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Ind. Eng. Chem. Res. 1998, 37, 2316-2322
Development of a Continuous Rotating Cone Reactor Pilot Plant for the Pyrolysis of Polyethene and Polypropene R. W. J. Westerhout, J. Waanders, J. A. M. Kuipers,* and W. P. M. van Swaaij Reaction Engineering Group, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands
A pilot plant for the high-temperature pyrolysis of polymers to recycle plastic waste to valuable products was constructed based on the rotating cone reactor (RCR) technology. The RCR used in this pilot plant, termed the continuous RCR ([C]RCR) was an improved version of the benchscale RCR ([B]RCR) previously used for the pyrolysis of biomass, Polyethene (PE), and Polypropene (PP). The improvements resulted in a higher total alkene yield in the [C]RCR compared to the [B]RCR for the pyrolysis of PE and PP. While the total alkene product yield amounts only to 51 wt % in the [B]RCR for PE, in the [C]RCR it could be increased to 66 wt %, which is comparable to the 65 wt % total alkene yield obtained in a bubbling fluidized bed (BFB) of similar scale. Together with the fact that almost no utilities are required for operation of a RCR, the product spectra obtained make this technology a good alternative to the reactor technologies presently applied in pyrolysis processes. Optimum total alkene yields are obtained at temperatures around 1023 K, as intermediate waxlike compounds are not converted at lower temperatures whereas too much aromatics and methane are formed at higher temperatures. The reactor and BFB temperature in the pilot plant have the largest impact on the product spectrum obtained, while the sand and polymer mass flow rates have a very limited effect. For PP pyrolysis the effect of the aforementioned parameters is more pronounced, because this polymer is more sensitive to thermal degradation. 1. Introduction 1.1. General Introduction. Recently, the rotating cone reactor (RCR) was successfully applied to the flash pyrolysis of biomass (Wagenaar et al., 1994). This reactor, termed the bench-scale RCR ([B]RCR), was also used for the high-temperature pyrolysis of plastics (Westerhout et al., 1998b). It offers a number of significant advantages over conventional reactors employed for these types of processes (bubbling fluidized bed (BFB) and circulating fluidized bed (CFB) reactors; see, for instance, Paisley and Litt, 1992; Sinn, 1974; Sinn et al., 1976), which require a fluidization agent for proper operation of the reactor. It was demonstrated that the [B]RCR is competitive with large-scale BFB reactors (Westerhout et al., 1998b). Despite these promising results, it was felt that significant technical improvements were still possible to enhance the technical and economical viability of the RCR technology. On the basis of this study, a new RCR, termed the continuous RCR ([C]RCR), was developed and tested. The results of this study will be described in this paper, but first the basic reaction scheme for polyethene (PE) and polypropene (PP) pyrolysis will briefly be discussed. 1.2. Reaction Scheme for 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 with a relatively high carbon number. In the gas phase these products are cracked further (secondary gas-phase reactions) to smaller hydrocarbons (for instance, ethene and propene). However, these lower alkenes and alkanes are thermodynamically unstable at these high temperatures * To whom correspondence should be addressed.
Figure 1. Reaction scheme for the pyrolysis of polymers.
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 reaction is given in Figure 1. For further details on the gas-phase reactions the interested reader is referred to Albright et al. (1983) or Westerhout et al. (1998a). 2. Experimental Equipment and Procedures On the basis of previous experience with the [B]RCR (Westerhout et al., 1998b) a new RCR was designed and constructed. The main technical improvements are as follows: (a) Operation with molten polymer feed. (b) Application of a continuous sand recycle and therefore continuous operation of the plant. (c) Improved sand/polymer distribution and mixing. (d) Reduction of dead volumes in the reactor. (e) Higher sand and polymer throughputs. (f) Improved data acquisition and control. (g) Better safety features.
S0888-5885(97)00703-3 CCC: $15.00 © 1998 American Chemical Society Published on Web 05/09/1998
Ind. Eng. Chem. Res., Vol. 37, No. 6, 1998 2317
Figure 2. (a) Schematic drawing of the [C]RCR pilot plant. (b) Cross-sectional view of the cone reactor. (c) Main dimensions of the cone.
