Settling Properties of Asphaltene Aggregates

Stokes law is the fundamental equation to describe the settling velocity of a particle for small Reynolds number flow and is applicable when the particle is spherical and impermeable. However, this equation is not suitable to describe the settling of porous aggregates found in many industrial applications, because of the nonsphericity and flow through the pores of the aggregates. Free-settling tests were used in the present work to estimate asphaltene aggregates porosity and effective density. The settling velocities of individual asphaltene aggregates are measured in a quiescent column via a photographic technique. Data analysis was conducted with a video frame grabber and digital imaging software, to determine the particle size, shape, and settling velocity. Results show that the aggregate size (area-equivalent diameter) determined from the video images ranged from 20 μm to 200 μm and the measured settling velocities varied from 100 μm/s to 564 μm/s. Settling velocity measurements indicated an inverse relationship between aggregate size and density. The reduced aggregate density with increasing size was attributed to the entrainment of fluid into the aggregate structure. These porous aggregates further collided with each other to form increasingly more-porous structures. An analysis was performed to understand the importance of aggregate fractal structure and its effective density on the settling velocity. The size-density fractal dimension of the asphaltene aggregates was estimated to be in the range of D 3 = 1.3−2.0. These relatively low fractal dimensions suggest that the asphaltene aggregates are highly porous and very tenuous.