gas hydrate - CCOP

Another important aspect of the present research is to explore the cage-specific distribution of
N 2 , CO 2 , and CH 4 , and their kinetic behavior during the replacement process. Two peaks
representing CH 4 in sI-S (2914 cm -1 ) and CH 4 in sI-L (2904 cm -1 ) continuously decreased
during a replacement period of 750 min, but subsequently no noticeable change occurred in
the peak intensity (Figure 2a). This kinetic pattern can be confirmed also by cross-checking
them with the corresponding Raman peaks of N 2 and CO 2 (Figure 2b and Figure 2c). The
quantitative analysis revealed that, 23% of CH 4 in hydrate is replaced with N 2 , while 62% of
CH 4 is replaced with CO 2 . Accordingly, approximately 85% of CH 4 encaged in saturated CH 4
hydrate is recovered. This recovery rate is expected to change with variations in external
variables such as pressure, temperature, and hydrate particle size. The overall results lead us
to conclude that the replacement of CH 4 with N 2 + CO 2 proceeds more effectively in
crystalline hydrate than is the case when using only pure CO 2 , because N 2 molecules are
confirmed to offer the excellent cage-guest interaction in an unusual configuration. Even for
the simple hydrate systems assessed in the present work, the unique cage dynamics drawn
from spectroscopic evidence should offer new insight into the inclusion phenomena,
particularly, the host lattice-guest molecule interaction and the guest-guest replacement
mechanism.
a b c
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t (min)
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Raman shift (cm -1 )
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t (min)
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Ram an shift (cm -1 )
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t (min)
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R am an sh ift (cm -1 )
1260
Figure 2. In-situ Raman spectra of CH 4 hydrate replaced with N 2 + CO 2 (80 mol% N 2 and 20
mol% CO 2 ) mixture. (a) C-H stretching vibrational modes of CH 4 molecules in the replaced
CH 4 hydrate. (b) N-N stretching vibrational modes of N 2 molecules in the replaced CH 4
hydrate. (c) C=O stretching and bending vibrational modes of CO 2 molecules encaged in the
large cages of the replaced CH 4 hydrate.
CO 2 Sequestration into sII CH 4 Hydrates
The sII hydrate, which is known to be formed by the influence of thermogenic hydrocarbon
and mainly includes oil-related C 1 -C 4 hydrocarbons, was discovered at shallow depth in sea
floor sediment at a few sites such as the Gulf of Mexico and outside the Caspian Sea (Yousuf,
2004; Sassen, 2000). Among major hydrocarbons, C 2 H 6 is specially selected to form a hydrate
with CH 4 . We note that both CH 4 and C 2 H 6 form simple crystalline sI hydrates with water.
However, when they are mixed within the limits of specific concentrations, they act as binary
guests causing the formation of a sII double hydrate (Subramanian, 2000). For experimental
convenience we only treated C 2 H 6, exclusive of propane and butanes, otherwise the use of
multi-guests makes spectroscopic identification too complicated. Accordingly, in the present
work, replacement of the mixed CH 4 + C 2 H 6 hydrate with CO 2 was performed in order to
investigate the swapping phenomena on sII CH 4 hydrate. Figure 3b shows the 13 C HPDEC
MAS NMR spectra of mixed CH 4 + C 2 H 6 hydrates that are replaced with CO 2 molecules at
274.15 K and 35 bars just below the equivalent CO 2 vapor pressure. Three peaks representing
CH 4 in sII-S, CH 4 in sII-L, and C 2 H 6 in sII-L appeared at chemical shifts of -3.95, -7.7, and
100
New Energy Resources in the CCOP Region - Gas Hydrates and Coalbed Methane