sustainability

Article
Effects of Heating Rate and Temperature on the Yield of
Thermal Pyrolysis of a Random Waste Plastic Mixture
José Manuel Riesco-Avila 1, * , James R. Vera-Rozo 1,2 , David A. Rodríguez-Valderrama 1 ,
Diana M. Pardo-Cely 1 and Bladimir Ramón-Valencia 2

1 Mechanical Engineering Department, Engineering Division, Campus Irapuato-Salamanca,

University of Guanajuato, Guanajuato 37320, Mexico; jr.verarozo@ugto.mx (J.R.V.-R.);
da.rodriguezvalderrama@ugto.mx (D.A.R.-V.); dm.pardocely@ugto.mx (D.M.P.-C.)
2 Mechanical Engineering Program, University of Pamplona, Pamplona 543050, Colombia;
hbladimir@unipamplona.edu.co
* Correspondence: riesco@ugto.mx

Abstract: Effects of heating rate and temperature on thermal-pyrolytic yield of a plastic-waste

mixture were studied in a semi-batch reactor. The temperature in the range of 380–460 ◦ C and heating
rates of 10, 19, and 28 ◦ C/min were evaluated through an experimental multi-level design. The
results show that higher temperatures or lower residence time reduce the yield of pyrolytic oil at the
expense of increasing the yield of gaseous products. The maximum liquid yield was 69%, obtained
at 410 ◦ C and a heating rate of 10 ◦ C/min. The composition of pyrolytic oil covers a wide range of
hydrocarbons; thus, a fractionation is necessary before using it as fuel in internal combustion engines.
The fractionation process yielded 21.12 wt% of light fraction (gasoline-like), 56.52 wt% of medium
fraction (diesel-like), and 22.36 wt% of heavy fraction (heavy diesel-like). The light fraction has an
Citation: Riesco-Avila, J.M.; octane index and caloric value within the range of the typical gasoline values. On the other hand,
Vera-Rozo, J.R.; Rodríguez- the cetane index and caloric value of the medium fraction meet the requirements of the standards
Valderrama, D.A.; Pardo-Cely, D.M.; for diesel.
Ramón-Valencia, B. Effects of
Heating Rate and Temperature on the Keywords: waste plastics; pyrolysis; fuels
Yield of Thermal Pyrolysis of a
Random Waste Plastic Mixture.
Sustainability 2022, 14, 9026. https://
doi.org/10.3390/su14159026 1. Introduction
Academic Editors: Shuangqiao Yang, Plastic is a light, flexible, versatile, and cheap material, so it is used to manufacture
Qingye Li and Dong Tian countless products in all kinds of sectors (home, commerce, industry, agriculture, construc-
Received: 19 June 2022
tion, automotive, etc.). However, the increase in the use of these products is causing the
Accepted: 16 July 2022
growing of plastic waste, together with its slow degradation, which represents a serious
Published: 23 July 2022
threat to the environment [1–3]. It is estimated that in Latin America and the Caribbean,
12% of urban solid waste is plastic, and despite the significant progress that has been
Publisher’s Note: MDPI stays neutral
made in waste management, the region still faces many challenges that require special
with regard to jurisdictional claims in
attention, such as low rates of recovery of urban solid waste, which are less than 4% [4]. The
published maps and institutional affil-
correct disposal of waste together with the recycling process helps to reduce the pollution
iations.
generated. In addition, it allows for revaluing the non-renewable resources. The ASTM D
5033 [5] and ASTM D 7209 [6] standards propose the reduction of waste, the recovery of
resources, and the use of recycled polymeric materials and products using mechanical or
Copyright: © 2022 by the authors.
thermal methods.
Licensee MDPI, Basel, Switzerland. Plastic recycling has received a lot of attention and many techniques have been de-
This article is an open access article veloped to improve it. Some of these procedures began to develop in the 1970s when
distributed under the terms and some countries began to incinerate their plastic waste. Since then, there have been many
conditions of the Creative Commons advances in the way plastics are recycled, resulting in four types of recycling: primary,
Attribution (CC BY) license (https:// secondary, tertiary, and quaternary [7–9].
creativecommons.org/licenses/by/ The primary recycling process is fundamentally the same for the different types
4.0/). of plastics. It consists of separating, cleaning, pelletizing, molding, injection molding,

Sustainability 2022, 14, 9026. https://doi.org/10.3390/su14159026 https://www.mdpi.com/journal/sustainability

