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

14-96 EQUIPMENT FOR DISTILLATION, GAS ABSORPTION, PHASE DISPERSION, AND PHASE SEPARATION
FIG. 14-88 Droplet-size distribution for three different types of nozzles. To convert pounds per square inch
gauge to kilopascals, multiply by 6.89; to convert gallons per minute to cubic meters per hour, multiply by
0.227. (Spraying Systems Inc.)
Entrainment due to Gas Bubbling/Jetting through a Liquid
Entrainment generally limits the capacity of distillation trays and is
commonly a concern in vaporizers and evaporators. Fortunately, it is
readily controllable by simple inertial entrainment capture devices
such as wire mesh pads in gravity separators.
In distillation towers, entrainment lowers the tray efficiency, and 1
pound of entrainment per 10 pounds of liquid is sometimes taken as
the limit for acceptable performance. However, the impact of entrainment
on distillation efficiency depends on the relative volatility of the
component being considered. Entrainment has a minor impact on
close separations when the difference between vapor and liquid concentration
is small, but this factor can be dominant for systems where
the liquid concentration is much higher than the vapor in equilibrium
with it (i.e., when a component of the liquid has a very low volatility, as
in an absorber).
As shown by Fig. 14-90, entrainment droplet sizes span a broad
range. The reason for the much larger drop sizes of the upper curve
is the short disengaging space. For this curve, over 99 percent of
the entrainment has a terminal velocity greater than the vapor
velocity. For contrast, in the lower curve the terminal velocity of the
largest particle reported is the same as the vapor velocity. For the
TABLE 14-20 Exponential Dependence of Drop Size on
Different Parameters in Two-Fluid Atomization
Relative Surface Atomizer
velocity tension Gas density 1 + L/G dimension
Jasuja (empirical for small
nozzle)
−0.9 0.45 −0.45 0.5 0.55
El-Shanawany and Lefebvre
(empirical for small nozzle)
−1.2 0.6 −0.7 1 0.40
Tatterson, Dallman, and
Hanratty (pipe flow)
−1 0.5 −0.5 0.5
Power/mass −1.2 0.6 −0.6 0.4 0.4
settling velocity to limit the maximum drop size entrained, at least
0.8 m (30 in) disengaging space is usually required. Note that even
for the lower curve, less than 10 percent of the entrainment is in
drops of less than 50 µm. The coarseness results from the relatively
low power dissipation per mass on distillation trays. This means that
it is relatively easy to remove by a device such as a wire mesh pad.
Over 50 percent is typically captured by the underside of the next
higher tray or by a turn in the piping leaving an evaporator. Conversely,
though small on a mass basis, the smaller drops are
extremely numerous. On a number basis, more than one-half of the
drops in the lower curve are under 5 µm. These can serve as nuclei
for fog condensation in downstream equipment.
Entrainment E is inherent in the bubbling process and can stem
from a variety of sources, as shown by Fig. 14-89. However, the
biggest practical problem is entrainment generated by the kinetic
energy of the flowing vapor rather than the bubbling process. As vapor
velocity approaches the flooding limit [Eq. (14-168)], the entrainment
rises approximately with (velocity) 8 .
Pinczewski and Fell [Trans. Inst. Chem Eng., 55, 46 (1977)] show
that the velocity at which vapor jets onto the tray sets the droplet size,
rather than the superficial tray velocity. The power/mass correlation
predicts an average drop size close to that measured by Pinczewski
and Fell. Combination of this prediction with the estimated fraction of
the droplets entrained gave a relationship for entrainment, Eq. (14-
202). The dependence of entrainment with the eighth power of velocity
even approximates the observed velocity dependence, as flooding
is approached.
constant(velocity)
E = (14-202)
8 (ρG) 4
���
(Af) 3 (ρL −ρG) 2.5 σ 1.5
(Here E is the mass of entrainment per mass of vapor and A f is the
fractional open area on the tray.)
When flooding is defined as the condition that gives E of 1, the
flood velocity is estimated by Eq. (14-203).