Abstract
This research involves the application of pyrolysis has been viewed as an efficient means for
the proper disposal of waste plastics in the environment amongst others. In this research, the
influence of two catalysts namely; Zeolite and Titanium (IV) oxide are examined. Zeolite as a
catalyst has obtained approval due to its ability to ensure the production of liquid hydrocarbons
when applied in the pyrolysis of waste plastics. It has also been used a bench mark to determine
the effectiveness of other catalyst in producing liquid hydrocarbons. Whereas, Titanium (IV)
oxide which is known for its many applications in paints, pharmaceuticals and cosmetics is
another catalyst which is used in catalytic pyrolysis of waste plastics for the first time. Titanium
(IV) oxide has several appealing properties such as its mesoporous structure, mechanical
resistance and high stability which are some of the reasons which prompted its use. This
comparative study between this two catal
INTRODUCTION
In our present-day society there has been a dynamic drive for new sources of energy possessing
the qualities of commercial viability and environmental sustainability. In contrast, there has
been an increased use in the consumption of materials involving plastics leading to an increased
amount of waste plastics and thus the problem of disposal. The disposal of waste plastics has
been a major contemporary issue all over the world today. The uses of plastics are broad and
for that reason, they can be seen everywhere. The problem which has been encountered in the
use of these plastics is in their disposal. Since the invention of plastic materials, there have not
been discoveries related to the biodegradable waste plastic material. For that reason various
suggestions have been proposed for the disposal of these waste plastics such as landfills,
incineration and burying.1 These plastic materials which are used so regularly just occupy space
in landfills; which would obviously one day get full. With the rise in demand for workable
land, such practices should not be supported. Another means which has been proposed for the
removal of these waste plastics is the process of incineration.1
Incineration involves the burning
of waste substances. The problem with this is that it leads to the release of harmful and toxic
gases into the environment such as CO2.
1 A method of waste disposal also popularly used in
rural areas is burying. The problem with this method lies in the fact that most plastics produced
are non-biodegradable. So, these waste plastic materials which are buried just stay beneath the
soil for hundreds and even thousands of years.1 All these methods of waste disposal affect the
environment and we humans in different ways and in order to curb these problems pyrolysis
was proposed.
Waste management is an important concept which had been developed in order to handle the
detriments which waste disposal poses to the environment. It involves the handling of discarded
resources. The main objective of waste management systems is to keep people and the
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environment benign of the potentially harmful effects of waste which could occur. These waste
materials are obtained from polymers such as low-density polyethylene (LDPE),
polypropylene (PP), polyethylene terephthalate (PET), polyvinyl chloride (PVC) and
polystyrene.1, 2 Waste materials are not usually dangerous but over time and if certain
conditions are kept in place such as intense heat and presence mixtures which could instigate
chemical reactions, harmful substances could be produced. This project highlights the need for
proper waste management by suggesting the conversion these waste plastics materials into
useful resources through the process of pyrolysis.
1.1 Some waste facts
In the last 50 years, there has been a global increase in the production of plastic. According to
some facts from the UN environmental program, around 22-43 % of plastic used are disposed
in landfills.2 This fact stated simply highlights pure waste of possible resourceful materials and
shows the possible effects which it poses to its surrounding communities to certain extents by
providing unconducive environments. It was also suggested that plastics produced all around
the world occupy a good 7% of the amount of crude oil produced yearly.2 This fact simply
implies that plastic production is important and its management should also be handled
similarly due to the fact that it possesses a variety of uses.
In that, the idea of pyrolysis comes in as a means for maximising waste plastic materials.
Pyrolysis is the decomposition brought about by high temperatures. It is a chemical reaction
which involves the molecular breakdown of large molecules into smaller molecules in the
presence of heat and the absence of oxygen. In the pyrolysis process, the heavier gases are
condensed to liquid oil while the lighter gases such as hydrogen and methane, which are gases
at room temperature, are called “syngas”.2, 4 During the process of pyrolysis changing the
temperature and duration of pyrolysis makes it possible to optimize for the production of more
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fractions. For example, slow pyrolysis under lower temperatures will produce more fuel gas
whereas fast pyrolysis at higher temperatures will produce more bio-oil.2, 4 With fast pyrolysis,
the syngas that is produced can be burned within the system to maintain the temperature,
resulting in hydrocarbon formation as the sole products of pyrolysis.
