Modeling crude oil droplet-sediment aggregation in nearshore waters

This paper describes a modeling approach that simulates changes in particle size distribution and density due to aggregation by extending the Smoluchowski aggregation kinetic model to particles of different density. Batch flocculation studies were conducted for clay, colloidal silica, crude oil, clay-crude oil, and silica-crude oil systems. A parameter estimation algorithm was used to estimate homogeneous collision efficiencies (alphaHOMO) for single-particle-type systems and heterogeneous collision efficiencies (alphaHET) for two-particle-type systems. Homogeneous collision efficiency values (alphaHOMO) were greater for clay (0.7) and for crude oil (0.3) than for silica (0.01). Thus, clay and crude oil were classified as cohesive particles while silica was classified as noncohesive. Heterogeneous collision efficiencies were similar for oil-clay (0.4) and oil-silica (0.3) systems. Thus, crude oil increases the aggregation of noncohesive particles. Data from the calibrated aggregation model were used to estimate apparent first-order flocculation rates (K') for oil, clay, and silica and apparent second-order flocculation rates (K'') for oil and clay in oil-clay systems and for oil and silica in oil-silica systems. For oil or clay systems, aggregation Damköhler numbers ranged from 0.1 to 1.0, suggesting that droplet coalescence and clay aggregation can occur on the same time scales as oil resurfacing and clay settling, respectively. For mixed oil-clay systems, the relative time scales of clay settling and clay-oil aggregation were also within an order of magnitude. Thus, oil-clay aggregation should be considered when modeling crude oil transport in nearshore waters.