CFD approach for simulation of bitumen froth settling process – Part I: Hindered settling of aggregates

Bitumen or heavy oil aggregates are formed when bitumen emulsions, consisting of emulsified water droplets dispersed solids and precipitated asphaltenes, are treated with aliphatic solvents. While settling, the aggregates exhibit zone settling mode with the development of a sharp oil and settling zone interface. Previous research [Long Y, Dabros T, Hamza H. Structure of water/solids/asphaltenes aggregates and effect of mixing temperature on settling rate in solvent-diluted bitumen. Fuel 2004;83:823–32] provides settling experimental data for bitumen aggregate settling and a Richardson–Zaki approximation was proposed by the authors with modified exponents for simulating the settling behavior of aggregates. However, the need for modified exponents and their dependence on aggregates and solvents used was not explained. Since the aggregates exhibit hindered settling, where the settling rate is different from that of individual particles or aggregates, numerous settling models have been proposed to correlate particle swarms to single particle via drag correlations [Richardson JF, Zaki WN. Sedimentation and fluidization: Part I. Trans Inst Chem Eng 1954;32:35–53; John G, Maan RA. Velocity–voidage relationships for fluidization and sedimentation in solid–liquid systems. Ind Eng Chem Proc Des Dev 1977;16:206–14]. The compaction zone, due to high concentrations of solids and the effect of their resultant weight during settling is also often ignored. In this work, the hindered settling behavior of bitumen aggregates is first studied in a CFD framework using two models: (a) modified Richardson–Zaki approximation by Long et al. [Richardson JF, Zaki WN. Sedimentation and fluidization: Part I. Trans Inst Chem Eng 1954;32:35–53] and (b) the Syamlal–O’Brien model [John G, Maan RA. Velocity–voidage relationships for fluidization and sedimentation in solid–liquid systems. Ind Eng Chem Proc Des Dev 1977;16:206–14]. To address the limitations of the two models, a new model is proposed that incorporates the irregular (fractal) structure of aggregates by considering the aggregates as porous liquid-filled solids that are fractal in nature. Results, from the new fractal model, are found to be in good agreement with empirical data.