Page 595 - Bird R.B. Transport phenomena

Problems 575
Well-mixed
stream of
A diffusing В diffusing BandS
through S through S

(reaction (outer edge Fig. 18B.11. Concentration profiles for dif-
plane) of stagnant fusion with rapid second-order reaction.
liquid film) The concentration of product P neglected.

(Hint: It is necessary to recognize that species A and В both diffuse toward a thin reaction
zone as shown in Fig. 18B.11.)
Answers: (a) N \ I; (b) N \ 1 +
Az z=0
V Az z=0 \
18B.12. A sectioned-cell experiment 9 for measuring gas-phase diffusivity (Fig. 18B.12). Liquid A is
allowed to evaporate through a stagnant gas В at 741 mm Hg total pressure and 25°C. At that
temperature, the vapor pressure of A is known to be 600 mm Hg. After steady state has been

Constant-
temperature bath
Sample ports -
in cell section

Gas manifold with
stream of pure gas В Gas manifold
Screen Screen

Diffusion cell
of diameter 2 cm. Diffusion cell
- Handle for rotating
Four rotatable — alternate cell Rotated —*
sections sections sections

Three stationary - Stationary-
sections sections
Height of liquid surface
Liquid A
added here Q Q
to maintain
constant • Liquid reservoir Liquid reservoir
liquid level
(я) (W
Fig. 18B.12. A sectioned-cell experiment for measuring gas diffusivities. (a) Cell configura-
tion during the approach to steady-state, (b) Cell configuration for gas sampling at the end of
the experiment.

i E. J. Crosby, Experiments in Transport Phenomena, Wiley, New York (1961), Experiment lO.a.