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A NNALS of Faculty Engineering Hunedoara – International Journal of Engineering
Tome XVII [2019] | Fascicule 3 [August]
159 | Fascicule3
1.Adewale George ADENIYI, 2.Saint O. OSEMWENGIE, 3.Joshua O. IGHALO
PRODUCTION OF SYNTHETIC FUELS FROM HIGH DENSITY POLYETHYLENE
(HDPE) WASTE THROUGH PYROLYSIS: EXPERIMENTAL AND SIMULATION
APPROACHES
1,3..
Chemical Engineering Department, Faculty of Engineering and Technology, University of Ilorin, Ilorin, NIGERIA
2
.Department of Chemical Engineering, Faculty of Engineering, University of Benin, NIGERIA
Abstract:
Production of synthetic fuels through the pyrolysis of high density polyethylene (HDPE) waste was examined. A semi-
batch reactor was designed and fabricated to pyrolyse HDPE waste in the absence of catalyst at a residence time of 60 minutes
and final temperature of 425oC. At the end of the process, 51.84%, 45.33%, and 2.83%, liquid product, char and evolved gaseous
product were obtained respectively. The temperature progression for the process was examined. The pyrolysis oil obtained was
light brown in colour, highly flammable, with a density of 772.6 kg/m3 and pH of 5.5. ASPEN Hysys was used to simulate the
pyrolysis process. Simulation results revealed an oil yield of 97%, a gas yield of about 2% and a <1% char yield. The simulation
revealed the commencement of reaction at about 325oC with an optimum reaction temperature of 450oC. If properly optimized
HDPE is revealed to be an excellent feedstock of pyrolysis.
Keywords:
Pyrolysis, HDPE, Synthetic fuels, ASPEN Hysys, Simulation
1. INTRODUCTION
Due to economic growth and changing consumption and production patterns, there has been a rapid increase in the
generation of waste plastic in the world. In the United States for example, municipal waste generation rose from 88.1
million tons in 1960 to 250.9 million tons in 2012 [1]. These figures rise at a rate of 5% yearly [2]. The increase in generation
has led to plastics waste becoming a major stream in solid waste. After food waste and paper waste, plastic waste is the
major constituent of municipal and industrial waste in cities [3]. Even cities with low economic growth have started
producing more plastic waste due to an explosion of the applicability of the different types of plastics. This increase has
now led to a major challenge for the authorities responsible for waste management. European Union (EU) has already
mandated that by 2020, all plastic waste must go to mechanical, thermal or chemical processing facilities. No more waste
will be allowed in landfills [2].
In countries where there is a lack of integrated solid waste management, most of the plastic waste is neither collected
properly nor disposed in appropriate manner to avoid its negative impacts on the environment and public health. Landfill
and incineration are two common means of plastic waste disposal which are currently being used which have severe
impact on the environment [4]. 60% of all plastic solid waste currently goes to landfill [2]. Landfill could result to plastic
additives such as phthalates and various dyes polluting ground water. Incineration results in the formation of
unacceptable emissions of gases such as nitrous oxide, Sulphur oxides, dusts, dioxins and other toxins. On the other hand,
plastic waste recycling can provide an opportunity to collect and dispose plastic waste in the most environmentally
friendly way and it can be converted into a resource.
Recycling of plastic already occurs on a wide scale. Extensive recycling and reprocessing of plastics are performed on
homogenous and contaminant free plastic wastes into new plastic products with a lower level of quality. The
disadvantage of such an approach is that the product no longer has the special characteristics of the plastics used to make
it. Thus, the product is less useful and end up competing for markets with cheap construction materials. The presence of
non-plastic contaminants leads to concern from potential buyers about product quality and consistency. For this reasons,
mechanical recycling of mixed plastic waste appears to have only a limited future. Tertiary recycling returns plastic to their
constituent monomers or to a higher value hydrocarbon feedstock and fuel oil. Tertiary recycling includes all those
processes which attempt to convert the plastic wastes to basic chemicals by the use of chemical reactions such as
hydrolysis, methanolysis and ammonolysis for condensation polymers, and to fuels with conventional refinery processes
such as pyrolysis, gasification, hydro-cracking catalytic cracking, coking and vis-breaking.
