Page 49 - Gas Adsorption Equilibria

1. Basic Concepts 35

helium neither to be adsorbed nor absorbed, the volume of the sorbent
material (mass: which is impenetrable to the helium molecules, i. e. the
so-called helium volume of the material, can be calculated from a mass
balance of this gas expansion experiment.

viz.

Here is the density of the helium gas which can be
determined from measured data of pressure (p) and temperature (T) normally
by using the ideal gas equation of state (EOS) or a standardized real gas EOS
[1.45].
This procedure is simple and effective and several types of He-gas
pycnometers are commercially available, cp. Tab. 1.5.
However, it does have certain disadvantages which may lead to serious
experimental errors in gas adsorption measurements [1.46-1.49]. This will be
demonstrated by a set of helium gas expansion experiments performed at the
Institute of Fluid- and Thermodynamics (IFT) at the University of Siegen,
Siegen during 1994 – 2002 as follows.

a) Gas expansion experiments were performed in a commercial gas
pycnometer to determine the known volume of a standard calibration
cylinder (stainless steel, electro-polished surface). Gases used were He
*)
(5.0), (5.0), (5.5) for pressures up to 0.14 MPa at T = 298 K.
Results are depicted in Figure 1.5. As can be seen, the volume
of the standardization sample was best reproduced by the
at p = 110 kPa. Neither He-measurements nor lead to
accurate results but show deviations of 1 ‰ and more, uncertainties of data
being about the size of the graphic symbols. The overshooting of the true
value of the sample volume by He-measurements may well be due to ab-
and adsorption of small amounts of helium in/on the walls of the adsorption
chamber (stainless steel) and the surface of the sample. The undershooting
of the sample volume by could not simply be
explained. Real gas effects can be excluded. However, sensitivity of the
pressure transducer system to may have caused a systematic
uncertainty of data [1.48].

* )
The numbers indicate the purity / quality of the gas. Example: 5.5 = 99.9995 %Vol.