Asphaltene micelles

Figure 3. Dynamic equilibrium of asphaltene micelle inversion process.

Mack (58, 59) points out that asphaltenes from different sources in the same petro-lenes give mixtures of approximately the same rheological type, but sols of the same asphaltenes in different petrolenes differ in flow behavior. Those in aromatic petrolenes show viscous behavior and presumably approach true solution. Those in paraffinic media show complex flow and are considered to be true colloidal systems. Pfeiffer and associates (91) consider that degree of peptization of asphaltene micelles determines the flow behavior. Thus, a low concentration of asphaltenes well peptized by aromatic petrolenes leads to purely viscous flow. High concentrations of asphaltenes and petrolenes of low aromatic content result in gel-type asphalts. All shades of flow behavior between these extremes are observed. 

Flo. 31. Proposed mechanism for asphaltene conversion (a) destruction of asphaltene micelle (b) depolymerization due to heteroatom removal (Asaoka a al1983). 

Vanadyl and nickel reactivity differences resulting from the chemistry of the oxygen ligand on vanadium were discussed in Section IV,A,l,c. Enhanced V reactivity could also arise from molecular size constraints. Beuther and co-workers (Beuther and Schmid, 1963 Larson and Beuther, 1966) speculate that nickel concentrates in the interior of asphaltene micelles while vanadium concentrates on the exterior. Thus a combination of stronger adsorption due to the oxygen ligand and inhibition of Ni reaction, coupled with the exposed position at the periphery of the asphaltene, may all contribute to the enhanced vanadium reactivity relative to nickel. 

A linear relationship is often observed between vanadium removal and sulfur removal, whereas the relationship between nickel and sulfur removal is linear but of smaller slope (Massagutov et al., 1967). For asphaltene-containing stocks, this phenomenon is interpreted on the basis of heteroatom distribution within the asphaltene micelles (Beuther and Schmid, 1963). Sulfur and vanadium are concentrated on the exterior, whereas nickel is concentrated in the interior. Conversion of the asphaltene generally leads to simultaneous removal of sulfur and vanadium, whereas nickel removal is more difficult. 

These fouling phenomena are believed to be due to the precipitation of asphaltenes from hydrocracked, effluent streams. Fragmentation reactions decrease the solubility power of effluent maltenes and the solubility of asphaltene micelles, thus facilitating precipitation [1]. 

It has been shown [21] that, at low concentrations (below the critical micelle formation concentration), asphaltenes in solution are in a molecular state. Above the critical micelle concentration, however, asphaltene micelle formation occurs in a manner similar to that in surfactant systems where surfactant monomers are more uniform in their structure and less polydisperse. Now, it is obvious that coke for-... 

E Y Sheu. Physics of asphaltene micelles and microemulsions - theory and experiment. J. Phys. Condens. Matter 8, A125-A141, 1996... 

Figure 6. The asphaltene micelle has been proposed as being composed of a stack of asphaltene molecules associated either by a bonds or through 7r-7r interactions.

Even in dilute solutions they associate (49, 50). Published sizes of the micelles vary from 2 to 4 run. Sophisticated analytical techniques such as small-angle X-ray diffraction (SAXS), small-angle neutron scattering (SANS), and NMR were used to study the asphaltene particle or micelle sizes (51). MacKay (15) reported that a MWtof 10,000 g/mol would correspond to a 2 to 4-nm cluster. This is very much smaller than a 1-pm water droplet, and considered to be 1/100 to 1/1000 the droplet diameter. This topic is worthy of a review on its own. However, the colloidal properties of asphaltenes, micelles, and... 

From data obtained by SANS analysis, Sheu and Storm (48) repotted that the sizes of asphaltene micelles in a good solvent such as toluene/pyridine fall in the range 3.0-3.2 nm. 

The schematic of Fig. 2 is obviously oversimplified nonetheless, there are several salient features illustrated which likely captiue the essence of asphal-tene-stabilized films. First, siuface adsorption of asphaltene molecules is probably driven by hydration of polar functional groups in the aromatic core of an individual asphaltene molecule. Second, resin molecules probably serve to solvate primary aggregates (asphaltene micelles) in the bulk phase, but these resins are likely shed and do not appreciably participate in the actual stabilizing film. In fact, as we will show later, resins are totally unnecessary in the stabilization of asphaltenic films. A missing detail in Fig. 2 is the means whereby individual asphaltene molecules crosslink to form... 

Many of the properties of asphalt are determined by the variety of chemical types and their divergent properties. The asphaltenes and saturates are immiscible. Mixtures of asphaltenes and naphthene aromatics are highly non-Newtonian at 100°F, but polar aromatics and asphaltene mixtures are Newtonian (63). It has long been proposed (74,75) that asphalt exists as asphaltene micelles or clusters solubilized by polar aromatics. 

MORPHOLOGICAL SIZE OF ASPHALTENE MICELLES IN ASPHALT AND HEAVY RESIDUE... 

Since the asphaltene micelle is close to a spherical shape... 

From the Table n, it can be found that no matter what type of asphalt, all basic units of asphaltene cells are the same size. No matter what solvent is used, the asphaltene micelle is ca. 10 nm, and is consistent with the results of other methods. 

Asphaltene particles are dispersed in saturates and aromatic hydrocarbons (gas-oil) with resins as peptizing agents in asphalt or heavy oil. The interaction between resin and asphaltene micelles is not well understood. In the present study, asphaltene has been dispersed into aromatic hydrocarbons (such as toluene), and the precipitations due to additions of paraffinic hydrocarbons (such as pentane) in the presence of a number of amphiphiles have been studied. These amphiphiles certainly affect the asphaltene precipitation, either by retardation or by enhancement, depending on the structural types and quantities of the amphiphiles. We have found that the nature of resin is that it behaves as an amphiphile, since the polar fractions of resin do contain amphiphiles. The solubility parameter spectra of these asphaltenes are discussed. 

Because the asphalt system is not a true solution, it can be fractionated into saturates, aromatics, resins, and asphaltenes by the solvent fraction method, SARA method, or TLC method. The polarity of these four fractions is increased in the order of saturates, aromatics, resins, asphaltenes. In crude oil, asphaltene micelles are present as discrete or dispersed particles in the oily phase. Although the asphaltenes themselves are insoluble in gas-oil (saturates and aromatics), they can exist as fine or coarse dispersions, depending on the resin content. The resins are part of the oily medium but have a polarity higher than gas-oil. This property enables the molecules to be easily adsorbed onto the asphaltene micelles and to act as a peptizing agent of the colloid stabilizer by charge neutralization. 

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