Molecular Dynamics Study on the Equilibrium and Kinetic Properties of Tetrahydrofuran Clathrate Hydrates

Abstract

Tetrahydrofuran (THF) is an effective promoter of methane hydrates, and itself with water can form clathrate hydrates even without the presence of methane gas. In this work, the stability limit and kinetic properties of THF hydrates were simulated using molecular dynamics (MD) simulations. The change in dissociation temperature of THF hydrates with pressure and concentration of THF in the aqueous phase were well reproduced with MD simulations. The rate of growth of THF hydrates is found to exhibit a maximum value when the liquid-phase THF concentration is about 0.3–0.8 times (depending on temperature) of the THF concentration in the hydrate phase. The existence of some optimal growth concentration explains the preferred lateral growth in experiments. The maximum growth rate is a result of two competing effects: the adsorption of THF molecules to the growing interface, which is the limiting step at low THF concentrations, and the desorption/rearrangement of THF molecules at the interface, limiting step at high THF concentrations. The large cages of structure II (sII) hydrate are fully occupied by THF molecules, regardless of the THF concentration in the aqueous phase, implying a strong stabilization effect of THF molecules to the cage structures of sII hydrates.

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

“…The offdiagonal LJ potential parameters between THF and water were taken from previous study. 34 The solubility of THF in water and the dissociation temperature of THF hydrate from these settings are in good agreement with experiment. (Note that the solubility of THF was underestimated and the dissociation temperature of THF hydrate was slightly overestimated if the geometric mean was used to determine the off-diagonal LJ parameters.)…”

“…Each HL can be further divided into four equally sized sublayers (SL) along the z-direction. 34 Before filling in guest molecules, the empty sII lattice was annealed by heating and cooling with the position of oxygen atoms fixed. This allows for the water molecules to rotate and reconstruct the hydrogen bond network such that a structure of zero net dipole moment can be obtained.…”

“…28 However, the rearrangement of THF at the interface is easier at high temperatures because of enhanced molecular mobility. 34 The different temperature dependence of these two competing factors results in a maximum growth rate at a temperature (about 290 K) that is slightly higher than the dissociation temperature of pure THF hydrate (276 K). It is reasonable to presume that between 276 and 290 K the growth is dominated by the rearrangement of THF.…”

“…This is because that the THF hydrates are not stable without the large cages being occupied. 34 The high concentration of THF results The Journal of Physical Chemistry C Article in fast adsorption of THF to the large cage sites, and thus the growth rate is dominated by slower processes, adsorption of CH 4 at high temperatures and THF desorption/migration at low temperatures. Also shown in Table 4 is the occupancy reported from NMR study at 4 MPa and 292 K. 16 The occupancy from our simulation at 5 MPa and 290 K agrees quite well with the experiment, which again confirms the reliability of our simulation.…”

Modeling the THF clathrate hydrate dynamics by combining molecular dynamics and quasi-elastic neutron scattering

“…There have been numerous fundamental studies on the THF clathrate hydrate. The thermodynamics properties have been the subject of various investigations going from the precise determination of its pressure-temperature phase diagram [17][18][19] to the formation kinetics and mechanisms [20][21][22][23]. The vibrational signatures of the THF clathrate hydrate have been worked over by means of Raman scattering [24][25][26][27][28] and IR absorption [29,30].…”