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Šolcová and Schneider
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In the transition diffusion region both mechanisms play a role and the contribu-
tion of each mechanism can be seen from the shape of the confidence region.
When the confidence region in the plane ψ vs. r ψ is nearly vertical (e.g., sam-
ples a, b, c) the influence of ψ is quite weak; hence, the contribution of bulk
diffusion is small. When the shape of the confidence region is nearly horizontal
(samples d, g, h) the influence of r ψ is weak. The minor role of Knudsen dif-
fusion mechanism and prevailing role of bulk diffusion is demonstrated. Shapes
of confidence regions for samples in which diffusion in the transition region oper-
ates are combinations of the above limiting shapes; thus, the contribution of each
diffusion mechanism can be qualitatively judged from the confidence region shape
(samples e, f).

14.4 DYNAMIC BINARY DIFFUSION
14.4.1 Gas Chromatography (Dynamic Diffusion)

One of the well-established techniques (Suzuki and Smith, 1975; Haynes, 1988),
the chromatographic method, permits diffusion study under dynamic condition. The
simplicity of the equipment as well as the speed of data acquisition are the main
reasons for the frequent use of these techniques.
Effective diffusivities are evaluated from response (chromatographic) curves of a
columns packed with the tested porous particles. The carrier-gas (C) flows through the
packed column at constant rate and a pulse of another gas (tracer – T) is injected into
the carrier stream at the column inlet. At the column outlet the tracer concentration
is monitored by a suitable detector and the recorded outlet response peak is then
analysed. Several processes occur during the passage of the tracer band through
the column: besides convection and axial dispersion, transport of tracer through the
laminar film around the packing particles takes place, followed by diffusion in the
pore structure and possibly by adsorption (for adsorbable tracers) on the internal
surface of the porous packing.
Porous particles can be packed in the column in two ways (i) a wide bed packed
with particles; the column to particle diameter ratio should be large enough, say
about 1:20, to satisfy conditions for axially dispersed plug-flow, or, (ii) particles
are packed one by one into a column with diameter that is only 10–20% larger than
the particle diameter. This arrangement is known as Single Pellet String Column
(SPSC) (Scott et al., 1974) and it is used usually for spherical or cylindrical porous
particles.

14.4.2 SPSC and Convolution
In SPSC arrangement (see Figure 14.5) high linear carrier-gas velocities – which
suppress the mass transfer resistance of the laminar film around particles and tracer
peak broadening due to axial dispersion – can be easily attained. This is significant
when inert (nonadsorbable) gases are used as tracers, which move through the column