4530
Ind. Eng. Chem. Res. 1997, 36, 4530-4534
Recycled Plastics in FCC Feedstocks: Specific Contributions Gabriela de la Puente,† Jose´ M. Arandes,‡ and Ulises A. Sedran*,† Instituto de Investigaciones en Cata´ lisis y Petroquı´micasINCAPE (FIQ,UNL-CONICET), Santiago del Estero 2654, 3000 Santa Fe, Argentina, and Departamento de Ingenierı´a Quı´mica, Universidad del Paı´s Vasco, Apartado 644, E-48080 Bilbao, Spain
Following a tertiary recycling option for plastics focused on the FCC process, styrene-based polymers were dissolved in benzene and injected over fresh and equilibrium samples of a commercial FCC catalyst. Reaction times were up to 12 s in a discontinuous fluidized bed reactor at 550 °C. The major reaction products were ethylbenzene, benzene, toluene, and styrene, and the production of coke was very significant. A reaction mechanism which includes polymer thermal cracking, surface oligomerization of styrene molecules, and further cracking, in parallel to hydrogen-transfer reactions, was delineated based on the comparison of the catalytic behaviors of the different samples. Increases in the yields of ethylbenzene, styrene, and coke are predictable for the industrial conditions. Introduction Driven by environmental and economic concerns, it has been of great interest during the last few years to develop techniques for a proper solution to the problem posed by the disposal of waste plastic materials (Fouhy et al., 1993). While direct combustion to take advantage of their elevated heats of combustion, or landfilling, also has a high environmental impact, polymer reuse is limited to a certain degree. Tertiary recycling, which means plastics conversion into fuels or chemicals, represents an interesting alternative, since it could be merged into standard petrochemical or petroleum refining industry operation (Shelley et al., 1992). In the case of condensation polymers, solvolysis reactions allow us to obtain raw materials, but addition polymers are usually subjected to pyrolysis to produce liquid hydrocarbon mixtures that could be upgraded (Shelley et al., 1992). However, an alternative approach may be developed based on the fact that it is possible to dissolve some plastic materials into hydrocarbon mixtures, namely, standard feedstocks for the process of catalytic cracking, FCC (Ng, 1995). This approach (direct addition of plastics into FCC feedstocks, as opposed to pyrolysis upgrading) seems feasible in current commercial FCC units with minor changes in technology, due to the high versatility of these units (King, 1992) that allows processing widely varying feedstocks; indeed, it is common practice to add different streams to compose them. In order to study the overall feasibility of this approach and due to the extreme complexity of feedstocks and products in FCC operation, it is then necessary to know about the specific contributions to the product slate that would be generated by the conversion of plastic materials, free from masking effects by the feedstock itself, thus complementing studies in which the plastics were dissolved into hydrocarbon mixtures (Arandes et al., 1997). As well, given the fundamental role played by coke in the FCC process, which sustains the thermal balance in the reactor-regenerator setups, coke deposition from plastic materials is a key issue which has to be considered in this approach. † INCAPE. Phone: 54-42-528062. Fax: 54-42-531068. Email:
[email protected]. ‡ Universidad del Paı´s Vasco. Phone: 34-4-464-7700. Fax: 34-4-464-8500. E-mail:
[email protected].
