Absorption - Introduction to Carbon Capture and Sequestration - page 212

Absorber

Compressor

14 kWh/tonne

22 kWh/tonne

N 2

CO 2

Condenser

34 kWh/tonne

Stripper

Heat exchanger

9 kWh/tonne

25 kWh/tonne

flue gas

Figure 5.7.3 Irreversibility in an absorption/stripping system
Irreversibility in an absorption/stripping system where the values represent ideal lost
work in kWh/tonnes CO 2 . Figure adapted from [5.13].

large. To estimate the minimum lost work, it is therefore important to
have some experience in the design of such equipment to make reason-
able assumptions on the typical performance and dimensions of the
equipment.
The numbers shown in Figure 5.7.3 show how much work we lose
for the different operations because our process is not reversible:

1. The absorber : to keep the absorber from being infi nitely long and still
have a reasonable mass-transfer rate, we need to have a signifi cant
driving force. We lose 14 kWh per tonne here.
2. The heat exchanger : a temperature gradient is the driving force for a
heat exchanger. Because of the temperature gradient we lose work,
typically 25 kWh per tonne.
3. The stripper : here we also cannot have an infi nite number of stages to
have an infi nitely small driving force, and we lose 9 kWh per tonne.
4. The condenser : the condenser requires cooling water and the tem-
perature difference gives a loss of reversibility, which equates to 34
kWh per tonne.
5. The compressor : the losses here are due to increases in temperature
during compression, and are about 22 kWh per tonne.