Feedstocks and Products of Crude Oil and Natural Gas R Efineries

21

2.1 NatureaNdCoNstitueNtsofPetroleumfluidsAs discussed in Chapter 1, petroleum fluids are mixtures of various hydrocarbons that may exist as gas or liquid in a petroleum reservoir. The principal elements of petroleum are carbon (C), hydrogen (H), and small quantities of het-eroatoms of sulfur (S), nitrogen (N), and oxygen (O). It is generally believed that the petroleum hydrocarbons have been derived from the conversion of organic compounds in some aquatic plants and animals. The most impor-tant factors that affect conversion of organic compounds to petroleum hydrocarbons are (1) heat and pressure, (2)radioactivity such as gamma rays, and (3) catalytic reac-tions. Vanadium and nickel species are the most effective catalysts in the formation of petroleum and are needed for the conversion reactions. For this reason, these metals may be found in small quantities in petroleum fluids. Occasion-ally traces of radioactive isotopes such as uranium and potassium can also be found in petroleum. The conditions required for converting organic compounds into petro-leum are (1) geological time frame in millions of years, (2)pressure up to 17 MPa (~2500 psi), and (3) temperature not exceeding 100120 C (~ 210250 F). In some cases, bacteria may have severely biodegraded the oil, destroying the light hydrocarbons. An example of such a case would be the large heavy oil accumulations found in Venezuela. Petroleum is a mixture of thousands of different identifi-able hydrocarbons that are discussed in the next section. Once petroleum is accumulated in a reservoir or in vari-ous sediments, hydrocarbon compounds may be converted from one form to another with time and varying geological conditions. The main difference between various oils from different fields around the world is the difference in their composition of hydrocarbon compounds and impurities [1].

Compounds that only contain elements of carbon and hydrogen are called hydrocarbons, and they form the largest group of organic compounds found in petroleum. There might be as many as several thousand different hydrocarbon compounds in petroleum reservoir fluids. Hydrocarbon compounds have a general closed formula of CxHy, where x and y are integer numbers. The lightest hydrocarbon is methane (CH4), which is the main compo-nent in natural gas. Methane is from a group of hydrocar-bons called paraffins. Hydrocarbons are generally divided into four groups: (1) paraffins, (2) olefins, (3) naphthenes, and (4) aromatics. Paraffins, olefins, and naphthenes are sometimes called aliphatic versus aromatic compounds.

The International Union of Pure and Applied Chemis-try (IUPAC), a nongovernmental organization, provides standard names, nomenclature, and symbols for chemical compounds, including hydrocarbons [2].

Paraffins are also called alkanes and have the general formula of CnH2n+2, where n is the number of carbon atoms in a given molecule. Paraffins are divided into two groups of normal and isoparaffins. Normal paraffins or normal alkanes are simply written as n-paraffins or n-alkanes, and they are open, straight-chain saturated hydrocarbons. Paraffins are the largest series of hydrocarbons found in petroleum and begin with methane, which is also shown by C1. Figure 2.1 shows several lighter paraffins found in petroleum fluids [3]. For example, the open formula for n-butane, n-C4, can be shown as CH3-CH2-CH2-CH3, and for simplicity in drawing, only the carbon-carbon bonds are drawn and most C-H bonds are omitted.

The second group of paraffins is called isoparaffins, which are branched-type hydrocarbons and they begin with isobutane (also called methylpropane), which has the same closed formula as n-butane (C4H10). Compounds of different structures with the same closed formula are called isomers.

As shown in Figure 2.1, there are two isomers for butane, three for pentane, and five isomers for hexane (only four are shown in Figure 2.1.) Similarly, octane (C8H18) has 18 and dodecane (C12H26) has 355 isomers, whereas octadecane (C18H38) has 60,523 and C40 has 62 10

12 isomers. The num-ber of isomers rapidly increases with the number of carbon atoms in a molecule because of the rapidly rising number of their possible structural arrangements, as shown in Figure 2.2 [1]. It should be noted that many of these isomers may not be found in petroleum because they are not thermody-namically stable. For the paraffins in the range of C5C12 the number of isomers is more than 600, although only approxi-mately 200400 of them have been identified in petroleum mixtures. Isomers have different physical and chemical properties. The same increase in number of isomers with molecular weight applies to other hydrocarbon series. As an example, the total number of hydrocarbons (from different groups) having 20 carbon atoms is more than 300,000 [5].

Under standard conditions of temperature and pres-sure (STP), the first four members of the alkane series (methane, ethane, propane, and butane) are in gaseous form, from C5H12 (pentane) to n-heptadecane (C17H36) are liquids, and n-octadecane (C18H38) or heavier compounds exist as wax-like solids at STP. Paraffins from C1 to C40 usu-ally appear in crude oil and represent up to 20 % of crude

2feedstocksandProductsofCrudeoilandNaturalGasrefineriesM.R. Riazi1 and Semih Eser2

1 Kuwait University, Kuwait2 The Pennsylvania State University, University Park, PA, USA

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Copyright by ASTM Int'l (all rights reserved); Wed Dec 10 04:33:39 EST 2014Downloaded/printed byIntertek Testing Services NA (Intertek Testing Services NA) pursuant to License Agreement. No further reproductions authorized.

22 PetroleumrefiNiNGaNdNaturalGasProCessiNG

by volume. Because paraffins are fully saturated (no double bond) they are stable and remain unchanged over long peri-ods of geological time.

Olefins are another series of noncyclic hydrocarbons, but they are unsaturated and have at least one double bond between carbon-carbon atoms. Compounds with one double bond are called mono-olefins or alkenes and include ethene (also named ethylene; CH2=CH2) and propene (or propylene; CH2=CH-CH3). In addition to the structural isom-erism connected with the location of double bond, there is another type of isomerism called geometric isomerism that indicates the way atoms are oriented in space. The configu-rations are differentiated in their names by the prefixes cis- and trans-, such as cis- and trans-2-butene. Mono-olefins

have the general formula of CnH2n. If there are two double bonds, the olefin is called a diolefin (or diene), such as butadiene (CH2=CH-CH=CH2). Unsaturated compounds are more reactive than saturated hydrocarbons (without double bond). Olefins are uncommon in crude oils because of their reactivity with hydrogen that saturates them; however, they can be produced in refineries through cracking reactions. Olefins are valuable products of refineries and are used as feedstocks for petrochemical plants to produce polymers such as polyethylene. Similarly compounds with triple bonds such as acetylene (CH CH ) are not found in crude oils because of their tendency to become saturated [1].

