Influence of Dynamic Mixing Energy on Dispersant Performance: Role of Mixing Systems

Abstract

Chemical dispersants can be used to mitigate the impacts of marine oil spills. Bench-scale testing is widely used to provide spill responders with information that can be used to determine which dispersants would be most effective for specific spills based on the type of oil and environmental conditions. Relative performance of specific oil-dispersant combinations can vary depending on the experimental system that was used, however, and results obtained in different systems are weakly correlated. The objective of this work was to evaluate and compare the factors that affect dispersant performance in two well-characterized mixing systems (baffled flask and paddle jar) under dynamic mixing conditions. Mixing time (p < 0.01), mixing energy (p < 0.001), and dispersant hydrophile–lipophile balance (HLB) (p < 0.001) all exerted statistically significant effects on dispersion effectiveness (defined as the mass fraction of oil entrained in the water column). For a fixed mixing time, effectiveness increased with mixing energy in both mixing systems, but the dispersion effectiveness reached its maximum at lower mixing energies in the paddle-jar system. Also, dispersant HLB exerted only minor effects on dispersion effectiveness in the baffled-flask system, but it was the dominant effect in the paddle-jar system. For all treatments, the droplet-volume distributions stabilized after mixing for less than 10 min, and the steady-state distributions consisted of several droplet-size modes that varied in relative importance among treatments. In general, the baffled-flask system produced dispersions that were dominated by relatively small oil droplets, whereas the paddle-jar system generated dispersions that consisted almost entirely of larger oil droplets. These differences indicate that factors other than mixing energy, such as fluid flow characteristics and system geometry, are also important in the process of oil dispersion and must be considered in the design of bench-scale dispersant performance tests.