Dissolution and Heavy Residue Sinking of Subsurface Oil Droplets: Binary Component Mixture Dissolution Theory and Model-Oil Experiments

Abstract

The processes by which oil-material appears suspended in the water column or becomes deposited on the sea floor is conventionally attributed to its attachment to marine solid particles. Since the 2010 Gulf oil spill, a significant volume has remained unaccounted for. The fraction volume of oil sinking resulting from dissolution of its lighter components is the subject of this work. An experimental study and theoretical development for the combined processes of oil-droplet dissolution/sinking (named SOLUTE-SINK) is based on a conceptual binary-component model for crude oil. The experiments were on buoyant pseudo-oils, a two-component (A and B) miscible hydrocarbon mixture formulated from a less-dense-than-water liquid hydrocarbon (Component A) and a more-dense-than-water hydrocarbon-like chemical (Component B). These were used in proof-of-concept experiments. A single, buoyant liquid drop surrounded by water was constrained from floating to the surface in a laboratory-scale microcosm dissolution tank. It was placed inside an inverted and submerged Petri dish so evaporation was not possible and weathering occurred by dissolution only. The heavier-than-water Component B was much less soluble; during the experiment, Component-A dissolution losses resulted in droplet sinking. This established proof-of-concept and the observed time period provided key data supporting the proposed oil binary-model theory. Experiments were performed with four binary chemical mixtures, each replicated 5–7 times. The dissolution time periods for achieving buoyancy inversion (i.e., sinking droplet) ranged from 2 to 4 days with the kinetics parameter dependent on the solubility of Component A. Concentration and bulk density measurements tracked the theoretical time-series behavior of the proposed model equations. Direct mathematical coupling the dissolution/buoyancy loss process for single liquid droplet in the water column is an original contribution of this work. In knowing the initial drop-size distribution, the field of oil-spill modeling is provided with an algorithm forecasting four oil-material produced fractions: dissolved, floating, suspended, and sinking. The SOLUTE-SINK model will find applications for deep-water blowout ejected droplets (constrained to dissolution weathering only) as well as breaking-wave-produced droplets for sea surface spills. In addition, it will aid the development of a laboratory method for the oil dissolution process.