Page 409 - Modelling in Transport Phenomena A Conceptual Approach

9.5. MASS TRANSFER WITH CONVECTION 389

Equation (9.5-27) can be expressed in terms of dimensionless numbers with the
help of Eq. (3.4-6). The result is

Sh
(kc)
StM = - --
-
ReSc (vz)
(9.528)
The use of Eq. (9.528) in Eq. (9.527) gives

(9.529)

As an engineer, we are interested in the rate of moles of species A transferred
to the fluid, i.e.,

Substitution of Eq. (9.5-25) into Eq. (9.530) results in

r 1

(9.531)

Note that Eq. (9.531) can be expressed in the form

nA = AM((IC,)(ACA)ch = (rDL)(kc) (ACA)LM (9.532)

which is identical with Eqs. (3.3-7) and (4.5-34).

Constant wall mass flux

Consider a circular pipe with a porous wall. If species A were forced through the
porous wall at a specified rate per unit area, then the molar flux of species A on
the pipe surface remains constant, i.e.,

NA~(~=~ = NA, = constant (9.533)
DAB
=
Substitution of Eq. (9.533) into Eq. (9.519) gives

(9.534)