Dewatering of Naphthenic Bitumen Froth

Bitumen froth is a water-in-bitumen emulsion (similar to 30 wt % water, 60 wt % bitumen, and 10 wt % of solids) stream obtained during the water-based extraction process of mined oil sands. The separation of water (to 2 wt % or less) and solids (to 0.5 wt % or less) from the froth is necessary to prevent corrosion, catalyst deactivation, and fouling in downstream processes. In naphthenic froth treatment (NFT), aromatic naphtha is added to reduce the density and viscosity of bitumen to aid in this separation, which often requires the addition of demulsifiers and centrifugation. This work looks at simulating the dewatering of froth using a bench-scale mixer and heptol 80/20 (80 vol % heptane; 20 vol % toluene) as a simulated naphtha solvent. Power dissipation during mixing, water contents, image analysis of micrographs, and acoustic spectroscopy were used to examine the dewatering process as a function of time for three froth samples with different compositions. Gravity drainage, in the absence of additives, led to a residual water content, after 2 h, ranging from 1.7 to 3.7 wt % for the three different samples, consistent with the typical residual water reported for these systems. Micrographs of the diluted froth show the eventual disappearance of large water drops and the prevalence of smaller emulsified drops (<10 mu m) in the residual water. An examination of this residual water using acoustic spectroscopy showed that up to 0.8 wt % water is in the form of similar to 0.3 mu m submicron drops that cannot be removed by gravity or centrifugation. A dewatering model using an initial drop size distribution (DSD) of water drops also supports the existence of a substantial amount of submicron drops. A low-shear dewatering test suggests that most of this submicron water was in the froth before simulated froth treatment, formed potentially during bitumen extraction and transportation, prior to solvent dilution and dewatering. Cryo-SEM imaging further supports this hypothesis. Studies of water solubilization in toluene-asphaltene and toluenenaphthenic acid systems suggest that up to 0.5 wt % of this submicron water could be originated from water-asphaltene association. The presence of high solid contents in the froth correlated with high residual water and submicron water contents, pointing to the potential role of solids in the formation of submicron drops.