Naphthenes or cycloalkanes are ring or cyclic saturated hydrocarbons with general formula of CnH2n. Cyclopentane (C5H10), cyclohexane (C6H12), and their derivatives such as n-alkylcyclopentanes are normally found in crude oils. Three types of naphthenic compounds are shown below.

Methylcyclopentane(C6H12)

Ethylcyclohexane(C8H16)

Cyclopentane(C5H10)

If there is only one alkyl group from n-paraffins (i.e., methyl, ethyl, propyl, n-butyl, etc.) attached to a cyclopen-tane hydrocarbon, the series is called n-alkylcyclopentanes, such as the two hydrocarbons shown above where on each junction of the ring there is a CH2 group, except on the alkyl group juncture, where there is only a CH group. Naphthenic hydrocarbons with only one ring are also called monocy-cloparaffins or mononaphthenes. In heavier oils, saturated multirings attached to each other called polycycloparaffins or polynaphthenes may also be available. Thermodynamic stud-ies show that naphthene rings with five and six carbon atoms are the most stable naphthenic hydrocarbons. The content of cycloparaffins in petroleum may vary up to 60 %. Generally, any petroleum mixture that has hydrocarbon compounds with five carbon atoms also contains naphthenic compounds.

Aromatics are an important series of hydrocarbons found in almost every petroleum mixture from any part of the world. Aromatics are cyclic but unsaturated hydro-carbons with alternating double bonds that begin with a benzene molecule (C6H6). The name aromatic refers to the fact that such hydrocarbons commonly have fragrant odors. A group of lighter aromatic hydrocarbons is shown in Figure 2.3. Although benzene has three carbon-carbon double bonds, it has a unique arrangement of electrons with resonance structures of the double bonds (aromaticity) that allow benzene to be relatively stable. However, benzene is known to be a cancer-inducing compound. For this reason, the amount of benzene allowed in petroleum products such as gasoline or fuel oil is limited by government regulations in many countries. Under standard conditions, benzene, toluene, and xylene are in liquid form whereas naphthalene is in a solid state.

Some of the common aromatics found in petroleum and crude oils are benzene and its derivatives with attached methyl, ethyl, propyl, or higher alkyl groups. This series of aromatics is called alkylbenzenes and compounds in this homologous group of hydrocarbons have the general

0Number of carbon atoms

Num

ber o

f isomers

1.0 1015

1.0 1010

1.0 105

1.0 100 10 20 30 40 50

figure2.2Numberofpossiblealkaneisomers[1].

figure2.1Lighterparaffinhydrocarbonspresent inpetroleumandnaturalgas[3].

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Chapter2nfeedstoCksaNdProduCtsofCrudeoilaNdNaturalGasrefiNeries 23

formula of CnH2n-6 (where n 6). Generally, an aromatic series with only one benzene ring is also called mono-aromatics or mononuclear aromatics. Naphthalene and its derivatives that have only two unsaturated rings are sometime called diaromatics. Crude oils and reservoir fluids all contain aromatic compounds. However, heavy petroleum fractions and residues contain unsaturated mul-tirings with many benzene and naphthene rings attached to each other. Such aromatics that are in solid form are also called polyaromatic hydrocarbons (PAHs) or polynuclear aromatics (PNAs). In this chapter, the terms of mono- and polyaromatics are used. Heavy crude oils usually contain more aromatics than light crudes. The amount of aromat-ics in coal liquids is usually high, and it could reach as high as 98 % by volume. It is common to have compounds with naphthenic and aromatic rings side by side, especially in heavy fractions. Monoaromatics with one naphthenic ring have the formula of CnH2n-8. There are many combinations of alkylnaphthenoaromatics [4,5].

Normally, high-molecular-weight polyaromatics con-tain several heteroatoms such as sulfur, nitrogen, or oxygen, but these compounds are still called aromatic compounds because their electronic configurations maintain the aro-matic character. Two types of these compounds are shown below [1].

S

NH

Dibenzothiophene Benzocarbazole (C16H11N)

Such heteroatoms in multiring aromatics are com-monly found in asphaltene compounds, as shown in Figure 2.4, where, for simplicity, carbon and hydrogen atoms are not marked on the rings or on the paraffinic chains attached to the ring systems.

Sulfur is the most important heteroatom in petroleum and it can be found in cyclic (e.g., thiophenes) and noncyclic compounds such as mercaptans (R-S-H) and sulfides (R-S-R), where R and R are alkyl groups. Sulfur in natural gas is usually found in the form of hydrogen sulfide (H2S). Some natural gases contain H2S as high as 30 % by volume. The amount of sulfur in a crude oil may vary from 0.05 to 6 % by weight. The presence of sulfur in finished petroleum prod-ucts is harmful. For example, the presence of sulfur in gaso-line can promote corrosion of engine parts. The amounts of nitrogen and oxygen in crude oils are usually less than the amount of sulfur by weight. In general, for petroleum oils the elemental composition varies within fairly narrow ranges, as shown below on a weight basis [5,6]:

Carbon (C), 83.087.0 %Hydrogen (H), 10.014.0 %Nitrogen (N), 0.12.0 %Oxygen (O), 0.051.5 %Sulfur (S), 0.056.0 %Metals (nickel, vanadium, and copper),


determine if a reservoir fluid is in the form of gas, liquid, or a mixture of gas and liquid. These factors are (1) composi-tion of reservoir fluid, (2) temperature, and (3)pressure. The most important characteristic of a reservoir fluid in addition to specific gravity (or API gravity) is its gas-to-oil ratio (GOR), which represents the amount of gas produced at standard conditions in standard cubic feet (scf) to the amount of liquid oil produced at the standard condition in stock tank barrels (stb). Other units of GOR and its calculation methods are discussed in Chapters 1 and 10 of ASTM Manual 50 [1]. Res-ervoir fluids are generally categorized into four or five types, the characteristics of which are given in Table 2.1. These five fluids in the direction of increasing GOR are black oil, volatile oil, gas condensate, wet gas, and dry gas.