Pyrolysis and catalytic conversion of plastics is a superior method of reusing the waste. The distillate product (pyrolysis oil)
is an excellent fuel and makes the process one of the best, economically feasible and environmentally sensitive recycling
systems in the world today. In the turn of the millennium, there were as much as 100 small scale pilot plants globally
processing more than 4000 tonnes of residue per year [5]. Pyrolysis plants are typically used to degrade carbon rich organic
and inorganic materials such as biomass, municipal and industrial waste.
A lot of work has already been done as regarding pyrolysis technology as it applies to plastic materials. Low, Connor [6]
described a simple pyrolysis reactor system, results of their pyrolysis tests showed that pure samples of polyolefinic and
polystyrenic resins can easily be pyrolysed to produce liquid yields in excess of 70%. Feng [7] optimized the processes of
ANNALS of Faculty Engineering Hunedoara – International Journal of Engineering
Tome XVII [2019] | Fascicule 3 [August]
160 | Fascicule3
plastic pyrolysis for maximizing the diesel range products and designed a continuous pyrolysis apparatus as a semi-scale
commercial plant. Caruso William, Danielle Sorenson [5] examined the viability of plastic pyrolysis as an alternative energy
technology that will serve as a solution for urban carbon reduction. They concluded that it is quite feasible and has good
prospects. Thorat, Sandhya Warulkar [8] prepared a report to show that pyrolysis oil is a truly sustainable waste solution
utilizing the embodied energy content of plastics and producing a highly usable commodity. Recently, Guarav, Madhukar
[3] pyrolysed low density polyethylene to get fuel oil at a yield of about 70% of the product with a simple reactor design.
As regards high density polyethylene (HDPE) in particular, studies has been carried out to understand the kinetics [9-14],
yield [15, 16], characteristics [10, 11, 17] and simulation [18, 19] of the pyrolysis process.
The aim of this research is to investigate the yield of high density polyethylene waste as feedstock for the production of
synthetic fuels via the pyrolysis process with a close investigation on the effect of some subtle factors like the reactor
design and temperature progression. A predictive simulation model using ASPEN Hysys 2006 was also developed for the
process and was used for a sensitivity analysis and a reference system in predicting product yield and process
behaviour/response to operating factors. This study comes at a time when concerted effort is made worldwide to curtail
the amount of plastic waste going to landfill and incineration. These processes have been considered harmful to the
environment. Plastic waste generation has been on the rise over the past few decades as more applications of plastics are
being explored. This has led to the current status quo of which, numerous processes for recycling these wastes are
currently investigated and optimized. Pyrolysis is
one of the most recent and promising recycling
techniques that researchers have come up with.
Any investigation into the pyrolysis process such as
this is by all means relevant as it would have
contributed to the cause of developing and
optimizing current recycling techniques for plastic
waste.
2. METHODOLOGY
Design of the semi-batch reactor
The first stage was the design and fabrication of a
miniature semi-batch plastic pyrolysis reactor for the
purpose of this study. A schematic diagram of the
design is presented in Figure 1 while the specification sheet for the reactor is presented in Table 1.
