improvement of tyres and plastics wastes pyrolysis through the

IMPROVEMENT OF TYRES AND PLASTICS WASTES
PYROLYSIS THROUGH THE PRESENCE OF
H-DONOR SOLVENTS
Miguel Mirandaa, Filomena Pintoa, Ibrahim Gulyurtlua, Arlindo Matosb, Isabel Cabritaa
a
INETI-DEECA,
Estrada do Paço do Lumiar, 1649-038, Lisboa-Portugal
b
Universidade de Aveiro,
Campus Universitário de Santiago, 3810-193, Aveiro Portugal
Abstract
Modern societies have widely used plastics and tyres and generated increasing amounts
of wastes with negative impact on environment. The natural raw material for both
plastics and tyres is petroleum which has a limited life time. Pyrolysis technology
applied to these wastes can be used as a recycling technology, allowing a better
management of the remaining petroleum resources and simultaneously decreasing the
amount of wastes, taking profit of theirs energetic content. Pyrolysis cracks the
polymeric structure of these wastes and produces a mixture of hydrocarbon molecules,
liquid and gas in nature, which can be used as fuels or as feedstock in petrochemical
industry.
Wastes pyrolysis is affected by experimental conditions and waste composition. When a
mixture of plastic wastes, containing (PE), polypropylene (PP) and polystyrene (PS) is
pyrolysed at 420°C, 0.41MPa and for 30 minutes liquid yields of around 80% (w/w) are
easily achieved. Waste tyres are more difficult to pyrolyse and for the experimental
conditions mentioned above, normally liquid yields are around 33% (w/w), being solid
residue about 51% (w/w). The conversion to liquid products may be increased by the
presence of suitable solvents which favours mass and heat transfer and consequently
pyrolysis reactions.
To improve liquid yields, experiments were conducted in a laboratory batch reactor with
hydrogen donor solvents (tetrahydronaphthalene, decahydronaphthalene and
tetrahydroquinoline), which greatly increased liquids yields. Due to the high price of
these solvents, some experiments were done in presence of some plastics wastes (PE,
PP and PS). Although lower liquids yields were obtained in presence of plastic wastes,
than those produced with hydrogen donor solvents, pyrolysis of tyres wastes mixed with
50% (w/w) of plastic wastes allowed increasing liquids yields of around 72%, whilst the
solid residue decreased about 46%.
Keywords: Solvents, Tyres, Plastics, Wastes, Pyrolysis
1. Introduction
The natural raw material for plastics and tyres production is petroleum, which is also the
greatest fossil fuel contributor to the worldwide energy production. As petroleum
reserves have a limited lifetime (Filomena Pinto, 2001, p. 27, Filomena Pinto, 2001, p.
39), it is advisable to have a better management of the remaining resources.
*
Miguel Miranda: Mail: [email protected]
Phone 351210924780
Fax351217166569
Besides, both plastics and tyres are used in many daily activities, which generate huge
amounts of wastes with a negative impact on environment, due to the fact that only a
small amount of these wastes can be recycled and most of them are biological resistant
to degradation. The solutions, used so far, to deal with these wastes present several
problems and do not seem to be the right ones. It has been increasingly difficult to find
suitable places to landfill wastes, due to the risk of underground waters contamination.
Moreover, landfilling of these wastes does not allow neither to take profit of the
energetic content, nor to recover theirs organic content that should be part of the organic
lifetime cycle. Although, incineration has the advantage of taking profit of waste
energetic content, the process also destroys wastes organic content that is converted
only into CO2 and H2O. Combustion of these wastes produces pollutants like light
hydrocarbons, nitrous and sulphur oxides, dusts and dioxins, which have highly
negative impacts on the environment (Filomena Pinto, 1999, p. 715, Filomena Pinto
1999).