A natural gas is called dry gas if it does not produce any liquid hydrocarbons after the surface separator under standard conditions. A natural gas that produces liquid hydrocarbons after production at the surface facilities is called wet gas. The word wet refers to the presence of hydrocarbon liquids in a natural gas that condense at surface conditions. In dry gases no liquid hydrocarbon is formed at the surface conditions. Volatile oils have also been called high-shrinkage crude oil and near-critical oils because the reservoir temperature and pressure are very close to the critical point of such oils, but the critical tem-perature is always greater than the reservoir temperature [1]. Gases and gas condensate fluids have critical tempera-tures that are less than the reservoir temperature. Black oils contain heavier compounds; therefore, the API gravity of stock tank oil is generally lower than 40 and the GOR is less than 1000 scf/stb. The specifications given in Table2.1 for various reservoir fluids, especially at the boundaries between different types, are somewhat arbitrary and may vary from one source to another. It is possible to have a reservoir fluid type with properties outside of the corre-sponding limits given above. Determination of a type of reservoir fluid by the above rule of thumb on the basis of the GOR, the API gravity of stock tank oil, or its color is not possible for all fluids. In general, oils produced from wet gas, gas condensate, volatile oil, and black oil increase in specific gravity (decrease in API gravity and quality) in the same order. Liquids from black oils are viscous and black in color, whereas the liquids from gas condensates or wet gases are clear and colorless. Volatile oils produce brown with some red/green color liquid. Wet gas contains less methane than a dry gas but a larger fraction of C2C6 components. The main difference between these reservoir fluids is obviously found in their molecular composition. An example of the composition of different reservoir fluids is given in Table 2.2 [1].

In this table, C7+ refers to all hydrocarbons having seven or more carbon atoms; this group is called the heptane-plus fraction. C6 refers to a group of all hydrocarbons with six carbon atoms (hexanes) that exist in the fluid. M7+ and SG7+ are the molecular weight and specific gravity, respectively, at 15.5 C (60 F) for the C7+ fraction of the mixture. It should be noted that molecular weight and specific gravity of the whole reservoir fluid are less than the corresponding values for the heptane-plus fraction. For example, for the crude oil sample in Table 2.2, the specific gravity of whole crude is 0.871, or an API gravity of 31. Details of such calculations are discussed in ASTM Manual 50 [1]. These compositions have been determined from a recombination of the compositions of the corresponding separator gas and stock tank liquid, which have been determined by various analytical tools (i.e., gas chromatography, mass spectrom-etry, etc.). Composition of reservoir fluids varies with the reservoir pressure and reservoir depth. In a producing oil field, the sulfur and amount of heavy compounds generally increase with production time. However, it is important to note that within an oil field, the concentration of light hydrocarbons and the API gravity of the reservoir fluid increase with the reservoir depth, whereas its sulfur and C7+ contents decrease with the depth [6]. The lumped C7+ fraction in fact is a mixture of many hydrocarbons up to C40 or higher. As an example, the number of pure hydrocarbons from C5 to C9 detected by chromatography tools in a crude oil from North Sea reservoir fluids was 70 compounds. Most recently, Mansoori has suggested that naturally found hydrocarbon petroleums can be categorized into seven groups, including two semi-solid forms of tar sands and oil shale [3]. The molecular weight distribution of these petro-leum fluids is shown in Figure 2.5.

Reservoir fluids from a producing well are introduced to two- or three-stage separators that reduce the pressure and temperature of the stream to atmospheric pressure and temperature. The liquid leaving the last stage is called stock tank oil (sto) and the gas released in various stages is called associated gas. The liquid oil after necessary field processing is called crude oil. The main factor in operation and design of an oil-gas separator is to find the optimum operating conditions of temperature and pressure so that the amount of produced liquid (oil) is maximized. Such conditions can be determined through phase behavior cal-culations, which are discussed in detail in ASTM Manual 50 [1]. Reservoir fluids from producing wells are mixed with free water. The water is separated through gravita-tional separators on the basis of the difference between densities of water and oil. The remaining water from crude can be removed through dehydration processes. Another

table2.1typesandCharacteristicsofvariousreservoirfluids[1]reservoirfluidtype Gor(scf/stb) CH4(mol%) C6+(mol%) aPiGravityofsto

Blackoil


surface operation is the desalting process, which is neces-sary to remove salt from crude oils. Separation of oil, gas, and water from each other and removal of water and salt from oil and any other process that occurs at the surface are called surface production operations and are discussed in Chapter 11.

In addition to the impurities (hetoroatoms and metals) discussed earlier, some impurities may result from com-pounds that have been added to petroleum fluids for vari-

ous reasons during their production, transportation, and storage. These include but are not limited to acids, alcohols, aromatic hydrocarbons, detergents, and polymers. Fur-thermore, petroleum fluids often contain compounds that result from the physical association with hydrocarbons; these may include colloids, crystalline solids, flocs, and slugs [3].

The crude oil produced from the atmospheric separa-tor has a composition different from the reservoir fluid

table2.2Composition(mol%)andPropertiesofvariousreservoirfluidsandaCrudeoil[1]Component dryGas(1) WetGas(2) GasCondensate(3) volatileoil(4) blackoil(5) Crudeoil(6)

CO2 3.70 0.00 0.18 1.19 0.09 0.00

N2 0.30 0.00 0.13 0.51 2.09 0.00

h2S 0.00 0.00 0.00 0.00 1.89 0.00

C1 96.00 82.28 61.92 45.21 29.18 0.00

C2 0.00 9.52 14.08 7.09 13.60 0.19

C3 0.00 4.64 8.35 4.61 9.20 1.88

iC4 0.00 0.64 0.97 1.69 0.95 0.62

nC4 0.00 0.96 3.41 2.81 4.30 3.92

iC5 0.00 0.35 0.84 1.55 1.38 2.11

nC5 0.00 0.29 1.48 2.01 2.60 4.46

C6 0.00 0.29 1.79 4.42 4.32 8.59

C7+ 0.00 1.01 6.35 28.91 30.40 78.23

100.00 100.00 100.00 100.00 100.00 100.00

GOr(scf/stb) 69,917 4428 1011 855

M7+ 113 143 190 209.8 266

SG7+(at15.5C) 0.794 0.795 0.8142 0.844 0.895

apI7+ 46.7 46.5 42.1 36.1 26.6

figure2.5Variouscategoriesofnaturalgasandliquidsnaturallyoccurringinpetroleumfluidsandtheirapproximatehydrocarbonmolecularweightdistributionsaccordingtotheircarbonnumbers[3,4].

light crude intermediatecrude

heavy oil tar sand oil shalenatural gas gas condensate(NGL)

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obtained from a producing well. The light gases are sepa-rated, and crude oils usually have almost no methane and a small C2C3 content whereas its C7+ content is higher than the original reservoir fluid. As an example, the composition of a crude oil produced through a three-stage separator from a reservoir fluid is also given in Table 2.2 in the last column. Actually this crude is produced from a black oil reservoir fluid, the composition of which is also given in Table 2.2 (column 5).

