Page 116 - Gas Adsorption Equilibria

102 Chapter 2

Temperature gradients during heating up and cooling down of the sample as
well as the time of activation should be chosen carefully as they may have
considerable impact on the properties of the activated sorbent material. After
this pre-treatment procedure is completed the whole instrument has to be
evacuated. Then sensor gas should be provided to the gas thermometers (1,2).
After this the instrument is placed within the thermostat. When thermal
equilibrium is reached valves (7,7A) are closed. At this moment the reading
of the difference pressure gauge corresponding to equal pressures on both
sides of the instruments membrane is a stable signal providing the baseline of
any pressure signal measurement and hence also of any calorimetric
measurement.

After filling the gas reservoir with an appropriate amount of sorptive gas
the adsorption process can be started by opening valve (V5). The sorptive gas
is flowing from the reservoir via capillary (3) into the (evacuated) sorption
vessel. During this process it is important to make sure that the velocity of the
gas flow is small enough to allow thermal equilibration of the gas temperature
to thermostat temperature (T*). Otherwise it would become difficult to set up
a concise energy balance of the adsorption process as the enthalpy of the
incoming sorptive gas would not be known exactly. As soon as the sorptive
gas reaches the sorbent material prepared inside the adsorption vessel
adsorption occurs and the heat of adsorption is released to the sorptive gas and
then transferred via the wall of the adsorption vessel to the sensor gas inside
the sensor gas vessel, cp. Fig. 1. As the temperature of the sensor gas is
increased due to this heat transfer so is its pressure compared to the
pressure in the reference vessel Hence there is a pressure difference
which is monitored and registered by the difference
pressure manometer (P3). Additionally, pressures at manometers (P1) and
(P2) also are monitored allowing to determine the amount of gas adsorbed i. e.
to evaluate the adsorption isotherm.

A numerical example for a pressure difference signal recorded during a
calibration process of an SGC is given in Fig. 2.11. It shows an increase in its
first part reaching a maximum value after which it again approaches the
baseline i. e. its initial value indicating thermal equilibrium in the overall
system. After this, sorptive gas again can be added to the adsorption vessel
increasing thus the inside pressure (p) and starting a new adsorption process
which finally will lead to a new adsorption equilibrium at some higher
pressure