 π/2 yes
PP 898 998 1 5 0.4 π/2 yes
Figure 4. Schematic drawing of the experimental setup for RTD experiments. Table 1. Standard Case for Residence Time Distribution Experiments value rotation frequency N2 purge gas outlet
600 rpm 1 g/s middle
value volume I volume II volume III
unblocked unblocked unblocked
started. This was done by starting the vibratory feeder. The pressure in the reactor was kept constant by using a vacuum pump connected to the exit of the product collection section. The gas samples and temperature measurements in the reactor showed that a stationary situation was reached after 4 min of operation, whereas each experiment lasted approximately 10 min. After each experiment, the integral mass balance was calculated and the yield of the liquid and gaseous products was determined. For most experiments the integral mass balance was satisfied within 90 wt %. The main reasons for this deviation are due to coke formation, which was not measured (estimated at approximately 1-2 wt %), and the slip of aerosols through the product collection system. In all figures presented below the composition of the gas phase is given, which has to be multiplied by the gas yield to obtain the actual yield of the gaseous product. 2.2. Residence Time Distribution Experiments. To study the residence time distribution characteristics of the [B]RCR, propane tracer gas experiments were conducted (Westerterp et al., 1984). The experimental equipment used in this study is shown in Figure 4. During the experiments 1 g/s of nitrogen gas was purged through the polymer feed pipe to simulate the generation of gas in the actual pyrolysis experiments. The effect of changing the reactor geometry could be simulated by blocking the three different dead volumes in the reactor as shown in Figure 4. Most of the experiments were done without sand as our first experiments showed that the sand flow had a very limited influence on the residence time distribution in the reactor. The parameters for the standard case for the residence time distribution (RTD) experiments are given in Table 1. 3. High-Temperature Pyrolysis Experiments of Polyethene 3.1. Definition of the Standard Case. To study the effect of different parameters, a standard case (I) for experiments with PE was defined based on literature data and our own initial experiments. For pyrolysis
Figure 5. Influence of the reactor temperature on the gas composition for the pyrolysis of PE.
Figure 6. Gas yield as a function of the reactor temperature for the pyrolysis of PE.
experiments involving PP, a second standard case (II) was defined with a lower temperature. The operating conditions for both standard cases are summarized in Table 2. The results of the experiments and the influence of different parameters will be discussed in the next sections for PE, whereas the results for PP will be presented in the next paragraph. 3.2. Influence of the Reactor Temperature. The influence of the reactor or sand temperature on the product spectrum is larger than that of any other parameter (Westerhout et al., 1998). The effect of the reactor temperature on the product spectrum obtained is shown in Figures 5 and 6. The reactor temperature is defined as the temperature in the reactor volume before each experiment. At elevated temperatures the methane fraction increases sharply, while the fractions of propene and butene decrease. The ethene fraction and the total gas yield are relatively insensitive to the reactor temperature. About 80 wt % of the polymer is converted to gas, while 20 wt % is converted to liquid/solid wax products at 923 K (see Figure 6). The liquid yield is the sum of partially converted intermediate waxlike components and aromatic products. The total liquid yield decreases slightly with an increasing reactor temperature. At higher temperatures there might also occur a shift in the composition of the liquids fraction from intermediate waxlike products to aromatic products. In the case of PE pyrolysis it might be possible or advantageous to recycle the unconverted intermediate waxlike products to the reactor to obtain higher gas yields.
