Membranes - Introduction to Carbon Capture and Sequestration - page 342

Figure 7.6.6 Effects of changes in the chemistry on the transport and thermodynamic
properties of a membrane
Effects of changes in the chemistry on the diffusion coeffi cient and Henry coeffi cient.
A decrease of U c corresponds to changing the interactions in the cavities (Figure 7.6.5b)
and a decrease of U w corresponds to changing the interactions in the window region
(Figure 7.6.5c) .
At this point, it is important to note that we did not make any
assumptions about the types of molecules that are adsorbed. If we
change the interactions with the walls these changes may have different
effects on, say, CO 2 compared to N 2 . For most materials, CO 2 will have a
higher permeability, and our reference material will have a selectivity that
is larger than one. We assume that we have such control of the chemistry
that our changes will mostly affect the interactions with CO 2 and only
minimally affect the interactions with N 2 . We have seen that to change
the permeation in our model membrane, we can modify the interactions
for the entire material or only for the window region.
If we make the barrier more positive, the diffusion coeffi cient will
decrease. This will also decrease the selectivity and CO 2 permeation.
Decreasing the energy of the barrier gives an increase of our diffusion coef-
fi cient and hence the permeation. If we change the chemistry of both the
cavity and the window, we can increase or decrease the Henry coeffi cient.
Both the selectivity and the CO 2 permeation will increase if we increase the
Henry coeffi cient and similarly both will decrease if we decrease the Henry
coeffi cient. In our model we can change the size of the cavities or the
dimensions of the window. This would give us a new reference material for