Page 208 - Gas Adsorption Equilibria

194 Chapter 4

representation of data of Tab. 4.1 in Fig. 4.3 is not exactly an adsorption
isotherm referring to a constant sorptive gas concentration, but in the three-
dimensional (p, n)-space a three-dimensional curve of slightly spiral
character within the concentration interval If data at
an exactly prescribed sorptive gas concentration are needed, it is
recommended to aim at 2 or (better) 3 equilibria states in the vicinity of the
wanted state, i. e. with neighboring gas concentrations, and use linear
interpolation.

The volumetric-gravimetric data presented in Tab. 4.1, Fig. 4.3 have been
checked by doing gravimetric-chromatographic measurements simultaneously,
i. e. by analyzing the sorptive gas phase by a gas chromatograph [4.11].
Coincidence always was satisfying, i. e. results of both methods were identical
within error bounds of data which were about 1 % for mole numbers and up to
2 % for sorptive gas concentrations.

To overcome several restrictions of the volumetric-gravimetric instrument
of Figs. 4.1, 4.2 which already have been mentioned at the end of Section 2.1
of this chapter, another instrument for volumetric, gravimetric, and
chromatographic measurements of multicomponent gas adsorption equilibria
has been designed at the authors’ institution in the years (1993–1998). A
schematic diagram of this instrument is given in Figure 4.4 below followed by
a snapshot, Fig. 4.5. The instrument mainly consists on two vessels for gas
storage, a circulation pump, microbalance, necessary valves and tubing and
supply systems for gases and vacuum. Details about design, materials, fittings,
etc. can be found in Ref. [2.5], [2.6], [4.3], [4.4], and [4.8].

The main advantage of this instrument compared to its predecessor,
Figs. 4.1, 4.2, was the use of a magnetic suspension decoupling the gas-filled
adsorption chamber from the microbalance. This allows measurements with
corrosive gases etc., in ranges of pressure (p < 100 MPa) and
temperature (T < 1500 K) which are considerably extended compared to those
where traditional two beam balances can be used. The magnetic suspension
already has been described in more detail in Chap. 3. It is manufactured by
RUBOTHERM-Präzisionsmesstechnik GmbH, Bochum, Germany and can be
used for many different types of high precision weighing experiments [4.9].