Page 240 - Gas Adsorption Equilibria

226 Chapter 4

2. Kinetics and Approach to Equilibrium

Use of a microbalance with a permanent recording program which – for
example – is able to take measurements of the weight of the sorbent sample
every second, allows one to get information concerning the kinetics of the
coadsorption process and to observe approach to adsorption equilibrium, cp.
example given in Fig. 4.15. However, it should be mentioned that for mixture
gas adsorption measurements – either volumetrically or gravimetrically or by
VGM – circulation of the sorptive gas mixture is always needed to avoid
concentration differences within the gas phase. That is during adsorption a gas
circulator should be turned on leading to certain, often small but basically not
neglectable dynamic effects in the microbalance reading and sometimes also
in the pressure transducer recordings, cp. remarks given at Fig. 4.15. The mass
uptake curve(s) may serve as a basis to calculate “effective” or “equivalent”
diffusion coefficients of the sorptive gas mixture within the sorbent material
[4.4]. This information may prove useful in industrial adsorption processes.

In practice a method proposed originally by O. Jäntti [2.4, 4.21] some
years ago and recently more elaborated by E. Robens et al. [4.22, 4.23] may
prove useful in gravimetric measurements of adsorption kinetics. It allows one
to curtail the microweighing time i. e. to avoid measurements of the approach
to equilibrium by calculating from 3 data points taken at 3 consecutive times
the asymptotic uptake of mass in the sorbent sample with fair accuracy.

3. Activation Procedure of the Sorbent Material

It is recommended to provide an electric heating system for the adsorption
chamber of any instrument of VGMs. This will allow one to activate the
sorbent material “on site” and thus avoid any pre-adsorption of – for example
– ambient air’s components, especially humidity, if the sample is activated
outside the instrument and then built in the adsorption chamber.

4.2 Disadvantages

VGMs do have all the disadvantages of volumetric / manometric
measurements which already have been discussed at the end of Chap. 2. These
are in brief:

1. Large amount of sorbent material needed.
2. Wall sorption may cause serious errors.
3. Uncertainties of measurements add up in step-up adsorption
(or step-down desorption) experiments.