Crude oil specification

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1.1 CRUDE OIL SPECIFICATION


1.1.1 Basics of Crude Oil

Crude oils are complex mixtures containing many different hydrocarbon compounds that vary in appearance and composition from one oil field to another. Crude oils range in consistency from water to tar-like solids, and in color from clear to black. An "average" crude oil contains about 84% carbon, 14% hydrogen, l% – 3% sulfur, and less than 1% each of nitrogen, oxygen, metals, and salts. Crude oils are generally classified as paraffmic, naphthenic, or aromatic, based on the predominant proportion of similar hydrocarbon molecules. Mixed-base crudes have varying amounts of each type of hydrocarbon. Refinery crude base stocks usually consist of mixtures of two or more different crude oils.Relatively simple crude oil assays are used to classify crude oils as paraffmic, naphthenic, aromatic, or mixed. One assay method (United States Bureau of Mines) is based on distillation, and another method (UOP "K" factor/ is based on gravity and boiling points. More comprehensive crude assays determine the value of the crude (i.e., its yield and quality of useful products) and processing parameters. Crude oils are usually grouped according to yield structure.Crude oils are also defined in terms of API (American Petroleum Institute) gravity. The higher the API gravity, the lighter the crude. For example, light crude oils have high API gravities and low specific gravities. Crude oils with low carbon, high hydrogen, and high API gravity are usually rich in paraffins and tend to yield greater proportions of gasoline and light petroleum products; those with high carbon, low hydrogen, and low API gravities are usually rich in aromatics.Crude oils that contain appreciable quantities of hydrogen sulfide or other reactive sulfur compounds are called "sour." Those with less sulfur are called "sweet." Some exceptions to this rule are West Texas crudes, which are always considered "sour" regardless of their H2S content, and Arabian high-sulfur crudes, which are not considered "sour" because their sulfur compounds are not highly reactive.
1.1.2 Basic Hydrocarbon Nomenclature
Petroleum is composed of two elements, hydrogen and carbon, Joined together in compound called hydrocarbons. Two simple ways of looking at these hydrocarbons is by ratio and by weight.The average ratio of hydrogen to carbon in petroleum hydrocarbons is 2 to 1. This means that although specific compounds may vary, crude have about two atoms of hydrogen for every atom of carbon.A single carbon atom weights 12 times as much as a hydrogen atom. Thus, despite the 2 to 1 ratio of hydrogen to carbon in 100 pounds (45 Kilograms) of crude oil, roughly 84 pounds (38 kilograms) will be carbon and 14 (6 Kilograms) will be hydrogen. The remaining 2 pounds (1 kilogram) are various impurities and must be greatly reduced because they are harmful to the environment and corrosive to both refining equipment and the machinery in which products must ultimately be used. These impurities include sulfur (0 – 3 lb.), Nitrogen (0 – 1 lb.), oxygen (0 – 0.5 lb.), and Chlorine. Nickel, vanadium, iron, copper, and other metals in traces so small they are measured in parts per million or parts per billion.

1.1.2.1 Classification of Hydrocarbons
There are so many different hydrocarbon compounds in crude oil that scientists can only guess at the exact number. Estimates range from 20,000 to 5,000,000 somewhere between 50,000 and 1,000,000 is a reasonable guess, with so many compounds, it’s necessary to have systematic ways to classify them into manageable groupings. The two basic systems used are by carbon number and by molecular structure.

Carbon Numbers

The simplest classification is by carbon number. This is based on the number of carbon atoms found in a given hydrocarbon molecule. For example, methane (CH₄) has one carbon atom per molecule and is C₁. Ethane (C₂H₆) and Ethylene (C₂H₄), through different compounds with different properties, are both classified C₂’s.The carbon number is important because it indicates the physical state of the compound. Basically, the higher the carbon number (i.e., the more carbon atoms per molecule), the higher the boiling point, the greater the viscosity (the rate at which it will flow through a small opening) and the higher the density (weight per volume).Compounds from C₁ to C₄ are gases at room temperature. Those from C₅ to C₁₇ are liquids, through some C₁₇’s may be solid. And those from C₁₇ to C₄₀ are solids. These solids are not like steel or concrete, but more like wax. They can be penetrated with a sharp instrument, but they won’t pour or flow unless heated.

