Asphaltenes, adsorption

Coming back to oil adsorption, we have evidence that the asphaltenes adsorption dissolved in the water saturated toluene, is more strongly on kaolinite than on illites. It is obvious that the size of an asphaltene molecule does not permit its access to the sites of adsorption that are responsible for the water adsorption. In other words, only basal surfaces are accessible to the large and planar asphaltene molecules that interact preferentially with the aluminol groups of the kaolinite while water deactivates the more energetic and certainly less accessible sites of illites. 

The G data were selected from 2—24 h frequency sweeps at 1 rad/s and plotted for each asphaltene concentration as a function of time. The moduli at short times tended to increase quickly and then shifted to a region of more gradual increase. The most rapid asphaltene adsorption likely occurred in the short period up to 6 or 8 h. The additional accumulation of asphaltenes was hindered by the material present at the interface and so the adsorption rate decreased. 

The results of flic interfacial rheological studies on asphaltene adsorption at oil-water interfaces teach us a great deal about the behavior of asphaltenes and their role in emulsion stabili2ation. The conclusions drawn are based largely on the assumption that the rheological properties measured, namely flic elastic film modulus G are directly related to the surface excess concentration of asphaltenes. F. It is understood diat die elastic modulus actually depends on both the surface excess concenlration and the relative conformation (i.e., coimectivity) of the adsorbed asphaltenes. It is further understood that a minimum adsorbed level is required to observe a finite value of G and that the relationship between G and G is not linear. With these caveats in mind, we can conclude die following ... 

The bulk concentration of asphaltenes appears to be an important variable driving asphaltene adsorption, higher concentrations leading to greater adsorption. 

Solvent plays a eritically important role in driving asphaltene adsorption, presumably related to the solvation of individual asphaltenie aggregates and molecules and the overall solubility parameter of the solvent versus that of the asphaltenes. Asphaltenes have their greatest tendency to adsorb and make the strongest interfacial films per unit mass at their limit of solubility. 

B. Factors Affecting Asphaltene Adsorption. and Self-Assembly... 

Table 3. Kinetic and thermodynamic parameters of asphaltene adsorption onto rock sands.

The rate of asphaltene adsorption is greater than desorption rate for studied concentration. Gibbs adsorption energy values define character of adsorption as physical, not chemical. Rate of asphaltene adsorption is less then resins (in two orders) reported earlier (Balabin Syunyaev, 2008). Parameters of resin adsorption from benzene solutions on quartz... 

The adsorption of asphaltenes is practically irreversible. Significantly larger masses and molecule sizes of asphaltenes appear to be the reason. Diffusion of such molecules to solid surface is embarrassing. The mechanism of diffusion limited adsorption is realized (Syunyaev et al., 2009 Diamant Andelman, 1996). Gibbs energy values are more or less the same for surfaces of all investigated materials quartz, dolomite, and mica. It is known that quartz and dolomite are the main components of oil reservoir framework rocks. The porosity has no influence on kinetic parameters of adsorption. Asphaltenes adsorption at the surfaces of quartz and dolomite is the most active. 

As a whole, the designed values of asphaltene adsorption parameters on metal surfaces are close to values obtained for mineral powders. Parameters of adsorption-desorption processes on metal surface are listed in Table 5. 

Table 5. Parameters of asphaltene adsorption on metal surfaces.

Menon, V.B. and Wasan, D.T (1986) Particle fluid interactions with application to solid-stabilized emulsions 1. The effect of asphaltene adsorption. Colloids and Surfaces, 19, 89—105. 

Asphaltenes seem to be relatively constant in composition in residual asphalts, despite the source, as deterrnined by elemental analysis (6). Deterrnination of asphaltenes is relatively standard, and the fractions are termed / -pentane, / -hexane, / -heptane, or naphtha-insoluble, depending upon the precipitant used (5,6,49). After the asphaltenes are removed, resinous fractions are removed from the maltenes-petrolenes usually by adsorption on activated gels or clays. Recovery of the resin fraction by desorbtion is usually nearly quantitative. 

Use of carefully selected surfactants in well treatment fluids is a way to accomplish this. Rock wettability can be altered by adsorption of polar materials such as surfactants and corrosion inhibitors, or by the deposition of polar crude oil components (173). Pressure appears to have little influence on rock wettability (174). The two techniques used to study wettability, contact and and relative permeability measurements, show qualitative agreement (175-177). Deposition of polar asphaltenes can be particularly significant in carbon dioxide enhanced oil recovery. 