Two important characteristics of a crude oil that deter-mine its quality are the API gravity (specific gravity) and sulfur content. Generally, a crude with an API gravity of less than 20 (specific gravity > 0.934) is called a heavy crude, and a crude with an API gravity of greater than 40 (specific gravity < 0.825) is called a light crude [1,5]. Crudes with an API gravity of less than 10 are considered as extra heavy oil, such as bitumen. Similarly, if the sulfur content of a crude is less than 0.5 wt % it is called sweet oil. On the other hand, the term sour oil refers to crudes that have more than 0.5 wt % sulfur. It should be noted that these ranges for the gravity and sulfur content are relative and may vary from one source to another. Further classification of crude oils will be discussed in Chapter 4.

2.3 refiNiNGProCessesaNdProduCtsfromCrudeoilrefiNeriesA crude oil produced after necessary field processing and surface operations is transferred to a refinery for process-ing and conversion into various useful products. Petroleum refining (or crude oil refining in more precise terms) has evolved from simple batch distillation in the late 19th cen-tury to todays complex processing schemes in modern refin-eries. Refining processes can be generally divided into three major types: (1) separation, (2) conversion, and (3) finishing.

Separation is a physical process that is carried out by using different techniques to fractionate crude oil or its derivatives. The most important separation process is distil-lation, which occurs in a distillation column to separate the constituent compounds on the basis of differences in their boiling points. Other major physical separation processes include absorption, stripping, and solvent extraction. In the gas plant of a refinery, absorption by a liquid solvent retains C3+ hydrocarbons from a gas mixture and allows methane and ethane to be sent overhead as fuel gas. The solvent is then regenerated in a stripping unit. The conversion pro-cesses involve chemical changes that occur with hydrocar-bons in reactors. The purpose of such reactions is to change the molecular weight and convert hydrocarbon compounds from one type to another. The most important reaction in modern refineries is cracking, which converts heavy hydrocarbons to lighter and more valuable hydrocarbons. Catalytic cracking and thermal cracking are commonly used for this purpose. Other types of reactions such as reforming, isomerization, and alkylation are used to produce high-octane-number gasoline. Finishing processes achieve the purification of various product streams by processes such as desulfurization or acid treatment to remove impu-rities and stabilize the fuels. Finishing processes that also include blending ensure that the refinery products meet the specifications dictated by performance characteristics and environmental regulations [68].

Crude oil in a refinery upon the desalting process enters the atmospheric distillation column where compounds are

separated with respect to their boiling points. Hydrocar-bons in a crude have boiling points ranging from 160 C (boiling point of methane) to more than 600 C (1100 F), which is the boiling point of the heaviest distillable com-pounds in the crude oil. However, the carbon-carbon bond in paraffinic hydrocarbons breaks down at temperatures near 350 C (660 F). This process is called cracking and it is undesirable during the distillation process because it changes the chemical composition of the crude feed. For this reason, compounds having boiling points above 350 C (660 F), constituting the residuum fraction, are removed from the bottom of the atmospheric distillation column and sent to a vacuum distillation column. Because by distillation it is not possible to completely separate the constituent compounds of the crude oil, a distillation col-umn does not produce pure hydrocarbon streams. Instead, distillate fractions are produced as defined according to the boiling point of the lightest and heaviest compounds in the mixtures of hydrocarbons. The lightest product of an atmospheric column is a mixture of methane and ethane (but mainly ethane), which has a boiling range of 180 to 80 C (260 to 40 F) corresponding to the boiling points of methane and ethane, respectively. This mixture, referred to as fuel gas in a refinery, is the lightest petroleum frac-tion. Fractions with a wider range of boiling points contain a greater number of hydrocarbons. All fractions from a distillation column have a known boiling range, except the residuum, the upper boiling point of which is not usually known. The boiling points of the heaviest components in a crude oil are not really known because many of them would undergo cracking or other chemical reactions at tempera-tures lower than their boiling points. Identification of the structure and determining the properties of the heaviest compounds found in crude oils and petroleum residuum still present a difficult challenge to researchers. Theoreti-cally, it can be assumed that the boiling point of the heavi-est compound in a crude oil is infinity. Atmospheric residue contains compounds with carbon numbers greater than 25, whereas vacuum residue has compounds with a carbon number greater than 50 (M > 800). Table 2.3 lists some petroleum fractions produced from distillation columns along with their boiling point ranges and applications. In this table, the boiling points and equivalent carbon number ranges are approximate and they may vary according to the desirable properties of specific products. For example, the light gas fraction consists mainly of a mixture of ethane, propane, and butane; however, some heavier compounds (C5+) may also exist in this fraction. The fraction is further fractionated to obtain ethane (a fuel gas), propane, and butane (petroleum gases). The petroleum gases are lique-fied under pressure to produce liquefied petroleum gas (LPG) that can be used as fuel for heating and cooking in dwellings or as autogas [http://www.worldlpgas.com/]. In addition, butane may be separated from the gas mixture to be used for improving the vapor pressure characteristics (volatility) of gasoline in cold weather. Petroleum fractions separated by distillation may undergo further processing to produce the desired products. For example, gas oil may go through a cracking process to produce more gasoline. The principal refinery processes are discussed in Chapter 5 of this manual. Because distillation is not a perfect separation process, the initial and final boiling points for each frac-tion are not exact and especially the endpoints represent

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table2.3someofthePetroleumfractionsProducedfromdistillationColumns[1]

PetroleumfractionapproximateHydrocarbonrange

approximateboilingrange

C f

Lightgases C2C4 90to1 130to30

Gasoline(lightandheavy) C4C10 1to200 30390

Naphthas(lightandheavy) C4C11 1to205 30400

Jetfuel C9C14 150255 300490

Kerosene C11C14 205255 400490

Dieselfuel C11C16 205290 400550

Lightgasoil C14C18 255315 490600

heavygasoil C18C28 315425 600800

Wax C18C36 315500 600930

Lubricatingoil >C25 >400 >750

Vacuumgasoil C28C55 425600 8001100

residuum >C55 >600 >1100

approximate values. Fractions may be classified as nar-row or wide depending on their boiling point range. As an example, the fractionation of an Alaskan crude oil into vari-ous products by distillation is graphically shown in Figure 2.6. The weight and volume percentages for the products are close to each other. It can be seen in Figure 2.6 that more than 50 % of the crude is processed in the vacuum distillation unit. The vacuum residuum consists mainly of resin- and asphaltene-type compounds containing high-molecular-weight multiring aromatics. The vacuum resid-uum may be further processed for upgrading or mixed with lighter petroleum fractions to obtain saleable products.

