gas hydrate - CCOP

a
+
CO 2 CH 4
C 2 H 6
b
CH 4 in sII-S or sI-S
c
CO 2 in sI
C 2 H 6 in sII-L
C 2 H 6 in sI-L
CH 4 in sII-L
CH 4 in sI-L
CO 2 in sI
0 hour
3 hour
9 hour
12 hour
18 hour
24 hour
CO 2 in sII
CO 2 in sII
5 0 -5 -10
Chemical shift (ppm)
1260 1280 1300 1320 1340 1360 1380 1400
Raman shift (cm -1 )
Figure 3. (a) Schematic diagram of CH 4 + C 2 H 6 hydrate replaced with CO 2 . Before the
swapping phenomena occurred, all the sII-S and most of sII-L cages were occupied by CH 4
(red ball). After the replacement was fully achieved, structure changed from sII to sI and most
of the CH 4 and C 2 H 6 (purple ball) molecules were replaced by CO 2 (blue ball). (b) The 13 C
HPDEC MAS NMR of mixed CH 4 + C 2 H 6 hydrate replaced with CO 2 . The samples were
measured at 203 K. (c) The Raman spectra of the mixed CH 4 + C 2 H 6 hydrate replaced with
CO 2 at 123 K. The peak intensity of CO 2 symmetric stretching and bending vibration increase
as the hydration goes to completion.
We note again that CO 2 molecules possess a sufficient enclathration power to be entrapped in
sI-S during change of sII to sI, while the CO 2 occupancy to sI-S of pure CH 4 hydrate is very
difficult to occur. The 30% or more CH 4 recovery enhancement in sII over 64% in sI is caused
by structure transition totally altering the host-guest interactions during swapping.
Furthermore, the naturally occurring sII hydrates contain a larger amount of CH 4 than the
laboratory-made sII hydrates used in these experiments (Sassen, 2000; Brooks, 1984; Milkov,
2004) and thus the actual limitation of recoverable CH 4 in sII hydrate would be higher than
the present outcome of 92%. Accordingly, with we might hope to better use the deep-ocean sII
CH 4 hydrate layers for effectively sequestrating CO 2 without any loss of recoverable CH 4
amount.
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New Energy Resources in the CCOP Region - Gas Hydrates and Coalbed Methane