Page 82 - Gas Adsorption Equilibria

68 Chapter 1

This is an algebraic equation for the unknown mass of the sorbate phase,
as are measured and hence well known quantities. Hence we
get by algebraic inversion of (1.58)

from which numerical values of can be calculated once have
been measured. Of course, as the explicit structure of the EOSs (1.50), (1.53)
often is not known, it is necessary to use series expansions, for example virial
expansions in pressure (p) and mass ratio To determine respective
coefficients quite a number of -data will be needed. Hence
measurements will be quite laborious and take time. A few combined
dielectric-calorimetric measurements of the adsorption of carbon dioxide
(4.5) on Wessalite (DAY) at T = 298 K have been performed during 2001-
2003 at IFT, U Siegen. A schematics of the experimental installation is shown
in Figure 1.23. It consists of a gas storage vessel combined with an adsorption
chamber provided with thermopiles and including a capacitor which holds the
sorbent material. Gas expansion experiments allow to determine the Gibbs
surface excess in the helium volume approximation. Combined dielectric-
calorimetric measurements allow one to determine the absolute mass
adsorbed. Results of 5 measurements are shown in Figure 1.24.

The absolute masses are always somewhat larger than the Gibbs excess
masses as it should be according to the underlying physical concepts. How-
ever, we should emphasize that the results presented only are preliminary, as
the data base is fairly small and much more data would be needed to show the
absolute – and the Gibbs excess-adsorption isotherm – over a considerable
range of pressure in order to finally answer the question in what range of
pressure the later is still a good approximation of the former. Experiments in a
so-called sensor gas calorimeter (SGC) are presently being performed at IFT
and should be published in due time, cp. also Chap. 2, Sect. 5.

Differences in the volumetrically and gravimetrically measured Gibbs
excess masses depicted in Fig. 1.24 are partly due to different activation
procedures of the zeolite samples used in the experiments. This of course
should be avoided, but as a matter of fact happened in our experiments and
therefore is brought to reader’s attention.