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Molecular Structure
A second, more complicated way to classify hydrocarbon compounds is by their molecular structure, the actual arrangement of the carbon and hydrogen atoms.

These atoms can combine in a number of ways to satisfy valence requirements. For convenience, these are separated into "families" or homologous series, each of which is given a name.

Every atom has the capacity to combine with a given number of other atoms; this is its valence number. The valence of hydrogen, for instance, is 1. One way of looking at that is to say it has a “hand” that can join with a “hand” from another atom to from a valance bond. Carbon has valence of 4 which means that each carbon atom can combine with four other atoms.
The carbon atoms can link together to form "chains" or "rings." Crude oil and natural gas mixtures consist primarily of "straight chain" hydrocarbon molecules, the bulk of which are paraffins.1.1.2.2 Paraffin Series Formula: C_nH_{2n + 2}Paraffins, also called alkanes, were named when early chemists through these compounds were relatively inactive (see Appendix A.2-1).Hydrocarbons in this series are saturated compounds - all four carbon bonds are connected either to another carbon atom or a hydrogen atom, with one such atom for each bond.
Notice that, all names end in -ane, the ending used for the paraffin series. In each case, the number of hydrogen atoms is two times the number of carbon atoms plus two more for the ends of the chain.The paraffin hydrocarbons are the most stable of the lot because all valence bonds are fully satisfied as indicated by the single line linkage. Most reactions involve the replacement of hydrogen atoms with other atoms; the carbon linkage remains stable.Each successive molecule in the paraffin series is created by adding a carbon and two hydrogens to the previous molecule. The incremental change in relative molecular weight is thus fourteen. Long chains con-taining scores of carbon atoms in series may be formed. However, the only ones normally identified by name contain ten or less carbons.

Name	Formula	Mol. Wt.	Name	Formula	Mol. Wt.
Methane	CH4	16	Hexane	C6H14	86
Ethane	C2H6	30	Heptane	C7H16	100
Propane	C3H8	44	Octane	C8H18	114
Butane	C4H10	58	Nonane	C9H20	128
Pentane	C5H12	72	Decane	C10H22	142

Table 1.1 Common Hydrocarbons M.W

In referring to a given paraffin hydrocarbon, the abbreviation C₃ for propane, C₄ for butane, etc. may be used. Statements like "propanes plus fraction (C₃⁺)" refer to a mixture composed of propane and larger atoms.Paraffin isomers: When the paraffin series molecule contains four or more carbon atoms there are different ways these can be connected without affecting the formula. Compounds which have the same chemical formula but a different atomic structure are called isomers. They possess different physical and chemical properties.There are only two isomers of butane. In the structural diagram shown below for i-butane we could draw the carbon atom below instead of above the carbon chain. But, this would be just a "mirror image" of the molecule as drawn. It is the same molecule with the same properties. The adjective "normal" is used to designate a molecule wherein all of the carbon atoms are in a straight line. An "isomer" has the same formula but a different arrangement of the carbon atoms. In an analysis, these are often abbreviated as "n" and "i" respectively.(n- C₄H₁₀) (i- C₄H₁₀)Even though these two compounds consist of the same number of carbon and hydrogen atoms they differ chemically and have different boiling points, densities, and refractive indices. Most important from our point of view is that the simple, straight chain paraffin has a much lower octane than the more compact, branched isoparaffins.As the carbon number increases, the number of possible permutations (isoparaffins) increases astronomically. There are three combinations for pentanes (C₅H₁₂), (normal pentane) and two isopentane (called, by convention, isopentane and neopentane).
Similarly, there are 9 possible combinations for the C₇ paraffin, heptanes, 355 for C₁₂ and 62, 491, 178, 805, 132 combinations for C₄₀.

