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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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