ABSTRACT
An integrated approach to study chemical dispersant effectiveness and dispersed oil toxicity is presented. Conventional lab scale effectiveness tests generally provide a measure of overall dispersant effectiveness. However, chemical dispersion can be viewed as two processes: (1) dispersant-oil slick mixing and (2) oil droplet transport into the water column. Inefficiencies in either process limit the overall dispersant effectiveness. This laboratory study centered on the latter process and was conducted to focus on the impacts of water column hydrodynamics on the resurfacing of dispersed oil droplets. Using a droplet coalescence model (Sterling et al., 2002), the droplet coalescence rates of dispersed crude oil was determined within a range of shear rates. A controlled shear batch reactor was created in which coalescence of dispersed oil droplets were monitored in-situ. Experimental dispersion efficiencies (C/C0) and droplet size distributions were compared to those predicted by Stokes resurfacing. Experimental C/C0 values were lower than that predicted from Stokes resurfacing. Experimental dispersion efficiency values (C/C0) decreased linearly with increasing mean shear rates due to increased coalescence rates. These results suggested that dispersed oil droplet coalescence in the water column can adversely impact overall dispersant efficiency. To avoid high control mortality in toxicity testing, the toxicity exposure chamber was designed with separate compartments for scaled mixing and organism exposure, respectively. Chamber design includes continuous re-circulation between mixing and exposure chamber. A 1-minute exposure compartment residence time was determined from tracer studies indicating virtually identical oil concentrations in the mixing and exposure compartments. In addition, the 96-hour mortality of 14-day oil Menidia beryllina varied from 2% in the no-oil control tests to 87% in the dispersed oil (200 mg/L) tests. These results show the effectiveness of the integrated vessel for the characterization and toxicity testing of oil dispersions.
Microfluidic Investigation of Crude Oil Droplet Coalescence: Effect of Oil/Water Composition and Droplet Aging
