271
Petroleum Analysis
Thus, it is not surprising that gas chromatography has been used extensively for individual component identification, as well as percentage composition, in the gaseous boiling ranges (ASTM
D2163, ASTM D2426, ASTM D2504, ASTM D2505, ASTM D2593, ASTM D2597, ASTM D2712,
ASTM D4424, ASTM D4864, ASTM D5303, ASTM D6159, IP 264, IP 318, IP 337, IP 345), in the
gasoline boiling range (e.g., ASTM D2426, ASTM D2427, ASTM D3525, ASTM D3606, ASTM
D3710, ASTM D4420, ASTM D4815, ASTM D5134, ASTM D5441, ASTM D5443, ASTM D5501,
ASTM D5580, ASTM D5599, ASTM D5623, ASTM D5845, ASTM D5986, IP 425), in higher boiling ranges such as diesel fuel (ASTM D3524), aviation gasoline (ASTM D3606), engine or motor oil
(ASTM D5480), and wax (ASTM D5442), as well as for the boiling range distribution of petroleum
fractions (ASTM D2887, ASTM D5307), light hydrocarbons in stabilized crude oil (IP 344), or the
purity of solvents using capillary gas chromatography (ASTM D2268). There are also recommendation for calibrating and checking gas chromatographic analyzers (IP 353).
The evolution of GLC has been a major factor in the successful identification of petroleum constituents. It is, however, almost impossible to apply this technique to the higher boiling petroleum
constituents because of the comparatively low volatility. It is this comparative lack of volatility in
the higher molecular weight, asphaltic constituents of petroleum that brought about another type of
identification procedure, namely, carbon-type analysis.
The technique has proved to be an exceptional and versatile instrumental tool for analyzing
compounds that are of low molecular weight and that can be volatilized without decomposition.
However, these constraints limit the principal applicability in petroleum science to feedstock identification when the composition is known to be in the low to medium boiling range. The use of this
technique for direct component analysis in the heavy fractions of petroleum is subject to many
limitations (Speight, 2001).
For example, the number of possible components of a certain molecular weight range increases
markedly with increasing molecular weight. Furthermore, there is a corresponding sharp decrease
in physical property differences between isometric structures as the molecular weight increases.
Thus, it is very difficult, and on occasion almost impossible, to separate and identify single components in the heavier fractions of petroleum by gas chromatography. Indeed, the molecular weights
of the constituents dictate that long residence times are necessary. This is inevitably accompanied
by the requirement of increased column temperature, which decreases the residence time on the
column but, at the same time, increases the possibility of thermal decomposition.
The instrumentation for GLC is fairly straightforward and involves passing a carrier gas passes
through a controller to the column (packed with an adsorbent) at the opening of which is a sample
injector. The carrier gas then elutes the components of the mixture through the column to the detector at the end of which may be another gas flow monitor. Any gas, such as helium, argon, nitrogen,
or hydrogen that is easily distinguishable from the components in the mixture may be used as the
carrier gas.
Column dimensions vary, but for analytic purposes a packed column may be 6 ft (2 m) long
by 3 in. (6 mm) in diameter. It is also necessary to use a dissolving liquid as part of the column
substance. This remains stationary on the adsorbent and effects partition of the components of the
mixture. The solid support is usually a porous material that allows passage of the gas. For example,
kieselguhr (diatomaceous earth), which can absorb up to 40% by weight of a liquid without appearing to be overly moist, is commonly used. The supporting material should not adsorb any of the
components of the mixture and must therefore be inert.
Individual components of mixtures are usually identified by their respective retention times,
that is, the time required for the component to traverse through the column under the specified
conditions. Although tables for retention time data are available, it is more common in practice to
determine the retention times of the pure compounds. The retention time of any component is itself
a function of the many variables of column operation, such as the flow rate of the carrier gas and
column temperature, and exact duplication of other operator’s conditions may be difficult, if not
impossible.
