Applied Catalysis B: Environmental 69 (2007) 145–153
www.elsevier.com/locate/apcatb
Catalytic conversion of commingled polymer waste into chemicals and
fuels over spent FCC commercial catalyst in a fluidised-bed reactor
Y.-H. Lin *, M.-H. Yang
Department of Biochemical Engineering & Graduate Institute of Environmental Polymeric Materials, Kao Yuan University, 821 Kaohsiung, Taiwan, ROC
Received 19 December 2005; received in revised form 29 June 2006; accepted 10 July 2006
Available online 14 August 2006
Abstract
A commingled post-consumer polymer (CPW#1) was pyrolysed over spent fluid catalytic cracking (FCC) commercial catalyst (ECat-1) using a
laboratory fluidised-bed reactor operating isothermally at ambient pressure. The influence of reaction conditions including catalyst, temperature,
ratios of commingled polymer to catalyst feed and flow rates of fluidising gas was examined. The conversion for spent FCC commercial catalyst
(82.7 wt%) gave much higher yield than silicate (only 14.2 wt%) and the highest yield (nearly 87 wt%) was obtained for ZSM-5. Greater product
selectivity was observed with ECat-1 as a recycled catalyst with about 56 wt% olefins products in the C3–C7 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 also 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.
# 2006 Elsevier B.V. All rights reserved.
Keywords: Polymer waste; Fluidised-bed reactor; Catalyst; Pyrolysis; Selectivity
1. Introduction
The recycling of polymer waste is important in the
conservation of resources and the environment [1]. The
destruction of wastes by incineration is prevalent, but is
expensive and often generates problems with unacceptable
emissions. It is also undesirable to dispose of waste plastics by
landfill due to high costs and poor biodegradability. 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 [2]. 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 [3].
Two main chemical recycling routes are the thermal and
catalytic degradation of waste polymers. Thermal cracking or
* Corresponding author. Tel.: +886 7 6077777; fax: +886 7 6077788.
E-mail address: [email protected] (Y.H. Lin).
0926-3373/$ – see front matter # 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcatb.2006.07.005
pyrolysis is a well-known technique and is often used in
petrochemical processing. The pyrolysis of waste polymers is
the thermal decomposition in the absence of oxygen and is
carried out in vessels, shaft kilns, autoclaves, rotary kilns, screw
conveyors or fluidised beds [4–8]. 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 8C and even up to 900 8C. These facts strongly
limit their applicability and especially increase the higher cost
of feedstock recycling for waste plastic treatment. Therefore,
catalytic degradation provides a means to address these
problems [9,10]. 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
reaction temperature, potentially leading to a cheaper process
with more valuable products.
Studies of the effects of catalysts on the catalytic
degradation of polymer has been performed by contacting
melted polymers with catalyst in fixed bed reactors [11–13],
heating mixtures of polymer and catalyst powders in reaction
vessels [14–16] and passing the products of polymer pyrolysis
146
Y.-H Lin, M.-H Yang / Applied Catalysis B: Environmental 69 (2007) 145–153
through fixed bed reactors containing cracking catalysts [17–
19]. 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. The use of fixed beds or adiabatic batch where polymer
and catalyst are contacted directly leads to problems of
blockage and difficulty in obtaining intimate contact over the
whole reactor. Without good contact the formation of large
amounts of residue are likely, and scale-up to industrial scale is
not feasible [20–22]. To compare the polymer cracking
properties of different catalysts, it is preferable to examine
the effects of catalysts without extensive complications due to
reactions of primary cracking products with unconverted
polymer waste by using techniques that minimize such
interactions. Hence, a laboratory fluidised-bed reactor has
been designed and applied to study catalytic degradation of
polymer waste by limiting the contact between primary volatile
products and the catalyst/polymer mixture.