Sustainability 2022, 14, 9026 2 of 12

compression molding and, thermoforming. This process produces a high-quality raw

material, similar to that of the original polymer. Secondary recycling differs from primary
recycling only in that here there is no separation of the plastics to be recycled, so the
products obtained have inferior properties to those of the original polymer. This method
eliminates the need to separate and clean, and thus the mixed plastics (including foil caps,
paper labels, powder, etc.), are ground and melted inside an extruder.
Tertiary recycling degrades the polymer to basic chemical compounds and fuels. This
type of recycling is different from the first two, fundamentally because it involves a chemical
change, not just a physical change. In tertiary recycling, the long polymer chains are broken
down into small hydrocarbons (monomers) or carbon monoxide and hydrogen. Currently,
tertiary recycling has three main methods: chemolysis/solvolysis (hydrolysis, alcoholysis,
glycolysis, and methanolysis), gasification or partial oxidation, and cracking or pyrolysis
(thermal and catalytic) [10].
Quaternary recycling consists of heating the plastic to use the thermal energy released
from this process to carry out other procedures, which means plastic is used as a fuel
to recycle energy. Incineration can be included in this classification, provided that the
recovery of thermal energy is accompanied by a steam generator or by the direct use of
high-temperature gases in a process that requires an external heat source.
Of the different methods described above, pyrolysis offers the possibility of overcom-
ing the limitations of mechanical recycling (primary and secondary), which requires large
quantities of clean, separated, and homogeneous plastic waste to guarantee the quality of
the final product [11]. In pyrolysis, the classification and cleaning of the different types of
plastic waste are not necessary, and it is possible to process waste contaminated with food
and chemical products, such as insecticides, herbicides, and fertilizers, reducing classifica-
tion and cleaning costs. Pyrolysis of plastic waste consists of chemical decomposition by
thermal degradation in the absence of oxygen. Pyrolysis can be thermal or catalytic [12].
During pyrolysis, plastic waste is subjected to high temperatures, where its macromolecules
break down into smaller molecules, resulting in the formation of a wide range of hydrocar-
bons. The products obtained from pyrolysis can be divided into a liquid fraction (consisting
of paraffins, olefins, naphthenes, and aromatics), a non-condensable gaseous fraction, and
solid waste. The precise composition of the liquid fraction depends on the properties of the
raw material and the conditions under which the process is carried out.
In this work, the results obtained from the thermal pyrolysis of a mixture of polypropy-
lene (PP), low-density polyethylene (LDPE), and high-density polyethylene (HDPE) residues
are presented. The current study aimed to evaluate the influence of the heating rate and
temperature on the pyrolytic-process yield.

2. Plastic-Waste Thermal Pyrolysis (PP, LDPE, HDPE)

According to the Organisation for Economic Co-operation and Development (OECD) [13],
global annual plastic waste was 353 million tons in 2019. Of those, only 9% were recycled,
19% were incinerated, and almost 50% went to sanitary landfills. The remaining 22% was
disposed of in uncontrolled dumpsites, burned in open pits, or leaked into the environment.
The generation of plastic waste is strongly related to how plastics are used. At the waste
stage, the ease of recycling and the potential mobility when lost to the environment are in-
fluenced by polymer type, dimensional shape, particle size, additive mix, and the items and
materials appended in assembly. The predominance of PP, LDPE, and HDPE has become
even greater in the waste stage because they are often used for packaging applications with
short lifetimes. In Mexico, The National Association of Plastics Industries (ANIPAC, by
its Spanish acronym) presented in 2019 the results of the first Quantitative Study of the
Plastics Recycling Industry [14], which reveal that, at national level, the total volume of
recycled materials amount to 1 million 913 thousand 710 tons per year. The report indicated
that the material that is most recycled in the country is polyethylene (LDPE and HDPE),
representing 51.2%, while polypropylene represents 18.2%.
Sustainability 2022, 14, 9026 3 of 12

PP, LDPE, and HDPE wastes have a great potential to be used in the pyrolytic process
since they can produce high liquid yield depending on the setup parameters. Many studies
have been conducted on pyrolysis of these plastics at different operating parameters to
investigate the product yield obtained. Table 1 summarizes the temperature ranges and
heating rates reported to optimize liquid-oil yield in PP, LDPE, and HDPE wastes in
thermal pyrolysis.

Table 1. Summary of studies on PP, LDPE, and HDPE wastes in thermal pyrolysis.

Type of Temperature Heating Rate Liquid Yield Gas Yield Solid Yield
Reference
Plastic [◦ C] [◦ C/min] [wt%] [wt%] [wt%]
PP 300–740 6–25 69.8–92.3 4.1–28.8 0.12–3.60 [15–17]
LDPE 425–600 3–10 51.0–95.0 5.0–24.2 0.16–7.50 [15,17,18]
HDPE 450–650 5–25 68.5–91.2 10.0–31.5 0.00–5.00 [16–18]

As summarized in Table 1, it can be concluded that LDPE produced the highest liquid
oil yield (95.0 wt%), followed by PP (92.3 wt%) and HDPE (91.2 wt%) in thermal pyrolysis.
The most effective temperature to optimize the liquid-oil yield in plastic pyrolysis would
be in the range of 500–550 ◦ C [15].
As previously mentioned, the pyrolytic process has an added advantage over the
recycling process since there is no need for sorting or cleaning the different types of plastic
waste and it is possible to process contaminated waste. The potential of mixed-plastic-
waste thermal pyrolysis has been explored by several researchers. Particularly, the thermal
pyrolysis of PP, LDPE, and HDPE mixtures has been studied by Donaj et al. [19] in a
lab-scale, bubbling, fluidized-bed reactor with a capacity of 1–3 kg/h and Papuga et al. [20]
in a fixed-bed pilot reactor with a capacity of 200 g. Table 2 summarizes the results obtained
in these investigations.

Table 2. Summary of thermal pyrolysis of PP, LDPE, and HDPE mixtures.

