Page 250 - Gas Adsorption Equilibria

236 Chapter 5

Hence the respective flow fields can be exactly calculated from the Navier
Stokes equations , at least for low frequencies, i. e. laminar flows. This is not
the case for linear oscillations at either low frequencies, i. e. the linear spring
in gas system [3.1], or high frequencies, namely piezo-quartz systems bearing
sorbent samples [5.10] or oscillating rod systems (SETARAM). Also at high
frequencies the state of the sorbate phase may become “transient”, i. e. deviate
considerably from a thermodynamic equilibrium state.

The torsional pendulum of a dense disk moving slowly in a viscous
medium has been developed by J. Kestin and co-workers about 1980 to
measure simultaneously the density and the viscosity of the fluid [5.11]. This
work was based on a couple of papers reporting on oscillating body
viscometers using cups, spheres, cylinders, etc. published already in the
period 1930-1960 [5.12]. Today pendulum viscosimeters are used to measure
the viscosity of both gases, liquids, and even molten metals and salts at
extreme temperatures (Ir(liquid melting state):2,800 K) and pressures
(100 MPa), [5.13-5.15].

The rotational pendulum does not seem to have been used for gas
adsorption measurements prior to 1993 when the respective theory was
developed by one of the authors [5.1], and a pendulum was designed and built
at the IFT, University of Siegen for this purpose [5.2, 5.7]. In this chapter we
are going to describe in brief this method, i. e. give some experimental
information, Sect. 2.1, present an outline of the theory, Sect. 2.2, and give
examples for gas adsorption measurements on activated carbon, i. e. a nearly
rigid sorbent material. In Section 3 we will discuss combined oscillometric –
gravimetric gas sorption measurements. These allow one to determine the
sorption of gases on non-rigid, i.e. swelling sorbent materials like polymers
[5.2, 5.7, 5.8]. In these measurements the mass and the volume of a polymer
sample in which gas has been dissolved can be determined independently
from each other, these data allowing one together with data of temperature
and pressure, to establish a thermal equation of state of the polymer – gas
system. In Section 4 combined oscillometric-volumetric measurements of gas
sorption equilibria are discussed. Pros and cons of these methods will
discussed in Sect. 5 briefly, followed by a list of symbols, Sect. 6, and
references.