Effect of temperature on heavy hydrocarbon crystallization in natural gas

Abstract

Understanding the mechanisms of nucleation and crystallization of heavy hydrocarbons in natural gas is significant for controlling alkane crystals, improving nucleation theory, and developing new processing technologies. However, the mechanism of heavy hydrocarbon crystallization at different temperatures is unknown. In this study, the effects of temperature on heavy hydrocarbon crystallization were investigated thermodynamically and kinetically using molecular dynamics simulations. Furthermore, the temperature dependence of the nucleation pathway was elucidated. The results suggest that a crystalline monolayer will first form on the supercooled heavy hydrocarbon droplet surface, followed by layer-by-layer bulk crystallization. As subcooling increases, the droplets and potential energy indicate four states: frustrated crystallization, rapid crystallization, surface crystallization, and vitrification. A temperature range of 130–110 K is ideal for heavy hydrocarbon droplet nucleation. Furthermore, bulk crystallization is determined by the competition between induced nucleation by the initial surface crystalline layer and homogeneous nucleation based on internal nucleation sites. And this depends on the degree of supercooling and determines the final morphology and molecular orientation distribution of crystals. The pathway of heavy hydrocarbon nucleation shifts from a non-classical two-step to a classical process as supercooling increases.