Plastic Mix Temperature Residence Time Liquid Yield Gas Yield Solid Yield Reference
%PP %LDPE %HDPE [◦ C] [h] [wt%] [wt%] [wt%]
650 3.25 48.40 36.90 15.70
24 46 30 [19]
730 2.98 44.70 42.40 13.90
400 18.89 41.24 39.86
500 1.0 30.66 67.91 1.43 [20]
450 26.68 47.87 25.46
40 35 25
475 28.26 59.99 11.75
500 0.75 32.80 65.75 1.46
525 28.80 69.98 1.23

As shown in Table 2, since these studies were carried out under different experimental
conditions, in different types of reactors, and with different percentages in the mixture,
the comparison could be quite complex. Nevertheless, some conclusions can be made. In
comparison to single-plastic pyrolysis, the pyrolysis of mixed plastics produced a lower
liquid yield of less than 50 wt%. High temperature and long residence time were the best
conditions to maximize gas production. However, these conditions are opposite to the
parameters to maximize oil production. For the fixed-bed reactor, the maximum liquid
yield is obtained at 500 ◦ C, which agrees with the single-plastic pyrolytic results.

3. Materials and Methods

3.1. Raw Material Characteristics
The raw material used in this work comes from a traditional mechanical recycling
company in the state of Guanajuato, Mexico. This company processes between 1200 and
1500 tons of plastic waste per month, mainly PP, LDPE, and HDPE. However, during
 1500 tons of plastic waste per month, mainly PP, LDPE, and HDPE. However, during re-
cycling processes, approximately 10% of the generated waste is no longer recoverable and
must be returned to the landfill. Figure 1 shows the plastic recycling process. The random-
ness of these residues makes it impossible to carry out homogeneous characterization
Sustainability 2022, 14, 9026 tests, for which a methodology is necessary to estimate the composition of said mixture. 4 of 12
However, thermogravimetry (TGA) and differential scanning calorimetry (DSC) have
been performed on the sample with a TA Instrument Q600 SDT. Nitrogen was used as an
inert gas and
recycling a heatingapproximately
processes, ramp of 20 °C/min.10% of the generated waste is no longer recoverable
First, the mixed waste
and must be returned to the waslandfill.
immersed in a1tank
Figure showswith
thewater
plasticatrecycling
4 °C (ρ = 1process.
000 kg/mThe) to
3
ran-
separate
domness theofplastic waste from
these residues makes theitdust particles
impossible to and
carryforeign material contained
out homogeneous in the
characterization
mixture. Then,
tests, for which thea mixture of plastic
methodology waste free
is necessary to of dust and
estimate the foreign material
composition wasmixture.
of said intro-
duced into athermogravimetry
However, container with a mixture
(TGA) and of ethyl alcoholscanning
differential and water (s = 0.93) to
calorimetry separate
(DSC) have the
been
HDPE (s = 0.94–0.97).
performed Finally,
on the sample theaseparated
with fractions
TA Instrument were
Q600 placed
SDT. in anwas
Nitrogen ovenusedat 120
as °C
an for
inert
8 hgas
to and
evaporate the ramp
a heating waterofand
20 alcohol
◦ C/min. residuals.

Figure 1. 1.
Figure Plastic recycling
Plastic process.
recycling process.

First, the mixed

3.2. Experimental waste
Setup and was immersed in a tank with water at 4 ◦ C (ρ = 1000 kg/m3 )
Procedure
to separate the plastic waste from the dust particles and foreign material contained in the
Pyrolytic-process investigation was carried out on a 2 L semi-batch reactor. A sche-
mixture. Then, the mixture of plastic waste free of dust and foreign material was introduced
matic description of the setup is presented in Figure 2. The reactor was made of stainless
into a container with a mixture of ethyl alcohol and water (s = 0.93) to separate the HDPE
steel and covered with an electric heater (4.5 kW) controlled by a solid-state relay (SSR).
(s = 0.94–0.97). Finally, the separated fractions were placed in an oven at 120 ◦ C for 8 h to
The temperature changes in the reactor can be traced to two characteristic points, the top
evaporate the water and alcohol residuals.
(Tt) and bottom (Tb) of the reactor. The temperatures were measured by using thermocou-
ples
3.2.K-type and recorded
Experimental Setup and using National Instrument card NI 9213. For all experiments,
Procedure
the thermocouple placed at the top of the reactor, Tt, was used as a control sensor for the
Pyrolytic-process investigation was carried out on a 2 L semi-batch reactor. A schematic
regulation of the electric heater. The heating rate was controlled with a precision of ± 0.25
description of the setup is presented in Figure 2. The reactor was made of stainless steel
°C/min and the set point temperature to ± 1.0 °C by proportional integral derivative (PID).
and covered with an electric heater (4.5 kW) controlled by a solid-state relay (SSR). The
The condensation
temperature system
changes wasreactor
in the made up
canof
betwo condensers
traced and two tanks,
to two characteristic and the
points, thetop
non-
(Tt )
condensable gaseous products reached a gas trap where they were washed.
and bottom (Tb ) of the reactor. The temperatures were measured by using thermocou-
plesThe fractions
K-type andof liquid oil
recorded and solid
using wereInstrument
National estimated on their
card NImass
9213.basis.
For The gas-phase
all experiments,
mass
the thermocouple placed at the top of the reactor, Tt , was used as a control raw
was determined from the mass balance as a difference between the mass of sensor ma-for
terial, mr, the sumofmass
the regulation of the liquid
the electric phase,
heater. ml, and the
The heating ratemass
was of the solidwith
controlled residues, ms :
a precision of
±0.25 ◦ C/min and the set point temperature to ±1.0 ◦ C by proportional integral derivative
(PID). The condensation system was made up of two condensers and two tanks, and the
non-condensable gaseous products reached a gas trap where they were washed.
The fractions of liquid oil and solid were estimated on their mass basis. The gas-phase
mass was determined from the mass balance as a difference between the mass of raw
material, mr , the sum mass of the liquid phase, ml , and the mass of the solid residues, ms :

