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Catalytic pyrolysis of polyolefin waste into valuable hydrocarbons over reused catalyst from refinery FCC units
Applied Catalysis A: General 328 (2007) 132–139 www.elsevier.com/locate/apcata
Catalytic pyrolysis of polyolefin waste into valuable hydrocarbons over reused catalyst from refinery FCC units Y.-H. Lin *, M.-H. Yang Department of Chemical and Biochemical Engineering, Kao Yuan University, 821 Kaohsiung, Taiwan, ROC Received 9 March 2007; received in revised form 18 May 2007; accepted 28 May 2007 Available online 6 June 2007
Abstract A commingled polyethylene/polypropylene (PE/PP) mixture was pyrolysed over used fluid catalytic cracking (FCC) commercial equilibrium catalyst (RCat-c1) using a laboratory fluidised-bed reactor operating isothermally at ambient pressure. The conversion at 390 8C for RCat-c1 catalyst (81.5 wt%) gave much higher yield than silicate (only 18.3 wt%). Greater product selectivity was observed with RCat-c1 as a post-use catalyst with about 53 wt% olefins products in the C3–C6 range. The selectivity could be further influenced by changes in reaction conditions. Valuable hydrocarbons of olefins and iso-olefins were produced by low temperatures and short contact times used in this study. It is demonstrated that the use of spent FCC commercial catalyst and under appropriate reaction conditions can have the ability to control both the product yield and product distribution from polymer degradation, potentially leading to a cheaper process with more valuable products. # 2007 Elsevier B.V. All rights reserved. Keywords: Pyrolysis; Polyolefin; Catalyst; FCC; Selectivity
1. Introduction Polymer waste can be regarded as a potential source of chemicals and energy. Methods for recycling polymer waste have been developed and new recycling approaches are being investigated [1]. The production of liquid hydrocarbons from polymer degradation would be beneficial in that liquids are easily stored, handled and transported. However, these aims are not easy to achieve. An alternative strategy is that of chemical recycling, known as feedstock recycling or tertiary recycling, which has attracted much interest recently with the aim of converting waste polymers into basic petrochemicals to be used as chemical feedstock or fuels for a variety of downstream processes [2]. The most widely used conventional chemical methods for waste polymer treatment are pyrolysis and catalytic reforming. Since thermal degradation demands relatively high temperatures and its products require further processing for their quality to be upgraded, catalytic degradation of polymer waste offers considerable advantages. Suitable catalysts have the ability to control both the product yield and
* Corresponding author. Tel.: +886 7 6077777; fax: +886 7 6077788. E-mail address: [email protected] (Y.H. Lin). 0926-860X/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2007.05.039
product distribution from polymer degradation as well as to reduce significantly the reaction temperature [3–5]. Some polymeric materials, e.g., polystyrene, can be decomposed thermally in high yields to the monomers. However, this is not true for polyethylene (PE) or polypropylene (PP), which are among the most abundant polymeric waste materials typically making up 60–70% of municipal solid waste. 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. Studies of the effects of catalysts on the catalytic degradation of polymer has been performed by contacting melted polymer with catalyst in fixed bed reactors [6–8], heating mixtures of polymer and catalyst powders in reaction vessels [9–11] and passing the products of polymer pyrolysis through fixed bed reactors containing cracking catalysts [12–14]. Catalytic pyrolysis has been carried out by considering a variety of catalysts with little emphasis on the reactor design, with only simple adiabatic batch and fixed bed reactors being used. However, the configuration of the pyrolysis-reforming reactors poses serious engineering and economics constraints. 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
Y.-H. Lin, M.-H. Yang / Applied Catalysis A: General 328 (2007) 132–139
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. Therefore, a fluidised-bed reactor has been used to study catalytic cracking of polymer waste by limiting the contact between primary volatile products and the catalyst/polymer mixture [15–18]. The catalytic degradation of polymeric materials has been reported for a range of catalysts centred around the active components in a range of different model catalysts, such as amorphous silica–aluminas, zeolites Y, ZSM-5 and various acidic catalysts and particularly the new family of MCM materials [6–18]. However, these catalysts have been used that even if performing well, they can be unfeasible from the point of view of practical use due to the cost of manufacturing and the high sensitivity of the process to the cost of the catalyst. An economical improvement of processing the recycling via catalytic cracking would operate in mixing the polymer waste with fluid catalytic cracking (FCC) commercial catalysts. These catalysts increase significantly the commercial potential of a recycling process based on catalytic degradation, as cracking catalysts could cope with the conversion of plastic waste co-fed into a refinery FCC unit [19–21]. Therefore, a more interesting approach is that of adding polymer waste into the FCC process, under suitable process conditions with the use of zero value of spent FCC catalysts, a large number of waste plastics can be economically converted into valuable hydrocarbons. The objective of this work is to explore the capabilities of a catalytic fluidised bed reaction system using spent FCC commercial catalysts for the study of product distribution and selectivity on the catalytic degradation of a post-consumer polyolefin waste (PP/PE mixture), and specifically for identification of suitable reaction conditions for enhancing the potential benefits of catalytic polymer recycling. 2. Experimental 2.1. Materials and experimental procedures The catalysts employed are described in Table 1. All the catalysts were pelleted, crushed and sieved to give particle sizes ranging from 75 to 180 mm. The catalyst (0.25–0.3 g) was then
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dried by heating in flowing nitrogen (50 ml min1) to 120 8C at 60 8C h1. After 2 h the temperature was increased to 520 8C at a rate of 120 8C h1 to active the catalyst for 5 h. In the case of the spent FCC catalyst, it could contain a certain amount of coke, and air was swept and maintained for 3 h at 600 8C to burn it off before activating. The polymer mixture used in this study was obtained from post-consumer plastic waste stream in South-Taiwan with the component of polyethylene (67 wt% PE = 40 wt% HDPE + 27 wt% LDPE), polypropylene (33 wt% PP). High purity nitrogen was used as the fluidising gas and 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 the incipient to fast fluidisation. Before catalytic pyrolysis experiments were started, several fluidisation runs were performed at ambient temperature and pressure to select: (i) suitable particle sizes (both catalyst and polymer waste) and (ii) optimise the fluidising gas flow rates to be used in the reaction. The particle size of both catalyst (75–180 mm) and polymer (75–250 mm) were chosen to be large enough to avoid entrainment but not too large as to be inadequately fluidised. High flow rates of fluidising 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 fluidisation and considerable entrainment of fines. 2.2. Experimental procedures and product analysis A process flow diagram of the experimental system is given elsewhere [15] and shown schematically in Fig. 1. 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 pre-heated nitrogen below the distributor and catalyst particles in the reaction volume could be effectively controlled to within 1 8C. 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 fluidised catalyst bed. Thus, commingled polyethylene/ polypropylene (PE/PP) polymer particles were purged under
Table 1 Catalysts used in the catalytic degradation of post-consumer PE/PP polymer waste Catalyst
Si/Al
Surfacea area (cm2/g) b
RCat-c1 ZSM-5 HUSY SAHA Silicalite a b c d e f g
2.1 17.5 13.6 3.6 >1000
Pore size (nm)
BET
Micro
External
147 426 547 268 362
103 263 429 21 297
44 128 118 247 65
–c 0.55 0.51 0.74 3.28f 0.55 0.51
Aciditya (mmol Py/g catalyst) Brwnsted
Lewis
4.8 33.4 33.4 0.1 –g
3.7 19.8 19.8 22.6
Commercial name
Equilibuium catalystsd ZSM-5 zeolitee Ultrastabilised Y zeolited Amorphous silica aluminad Synthesized in-house
Measured by IR spectroscopy and adsorption–desorption of pyridine (Py) at 623 K for the amount of adsorbed Py (mmol/g) on both acid sites of catalysts. Total surface area (BET). The catalyst was a mixture of zeolite, a silica–alumina matrix and binder, not determined. Chinese Petroleum Corp., CPC, Taiwan, ROC. BP Chemicals, Sunbury-on-Thames, UK. Single-point BET determined. The sodium form of siliceous ZSM-5 with very few or no catalytically active sites, not determined.
