Page 714 - Bird R.B. Transport phenomena

694 Chapter 22 Interphase Transport in Nonisothermal Mixtures

From this and Eqs. 22.4-40 and 22.4-9, we find that the correct value for k° /K? is
xm
xm
(22.4-44)

whereas the approximate value from Eq. 22.4-13 is
A: 0
• = 1 + • = 11 (22.4-45)
Thus the maldistribution of the liquid-phase mass transfer coefficient halves the rate of mass
transfer, even though the liquid phase resistance "on the average" is very low. The general
unavailability of such detailed information is one more reason for the uncertainty in predict-
ing the behavior of complex contactors.

§22.5 MASS TRANSFER AND CHEMICAL REACTIONS
Many mass transfer operations are accompanied by chemical reactions, and the reaction
kinetics can have a profound effect on transport rates. Important examples include ab-
sorption of reactive gases and reactive distillation. There are two situations of particular
interest:
(i) Absorption of a sparingly soluble substance A into a phase containing a second
reactant В in large concentration. Absorption of carbon dioxide into NaOH or
amine solutions is an industrially important example, and here the reaction
may be considered pseudo-first-order because reactant В is present in great
excess:
R = - = -K'c M (22.5-1)
A 1
An example of this type of problem was given in §18.4.
(ii) Absorption of a rapidly reacting solute A into a solution of B. Here to a first ap-
proximation it may be assumed that the two species react so rapidly that they
cannot coexist. An illustration of this was given in Example 20.1-2.
We shall be particularly interested in liquid boundary layers, and heat-of-reaction
effects tend to be modest because the ratio of Sc to Pr is usually very large. Macroscopic
heating effects do occur, and these are discussed in Chapter 23. Here we limit ourselves
to a few illustrative examples showing how one can use models of absorption with
chemical reaction to predict the performance of operating equipment. 1

EXAMPLE 22.5-1 Mass transfer measurements with irreversible first-order reaction have often been used to es-
timate interfacial area in complex mass transfer equipment. Show here how this method can
Estimation of the be justified.
Interfacial Area in a
Packed Column SOLUTION
The system we consider here is the absorption of carbon dioxide into a caustic solution, which
is limited by hydration of dissolved CO according to the reaction
2
CO (aq) + H O +± H CO (22.5-2)
2 2 2 3

1
T. K. Sherwood, R. L. Pigford, and С R. Wilke, Mass Transfer, McGraw-Hill, New York (1975),
Chapter 8.