Page 181 - Applied Process Design For Chemical And Petrochemical Plants Volume II

1 70 Applied Process Design for Chen iical and Petrochemical Plants

5. Calculate liquid entrainment, Wre, as pounds of liq- Bolles [3] recommends dynamic slot seals on bubble
uid per square foot of net tray area, equals (Step caps to check against calculated values. See Table 8-18.
3)/(Step 4), lbs/hr (ft2)
6. Calculate vapor velocity, vf, based on area of (Step Inlet Weir
4), ft/sec
The inlet weir, see Figure 8-92, for any tray, i.e., bubble
7. Calculate density factor, [p,/p~ - pJ I 1/2 cap, valve or sieve, is important in ensuring a seal on the
8. Calculate vf [pv/ (pL - p,) 1'12 inlet downcomer as well as maintaining a more uniform
9. From Figure 8-117, read Log We* liquid level across the flowing tray. The recessed seal pan,
10. Calculate S" from Figure 8-63, provides the same benefits plus it reduces
sieve tray leakage on the inlet side of the tray due to lower
Log W*, = Log W', + 2.59 Log Sff + Log p + 0.4 Log (J
immediate liquid head increase usually occurring at the
11. Assume a foam height, hf, of approximately twice the tray weir. It is necessary to drill weep holes for drainage of
height of the dynamic tray seal, in. This agrees with the tray at shut-down in the blocked-off inlet weir area, but
several investigators for mdium foaming systems. limit the number and size of holes to avoid excessive weep
12. Minimum tray spacing St = hf + S', in. drainage during tray operation.

where vf = vapor velocity based on free area above caps (not Bottom Tray Seal Pan
including two downcomers), ft/sec
We* = entrainment corrected for liquid properties and This seal acts like a typical downcomer seal from the tray
plate spacing above, and should be dimensioned approximately the
We = liquid entrainment mass velocity, lbs entrainment same, except:
per minute per/ft2, based on net tray area.
S' = effective tray spacing, distance between top of 1. To avoid liquid backup in the downcomer, provide a
foam, froth, or bubbles, and tray above, in. downcomer height that is about 1.5 times the select-
S" = clear height above foam or froth (equals tray spac- ed tray spacing in the column.
ing minus foam height above tray floor), ft 2. To ensure non-surging liquid flow under the down-
p = viscosity of liquid, centipoise comer, use a clearance, hdcl, of at least 3 times the
o = surface tension of liquid, dynes/cm design for the other trays, or a minimum of 2 in. to 4 in.
W,' = entrainment (based on assumed allowance) lbs 3. Enlarge the clearance between the downcomer face
liquid/(ft2 free plate area) (hr) and the inlet weir (or equivalent), (see Figure 8-92
hf = height of top of foam above tray floor, in.
or 63) to 1.5 to 2 times the dimensions used for the
other trays.
Free Height in Downcomer 4. Provide drainage holes in seal pan to allow adequate
drainage, flushing and cleaning, but not too large
F St + hw- Hd (8 - 246)
(number) to prevent liquid backup sufficient to
maintain a seal on the tray.
Slot Seal
Throw Over Outlet Segmental Weir
The static slot seal is the fixed distance between the top
of the outlet weir and top of the bubble cap slots. To ensure unobstructed vapor passage above the froth
The actual operating or dynamic slot seal is more and liquid in the downcomer from a tray, the liquid mixture
indicative of conditions pertaining to the tray in operation must not throw against the shell wall. The distance of throw
and is [5]: over the weir is given by Reference 5. See Figure 863.
= 0.8 [how (F)] llz, in. (8-248)

hf' = S, + h, - Hd, in. (8 - 249)
Note that this seal varies across the tray, although the
tray design must be such as to make the value of hds near- For center downcomers as in a two-pass design, the
ly the same for each row of caps. throw must be conservatively less than a distance that
In order to ensure good efficiencies and yet a definite would cause the opposing streams entering the same
seal consistent with allowable pressure drops, suggested downcomer to interfere with each other. Sometimes the
values for hds are modified from the references and shown installation of a splash baffle will help avoid conditions
in Table 8-18. leading to flooding and loss of tray efficiency.