Capillary Curves for Ex-situ Washing of Oil-Coated Particles

The oil removal efficiency for the ex situ extraction of bitumen from oil sands, or ex situ washing of oil-contaminated sand and related processes is determined by the balance of forces at the oil/water and solid/fluid interfaces. The objective of this work is to estimate the balance of forces at the interface using dimensionless numbers, and their use in evaluating and engineering ex situ soil washing processes. To this end, bitumen was removed from bitumen-coated sand particles using a two-step process. In the first step, the particles were mixed with a suitable solvent (toluene) used, primarily, to reduce the viscosity of bitumen. The particles were then mixed with water or an aqueous surfactant solution capable of producing low interfacial tensions with the solvent-bitumen mixture. The fraction of oil retained after washing was evaluated as a function of interfacial tension, solvent/bitumen ratio, mixing time, mixing velocity, and particle size. These ex situ washing conditions were normalized using dimensionless film and particle-based Weber and Capillary numbers. The fraction of oil retained by the particles was plotted against these dimensionless numbers to generate capillary curves similar to those used in enhanced oil recovery. These curves reveal the existence of a critical film-based Weber number and a particle-based Capillary number that can be used in the design or evaluation of soil washing processes. The film-based Weber number also explained literature data that associates interfacial tension with the removal of oil from oil-based drill cuttings, as well as field observations on the role that particle size plays on the removal of oil in soil washing operations.