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.


CHAPTER ONE


INTRODUCTION 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


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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




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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

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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,

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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).




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Table 1.1: Composition of Crude Oil.

Element Percentage range
   
Carbon  83.00 – 87.00%
Hydrogen    10.0 – 14.00 %
Nitrogen    0.10 – 2.00 %
Oxygen  0.10-1.50 %
Sulphur 0.50 – 6.00 %
Metals  < 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.











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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


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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).


        H                   H       H   H           H       H   H                      
                                                                                                       
                            H H                                             H H                                                     H
                                                                                                                                                                                                   
H       C                           C   C               C                               C   C                           C                  
                                                                                                                                                                                                                   
                                                            H                                                                   H                                  
        H               H       H       H       H                   Methane
                                                                                                                                                                   
Ethane                                                                                                                                                                                                 
                                                                                                                                            H                       H                   H
                                                                                                                                                                                                       
                                                                                                                            H                                                                                          
                                H           H               H               H                   C                       C                       C       H
                                                                                                                                                       
                                                                                                                                                                               
                                                                                                                                                                                                               
                H                                                                                                                           H                                                   H
                                C           C                   C                   C       H               H                       C                   H
                                                                                                                                                           
                                                                                                                                                                                                           
                                                                                                    H                                                                                                  
                                            H                                                                                                                                              
Propane H                       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).
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                H2              H2
                                C  CH
H C H   C  H    CH  H C
3   C       C   3   2               2
                                   
                    H2C CH2
                                   
                                   
    C           CH2         C
    H2                          H2
1,3-Dimethyl-cyclopentane   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).

                        H                                  
        H   HC  C   CH H C  H      
                                            3                  
HC  C  CH HC    CH  C   C  CH
                        C                                  
                                                           
HC  C  CH                           HC  C   C  CH
                                   
                CH  3          
        H                                       H   3
                                                           
Benzene Toluene 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,



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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 ).




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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.










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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 world-class 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

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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

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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





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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
 




C7 H16(l )

C6 H14(l )

C6 H12(l )
 



catalyst
 C7 H 8(l )
catalyst
 C6 H 6(l )
catakyst
 C6 H 6(l )
 



   4H 2( gas)  (1)

   H 2( g )  (2)

   3H 2( g )  (3)
 






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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:


C16 H 34 ( l )  catalystC8 H18( l )  C8 H16( l )  (3)

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).







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A typical reaction:        
C16H34  (l) catalyst  C8H18 (l) + C H  
                8   16
                    (l)



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
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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

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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).















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A typical reaction



CH3l               CH3


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).


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A typical reaction


CH3CH2CH2CH2CH3  CH3CH(CH3)CH2CH3 + CH3C(CH3)2CH3

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

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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



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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


2H2 S( l )   2O2( g )   SO2( g )   S( g )   2 H 2 O( l )   (5)



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


2H2 S( l )   SO2( g )   S 2( g )   3S( g )   2 H 2 O( l )   (6)



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

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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


104
 

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

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