R. B. Evans, Ill and 1. D. Love
Oak Ridge National Laboratory Oak Ridge, Tennessee 37830 and E. A. Mason Brown Universitv Providence, Rhode Island 02912
I II
I
Graham's Laws:
Simple Demonstrations
of Gases in Motion fed 11.
I n a previous paper (I) we presented a simple kinetic-theory exposition of the three different aspects of gas transport as embodied in Graham's studies of diffusion, effusion, and transpiration. I n this paper we present descriptions of two simple experiments on the flow and diffusion of gases in a porous medium, which illustrate the following points: (1) The three aspects of gas transport, and their occurrence together in each of the two experiments. (2) The analysis of rate experiments to obtaindesired rate coefficients, in a manner quite analogous to work in chemical kinetics. One of the present experiments is first order, but the other is more complicated and contains a logarithmic tenn. (3) Simple methods for characterizing a porous medium with respect to gas transport, which have some advantages over those in current use. This has practical applications for studies of such things as porous catalysts and nuclear fuel elements. The heart of the apparatus is a Graham ''diffusion tube" constructed from a small fritted-glass Biicher funnel fused to the graduated portion of a buret. All experiments were performed with the same diffusion tuhe used by Mason and Icronstadt (2),which by now may well be one of the most experimented-upon pieces of porous medium in existence. The frit had approximately a 3-cm diameter and 0.25-cm thickness with a nominal maximum pore size of 0.9-1.4 p. It is interesting that a common frit of ultrafine porosity a t 1 atm gas pressure gives results that are squarely in the so-called transition regime, where the gas can be described neither as a continuum fluid nor as a collection of completely independent molecules, but has some characteristics of each. We describe Graham's diffusion experiment first, since most of the interesting points can be illustrated even if it is assumed (erroneously) that the diffusion occurs entirely in the continuum regime. Correction for the transition regime requires knowledge of the Ihudsen diffusion coefficients, which are best found by permeability or transpiration measurements. These experiments are of considerable interest in their own right, but we feel the diffusion experiment has more kinetic-theory content.
Experiments
the buret was noted every few minutes. The pressure difference across the porous frit was kept zero by maintaining the water levels equal on the inside and outside of the buret. The gas on both sides of the frit was kept saturated with water vapor. A piece of dampened filter paper, placed above the frit, kept the outside air saturated; the method of filling the diffusion tuhe saturated the gas inside. The porosity/tortuosity ratio r/q will change if water contacts the frit. However, a sojourn in the drying oven readily restores the r/q to its original value.
RUBBER TUBING
- BURET HOSE CLAMP
Uniform-Pressure Diffusion
The arrangement used for Graham's diffusionexperiment is shown in Figure 1. Water was drawn up to the zero mark of the buret by pumping on the top of the funnel with an aspirator. Gas was then introduced by bubbling it in from the bottom, and the volume of gas in
Figure 1 . Diffusion tube and auxiliary equipment used to perform Graham's diffusion experiment. Gas is odded to h e buret through a piece of bent tubing. The inverted flask server as a water reservoir to mointain @qua1weter levels in the cylinder and buret.
Volume 46, Number 7, July 1969
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