Asphaltene molecules

The crude oil produced from the Main Zone of the Torrance Field has an API gravity of 18° and contains 5.3 weight percent asphaltenes. The solubility of the asphaltene molecules in Main Zone oil was measured by the Oliensis Test(35). In this test, the solubility parameter Qf ie oil was lowered by adding to the oil successively larger volumes of hexadecane, a poor solvent for asphaltene molecules. The minimum volume (in milliliters) of hexadecane, which when added to 5 g of crude oil, will cause the chromatographic separation of the asphaltene fraction is termed the Oliensis Number. The Oliensis Number for the Main Zone crude oil is 3, indicating that the asphaltene molecules are not well-solubilized in the oil. Small changes in the solubility parameter of the Main Zone oil can cause the asphaltenes to precipitate. 

In addition to adsorbing at mineral-oil interfaces, asphaltene molecules also adsorb at oil-water interfaces. Strong intermolecular dipole-dipole, hydrogen bonding, electron donor-acceptor and acid-base interactions cause the surface-adsorbed asphaltene molecules to form rigid skins" at oil-water interfaces (41 43). When water droplets are dispersed in an oil which contains asphaltene molecules, molecularly thick, viscous asphaltene films form around the water droplets, inhibit the drainage of intervening oil and sterically stabilize the water-inoil emulsion. 

The effect of conversion on the structure of an asphaltene molecule has been reported to depend on the operating conditions and on the presence or not of a catalyst. The effect of thermal processing reaction of a vacuum residue resulted in the selective cracking of the aliphatic or naphthenic side chains of the molecule, leaving the highly condensed aromatic core structure almost intact (see Fig. 16) [116]. 

Figure 36. Proposed structural models of asphaltene molecules.

The enormous amount of energetic resource found as asphaltene rich deposits justify the exploration of a bioaltemative for upgrading technologies, from which not even enzymatic cracking of asphaltene molecules should not be excluded. 

The covalent nature of the asphaltene molecules and the complex nature of the corresponding environment results in agglomeration. Based on the formulated structural models, asphaltenes are seen as an aromatic core which aggregates in concentrated solutions, > 1 %, comprising high-MW covalent molecules, which are surrounded by varying numbers of smaller ones held together by various intermolecular bonds [408], Such molecules are considered to be overlapped/stacked over each other in oil mainly due to... 

Fig. 7. Hypothetical molecular formula of an asphaltene molecule constructed from HNMR analysis of the Lagunillas oil asphaltene (Yen. 1972).

Figure 4-15 Hypothetical model for an amphoteric asphaltene molecule.

Figure 4-16 Hypothetical model for a neutral polar asphaltene molecule.

Oxidation studies on Athabasca and other oil sand asphaltenes have shown the presence of aliphatic sulfides in amounts of up to 25% of the total sulfur (25). The structure of these sulfides has been established using mild thermolysis to liberate them from the polymeric framework of the asphaltene molecules. The produced pyrolysis oil contains significant concentrations of the sulfides and can be readily subjected to analysis. SIR-GC/MS traces of the homologous series of sulfides identified are... 

The n-alkyl thianes and thiolanes are thermally interconvertible, with the equilibrium shifted toward the thiolane side. Also, the thiolane ring can move along the n-alkyl side chain hopping by three carbon atoms until it reaches a terminal position (11). For this reason the original thiane/thiolane distribution in the asphaltene molecule before its thermal breakup could have been somewhat different from that in the pyrolysis oil. 

Attempts have also been made to describe the total structures of asphaltenes (Figure 1) in accordance with NMR data and results of spectroscopic and analytical techniques, and it is difficult to visualize these postulated structures as part of the asphaltene molecule. The fact is that all methods employed for structural analysis involve, at some stage or another, assumptions that, although based on data concerning the more volatile fractions of petroleum, are of questionable validity when applied to asphaltenes. 

Finally, Figure 8 illustrates the molecular-weight distributions obtained by GPC of a number of n-heptane asphaltenes from various crude sources. As can be seen, a wide range of molecular weights can be obtained for asphaltenes from different crude sources. Furthermore, the peak in the distribution occurs at different values for the different crudes. It is well known that molecular association of asphaltene molecules can be a problem in molecular weight determination either by GPC or VPO. However, the real extent of this problem is quite problematical. Small angle x-ray scattering... 

The sizes determined in this work are the apparent molecular sizes and not necessarily the sizes of the asphaltene and maltene molecules at process conditions. Association efforts for asphaltene molecules have been observed for both vapor-phase osmometry molecular weight and viscosity measurements (14, 15). The sizes reported here were measured at 0.1 wt % in tetrahydrofuran at room temperature. Other solvent systems (chloroform, 5% methanol-chloroform, and 10% trichlorobenzene-chloroform) gave similar size distributions. Under these conditions, association effects should be minimized but may still be present. At process conditions (650-850°F and 5-30% asphaltene concentration in a maltene solvent), the asphaltene sizes may be smaller. However, for this work the apparent sizes determined can be meaningfully correlated with catalyst pore size distributions to give reasonable explanations of the observed differences in asphaltene and maltene process-abilities (vide infra). In addition, the relative size distributions of the six residua are useful in explaining the different processing severities required for the various stocks. Therefore, the apparent sizes determined here have some physical significance and will be referred to just as sizes. 

The effect of solvent on the solubility of the product can be explained on the basis of the general mechanism of a typical Friedel-Crafts reaction. Thus, asphaltene reacts with AlCl3 to form intermediate carbonium ions, which then undergo electrophilic substitution. If substitution occurs within the asphaltene molecule new bonds are formed and, depending on the size of initial fragments, the molecule may grow bigger and, thus, less soluble. 

Similar mechanisms resulting in the formation of new functional groups and simultaneous intramolecular alkylation with or without changing the molecular weight also can be readily envisaged as occurring in a complex asphaltene molecule ... 

Volatilized asphaltene molecules contain prevalently only one structural unit per molecule. Heavier molecules contain one to three of these units per molecule. 

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