Page 113 - Gas Adsorption Equilibria

2. Volumetry / Manometry 99

provided with a large number of thermocouples allowing to measure changes
in temperature due to transfer processes of the heat of adsorption flowing
from the sorbent/sorbate system to the (isothermal) heat bath covering the
sorption system. We here are not going to present this technique in detail as
this has been done in literature several times, [2.2], [2.25], [2.27], [2.28] and
the literature cited therein. Instead we would like to describe a new type of
calorimeter avoiding thermocouples but using instead an inert gas as sensor
medium. Hence, we will call it in what follows a sensor gas calorimeter
(SGC). In Sect. 5.1 the basic experimental information is provided. The
theory of the instrument is outlined in Sect. 5.2 followed by examples of
calibration measurements. Examples of simultaneous mass and heat of
adsorption measurements are presented in Sect. 5.3. These are compared to
results of analytical calculations of the heat of adsorption via Eq. (2.37) and
discussed to a certain extend.

5.1 Experimental

In 1994, W. Langer proposed to use a newly designed calorimeter for
simultaneous measurements of heats and isotherms of gas adsorption or
desorption processes [2.29]. This instrument may be called a sensor gas
calorimeter (SGC), as the heat flowing during a gas adsorption process from
the sorbent/sorbate system to the sorptive gas will then pass a gas jacket
surrounding the adsorption vessel. This gas, normally He or acts as a
sensor. The heat flux introduced changes its temperature and, as the gas
volume is constant, also its pressure. Actually, the time-dependent pressure
signal can be correlated with the heat flux passing the sensor gas and after
integration over time gives a measure of the total heat of adsorption released
from the adsorption vessel to the thermostat. A schematics of the instrument is
given in Fig. 2.9, followed by a photo of the instrument as designed and built
by W. Zimmermann in Lab PB-A 320, IFT, University of Siegen [2.23].

The sensor gas calorimeter (SGC) basically consists of a classical
volumetric gas adsorption device complemented by two gas thermometers
(cp. Fig. 2.9). The core of the instrument is an adsorption vessel which is
placed within a second vessel, the sensor gas jacket. This jacket vessel is
filled with gas at pressure acting as a sensor via a capillary (1)
connecting the vessel with a difference manometer (P3). Additionally, a
reference vessel also filled with the (same) sensor gas at pressure is
placed in the thermostat and connected via capillary (2) to the manometer
(P3). Upon opening the valves (V7, V7A) the pressures of the
sensor gases in the jacket vessel and the reference vessel are equalized, i. e.
we have Thermal equilibrium at temperature (T*) in the system