Petroleum Analysis
Thus, it is not surprising that gas chromatography has been used extensively for individual component identification, as well as percentage composition, in the gaseous boiling ranges (ASTM
D2163, ASTM D2426, ASTM D2504, ASTM D2505, ASTM D2593, ASTM D2597, ASTM D2712,
ASTM D4424, ASTM D4864, ASTM D5303, ASTM D6159, IP 264, IP 318, IP 337, IP 345), in the
gasoline boiling range (e.g., ASTM D2426, ASTM D2427, ASTM D3525, ASTM D3606, ASTM
D3710, ASTM D4420, ASTM D4815, ASTM D5134, ASTM D5441, ASTM D5443, ASTM D5501,
ASTM D5580, ASTM D5599, ASTM D5623, ASTM D5845, ASTM D5986, IP 425), in higher boiling ranges such as diesel fuel (ASTM D3524), aviation gasoline (ASTM D3606), engine or motor oil
(ASTM D5480), and wax (ASTM D5442), as well as for the boiling range distribution of petroleum
fractions (ASTM D2887, ASTM D5307), light hydrocarbons in stabilized crude oil (IP 344), or the
purity of solvents using capillary gas chromatography (ASTM D2268). There are also recommendation for calibrating and checking gas chromatographic analyzers (IP 353).
The evolution of GLC has been a major factor in the successful identification of petroleum constituents. It is, however, almost impossible to apply this technique to the higher boiling petroleum
constituents because of the comparatively low volatility. It is this comparative lack of volatility in
the higher molecular weight, asphaltic constituents of petroleum that brought about another type of
identification procedure, namely, carbon-type analysis.
The technique has proved to be an exceptional and versatile instrumental tool for analyzing
compounds that are of low molecular weight and that can be volatilized without decomposition.
However, these constraints limit the principal applicability in petroleum science to feedstock identification when the composition is known to be in the low to medium boiling range. The use of this
technique for direct component analysis in the heavy fractions of petroleum is subject to many
limitations (Speight, 2001).
For example, the number of possible components of a certain molecular weight range increases
markedly with increasing molecular weight. Furthermore, there is a corresponding sharp decrease
in physical property differences between isometric structures as the molecular weight increases.
Thus, it is very difficult, and on occasion almost impossible, to separate and identify single components in the heavier fractions of petroleum by gas chromatography. Indeed, the molecular weights
of the constituents dictate that long residence times are necessary. This is inevitably accompanied
by the requirement of increased column temperature, which decreases the residence time on the
column but, at the same time, increases the possibility of thermal decomposition.
The instrumentation for GLC is fairly straightforward and involves passing a carrier gas passes
through a controller to the column (packed with an adsorbent) at the opening of which is a sample
injector. The carrier gas then elutes the components of the mixture through the column to the detector at the end of which may be another gas flow monitor. Any gas, such as helium, argon, nitrogen,
or hydrogen that is easily distinguishable from the components in the mixture may be used as the
carrier gas.
Column dimensions vary, but for analytic purposes a packed column may be 6 ft (2 m) long
by 3 in. (6 mm) in diameter. It is also necessary to use a dissolving liquid as part of the column
substance. This remains stationary on the adsorbent and effects partition of the components of the
mixture. The solid support is usually a porous material that allows passage of the gas. For example,
kieselguhr (diatomaceous earth), which can absorb up to 40% by weight of a liquid without appearing to be overly moist, is commonly used. The supporting material should not adsorb any of the
components of the mixture and must therefore be inert.
Individual components of mixtures are usually identified by their respective retention times,
that is, the time required for the component to traverse through the column under the specified
conditions. Although tables for retention time data are available, it is more common in practice to
determine the retention times of the pure compounds. The retention time of any component is itself
a function of the many variables of column operation, such as the flow rate of the carrier gas and
column temperature, and exact duplication of other operator’s conditions may be difficult, if not
impossible.