5118
Ind. Eng. Chem. Res. 1997, 36, 5118-5124
Investigation of the Catalytic Pyrolysis of High-Density Polyethylene over a HZSM-5 Catalyst in a Laboratory Fluidized-Bed Reactor P. N. Sharratt* and Y.-H. Lin† 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.
High-density polyethylene (HDPE) was pyrolyzed over HZSM-5 catalyst using a specially developed laboratory fluidized-bed reactor operating isothermally at ambient pressure. The influence of reaction conditions including temperature, ratios of HDPE to catalyst feed, and flow rates of fluidizing gas was examined. The sodium form of siliceous ZSM-5, silicalite, containing very few or no catalytically active sites, gave very low conversions of polymer to volatile hydrocarbons compared with HZSM-5 (Si/Al ) 17.5) under the same reaction conditions. Experiments carried out with HZSM-5 gave good yields of volatile hydrocarbons with differing selectivities in the final products dependent on reaction conditions. Catalytic pyrolysis of HDPE performed in the fluidized-bed reactor was shown to produce valuable hydrocarbons in the range of C3-C5 carbon number with a high olefinic content. The production of olefins with potential value as a chemical feedstock is potentially attractive and may offer greater profitability than production of saturated hydrocarbons and aromatics. 1. Introduction The disposal of municipal and industrial waste is recognized to be a major environmental problem. Landfill is becoming much more expensive and of questionable desirability for many localities. The destruction of wastes by incineration is prevalent, but this practice is expensive and often generates problems with unacceptable emissions. Another alternative would be true recycling, i.e., to convert the waste material into products that can be reused and significantly reduce the net cost of disposal (Lee, 1995). Possible technologies for the conversion of waste to useful products have attracted research in the area of thermal degradation. Workers in Japan have developed a dual fluidized-bed process for obtaining mediumquality gases from municipal solid waste (Kagayama et al., 1980; Igarashi et al., 1984). Thermal cracking of waste polymer using kilns or fluidized beds has been piloted on a significant scale in Europe (Kaminsky et al., 1995; Kaminsky, 1995; Conesa et al., 1994). Other processes using a pilot-plant fluidized-bed reactor or an internally circulating-fluidized bed (ICFB) reactor to pyrolyze plastic waste have also been tried in North America (Scott et al., 1990; Sodero et al., 1996). 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. Catalytic pyrolysis provides a means to address these problems. Suitable catalysts can have the ability to control both the product yield and product distribution from polymer degradation as well as to reduce significantly the * Corresponding author. Telephone: +44 (161) 200 4367. Fax: +44 (161) 200 4399. E-mail:
[email protected]. † Current address: Department of R&D, Kaohsiung Chemistry, P.O. Box 90583, Chiu-Chu-Tang, 840 Kaohsiung, Taiwan, Republic of China. S0888-5885(97)00348-5 CCC: $14.00
reaction temperature, potentially leading to a cheaper process with more valuable products. In contrast to thermal degradation research, catalytic pyrolysis has been carried out by considering a variety of catalysts (Aguado et al., 1996; Audisio et al., 1992; Ohkita et al., 1993; Sakata et al., 1996) with little emphasis on the reactor design, with only simple adiabatic batch and fixed-bed reactors being used (Ishihara et al., 1993; Songip et al., 1993; Mordi et al., 1994). Also, even though catalysis has been used, this is often done by thermally degrading the polymer and passing the degradation products through the catalyst (Songip et al., 1993, Ohkita et al., 1993). The use of fixed beds where polymer and catalyst are contacted directly leads to problems of blockage and difficulty in obtaining intimate contact over a significant portion of the reactor volume. Without good contact the formation of large amounts of residue is likely, and scale up to industrial scale is not feasible. Much less is known about the performance of catalyst in polymer degradation using a fluidized-bed reactor. Scott et al. (1990) report the use of a fluidized bed containing sand, activated carbon, or an iron-loaded carbon as the solid medium. While the authors claim catalytic effects, the reactions were carried out at temperatures typical for pyrolysis: 500-790 °C. Hardman et al. (1993) used a fluidized bed containing quartz sand, silica, or other refractory materials. Again, relatively high operating temperatures are suggested, with 450-550 °C being preferred. Although zeolite catalysts were used in some trials, the results for those trials are sketchy and were carried out at temperatures in excess of 430 °C. The objective of this current work was to explore the capabilities of a laboratory catalytic fluidized-bed reaction system using a zeolite catalyst (i) for the study of product distributions and (ii) for identification of suitable reaction conditions for achieving waste polymer recycling. © 1997 American Chemical Society
Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997 5119
Figure 1. Schematic diagram of a catalytic fluidized-bed reactor system: 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 a three-zone furnace.
