Catalytic Conversion of Polyolefins to Chemicals and Fuels over

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Energy & Fuels 1998, 12, 767-774

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Catalytic Conversion of Polyolefins to Chemicals and Fuels over Various Cracking Catalysts Y.-H. Lin* and P. N. Sharratt Environmental Technology Centre, Department of Chemical Engineering, UMIST, P.O. Box 88, Manchester M60 1QD, U.K.

A. A. Garforth and J. Dwyer Centre for Microporous Materials, Department of Chemistry, UMIST, P.O. Box 88, Manchester M60 1QD, U.K. Received December 31, 1997

Catalytic pyrolysis of polymers over catalysts has the potential to recover valuable hydrocarbons from waste materials. Polyolefins were pyrolyzed over catalysts using a specially developed laboratory fluidized bed reactor operating isothermally at ambient pressure. The systematic experiments carried out with several catalysts show that under appropriate reaction conditions, catalysts may be selected to reduce the required reaction temperature, improve the yield of volatile products, and provide selectivity in the product distributions. A kinetic model based on a lumping reaction scheme for the observed products and catalyst deactivation has been investigated. The model gave a good representation of experiment results. This paper outlines some recent results relevant to the conversion of polyolefins to chemical feedstocks and fuels using various catalysts and also attempts to provide a framework for understanding the variety of influences on product distribution.

1. Introduction Polymer waste can be regarded as a potentially cheap source of chemicals and energy. The destruction of wastes by incineration is prevalent but is expensive and often generates problems with unacceptable emissions. It is also undesirable to dispose of waste plastics by landfill due to high costs and poor biodegradability. Methods for recycling polymer waste have been developed and new recycling approaches are being investigated.1,2 Chemical recycling, i.e., conversion of waste polymers into feedstock or fuels, has been recognized as an ideal approach and could significantly reduce the net cost of disposal.3 The most widely used conventional chemical methods for waste polymer treatment are pyrolysis and catalytic reforming. Workers in Japan have developed a dual fluidized bed process for obtaining medium-quality gases from pyrolysis of municipal solid waste.4,5 Thermal cracking of waste polymer using kilns or fluid beds has been piloted on a significant scale in Europe.6-9 Other processes using a pilot plant fluidized bed reactor * Corresponding author. Tel: +44 (161)2003975. Fax: +44 (161)2003988. E-mail: [email protected]. (1) Voss, D. Chem. Eng. Prog. 1989, 185, 67-72. (2) Brandrup, J.; Bittner, M.; Michaeli, W.; Menges, G. Recycling and Recovery of Plastics; Carl Hanser Verlag: Munich, 1996. (3) Lee, M. Chem. Br. 1995, 31, 515-516. (4) Kagayama, M.; Igarashi, M.; Fukada, J.; Kunii, D. ACS Symposium Series; American Chemical Society: Washington, DC, 1980; Vol. 130, pp 527-531. (5) Igarashi, M.; Hayafune, Y.; Sugamiya, R.; Nakagawa, Y. J. Energy Resour. Technol. 1984, 106, 377-382. (6) Kaminsky, W.; Rossler, H. Chemtech. 1992, 108-113.

