Page 329 - Modelling in Transport Phenomena A Conceptual Approach

8.5. MASS TRANSPOIET WITH CONVECTION 309

which indicates non-zero convective flux. Using Eq. (8.5-25) in Eq. (8.5-24) results
in
(8.526)

Substitution of Eq. (8.5-26) into Eq. (8.522) and rearrangement gives

(8.5-27)

where c> is the saturation concentration of species A in €3 at r = R in the gas
phase. Carrying out the integrations in Q. (8.5-27) yields

(8.5-28)

Example 8.16 A benzene droplet with a diameter of 8mm is swpended by a wire
in a laboratory. The temperature and pressure are maintained constant at 25°C
and latm, respectively. Estimate the digmion weflcient of benzene in air if the
variation of the droplet diameter as a function of time is recorded as follows:

t D
(min) (mm)
5 7.3
10 6.5
15 5.5
20 4.4
25 2.9

Solution

Physical properties

PA = 879 kg/ m3
For benzene (d)
:
Piat = 94.5mmHg

Assumptions

1. Pseudo-steadpstate behavior.
2. Air is insoluble in the droplet.