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180 3. Heterogeneous Processes and Reactor Analysis

external area that is wetted by the flowing liquid (Gianetto et al ., 1978). Here, the e xternal
wetting efficiency, defined as the fraction of the external catalyst area that is covered by the
flowing liquid film, is to be used (Burghardt et al ., 1995; W 1996; u, Al-Dahhan et al . 1997).

a LS
f w (3.410)
a u

where a LS is the efe masstransfer surface (liquid–solid interfacial area) per unit v fecti v ol-
ume of reactor and a u the total surface area of the particles per unit volume of reactor. The
part that is not coered by the flowing liquid is coered with a thin film of liquid, fre-
v
v
quently called the gas-coered part (Leung v et al ., 1987). Thus, this part is not completely
“dry”. Furthermore, capillary forces also hold liquid in the pores of the pellet surf ace
alerius exposed directly to the gas phase (V et al ., 1996). This w the reaction also tak ay , es
ilm of the “dry” place in pore openings, or in the liquid thin fpart of the catalyst, where gas
,
and liquid reactants can be found simultaneously the internal volume of the par-
. Finally
ticles has been shown to be completely filled with liquid (Leung et al ., 1987).
f
icienc
In a reactor completely filled with liquid, the wetting efy is 100% or in other ,
words, the external wetting of the catalyst is complete (Bur ghardt et al ., 1995). While it is
true that when a fixed bed is completely filled with liquid wetting is complete (wetting effi-
ciency is unity), the opposite is not true; in a trickle bed, a portion of the bed v oids will be
always occupied by the gas phase. Thus, while in a well-operated trickle bed the wetting
efficiency could be unity, its total liquid holdup based on the v wer w ays lo oid v olume is al
oidage, than the bed v i.e. the bed is neer completely filled with liquid. v
The analysis of partial wetting involves two scales—the bed and the particle size. At the bed
scale, def whereas iciencies in the liquid distributor design are responsible for partial wetting,
at the particle scale, the partial wetting is due to the liquid mass v icient f elocity being insuf
to cover the catalyst particles with a continuous liquid film (Dudukovic et al ., 2002). During
trickle flow, there are regions of nonirrigated, partially irrigated, and completely irrigated cat-
alyst particles. Almost complete wetting is established at high liquid flow rates.
icienc
f
The catalyst wetting efy of the external catalyst surface can be calculated at
atmospheric pressure using the correlation of El-Hisna wi et al . (1981; u, 1996): W
f w 1.617 Re L 0.146 Ga L 0.071 (3.411)

where
d 2 g
3
Ga L p L (3.412)

2
L
The Reynolds number is based on superficial velocity and SI units are used.
In Figure 3.48, the ef liquid density and liquid dynamic viscosity on fect of particle size, ,
wetting efficiency is presented. It is evident that by increasing particle size and liquid den-
sity, and decreasing liquid dynamic viscosity the wetting ef , f y is decreased. icienc