Published: Jan 2008
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CRUDE OIL IS A HIGHLY COMPLEX MIXTURE OF MOlecular species made up of hydrogen and carbon (commonly referred to as hydrocarbons), heterocyclic molecular species (hydrocarbons with cyclic structures containing other chemical species such as oxygen, nitrogen, or sulfur or a combination thereof), organometallic compounds (hydrocarbons containing metal chemical species), inorganic sediments, and water. Crude oil can vary greatly in composition, and can exhibit highly variable physical and chemical characteristics. This is due to the fact that crude oils are not pure materials, but usually contain colloidally suspended components, dispersed solids, and emulsified water. No two crude oils will be chemically or physically identical. The composition of crude oil can vary from highly flammable light ends to highly viscous and heavy tar-like materials. The chemical composition of crude oil varies from field to field, between regions, and even within the same geologic formation. The physical and chemical analyses of crude oil to determine its quality and processability is known as a crude oil assay. The results of a crude oil assay testing can provide extensive detailed data useful for refiners, oil traders, and producers. Assay data can help refineries determine if a crude oil is compatible for a particular petroleum refiner, or if the crude oil can cause yield, quality, production, environmental, and other problems. To determine the desired information, two different analytical schemes are commonly used, namely an inspection assay and a comprehensive assay. Inspection assays generally involve a few limited properties such as API gravity or density, sulfur content, sediment, and water. Occasionally pour point is determined to determine the fluidity of the crude oil. There is very limited agreement on what constitutes an inspection assay. Different oil companies generally do additional tests that will provide information that can be used in the processing of a given crude oil. Comprehensive assays involve more tests than what are performed on an inspection assay. The overriding issue in doing a comprehensive assay on a given crude oil is economics. Comprehensive assays of crude oil are done to determine: (a) the slate of products that can be produced with a given refinery's process technology, (b) processing difficulties that may arise as a result of the inherent composition and impurities of the crude oil, and (c) downstream processing and upgrading activities to optimize yields of high-value products. Since this manual is concerned mainly with distillation and vapor pressure measurements of crude oil, the focus of this chapter is on the distillation and vapor pressure data. There are a number of other tests which are performed during an inspection assay or a comprehensive assays. These are discussed in detail elsewhere .
Montemayor, Rey G.
Chief Chemist, Imperial Oil Ltd., Sarnia, Ontario