Investigation of catalytic cracking of waste polyethylene
Norbert Miskolczi*, László Bartha*, Gyula Deák*, Béla Jóvér**, Dénes Kalló***
*
University of Veszprém, Department of Hydrocarbon and Coal Processing, Hungary
** MOL Hungarian Oil and Gas PLC, R&D
*** Chemical Research Center Hungarian Academy of Sciences, Hungary
Abstract
Reduction of polymer wastes is important both from environmental and
energetic aspects. The degradation of polymer wastes is one possibility of industrial
utilizations. Hydrocarbon products of different boiling point ranges can be obtained in
this way. The decomposition processes could be enhanced by catalysts having notable
effects on the rate of the decomposition processes, and also on the structure and
properties of the reaction products.
Degradation of different types of waste polyethylenes was studied in a batch
reactor in absence and presence of catalysts. The influence of the catalysts on the
degradation products, i.e. on the amount and composition of gas, liquid and residue
components was determined. It was found that the properties of the products
significantly depend on the characteristics of applied catalysts. Presuming first order
kinetics the activation energies of thermal and catalytic degradation processes of the
waste polyethylenes were determined.
1. Introduction
In our modern world, with the extensive use of plastics the treatment of polymer
wastes become an important issue from environmental and energetic aspect. There are
several methods for reusing polymers. The thermal and catalytic degradations of
plastics into fuel oil is one of the possible routes among the various utilization of
plastic wastes. Recently, much attention has been paid to thermolysis and catalytic
polymer degradation techniques. In particular, polyethylene (PE) and polypropylene
(PP) have been targeted, because these are potential feedstocks for fuel producing
processes. A growing interest can also be observed in developing synthetic lubricants
from degradation products [1,4].
1
Most of the previous studies were performed on thermal degradation (pyrolysis)
of wastes. These processes require high temperature even up to 1000°C [2]. By
catalytic degradation of wastes the required energy might be decreased, because the
degradation temperature of wastes could be decreased with catalysts. The zeolites and
clay minerals are the most commonly used catalysts. They have effects also on the
structure of products, so when the target product is the diesel fuel isomerization
catalysts might be not advantageous [3]. The effects of the additive content in waste
PE on the degradation have not been reported in the literature yet. This paper will
focus on the degradation as a function of the type of the catalysts and additive content
of the waste polymer.
2. Experiments
2.1. Materials
Different additive containing waste low density polyethylene (LDPE) grains
were studied. One of them had filler content (FCPE) the other one not (FFPE). The
main physical properties of the wastes are summarized in Table 1.
Table. 1. Properties of polyethylene wastes
Polymer
FCPE
FFPE
Polymer type
LDPE
LDPE
TiO2, CaCO3
-
5
6
0,925
0,929
140-150
120-130
Filler material
Average grain size, mm
Density, g/cm3
Softening range, °C
One natural clay mineral (NCM) and an equilibrium FCC zeolite catalyst
(FCC-I) were used. The FCC-I catalyst has significant macro- and microporous
surface area (BET surface area: 184 m2/g, microporous surface area: 114 m2/g), while
the NCM catalyst has only significant macroporous surface area. Nitrogen was used
as inert atmosphere.
2.2. Apparatus
The cracking apparatus is shown in Fig. 1. 250 cm3 glass batch reactor was
fitted with a dip tube to introduce nitrogen and a thermocouple to measure the
2
temperature of the molten polymer in the reactor. Nitrogen bubbling resulted in
mixing the melted polymer and catalyst. Volatile polymer cracking products were
conducted through a condenser to separate liquid and gas products. The liquid was
sampled at set intervals, and the samples were analyzed.
T
Nitrogen
Waste PE
Gases
+Q
E-3
E-4
Liquid
product
E-2
Residue
Fig. 1. Apparatus for waste polyethylene cracking
2.3. Liquid product analysis
Liquid products formed in cracking reactions were analyzed with the
following methods.

iodine-bromine number determination (ASTM-D 149-60)

density measurement (MSZ EN ISO 12185)

determination of Engler distillation curve (ASTM-D 1078)
The olefin double bond distribution was determined with SHIMIDAZU IR-470
type spectrometer.
2.3. Reaction parameters
The following parameters were used:

Temperature: 420, 450°C

Residence time: 0-330 min.

