A Few Words on Flux and Permeability

Before we move on to review results of hydrogen permeation in various systems, we dwell on some terms of flux and permeability. 

The process of permeation through a membrane gives rise to a flux, or permeation. Commonly used units are mol s i and mL min = mb min (STP). In order to obtain (area specific) flux densities we divide by area and get, for eiample, units of mol s i cm 2 or mLn mim cm .  

In traditional membrane science, the permeability is the specific materials property coefficient that relates the pressure or concentration difference over the membrane to the flux density. The permeance is similarly the area-specific coefficient obtained after division by the membrane thickness. Typical units would be mol s i cm Pa for permeability and mol s cm Pa for permeance. 

For dense membranes it is more complicated, since the materials property itself can be pressure dependent, and the flux gets various pressure dependences depending on defect structure. It is therefore common to use the terms permeance and permeability for the flux including the actual pressures involved. Permeance is then the same as flux, typically given in area specific values like flux density, with units like mol s cm 2. Permeability is used for the materials specific - thickness independent - flux density obtained by multiplying by the membrane thickness, and with units of typically mol s cm or mL min cm .  

Alternatively, the permeance and permeability of a dense membrane can be expressed as a coefficient of flux density per unit pressure to a power depending on defect model (for instance, from the examples we have seen in Section 1.4, /i, A, 0, - /2, and - A). We will see examples of several ways of using these units 

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