Characterization of asphaltenes aggregation and fragmentation in a shear field

Aggregation and breakup of asphaltenes flocs in a Couette flow device are studied for various values of shear rate (G), volume fraction of particles (phi), and solvent composition (ratio of toluene to n-heptane in the solution X-s) The number average asphaltene aggregate size initially increases before achieving a maximum size and then decreases until reaching a steady-state value that reflects a balance between aggregation and fragmentation. Increasing the applied shear rate leads to higher aggregation and fragmentation rates, reducing the steady-state average aggregate size and the characteristic relaxation time required to achieve a steady state. For a fixed value of G, when either phi is increased or X-s is decreased, the steady-state floc size increases. The kinetics of the aggregation-fragmentation process and the attainment of steady state by asphaltenes aggregates were modeled using a population balance approach. The model describes the time evolution of the average floc size using fitted parameters (the breakup coefficient for shear fragmentation, b and the porosity of the aggregates epsilon). Binary breakage was included in calculations and collision efficiency was assumed to be equal to one.