Membranes - Introduction to Carbon Capture and Sequestration - page 323

1000

Upper Bound

100

10

1

0.0001 0.01 1 100 10,000
P(CO 2 ) Barrers

Figure 7.5.2 Robeson plot for CO 2 /N 2
The CO 2 /N 2 selectivity of a membrane versus CO 2 permeation. Each dot represents a
different material. If the permeation is large the selectivity is typically poor. The solid line
is the upper bound found in polymers experimentally. Figure reprinted from Robeson
[7.9], with permission from Elsevier .

To gain more insight into these Robeson plots it is interesting to look
at the plot for the separation of CO 2 and methane (see Figure 7.5.3 ). In
this plot we see that glassy polymers tend to have higher selectivities that
are closer to the upper bound. Glassy polymers are clearly performing
better [7.10]. Why?
First let us look at some simple properties of polymers. Figure 7.5.4
shows a diagram of the specifi c volume (volume/mass) of a polymer as a
function of temperature. Consider this plot carefully. The “occupied vol-
ume” represents the space occupied by the atoms that make up the
polymer. This volume only weakly increases with temperature (the hotter