ABSTRACT
Eighteen trace heavy metals were quantitatively analyzed using atomic
absorption spectroscopy; five samples of petroleum products were collected from
Kaduna Refining and Petrochemical Company. Three different preparation
methods were adopted for the determination of trace and heavy metals present
in the various petroleum products viz: Direct sample aspiration into the flame
after solvent dilution, Total acid (wet) digestion of the sample, Ashing of the
sample and dissolution with an appropriate acid. Sample treatment with organic
acid prior to aspiration proved to be more reliable and it gave good results for
trace and heavy metals in petroleum products except for kerosene sample,
where most of the elements responded positively to ashing preparation method.
The level of eighteen elements analyzed in Petrol (PMS), Kerosene (DPK), Gas
Oil (AGO), LPFO and Residual fuel are shown in table 3.7 which revealed that
Potassium(120 mg/l, 340 mg/l, 120 mg/l, 2900 mg/l, 2050 mg/l) and Sodium
(260 mg/l, 180 mg/l, 160 mg/l, 1800 mg/l, 1200 mg/l) are the most abundant
elements in both the five sample under study followed by Iron(1.20 mg/l, 1.74
mg/l, 1.25 mg/l, 0.60 mg/l), Manganese(1.28 mg/l, 1.04 mg/l, 1.46 mg/l, 6.6
mg/l4.8 mg/l), Lead(0.50 mg/l, 0.16 mg/l, 0.40 mg/l, 27.8 mg/l, 24.6 mg/l) and
Aluminium (0.42 mg/l, 1.06 mg/l, 0.32 mg/l, 0.82 mg/l, 24.6 mg/l). The
concentration ranges of trace heavy metals analyzed are within the permissible
levels set by World Health Organisation.
TABLE OF CONTENTS
Title Page i
Dedication ii
Certification iii
Acknowledgements iv
Table of contents v
List of figures vii
List of tables viii
Abstract ix
CHAPTER ONE
1.1 Introduction 1
1.1 Basic Principle/ Components of Atomic Absorption Spectroscopy 2
1.2 Literature review 3
1.3 Crude Oil / Composition 4
1.3.1Types of Hydrocarbon 6
1.3.2 Classification of Petroleum 7
1.3.3 Chemistry of Petroleum 7
1.3.4 Origin of Petroleum 10
1.3.5 Biogenic theory 10
1.3.6 Abiogenic theory 11
1.3.7 Petroleum movement/migration 11
1.4 History/Exploration of Petroleum in Nigeria 12
1.5 Petroleum refining process 13
1.5.1 Distillation (Fractionation) 13
1.5.2 Atmospheric distillation 13
1.5.3 Vacuum distillation 14
1.5.4 Reforming 15
1.5.5 Cracking 16
1.5.6 Catalytic cracking 16
1.5.7 Fluid catalytic cracking 16
1.5.8 Thermal cracking 17
1.5.9 Hydrocracking 18
1.5.10 Alkylation 19
1.5.11 Isomerization 20
1.5.12 Polymerization 21
1.5.13 Hydro-treating and sulphur plants 21
1.5.14 Sulphur recovery plants 22
7
1.6 Petroleum products 23
1.6.1Gasoline 23
1.6.2 Kerosene 24
1.6.3 Petroleum Diesel 24
1.6.4 Fuel Oil 25
1.6.5 Lubricating Oil 25
1.6.6 Wax 26
1.6.7 Petrolatum 26
1.6.8 Asphalt 26
1.7 Environmental effects 27
1.8 Aim and Objectives 28
1.9 Justification 28
CHAPTER TWO 29
2.0 Materials and Methods 29
2.1 Materials 29
2.2 Methods 29
2.2.1 Sampling 29
2.2.3 Chemicals /Reagents 31
2.2.4 Determination Physicochemical properties of Nig. Petroleum 32
products
Determination of Density/Specific Gravity/ API 32
Determination of Viscosity 32
Determination Aniline point 33
Determination of Total Sulphur 33
Determination of water Content 33
2.2.5 Sample Digestion method 35
2.2.6 Dilution of Sample in an Organic Solvent 35
2.2.7 Acid (Wet)Digestion 35
2.2.8 Ashing Digestion 36
2.2.9 Determination of Trace Metals 36
CHAPTER THREE
Results and Discussion 37
3.1 Results. 37
3.2 Discussion 43
CHAPTER FOUR 55
4.0 Conclusion and Recommendations 55
4.1 Conclusions 55
4.2 Recommendations 56
CHAPTER ONE
NTRODUCTION AND LITERATURE REVIEW
1.1 Introduction
Crude oil contains mainly hydrocarbons especially alkanes, naphthenes, and
aromatics (Odebunmi and Adeniyi,2004),It contains also some nitrogen, oxygen
and sulphur containing compounds along with trace amounts of elements
especially nickel, vanadium, titanium, iron, cadmium etc(Odebunmi and
Adeniyi,2004; Olajire and Oderinde,1996). The presence of trace metals and
non-metals in the crude oil and petroleum products is destructive, especially in
the refining process(Oderinde, 1989).Indigenous petroleum refineries, petroleum
depots and filling stations as well as environment in general require enough
information on the concentration of trace and heavy metals in Nigerian petroleum
products because of its detrimental effects on both equipment and environment.