2296 Ind. Eng. Chem. Res., Vol. 37, No. 6, 1998 Table 3. Comparison of the Product Spectra of PE Obtained in a Bench-Scale RCR and a Tubular Reactor product spectrum (wt %) compound
PE (RCR)
PE (TR, 923 K)
PE (TR, 1073 K)
methane ethene propene butene butadiene other alkanes >C4 total alkenes gas yield
38.9 31.6 10.0 6.1 0.2 11.5 1.7 48 87.8
6.6 41.6 23.6 24.1 0.0 4.1 0.0 89 89
11.0 52.5 19.0 13.5 0.0 4.0 0.0 85 99a
Table 4. Calculated Average Residence Time and Standard Deviation for Different Blocked Volumes blocked volume
average residence time (s)
standard deviation (s)
none I I/II I/II/III
4.91 5.00 4.46 2.42
22.76 23.04 21.14 13.86
a Formation of coke is necessary to satisfy mass balance but is not measured.
Figure 8. Gas composition as a function of the sand temperature for the pyrolysis of PE.
Figure 7. Influence of dead zones on the cumulative residence time distribution function F(t).
If the reactor temperature is increased, the total amount of valuable alkenes produced decreases, while the total amount of alkanes (especially methane) increases. It is therefore advantageous to operate the reactor at the lowest possible temperature to obtain a maximum yield of alkenes. However, if the temperature of the reactor becomes to low, the yield of unconverted intermediate waxlike compounds increases sharply, which leads to clogging of the product collection system. The temperature at which this occurs lies between 873 and 923 K. Below this temperature the unconverted intermediate waxlike product is the main product obtained and in this case we essentially deal with a back to fuel (BTF) process, while above this temperature the lower olefins constitute the main product, which means that at higher temperatures the process is a back to monomer (BTM) process. From an energetic point of view it is also advantageous to operate the reactor at lower temperatures. In an earlier study experiments were also conducted with LDPE1 in an isothermal tubular reactor with (near) plug flow characteristics (Westerhout et al., 1998). The yield of alkenes obtained in this reactor is significantly higher than the yield obtained in either [B]RCR (see Table 3) or BFB reactors, indicating that significant improvements in the product spectra are still possible. The decreased yield of alkenes is probably caused by the nonideal gas-phase residence time distribution and the existence of temperature gradients in the reactor, resulting in excessive cracking of intermediate products such as ethene, propene, etc., to methane and coke. In our earlier study it was found that the polymer throughput or the intermediate waxlike product concentration does not significantly influence the product spectrum obtained in the tubular reactor, indicating that the decreased yield of alkenes is not caused by the higher throughput and accompanying higher product concentration in the bench-scale RCR. Therefore, the gas-phase residence time distribution was measured with propane injection experiments.
The influence of the different dead reactor volumes on the cumulative residence time distribution function F(t) is presented in Figure 7. The corresponding average residence time and standard deviation are given in Table 4. It can be seen from this figure that in the base case (no volumes blocked) the cumulative residence time distribution function F(t) shows significant tailing, which can be reduced by blocking one or more dead volumes in the reactor. Especially, the blocking of reactor volume III causes a significant reduction in the extent of tailing of F(t). This implies that during normal reactor operation significant amounts of (product) gas are exchanged with reactor volume III. The relatively long residence time of the gas exchanged with this reactor volume probably causes excessive cracking of intermediate products (ethene and propene), which could be one of the main reasons for the relatively high methane and liquid yields observed in our [B]RCR compared to the tubular reactor. 3.3. Influence of the Sand Temperature. The effect of the sand temperature on the product spectrum of PE is the same as the influence of the reactor temperature, because the sand temperature directly influences the temperature in the reactor. The sand temperature is defined as the temperature of the sand in the bunker, before it is fed to the reactor. It can be concluded from Figure 8 that the sand temperature was chosen too high in the base case, which leads to excessive formation of methane (39 wt %). 3.4. Influence of the Gas-Phase Residence Time. The gas phase residence time in the reactor was altered by removing the inner cone from the interior reactor volume (see Figure 1). In the case where the inner cone is present the gas-phase volume is approximately 1.3 × 10-3 m3, while the total inner reactor volume is approximately 34.7 × 10-3 m3. These reactor volumes correspond with calculated average gas-phase residence times of respectively 0.4 (0.13 s in the bottom-plate volume) and 10 s in the standard case (see Table 2). It can be seen from Figure 9 that the influence of the average gas-phase residence time is minor compared to the influence of the reactor and/or sand temperature. A higher gas-phase residence time enhances methane formation, while the fractions of propene and butene produced decrease. This effect is similar to the effect of increased reactor and/or sand temperature. The product spectrum is therefore often correlated to the
Ind. Eng. Chem. Res., Vol. 37, No. 6, 1998 2297 Table 5. Product Spectrum for the Pyrolysis of PE as a Function of Polymer Mass Flow Rate product spectrum (wt %)
Figure 9. Influence of the gas-phase residence time on the gas composition of the pyrolysis of PE.
