Page 228 - Gas Adsorption Equilibria

214 Chapter 4

these mixtures always additional measurements of the sorptive gas
concentrations by gas-chromatography or mass-spectrometry are
recommended.

As already mentioned in Sect. 2.3 the dispersions of the balance
measurement of pressure and of system’s temperature are most
important to get accurate values of adsorbed masses whereas
the contributions of uncertainties in the compressibility factor (Z), often are
rather small and hence may be neglected.

3.4 Example

Densimetric-gravimetric measurements of binary coadsorption equilibria
have been performed at IFT using the instrument shown in Figs. 4.13, 4.14 in
2001-2002, [4.17]. The system chosen was carbon dioxide methane
and activated carbon (AC) D 47/3 at T = 293 K for pressures up to
1.4 MPa. Gas mixtures were provided by mixing components of purities
delivered by Messer Griesheim,
Düsseldorf. The AC is manufactured by German CarboTech company, Essen.
It is used as nearly cylindrical pellets (length: 3 mm, diameter: 1 mm) having
a bulk density of and a BET-surface of It
mainly is used in industrial adsorption processes for purification of gas
mixtures including either hydrogen or methane.

In Figure 4.15 an example of the kinetics of the coadsorption process of a
gas mixture on AC D47/3 is given. The equilibrium state realized
after 4 hours was at p = 0.71 MPa, T = 293.25 K, and sorptive gas
concentration The upper curve indicates the
increase of the total absolute mass of and adsorbed. As can be seen
from the figure, adsorption occurs rapidly in an initial period of about 5 min.
accompanied by a sharp increase of sorptive’s gas temperature (taken by a
thermocouple immediately below the sorbent sample) of ca. 2.7 K and an
almost instantaneous decrease of gas pressure from its initial value of about
1.1 MPa to 0.73 MPa. Then the uptake of mass slows down showing
relaxational behavior of order (1) within the next 1.5 hours and reaching 99 %
of its equilibrium value after 2 hours. At this time (135 min.) the gas circulator
was shut down. This is reflected in all 3 curves by a tiny fluctuation, most
clearly observable in the (upper) mass uptake curve. The relaxation in the
mass uptake is accompanied by a similar relaxation of the sorptive gas
temperature (lower curve) showing a small secondary decrease after circulator
shut down of about (0.1K).