Size distribution of primary submicron particles and larger aggregates in solvent-induced asphaltene precipitation in a model oil system

Abstract

Asphaltene precipitation is a crucial phase separation phenomenon in the oil industry, especially in paraffinic froth treatment to extract bitumen from oil sands ores. This work reveals the ubiquitous presence of particles at 0.2 to 0.4 μm in radius, defined as primary sub-micron particles (PSMPs) at early stage asphaltene precipitation from diffusive mixing with solvents. We observed PSMPs in a quasi-2D chamber where asphaltene solution (the model oil) is displaced by 3 paraffinic solvents, the blends of solvents, and solvents containing inhibitor at low concentrations. The yield and size distribution of asphaltene particles are affected not only by the Hildebrand solubility parameter of the precipitants but also by the diffusion coefficients of the asphaltene solution and the precipitant. Even though n-pentane is a stronger solvent than n-heptane, the yield of asphaltene in the diffusive mixing chamber is close because of the difference in the diffusion coefficient between n-pentane and n-heptane in asphaltene solution. The results of n-heptane and n-decane blends reveal that the increase of the ratio of PSMPs compared with pure n-heptane or n-decane. The inhibitors significantly decrease the aggregation rate of PSMPs, displayed as a higher ratio of PSMPs compared to a non-inhibitor group. The population balance model (PBM) with the Hildebrand solubility parameter has been used to model particle size distribution. Good agreement has been achieved between numerical predictions and the experimental data. It indicates that the colloid theory can describe the size distribution of PSMPs and larger aggregates in early stage of asphaltene precipitation. This study illustrates the importance of mixing dynamics on the size distribution in asphaltene precipitation. The findings can be useful for accurate modeling of the hydrocarbon separation or asphaltene precipitation.