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 [11–25] and particularly the new family of
MCM materials [26–28]. 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. Meanwhile, most studies have mainly
concentrated on the catalytic degradation of pure polymers
[11–31]. A more difficult task is recycling of commingled
post-consumer plastic waste since it consists of not only
hydrocarbons but also nitrogen and sulfur containing mixed
polymers as well as some modified materials. An economical
improvement of processing the recycling via catalytic
cracking would operate in mixing the polymer waste with
fluid catalytic cracking (FCC) commercial catalysts. To date,
the catalyst used in the FCC process comprises 5–40% zeolite
dispersed in a matrix of synthetic silica-alumina, semisynthetic clay-derived gel or natural clay. 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 [18,21]. However, much less is known about
performance of post-use FCC commercial catalysts in the
degradation of post-consumer polymer waste using a
fluidised-bed reactor. A fluidised-bed reactor has been used
for obtaining hydrocarbon products from the catalytic
degradation of different polymers over various model
catalysts (ZSM-5, USY, MOR, ASA and MCM-41) in our
previous studies [25,26]. The objective of the research
outlined in this paper 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 commingled postconsumer polymer, and specifically for identification of
suitable reaction conditions for enhancing the potential
benefits of catalytic polymer recycling.
2. Experimental
2.1. Materials and reaction preparation
The commingled polymer waste (CPW#1) used in this
study was obtained from post-consumer plastic waste streams
of several communities in South-Taiwan named as Kaohsiung
Plastic Recycling Center with the component of polyethylene
(62 wt% PE = 38 wt% HDPE + 24 wt% LDPE), polypropylene (30 wt% PP), polystyrene (7 wt% PS) and with
about 1 wt% poly(vinyl chloride) (PVC) mixtures. Typically,
the content of waste plastic sample tested by elemental
analysis was about 86.24% C, 12.95% H, 0.56% Cl, 0.05% O,
0.07% N and 0.13% S. The catalysts employed are described
in Table 1. Prior to use, 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 dried by heating, in the
reactor, 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. After 5 h at
520 8C, the reactor was cooled to the desired reaction
temperature.
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.
Table 1
Catalysts used in commingled polymer waste (CPW#1) degradation
Catalyst
Si/Al
Surface area (cm2 g1)
Metal
(ppm)
BETa Micropore External V
ECat-1
ND
b
147
Silicalite >1000
103
44
ND
USY
5.7
547
421
126
ZSM-5
ASA
17.5
3.6
391
268
263
21
128
247
a
b
c
d
Commercial
name
Ni
2560 870 Equilibuium
catalysts c
–
Synthesized
in-house
–
Ultrastabilised
Y zeolitec
–
ZSM-5 zeolited
–
Amorphous
silica-alumina c
Total surface area (BET).
Not determined.
Chinese Petroleum Corp., CPC, Taiwan, ROC.
BP Chemicals, Sunbury-on-Thames, UK.
Y.-H Lin, M.-H Yang / Applied Catalysis B: Environmental 69 (2007) 145–153
2.2. Experimental procedures and product analysis
A process flow diagram of the experimental system is given
elsewhere [32] and shown schematically in Fig. 1. The reactor
consists of a 400 mm long pyrex glass tube with a sintered
distributor in the middle section. The tube had an inverted bell
shape and was divided into three parts—an upper section, a
middle section and a lower section. 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 polymer
particles were purged under nitrogen into the top of the reactor
and allowed to drop freely into the fluidised bed at t = 0 min.
After selecting suitable particle parameters, the minimum
fluidisation 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. The
added polymer melts, wets the catalyst surface and is pulled
into the catalyst macropores by capillary action [24]. At
sufficiently low polymer/catalyst ratios (as used in this study)
the outside of the catalyst particles are not wet with polymer, so
the catalyst particles move freely.
147
Volatile products leaving the reactor were passed through a
glass-fibre filter to capture catalyst fines, followed by an ice–
acetone condenser (the ice–water and acetone was used and
gave an approximate temperature of 15 8C) to collect any
condensible liquid product. A de-ionised water trap was placed
in series after the condenser to catch any HCl produced by the
degradation of PVC component. 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 of
hydrocarbon production (Rgp, wt% min1) was defined by the
relationship:
Rgp ¼
hydrocarbon production rate ðg min1 Þ
100
total hydrocarbon product over the whole run 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 HCl in de-ionised water samples were
ayalysed using a Corning pH/ion meter with a chloride
electrode calibrated between 100 and 1000 ppm. A double
junction reference electrode filled with KNO3 with a fixed
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) de-ionised water trap, (8) 16-loop automated sample system, (9) gas bag, (10) GC and (11) digital controller for three-zone furnace.
