Effects of Physical Properties and Dispersion Conditions on the Chemical Dispersion of Crude Oil

Effects of oil type and mixing energy on the performance of chemical dispersants were investigated in a baffled flask mixing system using the commercial dispersant Corexit 9500. Effects of viscosity and interfacial tension were investigated by testing three crude oils (Arabian Light, Mars, and Lloyd) at two dispersant-to-oil ratios (DOR; 1:100 and 1:25) and three energy dissipation rates (0.00075, 0.016, and 0.16 W/kg). Dispersion effectiveness (i.e., fraction entrained as droplets in the water column) of all three oils was maximal at a mixing energy of 0.016 W/kg, which is similar to the energy dissipation rate in the surface layer of the open ocean. For Arabian Light and Mars, dispersion effectiveness was affected by DOR only at the lowest mixing energy, but it was proportional to DOR at all mixing energies for the more viscous oil, Lloyd. Droplet-size distributions were multimodal for all oil–dispersant combinations that were tested, indicating the involvement of multiple droplet-formation mechanisms. Diameter of mean volume of the major droplet-size modes was not sensitive to changes in mixing energy or DOR, but the fraction of dispersed oil in each mode was affected by these factors as well as by the oil type. Under all dispersion conditions (DOR and mixing energy), droplets produced by dispersion of more viscous oils were larger than those produced by less viscous oils. In general, higher dispersant concentration, favored the formation of smaller droplets, regardless of oil viscosity.