Chemical dispersant effectiveness testing: influence of droplet coalescence.

Thermodynamic and kinetic investigations were performed to determine the influence of coalescence of chemically dispersed crude oil droplets in saline waters. For the range of pH (4-10) and salinity (10 per thousand, 30 per thousand, 50 per thousand ) values studied, zeta-potential values ranged from -3 to -10 mV. As the interaction potential values calculated using Derjaguin-Landau-Verway-Overbeek (DLVO) theory were negative, the electrostatic barrier did not produce significant resistance to droplet coalescence. Coalescence kinetics of premixed crude oil and chemical dispersant were determined within a range of mean shear rates (Gm = 5, 10, 15, 20 s(-1)) and salinity (10 per thousand, 30 per thousand ) values. Coalescence reaction rates were modeled using Smoluchowski reaction kinetics. Measured collision efficiency values (alpha = 0.25) suggest insignificant resistance to coalescence in shear systems. Experimentally determined dispersant efficiencies (alpha = 0.35) were 10-50% lower than that predicted using a non-interacting droplet model (alpha = 0.0). Unlike other protocols in which the crude oil and dispersant are not premixed, salinity effects were not significant in this protocol. This approach allowed the effects of dispersant-oil contact efficiency eta(contact) to be separated from those of water column transport efficiency (eta(transport)) and coalescence efficiency (eta(coalescence)).