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products and coke. Catalytic pyrolysis products (P) are grouped together as hydrocarbon gases (95%. The term ‘‘yield’’ as used in this paper is defined by the relationship: yieldðwt%Þ ¼ Fig. 1. Schematic diagram of a catalytic fluidised-bed reactor system: (1) feeder, (2) furnace, (3) sintered distributor, (4) fluidised catalyst, (5) reactor, (6) condenser, (7) flow meter, (8) 16-loop automated sample system, (9) gas bag, and (10) GC 11. Digital controller for three-zone furnace.
nitrogen into the top of the reactor and allowed to drop freely into the fluidised bed at t = 0 min. At sufficiently low polymer/ catalyst ratios the outside of the catalyst particles are not wet with polymer, so the catalyst particles move freely. Volatile products leaving the reactor were passed through a glass-fibre filter to capture catalyst fines, followed by an iceacetone 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. The bags were replaced 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 analysed at various reaction times (t = 1, 2, 3, 5, 8, 12, 15, and 20 min). The rate (Rgp, wt% min1) of hydrocarbon production of gaseous products collected by automated sample system in each run was defined by the relationship:
Rgp
hydrocarbon production rate of gaseous products in each spot run ðg=minÞ 100 ¼ total hydrocarbon production of gaseous products over the whole spot runs ðgÞ
Gaseous hydrocarbon products were analysed using a gas chromatograph 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 ionisation 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 was used to help identify and quantify the gaseous products. The amount and nature of the residues was determined by thermogravimetric analysis (TGA). The remaining solid deposited on the catalyst after the polymer degradation was deemed ‘‘residues’’ and contained involatile
P ðgÞ 100 polymer fed ðgÞ
3.1. Degradation of polyolefin waste over RCat-c1 and silicalite Product distributions for PE/PP polymer waste degradation over silicalite (Si/Al > 1000) in the 330–450 8C range is summarised in Table 2. At temperatures below 390 8C, a large Table 2 Summary of products of PE/PP polymer waste over silicalite catalyst (fluidising N2 rate = 600 ml min1, catalyst particle size = 75–180 mm, polymer to catalyst ratio = 30 wt% and total time of collection = 60 min) Degradation results
Yield (wt% feed) Gaseous Liquida Residueb BTXc
Temperature (8C) 330
360
390
420
450
7.0 0.7 92.2 0.1
11.9 1.4 86.6 0.1
18.3 1.6 79.9 0.2
28.8 2.1 68.9 0.2
38.6 2.8 58.2 0.4
8.1 n.d n.d 0.2 0.1 3.3 0.6 3.9
12.2 n.d n.d 0.2 0.5 4.7 0.9 5.9
17.9 –d 0.1 0.3 0.8 5.9 1.2 9.6
23.2 0.1 0.1 0.5 0.9 7.8 1.5 12.3
3.8 –d 2.3 –d 1.2 n.d 0.3 n.d –d n.d
6.1 –d 3.6 –d 1.9 0.2 0.3 n.d 0.1 –d
10.9 0.3 4.8 0.1 3.6 0.3 1.4 –d 0.3 0.1
15.4 0.5 6.2 0.2 4.8 0.6 1.9 0.3 0.6 0.3
Distribution of gaseous products (wt% feed) P 4.5 Hydrocarbon gases ( C1–C4) C1 n.d n.d C2 C2= n.d C3 n.d 2.3 C3= C4 n.d C4= 2.2 P 2.5 Gasoline ( C5–C9) n.d C5 C5= 1.3 C6 n.d 1.0 C6= C7 n.d C7= 0.2 C8 n.d –d C8= P C9 n.d
n.d: not detectable. a Condensate in condenser and captured in filter. b Coke and involatile products. c Benzene, toluene and xylene. d Less than 0.01 (wt%).
Y.-H. Lin, M.-H. Yang / Applied Catalysis A: General 328 (2007) 132–139
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Table 3 Summary of products of PE/PP polymer waste degradation over RCat-c1 catalyst (fluidising N2 rate = 600 ml min1, catalyst particle size = 75– 180 mm, polymer to catalyst ratio = 30 wt%, and total time of collection = 30 min) Degradation results
Yield (wt% feed) Gaseous Liquida Residue b Involatile residue Coke BTXc
Temperature (8C) 330
360
390
420
450
78.1 6.7 14.8 12.9 1.9 0.4
79.6 6.9 13.3 11.2 2.1 0.7
81.5 5.1 12.2 9.8 2.4 1.2
83.9 4.2 10.2 7.5 2.7 2.0
85.4 3.0 9.1 6.1 3.0 2.5
22.8 n.d n.d 0.2 1.6 6.7 1.7 12.7
27.5 –d –d 0.1 1.9 8.9 2.5 14.1
29.9 –d –d 0.3 2.3 10.0 2.9 15.4
34.5 –d 0.2 0.4 2.7 11.3 3.2 16.5
56.8 2.3 18.8 3.4 16.1 4.9 6.1 1.3 3.0 3.0 0.7
54.0 2.7 16.5 3.5 13.9 4.6 7.3 1.5 2.7 2.7 1.3
52.7 3.2 16.1 3.3 11.8 3.7 8.1 1.8 2.8 2.8 1.8
50.9 3.5 13.7 4.7 10.3 3.2 8.8 2.0 3.3 3.3 1.5
54.3
53.4
53.3
51.8
Distribution of gaseous products (wt% feed) P 19.5 Hydrocarbon gases ( C1–C4) C1 n.d C2 n.d C2= –d C3 1.0 C3= 5.5 1.8 C4 C4= 11.2 P 58.6 Gasoline ( C5–C9) 1.8 C5 C5= 20.4 C6 3.5 C6= 18.1 3.6 C7 C7= 6.5 C8 1.3 C8= 2.9 2.9 C8= P C9 0.5 P 55.2 Olefins ( C3–C6)
n.d: not detectable. a Condensate in condenser and captured in filter. b Coke and involatile products. c Benzene, toluene and xylene. d Less than 0.01 (wt%).
amount of solid residue, presumably unconverted commingled polymer and high molecular weight degradation products, remained on the silicalite catalyst. The gaseous yield at 390 8C was only 18.3 wt% (Table 2) compared with 81.5 wt% (Table 3) when reused FCC commercial equilibrium catalyst (RCat-c1) was employed. Typically thermal degradation productions were observed with silicalite showing primary cracking products and an even spread of carbon numbers consisting of C3–C6 olefins with some isomerisation C4 and above of aromatics of BTX. At higher temperatures, product streams containing C1–C9 hydrocarbons were produced with gaseous yield 38.6 wt% of PE/PP polymer waste converted at 450 8C. As shown in Table 3, some similar trends in product yields were observed with RCat-c1 catalyst as the reaction temperature was increased. Gaseous and coke yields increased and involatile residues (unreacted or partially reacted PE/PP polymer waste) and liquids decreased. Product distributions with RCat-c1 catalyst contained more olefinic materials in the
Fig. 2. Comparison of hydrocarbon yields as a function of time at different reaction temperatures for the catalytic degradation of PE/PP polymer waste over RCat-c1 catalyst (rate of fluidisation gas = 600 ml min1, catalyst particle size = 75–180 mm, polymer to catalyst ratio = 30 wt%).
range of C3–C6 (about 53 wt% at 390 8C) with minor products, methane and ethane, only detectable at the higher reaction temperatures. The rate of hydrocarbon production as a function of time for PE/PP degradation over RCat-c1 catalyst at different reaction temperatures is compared in Fig. 2 and as expected, faster rates were observed at higher temperatures. At 450 8C, the maximum rate of hydrocarbon production (calculated as described in Section 2.2) was 36 wt% min1 after only 2 min, with all the polymer degraded after approximately 11 min. As the temperature of reaction was decreased, the initial rate of hydrocarbon production dropped and the time for PE/PP to be degraded lengthened. At 330 8C the rate of hydrocarbon production was significantly lower with the polymer being degraded more slowly over 20 min. 3.2. Product stream variation with reaction conditions The effect of reaction conditions including flow rates of fludising gas (270–900 ml min1), ratios of polymer to catalyst feed (0.1:1 to 0.6:1) and catalyst type (RCat-c1) has been investigated. The results shown in Fig. 3 illustrate that for efficient mixture of PE/PP polymer waste degradation good mixing is required, with a dramatic drop-off in the rate of degradation observed only at the lowest fluidising flow used (300 ml min1). After selecting suitable particle parameters, the minimum fluidation velocity of catalyst (Umf), at the different operating conditions was calculated. Fluidising gas velocities in the range 1.5–4 times the value of Umf were used in the course of this work. However, during the experiments, the actual particle density would vary according to the quantity of polymer present inside the catalyst, so the calculations were only indicative. Good mixing will both (i) favor rapid distribution of the polymer feed over the catalyst and (ii) reduce any mass-transfer to the escape of products from the catalyst surface. Furthermore, changing the fluidising flow rate influences the product distribution. At low flow rates (high contact times for primary products), secondary products are
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Y.-H. Lin, M.-H. Yang / Applied Catalysis A: General 328 (2007) 132–139 Table 5 Product distributions shown from RCat-c1 catalysed degradation of PE/PP polymer waste at different ratios of polymer to catalyst (reaction temperature = 390 8C, catalyst particle size = 75–180 mm, fluidising N2 rate = 600 ml min1, and total time of collection = 30 min) Degradation results
10
20
30
40
60
84.1 4.6 10.4 7.9 2.5 0.9
83.2 4.8 11.0 8.6 2.4 1.0
81.5 5.1 12.2 9.8 2.4 1.2
80.5 5.5 12.4 10.4 2.0 1.6
79.6 6.0 12.6 10.7 1.9 1.8
Distribution of gaseous products (wt% feed) P 26.9 27.6 Hydrocarbon gases ( C1–C4) P Gasoline ( C5–C9) 57.2 55.6
27.5 54.0
27.6 52.9
27.4 52.2
Yield (wt% feed) Gaseous Liquida Residueb Involatile residue Coke BTXc
Fig. 3. Comparison of hydrocarbon yields as a function of time at different fluidisation gas for the degradation of PE/PP polymer waste over RCat-c1 catalyst (reaction temperature = 390 8C, catalyst particle size = 75–180 mm, polymer to catalyst ratio = 30 wt%).
