EFFECT OF STOKES’ LAW SETTLING ON MEASURING OIL DISPERSION EFFECTIVENESS

Abstract

ABSTRACT Industry laboratory tests to measure dispersion effectiveness for oil spills on water measure only the volume percentage of oil dispersed and not the dispersed particle size. The effect of particle size on settling behavior is particularly pronounced in tests that use long settling times to superimpose a dispersion stability criterion on the effectiveness rating. The authors have studied the effect of settling time on the volume cumulative particle size distribution measured by the Coulter Multisizer II. Using Stokes’ law settling to analyze the results, we have demonstrated the effects of settling flask geometry and sample volume on measured effectiveness. These arbitrary test variables control the settling path height and vary markedly from test to test. The intrinsic variables that control settling vs time—initial particle size distribution, aqueous viscosity, and aqueous and oil densities—are functions of aqueous, oil, and dispersant compositions; temperature; and dispersion energy. The author's analysis shows that the effect of settling variables is to cut off the initial cumulative particle size distribution above a certain particle size, thereby fixing measured effectiveness. Stokes’ law provides a measure of this cutoff size. Experimental data have been developed to support this theoretical analysis. This analysis points to the variables that must be considered with different laboratory tests to rank dispersants when settling is part of the test procedure. Even with a single test, ranking may change with settling time given an initially large fraction of large particles and a sufficiently large difference between the densities of water and oil.