Solvent swelling of petroleum asphaltenes

Volumetric changes of crude oils and derived fractions can cause phase separation. Crude oil asphaltene swelling has been recently reported in the open literature. Solvent swelling processes have been formerly studied in greater detail for coal samples. Important structural and chemical information on coals have been gathered through the use of swelling as a characterization technique. In this study, crude oil isolated asphaltenes were successfully characterized by solvent swelling, employing polar as well as nonpolar solvents. Operationally stable and unstable crude oils, virgin and hydrocracked vacuum residua, and solid deposits from oil production tubings were the selected matrices for asphaltene isolation and characterization. The main factor that governs swelling appears to be dispersive forces, since swelling maxima were observed for alkane type solvents. Consistently, compositional asphaltene parameters like elemental hydrogen and aliphatic chain content significantly influenced the solvent swelling process. However, solvent donor properties and asphaltene acceptor properties also played an important role in the swelling phenomenom. Asphaltene swelling in nonpolar solvents was successfully fitted using the Flory-Rehner equation. This procedure allows to estimate the solubility parameters for the samples, which turned out relatively low. The former results allowed us to present a structural model for solid asphaltenes. This model proposes the solids to be composed of inner aromatic cores that are impermeable to solvents, surrounded by aliphatic moieties capable of incorporating solvents and swell. Kinetic dissolution experiments carried out with solvent swollen asphaltenes show consistency with the proposed model.