The trace/heavy metals composition in petroleum products can be used for the
identification of environmental fuel pollution. Exhaust from various machine
including cars, buses, generators, etc contributed immensely in so many
environmental problems due to the concentration of some trace/heavy metals in
it.Atomic Absorption Spectrophotometry (AAS) is a well-established extremely
valuable technique for the determination of trace amounts of metals. Since its
introduction by Walsh, the method has gone through a number of developmental
stages aiming at obtaining an increase in reliability, ease of operation and, above
all, improvement in the limit of detection .AAS is an analytical method based on
82
the absorption of electromagnetic radiation in the visible and ultraviolet regions of
the spectrum by gaseous atoms resulting in changes in electronic structure (Fritz
and Schenk, 1987). Atomic absorption spectrometry is one of the most widely
used techniques for the determination of trace and heavy metals in petroleum
products. Over sixty elements can be determined in almost any matrix.An
example includes petroleum products such as petrol, diesel, kerosene, fuel oil,
petrolatum, lubricating oil, etc. So many other samples that can be analyzed
using atomic absorption spectroscopy are body fluids, polluted water, foodstuffs,
soft drinks, beer, metallurgical and geochemical samples (Fifield and Kealey,
1995).
1.1.1 Basic Principle/Components of Atomic Absorption Spectroscopy
Atomic Absorption Spectroscopy is the determination of elemental composition
by its electromagnetic or mass spectrum. It can be divided by atomization source
or by the type of spectroscopy used. The basic principle is that, light is passed
through a collection of atoms. If the wavelength of the light has energy
corresponding to the energy difference between two energy levels in the atoms,
a portion of the light will be absorbed. The relationship between the
concentrations of atoms, the distance the light travels through the collection of
atoms, and the portion of the light absorbed is given by the Beer-Lambert
law.The main components of an atomic absorption spectrophotometry are
radiation source, an atomization cell, a wavelength selector and a wavelength
83
detector, both of which are very important components in the analyses of trace
metals using Atomic Absorption Spectroscopy technique.
1.2 Literature review
Various analytical methods have been reported for the determination of trace
metals in petroleum products. Traces of iron, nickel, and vanadium in petroleum
and petroleum products were analyzed using spectrophotometry method. The
sample was ashed and taking up with the potassium bisulphate. The
measurement was based on the development of coloured solutions by reagents
specific for each element. Frances (1964) and Henry and George (1975)
employed AAS to determined heavy metals in petroleum products. They used
two methods which are based on the decomposition and cold water vopour
atomic adsorption. Another method involved acid decomposition of the samples
in a closed system while the other method used oxy-hydrogen combustion to
decompose the sample.
In another report, Winston and Harry (1975) used AAS to determine trace
quantities of cadmium in petroleum and petroleum products in which the sample
was digested with sulphuric acid and then ashed. In another report, Oderinde
(1989) determined the vanadium and titanium contents of nine Nigerian crude
and petroleum products using a spectrometric method. He reported that in some
of the samples the Vanadium and Titanium content were high enough to cause
corrosion in turbines and refining processes line in the refinery. In a recent study,
Anthony (2005) reported in his comprehensive analysis of various metallic
elements in Nigerian petroleum products using Atomic Absorption Spectroscopy
84
technique, discussed the influence of these trace metals contaminants in the
refinery processes.
Farroha et al. (1984) used electrochemical method to determine trace levels of
sulphur in petroleum by constant current coulometry. The Tandem mass
spectrometer combined with chemical reaction was used to concentrate sulphur
containing poly-nuclear aromatic compound by wood et al.
(1984).Oderinde,(1984) thoroughly investigated the types of sulphur compounds
present in Ugheli Quality Control Centre(UQCC) of crude oil distillates fractions.
And in 2004, Odebunmi and Adeniyi, (2004) analyzed trace metals in petroleum
and petroleum products using AAS and they discovered that, the results
confirmed that the heavy crude oil contains trace metals higher than the medium
and light crudes oil and for the petroleum products shows lubricating oil which
has higher viscosity, followed by engine oil and the lubes oil was the least.
1.3 Crude Oil / Composition
Crude Oil is a naturally occurring, toxic, flammable liquid, consisting of a complex
mixture of hydrocarbons of various molecular weights, and other organic
compounds, that are found in geological formations beneath the Earth’s surface
(Mendham et al., 2000).
