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3.7 Three-Phase Fixed Beds: Trickle-Bed and Ped Bubble-Bed Reactors ack 179

ud 1
Re s p (3.404)
i,m

1
and h e,d is the dynamic liquid holdup based on the total volume of the empty bed (m 3 /m 3 ).
The following equation of Specchia and Baldi, deried for particles of seeral shapes, v v
can be applied to trickle beds for 0.3 < Re L < 300 (Specchia and Baldi, 1977;
Ramachandran and Chaudhari, 1984):

 ad up  0.65
h 3.86 Re Ga 0.545 0.42 (3.405)
v,d L L,m    

where
3
d g ( P Z )
Ga L,m p L L GL (3.406)

2 L

Here, the dynamic liquid holdup (in m 3 /m 3 ) refers to the portion of the void (available) bed
volume that has been occupied by the liquid. There are also correlations for the static
holdup, that is, when the flo w rate is zero after wetting. Dynamic liquid holdup is normally
between 0.03 and 0.25, whereas the static liquid holdup is between 0.01 and 0.05, and for
nonporous catalysts, usually h e,s 0.05 (see Section 3.6.3; Perry and Green, 1999).

ed b k Liquid holdup in pacubble bed r s eactor
The Achwal–Stepanek correlation can be used in a bubble flow regime (Ramachandran
and Chaudhari, 1984):
h
g,e 2 (3.407)
1 72.3
1 u 0.229 0.423 0.5
sL
where
u
sL (3.408)
u 2 sG

and h g,e is the total gas holdup based in the total volume of the empty bed (cm 3 /cm 3 ). Then,
the liquid holdup is gi en by v

h h (3.409)
e,t g,e
In these equations, CGS units should be used.

Wetting efficiency in trickle bed reactors
Two kinds of wetting must be defined for porous catalysts— internal wetting , which is the
amount of internal area wetted by the liquid, and external wetting , which is the amount of the