6
Petroleum
6.1 Physical tests
The performance of a petroleum product, whether it is a fuel or a lubricating oil, will depend as much
on its physical properties as its chemical composition.
Aniline Point
The aniline point of an oil is the lowest temperature at which the oil is completely miscible with an
equal volume of aniline (benzeneamine). The simplest and in most cases most appropriate method for
determining aniline point involves heating equal volumes of sample and aniline in a test tube until
miscibility occurs. Other methods are available for more volatile components.
The significance of this test is that provides a simple evaluation of the level of aromatic
compounds in the sample. The higher the aromatic level, the more miscible it will be with aniline, this
lowering the aniline point.
Calorific Value
The method of testing and the importance is the same as already described for coal.
Distillation Range
The fractions available from crude oil have been discussed in a previous subject. A laboratory
“simulation” of the fractionating column at the refinery gives a guide to the proportions of the various
boiling point ranges equivalent to each fraction.
Flash point
This may be defined as the lowest temperature at which enough vapour is given off a flammable
substance to allow ignition. Two tests are used to define flash points.
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for petroleum products having a flash point below 120°C, the Abel apparatus is used; this consists
of a temperature controlled oil bath which surrounds a flash cup (where the sample is contained).
The temperature of the oil is slowly raised in one degree increments and a flame exposed to the
sample after each increment.
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for petroleum products having a flash point above 120°C, the Pensky-Martens apparatus is used.
In this case heat is applied by a small oven which surrounds the sample container; the sample is
heated at a rate of 9-11°C per minute with the flame being exposed to the sample at two degree
intervals
The flash point is defined as the temperature at which flashing first occurs.
Viscosity
The most important property of lubricating oils is their viscosity. This is measured by the capillary
method, where a special and standard piece of glassware is employed. It has a fine capillary (a few
mm in diameter) through which the oil travels. The time taken for the oil to move between two
markers in the capillary is a measure of its viscosity. This can be related to the actual viscosity using a
complex equation, or the time can be used as the quality control measure.
Temperature control is absolutely critical, and all viscosity measurements of this type are carried
out in a thermostatically controlled water bath. The cleanliness of the capillary is also vital.
Octane rating
This is a measure of the performance of particular petrochemical products as motor fuels. As was
discussed in a previous subject, when straight chain hydrocarbons are used as fuel for internal
combustion engines they tend to burn too rapidly and cause the engine to knock. This problem may
6. Petroleum
be overcome by using mixtures of branched and straight chain hydrocarbons as fuels (as branched
hydrocarbons burn much more slowly than their straight chain analogues).
The octane rating of a particular petrochemical is most easily obtained by the engine test
method. This involves burning the sample in a standard engine under strict operating conditions and
measuring the "knock intensity" using a bouncing pin indicator and a knockmeter. The knock intensity
is then compared to the knocking characteristics of blends of reference fuels made from the straight
chain heptane and the branched 2,2,4-trimethylpentane (known as isooctane). The % of isooctane in
the standard is equal to the octane number for the sample. If the sample performs better than 100%
isooctane, tetraethyllead (TEL) is added to the isooctane standard until the two fuels are equal. Each
point above 100 is equal to a certain concentration of TEL. The octane rating is the average of the
octane numbers from two different standard engines.
6.2 Chemical tests
Petroleum products being organic compounds lend themselves to analysis by many standard organic
methods such as gas chromatography (which allows separation and identification of the many
components in petrochemical mixtures), infrared spectroscopy (which may be used for identifying the
aromatic components), and UV-visible spectroscopy (which may be used to identify alkene
components). Other methods such as NMR, mass spectrometry and X-ray analysis are also routinely
used.
One important application of gas chromatography involves a deliberately incomplete
separation, where a standard column and temperature program is used, and the product analysed by
the proportion of the eluting compounds that fall within certain retention time ranges. This is a
more sophisticated version of the distillation range test.
As petrochemicals also contain small amounts of many inorganic elements from the processing
or the original crude oil, it is common practice to analyse for elements such as sulfur and lead. Other
elements will exist at much lower levels and would only be checked for occasionally. These analysis
are generally carried out using flame or electrothermal AAS or ICP emission spectroscopy.
Sulfur
Sulfur in any fossil fuel is a problem because of the formation of the air pollutant, sulfur dioxide. Levels
are now required to be less than 200 ppm. In organic form, sulfur is not an easy element to analyse.
Under special conditions, it can determined by X-ray fluorescence, using standards prepared from Sfree mineral oil and an organic sulfur compound. An alternative method is to oxidise the sulfur to
sulfate (chemically or by combustion), and use the ICP emission spectrophotometer.
Lead
Lead has been added to petrol in the form of tetraethyl lead for many years to improve the octane
rating. It is now being phased out because of pollution concerns. It remains an element that must be
measured, whether as an additive or as a contaminant.
There are two possible approaches, both using either AAS or ICP emission. One leaves the lead
in organic form, and uses direct aspiration of the organic solution, using hydrocarbon-based standards.
Alternatively, the lead can be removed from the organic matrix by hydrochloric or nitric acid, and
analysed in aqueous solution by the same instruments.
What You Need To Be Able To Do
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outline the method of testing, and the significance of the property, for a range of physical
properties of petroleum products
describe how the chemical composition of petroleum products can be determined
outline how samples of different petroleum products can be obtained
Industrial Products (Testing)
6.2