Page 38 - Gas Adsorption Equilibria

24 Chapter 1

string gas structures. Similar arrangements of admolecules also may occur in
micropores (diameter: [1.18, 1.19], depending of course on
the size of the molecules adsorbed [1.20, 1.21].

At increasing amounts of molecules adsorbed, i. e. if the sorptive gas
pressure (p) is approaching the saturation pressure adsorbates often
form liquid like structures. These may occur as monolayer patches, liquid
films or pore fluids, especially in mesoporous systems, i. e. pores with
diameters 2 nm < d < 50 nm, [1.4, 1.11]. The density of these liquid like
phases (adliquids) may be higher than that of the bulk liquid phase in
saturation state at the same temperature. Also these adliquids can occur at
subtriple temperatures and high pressures where in bulk only solid phases
exist (surface melting [1.22]). An example for this is given by water which
even at temperatures of 77 K seems to form near the surface of mesoporous
solids a few, i. e. 2-4 molecular layers which are in a liquid like state, the
frozen solid state only starting above these [1.23].

Admolecules diffuse within the pore system of a solid sorbent. This process
can last many hours, days, and even weeks, as has been observed for
adsorption of helium in activated carbon (NORIT R1), [1.23]. As a
consequence it can take the same time till thermodynamic equilibrium
between the sorptive gas phase and the adsorbate is realized. In view of
practical and industrial needs it is therefore necessary to introduce the concept
of “technical equilibrium” defined as a state in which the relative uptake
of mass at total mass (m) due to adsorption is less than a given value
typically within a certain time interval typically
These data will allow, together with cycle periods of an industrial process,
one to define characteristic Deborah numbers

for this process. These allow one to approximately describe the “distance” of
an actual state of an adsorption system from its thermodynamic equilibrium
state at given temperatures and pressure [1.2, 1.4, 1.6, 1.15, 1.16]. For
the process is near equilibrium, whereas for essentially
non-equilibrium phenomena in heat and mass transfer should be taken into
account [1.24]. For more information about the kinetics of sorbate phases the
reader is referred to the (still growing) literature [1.4, 1.6, 1.7, 1.8, 1.25].

A few examples of gravimetrically investigated gas adsorption processes
will be graphically presented in the next Chapters 3, 4, 6.