Adsorption - Introduction to Carbon Capture and Sequestration - page 252

Optimal performance
From a research point of view an important question is: what is the ideal
adsorbent? There are a few obvious criteria — the material needs to be
selective toward CO 2 and the capacity needs to be reasonable. We have
seen that several additional parameters, such as working capacity and
heat of adsorption, come into play. This makes the selection of an opti-
mal adsorbent complicated. For example, one might focus on research
that improves the selectivity of a material for CO 2 , but different materials
may perform optimally at very different conditions. To address this prob-
lem, we propose to use the optimal parasitic energy as a metric to com-
pare the performance of different materials. The idea is to determine the
optimal performance, i.e., the lowest parasitic energy, for a given material
and then to compare this optimum for different materials. In the remain-
der of this section, we provide an example of the use of parasitic energy
as a metric to rank different materials for their performance in carbon
capture by adsorption.
In Figure 6.4.1 we schematically show how the working capacity of
a material depends on the Henry coeffi cient. For materials with a small
Henry coeffi cient, we expect a poor performance. The working capacity
is small, yet the entire system needs to be heated to the desorption con-
ditions, giving a high parasitic energy. In addition, for these materials the
adsorption of CO 2 is of the same order of magnitude as the adsorption of
N 2 and hence the selectivity of such a material is unusably low. Materials
with a larger Henry coeffi cient have a signifi cantly larger working capacity
and correspondingly lower parasitic energy. This trend continues until the
Henry coeffi cient of the material is so large that at fl ue gas conditions the
partial pressure is too high for the CO 2 adsorption to be in the linear
regime. Figure 6.4.1 shows that at these conditions the CO 2 loading at
the adsorbed state is not fully determined by a simple Henry coeffi cient,
and that materials with the same Henry coeffi cient have different working
capacities depending on the pore volume. Figure 6.4.1 illustrates that at
even larger Henry coeffi cients the adsorption of CO 2 becomes so strong
that it becomes increasingly diffi cult to regenerate the material. From this
analysis of the Henry coeffi cient, we hence expect to fi nd an optimal
Henry coeffi cient: not too big, not too small. Let us see whether we can
confi rm this by experiments.