Figure 10. Gas composition for the pyrolysis of PE in the benchscale RCR as a function of cone angle.
cracking severity of the pyrolysis conditions expressed by an intensity function IF (Sawaguchi et al., 1980, 1981):
IF ) Tτa
(1)
where a equals 0.03 for LDPE, while Sawaguchi et al. found a value of a of 0.04 for PP which indicates that the influence of the residence time is minor compared to the influence of temperature, which is generally found for the pyrolysis or cracking of hydrocarbons (Albright et al., 1983). 3.5. Influence of Other Parameters. 3.5.1. Type of PE. In the majority of the tests performed with the bench-scale RCR, LDPE was used, but some experiments were conducted under base case I conditions using HDPE and LLDPE. Tests were also done with another LDPE with the same average molecular weight but a different molecular weight distribution. Earlier pyrolysis experiments conducted in a tubular reactor using these polymer types (Westerhout et al., 1998) already revealed that the type of PE had no significant influence on the product spectrum obtained, a finding which was confirmed by the experiments conducted in the bench-scale RCR. The differences in the product spectra were minimal and certainly within experimental error. This means that the results found for the LDPE used in this study are also valid for other types of PE. 3.5.2. Cone Angle and Bottom Plate Size. In this study pyrolysis experiments were performed using three different cone angles. In the standard case a cone with a top angle of π/2 was used, while experiments with cone top angles of π/3 and π (i.e., a flat-plate reactor) were also performed. Experiments were conducted with the base case reactor temperature (998 K) and a slightly higher reactor temperature (1023 K). The experiments with the π/3 cone yielded slightly more ethene, propene, and butene and less methane than the experiments with the π/2 cone (see Figure 10), while the product spectrum with the flat plate did not differ significantly.
compound
0.59 g/s
0.98 g/s
1.65 g/s
methane ethene propene butene butadiene other alkanes >C4 total alkenes
49.7 27.1 11.2 5.6 0 6.4 0 43.9
38.9 31.6 10.0 6.1 0.2 11.5 1.7 48
41.5 23.2 15.8 8.9 0.1 8.8 1.7 48
In the case of the cone reactor with a top angle of π/3 the gas-phase volume in the cone reactor is 3.1 × 10-3 m3 if the inner cone is present, while the gas-phase volume equals 60.2 × 10-3 m3 without an inner cone. The corresponding calculated gas-phase residence times are respectively 0.89 (0.83 s in the bottom-plate volume) and 17.2 s. The main difference with the cone reactor with a top angle of π/2 is the much longer residence time in the bottom-plate volume (0.83 versus 0.13 s) and therefore less gas-phase residence time in the inclined part of the reactor. This implies less exchange of gas with dead volumes in the case of the cone reactor with a top angle of π/3 and therefore less gas-phase residence time distribution, leading to a slightly improved product spectrum. No clear trend concerning the effect of the cone angle could be observed, and therefore it was concluded that the cone angle itself has no significant influence on the product spectrum but that the variations in the product spectrum were caused by different factors, such as the gas flow patterns, mixing of sand and polymer, etc. 3.5.3. Polymer Mass Flow Rate. The polymer mass flow rate was changed from 0.6 to 1.7 g/s under standard case conditions. The results of these experiments are shown in Table 5. The polymer flow rate has a significant effect on the product spectrum obtained, because the polymer mass flow rate directly influences the actual temperature distribution in the reaction zone. The reactor temperature decreases with increasing polymer flow rate, because more heat is consumed by the endothermic pyrolysis reaction. Moreover, the gasphase residence time decreases with increasing polymer mass flow rate, because more gas is produced. The temperature effect is more important than the gasphase residence effect, as already mentioned earlier, which results in a decreased cracking severity with increasing polymer mass flow rate. Therefore, the total alkene yield