2. Experimental Section 2.1. Materials and Catalyst Preparation. The materials used were pure high-density polyethylene (unstabilized, MW ≈ 75 000, F ) 960.3 kg m-3, BASF), ammonium form of ZSM-5 (Si/Al ) 17.5, supplied by Conteka, ID No. CBV 3020), and the sodium, hydrogen form of silicalite (Si/Al > 1000, synthesized in-house). Silicalite is the purely siliceous analogue of ZSM-5 zeolite with a 3D pore structure consisting of straight (5.3 × 5.6 Å) and sinusoidal (5.1 × 5.5 Å) channels. Prior to use, NH4ZSM-5 and silicalite 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. Experimental Setup and Procedures. The Reactor. A process flow diagram of the experimental system is shown in Figure 1. The reactor consists of a 400 mm long pyrex glass tube with a sintered distributor (10 mm i.d.) in the middle section. The tube had an inverted bell shape and was divided into three parts: an upper section (170 mm × 20 mm i.d.), a middle section (30 mm × 10 mm i.d.), and a lower section (200 mm × 10 mm i.d.). A three-zone heating furnace with digital controllers was used, and the temperatures of the furnace in its upper, middle, and bottom zones were measured using three thermocouples. By these means the temperature of the preheated nitrogen below the distributor and catalyst particles in the reaction volume could be effectively controlled to within (1 °C. An additional thermocouple on a moveable joint connected to the reactor was used to measure the temperature at
any position in the reactor. Another thermocouple was located close to the middle of the furnace and was coupled directly to a high-temperature cutout. Fluidizing Gas Flow. High-purity nitrogen (BOC) was used as the fluidizing gas; the flow was controlled by a needle valve and preheated in the bottom section of the reactor tube. Flowmeters were used to measure the full range of gas velocities from incipient to fast fluidization. Before catalytic pyrolysis experiments were started, several fluidization runs were performed at ambient temperature and pressure (i) to select suitable particle sizes (both catalyst and polymer) and (ii) to optimize the fluidizing gas flow rates to be used in the reaction. The particle sizes of both catalyst (75180 µm) and polymer (75-250 µm) were chosen as to be large enough to avoid entrainment but not too large to be inadequately fluidized. Entrainment of polymer is only an issue as the polymer enters the reactor, as once it is in the bed it is effectively trapped (see the Reactor Performance section). High flow rates of the fluidizing stream improve catalyst-polymer mixing and external heat transfer between the hot bed and the cold catalyst. On the other hand, an excessive flow rate could cause imperfect fluidization and considerable entrainment of fines. After selecting suitable particle parameters, the minimum fluidization velocity of catalyst (Umf) at different operating conditions was calculated using the Ergun equation (Kunii and Levenspiel, 1991). The value for Umf working at 360 °C was found experimentally to be 1.03 cm s-1, which is in good agreement with that predicted by the Ergun equation (1.05 cm s-1). Fluidizing gas velocities in the range 1.5-4 times the value of Umf were used in the course of this work. Of course, during the experiments, the actual particle density would vary according to the quantity of polymer
