Page 138 - Modelling in Transport Phenomena A Conceptual Approach

118 CHAPTER 4. EVALUATION OF TRANSFER COEFFICIENTS

0.34 5 E 5 0.74
Pr M 0.7
All properties in Eq. (4.66) are evaluated at the average fluid temperature in the
bed.

4.6.2.1 Calculation of the heat transfer rate
Once the average heat transfer coefficient is determined, the rate of heat transfer
is calculated from
p=EGGl (4.68)

where V is the total volume of the packed bed and a, is the packing surface area
per unit volume defined by
6 (1 - E)
a, = - (4.69)
DP
4.6.3 Mass Transfer Correlation
Dwivedi and Upadhyay (1977) proposed a single correlation for both gases and
liquids in packed and fluidized beds in terms of the j-factor as

(4.610)

which is valid for 0.01 5 h;b _< 15,000. The terms j~,, and Re;, in Eq. (4.610)
are defined by
(4.6-11)

and
(4.612)
4.6.3.1 Calculation of the mass transfer rate

Once the average mass transfer coefficient is determined, the rate of mass transfer
of species d, rjz~, is given by

Example 4.21 Instead of wing a naphthalene pipe as in Example 4.17, it is sug-
gested to form a packed bed of porosity 0.45 in a pipe, 2.5 cm in internal diameter,
by wing naphthalene spheres of 5mm in diameter. Pure air at 40°C flows at a
superficial velocity of 9m/s through the bed. Determine the length of the packed
bed required for the average concentration of naphthalene vapor in the air to reach
25% of the satumtion value.