m g = mr − ( m l + m s ) (1)
Sustainability 2022, 14, x FOR PEER REVIEW 5 of 12

Sustainability 2022, 14, 9026 𝑚𝑔 = 𝑚𝑟 − (𝑚𝑙 + 𝑚𝑠 ) 5(1)

of 12

Figure 2. Schematics of the semi-batch reactor.

Figure 2. Schematics of the semi-batch reactor.
The influence of the heating rate and temperature on the pyrolytic process yield was
The influence
evaluated throughofan the heating ratemulti-level
experimental and temperature on the pyrolytic
design [21,22]. process in
The temperature yield
the was
range
evaluated
of 380–460 ◦ C andan
through experimental
heating rates of multi-level 28 ◦ C/min
10, 19, and design [21,22].
wereThe temperature
evaluated. in the
As response
range of 380–460
variables °C andthe
were defined heating
liquid,rates
solid,ofand
10,gaseous
19, andfractions,
28 °C/min andwere evaluated.
the carbons As re-
distribution
sponse
of the variables were defined the liquid, solid, and gaseous fractions, and the carbons
liquid fraction.
distribution
The yieldof theofliquid
liquidfraction.
oil was further characterized to study the physical properties
andThe their composition.
yield of liquid oilThe
wascomposition was analyzed
further characterized by Varian
to study 450GCproperties
the physical chromatograph,
and
with
their a column ofThe
composition. m × 0.25 mm
100composition × 0.25
was µm, flow
analyzed of 3 mL/min,
by Varian and C7 –C30 saturated
450GC chromatograph, with
alkanes as
a column of a100
standard.
m x 0.25The mm split-type
x 0.25 μm,injector
flow was
of 3 set at 280 ◦and
mL/min, C and
C7-Cthe
30 FID-type
saturateddetector
alkanes at
340 ◦ C. The heating ramp of the oven was 40 ◦ C for three minutes, followed by a rate
as a standard. The split-type injector was set at 280 °C and the FID-type detector at 340 °C.of
◦
20 heating
C/minramp until reaching 320was◦ C and kept
The of the oven 40 °C for for 8 min.
three Additionally,
minutes, followedan byidentification analysis
a rate of 20 °C/min
of paraffins,
until reaching iso-paraffins,
320 °C and kept aromatics,
for 8 min.naphthas, and olefins
Additionally, (PIANO) was
an identification carried
analysis ofout with
paraf-
the iso-paraffins,
fins, same chromatograph, aromatics,100naphthas,
m column,and andolefins
the method
(PIANO) proposed by ASTM
was carried outDwith
6729.the
This
allows
same determining the
chromatograph, components
100 m column,inand spark-ignition
the methodengine proposedfuels by
using
ASTMhigh-resolution
D 6729. This gas
chromatography. Kinematic viscosity was measured with a
allows determining the components in spark-ignition engine fuels using high-resolutionCannon-Fenske viscosimeter
gaswith a time and temperature
chromatography. Kinematicuncertainty
viscosity wasof ±measured ±0.1 a◦ C,
0.1 s and with respectively, according
Cannon-Fenske viscosim-to
the ASTM D 445 standard. Density was determined with a buoyant
eter with a time and temperature uncertainty of ± 0.1 s and ± 0.1 °C, respectively, according densimeter applying
ASTM D 1298. Heating values were determined with a calorimetric
to the ASTM D 445 standard. Density was determined with a buoyant densimeter apply- bomb IKA C3000, with
ingthe application
ASTM D 1298. ofHeating
the isoperibolic
values weremethod, followingwith
determined the aASTM D 240 standard.
calorimetric bomb IKA Distillation
C3000,
with the application of the isoperibolic method, following the ASTM D 240 standard. ASTM
temperatures were determined for 100 mL samples, at atmospheric pressure, under Dis-
D 86 standard.
tillation temperatures were determined for 100 mL samples, at atmospheric pressure, un-
der ASTM D 86 standard.
4. Results and Discussion
4.1. Raw Material Composition
4. Results and Discussion
Table 3 shows the results of the raw material characterization, according to the method-
4.1. Raw Material Composition
ology described in Section 3.1. As shown in Table 2, the raw material is mainly made up of
11.3Table
± 0.8% 3 shows
HDPE,the results of
a mixture ofLDPE
the rawandmaterial
PP (85.2characterization,
± 1.3%), and 3.5 ± according to the
0.5% of other meth-
materials.
odology described in Section 3.1. As shown in Table 2, the raw material is mainly made
upTable
of 11.3 ± 0.8%
3. Raw HDPE,
material a mixture of LDPE and PP (85.2 ± 1.3%), and 3.5 ± 0.5% of other
composition.
materials.
Type of Plastic wt%
HDPE 11.3 ± 0.8%
LDPE and PP 85.2 ± 1.3%
Other materials 3.5 ± 0.5%

