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TOLEDO (SPAIN)CONTINUOUS POLYOLEFIN CRACKING ON AN HZSM-5 ZEOLITE CATALYST IN A
CONICAL SPOUTED BED REACTOR
G. Elordi, M. Olazar, M. Artetxe, G. Lopez, and J. Bilbao
Department of Chemical Engineering, University of the Basque Country, P.O. Box 644, 48080 Bilbao, Spain.
e-mail: gorka.elordi@ehu.esAbstract
The effect of operating conditions on product yields and compositions has been studied in the catalytic cracking ofpolyolefins (high and low density polyethylene and polypropylene) in a conical spouted bed reactor. The catalyst has
been prepared by agglomerating an HZSM-5 zeolite (SiO2/Al2O3=30) with bentonite and alumina. The study has been
carried out in continuous mode, in the 450-570 ºC range, with polyolefin flow rates between 1 and 2 g min-1 and feeding
up to 1 kg of polyolefin into a 30 g catalyst bed. The result is an excellent performance of the reactor-catalyst pairing forcontinuous catalytic pyrolysis, given that the yields obtained in the pyrolysis of high density polyethylene at 500 ºC
without defluidization problems and very low catalyst deactivation are as follows: 60 wt% of C 2-C4 olefins, 15 wt% of nonaromatic C5-C11 hydrocarbons, 10 wt% of single-ring aromatics and 14 wt% of C4- alkanes. Only small differences areobserved in the pyrolysis of the different polyolefins.

Keywords: spouted bed, catalytic pyrolysis, waste plastic, HZSM-5 zeolite, polyolefin recycling

1. Introduction

2. Materials and Methods

The pyrolysis of waste plastics is an interesting route forthe large-scale production of fuels and monomer
recovery [1]. Different laboratory or pilot scale reactors
have been studied. The sticky nature of the fused plastic
and the limitations of heat and mass transfer between
phases are the main difficulties that limit the larger scale
implementation.

HDPE, LDPE, and PP polyolefins have been supplied by
Dow Chemical (Tarragona, Spain) in the form ofchippings (4 mm).

The conical spouted bed reactor performs well in termsof heat and mass transfer, and it allows for minimizing
the problem of defluidization caused by a very sticky
reaction medium. It allows for operating with the catalyst
in situ without segregation problems due to the rapid anduniform coating of sand particles with fused plastic. The
residence time of the volatiles is low (centiseconds),
minimizing their transformation into secondary products.
The use of acid catalysts (based on zeolites HY, HZSM5, Hβ, MCM-41) in situ allows for decreasing pyrolysis
temperature (essential for the viability of a highlyendothermic process) and increasing selectivity toproducts of high interest, as are olefins. The good
performance of the HZSM-5 zeolite in attenuating cokedeactivation is well-known in catalytic processes and it
has been confirmed in this case, in which a high flow rateof N2 helps the sweeping of coke precursors towards theoutside of the zeolite crystals. This phenomenon helps toattenuate deactivation by coke, which is lower than in thecase of the catalysts prepared based on HE and HYzeolites [2].

interiores libro.indd 87

The pyrolysis pilot plant and its dimensions have been
described elsewhere [3]. The system for feedingpolyolefins involves a hopper (2 L) with a three-way
hollow ball valve. The device is actuated by a frequency
programmed automaton every 7.5 s for a total feed of 1 g
min-1 into a bed of 30 g of catalyst. The pipe to feed thepolyolefin into the reactor (half an inch in diameter andcooled by tap water) is located over the fountain zone of
the spouted bed in order to facilitate the steps of plastic
fusion and coating of the catalyst particles prior tocracking. The on-line analysis of the outlet volatile stream(very diluted in N2) has been carried out by periodicallysending samples to a GC (Agilent 6890) equipped withan FID. Identification has been carried out by means of a
GC coupled with an MS (Shimadzu QP2010S).
A catalyst has been prepared based on an HZSM-5
zeolite with a ratio of SiO2/Al2O3=30. In order to obtain
particles of a suitable size (0.6-1.2 mm diameter range)
and mechanical resistance for the conical spouted bed
reactor, the zeolite particles (25 wt%) have been
agglomerated by wet extrusion with bentonite (30 wt%)
and inert alumina (45 wt%). The agglomeration
generates meso- and macropores in the particles,
attenuating deactivation. Prior to use, the catalyst has
been calcined at 575 ºC for 2 h in a N2 atmosphere.