An important thing to remember is that no matter how complicated or how simple, all Paraffins have the same ratio of two hydrogen atoms for every carbon atom, plus two more hydrogen, one at each end of the chain to fill the remaining valences.

Chemists express this as C_nH_2n+2, the formula for all Paraffins shown at the beginning of this section. If we have a two carbon atom paraffin, then n=2, thus there will be 2 x 2 + 2 = 6 atoms of hydrogen. C₂H₆ is, of course, ethane. You might try going back and checking one or two of the other Paraffins diagrammed above against the formula.

A final point to remember is that the valence bonds of Paraffins are saturated with hydrogen. That is, every carbon atom is holding as much hydrogen as it can; every valence not needed to link it to another carbon atom is linked to a hydrogen atom.

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Molecular Structure

A second, more complicated way to classify hydrocarbon compounds is by their molecular structure, the actual arrangement of the carbon and hydrogen atoms.

These atoms can combine in a number of ways to satisfy valence requirements. For convenience, these are separated into "families" or homologous series, each of which is given a name.

Every atom has the capacity to combine with a given number of other atoms; this is its valence number. The valence of hydrogen, for instance, is 1. One way of looking at that is to say it has a “hand” that can join with a “hand” from another atom to from a valance bond. Carbon has valence of 4 which means that each carbon atom can combine with four other atoms.
The carbon atoms can link together to form "chains" or "rings." Crude oil and natural gas mixtures consist primarily of "straight chain" hydrocarbon molecules, the bulk of which are paraffins.1.1.2.2 Paraffin Series Formula: C_nH_{2n + 2}Paraffins, also called alkaes, were named when early chemists through these compounds were relatively inactive .Hydrocarbons in this series are saturated compounds - all four carbon bonds are connected either to another carbon atom or a hydrogen atom, with one such atom for each bond.
Notice that, all names end in -ane, the ending used for the paraffin series. In each case, the number of hydrogen atoms is two times the number of carbon atoms plus two more for the ends of the chain.The paraffin hydrocarbons are the most stable of the lot because all valence bonds are fully satisfied as indicated by the single line linkage. Most reactions involve the replacement of hydrogen atoms with other atoms; the carbon linkage remains stable.Each successive molecule in the paraffin series is created by adding a carbon and two hydrogens to the previous molecule. The incremental change in relative molecular weight is thus fourteen. Long chains con-taining scores of carbon atoms in series may be formed. However, the only ones normally identified by name contain ten or less carbons.

Name	Formula	Mol. Wt.	Name	Formula	Mol. Wt.
Methane	CH4	16	Hexane	C6H14	86
Ethane	C2H6	30	Heptane	C7H16	100
Propane	C3H8	44	Octane	C8H18	114
Butane	C4H10	58	Nonane	C9H20	128
Pentane	C5H12	72	Decane	C10H22	142

Table 1.1 Common Hydrocarbons M.W

In referring to a given paraffin hydrocarbon, the abbreviation C₃ for propane, C₄ for butane, etc. may be used. Statements like "propanes plus fraction (C₃⁺)" refer to a mixture composed of propane and larger atoms.
Paraffin isomers: When the paraffin series molecule contains four or more carbon atoms there are different ways these can be connected without affecting the formula. Compounds which have the same chemical formula but a different atomic structure are called isomers. They possess different physical and chemical properties.There are only two isomers of butane. In the structural diagram shown below for i-butane we could draw the carbon atom below instead of above the carbon chain. But, this would be just a "mirror image" of the molecule as drawn. It is the same molecule with the same properties. The adjective "normal" is used to designate a molecule wherein all of the carbon atoms are in a straight line. An "isomer" has the same formula but a different arrangement of the carbon atoms. In an analysis, these are often abbreviated as "n" and "i" respectively.Even though these two compounds consist of the same number of carbon and hydrogen atoms they differ chemically and have different boiling points, densities, and refractive indices. Most important from our point of view is that the simple, straight chain paraffin has a much lower octane than the more compact, branched isoparaffins.As the carbon number increases, the number of possible permutations (isoparaffins) increases astronomically. There are three combinations for pentanes (C₅H₁₂), (normal pentane) and two isopentane (called, by convention, isopentane and neopentane).
Similarly, there are 9 possible combinations for the C₇ paraffin, heptanes, 355 for C₁₂ and 62, 491, 178, 805, 132 combinations for C₄₀.

