Page 491 - Modelling in Transport Phenomena A Conceptual Approach

PROBLEMS 471

10.4 In Section 10.3.1-2, the number of moles of species A transferred into the
semi-infinite medium, nA, is determined by integrating the molar transfer rate over
time, i.e., l3q. (10.3-32). It is also possible to determine nA from
I"
nA = A (CA - c~,)ds

Show that the substitution of Eq. (10.3-30) into the above equation leads to Eq.
(10.3-32).

10.5 A polymer sheet with the dimensions of 2 x 50 x 50mm is exposed to chlo-
roform vapor at 20°C and 5mmHg. The weight of the polymer sheet is recorded
with the help of a sensitive electrobalance and the following data are obtained:

Time Weight of polymer sheet
(h) ( g)
0 6.0000
54 6.0600
00 6.1200

Assuming that the mass transport of chloroform in the polymer sheet is described
by a Fickian type diffusion process, estimate the diffusion coefficient of chloroform
in the polymer sheet.
(Answer: 1.01 x m2/ s)
10.6 Consider an unsteady-state diffusion of species A through a plane slab with
the following initial and boundary conditions:

at t=O CA=O
at z=O CA =CA,
at a=L CA=O
a) In terms of the dimensionless quantities