Page 268 - Gas Adsorption Equilibria

254 Chapter 5

In Figure 5.7 the results of combined gravimetric and oscillometric
adsorption measurements of Norit R1 Extra powder in contact with helium
with purity for pressures up to 4 MPa at T = 293 K are
sketched. Data of the reduced mass determined gravimetrically show a
linear decrease with increasing gas density and thus can be used to calculate
the so-called helium volume of the AC given in Tab. 5.2, assuming helium
not to be adsorbed at the conditions mentioned above. The reduced mass
gained by oscillometric measurements shows a nearly linear increase
due to the increase of gas mass included in the free space between the powder
particles and moved along with the oscillations of the disk. From these data,
the Gibbs excess mass of helium adsorbed can be calculated by using
the helium volume of the AC determined gravimetrically and leading by
definition to
As can be seen the deviate only slightly, i. e. within
experimental uncertainties which are about three times the size of the
graphical symbols used, from the abscissa axis. Hence both measurement
methods lead to the same experimental results thus proving consistency of
oscillometric measurements, i. e. the key equation (5.39).

In Figure 5.8 data of gas adsorption equilibria of carbon dioxide on
activated carbon Norit R1 Extra taken at 293 K for pressures up to 7 MPa
(equivalent to gas densities of ca. are presented. Data have been
taken by both oscillometric and gravimetric measurements [5.2]. As the
adsorption capacity of the carbon at high pressures (> 5 MPa) is approaching
saturation, the reduced masses of both types of measurement can
be interpolated by straight lines, cp. Fig. 5.8. These reflect the fact that for
gravimetric measurements at adsorption saturation the buoyancy of the
sorbent-sorbate system increases linearly with increasing gas density reducing
the weight by whereas for oscillometric measurements the mass of gas
moved along with the pendulum in the interstitial space between the
pellets increases linearly by and the mass of the boundary layer
surrounding the pendulum at high pressures – according to calibration
measurements – nearly remains constant [5.7, 5.10].