Figure 3 shows the TGA and DSC of the non-homogeneous sample of 22.86 mg of the
random mixture of plastic waste. The TGA shows the onset of mass loss at approximately
380 ◦ C, reaching a mass loss of 94.66% at 515 ◦ C. These temperatures are within the range
reported for the raw materials identified in Table 3. [23,24]. In turn, the DSC analysis
 HDPE 11.3 ± 0.8%
LDPE and PP 85.2 ± 1.3%
Other materials 3.5 ± 0.5%
Figure 3 shows the TGA and DSC of the non-homogeneous sample of 22.86 mg of the
Sustainability 2022, 14, 9026 6 of 12
random mixture of plastic waste. The TGA shows the onset of mass loss at approximately
380 °C, reaching a mass loss of 94.66% at 515 °C. These temperatures are within the range
reported for the raw materials identified in Table 3. [23,24]. In turn, the DSC analysis pre-
presents ◦ C, which corresponds
sents twotwo variations
variations of the
of the heatheat
flow:flow:
the the
firstfirst variation
variation at 142.94
at 142.94 °C, which corresponds to
◦ C, which corresponds
atocharacteristic
a characteristic behavior
behavior of of
PP,PP,
andand thesecond
the secondvariation
variationatat484.84
484.84°C, which corresponds
to aa characteristic
to characteristic behavior
behavior ofof HDPE
HDPE [23,25].
[23,25]. This
Thisbehavior
behaviorproves
provesthe
thehypothesis
hypothesis that
that the
the
classification presented in Table 3 is valid, with the raw material tested being
classification presented in Table 3 is valid, with the raw material tested being an arbitrary an arbitrary
mixture of
mixture of these
these three
three plastics
plastics and
and some
some other
other non-plastic
non-plastic materials.
materials.

Figure
Figure3.
3. TGA
TGAand
andDSC
DSCof
ofthe
theraw
rawmaterial.
material.

4.2. Influence
4.2. Influenceofofthe
theTemperature
Temperature andand Heating
Heating Rate
Rate on
on the
the Pyrolytic-Process
Pyrolytic-Process Yield
Yield
The influence of the heating rate and temperature on the pyrolytic-process
The influence of the heating rate and temperature on the pyrolytic-process yield yield is
is
shown in Figure 4. Low temperature and high residence time (low
shown in Figure 4. Low temperature and high residence time (low heating rate) were the heating rate) were the
best conditions
best conditions toto maximize
maximize liquid
liquid production.
production.An Anincrease
increaseinintemperature
temperatureorora decrease
a decreasein
residence time (high heating rate) increases the yield of gaseous products
in residence time (high heating rate) increases the yield of gaseous products at the expense at the expense of
reducing
of reducing thethe
yield of pyrolytic
yield oil. oil.
of pyrolytic TheThemaximum
maximum liquid yield
liquid obtained
yield herehere
obtained is higher than
is higher
in others works, such as those presented in Table
than in others works, such◦ as those presented in Table 2. This 2. This maximum liquid yield was
maximum liquid yield was 69%
and was obtained at 410 C (T ) and a heating rate of 10 ◦ C/min. This higher maximum
69% and was obtained at 410 °Cb(Tb) and a heating rate of 10 °C/min. This higher maximum
liquidyield
liquid yieldobtained
obtainedcouldcouldbebe
due due to the
to the factfact
thatthat the mixture
the mixture usedused here contained
here contained a min-a
minimum percentage of HDPE, which is the plastic that produces the lowest liquid yield,
imum percentage of HDPE, which is the plastic that produces the lowest liquid yield, ac-
according to results presented in Table 1.
cording to results presented in Table 1.
The results show that complete conversion of raw materials was achieved under
The results show that complete conversion of raw materials was achieved under
practically all test conditions, since solid yield was minimum and this solid residue could
practically all test conditions, since solid yield was minimum and this solid residue could
be the non-plastic material present in the mixture. As shown in Table 3, 3.5% of the mixture
be the non-plastic material present in the mixture. As shown in Table 3, 3.5% of the mix-
consisted of other materials, which were non-plastic elements, such as paper, small rocks,
ture consisted of other materials, which were non-plastic elements, such as paper, small
dust, etc., that do not degrade at this temperature range since, as shown in the TGA
presented in Figure 3, at 515 ◦ C temperature, there was still 5.33% of the initial mass.
To know if the analysis is reliable, the response parameters (liquid, solid, and gaseous
fractions) obtained experimentally were evaluated, determining if they follow a normal
distribution. The comparison of the p-value with the level of significance (α = 0.05) was
used to determine the normal distribution of the data (p ≥ 0.05). When evaluating the
experimental data, p-values obtained were 0.086, 0.455, and 0.092 for liquid, solid, and
gaseous fractions, respectively. Therefore, this indicates a normal distribution, allowing the
liquid, solid, and gaseous yields to follow a continuous variable probability distribution,
and this pyrolytic phenomenon can be modeled with the factors of temperature and
heating rate.
 distribution. The comparison of the p-value with the level of significance (α = 0.05)
used to determine the normal distribution of the data (p ≥ 0.05). When evaluating the ex-
used to determine the normal distribution of the data (p ≥ 0.05). When evaluating th
perimental data, p-values obtained were 0.086, 0.455, and 0.092 for liquid, solid, and gas-
perimental data, p-values obtained were 0.086, 0.455, and 0.092 for liquid, solid, and
eous fractions, respectively. Therefore, this indicates a normal distribution, allowing the
eous fractions, respectively. Therefore, this indicates a normal distribution, allowin
liquid, solid, and gaseous yields to follow a continuous variable probability distribution,
liquid, solid, and gaseous yields to follow a continuous variable probability distribu
Sustainability 2022, 14, 9026 and this pyrolytic phenomenon can be modeled with the factors of temperature and heat- 7 of 12
and this pyrolytic phenomenon can be modeled with the factors of temperature and
ing rate.
ing rate.