5120 Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997 Table 1. Product Distributions from Silicalite-Catalyzed Pyrolysis of HDPE at Different Reaction Temperatures (Catalyst Particle Size ) 75-180 µm, Fluidizing N2 Rate ) 570 mL min-1, Polymer to Catalyst Ratio ) 40 wt %, Total Times of Collection ) 60 min)a
Table 2. Product Distributions Shown from HZSM-5-Catalyzed Pyrolysis of HDPE at Different Reaction Temperatures (Catalyst Particle Size ) 125-180 µm, Fluidizing N2 Rate ) 570 mL min-1, and Polymer to Catalyst Ratio ) 40 wt %)a
temperature (°C) pyrolysis results yield (wt % feed) gaseous liquidb residuec distribution of gaseous products (wt % feed) hydrocarbon gases (∑C1-C4) C1 C2 C2d C3 C3d C4 C4d gasoline (∑C5-C9) C5 C5d C6 C6d C7 C7d C8 C8d ∑C9 BTXd
360
430
460
490
temperature (°C) 520
4.1 1.5 94.4
24.2 1.8 74.0
52.6 3.0 44.4
61.5 5.0 33.5
75.1 5.5 19.5
3.2
13.4
26.8
29.4
34.2
n.d. n.d. n.d. n.d. 1.3 n.d. 1.9 0.9 n.d. 0.9 n.d. 0.01 n.d. n.d. n.d. n.d. n.d. n.d.
n.d. n.d. 0.2 0.2 5.4 0.4 7.2 10.8 0.07 6.7 0.3 3.4 n.d. 0.3 0.01 n.d. n.d.
n.d. 0.3 0.3 10.9 0.6 14.7 25.8 0.3 13.8 0.3 9.4 0.03 1.7 0.2 0.1 n.d. n.d.
0.01 0.1 0.6 0.6 11.8 0.6 15.7 32.1 0.1 16.0 0.3 12.0 0.03 3.0 0.4 0.2 0.01
0.05 0.1 0.5 0.6 13.4 0.9 18.6 40.9 0.6 18.8 1.2 13.8 0.6 4.6 0.1 1.2 0.02
a -: less than 0.01 (wt %). n.d.: not detectable. b Liquid: condensate in condenser and captured in filter, unidentified. c Residue: coke and unconverted polymer. d BTX: benzene, toluene, and xylene.
present inside the catalyst particles (see the Reactor Performance section), so the calculations were only indicative. Polymer Addition. The polymer feed system was designed to avoid plugging the inlet tube with melted polymer and to eliminate air in the feeder. The feed system was connected to a nitrogen supply to evacuate polymer into the fluidized-bed catalyst. Thus, HDPE particles were purged under nitrogen (>500 mL min-1) into the top of the reactor and allowed to drop freely into the fluidized bed at t ) 0 min. Reactor Performance. On addition of the polymer, the fluidized bed remains fluidized (Maegaard, 1997). 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 here) the outside of the catalyst particles are not wet with polymer, so the catalyst particles move freely. Product Analysis. Volatile products leaving the reactor were passed through a glass-fiber filter to capture catalyst fines, followed by an ice-water condenser to collect any condensible liquid product. A three-way valve was used after the condenser to route product 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 at intervals of 10 min throughout the course of reaction. The multiport sampling valve allowed frequent, rapid sampling of the product stream when required. Spot samples were collected and analyzed at various reaction times (t ) 0.5, 1, 2, 3, 5, 8, 11, and 15 min). The overall hydrocarbon gas yield (wt % based on feed) was calculated from both the gas bag average samples and spot samples. The rate of hydro-
pyrolysis results total time of collection (min) yield (wt % feed) gaseous liquidb residuec involatile products coke mass balance (%) distribution of gaseous products (wt % feed) hydrocarbon gases (∑C1-C4) C1 C2 C2) C3 C3) C4 C4) gasoline (∑C5-C9) C5 C5) C6 C6) C7 C7) C8 C8) ∑C9 BTXd
290
330
360
390
430
30
20
15
15
15
85.4 5.9 8.7 7.1 1.6 88.9
90.1 4.0 5.9 4.2 1.6 92.8
91.5 4.0 4.5 2.8 1.7 91.4
95.1 1.6 3.3 1.6 1.7 92.1
94.2 2.1 3.7 1.9 1.8 96.9
55.2
58.4
65.3
67.2
69.4
n.d. n.d. 0.5 0.7 17.4 6.0 30.6 30.1 2.6 16.9 2.4 6.1 1.6 0.5 0.04 n.d. 0.1
