Page 337 - Adsorbents fundamentals and applications

322 SORBENTS FOR APPLICATIONS

about 230 unit volumes of natural gas at 1 bar are compressed to one unit volume
of storage container, often designated as 230 V/V storage. For the same driving
range, the size of the CNG vessel is at least three times the volume of a gasoline
tank. Moreover, compression to 200 bar requires a four-stage compressor. For
simplicity and reduced cost, a storage system using a single-stage compressor
is attractive, which puts a pressure limit of about 5 MPa. This limit poses no
problem for ANG since adsorption of methane on known sorbents has reached
an isotherm plateau at this pressure range. A benchmark pressure of 3.5 MPa
(35 bar) has been widely adopted for comparing different sorbents for ANG use.
A number of practical problems need to be considered in ANG technology:
higher hydrocarbons and impurities in the natural gas (Mota, 1999), mass transfer
rates, and heat effects. The first problem has been solved by the use of guard
beds, which is well-known in the PSA technology. The guard bed traps these
impurities during charging and releases them during discharge. The mass transfer
and heat effects (heating during charging and cooling during discharge) have
been studied and are well understood (Mota et al., 1997; Biloe et al., 2001;
Mota and Rodrigo, 2002; Biloe et al., 2002). These problems have also been
minimized with clever designs of monolithic sorbent and storage vessels (Cook
et al., 1999). An excellent review of ANG has been given by Cook et al. (1999).
Reviews on sorbents for ANG are also available (Mullhaupt et al., 1992; Menon
and Komarneni, 1998).
For sorbent development, the U.S. Department of Energy set the target of
◦
150 V/V deliverable capacity at 3.5 MPa and 25 C (Wegrzyn et al., 1992). This
deliverable amount is the total amount between 35 bar and 1 bar (isothermal),
including the gas phase.
To date, the best sorbents are carbons. The theoretical limits for storage in
carbons as well as the optimal pore dimension of the carbon have been stud-
ied extensively (Tan and Gubbins, 1990; Mastranga et al., 1992; Cracknell and
Gubbins, 1992). Tan and Gubbins (1990) used GCMC and DFT to calculate
methane adsorption in model porous carbons for a wide range of pore sizes, and
determined that the optimal dimension for a slit pore is 11.4 ˚ A. This is the center-
to-center distance between the two graphite layers. Thus the free spacing is less
than 11.4 ˚ A, and is ∼8 ˚ A. Mastranga et al. (1992) reached the same conclusion
as Tan and Gubbins. Using this slit width and assuming that each two slits are
separated by a single layer of graphite, the theoretical limits (e.g., at 209 V/V)
are substantially higher than the DOE target. Myers and Glandt (1993) concluded
that the theoretical limit is 220 V/V.
The two key factors for methane storage are micropore volume and sorbent-
packing density. Optimal storage will occur when the micropore volume is
maximized. However, for fast mass-transfer rates, some mesopores and macro-
pores are also needed as feeder pores. The micropore volumes of carbons are
correlated well with the BET surface areas measured with N 2 at 77 K. In fact, a
linear correlation was obtained for the amount of methane adsorbed at 3.5 MPa
◦
and 25 C and the BET surface area based on the data on 35 commercial carbons
(Mullhaupt et al., 1992).