Kinetic modeling of asphaltene nano-aggregates formation using dynamic light scattering technique

Abstract

Asphaltene is known as a class of polar poly-nuclear aromatic compounds of crude oil which is extracted by its solubility difference in normal heptane and toluene. Precipitation of this hydrocarbon compound by changing in fluid thermodynamic conditions decelerates flow when passing through up-stream and down-stream processes. In order to understand the thermodynamics of the deposition phenomenon, one needs to pay attention to the kinetics of the aggregation of these nano-particles. In this study, the kinetics phenomenon of aggregation of asphaltene clots in toluene is investigated using dynamic light scattering (DLS) technique. The dynamic light scattering results for super dilute solutions showed that at high and low sonication time, the asphaltene is monomeric and polymeric forms, respectively. Moreover, the experiments revealed that the relationship between the amount of sonication energy given to the solutions and the particle size is logarithmic. The DLS spectra obtained from the lower-sonication solutions were decomposed to the four normal distributions using the deconvolution technique, indicating four different clusters in the solution. The ratio of the area of each normal distribution to the area of the spectrum indicates the fraction of the presence of each cluster in the solution. For the lower and the higher concentrations of the asphaltene solution, the percentage of nano-aggregates is between 8%-41% and 4%–51%, respectively. By assuming the asphaltene molecule to be on a coin-shaped plate and forming a π bond with a minimal coinage overlap, the coin model of asphaltene aggregation kinetics has been developed. The concentration of each nano-aggregates in equilibrium solution is calculated according to the fraction of its presence in the medium and the coin model. Then, these equilibrium concentrations are used to calculate the equilibrium reaction constants. The correlation coefficient between the reaction constants and the diffusion coefficients of the clots in the solutions for all reactions was found to be equal to 0.42. This same value indicates that this coefficient is independent of the concentration of asphaltene in the solution and is dependent on the nature of the solution.