Figure 4. Pyrolytic-process
Pyrolytic-process yield.
Figure 4. Pyrolytic-process yield.
standardized Pareto diagram for the pyrolytic-process yield. This
Figure 5 shows the standardized
shows that Figure
that 5 shows
heating the standardized
rate (factor
(factor A) and Pareto diagram
and temperature (factorfor the pyrolytic-process
important yield.
diagram shows heating rate A) (factor B) have an important
diagram shows that heating rate (factor A) and temperature (factor ofB)the
have
fac-an impo
effect on product
productyield,
yield,but
butititisishigher
higherforforthe
thegasgasyield.
yield.OnOnthetheother hand,
other none
hand, none of the
tors influenced effect
solidon product
yield, so the yield, but it is higher for the gas yield. On the other hand, none o
factors influenced solid yield, so fraction of this
the fraction of product is indifferent
this product to theto
is indifferent experimental
the experi-
conditions. factors influenced
This confirms solid
that complete yield, so
conversionthe fraction of this
of raw materials product is indifferent
is achieved, and the the ex
to
mental conditions. This confirms that complete conversion of raw materials is achieved,
solidthe
residue mental conditions.material
This confirms
presentthat complete conversion of raw materials is achie
and solid is the non-plastic
residue is the non-plastic material in the
present mixture.
in the mixture.
and the solid residue is the non-plastic material present in the mixture.

Figure 5.
Figure Standardized Pareto
5. Standardized Pareto chart
chart for
for response
response parameters.
parameters.
Figure 5. Standardized Pareto chart for response parameters.
4.3. Liquid Product Analysis
Table 4 shows the carbon number distribution of pyrolytic liquid fraction for heating
rate and temperatures tested. As shown in Table 4, the liquid products from pyrolysis of
plastic wastes are a mix of hydrocarbon-light, -medium, and -heavy fractions. Regardless of
the heating rate or the pyrolytic temperature, the carbon number distribution of the liquid
fraction is almost the same: 57.5% C7 –C10 , 23% C11 –C14 , and 19.5% C15 –C30 , approximately.
Sustainability 2022, 14, 9026 8 of 12

Table 4. Carbon number distribution (%).

Temperature
Heat Rate
[◦ C] C7 –C10 C11 –C14 C15 –C30
[◦ C/min]
Tt Tb
380 410 57.23 24.84 17.94
400 430 57.47 22.09 20.44
10 420 450 54.41 23.31 22.28
440 500 59.37 20.88 19.75
460 550 54.39 23.86 21.76
380 410 57.98 23.47 18.55
400 430 56.53 21.72 21.77
19 420 450 61.65 20.35 17.99
440 500 60.40 23.79 15.81
460 550 58.36 21.07 20.57
380 410 62.15 18.83 19.02
400 430 55.54 23.82 20.63
28 420 450 58.36 23.33 18.30
440 500 55.17 22.70 22.14
460 550 55.64 29.08 15.28

One of the important properties of fuel is its calorific or heating value, which is
defined as the magnitude of the heat of reaction at constant pressure or constant volume
at a standard temperature (usually 25 ◦ C) for the complete combustion of a unit mass of
fuel [26]. The pyrolytic liquid fraction produced has a heating value of 45.85 ± 0.28 MJ/kg,
as shown in Table 5, which is like one of the commercial fuels, such as gasoline and
diesel [27]. These values are also like those reported by many studies which are within the
range of 38.3–46.04 MJ/kg, depending on the original plastic polymer composition [28].

Table 5. Liquid fraction heating values.

Temperature Heating Values

[◦ C]
HR10 HR19 HR28
Tt Tb
380 410 46.05 46.14 46.03
400 430 46.09 45.96 45.92
420 450 46.29 45.58 45.81
440 500 45.88 45.17 46.19
460 550 46.02 45.57 44.98

