Effect of precipitation time and solvent power on asphaltene characteristics

- Composition of precipitated asphaltenes changes widely as a function of time.
- Asphaltene precipitation involves reorganization of the aggregates.
- Reorganization of aggregates includes expulsion and incorporation of molecules.
- 24 h is not enough time to reach a steady composition of the asphaltenes.

The kinetics of asphaltene precipitation is studied regarding the characterization of the precipitated material. The association of different components of the crude oil depends on the solvent power of the blend heptane/oil and produces aggregates with a variable composition that change as a function of time. It was found that the precipitate contains a significant amount of maltenes whose relative content decreases as a function of time. In agreement with this behavior, the precipitate becomes more hydrogen deficient. These changes also produced a significant increase in the apparent average molecular weight of the precipitate as measured using size exclusion chromatography. This increase correlated with the amount of asphaltenes in the precipitated material independently of time or heptane/oil ratio. Solubility profile measurements showed that the average characteristics of the asphaltenes found in the precipitate varied as a function of time. As more asphaltene molecules precipitated, the distributions of these molecules changed decreasing its average solubility parameter. Considerable differences in the amount collected at low and high heptane/oil ratios as a function of time were also found. Based on the measurements, a new model was developed that describes the complex aggregation behavior involved in asphaltene precipitation.