The new pilot plant, termed the continuous rotating cone reactor ([C]RCR) pilot plant, is schematically shown in Figure 2a-c. The new pilot plant consists of three parts (see Figure 2a): the actual cone reactor, a bubbling fluidized bed (BFB) reactor to heat up the sand, and a riser to transport the sand back to the cone. The three parts are required to enable the continuous operation of the plant, as the sand has to be heated continuously to supply heat for the pyrolysis reaction, which is highly endothermic (estimated at approximately 4 MJ/kg). For this purpose a BFB reactor is used, which was heated electrically. On an industrial scale, air will be used in the BFB reactor to burn off the coke generated during the pyrolysis reaction together with additional fuel to generate heat for the operation of the plant, but this feature was not implemented in the present reactor to reduce the complexity and costs of the pilot plant. The reactor and BFB temperatures used in this study are defined as the temperature of the cone reactor and the BFB set before the experiment. The geometry of the [C]RCR was altered to reduce the number and size of the dead volumes in the reactor, which was a major drawback of the [B]RCR. To achieve this goal, no inner cone was used but the reactor volume was decreased using cone-shaped reactor walls (see Figure 2b), eliminating most of the dead volumes in the reactor. A rotation frequency of 600 rpm was used during all experiments. The rotation frequency is not expected to have a large influence on the product and gas yields and was therefore not varied during this study. The polymer was fed to the reactor by heating the polymer to 548-575 K and pumping the molten polymer with a rotary gear pump to the reactor through 10-mm pipes, which were traced. Close to the reactor, gas was used to heat/cool the feeding pipe (see Figure 2b). Using a molten polymer feed has the major advantage that the polymer particles do not have to be grinded to the desired size (C4 total alkenes gas yield
18.0 16.8 33.3 18.4 9.1 4.4 68.5 74.8
3.9 17.7 47.8 27.1 3.5 0.0 93 99a
15.7 32.2 28.5 16.5 7.1 0.0 77 99a
a Formation of coke must be necessary to satisfy mass balance. This could not be measured due to the experimental setup.
Figure 12. Comparison of different large-scale pyrolysis reactors for PP. Table 6. Comparison of the Product Spectra of PE Obtained in the [C]RCR and a Tubular Reactor component
[C]RCR, 1023 K (wt %)
TR, 923 K (wt %)
TR, 1073 K (wt %)
methane ethene propene butene other alkanes >C4 total alkenes gas yield
13.2 33.7 23.7 13.6 8.7 7.1 71 93
6.6 41.6 23.6 24.1 4.1 0.0 89 89
11.0 52.5 19.0 13.5 4.0 0.0 85 99a
a Formation of coke must be necessary to satisfy mass balance. This could not be measured due to the experimental setup.
4.2. Comparison of Results with Literature and Other Reactors. Almost no data are available in the literature for the pyrolysis of PP in large-scale reactors. To our knowledge, no experiments have been conducted with PP and N2 as the fluidization gas in a BFB on a scale comparable with that of the [B]RCR and the [C]RCR. For this reason only the [B]RCR and [C]RCR are compared in Figure 12. This figure shows that the [C]RCR produces a higher total alkene yield, which was also found for PE. The elimination of the dead volumes in the [C]RCR leads to better residence time distribution characteristics compared to the [B]RCR, which is probably the main reason that the product spectrum obtained in the [C]RCR is superior to the one obtained in the [B]RCR. The comparison between the reactors was made at optimal conditions for each reactor. 5. Comparison of Results with Optimal Yield To study the yield of the pyrolysis of a forementioned polymers under ideal conditions, a small-scale tubular reactor (TR) with (near) plug-flow characteristics and negligible temperature gradients was used (Westerhout et al., 1998a). The yields in the TR are compared with the yields achieved in the [C]RCR in Tables 6 and 7 for respectively PE and PP. The temperature is not welldefined in large-scale reactors as significant temperature gradients may exist in the reactor and therefore