148
Y.-H Lin, M.-H Yang / Applied Catalysis B: Environmental 69 (2007) 145–153
potential was used in conjunction with the chloride. The
remaining solid deposited on the catalyst after the polymer
degradation was deemed ‘‘ residues’’ and contained involatile
products and coke. The amount and nature of the residues was
determined by thermogravimetric analysis as described elsewhere [33].
3. Results and discussion
The reactor and various units of the collection system were
weighted before and after the runs to determine the mass
balance. Catalytic pyrolysis products (P) are grouped together
as hydrocarbon gases (<C5), gasoline up to C9 (C5–C9), liquids
(condensate in condenser and filter), HCl (trapped in de-ionised
solutions) and residues (coke and products, involatile at
reaction temperature and deposited on catalyst) to enable the
overall pyrolysis processes to be described more easily. A
number of runs were repeated in order to check their
reproducibility. It was found that the experimental error was
within 5%. The term ‘‘yield’’ as used in this paper is defined
by the relationship:
Yield ðwt%Þ ¼
P ðgÞ
100
polymer fed ðgÞ
Due to the high nitrogen flow rates used in this study, it is
difficult to completely recover all the lower molecular weight
material, and this results in some loss in the mass balance. The
mass balances in this paper are a matter still to be resolved fully,
though it is clear that the missing material is not very high
molecular weight material that is unreacted or deposited in the
system. Mass balances of 90 5% were obtained for all
experiments.
3.1. Degradation of CPW#1 over silicalite and spent FCC
commercial catalyst
Product distributions for commingled polymer (CPW#1)
degradation over silicalite (Si/Al > 1000) in the 340–460 8C
range is summarised in Table 2. At temperatures below 400 8C,
a large amount of solid residue, presumably unconverted
commingled polymer and high molecular weight degradation
products, remained on the silicalite catalyst. The gaseous yield
at 400 8C was only 14.2 wt% (Table 2) compared with
82.7 wt% (Table 3) when spent FCC commercial equilibrium
catalyst (ECat-1) was used. The effect of silicalite on gaseous
yield is consistent with the cracking of high density
polyethylene (HDPE) in our previous study [23]. Typically
thermal degradation productions were observed with silicalite
showing primary cracking products (HCl and styrene) and an
even spread of carbon numbers consisting of C3–C6 olefins
products with some isomerisation of BTX. The chlorine (0.5–
0.7 wt%) was chemically separated from the PVC component
and as a hydrochloric acid (HCl) in de-ionised water system. At
higher temperatures, product streams containing C1–C9
hydrocarbons were produced with gaseous yield 34.6 wt% of
polymer converted at 460 8C.
Table 2
Summary of products of CPW#1 polymer degradation 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
HCl
Temperature (8C)
340
370
400
430
460
5.1
0.3
94.1
0.5
9.3
1.2
88.9
0.6
14.2
1.5
83.7
0.6
23.2
1.7
74.4
0.7
34.6
2.8
62.0
0.6
6.3
n.d.
n.d.
0.1
–c
2.5
0.3
3.4
8.8
n.d.
n.d.
0.1
0.3
3.2
0.5
4.7
13.6
n.d.
–c
0.2
0.5
4.1
0.6
8.2
19.5
0.1
0.1
0.3
0.9
5.7
1.2
12.6
2.2
n.d.
1.4
–c
0.6
n.d.
0.2
n.d.
–c
n.d.
4.1
n.d.
2.7
–c
0.8
0.1
0.4
n.d.
0.1
–c
7.8
0.3
3.4
0.1
2.3
0.2
1.2
–c
0.2
0.1
11.1
0.5
5.4
0.2
2.6
0.3
1.4
0.2
0.3
0.2
0.7
0.1
1.1
0.2
1.6
0.2
2.2
0.4
Distribution of gaseous products (wt% feed)
P
3.2
Hydrocarbon gases ( C1–C4)
C1
n.d.
n.d.
C2
C¼
n.d.
2
n.d.
C3
1.4
C¼
3
n.d.
C4
C¼
1.8
4
P
1.3
Gasoline ( C5–C9)
C5
n.d.
C¼
0.8
5
n.d.
C6
C¼
0.4
6
n.d.
C7
C¼
0.1
7
n.d.
C8
C¼
–c
P8
n.d.
C9
Styrene
BTXd
a
b
c
d
0.5
0.1
Liquid: condensate in condenser and captured in filter.
Residue: coke and involatile products.
