Page 208 - Applied Process Design For Chemical And Petrochemical Plants Volume II

Distillation 197

Estimated Tower Diameter Total tower area = x (2.5)2/4 = 4.91 ft2
Souders-Brown method Percent of tower area = 0.492 (100)/4.91 = 10.01%

Using Figure 8100 for segmental downcomers, at 10%
downcomer area, the weir length is '72.8% of tower diameter.
From Figure 8-82 C = 100 for %in. tray spacing
In this case rates are close and pv does not change much Weir length = 72.8 (30)/100 = 21.8 in.
from bottom to top of tower.
Since standard details for fabrication are already available
W = 100 [0.674 (85 - 0.674)]1/2 = 753 lbs/hr (ft2) for a tray with a 19.5-in. weir in a 30-in. tower (65% of dia.),
try this as first tray examined. This is 6.8% of tower cross-
= 753 = 0.36 fts/sec (ft2) sectional area. Downcomer area = 0.068 (4.91) = 0.334 f9.
(3600) (0.582)
Hole Size
Tower cross-section area = 5.23/0.36 = 14.3 ft2
Try %in. dia. on %in. pitch
Diameter = [ (4/X) (14.5)]'12 = 4.28 ft
This is spacing of 2.66 do, and is as close as good design
Using Hunt equation: would suggest. Use %in. tray thickness.
Ratio do/c = %e/% = % = 0.375
Assume: h, -1- h,,+ = 1.5 in. Percent hole area = 12.8% (of perforation area only) as
shown in Figure 8-143.
S' = St - 2.3 h, = 9 - 2.5 (1.5) = 5.25 in.
Minimum Hole Trelocity: Weeping
At surface tension = 20 dyties/cm,
Assume: v, (P,)'/~ = 13
For e, = 3% = 0.0.5
Assume: Submergence = 1.5-in. = h,l = hdl (neglecting A/2)
Figure 8-121 reads: allowable tower velocity = 2.2 ft/sec
Dry Tray Pressure drop, hdt
Required tower area = 3.38/2.2 = 2.54 f$ (bottom, largest)

Diameter = [(4/x) (2.54)I1I2 = 1.8 ft

Select: tower diameter = 2.5 ft
The hole diam./tray thickness ratio = %fi/X = 1.5
From Figure 8128, orifice coefficient, C, = 0.78, p = 0.128
A 2-ft tower would be expected to perform satisfactorily
with properly designed trays. However, a 2.5-ft tower is the
minimum diameter suitable for internal inspection and hdt = 0.003 (13)2 (62.3/85) (1 - (0.128y) = 0.608 in. liquid
maintenance. The cost of a tray tower of 2.5-ft has been (0.78f2
found to be no more, and from some bids 5 percent less,
than the smaller 2-ft. tower. A 2-ft. tower would either be Effective head
used with packing or with trays inserted from the top on For h,l = 1.5, F, < 14
rods with spacers. This would allow removal of the trays for Read Figure 8-130; effective head = 1.58 in. liquid
inspection and maintenance.
Total Wet Tray Pressure Drop
Tray Layout Eased on 2.59 Diameter Tower
ht = 0.608 + 1.38 = 2.188 in. liquid
Use a segmental downcomer on a cross-flow tray.
From the residence time in downcomers for bubble cap Weep Point
trays, and at the very low tray spacing of 9 inches, select an
allowable liquid velocity of 0.1 ft/sec. Using Figure 8-131 Curve A, and h, = 2.19 in. liquid
Read weep point velocity = 12.5 = vom (P,,)'/~
22.1 gpm Curve A is used when in doubt, and it gives a higher
Downcomer area = = 0.492 ft2
7.48 (60) (0.1) minimum yo,, which is on safer side for design.