Page 236 - Gas Adsorption Equilibria

222 Chapter 4

Numerical examples easily show that in order to get accurate masses of
adsorbate it is essential to have precise measurements of the
system pressure and temperature at hand. Also, as already has been
mentioned in Sect. 3.3, the molar masses of the sorptive gas
components should be “fairly different”, i. e.
If this condition does not hold, uncertainties (4.72) may well exceed 10 %. In
this case again additional measurements of sorptive gas concentrations in the
equilibrium state considered are necessary [4.17].

Densimetric-volumetric measurements have been performed recently at
our Institute determining again coadsorption equilibria of on AC
D47/3 at T = 293 K for pressures up to 1.4 MPa, cp. Sect. 3.4. Results are
identical within experimental uncertainties with those received by
densimetric-gravimetric measurements, Sect. 3.4. Hence discussion of this
method can be postponed to Sect. 4 where all experimental methods outlined
in this chapter will be evaluated from both the experimental and the theoretical
point of view.

3.6 Volumetric-Densimetric Measurements of Wall
Adsorption

In this section we would like to address in brief the problem of wall
adsorption. By this we understand the phenomenon that any gas or liquid
contacting the inner surfaces of vessels, tubes, valves etc. of any device will
be adsorbed to a certain, though normally only tiny, amount on them. This
phenomenon first has observed in gas thermometers at very low temperatures
i. e. low gas pressures [4.20]. Here it added considerably to the uncertainties
of measurements. The same can be true for volumetric or volumetric-
gravimetric adsorption measurements at low pressures or if only very small
amounts of sorbent material – say 100 mg or less – are available, cp. Chap. 2.
Therefore it sometimes may be necessary in setting up mass balances for
adsorption measurements to take this phenomenon into account. A possibility
to do this is to measure it. This on principle can be done by using the
instrument sketched in Fig. 4.20 below. It mainly consists on three parts: a gas
)
*
storage vessel (SV) equipped with a piston to change its volume and
*) For this volume changing vessel it is recommended to use a sapphire cell equipped with a
smoothly movable piston. Detailed information about such a device is available at the