An important thing to remember is that no matter how complicated or how simple, all Paraffins have the same ratio of two hydrogen atoms for every carbon atom, plus two more hydrogen, one at each end of the chain to fill the remaining valences.

Chemists express this as C_nH_2n+2, the formula for all Paraffins shown at the beginning of this section. If we have a two carbon atom paraffin, then n=2, thus there will be 2 x 2 + 2 = 6 atoms of hydrogen. C₂H₆ is, of course, ethane. You might try going back and checking one or two of the other Paraffins diagrammed above against the formula.

A final point to remember is that the valence bonds of Paraffins are saturated with hydrogen. That is, every carbon atom is holding as much hydrogen as it can; every valence not needed to link it to another carbon atom is linked to a hydrogen atom.

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.1.2.3 Olefin or Ethylene Series (Alkenes) Formula: C_nH_2n
Olefins are the second type of hydrocarbon important to us. Olefins are not found naturally in crude oil, but are the product of the refining process.

The olefin group of compounds is a simple straight chain series in which all the names end in -ene. Ethylene (ethene) C₂H₄ is the simplest molecule in the series.Hydrocarbons in this series combine easily with other atoms like chlorine and bromine, without the replacement of a hydrogen atom. Since they are so reactive, they are called unsaturated hydrocarbons.Unlike the paraffins, the maximum bonding capacity of the carbon atom is not fully satisfied by hydrogen or carbon atoms. Two adjacent carbon atoms form a "temporary" bond (in the absence of other available atoms) to meet bonding requirements fixed by valence. It is a necessary but unstable alliance. The structural formula for the olefins uses a double line to indicate the double carbon-carbon linkage, the most reactive point in the molecule.

What distinguishes olefins from Paraffins is that olefins are unsaturated. C₂ paraffin, ethane, was saturated because it contained six hydrogen atoms. If we remove two of the hydrogen’s and bend the two vacated carbon valence bonds around to join with each other, we create a double bond or unsaturated bond. The resulting compound is ethylene, the C₂ olefin.

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1.1.2.4 Acetylene or Alkyne Series Formula: C_nH_2n-2
This series is of basic importance only in certain refining and petrochemical applications. Acetylene is the most important member of this series. It has the formula C₂H₂. The structural formula for acetylene is:

H - C--- C - H

There is a triple bond between the carbon atoms. This satisfies the valence requirements but the carbon linkage is very weak.Acetylene is even more reactive than the olefins. Carbon likes the sharing of three valence linkages even less than sharing two. Acetylene not only is unsaturated, it is almost unstable chemically. In the liquid state it is explosive if subjected to a sudden shock.

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1.1.2.5 Diolefins Formula: C_nH_2n-2
The diolefines have the same formula as acetylene. The members of this series contain two double linkages. They normally are named by replacing the -ane for paraffins by -diene.
Diolefins are primarily of concern in petrochemical plants. Butadiene is possibly the most interesting and useful since it is a primary ingredient in synthetic rubber compounds. It has the formula:

CH₂ = CH – CH = CH₂

All of these unsaturated compounds are reactive. They may be hydrogenated. Liquid cooking oils (unsaturated) may be hydrogenated to form solid fats.These compounds also polymerize - the process wherein a very large molecule is built up from the self-addition reaction of small identical molecules (monomers). Ethylene and propylene polymerize to form polyethylene and polypropylene, the basic ingredients in plastic materials. Acetylene polymerizes to form benzene, a cyclic hydrocarbon.