Methods of Measuring Permeability

Conceptually, measuring the gas permeability of polymeric membranes is simple although fraught with experimental pitfalls related to establishing steady state flow. The experimental parameters are given in eqn [2]. Several methods and apparatus have been developed to conduct permeability measurements. For obtaining the permeation pure gases one

barrers

Figure 8 Relationship between the upper bound slope (n) and kinetic diameter difference of gas pairs ij.

Table 2 Upper bound parameters

P" = kanj

Gas pair (ij)

k (barrer)

n

O2/N2

389 224

- 5.800

H2/CH4

18 500

- 1.2112

co2/ch4

1 073 700

- 2.6264

He/N2

12 500

- 1.0242

Figure 8 Relationship between the upper bound slope (n) and kinetic diameter difference of gas pairs ij.

measurers either the rate of permeability pressure rise (usually from a vacuum) in a constant volume/temperature receiver or the volume of permeate gas at a fixed pressure (Figure 5). Commercially available test cells are known as the volumetric or 'Linde cell' and the manometric or 'Dow' cell. The American Society of Testing and Materials has published a method for measuring gas permeability (ASTM Method D1434-82). It is possible to obtain the permeability coefficient Po and the diffusion coefficient D through a time lag measurement as shown in Figure 6; subsequent calculation of the solubility term though an independent measurement of S is advised.

Measuring the permeability of gas mixtures is more complex. Usually a flow of a pre-blended source is passed over the feed side of the membrane. The steady state permeate flux can be measured by employing an inert gas such as helium to sweep the permeating components into a gas chromatograph, for example, for compositional analysis. Preferably the experiment is conducted such that back-diffusion of the helium sweep is insignificant and where the feed gas composition is not altered by permeation. Additionally, nuances in the flow patterns within test cells not specifically designed for mixed gas experiments can lead to erroneous results.

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Solar Panel Basics

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