Page 216 - Gas Adsorption Equilibria

202 Chapter 4

(1, 2). Then the question can be raised whether the state of equilibrium
attained will be the same after reversing the order of components 1 and 2, that
is, first to realize adsorption equilibrium with pure component 2 and then
adding component 1 thus approaching an equilibrium state Z (2, 1). This
problem is by no way trivial in view of so-called “ink bottle effects” and
similar phenomena in micro- and mesoporous solids [2.2]. It also is
fundamental for applying the principles of multicomponent thermodynamics
[4.15], [4.16] to multicomponent adsorption phenomena. To check whether Z
(2, 1) = Z (1, 2) a special experiment in the system AC Norit R 1,
(component 1), (component 2) at 298 K was performed. First the
activated carbon was exposed to pure for 20 hs. Then was added and
the approach to equilibrium monitored for another 20 hs. The respective data
of the total mass adsorbed are shown in the lower curve of Figure 4.9 below.
After this, the experiment was repeated with components reversed.
Corresponding data are shown in the upper curve of Figure 4.9. The gas
pressure approached in both experiments was about As can be
seen from the curves in Figure 4.9, in both experiments the same final
equilibrium state is approached asymptotically, i. e. after 40 hs. This also has
been confirmed by analysis of adsorbates’ partial loads of and which
turned out to be the same in both experiments [4.4].

The instrument sketched in Figure 4.4 above also can be used to
investigate changes of adsorption capacities of sorbent materials due to pre-
adsorption of gases or vapors. An example for this phenomenon is given
below. Figure 4.10 shows data of Gibbs excess amounts of carbon dioxide
adsorbed on dry molecular sieve (MS) NAX13 (Linde, UOP) at 323 K
and the corresponding data for this sorbent material including 43 mg/g MS
and 56 mg/g MS pre-sorbed water respectively. As can be seen from
the data, the adsorption capacity of the molecular sieve for is reduced by
40 % due to the pre-adsorbed water, the amount of which is only slightly
changed during the adsorption process of [4.15].

The instrument for combined volumetric – gravimetric measurements of
binary coadsorption measurements, Figs. 4.4, 4.5 has been redesigned and
automated by BEL-Japan Inc., Osaka, Japan. It is now available commercially
from BEL-Japan Inc. or Rubotherm GmbH. A schematic diagram of this
instrument including a magnetic suspension balance is shown in Fig. 4.11 a,
followed by a snapshot taken at lab PBA-0126/1 of IFT, Fig. 4.11 b. This
instrument is used today by several institutions in Japan, Germany and the
USA. Examples of measurements of coadsorption equilibria of not only gases
but also (subcritical) vapors are given in the literature [4.16 a, b].