Crude Oil:
Crude oil is the liquid form of petroleum. It is a mixture of hydrocarbons that exists in the liquid phase in the underground reservoir as well as when produced at normal surface conditions. It is composed of hydrocarbon molecules with five or more carbon atoms.
Nature:
It varies in colour from straw yellow, green, brown, and dark brown to black. Most oils are lighter than water and vary widely in viscosity.
Composition:
Compositionally, It is a mixture of hundreds of hydrocarbon molecules. Hydrocarbon compounds in crude oil are divided into chemical groups named Paraffins (CnH2n+2), Naphthene (CnH2n), aromatic (CnHn), and asphaltites. The relative abundance of those groups of molecules determines the physical and chemical properties of crude oil.
It has some hetero compounds that contain nitrogen (N), Sulphur (s), and heavy metals. Sulfur compounds range from 0.1% to 0.7% by weight, and they lower the quality of the crude oil.
Classification of crude oil:
Depending on the relative abundance of hydrocarbon molecules:
It is classified into,
- Paraffinic oil: when rich in paraffin compounds
- Napthenic oil: when rich in naphthenic compound
- Aromatic oil: when rich in aromatic compounds
Paraffinic oil is waxy, napthenic oil tends to have high asphalt content that reduces the value of oil, and aromatic oil has a fruity Oduor and has high gasoline content.
Based on density:
It is also classified on the basis of its density. A commonly used density scale is. API gravity. The API gravity scale is inversely proportional to density, ie, lower the gravity higher the density is vice versa. Most crude oil is lighter than water, and average crude oil has a density in the range of 30 to 40 API gravity. It is categorized as,
- Light oil = 731° API
- Medium oil = 22-31° API
- Heavy oil = 10-21° API
- Extra heavy oil = <10° API
The lighter oils are rich in gasoline and are more valuable, whereas heavy oils are rich in asphalt and less valuable.
According to the amount of sulfur compound :
Sweet Oil: Sulfur content is less than 1%. Light crude oils are sweet.
Sour Oil: Sulfur content is higher than 1%. Heavy viscous oils are sour.
