Page 139 - Modelling in Transport Phenomena A Conceptual Approach

4.6. FLOW IN PACKED BEDS 119

Solution
Physical properties

Diif3pzion coefficient of naphthalene (A) in air (B) at 40 "C (313 K) :

312
= (0.62 x 10-5) (g) = 6.61 x m2/ s

For air at 40°C (313K) : u = 16.95 x m2/s

The Schmidt number is
sc = -
U
DAB
- 16.95 x = 2.56
-
6.61 x
Assumptions

1. Steady-state conditions prevail.
2, The system is isothermal.

3. The diameter of the naphthalene spheres does not change appreciably.

Analysis
System: Air in the packed bed

Under steady conditions, the consemation statement for naphthalene, species A,
becomes
Rate of moles of A in = Rate of moles of A out (1)
The terms an Eq. (1) are expressed by

Rate of moles of A in = a,V(k,)(AcA)LM (2)
Rate of moles of A out = Q (cAb)OZlt = (7r02/4) vo(~Ab)out (3)

Since the concentration at the surface of the naphthalene spheres is constant, the
expression for (ACA)LM, Eq. (4.5451, becomes