S0888-5885(97)00142-5 CCC: $14.00
Since polystyrene-based materials are very important in the spectrum of polymers with high consumption that could be subjected to this utilization when wasted and considering the lack of publications about this recycling option, it is the objective of this paper to report results concerning product distributions and coke formation that were obtained in the conversion of the said type of polymers, dissolved in an inert solvent, on fresh and equilibrium samples of a commercial FCC catalyst. Possible reaction mechanisms are also considered. Experimental Section The catalyst used was a commercial FCC catalyst (Octydine BR 1160, from Engelhard, 1.30% rare-earth metal oxides) in the fresh form (specific surface area, 342 m2/g; UCS, 24.72 Å; Al2O3, 48%), and equilibrated in a presently running FCC unit (specific surface area, 175 m2/g; UCS, 24.31 Å; Ni, 300 ppm; V, 1300 ppm). Additional characterization data are presented elsewhere (Arandes et al., 1997). Two commercial plastics polystyrene (average molecular weight 312 000; polydispersity, 2.4) and polystyrene-polybutadiene (8.8 wt %; average molecular weight, 184 000; polydispersity, 2.8), were dissolved to 6.5 wt % in benzene (Merck, 99.7%). The solution was injected at atmospheric pressure and 550 °C in a discontinuous stirred tank reactor with a fluidized bed regime (de Lasa, 1992), reaction times being 3, 6, 9, and 12 s. The mass of catalyst was always 0.8 g, and the volume of solution injected was 0.15 mL. After reaction time was elapsed, products were rapidly evacuated into a heated vacuum chamber from which a heated sampling valve was loaded. Reaction products were analyzed by gas chromatography. Benzene was checked to be essentially inert under the conditions mentioned, and duplicate reference experiments with pure toluene (Merck, >99.5%) and the plastics dissolved in toluene (6.5 wt %) were completed on both samples under the same set of experimental conditions to estimate benzene yields from the plastics. Styrene (Merck, >99%) dissolved in benzene (6.5 wt %) was also cracked on both catalyst samples. Experiments to assess the extent and products of thermal cracking were performed under the same conditions, the reactor being loaded with inert silica particles. Independent runs were devoted to the evaluation of coke deposition. After attaining the selected reaction time, products were rapidly evacuated while a nitrogen © 1997 American Chemical Society
Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997 4531
purging stream was fed simultaneously to the reactor. The amount of coke on the catalyst was estimated by a weight difference method after burning off the carbonaceous deposits. Identical reference coke experiments were performed with pure benzene in order to determine the amount of coke formed by the solvent itself, to be deduced from the plastic (or styrene)-benzene experiments. Results and Discussion As mentioned, it is necessary to know specifically how polymers added to standard feedstocks contribute to the complex product distributions of conventional FCC processes. So far, the conversion of styrene-based polymers on commercial FCC catalysts, composed by zeolite Y on a certain matrix, has not been reported in the related literature. Some other acidic catalysts, like silica-alumina, ZSM-5, and SO4/ZrO2, were tested in different approaches that comprise firstly thermal decomposition and then catalytic conversion (Zhibo et al., 1996) or the coating of catalyst particles by plastics (Lin and White, 1996). The catalyst used for this study can be considered as representative of those presently used in standard FCC operation, with a certain degree of rare-earth metal exchange on the Y zeolite component. Both the fresh and equilibrium samples show properties (unit cell size, specific surface area, zeolite and rare-earth metal content) that can be considered typical (Biswas and Maxwell, 1990). The singular reactor’s design, as well, allows for a close simulation of working conditions in commercial riser units, thus making the results more reliable (de Lasa, 1992). For all the experiments performed, both thermal and catalytic, the conversion of plastics was complete, as shown by careful mass balance calculations. It has been shown that the thermal decomposition of these types of materials (polystyrene or polystyrene-polybutadiene) is complete at 450 °C (Lin and White, 1996; Arandes et al., 1997); also, mass balance considerations in the thermal conversion of polymer-light cycle oil feedstocks in the 450-550 °C range proved total conversion (Arandes et al., 1997). All detected products can be considered to come from the conversion of plastic materials because, as observed from specific experiments, benzene did not produce gasphase products under the experimental conditions used. Since the solvent benzene is also produced by the conversion of the polymers, its yields were assessed from separate experiments with toluene as the solvent under the same conditions (polymer concentration, catalysts, and reaction times): the comparison between the yields of benzene from pure toluene, and plastics-toluene conversions, respectively, allowed us to determine how much benzene is formed directly from the plastics. Table 1 shows a typical product distribution (coke excluded) observed in the thermal conversion of polystyrene at a temperature level representative of those employed in FCC. It can be seen that thermal cracking induces mainly the formation of styrene, the other products being ethylbenzene, benzene, toluene, and C9-12 aromatics. Very small amounts of light C5products are formed. In the thermal cracking of polystyrene, other authors observed dimers at 350 °C (Zhibo et al., 1996), and dimers and trimers at 400 °C (Lin and White, 1996). When polystyrene-polybutadiene is cracked thermally, essentially the same product slate is obtained, though higher amounts (about 1.60%) of C4 hydrocarbons were observed.
