Page 325 - Adsorbents fundamentals and applications

310 SORBENTS FOR APPLICATIONS

Experimental Techniques and Pitfalls. The volumetric technique is the most
commonly used method for measuring high-pressure isotherms (Yang, 1987). The
apparatus involves a sample cell and a reservoir section. A pressure drop is mea-
sured when the pressurized reservoir is connected to the evacuated sample cell.
The dead volumes of both compartments are measured by helium displacement
using ideal gas law. Any additional pressure drop over that of He is attributed to
adsorption. The amount adsorbed can be obtained from the dead volumes and a
P-V-T relationship. Desorption can be measured by reversing the process.
This technique is prone to artifacts, particularly for H 2 . Leakage through fit-
tings and valves is a particular problem for H 2 because of its small molecular size.
A number of pitfalls were discussed by Tibbetts et al. (2001). In a typical high-
pressure apparatus (at 100 bar), the leakage of a few psi is equivalent to 1 wt %
adsorption. A second source of error is temperature variation, by either ambient
temperature fluctuation or heat of adsorption. In a typical setup, an artifact of
◦
2.6 wt % adsorption can be caused by a 1 C temperature rise (Tibbetts et al.,
2001). Another source of artifact stems from the thermodynamic principle that a
chamber being pressurized experiences a temperature rise. The temperature drop
upon returning to the initial temperature can cause an artifact in adsorption. These
errors could be associated with several reported results (Tibbetts et al., 2001). A
similar temperature effect could be caused by the exothermic heat of adsorption
(Darkrim et al., 2002). Here the temperature rises quickly, followed by a slow
return to the ambient temperature (and the associated pressure drop), which could
be erroneously taken as adsorption. Using a low value for the heat of adsorption
for H 2 , the temperature rise due to adsorption would amount to a couple degrees.
The gravimetric technique is relatively trouble-free because it measures the
weight changes. However, because of the high sensitivities, careful calibration is
necessary for changes in buoyancy and friction loss from gas flow upon changes
of gas composition and temperature. High-pressure gravimetric (TGA) systems
are available, but costly. With flow systems, contaminants in the supply gas could
lead to artifacts because they could accumulate on the sorbent. This would not
be a problem with static or volumetric systems due to the finite amount of gas
that is in contact with the sorbent.
Temperature-programmed desorption (TPD) was used by Dillon et al. (1997)
and Hirscher et al. (2001). This technique is also relatively trouble free, although
its accuracy depends on the detector. Mass spectrometry was used in the work
cited above.

Activated Carbon. Among the commercially available sorbents, activated car-
bon adsorbs the largest amounts of hydrogen. Activated carbon has been consid-
ered for on-board applications (Schwartz, 1993; Chahine and Bose, 1994; Hynek
et al., 1997; Dillon and Heben, 2001). The isotherms of hydrogen on a typical
activated carbon are shown in Figure 10.26. The low-temperature isotherms on a
super-activated carbon are shown in Figure 10.27 (Zhou and Zhou, 1998). These
amounts (∼0.5 wt % at 100 atm and ambient temperature) are clearly not useful
for meeting the DOE targets for on-board storage.