Nature of Asphaltene Aggregates

The size and fractal dimension of asphaltene aggregates were investigated for two Western Canadian bitumens diluted with n-heptane at concentrations above the onset of precipitation. Asphaltene aggregate size distributions were measured over time using focused beam reflectance and micrographic methods in a series of batch experiments at different n-heptane contents and shear rates. The fractal dimensions of the aggregates were determined from the volume of the settled aggregates. The asphaltenes formed approximately log-normal size distributions with volume mean diameters of tens to hundreds of micrometers, depending mainly upon the n-heptane content. The distributions were established in less time than the first measurement could be obtained (about 30 s) and changed relatively little afterward. The average aggregate size increased with an increasing n-heptane content but reached a plateau value at 70–80 wt % n-heptane. The fractal dimension reached a maximum at a similar n-heptane content. Micrographic i...