Table 1. Product Yields (wt %, Excluding Coke) from the Reaction of Polystyrene (6.5 wt % in Benzene). Catalytic Cracking on Fresh and Equilibrated Catalyst Samples and Thermal Cracking (Temperature, 550 °C; Reaction Time, 9 s) cracking catalytic product
fresh catalyst
equilib catalyst
thermal
methane ethylene ethane propylene propane isobutane isobutene + 1-butene benzene toluene ethylbenzene xylenes styrene C9 aromatics C10+ aromatics
0.40 0.31 0.78 1.66 2.01 0.51 0.30 34.09 35.36 6.42 1.24 14.90 0.47 1.55
nil nil nil nil 1.76 nil nil 9.85 13.10 36.41 nil 38.88 nil nil
nil nil 0.12 nil 0.16 0.06 nil 4.54 2.90 7.84 nil 75.59 3.20 5.52
Example product distributions of the conversion of the same polymer when catalysts are present are also included in Table 1. Since the products were essentially the same in all the cases (plastics and reaction times) and most important changes were observed in the relative yields, as discussed below, these listings can be considered as representative for both plastics. It can be observed that in the case of the fresh catalyst, the main products are benzene, toluene, ethylbenzene, and styrene, with smaller amounts of products lighter than 5 carbon atoms per molecule (especially propane and propene) and aromatic compounds with 9 and 10 carbon atoms per molecule (indane and methylindanes). The equilibrium catalyst only produces benzene, toluene, ethylbenzene, and styrene, the other products showing negligible yields. Noticeable among the products is an important change in the relative amounts of benzene and toluene (most important in the fresh catalyst) and ethylbenzene (most important in the equilibrium catalyst) that will be discussed later. When polystyrenepolybutadiene was cracked, a somewhat higher amount of gases was observed. It has to be pointed out that for all the polymers and catalysts, no styrene dimers and trimers were detected in any case, as also reported by Lin and White (1996) at 400 °C. In order to study the evolution of products from the cracking of this type of styrene-based plastic materials, it will be assumed that the major first step in the catalytic conversion at 550 °C is the production of monomer styrene by thermal cracking. This assumption is supported by the results of thermal cracking, showing high yields of styrene (Ogawa et al., 1981; Ide et al., 1984; refer also to Table 1, this work, considering that yields are even higher at shorter reaction times) and styrene oligomers (Lin and White, 1996, reaction temperature 400 °C; Zhibo et al., 1996, reaction temperature 350 °C). Moreover, it can also be shown that if styrene is reacted on a given catalyst, it will exhibit essentially the same products and conversion patterns as polystyrene. As an example, refer to Figure 1 for the fresh sample, in which the various products selectivities from the reaction of styrene (defined as the individual yields divided by the total products yield, coke excluded) are compared with the observations from the reaction of polystyrene. It can be seen that the evolution of the selectivity curves is the same. Thus, conversion was defined as a function of styrene remaining in the reactant mixture; for all the plastics and catalysts, as
4532 Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997
Figure 1. Selectivities of benzene (O), toluene (0), ethylbenzene (∆), and cracking products (∇) as a function of conversion. Fresh catalyst, 550 °C. Dashed line and open symbols: polystyrene (6.5 wt % in benzene); filled line and closed symbols: styrene (6.5 wt % in benzene).
Figure 3. Conversion of polystyrene (6.5 wt % in benzene) at 550 °C on equilibrium catalyst. Yields of main products as a function of conversion. Symbols as in Figure 1. Table 2. Coke on Catalyst and Coke Yield from the Reaction of Polystyrene and Polystyrene-Polybutadiene (6.5 wt % in Benzene) on Fresh and Equilibrium Catalyst Samples (Temperature: 550 °C) at reaction time, s polystyrene coke on catalyst, wt % 3
9
polystyrene-polybutadiene
coke yield, wt % 3
coke on catalyst, wt % 9
3
fresh 0.36 0.64 33.49 58.85 0.14 equilibrium 0.20 0.37 18.13 33.80 0.08
Figure 2. Conversion of polystyrene (6.5 wt % in benzene) at 550 °C on fresh catalyst. Yields of main products as a function of conversion. Symbols as in Figure 1.
