Page 39 - Gas Adsorption Equilibria

1. Basic Concepts 25

In conclusion it can be said that due to the complexity of the interactions of
a) admolecule – surface atoms of sorbent,
b) admolecule – admolecule
and also due to the energetic and geometric heterogeneity of most sorbent
surfaces, adsorbates can have many different structures. Hence a unifying
model that describes all the different structures probably does not exist.
Needless to say that the situation becomes even more complicated if mixture
adsorption phenomena or kinetic processes within adsorbed phases have to be
considered. Consequently, all models for equilibria and non-equilibria states
of single or multicomponent adsorbates presently discussed in the literature
have practical limitations which should be taken into account [1.1-1.5]. To
investigate the structure and properties of adsorbed phases joint efforts of
classical and new experimental measurement methods, refined thermo-
analytic models and molecular simulation models are needed. Exchange of
results, gained by the various approaches, undoubtedly will lead to progress in
understanding the phenomenon of porosity of solids and the design of
industrial adsorption processes.

3. SORBENT MATERIALS

Today there are many different types of materials available designed for
adsorption of molecules from gases and liquids, i. e. having considerable
internal surfaces which are – based on the BET surface * ) - mostly in the range
[1.2-1.3, 1.26].

For industrial purposes the most important sorbents are activated carbons
and zeolites which are available in a great variety of different forms (powder,
pellets, fibers, membranes etc.) having different properties [1.27, 1.28].
Besides many other sorbents are investigated and synthesized today being
based on either natural materials like peat or coal or natural gas and crude oil
leading – for example – finally to porous polymeric materials etc. [1.26].

In view of the abundance of porous materials already available and actual
space limitations, here the purpose of this section only can be to provide the

* ) This surface is usually determined by the amount of adsorbed on the surface of the pores
at the boiling temperature of at 1 atm, i. e. 77 K. Experimental data are correlated by use
of a special adsorption isotherm due to Brunauer, Emmett and Teller, cp. Chap 7, Sect. 3.4.
From this curve the BET surface is determined assuming the molecules form a
monolayer, each molecule occupying an area of Sometimes instead of
Ar at T = 87 K is measured. In this case the respective area should
be used [1.3].