Biomedical Engineering Reference
In-Depth Information
8.4.2.1
Methane Physical Adsorption in Carbon
Nanostructures
Methane uptake in CNT bundles has been studied by Stan and co-
workers for rigid nanotubes following the approach discussed
previously [92, 93]. LJ parameters and Lorentz-Berthelot rules
are employed to calculate the ideal uptake curves
) (for
endohedral and interstitial sites) at low coverage for a threshold gas
density and fixed chemical potential and temperature; it is shown
that, in spite of the deep potential energy wall (
s
=
s
(
e
gg
gg
gg
e
= 145 K), the
interstitial sites have low uptake at moderate pressure due to the
molecular size.
GCMC simulations of CH
gg
adsorption in nanotube arrays have
been performed to predict the adsorption excess for both the
endohedral and interstitial sites for different pressure values and
van der Waals gaps between the tubes [17, 94 95]. The decreasing
behavior of the interstice excess adsorption reveals that the outer
uptake saturates while the gas density increases linearly for
compression. The usable capacity ratio (UCR), that measures the
available fuel upon adsorption-desorption cycles with respect
to the available fuel in a storage vessel, is calculated for different
loading pressures as the van der Waals gap varies (see Fig. 8.6): it
is demonstrated that the compressed natural gas (CNG) value is
reached for much lower pressure. Kowalczyk and co-workers [57]
have found that carbon tubular worm-like structures are potentially
advantageous for methane storage.
The isosteric heat of methane adsorption at zero loading in
various CNT arrays has been calculated also by Cruz and co-
workers [25] and various uptake sites have been considered
such as interstitial, surface, groove, intratubular, etc. If allowed,
the interstitial adsorption site is the most favorable followed by
intratubular, groove, and surface sites.
Hydrogen and methane mixtures (hythane) are also considered
for adsorption in CNT arrays, slit-like carbon nanopores, and
mesoporous carbon. This is aimed, for instance, for hydrogen and
methane separation in synthetic gas obtained from steam reforming
of natural gas or for storage of alternative clean fuel on vehicles
[55, 56, 81].
Ideal absorbents such as slit-like pores and CNTs have revealed
to be suitable for hythane storage with respect to the 2010
volumetric stored energy target of 5.4 MJ/cm
4
3
established by the
