Page 198 - Gas Adsorption Equilibria

184 Chapter 4

To measure binary coadsorption equilibria by the volumetric-gravimetric
method one proceeds as follows: A sorbent sample of 1 g – 3 g and
appropriate counterweights, typically lead or silver balls, are placed to the
buckets of the microbalance. Then the sorbent is “activated” by exposing it to
helium gas at higher temperatures, i. e. 433 K for activated carbons, 673 K for
zeolites and inorganic molecular After cooling down and evacuation
(< 10 Pa) the adsorption chamber is prepared for an adsorption experiment.

Next a gas mixture of known molar concentrations i = 1,2),
pressure temperature T, is prepared in the storage vessel of volume
Then the valves between the storage and the adsorption vessels are opened and
the circulation pump is turned on. The adsorption gas mixture is circulated
until adsorption equilibrium is reached, i. e. the gas pressure (p) within the
system and the signal from the microbalance remain “constant” after
stopping the gas circulator. In practice this state often has to be defined in
technical terms, i. e. by stating that the microbalance reading does not change
more than a certain prescribed amount within a certain period of time
Such a state may be called “technical equilibrium” which for i.
e. hour and i. e. tends to physical or exact
thermodynamic equilibrium. We experienced very quick adsorption processes
where equilibrium was reached within a few minutes, but also very slow ones
which took days and even weeks as for example the penetration process of
helium into porous materials, Chap. 1. Hence it is a matter of practice to
choose proper values of and according to the nature and behavior of
the adsorption system and the accuracy of the data needed. From recorded
data p, T, i = 1. 2, the masses of both components adsorbed
can be calculated as will be outlined in the next section.

If the mass of the sorbent material attached to the microbalance is
very small, it may happen that the change of pressure caused by gas
adsorption is tiny compared to that caused by pure gas expansion from volume
to In this case it is helpful to place a considerable amount
of physico-chemically identical sorbent material on the bottom of the
adsorption chamber to increase the mass of gas adsorbed and also the change
of pressure which is due to gas adsorption, cp. Fig. 4.1.

The experiment itself may be continued by adding more gas to the storage
vessel and expanding it again to the adsorption vessel to realize a new state of
adsorption equilibrium at a higher pressure. As uncertainties of measured
quantities are additive, it is not recommended to perform this step up (or

*) Two beam microbalances have to be cooled during this process as their thermal range of
operation normally is restricted to temperatures below 350 K.