Absorption - Introduction to Carbon Capture and Sequestration - page 197

gas liquid film bulk liquid

c co 2

c co 2

z=0 z=L

z

Figure 5.4.2 Concentration profi le at the gas liquid interface
C CO 2 ,i is the equilibrium concentration in the fl uid phase and C* CO 2 is the actual (non-
equilibrium) concentration. L is the typical thickness of the interfacial layer.

approximate concentration profi le at the liquid-vapor interface. In this
scenario, we see that the gas phase is well mixed, so the gas concentra-
tion is constant in the gas phase. The liquid phase however is not well
mixed and we see that next to the gas interface the CO 2 concentration is
higher compared to the concentration in the bulk liquid. Because of this
concentration gradient, CO 2 will diffuse from the interface to the bulk.
This diffusion process follows Fick's law , which states that the fl ux (in
molecules per unit area per unit time) is proportional to the gradient in the
concentration:

dc

CO

2

j =− D ,

CO CO

dz

2 2

where D CO 2 is the diffusion coeffi cient of CO 2 in the liquid phase. If this
diffusion coeffi cient is suffi ciently high then the fl ux of CO 2 can keep up
with the fl ux of CO 2 from the fl ue gas. If, however, the diffusion coeffi cient
is small it can be a limiting factor in the process.
To quantify this effect let us consider a mass balance ( Figure 5.4.3 )
on a small volume across the interfacial region:

accumulation = fl ow in - fl ow out