Page 252 - gas transport in porous media

Chapter 14: Experimental Determination of Transport Parameters
249
14.2 TRANSPORT PARAMETERS
The unknown real pore structure makes an a priori determination of transport char-
acteristics unfeasible. The pore structure characteristics relevant to transport in pores
have to be determined experimentally. Two approaches are used in this respect:
(i)texturalanalysisoftheporoussolidand(ii)evaluationofsimpletransportprocesses
taking place in the porous solid in question.
The advantage of textural analysis of the porous solid derives from the wealth of
available experimental methods and evaluation procedures (physical adsorption of
gases, high-pressure mercury porosimetry, etc.). These methods are frequently used,
but they are far from being the best choice; intrusion of a liquid metal into pores or
multilayer physical adsorption and condensation for example, of nitrogen at 77 K
are governed by completely different laws than gas transport (Schneider and Gelbin,
1984).
The relevance of evaluation of transport parameters from simple transport pro-
cesses which take place in the porous solid in question stems from the possibility to
use the same pore-structure model both for evaluation of transport parameters and for
description of the process in question. It is a good idea to use a mass transfer process,
which is similar to the gas transport process under consideration. It is of advantage
to choose for determination of transport parameters a (simple) process that can be
easily followed at near-laboratory conditions, which does not require sophisticated
instrumentation.
Several choices can be made:
(a) Pure counter-current diffusion of binary gas mixtures under steady-state
conditions.
(b) Binary diffusion under dynamic conditions.
(c) Dynamic or steady-state permeation of individual gases.
(d) Combined diffusion and permeation gas transport.
At the same time it is good choice to use inert (i.e., nonadsorbale) gases; this
eliminates transport of adsorbed gas along the surface of pores (surface diffusion) the
nature of which is not very well understood.

14.3 STEADY-STATE BINARY COUNTER-CURRENT DIFFUSION

14.3.1 Wicke-Kallenbach Cell
The classic Wicke-Kallenbach cell (Figure 14.1) (Wicke and Kallenbach, 1941) con-
sists of upper and lower compartment and a metallic disc with cylindrical holes
sandwiched between both compartments. A porous pellet is forced into undersized
rubber tube and the pellet-tubing assemblies are then forced into holes of the metallic
disc. The absence of gaps between porous pellets and rubber tubes as well as between
rubber tubes and metallic surfaces of disc holes can be verified by replacing porous
pellets by identically sized metallic cylinders. One gas flows steadily through one cell
compartment and another gas through the other cell compartment; both chambers are