Crystallization and nucleation mechanism of heavy hydrocarbons in natural gas

Abstract

In natural gas liquefaction process, the pre-crystallization of heavy components contributes to the heterogeneous nucleation of methane, which may provide insights for the optimization of traditional processing and liquefaction process. However, the crystallization mechanism of heavy hydrocarbons has not been solved yet and the applicability of existing nucleation models to heavy hydrocarbons remains unclear. To tackle this mystery, molecular dynamics simulations were conducted to examine the crystallization of heavy hydrocarbons thermodynamically and kinetically, and explore the nucleation pathway directly related to the crystallization kinetics. The results indicate that the energy cost of heavy hydrocarbon crystallization is the reduction of Lennard-Jones energy and torsion angle energy, and the crystallization is intrinsically a process of orientational symmetry breaking in heavy hydrocarbon structure. Besides, the orientational symmetry breaking is found to be strongly size-dependent and it is more marked for droplets with less than 1000 molecules. A unified picture was proposed for the crystallization pathway, as manifest in the preordering of surface molecules, surface crystallization and internal layer-by-layer crystallization, respectively. Further, the occurrence of internal crystallization and the orientational symmetry breaking are coupled and competitive. Moreover, a non-classical two-step model for the homogeneous nucleation of heavy hydrocarbon droplets was established.