expected, the longer the reaction time, the less styrene remains, that is to say, the higher the conversion. According to this definition, the results also show that, as predictable, the fresh catalyst is more active than the equilibrium sample in all the conditions (refer to Table 1 as an example). The yield curves of the major products from the catalytic conversion of polystyrene on the fresh and equilibrium samples are presented in Figures 2 and 3. In the case of polystyrene-polybutadiene, the results were very similar. For both plastics on the fresh sample (refer to Figure 2), the most important products, benzene and toluene, show increasing profiles in the whole conversion range, with somewhat lower benzene yields in the case of polystyrene-polybutadiene. Ethylbenzene yields increase until conversion is about 70% and then decrease slightly, suggesting further conversion; in turn, cracking products show an inverse behavior, accelerating their yields at about the same point. Minor amounts of C9-C10 aromatics (not shown) balance the total sum of products. The conversion of both plastics on the equilibrium sample (refer to Figure 3) indicates that all important products, ethylbenzene (the major one), toluene, and benzene, show continuously increasing profiles in the conversion range. In comparison to the fresh sample,
coke yield, wt %
9
3
9
0.44 0.27
13.92 8.18
43.20 26.97
the most important feature is the shift between benzene and toluene and ethylbenzene, as the predominant products from plastics conversion. The deposition of coke is an important issue concerning the cracking of plastics following a FCC approach, since it might have an important impact on the heat balance and, subsequently, on the overall operation of the FCC units. Coke yields are shown in Table 2, where it can be seen that they are very significant. Considering that the results obtained over the equilibrium catalyst would be more representative of commercial operation, since it is an actual running catalyst, then coke yields as high as about 30 wt % are to be taken into account. Consistently, the more active fresh catalyst yields about 2-fold more coke than the equilibrium catalyst for each of the plastics. In all the cases, polystyrene produces more coke than polystyrenepolybutadiene, but its rate of formation seems to flatten off faster as a function of reaction time. When cracking styrene over the equilibrium catalyst, coke yields were 9.11% and 21.73% at 3 and 9 s, respectively. The only published results concerning coke formation when plastics were added to standard feedstocks that could be compared to ours are those from Ng (1995), who used polyethylene dissolved in vacuum gas oil to report coke yields of about 40 wt %, although the experiments were of the microactivity test (MAT) type. The results presented so far can be rationalized in terms of the following discussion which, due to reported similarities, is applicable to both plastics, and even though matrices in FCC catalysts may have some activity (Scherzer, 1989), the analysis of polymers conversion will be focused on the main component of these compound catalysts, which is the Y zeolite. The experiments seem to support that the formation of
Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997 4533
Figure 4. Mechanism proposed to account for major products in polystyrene-type plastics.
