Catalogue of Plausible Molecular Models for the Molecular Dynamics of Asphaltenes and Resins Obtained from Quantitative Molecular Representation

Computer simulation studies aimed at elucidating the phase behavior of crude oils inevitably require atomistically-detailed models of representative molecules. For the lighter fractions of crudes, such molecules are readily available, as the chemical composition can be resolved experimentally. Heavier fractions pose a challenge, on one hand due to their polydispersity and on the other due to poor description of the morphology of the molecules involved. The Quantitative Molecular Representation (QMR) approach is used here to generate a catalogue of 100 plausible asphaltene and resin structures based on elemental analysis and 1H – 13C NMR spectroscopy experimental data. The computer-generated models are compared in the context of a review of previously proposed literature structures and categorized by employing their molecular weights, double bond equivalents (DBE) and hydrogen to carbon (H/C) ratios. Sample atomistic molecular dynamics simulations were carried out for two of the proposed asphaltene structures with contrasting morphologies, one island-type and one archipelago-type, at 7 wt% in either toluene or heptane. Both asphaltene models, which shared many characteristics in terms of average molecular weight, chemical composition and solubility parameters showed marked differences in their aggregation behavior. The example showcases the importance of considering diversity and polydispersity when considering molecular models of heavy fractions

Crude Oil Fouling: Fluid Dynamics, Reactions and Phase Change

In the present study, the fluid dynamics and phase behavior of crude-oil fouling in a closed-end heat-exchanger is studied. The deposition process associated with fouling is assumed to be due to two routes: asphaltene precipitation, and a two-step chemical reaction. The SAFT-γ Mie theory is employed to describe the phase behavior of an asphaltene-containing crude oil system, which comprises pseudo-components (C13 ~ C20+). The predicted phase equilibrium constants are used to quantify the asphaltene precipitation rate. A computational fluid dynamics framework is then used to simulate the fouling process, accounting for the multiphase flow dynamics, heat transfer, and the two deposition routes. Fouling is simulated due to the two routes individually and in concert. In the latter case, it is found that the interaction of the two routes is due to the fouling layer adhering to the heatexchanger walls, which influences heat transfer from the hot walls to the cooler oil in the bulk. The delicate interplay between heat transfer and fluid dynamics, which accompanies the flow, leads to enhancement and suppression of chemical reaction- and precipitation-driven fouling, respectively, and an overall rise in the fouling rate

Extension of the effective solid-fluid Steele potential for Mie force fields

Molecular simulation of fluid systems in the presence of surfaces require computationally expen-sive calculations due to the large number of solid–fluid pair interactions involved. Representingthe explicit solid as a continuous wall with an effective potential can significantly reduce thecomputational time and allows exploring larger temporal and spatial scales. Different ana-lytical expressions can be found in the literature depending on the structural characteristicsof the solid and the approximations adopted in the derivation. The well-known (10-4-3) Steelepotential is one such analytic expression that faithfully represents the effective solid–fluid inter-actions for homonuclear crystalline solids with hexagonal lattice symmetry. However, this andmost of the effective potentials found in the literature have been developed for fluids and solidsinteracting exclusively through Lennard-Jones potentials. In this work, we extend the Steelemodel to obtain the effective wall–fluid potentials for Mie force fields. We perform moleculardynamics simulations of coarse-grained fluids modelled via the SAFT force field approach inthe presence of explicit and implicit surfaces to compare structural and dynamic properties inboth representations. Also, we study the adsorption of ethane into slit-like pores with explicitand implicit surfaces via grand canonical Monte Carlo simulations. We explore the validityand the improvement in the simulation performance as well as the limitations of the proposedexpression

Fluid-solid phase transition of n-alkane mixtures: coarse-grained molecular dynamics simulations and diffusion-ordered spectroscopy nuclear magnetic resonance

