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3.7 Three-Phase Fixed Beds: Trickle-Bed and Ped Bubble-Bed Reactors ack 185

Burghardt et al . (1995) studied, among others, the liquid distribution using needle-type
distributors in trickle beds and found that the density of the liquid feed points does ha e v
an important effect on the value of the liquid holdup, and thus on the performance of the
reactor. They concluded that for a density of more than 5000 feeding points per square
meter, the liquid holdup was stabilized.
Another typical liquid distributor, which is sho essentially consists of wn in Figure 3.50,
a single parting box and weir troughs. Liquid loading, gas loading, and the column diam-
eter determine the number of parting boxes and weir troughs.

3.7.4 External Mass transfer

In trickle beds, the gas-to-liquid, k a GL , and liquid-to-particle, k a f LS , coef icients are used f
fg
to represent the effect of the external mass transfer resistances. The interfacial areas a GL
and a LS refer to the efe mass transfer surface per unit volume of empty reactor fecti v . Due
to the fact that the coef f icients k fg and k fL cannot be easily estimated independently from
the corresponding interfacial areas a GL and a LS respectively, by simple experimental tech-
niques, correlations are normally reported for the products k a GL and k a f LS (Smith, 1981).
fg
eactor Gas-liquid mass transfer in trickle-bed r s

The mass transfer coeficient in the gas-to-gas interf f ace k a g GL is ealuated using the fol- v
lowing correlation (F, 1999): ogler

ka 23 (3.424)
2 0.91
gG L
 P G 
20.96   Z   u sG (3.425)

a/m,
where the pressure drop of the gas phase is in kP the superficial gas velocity in m/s,
and the mass transfer coeficient in s f 1 . As in the agitated three-phase mass transfer the ,
f
liquid-side mass transfer coeficient is the most significant coeficient in trickle beds,
f
because the gas phase is often either a nearly pure component, e.g. hydrogenations, or a
slightly soluble gas, e.g. oxidations, in these reactors (Smith, 1981).

Figure 3.50 Liquid distributors with single parting box and weir troughs.