or internally circulating fluidized bed (ICFB) reactor to pyrolyze plastic waste have also been tried in North America.10,11 However, the thermal degradation of polymers to low molecular weight materials has a major drawback in that a very broad product range is obtained. In addition, these processes require high temperatures, typically more than 500 °C and even up to 900 °C. In contrast to thermal degradation research, studies of the effects of catalysts on the catalytic degradation of polymer have been performed by (i) contacting melted polymer with catalyst in fixed bed reactors,12-15 (ii) heating mixtures of polymer and catalyst powders in reaction vessels,16-18 and (iii) passing the products of (7) Hardman, S.; Leng, S. A.; Wilson, D. C. Eur. Patent Appl. 567292, 1993. (8) Conesa, J. A.; Font, R.; Marcilla, A.; Garcia, A. N. Energy Fuels 1994, 8, 1238-1246. (9) Kaminsky, W.; Schlesselmann, B.; Simon, C. J. Anal. Appl. Pyrolysis 1995, 32, 19-27. (10) Scott, D. S.; Czernik, S. R.; Piskorz, J.; St. A. G.; Radlein, D. Energy Fuels 1990, 4, 407-411. (11) Sodero, S. F.; Berruti, F.; Behie, L. A. Chem. Eng. Sci. 1996, 51, 2805-2810. (12) Uemichi, Y.; Kashiwaya, Y.; Tsukidate, M.; Ayame, A.; Kanoh, H. Bull. Chem. Soc. Jpn. 1983, 56, 2768-2773. (13) Vasile, C.; Onu, P.; Barboiu, V.; Sabliovshi, M.; Ganju, D.; Florea, M. Acta Polym. 1985, 36, 543-550. (14) Audisio, G.; Bertini, F.; Beltrame, P. L.; Carniti, P. Makromol. Chem. Macromol. Symp. 1992, 57, 191-209. (15) Mordi, R. C.; Fields, R.; Dwyer, J. J. Anal. Appl. Pyrolysis 1994, 29, 45-55. (16) Ishihara, Y.; Nanbu, H.; Saido K.; Ikemura T.; Takesue T.; Kuroki T. Fuels 1993, 72, 1115-1119. (17) Beltrame, P. L.; Carniti, P.; Audisio, G.; Bertini, F. Polym. Deg. Stab. 1989, 26, 209-219.

S0887-0624(97)00233-8 CCC: $15.00 © 1998 American Chemical Society Published on Web 05/20/1998

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polymer pyrolysis through fixed bed reactors containing cracking catalysts.19,20 However, the configuration of the pyrolysis-reforming reactors poses serious engineering and economics constraints. For the study of these pyrolysis-reforming reactions, it is difficult to measure the exact mass flow rate of the reactant from the pyrolysis to catalytic zones, and consequently it is virtually impossible to identify and quantify the reactant and to control its quality. It is difficult to develop a kinetic model of the reaction which is an integral requirement in the design and scale-up of the catalytic reactor. Also, the use of fixed beds or adiabatic batch where polymer and catalyst are contacted directly leads to problems of blockage and difficulty in obtaining intimate contact over the whole reactor. Without good contact the formation of large amounts of residue are likely, and scale-up to industrial scale is not feasible. To compare the polymer cracking properties of different catalysts, it is preferable to examine the effects of catalysts without extensive complications due to reactions of primary cracking products, e.g., olefins, with unconverted polymer by using techniques that minimize such interactions. For this purpose, a laboratory fluidized bed reactor has been successfully designed to study catalytic cracking of polymers by limiting the contact between primary volatile products and the catalyst/ polymer mixture. The literature available involving the catalytic degradation of polymers using a fluidized bed reactor is rather scarce. Scott et al.10 reported the use of a fluidized bed containing sand, activated carbon, or an iron-loaded carbon as the solid medium operated at temperatures typical for pyrolysis (500-790 °C). Hardman et al.7 used a fluidized bed containing quartz sand, silica, or other refractory materials. Again, relatively high operating temperatures are suggested, with 450550 °C being preferred. Although zeolite catalysts were used in fluidized beds in some trials, the results for those trials are sketchy and were carried out at temperatures in excess of 430 °C. Some polymeric materials, e.g., polystyrene, can be decomposed thermally in high yields to the monomers. However, this is not true for polyethylene or polypropylene, which are among the most abundant polymeric waste materials typically making up 60-70% of municipal solid waste.2 It would be desirable to convert these waste polyolefins into products of value other than the monomers, because the products could be of sufficient value to offset the collection and pyrolysis costs. The objective of this paper is to use this newly developed laboratory fluidized bed reactor to study product distributions arising from the reaction of different polyolefins with various catalysts. Additionally, a model for catalytic degradation of polymers based on lumping techniques has been investigated to describe the results and to provide some basis for the optimization of the potential benefit of catalytic polymer recycling.