n.d. 1.3 1.6 20.4 8.5 26.6 30.3 3.6 14.7 2.2 5.1 2.0 1.9 0.7 0.08 0.01 1.4
n.d. 2.5 1.8 25.9 8.1 27.0 25.1 3.5 13.9 1.7 4.1 0.8 0.6 0.4 0.06 0.01 1.1
n.d. 0.01 3.9 2.5 26.1 9.7 24.9 24.9 4.4 11.6 2.6 3.6 1.3 0.9 0.4 0.07 0.01 3.0
0.01 0.03 3.1 2.6 26.5 10.3 26.9 21.9 3.9 10.3 1.5 3.8 1.2 1.1 0.1 0.02 0.01 2.9
a -: less than 0.01 (wt %). n.d.: not detectable. b Liquid: condensate in condenser and captured in filter, unidentified. c Residue: coke and involatile products. d BTX: benzene, toluene, and xylene.
carbon production (Rgp, wt % min-1) 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 (TCD) fitted with a 1.5 m × 0.2 mm i.d. molecular sieve 13× packed column and (ii) a flame ionization detector (FID) 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. Residue Analysis. The remaining solid deposited on the catalyst after the polymer degradation was deemed “residues” and contained involatile products (such as partially depolymerized HDPE) and coke. The amount and nature of the residues were determined by thermogravimetric analysis (TA Instruments, SDT 2960 simultaneous DTA-TGA). First, the post reaction catalyst and residue were heated to 500 °C in nitrogen (to determine the amount of products volatilized between reaction temperature and 500 °C). The sample was then allowed to cool to 300 °C before switching to an air stream and heating to 600 °C to burn off carbonaceous deposits (coke) on the catalyst. It is worth noting that this procedure regenerated the catalyst. Other work
Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997 5121
Figure 2. Comparison of 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, polymer to catalyst ratio ) 40 wt %).
has shown that zeolitic catalysts are readily regenerated by such a procedure with only a small loss in activity (Lin et al., 1997). Data analysis was carried out using a Microsoft Excel spreadsheet. The reactor and various units of the collection system (condenser, trap, and filter tube) were weighed before and after the runs to determine the mass balance. 3. Results and Discussion For simplicity, catalytic pyrolysis products (P) are grouped together as hydrocarbon gases (65 wt %, Table 2) with minor products, methane and ethane, only detectable at the higher reaction temperatures. The observed yields of olefins with HZSM-5 were expected since bimolecular reactions which lead to the production of saturated compounds are sterically limited within its structure. Equilibrium ratios of i-butane/n-butane and i-butene/ Σbutenes were predicted using Gibbs free energy minimization on the PRO/II flowsheets package (Version 3.02) for the temperatures used experimentally and are presented alongside the corresponding experimental results in Table 3. The i-butane/n-butane ratio reflects the involvement of tertiary C4 carbenium ions in bimolecular hydrogen-transfer reactions, and since tertiary carbenium ions are more stable than primary ions, a higher yield of i-butane would be expected. The observed i-butane/n-butane ratios are well above calculated equilibrium values consistent with the cracking of longer chain hydrocarbon molecules to yield isobutylcarbenium ions which provide a source for i-butane, via hydrogen transfer, or i-butene. The features of i-butane versus n-butane in cracking of high molecular weight hydrocarbon feedstock in both medium- and