Table 1. The specification sheet for the reactor
Identification: Semi-Batch Plastic Pyrolysis Reactor
By: Adeniyi, A.G., Osemwengie S. O. and Ighalo Joshua O
Function; Non-catalytic pyrolysis of high and low density polyethylene (HDPE & LDPE)
Operation; Semi-batch
Design Data
Reactor vessel
Material Handled; HDPE
Temperature; ≤4500C
Material of construction; Aluminum
Pressure; 2 bars
Shape; Cylindrical body with a tori-spherical top
Diameter; 26cm
Height; 17cm
1product escape route
Condenser
Materials handled: Product vapour
Temperature; ≤4500C
Material of construction: Copper
Pipe diameter; 3/8in
3 rings
8cm wide
17cm high
Product escape Pipe
Material of construction: Steel
Material handled; Product vapour
Length; 75cm
Pipe diameter; 3cm
Peripherals: Globe valve, pressure gauge, thermometer & cork, Nitrogen
Additional information: Personal protective equipment (PPE) includes Chemical respirator and high temperature gloves
Method for the Experiment
The second step was to source for samples of high density polyethylene (HDPE) present in municipal waste and practically
pyrolysing them. Samples of high density polyethylene (HDPE) in the form of jerry cans was picked from different dump
sites of municipal plastic waste in University of Benin, Benin-city, Edo State. The HDPE jerry cans were broken into large
pieces and then thoroughly washed with detergent and water to remove dirt. They were afterwards rinsed with clean
water. The HDPE samples were sundried from one (1) day. They were then cut into smaller sizes of about 1mm by 1mm.
This was done to conserve the effective density of the plastic and to ensure heat flow through the sample at the initial
stage of heating. The samples were then weighed with the Mettler PM4800 electric weighing balance.
The reacting vessel was cleaned and 1kg of high density polyethylene (HDPE) samples was introduced. The reactor was
sealed. With the product escape valve open, a stream of gaseous nitrogen was introduced into the system for 60 seconds.
This was to purge out the oxygen present in the reactor and provide an inert environment for pyrolysis to take place. The
product escape valve was closed and the thermometer and cork for temperature measurement was introduced. The entire
Table 1: The specification sheet for the reactor
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Tome XVII [2019] | Fascicule 3 [August]
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set up (of the reactor) was heated by an LPG burner. The rate of heating of the reactor was approximately 7OC/min and
constant monitoring of the process was ensured. The product escape valve was opened at 270OC though pyrolysis
commences at a higher temperature, this was done to avoid any pressure buildup within the reactor. A slight positive
pressure was noticed throughout the experiment. The experiment
lasted for one (1) hour after no significant formation of product was
noticed signaling the end of the pyrolysis process.
Method for the Simulation Study
ASPEN Hysys 2006 was used in modelling the pyrolysis of high density
polyethylene. The reaction stoichiometry can be used to represent
the reaction sequence. The sequence is represented in Figure 2.
As observed from the sequence, secondary reactions are considered
to be absent as the process is approximated as a direct conversion of
the feedstock to the respective products. The equation of reaction
utilized is chosen in such a way that the above sequence can be easily represented. The equation ids presented below and
is a modification of that of Alla and Ali [19]. The reaction was stipulated to occur in the vapour phase alone.
LDPE = H2+ C1+ C2+⋯+ C10 + C11 +⋯+ C25 +20C
The kinetics of waste HDPE has been extensively studied by numerous researchers [9-14]. For this study, the kinetic
parameters for waste HDPE pyrolysis obtained by Kayacan and Doğan [14] at a heating rate of 5 K/min was utilized. The
reported values are A = 3.42 E27 s-1 and E = 420.86 KJ/mol. The Arrhenius rate equation is given by the expression in
equation 1
K = A Exp(−E
RT) (1)
where K (s-1) is the rate constant, A (s-1) is the pre-exponential factor, E (KJ/mol) is the activation energy, T (K) is temperature
and R is the universal gas constant 8.314 KJ/molK.
The Peng-Robinson (PR) property package was used utilised in the simulation. The databank in Aspen HYSYS 2006 does
not contain any polymers. The method used was to represent the HDPE polymer feed as a hypothetical component. The
chemical species used to model the feed is ethylene. This information informs the software of the elemental composition
of the feedstock. However, to completely predict all other physical and chemical properties of the feedstock, Aspen HYSYS
requires the stipulation of three properties; density, molecular weight and normal boiling point. These properties were
inputted into the hypothetical component
manager and they served as the basis with
which the software estimated the other
necessary information about the polymer. The
properties of HDPE inputted into the
hypothetical component manager includes a
Density of 940 kg/m3 [20], Molecular weight of
46200 g/mol [20], Normal boiling point of
2700C (predicted from experiment) and
Normal melting point of 1250C (predicted from
experiment). Other reaction products (H2,
carbon, C1-C24) were added as conventional
components in the simulation. The process is
represented in Figure 3.
