Page 263 - gas transport in porous media

Šolcová and Schneider
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PERMEATION OF INDIVIDUAL GASES
14.5
2
Transport parameters r ψ and r ψ can be evaluated from permeation measure-
ments (single gas flow-rate under controlled pressure gradient) performed with pure
gases. The corresponding permeation cells can be designed for measurements under
steady state as well under dynamic conditions. To gather sufficient experimental infor-
mation measurements must be repeated for different pressures and different gases.
The steady-state permeation requires control of pressures and determination of flow-
rates at these pressures. The last task is by no means simple. It is therefore reasonable
to use pseudo-stationary or dynamic permeation cells.

14.5.1 Pseudo-Stationary and Dynamic Permeation
Both the pseudo-stationary and dynamic cells (Figure 14.9) consist of upper and
lower compartments and a metallic disc with cylindrical holes filled by pellets of
porous solid, same as in the Wicke-Kallenbach and Graham diffusion cells. The cells
differ mainly in compartment volumes, inlet and outlet gas lines (with valves) and
the attached pressure-/differential pressure-transducers.
The number of pellets in the metallic disc and the volumes of cell compartments
affect the permeation rate. Thus, their change can be used for obtaining pressure
responses compatible with capabilities of the used pressure transducer.

14.5.2 Pseudo-Stationary Cell (Figure 14.9a)

Identical compartment volumes V = V L = V U are the basic precondition. After
evacuation both compartments are filled with permeation gas up to initial pressure
o
P . At run start the gas pressure in the upper cell compartment is increased by a small

6
7 5 7

1 1

4 4
9
3 3
2 2
7 7
8 8

(a) (b)
Figure 14.9. Pseudo-stationary (a) and dynamic (b) permeation cells 1, 2 upper and lower cell compart-
ment, 3 cylindrical porous pellets, 4 metallic disc with holes for pellets, 5 capillary gas inlet, 6 100 cm 3
gas volume, 7 vacuum valves, 8 pressure gauge, 9 differential pressure gauge