Petroleum
Petroleum
Crude
petroleum (mineral oil) is the term usually applied to the gases occurring
naturally in the oilfields, the liquid from the wells, and the solids which are
dissolved in, or have been separated from, the liquid. The composition of crude
petroleum varies with the locality of occurrence. But all contain alkanes
(straight- and branched-chain from about C1 to C40), cycloalkanes or
naphthenes, and aromatic hydrocarbons. The low-boiling fractions of almost all
petroleums are composed of alkanes; it is the composition of the higher-boiling
fractions which differs according to the source of the petroleum. In addition
to hydrocarbons, there are also present compounds containing oxygen, nitrogen,
sulphur and metallic constituents.
Composition of crude petroleum
If the
residue of petroleum, after removal of volatile compounds, contains a large
amount of alkanes or wax, the petroleum is classified as paraffinic or paraffin
base oil. If naphthenes predominate, the petroleum is classified as asphaltic
or asphalt base oil. The crudes from the well in Pennsylvania, Irak and Rumania
are paraffinic; those from Baku and Venezuela are asphaltic; and those from
Oklahoma, Taxas and Mexico are intermediate in composition, and may be
classified as paraffinic and asphaltic.
Distillation of petroleum
The crude
oil is nearly always associated with water and sand; hence the crude petroleum
discharged from the top of the well contains water and sand in suspension. The
mixture is passed, under pressure, into cylindrical tanks, and the gas, oil and
solids are drawn off separately.
Except for
the low-boiling hydrocarbons, no attempt is made to separate the individual
hydrocarbons. The crude oil is fractionated by continuous distillation into
four main fractions : Petrol (gasoline), Kerosene (Kerosine,
paraffin oil), gas oil (heavy oil) and lubricating oil.
The residue may be fractionated by means of vacuum-distillation to give light,
medium and heavy lubricating oils, paraffin wax, and asphaltic bitumen. Each of
the four main fractions may be further split up by batch distillation into
fractions of narrow boiling range. Recently, it has been possible to isolate
individuals by ‘superfractionation’. The final number of fractions taken
depends on the purpose in view. Below table shows one set of fractions that may
be obtained.
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