Environmental Engineering Reference
In-Depth Information
valves. The apparatus that employs this technology is called a simulated
moving bed ( Figure 6.2.2 ).
Temperature and pressure swing adsorption
Let us now look at how we can operate a fi xed bed adsorption column.
As in absorption, the key thermodynamic property is the amount of cap-
tured CO 2 , in this case the amount that adheres to the adsorbent. This is
given by adsorption isotherms. We will discuss the thermodynamics of
adsorption in detail in a moment. For the time being, we simply assume
that the pressure of the fl ue gas is suffi ciently low such that adsorption
is proportional to the partial pressure of CO 2 (according to Henry's law).
The proportionality constant is called the Henry coeffi cient (see Box 5.2.2 ),
and it is temperature-dependent.
In Figure 6.2.3 , we illustrate temperature and pressure swing
adsorption . We assume that we adsorb our fl ue gasses at 40ºC and that
we have a 14% CO 2 and 86% N 2 mixture at 1 atm. The bottom half of
Figure 6.2.3 shows the amount of CO 2 we adsorb ( σ CO 2 ) for a typical
material for which Henry's law holds. Additionally, we assume that the
material has good selectivity and the adsorption of N 2 is small. As the
total pressure is 1 atm, the partial pressure of CO 2 is 0.14 atm. To desorb
the CO 2 we heat our adsorbent to T fi nal , for which we desorb nearly pure
CO 2 at 1 atm. At this point, the adsorption isotherm shows that there is
still some CO 2 left in the material. The total amount of CO 2 we remove in
an adsorption and desorption cycle is defi ned as the working capacity of
the material. This process is called a temperature swing operation.
An alternative form of operation is pressure swing adsorption. In this
mode we do not change the temperature. Rather, we lower the partial
pressure. This can be done by fl owing a purge gas over the adsorber. In
the purge gas, the partial pressure of CO 2 is very low, therefore most of
the CO 2 will desorb. Of course, we can also change both the pressure
and temperature and use a combined pressure and temperature swing
operation.
The working capacity of a material can be changed by modifying the
operational conditions. For example, if we operate the desorber at a higher
temperature, we will get a larger working capacity. Increasing the working
capacity seems to be a good thing, as we need less adsorbent to capture
the same amount of material. However, both heating and increasing the
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