Adsorption - Introduction to Carbon Capture and Sequestration - page 240

pressure cost energy; so in the end, the benefi t of using a material more
effi ciently may be completely lost by the penalty of a higher energy bill.
This issue of working capacity leads us to another important
question: what is the ideal adsorbent for our purposes? In the following
section, we will try to partially answer to this question.

Section 3
Adsorption design

Adsorption thermodynamics

For an adsorption process to work, we need a material that selectively
takes up CO 2 . The adsorption behavior of a material is described by the
adsorption isotherms, which give the amount of CO 2 and N 2 adsorbed as
a function of the partial pressure. As these isotherms play a central role
in the design of an adsorber, we discuss in this section the underlying
thermodynamics of adsorption.
A typical isotherm is shown in Figure 6.3.1 . Initially the adsorption
is proportional to the pressure, following Henry's law as described in
the previous section, but as our material has a limited capacity it will
require increasingly higher pressures to further increase the loading
until saturation. The simplest mathematical formula that describes this
behavior is:

p bp

σ
θ=

()

()

p = ,

1 bp

σ +

max

where p is the (partial) pressure, θ is the fractional occupancy, σ is the
loading (in mol/kg adsorbent) and σ max is the saturation loading. This
equation is a Langmuir isotherm , named after Irving Langmuir, who won
the Nobel prize for his work in 1932. In Box 6.3.1 , we give a simple kinetic