Petroleum, meaning literally “rock oil,” is the term used to describe a myriad of hydrocarbon-rich fluids that have accumulated in subterranean reservoirs. (also called crude oil) varies dramatically in color, odor, and flow properties that reflect the diversity of its origin.
Petroleum products are any petroleum-based products that can be obtained by refining and comprise refinery gas, ethane, liquefied petroleum gas LPG, naphtha, gasoline, aviation fuel, marine fuel, kerosene, diesel fuel, distillate fuel oil, residual fuel oil, gas oil, lubricants, white oil, grease, wax, asphalt, as well as coke.
Crude oils are complex mixtures of these hydrocarbons. Oils containing primarily paraffin hydrocarbons are called paraffin-based or paraffinic. Traditional examples are Pennsylvania grade crude oils. Naphthenic-based crudes contain a large percentage of cycloparaffins in the heavy components. Examples of this type of crude come from the United States midcontinent region. Highly aromatic crudes are less common but are still found around the world.
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Crude Oil Composition Crude oils tend to be a mixture of paraffins, naphthenes, aromatics, with paraffins and naphthenes the predominant species. Resins and asphaltenes may also be present in crude oil. Resins and asphaltenes are the colored and black components found in oil and are made up of relatively high-molecular weight, polar, polycyclic, aromatic ring compounds. Pure asphaltenes are nonvolatile, dry, solid, black powders, while resins are heavy liquids or sticky solids with the same volatility as similarly sized hydrocarbons. High-molecular-weight resins tend to be red in color, while lighter resins are less colored. Asphaltenes do not dissolve in crude oil but exist as a colloidal suspension. They are soluble in aromatic compounds such as xylene, but will precipitate in the presence of light paraffinic compounds such as pentane. Resins, on the other hand, are readily soluble in oil.
Petroleum products are highly complex chemicals, and considerable effort is required to characterize their chemical and physical properties with a high degree of precision and accuracy. Indeed, the analysis of petroleum products is necessary to determine the properties that can assist in resolving a process problem as well as the properties that indicate the function and performance of the product in service.
Crude petroleum and the products obtained there from contain a variety of compounds, usually but not always hydrocarbons. As the number of carbon atoms in, for example, the paraffin series increases, the complexity of petroleum mixtures also rapidly increases. Consequently, detailed analysis of the individual constituents of the higher boiling fractions becomes increasingly difficult, if not impossible.
Additionally, classes (or types) of hydrocarbons were, and still are, determined based on the capability to isolate them by separation techniques. The four fractional types into which petroleum is subdivided are paraffins, olefins, naphthenes, and aromatics (PONA). Paraffinic hydrocarbons include both normal and branched alkanes, whereas olefins refer to normal and branched alkenes that contain one or more double or triple carbon-carbon bonds. Naphthene (not to be confused with naphthalene) is a term specific to the petroleum industry that refers to the saturated cyclic hydrocarbons (cycloalkanes). Finally, the term aromatics includes all hydrocarbons containing one or more rings of the benzenoid structure.
Although not directly derived from composition, the terms light and heavy or sweet and sour provide convenient terms for use in descriptions. For example, light petroleum (often referred to as conventional petroleum) is usually rich in low-boiling constituents and waxy molecules whereas heavy petroleum contains greater proportions of higher-boiling, more aromatic, and heteroatom-containing (N-, O-, S-, and metal containing) constituents. Heavy oil is more viscous than conventional petroleum and
requires enhanced methods for recovery. Bitumen is near solid or solid and cannot be recovered by enhanced oil recovery methods.
conventional (light) petroleum is composed of hydrocarbons together with smaller amounts of organic compounds of nitrogen, oxygen, and sulfur and still smaller amounts of compounds containing metallic constituents, particularly vanadium, nickel, iron, and copper. The processes by which petroleum was formed dictate that petroleum composition will vary and be site specific, thus leading to a wide variety of compositional differences.
The term site specific is intended to convey that petroleum composition will be dependent on regional and local variations in the proportion of the various precursors that went into the formation of the protopetroleum as well as variations in temperature and pressure to which the precursors were subjected.
Thus the purely hydrocarbon content may be higher than 90% by weight for paraffinic petroleum and 50% by weight for heavy crude oil and much lower for tar sand bitumen. The nonhydrocarbon constituents are usually concentrated in the higher-boiling portions of the crude oil. The carbon and hydrogen content is approximately constant from crude oil to crude oil even though the amounts of the various hydrocarbon types and of the individual isomers may vary widely. Thus the carbon content of various types of petroleum is usually between 83% and 87% by weight and the hydrogen content is in the range of 11–14% by weight.
General aspects of petroleum quality (as a refinery feedstock) are assessed by measurement of physical properties such as relative density (specific gravity – which affects on Oil Price), refractive index, or viscosity, or by empirical tests such as pour point or oxidation stability that are intended to relate to behavior in service. In some cases the evaluation may include tests in mechanical rigs and engines either in the laboratory or under actual operating conditions.
Measurements of bulk properties are generally easy to perform and, therefore, quick and economical. Several properties may correlate well with certain compositional characteristics and are widely used as a quick and inexpensive means to determine those characteristics. The most important properties of a whole crude oil are its boiling-point distribution, its density (or API gravity), and its viscosity. The boiling-point distribution, boiling profile, or distillation assay gives the yield of the various distillation cuts, and selected properties of the fractions are usually determined.
It is a prime property in its own right that indicates how much gasoline and other transportation fuels can be made from petroleum without conversion.
Density and viscosity are measured for secondary reasons.
The former helps to estimate the paraffinic character of the oil, and the latter permits the assessment of its undesirable residual material that causes resistance to
flow. Boiling-point distribution, density, and viscosity are easily measured and give a quick first evaluation of petroleum oil. Sulfur content, another
crucial and primary property of a crude oil, is also readily determined. Certain composite characterization values, calculated from density and mid-boiling point, correlate better with molecular composition than density alone.
The acceptance of heavy oil and bitumen as refinery feedstocks has meant that the analytical techniques used for the lighter feedstocks have had to evolve to produce meaningful data that can be employed to assist in defining refinery scenarios for processing the feedstocks. In addition, selection of the most appropriate analytical procedures will aid in the predictability of feedstock behavior during refining. This same rationale can also be applied to feedstock behavior during recovery operations. Indeed, bitumen, a source of synthetic crude oil, is so different from petroleum that many of the test methods designed for petroleum may need modification.
References:
1. Petroleum & Gas Field Processing, H K. Abdel-Alal and Mohamed Aggour, King Fahd University of Petroleum & Minerals
2. Petroleum Engineering Handbook, L.W.Lake, Vol.1 “General Engineering”
