Absorption - Introduction to Carbon Capture and Sequestration - page 169

Box 5.2.1 Concurrent versus countercurrent

In the design of an absorber unit we have two options: we can inject the solvent and
the gas at the same place on the column, or we can inject one at the top and the other
at the bottom. If both are injected in the same place, the gas and the liquid will fl ow in
the same direction and we have concurrent fl ow; otherwise the fl ow is countercurrent .
The mechanics of the problem make it easier for a gas to be injected at the bottom
and the fl uid at the top, as gravity naturally will cause gas to rise and liquid to fall.
However, there is a more compelling reason to favor countercurrent fl ow.
In the countercurrent fl ow, the concentration of the CO 2 in the fl ue gas will
decrease as a function of the height in the column. As the regenerated solvent is
injected at the top, the concentration of CO 2 in the solvent is the lowest at the top. The
concentration profi le shows that the difference between the concentration of CO 2 in
the liquid and gas phases remains large throughout the column. In the concurrent fl ow,
the difference in concentration of CO 2 in the solvent (fresh) and input fl ue gas is large.
But as you go up the absorber, the concentrations of CO 2 in the gas and solvent phase
get closer, so the difference in concentration between the two phases gets increas-
ingly smaller.
Why do such differences matter? A fundamental axiom of mass transport
between phases is that the rate of the mass transfer between phases is proportional
to the concentration differences between the two phases. Simple designs for absorber
plate towers assume that in the bubbling froth above each plate the rate of mass
transfer between gas and liquid phases is rapid — so rapid as to achieve thermody-
namic equilibrium at each plate. By maximizing the concentration differences between
the two phases in all plates in the absorber, the countercurrent design maximizes the
chances that equilibrium is achieved at each plate.

Countercurrent Concurrent

solvent
(regenerated)

solvent
(loaded)

exhaust

exhaust

top

flue gas

flue gas

solvent
(loaded)

solvent
(regenerated)

bottom

concentration