Adsorption - Introduction to Carbon Capture and Sequestration - page 251

to overcome the heat of absorption ( Q des ). The thermal energy require-
ment ( Q ) per unit mass of CO 2 produced ( ∆σ CO 2 ) is given by:
+
∆σ

QQ

sen des

Q = ,

CO

2

with:

( )

QCmT = − T ,

sen p ads des ads

where C p is the specifi c heat capacity of the adsorbent, m ads is the total
mass of the adsorbent, ( T des -T ads ) is the temperature difference between
the adsorption and desorption conditions. We also have:

ads ads ads ads

Q = ∆ h ∆σ + ∆ h ,

NN

∆σ

des

CO CO

2 2 2 2

where ∆σ i is the difference in mass loading for each species between the
beginning and end of regeneration, and ∆ h i is the heat of adsorption for
each species. The loading at various conditions is calculated from the
adsorption isotherms. As with our analysis for absorption , we assume
that this thermal energy is supplied by diverting steam from the power
cycle. Diverting steam effectively imposes a parasitic load on the power
plant that can be represented as the thermal energy requirement dis-
counted by 75%, which represents the typical effi ciency of a turbine,
times the Carnot effi ciency η of the extracted steam. To obtain the para-
sitic energy, we also need to add the energy required for the compres-
sion of CO 2 to 150 bar ( W comp ):

E = 0.75 × Q × η + W

par comp

We see that our expression for the parasitic energy is nearly identical
to the one we derived for the absorption process. Nevertheless there are
important differences. In our liquid absorption process, we can use heat
exchangers such that the heat we supply for the desorption is not all lost.
For solid adsorption, heat exchange is much more diffi cult; therefore we
have to assume that the heat we supply is lost. As a consequence it is
much more important that we choose the material that will yield the low-
est parasitic energy. At this point it is also important to emphasize that
the above estimate is a lower bound on the parasitic energy; a real pro-
cess involves many more operations that cost energy and the actual
parasitic energy will be about 30% higher.