In its strictest sense, petroleum includes only crude oil, but in common usage it
includes both crude oil and natural gas. Both crude oil and natural gas are
predominantly a mixture of hydrocarbons. Under surface pressure and
temperature conditions, the lighter hydrocarbons such as methane, ethane,
85
propane and butane occur as gases, while the heavier ones from pentane and up
are in the form of liquids or solids. However, in the underground oil reservoir the
proportion which is gas or liquid varies depending on the subsurface conditions
and on the phase diagram of the petroleum mixture (Mendham et al., 2000).An
oil well produces predominantly crude oil, with some natural gas dissolved in it.
Because the pressure is lower at the surface than underground, some of the gas
will come out of solution and be recovered (or burned) as associated gas or
solutiongas. A gas well produces predominately natural gas. However, because
the underground temperature and pressure are higher than at the surface, the
gas may contain heavier hydrocarbons such as pentane and hexane in the
gaseous state. Under surface conditions these will condense out of the gas and
form natural gas condensate, often shortened to condensate (Condensate
resembles gasoline in appearance and is similar in composition to some volatile
light crude oils).The proportion of light hydrocarbons in the petroleum mixture is
highly variable between different oil fields and ranges from as much as 97% by
weight in the lighter oils to as little as 50% in the heavier oils and bitumen (Mall,
2007). The hydrocarbons in crude oil are mostly alkanes, cycloalkanes and
various aromatic hydrocarbons while the other organic compounds contain
nitrogen, oxygen and sulphur, and trace amounts of metals such as iron, nickel,
copper and vanadium. The exact molecular composition varies widely from
formation to formation but the proportions of chemical elements vary over fairly
narrow limits as shown in Table 1 (Mall, 2007).
86
Table 1.1: Composition of Crude Oil.
Element Percentage range
Carbon
Hydrogen
Nitrogen
Oxygen
Sulphur
Metals
83.00 – 87.00%
10.0 – 14.00 %
0.10 – 2.00 %
0.10-1.50 %
0.50 – 6.00 %
< 0.10 %
Source: (Mall, 2007)
1.3.1 Types of Hydrocarbon
Generally there are four different types of hydrocarbon molecules in crude oil.
The relative percentage of each varies from oil to oil, depending on the properties
of each oil. Table 2 presents the composition of hydrocarbon by weight.
Table 1.2: Composition by weight of Hydrocarbon
Hydrocarbon Composition by weight Average Range
Paraffins 30.00 15.00 – 60.00
Naphthenes 49.00 30.00 – 60.00
Aromatic 15.00 3.00 – 30.00
Asphaltic 6.00 Remainder
Source: Mall, 2007.
87
1.3.2 Classification of Petroleum
The oil industry classifies “crude” by the location of its origin (e.g., “West Texas
Intermediate, WTI” or “Brent”) and often by its relative weight (API gravity) or
viscosity (“light”, “intermediate” or “heavy”); refiners may also refer to it as
“sweet”, which means it contains relatively little sulphur, or as “sour”, which
means it contains substantial amounts of sulphur and requires more refining in
order to meet current product specifications (Speight,1999 ).
a) Paraffin Base: This classification was based on the fact that some petroleum
oils separated paraffin wax on cooling leading to the conclusion that, these
consisted mainly of paraffins (e.g. methane, ethane, propane, etc. with
general formula (CnH2n+2).
b) Asphaltic Base: These were the petroleum oils which gave no separation of
paraffin wax on cooling again leading to the conclusion that these
predominantly contained cyclic (or naphthenic) hydrocarbons.
c) Mix Base: These petroleum oils leave a mixture of paraffin wax and asphaltic
bitumen when subjected to nondestructive distillation, hence the name.
d) Hybrid Base: These are basically asphaltic oils that contain a small amount of
wax.
1.3.3 Chemistry of Petroleum
Petroleum is a mixture of a very large number of different hydrocarbons; the most
commonly found molecules are alkanes (linear or branched), cycloalkanes,
aromatic hydrocarbons, or more complicated chemicals like asphaltenes. Each
88
petroleum variety has a unique mix of molecules, which define its physical and
chemical properties, like color and viscosity (Speight, 1999).
Paraffin
These consist of straight or branched carbon rings saturated with hydrogen
atoms, the simplest of which is methane (CH4) the main ingredient of natural gas.
Others in this group include ethane (C2H6), and propane (C3H8). (Mall, 2007).
C H
H
H
H
C C C
H H
H
H H
H H
H
C C C
H H
H
H H
H H
H
Methane
Ethane
Propane
C C C
H H
H
H
H H
H
C
H
H
H
C C C
H H
H
H H
H
H
H C H
H Normal
Butaneiso-butane
Figure 1.1: Structures of selected paraffins
Naphthenes
Naphthenes consist of carbon rings, sometimes with side chains, saturated with
hydrogen atoms. Naphthenes are chemically stable; they occur naturally in crude
oil and have properties similar to paraffins (Mall, 2007).
