Simulation of Asphaltene Aggregation through Molecular Dynamics: Insights and Limitations

Abstract

We report classical atomistic molecular dynamics simulations of four structurally diverse model asphaltenes, a model resin, and their respective mixtures in toluene or heptane under ambient conditions. Relatively large systems (∼50 000 atoms) and long time scales (>80 ns) are explored. Wherever possible, comparisons are made to available experimental observations asserting the validity of the models. When the asphaltenes are dissolved in toluene, a continuous distribution of cluster sizes is observed with average aggregation number ranging between 3.6 and 5.6, monomers and dimers being the predominant species. As expected for mixtures in heptane, the asphaltene molecules tend to aggregate to form a segregated phase. There is no evidence of the distinct formation of nanoaggregates, and the distribution of clusters is found to be continuous in character. Analysis of the shape of the clusters of asphaltenes suggests that they are generally spherical in character, with the archipelago models favoring longer prolate structures and the continental model tending toward oblate structures. The aggregates are seen to be diffuse in nature, containing at least 50% solvent on average, being denser in heptane than in toluene. Mixtures of asphaltenes with different architecture are found to have cluster properties that are intermediate between those of the individual components. The presence of resins in the mixture does not appear to alter the shape of the asphaltene aggregates or their size or density when toluene is the solvent; on the other hand, the resins lead to an increase in the density of the resulting aggregates in heptane. Quantification of these observations is made from histograms of the cluster distributions, potential of mean force calculations, and an analysis of the shape factors. We illustrate how the time scales for complete aggregation of molecules in heptane are longer than the longest of the simulations reported in the open literature and as an example report a long simulation (0.5 μs) that fails to reach an equilibrium state, suggesting that acceleration techniques (e.g., using coarse-grained models) are needed to appropriately explore these phenomena.

Asphaltene aggregation studied by molecular dynamics simulations: role of the molecular architecture and solvents on the supramolecular or colloidal behavior

“…These findings indicate that aggregation among individual asphaltene molecules is primarily driven by π − π interactions, in a hierarchical way. Oppositely, experimental (Varadaraj and Brons 2012;Schulze et al 2015;Subramanian et al 2017) as well as simulation-based evidence (Headen et al 2009(Headen et al , 2017 exist corroborating the supramolecular, Gray's model. These works showed that asphaltene self-interaction depends also on acid-base and polar interactions.…”

“…They can also be responsible for coke deposits, catalysis deactivation, low yield in fuels or chemicals, significant content of sulfur or metals in the output and energy consumption during the upgrading process (Merdrignac and Espinat 2007). Both the chemical composition and the molecular cartography (i.e, how the chemical elements form the various structures) play a key role in the understanding of this problematic phenomenon (Headen et al 2017). In this way, a clear and well-determined structure-function link between molecular structure and aggregation behavior of asphaltenes is highly sought after so that these nasty effects can be avoided, and new refining techniques, asphaltene inhibitors and catalysts can be developed, and so on (Murgich et al 1996;Takanohashi et al 2003;Ortega-Rodriguez et al 2004).…”

Predicting Asphaltene Aggregate Structure from Molecular Dynamics Simulation: Comparison to Neutron Total Scattering Data

“…These structures have been previously studied extensively by atomistic simulation 1,11,27 respectively. These structures have a very similar atomic composition to that obtained from the elemental analysis of the asphaltene sample used in this study, as shown in Table 1.…”

“…The direct comparison of simulated and experimental structures provides novel insight into the extent to which existing simulation strategies successfully predict asphaltene structures and provides pathways for additional study and improvement. Herein we present a preliminary simulation study of three types of asphaltene molecular structures as used in our previous works 26,27 . The structures used have been chosen to exemplify the range of molecular architecture and functionalities present within asphaltenes.…”

Addressing hysteresis and slow equilibration issues in cavity-based calculation of chemical potentials

“…Asphaltenes are prone to aggregation that is driven by changes in condition, such as pressure, temperature or compositional changes by blending/incorporating with other elements such as other incompatible oils or gases. [16][17][18][19][20][21] .…”