Settling of Asphaltene Aggregates in n-Alkane Diluted Bitumen

The settling rate of asphaltene aggregates is a key design parameter for partial deasphalting processes, and yet few data and models are available for these systems. The settling rates of asphaltene aggregates were measured in two Western Canadian bitumens diluted with n-heptane or n-pentane at 21 degrees C and atmospheric pressure. The density and viscosity of the mixtures, the size distributions of the aggregates, and the fractal dimensions of the aggregates were also measured. The asphaltene aggregates settled as a zone, that is, all aggregates settled at the same rate. The settling rates increased with increasing n-alkane content, reached a maximum at approximately 75 wt % n-alkane and then decreased at higher dilutions. The maximum settling rate corresponded to the maximum aggregate diameter and fractal dimension. The maximum settling rate in n-pentane diluted bitumen was 2 orders of magnitude greater than in the same bitumen diluted with n-heptane. The difference was attributed to the lower density and viscosity of the medium, larger aggregates, and higher fractal dimensions in n-pentane versus n-heptane. The settling rates were modeled with Stokes' law modified to include the fractal dimension of the aggregates. Since zone settling was observed, the settling rate was determined from a single average diameter applied to all of the aggregates. The Sauter mean diameter was found to provide the most consistent results for the diluted bitumen systems in this study. No other form of hindering was required to match the data. The model matched the measured settling rates to within 20% of the maximum settling rate. The model is sensitive to the fractal dimension of the aggregates, and therefore precise determination of the fractal dimension is critical or it must be tuned to match the settling data.