89
C H
CH
2
CH2
C H
C
H2
H C CH3 3
1,3-Dimethyl-cyclopentane
H C 2
H C 2 CH
2
CH2
CH2
C
H2
Cyclohexane
Figure: 1. 2: Structures of some Naphthenes
Aromatics
Aromatic hydrocarbons are compounds that contain a ring of six carbon atoms
with alternating double and single bonds and six attached hydrogen atoms. This
type of structure is known as a benzene ring. They occur naturally in crude oil,
and can also be created by the refining process (Mall, 2007).
HC
HC
CH
CH
CH
C H
Benzene
HC
HC
C
CH
CH
C H
CH3
Toluene
C
HC
CH
C
CH
C H
H C 3
CH3
p-Xylene
Figure: 1.3: Structures of some Aromatics
The more carbon atoms a hydrocarbon molecule has, the “heavier” it is (the
higher is its molecular weight) and the higher is its boiling point. Small quantities
of a crude oil may be composed of compounds containing oxygen, nitrogen,
90
sulphur and metals. Sulphur content ranges from traces to more than 5 per cent
(Speight, 1999).
1.3.4 Origin of Petroleum
Most scientists agree that hydrocarbons (oil and natural gas) are of organic
origin. A few, however, maintain that some natural gas could have formed deep
within the earth, where heat melting the rocks may have generated it
inorganically (Gold and Soter,1980). Nevertheless, the weight of evidence
favours an organic origin, most petroleum coming from plants and perhaps also
animals, which were buried and fossilized in sedimentary source
rocks(Levorsen,1967). The petroleum was then chemically altered into crude oil
and gas (Tissot and Welte, 1984). The chemistry of oil provides crucial clues as
to its origin. Petroleum is a complex mixture of organic compounds. One such
chemical in crude oils is called porphyrin. This compound have been identified in
a sufficient number of sediments and crude oils to establish a wide distribution of
the geochemical fossils, it’s also found in and animals (McQueen, 1986).
1.3.5 Biogenic theory
Most geologists view crude oil, like coal and natural gas, as the product of
compression and heating of ancient vegetation over geological time scales.
According to this theory, it is formed from the decayed remains of
prehistoricmarineanimals and terrestrial plants. ( Alboud warej,2006 ).
91
1.3.6 Abiogenic theory
This theory suggests that large amounts of carbon exist naturally in the planet,
some in the form of hydrocarbons.Thermodynamic calculations and experimental
studies confirm that n-alkanes (common petroleum components) do not
spontaneously evolve from methane at pressures typically found in sedimentary
basins, and so the theory of an origin of hydrocarbons suggests deep generation
below 200 km (Dean, 1993).
1.3.7 Petroleum movement/migration
Petroleum Migration is a process (or processes) whereby petroleum moves from
place of its origin, the source rock to its destruction at the earth’s surface. A Long
the route, the petroleum’s progress may be temporarily arrested and the
petroleum may “rest” on its journey within the trap .The process of migration can
be divided in to three stages;
a) Primary migration is the expulsion of the petroleum products from the
source rock
b) Secondary migration is the journey from the source rock to the trap
c) Tertiary migration is the leakage and dissipation of petroleum at the
earth’s surface.
92
1.4 History/Exploration of Petroleum in Nigeria
The Nigerian oil and gas industry, taken from when the first known Mineral
Survey was carried out (Araromi, present Ondo State) in 1905, is just a 107
years today. Real exploration of the hydrocarbon potentials of the country
commenced, however, in 1908. The efforts of the Nigerian Bitumen Corporation
(NBC), a German concern, in that year were only able to accomplish some 16
shallow boreholes, confirming a line of oil seepage in the Eastern Dahomey
Basin in Okitipupa, Western Region of Nigeria.
Although the results were not too encouraging and the resultant First World War
did stall the efforts, the attempts by NBC nevertheless spurred subsequent
efforts by the Shell Overseas Exploration Company and D’Arcy Exploration to
open up the country, particularly the subsequently prolific Niger Delta as a worldclass
hydrocarbon prospective region. Although the Royal/Dutch Company
initially got the whole of Nigeria as one huge concession, the search was to be
narrowed to the Niger Delta where in 1956, after having drilled some 15 dry
holes, beginning with the lho-1 NW in Owerri, the first successful well was
spudded at Oloibiri. With that milestone, Nigeria’s first shipment of crude oil
(5,000 barrels) hit the international market in 1958.
By the late sixties and early seventies, Nigeria had attained a production level of
over 2 million barrels of crude oil a day. Although production figures dropped in
the eighties due to economic slump, 2004 saw a total rejuvenation of oil
production to a record level of 2.5 million barrels per day. Current development
93
strategies are aimed at increasing production to 4million barrels per day by the
year 2010.
