Page 177 - Gas Adsorption Equilibria

3. Gravimetry 163

adsorbed and thermodynamic equilibrium conditions, i. e. equality of
chemical potentials of the sorptive and the sorbate phase, the mass of all
components adsorbed (van Ness model). Though this method sometimes
seems to work very well, it cannot be recommended as coadsorption
equilibria are fairly complex phenomena which only for sorptive gas mixtures
with similar molecules approximately can be calculated by this or related
methods (IAST), [3.38].

As a supplement we add pure and binary mixture data taken for adsorption
equilibria in a liquid-solid system. This is to be understood as an example
demonstrating that microbalances can also provide information on liquid
adsorption processes, a field which in view of protein adsorption phenomena
for separation certainly has potential for future development.

Example 1

Adsorption of ternary gas mixtures on activated carbon
(AC) NORIT R1, [3.22, 3.27]. Corresponding data of binary mixtures are
presented in Chap. 4, Sect. 2.4.

Data have been measured gravimetrically (Sartorius) aided by gas phase
chromatography (Perkin Elmer). For AC’s activation procedure and some of
its microscopic properties we refer to Sect. 2.3 of this chapter. Time for
equilibration depended considerably on the methane concentration of the
sorptive gas and ranged between 1 h for high and 8 h for low concentrations.
In Figure 3.26 coadsorption equilibria data of the amounts of
and being adsorbed on AC NORIT R1 at 298 K for sorptive
gas concentrations in the pressure range
p < 6 MPa are sketched. Uncertainties of data are about 2-3 the size of the
graphical symbols chosen. The full lines indicate adsorption isotherms
calculated by fitting the data to a 2-site generalized AI of Langmuir type, cp.
Chap. 7 and [3.27].

Data show that the AC has a good selectivity for although its gas
concentration is only its concentration in the adsorbate is about
Hence this AC can be considered as a sorbent for purification of
low energy gas consisting of from

For practical purposes so-called Gibbs ternary diagrams are very useful to
present ternary adsorption equilibria. An example for this is given in the next
Figure 3.27. It again refers to gas mixtures being adsorbed at
four different concentrations on AC NORIT R1 at T = 298 K and a total gas
pressure p = 1 MPa, [3.27].