styrene monomer can easily proceed via thermal cracking; this would be an important first step for the subsequent catalytic molecular reordering according to the properties of the catalysts. Styrene dimers or trimers were not observed in any of the runs, either thermal or catalytic, a fact that can be assigned to the high temperature used and the very efficient contact between reactants and catalyst particles that can be achieved in the reactor employed (Pekediz et al., 1992). The set of reactions leading to observed product distributions can be described based on the further occurrence of styrene oligomerization. It is known that oligomerization reactions are able to proceed over acidic catalysts of this type; both acid amount and strength are enough to allow these reactions (Scherzer, 1989). They are electrophilic reactions, with a carbocationbased mechanism (Olah et al., 1985). Then, a simple scheme contemplating mainly oligomerization, cracking, and hydrogen-transfer reactions can be considered to explain the formation of the most important products (benzene, toluene, and ethylbenzene). For the sake of clarity, only some examples will be considered on the basis of styrene dimers, but it can be shown (Ogawa et al., 1981; Ide et al., 1984) that other carbocations from higher molecular weight oligomers lead to the same products: benzene, alkylbenzenes, and indane-type molecules. For the case example of dimers, then, all three main products (benzene, toluene, and ethylbenzene) can be formed from the direct β-scission cracking of these carbocations, as shown in Figure 4a, and in the particular cases of benzene and toluene, additional methylindanes and indane molecules, respectively, are formed. The experimental results show that there exists a lack of molar balance between benzene and toluene and indane-type molecules (see Table 1), which can be explained by the high coke-forming tendency of this type of aromatic compounds, based on the existence of rings with five carbon atoms and conjugated double bonds (Parera et al., 1984). It was also observed that the higher the composite yields of benzene and toluene, the
higher the coke yields (refer to Figures 2 and 3 and Table 2). It should also be considered that benzene and alkylbenzenes can be formed from proton electrophilic attack on the aromatic rings of the chain of the polymers (Lin and White, 1996). Ethylbenzene could also be the result of direct hydride transfer to styrene molecules (refer to Figure 4b). This reaction would be sustained by the relatively high concentration of hydride donor molecules, simultaneously leading to coke deposition (Sedran, 1994). When comparing the performances of the catalysts in terms of product yields, it is apparent from the results in Figures 2 and 3 that there exists an inversion of the relative importance of the reactions schematized in Figure 4, since benzene and toluene (mainly produced by reactions described in Figure 4a) prevail in the case of the fresh catalyst, while there is more ethylbenzene (mainly produced by reaction described in Figure 4b) in the case of the equilibrium catalyst. Certainly, the ratio of the yield of ethylbenzene to the total yield of benzene plus toluene has an average value of 0.13 for the fresh catalyst and 1.68 for the equilibrium one (at conversions over 40%) for both plastics. This fact is also observed in the comparison of product selectivities when styrene alone is cracked on both catalysts, as shown in Figures 1 and 5. Since hydrogen-transfer reactions follow a bimolecular mechanism that strongly depends on the density of paired acid sites (Sedran, 1994), they should be restricted on the equilibrium sample, which has lower acid site density than the fresh one. However, the inhibition factor imposed by the loss of acid sites due to zeolite dealumination seems to be significantly higher for the reactions of oligomerization, which are also known to depend on acid site concentration (Pines, 1981), thus severely limiting the course suggested by Figure 4a and justifying the observed shift in the prevalence of reaction products. Figure 5 shows the evolution of the selectivity curves in the cracking of styrene and polystyrene on the equilibrium catalyst. It can be seen again that styrene
4534 Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997
Acknowledgment This work was performed with the financial assistance of University of Litoral, Secretary of Science and Technology (Santa Fe, Argentina), Project 167; the project is within a Joint Study Project CENACA-JICA in the field of catalysis. The financial assistance of Antorchas Foundation (Argentina) is gratefully acknowledged. Literature Cited
Figure 5. Selectivities of benzene, toluene, ethylbenzene, and cracking products as a function of conversion. Equilibrium catalyst, 550 °C. Symbols and lines as in Figure 1.
produces the same conversion patterns as polystyrene, which, in addition to similar values of coke yields, gives additional support to the proposed mechanism requiring the oligomerization of styrene molecules. Conclusions The conversion of styrene-based polymers dissolved in benzene over fresh and equilibrium samples of a commercial FCC catalyst allowed us to determine their specific contributions to product distribution, under the approach of tertiary recycling of these plastics by addition to standard FCC feedstocks. The experimental technique adopted (plastics dissolution, fluidized bed discontinuous reactor, short contact times) gives a sounder basis to the results of the research. The products are essentially the same on both catalysts, with changes in the yields that depend on properties of the catalysts. For both plastics, the more representative equilibrium catalyst yields ethylbenzene, benzene, toluene, and some styrene monomer as the only important products. At the high temperatures of FCC operation, thermal cracking, which is assumed to be the major initial step for the catalytic conversion, was complete to yield mainly styrene. Then, according to catalyst attributes, the resulting product slate can be accounted for by a simple mechanistic scheme considering the reactions of oligomerization, β-scission cracking, and hydride transfer. Oligomerization and cracking prevail against hydride transfer on the fresh catalyst, while the contrary was observed for the equilibrium catalyst, since the loss of active acid sites would inhibit oligomerization reactions more severely. Coke formation was very significant, with yields of up to about 30 wt % on the equilibrium catalyst, which could have a strong impact on the technological implementation of this polymer-recycling alternative. The proposed mechanism also accounts for the high cokeforming trend, based on the existence of coke-precursor, indane-derived molecules. Under conditions of FCC commercial operation, then, increases in the yields of ethylbenzene, styrene, and coke are foreseeable.