The feed (1kg/hr) is sent into the reactor at ambient conditions (250C) and 1atm. The continuous stirred tank reactor (CSTR)
was chosen as the pyrolyser in the simulation as the reaction kinetics and stoichiometry could be easily implemented on
it. The pyrolysis temperature was set 4250C. The bottom product from the reactor was removed as the char stream while
the vapour stream was sent to a condenser unit operating at 250C and ambient presssure. The non-condensable gases
leave the condenser as top product synthesis gas stream while the condensate pyrolysis oil leave in a bottom stream.
Several assumptions were taken in developing the simulation model. The pyrolysis reaction was considered to take place
only in the vapour phase. The process was studied at steady state hence time is not included as a factor. The char was
assumed to be composed solely of elemental carbon. In practical scenarios, bio-char from pyrolysis sometimes contain
heavy metals but this will not be implemented in the simulation
3. RESULTS AND DISCUSSION
Considering the Table 1, it can be observed that about half the high density polyethylene sample was converted to liquid
product. Just over 45% was residual char while the rest was gaseous product. Looking at this result, it must be said that
the amount of char produced by the process is worryingly large. This is due to the design intricacies of the reactor. When
Figure 2: The Reaction Stoichiometry Sequence
Figure 3: The Pyrolysis of HDPE Waste
ANNALS of Faculty Engineering Hunedoara – International Journal of Engineering
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the heating rate is not rapid and product elutriation is
not optimal due to reactor shape, the pyrolysis
process tends to be slow hence favouring higher char
formation. Several other factors may likely have
contributed to this but a clearer picture will be
obtained when these results are juxtaposed with
those of other researchers.
Low, Connor [6] pyrolysed High Density Polyethylene
(HDPE) using a semi-batch laboratory apparatus
setup at a temperature of about 400OC for 80 minutes.
They obtained 70% liquid product, 28% gas and 2%
residue. Residue obtained was characteristically low
too. Though both experiments are similar in terms of
catalysis, residence time and temperature, the type of
reactor used are quite dissimilar. An experimental
laboratory setup will always perform higher than
larger scale setup. There is minimal temperature variation in an experimental apparatus compared to larger scale setup.
Also, the removal of product from the reaction chamber is more efficient as it approximates ideality far more than larger
scale setup. A deviation from ideality usually occurs in reactors in the form of stagnant regions, short-circuiting and
channeling [21]. Feng [7] pyrolysed virgin HDPE in a batch
apparatus at an end temperature of about 800OC in an
experiment lasting for 60 minutes. He obtained 83.5% liquid
product, 0.5% char and 16% gaseous product. The result of
this experiment is quite similar to that obtained by Low,
Connor [6] which displays a fantastic amount of liquid
product yield and very little amount of left over residue. The results of Williams and Williams [15] were also quite similar
to those of Feng [7] albeit at a heating rate of 250C/min and a final temperature of 7000C. We notice that pyrolysis char
formation can be minimized by utilizing very high temperatures for the process. However, considering the energy
requirement of such a process, economic considerations should be considered and low temperature optimization of the
process becomes a viable option.
The result of Achilias, Roupakias [16] (Table 3) for waste HDPE at 4500C is fairly consistent with that of the current study.
Though utilizing a fixed bed reactor, their process was catalytic (FCC) and the setup was laboratory scale. There was a
constant stream of nitrogen purge gas has product elutriation was faster. Reported residence time was 17 minutes. Alla
and Ali [19] developed a simple simulation model for polyethylene pyrolysis and for a temperature of 4500C obtained
95.2% oil, 4.6% gas and <1% char. A different and more detailed and predictive model was developed in this study.
Table 3: Comparison of this Work with Previous Work
Product Current study
Achilias et al.
