Page 323 - Modelling in Transport Phenomena A Conceptual Approach

8.5. MASS TRANSPORT WITB CONVECTION 303

Figure 8.32 Evaporation from a tapered tank.

4. The entire system is maintained at a constant temperature and pressure, i.e.,
the total molar concentration in the gas phase, c = P/RT, is constant.
5. There is no chemical reaction between species A and B.

If the taper angle is small, mass transport can be considered onedimensional in
the r-direction and the conservation statement for species A, Eq. (8.41), can be
written over a differential volume element of thickness AZ as

(8.57)

(8.5-8)

or,
dE(ANAz 1 =O (8.5-9)
dz
Equation (8.59) indicates that
ANA, = jl~ constant (8.5-10)
=
In a similar way, the rate equation for the conservation of species B leads to
A NB, = constant (8.5-11)

Since species B is insoluble in liquid d, i.e., NB, lz=o = 0, this implies that
NB. =O for OSzSL (8.5-12)

From Table 8.12, the total molar flux of species d is given by

(8.5-13)