2.1. Materials and Catalyst Preparation. The polymers used in these experiments were pure high-density polyethylene (HDPE; unstabilized, MW ≈ 75 000, F ) 960.3 kg m-3, BASF) and pure polypropylene (PP; isotactic, MW ≈ 330 000, F ) 851.6 kg m-3, Aldrich). Both HDPE and PP were pyrolyzed over zeolites (HZSM-5, HUSY, and HMOR) and nonzeolitic (SiO2-Al2O3 and MCM-41{Al}) catalysts. The catalysts employed are described in Table 1. Prior to use, all the catalysts were pelleted using a press (compression pressure ) 160 MPa), crushed, and sieved to give particle sizes ranging from 75 to 180 µm. The catalyst (0.25-0.3 g) was then activated by heating in the reactor in flowing nitrogen (50 mL min-1) to 120 °C at 60 °C h-1. After 2 h the temperature was increased to 520 °C at a rate of 120 °C h-1. After 5 h at 520 °C the reactor was cooled to the desired reaction temperature. 2.2. The Reactor, Fluidizing Reaction, and Product Analysis. A detailed description of the experimental system is given elsewhere23 and shown schematically in Figure 1. The reactor consists of a Pyrex glass tube with a sintered distributor and a three-zone heating furnace with digital controllers. High-purity nitrogen (BOC) was used as the fluidizing gas, and the flow was controlled by a needle valve. The feed system was designed to allow both HDPE and PP particles, purged under nitrogen, to enter the top of the reactor and to drop freely into the fluidized bed at t ) 0 min. The particle size of both catalyst (75-180 µm) and polymer (75-250 µm) were chosen as being large enough to avoid entrainment. The catalyst was small enough to be adequately fluidized. The added polymer melts, wets the catalyst surface, and is pulled into the catalyst macropores by capillary action. At sufficiently low polymer/catalyst ratios (as used in this study), the outsides of the catalyst particles are not wet with polymer, so the catalyst particles move freely.24 Volatile products leaving the reactor were passed through a glass-fiber filter to capture catalyst fines, followed by an icewater condenser to collect any condensable liquid product. Gases were routed either into a sample gas bag or to an automated sample valve system with 16 loops. The Tedlar bags, 15 L capacity, were used to collect time-averaged gaseous samples and were replaced every 10 min throughout the reaction. The multiport sampling valve allowed “spot” sampling of the product stream at 0.5 or 1 min intervals. The overall hydrocarbon gas yield (wt % based on feed) was calculated from both the gas bag average samples and spot samples. The rate of hydrocarbon production (Rgp, wt % min-1)

(18) Ishihara, Y.; Nanbu, H.; Saido, K.; Ikemura, T.; Takesue T. J. Appl. Polym. Sci. 1989, 38, 1491-1501. (19) Ohkita, H.; Nishiyama, R.; Tochihara, Y.; Mizushima, T.; Kakuta, N.; Morioka, Y.; Ueno, A.; Namiki, Y.; Tanifuji, S.; Katoh, H.; Sunazyka, H.; Nakayama, R.; Kuroyanagi, T. Ind. Eng. Chem. Res. 1993, 32, 3112-3116. (20) Songip, A. R.; Masuda, T.; Kuwahara, H.; Hashimoto, K. Appl. Catal. B: Environmental 1993, 2, 153-164.

(21) Beck, J. S.; Chu, C. T. W.; Johnson, I. D.; Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C. WO 91/11390, 1991. (22) Beck, J. S.; Calabro, D. C.; Mccullen, S. B.; Pelrine, B. P.; Schmitt, K. D.; Vartuli, J. C. US Patent 5145816, 1992. (23) Sharratt, P. N.; Lin, Y.-H.; Garforth, A.; Dwyer, J. Ind. Eng. Chem. Res. 1997, 5118-5124. (24) Maegaard S. Studies of the Interface between Zeolite Catalyst and Degrading Polymer. MSc Dissertation, UMIST, 1997.

Table 1. Catalysts Used in Fluidized Bed Reactor for HDPE and PP Degradation catalyst

micropore size (nm)

BET area Si/Al (cm3/g) ratio

HZSM-5 HUSY

0.55 × 0.51 0.74

391 603

HMOR SAHA

0.65 × 0.70 3.15d

561 274

MCM-41{Al} 4.2∼5.2d

845

commercial name

17 H-ZSM-5 zeolitea 6.0 H-ultrastabilized Y zeoliteb 6.3 H-mordenitec 2.6 synclyst 25b (silica-alumina) 17.5 e

a

BP Chemicals, Sunbury-on-Thames, U.K. b Crosfield Chemicals, Warrington, U.K. c Laporte, Warrington, U.K. d Single-point BET determination. e Synthesized by procedure outlined by Beck.21,22

