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Simulation of asphaltene aggregation through molecular dynamics: insights and limitations

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Title: Simulation of asphaltene aggregation through molecular dynamics: insights and limitations
Authors: Headen, T
Boek, E
Jackson, G
Totton, T
Muller, EA
Item Type: Journal Article
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 at ambient conditions. Relatively large systems (~50,000 atoms) and long timescales (> 80 ns) are explored. Where ever possible , comparisons are made to available experimental observations asserting the validity of the models. W hen the asphaltenes are dissolved in toluene, a continuous distribut ion 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 a distinct formation of nanoaggregates, the distributions of clusters is found to be continuous in character. The analysis of the shape of the clusters of asphaltenes suggests that they are generally spherical in character, with the archipelago models favouring longer prolate structures and the continental model tending towards 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. M ixtures 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 a lter the shape of the asphaltene aggregate s , 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 the hi stograms of cluster distributions, the p otential 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 larger than the longest of the simulations reported in the open literature and as an example report a long simulation (0.5 μs) which fails to reach an equilibrium state , suggesting that acceleration techniques , for example , using coarse grained models , are needed to appropriately explore these phenomena.
Issue Date: 3-Jan-2017
Date of Acceptance: 3-Jan-2017
URI: http://hdl.handle.net/10044/1/43754
DOI: https://dx.doi.org/10.1021/acs.energyfuels.6b02161
ISSN: 1520-5029
Publisher: American Chemical Society
Start Page: 1108
End Page: 1125
Journal / Book Title: Energy & Fuels
Volume: 31
Issue: 2
Copyright Statement: © 2017 American Chemical Society. This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes (http://pubs.acs.org/page/policy/authorchoice_termsofuse.html)
Sponsor/Funder: Engineering & Physical Science Research Council (EPSRC)
Engineering & Physical Science Research Council (EPSRC)
BP International Limited
Funder's Grant Number: EP/E016340/1
EP/J014958/1
75195/ICAM15 (All) WS2
Keywords: Science & Technology
Technology
Energy & Fuels
Engineering, Chemical
Engineering
ATOMIC-FORCE MICROSCOPY
ANALYZING PETROLEUM ASPHALTENES
SIZE-EXCLUSION CHROMATOGRAPHY
ANGLE NEUTRON-SCATTERING
CRUDE OILS
MODEL
SYSTEMS
NANOAGGREGATION
REPRESENTATION
MECHANICS
Energy
03 Chemical Sciences
09 Engineering
Publication Status: Published
Appears in Collections:Faculty of Engineering
Chemical Engineering