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6th International Symposium on Feedstock Recycling of Polymeric Materials

3. Results and Discussion
Figure 1 shows the effect of temperature, in the 450-570ºC range for the transformation of HDPE. Productfractions have been classified as follows: light olefins (C2C4), light alkanes (C4-), light liquid fraction (C5-C11,
without aromatic components), single-ring aromatics,
heavy liquid fraction (C12-C20), and waxes (C21+).
60

450 ºC
500 ºC
525 ºC
550 ºC
570 ºC50Yields (wt%)

40
30

When the flow rate of HDPE is doubled at 500 ºC, theyield of C2-C4 olefins decreases slightly (from 58 wt% for
1.0 g min-1 to 54 wt% for 2.0 g min-1), with propylene
being the one more affected, and the yields of single-ringaromatics and light alkanes increase slightly.
A slightly lower yield of light olefins and a higher yield of
light alkanes and aromatics are observed for LDPE.
These differences are attributed to the shorter chainswith a higher branching degree for LDPE, which makes
its cracking easier and, consequently, facilitates a more
rapid formation of olefins and their condensation steps toform aromatics or C12+ hydrocarbons. The higher yield ofC12+ in the pyrolysis of PP is associated with thepolymerizing capacity of propylene.

204. Conclusions
100LightOlefins

Light Non-Arom.
Aromatics
C5-C11Alkanes

C12+Fig. 1. Effect of temperature on product fraction yields inthe pyrolysis of HDPE.
It is noteworthy that an increase in temperature gives
way to a decrease in the yield of light olefins, from 57.8
wt% at 450 ºC to 50.3 wt% at 570 ºC, whereas the yieldof single-ring aromatics increases, from 9.1 wt% to 20.7
wt%, which is due to the enhancement of Diels-Aldercondensation reactions. A decrease is observed in theyield of the non-aromatic C5-C11 fraction, from 16.6 wt%
at 450 ºC to 11.5 wt% at 570 ºC. Nevertheless, there isno clear trend for the effect of temperature on the yields
of light alkanes and the C12-C20 fraction.
Regarding the gaseous product fraction, C2-C4 olefins
are the main products according to the following yieldorder: propylene > butenes > ethylene. As temperature isincreased the yield of ethylene increases, but it is steadyabove 525 ºC, around a value of 11.0 wt%. The yield of
propylene peaks around 500 ºC with a yield of 28.7 wt%.
The yield of butenes decreases as temperature isincreased and reaches a steady value of around 17.5
wt% above 525 ºC. This re-distribution of olefins is due tothe oligomerization-cracking mechanisms.
Analyzing the C5-C11 fraction, as the temperature israised the yield of C5 components decreases, butincreases in the cases of C7, C8, and C9, which isexplained by the olefinic nature of C5 compounds and thearomatic character of C7-C9 compounds. This effect oftemperature is explained by the cracking of C5+ olefins toform C2-C4 light olefins and the formation of aromatics by
the condensation of C2-C4 olefins. Nevertheless, thearomatic components are not cracked under these
conditions. Likewise, there is no significant effect oftemperature on the yields of paraffins, isoparaffins, andnaphthenes in the C5-C11 fraction.

interiores libro.indd 88The conical spouted bed reactor is suitable for thecatalytic pyrolysis of polyolefins (HDPE, LDPE, PP) in
continuous mode for obtaining high yields of olefins andhydrocarbons of interest as fuel. The small difference inthe results shows that this technology with an HZSM-5
zeolite catalyst is suitable for the pyrolysis of different
polyolefins.
In order to avoid the operational problems observed attemperatures below 450 ºC, operation should be carriedout at 500 ºC. The yields for C2-C4 olefins, non-aromatic
fraction, single-ring aromatics, and C4- alkanes are 60wt%, 15 wt%, 10 wt%, and 14 wt%, respectively.
The variation of the flow rate of HDPE fed into the reactor
shows the versatility of the spouted bed reactor, i.e.,there are small differences in the yields as the flow rateof HDPE is increased and the transformation into light
products is completed thanks to the efficient coating ofcatalyst particles with the fused plastic. The yields arevery similar for the different polyolefins (HDPE, LDPE,PP), albeit lower for olefins (due to a lower yield of
propylene and butenes), and higher for light alkanes andaromatics when LDPE is in the feed.

References[1] J. Aguado and D.P. Serrano. Feedstock recycling ofplastic wastes, The Royal Society of Chemistry:
Cambridge, 1999.
[2] G. Elordi, M. Olazar, G. Lopez, M., P. Castaño, J.
Bilbao. Role of pore structure in the deactivation ofzeolites (HZSM-5, HE and HY) by coke in the pyrolysis ofpolyethylene in a conical spouted bed reactor. Appl.
Catal. B: Environ. 102 (2011) 224-231.
[3] G. Elordi, M. Olazar, G. Lopez, M. Amutio, M. Artetxe,R. Aguado, and J. Bilbao. Catalytic pyrolysis of HDPE incontinuous mode over zeolite catalysts in a conicalspouted bed reactor. J. Anal. Appl. Pyrolysis 85 (2009)345-351.

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