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Simulation of asphaltene aggregation through molecular dynamics: Insights and limitations Thomas F Headen, Edo Sicco Boek, George Jackson, Tim S. Totton, and Erich A Muller Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02161 • Publication Date (Web): 03 Jan 2017 Downloaded from http://pubs.acs.org on January 4, 2017
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Simulation of asphaltene aggregation through molecular dynamics: Insights and limitations T. F. Headen1.2, E. S. Boek1,3, G. Jackson1, T.S. Totton4, E. A. Müller1,* 1
2
) Department of Chemical Engineering, Imperial College London, U.K.
) ISIS Neutron and Muon Source, Science and Technology Facilities Council, Rutherford Appleton Laboratory, Harwell Campus, Didcot, Oxon, U.K.
3
) Department of Applied Mathematics and Theoretical Physics, University of Cambridge, U.K. 4
) BP Exploration Operating Co. Ltd, Sunbury-on-Thames, U.K.
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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. 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 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. 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, 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 histograms of cluster distributions, the 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 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.
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1. Introduction Asphaltenes are a solubility fraction of crude oils, defined as being the part of the crude insoluble in n-alkanes (usually heptane) and soluble in aromatic solvents (usually toluene). Asphaltenes are one of the most polar and heaviest (in terms of their molecular size) components of mixtures of carbonaceous fluids and are hence prone to aggregation, flocculation, and deposition. Aggregation is an extremely complex process and may be triggered by reductions in pressure and temperature and/or by blending with other incompatible oils and/or by the incorporation of gases such as CO2. The study of the phase behaviour and the aggregate structures of carbonaceous deposits is an issue of continuous importance throughout the oil industry particularly in the area of flow assurance1. The complexity of the crude oil mixture in terms of number of possible individual components, make the experimental analysis an ongoing tour de force which, although enlightening, has not yet provided the community with unequivocal answers as to the aggregation mechanisms and physical behaviour of asphaltenes in solution. Models for asphaltene aggregation are plentiful, conflicting, and span decades of research. For example, the “Yen-Mullins” model2 is a well-cited interpretation of asphaltene aggregation which predicts that asphaltenes form small and dense nanoaggregates, which in turn aggregate loosely into clusters. While the hierarchical structure of two aggregate sizes seems to explain, for example, the discrepancy between some experimental results3,4 that indicate an aggregation number of ~4 to 8 and X-ray and neutron scattering results5,6,7 which show larger more diffuse clusters, the model has not been universally accepted or confirmed. The effect of concentration on the phase behavior is also non-trivial. Asphaltenes may be present at ppm levels in the live unstable crudes generating troublesome organic deposits, however Boscan crudes are known to contain 17% asphaltenes and are stable with no deposits formed at any stage of production while highly paraffinic crudes with 0.1% or less asphaltene content present serious production issues. It is surprising that in spite of more than half a decade of experimental and theoretical modelling, questions still remain in terms of the description and interpretation of the most fundamental details of asphaltene precipitation. Molecular simulation provides a potential capability to expand our knowledge of the aggregation and deposition mechanisms of asphaltenes through an increased understanding of aggregate structure and energetics and is the topic of our current contribution.
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To be able to successfully simulate systems containing asphaltenes it is vital to have an accurate description of the molecular structure of asphaltenes. This is a challenging step, as asphaltenes represent a whole class of molecules, rather than an individual species. In general an assumption is made that a molecule, or a small group of molecules, that have similar chemical characteristics to asphaltenes, can represent the continuum of asphaltene molecular structures. The crux is to obtain molecular structures that faithfully represent the entire chemical family. There are very few systematic methodologies to propose sensible asphaltene structures and the folklore is based on experimental information that is only marginal and in many instances inconclusive and contradictory. Special mention is made of the Quantitative Molecular Representation (QMR) methods that have been used to propose representative structures of asphaltenes8 to remove some of the empiricism surrounding this aspect of the problem. We first review the state of the art of asphaltene simulation, and the types of molecular structures that have been used to date. Atomistic simulation of asphaltene aggregation is then presented at a size and timescale that is at the limit of what can be reasonably achieved using modern multi-core processors use with GPU acceleration of a parallel MD code. Finally, we discuss the insight that simulation can give regarding the aggregation mechanisms for asphaltene in good and bad solvents, the limitations of this atomistic approach, and potential methods for increasing the size and length scales accessible through simulation.