Arandes, J. M.; Abajo, I.; Lo´pez-Valerio, D.; Fernandez, I.; Azkoiti, M. J.; Olazar, M.; Bilbao, J. Transformation of Several Plastic Wastes to Fuels by Catalytic Cracking. Submitted to Ind. Eng. Chem. Res. 1997. Biswas, J.; Maxwell, I. E. Recent Process- and Catalyst-Related Developments in Fluid Catalytic Crackingsa Review. Appl. Catal. 1990, 63, 197. de Lasa, H. I. Riser Simulator for Catalytic Cracking Studies. U.S. Patent 5,102,628, 1992. Fouhy, K.; Kim, I.; Moore, S.; Culp, E. Plastics Recycling’s Diminishing Returns. Chem. Eng. 1993, 100 (Dec), 30. Ide, S.; Ogawa, T.; Kuroki, T.; Ikemura, T. Controlled degradation of Polystyrene. J. Appl. Polym. Sci. 1984, 29, 2561. King, D. Engineering of Fluidized Catalytic Crackers. In NATO ASI Series E: Chemical Reactor Technology for Environmentally Safe Reactors and Products; de Lasa, H. I., Dogu, G., Ravella, A., Eds.; Kluwer: Dordrecht, 1992; Vol. 225, p 17. Lin, R.; White, R. L. Catalytic cracking of polystyrene. ACS Symp. Meeting Abs.; American Chemical Society: Washingon, DC, 1996; Vol. 41, p 1165. Ng, S. H. Conversion of Polyethylene Blended with VGO to Transportation Fuels by Catalytic Cracking. Energy Fuels 1995, 9, 216. Ogawa, T.; Kuroki, T.; Ide, S.; Ikemura, T. Recovery of indane derivative from polystyrene waste. J. Appl. Polym. Sci. 1981, 27, 657. Olah, G. A.; Prakash, G. K. S.; Sommer, J. Superacids; J. Wiley & Sons: New York, 1985. Parera, J. M.; Fı´goli, N. S.; Beltramini, J. N.; Churı´n, E. J.; Cabrol, R. A. Mechanism of coke formation during naphtha reforming. In Proc. 8th. Internat. Congr. Catal.; Verlag Chemie: Berlin, 1984; Vol. II, p 593. Pekediz, A.; Kraemer, D. W.; Chabot, J.; de Lasa, H. I. Mixing Patterns in a Novel Riser Simulator. In NATO ASI Series E: Chemical Reactor Technology for Environmentally Safe Reactors and Products; de Lasa, H. I., Dogu, G., Ravella, A. Eds.; Kluwer: Dordrecht, 1992; Vol. 225, p 133. Pines, H. The Chemistry of Catalytic Hydrocarbon Conversions; Academic: New York, 1981. Scherzer, J. Octane-Enhancing Zeolitic FCC Catalysts: Scientific and Technical Aspects. Catal. Rev. Sci. Eng. 1989, 31, 215. Shelley, S.; Fouhy, K.; Moore, S. Plastics reborn. Chem. Eng. 1992, 99 (July), 30. Sedran, U. A. Laboratory Testing of FCC Catalysts and Hydrogen Transfer Properties Evaluation. Catal. Rev. Sci. Eng. 1994, 36, 405. Zhibo, Z.; Nishio, S.; Morioka, Y.; Ueno, A.; Ohkita, H.; Tochihara, Y.; Mizushima, P.; Kakuta, N. Thermal and chemical recycle of waste polymers.Catal. Today 1996, 29, 303.
Received for review February 17, 1997 Revised manuscript received June 9, 1997 Accepted July 28, 1997X IE970142A
Abstract published in Advance ACS Abstracts, October 1, 1997. X