2. Experimental Section

Catalytic Conversion of Polyolefins

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Figure 1. Schematic diagram of catalytic fluidized bed reactor system. Keys: 1, feeder; 2, furnace; 3, sintered distributor; 4, fluidized catalyst; 5, reactor; 6, condenser; 7, flow meter; 8, 16-loop automated sample system; 9, gas bag; 10, GC; 11, digital controller for three-zone furnace. was defined by the relationship:

Rgp ) hydrocarbon production rate (g/min) × 100/total hydrocarbon product over the whole run (g) Gaseous products were analyzed using a gas chromatograph (Varian 3400) equipped with (i) a thermal conductivity detector fitted with a 1.5 m × 0.2 mm i.d. molecular sieve 13X packed column and (ii) a flame ionization detector fitted with a 50 m × 0.32 mm i.d. PLOT Al2O3/KCl capillary column. A calibration cylinder containing 1% C1-C5 hydrocarbons (Linde Gas Ltd, U.K.) was used to help identify and quantify the gaseous products. The solids remaining deposited on the catalyst after the catalytic degradation of the polymer were deemed “residues” and contained involatile products and coke. The amount and nature of the residues were determined by thermogravimetric analysis as described elsewhere.25 2.3. Experimental Results and Discussion. For simplicity, catalytic pyrolysis products (P) are grouped together as hydrocarbon gases ( HUSY ≈ HMOR) gave higher yield than nonzeolitic catalysts (SAHA ≈ MCM-41), and the highest yield (nearly 95 wt %) was obtained for HZSM-5. Overall, the bulk of the products observed was in the gas phase with less than 7 wt % liquid collected. The highest level of unconverted polymer was observed with SAHA and MCM-41, while the highest coke yields were observed with HUSY and HMOR. The differences in the product distributions between the zeolites can be seen with HUSY producing a wider molecular weight range than HZSM-5 and HMOR. Some similarities were observed between HMOR and HZSM-5 with C1-C4 and C5-C9 yields, which were approximately 60 wt % and 30 wt %, respectively, for both HDPE and PP degradation. However, with HUSY the C1-C4 and C5-C9 yields were approximately 30 and 55 wt %, respectively. The volatile hydrocarbon products of HDPE and PP degradation over various catalysts are also listed in Table 2. Both HMOR and HUSY produced more paraffinic streams with large amounts of C4. The results of a comparison of the products of HDPE and PP degradation over HZSM-5, SAHA, and MCM-41 reflect the differing acidities of the zeolite compared with the nonzeolitic materials. Product distributions with HZSM-5 contained more olefinic materials with over 80 wt % olefins in the range C3-C5. Both SAHA and MCM41 gave similar product distributions with very high yields of olefins (>85%) over a wide molecular weight range. Effect of Reaction Conditions on HDPE and PP Degradation. The rate of hydrocarbon production as a function of time for HDPE degradation over HZSM-5 at different reaction temperatures is compared in Figure 2, and, as expected, faster rates were observed at higher temperatures. At 430 °C, the maximum rate of hydrocarbon production was 48 wt % min-1 (calculated as described in section 2.2) after only 0.5 min with all the polymer degraded after approximately 3 min. As the temperature of reaction was decreased, the initial rate of hydrocarbon production dropped and the time for HDPE to be degraded lengthened. At 290 °C the rate of hydrocarbon production was significantly lower with the polymer being degraded more slowly over 15 min. The change in the hydrocarbon yield with temperature was similar, for all

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Table 2. Summary of Products of HDPE and PP Degradation over Various Catalysts HZSM-5 PE

PP

HUSY PE

HMOR PP

SAHA

PE

PP

PE

PP

PE

PP

Yield (wt % Feed) 90.3 88.6 37.1 60.8 52.3 27.8 3.5 2.5 6.2 8.9 2.4 6.1 3.7 2.8 93.4 89.7

87.0 58.4 28.6 4.4 8.6 5.8 3.7 93.6

84.5 30.3 54.2 3.4 12.1 9.6 2.5 88.2

86.1 22.6 63.5 4.5 9.4 7.6 1.8 90.2

84.8 23.4 61.4 6.1 9.1 6.9 2.2 86.1

85.5 26.2 59.3 5.0 9.5 7.8 1.7 89.3

n.d. n.d. 0.1 7.6 1.7 20.9 1.4 23.3 2.1 16.0 2.9 7.2 0.5 0.6 0.2 n.d.