The process of pyrolysis can occur in two different ways which are; catalytic pyrolysis and
thermal pyrolysis.4 Thermal pyrolysis involves the use of intense heat to breakdown molecular
chains in order to produce hydrocarbon yield. This process occurs at extremely high
temperature conditions. The difficulty here lies in the fact that the product of this process has
low liquid yield.4 This is partly due to the fact that intense heating condition would lead to the
production of more gases. The other form of pyrolysis is the catalytic pyrolysis and this form
of pyrolysis also involves heating too but at lower temperatures.4, 6 The key difference here is
in the application of a catalyst and this helps reduce the reaction time and the reaction
temperature. The advantage of this process is that high liquid yield is gotten and for that reason,
catalytic pyrolysis is applied in this research.
1.2 History of Pyrolysis
Some thousands of years, the practise of pyrolysis was believed to have begun somewhere in
the Amazon rainforest. The substances which were produced were bio char which was a
charcoal like substance that was applied to improve and stabilize the nutrient poor rainforest
soils.3
Individuals who lived in these areas started fires and when it became too hot, the fire
was covered with earth materials to prevent oxygen from coming in contact with the fire. The
intensity of the fire increased and temperature became higher so long as the source of the fire
was present. The fuel was broken down in the absence of oxygen and bio char was produced
rather than ash which turned out to be somewhat of a new innovation. An even more recent
study suggested that pyrolysis was used with wood waste feedstock in World War 1 & 2 to
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produce transportation fuel when fossil fuels were unavailable.3
In 1945, vehicular machines
such as trucks, buses and various agricultural machines were all powered by gasification. After
an increase in the production of such fuels, it was estimated that there were nearly 9 million
vehicles running on pyrolysis gas in many places around the world.3Due to modernisation, the developments related to pyrolysis were made and the process
emerged on a number of fronts in the 1950s. In 1958, a laboratory in the United States known
as the Bell Laboratories alongside a number of universities, institutions and establishments
around the world started the R&D programs to examine the usefulness of pyrolysis.3 The main
focus of the program was to produce gas from waste materials found in the environment.
So initially, the first pyrolytic gasification systems were firebrick ovens which applied heat
indirectly in a low oxygen environment. The early systems were batch processes: ovens were
filled, sealed and then heat was applied. After each batch, the oven would be cleaned and
readied for the next batch. It was quite a process.3
In the early 1970s, the first commercial The
first commercial forms of pyrolysis batch systems for gasification were introduced in the health
sector in hospitals but due to low volume capacity and issues with the mortar used in the kiln
construction, little commercial success was observed. In the late 1970s and early 1980s the
batch systems gave way to continuous feed systems with a cone design that made the
evacuation of the gasses more efficient.3
The continuous feed cone design first showed up in
England then the US, Germany, Japan, Canada and the Netherlands. The pyrolysis gradually
became a major process for the production of fuels and prevention of waste was avoided.3
1.3 Catalyst
A catalyst is a substance which changes the way a reaction occurs by creating new pathways
and thereby lowering its activation energy and speeding up the reaction. A catalyst can either
be homogenous or heterogeneous in nature. 5 A homogeneous catalyst is one which exists in
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only one phase while a heterogeneous catalyst is one which exists in more than one phase. A
concept which is usually discussed when catalysis is involved is activation energy. Activation
energy is the minimum quantity of energy that the reactants must possess in order for a reaction
to occur. A catalyst works by lowering activation energy for a reaction.5, 6 Catalysts lower
activation energy by providing simple and less energy-intensive means for reactant molecules
to break bonds and create new temporary pathways.
1.3.1 TiO2
Fig 1: Showing TiO2 sample
Titanium is the ninth most abundant metal found on earth. It was discovered by William Gregor
in 1791. It occurs naturally in the environment. It is a group 4, period 4 of the periodic table. It
is a d block transition element. It has its electronic configuration to be [Ar] 3d2
4s2
.