The need of reducing these wastes to minimize environmental impact and finding new
sources of alternative fuels led to the idea of applying pyrolysis technology to plastics
and tyres wastes, as an approach to produce economical valuable products (Filomena
Pinto, 2001, p. 363, Kaminsky W., 1995, p. 19, Scott D., 1990, p. 407, Conesa J., 1997,
p.419). Pyrolysis process promotes the thermal decomposition of these wastes, under
moderate conditions of temperature and pressure and in presence of an inert atmosphere
(Elizabeth A., 1997, p. 347, Kaminsky W., 1997, p. 365), breaking down initial
structure and producing smaller intermediate species (radicals or ions). These fragments
can further react to produce a mixture of smaller hydrocarbon molecules, being liquid
and gas in nature, which may be used as fuel or as raw material in several industries
(Filomena Pinto, 2001, p. 363).
Yields and composition of pyrolysis products are affected by experimental conditions,
such as: run temperature, heating rate, initial pressure, residence time and waste
characteristics and composition.
Pyrolysis of only tyres wastes at moderate conditions of temperature and pressure
presented low liquid yields, around 33% (w/w). Liquid yields were found to increase
when tyres were mixed with plastic wastes, probably, because plastic wastes were easier
to pyrolyse and led to a liquid medium that favoured heat and mass transfer tyres
pyrolysis reactions. In fact, pyrolysis of mixtures of tyres and plastic wastes allowed to
rise liquid yields. A mixture of the three more used plastics was chosen, with
composition similar to the one existing in Portuguese municipal solid waste (MSW). In
order to improve the liquid fraction yields, it was also studied the effect of adding
several hydrogen donor solvents to reactional medium, like decahydronaphthalene,
tetrahydroquinoline and tetrahydronaphthalene, which proved to increase liquids yields,
by decreasing gases and solid yields.
2. Experimental Part
All experiments were carried out in a 1 liter autoclave, built of Hastelloy C276, by Parr
Instruments. In Figure 1 it is shown a schematic representation of the experimental
installation used for waste pyrolysis studies. Experimental procedure was described in
previous work (Filomena Pinto, 2001, p. 27, Filomena Pinto, 2001, p. 363). In the
previous studies it was found the following set of experimental conditions for pyrolysis
parameters: run temperature - 420ºC, initial pressure - 0.41 MPa and reaction time - 30
minutes. These experimental conditions were used in the present work.
Mixtures of tyres wastes and solvents with different H-donor capacities were pyrolysed.
Two tyres wastes contents were tested: 50% (w/w) and 33% (w/w). The H-donor
capacities solvents studied were: decahydronaphthalene, tetrahydroquinoline and
tetrahydronaphthalene. Due to the fact that liquid solvents used in pyrolysis of tyres
wastes are extremely expensive a mixture of plastics wastes were used as a solid solvent
which is less expensive and can be easily found in great amounts. Therefore, a mixture
of plastic waste composed by PE, PP and PS (all of them in identical content for the
different tyre waste content tested) were also used as a “solid solvent”.
Mixtures of tyres and plastic wastes with different compositions were studied. The
mixture of plastic waste tested contained: polyethylene (PE) around 20% (w/w),
polypropylene (PP) about 30% (w/w) and polystyrene (PS) around 20% (w/w). Tyres
wastes main components were: natural rubber (NR), styrene butadiene rubber (SBR)
and butadiene rubber (BR) in an amount of 30% (w/w).
LEGEND
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N2
H2
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373 C
OUT 1100
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1- Autoclave
2 - Furnace
3 - Sttiring system
4 - Internal cooling coil
5 - Liquid sampling tube
6 - Thermocouple
7 - Gas inlet tube
8 - Tube connected to safety r
9 - Gas release tube
10- Cooling bath
11- Pressure reduction
12- Pressure gage
13- Gas meter
14- Controller
15- Furnace temperature mea
16- Autoclave temperature me
17- Sttiring speed measureme
18- Cooling coil valve control
19- Furnace temperature contr
20- PC for data acquisition
21- Gas sampling valve
20
2
3
1
10
15
Figure 1. Schematic representation of the solid waste pyrolysis installation.
Gases produced in this process were measured and analysed by gas chromatography
(GC). Liquid hydrocarbons were distilled and separated into three fractions; the lighter
one distilled from room temperature till 150°C, the other presented a distillation range
between 150ºC up to 270°C and the remaining fraction was the distillation residue. The
two lighter fractions were analysed using gas chromatography with a capillary column
associated to a mass spectrometry (GC-MS) to identify their main compounds. The
solid products were extracted, first with dicloromethane and than with tetrahydrofuran.