n.d. 0.1 0.1 3.8 0.7 17.9 0.4 21.3 0.1 20.2 3.0 10.9 2.8 4.5 0.3 n.d.

n.d. 0.1 6.6 0.8 15.9 0.3 21.6 1.7 19.4 2.8 10.8 1.8 2.6 0.4 n.d.

n.d. 0.1 7.3 1.1 17.7 1.0 20.8 1.2 19.0 2.1 11.7 1.8 1.6 0.1 n.d.

gases total (∑C1-C9) gases (∑C1-C4) gasoline (∑C5-C9) liquida residue involatile products coke mass balance (%)

93.2 64.1 27.1 2.3 4.5 2.8 1.7 90.3

94.0 67.4 25.1 2.3 3.7 2.6 1.1 94.8

C1 C2 C2) C3 C3) C4 C4) C5 C5) C6 C6) C7 C7) C8 C8) ∑C9 BTXb

n.d. 2.7 3.8 20.7 10.2 26.7 5.0 12.5 2.1 4.2 1.7 1.1 0.4 0.1 2.0

Distribution of Gaseous Products (wt % Feed)c n.d. n.d n.d. 3.2 0.3 0.2 1.4 1.2 3.3 0.6 0.2 3.9 2.4 23.5 8.8 8.1 16.5 15.6 12.7 13.8 11.0 19.3 17.1 24.7 11.3 17.6 19.7 22.1 5.2 14.5 11.0 7.9 6.1 11.6 7.5 9.4 11.4 14.3 2.7 13.0 13.9 3.2 2.6 3.5 4.3 7.3 3.5 3.6 1.1 7.4 5.5 0.5 0.5 0.9 1.1 1.6 0.3 0.6 0.1 4.0 3.0 0.1 0.2 1.2 0.4 0.1 0.1 0.7 0.2 0.8 0.6 1.5 1.2 0.9 -

89.7 34.8 53.7 3.2 7.1 3.1 3.9 91.8

MCM-41

a Condensate in condenser and captured in filter, not identified. b Benzene, toluene, and xylene. c Dash (-) represents less than 0.1 (wt %); n.d., not detectable.

Figure 2. Hydrocarbon yields as a function of time at different reaction temperatures for the catalytic degradation of HDPE over HZSM-5 (rate of fluidizing gas ) 570 mL/min, ratio of polymer to catalyst ) 40 wt %/wt, catalyst particle size ) 75∼120 µm). catalysts used and for different polymer fed (HDPE and PP), with faster rates observed at higher temperatures. The rate of gaseous hydrocarbon evolution further highlights the slower rate of degradation over nonzeolitic catalysts (SAHA and MCM-41), as shown in Figure 3 when comparing all catalysts under identical conditions at 360 °C. Overall, the maximum rate of generation was observed after 1 min with the zeolite catalysts whereas the maximum was observed after 2 min with SAHA and MCM-41. For the comparison of parts a and b of Figure 3, it can be seen that both of the larger pore structure zeolites (HUSY and HMOR) result in a similar catalytic effect on the structural feed of PP and HDPE, while small channels present in HZSM-5 presumably give lower initial cracking rates for PP than HDPE. Relation of Transient Change in Catalytic Activity to Catalyst Deactivation. The deactivation of the catalyst was examined by the transient change in the amount of gaseous compounds produced. Rapid deactivation of both HMOR and HUSY was observed (Figure 4) when the spot samples, taken

Figure 3. Comparison of hydrocarbon yields as a function of time for the catalytic degradation of (a) HDPE and (b) PP over various catalysts (polymer-to-catalyst ratio ) 40 wt %/wt, rate of fluidizing gas ) 570 mL/min, reaction temperature ) 360 °C). during the course of the reaction, were analyzed. The deactivation is reflected in the decrease of the amount of isobutane (i-C4) produced (product of bimolecular reaction) and the relative increase in olefins (product of monomolecular reaction), exemplified by C4) and C5). The larger-pore openings

