Asphaltene precipitation

Black, viscous residuum direc tly from the still at 410 K (390°F) or higher serves as fuel in nearby furnaces or may be cooled and blended to make commercial fuels. Diluted with 5 to 20 percent distillate, the blend is No. 6 fuel oil. With 20 to 50 percent distillate, it becomes No. 4 and No. 5 fuel oils for commercial use, as in schools and apartment houses. Distillate-residual blends also serve as diesel fuel in large stationaiy and marine engines. However, distillates with inadequate solvent power will precipitate asphaltenes and other high-molecular-... 

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. 

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. 

Determining the molecular weights of asphaltenes is a problem because they have a low solubility in the liquids often used for determination. Also, adsorbed resins lead to discrepancies in molecular-weight determination, and precipitated asphaltenes should be reprecipitated several times prior to the determination (12). Thus, careful precipitation and careful choice of the determination method are both very important for obtaining meaningful results. 

In addition to the effects of crystal lattice energy, choice of solvent other than n-paraffins can also be important. For example, Corbett and Swarbrick (10) have shown that a number of oxygenated compounds can precipitate asphaltenes from petroleum residua in quantities varying from 12 wt % to 100 wt % on resid. Clearly, the shape of the precipitation curves for the different solvents should be different from that for n-paraffins and from each other. However, the use of a solubility parameter type of polarity scale should permit rationalization of the results when considered along with the solubility parameter of the particular solvent. 

A study of MW distribution for precipitated asphaltenes and the derivation of conclusions about bitumen or asphalt properties from it has severe limitations since this complex mixture exhibits a considerable overlap of GPC curves for all the fractions obtained in a conventional separation procedure. Similarly, the resins separated on clay and the eluted hydrocarbons exhibit overlap, as shown by Figures 5 and 6. Figure 5 demonstrates the GPC profiles of Athabasca asphaltenes (nC5) and resins (Attapulgus clay—total resin eluent)... 

EMULSION IS USUALLY DEFINED as a system consisting of a liquid dispersed with or without an emulsifier in an immiscible liquid, usually in droplets of larger than colloidal sizes. In petroleum emulsions, solids play an extremely important role in both the formation and stability of emulsions. These solids can be oil-phase components such as wax crystals or precipitated asphaltenes, or mineral components that are partially oleophilic, a property that allows them to act as stabilizers between the oil and water phases. 

H-Oil cracking appears to significantly increase the readiness of an atmospheric residue to precipitate asphaltenes. The increased readiness can be explained on a molecular and colloidal... 

Reactor deposit analyses do not reflect compositions consistent with asphaltene deposition the V, Ni, and S values are high., and the LOI value is low. The current authors doubt that the increased readiness of the effluent to precipitate asphaltenes is of relevance at reactor conditions. 

Electrostatic Properties in Nonaqueous Media. Although suspensions most commonly comprise particles dispersed in aqueous media, the petroleum industry contains many examples of particles dispersed in nonaqueous media. Examples include precipitated asphaltenes in oil (see Chapter 8) and mineral solids dispersed in diluted froth in oil sands processing (see Chapter 13). Particles can be electrostatically stabilized in nonaqueous media, although the charging mechanism is different (7, 34). In a recent review Morrison (34) emphasized that many models are... 

If sand is being produced, pore blocking by fines or precipitated asphaltenes does not arise. Sand removal creates a softened zone, stresses are redistributed outward from the soft zone, and vertical stress concentrations, combined with lateral stress reduction because of sand removal, bring the force of gravity to bear, aiding destabilization and extrusion of sand toward the producing wellbore (Fig. 8). 

Finely divided sohds such as clay, sand, shale, silt, gilsonite, drilling muds, workover fluids, cor ro-sion products, crystallized paraffins or waxes, and precipitated asphaltenes and resins. 

Generally, for water-in-crude oil emulsions the indigenous component thought to have the largest effect on stability is the asphaltenes. Therefore, model systems based on asphaltenes were prepared for selected oils. From Table 1 one can observe that not all precipitated asphaltene fractions could stabilize a model emulsion. However, many model emulsions had a similar stability as the original crude oil-based emulsion, indicating that the fraction extracted from the oil plays a central role in the stabilization. 

The solubility of asphaltenes is highly dependent on the medium in which they are placed (81). The presence of dissolved asphaltenes in crude oil is mediated by a combination of crude aromaticity and petroleum resins that act to solvate asphaltene aggregates. Adding an excess of aliphatic solvent, namely n-hep-tane, sufficiently reduces the solubility of asphaltenes in crude oil and causes precipitation. To perform subsequent film experiments these precipitated asphaltenes were then redissolved in toluene. As n-heptane was added to the asphaltene—toluene solutions. 

Figure 10 Fraction of precipitated B6 asphaltenes as a function of toluene content (v/v) in heptane. Experiments were performed gravimetrically by weighing the % of precipitated asphaltenes which were retained by a 1.6- xm filter.

Table 2 (63) shows additional stmctural data estimated for the fractions. These results are all dependent on the composition of the source crude oil, particularly heteroatom content and metals. Both Ni and V are found primarily in the heptane-precipitated asphaltenes and are evenly distributed without regard to molecular size. They seem to be interchangeable in structure in that in fractions of a given asphalt the ratio of V to Ni is constant over wide ranges of composition. These metals often exist in porphyrin structures and have been implicated in higher rates of asphalt oxidation. 

First, traditional solvent methods were used to remove soluble impurities from precipitates (asphaltenes and preasphaltenes). The precipitates were then dissolved in toluene solution to remove the precipitated preasphaltenes. Finally, relatively pure asphaltenes were re-dissolved in toluene to obtain a 100 ppm concentration of asphaltene solution . 

Figure 5.15 Amount of precipitated asphaltene and resin from the Weyburn reservoir fluid (seelhble 5.7) and CO2 mixture at 160 bar and 332 K (from Pan and Firoozabadi, 1997a).

Example 5.7 Derive the Gibbs free energy expression for the precipitated asphaltene phase when it is in a solid state. 

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