5-7 OCTOBERISFR2011

95

TOLEDO (SPAIN)

INFLUENCE OF TEMPERATURE IN THE CATALYTIC PYROLYSIS OF HDPE IN TWO
STEPS
M. Artetxe, G. Lopez, M. Amutio, G. Elordi, J. Bilbao and M. Olazar
University of the Basque Country, Department of Chemical Engineering, P.O. Box 644 - E48080 (Bilbao) Spaine-mail: maite.artetxe@ehu.es

Abstract
Continuous pyrolysis of high density polyethylene was carried out in a two step reaction system consisting of an initialpyrolytic spouted bed reactor followed by a catalytic fixed bed reactor. The pyrolysis has been carried out at 500 ºC andHDPE continuous feed rate was 1g min-1. The effect of the catalytic step temperature on product yields and composition
have been studied using a catalyst based on HZSM-5 (SiO2/Al2O3= 30) zeolite. 8 g of catalyst have been used in eachrun and the influence of the catalytic step temperature was studied in the 350-550 ºC range. The products have beengrouped into five lumps: light olefins (C2-C4), light alkanes (C1-C4), non aromatic C5-C11 compounds, single-ring aromaticcompounds and C12-C21 hydrocarbons. An increase in temperature gives way to an increase in the yield of light olefins,from 31.5 wt% at 350 ºC to 63 wt% at 550 ºC, and an increase in the yield of single-ring aromatics, from 3 wt% at 350 ºC
to 10 wt% at 550 ºC. On the other hand, a great decrease is observed in the yield of non-aromatic C5-C11 compounds,
from 50 wt% at 350 ºC to 21 wt% at 550ºC.

Keywords: plastic waste, catalytic pyrolysis, two step process, spouted bed, HZSM-5 zeolite, HDPE recycling

1. Introduction
The thermal and catalytic cracking of plastic waste hasbeen regarded as one of the most feasible plastic
recycling method at industrial scale, since used plastic
could become a valuable source of chemicals, and gas
and liquid fuels [1]. The main difficulties that limit thelarger scale implementation of the process are the sticky
nature of the fused plastic and the low thermal
conductivity of plastics, which restricts heat and masstransfer and hinders be isothermicity. Therefore, anadequate choice of the pyrolysis reactor is essential. The
conical spouted bed reactor has been used in polyolefin
pyrolysis under conditions of maximum particle
stickiness. The cyclic movement of sand particles
enhances their uniform coating with fused plastic andavoids bed agglomeration [2]. Furthermore, it is a simple
device with isothermal bed and suitable for continuous
operation and scaling-up.
The use of acid catalysts (HZSM-5, HY and Hβ) in situ
allows for increasing the selectivity of high interest
products, such as olefins in the pyrolysis of polyolefins,and decreasing pyrolysis temperature [3]. Two-step
catalytic pyrolysis not only allows using differenttemperatures in the pyrolytic and catalytic reforming
stages but also ensuring the contact between pyrolysis
volatile products and the catalyst. Furthermore, thisstrategy may be implemented with cracking catalysts for
obtaining the monomer or fuels and with reforming
catalysts for obtaining H2 [4].
This work approaches continuous thermal cracking of
HDPE in a spouted bed reactor followed by a catalyticinteriores libro.indd 95

step in a fixed bed reactor in order to study the effect of
the catalytic step temperature on product yields andcompositions.

2. Materials and Methods
The high density polyethylene (HDPE) used has been
supplied by Dow Chemical (Tarragona, Spain) in the formof cylindrical pellets (4 mm).
The catalysts consists of 25 wt% of HZSM-5 zeolite
provided by Zeolyst International (Kansas, USA),
agglomerated by wet extrusion with bentonite (30 wt%)
and with inert alumina (45 wt%). The catalyst wascalcined at 575 ºC for 2 h in order to achieve resistanceto irreversible deactivation by dealumination. The mostimportant properties of the catalyst have been measured;
BET surface area is 184 m2 g-1, total acidity 0.2 mmol
NH3 g-1 and average acid strength 100 kJ (mol NH3)-1.
Continuous pyrolysis of HDPE was carried out in aconical spouted bed reactor at 500 ºC. The bed wasmade up with 50 g of sand (0.3-0.4 mm) and the plastic
was fed continuously with a feedrate of 1 g min-1.
Nitrogen was used to fluidize the bed and the flowrate
was 20 % in excess of that required for minimum
spouting velocity (5 L min-1 at room conditions). The
pyrolysis volatiles (mainly waxes) formed are fed in line
to a catalytic fixed bed reactor. The bed consists of 8 g ofcatalyst (1-2 mm) and the experiments have been carriedout at 350, 400, 450, 500 and 550 ºC. Each continuous
run lasts for around 10 min, and the experiments havebeen repeated several times to ensure a good
reproducibility of the results. The product stream has
been analyzed by a Varian 3900 chromatograph and a

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6th International Symposium on Feedstock Recycling of Polymeric Materials

μGC (Varian Q4900) connected on-line to the reactor. Atthe same time, a mass spectrometer (ShimadzuQP2010S) and a microGC-MS spectrometer (AgilentMSD5975B) have been used to identify the compounds.

