Recently, we proposed a new technique for the determination of the onset of asphaltene flocculation. This new method is based on viscosity measurements of a crude oil upon titration with a precipitating solvent (e.g. n-pentane, n-heptane, etc.). The onset of asphaltene flocculation is detected by a sharp increase in the relative viscosity of the suspension in which asphaltene particle aggregation occurs. The key point in this development is the phenomenon of asphaltene flocculation induced by the addition of an n-paraffin hydrocarbon to a crude oil. The proposed method can also be applied to detect the onset of micellization of asphaltenes in aromatic solvents.
In this paper, we validate the proposed new technique from a theoretical standpoint.
Numerous experimental works have revealed the colloidal nature of the asphaltene fraction of a crude oil. Kawanaka et al. consider the asphaltenes to exist in crude oil as both dissolved and suspended particles. Dispersed asphaltenes are sterically stabilized by neutral resins, electrically charged, and have a diameter of 30 to 40 Å. The stability of these particles can be disrupted by addition of solvents that cause desorption of the resin molecules from the asphaltene-particle surface (e.g. n-heptane). When two asphaltene particles collide on their resin-uncovered spaces, aggregation occurs; therefore, we may expect that at a particular solvent concentration the aggregation process will be initiated rather vigorously. This point is defined as the onset of asphaltene flocculation. The amount of solvent needed to arrive at the onset depends on the type of crude oil being analyzed, and particularly on the resin content. It has been demonstrated that after the onset the amounts of material flocculated out of solution increase very rapidly. This is shown in Fig, 1. Furthermore, it is well understood that asphaltenes in fact show high polydispersity and nonsphericity when they are precipitated from crude oils. The actual shape of asphaltenes is not known; however, researchers have proposed a somewhat elongated shape, which could well be polydisperse cylinders or ellipsoids. Studies on the colloidal properties and the self-association character of asphaltenes in polar solvents (i.e. toluene) have suggested that asphaltene/toluene systems are Newtonian. When asphaltene molecules are exposed to excess amounts of an aromatic solvent (e.g. toluene), there is a possibility of micelle formation. Thus, the present viscometric technique can be equally applicable to detect the onset of asphaltene micellization.