Introduction:
Petroleum is considered as a complex mixture of hydrocarbons
compounds that are essentially composed of carbon and hydrogen in a wide
variety of molecular arrangements. These hydrocarbons are divided into three
main classes – alkanes, cycloalkanes and aromatics. Their average range in
crude oil is C5H12 to C18H38 along with heteroatom-containing, oxygen, nitrogenand sulfur. In addition to the previous contents, petroleum oil contains trace
amounts of metals such as iron, nickel, copper and vanadium. The physical
properties, particularly specific gravity, of the crude oil play a big role in
the global petroleum trading. It is defined as the ratio of the mass of a given
volume of the oil at 15°C to the mass of an equal volume of pure water at the
same temperature.
It varies from 0.8 (45.3 API) for the lighter crude oils to
over 1.0 (less than 10 API) for heavy crude oil and bitumen . API gravity
which is an arbitrary figure and more popular term, can be defined as the
inverse measure of the relative density of a petroleum liquid and the densityof water with the formula (API=(141.5/Specific Gravity at 60°F)-131.5). It
is clearly seen from the previous relation that the API gravity of the oil
increases with the decrease of the specific gravity and vice versa. The API
gravity term reflects directly the economic value of the oil and it is also an
important factor in shipping and volume calculations.
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