Catalytic Conversion of Polyolefins

Energy & Fuels, Vol. 12, No. 4, 1998 771

Figure 4. Comparison of some of the main hydrocarbon products (i-C4, isobutane; C5), pentenes; tot C4), butenes) as a function of time for the catalytic degradation of PP over (a) HZSM-5, (b) HUSY, (c) HMOR, (d) SAHA, and (e) MCM-41 catalysts (reaction temperature ) 360 °C, rate of fluidizing gas ) 570 mL/min, ratio of polymer to catalyst ) 40 wt %/wt). and channel system of HMOR allow bulky bimolecular reactions to occur, ultimately leading to the generation of coke and subsequently deactivation of the catalyst. The deactivation was more exaggerated in the case of HUSY with its largerpore openings and internal supercages. In contrast, HZSM-5 and SAHA product streams remain virtually unchanged throughout the degradation of PP. HZSM-5 is resistant to coking when coke builds up on the outersurface and the product stream remains essentially unchanged, whereas the weakness and lower density of the acid sites in SAHA and MCM-41 along with the increased tolerance to “coke” in the larger mesopore systems provide the most likely reasons for the lack of variation in the product stream over these catalysts.

gasoline, light gases, and coke lumps and is considered to represent the product distributions.28,29 On the basis of the proposed reaction pathway shown in Figure 5, a four-lump model can be adopted for simulation of the fluidized bed reactions. The individual rate constants are conveniently grouped as follows

3. Kinetic Modeling

where k0 is the overall cracking rate constant and k1 and k3 are the individual disappearance rate constants from the four-lump scheme. The kinetic expression of

3.1. The Lumped Kinetic Model. Lumping techniques have been used to develop kinetic models for catalytic cracking.26,27 Since the amount of coke deposition on a Y zeolite is significant as compared to ZSM-5 and silica-alumina (SA), coke had to be taken separately as an independent product to facilitate analysis of catalyst deactivation caused by coke deposition. Considering this, a four-lump model was proposed which separately takes into account the unconverted feedstock,

k0 ) k1 + k3

(1)

k1 ) k11 + k12

(2)

k2 ) k21 + k22

(3)

(26) Weekman, V. M. Ind. Eng. Chem. Process Des. Dev. 1968, 7, 90-95. (27) Wei, J.; Kuo, J. C. W. Ind. Eng. Chem. Fundam. 1969, 8 (1), 114-123. (28) Farag, H. I.; Ng, S.; de Lasa H. I. Ind. Eng. Chem. Res. 1993, 32, 1071-1080. (29) Songip, A. R.; Masuda T.; Kuwahara H.; Hashimoto K. Energy Fuels 1994, 8, 131-135.

772 Energy & Fuels, Vol. 12, No. 4, 1998

Lin et al. Table 3. Comparison of Product Selectivity for the Degradation of HDPE over Different Particle Sizes of HZSM-5 Using a First-Order Lumping Model HZSM-5-S (75-120 µm)

HZSM-5-L (125-180 µm)

temp (°C)

k11/k0a (%)

k12/k0 (%)

k3/k0 (%)

k11/k0 (%)

k12/k0 (%)

k3/k0 (%)

330 360 390 430

41.5 34.6 34.3 36.9

57.7 64.7 65.0 62.6

0.8 0.6 0.7 0.6

40.6 33.9 31.7 37.1

58.7 65.6 67.7 62.4

0.7 0.6 0.6 0.5

a

k0 ) k11 + k12 + k3.

activity decaying as function of coke on catalyst was employed for HDPE on HUSY30

η ) exp[-RC(c)] Figure 5. Reaction scheme proposed in the four-lump model.

(5)

where C(c) is coke content deposited on the catalyst HUSY and R is a constant, calculated to be 0.163 (wt %/wt)-1 for HDPE. For this initial modeling we consider, as a first approximation, that R, in the deactivation process, can be taken as constant for both polymers and that the active sites are deactivated at the same rate for the acid catalysts studied. It is assumed that all sites leading to the generation of unconverted polymer, gasoline, light gases, or coke are deactivated at the same rate:

η ) ηA ) ηB ) ηC ) ηD

(6)

Equations 7-10 were obtained assuming that reaction rates can be represented by simple first-order processes (i.e., ni ) 1) and catalyst deactivation involving four simultaneous equations describing the evolution of unconverted polymer lump (A), the gasoline lump (B), the light gases lump (C), and the coke lump (D).

