Page 140 - Modelling in Transport Phenomena A Conceptual Approach

120 CHAPTER 4. EVALUATION OF TRANSFER COEFFICIENTS

Substitution of Eqs. (2)-(4) into Eq. (1) and noting that V = (rD2/4)L gives

Note that for a circular pipe, i.e., a, = 4/D, the above equation reduces to Eq. (5)
in Example 4.17.
The interfacial area per unit volume, a,, is calculated from Eq. (4.6-9) as

6 (1 - E)
a, = -
DP

To determine the average mass transfer coefficient from Eq. (4.6-10)) first it is
necessary to calculate the Reynolds number

- (0.005)(9) = 2655
-
16.95 x
Substitution of this value into Eq. (4.6-10) gives

0.765 0.365
€.?M,,I, =
(Re;lb)o’82 -I- (R€&,) 0*386
-
- 0.765 0.365
o.386 = 0.0186
(2655)0.82 + (2655)
in which ejMpb is given by Eq. (4.6-11). Therefore, the average mass transfer
coefficient is
(kc) = 0.0186 -
VO
€ SC213
- (0.0186)(9)
- = 0.2 m/
s
(0.45)(2.56)2/3
The length of the bed is calculated from Eq. (5) as

L=- 9 In(1 - 0.25) = 0.02m
(0.2) (660)

Comment: The use of a packed bed increases the mass transfer area between air
and solid naphthalene. This in turn causes a drastic decrease in the length of the
equipment.