Structure of water/solids/asphaltenes aggregates and effect of mixing temperature on settling rate in solvent-diluted bitumen

Abstract

Emulsified water droplets (WD), dispersed solids (DS), and precipitated asphaltenes (PA) form aggregates when bitumen emulsions are treated with aliphatic solvents. The WD/DS/PA aggregates in solvent-diluted bitumen exhibit zone settling and the settling rate is strongly influenced by aggregate structure. In this work, procedures were developed for determining structural parameters of the aggregates. The average porosity of the aggregates was found to be 0.40–0.45 for a system using a 50/50 by wt light n-pentane/ n-hexane solvent mixture (C5C6) at a solvent-to-bitumen ratio (S/B, by wt) of 2.0 and 0.70–0.75 for a system using heavier n-heptane (C7) solvent at S/B=3.0. Consequently, the effective aggregate density for the C5C6 solvent system (approximately 1.00 g/ml) is higher than that for the C7 solvent system (approximately 0.87 g/ml). The aggregates formed in the C5C6 solvent contain embedded or attached water droplets larger than 20 μm, while the water droplets in the aggregates using the C7 solvent are generally smaller than 10 μm. The average volume fraction of the aggregates in the hindered settling zone is 0.12–0.13 at all studied conditions. Higher temperature (e.g. 50–120 °C) for mixing of the bitumen emulsion and solvent leads to larger aggregates and results in a significant increase in the settling rate of the aggregates. Mixing at elevated temperature provides an efficient means of increasing settling flux in settlers that are used to treat the solvent-diluted bitumen emulsions, especially when the settlers are preferably operated at near-ambient conditions (e.g. about 30 °C and less than 350 kPa).