-dWA/dt ) ηk0WA ) η(k1 + k3)WA

(7)

dWB/dt ) η(k11WA - k2WB) ) η(k11WA - k21WB k22WB) (8) dWC/dt ) η(k12WA + k21WB)

(9)

dWD/dt ) η(k3WA + k22WB)

(10)

The mass balance can be written as follows

-dWA/dt ) dWB/dt + dWC/dt + dWD/dt

Figure 6. Comparison of calculated and experimental products using the four-lump model for conversion of HDPE over (a) HZSM-5, (b) HUSY, and (c) SAHA at 360 °C. X(A), unconverted polymer; X(B), gasoline; X(C), light gases; X(D), coke.

the four-lump reaction may be written as

ri(i ) A, B, C, D) ) kiWni iηi

(4)

where ri is the rate of consumption of the ith lump, ki is the rate constant of the ith lump, Wi is the weight fraction of the ith lump, ηi is the catalyst activity decay of the ith lump, ni is the reaction order, and suffixes A, B, C, and D refer to unconverted polymer, gasoline, light gases, and coke. An exponential decay function with

(11)

Equations 7-11 were numerically integrated by a fourth-order Runge-Kutta algorithm for the four lumps linked with the calculation program using a Microsoft Excel (version 5.0) macro to minimize the sum of the square deviations between the calculated and the experimental results. This gave values for the apparent rate constants. 3.2. Kinetic Results and Discussion. The fourlump model was used to predict product distributions for the degradation of polymers over HZSM-5 under the operating conditions of the fluidized bed reaction. As shown in Figure 6, it was found that the experimental data using various catalysts were in good agreement with the calculated values. While k2 ) k21 + k22 reflects (30) Lin, Y.-H.; Sharratt, P. N.; Garforth, A.; Dwyer, J. Thermochim. Acta 1997, 294, 45-50.

Catalytic Conversion of Polyolefins Table 4.

HZSM-5 HUSY SAHA a

Energy & Fuels, Vol. 12, No. 4, 1998 773

Comparison of Apparent Rate Constants and Product Selectivity Using Lumping Model for HDPE Cracking over Different Catalysts at 360 °C for Large (125-180 µM) Particles k11 × 10-2 (min-1)

k12 × 10-2 (min-1)

k3 × 10-2 (min-1)

k21 × 10-2 (min-1)

k11/k0a (min-1)

k12/k0 (%) (min-1)

k3/k0 (min-1)

21.6 22.7 17.7

41.8 14.1 9.5

0.36 1.37 0.80

0.20 0.14 0.02

33.9 59.6 63.2

65.6 36.8 33.9

0.6 3.6 2.9

k0 ) k11 + k12 + k3.

gasoline overcracking, those two constants have in practice very different magnitudes, k21 . k22, and consequently, k21 was the only overcracking constant considered further in the analysis. Selectivity Changes with Operating Conditions. The rate constants for the catalytic degradation of HDPE over HZSM-5 with particle sizes of 125-180 µm and 75-120 µm at four different temperatures are listed in Table 3. The selectivity toward the gasoline fraction (k11/k0) as a function of reaction temperature shows that the amount of gasoline formation decreased with increasing reaction temperature from 330 to 390 °C and then increased at 430 °C. On the other hand, the results show the highest amount of light gases (k12/k0) occurred at 390 °C rather than at the higher temperature of 430 °C. It could be the case that selectivity products of light gases increased with increasing reaction temperature and gasoline decreased with an increase in temperature below 400 °C. But at higher temperature (430 °C), the degradation of HDPE to volatile products may proceed both by catalytic and thermal reactions leading to the variation of product distributions. Previous study shows that for the degradation of HDPE over silicalite (containing very few or no catalytically active sites) about 26 wt % of polymer is converted at 430 °C at similar residence times.23 Of course, the classification of products into “lumps” does not lead to unique predictions of changes in product slate arising from changes in reaction conditions. From values of k11/k0 and k3/k0, the larger particle HZSM-5-L catalyst (125-180 µm) shows a higher selectivity for light gases (57.7∼65% vs 58.7∼67.7%) and a lower selectivity for coke (0.5∼0.7% vs 0.6∼0.8%) than the smaller particle HZSM-5-S catalyst (75-120 µm). Moreover, the relatively low values of the k21 constant point toward only a moderate contribution to light gases by overcracking of gasoline for all the conditions studied for both particle size ranges of HZSM-5. Selectivity Dependence on Catalyst Structure. The effect of different catalysts on the kinetic parameters obtained from the lumping scheme is summarized in Table 4. The lumping rate constants for each reaction step are found to be affected by the microstructure of catalysts with the larger pore size material resulting in smaller rate constants, under the same reaction conditions. The k11 value for the catalysts used in this study was higher for zeolites (HZSM-5 and HUSY) than for SAHA, while the k12 value for HUSY was higher than that for SAHA, but much lower than that for HZSM-5. This suggests that the value of both k11 and k12 for the catalysts used in this study for the given reaction stream was dependent on the nature of the acid sites, which are much stronger in zeolites than in SAHA. Table 4 also shows that the selectivity to gasoline (k11/ k0) was similar for SAHA (63.2%) and HUSY (59.6%), but much higher than that for HZSM-5 (33.9%). SAHA

