Asphaltenes aromatic sheets

The heptane insoluble (ASTM D-3279) method is commonly used to measure the asphaltene content of the feed. Asphaltenes are clusters of polynuclear aromatic sheets, but no one has a clear understanding of their molecular structure. They are insoluble in C3 to paraffins. The amount of asphaltenes that precipitate varies from one solvent to another, so it is important that the reported asphaltene values be identified with the appropriate solvent. Both normal heptane and... 

Tynan and Yen (1969) have suggested that the association of aromatic sheets in the asphaltene macrostructure may occur through coordination of heterocycles. The aromatic sheets may have defect centers with heteroelements providing coordination centers for metals (Yen, 1974). 

Porphyrin and nonporphyrin metals associated with asphaltenes have not been easy to identify in terms of molecular structure. This is partly due to the fact that the characteristics (i.e., spectra) of all possible model nonporphyrin compounds have not been studied. Nonporphyrin metals are probably small polar molecules that precipitate as asphaltenes (Filby, 1975) or complex at defect sites in large aromatic sheet structures of the type shown in Fig. 10. Porphyrins with increased aromaticity and systems with low aromaticity due to discontinued ring conjugation are both characterized as nonporphyrin species. These compounds do not have the characteristic visible absorption spectra and hence are not readily identified. It is also possible that some of the porphyrin in the residuum may not be extracted and identified due to intermolecular association with the asphaltene-generating molecules. 

Fig. 10. Possible defect site in an aromatic sheet of the asphaltene (Tynan and Yen, 1969).

Vanadyl salen provides a model of mixed heteroatom metal coordination characteristic of Ni and VO in the maltenes and asphaltenes. Approximately 50-90% of the metals in petroleum are not contained in the free porphyrin fraction. Yen (1975, 1978) has postulated that these metals exist in a variety of environments such as highly aromatic bound porphyrins, complexed to tetradentates of mixed N, S, and O ligands, or defect sites in large aromatic sheets. Analytical work by Fish et al. (1984) has indicated the presence of metals complexed to salen-type ligands in petroleum. 

Investigations of the x-ray diffraction patterns of various low temperature (450°-750°C) synthetic carbons (18)t carbon black blended with polyethylene (15), condensed aromatics of known structure where the maximum diameter of the sheets is approximately 14 A (15) as well as mixtures of condensed aromatics and porphyrins (19) indicate that the x-ray diffraction patterns can be reproduced thereby supporting the concept of condensed aromatic sheets (having a tendency to stack) as the structure of asphaltenes. However, it is perhaps this ease with which the x-ray diffraction of the asphaltenes can be reproduced which dictates that caution is necessary in the interpretation of the data. Indeed, any empty polyethylene sample holder will exhibit a similar... 

Galtsev et al. (29) studied asphaltene and resin association in real crude oil by using electron nuclear double-resonance spectroscopy. They found that most of the asphaltene moleeules are assoeiated with each other from room temperature up to 90°C, and have a core of a stack of condensed aromatic sheets with a radius of 1 nm. 

Fig. 6. Model of asphaltene structure (wavy lines represent the zigzag configuration of a saturated carbon chain or a loose net of naphthenic rings and straight lines represent the edge of flat sheets of condensed aromatic rings) (Yen et at., 1961).

Hg and Zn have similar fractional patterns. Antimony is strongly concentrated in the asphaltene,s and As shows the lowest enrichment of the elements studied. The asphaltenes and resins exist in the oil in colloidal form, and the crude oil system may be regarded as a transition from the polar aromatic micelle of the asphaltenes to the less polar resins to the nonpolar hydrocarbons of the bulk crude oil. The trace elements concentrated in the asphaltenes may be present in small highly polar molecules, which would precipitate with the asphaltenes or might complex in the asphaltene sheet structure at sites bounded by hetero atoms such as 0, N, or S. Gel permeation chromatography was used to investigate this. 

Structurally, asphaltene contains flat sheets of condensed aromatic systems that may be interconnected by sulfide, ether, aliphatic chains or naphthenic ring linkages. Gaps and holes appear as defect centers in the aromatic systems with heterocyclic atoms coordinated to transition metals such as vanadium and nickel, most likely caused by free radicals. Due to the complexity and the large size of asphaltene molecules, asphaltene particles conveniently faU within the colloidal range. The stmcture of asphaltene has been determined previously by the x-ray diffraction method and is shown as Figure 2. 

---