Nucleation and crystallization mechanism of heavy hydrocarbons in natural gas under flow field

Abstract

The understanding of the crystallization mechanism of heavy hydrocarbons in natural gas directly determines the modification of nucleation theory and the development of new liquefaction processes. However, the mechanism of heavy hydrocarbon crystallization in the flow field is still not well understood. In this paper, molecular dynamics (MD) simulations were adopted to investigate heavy hydrocarbon crystallization under shear flow, and the nucleation pathway related to crystallization kinetics was explored. The results show that shear rates less than 1 × 108 s−1 promote the rapid growth of heavy hydrocarbon crystals with high final crystallinity, whereas higher shear rates cause crystals to grow slowly ending up with lower crystallinity. Compared with quiescent nucleation, the presence of a shear flow promotes the heavy hydrocarbon stretching and consequently accelerates the kinetic process of phase transition. In addition, molecular orientation fluctuations arising from the flow field are accompanied by the elastic deformation of solid nucleus, increasing the nucleation free energy barrier. Therefore, the nucleation rate of heavy hydrocarbon liquids peaks at the optimal shear rate 1 × 107 s−1. Moreover, a unified pathway for heavy hydrocarbon nucleation under shear flow was proposed.