Settling of Asphaltene Aggregates inn-Alkane Diluted Bitumen

Abstract

The settling rate of asphaltene aggregates is a key design parameter for partial deasphalting processes, and yet few data and models are available for these systems. The settling rates of asphaltene aggregates were measured in two Western Canadian bitumens diluted with n-heptane or n-pentane at 21 °C and atmospheric pressure. The density and viscosity of the mixtures, the size distributions of the aggregates, and the fractal dimensions of the aggregates were also measured. The asphaltene aggregates settled as a zone, that is, all aggregates settled at the same rate. The settling rates increased with increasing n-alkane content, reached a maximum at approximately 75 wt % n-alkane and then decreased at higher dilutions. The maximum settling rate corresponded to the maximum aggregate diameter and fractal dimension. The maximum settling rate in n-pentane diluted bitumen was 2 orders of magnitude greater than in the same bitumen diluted with n-heptane. The difference was attributed to the lower density and viscosity of the medium, larger aggregates, and higher fractal dimensions in n-pentane versus n-heptane. The settling rates were modeled with Stokes' law modified to include the fractal dimension of the aggregates. Since zone settling was observed, the settling rate was determined from a single average diameter applied to all of the aggregates. The Sauter mean diameter was found to provide the most consistent results for the diluted bitumen systems in this study. No other form of hindering was required to match the data. The model matched the measured settling rates to within 20% of the maximum settling rate. The model is sensitive to the fractal dimension of the aggregates, and therefore precise determination of the fractal dimension is critical or it must be tuned to match the settling data.

Primary submicron particles from early stage asphaltene precipitation revealed in situ by total internal reflection fluorescence microscopy in a model oil system

“…The settling rate depends on the type of solvent precipitant. For example, the rate of n-pentane precipitated asphaltene is two magnitudes higher than that of n-heptane precipitated asphaltene [4]. For a given type of precipitant, settling rate increases with the concentration of precipitant and then decreases at approximately 75% [4].…”

“…The settling behavior suggests that asphaltene precipitates are porous and tenuous fractal structures with fluid trapped inside. A quicker settling rate represents a larger fractal dimension and more compact particles [4]. The settling rate depends on the type of solvent precipitant.…”

“…For example, the rate of n-pentane precipitated asphaltene is two magnitudes higher than that of n-heptane precipitated asphaltene [4]. For a given type of precipitant, settling rate increases with the concentration of precipitant and then decreases at approximately 75% [4]. Based on the mean diameter of the precipitates, two classical models are applied to approximately describe the growth dynamics: diffusionlimited aggregation (DLA) and reaction-limited aggregation (RLA) [5,6].…”

Size distribution of primary submicron particles and larger aggregates in solvent induced asphaltene precipitation

“…Settling behavior suggests that the morphology of asphaltene precipitate is also affected by the precipitant composition. The settling rate of asphaltene particles in pentane is two magnitudes higher than in heptane [23], indicating a higher fractal dimension and larger aggregates.…”

Dynamics of Viscous Fingering in Porous Media in the Presence of In Situ Formed Precipitates and Their Subsequent Deposition

“… ξ 1 and ξ 2 are matching parameters that can be obtained based on the experimental data. Fitting of mathematical equations similar to equation () for predicting the local equilibrium conditions has also been implemented by previous researchers (Casas et al, 2019). It should be noted that equation () is not used when w A = 1 simply because, in this case, only fluid A exists in the system.…”