Microscopic Mechanism of Carbon-Nanotube-Modified Ethylene Vinyl Acetate as an Inhibitor for Weakening Wax Crystallization: From the Perspective of Molecular Simulation.

The wax crystallization in crude oil seriously affects the flow of crude oil. As crude oil flow improvers, nanocomposite materials and ethylene vinyl acetate (EVA) are the focus of research; however, the sensitivity of EVA to wax molecules is limited. In this study, we grafted EVA onto carbon nanotubes (CNT) to obtain CNT-EVA, observed the effects of CNT-EVA on the crystallization behavior of straight-chain C30 alkanes using the molecular dynamics (MD) simulations, and thus revealed the microscopic mechanism of CNT-EVA. Three main findings were made: (1) CNT-EVA can better inhibit the crystallization of C30 alkanes than EVA, effectively reduce the peak value of the radial distribution function (RDF) of wax from 29 to 12, and weaken the aggregation degree of C30 alkanes. (2) CNT-EVA has a stronger van der Waals (vdW) interaction with C30 alkanes, making the C30 alkanes have a less end-to-end mean square distance, larger radius of gyration (Rg), and narrower frequency distribution. In this case, the straight-chain C30 alkanes are more difficult to transition from one equilibrium conformation to another. (3) CNT-EVA provides multiple heterogeneous nuclei during the nucleation of C30 alkanes, so that the C30 alkanes first break away from the clusters formed with CNT-EVA and then straighten and close to the wax crystal nucleus to complete the crystallization, further slowing down the wax crystallization rate and reducing the wax accumulation degree. This study reveals the microscopic influence mechanism of the carbon-nanotube-modified EVA on wax crystallization at the molecular level and provides a theoretical basis for using carbon-nanotube-based nanocomposites as a crude oil pour point depressant.