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116 3. Heterogeneous Processes and Reactor Analysis

elocities,
bubbles travel through and up the column at high v in the range 1–2 m/s, in near
ubbles,
plug-flow motion. The smaller gas b on the other hand, are entrained within the
liquid recirculation (Krishna et al ., 2000; Krishna, 2000). In general, the transition between
the homogeneous and heterogeneous regime is difficult to be characterized. Furthermore, in
small diameter columns, as the gas vgas b elocity increases, ubbles tend to coalesce to form
slugs, whose diameters can be as lar ge as the column diameter. This regime is called as the
slug-flow regime. Bubble slugs can be observed in columns of diameters lower than 0.15 m
(Shah et al ., 1982). ubble flow (homogeneous flow regime) is used in some applica- While b
tions (gas velocities smaller than 2–3 cm/s), churn-turbulent flow (gas velocities between 10
and 50 cm/s) is of current industrial interest (Dudukovic et al ., 1999).
Koide (1996) recommended that for air–water systems, if D
≥2 ×10 4 m 2 , the flo w can
be considered to be in the heterogeneous regime. In this relationship, D is the column
diameter and the nozzle or hole diameter of the gas distrib gion can The transition re . utor
be defined in terms of gas holdup by using Marrucci’s and Akita–Yoshida equations as pre-
oide, sented in Figure 3.28 (K 1996).

Gas distrib ution
Gas distributors are an integral part of the design and scale-up of bubble columns and
SBCRs. There are numerous types of gas distrib which differ significantly in their
utors,
size and number of orifices. Porous plates, perforated plates (sie mul- e plate/sie v e tray), v
tiple/single-orifice nozzles, bubble caps, perforated rings, annular shears, spider -type,
injectors, and hollow fibers among others, account for the most commonly used spar gers
in bubble and slurry bubble column reactors. Figure 3.29 illustrates some of these gas dis-
tributors. Opening size, number of openings, sparger positioning, and nozzles position/
orientation are the most important characteristics of a gas distrib ws a . Figure 3.30 sho utor
sparger-type gas distributor.
The porous plate usually consists of microsize pores with mean diameters ranging
between 1.7 and 300 µm (Smith et al ., 1996; V ial et al ., 2001; Bouaif i et al ., 2001). The
perforated plate design, hotakpitch, and diameter of the , es into account the number
,
we
v
er
0 25
Marucci
0 20 equation
Transition
region Akita and Yoshida
0 15
equation
g
h
0 10
0 05

0 00
0 0 02 0 04 0 06 0 08 0 1 0 12 0 14 0 16
U sG (m/s)

Figure 3.28 Transition region in two-phase water–air bubble columns (K 1996). oide,