The effect of dispersant on the oil droplet size distribution in multiphase plume from an accidental release: CFD with Population Balance model

Abhijit Rao, Rupesh K. Reddy and K. Nandakumar
Department of Chemical Engineering, Louisiana State University, Baton Rouge, LA, 70803
During the â??Deepwater Horizonâ?? accident in the deep sea in 2010, about 4.9 million barrels (www.oilspillcommission.gov) of oil was released into the Gulf of Mexico, making the spill one of the worst ocean spills in recent times. The accidentally released oil mass loses its momentum to surrounding mass of water which is relatively quiescent and results in entrainment of surrounding water to form a plume. A typical plume is thus a multiphase mixture of oil, gas and ambient water. The shear interaction of oil phase with the surrounding medium results in generation of droplets. Dispersants are often added as a method of oil spill remediation, to disperse the oil mass in water column. The surfactants in a dispersant get adsorbed on to the surface of oil droplets and lower the interfacial tension and the existing turbulence causes the disintegration of larger droplets into finer ones. Thus, in the water column, there exists plethora of droplets with a wide size distribution. The fate of oil droplets is highly dependent on its size. The droplet size distribution also determines the overall interfacial area available for the dissolution process of soluble (lighter) hydrocarbons from droplet phase to the surrounding
water phase.
We have developed a comprehensive multiphase computational model which is able to predict the size distribution of droplets in a system under consideration. The oil droplets rising in the column, are free to interact with each other and as a consequence, may undergo coalescence and disintegration. The local turbulence and the varying interfacial tension dictate the coalescence and breakage
mechanisms. So, along with the continuity, momentum, and turbulence models, it is essential to include
a model which has the capability of handling these changes in population of droplets of varying sizes. In this work we achieve this by integrating the multiphase Eulerian-Eulerian CFD model with the Population balance Model.
The spreading of plume in the downstream of release point increases the distances between the droplets and hence disintegration(breakage) process becomes more important over coalescence process. Appropriate breakage kernels representative of the system under consideration were included in the numerical model. We have used the experiment described in Branvik et al1 , to build our numerical model. The oil phase was injected into to computational domain with a flow rate between
1.2 L/min to 1.5 L/min through a nozzle with ID of 1.5mm. The simulation results revealed a trend similar to that observed in the experiment ; the lowering of interfacial tension between oil/water interface form 15.5 mN/m to 1.5 mM/m produced much finer droplets and mean droplet diameter reduced nearly by two folds.

1. Johansen Ã?, Brandvik PJ, Farooq U. Droplet breakup in subsea oil releases â?? Part 2: Predictions

of droplet size distributions with and without injection of chemical dispersants. Marine Pollution

Bulletin. 8/15/ 2013;73(1):327-335.