the product spectrum in the tubular reactor is given for pyrolysis of two different temperatures. The data in the tables show that the yields obtained in the [C]RCR for PE and PP are not optimal but are reasonably close to the “optimal yields” considering the scale of the reactor. The difference is larger for PP, because this polymer is more sensitive to thermal degradation. The comparison of the product spectra obtained in both reactors shows that the pyrolysis conditions in the [C]RCR are not optimal, but this is inherently the case in large-scale reactors in which the pyrolysis reaction causes temperature gradients and a nonideal residence time distribution. 6. Conclusions The rotating cone reactor used by Wagenaar et al. (1994), which was also employed for the high-temperature pyrolysis of polymers (Westerhout et al., 1998b), was modified and improved to obtain better product spectra. The new [C]RCR features a molten polymer feed (no grinding of particles), a continuous sand recycle, higher throughputs, better polymer/sand mixing and distribution, and less dead volumes compared to the [B]RCR. Pyrolysis experiments using PE confirmed that the product spectrum obtained in the [C]RCR is significantly better than that obtained in the [B]RCR. Several experiments were performed to study the influence of the different parameters on the product spectra obtained. If the reactor or BFB temperature are increased, higher ethene and methane yields are found at the expense of the propene and butene yields, which is in agreement with results obtained with other reactors. An increase in the cracking severity (higher temperature and/or longer gas-phase residence time) leads to the formation of more ethene and methane. The influence of other parameters (mass sand flow rate, polymer mass flow rate) is limited or indirect. Experiments were also performed with PP, which confirmed the results found for PE. However, PP and its product spectrum are more sensitive to thermal degradation, which means that the influence of different parameters on the product spectra obtained is larger compared to PE. If the [C]RCR is compared to a BFB reactor of similar scale, it can be concluded that the product spectrum produced by the [C]RCR is better or comparable, while the reactor requires less utilities, which means that the reactor is more energy efficient. This makes the RCR a good alternative to more conventional reactors (BFB and CFB) for the high-temperature pyrolysis of polymers.
2322 Ind. Eng. Chem. Res., Vol. 37, No. 6, 1998
A comparison of the product spectra of the [C]RCR and a small-scale tubular reactor with (near) plug-flow characteristics and negligible temperature gradients showed that the product spectrum produced in the [C]RCR is close to the “optimal” product spectrum produced in the tubular reactor. The difference is due to the inherent temperature gradients and residence time distribution associated with large-scale reactors. Symbol List Abbreviations BFB: bubbling fluidized bed CFB: circulating fluidized bed PE: polyethene PP: polypropene [B]RCR: bench-scale rotating cone reactor [C]RCR: continuous rotating cone reactor TR: tubular reactor
Literature Cited Albright, L. F.; Crynes, B. L.; Corcoran, W. H. Pyrolysis, Theory and Industrial Practice; Academic Press: New York, 1983. Paisley, M. A.; Litt, R. D. Monomeric Recovery from Polymeric Materials. U.S. Patent 5,136,117, August 1992.
Seeger, M.; Cantow, H.-J. Thermische Spaltungsmechanismen in Homo- und Copolymeren aus R-Olefinen. Macromol. Chem. 1975, 176, 1411-1425. Seeger, M.; Ritter, R. J. Thermal Decomposition and Volatilisation of Poly(R-olefins). J. Polym. Sci. 1977, 15, 1393-1402. Sinn, H. Recycling of Polymers (Recycling der Kunststoffe). Chem.Ing.-Tech. 1974, 46 (14), 579-589. Sinn, H.; Kaminsky, W.; Janning, J. Processing of Polymer Waste and Used Tires for Chemical Feedstock Production Using Pyrolysis (Verarbeitung von Kunstoffmull und Altreifen zu ChemiesRohstoffen, besonders durch Pyrolyse). Angew. Chem. 1976, 88 (22), 737-750. Wagenaar, B. M.; Prins, W.; van Swaaij, W. P. M. Pyrolysis of Biomass in the Rotating Cone Reactor: Modelling and Experimental Justification. Chem. Eng. Sci. 1994, 49 (24), 5109-5126. Westerhout, R. W. J.; Kuipers, J. A. M.; van Swaaij, W. P. M. Experimental Determination of the Yield of Pyrolysis Products of PE and PP. Influence of Reaction Conditions. Ind. Eng. Chem. Res. 1998a, 37 (3), 841-847. Westerhout, R. W. J.; Waanders, J.; Kuipers, J. A. M.; van Swaaij, W. P. M. Recycling of (Mixed) Plastic Waste in a Novel Bench Scale Rotating Cone Reactor by High-Temperature Pyrolysis. Ind. Eng. Chem. Res. 1998b, to be published.
Received for review September 11, 1997 Revised manuscript received March 30, 1998 Accepted April 9, 1998 IE970703Y
