Microscale dynamics of dilution-induced asphaltene precipitation
Creators: Meng, Jia
Publisher: name: University of Alberta Library
Year: 2022

<p>Phase separation by dilution with poor solvents is important for many separation processes, notably asphaltene precipitation in paraffinic froth treatment (PFT) in oil sands extraction. PFT is a step of bitumen froth treatment in which paraffinic solvent is added to the bitumen froth to reduce the viscosity and remove impurities along with asphaltene precipitation. It remains challenging to quantitatively understand the early stage dynamics of asphaltene precipitation induced by paraffinic solvent addition. The mixing conditions in the conventional bulk system were not well controlled, which might have a great effect on asphaltene precipitation, and further affects oil sands extraction efficiency. </p>

<p>This thesis studied asphaltene precipitation under controlled mixing conditions. A microfluidic device is designed and fabricated to control the mixing under diffusive dominant between model oil and the paraffinic solvent to study asphaltene precipitation without convective flows. The total internal reflection fluorescence (TIRF) microscope, which has a high spatial resolution (~ 200 nm) and temporal resolution (4 frames per second) is used to follow the growth dynamics of individual asphaltene domains. We observed the basic units of asphaltene precipitation in the diffusive mixing and controlled external mixing processes, which are named PSMPs. The radius of which is 200 - 400 nm and independent of the change of solvents. The asphaltene first precipitates as primary submicron particles (PSMPs) in the early stage of precipitation and then PSMPs further aggregate, resulting in the increase of asphaltene particle size. This study direct visualizes asphaltene precipitation and aggregation under controlled mixing at the submicron scale, which was absent in the previous studies.</p>

<p>Afterwards, we found that the relative frequency of PSMPs to the total quantity of the particles is affected by the type and concentration of the precipitants. The population balance model (PBM) based on Smoluchowski aggregation model can describe the aggregation process based on the collision frequency and efficiency of particles. The experimental and simulation results of size distribution agree well with each other in our examined 23 types of precipitants. Unlike the conventional bulk system, in the diffusive mixing system, the yield of asphaltene is not only dependent on the Hildebrand solubility parameter but also affected by the diffusion coefficient of solvents to the model oil. Compared with pure n-heptane and n-decane, the mixture of them produces a higher proportion of PSMPs due to the difference in diffusion coefficients. The above findings prove the influence of the hydrodynamics factor (i.e., diffusion coefficient) on asphaltene precipitation and lead to the important role of mixing conditions, which was neglected in the previous research.</p>

<p>To investigate the influence of mixing conditions, a related system was studied as a basis for studying the influence of mixing conditions on asphaltene precipitation: dilution-induced nanodroplets formation at high viscosity. Asphaltene precipitation and nanodroplets formation are comparable because they both are phase separation processes due to the addition of poor solvents. However, the viscosity of the asphaltene precipitation system is higher than the aqueous system. The total volume of the nanodroplets formed by the dilution-induced phase separation increases with the flow rate and reaches a plateau after a critical value. This is because the flow boundary layer becomes thicker than the concentration boundary layer due to the increase of viscosity, leading to the vanish of flow velocity on mass transfer at a high flow rate. This work provides a fundamental understanding of the influence of mixing conditions on the dilution-induced phase separation process.</p>

<p>The following exploration used a model oil system to study the growth dynamics of asphaltene particles under 20 different mixing conditions. The morphology of the asphaltene particles, including surface coverage and particle size distribution in submicron scale and micron-scale are affected by the geometry of the microfluidic chamber, orientation, flow rate of mixing and temperature. COMSOL is used to simulate the flow profile to explore the mechanism behind the phenomenon. Shear force and local concentration are the two most important reasons for the influence of mixing dynamics on dilution-induced phase separation. The findings may potentially lead to technology innovation based on controlled solvent addition to achieve reduced use of solvents in the PFT process.</p>