Packed Bed flooding.pdf - Youngstown State University's Personal ...

FIG. 14-97 Installation and dimensions of a tube stirrer: h/d = 1; H/D ≈ 1; D/δ = 10; A = 1.5 d w 2 ; D/dN = 10;
d/d N = 3; d/d ri = 7.5; d/d ra = 6. [Zlokarnik, Ullman’s Encyclopedia of Industrial Chemistry, Sec. 25, VCH, Weinheim,
Germany, 1988.]
guidelines are given as to what equipment might be feasible and what
equipment might prove most economical.
For producing foam for foam-separation processes, perforated-plate or
porous-plate spargers are normally used. Mechanical agitators are often
not effective in the light foams needed in foam fractionation. Dissolvedair
flotation, based on the release of a pressurized flow in which oxygen
was dissolved, has been shown to be effective for particulate removal
when sparged air failed because the bubbles formed upon precipitation
are smaller—down to 80 µm—than bubbles possible with sparging, typically
1000 µm [Grieves and Ettelt, AIChE J., 13, 1167 (1967)]. Mechanically
agitated surface aerators such as the Wemco-Fagergren flotation
unit (Fig. 14-102) are used extensively for ore flotation.
To produce foam in batch processes, mechanical agitators are used
almost exclusively. The gas can either be introduced through the free
surface by the entraining action of the impeller or alternatively
sparged beneath the impeller. In such batch operation, the liquid level
gradually rises as the foam is generated; thus, squatly impellers such as
turbines are rapidly covered with foam and must almost always be
sparged from below. Tall impellers such as wire whips (Fig. 14-103)
are especially well suited to entrain gas from the vapor space. Intermeshing
wire whips are standard kitchen utensils for producing
foamed meringues, consisting of air, vegetable oil, and egg whites. For
a new application, generally some experimentation with different
impellers is necessary in order to get the desired fine final bubble size
without getting frothing over initially. For producing foams continually,
an aspirating venturi nozzle and restrictions in pipes such as baffles
and metal gauzes are generally most economical.
For gas absorption, the equipment possibilities are generally packed
columns; plate distillation towers, possibly with mechanical agitation
on every plate; deep-bed contactors (bubble columns or sparged
lagoons); and mechanically agitated vessels or lagoons. Packed towers
and plate distillation columns are discussed elsewhere. Generally these
FIG. 14-98 The Cyclox surface aerator. (Cleveland Mixer Co.)
PHASE DISPERSION 14-107
devices are used when a relatively large number of stages (more than
two or three) is required to achieve the desired result practically.
The volumetric mass-transfer coefficients and heights of transfer
units for bubble columns and packed towers have been compared for
absorption of CO2 into water by Houghton et al. [Chem. Eng. Sci., 7,
26 (1957)]. The bubble column will tolerate much higher vapor velocities,
and in the overlapping region (superficial gas velocities of 0.9 to
1.8 cm/s), the bubble column has about three times higher masstransfer
coefficient and about 3 times greater height of transfer unit.
The liquid in a bubble column is, for practical purposes, quite well
mixed; thus, chemical reactions and component separations requiring
significant plug flow of the liquid cannot be effected with bubble
columns. Bubble columns and agitated vessels are the ideal equipment
for processes in which the fraction of gas absorbed need not be
great, possibly the gas can be recycled, and the liquid can or should be
well mixed. The gas phase in bubble columns is not nearly so well
back-mixed as the liquid, and often plug flow of the gas is a logical
assumption, but in agitated vessels the gas phase is also well mixed.
The choice of a bubble column or an agitated vessel depends primarily
on the solubility of the gas in the liquid, the corrosiveness of the
liquid (often a gas compressor can be made of inexpensive material,
whereas a mechanical agitator may have to be made of exotic, expensive
materials), and the rate of chemical reaction as compared with the
mass-transfer rate. Bubble columns and agitated vessels are seldom
used for gas absorption except in chemical reactors. As a general rule,
if the overall reaction rate is five times greater than the mass-transfer
rate in a simple bubble column, a mechanical agitator will be most
economical unless the mechanical agitator would have to be made
from considerably more expensive material than the gas compressor.
In bubble columns and simply sparged lagoons, selecting the
sparger is a very important consideration. In the turbulent regime
(superficial gas velocity greater than 4.6 to 6 cm/s), inexpensive