(325b) Asphaltene Mesoscale Aggregation Behavior in Organic Solvents: A Brownian Dynamics Study

Crude oil is generally defined as a naturally occurring mixture consisting of very complex hydrocarbons. A fraction of this complex mixture is called asphaltene, which contains the heaviest and most polar components of crude oil and is responsible for many serious problems during oil production, transportation, and refining primarily due to their aggregation and deposition. For this reason, asphaltenes science has received significant attention in recent years, to shed light on the main parameters controlling their stability and to develop predictive models, as the related literature offers tremendous advances in the asphaltene molecular and colloidal properties, the asphaltene phase behavior, and interfacial science of asphaltenes.

In 2010, Mullins demonstrated that the island structure is the most dominant and stable architecture for asphaltene molecules. Based on this model, known as the Yen-Mullins model, asphaltene molecules consist of a single polycyclic aromatic hydrocarbon (PAH) enclosed by alkanes (~1.5 nm). These molecules can form nanoaggregates (~2 nm) with an aggregation number of 4-10 capable of forming clusters (~5 nm) of nanoaggregates with an aggregation number around eight. According to this model, aggregation of asphaltene molecules occurs mainly because of London dispersion forces whose principal site is the PAH ring system at the center of the asphaltene molecule. However, the peripheral alkanes produce steric (structural) repulsion which is responsible for the colloidal stability of asphaltenes in crude oil solution. This model has been widely supported by recent experimental and theoretical studies.

Although, molecular dynamics and dissipative particle dynamics simulation techniques have provided a clear picture of both the molecular scale mechanisms leading to asphaltene aggregation and the intermolecular forces governing this phenomenon, their computational deficiency restricted their application to aggregating systems in the order of a few nanometers where the structural evolution of asphaltene particles can hardly proceed towards the metrics larger than the cluster of nanoaggregates. Here, we aim at studying asphaltenes aggregation at mesoscales wherein the nanoaggregates are forming the primary building blocks of large aggregates. For this purpose, we used the existing detailed information− which are taken from molecular simulations provided by the literature− on the size of asphaltene nanoaggregates and their intermolecular forces to construct our primary particles representing the asphaltene nanoaggregates. Then, Brownian dynamics (BD) simulation approach was employed to investigate the asphaltene aggregation behavior in organic solvents at different volume fractions (ϕ = 1-7%) and comparisons are made with respect to available experimental results.

Our simulation results confirmed that asphaltene nanoaggregates form small clusters with an average aggregation number and gyration radius of 7.47 and ~4 nm, respectively, that are generally oblate spheroid in shape. These clusters can establish fractal aggregates with a fractal dimension of 1.9-2.04. This finding concurred well with the hierarchal self-assembly model of Yen-Mullins and the available experimental observations. The calculated self-diffusion coefficients of asphaltene in the employed solvents at the volume fractions of (Ï• = 1-7%) ranges from 1.5×10^-10 to 0.07×10^-10 m²/s which is also in a good agreement with the experimental values reported in the literature.