Wax appearance temperature (WAT), defined as the temperature at which the first solid paraffin crystal appears in a crude oil, is one of the key flow assurance indicators in the oil industry. Although there are several commonly-used experimental techniques to determine WAT, none provides unambiguous molecular-level information to characterize the phase transition between the homogeneous fluid and the underlying solid phase. Molecular Dynamics (MD) simulations employing the statistical associating fluid theory (SAFT) force field are used to interrogate the incipient solidification states of models for long-chain alkanes cooled from a melt to an arrested state. We monitor the phase change of pure long chain n-alkanes: tetracosane (C24H50) and triacontane (C30H62), and an 8-component surrogate n-alkane mixture (C12-C33) built upon the compositional information of a waxy crude. Comparison to Diffusion Ordered Spectroscopy Nuclear Magnetic Resonance (DOSY NMR) results allows the assessment of the limitations of the coarse-grained models proposed. We show that upon approach to freezing, the heavier components restrict their motion first while the lighter ones retain their mobility and help fluidize the mixture. We further demonstrate that upon sub-cooling of long n-alkane fluids and mixtures, a discontinuity arises in the slope of the self-diffusion coefficient with decreasing temperature, which can be employed as a marker for the appearance of an arrested state commensurate with conventional WAT measurements

A multiscale method for simulating fluid interfaces covered with large molecules such as asphaltenes

The interface between two liquids is fully described by the interfacial tension only for very pure liquids. In most cases the system also contains surfactant molecules which modify the interfacial tension according to their concentration at the interface. This has been widely studied over the years, and interesting phenomena arise, e.g. the Marangoni effect. An even more complicated situation arises for complex fluids like crude oil, where large molecules such as asphaltenes migrate to the interface and give rise to further phenomena not seen in surfactant-contaminated systems. An example of this is the “crumpling drop” experiments, where the interface of a drop being deflated becomes non-smooth at some point. In this paper we report on the development of a multiscale method for simulating such complex liquid–liquid systems. We consider simulations where water drops covered with asphaltenes are deflated, and reproduce the crumpling observed in experiments. The method on the nanoscale is based on using coarse-grained molecular dynamics simulations of the interface, with an accurate model for the asphaltene molecules. This enables the calculation of interfacial properties. These properties are then used in the macroscale simulation, which is performed with a two-phase incompressible flow solver using a novel hybrid level-set/ghost-fluid/immersed-boundary method for taking the complex interface behaviour into account. We validate both the nano- and macroscale methods. Results are presented from nano- and macroscale simulations which showcase some of the interesting behaviour caused by asphaltenes affecting the interface. The molecular simulations presented here are the first in the literature to obtain the correct interfacial orientation of asphaltenes. Results from the macroscale simulations present a new physical explanation of the crumpled drop phenomenon, while highlighting shortcomings in previous hypotheses

SAFT‑γ force field for the simulation of molecular fluids. 5. Hetero Group coarse-grained models of linear alkanes and the importance of intramolecular interactions

The SAFT-γ Mie group-contribution equation of state [Papaioannou J. Chem. Phys. 2014, 140, 054107] is used to develop a transferable coarse-grained (CG) force-field suitable for the molecular simulation of linear alkanes. A heterogroup model is fashioned at the resolution of three carbon atoms per bead in which different Mie (generalized Lennard-Jones) interactions are used to characterize the terminal (CH3–CH2–CH2−) and middle (−CH2–CH2–CH2−) beads. The force field is developed by combining the SAFT-γ CG top-down approach [Avendaño J. Phys. Chem. B 2011, 115, 11154], using experimental phase-equilibrium data for n-alkanes ranging from n-nonane to n-pentadecane to parametrize the intermolecular (nonbonded) bead–bead interactions, with a bottom-up approach relying on simulations based on the higher resolution TraPPE united-atom (UA) model [Martin; , Siepmann J. Phys. Chem. B 1998, 102, 2569] to establish the intramolecular (bonded) interactions. The transferability of the SAFT-γ CG model is assessed from a detailed examination of the properties of linear alkanes ranging from n-hexane (n-C6H14) to n-octadecane (n-C18H38), including an additional evaluation of the reliability of the description for longer chains such as n-hexacontane (n-C60H122) and a prototypical linear polyethylene of moderate molecular weight (n-C900H1802). A variety of structural, thermodynamic, and transport properties are examined, including the pair distribution functions, vapor–liquid equilibria, interfacial tension, viscosity, and diffusivity. Particular focus is placed on the impact of incorporating intramolecular interactions on the accuracy, transferability, and representability of the CG model. The novel SAFT-γ CG force field is shown to provide a reliable description of the thermophysical properties of the n-alkanes, in most cases at a level comparable to the that obtained with higher resolution models