Precipitation, Aggregation, and Settling of Asphaltenes from n-Alkane Diluted Bitumens
Date
2019-09-13
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Abstract
Several industrial processes involve the precipitation, aggregation, and settling of asphaltenes from solvent diluted bitumen. Models and data that include the effect of solvent type, concentration, shear, and contact time on asphaltene precipitation and settling are required to design and optimize these processes but are lacking or are specific for conventional oils. In this thesis, the kinetics of asphaltene precipitation, aggregation and settling were investigated for n-heptane and n-pentane diluted bitumens at ambient conditions. The onset and amount of precipitation were measured in air and nitrogen. The size distribution, fractal dimension, and settling rates of the aggregates were also measured. Asphaltene aggregates were found to be fused fractal structures that are insensitive to shear once formed. A semi-empirical correlation for their fractal dimension was proposed. Asphaltene precipitation in a nitrogen atmosphere was found to be time-dependent but reached an apparent equilibrium after tens of hours. A previously developed population balance model was adapted to include both nucleation and flocculation mechanisms and was tuned to match both precipitation amounts and aggregate sizes. Asphaltene precipitation in air did not reach an equilibrium likely because oxygen altered the asphaltenes. An oxidation rate term was introduced into the population balance model to match the precipitation data in air. The proposed model matched both yields and aggregate sizes over time for several bitumens and conventional oils to within 15% in nitrogen and 20% in air. Asphaltene aggregates were found to have an approximately log normal size distribution but settled as a zone; that is, all aggregates settled at the same rate. The average aggregate size and fractal dimension reached a maximum at an n-alkane content of approximately 75 wt%. Settling rates are a function of the aggregate size and fractal dimension and reached a maximum at approximately the same n-alkane content. The settling rates were modeled with Stokes’ law modified to include the fractal dimension of the aggregates and using the Sauter mean diameter for all aggregates. The model matched settling rates to within 20% of the maximum settling rate.
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asphaltene precipitation, bitumen, aggregation, size distribution, fractal dimension, settling
Citation
Duran, J. A. (2019). Precipitation, Aggregation, and Settling of Asphaltenes from n-Alkane Diluted Bitumens (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca.