Page 134 - Modelling in Transport Phenomena A Conceptual Approach

114 CHAPTER 4. EVXLUATION OF TRANSFER COEFFICIENTS

c) The mean bulk temperature is (25 + 50)/2 = 37.5"C. At this temperature

k = 628 x W/ m. K and Pr = 4.62

The use of the Dittus-Boelter correlation, Eq. (4.5-26), gives

Nu = 0.023 Re:/5
= 0.023 (64, 548)4/5(4.62)0.4 = 299

Therefore, the average heat transfer coefficient is

= (299) ( 628 ) = 16,257W/m2.K
1.155 x

4.6 FLOW IN PACKED BEDS

The chemical and energy industries deal predominantly with multiphase and mul-
ticomponent systems in which considerable attention is devoted to increasing the
interfacial contact between the phases to enhance property transfers and chemical
reactions at these extended surface interfaces. As a result, packed beds are exten-
sively used in the chemical process industries. Some examples are gas absorption,
catalytic reactors, and deep bed filtration.

4.6.1 Friction Factor Correlations

The friction factor for packed beds, fpb, is defined by

(4.6-1)

where E is the porosity (or, void volume fraction), Dp is the particle diameter, and
vo is the superficial velocity. The superficial velocity is obtained by dividing the
volumetric flow rate to the total cross-sectional area of the bed. Note that the
actual flow area is a fraction of the total cross-sectional area.

Example 4.19 Water Bows through an annulw at a volumetric flow rate of
5m3/ min. The diameters of the inner and the outer pipes are 30cm and 50cm,
respectively. Calculate the superficial velocity.