Page 241 - Gas Adsorption Equilibria

4. Volumetric – Gravimetric Measurements 227

Commercially available instruments for VGMs usually need several grams of
sorbent material to perform reasonably accurate measurements.
For coadsorption equilibria of binary mixtures of unsaturated gases (with or
without a supercritical carrier gas), wall adsorption in instrument’s tubes,
valves, and vessel may cause a serious problem. Calibration experiments in
the empty instrument, i. e. without a sorbent, may solve the problem but not
always. In these cases additional measurements of sorptive gas’ concentrations
are indispensable.
In VGMs uncertainties of measurements also add up. Hence it is
recommended not to do more than 3 steps in a step up (or step down) sorptive
gas pressure experiment.

4. Isomeric Sorptive Gas Components

An intrinsic disadvantage of VGMs is that they fail for sorptive gas mixtures
with isomeric components as for example CO). In this case the resulting
equations become linearly dependent and can not be solved algebraically.
Hence chromatographic determination of sorptive gas concentrations is
necessary. If these quantities are available, masses of the adsorbate’s
components can be calculated as outlined in Sect. 2.2.

In practice an instrument for volumetric-gravimetric coadsorption
measurement requires a considerable amount of equipment and expertise to
operate it properly. Hence the instrument will be quite expensive, especially if
it is designed and equipped for automatic measurements.

4.3 Comparison of Densimetric-Gravimetric and
Densimetric-Volumetric Binary Coadsorption
Measurements

Prior to comparing densimetric-gravimetric measurements (DGMs) of
binary coadsorption equilibria, cp. instrument Fig. 4.14, and densimetric-
volumetric measurements (DVMs), cp. instrument Fig. 4.16, it seems to be
useful to compare both methods with basic volumetric-gravimetric
measurements (VGMs) discussed in Sects. 1, 2. For this it should be observed
that the instruments sketched in Figs. 4.14, 4.16 are designed to use premixed
sorptive gas mixtures. Consequently, adsorption equilibria will be attained at
different sorptive gas concentrations i = 1,2), depending on the selectivity
of the sorbent material used. If adsorption equilibria data at prescribed gas
concentrations i = 1,2) are needed, instruments in Figs. 4.14, 4.16 have to
be equipped with two separate gas storage vessels for pure components (1,2).