Page 294 - Handbook Of Multiphase Flow Assurance

Experimental and computer study of the effect of kinetic inhibitors on clathrate hydrates 293

Translational and vibrational spectra
The frequency distribution functions obtained by Fourier transformation from time correla-
tion functions can be compared to experimental spectra as these become available. Simulated
translational (motion) and librational (rotational oscillation) spectra were compared for ice,
empty hydrate lattice and methane hydrate using MD simulation (Tse et al., 1983a). It was
noted that long-range electrostatic interactions are important for correct calculation of a trans-
lational spectrum. Vibrational (translational oscillation) spectrum for methane guest was par-
titioned into contributions from large and from small cavities.
In a MD simulation of xenon sI hydrate vibrational and librational spectra (Tse et al., 1983b)
were calculated. Simulators were able to predict the gap separating the two modes of motion
in spectra obtained by infrared (IR) spectroscopy.
Tse reports (Tse et al., 1984) that the vibrational frequency distribution functions for the
host molecules are broadly similar for hydrates enclathrating different guest molecules. The
oscillatory motions correlation time for tetrafluoromethane of 5 ps are remotely comparable
with experimental value of 13.6 ps (Davidson et al., 1977). The same modeling work reports
the existence of preferred orientations for ethylene oxide and cyclopropane molecules in large
sII cavities. A recent polarized Raman spectroscopy work (Tomoko, 1996) reports the exis-
tence of preferred orientation for CO 2 molecules in hydrate cages.
In 1987 the simulated vibrational spectrum of a sII hydrate lattice with krypton guests (Tse
and Klein, 1987) was found to be similar to that of a sI host lattice; the translational spectrum
was different. The frequencies of krypton translation in hydrate cavities were calculated to be
9 and 34 wavenumbers. These values were compared to the experimental frequency of guest's
rattling motion in β-quinol hydrate of 36 wavenumbers.
One of the most recent MD works from Hokkaido University (Itoh et al., 1996) studies
the stretching and bending frequencies of CO 2 in large and small cavities of sI hydrate. The
Kumagai (Kumagai et al., 1994) potential was used. The higher frequencies in small cavities
were attributed to guest-host van der Waals interactions. Only qualitative agreement with
experimental spectra was achieved.

Stability of gas hydrates

As simulated by Rodger (1991, 1992) the stability of hydrate lattice analyzed in terms of
mean square displacement (MSD) of oxygen atoms of waters and cavity radial distribution
function (CRDF) increased with the number of cavities filled with guests. In one simulation
melting of the empty hydrate lattice was observed. The importance of guest-host repulsive
interactions for hydrate stability was emphasized.
Tanaka and Kyohara (1993a) have studied the contributions of guest-host interactions, the
free energy of guest vibration, and change in energy of host vibrations through coupling with
guest on the stability of hydrate. They found that the presence of guests causes an increase
in most of the vibrational frequency peaks. These shifts are reported to be thermodynami-
cally unfavorable to hydrate stability as the chemical potential of water in host lattice was
decreased.