Less than 0.01 (wt%); n.d.: not detectable.
BTX: benzene, toluene and xylene.
As also can be seen in Table 3, some similar trends in
product yields were observed with spent FCC catalyst (ECat-1)
as the reaction temperature was increased. Gaseous and coke
yields increased and involatile residues (unreacted or partially
reacted CPW#1) and liquids decreased. Product distributions
with ECat-1 catalyst contained more olefinic materials in the
range of C3–C7 (about 56 wt% at 400 8C) with minor products,
methane and ethane, only detectable at the higher reaction
temperatures. The major products of polystyrene cracking over
ECat-1 were styrene at about 3–4 wt% with light aromatics
(such as benzene, toluene, ethyl-benzene, xylenes, etc.) and
smaller chain olefins and paraffins, and with some amount of
unindentified products (unconverted polystyrene and coke
formation over the reaction) deposited on the catalyst. The
results indicate that although the initial cracking of polymer
waste must be confined to the external surface and pore mouths
of the cracking catalysts, the resultant initial cracked products
are then degraded further within the catalyst. The rate of
hydrocarbon production as a function of time for CPW#1
degradation over ECat-1 catalyst at different reaction temperatures is compared in Fig. 2 and, as expected, faster rates
were observed at higher temperatures. At 460 8C, the maximum
rate of hydrocarbon production was 37 wt% min1 after only
Y.-H Lin, M.-H Yang / Applied Catalysis B: Environmental 69 (2007) 145–153
149
Table 3
Summary of products of CPW#1 polymer degradation over ECat-1 catalyst
(fluidising N2 rate = 600 ml min1, catalyst particle size = 75–180 mm, polymer to catalyst ratio = 30 wt% and total time of collection = 20 min)
Degradation results
Temperature (8C)
340
370
400
430
460
80.1
6.9
81.3
6.1
82.7
5.3
84.3
4.7
85.7
3.9
Residue b
Involatile residue
Coke
12.6
10.7
1.9
12.2
10.1
2.1
11.5
9.2
2.3
10.4
7.8
2.6
9.7
6.8
2.9
HCl
0.4
0.4
0.5
0.6
0.7
86.3
90.1
91.5
93.7
92.8
20.8
n.d.
n.d.
0.1
1.6
5.0
1.6
12.5
23.8
n.d.
–c
0.1
1.6
6.7
2.3
13.1
26.5
n.d.
–c
0.2
1.8
7.2
3.0
14.3
28.7
–c
0.1
0.2
1.8
8.2
2.8
15.6
56.3
2.3
18.8
3.4
16.5
4.2
6.7
0.9
3.3
0.2
53.9
2.6
16.4
3.6
13.5
4.5
7.7
1.3
3.2
1.1
51.5
3.5
14.4
3.4
11.6
3.6
8.7
1.9
2.6
1.8
50.6
3.7
13.1
4.2
10.6
3.5
9.4
2.2
2.2
1.7
3.3
0.9
3.6
1.4
4.1
2.2
3.7
2.7
Yield (wt% feed)
Gaseous
Liquida
Mass balance (%)
Distribution of gaseousP
products (wt% feed)
18.2
Hydrocarbon gases ( C1–C4)
n.d.
C1
C2
n.d.
C¼
–c
2
1.2
C3
C¼
3.8
3
1.4
C4
C¼
11.8
4
P
57.7
Gasoline ( C5–C9)
C5
1.9
C¼
20.4
5
3.7
C6
C¼
17.2
6
4.1
C7
5.7
C¼
7
1.5
C8
C¼
2.7
P8
0.5
C9
Styrene
BTXd
a
b
c
d
3.8
0.4
Fig. 2. Comparison of hydrocarbon yields as a function of time at different
reaction temperatures for the catalytic degradation of commingled polymer
(CPW#1) over spent FCC commercial catalyst (ECat-1) (rate of fluidisation
gas = 600 ml min1, catalyst particle size = 75–180 mm and polymer to catalyst
ratio = 30 wt%).
Liquid: condensate in condenser and captured in filter.
Residue: coke and involatile products.
Less than 0.01 (wt%); n.d.: not detectable.
BTX: benzene, toluene and xylene.
2 min with all the polymer degraded after approximately 8 min.
