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The Chemistry and Technology of Petroleum
Despite this, there are still reports of asphaltenes being isolated from crude oil by much lower
proportions of the precipitating medium, which can lead to errors not only in the determination
of the amount of asphaltenes in the crude oil but also in the determination of the compound type.
For example, when insufficient proportions of the precipitating medium are used, resins may also
appear within the asphaltene fraction by adsorption onto the asphaltenes from the supernatant liquid
and can be released by reprecipitation (Speight et al., 1984; Ali et al., 1985). These resins would be
isolated at a later stage of the separation procedure by adsorption chromatography if standardized
parameters were employed for the separation of the asphaltenes (Speight et al., 1984; ASTM D3279;
ASTM D4124). Questionable isolation techniques throw serious doubt on any conclusions drawn
from subsequent work on the isolated material (Andersen and Speight, 2001).
In fact, it is now accepted that to ensure stable asphaltene yields, it is necessary to employ the
following parameters:
1. An excess of the liquid hydrocarbon (>30 mL hydrocarbon per gm feedstock).
2. Use of n-pentane or n-heptane as the liquid hydrocarbon; volatility constraints and consistency of the asphaltene fraction favor the use of n-heptane.
3. A prolonged contact time (8–10 h) is preferable (Chapters 9 and 10).
4. A precipitation sequence to remove any constituents that separate with the asphaltene
constituents.
The precipitation sequence involves dissolution of the asphaltenes in benzene or toluene (10 mL/g
asphaltene) followed by the addition of the hydrocarbon (50 mL precipitant per mL toluene or
benzene) to the solution. This sequence should be repeated three times to remove adsorbed lower
molecular weight resin material and to provide consistency of the asphaltene fraction.
Briefly, the precipitation of asphaltenes can be ascribed to changes in crude oil composition
caused by the addition of lower boiling components that the complex equilibrium keeping the
asphaltenes in solution or in a peptized state. In the case of addition of low-boiling liquid hydrocarbons to the feedstock, the hydrocarbon causes a change in the solubility parameter of the oil
medium that, in turn, changes the tolerance of the medium for the complex micelle structure. As
this occurs, the lower molecular weight and less polar constituents of the micelle are extracted
into the liquid, leaving the asphaltene constituents without any surrounding (dispersing) sheath.
Separation then ensues. If the process is thermal (i.e., in excess of the thermal decomposition temperature), a different sequence of events ensues leading to the formation of an insoluble phase and
ultimately to coke (Chapter 18).
The focal point of the asphaltene separation method is the isolation of the asphaltene constituents as a discrete fraction. However, a question that often arises is related to whether or not the
asphaltenes self-associate in the crude oil or whether the asphaltene constituents interact directly
with the other crude oil constituents. In addition, there is also a question related to precipitation of
the asphaltene constituents or extraction of the nonasphaltene constituents from the micelle that is
responsible for the dispersion of the asphaltenes in the oil.
The term resin generally implies material that has been eluted from various solid adsorbents,
whereas the term maltenes (or petrolenes) (Chapter 1) indicates a mixture of the resin constituents and oil constituents obtained in the filtrates from the asphaltene precipitation. Thus, after the
asphaltenes are precipitated, adsorbents are added to the n-pentane solutions of the resins and oils,
by which process the resins are adsorbed and subsequently recovered by use of a more polar solvent
and the oils remain in solution (Chapters 9 and 10).
Resin constituents are soluble in the liquids that precipitate asphaltene constituents and are usually
soluble in most organic liquids, except in the lower alcohols and acetone, but they are precipitated
by liquid propane and liquid butanes. The resin constituents often coprecipitate with the asphaltenes
in controlled propane deasphalting procedures, and the product, called propane asphalt, contains
appreciable amounts of adsorbed resins and has the properties of a low-melting-point asphalt.
The Chemistry and Technology of Petroleum
Despite this, there are still reports of asphaltenes being isolated from crude oil by much lower
proportions of the precipitating medium, which can lead to errors not only in the determination
of the amount of asphaltenes in the crude oil but also in the determination of the compound type.
For example, when insufficient proportions of the precipitating medium are used, resins may also
appear within the asphaltene fraction by adsorption onto the asphaltenes from the supernatant liquid
and can be released by reprecipitation (Speight et al., 1984; Ali et al., 1985). These resins would be
isolated at a later stage of the separation procedure by adsorption chromatography if standardized
parameters were employed for the separation of the asphaltenes (Speight et al., 1984; ASTM D3279;
ASTM D4124). Questionable isolation techniques throw serious doubt on any conclusions drawn
from subsequent work on the isolated material (Andersen and Speight, 2001).
In fact, it is now accepted that to ensure stable asphaltene yields, it is necessary to employ the
following parameters:
1. An excess of the liquid hydrocarbon (>30 mL hydrocarbon per gm feedstock).
2. Use of n-pentane or n-heptane as the liquid hydrocarbon; volatility constraints and consistency of the asphaltene fraction favor the use of n-heptane.
3. A prolonged contact time (8–10 h) is preferable (Chapters 9 and 10).
4. A precipitation sequence to remove any constituents that separate with the asphaltene
constituents.
The precipitation sequence involves dissolution of the asphaltenes in benzene or toluene (10 mL/g
asphaltene) followed by the addition of the hydrocarbon (50 mL precipitant per mL toluene or
benzene) to the solution. This sequence should be repeated three times to remove adsorbed lower
molecular weight resin material and to provide consistency of the asphaltene fraction.
Briefly, the precipitation of asphaltenes can be ascribed to changes in crude oil composition
caused by the addition of lower boiling components that the complex equilibrium keeping the
asphaltenes in solution or in a peptized state. In the case of addition of low-boiling liquid hydrocarbons to the feedstock, the hydrocarbon causes a change in the solubility parameter of the oil
medium that, in turn, changes the tolerance of the medium for the complex micelle structure. As
this occurs, the lower molecular weight and less polar constituents of the micelle are extracted
into the liquid, leaving the asphaltene constituents without any surrounding (dispersing) sheath.
Separation then ensues. If the process is thermal (i.e., in excess of the thermal decomposition temperature), a different sequence of events ensues leading to the formation of an insoluble phase and
ultimately to coke (Chapter 18).
The focal point of the asphaltene separation method is the isolation of the asphaltene constituents as a discrete fraction. However, a question that often arises is related to whether or not the
asphaltenes self-associate in the crude oil or whether the asphaltene constituents interact directly
with the other crude oil constituents. In addition, there is also a question related to precipitation of
the asphaltene constituents or extraction of the nonasphaltene constituents from the micelle that is
responsible for the dispersion of the asphaltenes in the oil.
The term resin generally implies material that has been eluted from various solid adsorbents,
whereas the term maltenes (or petrolenes) (Chapter 1) indicates a mixture of the resin constituents and oil constituents obtained in the filtrates from the asphaltene precipitation. Thus, after the
asphaltenes are precipitated, adsorbents are added to the n-pentane solutions of the resins and oils,
by which process the resins are adsorbed and subsequently recovered by use of a more polar solvent
and the oils remain in solution (Chapters 9 and 10).
Resin constituents are soluble in the liquids that precipitate asphaltene constituents and are usually
soluble in most organic liquids, except in the lower alcohols and acetone, but they are precipitated
by liquid propane and liquid butanes. The resin constituents often coprecipitate with the asphaltenes
in controlled propane deasphalting procedures, and the product, called propane asphalt, contains
appreciable amounts of adsorbed resins and has the properties of a low-melting-point asphalt.