Effect of Solvents on the Stability of Water-in-Bitumen Emulsions

In this study, the role of the solvent in stabilizing water-in-oil emulsions was investigated below and above the onset of asphaltene precipitation. Two Western Canadian bitumens with significantly different emulsion-stabilizing capacities were evaluated. Water-in-asphaltene/solvent and water-in-diluted bitumen emulsions were prepared with extracted n-heptane-insoluble asphaltenes, toluene, and an alkane (n-pentane, n-heptane, n-decane, n-hexadecane, and cyclohexane). The organic phases were prepared with and without removing any precipitated asphaltenes. The stability of the settled emulsions was assessed in terms of the free water evolved after treatment with heating and centrifugation. Factors that could contribute to emulsion stability were measured including the drop size distribution, water volume fraction, and asphaltene mass surface coverage of the settled emulsions. Surface pressure isotherms of asphaltene films were measured to assess irreversible interfacial film formation in each solvent. In precipitate-free emulsions, emulsion stability was found to increase significantly at the onset of precipitation. The increase in stability was attributed to the increased irreversible adsorption of asphaltenes at the interface as their solubility in the oil phase decreased. Most of the emulsions became less stable at high n-alkane contents as more asphaltenes were precipitated out of solution and the supply of soluble emulsion stabilizers was depleted. The results demonstrate that oils at conditions near the onset of asphaltene precipitation are more likely to experience problematic emulsions. However, it was found that, with low-viscosity solvents, this effect may be mitigated by the presence of the precipitated particles in the emulsion.