5-7 OCTOBERISFR2011

61

TOLEDO (SPAIN)PYROLYSIS OF HIGH DENSITY POLYETHYLENE CARRIED OUT IN CONTINUOUS
MODE IN A CONICAL SPOUTED BED REACTOR IN THE 500-700 ºC RANGE
G. Elordi, M. Artetxe, G. Lopez, J. Bilbao, and M. Olazar
Department of Chemical Engineering, University of the Basque Country, P.O. Box 644, 48080 Bilbao, Spain.
e-mail: martin.olazar@ehu.esAbstract
The pyrolysis of high density polyethylene (HDPE) carried out in continuous mode in a conical spouted bed reactor has beenstudied in the range between 500 and 700 °C. The products have been grouped into the lumps of gas (C4-), gasoline (C5C11), diesel (C12-C20) and waxes (C21+). The product yields and compositions of these fractions have been compared both to
those previously obtained in the pyrolysis performed in discontinuous mode and to those obtained by other authors influidized bed reactors. The results confirm the optimal performance of the conical spouted bed reactor (CSBR) in obtaining
high yields of waxes and fuels with low aromatic content, which is explained by the appropriate conditions of the CSBRneeded to enhance heat and mass transfer between phases (capacity for coating the sand with plastic) and minimize
secondary reactions (short residence time of the volatiles).

Keywords: pyrolysis, plastic wastes, spouted bed, waxes, fuels

1. Introduction
The pyrolysis of plastics arouses great interest for theirvalorization, as it allows obtaining high yields of fuels andraw materials for the petrochemical industry. Moreover,the pyrolysis process has few environmental issues [1].
The pyrolysis of polyolefins takes place through acomplex free-radical mechanism, which gives way to awide product distribution that depends on pyrolysis
conditions: heating rate, temperature, and residence
time. The selection of reactor is a key factor for thekinetics of pyrolysis as well as for the product distribution
obtained. The following steps take place from themoment that the waste plastic particles are fed into thereactor: i) Fusion of the plastic; ii) fused plastic pyrolysis.
The design of the reactor, with a continuous feeding
system, must attain the fast fusion of the plastic in orderfor pyrolysis (the second step) to be the controlling one.
The yields of products of commercial interest may be
optimized by selecting appropriate pyrolysis conditions.
Solid materials are used in the reactors in order to becoated with the fused plastic, facilitating heat and masstransfer between the plastic and the gas. The main
advantages of the CSBR, Figure 1, are: i) Cyclic
movement of the particles suitable for fast and uniform
coating of the sand or catalyst particles with the fusedplastic. This movement is characteristic of spouted beds.
ii) More vigorous particle circulation than in bubbling
fluidized bed reactors, thus minimizing defluidizationproblems, which is a well-known limitation for FBRs. iii)
Short residence time of the volatiles (in the order ofcentiseconds), which minimizes the formation ofsecondary products.

interiores libro.indd 61

Fig. 1. Reactor diagram.

2. Materials and Methods
The HDPE has been provided by Dow Chemical in theform of 4 mm size chippings. The pyrolysis pilot plant has
previously been described [2]. Pyrolysis has been carriedout in the 500 to 700 °C range. A quantity of 30 g of sandwith a particle diameter in the 0.6-1.2 mm range hasbeen determined as the most suitable bed.
The on-line analysis of the outlet volatile stream has
been carried out by a GC (Agilent 6890) provided with anFID. Identification of the liquid fraction components andwaxes (diluted in tetrahydrofuran at 55 °C) has been
carried out by means of a GC coupled with an MS(Shimadzu QP2010S). The simulated distillation of thecondensed liquid and waxes has been developed
according to the ASTM-D2887-04 standard. The waxes
have also been analysed by means of high resolution
liquid chromatography (HP 1100) provided by a GPC

26/9/11 13:12:54

62

6th International Symposium on Feedstock Recycling of Polymeric Materials

column. The nature of the bonds has been studied by
FTIR spectrophotometry in a Nicolet 6700.

