Absorption - Introduction to Carbon Capture and Sequestration - page 176

Figure 5.2.4 Mass balance over the top of the absorber
The dotted line gives the boundaries for the mass balance over the top and plate P.

the system. Let's look at mass conservation as engineers do, by drawing
a system of boxes around what is entering and leaving the system.
We start at the top of an absorber (see Figure 5.2.4 ), where gas is
exiting the absorption column. Even though this gas contains less CO 2
than when it entered the column, not all of the CO 2 from the original fl ue
gas will have been absorbed. The exhaust vapor will still contain some
amount of CO 2 as it exits into the atmosphere. We will represent this
residual CO 2 fl ux as the mole fraction of CO 2 ( y exh

CO 2 ) times the fl ow rate
n fl ue . Entering the top of the column, we have nearly pure water, with
some residual CO 2 left after the regeneration. We will represent this
residual CO 2 fl ux as the fl ow of the solvent times the mole fraction of CO 2
in the solvent ( n sol x reg

CO 2 ).
If we apply the mass balance to the entire column, we have:

inlet = outlet,

where we have for the inlet:

reg flue

inlet = nx + n y ,

sol flue

CO CO

2 2

and for the outlet:

exh sat

outlet = ny + nx

exh sol

CO CO

2 2