Acoustic and Electroacoustic Spectroscopy of Water-in-Diluted-Bitumen Emulsions
Water-in-oil emulsions of Athabasca bitumen diluted with toluene have been studied using the latest advances in acoustic and electroacoustic spectroscopy. From the sound attenuation spectra of emulsions, the water droplet size distribution is measured. The electrical surface charge density of the water droplets is obtained from the colloid vibration current. In the case of freshly prepared water-in-oil emulsions, the droplet size increased while; the surface charge density decreased with time. The time-dependent relaxation of the surface charge ranges from several hours to 3 days, and it is probably due to the slow adsorption/ desorption kinetics of bituminous components at the water—oil interface. This study illuminates the contribution of the electrostatic interactions to the stability of water-in-oil emulsions.
Oil sands deposits in northern Alberta are mixtures of bitumen, silica sands, and fine clays.1 Bitumen is a form of heavy oil which needs to be upgraded to obtain commercial fuels. The lesser deep oil sands ores typically contain 9% to 13 wt % bitumen, which is recovered through surface mining operations, followed by a water-based extraction process.1 At some stage of the process, the extracted bitumen is in the form of bitumen froth, containing about 60% bitumen, 30% water, and 10% solids.1 Water and solids must be separated from the bitumen prior to upgrading (in order to avoid serious corrosion and plugging problems in upgrading plant operations*, and this is done through the addition of an organic solvent (diluent) to the bitumen froth, to lower the density and viscosity of the oil phase.1 Such a mixture of bitumen froth with organic solvent forms a water-in-oil emulsion. They are referred to as water-in-diluted-bitumen emulsions.
Asphaltenes, fine solids, and natural surfactants present in bitumen have been identified as stabilizing compounds of water-in-diiutedbitumen emulsions. Among them, asphaltenes and fine solids are recognized as the main emulsion stabilizers.2 They tend to gather at the water-oil interface-resulting in a film which acts as a structural barrier to the coalescence of the water droplets. Most experimental work, to date, suggests that the stability of water-in-diluted-bitumen emulsions is mainly due to a steric repulsion mechanism. Repulsive electrical interactions might have a role in the emulsion stability,1 as was suggested for instance by Marinova et al' and Karraker and Radke4 who described electric charge generation at water-oil interfaces. However, the electrical repulsion mechanism has been clearly not sufficiently experimentally investigated. The advent of commercially available ultrasound based techniques during the past decade offers a new opportunity for studying water-in-diluted-bitumen emulsions. They can provide information about size distribution and electrosurface properties of particles or droplets dispersed in nontransparent and nonpolar media, in moderately concentrated dispersions.'1 7 The theory of electroacoustic spectroscopy by Shilov and co-workers based on measurements of the colloid vibration current (CVO provides the surface charge density of the dispersed phase under the assumption of strongly overlapped double layers.8 ,0 Acoustic spectroscopy derives the particle size distribution from measurements of the sound atten uation in the dispersion.51' Ir- These features are incorporated in the DT-1200 Acoustic and Electroacoustic Spectrometer from Dispersion Technology Inc.5 The purpose of the present study is to investigate the surface charge density of water droplets dispersed in diluted bitumen by means of this technique and associated theory.