(2007)
Low et al. (2001) Feng (2010)
Williams and Williams
(1997)
Temp
4250C
4500C
400OC
800OC
700OC
Char
45.33%
52.50%
2%
0.50%
0.00%
Liquid
51.84%
44.20%
70%
83.50%
79.72%
Gas
2.83%
3.30%
28%
16%
16.77%
Temperature-Progression Profile for HDPE Pyrolysis
After the first 2-3 minutes of heating, no significant change occurred in the reactor. The metal of the reactor body is still
heating up and little energy reaches the polymer. Subsequently, the temperature of the polymer rose at a steady rate
(approx. 7OC/min) for about 13 minutes till the 125OC threshold was attained. There was an interesting thermal behavior
at this point because upon heating, little or no change occurred in the temperature of the system for a while. This was
due to the fact that the polymer samples melted somewhere within 125OC to 130OC. The latent heat of fusion makes it
impossible for the temperature to climb, as extra work needs to be done (in the form of heat) to change the state of matter
from solid to liquid. Beyond this temperature, a steady rise was re-established and similar to the pre-melting point
temperature rise of about 7OC/min. Once again, after about 30 minutes, the lag was observed again. It occurred at about
270OC. This is also likely to be the vapourisation temperature. At temperatures beyond 270OC, a lot began to happen
within short time intervals. As the temperature rose, the initial stage of pyrolysis commenced. The long-chain polymers
break into compounds of shorter chain length and the first pyrolysis products was evolved. The initial liquid product
appeared to be waxy because the polymer chains in the product are still quite long as only primary pyrolysis has taken
place. Beyond 350OC, the temperature climbs very rapidly (1000C in about 10 minutes). It is during this period that the
Table 2: Product yield from the pyrolysis experiment
Product
Yield
Char
45.33%
Liquid
51.84%
Gas
2.83%
Pyrolysis temperature; 425
0
C, Residence time; 1 hour
Figure 4: Temperature-Progression Profile for HDPE Pyrolysis
ANNALS of Faculty Engineering Hunedoara – International Journal of Engineering
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main bulk of the product was collected. The oil produced during this period was rather clear. Beyond 400OC, there was
no longer any significant rise in the temperature of the reactor with time. Most of the products have being collected and
only a little amount of the oil was being evolved. The process was finally stopped at about 4250C when it was noticed that
no significant amount of oil and synthesis gas was being produced by the system.
Feng [7] also monitored the temperature profile of plastic pyrolysis albeit with mixed polymer samples. Though allowing
his experiment to run for about 300 minutes, temperature rise only occurred approximately within the first 80 minutes of
reaction, and he obtained a profile quite similar to the one obtained in this research work. However, due to his
experimental methodology and other factors (such as heating rate), the latent heats (of fusion and of vapourisation) were
not noticed. However, Low, Connor [6] using a heating rate of 33OC/min for a residence time of 80 minutes obtained
similar results. Several factors affect the nature of temperature progression in pyrolysis reactions of paramount significance
is the reactor type. The design intricacies of the reactor will determine where the temperature readings will be taken from.
It can be from the liquid, the vapour, the inner wall of the reactor, reflux inlet or outlet etc. Another factor is the starting
polymer material; different polymers will pyrolyse at different temperature ranges. Heating rate and the presence of
catalyst are also important factors.
Basic Properties of the Pyrolysis Oil
The pyrolysis oil obtained this research work was observed to be of light brown colour. This result (Table 4) is consistent
other experimental results. It is highly flammable as expected of any combustion fuel and burns in air without any residue.
The pH was found to be 5.5. Cole [22] noted that pyrolysis fuels are generally acidic hence tends to corrode most steel
storage tanks, fuel lines and engine parts. The density of the pyrolysis oil obtained is 772.6kg/m3. Guarav, Madhukar [3]
obtained a liquid product with a density of 702.5 kg/m3 and Low, Connor [6] obtained a liquid product with a density of
750 – 800 kg/m3 range. The density of conventional diesel
is 830kg/m3. The density obtained in this research work is
consistent with those obtained by other researchers. As
revealed by Wongkhorsub and Chindaprasert [23] this oil
can serve as a suitable substitute to diesel, albeit at a lower
performance level and higher production cost.