1.5 Petroleum refining process
Refinery processes have developed in response to changing market demands for
certain products. The quantities of petrol available from distillation alone were
insufficient to satisfy consumers demand, therefore, the main task of refineries
became the production of petrol, and as a result the refineries began to look for
ways to produce more and better quality petrol (Speight, 2009). As a result of
that, two processes were developed thus, breaking down large, heavy
hydrocarbon molecules via catalytic and thermal cracking process and
Reshaping or rebuilding hydrocarbon molecules via reforming process.
1.5.1 Distillation (Fractionation)
Because crude oil is a mixture of hydrocarbons with different boiling
temperatures, it can be separated by distillation into groups of hydrocarbons
that boil between two specified boiling points. Two types of distillation are
performed: atmospheric and vacuum distillation (Hobson, 1986).
1.5.2 Atmospheric distillation
Atmospheric distillation takes place in a distilling column at or near atmospheric
pressure. The crude oil is heated to 350 – 400oC and the vapour and liquid are
piped into the distilling column. The liquid falls to the bottom and the vapour
94
rises, passing through a series of perforated trays (sieve trays). Heavier
hydrocarbons condense more quickly and settle on lower trays and lighter
hydrocarbons remain as a vapour longer and condense on higher trays. Liquid
fractions are drawn from the trays and removed. In this way the light gases,
methane, ethane, propane and butane pass out the top of the column, petrol is
formed in the top trays, kerosene and gas oils in the middle, and fuel oils at the
bottom. Residue drawn of the bottom may be burned as fuel, processed into
lubricating oils, waxes and bitumen or used as feedstock for cracking units. To
recover additional heavy distillates from this residue, it may be piped to a second
distillation column where the process is repeated under vacuum, called vacuum
distillation (Hobson, 1986).
1.5.3 Vacuum distillation.
This allows heavy hydrocarbons with boiling points of 450OC and higher to be
separated without them partly cracking into unwanted products such as coke and
gas. The heavy distillates recovered by vacuum distillation can be converted into
lubricating oils by a variety of processes. The most common of these is called
solvent extraction. In one version of this process the heavy distillate is washed
with a liquid which does not dissolve in it but which dissolves (and so extracts)
the non-lubricating oil components out of it. Another version uses a liquid which
does not dissolve in it but which causes the non-lubricating oil components to
precipitate (as an extract) from it. Other processes exist which remove impurities
95
by adsorption onto a highly porous solid or which remove any waxes that may be
present by causing them to crystallize and precipitate out (Dhariaet al., 1992)
1.5.4 Reforming
Reforming is a process which uses heat, pressure and a catalyst (usually
containing platinum) to bring about chemical reactions which upgrade naphtha
into high octane petrol and petrochemical feedstock. The naphtha is hydrocarbon
mixtures containing many paraffins and Naphthenes. In Australia, this naphtha
feedstock comes from the crudes oil distillation or catalytic cracking processes,
but overseas it also comes from thermal cracking and Hydrocracking processes.
Reforming converts a portion of these compounds to isoparaffins and aromatics,
which are used to blend higher octane petroleum product (Dhariaet al.,1992).
paraffins are converted to isoparaffins
paraffins are converted to naphthenes
naphthenes are converted to aromatics
Equation to illustrate
4 (1)
7 16( ) 7 8( ) 2( )
(2)
6 14( ) 6 6( ) 2( )
3 (3)
6 12( ) 6 6( ) 2( )
catalyst
C H C H H
l l gas
catalyst
C H C H H
l l g
catakyst
C H C H H
l l g
96
1.5.5 Cracking
Cracking processes break down heavier hydrocarbon molecules (high boiling
point oils) into lighter products such as petrol and diesel. These processes
include catalytic cracking, thermal cracking and Hydrocracking.
A typical reaction:
16 34( ) 8 18( ) 8 16( ) (3) catalyst
l l l C H C H C H
1.5.6 Catalytic cracking
Catalytic cracking is used to convert heavy hydrocarbon fractions obtained by
vacuum distillation into a mixture of more useful products such as petrol and light
fuel oil. In this process, the feedstock undergoes a chemical breakdown, under
controlled heat (450 – 500oC) and pressure, in the presence of a catalyst (a
substance which promotes the reaction without itself being chemically changed).
Small pellets of silica – alumina or silica – magnesia have proved to be the most
effective catalysts. The cracking reaction yields petrol, LPG, unsaturated olefin
compounds, cracked gas oils, a liquid residue called cycle oil, light gases and a
solid coke residue. Cycle oil is recycled to cause further breakdown and the
coke, which forms a layer on the catalyst, is removed by burning. The other
products are passed through fractionators to be separated and separately
processed (Habsi-Halabiet al., 1997).
