Page 101 - Gas Adsorption Equilibria

2. Volumetry / Manometry 87

Analysis of numerical data shows that measurements of pressures (p, p*), i. e.
values of are the most crucial for accurate measurements of

followed of course by temperature measurements and those for basic
data of the equipment and of the sorbing material To
achieve useful results, i. e. limiting values of variances
should hold as follows [2.7]:

2.4 Example

In Figure 2.2 equilibrium adsorption data for carbon dioxide on
zeolite Na 13X (Linde, UOP) are presented for temperatures 298 K and 303
K. The mol numbers of the Gibbs surface excess amounts per unit mass of
sorbent are depicted as function of the sorptive gas pressure and temperature.
Relative uncertainties of measurements are about
The subcritical isotherms are in the range of pressure
measured of Type I – IUPAC classification [2.20].

However, the adsorption isotherms exhibit Type II structure if the
saturation pressure is approached, cp. Chapter 3. Prior to adsorption,
the zeolite has been “activated” at 623 K for 3 hours under vacuum conditions
(p < 15 Pa). Some characteristic data of the zeolite also have been measured
in our laboratory [2.19], namely the BET surface at 77 K being
(UOP-date: the total pore volume being the He-
expansion density being

Equilibration times varied from 15 minutes at low pressures up to (nearly)
1 hour at the higher pressures considered, i. e. This
information was gained from time dependent temperature signals produced by
the sorbent sample and gained simultaneously with adsorption equilibria
measurements [2.23], cp. Sect. 5.