Page 165 - Applied Process Design For Chemical And Petrochemical Plants Volume II

I54 Applied Process Design for Chemical and Petrochemical Plants

Table 8-16
General Purpose Distillation Trays Using 3%~ I.D. Pressed Metal Bell Caps on 5g-h. Centers

~~~
Inside Diameter 2 ft-6 in. 3 ft-0 in. 3 ft-6 in. 4 ft-0 in. 5 ft-0 in. 6 ft-0 in. 6 ft-6 in. 7 ft- Oh. 8 ft-0 in.
_-- - -- - -__ _ - __ __ - __ - __
-.
Total Area, ft2 4.91 7.07 9.62 12.56 19.63 28.28 33.18 38.48 50.26
Liquid Flow Cross Cross Cross Cross Cross Cross Cross Cross Cross
No. Downflow Weirs (1) One One One One One One One One One
No. Downflow Seals (2) One One One One One One One One One
No. Caps & Risers/Tray 18 27 37 51 79 129 130 173 223
No. Rows/Tray 4 5 6 7 9 11 12 13 13
Total Slot Area, ft2 (3) 1.15 1.73 2.38 3.27 5.08 8.29 9.64 11.1 14.29
Percent of Tower Area 23.4 24.5 24.8 26.0 25.9 29.3 29.1 28.9 28.50
Total Riser Area, ft2 (4) 0.683 1.04 1.42 1.96 3.00 4.95 5.36 6.64 8.57
Percent of Tower Area 13.9 14.7 14.8 13.6 15.3 17.5 16.75 17.25 17.05
Overflow Weir Length, ft (1) 1.625 2.05 2.35 2.686 3.35 4.0 4.31 4.62 5.21
Percent of Tower Diameter 65.0 68.0 67.1 67.0 67.0 66.7 66.0 66.05 65.1
Downflow Segment Area (5)
Maximum Area, ft2 0.338 0.642 0.80 0.96 1.41 2.12 2.39 2.85 3.50
Minimum Area, ft2 0.167 0.225 0.33 0.45 0.69 0.886 1.027 1.14 1.47
Under Flow Clearance, in. 2% 2% 2% 2% 2% 2% 3 3 3
Under Flow Area, ft2 0.274 0.354 0.426 0.473 0.604 0.710 0.750 0.887 1.01
Up Flow Area,
0.352
0.795
0.335
Minimum ft2 (6) 0.137 0.365 _-__ - __- _ - 0.740 0.882 1 .ooo 1.21
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(1) 2% in. minimum height above tray floor, with adjustable weir 0 to 3% in. additional.
(2) 3% in. high above tray floor.
(3) Slots are 50-M in. x 1% in. per cap. Caps on triangular pitch.
(4) Riser inside diameter = 2.68 in.
(5) Design uses tapered segmental downcomer and inlet weir.
(6) Area between downcomer and inlet weir for upflow of inlet liquid. All trays included access manway.
(text continued from page 135) and minus area of segmental or other downcomer
at outlet of tray, ft2
p = liquid density, grams/cc at liquid temperature in
resulting low tray efficiencies. Table &15 gives recommen- tower
dations for layout. . p = viscosity of liquid at tower temperature, centipoise
dh = weep hole diameter, in. Note that this is the diame-
Liquid Drainage or Weep Holes ter equivalent to the area of all the weep holes/tray
h' = height of overflow weir or bubble cap riser,
Holes for drainage must be adequate to drain the col-
whichever is smaller, in.
umn in a reasonable time, yet not too large to interfere N = total number of actual trays in tower
with tray action. Draining of the column through the trays
is necessary before any internal maintenance can be start- The accuracy of this relation is given as 14% maximum,
ed or before fluid services can be changed, when mixing is 6% average.
not desirable. The majority of holes are placed adjacent to A general recommendation [5] is to provide four
the outlet or downcomer weir of the tray. However, some square inches of weep hole area per 100 ft2 of net open
holes are placed in the downcomer inlet area or any sus- liquid tray area in the tower. This latter refers to the total
pected low point in the mechanical layout of the column. of all trays in the tower.
The study of Broaddus et al. [7] can be used to develop
the following drainage time relation, and is based on flu- Bottom Tray Seal Pan
ids of several different densities and viscosities:
The bottom tray of a tower must have its downcomer
e= (0.18 N + 0.15) (p)'.'' (An ) (8- 221) sealed to prevent upflow of reboiled vapors. The down-
(p)lI4 (dh / h')'.' comer of this tray is usually equal to or 6 in. longer than
the other downcomers to ensure against bottom vapor
where 8 = time to drain tower, minutes surges or pulses in pressure breaking the seal. The sed
An = net open liquid area of one tray, equal to total pan is designed to avoid liquid back pressure and mini-
tower cross section minus area occupied by caps mum restrictions to liquid flow.