Page 172 - Gas Adsorption Equilibria

158 Chapter 3

the system, a thermostat and thermometers and manometers. The sample
vessel should be connected to a gas chromatograph allowing one to analyze
the concentrations of the sorptive gas in equilibrium. Helium at about 400 K-
500 K is recommended as carrier gas. Materials of vessels, tubes, and valves
should be chosen according to recommendations already given in Chap. 2,
Section 2.1. Further information on design and operation of such an
instrument can be found in [3.16, 3.20, 3.41].

An instrument for gravimetric measurements of multicomponent gas
adsorption equilibria has been designed and operated during 1993-2003 at the
IFT, University of Siegen. It was part of a multipurpose instrument for
different types of gas coadsorption measurements. A photo of it is included in
Chap. 4, cp. Fig. 4.2.

4.2 Theory

We consider the microbalance installation in Fig. 3.22 and assume that a
certain mass (m*) of sorptive gas mixture at known mass concentrations
i = 1...N) has been supplied to the (originally evacuated) adsorption
vessel with volume The vessel includes a certain amount of sorbent
material of mass fixed to the balance. Adsorption of gas is enhanced by
the circulation pump thus avoiding concentration differences within the
vessel. If adsorption equilibrium is attained, a sample of the sorptive gas
mixture may be taken and its (molar and mass) concentrations be
determined in a gas chromatograph. In order to determine the masses of all
components adsorbed i = 1...N) from this information, we proceed as
follows: First we calculate the total mol number of the sorptive gas from
its equation of state (EOS)

Here we assume that the real gas factor or compressibility (Z) of the sorptive
gas in the adsorption equilibrium state considered,

is a known function of pressure p, temperature T, and molar concentration
i = 1...N. The volume of the sorptive gas inside the adsorption vessel