Absorption - Introduction to Carbon Capture and Sequestration - page 201

gas phase interface liquid phase

CO 2 CO 2 CO 2 +B BCO 2

Step 1:
diffusion

Step 2:
dissolution

Step 3:
diffusion

Step 4:
chemical reaction

Figure 5.4.4 Mass transfer from the gas to liquid phase

As we think about the reaction rate limitation, we can use a similar
approach to the one we used for diffusion limitation. If we construct
another mass balance for our control volume, we get:

()

2

()

dc z d c z

CO CO

2 2

= D

CO

dt

2

2

dz

Due to the fact that we now have a chemical reaction, CO 2 can disappear
in our volume, according to the rate equation:

()

dc z

CO

2

= kc c

rB CO

dt

2

In this rate equation, k r is the reaction rate of CO 2 with B, which we
assume to be irreversible. Substitution in our mass balance equation
gives us:

()

2

dc z

kc CO

rB 2

= c = k c ′ ,

CO CO

2

D

2 2

dz

CO

2

c CO 2 and c CO 2 (L) c CO 2 (see with boundary conditions c CO 2 (0) = =

Figure 5.4.2 ). This equation has as solution:

( ) ( )

()

c z = A exp k z ′ + B exp − k z ′

CO

2