Methane Nucleation Research Articles

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.

Adsorption isotherms and nucleation of methane and ethane on an analog of Titan’s photochemical aerosols

In planetary atmospheres, adsorption of volatile molecules occurs on aerosols prior to nucleation and condensation. Therefore, the way adsorption occurs affects the subsequent steps of cloud formation. In the classical theory of heterogeneous nucleation, several physical quantities are needed for gas condensing on a substrate like aerosols, such as the desorption energies of the condensing gases on the substrate and the wetting parameters of the condensed phases on the substrate. For most planetary atmospheres, the values of such quantities are poorly known. In cloud models, these values are often approximately defined from more or less similar cases or simply fixed to reproduce macroscopic observable quantities such as cloud opacities. In this work, we used the results of a laboratory experiment in which methane and ethane adsorption isotherms on tholin, an analog of photochemical aerosols, are determined. This experiment also permits determination of the critical saturation ratio of nucleation. With this information we then retrieved the desorption energies of methane and ethane, which are the quantitative functions describing the adsorption isotherms and wetting parameters of these two condensates on tholin. We find that adsorption of methane on tholin is well explained by a Langmuir isotherm and a desorption energy ΔFo = 1.519 ± 0.0715 × 10−20 J. Adsorption of ethane tholin can be represented by a Brunauer-Emmett-Teller isotherm of type III. The desorption energy of ethane on tholin that we retrieved is ΔFo = 2.35 ± 0.03 × 10−20 J. We also determine that the wetting coefficients of methane and ethane on tholin are m = 0.994 ± 0.001 and m = 0.966 ± 0.007, respectively. Although these results are obtained from experiments representative of the Titan case, they are also of general value in cases of photochemical aerosols in other planetary atmospheres.

Исследование процесса роста метанового зародыша в линейном приближении

Исследование процесса роста метанового зародыша в линейном приближении

Molecular dynamics study on the nucleation of methane + tetrahydrofuran mixed guest hydrate

The nucleation of methane (CH4), tetrahydrofuran (THF), and CH4 + THF hydrates are investigated by microsecond MD simulations. These three systems exhibit distinct structural developments in the aqueous phase quantified by the formation of cage structures of hydrogen bonded water molecules. The development of a cluster of cages in the CH4 system is limited by the scarce CH4 molecules in the solution, while in the THF system it is limited by the short lifetime of cages. In the CH4 + THF mixed guest system, a small cluster of caged CH4 molecules can be rapidly stabilized by abundant neighboring cages of THF molecules. Therefore, the induction time of the CH4 + THF mixed guest system is found to be significantly shorter than that of the pure CH4 and pure THF systems. Furthermore, the structure of cages found in the initially formed cage clusters are often different from the typical 5(12)6(n) (n = 0, 2, 3, 4) cages observed in clathrate hydrate systems. The cluster of cages may grow or transform into structure I or II clathrate hydrate in the later stages.

Three-Dimensional Modeling of the Tropospheric Methane Cycle on Titan

The tropospheric methane cycle on Titan including advection, diffusion, condensation, precipitation, evaporation, and surface source is simulated by a three-dimensional general circulation model. The model takes into account the different nucleation of methane in Titan's troposphere, and assumes condensation whenever the saturation ratio (relative humidity) exceeds a critical value of either 100 or 150%. The simulation with the latter critical saturation is shown to be more compatible with the Voyager data. The meridional circulation accounts for a seasonal methane transport from the winter to summer hemisphere in the mid-troposphere, causing a higher supersaturation in late summer and early autumn. Condensation preferentially occurs in this season at 15 km altitude at low latitudes. The occasional character may be explained by the high critical saturation ratio, which cannot be often exceeded. Falling rain evaporates during the descent, so no rain arrives at the surface. The hydrological cycle of methane on Titan takes place mostly within the atmosphere. The latitudinal methane distribution near the surface cannot be reproduced by transport, evaporation, condensation, and combinations of them, indicating some undetermined surface processes affecting the methane distribution. The zonal and meridional circulation is sensitive to the methane distribution.