7 Titanium
has low density and for that reason, it is applied in the creation of aircrafts and missiles. One
of the largest uses of Titanium can be found in the form of titanium (IV) oxide. TiO2 exists in
3 crystalline forms which are; anastase, rutile and brookite.7 The rutile is the most thermally
stable amongst the other forms. It has a molecular weight of 79.938 g/mol. TiO2 has no odour
and has no taste. It is also insoluble in water.7
In the area of catalysis there has been the search for catalysts with high stability. During
catalytic operations, various particles have the ability to enclose the active sites of the catalyst
and thus causes instability. TiO2 as a catalyst possesses a high surface area and prevents that
enclosure by particles because of its mesoporous structure.7, 8 In recent times, TiO2 metal
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catalysts have gotten added interest as a result of high activity nanoparticles for various
reduction and oxidation in suitable conditions such as at low pressures and temperatures. TiO2
has gotten has gotten a lot of recognition in the field of science because of its high stability in
acidic and basic media.7 The availability and mode of synthesis is an important factor to
consider in the selection of a catalyst. Due to its non-toxicity, high effectiveness and long-term
photo stability TiO2 has been applied in the mineralizing of non-biodegradable and toxic
environmental contaminants.7
It also possesses a suitable mechanical resistance in oxidative
and acidic media. Generally, TiO2 has been believed to be a major upgrade to as it allows
modulation of catalytic actions in reactions which include hydrodesulphurisation, water gas
shift and thermal catalytic decomposition.
7 Despite the many benefits, there are some draw
backs which have been encountered in the use of metal oxide catalysts.
1.3.2 Zeolite
Fig 2: Showing a Zeolite Sample
Zeolites are compounds that exist in nature. These zeolites have been known for over 250 years.
They are basically alumina silicate materials. Some examples of zeolites include; faujasite,
modernite and chabazite. In recent times, the use of naturally occurring zeolites have been
reduced due to the presence of impurities that are not required and the inability of the zeolites
to acts as catalysts. Between 1948 and 1955, Barrer and Milton pioneered the first group of
synthetic zeolites which were porous materials that played major roles in catalysis.8
7
In 1962, application of synthetic Faujasites (zeolite X and Y) on an industrial scale were used
on heavy hydrocarbon distillates through fluid catalytic cracking (FCC). With time, the zeolite
catalyst captured the attention of the petroleum refining and petroleum chemistry fields. Zeolite
was applied in some important processes in the chemical industry such as hydrocracking of
heavy petroleum distillates, isomerization (octane number enhancement of light gasoline) and
in the synthesis of ethyl benzene from benzene and ethene in the Mobil-Badger process.8
In
zeolites compounds, SiO4 and AlO4 are the main building blocks. The building principle behind
zeolite involves a combination of linked tetrahedral structures. In the framework of zeolites,
there exist some channels, channel intersections and with sizes ranging from 0.2 to 1nm.8
In
zeolite, there exists void systems which hold water molecules and pore dimensions that indicate
size.
There are so many types of synthetic zeolites but for this research, the ZSM-5 is used due to its
properties which favour it. Zeolite ZSM-5 has an all-silica analogue silicate-1 built from a
pentasil unit which contain intersecting system of ten-membered ring pores.8
In the pore
system, one is straight while the other is sinusoidal. ZSM-5 is a very important heterogeneous
catalyst.8
1.4 Polymers: Polypropylene and LDPE (Low Density Polyethylene)
In the polymer industry, most plastics used are made from LDPE and Polypropylene or even a
combination of both due to their various applications. Polypropylene is a thermoplastic which
is made from monomers of propylene.
9, 17 It is formed through addition polymerisation. It can
be manufactured from propylene gas in the presence of a catalyst. Polypropylene is a plastic
which is widely used because it has various useful properties such as high melting point.9
Its
high melting point explains its application in food packaging. Also, it helps prevent
dehydration and evaporation in order to ensure preservation of fresh food substances.
8
Polypropylene is a lightweight and very flexible plastic. Polypropylene bags which can be seen
in most places have a high barrier of protectiveness which help prevent moisture and vapour.17
Some examples of Polypropylene substances include; Tapes and shopping bags.
Polymers made from ethylene are important in the production of materials such as plastic bags,
packages for clothing materials, sandwich bags and various others uses. Ethylene is the
monomer which is used to form polyethylene through polymerisation. Low density
polyethylene (LDPE) is also a thermoplastic which is made from ethylene monomer.9, 17 LDPE
is not reactive at room temperatures. It is has a low density with high resilience which prevents
it from easily tearing apart. Some of examples of LDPE include; plastics raps on new products
like computers and tubing materials.