increases with increasing polymer mass flow rate, resulting in an increased yield of propene and butene and a decreased yield of methane. No information could be obtained on the maximum possible throughput of polymer, due to the limitations of the polymer and sand feeding systems. Cold flow experiments showed that up to 10 kg/s sand could be handled easily by the reactor, which corresponds to a possible polymer mass flow rate of 0.5 kg/s. The only major problem that would emerge is the large reactor volume required to achieve the desired average gasphase residence time of about 0.5 s (Westerhout et al., 1998) for complete conversion of intermediate waxlike products. 3.5.4. Sand Mass Flow Rate. A reduction of the sand mass flow fed to the reactor causes a lower temperature in the reactor zone, because less heat is supplied to the reactor. This implies a decrease in
2298 Ind. Eng. Chem. Res., Vol. 37, No. 6, 1998 Table 6. Comparison of Product Spectra for PE and PP Pyrolysis under Standard Case I Conditions product spectrum (wt %)
product spectrum (wt %)
compound
PE
PP
compound
PE
PP
methane ethene propene butene butadiene
38.9 31.6 10.0 6.1 0.2
41.5 23.2 15.8 8.9 0.1
other alkanes >C4 total alkenes gas yield
11.5 1.7 48 87.8
8.7 1.7 48 86.7 Figure 12. Gas yield as a function of reactor temperature for the pyrolysis of PP. Table 7. Comparison of the Gas Product Composition for Pyrolysis of PP in the Bench-Scale RCR and a Tubular Reactor (Base Case II) gas product composition (wt %)
Figure 11. Gas composition as a function of reactor temperature for the pyrolysis of PP.
cracking severity, resulting in a larger alkene (especially propene and butene) yield. This effect was also found experimentally. 3.5.5. Gas Outlet Position. During all experiments presented so far the gas outlet position was located on the bottom plate of the reactor (see Figure 1). To study the effect of the location of the gas outlet on the product spectrum, an inner cone with four gas outlets located halfway up the inclined cone wall was inserted into the reactor to change the gas flow pattern in the cone reactor. This modification, however, had no significant influence on the product spectrum obtained. This result is in accordance with findings from earlier experiments, which revealed that the influence of the gas-phase residence time on the product spectrum is limited. Tracer experiments (volumes I-III not blocked) confirmed that the influence of this modification on the gasphase residence time distribution is also limited. 4. High-Temperature Pyrolysis Experiments of Polypropene Preliminary experiments with PP conducted under standard case I conditions revealed that the yield of methane was very high (see Table 6), with ethene as the main product and not propene. PP and its main product (propene) apparently degrade significantly faster compared to PE with its main product ethene (Westerhout et al., 1997; Kunugi et al., 1969, 1970). This implies that the reaction conditions of standard case I correspond to a cracking severity in the reactor, which is too high for PP and therefore leads to a diminished yield of alkenes (see experiments with PE). Therefore, a standard case with more favorable conditions was defined with a lower reactor (898 K) and sand temperature (998 K) compared to the initial experiments. Under the new standard case conditions, first pyrolysis experiments of PP were conducted in which the reactor temperature was varied. The results of these experiments are presented in Figures 11 and 12. Under base case conditions the main pyrolysis product of PP is propene, but with increasing reactor temperature the extent of ethene and methane formation increases sharply at the expense of propene and butene formation. In the case of PP approximately 80 wt % of the polymer
compound
PP (RCR, 898 K)
PP (TR, 923 K)
PP (TR, 1073 K)
methane ethene propene butene butadiene other alkanes >C4 gas yield total alkenes
19.8 14.3 27.1 16.6 0.5 13.1 8.6 79.6 59
3.9 17.7 47.8 27.1 0.0 3.5 0.0 99a 93
15.7 32.2 28.5 16.5 0.0 7.1 0.0 99a 77
a Formation of coke must be necessary to satisfy mass balance. This could not be measured due to the experimental setup.
