Membranes - Introduction to Carbon Capture and Sequestration - page 291

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(a) (b)

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(c)

Figure 7.2.4 Diffusion mechanisms
(a) Molecular sieving : the pores are suffi ciently small that only one component can
pass.
(b) Knudsen diffusion : molecules with smaller mass have a higher average velocity and
move faster through the membrane.
(c) Diffusion solubility : separation is due to a difference in the Henry coeffi cient and
diffusion coeffi cient of the two components in the membrane.

Molecular sieving

The simplest concept of a membrane is a molecule sieve (see
Figure 7.2.4 (a) ); molecules that are bigger than the pore diameter of the
sieve will not pass through. From a practical point of view, the main dis-
advantage is that one needs to use relatively narrow pores. Also, for
those molecules that do pass through the membrane, the fl ux can be