With the qualities of these polymer materials in mind, the research was performed through
pyrolysis in order to produce useable hydrocarbon yields.
1.4 Mechanism of the Reaction
The mechanism of the pyrolysis reaction involves the thermal decomposition of naturally
occurring carbonaceous materials in the absence of oxygen. Pyrolysis can be broken down into
in to two different stages which are primary and secondary pyrolysis. The latter involves the
fragmentation and shrinkage while the former involves a series of steps which are; reforming,
dehydration, cracking, polymerization, oxidation and gasification. The primary reaction in this
process are endothermic while the secondary which is the final is exothermic. It involves a
process of hydro charring. In a pyrolysis reaction after the feedstock which has been shredded
is assembled to be pyrolyzed is placed in the Pyrex round bottom flask, combustion occurs.
The product of the combustion are usually charcoal, condensable liquids and non-condensable
gases which are usually kept or aired out. Various kinetic of the reaction are also studied such
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as the heating rate and the residence time. After which the yield is obtained and its properties
ascertained. The rate of pyrolysis reaction is determined with the aid of the formula below.
?? = ???
???
?? ?
Where A represents the pre-exponential factor [1/s], E represents the activation energy in
[kJ/mol] and R represents the gas constant = 8.314 [J/mol/K].
In this catalytic pyrolysis reaction, between two catalysts and two polymers which are zeolite
and TiO2. And also, the PP and LDPE polymers. The two catalyst are mesoporous and have
good surface area stability. So in the reaction with these polymer there involves a large amount
of heat which causes the breakdown of large molecule into smaller ones. In the heating process,
the mesoporous structures of both does not get enclosed unlike in other catalytic pyrolysis
reaction. These structures act as barriers made of series of crosslinking matrix which exist both
in the polymer and catalyst. These mesoporous structures on contrast with the high surface are
stability give for the format of liquid hydrocarbons. Various other concepts to these would be
investigated in the course of the research.
1.5Problem Statement
The population in Nigeria has increased astronomically over the past few years. As of the year
2015, the Nigerian population was about 180 million and in our present year it has been
estimated to about 190million.4, 5 This statistic shows huge change in population by a difference
of 10 million in just 3 years. Relating such facts to material production indicates a rise in the
amount of plastics being used yearly. An increase in the amount of plastics also signifies an
increase in the amount of waste plastics are found everywhere in our surroundings. There have
been various means set out to curb this problem but some have proved to be inefficient while
others have not been eco-friendly. In that, the conversion of these waste plastics into useful
resources has been proposed.
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1.6Significance of Study
In Nigeria, the use of plastics has increased over time due to innovation and the need for
packaging and preservation of materials. However, the need for the disposal of these plastic
materials comes into light in the fact that as they are just left as refuse on road sides in large
amounts causing an unhealthy sight and making the environment unconducive for living.
Various methods of have been proposed to ensure safe disposal of these waste plastics and have
helped curb the problem to a certain extent. Thus, the application of pyrolysis as a means to
curb waste plastic disposal through turning them into useful resources such as light
hydrocarbons has been put forward.
1.7Aim of the Research
The aim of the research is to perform a comparative study on the benefits of TiO2 and zeolite
as a catalyst in the pyrolysis of waste plastic polymers in other to form liquid hydrocarbons.
These results would be examined as successful if;
? An alternative use of Titanium oxide is gotten through the results of the pyrolysis.
? The polymer used could be able to provide much yield and hydrocarbon qualities.
? The thermodynamic reaction conditions are good enough to produce suitable
hydrocarbon yield.
? Determine if TiO2 could be used as a suitable pyrolysis alternative to zeolite through
the observances gotten.
? The amount of time used in production of yield is efficient.
? Analysing the product yield to know if it possesses suitable hydrocarbon quality.
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1.8Hypothesis
Pyrolysis has proved to be a good enough method for maximally utilising waste plastics which
we find in our environment. Several countries in the world apply the use of pyrolysis in
maximising waste plastic in our environment such as India asides the process of recycling. The
use of catalyst on the area of pyrolysis has helped to produce suitable Hydrocarbon yield in
shorter amount of times. In that, I hypothesize that through the use of TiO2 as a catalyst in
pyrolysis, efficiency of could be achieved through the production of appropriate hydrocarbon
yield