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14-106 EQUIPMENT FOR DISTILLATION, GAS ABSORPTION, PHASE DISPERSION, AND PHASE SEPARATION
TABLE 14-23 Characteristics of Porous Septa
Air-permeability data
Avg. Diaphragm Pressure Air flow,
Avg. % pore thickness, differential, cu ft/
Grade porosity diam. in in water (sq ft)(min)
Alundum porous alumina*
P2220 25 1 2 0.35
P2120 36 60 1 2 2
P260 35 164 1 2 15
P236 34 240 1 2 40
P216 720 1 2 110
National porous carbon†
60 48 33 1 2
45 48 58 1 2 2
25 48 120 1 2 13
Filtros porous silica‡
Extra fine 26.0 55 1.5 2 1–3
Fine 28.8 110 1.5 2 4–8
Medium fine 31.1 130 1.5 2 9–12
Medium 33.7 150 1.5 2 13–20
Medium coarse 33.8 200 1.5 2 21–30
Coarse 34.5 250 1.5 2 31–59
Extra coarse 36.5 300 1.5 2 60–100
Porous plastic§
Teflon 9 0.125 1.38 5
Kel-F 15 0.125 1.38 13
Micro Metallic porous stainless steel§,¶
H 45 5 0.125 1.38 1.8
G 50 10 0.125 1.38 3
F 50 20 0.125 1.38 5
E 50 35 0.125 1.38 18
D 50 65 0.125 1.38 60
C 55 165 0.125 27.7 990
*Data by courtesy of Norton Co., Worcester, Mass. A number of other grades
between the extremes listed are available.
†Data by courtesy of National Carbon Co., Cleveland, Ohio.
‡Data by courtesy of Filtros Inc., East Rochester, N.Y.
§Data by courtesy of Pall Corp., Glen Cove, N.Y.
¶Similar septa made from other metals are available.
In a vessel with baffles extending only halfway to the liquid surface the
optimum impeller submergence increased with agitator speed
because of the vortex formed. At optimum depth, the following correlation
is recommended for larger vessels:
Q = 0.00015(N/10) 2.5 (D/0.1) 4.5 (14-214)
with Q in m 3 /s, N in rps, and D in m. Thus, a typical 3-m-diameter
plant-size vessel, with four-blade pitched impeller (D = 1 m) operating
at 2 rps will give gas dispersion from the headspace into the batch of
Q = 0.00015(2/10) 2.5 (2/0.1) 4.5 = 1.9 m 3 /s (4000 ft 3 /min).
Borema et al. (op. cit.) recommend headspace gas dispersion with
partial baffling for fatty oils hydrogenation in stirred reactors: “The
hydrogen in the head-space of the closed reactor can again be brought
into contact with the liquid by a stirrer under the liquid level, . . .” And
Penney in Paul et al. (loc. cit.) says, “partial baffling can be very effective
to produce a vortex, which can effectively drawdown gas from the
headspace. . . .” However, Middleton et al. in Paul et al. (op. cit) say,
“The simplest self-inducer for an agitated vessel is an impeller located
near the surface, sometimes with the upper part of the baffles removed
so as to encourage the formation of a vortex. This is, however, a sensitive
and unstable arrangement. It is better, although probably more
expensive, to use a self-inducing impeller system in which gas is drawn
down a hollow shaft to the low-pressure region behind the blades of a
suitable, often shrouded impeller. ...Various proprietary designs are
available, such as the Ekato gasjet, Prasair AGR and the Frings Friborator.
. . .” Figure 14-97 illustrates a gas-inducing hollow shaft/hollow
impeller agitator. In many hydrogenation reactors, the impeller just
FIG. 14-96 Pressure drop across porous-carbon diffusers submerged in water
at 70°F. To convert feet per minute to meters per second, multiply by 0.0051; to
convert inches to millimeters, multiply by 25.4; °C = 5 ⁄9 (°F − 32). (National
Carbon Co.)
underneath the free surface has, without any doubt, performed
admirably; consequently, one must consider this fact very carefully
before using a self-inducing impeller.
Gas dispersion through the free surface by mechanical aerators is
commonplace in aerobic waste-treatment lagoons. Surface aerators
are generally of three types: (1) large-diameter flow-speed turbines
operating just below the free surface of the liquid, often pontoonmounted;
(2) small-diameter high-speed (normally motor-speed)
propellers operating in draft tubes, the units of which are always pontoon-mounted;
and (3) hollow-tube turbines (Fig. 14-97). An example
of the turbine type is illustrated in Fig. 14-98 and the propeller
type is illustrated in Fig. 14-99. There are several other styles of the
turbine type; for instance, Mixing Equipment Co., Inc., (www.lightninmixers.com),
uses an unshrouded 45° axial-flow turbine [see Dykman
and Michel, Chem. Eng., 117 (Mar. 10, 1969)], and Infilco
(www.infilcodegremont.com) makes a unit that has a large-diameter
vaned disk operating just below the free surface with a smaller-diameter
submerged-disk turbine for additional solids suspension.
Aeration injectors, like the one shown in Fig. 14-100 by Penberthy
[a division of Houdialle Industries (penberthy-online.com/jet1.asp)],
are used to provide mass transfer in gas-liquid applications, and simple
impingement aerators (Fig. 14-101) are sometimes used for masstransfer
applications.
Equipment Selection Ideally, selection of equipment to produce
a gas-in-liquid dispersion should be made on the basis of a complete
economic analysis. The design engineer and especially the
pilot-plant engineer seldom have sufficient information or time to do
a complete economic analysis. In the following discussion, some
TABLE 14-24
1 in Thick*
Wet Permeability of Alundum Porous Plates
Dry permeability at 2 in Pressure differential Air flow through
of water differential, across wet plate, wet plate,
cu ft/(min)(sq ft) in of water cu ft/(min)(sq ft)
4.3 20.67 2.0
21.77 3.0
22.86 4.0
23.90 5.0
55.0 4.02 1.0
4.14 2.0
4.22 3.0
4.27 4.0
4.30 5.0
*Data by courtesy of Norton Company, Worcester, Mass. To convert inches to
centimeters, multiply by 2.54; to convert feet per minute to meters per second,
multiply by 0.0051.