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3.7 Three-Phase Fixed Beds: Trickle-Bed and Ped Bubble-Bed Reactors ack 183

eactor Axial dispersion in trickle-bed r s
f
The values of the axial dispersion coeficients in trickle beds are 1/3 – 1/6th those of the
liquid flow alone at the same Reynolds numbers. A correlation by Michell and Furzer is
available (Satterfield, 1975; Perry and Green, 1999):

Re  L  0.7 
2  0.32
Pe    L 2  (3.417)
L    dg p 3 L 

where


2  0.44
0.68 Re 0.8  L 2  ad (3.418)
L 3 up
 dg p 
L
The Reynolds and Peclet numbers are based on the superficial liquid v whereas , elocity d p
and a u are expressed in cm and cm 2 /cm 3 , respecti. For gas-phase dispersions, the
ely
v
Hochman–Effron correlation is a 1975): v ield, ailable (Satterf
Pe Re 1.8 10 0.7 (3.419)
G G
where

0.005 Re L (3.420)

Here, the Reynolds and Peclet numbers are based on the superficial liquid v . This elocity
equation holds for 11 Re G 22 and 5 Re L 80.
Gas-phase dispersions hae also been found to be one or two orders of magnitude less v
,
than in single-phase gas flows. Normally in trickle beds, both phases are substantially in
plug flow (Perry and Green, 1999). According to Satterf the gas-phase disper- ield (1975),
sion is not ordinarily of concern in trickle-bed processing.
In trickle beds, the criterion of Mears can be used (Satterf 1975): ield,

Z 20 n C  
ln  i  (3.421)
d p Pe L C  o 

where C i and C o are the feed and outlet concentrations, respecti and , v ely n , the reaction
order. This criterion gies the minimum v Z / d p ratio required to hold the reactor length
within 5% of that needed for plug flo . w