Waste polyethylenes: FFPE
FCPE

Catalysts: FCC-I
NCM

Catalyst concentration: 1 (w/w%)
3
3. Results
3.1. Yields
The thermal and catalytic degradation behavior of waste LDPE was
investigated. Figs. 2 and 3 show the liquid yields as functions of cracking time at
420 and 450 °C.
90
80
Liquid product, %
70
60
50
40
Thermal (FCPE)
Catalytic (FCC-I), (FCPE)
Catalytic (NCM), (FCPE)
Thermal, (FFPE)
Catalytic (FCC-I), (FFPE)
Catalytic (NCM), (FFPE)
30
20
10
0
0
50
100
150
200
Degradation time, min.
250
300
350
Fig. 2. Effect of reaction time on yields at 420°C
100
90
80
Liquid product, %
70
60
50
40
Thermal (FCPE)
Catalytic (FCC-I), (FCPE)
Catalytic (NCM), (FCPE)
Thermal, (FFPE)
Catalytic (FCC-I), (FFPE)
Catalytic (NCM), (FFPE)
30
20
10
0
0
10
20
30
40
Degradation time, min.
50
60
70
Fig. 3. Effect of reaction time on yields at 450°C
4
Liquid yields increased with the cracking time for both types of waste PE as
shown in Figs. 2 and 3. While the degradation was practically complete within one
hour at 450°C, it took more than five hours to reach a liquid yield of 80% at 420 oC.
In the presence of catalysts higher yields were observed at both temperatures. The
catalysts significantly affected the product yields taking into account that thermal
reactions proceed in the entire volume (in three dimensions) while the catalytic
transformations on the contact surfaces (in two dimensions), only. At 450°C a higher
coke formation was observed. Lighter liquid colors were observed in the presence of
catalysts becoming dark yellow by the end of the reaction. At higher temperatures the
yields were higher and the gap between thermal and catalytic degradations is
significantly smaller than at 420°C (due to the greater activation energy of the former,
vide infra). The activity of the FCC-I catalyst was higher than the activity of the NCM
at 450°C. The activity of the catalysts decreased with time.
Some degradation reactions were considered of first order in the literature 5.
Thus we used the following rate equation:

