WHAT ARE ASPHALTENES ?
The discussion below, on this page, is a general discussion of asphaltenes which focuses on some of the problems they cause. Their properties are also useful, and have long contributed to some of the important end user products deriving from the Lloydminster region. To learn more about this positive side ... click here ... otherwise, read on below.
Asphaltenes constitute a general class of aromatic-type substances which are defined on the basis of their solubility. Asphaltenes are soluble in carbon disulfide but insoluble in light alkanes such as n-pentane and n-heptane. Asphaltene molecules carry a core of stacked, flat sheets of condensed (fused) aromatic rings linked at their edges by chains of alipathic and/or naphthenic-aromatic ring systems. The condensed sheets contain NSO atoms and probably vanadium and nickel complexes. (See Figure 1 below).
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Figure 1 |
Figure 2 |
The present trend in the petroleum industry is an increasing demand for light products. In
order to meet the market demand, refineries convert a portion of their residuals into
light fractions. This conversion results in the production of modern heavy fuels which
contain a greater concentration of sulphur, vanadium, and asphaltenes.
Asphaltenes are considered as part of the ¨bottom of the barrel¨. They constitute the non-volatile, high molecular weight fraction of petroleum. In addition, since they are non-soluble in heptane, they remain in solid form in the crude as well.
The condensed aromatic rings exist in the form of a non-homogeneous flat sheet (See Figure 2 above). In the crude, the asphaltene sheets remain dispersed. However, they have the tendency to be attracted towards each other thus resulting in the formation of an agglomeration. The structure of the agglomeration is similar to that of a book: a compact stack of thin sheets (See Figure 3 below).
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Figure 3 |
Asphaltenes which exist in crudes in a dispersed state, are held in this condition by resins. They have three (3) characteristics that make them problematic to a refinery system :
1- they constitute the largest aromatic fraction in petroleum as well as being the highest molecular weight component,
2- they have no definite melting point and therefore remain in solid form thus contributing to carbon residue.
3- they agglomerate to form a book-like structure.
To be refined, the liquid crude must be converted to a gaseous state. In order to do this, it is first necessary to atomize the feedstock to form a mist of very small droplets. As these droplets move toward the heat source, the temperature rises rapidly resulting in the vaporization of the light fraction of the droplets. The solid asphaltenes, which make up the heavy fraction, agglomerate into compact, book-like structures as vaporization of the volatiles takes place (See figure 4 below).
Figure 4
Unburned carbon residues contribute to significant problems in the refining process. For example, the can supply anywhere from 40% to 80% of the total particular emissions.