observed with increased amounts of coke precursors (BTX) although the overall degradation rate is slower as shown by increasing amounts of partially depolymerised products (Table 4). As the polymer to catalyst ratio increases, the possibility of PE/PP polymer waste adhesion to the reactor wall increases as the amount of unreacted polymer waste in the reactor rises. The total product yield after 20 min showed only a slight downward trend even after a sixfold increase in added polymer waste. This can be attributed to the sufficient cracking ability of RCat-c1 and excellent contact between polyolefin waste and catalyst particles. As more PE/PP polymer was added, lower C5–C9 gasoline yields but higher liquid yields and involatile products were observed (Table 5). More BTX (coke precursor) was produced but increasing the PE/PP polymer waste to catalyst ratio had only virtually no effect on C1–C4 hydrocarbon gases production. The amount of RCat-c1 used in the degradation of
Ratio of polymer to catalyst (wt%/wt)
a b c
Condensate in condenser and captured in filter. Coke and involatile products. Benzene, toluene and xylene.
PE/PP polymer waste remained constant and, therefore, as more waste polymers was added to the reactor then fewer catalytic sites per unit weight of catalyst were available for cracking. The overall effect of increasing the polymer to catalyst ratio from 0.1:1 to 0.6:1 on the rate of hydrocarbon generation was small but predictable (Fig. 4). Both the carbon number distribution of the products of PE/ PP polymer cracking at 390 8C over the various catalysts used in this study (Table 6) and the nature of the product distribution were found to vary with the catalyst used. Overall, the bulk of the products observed with these acidic cracking catalysts (RCat-c1, ZSM-5, HUSY and SAHA) were in the gas phase with less than 5 wt% liquid collected. The highest level of unconverted polymer was observed with silicalite, while the highest coke yields (4.9 wt%) were observed with HUSY. As shown in Table 6, the yield of volatile hydrocarbons for zeolitic catalysts (ZSM-5 HUSY) gave higher yield than used FCC
Table 4 Product distributions shown from RCat-c1 catalysed degradation of PE/PP polymer waste at different fluidising N2 rates (reaction temperature = 390 8C, catalyst particle size = 75–180 mm, polymer to catalyst ratio = 30 wt%, and total time of collection = 30 min) Degradation results Yield (wt% feed) Gaseous Liquida Residueb Involatile residue Coke BTXc
fluidizing N2 rates (ml/min) 900 750 600 450
300
84.7 5.1 9.7 7.6 2.3 0.5
83.5 4.9 10.8 8.5 2.3 0.8
81.5 5.1 12.2 9.8 2.4 1.2
80.9 4.6 12.9 10.5 2.5 1.6
79.8 4.3 13.8 11.2 2.6 2.1
Distribution of gaseous products (wt% feed) P 29.8 28.9 Hydrocarbon P gases ( C1–C4) Gasoline ( C5–C9) 54.9 54.6
27.5 54.0
26.7 54.2
26.3 53.5
a b c
Condensate in condenser and captured in filter. Coke and involatile products. Benzene, toluene and xylene.
Fig. 4. Comparison of hydrocarbon yields as a function of time at different ratios of polymer to catalyst for PE/PP polymer waste degradation over RCat-c1 catalyst (reaction temperature = 390 8C, catalyst particle size = 75–180 mm, rate of fluidisation gas = 600 ml min1).
Y.-H. Lin, M.-H. Yang / Applied Catalysis A: General 328 (2007) 132–139 Table 6 Summary of products of PE/PP polymer waste degradation over various commercial cracking catalysts (reaction temperature = 390 8C, fluidising N2 rate = 600 ml min1, polymer to catalyst ratio = 30 wt%, and total time of collection = 30 min) Degradation results
Catalyst type RCat-c1 USY ZSM-5 SAHA Silicalite
Yield (wt% feed) Gaseous Liquida Residue b Involatile residue Coke BTXc
88.1 89.6 3.4 3.5 8.7 5.0 2.9 3.1 4.9 1.9 0.7 1.7
83.1 3.7 12.9 10.5 2.4 0.3
18.3 1.6 79.9 76.7 3.2 0.2
Distribution of gaseous products (wt% feed) P 33.4 56.1 Hydrocarbon gases ( C1–C4) 27.5 P Gasoline ( C5–C9) 54.0 54.7 33.5
30.2 52.9
12.2 6.1
a b c
81.5 5.1 12.2 9.8 2.4 1.2
Condensate in condenser and captured in filter. Coke and involatile products. Benzene, toluene and xylene.
commercial catalyst (RCat-c1) and non-zeolitic catalysts (RCat-c1 SAHA) and the highest was obtained for ZSM-5 (89.6 wt%). The differences in the product distributions between those catalysts can be seen with ZSM-5 producing a much more C1–C4 hydrocarbon gases (56 wt%) than RCat-c1, HUSY and SAHA catalysts. Some similarities were observed between RCat-c1 and SAHA with C1–C4 and C5–C9 yields, which were approximately 28–30 and 53–54 wt%, respectively. The rate of gaseous hydrocarbon evolution further highlights the slower rate of degradation over silicalite catalyst as shown in Fig. 5 when comparing all catalysts under identical conditions. The results of the products of PE/PP mixture degradation reflect the differing cracking effect of RCat-c1 catalyst compared with the zeolite and non-zeolitic materials. The maximum rate of generation was observed after 2 min with the zeolite catalysts whereas the maximum was observed after 3 min with RCat-c1 and SAHA. The RCat-c1 catalyst used in this study is a spent equilibrium catalyst obtained from a
Fig. 5. Comparison of hydrocarbon yields as a function of time for the catalytic degradation of PE/PP polymer waste at 390 8C over different catalysts (polymer to catalyst ratio = 30 wt%, rate of fluidisation gas = 600 ml min1).