3. Results and Discussion
Figure 2 shows that the waxes are the fraction of highest
yield from 500 to 650 °C. Their yield accounts for almost
70 wt% at 500 °C, and when added to the yield of thediesel fraction, the joint yield accounts for more than 90
wt%. The yield of waxes diminishes as temperature israised, whereas the yields of gasoline and gas increase
and that of diesel peaks at 30 wt% at 550 °C. For
temperatures above 675 °C, the highest yieldcorresponds to the gas fraction.
70Gas
C5-C1160

C12-C20Yields (wt%)

50Waxes

40
3020
100

500

550

600

650

700

Temperature (ºC)

Fig. 2. Effect of temperature on the yields of productfractions (wt%).
Thermal pyrolysis above 550 °C gives way to high yieldsof C2-C4 olefins (37 wt% at 700 °C) and a low aromatic
content in the gasoline fraction is obtained. The lightalkanes are by-products of low interest, which are bestused to supply energy to the process.
Regarding the gas fraction, the yield of ethylene is theone that increases in a more pronounced way, from 0.08
wt% at 500 °C to 17.82 wt% at 700 °C. This highincrease in the yield of ethylene is in line with the resultsin the literature obtained in fluidized bed reactors. Theyield of propylene also increases considerably as
temperature is raised.
The yields of the components in the gasoline fraction
increase as temperature is raised. The distribution of theyields for the components with a different number ofcarbon atoms is fairly uniform in the 500-650 °C range.
At 700 °C, the yields of C5-C7 components are higher,
especially that of C6.
An analysis of the nature of the bonds reveals that thehighest yield is that of olefins, and it increases
considerably as temperature is raised. The yield ofisoparaffins is also significant, although it increases withtemperature to a lesser extent than that of olefins, andeven decreases at 700 °C. The yield of aromaticcomponents also increases with temperature, but to alesser extent than that of olefins. The yields of both linear
paraffins and naphthenes are low and hardly vary withtemperature.

interiores libro.indd 62

In the diesel fraction, the yields of the diolefin
components are more significant than in the case of thegasoline fraction. The yield of the heaviest components
decreases as temperature is increased and the yield
distribution obtained becomes more uniform. The yield ofolefins is the highest one for the whole temperature
range studied, peaking at 600 °C. The yield of diolefin
components increases by increasing temperature from500 to 600 °C, whereas that of paraffins decreases
linearly when temperature is raised.
Two waxes fractions have been quantified: i) Lightwaxes, i.e., the C21-C40 fraction; ii) Heavy waxes (theremaining waxes). As the reaction temperature is raised,
the content of light fraction compounds (C20-) dissolved in
the waxes increases. An increase in reaction
temperature involves the production of lighter waxes.

4. Conclusions
The conical spouted bed reactor performs well forpolyolefin pyrolysis carried out in continuous mode. The
cyclic movement of the particles facilitates their uniform
coating with fused plastic, improving the heat and masstransfer rate between the phases. Besides, the short
residence time of the volatiles minimizes by-productformation (methane and polyaromatic hydrocarbons).
The HDPE pyrolysis process is versatile in the 500-700°C range. The yield of waxes (C21+) accounts for 67 wt%
at 500 °C and decreases to 12 wt% in this range. An
increase in temperature involves an increase in thegases (C4-) and gasoline fraction (C5-C11), which accountfor 39 wt% and 33 wt% at 700 °C, whereas that of diesel(C12-C20) decreases (16 wt% at 700 °C). Consequently,
the CSBR is an excellent technology for obtaining waxes
at low temperatures (500 °C) without defluidization
problems, making them a suitable feed for FCC units. On
the other hand, the operation at high temperature (700°C) is promising for obtaining gasoline (33 wt% with asignificant content of C5-C7 olefins), together with a highyield of C2-C4 olefins (37.7 wt%).
The results obtained in the literature in fluidized bed [3]are consistent with those obtained in this study.

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. 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.
[3] M. Predel and W. Kaminsky. Pyrolysis of mixed
polyolefins in a fluidised-bed reactor and on a PyroGC/MS to yield aliphatic waxes. Polym. Degrad. Stab.
70 (2000) 373-385.

26/9/11 13:12:54