97
A typical reaction:
C16H34
C8H (l) 18(l) C8H16(l)
catalyst +
1.5.7 Fluid catalytic cracking
Fluid catalytic cracking uses a catalyst in the form of a very fine powder which
flows like a liquid when agitated by steam, air or vapour. Feedstock entering the
process immediately meets a stream of very hot catalyst. The resulting vapours
keep the catalyst fluidized as it passes into the reactor, where the cracking takes
place and where it is fluidized by the hydrocarbon vapour. The catalyst next
passes to a steam stripping section where most of the volatile hydrocarbons are
removed. It then passes to a regenerator vessel where it is fluidized by a mixture
of air and the products of combustion which are produced as the coke on the
catalyst is burnt off. The catalyst then flows back to the reactor. The catalyst thus
undergoes a continuous circulation between the reactor, stripper and regenerator
sections.The catalyst is usually a mixture of aluminium oxide and silica. Most
recently, the introduction of synthetic zeolite catalysts has allowed much shorter
reaction times and improved yields and octane numbers of the cracked gasoline
(Krishner, 1991).
1.5.8 Thermal cracking
Thermal cracking uses heat to break down the residue from vacuum distillation.
The lighter compounds produced from this process can be made into distillate
fuels and petrol. Cracked gases are converted to petrol blending components by
98
alkylation or polymerization. Naphtha is upgraded to high quality petrol by
reforming. Gas oil can be used as diesel fuel or can be converted to petrol by
Hydrocracking. The heavy residue is converted into residual oil or coke which is
used in the manufacture of electrodes, graphite and carbides (Elliot, 1992).
A typical equation:
C13H38(l) C3H6(g) + C10H22(l)
1.5.9 Hydrocracking
Hydrocracking can increase the yield of petroleum components, as well as being
used to produce light distillates. It produces no residues, only light oils.
Hydrocracking is catalytic cracking in the presence of hydrogen. The extra
hydrogen saturates, or hydrogenates the chemical bonds of the cracked
hydrocarbons and creates isomers with the desired characteristics.
Hydrocracking is also a treating process, because the hydrogen combines with
contaminants such as sulphur and nitrogen, allowing them to be removed.Gas oil
feed is mixed with hydrogen, heated, and sent to a reactor vessel with a fixed
bed catalyst, where cracking and hydrogenation take place. Products are sent to
a fractionators to be separated. The hydrogen is recycled. Residue from this
reaction is mixed again with hydrogen, reheated, and sent to a second reactor for
further cracking under higher temperatures and pressures. In addition to cracked
naphtha for making petrol, Hydrocracking yields light gases useful for refinery
fuel, or alkylation as well as components for high quality fuel oils, lube oils and
99
petrochemical feedstock.Following the cracking processes it is necessary to build
or rearrange some of the lighter hydrocarbon molecules into high quality petrol or
jet fuel blending components or into petrochemicals. The former can be achieved
by several chemical processes such as alkylation and isomerization. Equation of
the reaction:
H2
phenanthrene naphthenonaphthalene
+
1.5.10 Alkylation
Alkylation refers to the chemical bonding of these light molecules with 99 Carbon
atoms to form larger branched-chain molecules (isoparaffins) that make high
octane petrol. Olefins are mixed with an acid catalyst and cooled. They react to
form alkylate, plus some normal butane and propane. The resulting liquid is
neutralized and separated in a series of distillation columns. Isobutane is
recycled as feed and butane and propane sold as liquid petroleum gas (LPG).
100
A typical reaction
CH CH3 3l
AlCl3 catalyst,reflux
Benzene Toluene; compound with methane
1.5.11 Isomerization
Isomerization refers to chemical rearrangement of straight-chain hydrocarbons
(paraffins), so that they contain branches attached to the main chain
(isoparaffins). This is done for two reasons:
They create extra 100 feed for alkylation
They improve the octane of straight run pentanes and hexanes and hence
make them into better petrol blending components.
Isomerization is achieved by mixing normal butane with a little hydrogen and
chloride and allowed to react in the presence of a catalyst to form
100ersian100e, plus a small amount of normal butane and some lighter gases.
Products are separated in fractionators. The lighter gases are used as refinery
fuel and the butane recycled as feed. Pentanes and hexanes are the lighter
components of petrol. Isomerisation can be used to improve petrol quality by
converting these hydrocarbons to higher octane isomers. The process is the
same as for butane isomerization (Matar, 2002).
101
A typical reaction
CH CH3C(CH3)2CH3 3CH2CH2CH2CH3 CH3CH(CH3)CH2CH3+
Pentane 2-methyl butane 2,2-dimethyl propane
1.5.12 Polymerization
Under pressure and temperature, over an acidic catalyst, light unsaturated
hydrocarbon molecules react and combine with each other to form larger
hydrocarbon molecules. Such process can be used to react butenes (olefin
molecules with four carbon atoms) with iso-butane (branched paraffin molecules,
or isoparaffins, with four carbon atoms) to obtain a high octane olefinic petrol
blending component called polymer gasoline (Mill, 2007).
