Page 335 - Modelling in Transport Phenomena A Conceptual Approach

8.5. MASS TRANSPORT WITH CONVECTION 315

The values of the total molar concentration at z = 0 and z = L are calculated from
Eq. (8.5-36) as

c, = - 1
VB + (PA - VB) XA,
-
- 1 = 10.9 x mol/ cm3
88.7 + (107.8 - 88.7)(0.15)

1
- = 11.2 x mol/cm3
-
88.7 + (107.8 - 88.7)(0.05)
Therefore, the use of Eq. (8.5-38) gives the molar flux of cyclohexane through
benzene layer as

1
- 2.09 x 10-5 (11.2 x 10-~)(1- 0.05) = 1.8 10-7mol~cm2~s
-
(0.15)(107.8) In [ (10.9 x 10-3)(1 - 0.15)
8.5.3 Diffusion With Heterogeneous Chemical Reaction

An ideal gas A diffuses at steady-state in the positive z-direction through a flat
gas film of thickness 6 as shown in Figure 8.37. At z = 6 there is a solid catalytic
surface at which A undergoes a first-order heterogeneous dimerization reaction

As an engineer, we are interested in the determination of the molar flux of species
A in the gas film under steady conditions. The gas composition at z = 0, Le., XA~,
is known.
The conservation statement for species A, &. (8.41), can be written over a
differential volume element of thickness Az as

(8.5-46)

Dividing Eq. (8.546) by A AI and letting Az -+ 0 gives

(8.5-47)

or,
dNA
-
2- NA, = constant (8.5-48)
0
j
dz