with large mesopores and low acidity (very small value of k21 with less gasoline overcracking) gave rise to the broadest carbon range (C5-C9). HUSY with 12-ring pore openings and large internal supercages allowed significant bimolecular reactions and yielded a wide carbon number distribution and substantial coke levels (3.6%). A much higher selectivity to light gases (k12/k0) for HZSM-5 (65.6%) compared with that for HUSY and SAHA (36.8% and 33.9%) was obtained, which reflects, presumably, increased cracking of pregasoline products to light gases with HZSM-5 as compared to the other catalysts. The difference in the value of k21 for different catalysts is in the order HZSM-5 > HUSY > SAHA. This order presumably reflects the differences in the activity of the catalyst for the cracking of molecules in the gasoline fraction. Additionally, the smaller pores in HZSM-5 generate a higher concentration of smaller gasoline molecules, e.g., C6-C8, which also enhances production of light gases. The feature of overcracking of gasoline when HZSM-5 is added to FCC catalysts has been reported previously.31 The relatively low value of k3/k0 (coke selectivity) and the fact that formation of coke shows only a very small increase with increasing reaction temperature (Table 3) suggest that the pore structure of HZSM-5 restricts coke. 4. Conclusions The systematic experiments reported in this paper show that the use of a catalyst reduces significantly the required reaction temperature and improves the yield of volatile products as well as providing better selectivity in the product distributions. Reaction in a fluidized bed reactor was found to be a useful method for the conversion of polyolefins to chemicals and fuels. The acidic zeolite catalysts, HZSM-5, HMOR, and HUSY, are more effective in converting polyolefins to volatile hydrocarbons than the less acidic, amorphous SAHA and MCM-41. Product selectivity was observed with HZSM-5 that resulted in more olefinic materials with over 80 wt % olefins in the range C3-C5, while HMOR generated the highest yield of C4 paraffins at all catalysts studied. Both the larger-pore zeolites (HUSY and HMOR) showed rapid deactivation in contrast to the more restrictive HZSM-5 and the nonzeolitic catalysts (SAHA and MCM-41). A kinetic model with four lumps, representing unconverted polymer, light gases, gasoline, and coke (i.e., taking into account catalyst deactivation), was used to determine the kinetics of catalytic degradation of polyolefins. A good fit between calculated and experimental results was obtained. This model provides the benefits (31) Rawlence, D. J.; Dwyer J. Fluid Catalytic Cracking II; American Chemical Society: Washington, DC, 1991; Vol. 452, pp 56-78.

774 Energy & Fuels, Vol. 12, No. 4, 1998

of lumping product selectivity, in each reaction step, in relation to the effect of reaction temperature, the performance of the catalyst used, and the particle size selected. Acknowledgment. The financial support of the EPSRC (Grant No. GR/J 10730) is acknowledged, as is

Lin et al.

the support of Kaohsiung Chemistry Co. and the Government of the Republic of China (R.O.C.) for Y.-H. Lin. Also, the Centre for Microporous Materials acknowledges the support of BNFL, BOC, Engelhard and ICI. EF970233K