Shear-induced growth of asphaltene aggregates

A model system was studied to investigate asphaltenes coagulation in toluene-heptane solvent mixtures. The effects of shear rate (G ), solids volume fraction (ϕ), and toluene-to-heptane ratio (T:H) in solvent mixtures can be identified as the theme of the first part of the dissertation. The evolution of aggregates size distribution in an aggregating and fragmenting suspension of asphaltene particles is studied experimentally in a Couette device and theoretically using a population balance approach. Initially, aggregation rapidly depletes the primary particles, forming larger aggregates. The larger aggregates become more susceptible to breakage and the primary particles diminish until aggregation and fragmentation balance each other and a steady state is attained. Raising the shear rate, G, contributes to higher collision rate, but, eventually, fragmentation mode also increases. As a result, higher G gives smaller steady-state average aggregate sizes and reduces time before the steady state is reached. Higher ϕ and lower T:H produces aggregates with larger average size.; The second part of the thesis work involves experiments that were performed in a bench scale stirred tank using a photometric dispersion analyzer (PDA) to examine asphaltenes aggregation. It was shown that the presence of maltenes in the bitumen solution retards the aggregation kinetics significantly, though the steady-state aggregate size reaches the same size as found with asphaltenes alone. Aggregates formed at a certain shear rate are broken at a higher shear rate, and can reform (re-grow) if a lower shear rate is applied. But, the aggregates do not attain the previous size. Asphaltenes flocculation in toluene-heptane mixture is not fully reversible and the final, steady-state floc size depends on the shear history. Cyclic- and tapered-shear flocculation appear more advantageous than constant-shear flocculation for production of denser, and more compact particles beneficial for faster settling.; Free settling test results show that the asphaltene aggregates are highly porous, higher shear rate produces more compact and denser aggregates, and porosity increases with aggregate size. Larger aggregates with smaller effective density exhibited higher settling velocities due to their fractal structure and flow through effect. The dynamic evolution of boundary fractal dimension exhibits the presence of shear-induced breakage/restructuring. The lower values of 3-D fractal dimension (D < 2) indicate tenuous aggregate structure with very low space-filling capacity.