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commercial FCC unit with different levels of metal contamination and a mixture of Y zeolite, a silica–alumina matrix and binder, which indicates that a high amount of catalyst of zero market value can be used, mainly when compared with FCC processes working with quite higher plastic/catalyst ratios. The use of this reaction system coped with a spent FCC equilibrium catalyst can be a better option since it can gives a good conversion compared with silicalite, and even its activity is lower than that of the zeolites (ZSM-5 and USY) and silica– aluminas (SAHA), this can be compensated by increasing the catalyst to PP ratio. As shown in Table 6, the production distribution for the catalytic degradation of PE/PP at 390 8C with zeolites (HUSY and ZSM-5) and SAHA are presented and compared that of the spent RCat-c1 catalyst. The result appears that the feed can be cracked at or close to the external surface of the catalysts and therefore, the controlling catalytic parameters will be not only the total number of acid sites but also the number of accessible ones. It can be seen there that differences of yields of conversion and product selectivity are considerable, supporting the suitability of using spent commercial catalyst from FCC units can be a better approach for the catalytic recycling of polymer waste. 3.3. Variation of product stream with product selectivity and time P P Equilibrium ratios of i-butene/ butenes (i-C4=/ C4=) and i-butane/n-butane (i-C4/n-C4) were predicted using Gibbs free energy minimisation on the PRO/II package for the temperatures used experimentally and are presented alongside the corresponding experimental results in Table 7. The i-butene/ P butenes 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 reused FCC commercial equilibrium catalyst (RCat-c1), and their ratio is primarily equilibrium controlled. 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 iso-butane would be expected. Polymer cracking is known to proceed over acidic catalysts by carbocation mechanisms, where the initially formed ions undergo chain reactions via processes, such as scission or b-scission and isomerisation and hydrogen-transfer alkylation and oligomerisation, to yield typically smaller cracked products. Much higher i-C4/n-C4 have been observed in PE/PP cracking for RCat-c1 catalyst compared to zeolites, probably because in the absence of the constraints of the zeolitic structures the formation bulky bimolecular reaction intermediates is not restricted. As also can be seen in Table 7, the observed ibutane/n-butane ratios at 390 8C are well above calculated equilibrium values consistent with the cracking of long chain hydrocarbon molecules to yield iso-butylcarbenium ions which provide a source for i-butane, via hydrogen transfer, or i-butene. The selectivity could be varied by changes in different operating conditions used in this study. Further evidence of the increase in the reaction of bimolecular
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Table 7 Influence of reaction conditions on product selectivity for the catalysed degradation of PE/PP polymer waste over RCat-c1 catalyst: experimental and predicted equilibrium results Ratio
Reaction conditions a Temperatureb (8C)
P i-butene/Pbutenes i-butene/ butenes e i-butane/n-butane i-butane/n-butane e P P olefins/ paraffinsf a b c d e f
N2 rate d(ml min1)
P/C ratioc (wt%)
330
390
450
10
30
60
300
600
900
0.55 0.54 3.31 1.04 4.46
0.49 0.50 2.52 0.89 3.85
0.44 0.45 1.85 0.77 2.82
0.56
0.52
0.54
0.45
0.50
0.57
3.44
2.52
2.35
2.27
2.52
3.41
3.55
3.87
3.37
3.74
3.96
4.23
Represents a series of runs with different range of reaction temperature, polymer waste to catalyst ratio and fluidising rate. Polymer to catalyst ratio = 30 wt% and 600 ml min1 N2 fluidising rate. Reaction temperature = 390 8C and fluidising N2 = 600 ml min1. Polymer to catalyst ratio = 30 wt% and reaction temperature = 390 8C. Predicted equilibrium data. Denotes the ratio of the sum all olefinic to paraffinic products.
hydrogen transfer,P in the experimental range, was seen in the P lowering of the olefin/ paraffin (o/p = 4.46 at 330 8C versus o/p = 2.82 at 450 8C) and i-butane/n-butane (i-C4/nC4 = 3.31 at 330 8C versus i-C4/n-C4 = 1.85 at 450 8C) ratios as temperature increases. At fast flow rates, primary cracking products P are favoured as evidenced by the increasing ratios of P i-butene/ butenes (i-C4=/ PC4= = 2.27 in 300 ml min1 N2 = fluidising rate versus i-CP C4=P = 3.41 in 900 ml min1 N2 4 / fluidising rate) and olefin/ paraffin (o/p = 3.74 in 300 ml min1 N2 fluidising rate versus o/p = 4.23 in 900 ml min1 N2 fluidising rate).
Rapid variation in the product stream of RCat-c1 and HUSY catalysts was observed (Fig. 6) when the spot samples, taken during the course of the reaction, were analysed. The transient change in the amount of volatile hydrocarbon products 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 post-use RCat-c1 catalyst containing cracking activity with bimodal pore structures, which is composed of both the micropore of zeolite and the mesopore of silica–alumina used in the FCC catalyst matrix, may allow bulky 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 large pore openings and internal supercages. In contrast, ZSM-5 is resistant to coking when coke builds up on outersurface and the product stream remains essentially unchanged, whereas the weakness and lower density of the acid sites in SAHA along with the increased tolerance to coke in the amorphous structure is most likely the reason for the lack of variation in the product stream over this catalyst. 4. Conclusions
P Fig. 6. Some products of iso-butene (i-C4), butanes (tot P of main hydrocarbon = = C4 ) and pentanes (tot C5 ) as a function of time for mixture of PE/PP polymer waste degradation over (a) RCat-c1, (b) HUSY, (c) ZSM-5, and (d) SAHA cracking catalysts.
A laboratory catalytic fluidised bed reactor has been used to obtain a range of volatile hydrocarbons by catalytic degradation of polyolefin waste in the temperature range 330–450 8C. The catalytic degradation of mixture of PE/PP polymer waste over post-use FCC commercial catalyst performed in fluidised-bed reactor was shown to be a useful method for the production of potentially valuable hydrocarbons. From a practical point of view, the use spent equilibrium catalyst from FCC units can be the most adequate solution. Silicalite give very low conversions of polymer waste to volatile hydrocarbons compared with reused FCC commercial catalyst (RCat-c1) under the same reaction conditions. Product distributions with RCat-c1 catalyst contained more olefinic materials in the range of C3–C6 (about 53 wt% at 390 8C). Experiments carried out with RCat-c1 catalyst gave good yields
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of volatile hydrocarbons with differing selectivity in the final products dependent on reaction conditions. The selectivity could be further influenced by changes in operating conditions; in particular, olefins and iso-olefins were produced by low temperatures and short contact times. It is concluded that a post-use catalyst system under appropriate conditions the resource potential of polymer waste can be economically recovered and also can provide an alternative practically to solve a major environment problem. Acknowledgements The authors would like to thank the National Science Council (NSC) of the Republic of China (ROC) for financial support (NSC 95-221 1-E-244-013). Thanks also are due to Professor M.D. Ger and Dr. C.-M. Chiu for samples of ASA, spent FCC commercial catalyst and surface area/pore size measurements. References [1] N.C. Billingham, Polymers and the Environment, Gerald Scott, Royal Society of Chemistry, London, 1999. [2] J. Brandrup, M. Bittner, W. Michaeli, G. Menges, Recycling and Recovery of Plastics, Carl Hanser Verlag, Munich, New York, 1996. [3] P.N. Sharratt, Y.H. Lin, A. Garforth, J. Dwyer, Ind. Eng. Chem. Res. 36 (1997) 5118.
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[4] Y.H. Lin, P.N. Sharratt, A.A. Garforth, J. Dwyer, Energy Fuels 12 (1998) 767. [5] A.A. Garforth, Y.H. Lin, P.N. Sharratt, J. Dwyer, Appl. Catal. A: Gen. 169 (1998) 331. [6] S.Y. Lee, J.H. Yoon, J.R. Kim, D.W. Park, Polym. Degrad. Stab. 74 (2) (2001) 297. [7] A. Dawood, K. Miura, Polym. Degrad. Stab. 76 (1) (2002) 45. [8] Q. Monos, I.Y. Yusof, N.H. Gangas, N.K. Papayannakos, Energy Fuels 16 (2002) 485. [9] A. Marcilla, A. Gomez, A. Reyes-Labrta, A. Giner, Polym. Degrad. Stab. 80 (2003) 233. [10] Z. Gao, K. Ksuyoshi, I. Amasaki, M. Nakada, Polym. Degrad. Stab. 80 (2003) 269. [11] A. Durmus, S.N. Koc, G.S. Pozan, A. Kasgoz, Appl. Catal. B: Environ. 61 (2005) 316. [12] H. Ohkita, R. Nishiyama, Y. Tochihara, T. Mizushima, N. Kakuta, Y. Morioka, Y. Namiki, H. Katoh, H. Sunazyka, R. Nakayama, T. Kuroyanagi, Ind. Eng. Chem. Res. 32 (1993) 3112. [13] G. Puente, U. Sedran, Appl. Catal. B: Environ. 19 (3) (1998) 305. [14] Y. Uemichi, J. Nakamura, T. Itoh, sugioka, A.A. Garforth, J. Dwyer, Ind. Eng. Chem. Res. 38 (1999) 385. [15] Y.H. Lin, W.H. Hwu, M.D. Ger, T.F. Yeh, J. Dwyer, J. Mol. Catal. A: Chem. 171 (2001) 143. [16] Y.H. Lin, M.H. Yang, T.F. Yeh, M.D. Ger, Polym. Degrad. Stab. 86 (2004) 121. [17] Y.H. Lin, H.Y. Yen, Polym. Degrad. Stab. 89 (2005) 101. [18] Y.H. Lin, M.H. Yang, J. Mol. Catal. A: Chem. 231 (2005) 113. [19] G. Puente, C. Klocker, U. Sedran, Appl. Catal. B: Environ. 36 (4) (2002) 279. [20] C.C. Salvador, C. Avelino, Appl. Catal. B: Environ. 25 (2000) 151. [21] K.H. Lee, N.S. Noh, D.H. Shin, Y.H. Seo, Polym. Degrad. Stab. 78 (2002) 539.
