Homogeneous nucleation and condensation mechanism of methane gas: A molecular simulation perspective

Liquefied natural gas (LNG) occupies an increasing proportion in global natural gas industry. However, there is a lack of microscopic understanding of methane condensation mechanism. Furthermore, the accuracy of existing nucleation theories for alkane gases remains unclear. Herein, methane nucleation and growth pathways are elucidated and the influence mechanisms of initial conditions on nucleation thermodynamics and kinetics are analyzed using molecular dynamics (MD) simulations. It is discovered that a system with a controlled carrier gas temperature is more consistent with the actual condensation process. For such a system, the heat transfer processes between monomers and clusters make the condensed molecules easier to vaporize and its nucleation and growth stages last longer. Besides, the condensation processes end earlier with higher nucleation rates and liquefaction degrees at high initial pressures and low cooling temperatures. Higher pressures lead to higher temperatures of monomers and clusters during nucleation, while avoiding secondary evaporation. Furthermore, as initial pressures and cooling temperatures increase, the effects of system quenching on pressure relieve, and the fall in pressure mainly depends on the liquefaction degree. Compared with Classical Nucleation Theory (CNT), Internally Consistent Classical Theory (ICCT) has higher accuracy for the nucleation calculation of methane under high-pressure and low-temperature conditions.