Page 134 - Gas Adsorption Equilibria

120 Chapter 3

2. GRAVIMETRIC MEASUREMENTS OF PURE GAS
ADSORPTION EQUILIBRIA (N = 1)

2.1 Two Beam Balances

2.1.1 Experimental

Two beam microbalances can be used to measure adsorption equilibria of
pure non-corrosive gases like (He, Ar, Ne, etc.) on porous solids.
Also the uptake (or release) of moisture from (or to) humid air in
hygroscopic substances can be determined fairly well (Hiden-Balance). A
schematic diagram of an instrument designed for this purpose is given in
Figure 3.1 followed by a photo taken in lab PB-A 318 of IFT, 2003. As
clearly can be seen, the two vessels beneath the balance including a sorbent
sample and a ballast or tare fixed to the balance respectively are placed in a
thermostat. The vessels are also provided with tubes for gas supply and
evacuation as well as with manometer(s) and thermometer(s) to measure the
pressure (p) and temperature (T) inside. A gas circulator can be helpful in
enhancing adsorption and reducing waiting times for adsorption equilibria but
is not mandatory for pure gas experiments. On the other hand the circulator
may cause problems for fine grained sorbent materials like powders or
activated carbon fibers which may be removed or simply blown away from
the sorbent vessel. By this the sorbent may be distributed all over the
instrument and causes problems – especially in valves.

For design and building a gravimetric adsorption instrument the technical
hints regarding choice of materials etc. given here should be observed, cp.
also Chap. 4 (2.1), [3.20].

A gravimetric adsorption experiment normally is started by mounting a
sample of the sorbent material in the microbalance. Then activation by
evacuation, heating and flushing with helium follows. At the end the balance
again has to be evacuated to measure the mass of the sorbent sample. After
this the sorptive gas can be introduced leading to a change of the balance’s
recording due to
a) adsorption of gas on the sorbent’s surface and
b) buoyancy effects of the sample in the surrounding gas.
As the sorption process of the gas in the sample sometimes may last hours and
even days, one has to define “technical equilibrium” as a practical term to
finish the measurement.