Asphaltene Molecular Representation: Impact on Aggregation Evaluation

Abstract

Molecular dynamics simulations were used to evaluate the effect of the asphaltene molecular representation on calculations of the aggregate size and aggregation behavior of asphaltene/solvent systems. Three different asphaltene representations were studied, namely, a mixture of four molecules, an island-type molecule and an archipelago-type molecule. Calculations were conducted for pure asphaltene systems and in solutions of n-heptane and toluene. For pure asphaltene systems, the island-type representation allows for the formation of extremely large aggregates, whereas for the mixture and archipelago representations, the aggregates contained up to four molecules. For asphaltene/solvent systems, the mixture representation was consistent with the expected solubility behavior of asphaltenes in both n-heptane and toluene. With this representation, the final configuration in n-heptane consisted of up to fourmolecule aggregates, whereas in toluene, the observed aggregates were dimers, at most. The structural configuration of the island-type molecule misrepresented the aggregation behavior of the asphaltenic phase. The representation of the asphaltene phase, exclusively with the archipelago architecture, also fails to correctly describe the asphaltene aggregation since almost no aggregation was observed. In n-heptane, the asphaltene aggregates were compact and stable with time, and their behavior resembled that of solid particles suspended in a fluid phase. In toluene, the aggregates were of a porous nature, forming viscoelastic networks and reducing the mobility of the fluid phase. The results indicate that the mixture representation is a more appropriate choice for the evaluation of asphaltenic system behavior.