Catalytic pyrolysis of polyolefin waste into valuable hydrocarbons over reused catalyst from refinery FCC units Y.-H. Lin *, M.-H. Yang Department of Chemical and Biochemical Engineering, Kao Yuan University, 821 Kaohsiung, Taiwan, ROC Received 9 March 2007; received in revised form 18 May 2007; accepted 28 May 2007 Available online 6 June 2007
Abstract A commingled polyethylene/polypropylene (PE/PP) mixture was pyrolysed over used fluid catalytic cracking (FCC) commercial equilibrium catalyst (RCat-c1) using a laboratory fluidised-bed reactor operating isothermally at ambient pressure. The conversion at 390 8C for RCat-c1 catalyst (81.5 wt%) gave much higher yield than silicate (only 18.3 wt%). Greater product selectivity was observed with RCat-c1 as a post-use catalyst with about 53 wt% olefins products in the C3–C6 range. The selectivity could be further influenced by changes in reaction conditions. Valuable hydrocarbons of olefins and iso-olefins were produced by low temperatures and short contact times used in this study. It is demonstrated that the use of spent FCC commercial catalyst and under appropriate reaction conditions can have the ability to control both the product yield and product distribution from polymer degradation, potentially leading to a cheaper process with more valuable products. # 2007 Elsevier B.V. All rights reserved. Keywords: Pyrolysis; Polyolefin; Catalyst; FCC; Selectivity
1. Introduction Polymer waste can be regarded as a potential source of chemicals and energy. Methods for recycling polymer waste have been developed and new recycling approaches are being investigated [1]. The production of liquid hydrocarbons from polymer degradation would be beneficial in that liquids are easily stored, handled and transported. However, these aims are not easy to achieve. An alternative strategy is that of chemical recycling, known as feedstock recycling or tertiary recycling, which has attracted much interest recently with the aim of converting waste polymers into basic petrochemicals to be used as chemical feedstock or fuels for a variety of downstream processes [2]. The most widely used conventional chemical methods for waste polymer treatment are pyrolysis and catalytic reforming. Since thermal degradation demands relatively high temperatures and its products require further processing for their quality to be upgraded, catalytic degradation of polymer waste offers considerable advantages. Suitable catalysts have the ability to control both the product yield and
* Corresponding author. Tel.: +886 7 6077777; fax: +886 7 6077788. E-mail address: [email protected] (Y.H. Lin). 0926-860X/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2007.05.039
product distribution from polymer degradation as well as to reduce significantly the reaction temperature [3–5]. Some polymeric materials, e.g., polystyrene, can be decomposed thermally in high yields to the monomers. However, this is not true for polyethylene (PE) or polypropylene (PP), which are among the most abundant polymeric waste materials typically making up 60–70% of municipal solid waste. 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. Studies of the effects of catalysts on the catalytic degradation of polymer has been performed by contacting melted polymer with catalyst in fixed bed reactors [6–8], heating mixtures of polymer and catalyst powders in reaction vessels [9–11] and passing the products of polymer pyrolysis through fixed bed reactors containing cracking catalysts [12–14]. Catalytic pyrolysis has been carried out by considering a variety of catalysts with little emphasis on the reactor design, with only simple adiabatic batch and fixed bed reactors being used. However, the configuration of the pyrolysis-reforming reactors poses serious engineering and economics constraints. 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
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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. Therefore, a fluidised-bed reactor has been used to study catalytic cracking of polymer waste by limiting the contact between primary volatile products and the catalyst/polymer mixture [15–18]. The catalytic degradation of polymeric materials has been reported for a range of catalysts centred around the active components in a range of different model catalysts, such as amorphous silica–aluminas, zeolites Y, ZSM-5 and various acidic catalysts and particularly the new family of MCM materials [6–18]. However, these catalysts have been used that even if performing well, they can be unfeasible from the point of view of practical use due to the cost of manufacturing and the high sensitivity of the process to the cost of the catalyst. An economical improvement of processing the recycling via catalytic cracking would operate in mixing the polymer waste with fluid catalytic cracking (FCC) commercial catalysts. These catalysts increase significantly the commercial potential of a recycling process based on catalytic degradation, as cracking catalysts could cope with the conversion of plastic waste co-fed into a refinery FCC unit [19–21]. Therefore, a more interesting approach is that of adding polymer waste into the FCC process, under suitable process conditions with the use of zero value of spent FCC catalysts, a large number of waste plastics can be economically converted into valuable hydrocarbons. The objective of this work is to explore the capabilities of a catalytic fluidised bed reaction system using spent FCC commercial catalysts for the study of product distribution and selectivity on the catalytic degradation of a post-consumer polyolefin waste (PP/PE mixture), and specifically for identification of suitable reaction conditions for enhancing the potential benefits of catalytic polymer recycling. 2. Experimental 2.1. Materials and experimental procedures The catalysts employed are described in Table 1. All the catalysts were pelleted, crushed and sieved to give particle sizes ranging from 75 to 180 mm. The catalyst (0.25–0.3 g) was then
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dried by heating in flowing nitrogen (50 ml min1) to 120 8C at 60 8C h1. After 2 h the temperature was increased to 520 8C at a rate of 120 8C h1 to active the catalyst for 5 h. In the case of the spent FCC catalyst, it could contain a certain amount of coke, and air was swept and maintained for 3 h at 600 8C to burn it off before activating. The polymer mixture used in this study was obtained from post-consumer plastic waste stream in South-Taiwan with the component of polyethylene (67 wt% PE = 40 wt% HDPE + 27 wt% LDPE), polypropylene (33 wt% PP). High purity nitrogen was used as the fluidising gas and 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 the incipient to fast fluidisation. Before catalytic pyrolysis experiments were started, several fluidisation runs were performed at ambient temperature and pressure to select: (i) suitable particle sizes (both catalyst and polymer waste) and (ii) optimise the fluidising gas flow rates to be used in the reaction. The particle size of both catalyst (75–180 mm) and polymer (75–250 mm) were chosen to be large enough to avoid entrainment but not too large as to be inadequately fluidised. High flow rates of fluidising 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 fluidisation and considerable entrainment of fines. 2.2. Experimental procedures and product analysis A process flow diagram of the experimental system is given elsewhere [15] and shown schematically in Fig. 1. 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 pre-heated nitrogen below the distributor and catalyst particles in the reaction volume could be effectively controlled to within 1 8C. 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 fluidised catalyst bed. Thus, commingled polyethylene/ polypropylene (PE/PP) polymer particles were purged under
Table 1 Catalysts used in the catalytic degradation of post-consumer PE/PP polymer waste Catalyst
Si/Al
Surfacea area (cm2/g) b
RCat-c1 ZSM-5 HUSY SAHA Silicalite a b c d e f g
2.1 17.5 13.6 3.6 >1000
Pore size (nm)
BET
Micro
External
147 426 547 268 362
103 263 429 21 297
44 128 118 247 65
–c 0.55 0.51 0.74 3.28f 0.55 0.51
Aciditya (mmol Py/g catalyst) Brwnsted
Lewis
4.8 33.4 33.4 0.1 –g
3.7 19.8 19.8 22.6
Commercial name
Equilibuium catalystsd ZSM-5 zeolitee Ultrastabilised Y zeolited Amorphous silica aluminad Synthesized in-house
Measured by IR spectroscopy and adsorption–desorption of pyridine (Py) at 623 K for the amount of adsorbed Py (mmol/g) on both acid sites of catalysts. Total surface area (BET). The catalyst was a mixture of zeolite, a silica–alumina matrix and binder, not determined. Chinese Petroleum Corp., CPC, Taiwan, ROC. BP Chemicals, Sunbury-on-Thames, UK. Single-point BET determined. The sodium form of siliceous ZSM-5 with very few or no catalytically active sites, not determined.