Simulation results
The results in Table 5 show the oil yield of HDPE pyrolysis
from the simulation. Results of Alla and Ali [19] for
polyethylene pyrolysis is noticeably similar than those of
the simulation. Simulation results will always
demonstrate above average oil yields because they are
essentially idealized and many of the extraneous factors
obtainable in conventional experiments cannot be
incorporated into these softwares. Accuracies are also
determined by the chosen reaction scheme, kinetics
and approximate stoichiometry. These results however
prove that if properly optimized, high density
polyethylene is an excellent feedstock for pyrolysis will
give very good yield of bio-oil.
The reaction conversion from the CSTR was monitored
with the temperature of reaction to obtain the plot
given in Figure. It reveals the commencement of HDPE pyrolysis at about 3250C and optimal reaction conversion at 4500C.
From experiment, a majority of the products was evolved from the system between 3500C and 4000C. We can surmise that
the optimum reaction temperature for high density polyethylene pyrolysis is 4500C. The above profile is consistent with
experimental results for obtained for other plastics by researchers [24, 25].
4. CONCLUSIONS
This research work is simply an attempt to pyrolyse high density polyethylene (HDPE) waste with an eye to obtain very
valuable pyrolysis oil while simultaneously proffering solution to the malignant issue of plastic waste disposal. This aim
was to be achieved by estimating the amount of pyrolysis oil obtained as well as examining the temperature progression
for the process. A semi-batch plastic pyrolysis reactor was designed and fabricated for this purpose. HDPE samples were
pyrolysed in the absence of catalyst at a final temperature of 425OC and a residence time of 60 minutes. 51.84% of liquid
product was obtained, 45.33% char remained in the reactor while the remaining 2.83% was gaseous product. The pyrolysis
oil obtained was a light brown highly flammable liquid with a density of 772.6 kg/m3 and a pH of 5.5. ASPEN Hysys was
used to simulate the pyrolysis process. The reactor was modelled by a continuous stirred tank reactor requiring both
stoichiometric and kinetic information. The model functioned as a reference system in predicting product yield and
Table 4: Table Basic Properties of the Pyrolysis Oil
Property
Value
Colour
Light brown
Flammability
Highly Flammable. Burns without residue
pH
Acidic (5.5)
Density
772.6kg/m3
Table 5: Oil yield of HDPE Pyrolysis from the Simulation
Products
Current Study
Alla and Ali (2014)
Temperature
4500C
4500C
Char
0.20%
<1%
Liquid
97.43%
95.20%
Gas
2.37%
4.60%
Figure 5: The reaction conversion profile
0
20
40
60
80
100
250 275 300 325 350 375 400 425 450 475 500 525 550
Conversion (%)
Temp (
O
C)
ANNALS of Faculty Engineering Hunedoara – International Journal of Engineering
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process behaviour/response to operating factors (such as temperature). Simulation results revealed a bio-oil yield of 97%,
a gas yield of about 2% and a <1% char yield. The simulation revealed the commencement of reaction at about 3250C
with an optimum reaction temperature of 4500C. If properly optimized high density polyethylene is revealed to be an
excellent feedstock for the pyrolysis process.
Due to the fact that a good product yield is obtainable, waste plastic pyrolysis can be a lucrative venture. Entrepreneurs
are hereby encouraged to explore this possibility. Investors should make concerted effort to construct large-scale pyrolysis
plants (especially in the major cities where plastic wastes are readily available) to help deal with the issue of plastic waste
disposal. A major issue noticed is a lack of market (worldwide) for pyrolysis oil because it is considered unstable and acidic
[26]. Researchers should investigate means of modification and refinements of the oil so as to enable it compete favourably
in the market.
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copyright © University POLITEHNICA Timisoara, Faculty of Engineering Hunedoara,
5, Revolutiei, 331128, Hunedoara, ROMANIA
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