Transport Properties of Waxy Crude Oil: A Molecular Dynamics Simulation Study

Abstract

To study the effects of paraffin on viscosity of waxy crude oil and transport properties of small molecules, light and waxy crude oil models were investigated at atmospheric pressure and 293–323 K temperature range using atomistic molecular dynamics simulations. The optimized parameters for liquid simulations all-atom (OPLS-AA) and atomistic polarizable potential for liquids, electrolytes, and polymers (APPLE&P) force fields were employed. The self-diffusion coefficients, viscosity, and paraffin configurations were compared for two oil models and between the two employed force fields. However, the behavior of paraffin molecules predicted by two force fields was quite different. Simulations using the OPLS-AA force field showed crystallization of longer paraffin molecules below 323 K, while simulations with the APPLE&P force field demonstrated a homogeneous mixture down to 293 K. To provide additional validation of the employed force fields, the density, diffusion coefficient, and crystallization of pure alkanes were compared with experimental data. The density and diffusion coefficients of n -C 6 and n -C 14 simulated with the APPLE&P force field were found to be in much closer agreement with the experimental data. The OPLS-AA force field was found to overestimate the crystallization temperature of pure alkanes. Therefore, simulations with the APPLE&P provide more realistic description of the waxy oil structure and transport properties. In this temperature range, the paraffin molecules are homogeneously distributed in the mixture, and viscosity of the system increased by a factor of two compared to light oil. Crystallization of paraffins requires lower temperatures or/and the presence of other components such as nanoparticles or asphaltene molecules to facilitate nucleation.

Influence of Polarizability on the Structure, Dynamic Characteristics, and Ion-Transport Mechanisms in Polymeric Ionic Liquids

“…More recently, significant interest has arisen with respect to the influence of polarizability on the structure and dynamics of soft matter systems. − Broadly, there are three primary methods for explicitly accounting for the effects of electronic polarization in classical simulations, namely, the induced point dipole, fluctuating charge, and the Drude oscillator models. ,, The total electrostatic energy of the induced point dipole system contains the charge–charge, charge–dipole, and dipole–dipole interactions, with the point dipole on a given atom calculated using an iterative self-consistent field (SCF) procedure. − The fluctuating charge model allows the charges on individual molecules to dynamically change according to the electronegativity of each atom in the molecule. − The Drude oscillator model attaches a ghost (Drude) particle to its host (core) particle with a harmonic spring, and then the mass and partial charge of the true atom are divided into the Drude and core portions. ,,− In general, in comparison to the traditional nonpolarizable force field (fixed charge) model, all of the polarizable models introduce additional degrees of freedom to be solved self-consistently during the simulation, which increases the computational expense and serves to explain the relatively fewer number of studies employing such a framework.…”

Crystalline Behavior of Paraffin Wax

“…Molecular dynamics simulation provides the capability of revealing both the structural and dynamic details on the atomic scale, which is complementary to the experimental research. − For example, Glova et al chose n-eicosane as the paraffin model, and they further evaluated the performance of 10 different force fields in describing the physical properties of n-eicosane . Papavasileiou et al and Shahruddin et al mainly evaluated the performance of the coarse-grained force field in describing the physical properties of n-alkane with different carbon atom numbers, and their binary or multicomponent mixture. , Gan et al and Chen et al studied the influence of paraffin crystal (n-alkane binary mixture) on the viscosity of crude oil and the dynamics properties of other molecules. − Zeng et al explored the mechanism of crystallization behavior of the n-alkane binary mixtures (C 20 and C 30 ) and the influence of the crystal morphology on thermal conductivity . However, there is still inadequacy in the theoretical study of the crystallization behavior of paraffin wax.…”

The effect of bitumen molecular fractions on diffusivity and rheology of bitumen under high‐temperature conditions: Molecular dynamics (MD) simulation study

“…[35] Molecular dynamics (MD) simulation has effectively been applied to predict a wide range of parameters in heavy oil and bitumen systems, for example, interfacial behaviour between mineral aggregates and bitumen, [36][37][38] elastic constants of hydrocarbons, [39] heavy oil sticking behaviour on pipe walls, [40] dissolution behaviour of heavy oil in supercritical water, [41,42] density, solubility parameters, and surface energy. [43,44] Chen et al [45] applied MD simulations in a molecular model of waxy oil to calculate its transport properties, for example, the viscosity and self-diffusion coefficients of representative molecules. They also studied the effects of temperature and paraffin molecules on transport coefficients, especially diffusion coefficients and viscosity.…”

“…Chen et al [ 45 ] applied MD simulations in a molecular model of waxy oil to calculate its transport properties, for example, the viscosity and self‐diffusion coefficients of representative molecules. They also studied the effects of temperature and paraffin molecules on transport coefficients, especially diffusion coefficients and viscosity.…”