Distillation of a crude oil can also be performed in the laboratory to divide the mixture into many narrow boiling point range fractions with a boiling range of approximately 10 C. Such narrow range fractions are sometimes referred to as petroleum cuts. When boiling points of all of the cuts in a crude are known, then the boiling point distribution (distillation curve) of the whole crude can be obtained. In a petroleum cut, hydrocarbons of various types are lumped together in four groups of paraffins (P), olefins (O), naph-thenes (N), and aromatics (A). For olefin-free petroleum cuts, the composition is represented by the PNA content. Crude oils are generally free of olefins.

As mentioned earlier, the petroleum fractions pre-sented in Table 2.3 are not the final products of a refinery. They go through further separation (physical), conversion (chemical), and finishing processes to achieve the product specifications set by the market and government regula-tions. Through refining processes (discussed in Chapter 5), the petroleum fractions shown in Table 2.3 are converted to petroleum products. The terms petroleum fraction, petroleum cut, and petroleum product are usually used interchangeably, but this is not appropriate because each term has a specific meaning that is different from the other two. In general, the petroleum products that are obtained in a refinery can be divided into two groups fuel prod-ucts and nonfuel productsas discussed in the following sections.

2.3.1 PetroleumFuelProductsThe major petroleum fuel products of a refinery are LPG, gasoline, jet fuel, diesel and heating oil, residual fuel oil, and petroleum coke as described below [1,710]. The specifica-tions of these fuels are discussed in Chapter 4 of this manual.1. LPGs are mainly used for domestic heating and cook-

ing (50 %), industrial fuel (clean fuel requirement) (15 %), feedstock for steam cracking (25 %), and as a motor fuel (autogas) for spark ignition engines (10 %). LPG is produced by crude oil refining or natural gas fractionation. The estimated world production in 2005 was 250 million tons per year (8 million bbl/day) [10]. LPG consists mainly of a mixture of propane (C3H8) and n-butane (C4H10), but it may also include ethane (C2H6), ethylene (C2H4), propylene (C3H6), butylene (C4H8), isobutane, and isobutylene in small concen-trations. Propane, butane, or propane/butane mix-tures can be liquefied at ambient temperature under moderate pressure. LPGs are considered ideal fuels because they can be transported and stored in liquid form and used as a gas or a liquid. Propane can be safe-ly used at ambient temperatures from approximately 40C (104F) to 45C (113F), whereas butane can be used at temperatures from 0C (32F) to approximately 110C (230F) [8]. They have high energy density, low sulfur content, and they burn cleanly.

LPGs have been used increasingly as auto fuel under the generic name autogas. The composition of autogas varies depending on the prevailing ambient temperatures in the countries it is used. At moderate ambient temperatures, it consists of 6070 % propane and 3040 % butane [9]. The advantages of using LPG compared with gasoline and diesel include lower fuel and maintenance cost and lower engine emissions. See Chapter 4 for specifications on autogas and variations in specifications in different countries.

2. Gasoline is perhaps one of the most important prod-ucts of a refinery. In the United Kingdom it is referred to as petrol. Gasoline is obtained by blending various

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streams obtained from different refinery operations, including crude oil distillation, catalytic cracking, and catalytic reforming. It contains hydrocarbons from C4 to C11 (molecular weight of ~100110). It is used as a fuel for cars with spark-ignition engines. Its main char-acteristics include anti-nock (octane number), volatility (distillation data and vapor pressure), stability, and density. The main evolution in gasoline production has been the introduction of nonleaded gasoline (referred to as unleaded gasoline, which excludes using tetra-ethyl lead as an additive to increase the octane number) in many parts of the world and the use of reformulated gasoline (RFG) in the United States. The RFG has less butane, less aromatics, and more oxygenates. Sulfur content of gasoline should not exceed 0.03 % by weight. Further properties and characteristics of gasoline will be discussed in Chapter 4. The U.S. gasoline demand in 1964 was 4.4 million bbl/day and increased from 7.2 to 8.0 million bbl/day in a period of 7 years from 1991 to 1998 [1]. In the 1990s, gasoline was approximately

one-third of the refinery products in the United States, whereas in July 2007 gasoline production was approxi-mately 9.33 million bbl/day, or 37.5 % of total products according to the API report.

3. Kerosene is a distillate fraction of crude oil that boils between 150C and 250C and is primarily used for producing jet fuel to power gas turbine or jet engines. To a much smaller extent, kerosene is used as fuel for lighting and cooking, particularly in rural areas where access to natural gas, LPG, and electricity is limited. Jet fuel, which is also called aviation turbine fuel, is a premium fuel that has shown a faster increase in demand than any petroleum fuel because of expanding civil and military aviation. In 2007, an estimated con-sumption for jet fuel was 205 million t [10]. The main characteristics of jet fuel include sulfur content, cold resistance (more stringent performance for military jet fuel), density, aromatics content, and ignition quality. ASTM and the International Air Transport Association (IATA) have issued specifications for commercial (e.g.,

figure2.6productsandcompositionofalaskacrudeoil[1].

0

10

20

30

40

50

60

0 20 40 60 80 100

Naphtha

Kerosene

Light Gas Oil

Heavy Gas Oil

Vacuum Gas Oil

Vacuum Residuum

- 655

- 455

- 345

- 205

- -90

Volume Percent

Vacuum Distillation 53.1 %

Carbon Num

ber

Boilin

g Po

int,

C

Atmospheric Distillation 46.9 %

Light Gasoline Light Gases

AST-MNL58-11-0801-002.indd 28 3/12/13 12:37 PM



Jet A, Jet A-1, the Russian TS-1) and military jet fuel (JP-8) that differ only in freezing point [9].