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products and coke. Catalytic pyrolysis products (P) are grouped together as hydrocarbon gases (
nitrogen into the top of the reactor and allowed to drop freely into the fluidised bed at t = 0 min. At sufficiently low polymer/ catalyst ratios the outside of the catalyst particles are not wet with polymer, so the catalyst particles move freely. Volatile products leaving the reactor were passed through a glass-fibre filter to capture catalyst fines, followed by an iceacetone 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. The bags were replaced 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 analysed at various reaction times (t = 1, 2, 3, 5, 8, 12, 15, and 20 min). The rate (Rgp, wt% min1) of hydrocarbon production of gaseous products collected by automated sample system in each run was defined by the relationship:
Rgp
hydrocarbon production rate of gaseous products in each spot run ðg=minÞ 100 ¼ total hydrocarbon production of gaseous products over the whole spot runs ðgÞ
Gaseous hydrocarbon products were analysed using a gas chromatograph 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 ionisation 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 was used to help identify and quantify the gaseous products. The amount and nature of the residues was determined by thermogravimetric analysis (TGA). The remaining solid deposited on the catalyst after the polymer degradation was deemed ‘‘residues’’ and contained involatile
P ðgÞ 100 polymer fed ðgÞ
3.1. Degradation of polyolefin waste over RCat-c1 and silicalite Product distributions for PE/PP polymer waste degradation over silicalite (Si/Al > 1000) in the 330–450 8C range is summarised in Table 2. At temperatures below 390 8C, a large Table 2 Summary of products of PE/PP polymer waste over silicalite catalyst (fluidising N2 rate = 600 ml min1, catalyst particle size = 75–180 mm, polymer to catalyst ratio = 30 wt% and total time of collection = 60 min) Degradation results
Yield (wt% feed) Gaseous Liquida Residueb BTXc
Temperature (8C) 330
360
390
420
450
7.0 0.7 92.2 0.1
11.9 1.4 86.6 0.1
18.3 1.6 79.9 0.2
28.8 2.1 68.9 0.2
38.6 2.8 58.2 0.4
8.1 n.d n.d 0.2 0.1 3.3 0.6 3.9
12.2 n.d n.d 0.2 0.5 4.7 0.9 5.9
17.9 –d 0.1 0.3 0.8 5.9 1.2 9.6
23.2 0.1 0.1 0.5 0.9 7.8 1.5 12.3
3.8 –d 2.3 –d 1.2 n.d 0.3 n.d –d n.d
6.1 –d 3.6 –d 1.9 0.2 0.3 n.d 0.1 –d
10.9 0.3 4.8 0.1 3.6 0.3 1.4 –d 0.3 0.1
15.4 0.5 6.2 0.2 4.8 0.6 1.9 0.3 0.6 0.3
Distribution of gaseous products (wt% feed) P 4.5 Hydrocarbon gases ( C1–C4) C1 n.d n.d C2 C2= n.d C3 n.d 2.3 C3= C4 n.d C4= 2.2 P 2.5 Gasoline ( C5–C9) n.d C5 C5= 1.3 C6 n.d 1.0 C6= C7 n.d C7= 0.2 C8 n.d –d C8= P C9 n.d
n.d: not detectable. a Condensate in condenser and captured in filter. b Coke and involatile products. c Benzene, toluene and xylene. d Less than 0.01 (wt%).
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Table 3 Summary of products of PE/PP polymer waste degradation over RCat-c1 catalyst (fluidising N2 rate = 600 ml min1, catalyst particle size = 75– 180 mm, polymer to catalyst ratio = 30 wt%, and total time of collection = 30 min) Degradation results
Yield (wt% feed) Gaseous Liquida Residue b Involatile residue Coke BTXc
Temperature (8C) 330
360
390
420
450
78.1 6.7 14.8 12.9 1.9 0.4
79.6 6.9 13.3 11.2 2.1 0.7
81.5 5.1 12.2 9.8 2.4 1.2
83.9 4.2 10.2 7.5 2.7 2.0
85.4 3.0 9.1 6.1 3.0 2.5
22.8 n.d n.d 0.2 1.6 6.7 1.7 12.7
27.5 –d –d 0.1 1.9 8.9 2.5 14.1
29.9 –d –d 0.3 2.3 10.0 2.9 15.4
34.5 –d 0.2 0.4 2.7 11.3 3.2 16.5
56.8 2.3 18.8 3.4 16.1 4.9 6.1 1.3 3.0 3.0 0.7
54.0 2.7 16.5 3.5 13.9 4.6 7.3 1.5 2.7 2.7 1.3
52.7 3.2 16.1 3.3 11.8 3.7 8.1 1.8 2.8 2.8 1.8
50.9 3.5 13.7 4.7 10.3 3.2 8.8 2.0 3.3 3.3 1.5
54.3
53.4
53.3
51.8
Distribution of gaseous products (wt% feed) P 19.5 Hydrocarbon gases ( C1–C4) C1 n.d C2 n.d C2= –d C3 1.0 C3= 5.5 1.8 C4 C4= 11.2 P 58.6 Gasoline ( C5–C9) 1.8 C5 C5= 20.4 C6 3.5 C6= 18.1 3.6 C7 C7= 6.5 C8 1.3 C8= 2.9 2.9 C8= P C9 0.5 P 55.2 Olefins ( C3–C6)
n.d: not detectable. a Condensate in condenser and captured in filter. b Coke and involatile products. c Benzene, toluene and xylene. d Less than 0.01 (wt%).
amount of solid residue, presumably unconverted commingled polymer and high molecular weight degradation products, remained on the silicalite catalyst. The gaseous yield at 390 8C was only 18.3 wt% (Table 2) compared with 81.5 wt% (Table 3) when reused FCC commercial equilibrium catalyst (RCat-c1) was employed. Typically thermal degradation productions were observed with silicalite showing primary cracking products and an even spread of carbon numbers consisting of C3–C6 olefins with some isomerisation C4 and above of aromatics of BTX. At higher temperatures, product streams containing C1–C9 hydrocarbons were produced with gaseous yield 38.6 wt% of PE/PP polymer waste converted at 450 8C. As shown in Table 3, some similar trends in product yields were observed with RCat-c1 catalyst as the reaction temperature was increased. Gaseous and coke yields increased and involatile residues (unreacted or partially reacted PE/PP polymer waste) and liquids decreased. Product distributions with RCat-c1 catalyst contained more olefinic materials in the
Fig. 2. Comparison of hydrocarbon yields as a function of time at different reaction temperatures for the catalytic degradation of PE/PP polymer waste over RCat-c1 catalyst (rate of fluidisation gas = 600 ml min1, catalyst particle size = 75–180 mm, polymer to catalyst ratio = 30 wt%).
range of C3–C6 (about 53 wt% at 390 8C) with minor products, methane and ethane, only detectable at the higher reaction temperatures. The rate of hydrocarbon production as a function of time for PE/PP degradation over RCat-c1 catalyst at different reaction temperatures is compared in Fig. 2 and as expected, faster rates were observed at higher temperatures. At 450 8C, the maximum rate of hydrocarbon production (calculated as described in Section 2.2) was 36 wt% min1 after only 2 min, with all the polymer degraded after approximately 11 min. As the temperature of reaction was decreased, the initial rate of hydrocarbon production dropped and the time for PE/PP to be degraded lengthened. At 330 8C the rate of hydrocarbon production was significantly lower with the polymer being degraded more slowly over 20 min. 3.2. Product stream variation with reaction conditions The effect of reaction conditions including flow rates of fludising gas (270–900 ml min1), ratios of polymer to catalyst feed (0.1:1 to 0.6:1) and catalyst type (RCat-c1) has been investigated. The results shown in Fig. 3 illustrate that for efficient mixture of PE/PP polymer waste degradation good mixing is required, with a dramatic drop-off in the rate of degradation observed only at the lowest fluidising flow used (300 ml min1). After selecting suitable particle parameters, the minimum fluidation velocity of catalyst (Umf), at the different operating conditions was calculated. Fluidising gas velocities in the range 1.5–4 times the value of Umf were used in the course of this work. However, during the experiments, the actual particle density would vary according to the quantity of polymer present inside the catalyst, so the calculations were only indicative. Good mixing will both (i) favor rapid distribution of the polymer feed over the catalyst and (ii) reduce any mass-transfer to the escape of products from the catalyst surface. Furthermore, changing the fluidising flow rate influences the product distribution. At low flow rates (high contact times for primary products), secondary products are
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Y.-H. Lin, M.-H. Yang / Applied Catalysis A: General 328 (2007) 132–139 Table 5 Product distributions shown from RCat-c1 catalysed degradation of PE/PP polymer waste at different ratios of polymer to catalyst (reaction temperature = 390 8C, catalyst particle size = 75–180 mm, fluidising N2 rate = 600 ml min1, and total time of collection = 30 min) Degradation results
10
20
30
40
60
84.1 4.6 10.4 7.9 2.5 0.9
83.2 4.8 11.0 8.6 2.4 1.0
81.5 5.1 12.2 9.8 2.4 1.2
80.5 5.5 12.4 10.4 2.0 1.6
79.6 6.0 12.6 10.7 1.9 1.8
Distribution of gaseous products (wt% feed) P 26.9 27.6 Hydrocarbon gases ( C1–C4) P Gasoline ( C5–C9) 57.2 55.6
27.5 54.0
27.6 52.9
27.4 52.2
Yield (wt% feed) Gaseous Liquida Residueb Involatile residue Coke BTXc
Fig. 3. Comparison of hydrocarbon yields as a function of time at different fluidisation gas for the degradation of PE/PP polymer waste over RCat-c1 catalyst (reaction temperature = 390 8C, catalyst particle size = 75–180 mm, polymer to catalyst ratio = 30 wt%).