is converted into gas at 898 K, while 20 wt % of the polymer is converted into liquids. The gas yield increases with increasing temperature, as is evident from Figure 12. The gas and liquid yields obtained are comparable to those found for PE, but the composition of the liquids is clearly different. Analysis showed that the liquid formed during the pyrolysis of PP consists mainly of aromatics (benzene, toluene), while the liquid formed during PE pyrolysis consists mainly of waxlike intermediate compounds (long alkanes and alkanes). The total yield of alkenes is higher at lower reactor temperatures (lower cracking severity), which was also found for PE. The total alkene yield drops significantly from 59 wt % (gas composition) at a reactor temperature of 898 K to 47 wt % at 998 K. The conclusion, which was drawn on the basis of the PE pyrolysis experiments, namely, that the reactor and sand temperature should be chosen as low as possible to optimize the total alkene yield, is confirmed by the PP pyrolysis experiments. However, the product spectrum of PP is more sensitive to the temperature, compared to the product spectrum of PE, which is in accordance with literature data and results of earlier kinetic experiments (Westerhout et al., 1997). In Table 7 the product spectrum obtained from PP pyrolysis in an isothermal tubular reactor with (near) plug flow reactor characteristics is compared with the product spectrum of PP obtained from the bench-scale rotating cone reactor. It can be concluded that the yield of valuable alkenes is significantly lower in the benchscale RCR, indicating that improvements in the product spectra are still possible by modifying the reactor characteristics (i.e., plug flow behavior and improved control of the temperature). 5. High-Temperature Pyrolysis Experiments with Mixtures of Polyethene and Polypropene In many situations it is very difficult and expensive to separate two types of polymers from each other. In
Ind. Eng. Chem. Res., Vol. 37, No. 6, 1998 2299
Figure 13. Gas composition as a function of the fraction PE in a PE/PP mixture.
the product spectrum obtained from a BFB is more favorable compared to the one produced by the [B]RCR (less methane and more ethene and propene), but the difference is small. However, the use of utilities (steam or nitrogen and energy) in a process based on a BFB reactor leads to larger reactor dimensions and higher separation costs (larger gas stream). However, the bench-scale RCR is probably more expensive to construct (higher investment costs). Which reactor type offers an economic advantage over the other is therefore hard to assess without a detailed economical evaluation. It must also be kept in mind that BFB technology has been fully developed during 20 years of experimentation by the Kaminsky group at University of Hamburg, while the bench-scale RCR is the first reactor of its kind, which can be developed further. 7. Conclusions and Future Work
Figure 14. Comparison of the product spectrum of [B]RCR to spectra of other reactors for PE.
waste streams encountered in practice the polymers will most likely be present in a mixed state and will therefore undergo simultaneous pyrolysis in a reactor. It is therefore important to know whether the product spectrum of the different polymers is influenced by the presence of a second or third polymer. Kinetic experiments (Westerhout et al., 1997) and experiments conducted in a tubular reactor (Westerhout et al., 1998) revealed that at very short gas-phase residence times the aforementioned mixing effect could not be observed. These earlier results were confirmed by experiments conducted in the [B]RCR. LDPE and PP were thoroughly mixed in an extruder, and the mixture was ground to the desired size (
