Page 180 - Applied Process Design For Chemical And Petrochemical Plants Volume II

Distillation 169

The residence time in the downcomers is another crite-
rion of adequate tray spacing.
Residence Time in Downcomers

To provide reasonably adequate time for disengage-
ment of foam and froth from the liquid in the downcom-
er, the total downcomer volume is checked against a min-
imum allowable average residence time of 5 seconds.
For various foaming characteristics of the liquid of the
system, Ester [190] reports a recommendation of W. L
Bolles based on course lectures at the University of New
South Wales and the University of Sydney as follows:

Foaming Residence Time,
Tendency Example Seconds

LOW Low MW hydrocarbons, 3
and alcohols Figure 8-116. Correlation of entrainment for bubble caps. Used by
Medium Medium MW hydrocarbons 4 permission, Bolles, W. L., Pet. Processing, Feb. thru May (1956),
High Mineral oil absorbers 5 using data of Simkin et al. [64].
Very High Amines, glycols 7

Table 8-20 gives suggested downcomer clear liquid
velocities based on relative foaming characteristics of the
fluid on the tray at tray conditions.

Liquid Entrainment from Bubble Cap nays
The work of Simkin, Strand, and Olney [64] correlates
most of the work of other investigators, and can be used
for estimation of probable entrainment from bubble cap
trays as shown in Figure 8-116. It is recommended that
the liquid entrainment for design be limited to 0.10
mols/mol dry vapor.
Eduljee's [19] correlation of literature data appears to
offer a route to evaluating the effect of entrainment on
tray spacing and efficiency. It is suggested as another
check on other methods. Figure 81 17 may be used as rec-
ommended:
Figure 8-1 17. Eduljee's entrainment correlation for bubble caps.
Table 8-20 Used by permission, Eduljee, H. E., British Chemical Engineer, Sept.
Suggested Downcomer Velocities (1 958).
-. - - .- - - - - - - - - - - - - - -
-.
-.
-.
Approximate System Foaming Characteristics
Tray Spacing, Allowable Clear Liquid Velocities, ft/sec 1. Assume or establish a level of acceptable entlain-
In. High Medium Low ment, such as 10 mols liquid/100 mols vapor.
-. __ - - -. ___ - - -
-.
18 0.15-0.2 0.35-0.42 0.45-0.52 2. Determine effect on efficiency by Colburn's relation
24 0.25-0.32 0.48-0.52 0.55460 (see Efficiency section).
0.65-0.70
_-
-.
.-
.-
. . -. . -30 -. - -_ 0.30-0.35 0.48-0.52 - - .- - 3. Calculate total entrainment as pounds of liquid per
-. - ._
Typical Repre Amine, Oil systems Gasoline, hour, based on total vapor flow in tower.
sentative System Glycerine Light Hydro- 4. Determine area of tray above caps (equals tower
carbons area minus area of two downcomers for cross-flow
-
- -. .- - -. .- - - - - - - - - - - -
.
.-
.-
Used by permission, Pet. Processing, Bolles, W. L., Feb. thru May (1956). tower).