observed with increased amounts of coke precursors (BTX) although the overall degradation rate is slower as shown by increasing amounts of partially depolymerised products (Table 4). As the polymer to catalyst ratio increases, the possibility of PE/PP polymer waste adhesion to the reactor wall increases as the amount of unreacted polymer waste in the reactor rises. The total product yield after 20 min showed only a slight downward trend even after a sixfold increase in added polymer waste. This can be attributed to the sufficient cracking ability of RCat-c1 and excellent contact between polyolefin waste and catalyst particles. As more PE/PP polymer was added, lower C5–C9 gasoline yields but higher liquid yields and involatile products were observed (Table 5). More BTX (coke precursor) was produced but increasing the PE/PP polymer waste to catalyst ratio had only virtually no effect on C1–C4 hydrocarbon gases production. The amount of RCat-c1 used in the degradation of
Ratio of polymer to catalyst (wt%/wt)
a b c
Condensate in condenser and captured in filter. Coke and involatile products. Benzene, toluene and xylene.
PE/PP polymer waste remained constant and, therefore, as more waste polymers was added to the reactor then fewer catalytic sites per unit weight of catalyst were available for cracking. The overall effect of increasing the polymer to catalyst ratio from 0.1:1 to 0.6:1 on the rate of hydrocarbon generation was small but predictable (Fig. 4). Both the carbon number distribution of the products of PE/ PP polymer cracking at 390 8C over the various catalysts used in this study (Table 6) and the nature of the product distribution were found to vary with the catalyst used. Overall, the bulk of the products observed with these acidic cracking catalysts (RCat-c1, ZSM-5, HUSY and SAHA) were in the gas phase with less than 5 wt% liquid collected. The highest level of unconverted polymer was observed with silicalite, while the highest coke yields (4.9 wt%) were observed with HUSY. As shown in Table 6, the yield of volatile hydrocarbons for zeolitic catalysts (ZSM-5 HUSY) gave higher yield than used FCC
Table 4 Product distributions shown from RCat-c1 catalysed degradation of PE/PP polymer waste at different fluidising N2 rates (reaction temperature = 390 8C, catalyst particle size = 75–180 mm, polymer to catalyst ratio = 30 wt%, and total time of collection = 30 min) Degradation results Yield (wt% feed) Gaseous Liquida Residueb Involatile residue Coke BTXc
fluidizing N2 rates (ml/min) 900 750 600 450
300
84.7 5.1 9.7 7.6 2.3 0.5
83.5 4.9 10.8 8.5 2.3 0.8
81.5 5.1 12.2 9.8 2.4 1.2
80.9 4.6 12.9 10.5 2.5 1.6
79.8 4.3 13.8 11.2 2.6 2.1
Distribution of gaseous products (wt% feed) P 29.8 28.9 Hydrocarbon P gases ( C1–C4) Gasoline ( C5–C9) 54.9 54.6
27.5 54.0
26.7 54.2
26.3 53.5
a b c
Condensate in condenser and captured in filter. Coke and involatile products. Benzene, toluene and xylene.
Fig. 4. Comparison of hydrocarbon yields as a function of time at different ratios of polymer to catalyst for PE/PP polymer waste degradation over RCat-c1 catalyst (reaction temperature = 390 8C, catalyst particle size = 75–180 mm, rate of fluidisation gas = 600 ml min1).
Y.-H. Lin, M.-H. Yang / Applied Catalysis A: General 328 (2007) 132–139 Table 6 Summary of products of PE/PP polymer waste degradation over various commercial cracking catalysts (reaction temperature = 390 8C, fluidising N2 rate = 600 ml min1, polymer to catalyst ratio = 30 wt%, and total time of collection = 30 min) Degradation results
Catalyst type RCat-c1 USY ZSM-5 SAHA Silicalite
Yield (wt% feed) Gaseous Liquida Residue b Involatile residue Coke BTXc
88.1 89.6 3.4 3.5 8.7 5.0 2.9 3.1 4.9 1.9 0.7 1.7
83.1 3.7 12.9 10.5 2.4 0.3
18.3 1.6 79.9 76.7 3.2 0.2
Distribution of gaseous products (wt% feed) P 33.4 56.1 Hydrocarbon gases ( C1–C4) 27.5 P Gasoline ( C5–C9) 54.0 54.7 33.5
30.2 52.9
12.2 6.1
a b c
81.5 5.1 12.2 9.8 2.4 1.2
Condensate in condenser and captured in filter. Coke and involatile products. Benzene, toluene and xylene.
commercial catalyst (RCat-c1) and non-zeolitic catalysts (RCat-c1 SAHA) and the highest was obtained for ZSM-5 (89.6 wt%). The differences in the product distributions between those catalysts can be seen with ZSM-5 producing a much more C1–C4 hydrocarbon gases (56 wt%) than RCat-c1, HUSY and SAHA catalysts. Some similarities were observed between RCat-c1 and SAHA with C1–C4 and C5–C9 yields, which were approximately 28–30 and 53–54 wt%, respectively. The rate of gaseous hydrocarbon evolution further highlights the slower rate of degradation over silicalite catalyst as shown in Fig. 5 when comparing all catalysts under identical conditions. The results of the products of PE/PP mixture degradation reflect the differing cracking effect of RCat-c1 catalyst compared with the zeolite and non-zeolitic materials. The maximum rate of generation was observed after 2 min with the zeolite catalysts whereas the maximum was observed after 3 min with RCat-c1 and SAHA. The RCat-c1 catalyst used in this study is a spent equilibrium catalyst obtained from a
Fig. 5. Comparison of hydrocarbon yields as a function of time for the catalytic degradation of PE/PP polymer waste at 390 8C over different catalysts (polymer to catalyst ratio = 30 wt%, rate of fluidisation gas = 600 ml min1).
