Molecular Dynamics Simulation On The Crystallization Process Of Wax Crystal

The microstructure of wax crystal particles in crude oil has a significant influence on its macroscopic rheological properties.Exploring the crystallization and growth mechanism of wax crystal particles has great theoretical significance for studying the macroscopic rheological behavior of oil products.Predecessors have carried out a large number of studies on the microscopic characteristics of wax crystal particles by microscopic microscopy.However,the experimental tests are limited to the micron scale.The crystallization and growth of wax crystals are essentially driven by the molecular dynamics characteristics of their composites.Currently,there is little discussion on the mechanism of wax crystal formation at the molecular level.Because the wax crystal composition of crude oil is complex,the alkane system of ideal white oil is constructed to simulate the wax crystal growth process.The molecular dynamics simulation of the aggregation behavior of the cooling process is carried out by using LAMMPS software.The effects of composition,cooling time,and heat treatment temperature on wax crystal crystallization were discussed,and the mechanism was explained by radial distribution function and intermolecular force.The main contents and results are as follows:In this paper,the pure and multi-component models of alkanes and cycloalkanes with different carbon numbers were constructed by molecular simulation method,and the molecular aggregation behavior in the cooling process was studied.It is proved that when the temperature of each system is controlled at the temperature of its phase transition point,the "RDF peak" representing the degree of molecular aggregation of alkanes decreases with the increase of carbon number,namely,low carbon number molecules tend to form clusters.For alkanes,the length of the carbon chain has a significant effect on the "agglomeration / free" characteristics of molecules in the cooling process.In contrast,the "RDF peak" of the naphthenic hydrocarbon system maintains constant with the increase of carbon number,and the molecules in the system are only "agglomerated".Under the condition of the same carbon number,cycloalkanes are easier to agglomerate than alkanes at their phase transition points.It is also verified that the intermolecular forces of cycloalkanes are greater than that of alkanes in all dimensions.The crystallization simulation of the alkane system was carried out by controlling different cooling time(cooling rate)and heat treatment temperatures.Typically,the cluster standard deviation,mean square displacement,and intermolecular force were tracked,and the effects of cooling time and heat treatment temperature on the crystallization process were discussed.It is found that the cooling rate will affect the "mean square displacement" of the molecules of the system,thus,determine the characteristics of molecular clustering.Conclusively,the faster the cooling rate is,the smaller the mean square displacement of the molecules of the system is,obviously,the lower the degree of molecular diffusion.At a high cooling rate,the molecules tend to form clusters with their closest neighbors.The smaller the volume and volume standard deviation of the clusters,the more uniform and dense the cluster distribution is.The cooling rate will affect the initial structure of systems at different heat treatment temperatures.For example,when the heat treatment temperature is decreased from 355 K to345K,the cooling rate of this system is higher than that of the system that maintaining heat treatment temperature at 345 K.The standard deviation between the volume,and the volume of the clusters are both smaller for the first system.Correspondingly,the distribution of clusters in the first system is more uniform and dense.This paper provides a molecular-scale display and explanation of the microstructure and cohesive behavior of wax crystals,which is very helpful to understand the properties of wax crystals from the physical mechanism.Also,this research provides a primary reference for the study of micro-properties of wax crystals in the future.