Absorption - Introduction to Carbon Capture and Sequestration - page 168

possible. The other important design consideration is that the driving
force for CO 2 to move from the gas to the solvent is determined by the
difference between the actual concentration and the equilibrium concen-
tration. In the 1930 design, we see that the fl ue gas and the solvent move
in opposite directions; the fl ue gas is injected at the bottom and removed
as exhaust from the top, while the solvent is injected at the top and
removed at the bottom. The importance of countercurrent fl ow com-
pared to concurrent fl ow is further discussed in Box 5.2.1.
On the outside, an absorber almost universally looks like a tower. But
several types of absorber designs differ on the inside in the way they
increase the surface area between the liquid and the gas. The most
straightforward type of absorber is a plate tower (see Figure 5.2.1 ). This
is a cylinder in which plates are placed at regular intervals. A plate can be
any metal disk with small holes that retain the liquid (see Figure 5.2.2 ),
but allow gas to pass through. A packed column looks very similar to a
plate tower from the outside, but rather than plates it is fi lled with material
that looks somewhat like packing material. The idea is for the packing
material to increase the liquid-gas surface when liquid fl ows over it.
Another way of increasing the interface area is to spray small droplets of
liquid in the column. The most important equipment in a spray column
therefore resembles a shower head. In a bubble column , the area is
increased by injecting small bubbles of gas in the bottom of the column.
Let's zoom in to see what's going on in one of the plate tower trays.
Liquid comes down from above via a funnel and then falls down to the
next plate (see Figure 5.2.2(c) ) through the holes. In this system, we have
vapor rising and liquid falling. There are several types of plates with
slightly different formal geometries, such as bubble caps, to facilitate
vapor/liquid mixing, but the most common form is a metal plate with the
holes punched in it and a weir, or dam-like structure, that maintains a thin
layer of liquid on each tray. The idea is to ensure optimal contact between
the solvent and the fl ue gas such that the system is as close to equilib-
rium as possible (see Movie 5.2.1 ).
Now back to our task of designing a separation unit on the scale of
70 × 10 8 g of CO 2 per day of fl ue gas using water as a solvent. For this
we need to learn a little more about absorber design. We need to address
a few questions: what are the dimensions of our plate absorber? How
many plates do we need to install? And how much water will we need?
Of course, if we were to make a design in real life, we would need many
more details. But in our case, answering these elementary questions will