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commercial FCC unit with different levels of metal contamination and a mixture of Y zeolite, a silica–alumina matrix and binder, which indicates that a high amount of catalyst of zero market value can be used, mainly when compared with FCC processes working with quite higher plastic/catalyst ratios. The use of this reaction system coped with a spent FCC equilibrium catalyst can be a better option since it can gives a good conversion compared with silicalite, and even its activity is lower than that of the zeolites (ZSM-5 and USY) and silica– aluminas (SAHA), this can be compensated by increasing the catalyst to PP ratio. As shown in Table 6, the production distribution for the catalytic degradation of PE/PP at 390 8C with zeolites (HUSY and ZSM-5) and SAHA are presented and compared that of the spent RCat-c1 catalyst. The result appears that the feed can be cracked at or close to the external surface of the catalysts and therefore, the controlling catalytic parameters will be not only the total number of acid sites but also the number of accessible ones. It can be seen there that differences of yields of conversion and product selectivity are considerable, supporting the suitability of using spent commercial catalyst from FCC units can be a better approach for the catalytic recycling of polymer waste. 3.3. Variation of product stream with product selectivity and time P P Equilibrium ratios of i-butene/ butenes (i-C4=/ C4=) and i-butane/n-butane (i-C4/n-C4) were predicted using Gibbs free energy minimisation on the PRO/II package for the temperatures used experimentally and are presented alongside the corresponding experimental results in Table 7. The i-butene/ P butenes 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 reused FCC commercial equilibrium catalyst (RCat-c1), and their ratio is primarily equilibrium controlled. 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 iso-butane would be expected. Polymer cracking is known to proceed over acidic catalysts by carbocation mechanisms, where the initially formed ions undergo chain reactions via processes, such as scission or b-scission and isomerisation and hydrogen-transfer alkylation and oligomerisation, to yield typically smaller cracked products. Much higher i-C4/n-C4 have been observed in PE/PP cracking for RCat-c1 catalyst compared to zeolites, probably because in the absence of the constraints of the zeolitic structures the formation bulky bimolecular reaction intermediates is not restricted. As also can be seen in Table 7, the observed ibutane/n-butane ratios at 390 8C are well above calculated equilibrium values consistent with the cracking of long chain hydrocarbon molecules to yield iso-butylcarbenium ions which provide a source for i-butane, via hydrogen transfer, or i-butene. The selectivity could be varied by changes in different operating conditions used in this study. Further evidence of the increase in the reaction of bimolecular
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Table 7 Influence of reaction conditions on product selectivity for the catalysed degradation of PE/PP polymer waste over RCat-c1 catalyst: experimental and predicted equilibrium results Ratio
Reaction conditions a Temperatureb (8C)
P i-butene/Pbutenes i-butene/ butenes e i-butane/n-butane i-butane/n-butane e P P olefins/ paraffinsf a b c d e f
N2 rate d(ml min1)
P/C ratioc (wt%)
330
390
450
10
30
60
300
600
900
0.55 0.54 3.31 1.04 4.46
0.49 0.50 2.52 0.89 3.85
0.44 0.45 1.85 0.77 2.82
0.56
0.52
0.54
0.45
0.50
0.57
3.44
2.52
2.35
2.27
2.52
3.41
3.55
3.87
3.37
3.74
3.96
4.23
Represents a series of runs with different range of reaction temperature, polymer waste to catalyst ratio and fluidising rate. Polymer to catalyst ratio = 30 wt% and 600 ml min1 N2 fluidising rate. Reaction temperature = 390 8C and fluidising N2 = 600 ml min1. Polymer to catalyst ratio = 30 wt% and reaction temperature = 390 8C. Predicted equilibrium data. Denotes the ratio of the sum all olefinic to paraffinic products.
hydrogen transfer,P in the experimental range, was seen in the P lowering of the olefin/ paraffin (o/p = 4.46 at 330 8C versus o/p = 2.82 at 450 8C) and i-butane/n-butane (i-C4/nC4 = 3.31 at 330 8C versus i-C4/n-C4 = 1.85 at 450 8C) ratios as temperature increases. At fast flow rates, primary cracking products P are favoured as evidenced by the increasing ratios of P i-butene/ butenes (i-C4=/ PC4= = 2.27 in 300 ml min1 N2 = fluidising rate versus i-CP C4=P = 3.41 in 900 ml min1 N2 4 / fluidising rate) and olefin/ paraffin (o/p = 3.74 in 300 ml min1 N2 fluidising rate versus o/p = 4.23 in 900 ml min1 N2 fluidising rate).
Rapid variation in the product stream of RCat-c1 and HUSY catalysts was observed (Fig. 6) when the spot samples, taken during the course of the reaction, were analysed. The transient change in the amount of volatile hydrocarbon products 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 post-use RCat-c1 catalyst containing cracking activity with bimodal pore structures, which is composed of both the micropore of zeolite and the mesopore of silica–alumina used in the FCC catalyst matrix, may allow bulky 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 large pore openings and internal supercages. In contrast, ZSM-5 is resistant to coking when coke builds up on outersurface and the product stream remains essentially unchanged, whereas the weakness and lower density of the acid sites in SAHA along with the increased tolerance to coke in the amorphous structure is most likely the reason for the lack of variation in the product stream over this catalyst. 4. Conclusions
P Fig. 6. Some products of iso-butene (i-C4), butanes (tot P of main hydrocarbon = = C4 ) and pentanes (tot C5 ) as a function of time for mixture of PE/PP polymer waste degradation over (a) RCat-c1, (b) HUSY, (c) ZSM-5, and (d) SAHA cracking catalysts.
A laboratory catalytic fluidised bed reactor has been used to obtain a range of volatile hydrocarbons by catalytic degradation of polyolefin waste in the temperature range 330–450 8C. The catalytic degradation of mixture of PE/PP polymer waste over post-use FCC commercial catalyst performed in fluidised-bed reactor was shown to be a useful method for the production of potentially valuable hydrocarbons. From a practical point of view, the use spent equilibrium catalyst from FCC units can be the most adequate solution. Silicalite give very low conversions of polymer waste to volatile hydrocarbons compared with reused FCC commercial catalyst (RCat-c1) under the same reaction conditions. Product distributions with RCat-c1 catalyst contained more olefinic materials in the range of C3–C6 (about 53 wt% at 390 8C). Experiments carried out with RCat-c1 catalyst gave good yields
Y.-H. Lin, M.-H. Yang / Applied Catalysis A: General 328 (2007) 132–139
of volatile hydrocarbons with differing selectivity in the final products dependent on reaction conditions. The selectivity could be further influenced by changes in operating conditions; in particular, olefins and iso-olefins were produced by low temperatures and short contact times. It is concluded that a post-use catalyst system under appropriate conditions the resource potential of polymer waste can be economically recovered and also can provide an alternative practically to solve a major environment problem. Acknowledgements The authors would like to thank the National Science Council (NSC) of the Republic of China (ROC) for financial support (NSC 95-221 1-E-244-013). Thanks also are due to Professor M.D. Ger and Dr. C.-M. Chiu for samples of ASA, spent FCC commercial catalyst and surface area/pore size measurements. References [1] N.C. Billingham, Polymers and the Environment, Gerald Scott, Royal Society of Chemistry, London, 1999. [2] J. Brandrup, M. Bittner, W. Michaeli, G. Menges, Recycling and Recovery of Plastics, Carl Hanser Verlag, Munich, New York, 1996. [3] P.N. Sharratt, Y.H. Lin, A. Garforth, J. Dwyer, Ind. Eng. Chem. Res. 36 (1997) 5118.
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[4] Y.H. Lin, P.N. Sharratt, A.A. Garforth, J. Dwyer, Energy Fuels 12 (1998) 767. [5] A.A. Garforth, Y.H. Lin, P.N. Sharratt, J. Dwyer, Appl. Catal. A: Gen. 169 (1998) 331. [6] S.Y. Lee, J.H. Yoon, J.R. Kim, D.W. Park, Polym. Degrad. Stab. 74 (2) (2001) 297. [7] A. Dawood, K. Miura, Polym. Degrad. Stab. 76 (1) (2002) 45. [8] Q. Monos, I.Y. Yusof, N.H. Gangas, N.K. Papayannakos, Energy Fuels 16 (2002) 485. [9] A. Marcilla, A. Gomez, A. Reyes-Labrta, A. Giner, Polym. Degrad. Stab. 80 (2003) 233. [10] Z. Gao, K. Ksuyoshi, I. Amasaki, M. Nakada, Polym. Degrad. Stab. 80 (2003) 269. [11] A. Durmus, S.N. Koc, G.S. Pozan, A. Kasgoz, Appl. Catal. B: Environ. 61 (2005) 316. [12] H. Ohkita, R. Nishiyama, Y. Tochihara, T. Mizushima, N. Kakuta, Y. Morioka, Y. Namiki, H. Katoh, H. Sunazyka, R. Nakayama, T. Kuroyanagi, Ind. Eng. Chem. Res. 32 (1993) 3112. [13] G. Puente, U. Sedran, Appl. Catal. B: Environ. 19 (3) (1998) 305. [14] Y. Uemichi, J. Nakamura, T. Itoh, sugioka, A.A. Garforth, J. Dwyer, Ind. Eng. Chem. Res. 38 (1999) 385. [15] Y.H. Lin, W.H. Hwu, M.D. Ger, T.F. Yeh, J. Dwyer, J. Mol. Catal. A: Chem. 171 (2001) 143. [16] Y.H. Lin, M.H. Yang, T.F. Yeh, M.D. Ger, Polym. Degrad. Stab. 86 (2004) 121. [17] Y.H. Lin, H.Y. Yen, Polym. Degrad. Stab. 89 (2005) 101. [18] Y.H. Lin, M.H. Yang, J. Mol. Catal. A: Chem. 231 (2005) 113. [19] G. Puente, C. Klocker, U. Sedran, Appl. Catal. B: Environ. 36 (4) (2002) 279. [20] C.C. Salvador, C. Avelino, Appl. Catal. B: Environ. 25 (2000) 151. [21] K.H. Lee, N.S. Noh, D.H. Shin, Y.H. Seo, Polym. Degrad. Stab. 78 (2002) 539.