The Paradox of Asphaltene Precipitation with Normal Paraffins

For bitumens and crude oils, the volume of n-paraffin at the flocculation point, which is the point of incipient asphaltene precipitation, increases as the n-paraffin carbon number increases, reaching a maximum at a carbon number of 9 or 10, and then decreases. Thus, asphaltenes often can begin precipitating with a smaller volume of n-hexadecane than with n-pentane, even though large volumes of n-hexadecane precipitate much less (and more aromatic) asphaltenes than large volumes of n-pentane. How can n-hexadecane be both a better and a poorer solvent than n-pentane for asphaltenes? This paradox of solvent quality can be resolved by combining the entropy of mixing of molecules of different sizes with the heat of mixing from solubility parameters, as expressed by the regular Flory−Huggins model. With sufficient characterization data, the approximations and methods of Yarranton et al. can quantitatively describe asphaltene precipitation from the flocculation point to large excesses of n-paraffins from pentane to hexadecane. To describe only the flocculation point data of bitumens and crude oils, the oil compatibility model of Wiehe can be used. Although the oil compatibility model was derived on the basis that the solubility parameter is constant for a given oil at the flocculation point, using “effective” solubility parameters, flocculation points can be predicted with little characterization data.