Kinetics of asphaltene precipitation/aggregation from diluted crude oil

Asphaltene precipitation kinetics were investigated for four crude oils with different asphaltene contents, each diluted with n-heptane at room conditions. The yields of precipitated asphaltenes (mass of precipitated asphaltenes divided by mass of bitumen in feed) were measured gravimetrically and the diameters of the aggregated asphaltene particles were measured using both microscopy and the focused beam reflectance method. The measurements were performed at anaerobic conditions. The data were first modeled with the geometric population balance proposed by Maqbool et al. (2011) [1]. This approach matched the yield data but not the aggregate diameters, nor did it account for the rapid (almost instantaneous) precipitation and aggregation observed in heavy oils. The model was adapted to include a simultaneous nucleation/growth/aggregation mechanism as follows: 1) the primary particles size was increased to the micrometer scale; 2) the fraction of instantaneously unstable asphaltenes was explicitly accounted for; 3) a nucleation term was introduced for more slowly precipitating material; 4) the collision frequency was modified to include interparticle attraction, and; 5) the experimentally observed loss of interparticle adhesion over time was explicitly accounted for in the collision efficiency. Correlations were proposed for most of the model parameters. The minimum required inputs for the model are the temperature, precipitant content, and the equilibrium asphaltene yield. The proposed model matched both yield and aggregate size kinetic data from this study and the literature.