As the temperature of reaction was decreased, the initial rate of
hydrocarbon production dropped and the time for CPW#1
polymer to be degraded lengthened. At 340 8C the rate of
hydrocarbon production was significantly lower with the
polymer being degraded more slowly over 20 min.
3.2. Effect of reaction conditions on CPW#1 degradation
over spent FCC catalyst
The effect of reaction conditions including flow rates of
fludising gas (270–900 ml min1), ratios of commingled
polymer (CPW#1) to catalyst feed (1:1–1:6) and catalyst
type (ECat-1, ZSM-5, USY and ASA) has been investigated
in this paper. The results shown in Fig. 3 illustrate that for
efficient commingled polymer (CPW#1) 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). Furthermore, changing the fluidising flow rate
Fig. 3. Comparison of hydrocarbon yields as a function of time at different
fluidisation gas for the degradation of CPW#1 polymer over ECat-1 catalyst
(reaction temperature = 400 8C, catalyst particle size = 75–180 mm and polymer to catalyst ratio = 30 wt%).
influences the product distribution. At low flow rates (high
contact times for primary products), 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 depolymerised products
(Table 4).
The amount of ECat-1 used in the degradation of CPW#1
polymer 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
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Y.-H Lin, M.-H Yang / Applied Catalysis B: Environmental 69 (2007) 145–153
Table 4
Product distributions shown from ECat-1 catalysed degradation of CPW#1
polymer at different fluidising N2 rates (reaction temperature = 400 8C, catalyst
particle size = 75–180 mm, polymer to catalyst ratio = 30 wt% and total time of
collection = 30 min)
Degradation results
Fluidizing N2 rates (ml min1)
Table 5
Product distributions shown from ECat-1 catalysed degradation of CPW#1
polymer at different ratios of polymer to catalyst (reaction temperature = 400 8C, catalyst particle size = 75–180 mm, fluidising N2 rate = 600 ml min1 and total time of collection = 30 min)
Degradation results
900
750
600
450
300
84.3
4.8
83.3
5.2
82.7
5.3
82.5
4.9
81.6
5.1
10.3
8.2
2.1
10.9
8.5
2.4
11.5
9.2
2.3
12.1
9.8
2.3
12.8
10.3
2.5
0.6
0.6
0.5
0.5
0.5
89.2
91.6
92.6
93.1
92.5
Mass balance (%)
Distribution of gaseousP
products (wt% feed)
28.2
25.8
Hydrocarbon gases ( C1–C4)
P
Gasoline ( C5–C9)
52.1
52.6
Styrene
3.5
3.8
BTXc
0.5
1.1
23.8
53.9
3.6
1.4
24.3
53.4
3.1
1.7
23.5
52.7
3.1
2.3
Yield (wt% feed)
Gaseous
Liquida
Residueb
Involatile residue
Coke
HCl
Mass balance (%)
a
b
c
Liquid: condensate in condenser and captured in filter.
Residue: coke and involatile products.
BTX: benzene, toluene and xylene.
predictable (Fig. 4). As the polymer to catalyst ratio increases,
the possibility of CPW#1 polymer adhesion to the reactor wall
increases as the amount of unreacted polymer waste in the
reactor rises. However, for the work carried out in this paper no
such problems were observed. 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 ECat-1 and excellent contact
between CPW#1 polymer and catalyst particles. As more
CPW#1 was added, lower C5–C9 gasoline yields but higher
liquid yields and involatile products were observed (Table 5).
Fig. 4. Comparison of hydrocarbon yields as a function of time at different
ratios of polymer to catalyst for the degradation of CPW#1 polymer over ECat-1
catalyst (reaction temperature = 400 8C, catalyst particle size = 75–180 mm and
rate of fluidisation gas = 600 ml min1).
Ratio of polymer to catalyst (wt%)
10
20
30
40
60
85.4
4.2
83.9
4.8
82.7
5.3
81.5
6.3
80.3
6.9
Residueb
Involatile residue
Coke
9.7
7.3
2.4
10.8
8.7
2.1
11.5
9.2
2.3
11.8
9.8
1.7
12.3
10.7
1.6
HCl
0.5
0.5
0.5
0.4
0.5
89.4
91.4
92.6
92.5
90.4
Distribution of gaseousP
products (wt% feed)
24.7
24.7
Hydrocarbon gases ( C1–C4)
P
Gasoline ( C5–C9)
56.5
54.4
Styrene
3.4
3.6
BTXc
0.8
1.2
23.8
53.9
3.6
1.4
23.1
52.8
3.5
2.1
24.1
51.2
3.2
2.4
Yield (wt% feed)
Gaseous
Liquida
a
b
c
Liquid: condensate in condenser and captured in filter.
