(492e) Enhanced Asphaltene Precipitation in Paraffinic Froth Treatment Using Carbon Dioxide

Authors: 
Paraffinic froth treatment is an essential step in oil sand surface mining operations for the removal of water and mineral solids from bitumen froth. In this process some of the asphaltenes present in the bitumen precipitate out and form aggregates with emulsified water droplets and dispersed mineral solids, accelerating the separation of solids and water from the bitumen and producing a clean bitumen product. In order to achieve the required asphaltene separation, a large volume of paraffinic solvent such as pentane or hexane is added to bitumen froth.

It has been reported that CO2 dissolved in heavy oil results in asphaltene precipitation during enhanced oil recovery. Because asphaltene precipitation is considered a key step of paraffinic froth treatment, we conducted a lab-scale investigation to explore whether adding CO2 to paraffinic froth treatment solvent could enhance asphaltene precipitation.

Froth treatment tests were conducted using both supercritical fluid CO2 and liquid CO2 mixed with pentane-diluted bitumen froth in an autoclave. In these tests both pentane and CO2 concentrations were varied. The results indicated that when mixing CO2 with n-pentane-diluted bitumen froth significant asphaltene precipitation was observed. At a pentane-to-bitumen ratio of 1.0 and lower, which is below the onset asphaltene precipitation point with solvent alone, adding CO2 to the diluted bitumen produced significant asphaltene precipitation and resulted in a much reduced asphaltene content in the bitumen product. The combined solvent-to-bitumen ratio (CO2 plus solvent) was much lower than that using n-pentane alone when precipitating the same amount of asphaltenes. For example, at the combined solvent-to-bitumen ratio of about 1.4, the asphaltene content in the product bitumen decreased to about 5-7 wt% from an initial 17 wt%. To reach these levels of asphaltene content using pentane alone, pentane-to-bitumen ratios of 2.2-2.5 are needed. Low water content (below 0.3 wt%) in the diluted bitumen phase was also observed. These results suggest that CO2, when dissolved into diluted bitumen, may enhance the paraffinic froth treatment process.