Suspension Model of Interaction of Asphaltene and Oil This model is based upon the concept that asphaltenes exist as particles suspended in oil. Their suspension is assisted by resins (heavy and mostly aromatic molecules) adsorbed to the surface of asphaltenes and keeping them afloat because of the repulsive forces between resin molecules in the solution and the adsorbed resins on the asphaltene surface (see Figure 4). Stability of such a suspension is considered to be a function of the concentration of resins in solution, the fraction of asphaltene surface sites occupied by resin molecules, and the equilibrium conditions between the resins in solution and on the asphaltene surface. Utilization of this model requires the following (12) 1. Resin chemical potential calculation based on the statistical mechanical theory of polymer solutions. 2. Studies regarding resin adsorption on asphaltene particle surface and... 

The asphaltenes are nonvolatile and remain in the residue when the crude is subjected to distillation. The resins are partially volatile and therefore may be present in the lubricating oil fractions of higher boiling point as well as in the residue. Among the many methods employed for the separation of these materials from the oil fractions are distillation, adsorption, chemical treatment, and precipitation by special solvents. 

Furby (12) has developed a method for evaluating stocks in the lubricating oil range that results in a breakdown of components into asphaltenes, resins, wax, and dewaxed oil and provides a yield-viscosity index relationship for the dewaxed oil. The author has found such analyses very useful and inexpensive for evaluating a large number of potential lubricating oil stocks. Furby s method utilizes petroleum ether to precipitate asphaltenes, a fuller s earth-petroleum ether fractionation to isolate resins, methyl ethyl ketone-benzene dewaxing on the deasphalted-deresinified material to separate wax, and an adsorption fractionation to provide cuts from which the yield-viscosity index relationship for dewaxed, solvent-refined oil is obtained. 

The early methods for determining wax in asphaltic products employed high temperature distillation, vigorous chemical treatment, or selective adsorption to eliminate interference of asphaltenes and resins before the wax could be determined by crystallization methods (2). The Holde method, involving destructive distillation of sample to coke, was best known and most widely used. Since solid paraffins may be decomposed or altered by such vigorous treatment, the reliability of the results was in doubt. A new ap-... 

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. 

Despite claims by Spry and Sawyer (1975) of analytical measurements verifying asphaltene molecular sizes in the 100 A range at ambient conditions, it is unlikely that molecules this bulky exist at reaction conditions. The good predictive capability of the model may therefore result from a compensation effect. Electrostatic and adsorption interactions between solute molecules and the pore walls not explicitly accounted for with the purely geometric partition coefficient may result in the diffusing molecules appearing larger than they are at reaction conditions. 

It appears that the high molecular weight species originally present in the feedstock (or formed during the process) are not sufficiently mobile (or are too strongly adsorbed by the catalyst) to be saturated by the hydrogenation components and, hence, continue to condense and eventually degrade to coke. These deposits deactivate the catalyst sites and eventually interfere with the hydrodesulfurization process. Thus, the deposition of coke and, hence, the rate of catalyst deactivation, is subject to variations in the asphaltene (and resins) content of the feedstock as well as the adsorptive properties of the catalyst for the heavier molecules. 

After removal of the asphaltene fraction, further fractionation of petroleum is also possible by variation of the hydrocarbon solvent. For example, liquehed gases, such as propane and butane, precipitate as much as 50% by weight of the residuum or bitumen. The precipitate is a black, tacky, semisolid material, in contrast to the pentane-precipitated asphaltenes, which are usually brown, amorphous solids. Treatment of the propane precipitate with pentane then yields the insoluble brown, amorphous asphaltenes and soluble, near-black, semisolid resins, which are, as near as can be determined, equivalent to the resins isolated by adsorption techniques. 

Bulk composition the make-up of petroleum in terms of bulk fractions such as saturates, aromatics, resins, and asphaltenes, separation of petroleum into these fractions is usually achieved by a combination of solvent and adsorption (q.v.) processes. 

Barranco et. al. (1999) hypothesized that film formation results from the adsorption of asphaltenes at the coal tar-silica interface. Barranco et. al. (1999) further postulate that these asphaltene components are responsible for pH-dependent interfacial properties observed in coal tar-water-quartz systems. 

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