Kinetics of Asphaltene Aggregation in Crude Oil Studied by Confocal Laser-Scanning Microscopy

This paper presents results of our study of the kinetics of asphaltene aggregation as a function of different precipitants, solvents, and the Hildebrand solubility parameter δ. The aggregation was studied in situ in the crude oil using confocal laser-scanning microscopy. We observe that the kinetics of asphaltene aggregation strongly depends upon the Hildebrand solubility parameter of the mixture. The aggregation process, which occurs when adding precipitant close to the precipitation onset point, can be described by a diffusion-limited aggregation (DLA) process. A slight change of the Hildebrand solubility parameter away from the precipitation onset point into the precipitation regime leads to a different kinetic mechanism. In this regime, we observe that the aggregate size increases almost linearly over time, which is an indication of the crossover between DLA and reaction-limited aggregation (RLA). The maximum mean aggregate diameter increases from 18 μm near the onset point to 90 μm away from the onset point. Precipitating the asphaltenes with n-heptane leads to a faster aggregation rate with bigger flocs at a short time when compared to n-hexane.