3. Results and Discussion
Product streams have been grouped into five lumps: lightolefins (C2-C4), light alkanes (C1-C4), single-ringaromatics, non-aromatic C5-C11 compounds and C12-C21hydrocarbons. The yield of waxes, which is the main
product in the first step, is negligible.
Figure 1 shows the effect of temperature on productfraction yields. An significant effect of temperature has
been observed, increasing the yield of light olefins from31.5 wt% at 350 ºC to 63 wt% at 550 ºC and decreasing
the yield of non-aromatic C5-C11 compounds from 50 wt%
at 350 ºC to 21 wt% at 550 ºC. Moreover, the yield ofsingle-ring aromatic compounds increases whentemperature is raised, from 3 wt% at 350 ºC to 10 wt% at550 ºC. The effect of temperature on light alkanes andC12-C20 hydrocarbons is less significant. Elordi et al.
observed similar product yields in a study carried out in a
conical spouted bed reactor at 500 ºC using HZSM-5zeolite based catalyst in situ [3].
70350 ºC

60
400 ºC450 ºC
50Yield (wt %)500 ºC
550 ºC40

aromatic compounds increase. This effect of temperature
is explained by the cracking of C5+ olefins to form C2-C4
light olefins and the formation of aromatics by thecondensation of C2-C4 olefins. The main compounds inthe single-ring aromatic fraction are toluene and xylenes,
with a yield of 4 wt% and 3 wt% at 550 ºC, respectively.
Furthermore, the yields of paraffins, isoparaffins andnaphthenes in the C5-C11 fraction decrease steadily as
temperature is increased.
The main products in the C12-C21 fraction are paraffins
and the yield of this fraction decreases as thetemperature increases from 350 ºC to 400 ºC, whereas
no significant effect of temperature has been observedon the yield of C12-C21 hydrocarbons in the 400-550 ºC
range.

4. Conclusions
The conical spouted bed reactor is a suitable technology
for HDPE continuous pyrolysis, given that high heattransfer and bed turbulence avoid defluidization
problems.
The catalyst based on HZSM-5 zeolite exhibits highactivity transforming waxes into C20- hydrocarbons evenat 350 ºC. Besides, the catalyst used is highly selectivefor producing light olefins, reaching a value of 63 wt% at550 ºC, with propylene being the main product with ayield of 35.5 wt%.
The increase in the temperature in the 350-500 ºC range
gives way to an increase in the yield of light olefins and adecrease in the yield of non-aromatic C5-C11 compounds.
Furthermore, temperature also affects single-ringaromatic compounds by increasing their yield when thisvariable is increased.

30

References
20

100light olefins

light alkanes

single ringaromaticsnon aromatic C5C11C12-C20

Fig.1. Effect of temperature on product fraction yields.
Regarding the gas fraction, light olefin fraction is mainlycomposed of propylene. The yield of propylene increases
as temperature is increased, reaching a value of 35.5
wt% at 550 ºC. Furthermore, the yield of ethylene
increases when temperature is increased, from 0.8 wt%
at 350 ºC to 10 wt% at 550 ºC, whereas the yield ofbutene (18 wt%) remains almost constant. The main
compound in the lump of light alkanes is butane, with ayield of 6.5 wt% at 550 ºC. The yields of methane andethane are very low (below 0.4 wt%).
In the gasoline fraction, which is made up of single-ringaromatic fraction and non-aromatic C5-C11 compounds,
C5 and C6 olefins are the main products at lowtemperature. As temperature is raised, the yields of C5
and C6 olefins decrease and the yields of C6, C7 and C8interiores libro.indd 96[1] J. Aguado, D.P. Serrano and J.M. Escola. Fuels from
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[3] G. Elordi, M. Olazar, G. opez, M. Amutio, M. Artetxe,
R. Aguado and J. Bilbao. Catalytic pyrolysis of HDPE incontinuous mode over zeolite catalysts in a conicalspoted bed reactor. Journal of Analytical and Appliedpyrolysis 85(2009) 345-351[4] Ch. Wu and P.T. Williams. Pyrolysis-gasification ofplastics, mixed plastics and real-world plastic waste withand without Ni-Mg-Al catalyst. Fuel 89(2010) 3022-3032.

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