5122 Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997 Table 3. Influence of Operating Conditions on Product Selectivity for the Catalyzed Pyrolysis of HDPE Using HZSM-5: Experimental and Predicted Equilibrium Results reaction conditions reaction temperaturea (°C)
polymer to catalyst ratiob (% wt)
fluidizing N2 ratec (mL min-1)
ratio
290
360
430
10
60
100
270
500
720
i-butane/n-butane i-butane/n-butaned i-butene/∑butenes i-butene/∑butened ∑olefins/∑paraffinse
5.4 1.17 0.52 0.56 5.4
4.8 0.95 0.49 0.52 4.5
2.5 0.81 0.43 0.48 3.6
4.8
4.8
4.9
4.2
4.8
5.0
0.49
0.48
0.44
0.48
0.49
0.49
5.5
5.2
3.8
3.3
5.1
5.6
a
min-1. b
Represents a series of runs where polymer to catalyst ratio ) 40 wt % and fluidizing N2 rate ) 570 mL Reaction temperature ) 360 °C, fluidizing N2 rate ) 570 mL min-1. c Reaction temperature ) 360 °C, polymer to catalyst ratio ) 40 wt %. d Predicted equilibrium e data. ∑olefins denotes the sum of all olefinic products. ∑paraffins denotes the sum of all paraffinic products.
Figure 3. Comparison of hydrocarbon yields as a function of time at different ratios of polymer to catalyst for catalytic pyrolysis over HZSM-5 (temperature ) 360 °C, rate of fluidization gas ) 570 mL/min).
large-pore zeolites have been discussed previously (Rawlence and Dwyer, 1991). A second paper will discuss mechanistic aspects, in relation to catalyst type and polymer feed, in more detail. The i-butene/Σbutenes ratio is very close to the predicted equilibrium values and thus the reactions involved in the production and interconversion of butenes are very fast over ZSM-5, and their ratio is primarily equilibrium controlled. The yield of smaller cracked products increased with temperature as did the yield of BTX and coke. Further evidence of the increase in secondary reactions, for example, bimolecular hydrogen transfer, was seen in the lowering of the Σolefins/ Σparaffins ratios as temperature increases, in the experimental range. The pore structure of HZSM-5 restricts the formation of bulky intermediates and consequently the catalyst is resistant to coke formation (Holmes et al., 1997), explaining the relatively low values observed at high conversions and the very small increase with increasing reaction temperature (Table 2). 3.3. Pyrolysis of HDPE over HZSM-5. Influence of HDPE to Catalyst Ratio. In the present study, the amount of catalyst used in the degradation of HDPE remained constant and, therefore, as more polymer was added to the reactor, fewer catalytic sites per unit weight of catalyst were available for cracking. The overall effect of increasing the polymer to catalyst ratio from 1:10 to 1:1 on the rate of hydrocarbon generation
Table 4. Product Distributions Shown from HZSM-5-Catalyzed Pyrolysis of HDPE at Different Ratios of Polymer to Catalyst (Reaction Temperature ) 360 °C, Catalyst Particle Size ) 125-180 µm, Fluidizing N2 Rate ) 570 mL min-1, and Total Time of Collection ) 15 min) ratio of polymer to catalyst (% wt) pyrolysis results yield (wt % feed) gaseous liquida residueb involatile products coke mass balance (%) distribution of gaseous products (wt % feed) hydrocarbon gases (∑C1-C4) gasoline (∑C5-C9) BTXc
10
20
30
60
80
100
94.8 2.3 4.9 2.4 2.5 87.1
93.4 2.0 4.6 2.5 2.1 91.1
92.2 3.5 4.3 2.5 1.8 90.3
90.8 4.6 4.6 3.0 1.6 89.2
88.9 4.6 6.5 5.1 1.4 87.3
87.1 5.5 7.4 6.2 1.2 85.1
69.2
67.5
65.8
64.9
62.3
60.9
25.2 0.4
25.6 0.3
25.4 1.0
25.0 0.9
25.6 1.0
24.9 1.3
a Liquid: condensate in condenser and captured in filter, unidentified. b Residue: coke and involatile products. c BTX: benzene, toluene, and xylene.
