Molecular Dynamics Study on the Growth Mechanism of Methane plus Tetrahydrofuran Mixed Hydrates

Abstract

Molecular dynamics (MD) simulations are performed to analyze the dominating factors for the growth of CH 4 + THF mixed hydrates, and the results are compared with the growth of single guest CH 4 and THF hydrates. While CH 4 hydrate has a type I crystalline structure, the presence of THF in the aqueous phase results in the growth the type II structure hydrate. Compared to THF hydrates, the presence of CH 4 in the system enhances the dissociation temperature. The growth rate of CH 4 + THF mixed exhibits a maximum value at about 290 K at 10 MPa. The growth rate is found to be determined by two competing factors: (1) the adsorption of CH 4 at the solid−liquid interface, which is enhanced with decreasing temperature, and (2) the migration of THF to the proper site at the interface, which is enhanced with increasing temperature. Above 290 K, which is about 10 K higher than the dissociation temperature of pure THF hydrate, the growth of cage can proceed only when a sufficient amount of CH 4 is adsorbed at the interface. The growth rate is dominated by the uptake of CH 4 at the interface, as in the case of pure CH 4 hydrate. Below 290 K, the growth is not much affected by the presence of CH 4 . Instead, the growth rate is determined by the rearrangement of THF molecules at the interface, as in the case of pure THF hydrate.

Tetrahydrofuran (THF)-Mediated Structure of THF·(H2O)n=1–10: A Computational Study on the Formation of the THF Hydrate

“…Based on computer simulation methods [23], several hypotheses have been proposed to illustrate the microprocess of hydrate nucleation and growth, including a labile cluster hypothesis [24,25], a local structuring hypothesis [26], a blob mechanism [27], and a cage adsorption hypothesis [28]. Further, Wu and coworkers [29] have carried out a molecular dynamics study of the growth of the THF-CH 4 binary hydrate, and found that the growth rate is dominated by the adsorption of CH 4 to the growing interface and the migration and rearrangement of THF at the interface. Alavi and coworkers [30] have studied the hydrogen bonding of THF in binary sII hydrates via molecular dynamics simulations, and suggested that the number and nature of the second guests can affect the probability of hydrogen bonding of THF.…”

Aggregation Behavior of Asphalt on the Natural Gas Hydrate Surface with Different Surfactant Coverages

“…Then, the simulations are performed in the NPT ensemble at 270 K and 20 MPa (at this stage, all of the molecules of hydrate are free). ,, The total calculation time is 200 ns. The Nosé–Hoover thermostat is used to control the temperature, and the Parrinello–Rahman barostat is used to control the pressure. − The temperature and pressure time constants are 4 and 10 ps, respectively. The compressibility is 4.5 × 10 –4 MPa –1 .…”

Hydrophobic Hydration of Xenon and Tetrahydrofuran in Amorphous Solid Water

“…The solvent water structure is enhanced near the hydrophobic entity, leading to a net loss of entropy . The structural ordering of liquid water to form cages around hydrophobic solute species is explored using various experimental techniques. − In this respect, the interactions of water with hydrophobic additives are also discussed in terms of the clathrate hydrate (CH) formation. − CHs are inclusion compounds in which small guest molecules are confined in cages formed by hydrogen-bond networks of water molecules. In contrast with crystalline ices, an open frame of polyhedral water cages is formed in CH because of the predominance of pentagonal rings relative to hexagonal ones, and the cage is stabilized by interactions of water with guest molecules. − Because of the low solubility of hydrophobic species, the development of CHs from liquid water requires a very long time, ranging from hours to days .…”

“…The ASW film formed at temperatures <70 K is characterized by a microporous structure; the effective surface area of porous ASW films has been determined from the TPD spectra of trapped N 2 molecules in the film interior. In contrast with this picture, it has been depicted that hydrophobic molecules are not simply physisorbed on the surface of ASW but are compressed and caged microscopically by the hydrogen-bonded water molecules. , The bulk properties of ASW can be modified by water-miscible polar molecules because they reduce the water activity via disruption of the water’s hydrogen-bond network. , Alcohols are known to be the most popular hydrate formation inhibitors, , but some weakly polar molecules like cyclic ethers rather act as promoters. , …”