4. Diesel and heating oil are used for motor fuel and domestic purposes. Diesel is obtained from fractional distillation of crude oil between 200C and 350C. The main characteristics are ignition (for diesel oil), volatility, viscosity, cold resistance, density, sulfur con-tent (corrosion effects), and flash point (safety factor). There are basically three kinds of diesel fuel: No. 1, No. 2, and No. 4. Diesel No. 1 is for use in farm and city buses, whereas diesel No. 2 is for use in automo-bile, truck, and railroad vehicles. Diesel No. 4 is for use in railroad, marine, and stationary engines [9]. Diesel fuels used in city buses have a lower endpoint, lower sulfur content, and higher cetane number.

5. Residual fuel oil is used for industrial fuel, thermal pro-duction of electricity, and motor fuel (low speed diesel engines). Its main characteristics are viscosity (good atomization for burners), sulfur content (corrosion), stability (no decantation separation), cold resistance, and flash point (for safety). Basically there are five types of fuel oils in commercial use: No. 1, No. 2, No. 4, No. 5, and No. 6. Fuel oil No. 1 is used for stoves and farms, fuel oil No. 2 is for home heating uses, No. 4 is used for light industrial uses, No. 5 is used for medium industrial applications, and No. 6 is used for heavy industrial and marine applications [9]. Fuel oil No. 1 has the lowest density, boiling point, flash point, pour point, viscosity, and sulfur content, whereas fuel oil No. 6 is the heaviest fuel oil, with high sulfur content and high viscosity.

6. Petroleum coke, which is a solid byproduct obtained from delayed coking or fluid coking of vacuum distilla-tion residue, may be used as industrial fuel depending on its sulfur and metal contents [11]. It contains less than 1 %wt ash, but it needs to be burned in industrial furnaces with strict controls on emissions. Important properties of fuel coke include grindability, volatile matter content, sulfur content, and nickel and vana-dium contents. Nonfuel uses of petroleum coke are described in the next section.

2.3.2 NonfuelPetroleumProductsThe major nonfuel petroleum products include solvents, naphthas, petrochemical feedstocks, lubricating oils, waxes, asphalts, and petroleum cokes [1,79,11]. Brief descriptions of the nonfuel products and their uses are given below. 1. Solvents are light petroleum cuts in the C4C14 range

that have numerous applications in industry and agriculture. For example, white spirits that have boil-ing point ranges between 135 and 205 C are used as paint thinners. The main characteristics of solvents are volatility, purity, odor, and toxicity. Benzene, toluene, and xylenes (BTX) are used as solvents for glues and adhesives. Naphthas constitute a special category of petroleum solvents with boiling ranges corresponding to those of white spirits. Similar to BTX, naphthas may be used as raw materials for producing petrochemical feedstocks, as described below. Therefore, naphthas are considered to be industrial intermediates that are subject to commercial specifications

2. Petrochemical feedstocks that are produced in the refinery include C6 to C8 aromatics (BTX and ethyl

benzene) and C2 to C4 olefins. In petrochemical plants, these feedstocks are used to produce plastics and res-ins, pharmaceuticals, antifreeze agents, detergents, solvents, dyes, and agricultural chemicals such as fertilizers, pesticides, and herbicides. BTX and ethyl benzene are produced in refineries [in fluid catalytic cracking (FCC) and catalytic reforming units] and in petrochemical plants through reforming of naphtha. The C3 to C4 olefins are produced in FCC units, and C2 and C3 olefins are produced by coking processes in a refinery and steam cracking of naphtha or gas oils in petrochemical plants.

3. Lubricants are composed of a main base stock obtained from dearomatized and dewaxed vacuum gas oils for controlling the viscosity and freezing point and are combined with additives to obtain the desired perfor-mance characteristics. Among the most important char-acteristics of lubricants are thermal stability, viscosity, and the viscosity index, which reflects the change of viscosity with temperature. Aromatics are usually elim-inated from lubricants to improve their viscosity index. Lubricants consist mostly of isoparaffinic compounds. Additives used for lubricants include viscosity index additives such as polyacrylates and olefin polymers, antiwear additives (i.e., fatty esters), antioxidants (i.e., alkylated aromatic amines), corrosion inhibitors (i.e., fatty acids), and antifoaming agents (i.e., polydimethyl-siloxanes). Lubricating greases constitute another class of lubricants that are semisolid. The specifications for lubricants include viscosity index, freezing points, ani-line point (indication of aromatic content), volatility, and carbon residue (indication of thermal stability).

4. Petroleum waxes are of two types: the paraffin waxes in petroleum distillates and the microcrystalline waxes in petroleum residua. In some countries such as France, paraffin waxes are simply called paraffins. Paraffin waxes have high melting points; they are removed by dewaxing of vacuum distillates to control the pour points of lubricating oil base stocks. Paraffin waxes are mainly straight-chain alkanes (C18 to C36) with a very small proportion of isoalkanes and cycloalkanes. Their freezing point is between 30 and 70 C, and the average molecular weight is approximately 350. When pres-ent, aromatics appear only in trace quantities. Waxes from petroleum residua (microcrystalline form) are less defined aliphatic mixtures of n-alkanes, isoalkanes, and cycloalkanes in various proportions. Their average molecular weights are between 600 and 800, their car-bon number range is C30 to C 60,

and the freezing point range is 6090 C. Paraffin waxes (when completely dearomatized) have applications in food industry and food packaging. They are also used in the production of candles, polishes, cosmetics, and coatings [6,8]. Waxes at an ordinary temperature of 25 C are in solid states, although they contain some hydrocarbons in liquid form. When melted, they have relatively low viscosity.

5. Asphalt is produced from vacuum distillation residues by solvent deasphalting. Asphalts contain nonvolatile high-molecular-weight polar aromatic compounds such as asphaltenes and cannot be distilled even under very high vacuum conditions. In some countries asphalt is called bitumen, although this is not a strictly correct use of the term bitumen. Asphaltic materials (containing

AST-MNL58-11-0801-002.indd 29 3/12/13 12:37 PM



asphaltenes and resins) are used as binders for paving the roads. The major properties of asphalt that deter-mine its quality include flash point (for safety), compo-sition (wax content), viscosity, softening point, weather-ing properties (resistance to oxidation or degradation), specific gravity, and stability or chemical resistance.

