The Effect of Convective Currents on the Drying of Reconstituted Alberta Oil Sands Gangue

Abstract

Oil sands account for over 98% of Canada’s oil reserves and current commercial extraction methods have several shortcomings, including significant water and energy consumption, accumulation of tailings ponds, being impractical for low grade ores, and substantial greenhouse gas emissions. Non-aqueous or solvent extraction has the potential to replace the current hot-water process, thus also addressing these concerns. In non-aqueous extraction, bitumen is recovered from the ore using an organic solvent and the resulting solid mixture, also called “gangue”, consists of residual bitumen, solvent, and any water initially in the ore. Despite the many advantages, non-aqueous extraction methods have not been implemented on a commercial scale due to the difficulty of removing solvent from the extracted gangue while maintaining high bitumen recovery. In this study the effects of an induced convective current on the recovery of solvent was examined for samples of varying bitumen and water content. The horizontal air velocity over the samples were 0.9, 2.3, 3.0, and 3.5 m/s. Reconstituted gangue samples were prepared with known bitumen, water, and cyclohexane fractions to eliminate the variability associated between batches of real gangue. To investigate the effect of current velocity on samples of varying bitumen, reconstituted gangue samples with 0, 0.5, 1.0, 1.5, and 2.0 wt.% bitumen carbon, 3.7 wt.% water and 12 wt.% cyclohexane were prepared. To determine the role of air velocity on gangue samples of varying water, samples with 0, 3.7, 6, and 8 wt.% water, 1 wt.% bitumen carbon, and 12 wt.% cyclohexane were prepared. Samples were dried under a fume hood at the various air velocities and at ambient temperature and pressure for two hours. The drying curve of all samples exhibited two stages, a faster initial stage and a slower final stage separated by a breakpoint. The initial stage is dominated by cyclohexane liquid films that maintain connectivity between the bulk solution and the external surface. The final stage is characterized by water evaporation, where the flux is limited by diffusion within the gangue. Any increase in the initial flux corresponds to increased solvent removal. Increasing air velocity was observed to increase initial flux but only to a certain extent, after which the initial flux becomes independent of flow velocity. The optimal flow velocity for samples with less than 1.5% bitumen carbon was observed to be 3 m/s. The relative increase in initial flux was more prominent for samples with less residual bitumen given the same flow velocity increase. For samples with higher amounts of residual bitumen, higher flow rates are required to maximize initial flux. The positive effect of increased flow velocity on solvent removal is not diminished as long as the fraction of water remains between 4 – 7 wt.%. In the absence of water, cyclohexane liquid film formation is suppressed, resulting in significantly lower initial fluxes at all flow velocities. Shorter transition times were observed when the initial flux increases. Despite comparable increases in the initial flux, the transition time decease for samples with 6 wt.% water was smaller than for samples with 8 wt.% water. This study provides new insight on the factors governing cyclohexane removal from gangue, but also acknowledges that additional work is required to develop a comprehensive model that accurately accounts for all parameters and mechanisms of the drying gangue.