was small but predictable (Figure 3). The maximum rate observed dropped slightly, and the time taken to generate the maximum rate extended from 1 to 3 min. The total product yield after 15 min (Table 4) showed only a slight downward trend even after a 10-fold increase in added polymer. This can be attributed to
Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997 5123
Figure 4. Comparison of hydrocarbon yields as a function of time at different fluidizing gases for the catalytic degradation of HDPE over HZSM-5 (temperature ) 360 °C, polymer to catalyst ratio ) 40 wt %).
the high activity of HZSM-5 and excellent contact between HDPE and catalyst particles. Consequently, as more polymer was added, lower C1-C4 hydrocarbon gases and coke yields but higher liquid yields and involatile products were observed (Table 4). In addition, more BTX (coke precursor) was produced, but increasing the polymer to catalyst ratio had virtually no effect on gasoline production. As the polymer to catalyst ratio increases, the possibility of HDPE adhesion to the reactor wall increases as the amount of unreacted polymer in the reactor rises. However, for the work carried out in this paper no such problems were observed. Higher than 1:1 polymer to catalyst ratios have not yet been investigated. 3.4. Pyrolysis of HDPE over HZSM-5. Influence of Fluidizing Gas Rate. The results shown in Figure 4 illustrate that for efficient polymer degradation good mixing is required, with a dramatic dropoff in the rate of degradation observed only at the lowest fluidizing flow used (270 mL min-1). Good mixing will both (i) favor rapid distribution of the polymer feed over the catalyst and (ii) reduce any mass-transfer resistance to the escape of products from the catalyst surface. Thus, good mixing will increase the rate of polymer degradation. Furthermore, changing the fluidizing flow rate influences the product distribution (Table 5). At fast flow rates (short contact times), primary cracking products are favored as evidenced by the increasing Σolefins/Σparaffins ratios (Table 3). At low flow rates (longer contact times), secondary products are observed with increased amounts of coke precursors (BTX), although the overall degradation rate is slower, as shown by increasing amounts of partially depolymerized products. 3.5. Comparison with Other Reactor Types. The results obtained can be compared with some of the results published for other reactor types. This comparison is not straightforward as reaction conditions are not perfectly matched: catalyst compositions, particle sizes, and internal voidage will be different, and the
Table 5. Product Distributions Shown from HZSM-5-Catalyzed Pyrolysis of HDPE at Different Fluidizing N2 Rates (Reaction Temperature ) 360 °C, Catalyst Particle Size ) 125-180 µm, Polymer to Catalyst Ratio ) 40 wt %, and Total Time of Collection ) 15 min) fluidizing gas rate (mL min-1) pyrolysis results yield (wt % feed) gaseous liquida residueb involatile products coke mass balance (%) distribution of gaseous products (wt % feed) hydrocarbon gases (∑C1-C4) gasoline (∑C5-C9) BTXc
720
630
500
440
365
270
92.0 2.3 5.7 3.9 1.8 87.1
90.9 3.7 5.3 3.5 1.8 91.1
90.8 4.4 4.8 3.1 1.7 87.4
91.1 4.1 4.8 3.2 1.6 89.2
89.3 5.5 5.2 3.7 1.5 87.3
88.1 5.4 6.5 5.0 1.5 85.1
67.2
65.8
64.8
64.6
62.5
63.8
24.4 0.4
24.8 0.3
24.4 1.0
25.6 1.0
25.5 1.4
22.6 1.7
a Liquid: condensate in condenser and captured in filter, unidentified. b Residue: coke and involatile products. c BTX: benzene, toluene, and xylene.
