Influence of Temperature on Aggregation and Stability of Asphaltenes. II. Orthokinetic Aggregation

The effect of temperature on asphaltene aggregation has not been investigated at different shear rates. Following our previous work, the effects of temperature and shear rate on the evolution of asphaltene aggregate size distribution in a heptane–toluene mixture are experimentally studied within a Couette flow device. At fixed temperature and shear rate, the average diameter of flocs initially increased with time until it reached a maximum value and then declined to a constant steady state size as a result of the balance between the aggregation and fragmentation. Increasing the temperature resulted in a smaller steady state average diameter. Results of experiments indicated that the agitation rate affects the evolution kinetics of asphaltene aggregates less than temperature. The effect of shear rate on the fragmentation rate was found to be higher than that on aggregation which resulted in a maximum in the average steady state diameter versus shear rate. A fractal geometric population balance model was developed to simulate the experimental results and to extract the kinetics of the aggregation and fragmentation processes. In the proposed model, three fitting parameters, including the collision efficiency, the fractal dimension, and the breakup rate coefficient, were used to match the model on experimental data. The calculated collision efficiency was strongly dependent on temperature. The results showed that the collision efficiency decreases with an increase in temperature in both perikinetic and orthokinetic aggregations. On the basis of the obtained breakup rate coefficients, the floc strength decreases with temperature.