Page 25 - Gas Adsorption Equilibria

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4. WHAT IS NOT CONSIDERED

In view of limitations in time and number of printed pages not all of the
experimental methods to measure gas adsorption equilibria, which are
discussed in today’s literature, could be taken into account. To give reason for
this the following remarks should be helpful.

1. Dynamic methods using sorbent material filled columns with open gas
flows are not considered. Their main advantages are that apparatus and
measurements are fairly simple, cp. Tab. 2.1, [0.32, 0.33] and pressure (p)
and Temperature (T) of the sorptive gas can be measured directly.
However, the amount of gas adsorbed cannot be determined directly from
measured data but models of both equilibria and kinetics of the adsorption
column have to be introduced. Naturally, results will depend on the
respective models which makes it difficult to compare them to other
experimental data. However, this method does have the advantage that by a
single, fairly simple experiment information not only on adsorption
equilibria but also on the kinetics of the adsorption process may be gained.

2. Spring balances for gravimetric and/or oscillometric measurements are not
considered. Uncertainties of measurements often are to large and, in case of
oscillations, the flow field of the surrounding gas becomes turbulent, i. e.
the friction forces exerted by the gas on the sorbent sample cannot reliably
be calculated from the Navier-Stockes-equations, cp. Chap. 5.

3. High frequency oscillating disks or rods using sometimes Piezo-effects are
not considered [0.34, 0.35]. Here again the geometry of the oscillating
elements is too complicated to allow calculation of the gas flow field
surrounding it. Hence, the friction force exerted by the gas on the
instrument including the sorbent sample cannot be exactly calculated and
hence masses adsorbed cannot be determined.

4. The zero length column (ZLC) method is not considered here [0.36]. This
is a fairly new and interesting measurement method allowing in principle to
get information of adsorption equilibria as well as of adsorption kinetics, i.
e. diffusion coefficients by fairly simple experiments. However, there are
still open questions about the actual state of the sorbent material filled with
adsorbed gas in the surrounding gas flow. This state often will be kind of
transient non-equilibrium state, i. e. a corresponding equilibrium state is not
directly observed but data are gained by extrapolation which sometimes
may be misleading. Also thermal polarization of the sorbent sample in the
gas flow may occur, i. e. small temperature differences between its front