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

1.25(Af)
Uflood = (14-203)
3/8 [g(ρL −ρG)] 0.3125 σ 0.1875
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The relationship is dimensionally consistent; any set of consistent
units on the right-hand side yields velocity units on the left-hand side.
It is similar in form to Eq. (14-168) and provides a conceptual framework
for understanding the ultimate distillation column capacity concept.
“Upper Limit” Flooding in Vertical Tubes If, instead of a gas
jet being injected into a liquid as in distillation, the liquid runs down
the walls and the gas moves up the center of the tube, higher velocities
can be achieved than shown by Eq. (14-168) or (14-203). This
application is important in the design of vertical condensers.
Maharudrayya and Jayanti [AIChE J., 48(2), 212–220 (2002)] show
that peak pressure drop in a 25-mm vertical tube occurs at a value
close to that predicted by Eq. (14-168) or (14-203). At this velocity
about 20 percent of the injected liquid is being entrained out the top
of the tube. However, the condition where essentially all liquid was
entrained didn’t occur until a velocity over twice the value estimated
from Eq. (14-168) and Eq. (14-203).
The higher velocities at modest entrainment observed by Maharudrayya
and Jayanti were obtained with special smooth entry of gas
(and exit of liquid) at the bottom of the tube. Hewitt (Handbook of
Heat Exchanger Design, pp. 2.3.2-23, 1992) suggests that these values
should be derated by at least 35 percent for more typical sharp heat
exchanger tube entry. Similar to the smooth entry effect, other data
suggest that countercurrent capacity can be increased by providing
an extension of the tube below the tube sheet, with the bottom of the
extension cut on a steep angle (>60°) to the horizontal. The tapered
extension facilitates drainage of liquid.
An extensive data bank correlated by Diehl and Koppany [Chem. Eng.
Prog. Symp. Ser., 65, 77–83 (1965)] also gave higher allowable entry
velocities than Eq. (14-168) or (14-203). Dielhl and Koppany’s correlation
[Eq. (14-204)] is dimensional, and appears to give a much higher
dependence on σ than the more recent work. However, for many fluids,
σ 0.5 is essentially the same as the combination σ 0.1875 (ρ l − ρ g) 0.3125 that
appears in Eq. (14-203). Hence Eq. (14-204) gives a similar physical
property dependence.
where U f = flooding gas velocity, m/s
F 1 = 1.22 when 3.2 d i/σ>1.0
= 1.22 (3.2 d i/σ) 0.4 when 3.2 d i/σ