Page 397 - Modelling in Transport Phenomena A Conceptual Approach

9.4. MASS TRANSFER WITHOUT CONVECTION 377

9.4 MASS TRANSFER WITHOUT
CONVECTION

Under steady conditions) the conservation statement for species A is expressed by

Rate of Rate of
( species A in ) - ( speE7iLut ) + ( species A generation ) = 0 (9.41)

In this section we restrict our analysis to cm in which convection is negligible
and mass transfer takes place mainly by diffusion.

9.4.1 Diffusion in a Liquid With Homogeneous Reaction
Gas A dissolves in liquid 8 and diffuses into the liquid phase as shown in Figure
9.14. As it diffuses) species A undergoes an irreversible chemical reaction with
species B to form AB, i.e.,
A+B+AB
The rate of reaction is expressed by

r=kcA
We are interested in the determination of concentration distribution within the
liquid phase and the rate of depletion of species A.

Figure 9.14 Diffusion and reaction in a liquid.

The problem will be analyzed with the following assumptions:

1. Steady-state conditions prevail.

2. The convective flux is negligible with respect to the molecular flux, i.e.,
v; 2( 0.

3. The total concentration is constant, i.e.,