concentrations of inerts are different. The following comparisons should thus only be considered as indicative. Ohkita et al. (1993), using a fixed-bed reactor for the H-ZSM5-catalyzed cracking of the volatiles from polymer pyrolysis, obtained lower yields of C1-C4 gases: 50 wt % at 400 °C compared with 67.2 wt % (at 390 °C) in this work. On the other hand, they obtained much higher yields of aromatics (35 wt % at 400 °C versus 3.0 wt % at 390 °C in this work). It also seems, although insufficient data are available for full comparison, that their olefin yields were much lower than those in the present work. Their reaction time was relatively long, up to 3 h, and was presumably set by the relatively low polymer pyrolysis rate at this temperature. Aguado et al. (1996) using a batch reactor at 400 °C obtained C1-C4 yields of 52-58% over H-ZSM5 and
5124 Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997
again relatively high aromatics yields (6.8-17.8%, depending on the catalyst/polymer ratio). Data are provided by Songip et al. (1993) on the products from the catalytic cracking in a fixed bed at 400 °C of a heavy oil arising from polyethylene pyrolysis. Their C1-C4 yields were similar to those in this work but contained more C1-C3 and less C4 by weight. However, they had to remove a residue from the feed oil before catalytic processing that represented 20-30 wt % of the original polymer, making their proposed process rather wasteful. 4. Conclusions In this work, the fluidized bed has been shown to have a number of advantages in the pyrolysis of HDPE; it is characterized by excellent heat and mass transfer, is much less prone to clogging with molten polymer, and gives a nearly constant temperature throughout the reactor. A fluidized-bed reactor has been developed and demonstrated to be particularly suitable to obtain hydrocarbon products from the catalytic pyrolysis of HDPE polymer in the temperature range from 290 to 520 °C. HZSM-5-catalyzed degradation resulted in much greater amounts of volatile hydrocarbons compared with degradation over silicalite. In the presence of the HZSM-5 catalyst at 360 °C, conversion to volatile hydrocarbons in the catalytic fluidized-bed reactor was more than 90 wt % of feed in 15 min, while silicalite yielded less than 6 wt % of feed after 60 min. The systematic experiments carried out with HZSM-5 show that the use of catalyst reduces the required reaction temperature, improves the yield of volatile products, and provides selectivity in the product distributions. The selectivity could be further influenced by changes in reactor conditions; in particular, olefins and i-olefins were produced by low temperatures and short contact times. The catalytic pyrolysis of polymer in the fluidized-bed reactor was a useful method for the production of potentially valuable hydrocarbons. It is concluded that under appropriate conditions the resource potential of polymer waste can be recovered. Acknowledgment The financial support of EPSRC (Grant No. GR/J 10730) is acknowledged, as is the support of the Government of the Republic of China for Y.-H.L. Also, the Centre for Microporous Materials acknowledges the support of BNFL, BOC, Engelhard, and ICI. The authors also thank Professor Yoshio Uemichi of Muroran Institute of Technology (Japan) for his useful discussions during his sabbatical stay at the Centre’s laboratory and Dr. C. S. Cundy and R. J. Plaisted for helpful discussions and for providing the sample of silicalite. Literature Cited Aguado, J.; Serrano, D. P.; Romero, M. D.; Escola, J. M. Catalytic conversion of polyethylene into fuels over mesoporous MCM41. Chem. Commun. 1996, 725-726. Audisio, G.; Bertini, F.; Beltrame, P. L.; Carniti, P. Catalytic degradation of polyolefins. Makromol Chem., Macromol Symp. 1992, 57, 191-209.
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Received for review May 15, 1997 Revised manuscript received August 20, 1997 Accepted August 21, 1997X IE970348B X Abstract published in Advance ACS Abstracts, October 15, 1997.
