Understanding the Kinetics of Asphaltene Precipitation from Crude Oils

Abstract

The precipitation of asphaltenes from crude oils can lead to serious challenges during oil production and processing. This study investigates the kinetics of asphaltene precipitation from crude oils using n-alkane precipitants. For several decades, it has been understood that the precipitation of asphaltenes is a solubility driven phenomenon and the previous studies on the effect of time are usually limited to short time scales. By using optical microscopy and centrifugation based separation, we have demonstrated that the time required to precipitate asphaltenes can actually vary from a few minutes to several months, depending on the precipitant concentration used. Our results demonstrate that no single concentration can be identified as the critical precipitant concentration for asphaltene precipitation. We have also been able to establish the solubility of asphaltenes as a function of the precipitant concentration and it is shown that the short-term experiments over-predict the solubility.

The effect of temperature on the precipitation kinetics of asphaltenes is also investigated and different competing effects have been identified. We demonstrate that at higher temperatures the precipitation onset time for asphaltenes is shorter and their solubility is higher. We also present a hypothesis to explain these results and demonstrate that the viscosity difference resulting from a change in temperature in the key parameter in the aggregation of asphaltenes.

In order to simulate the growth of asphaltene aggregates from the nanometer scale to micron-size particles a generalized geometric population balance model has been successfully developed. The Smoluchowski kernel has been incorporated to describe the aggregation of the asphaltene nanoaggregates that is induced by the addition of a precipitant e.g. heptane. The model has been validated with experimental data for various heptane concentrations and a good fit has been observed in each case.

Finally, it is shown that the asphaltenes that precipitate earliest in the precipitation process are the most unstable fraction. They have a higher dielectric constant and contain greater quantities of metals like Ni and V than other asphaltenes. Additionally, they also contain relatively larger quantities of the high polarity fractions as compared to the asphaltenes that precipitate later.