Residue: coke and involatile products.
BTX: benzene, toluene and xylene.
Additionally, more BTX (coke precursor) was produced but
increasing the polymer to catalyst ratio had only virtually no
effect on C1–C4 hydrocarbon gases production. 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. Since the
product distributions will alter over the course of the reaction as a
sequence of lump selection, the distribution calculations can be
considered comparison. The present results indicate that a
generation of secondary reaction and oligomerisation is followed
by a faster formation of unzipped intermediates via thermal and
catalytic cracking reactions, within the different degree of
CPW#1, to give relatively high conversion with similar yields of
C1–C4 hydrocarbons. For this mechanism used in polyethylene
cracking [24], the polymer coating the particles is stated to be
liquid, and for the reactions that occur on the interior pore surface
the situation would seem to be completed. Gaseous products are
forced out to have diffused and produced on the interior surface.
A fuller paper is being developed from the mass and heat transfer
effects on different reaction conditions and the behaviors of
catalyst deactivation as related to the structure of catalysts and
their acid sites.
Both the carbon number distribution of the products of
CPW#1 polymer cracking at 400 8C over ECat-1 catalyst,
zeolitic catalysts (ZSM-5 and USY) and non-zeolitic amorphous silica-alumina (ASA) used in this study and the nature of
the product distribution were found to vary with the catalyst
used. As can be seen in Table 6, the yield of volatile
hydrocarbons for zeolitic catalysts (ZSM-5 USY) gave
higher yield than spent FCC commercial catalyst (ECat-1)
and non-zeolitic catalysts (ECat-1 ASA) and the highest was
Y.-H Lin, M.-H Yang / Applied Catalysis B: Environmental 69 (2007) 145–153
151
Table 6
Summary of products of CPW#1 polymer degradation over various commercial
catalysts (reaction temperature = 400 8C, fluidising N2 rate = 600 ml min1,
polymer to catalyst ratio = 30% (w/w) and total time of collection = 30 min)
Degradation results
Catalyst type
ECat-1 USY ZSM-5 ASA Silicalite
Yield (wt% feed)
Gaseous
Liquida
82.7
5.3
85.5
3.3
86.9
4.7
81.5
4.6
14.2
1.5
11.5
9.2
2.3
10.6
4.7
5.9
7.8
5.1
2.7
13.4
8.3
3.1
83.7
74.2
9.5
0.5
0.6
0.6
0.5
0.6
91.5
89.3
94.5
90.7
93.3
Distribution of gaseousP
products (wt% feed)
31.3
Hydrocarbon
gases
(
C1–C4) 23.8
P
53.9
52.2
Gasoline ( C5–C9)
Styrene
3.6
3.3
BTXc
1.4
0.7
53.4
33.5
3.9
2.1
26.8
52.0
4.2
0.5
8.8
4.1
1.1
0.2
Residue b
Involatile residue
Coke
HCl
Mass balance (%)
a
b
c
Fig. 5. Comparison of hydrocarbon yields as a function of time for the catalytic
degradation of CPW#1 polymer at 400 8C over different catalysts (waste
polymer to catalyst ratio = 30% (w/w) and rate of fluidisation
gas = 600 ml min1).
Liquid: condensate in condenser and captured in filter.
Residue: coke and involatile products.
BTX: benzene, toluene and xylene.
of CPW#1 degradation reflect the differing cracking effect of
ECat-1 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 ECat-1 and ASA.