These fractions were also analysed by GC-MS. Pyrolysis products were characterised
by some of theirs chemical and physical properties using ASTM standard D 86 (J.
Annual Book, 1994).
3. Discussion of Results
In Figure 2 is shown the H-donor solvent effect on tyre waste pyrolysis. Solvent content
was 50% (w/w) (solvent/tyres ratio - 1). Yield values refer to initial feedstock, including
solvent amount. Pyrolysis of only tyres wastes under moderate conditions of
temperature and pressure led to a liquid yield of around 33% (w/w), which was
potentially increased by the presence of plastics wastes. In fact, pyrolysis of mixtures of
tyres and plastic wastes allowed to rise liquid yields of about 72%, probably because in
plastics wastes pyrolysis a liquid medium was formed, which favoured heat and mass
transfer process and, therefore, tyres wastes pyrolysis.
Gas
Liquid
Solid
100
90
Run temperature = 420ºC - Initial pressure = 0.41M Pa
Rection time = 30 min - Solvent content = 50% (w/w)
80
Product Yields (%)
70
60
50
40
30
20
10
0
e
line
ene
len
hal
i no
tha
t
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h
h
q
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p
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ahy
yd
y
r
h
t
h
ca
Te
tra
De
Te
sti
Pla
cs
th
Wi
ou
o
tS
n
lve
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Type of H-donor Solvent
Figure 2. Effect of the type of H-donor solvent on tyre wastes pyrolysis. Solvent content was 50%
(w/w) (solvent/tyres ratio - 1), plastics contained PE, PP and PS.
For all experiments, liquids yields were favoured by the presence of a solvent,
especially for solvents with H-donor capacities. The highest value of liquid yield was
achieved with tetrahydronaphthalene reaching 83% (w/w), which mean an increase of
about 150% in relation to the absence of solvent. The other solvents tested,
decahydronaphthalene and tetrahydroquinoline led to similar liquid yields.
Decahydronaphthalene is the liquid solvent with the highest H-donor capacity, when
converted into tetrahydronaphthalene, 3 hydrogen molecules could be released, while if
naphthalene was formed 5 hydrogen molecules would be supplied to reactional medium.
However, pyrolysis liquid yield was lower than the one obtained in presence of
tetrahydronaphthalene.
For all experimental runs, the conversion of decahydronaphthalene into either
tetrahydronaphthalene or naphthalene were probably less favoured than
tetrahydronaphthalene conversion into naphthalene, as higher liquid yields were
obtained, even if only 2 hydrogen molecules were released to reactional medium. These
hypotheses agree fairly well with the analysis of liquid fractions by GC-MS. In fact,
when decahydronaphthalene was used both tetrahydronaphthalene and naphthalene
were detected, but only in small concentrations. When tetrahydronaphthalene was used,
the amount of naphthalene detected in liquid fraction, was much higher, which agrees
with the previous hypothesis.
In Figure 3 can be analysed the effect of using higher solvent contents, 67% (w/w),
(solvent/tyres ratio - 2). The increase of solvent content presented no great changes in
liquids yields. The use of a higher content of decahydronaphthalene led to an increase of
around 13%, in liquids yield. The same rise in liquids yield was reached when a higher
plastic waste content was tested. Liquids yield obtained for these conditions
corresponded to an increase of around 93%, regarding the single pyrolysis of tyres
wastes. All the three tested solvents with H-donor capacities led to similar liquid yields,
around 86% (w/w).
Product Yields (%)
Gas
Liquid
Solid
Run temperature = 420ºC - Initial pressure = 0.41M Pa
Rection time = 30 min - Solvent content = 67% (w/w)
100
90
80
70
60
50
40
30
20
10
0
yd
ca h
e
D
ro
a
hth
nap
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dro
line
h
pht
n
al e
e
s
sti c
Pla
a
on
ydr
h
a
tr
Te
Type of H-donor Solvent
th
Wi
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tS
lve
nt
Figure 3. Effect of the type of H-donor solvent on tyre wastes pyrolysis. Solvent content was 67%
(w/w) (solvent/tyres ratio - 2), plastics contained PE, PP and PS.
