(66a) Investigation on the Nature of Asphaltene Flocs

Asphaltene flocculation is an important step in some settling processes involving solvent diluted bitumen and is a contributing step to asphaltene deposition in pipelines and oil processing facilities. Asphaltene flocculation occurs when asphaltenes precipitate from an oil upon depressurization or the addition of a poor solvent. Asphaltene precipitation is considered by some to be a colloidal flocculation process (Eyssautier et al., 2012) and by others to be a phase transition (Johnston et al., 2017; Dini et al., 2016). Once an oil is destabilized or undergoes a phase transition, asphaltenes grow from nano-aggregates of a few nanometers in diameter to particles of approximately 1 micrometer in diameter which in turn form flocs that are hundreds of micrometers in diameter (Angle et al., 2006). These flocs are assumed to be held together by electrostatic and van der Waals forces and therefore their size distribution is expected to be shear sensitive. Population balance models have been used to model the kinetics of asphaltene flocculation based on the assumption that there is a dynamic equilibrium between flocculation and shattering as found in other chemical systems (Maqbool et al., 2011). The objectives of this study are to determine if the underlying assumptions of the proposed flocculation process are in fact correct and to propose an appropriate model for the observed behavior.

Asphaltene flocs were examined from two Western Canadian bitumen samples diluted with n-pentane or n-heptane at concentrations above the onset of precipitation. The asphaltene floc size distribution was measured over time in a series of batch experiments at different diluent contents, shear rates, and contact times. All experiments were carried out at room temperature and atmospheric pressure. The floc size distributions were measured while mixing using a focused beam reflectance method apparatus. These measurements were validated with micrographic images of samples collected in situ. The fractal dimension of the flocs was determined from the volume of the settled flocs.

In all cases, the asphaltenes formed particle (or floc) size distributions with number average diameters of tens to hundreds of micrometers. The distributions were established in less time than the first measurement could be obtained (about 30 seconds) and only changed slowly, if at all, afterwards. The initial floc size distribution was shear dependent. The average floc size increased with diluent content but reached a plateau value at 75 to 85 wt% diluent.

Micrographic images and fractal dimensions indicated that, near the onset of precipitation, compact linear and planar floc structures dominated. At higher diluent contents up to approximately 85 wt% diluent the flocs remained compact but became more three-dimensional. Above 85 wt% diluent, the flocs became more porous and branched. The fractal dimensions increased from approximately 2.3 near the onset to approximately 2.8 as the diluent content increased to 80 wt% and then decreased to 2.2 at higher diluent contents. Higher fractal dimensions indicate more spherical and/or more compact flocs. Near the onset of precipitation, no significant change in floc size was observed over time (up to 5 hours). At dilutions above 80 wt% solvent, the average floc size decreased with shear while the number of flocs increased, consistent with shattering. No re-flocculation was observed at any conditions even when the shear rate was reduced.

We hypothesize that the flocs first form via a rapid nucleation and growth process that can be modeled as flocculation. The asphaltenes are sticky in this phase and the flocs are fused structures. We further hypothesize that the asphaltenes rapidly lose their stickiness such that no further flocculation occurs. After the first few seconds, only shattering can occur. The compact structures formed near the onset are difficult to shatter and therefore shear insensitive while the looser structures formed at high dilution can be partially shattered under the shear conditions of these experiments. Further tests to validate these hypotheses will be discussed. A shear-dependent Smoluchowski aggregation model is proposed for the steep growth of the flocs in the first seconds. A second shattering-only model is proposed to account for the effect of shear over time.

Keywords: asphaltene, flocculation, shattering, bitumen, fractal dimension, population balance.

References

Angle, C.W., Long, Y., Hamza, H., Lue, L. Precipitation of asphaltenes from solvent-diluted heavy oil and thermodynamic properties of solvent-diluted heavy oil solutions. Fuel, 85, 2006, 492-506.

Dini, Y., Becerra, M., Shaw, J.M. Phase behavior and thermophysical properties of Peace River bitumen + propane mixtures from 303 K to 393 K. Journal of Chemical Engineering Data, 61, 2016, 2659-2668.

Eyssautier, J., Frot, D., Barré, L. Structure and Dynamics of Colloidal Asphaltene Aggregates. Langmuir, 28, 2012, 11997-12004.

Johnston, K., Schoeggl, F.F., Satyro, M.A., Taylor, S.D., Yarranton, H.W. Phase Behavior of Bitumen and n-Pentane, Fluid Phase Equilibria, 442, 2017, 1-19.

Maqbool, T., Raha, S., Hoepfner, M.P., Fogler, H.S. Modeling the aggregation of asphaltene nanoaggregates in crude oil-precipitant systems. Energy & Fuels, 25, 2011, 1585-1596.