obtained for ZSM-5 (nearly 86 wt%). Overall, the bulk of the
products observed with these acidic cracking catalysts (ECat-1,
ZSM-5, USY and ASA) were in the gas phase with less than
6 wt% liquid collected. The differences in the product
distributions between those catalysts can be seen with ZSM5 producing a much more C1–C4 hydrocarbon gases (53 wt%)
than ECat-1, USY and ASA catalysts. Some similarities were
observed between ECat-1 and ASA with C1–C4 and C5–C9
yields, which were approximately 24–27 and 50–54 wt%,
respectively. The highest level of unconverted polymer was
observed with ECat-1 and ASA, while the highest coke yields
were observed with USY. 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
3.3. Product stream variation with operating conditions
P
P ¼
Equilibrium ratios of i-butene/ butenes (i-C¼
C4 ) and
4=
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
Table 7
Influence of reaction conditions on product selectivity for the catalysed degradation of CPW#1 polymer over ECat-1 catalyst: experimental and predicted equilibrium
results
Ratio
Reaction conditions a
Temperatureb (8C)
P
i-Butene/ butenes
P
i-Butene/ butenese
i-Butane/n-butane
e
i-Butane/n-butane
P
P
Olefins/ paraffinsf
a
N2 rated (ml min1)
P/C ratioc (wt%)
340
400
460
10
30
60
300
600
900
0.54
0.53
3.78
1.02
4.48
0.50
0.49
2.35
0.87
3.86
0.41
0.44
1.83
0.76
2.94
0.57
0.52
0.54
0.46
0.51
0.59
3.72
3.41
3.66
2.11
2.35
3.46
3.48
3.86
3.61
3.42
3.86
4.37
1
Represents a series of base runs where reaction temperature = 400 8C, 30 (w/w) commingled polymer mixture to catalyst feed and 600 ml min
rate.
b
Polymer mixture to catalyst ratio = 30 wt% and 600 ml min1 N2 fluidising rate.
c
Reaction temperature = 400 8C and fluidising N2 = 600 ml min1.
d
Polymer mixture to catalyst ratio = 30 wt% and reaction temperature = 400 8C.
e
Predicted equilibrium data.
f
Denotes the ratio of the sum all olefinic to paraffinic products.
N2 fluidising
152
Y.-H Lin, M.-H Yang / Applied Catalysis B: Environmental 69 (2007) 145–153
interconversion of butenes are very fast over ECat-1, 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. As can be seen in
Table 7, the observed i-butane/n-butane ratios at 400 8C are
well above calculated equilibrium values consistent with the
cracking of long 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 polypropylene in both medium
and large-pore zeolites have been discussed previously [21].
Much higher i-C4/n-C4 have been observed in polyethylene
cracking for mesoporous catalysts, such as MCM-41 and silicaaluminas, compared to microporous catalysts [25], because in
the absence of the constraints of the zeolitic structures the
formation bulky bimolecular reaction intermediates is not
restricted. The selectivity could be varied by changes in
different operating conditions used in this study. The yield of
smaller cracked products increased with temperature as did the
yield of BTX and coke (Table 3). Further evidence of the
increase in secondary reactions, for example, bimolecular
P
hydrogen transfer, was seen in the lowering of the olefin/
P
paraffin (o/p = 4.48 at 340 8C versus o/p = 2.94 at 460 8C)
and i-butane/n-butane (i-C4/n-C4 = 3.78 at 340 8C versus i-C4/
n-C4 = 1.83 at 460 8C) ratios as temperature increases, in the
experimental range. At fast flow rates (short contact times),
primary cracking products are
P favoured as evidenced
P ¼ by the
increasing ratios of i-butene/ butenes (i-C¼
4 =P C4 = 2.11 in
300 ml min1 N2 fluidising rate versusP
i-C¼
C¼
4= P
4 = 3.46 in
1
900 ml min N2 fluidising rate) and olefin/ paraffin (o/
p = 3.42 in 270 ml min1 N2 fluidising rate versus o/p = 4.37 in
900 ml min1 N2 fluidising rate).
