Page 340 - Adsorbents fundamentals and applications

METHANE STORAGE 325

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mainly <2 nm. The BET surface area is typically 2800 m /g, reaching as high
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as 4057 m /g, depending on the activation condition (Wennerberg and O’Grady,
1978). The mechanism of activation is not understood. This carbon was first
marketed by Anderson Development Company as Anderson AX-21, and is now
made by Kansai Coke & Chemicals Company under license from Amoco and is
marketed as Maxsorb. The original process was aimed at petroleum coke as the
precursor. Kansai has developed carbons by using coconut shell (Otowa, 1991).
Similar products using cellulose-based precursors were developed by Westvaco
for ANG (Baker, 1997).
As shown in Table 10.8, the V/V storages in the AX-21 type carbons are
very high. Good methane storage capacities were also reported on KOH-activated
anthracite (Lozano-Castello et al., 2002, who also optimized the activation condi-
tions) and KOH-activated bituminous coal (Sun et al., 1996). Monolithic sorbents
of AX-21 prepared with polymer binders showed extremely high V/V storage
capacities (Bose et al., 1991; Chen and McEnaney, 1995, see Table 10.8). Manzi
et al. (1997) measured V/V storages for Kansai Maxsorb in three forms: powder,
pellets, and disks. The pellets and disks were prepared by compaction alone.
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The bulk density increased with compaction: 0.28 g/cm (powder), 0.32 g/cm 3
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(pellets), and 0.56 g/cm (disks). The V/V storage capacities were, respectively,
110, 100, and 155. For the pellets, the increase in packing density by compaction
was not large enough to offset the decrease in surface area. Comparing these
results, it is clear that it is more effective to use binder (then compaction alone)
for forming monoliths. Based on the data shown in Table 10.8, the DOE target
of 150 V/V delivered methane is well within reach or has already been reached
with the AX-21 type and ACF sorbents.
Effects of chemical modification of carbons on their methane storage capac-
ities have been reported. Kaneko and Murata (1997) modified AX-21 by MgO
impregnation and showed a substantial increase in methane adsorption at 10 MPa
and 303 K. However, as pointed out by Cook et al. (1999), on a V/V basis there
would be no advantage as the micropore volume is significantly reduced. Surface
treatment by oxidation and reduction of activated carbon (BPL) showed essential
no effects on methane adsorption (Barton et al., 1991).
Zeolites have higher packing densities than carbons. The interactions of meth-
ane with zeolites are stronger than that with carbon. For example, the isosteric
heat of adsorption on CaX is 24.6 kJ/mol (Zhang et al., 1991), compared with
13.3 kJ/mol for 11.4 ˚ A slit pores of graphite (Tan and Gubbins, 1990). However,
the isotherms of methane on zeolites level off quickly (Zhang et al., 1991), and
hence do not yield high V/V capacities at 3.5 MPa. The strong adsorption of
water in zeolites is a practical problem. The methane capacities on high-surface-
area polymeric resins and silicas (such as MCM-41) are not high, as shown in
Table 10.8.
The search for sorbents for methane storage remains to be an active research
area worldwide. Methane storage could play a key role in fuel cell technology
if an effective catalyst is developed for direct methane fuel cell (i.e., methane is
fed directly to the fuel cell).