In Figure 4 may be analysed the effect of each individual type of plastics waste on
pyrolysis products yields. These results may also be compared with the ones referring to
the absence of plastic waste. Both PE and PS favoured liquid yields, leading to values
near 60% (w/w), whilst in presence of PP, liquid yield was around 50% (w/w). Both PE
and PS led to an increase in liquid yields of about 76%, in relation to the absence of
plastic waste. In presence of PE the liquid yield was achieved, probably, at the expenses
of solid yield, which was about 24% (w/w), as similar gas yields were obtained either in
presence or in absence of PE. On the other hand, in presence of PS liquid yield might be
a major consequence of a substantial decrease in gas yield, around 66%, whilst solid
yield decreased about 30%, in relation to the absence of plastic.
Gas
Liquid
Solid
100
Run temperature = 420ºC
Initial pressure = 0.41M Pa
Rection time = 30 min.
90
Product Yields (%)
80
70
60
50
40
30
20
10
0
PE (50%)
PP (50%)
PS (50%)
Without
Type of Plastic
Figure 4. Effect of H-donor individual solid solvent on 50% tyre waste content products yields
Gas
Liquid
Solid
100
Run temperature = 420ºC
Initial pressure = 0.41M Pa
Rection time = 30 min.
90
Product Yields (%)
80
70
60
50
40
30
20
10
0
0
10
20
30
40
50
60
70
80
90
100
Tyre (% w/w)
Figure 5. Effect of tyre content on products yields.
Figure 5 illustrates the effect of tyre content on products yields. These experiments were
done with a plastic composition of 55% (w/w) PE, 30% (w/w) PP, and 15% (w/w) PS
and clearly show that the presence of plastics wastes in pyrolysis of tyres favoured the
formation of liquid compounds, reaching yields higher than 60% (w/w) in the case of
pyrolysis single plastic wastes. Also, it is possible to observe that the lowest value of
solid yield (12% w/w) was achieved for the same conditions. When tyres wastes content
increased, solid yields were found to rise, reaching the highest value of 51% (w/w),
when only tyres wastes were pyrolysed. It is also possible to observe a substantially
decrease in both liquids and gases yields, of around 45%, when tyres wastes increased
from 0 to 100% (w/w).
4. Conclusions
The work done so far on tyre wastes pyrolysis has shown that the presence of a proper
solvent favoured higher liquid yields and lower solid yields. The liquid H-donnor
solvent which led to the highest yield (83% w/w) was tetrahydronaphthalene. Regarding
other solvents with H-donor capacities, both decahydronaphthalene and
tetrahydroquinoline led to similar liquid yields, , although decahydronaphthalene is
theoretically the liquid solvent with the highest H-donor capacity. However, analysis of
liquid fractions by GC-MS showed that in presence of this solvent only small
concentrations of tetrahydronaphthalene and naphthalen were detected. When
tetrahydronaphthalene was used, the amount of naphthalene detected in liquid fraction
was much higher, which helps to explain the higher liquid yields obtained and seams to
show that tetrahydronaphthalene presents the highest H-donor capacity to the reactional
pool, under the specific conditions tested for the pyrolysis.
When a higher amount of solvent was added to the pyrolysis medium only small
increases in liquid yields were detected and all the tested solvents with H-donor
capacities led to similar liquid yields.
The addition of plastics wastes (PE, PP, PS) into pyrolysis of tyres wastes was
beneficial as higher liquid yield were produced, even if lower than those obtained in
presence of H-donor solvents; in fact, the rise of liquid yield was around 72% in relation
to pyrolysis of only tyres wastes. Plastics wastes are significantly less expensive than Hdonor solvents and can be easily found in great amounts; therefore they can be
successfully used to increase tyres wastes performance.
References
Conesa J. A., R. Font, A. Marcilla, J. A. Caballero, 1997, J. Anal. Appl. Pyrolysis, Vol. 40-41,
419.
Elizabeth A. Williams, Paul T. Williams, 1997, Anal. Appl. Pyrolysis Vol.40-41, p.347.
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