Predicting the Onset of Asphaltene Precipitation from Refractive Index Measurements

Abstract

Despite the complexity of asphaltene chemistry, a simple relationship exists between the onset of precipitation and the refractive index of the mixture in which precipitation is first observed. Separation of asphaltenes from oil is a colloidal phenomenon. Because aggregation and separation of the asphaltenes are primarily dependent on the magnitude of van der Waals forces between nonpolar species, the mixture refractive index can be used to quantify the ability of a crude oil to disperse its asphaltene fraction. In some crude oils, asphaltenes are very close to the onset of precipitation, while in other oils, the asphaltenes are relatively stable. The value of the refractive index of an oil sample and the refractive index at which precipitation is first observed are distinctive properties of each crude oil. The difference between these two values is a measure of the stability of the asphaltenes in their respective crude oils. Mixture refractive index correctly predicts the onset of asphaltene precipitation in solutions with varying concentrations of asphaltenes.