dm
 km
dt
where m: weight of the waste polymer; k: reaction rate constant.
The calculated activation energies are given in Table 2. Using catalysts
resulted in a decrease of the phenomenological activation energy.
Table 2. – Activation energies of cracking
Activation energy, kJ/mol
Thermal
Catalytic (FCC-I)
Catalytic (NCM)
FFPE
368
357
359
FCPE
369
359
360
3.2. Structure of products
Two methods were used for the determination of the olefin content of the liquid
fractions: one based on the iodine-bromine number, and the other one on the IR
spectra. Fig. 4 shows the iodine-bromine numbers of liquid fractions as functions of
the cracking time at 420°C. The olefin content of the liquid product decreased with
time. The changes during the catalytic cracking were more significant than during the
5
thermal cracking. In the case of
190
IBr number, g I/100g product
thermal
Thermal (FCPE)
Catalytic (FCC-I), (FCPE)
Catalytic (NCM), (FCPE)
Thermal, (FFPE)
Catalytic (FCC-I), (FFPE)
Catalytic (NCM), (FFPE)
170
150
bromine
the
numbers
iodine-
of
liquid
fractions decreased slightly with
130
the reaction time. The iodinebromine
110
numbers
of
liquid
fractions as functions of cracking
90
time
70
0
50
100
150
200
250
300
350
Degradation time, min.
Fig. 4. – The IBr number of liquids vs. cracking time at 420°C
at
450°C
are
shown
in Fig. 5.
Smaller
found
differences
among
were
iodine-bromine
numbers at 450°C than at 420°C.
115
110
IBr number, g I/100g product
cracking
Thermal (FCPE)
Catalytic (FCC-I), (FCPE)
Catalytic (NCM), (FCPE)
Thermal, (FFPE)
Catalytic (FCC-I), (FFPE)
Catalytic (NCM), (FFPE)
105
100
95
The olefin content was also lower
at 450°C. Figs. 4 and 5 show that
the catalytic activities decreased
with the reaction time, and
90
85
greater activity was observed at
80
lower
75
temperatures.
At
both
temperatures the natural clay
70
0
5
10
15
20
25
30
Degradation time, min.
35
40
45
mineral catalyst is of lower
Fig. 5. – The IBr number of liquids vs. cracking time at 450°C
activity. The decrease of the
catalyst
activity
could
be
attributed to the formation of
Vinyl
Internal
1,1-disubstitued
carbonaceous deposits blocking
80
Olefin content, (V/V%)
70
the active centers of the catalyst.
60
50
Different
filler
and
pigment
40
containing waste polyethylenes
30
20
were used in the experiments.
10
The additive content of polymer
0
30
120
300
30
FCPE
120
300
FFPE
Degradation time, min.
Fig. 6. – The distribution of olefin double bonds of liquids
vs. cracking time at 420°C without catalyst
did not significantly influence the
characteristics of end products.
Double bond isomerization was
observed in catalytic processes.
6
Vinyl
Internal
Figs. 6, 7 and 8 show the
1,1-disubstitued
90
distribution
of
olefin
double
Olefin content, (V/V%)
80
bonds at different cracking times.
70
60
The olefin content of liquid
50
fractions decreased with time (in
40
30
accordance
20
10
with
the
iodine-
bromine numbers in Figs. 4 and
0
30
120
300
30
120
FCPE
300
5). The double bond content of
FFPE
the products of thermal cracking
Degradation time, min.
Fig. 7. – The distribution of olefin double bonds of liquids
vs. cracking time at 420°C with catalyst (FCC-I)
is
lower than that of catalytic
cracking. The liquid products
had significant vinyl and -olefin
Vinyl
Internal
1,1-disubstitued
content,
90
Olefin content, (V/V%)
80
approximately
50(V/V%). The distributions of
70
olefin double bond at different
60
50
times of catalytic cracking are
40
30
given
in
Figs.
7
and
8.
20
Differences
10
0
30
120
300
30
120
FCPE
300
FFPE
can be
observed
between thermal and catalytic
cracking. Catalysts stimulated the
Degradation time, min.
Fig. 8. – The distribution of olefin double bonds of liquids
vs. cracking time at 420°C with catalyst (NCM)
olefin double bond isomerization.
Using catalyst the significant
vinyl content observed in thermal
450
cracking
Temperature, °C
400
decreased
from
350
40(V/V%) to 20 (V/V%), while
300
the internal double bond content
250
increased approximately from 20
200
(V/V%) to 40(V/V%). Using
Thermal (FCPE)
Catalytic (FCC-I), (FCPE)
Catalytic (NCM), (FCPE)
Thermal (FFPE)
Catalytic (FCC-I), (FFPE)
Catalytic (NCM), (FFPE)
150
100
50
catalysts
resulted
in
liquid
fractions of higher olefin content.
0
0
10
20
30
40
50
60
70
80
90
100
At higher temperatures the olefin
Distillation volume, (V/V%)
Fig. 9. – The Engler distillation curve of liquid products
contents were lower as well as
the vinyl olefin content.
7
Fig. 9 shows the Engler distillation curves of the liquids produced at 420°C.
The catalysts influenced the average molecular weight of the liquid products. Using
different additive containing polyethylene wastes the Engler curves did not differ
from each other. The molecular weight reducing effect of the catalysts can be seen in
this Figure, but the type of the catalyst had no significant effect on the average
molecular weight of the liquids.
4. Conclusions
The cracking of different additive containing waste polyethylenes was
investigated in presence of different catalysts and in absence of catalyst at two
temperatures. The temperature and catalysts had significant effect on yields of the
liquid products. Using catalysts the yields are higher, and significant change in the
product composition can be observed (olefin double bond isomerization, higher olefin
content). The temperature of the cracking affected the yields and to a less degree the
product composition. The catalyst activity decreased with degradation time, and the
activation energy of cracking could be decreased by using catalyst.
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