Molecular Dynamics Simulation of Waxy Crude Oil Multiphase System Depositing and Sticking on Pipeline Inner Walls and the Micro Influence Mechanism of Surface Physical–Chemical Characteristics

Abstract

For ensuring the safe and stable operation of waxy crude oil pipeline transportation, in this research, the molecular dynamics model was established to characterize the deposition and wall sticking behavior of waxy crude oil multiphase system pipeline transportation. The equal density interpolation fitting method was proposed to determine the wall contact angle of simulation results. Through verification, the error between the simulation results and the experimental results measured by Dos Santos et al. (2006) was less than 5%, which showed that the established model was accurate and reliable. Using the established model, the deposition and wall sticking behavior of waxy crude oil nucleated clusters was simulated. It was found that the nucleated clusters would first adhere to the wall surface to form the solidified oil layer. Then, the wax and asphaltene molecules would diffuse to the deposit layer, and the oil molecules in the solidified oil layer reverse diffused to the direction of the oil flow. With the adhering and spreading degree of clusters on the wall surface increasing, the deposit layer gradually aged, and the gelled deposit layer with a higher density and hardness would form. On this basis, the micro influence mechanism of the surface free energy was studied. It was found that the higher the surface free energy, the more hydrophilic the pipeline wall was, and the higher the adhesion degree would be. Moreover, based on the ABF sampling and the potential of mean force calculations, the selective deposition process of waxy crude oil deposited on the sedimentary layer was studied. The micro information on the deposition sites, the binding conformation, and the binding energy of different molecules in clusters deposited on different molecules in sedimentary layer were analyzed. The investigations in this study could provide theoretical support for paraffin removal and control, which could ensure the safe and stable operation of the waxy crude oil production system.

Effect of a Coated Wall on Wax–Asphaltene Molecule Diffusion, Aggregation, and Adhesion Mechanism: A Molecular Dynamics Simulation Study

“…The radial distribution function (RDF) was introduced to reveal the microscopic aggregation properties of wax–asphaltene molecules on the non-coated wall surface. It calculated the ratio of the probability of free wax–asphaltene molecules to the random distribution in the hemispherical shell along the Z -axis (taking the central atom Fe as the center, with the radius of r , the thickness of d r ). , The g ( r ), RDF of different molecules relative to the central Fe atom, can be expressed as where ρ other is the number density of other molecules, Å –3 ; N Fe‑other is other molecular number densities around Fe atoms in the range from r to r + d r along the Z -axis direction, Å –3 ; and r Fe‑other is the radius of coordination ring from r to r + d r , Å. It could be seen from Figure a that the peaks occurred at the same position at different temperatures, but the difference was in the peak values.…”

“…Finally, for the optimized model, it was subjected to a 1000 ps dynamics simulation in the NVT ensemble with temperatures of 318.15, 308.15, 298.15, 288.15, and 278.15 K. The convergence criterion was that the temperature and energy profile of the model fluctuated within 5%, and then, the model was simulated in the NVE ensemble for 400 ps to use the calculated results. The potential functions of all atoms and the parameters of the interatomic interactions in the model were assigned by the COMPASS force field. ,, The initial velocity of each molecule in the starting state was given randomly according to the Boltzmann distribution, and periodic boundary conditions were used in each direction of the model. The inter-particle van der Waals interactions were determined using an atom-based algorithm with a truncation radius of 15.5 Å.…”

“…The potential functions of all atoms and the parameters of the interatomic interactions in the model were assigned by the COMPASS force field. 41,45,46 The initial velocity of each molecule in the starting state was given randomly according to the Boltzmann distribution, and periodic boundary conditions were used in each direction of the model. The inter-particle van der Waals interactions were determined using an atom-based algorithm with a truncation radius of 15.5 Å.…”

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.…”

Molecular Dynamics Study on the Radial Deposition and Adhesion Process for the Waxy Crude Oil Tube Transport System and Effect-Principle of Features in Construction

“…As the clusters piled up layer by layer, the tube transportation would be blocked. Our research group had explored the deposition and adhesion process of the heterogeneous cluster particles of waxy crude oil and analyzed the effect-principle of surface physical–chemical properties . However, for the overall MD simulation and the characteristic comparison of the waxy crude oil homogeneous and heterogeneous clusters, our research group had not carried it out yet.…”