Page 254 - gas transport in porous media

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Chapter 14: Experimental Determination of Transport Parameters
The disadvantage of this cell is: (a) the necessity to monitor composition of the two
outlet gas streams, which requires analytical instruments and (b) the strict requirement
of equal pressure in both cell compartments (even small pressure differences between
compartments can cause large errors due to the intrusion of permeation flow).
14.3.2 Graham Cell

The two-component version of the general Graham’s law (Eq. (14.9)) reads

N d M B
A =− (14.23)
N d M A
B
where M A and M B are molecular weights of A and B. Thus, in binary countercurrent
diffusion the diffusion fluxes are not equimolar as is often assumed. The minus sign in
Eq. (14.23) reflects the opposite directions of molar diffusion flux densities. For gases
with different molecular weight this ratio is far from unity. Thus, the net diffusion
d
d
d
flux, N = N + N , is nonzero. This offers a simple way for determination of flux
A B
d
densities of individual gases simply by following the net diffusion flux, N .
The diffusion cell based on Graham’s law (Figure 14.2) (Valuš and Schneider, 1981,
1985) employs the easily determinable nonzero net molar diffusion flux densities,
d
N (Eqs. (14.1) or (14.2)). The cell design is similar as in the Wicke-Kallenbach
O2
G3 FMC
P
M
D
G2 FMC
V1
V3
V2
O1

G1 FMC
B

Figure 14.2. Graham diffusion cell. G1, G2, G3 pressure cylinders with gases 1, 2, 3; FMC flow-meter-
controllers; O1, O2 gas outlets; D diffusion cell, M metallic disc, P porous pellets, B optical digital
flowmeter; V1, V2, V3 valves