Applications of Chemical Kinetics in Environmental Systems - Elements of Environmental Engineering: Thermodynamics and Kinetics - page 284

The above rate is plotted in Figure 6.10. There are two distinct regions. In the first, the
rateisproportionalto (u/D p ) 1 / 2 andiscalledthe diffusion-limitedregion. Inthisregion,
r increases with increasing D p and decreasing u . In the second region, r is independent
of (u/D p ) 1 / 2 and is called the reaction-limited region. Diffusion-limited reactions are
important in several important environmental processes.

6.1.4.1 Kinetics and Transport at Fluid-Fluid Interfaces

Consider a gas and a liquid in contact (Figure 6.11). Let us consider a component i
from the gas phase that exchanges with the liquid phase. This component can either
be reactive in the liquid phase or not. Let us first consider the base case where there
is no reaction in either phase. The two bulk fluids are completely mixed so that
the concentration of species is C G i and C L i . Note that we can also represent the
gas-phase composition by a partial pressure P i . Mixing and turbulence in the bulk
phase quickly disperse the species in the solution. However, near the interface on
both sides there is insignificant mixing (low turbulence). Hence, diffusion through
the interfacial films limits mass transfer. The interface, however, is at equilibrium and
air-water partitioning equilibrium applies so that

C int K aw C int

G i = L i .

(6.46)

Interface
(at equilibrium)

C G ∞

P i

int

C Gi

Bulk gas
(mixed)

Bulk liquid
(mixed)

int

C Li

C L ∞

Gas film

Liquid film

FIGURE 6.11 Schematic of the two-film theory of mass transfer for transfer of a solute from
the gas to the liquid, namely absorption. Note that equilibrium exists only at the arbitrary
dividing plane called the interface. Mixing is complete in both bulk phases at distances away
from the interface.