3.4. Comparison with other studies on polymer
degradation over spent FCC catalyst
The results obtained can be compared with some of the results
published for the catalytic degradation of polymer waste using
spent FCC catalysts. This comparison is not straightforward as
reaction conditions are not perfectly matched: catalyst composition, reactor types, particle sizes, internal voidage and the
concentration of inerts will be different, and especially the
polymer types involved are different. The following comparison
should thus only be considered as indicative. Cardona and Corma
[18] using a semi-batch stirred reactor for PP degradation
obtained lower yields of gases: 7–10 wt% (variation with
reaction time) at 380 8C compared with 21 wt% (at 370 8C) in
this work. It seems, although insufficient data are available for
full comparison olefin yields were much lower those in the
present work. Additionally, they had to remove a residue from the
reactor before catalytic processing that represented 10–20 wt%
of the original polymer, making their proposed process rather
waste. Much higher yields of liquids with aromatics (15–
20 wt%) in the 100 of molecular range weight were provided by
Lee et al. [30] from the catalytic cracking of a pure polyolefin
(PE, PP and PS) over spend FCC catalyst in a stirred semi-batch
at 400 8C. The reaction time was relatively long, up to 3 h, and
was presumably set by the relative low polymer degradation rate
at this temperature. Although catalysis has been used, this often
involves thermal cracking of the polymer followed by catalytic
conversion of the degradation products. However, the configuration of the pyrolysis-reforming reactors poses serious
engineering and economics constraints. Problems associated
with blockage and limited polymer/catalyst contact within the
reactor make continuous processing difficult in fixed-bed
reactors. A BP process using a thermal cracking reactor over
fluidised sand with a reaction temperature of 500 8C to crack
mixed polymer waste for yielding a product is range up to C50.
However, this process is estimated to be uneconomic [29].
Another example was the attempt to use a fluidised-bed reactor
containing activated carbon or an iron-loaded carbon. It was also
ineffectual and seems to have little catalytic effects [34]. Again,
relatively high operating temperatures are suggested, with 500–
790 8C being performed. In this work, the fluidised bed has been
shown to have a number of advantages in the pyrolysis of
commingled polymer waste; it is characterised by minimisation
of mass transfer resistance fro product removal, much less prone
to clogging with molten polymer and gives a nearly constant
temperature throughout the reactor.
Data are provided by de la Puente and Sedran [31], using a
riser simulator reactor for the catalytic cracking of the LDPE
dissolved into toluene, obtained gases yield 20 wt% and again
relative high aromatics yields (25 wt%) and coke content (nearly
10 wt%) deposited on the catalyst. However, catalytic pyrolysis
of polymer waste performed in the fluidised-bed reactor used in
this paper was shown to produce valuable hydrocarbons in the
range of C3–C7 carbon number with a high olefinic content.
Moreover, 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. Although spend FCC catalysts were used in some
trails [18,30,31] the results are not feasible for scaling-up. In the
presence of the spent FCC commercial catalyst at 400 8C used in
this work, conversion post-consumer polymer waste to volatile
hydrocarbons in the catalytic fluidised-bed reactor was more than
82 wt% of feed in 20 min, while silicalite yielded less than
15 wt% of feed after 60 min. Also, from an economic point of
view and taking into account the reaction conditions needed, a
process could be the most favourable solution if the catalyst cost
is practically zero for the catalytic conversion of polymer waste.
Therefore, a post-use catalyst system with both post-consumer
polymer wastes and reaction conditions that has been used to
address the recycling desire to see an alternative to solve a major
environment problem further strengthens the interesting results
of this research.
4. Conclusions
Polymer waste can cause serious pollution but also could be
regarded as a cheap and abundant source of chemicals and
energy. A laboratory catalytic fluidised-bed reactor has been
used to obtain a range of volatile hydrocarbons by catalytic
Y.-H Lin, M.-H Yang / Applied Catalysis B: Environmental 69 (2007) 145–153
degradation of post-consumer polymer waste in the temperature range 340–460 8C. The catalytic degradation of commingled polymer mixture (PE/PP/PS/PVC) over spent FCC
commercial catalyst performed in fluidised-bed reactor was
shown to be a useful method for the production of potentially
valuable hydrocarbons.
The sodium form of siliceous ZSM-5, silicalite, containing
very few or no catalytically active sites, give very low
conversions of polymer waste to volatile hydrocarbons
compared with spent FCC catalyst (ECat-1) under the same
reaction conditions. Product distributions with ECat-1 catalyst
contained more olefinic materials in the range of C3–C7 (about
56 wt% at 400 8C). Experiments carried out with ECat-1
catalyst gave good yields 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 under appropriate
conditions the resource potential of polymer waste can be
recovered.
Acknowledgements
The authors would like to thank the National Science
Council (NSC) of the Republic of China (ROC) for financial
support (NSC 94-2211-E-244-008). In addition, thanks also are
due to Dr. F.-S. Lee and Mr. C.-M. Chiu for samples of ASA,
spent FCC commercial catalyst and surface area/pore size
measurements and to Professor M.D. Ger for helpful discussion
in the preparation of this paper.
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