1.5.13 Hydro-treating and Sulphur plants
A number of contaminants are found in crude oil. As the fractions travel through
the refinery processing units, these impurities can damage the equipment, the
catalysts and the quality of the products. There are also legal limits on the
contents of some impurities, like sulphur in products. Hydrotreating is one way of
removing many of the contaminants from many of the intermediate or final
products. In the hydrotreating process, the entering feedstock is mixed with
hydrogen and heated to 300 – 380oC. The oil combined with the hydrogen then
102
enters a reactor loaded with a catalyst which promotes several reactions (Goar
and Goar, 1986).
Hydrogen combines with sulphur to form hydrogen sulphide (H2S)
Nitrogen compounds are converted to ammonia (NH3)
any metals contained in the oil are deposited on the catalyst
Some of the olefins, aromatics or naphthenes become saturated with
hydrogen to become paraffins and some cracking takes place, causing the
creation of some methane, ethane, propane and butanes.
1.5.14 Sulphur recovery plants
The hydrogen sulphide created from hydrotreating is a toxic gas that needs
further treatment. The usual process involves two steps:
the removal of the hydrogen sulphide gas from the hydrocarbon stream
The conversion of hydrogen sulphide to elemental sulphur, a non-toxic
and useful chemical.
Solvent extraction, using a solution of diethanolamine (DEA) dissolved in water,
is applied to separate the hydrogen sulphide gas from the process stream. The
hydrocarbon gas stream containing the hydrogen sulphide is bubbled through a
solution of diethanolamine solution (DEA) under high pressure, such that the
hydrogen sulphide gas dissolves in the DEA. The DEA and hydrogen mixture is
then heated at a low pressure and then dissolved hydrogen sulphide which is
103
then released as a concentrated gas stream which is sent to another plant for
conversion into sulphur (Goar and Goar, 1986).
Conversion of the concentrated hydrogen sulphide gas into sulphur occurs in two
stages.
1. Combustion of part of the H2S stream in a furnace, producing sulphur dioxide
(SO2) water (H2O) and sulphur (S).
Equation of the reaction
2 2 ( ) 2 2( ) 2( ) ( ) 2 2 ( ) (5) l g g g l H S O SO S H O
2. Reaction of the remainder of the H2S with the combustion products in the
presence of a catalyst. The H2S reacts with the SO2 to form sulphur.
Equation of the reaction
2 ( ) 2( ) 2( ) ( ) 2 ( ) 2 3 2 (6) l g g g l H S SO S S H O
As the reaction products are cooled the sulphur drops out of the reaction vessel
in a molten state, this can be stored and shipped in either a molten or solid state.
1.6 Petroleum products:
Major products of oil refineries are usually grouped into three categories: light
distillates (LPG, gasoline, naphtha), middle distillates (kerosene, diesel), heavy
distillates and residuum (fuel oil, lubricating oils, wax, tar).This classification is
104
based on the way crude oil is distilled and separated into fractions (called
distillates and residuum). Liquid petroleum gas (LPG),Gasoline (also known as
petrol), Naphtha Kerosene and related jet aircraft fuels, Diesel fuel, Fuel oils,
Lubricating oils, Paraffin wax, Asphalt and Tar, Petroleum coke.
1.6.1 Gasoline
Gasoline or Petrol is a petroleum-derived liquid mixture which is primarily used
as a fuel in internal combustion engines. It is also used as a solvent, mainly
known for its ability to dilute paints. It consists mostly of aliphatic hydrocarbons
obtained by the fractional distillation of petroleum, enhanced with iso-octane or
the aromatic hydrocarbons e.g. toluene and benzene to increase its octane
rating. (Fessenden and Fessenden, 1991).
1.6.2 Kerosene
Kerosene is a thin, clear liquid formed from hydrocarbons, with density of 0.78–
0.81 g/cm3. It is obtained from the fractional distillation of petroleum between
150 °C and 275 °C, resulting in a mixture of carbon chains that typically contain
between 6 and 16 carbon atoms per molecule. Kerosene is widely used to power
jet-engine aircraft (jet fuel) and some rockets, but is also commonly used as a
heating fuel and for fire toys(Russell, 2003).
1.6.3 Petroleum Diesel
Petroleum diesel, also called petro-diesel, or fossil diesel is produced from the
fractional distillation of crude oil between 200 °C (392 °F) and 350 °C (662 °F) at
105
atmospheric pressure resulting in a mixture of carbon chains that typically
contain between 8 and 21 carbon atoms per molecules. Petroleum-derived diesel
is composed of about 75% saturated hydrocarbons (primarily paraffins including
n, iso and cycloparaffins) and 25% aromatic hydro carbon (including
naphthalene, alkyl benzene) (Matar, 2002).
