Membranes - Introduction to Carbon Capture and Sequestration - page 290

Table 7.2.1 Permeabilities of gasses for some common polymer membrane
materials

Polymer H 2 He CH 4 N 2 O 2 CO 2

Silicone 940 560 1,370 440 930 4,600

Natural rubber 49 30 29 8.7 24 134

Polycarbonate — 14 0.28 0.26 1.5 6.5

Polyimide 2.3 — 0.007 0.018 0.13 0.41

Permeabilities given in Barrer at 25-30ºC.

Here “cm 3 gas (STP)” represents the quantity of gas that would take up
1 cm 3 at standard temperature and pressure (STP) as calculated via the
ideal gas law, i.e., the molar volume. The “cm thickness” represents the
thickness of the material whose permeability is being measured, and
“cm 2 membrane area” is the surface area of that material. The conversion
to SI units is 1 Barrer 3.348 × 10 -19 kmol m/(m 2 s Pa). In Table 7.2.1 ,

the permeabilities of some common materials are shown.
Let us now consider a mixture of two components. For fl ue gasses
we can assume that the pressure is suffi ciently low that adsorption in the
membrane is still in the Henry regime. In this case the two components
permeate independently and we can use the same formula to describe
the permeation of each component. We do need to replace the pressure
by the partial pressure of the corresponding component, and we use the
permeabilities for the pure components. Also, here we again stress that
the more general case is more complex, the molecules do interact in the
membranes, and understanding the effects of these interactions on the
permeation of the two components is important in the design of a real
membrane.

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Diffusion mechanisms

In one of the following sections, we will look in much more detail at the
molecular aspects of membranes. In this section, we give a brief over-
view of the different types of diffusion mechanisms one can fi nd in the
literature (see Figure 7.2.4 ).