Droplet Size Distribution as a Measure of Dispersant Effectiveness
Citation:
Sundaravadivelu, D., R. Conmy, R. Grosser, A. Burkes, AND E. Holder. Droplet Size Distribution as a Measure of Dispersant Effectiveness. In Proceedings, International Oil Spill Conference, Cincinnati, OH, May 10 - 13, 2021. Allen Press, Inc., Lawrence, KS, 1141617, (2021). https://doi.org/10.7901/2169-3358-2021.1.1141617
Impact/Purpose:
The objective of this study is to evaluate DE and the corresponding Droplet Size Distribution (DSD) for seven petroleum oils and eight commercially- available dispersants at two temperatures using the standard baffled flask test at 34 ppt salinity.Presenting this research paper at the annual AMOP oil conference in Canada.
Description:
Laboratory evaluation of dispersion effectiveness (DE) typically measures the total mass of oil dispersed into the aqueous phase using known oil concentrations. Studying dispersant performance using droplet size distribution methods can offer important information regarding the mass of small droplets produced during dispersant addition, providing potentially useful information for oil spill response decision-making. In this study, seven crude oils and eight dispersants were evaluated for DE using the Baffled Flask Test (BFT) at 5 and 25 °C at a Dispersant to Oil Ratio (DOR) of 1:25. The Crude oils with densities from 0.85 to 0.96 g/cm3 (at 25 °C) and dispersants with varying effectiveness values listed on the National Contingency Plan (NCP) Product Schedule were used. A UV spectrophotometer was used to measure the percent of total oil dispersed (mg oil dispersed/mg oil added) or DE. In addition to the BFT DE measure of total oil dispersed into solution, the size of the particulate fraction is critical for better understanding the ultimate transport and fate in the ocean. The Droplet Size Distribution (DSD), Total Volume Concentration of particles <250 µm (TVC, µL/L), and Mean Diameter (MD, µm) were measured using a Laser In Situ Scattering and Transmissometry probe (LISST-100X, Sequoia Scientific, Inc.). The interfacial tension (IFT) between the oil droplet and the artificial seawater for varying oil-dispersant combinations was also measured with a spinning drop tensiometer. Results indicate that DE for a given oil-dispersant combination depended on the ability of the dispersant to decrease the IFT. While the DE measured by UV did not show a significant distinction in product effectiveness, LISST measurements (percent of droplets under 70 µm, MD and TVC) were better indicators of dispersant performance. Furthermore, the distribution data using the LISST produced reliable estimates of oil mass dispersed even though LISST captures particles in a limited size range. These predictive estimates of DE based on LISST measurements would provide effectiveness values almost real-time in comparison to the standard BFT, for which the responses vary from hours to days. Also, the results provide a more realistic estimate of how much and how long the dispersed droplets would stay in the water column and thus would be amenable to naturally occurring biodegradation processes.
