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

hole diameters [Eq. (14-163)]. Lack of the drip tubes eliminates the
possible in-tube boiling issue (above).
Multistage orifice trough distributors (Fig. 14-69f) also attempt to
provide good irrigation at low liquid rates without resorting to plugging-prone
small holes. The primary stage uses fewer irrigation points.
Liquid from the primary stage is further split at the secondary stage.
The secondary stage is of small size, so leveling and small flow variations
are not of great concern. The secondary stage may use the same
or a different liquid splitting principle as the primary stage. Even
short layers of structured packings have been used as a secondary distribution
stage.
Notched trough distributors (Fig. 14-69g) consist of parallel
troughs with side V notches. This distributor obeys the triangular
notch equation instead of the orifice equation, which makes the
flow proportional to h 2.5 [instead of h 0.5 in Eq. (14-163)]. This high
power renders the distributor highly sensitive to out-of-levelness and
EQUIPMENT FOR DISTILLATION AND GAS ABSORPTION: PACKED COLUMNS 14-75
(a) (b)
hydraulic gradients and makes it difficult to incorporate a large number
of distribution points. Since the liquid issues sideways, it is difficult
to predict where the liquid will hit the packings. Baffles are
sometimes used to direct the liquid downward. Overall, the quality of
distribution is inferior to that of orifice distributors, making notchedtrough
distributors unpopular. Their strength is their insensitivity to
fouling and corrosive environments and their ability to handle high
liquid rates at good turndown.
With any trough distributor, and especially those with V notches,
excessive hydraulic gradients must be avoided. This is often achieved
by using more parting boxes.
The hydraulic gradient is highest where the liquid enters the
troughs, approaching zero at the end of the trough. The hydraulic gradient
(between entry point and trough end) can be calculated from
[Moore and Rukovena, Chemical Plants and Processing (European
ed.), p. 11, August 1987]
(c) (d)
FIG. 14-69 Gravity liquid distributors. (a) Orifice pan. (b) Orifice tunnel. (c) Orifice tube, using external drip tubes. (d) Orifice trough, using internal drip tubes.
(e) Splash plate orifice trough. (f) Two-stage orifice trough. (g) Notched trough. (Parts a–c, g, courtesy of Koch-Glitsch LP; parts d–f, courtesy of Sulzer Chemtech.)