Biomedical Engineering Reference
In-Depth Information
1
microbalance chamber. In the more favorable isobaric method,
further gas is delivered to the apparatus, as the sample adsorbs, to
maintain constant pressure conditions; however, this is not strictly
necessary and so the system can also be sealed while the sample
reaches equilibrium. Once the sample has reached a sufficient
equilibrium (termed
by Keller and Staudt
[4]), the weight reading and the gas pressure are recorded. A higher
pressure is then applied to the microbalance chamber and the
approach to equilibrium is again monitored. Upon equilibration,
the weight is recorded and this stepwise process continues until
a complete isotherm has been determined. After reaching the
maximum measurement pressure, a desorption isotherm can be
determined by reducing the pressure in a stepwise fashion and
monitoring the sample weight change until equilibration. The
stepwise sequence described above is then repeated in reverse
until vacuum or a sufficiently low pressure has been reached. In
order to calculate the uptake versus pressure, the buoyancy effect
corrections covered in Section 1.5.10 of this chapter must be applied
to the raw weight data.
Another form of gravimetric measurement was presented
recently by Zielinski
technical equilibrium
[5], which allows more rapid screening
of potential hydrogen and methane storage materials than the
microbalance-based method described above. Their straightforward
and cost-effective approach involves the measurement of the mass
change due to pressurization of a detachable pressure cell filled
with adsorbent. Their method was demonstrated by performing
hydrogen and methane adsorption measurements on a 1.8 g
activated carbon sample. The technique is not as accurate as the
typical gravimetric measurements described above and determines
the total amount of gas contained in the sample cell rather than the
adsorbed quantity. However, it has potential as a rapid screening
method, providing large enough sample quantities are available.
et al.
1..
Volumetric Techniques
Volumetric techniques determine the amount of gas adsorbed by a
sample using the real gas law,
1
The isobaric method maintains a constant chemical potential on the external
surface of the material, thus simplifying kinetic data analysis, and ensures that gas
adsorption occurs under more stable conditions of temperature and pressure.
