Non-equilibrium Molecular Dynamics Simulation Of Self-Aggregation Behavior And Viscosity Of Asphaltene Molecules

In the context of reduction of conventional crude oil,it is a pivotal concern for petroleum industry to know how to make the most use of heavy crude oil.However,the problems of self-accumulation and sedimentation caused by asphaltene components in heavy crude oil have been blocking the process of exploration,production,storage,transportation and processing in the petroleum industry.Therefore,the study of the intrinsic mechanism of the self-aggregation of asphaltene molecules is conducive to boost the availability of heavy crude oil.Although researchers have done a lot of experimental study on the aggregation behavior of asphaltenes owing to the limitations of experimental means,they cannot directly observe the aggregation process and morphology of asphaltenes molecules.With the development of computational simulation,molecular dynamics simulation could be used to study the interaction mechanism of asphaltene molecules on the atomic or molecular level.The results of molecular simulation are not only a fruitful supplement to the experimental results,but also can obtain the molecular mechanism of the self-aggregation behavior of asphaltene molecules.A string of molecular models of asphaltene have been constructed,based on the molecular structure of asphaltene observed by AFM.A series of non-equilibrium molecular dynamics(NEMD)simulations were carried out by means of LAMMPS software and PCFF force field in order to delve into the effect of shear field on the aggregation behavior and the viscosity of the system.The major research contents and conclusions are as follows:In the first part of the paper,non-equilibrium molecular dynamics simulations of five island asphaltene molecules were carried out at a temperature of 300 K,a pressure of 1 bar,and a shear rate of 1E-7/fs.By studying the effects of toluene solvent concentration,polycyclic aromatic cores structure,aliphatic side-chain of asphaltene molecules,heteroatoms in asphaltene molecules,and shear field on the aggregation behavior and viscosity of asphaltene molecules,the results show that asphaltene has the best dissolution effect as the concentration of toluene solvent is 20Wt%,.When the structure of polycyclic aromatic cores is the cluster-shaped aromatic ring,the intermolecular molecules are dominated by ?-? interaction.When the structure of polycyclic aromatic cores is the chain-shaped aromatic ring,the intermolecular molecules are assembling behaviors caused by structural obstruction.Under the condition of 20Wt% toluene solvent,the viscosity of asphaltene molecules with only polycyclic aromatic cores is between 75 c P and 120 c P.The viscosity of asphaltene molecules of polyaromatic hydrocarbons with increased alkyl side chains increase by 0.5 times.With asphaltenes of polyaromatic hydrocarbons of heteroatoms and alkyl side chains advancing,the viscosity increases by twice.Aliphatic side-chain and heteroatoms can make asphaltene self-aggregates take on different sizes and numbers.The shear field will lead to the rupture and reorganization effect of asphaltene aggregates.In the second part of the paper,non-equilibrium molecular dynamics simulations of four archipelago asphaltene molecules was carried out at a temperature of 300 K,a pressure of 1 bar and a shear rate of 1E-7/fs.The effects of the polycyclic aromatic cores,bridged aliphatic side chains,and bridged polyaromatic hydrocarbon cores of island-type asphaltene molecules on the self-aggregation behavior and viscosity of asphaltene molecules were studied.When the polycyclic aromatic cores are the chainshaped aromatic ring structure,the asphaltene molecules have higher viscosity and complex self-assembling morphology.The length of the bridged aliphatic side-chain and the number of rings of the bridged polycyclic aromatic cores are positively correlated with the viscosity of the asphaltene molecule.In this paper,according to a series of studies on asphaltene molecules with different structures in the shear field,the mechanism of dispersion and sedimentation that may reduce asphaltene viscosity and self-aggregation is proposed.