6. There are some other products such as white oils (used in pharmaceuticals or in the food industry), aromatic extracts (used in the paint industry or the manufacture of plastics), and coke (as a fuel or to produce carbon electrodes for aluminum refining). Aromatic extracts are black materials composed essentially of condensed PNAs and heterocyclic nitrogen or sulfur compounds, or both. Because of this highly aromatic structure, the extracts have a good solvent power. Petroleum cokes produced by delayed coking of vacuum distillation

residue can be specified as sponge, or shot cokes, depending on their microstructure [11]. Sponge cokes that have low ash, low sulfur, and low metal contents can be used for making carbon anodes that are used in electrolysis of alumina to manufacture aluminum. Shot cokes that are much harder than sponge cokes have a niche application for producing titanium diox-ide [11]. Delayed coking of FCC decant oils produces a special coke called needle coke that is used to pro-duce graphite electrodes for electric-arc furnaces for recycling scrap iron and steel. Important properties of calcined needle cokes include density, ash content, and the coefficient of thermal expansion [11].In general, more than 2000 petroleum products within

some 20 categories are produced in refineries in the United States [6,8]. Some of these products obtained from a

figure2.7Someproductsproducedfromcrudeoilprocessing[12].

-CH2-

CO H2

HCOOH CH3OH

CH3(CH)nOHCH3NH2CH4 HCHOHCOOCH3

HCON(CH3)2 HCONH2

HO(CH2)2OH

HCN

Gas and Naphthas

Oil

Keroseneand Gas OilResidue

Gas

Heavy Gasoline

CommercialEnergy

Car Fuel

AviationFuel

Bitumen,Lube Oil, etc,

PetrochemicalIndustry

ClCN

Cyanuric Chloride

Melamine1

H2NCO2(CO)NH

ChloronatedMethanes

FluoronatedMethanes

C2H2

HO(CH2)4OH

Tetrahydrofuran

C4

n-alkanes/alkenes

Butadiene

n-alkyl carboxylic acids, e.g., acetic acid

Rubbers

Adiponitrile

OxalicAcid

CH3X

Various Methyl Esterse.g., Methyl Methacrylate

C(CH2OH)4

Ethylene

VinylAcetate

Vinyl Chloride

Vinyl Acetate

Ethylene Oxide

Butyrolactone

H3CCOOH

EthylAcetate

AceticAnhydride

Ethanol

Carbohydrate

EthylChloride

Cyclohexane Cyclohexanone

Caprolactone

Adipic Acid

Phenol

Hexane-diamine

VinylChloride

Butyrolactam

NMP & NVP

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typical crude oil are shown in Figure 2.7 as presented by de Jong et al. [12]. In this figure, fuel products directly produced in refineries are marked in color, whereas many chemicals may be produced in the follow-up processes in a petrochemical plant. Blending techniques are used to make multiple products according to the desired properties or to improve product quality. The product specifications must satisfy customers requirements for good performance and government regulations for safety and environmental pro-tection. Therefore, to be able to plan refinery operations, the availability of a set of product quality prediction meth-ods is very important [1].

2.4 NaturalGasaNditsProduCtsThe typical composition of natural gas is given in Table 2.2. Usually natural gases contain CO2 and H2S known as acid gases, but the main components are methane, ethane, and propane, although hydrocarbons as heavy as C11 may be present. Natural gases may also contain inert gases such as nitrogen and helium. Pipeline gases containing mainly nitrogen, helium, C1, C2, and C3 in liquefied form are called LNG. The liquefied form of gases C2, C3, and C4 is called LPG. Pentanes and heavier including isobutane can be separated from natural gas as natural gasoline. Natural gas liquids (NGLs) and light and heavy naphthas may also be separated naturally from natural gas. At normal pressure conditions, only C5 and heavier components are in liquid form. Methane needs to be refrigerated to 259F to have it as liquid. For storage of natural gas at normal temperatures (above boiling point), it is necessary to compress it, which is known as compressed natural gas (CNG). Liquid mixtures of C3 and C4 are ideal fuel for many applications. They are stable, high-energy content, relatively low sulfur, and clean burning fuels that can be transported as liquid and used as liquid or gas. LPG can be produced from natural gas and crude oil. LPG is also a preferred feedstock for petrochemi-cals, gas cracking, and plastics. The first commercial use of LPG from crude oil or natural gas was in 1912. Propane used in LPG is not suitable for gasoline (it is very volatile) or for use in natural gas (heavy component in natural gas pipe-line), so its best application is in LPG. The ratio of C3C4 in LPG mainly depends on the temperature because at high temperatures (summer) more C4 and at low temperatures (winter) more C3 is used in the mixture. Tanks containing LPG should never be filled with liquids to allow space for vapors and volume expansion for safety reasons [8].

Natural gas and NGLs are also the main feedstocks for petrochemical plants. Through absorption processes, H2S can be separated from natural gas, and upon oxidation of H2S sulfur can be produced. Through distillation/extraction processes, components such as C2, C3, C4, and heavier com-pounds are separated. Methane as the main component of natural gas can be used through processes such as reform-ing and oxidation to produce a group of chemicals such as CO2, hydrogen, ammonia,, methyl chloride, acetylene, methanol, nitric acid, urea, acrylonitrile, vinyl chloride, ethanol, propanol, butanol, formaldehyde, pharmaceuticals and feeds to pharmaceutical industries, carbon tetrachlo-ride, acetaldehyde, vinyl resins, etc.

The next main components of natural gas are eth-ane and propane. These components can be converted to ethylene and propylene through cracking processes. Ethylene can be used to produce many products such

as polyethylene, ethylene oxide, ethyl chloride, etha-nolamine, ethylene glycol, acetaldehyde, styrene, ethyl benzene, detergents, etc. Propylene is used to produce a group of compounds through processes such as oxidation, hydration, polymerization, and alkylation. These products include cumene, polymers, isopropyl alcohol, allyl chloride, acetone, glycerin, epoxy resins, isobutanol, acetic acid, nitro glycerin, etc.

