Membranes - Introduction to Carbon Capture and Sequestration - page 282

Section 1
Introduction
In the previous chapters we have discussed absorption and adsorption
as mechanisms to capture CO 2 . These technologies are based on differ-
ences in solubility or adsorption of the different components of the fl ue
gas in a solvent or solid material, respectively. An alternative technology
is to use a membrane. Membranes are based on the difference in perme-
abilities of different gasses in a material. The higher the permeability the
higher the fl ux, the number of molecules that fl ow through the membrane
per unit time. Hence, if these permeabilities are different enough and suf-
fi ciently large we can separate the components of the fl ue gas by passing
them through a membrane. The driving force for membrane separation is
the difference in pressure, or more accurately, the difference in chemical
potential between the two sides of the membrane.
In this chapter, we aim to discuss two aspects of membrane separa-
tions. The fi rst aspect is engineering. For many years people have argued
it is useless to consider membranes to separate fl ue gasses. So why a
chapter on membranes? The answer is: very clever engineering.
A second aspect concerns how to design an ideal membrane. We
show that understanding membranes requires knowledge of adsorption
and diffusion. Transport through a membrane requires molecules to
adsorb and diffuse through the material. For this understanding, we go to
the molecular level and see how diffusion and adsorption depend on the
interactions with the materials. This will require us to obtain some under-
standing of how entropy, energy, and chemical potential are related to
the chemical structure of a material.
Membrane separations are used in several applications (see
Figure 7.1.1 ). Reverse osmosis membranes exclude salt from sea water,
which is becoming an important technology in places that lack potable
water. Dialysis is another common application of membranes. Dialysis is
based on a membrane system that is designed to remove salts from the
blood stream.
Another application of membranes is in fi ltration. Here the membrane
acts as a sieve; depending on the size of the objects we would like to
separate, we select a certain well-defi ned pore diameter for the sieve.