2. Background 2.1 Atomistically-detailed simulation studies In recent years, as high-performance computing has become more prevalent, there has been an increasing effort to understand the structure of asphaltene aggregation at the molecular level using molecular simulation. The earliest simulation papers9,10,11 were constrained by the computational resources available and as a consequence were limited to considering single model asphaltene molecules or asphaltenes without explicit solvent molecules in what today would be considered rudimentary systems, not because of the scientific content, but rather as result of the inexorable advance of Moore’s law. In addition, only relatively short simulation times (up to ~500 ps) were achievable and therefore dynamic properties such as diffusion or the observation of spontaneous aggregation were not possible. Instead, the early studies tended to use a combination of energy minimization and “manual docking” to explore different
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conformations for asphaltene aggregation and examined data during a short molecular dynamics (MD) run in this conformation. One of the earliest studies, by Rogel11, investigated asphaltene aggregation through molecular dynamics simulation of two model asphaltene structures. Asphaltene aggregates were formed through a combination of molecular mechanics and dynamics to produce reasonable low-energy structures. Solubility parameters where calculated from monomers and aggregates of the model asphaltenes, showing a reduction in the solubility parameter with increasing aggregate size. Some simulations were also carried out with a small number (45) of toluene or heptane solvating molecules. Interaction energies between asphaltene molecules were calculated as -160 kJ/mol and -40 kJ/mol in heptane and toluene, respectively, showing qualitative agreement with the solubility definition, but suggesting a rather large difference ( > 100 kJ/mol) in the solvation energies of the respective solvents. A later study considered a larger diversity of asphaltene model molecules in order to estimate asphaltene densities. The simulations tended to overestimate the densities when compared to experiment12. It should be noted that all the asphaltene structures used had a relatively large number (8-22) of condensed aromatic rings. In another pioneering study, Murgich et al.9 conducted molecular mechanics calculations using relatively large “continental” asphaltene structures (with 24 aromatic rings) in conjunction with resins and showed that aggregation mainly occurred as a result of the stacking of the polyaromatic cores of the asphaltene. A later study using a smaller model asphaltene molecule with 7 fused rings in its poly-aromatic core13 lead to similar findings. The structures used from both these studies were built to be consistent with measured spectroscopic data for asphaltenes from Venezuelan crude and Athabasca bitumen. Pacheco-Sánchez et al. used MD to show that asphaltene aggregation occurs spontaneously even for smaller asphaltene molecules forming dimers, trimers and tetramers during a short 100ps simulation14. The structure of the aggregates formed displayed no overriding structure type: face-to-face, offset stacked and T-shaped aggregates were observed. An asphaltene structure in the solid phase was also examined, comparing the simulated structures to experimental scattering data15. Four different asphaltene structures were chosen from the literature: models extracted from the work of Groenzin and Mullins16, Murgich9, Zajac et al.17, and Speight18 each respectively formulated from analytical study of asphaltenes. All four simulation studies replicated the experimentally determined structure factor to reasonable
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accuracy. In a further study the Groenzin-Mullins asphaltene model was simulated in a box of explicit solvent and the effect of pressure on aggregation was investigated19. Finally, Luis Vicente et al., adopted the same simulation approach to provide the enthalpy of mixing and cohesive energies for the Groenzin-Mullins model molecule in order to determine the Hildebrand solubility parameter, closely matching experimentally determined results20. Simulations of asphaltene dimers with explicit solvent molecules conducted over 100 ps by Carauta et al.21 have shown that the asphaltene dimers bind face-to-face at a distance 3.6 Å in heptane and 5 Å in toluene, matching solubility trends. Furthermore, they show that the effect of increasing temperature (from 323 K to 573 K) is to decrease the distance between the asphaltene dimer. A further study used molecular mechanics followed by quantum mechanical calculations to study the structure of asphaltene dimers in solution using smaller model compounds (~6 aromatic rings) and containing oxygen heteroatoms22. In general, these early studies are limited either by the lack of explicit solvent, or the short timescales over which structural and thermodynamic data is collected. The results are however qualitatively consistent with the solubility behavior of asphaltenes, and indicate that face-to-face stacking of the aromatic cores is a primary structural characteristic − as shown through X-ray diffraction from solid asphaltene samples23. In more recent years, increased computing power has led to the study of larger model systems being simulated for longer periods of time, bringing the simulations a step closer to the experimental systems. Greenfield and co-workers have written a series of papers on the simulation of model asphalt described
as
a
ternary
mixture
containing
model
asphaltenes,
resins
and
maltenes24,25,26,27,28,29,30,31,32 . The initial studies24-26 used model asphaltenes from Groenzin and Mullins16 and from a study by Artok et al. employing pyrolysis-GC-MS, NMR, gel permeation chromatography and MALDI-TOF-MS to study asphaltene structure and derive model asphaltenes33. The other components of the asphalt were modelled as dimethylnapthalene and the alkane n-C20 for the aromatic and saturate fractions of the crude oil, respectively. The simulations were used to estimate the dynamical properties of the model asphalt such as bulk viscosity and diffusion of the molecular components25. This relatively simplistic simulation was able to predict experimentally determined viscosities to within an order of magnitude. A detailed analysis of the orientational structure within the mixture was obtained by considering the angle between the asphaltene aromatic planes as a function of the radial distribution function26. The
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addition of polymer modifiers (polystyrene) was also examined27. More recent studies have used a modified model system. Adapted versions of asphaltene molecules from Mullins2 were used as the asphaltene component – the positions of the alkyl chains on the asphaltene core were changed to reduce the high internal energies caused by steric repulsion30. Polar molecules were also used to more realistically model the aromatic component of the asphalt31. A similar analysis for the dynamical properties of the asphalt were conducted with this improved model system32. Given that there is a continuing level of uncertainty in the general molecular structural characteristics of asphaltenes, i.e., the recognition that asphaltenes have a wide variety of different structures; it makes sense to try and understand the effects that different structural characteristics have on the physical properties. Along this vein the separate studies by Kuznicki et al.34 and Ungerer et al.35 focused on the study of the differences amongst these geometries. Kuznicki et al. used three different molecular structures of asphaltenes, one archipelago structure with two aromatic cores and two similar island molecules – one of them with a carboxylate group (COO-) terminating a side chain34. Simulations of 12 continental asphaltene with 12 archipelago asphaltenes were then conducted in three different solvents (toluene, heptane and water). The study also considered a two-phase system of water and toluene and compared the differing aggregation at the interface with and without the anionic carboxyl group. In the single solvent simulations, similar structural features for aggregation were seen in all solvents. However, the strength of the aggregation varied, indicated by identical peak positions in the radial distribution function but with peak heights, in the order toluene < heptane