Butanes in natural gas may be in the form of isobutene or n-butane, which can be separated through a distillation process. These components can be converted to products such as isobutylene, tert-butyl alcohol, butadiene, polybu-tadiene, nylon, methyl ethyl ketone, synthetic resins, lube oil additives, tert-butyl phenol, etc., through dehydrogena-tion, polymerization, and copolymerization processes.

2.5 biofuelsBiofuels represent a group of fuels derived from biomateri-als such as vegetable oil or biomass. A good example of a bio-fuel is biodiesel, which is a cleaner fuel than petrodiesel and can be produced from renewable sources such as vegetable oil, palm oil, cooking oil, or animal fat. These oils undergo a process called transesterification, in which they react with an alcohol such as methanol or ethanol with sodium hydrox-ide or potassium hydroxide as catalyst [1316]. Transesterifi-cation converts fats and oils (triglycerides) into alkylesters of fatty acids that have similar properties to those of petroleum diesel. The process produces large quantities of glycerol as a byproduct. Biodiesel does not contain any sulfur or aro-matics. Therefore, in comparison to petroleum diesel, the combustion of biodiesel results in a reduction in unburned hydrocarbons, carbon monoxide, and particulate matter emissions. Because it has a higher flash point it is safer to store and to handle [1517]. Biodiesel can be used in its pure form (B100) or in blends with petroleum diesel in a wide range of concentrations (e.g., B2, B5, B20) in diesel engines.

Another group of biofuels comprises bioalcohols, which are biologically produced alcohols. The most com-monly used bioalcohols are ethanol, propanol, and butanol. Butanol can be used directly in spark-ignition (gasoline) engines without any alteration. Butanol can produce more energy than ethanol and is less corrosive because it is less soluble in water. However, ethanol is the most commonly used biofuel in the world and in particular in Brazil. Etha-nol can also be mixed with gasoline at any ratio, but use of 15 % bioethanol in gasoline (marked by E15) is common. Mixtures of gasoline and ethanol produce less pollution than gasoline upon combustion, especially in cold winters and high altitudes. However, ethanol has a lower heating value than gasoline [13].

Other types of biofuels include biogas and solid biofu-els. Biogas is produced when organic material isanaerobi-cally digested by anaerobes. Biogas consists of methane, and landfill gas is created in landfills because of natural anaerobic digestion. Charcoal and wood are examples of solid biofuels. The combined processes of gasification, combustion, and pyrolyis can produce syngas, which is a biofuel. This syngas can be directly burned in internal com-bustion engines. Syngas can be used to create hydrogen and methanol. Syngas can be transformed to a synthetic petro-leum substitute using the FischerTropsch process. Finally, a third-generation biofuel is produced from algae, which is called oilage [13].

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refereNCes[1] Riazi, M.R., Characterization and Properties of Petroleum

Fractions, MNL50, ASTM International, West Conshohock-en, PA, 2005.

[2] IUPAC: International Union of Pure and Applied Chemistry (IUPAC), http://www.iupac.org (accessed July 7, 2009).

[3] Mansoori, G.A., A Unified Perspective on the Phase Behavior of Petroleum Fluids, Int. J. Oil, Gas Coal Technol., Vol. 2, 2009, pp. 141167.

[4] Riazi, M.R., Energy, Economy, Environment and Sustainable Development in the Middle East and North Africa, Int. J. Oil, Gas, Coal Technol., Vol. 3, 2010, pp. 301345.

[5] Altagelt, K.H., and Boduszynski, M.M., Composition and Analysis of Heavy Petroleum Fractions, Marcel Dekker, New York, 1994.

[6] Speight, J.G., The Chemistry and Technology of Petroleum, 3rd ed., Marcel Dekker, New York, 1998.

[7] Wauquier, J.-P., Petroleum Refining. Vol. 1 Crude Oil. Petro-leum Products. Process Flowsheets, Editions Technip, Paris, 1995.

[8] Gary, J.H., Handwerk, G.E., and Kaiser, M.J., Petroleum Refin-ing, Technology and Economics, 5th ed., Marcel Dekker, New York, 2007.

[9] Totten, G.E., Westbrook, S.R., and Shah, R.J., Fuels and Lubri-cants Handbook: Technology, Properties and Testing, MNL37, ASTM International, West Conshohocken, PA, 2003.

[10] Parkash, S., Petroleum Fuels Manufacturing Handbook, McGraw-Hill, New York, 2010.

[11] Eser, S., and Andresen, J., Properties of Fuels, Petroleum Pitch, Petroleum Coke, and Carbon Materials, in Fuels and Lubricants Handbook: Technology, Properties, Performance, and Testing, MNL37, G.E. Totten, R.J. Shah, and S.R. West-brook, Eds., ASTM International, West Conshohocken, PA, 2003, pp. 757786.

[12] de Jong, E., van Ree R., van Tuil, R., and Elbersen, W., Biorefineries for the Chemical IndustryA Dutch Point of View, A Joint Research Report from Agrotechnology and Food Innovations, Wageningen, The Netherlands and the Energy Research Centre of the Netherlands (ECN)Biomass Depart-ment, Petten, The Netherlands, http://www.biorefinery.nl/fileadmin/ biorefinery/docs/biorefineries_for_the_chemical_industry_a_dutch_point_of_view.pdf (accessed July 2009).

[13] Biofuels: The Fuel of the Future, http://biofuel.org .uk/types-of-biofuel.html (accessed July 7, 2009).

[14] Srivastava, A., and Prasad, R., Triglycerides-Based Diesel Fuels, Renew. Sustain. Energy Rev., Vol. 4, 2000, pp. 111133.

[15] Natural Resources Canada: Biodiesel, http://oee .nrcan.gc.ca/transportation/alternative-fuels/index.cfm?attr=8 (accessed July 7, 2009).

[16] Meher, L.C., Vidya Sagar, D., and Naik, S.N., Technical Aspects of Biodiesel Production by TransesterificationA Review, Renew. Sustain. Energy Rev., Vol. 10, 2006, pp. 248268.

[17] Morf, O, BIODIESEL: A Guide for Policy Makers and Enthusiasts, National Agricultural and Environmental Forum, Siddharthanagar, Bhairahawa, Nepal, http://www .naef-nepal.org (accessed July 7, 2009).

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Feedstocks and Products of Crude Oil and Natural Gas R Efineries