Page 176 - Gas Adsorption Equilibria

162 Chapter 3

As can be recognized from eqs. (3.55a, b, 3.56a, b) accurate measurements of
gas concentrations or, equivalently are very important in
order to get small dispersions of adsorbate’s mass Due to experience it
also can be said that accurate measurements of the system’s pressure (p) and
temperature (T) are essential, whereas the influence of the real gas factor (Z)
and its uncertainly often is rather small. Given MSDs of gas concentrations
of microbalance measurement and of all the
other quantities to be measured at the orders of magnitude mentioned in Sects.
2.3, 4.3 of Chap. 2, one can expect relative uncertainties of adsorbed masses
for binary systems (N = 2) of about 2 % and for ternary systems
(N = 3) of ca. 5 %. Often the less adsorbed component poses a real problem as
it is very difficult to determine small changes of gas concentrations with
satisfying accuracy [3.20, 3.22, 3.27]. An example for this experimental
problem is given in the next section.

4.4 Examples

Coadsorption equilibria of gas mixtures on porous solids normally are
measured using the volumetric/manometric method described in Chap. 2,
Sect. 4, cp. also [3.42-3.44]. Despite the fact that purely gravimetric-
chromatographic measurements of coadsorption equilibria today are scarce, it
should be emphasized that this method still has the advantage that, contrary to
the manometric procedure, the approach to equilibrium can be accurately
observed from the microbalance signal. Hence it can be checked whether or
not the adsorption system investigated is in equilibrium or still in a transient
non-equilibrium state. For this reason several gravimetric-chromatographic
measurements of binary (N = 2) and ternary (N = 3) coadsorption equilibria of
non-corrosive gases on both activated carbons and zeolites have been
performed during the last 10 years at IFT, University of Siegen. In the
following we first present adsorption equilibria of a ternary gas mixture on
activated carbon. Data were taken by gravimetric measurements of the total
mass adsorbed and analyzing the sorptive gas phase, cp. Sect. 4.2. This
method is cumbersome but reliable from an experimenter’s point of view. To
avoid lengthy and therefore expensive gas analyzing procedures it has been
suggested in the literature [3.45], to complement gravimetric measurements
with a thermodynamic model allowing one to calculate from the total mass