The properties of pyrolytic oil, which are presented in Table 6, make it suitable for use
in thermal devices, such as boilers, incinerators, ovens, etc.; however, for them to be used in
internal combustion engines, they must meet certain specifications to ensure proper engine
operation. Thus, a fractionation is necessary. A light fraction should be collected to be used
on gasoline engines, while a medium fraction will be used on diesel engines [29]. In this
research, the pyrolytic oil was subjected to a fractionation process to obtain better quality
fuels. Figure 6 shows the visual appearance of products obtained from this fractionation
process. In turn, Table 6 shows a chromatographic analysis of the chemical composition of
pyrolytic oil, gasoline, and diesel. This shows a high percentage of iso-paraffins of 38.06%
and 37.44% unknown in the pyrolytic oil. For gasoline, there is a large load of olefins of
40.6% and oxidized additives of 17%. Finally, for diesel, there is about 20% of aromatics
and a high percentage of the unknown; this is due to the lack of identification data with
which the compounds are identified since it only has up to molecules of 20 carbons, in
addition to being a specific analysis of gasoline or fuels of low evaporation temperatures.
 a large load of olefins of 40.6% and oxidized additives of 17%. Finally, for diesel, there is
about 20% of aromatics and a high percentage of the unknown; this is due to the lack of
identification data with which the compounds are identified since it only has up to mole-
cules of 20 carbons, in addition to being a specific analysis of gasoline or fuels of low
evaporation temperatures.
Sustainability 2022, 14, 9026 9 of 12
Table 6. Fuel properties of plastic pyrolytic oil and standard parameters of gasoline and diesel.

Properties Pyrolytic Oil Gasoline [27] Diesel [27]

Table 6. Fuel properties of plastic pyrolytic oil and standard parameters of gasoline and diesel.
Calorific value [MJ/kg] 45.85 45.6 43.5–55.7
Kinematic viscosityProperties
at 40 °C [mm2/s] 3.69
Pyrolytic Oil 1.3–2.4
Gasoline [27] 1.9–5.5
Diesel [27]
DensityCalorific
[kg/m3]value [MJ/kg] 790 45.85 780 45.6 80743.5–55.7
index (CI)viscosity at 40 ◦ C [mm2 /s]
Cetane Kinematic 62.873.69 – 1.3–2.4 Min. 301.9–5.5
*
3] 790 780
Density [kg/m
Research octane number (RON) – 90.2–107.1 – 807
Cetane index (CI) 62.87 – Min. 30 *
Motor octane number (MON) – 82.6–103.1 –
Research octane number (RON) – 90.2–107.1 –
Motor octane Chromatographic
number (MON) analysis (ASTM– D 6729) [wt%]
82.6–103.1 –
Paraffins 0.00
Chromatographic analysis (ASTM D 0.00
6729) [wt%] 1.84
Paraffins
Iso-Paraffins 38.060.00 19.41 0.00 4.87 1.84
Iso-Paraffins 38.06 19.41
Aromatic 10.74 6.34 19.22 4.87
Aromatic 10.74 6.34 19.22
Naphthas
Naphthas 2.44 2.44 9.25 9.25 1.39 1.39
Olefines
Olefines 11.3111.31 40.6040.60 0.48 0.48
Oxygenated
Oxygenated 0.00 0.00 17.0017.00 0.00 0.00
UnknownUnknown 37.4437.44 7.39 7.39 72.2 72.2
* Cetane number (CN).
* Cetane number (CN).

(a) (b) (c) (d)

FigureFigure
6. Color and visual
6. Color appearance
and visual of pyrolytic
appearance oil obtained
of pyrolytic fromfrom
oil obtained (a) mixed plastic
(a) mixed waste,
plastic frac-
waste, fraction-
tionation of mixed plastic waste at (b) 150 °C◦(light), (c) 320 °C◦(medium), and (d) 460 °C ◦(heavy).
ation of mixed plastic waste at (b) 150 C (light), (c) 320 C (medium), and (d) 460 C (heavy).

Table Table
7 shows the yields
7 shows and properties
the yields of theofobtained
and properties fractions.
the obtained The fractionation
fractions. The fractionation
process yielded
process 21.12 21.12
yielded wt% ofwt% light
of fraction (gasoline-like),
light fraction 56.52 56.52
(gasoline-like), wt% of wt%medium fraction
of medium fraction
(diesel-like), and 22.36
(diesel-like), wt% of
and 22.36 wt% heavy fraction
of heavy (heavy
fraction diesel-like).
(heavy One of
diesel-like). theofimportant
One the important
properties
properties of gasoline
of gasoline is the is the octane
octane number,number,
whichwhich is a measure
is a measure of a fuel’s
of a fuel’s abilityability to resist
to resist
“knock”.
“knock”. The higher
The higher the octane
the octane number,number, the greater
the greater the fuel’s
the fuel’s resistance
resistance to knocking
to knocking or or
pingingpinging
during during combustion.
combustion. Two methods
Two methods for measuring
for measuring octaneoctane
number number
are theare the Research
Research
Method Method
(ASTM (ASTM
D-2699) D-2699)
and theand the Method
Motor Motor Method (ASTM D-2700).
(ASTM D-2700). With these With these methods,
methods, the
the octane
research research octane(RON)
number number (RON)
and and the
the motor motor
octane octane(MON)
number numberare (MON) are obtained,
obtained, re-
respectively.
spectively. Both methods
Both methods use a standardized
use a standardized single-cylinder
single-cylinder engineengine developed
developed underunder
the the
auspices
auspices of theof the Cooperative
Cooperative Fuel Research
Fuel Research Committee
Committee in 1931—the
in 1931—the CFR engine
CFR engine [23]. The
[23]. The
term octane
term octane index index
(OI) is(OI)
oftenis used
often to
used toto
refer refer
the to the calculated
calculated octaneoctane
qualityquality in contradis-
in contradis-
tinction
tinction to theto(measured)
the (measured)
researchresearch or motor
or motor octaneoctane numbers.
numbers. The fraction
The light light fraction
has anhas an
octane index of 96.6/92.2 (depending on the method used), which is within the range of the
gasoline values (see Table 6). Concerning the chemical composition of the light fractions
when compared to commercial gasoline using a PIANO analysis, it can be observed that
the light fraction does not have oxygenates since it has not been reformulated. Typically,
additives are added to commercial gasolines to prevent corrosion, increase the octane
number, and make them more resistant to low temperatures, and this light fraction has only
been distilled [30]. The other compositions are very similar, with olefins with seven and
eight carbon compounds being the most present in both fuels (as seen in Tables 6 and 7).
Sustainability 2022, 14, 9026 10 of 12

Table 7. Fuel properties of distillate products.