1.6.4 Fuel Oil
Fuel oil is a fraction obtained from petroleum distillation, either as a distillate or a
residue. Broadly speaking, fuel oil is any liquid petroleum product that is burned
in a furnace or boiler for the generation of heat or used in an engine for the
generation of power, except oils having a flash point of approximately 40 °C
(104 °F) and oils burned in cotton or wool-wick burners(Matar, 2002)
1.6.5 Lubricating Oil
Lubricating oil or engine oil is oil used for lubrication of various internal
combustion engines. While the main function is to lubricate moving parts,
lubricating oil also cleans, inhibits corrosion, improves sealing, and cools the
engine by carrying heat away from moving parts (Chris, 2007).Lubricating oils
are derived from petroleum-based and non-petroleum-synthesized chemical
compounds .But are today mainly blended by using base oils composed of those
organic compound consisting entirely of carbon and hydrogen (Corsico, 2009).
106
1.6.6 Wax
A wax is a type of hydrocarbon that typically contains long-chain alkanes often
containing ester, carboxylic acid, or alcohol groups. The structure and molecular
weight of the hydrocarbon chain and the relative concentration of the functional
groups determine the hardness of the wax. Waxes are used to make wax paper,
impregnating and coating paper and card to waterproof it or make it resistant to
staining, or to modify its surface properties. Waxes are also used in shoe
polishes, wood polishes, and automotive polishes, as mold release agents in
mold making, as a coating for many cheeses, and to waterproof leather and
fabric (Corsico, 1999).
1.6.7 Petrolatum
Petroleum jelly, petrolatum or soft paraffin, is a semi-solid mixture of
hydrocarbons “with carbon numbers mainly higher than 25” (Schlosberg, 2001),
originally promoted as a topical ointment for its healing properties. Its folkloric
medicinal value as a “cure-all” has since been limited by better scientific
understanding of appropriate and inappropriate uses. However, it is recognized
by the U.S. Food and Drug Administration (FDA) as an approved over-thecounter
(OTC) skin protectant and remains widely used in cosmetic skin care.
(Muhammad, 1992).
1.6.8 Asphalt
Asphalt is a sticky, black and highly viscous liquid or semi-solid that is present in
most crude petroleum and in some natural deposits sometimes termed
asphaltum (Abraham, 1938). Asphalt can be separated from the other
107
components in crude oil (such as naphtha, gasoline and diesel) by the process of
fractional distillation, usually under vacuum conditions. A better separation can
be achieved by further processing of the heavier fractions of the crude oil in a deasphalting
unit, which uses either propane or butane in a supercritical phase to
dissolve the lighter molecules which are then separated (Abraham, 1938).
1.7 Environmental effects
The presence of oil has significant social and environmental impacts, from
accidents and routine activities such as seismic exploration, drilling, and
generation of polluting wastes. Oil extraction is costly and sometimes
environmentally damaging, although Dr. John Hunt from Woods Hole pointed out
in a 1981 paper that over 70% of the reserves in the world are associated with
visible macroseepages, and many oil fields are found due to natural leaks.
Offshore exploration and extraction of oil disturbs the surrounding marine
environment. Extraction may involve dredging, which stirs up the seabed, killing
the sea plants that marine creatures need to survive. Crude oil and refined fuel
spills from tanker ship accidents have damaged fragile ecosystems in Alaska, the
Galapagos Islands, Spain, and many other places. Burning oil releases carbon
dioxide into the atmosphere, which is thought to contribute to global warming.
Petroleum produces less CO2 than coal, but more than natural gas. However,
oil’s unique role as a transportation fuel makes reducing its CO2 emissions a
particularly thorny problem; amelioration strategies such as carbon sequestering
are generally geared for large power plants, not individual tailpipes (Wilhelm
andHermann, 2005).
108
1.8 Aim and Objectives
This research work is aimed at determining and comparing the concentration of
trace metals in different Nigerian petroleum products. This will contribute a lot in
assessing the Nigerian petroleum products’, impact in the environment,
technological processes and economic potentiality.
The objectives of this research work are to:
a) Identify the trace metals in the various petroleum products sample under
study.
b) Compare the result of the different digestion methods adopted.
c) Compare the different petroleum products samples used in terms of trace
metal concentration.
1.9 Justification
The analyses of trace metals in Nigerian petroleum products is very important
considering the detrimental effect of these metals in the refining processes and
environment in general. And also, the data on the concentration of trace metals
in our indigenous petroleum products are lacking, this and other factors
necessitates the analyses of trace and heavy metals in this research work.
IF YOU CAN'T FIND YOUR TOPIC, CLICK HERE TO HIRE A WRITER»
