The amount of petroleum can be measured by weight or by volume. It is measured by weight when the petroleum is transported by tankers and the unit used is the metric ton. It is measured by volume when it is transported by pipeline or railway and the unit used is the barrel.
In the beginning of petroleum exploration in Pennsylvania it was transported in 46-50 gallons wine barrels. On receiving the barrel were paid to 42 gallons per barrel to compensate for losses by spillage and leakage in transportation and so is until today. To avoid financial loss the sellers also compensated placing 42 gallons per barrel. So,the volume of the petroleum barrel is now 42 gallons.
The density is a parameter which allows the conversion of volume to weight and vice-versa. The petroleum exploration and refining have peculiarities that even this very simple conversion may become a complicate issue. Anyway, there are two densities: absolute density and relative density.
The absolute density is the ratio between the mass and the volume of a material. The SI mass unit is the kilogram and the volume unit is the cubic meter. So, the SI absolute density unit is the kilogram per cubic meter (kg/m3). The reasons why the SI unit for mass is the kilogram instead of gram is a good meditation theme.
The water absolute density reaches a maximum at 3.97ºC. At this temperature its absolute density is 1000kg/m3 decreasing as temperature varies in increasing or decreasing. The petroleum absolute density is lower than water. The absolute density of degassed petroleum varies from 700 kg/m3 to near 1000 kg/m3. Petroleum with absolute density higher than water is rare, but some heavier petroleum fraction may be denser than water.
The relative density is the ratio between the absolute density of a given material end the absolute density of the reference material both measured at the same temperature and pressure. The universal reference material for liquids is the water and the reference material for gases is or the air or the hydrogen. The petroleum industries prefer to measure the relative density at 60ºF which corresponds to 16ºC. This density is represented by D60/60. Finally, the relative density is dimensionless parameter and its numerical value independs of the unit system used.
The relative density of petroleum varies from 0.8 to 0.9. However, the relative density of the lightest petroleum may reach 0.7 and of the heaviest one 1.0.
In the refinery the petroleum and its fractions densities is given in APIº. The API density is calculated from D60/60 using the relationship
To get the relative density 60/60 of the petroleum from API density the following expression may be used
The API density decreases as the relative density increases. The higher de API density value the lighter is the petroleum and vice-versa. The API density of water is 10º and the API density of the petroleum usually varies from 26º up to 45ª. However, the lightest petroleum may reach 70ºAPI and the heaviest one approach 10ºAPI but this is a Guinness Book issue. .
At beginning the petroleum densities were given in the Baumé scale as usual for liquids lighter than water. Much later it was discovered that the measurements were incorrect because the instrument used was calibrated in 141.5 instead of 140 as required. The error was so ingrained that the API decided to enact the error by creating the API density in 1921.
The API density allows a density based classification of petroleum.
Light: acima de 30ºAPI
Intermediate: entre 20ºAPI e 30ºAPI
Heavy: abaixo de 20ºAPI
The commonest is petroleum with density between 25ºAPI and 35ºAPI.
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