Fraction Reference
Properties Light Medium Heavy
(150 ◦ C) (320 ◦ C) (460 ◦ C)
Yield [wt%] 21.12 ± 0.01 56.52 ± 0.01 22.36 ± 0.01 –
Density at 20 ◦ C [kg/m3 ] 737 ± 0.01 784.00 ± 0.01 – ASTM D 1298
Kinematic viscosity at 40 ◦ C [mm2 /s] 0.66 ± 0.01 1.58 ± 0.01 – ASTM D 445
Initial Boiling Point 77.8 ± 0.1 134.3 ± 0.1 371.2 ± 0.1
T10 (◦ C) 84.4 ± 0.1 154.4 ± 0.1 381.7 ± 0.1
T50 (◦ C) 117 ± 0.1 215.3 ± 0.1 425.4 ± 0.1 ASTM D 86
T90 (◦ C) 156.2 ± 0.1 309.0 ± 0.1 471.8 ± 0.1
Final Boiling Point 202.2 ± 0.1 330 ± 0.1 483.9 ± 0.1
Caloric value [MJ/kg] 44.40 ± 0.01 46.17 ± 0.01 – ASTM D 240
Octane index (OI) 96.6 a /92.2 b – – [31]
Cetane index (CI) – 57.2 ± 0.1 – ASTM D 4737
Chromatographic analysis [wt%]
Paraffins 0.52 0.00 –
Iso-Paraffins 28.1 17.05 –
Aromatic 4.66 34.19 –
ASTM D 6729
Naphthas 2.92 1.65 –
Olefines 57.21 1.54 –
Unknown 6.5 45.56 –
a 2 − 113.2 × 108 · T −3 .
OI = −356.5 + 620.7·s + 560.9·s2 − 782.9·s3 ; b OI1 = 179.5 − 0.1364· T90 − 0.001307· T90 90

On the other hand, the medium fraction has two important characteristics: the cetane
number and its composition. The cetane number (CN) is an empirical parameter associated
with the ignition delay time of diesel fuels. The cetane index (CI) is used as a substitute
for the cetane number of diesel fuel. The cetane index is calculated based on the fuel’s
density and distillation range (ASTM D 4737). The medium fraction has a cetane index
of 57.2, which meets the requirements of the standard (as seen in Table 6). Regarding
the composition, the analysis shows a high content of aromatics in both fuels, diesel with
19.23% and the medium fraction with 34.19%, as well as unknowns of 72.2% for diesel
and 45.56% for the medium fraction. The medium fraction presents a higher percentage of
iso-paraffins concerning diesel but does not show the presence of paraffins, while naphthas
and olefins for both fuels are the lowest percentages (as seen in Tables 6 and 7).

5. Conclusions
A study has been conducted to investigate the effects of heating rate and temperature
on the plastic-waste random-mixture pyrolysis (PP, LDPE, HDPE). The results show that
complete conversion of raw material was achieved with a maximum liquid yield of 69 wt%.
This means that for 1 kg of waste, about 0.85 L of pyrolytic oil are obtained.
Higher temperatures or lower residence time (high heating rate), reduce the yield of
pyrolytic oil at the expense of increasing the yield of gaseous products.
Pyrolytic oil covers a wide range of hydrocarbons; thus, a fractionation is necessary
before using it as fuel in internal combustion engines. The fractionation process yielded
21.12 wt% of light fraction (gasoline-like), 56.52 wt% of medium fraction (diesel-like), and
22.36 wt% of heavy fraction (heavy diesel-like). The light fraction has an octane index and
caloric value within the range of the typical gasoline values. On the other hand, the cetane
index and caloric value of the medium fraction meet the requirements of the standards
for diesel.
Finally, this work shows that pyrolysis is a good alternative for plastic-waste upgrad-
ing, which is no longer recoverable by traditional mechanical recycling.

Author Contributions: Conceptualization, methodology, and analysis results, J.M.R.-A.; experi-

mentation, and analysis results, J.R.V.-R. and D.A.R.-V.; experimental multi-level design, D.M.P.-C.;
writing—review and editing, B.R.-V. All authors have read and agreed to the published version of
the manuscript.
Sustainability 2022, 14, 9026 11 of 12

Funding: This research was funded by The Secretary of Innovation, Science and Higher Education
of the state of Guanajuato (SICES), and the company RECICLA.LO, S.A. DE C.V., grant number
FINNOVATEG: MA-CFINN0760.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Acknowledgments: We acknowledge the University of Guanajuato for sponsorship of this paper.
Conflicts of Interest: The authors declare no conflict of interest.

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