Page 207 - Gas Adsorption Equilibria

4. Volumetric – Gravimetric Measurements 193

As immediately can be seen from (4.29), dispersions become large if
the difference between the molar masses of the adsorbed components
becomes small. Hence this method is not recommended for
example to investigate cosorption phenomena of isotopes. Even for mixtures
of nitrogen and oxygen (air) it may fail as may be of the order of
magnitude of itself. According to our experience the most important
dispersions within the set mentioned in (4,37) are i. e. the
balance measurements and measurements of the equilibration pressure (p) and
thermostat’s temperature (T) are most important to get accurate values of
i. e. dispersions Also it should be noted that for
supercritical sorptive gas mixtures far away from saturation in (4.26) the ideal
gas approximation may be used, i. e. one may assume the compressibility
factor Z = 1 and hence

2.4 Examples

In this section we want to present data for binary coadsorption equilibria of
gas mixtures on activated carbon (NORIT R 1) which have been taken by
combined volumetric-gravimetric measurements. We start with a set of data
which have been measured with the instrument depicted in Figs. 4.1, 4.2. Then
we describe in brief a new type of volumetric-gravimetric instrument
including a magnetic suspension balance allowing also measurements with
corrosive sorptive gases. Equilibria as well as kinetic data taken at this
instrument will be presented. Finally we hint at a commercialized version of
this instrument offered by BEL – Japan company.

A set of data describing the coadsorption equilibria of methane -
nitrogen gas mixtures at T = 298 K in the pressure range p = 0-12 MPa
on activated carbon NORIT R 1 is given in Table 4.1 below [4.11]. It includes
consecutively the sorptive gas pressure (p), the molar fraction of in the
sorptive gas the density of the sorptive gas the number of moles of
and adsorbed per unit mass of sorbent and respectively
and the total mol number of gases adsorbed per unit mass of sorbent
As can be seen from the table, the molar fraction of in the
sorptive gas is changing from about 18.1 % at p = 0.11 MPa to 23.7 % at
p = 7.93 MPa. This is due to the experimental technique applied: As
coadsorption equilibria basically are unknown, the amount and concentrations
of the sorptive gas mixture to be prepared in the storage vessel of the
instrument, Fig. 4.1, are unknown and have to be chosen either according to
approximate calculations from pure component adsorption data using for
example the IAST-formalism [4.10], or by experience. Hence the graphical